Exhumation of the North Atlantic Margin: Timing, Mechanisms and Implications for Petroleum Exploration (Geological Society Special Publication, No. 196)

Exhumation of the North Atlantic Margin : Timing, Mechanisms an d Implications fo r Petroleum Exploratio n

Geological Society Special Publication s Society Book Editors A. J . FLEE T (CHIE F EDITOR ) P. DOYL E F. J . GREGOR Y J. S . GRIFFITH S A. J . HARTLEY R. E . HOLDSWORT H

A. C . MORTO N N. S . ROBIN S M. S . STOKE R J. P . TURNE R

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It is recommended that referenc e to all or part of this boo k shoul d be made i n one of the followin g ways: DORE, A . G. , CARTWRIGHT , J . A. , STOKER , M . S. , TURNER , J . P . & WHITE , N . (eds ) 2002 . Exhumation of th e North Atlantic Margin: Timing, Mechanisms an d Implications for Petroleum Exploration. Geo logical Society , London , Specia l Publications , 196 . BLUNDELL, D . J . 2002 . Cenzoi c inversio n an d uplif t o f souther n Britain. In: DORE , A . G., CARTWRIGHT , J. A. , STOKER , M . S. , TURNER , J . P . & WHITE , N . (eds ) Exhumation o f th e North Atlantic Margin: Timing, Mechanisms an d Implications fo r Petroleum Exploration. Geologica l Society , London , Specia l Publications, 196 , 85-101 .

GEOLOGICAL SOCIET Y SPECIA L PUBLICATION No. 196

Exhumation o f the North Atlantic Margin: Timing, Mechanisms and Implications fo r Petroleu m Exploration EDITED B Y

A. G. DORE Statoil, UK

J. A. CARTWRIGHT Cardiff University , UK

M. S. STOKE R

British Geological Survey, Edinburgh, UK

J. P. TURNE R

University of Birmingham, UK and

N. WHIT E Bullard Laboratories, Cambridge, UK

2002 Published by The Geological Societ y London

THE GEOLOGICAL SOCIETY

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Contents DORE, A. G. , CARTWRIGHT , J. A. , STOKER , M . S. , TURNER , J . P . & WHITE , N. J . Exhumatio n of 1 the North Atlanti c margin : introductio n an d background Mechanisms JONES, S . M. , WHITE , N. , CLARKE , B . J. , ROWLEY , E . & GALLAGHER, K . Presen t an d pas t 1 influence o f the Iceland Plum e on sedimentation

3

ROHRMAN, M. , VA N DE R BEEK , P . A. , VA N DER HILST , R . D . & REEMST , P . Timin g an d 2 mechanisms of North Atlantic Cenozoic uplift : evidenc e fo r mantle upwelling

7

NIELSEN, S . B. , PAULSEN , G . E. , HANSEN , D . L. , GEMMER , L. , CLAUSEN , O . R. , JACOBSEN , B . 4 H., BALLING , N. , HUUSE , M . & GALLAGHER , K . Paleocen e initiatio n o f Cenozoi c uplif t i n Norway

5

GRAVERSEN, O . A structura l transec t betwee n th e centra l Nort h Se a Dom e an d th e Sout h 6 Swedish Dome : Middl e Jurassic-Quaternar y uplift-subsidenc e reversa l an d exhumatio n acros s the eastern Nort h Se a Basin

7

BLUNDELL, D. J. Cenozoic inversio n an d uplif t o f souther n Britain 8

5

Scandinavia, Greenland an d adjacent margi n LlDMAR-BERGSTROM, K. & NASLUND , J. O . Landforms an d uplif t i n Scandinavi a 10

3

HENDRIKS, B . W . H . & ANDRIESSEN , P . A . M . Patter n an d timin g o f th e post-Caledonia n 11 denudation of northern Scandinavi a constraine d by apatite fission-track thermochronology

7

EVANS, D. , McGiVERON , S. , HARRISON , Z. , BRYN , P. & BERG , K . Along-slop e variatio n i n th e 13 late Neogene evolutio n of the mid-Norwegian margin in response to uplift an d tectonism

9

STROEVEN, A . P. , FABEL , D. , HARBOR , J. , HATTESTRAND , C . & KLEMAN , J . Reconstructin g th e 15 erosion histor y o f glaciate d passiv e margins : application s o f i n situ produce d cosmogeni c nuclide technique s

3

CEDERBOM, C . Th e thermotectoni c developmen t o f souther n Swede n durin g Mesozoi c an d 16 Cenozoic tim e

9

JAPSEN, P. , BIDSTRUP , T . & LIDMAR-BERGSTROM , K . Neogen e uplif t an d erosio n o f souther n 18 Scandinavia induced by the rise of the Sout h Swedish Dom e

3

HUUSE, M . Cenozoi c uplif t an d denudatio n o f souther n Norway : insight s fro m th e Nort h Se a 20 Basin

9

FALEIDE, J . L , KYRKJEB0, R., KJENNERUD , T., GABRIELSEN , R. H. , JORDT, H. , FANAVOLL, S. & 23 BJERKE, M . D . Tectoni c impac t o n sedimentar y processe s durin g Cenozoi c evolutio n o f th e northern Nort h Se a and surrounding area s

5

UK, Ireland and adjacent margin HALL, A . & BISHOP , P . Scotland' s denudationa l history : a n integrate d vie w o f erosio n an d 27 sedimentation a t an uplifted passiv e margin

1

ANDERSEN, M . S. , S0RENSEN , A. B. , BOLDREEL, L . O . & NIELSEN, T . Cenozoi c evolutio n o f th e 29 Faroe Platform , comparing denudatio n and depositio n

1

STOKER, M. S. Late Neogen e developmen t o f the UK Atlantic margi n 31

3

GREEN, P . F. , DUDDY , I . R . & HEGARTY , K . A . Quantifyin g exhumatio n fro m apatit e fission - 33 1 track analysi s an d vitrinit e reflectanc e data : precision , accurac y an d lates t result s fro m th e Atlantic margin of NW Europe

WARE, P. D. & TURNER, J . P. Sonic velocity analysi s o f the Tertiar y denudatio n o f the Iris h Se a 35 basin

5

ALLEN, P . A. , BENNETT , S . D. , CUNNINGHAM , M . J . M. , CARTER , A. , GALLAGHER , K. , 37 LAZZARETTI, E. , GALEWSKY , J. , DENSMORE , A . L. , PHILLIPS , W . E . A. , NAYLOR , D . & HACH , C. S. The post-Varisca n thermal an d denudational histor y o f Ireland

1

Implications for petroleum exploration DORE, A . G. , CORCORAN , D . V . & SCOTCHMAN , I . C . Predictio n o f th e hydrocarbo n syste m i n 40 exhumed basins , an d applicatio n t o the NW European margi n

1

PRICE, L . C . Geologica l an d geochemica l consequence s o f basi n exhumation , and commercia l 43 implications

1

PARNELL, J. Diagenesis an d fluid flow in response t o uplift an d exhumation 43

3

CRAMER, B. , SCHLOMER , S . & POELCHAU , H . S . Uplift-relate d hydrocarbo n accumulations : th e 44 release o f natural gas from groundwate r

7

CORCORAN, D . V . & DORE , A . G . Depressurizatio n o f hydrocarbon-bearin g reservoir s i n 45 exhumed basin settings : evidenc e fro m Atlanti c margin and borderland basin s

7

Index 48

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Exhumation o f the North Atlanti c margin : introduction and background A. G. DORE 1, J. A. CARTWRIGHT 2, M. S. STOKER 3, J. P. TURNER4 & N. J. WHITE 5 l Statoil (UK) Ltd, lla Regent Street, London SW1Y 4ST, UK (e-mail: agdo@ statoil.com) Department of Earth Sciences, Cardiff University, PO Box 914, Cardiff CF10 BYE, UK ^British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 SLA, UK ^University of Birmingham, School of Earth Sciences, Edgbaston, Birmingham B15 2TT, UK 5 Bullard Laboratories, Madingley Rise, Madingley Road, Cambridge CB3 OEZ, UK

Since consolidatio n durin g th e Caledonia n an d Variscan orogenies , N W Europ e ha s undergon e repeated episode s o f exhumatio n (th e exposur e of formerl y burie d rocks ) a s a resul t o f suc h factors a s post-orogenic unroofing , rift-shoulde r uplift, hotspo t activity , compressiv e tectonics , eustatic sea-leve l change , glaciatio n an d iso static readjustment . Modern measuremen t tech niques, suc h a s apatit e fission-trac k analysis , have helpe d t o establis h usefu l denudatio n chronologies fo r thi s entir e tim e span . How ever, th e mai n observationa l legac y o f exhumation aroun d th e Nort h Atlanti c i s preserve d i n the comparativel y youn g (Mesozoi c an d Cenozoic) geologica l recor d o f thi s region . This i s clearl y reflecte d b y th e unifyin g them e of thi s volume , whic h document s evidenc e fo r the widesprea d uplif t an d emergenc e o f larg e sections o f th e Nort h Atlanti c margi n i n Cenozoic time . All student s o f N W Europea n geolog y ar e aware o f th e compellin g palaeogeographica l evidence fo r th e transitio n a t th e en d o f th e Cretaceous fro m shel f sea s an d low-relie f landmasses t o a n are a dominate d b y highland s and newl y emergen t landmasses , flanke d b y shelves dominate d b y rejuvenate d clasti c depo sition. Similarly, it is also widely known that the highlands o f Norwa y an d Scotlan d d o no t represent th e origina l Caledonia n mountai n range bu t mus t b e instea d a produc t o f lat e emergence o r uplift . The Cenozoi c uplif t o f Fennoscandi a i n particular ha s a lon g histor y o f study . I t i s arguably one of the oldest debate s i n the history of systemati c geolog y an d feature d prominently in Lyell' s Principles o f Geology (Lyel l 1830-1875). Al l o f thi s earl y wor k was , o f course, base d o n onshor e observations. B y th e late 19t h century , i t wa s realize d tha t Norwa y

was essentially a tableland, a plain that had been reduced t o som e bas e leve l an d subsequentl y uplifted (e.g . Beete-Juke s 1872) . Th e ancien t land surface , no w considerabl y modifie d an d incised b y recent glacia l an d fluvial erosion, was termed th e Paleic Surfac e by Reusch (1901) and subsequently describe d i n detai l b y Gjessin g (1967). Base d solel y o n regiona l evidence , principally th e Alpine-relate d uplif t o f larg e parts of central Europe, it was inferred that such a surface mus t have been formed in late Mesozoi c or early Cenozoic time , and that uplift mus t have taken place a t some later stag e of Cenozoic time (see, e.g . Gregory 1913) . Overlapping wit h this work, similar planatio n surfaces an d episode s o f Cenozoi c uplif t wer e inferred i n Scotlan d (se e e.g . Godar d 1962 ; George 1966 ; Hal l 1991) , an d th e Cenozoi c emergence o f souther n Britai n wa s obvious , based o n widesprea d outcrop s o f Jurassic , Cretaceous and Eocene marin e rocks . Holtedahl (1953 ) made the critical observatio n that som e o f th e highland s bordering th e Nort h Atlantic wer e probabl y complementar y t o area s of downwar p an d depositio n o n th e adjacen t shelves. Althoug h b y n o mean s obviou s a t th e time, the hypothesi s was quickl y teste d by the explosion i n offshor e hydrocarbo n exploration , which confirme d tha t mos t o f th e surroundin g shelves wer e characterize d b y Mesozoic-Cen ozoic sedimentar y basins . Consequently , a n attempt coul d b e mad e t o matc h th e suppose d Cenozoic evolutio n o f th e lan d areas , includin g denudation i n response t o uplift , t o th e offshor e sedimentary response . Furthermore , recognitio n of th e importanc e o f th e Cenozoi c evolutio n in the formation of the offshore hydrocarbo n riches (e.g. Parke r 1975 ) provide d a commercial , a s well as academic , motivatio n for continue d research.

From: DORE , A.G. , CARTWRIGHT , J.A., STOKER , M.S. , TURNER , J.R & WHITE , N . 2002 . Exhumation o f th e North Atlantic Margin: Timing, Mechanisms an d Implications for Petroleum Exploration. Geologica l Society , London, Specia l Publications , 196 , 1-12 . 0305-8719/027 $ 15.00 © The Geological Societ y o f London 2002 .

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A. G . DORE ETAL.

Historical description s o f th e numerou s strands o f subsequen t investigatio n hav e bee n given b y Gabrielse n & Dor e (1995) , Stuevol d & Eldhol m (1996 ) an d Japse n & Chalmer s (2000). A summar y o f th e ke y finding s i s a s follows. • A s well a s in mainland Norwa y an d Britain, Cenozoic uplif t and/o r emergenc e too k plac e in Spitsberge n (Harlan d 1969) , Swede n an d part o f Denmar k (e.g . Japse n & Chalmer s 2000), Irelan d (e.g . Naylo r 1992) , Eas t Greenland (e.g . Johnso n & Gallaghe r 2000 ) and West Greenland (e.g . Mathiesen 1998) . In other words , i t i s a circum-Nort h Atlanti c phenomenon. • Cenozoi c exhumatio n als o too k plac e i n basins periphera l t o th e landmasses . I t wa s recognized earl y that the huge expanse of shelf forming th e Barent s Se a wa s expose d subaerially an d erode d durin g lat e Cenozoi c time (Nanse n 1904 ; Harlan d 1969) . T o thi s have subsequentl y bee n adde d th e Hord a Platform, Stor d Basi n an d Farsun d Basi n o f offshore Norwa y (e.g . Ghaz i 1992 ; Jense n & Schmidt 1993) , th e Wes t Shetlan d Inne r Moray Firt h an d East Iris h Se a Basins i n UK waters (described by, for example, Lewis el al (1992), Parnel l e t a l (1999) , Hilli s e t a l (1994) an d Rowley & White (1998) , respect ively), th e Slyne-Erri s an d Nort h Celti c Se a Basins of f th e Iris h coas t (describe d b y Scotchman & Thoma s (1995 ) an d Murdoc h et a l (1995) , respectively ) an d numerou s others. Earl y Cenozoi c uplif t an d subaeria l exposure als o too k plac e alon g th e volcani c highs margina l t o th e newl y developin g North Atlantic , area s no w submerge d t o depths o f a kilometre o r mor e (e.g . Eldhol m et a l 1989) . • Fro m the numerous studie s now carried out , both loca l an d regional , i t i s clea r tha t th e circum-North Atlanti c uplif t an d erosio n wa s variable in magnitude, locatio n an d timing. I t could thu s perhap s b e argue d tha t th e phenomenon represent s a patchwork o f effect s deriving fro m man y unrelate d causes . Never theless, a s integratio n betwee n th e variou s studies improves , i t seem s tha t a t leas t tw o events ha d regiona l significance : (1 ) a Paleocene episod e o f widesprea d emergenc e in NW Europe coinciden t wit h North Atlantic opening an d th e initia l effect s o f th e Icelan d Plume (e.g. White 1988;Brodie&White 1995); (2) a Neogen e (mainl y Plio-Pleistocene ) episode wit h n o obviou s tectoni c cause , emphasized b y rapi d glacia l erosio n an d isostatic adjustmen t o f landmasse s an d

bordering shelve s (e.g . Solhei m e t a l 1996 ; Japsen & Chalmer s 2000) , togethe r wit h redeposition an d a widesprea d chang e i n th e deep-water circulatio n pattern i n the adjacen t basins (Stratage m Partner s 2002) . Th e Paleo cene even t appear s t o hav e bee n particularl y significant in the British Isles, whereas the late Neogene even t seem s t o hav e extende d fro m Scandinavia t o the Atlanti c margin of Britai n and /Ireland. Additionally , compressiona l upli/t (inversion) associated with Alpine stress and/or ridge-pus h fro m Atlanti c spreadin g i s common throughou t muc h o f th e area , although localize d an d ver y variabl e i n effec t (e.g. Murdoc h e t a l 1995 ; Dor e & Lundi n 1996). • Criticall y fo r the petroleum industry , severa l North Atlanti c basin s containin g commer cially significan t hydrocarbo n resource s wer e both uplifte d an d exhume d durin g Cenozoi c time, wit h profoun d implication s fo r th e quantity an d natur e o f th e hydrocarbon s discovered. Thes e effect s hav e bee n system atically studie d i n th e Barent s Se a (Nylan d et al 1992 ) and to some extent in the east Irish Sea (Cowa n e t a l 1999) , bu t i n term s o f overall applicabilit y t o uplifte d terrane s ar e still underestimated. Despite thi s rapi d increas e i n th e under standing o f th e exhumatio n o f th e Nort h Atlantic borderlands , ther e are stil l man y unknowns. Th e relativ e intensit y o f th e variou s phases, an d thei r variatio n i n importanc e geographically, ar e stil l onl y understoo d i n a very genera l sense . Althoug h ther e i s n o shortage o f postulate d uplif t mechanisms , ther e is stil l a scarcit y o f observationa l evidenc e an d modelling studie s t o establis h beyon d reason able doub t whic h ar e th e primar y cause s o f exhumation, an d ho w thes e ma y var y fro m area t o area . Tie d t o thes e problem s i s th e larger-scale questio n o f whethe r th e circum North Atlanti c i s uniqu e o r whethe r it s behaviour i s typica l fo r passiv e margins . There hav e bee n severa l attempt s i n recen t years t o brin g togethe r researcher s t o addres s these questions , an d compilation s hav e bee n published tha t ar e th e direc t antecedent s o f thi s book (se e particularl y Jense n e t a l (1992) , Solheim e t al (1996 ) and Chalmers & Cloetingh (2000)). Thes e proceedings , however , hav e tended t o focu s o n on e particula r geographica l area or one particular exhumation phase. There is an acknowledge d nee d t o brin g togethe r disciplines tha t hav e traditionall y remaine d apart (e.g . geomorphologist s an d offshor e seismic interpreters ; Paleogen e an d Neogen e

INTRODUCTION

3

Fig. 1 . Topographi c an d bathymetri c ma p o f th e easter n Nort h Atlantic , showin g th e locatio n o f studie s represented i n this volume. The papers are numbered as follows: 1, Jones et al.\ 2, Rohrman et al.;3, Nielsen et al.\ 4, Graversen ; 5 , Blundell ; 6 , Lidmar-Bergstro m & Naslund ; 7 , Hendrick s & Andriessen ; 8 , Evan s e t al.\ 9 , Stroeven e t al.\ 10 , Cederbom; 11 , Japsen et aL\ 12 , Huuse; 13 , Faleide et aL\ 14 , Bishop & Hall; 15 , Andersen et al\ 16 , Stoker; 17 , Green et al; 18 , Ware & Turner; 19 , Allen et al\ 20, Dore et al\ 21, Price; 22, Parnell; 23, Cramer e t al.; 24 , Corcora n & Dore . Fo r paper s tha t attemp t onshore-of f shore even t correlation , th e are a o f interest i s represented b y the sam e numbe r onshore an d offshore .

workers; Scandinavia n an d British-Iris h research schools ) befor e a n integrate d stor y ca n emerge. B y providin g a n interdisciplinar y se t of studies ove r a wide latitudina l rang e o f th e N W European margin (Fig. 1) , this volume represents an initia l ste p i n this direction .

Exhumation and other terms: some definitions Terms suc h a s exhumation, erosio n an d denuda tion hav e a varied usag e in the literature an d are often use d interchangeably . Numerou s attempt s

4

A. G . DORE ETAL.

have been made at deriving forma l definitions for these terms , an d th e subtl e difference s betwee n these definition s ca n rende r communicatio n difficult. Furthermore , th e wa y suc h term s ar e defined may depend on the concerns an d research orientations o f th e users . So , fo r example , scientists primaril y concerne d wit h th e litho spheric unloadin g o f orogeni c belt s an d cor e complexes (e.g . Ring et al. 1999) may emphasize different factors , an d hav e differen t definitions , from thos e concerne d wit h th e behaviou r o f passive margin s (e.g . thos e contributin g t o thi s volume). We hav e no t attempte d t o impos e a rigi d standardization on the papers in this volume, and have deliberatel y use d th e descriptiv e an d les s rigorously defined ter m 'exhumation ' in the title. Nevertheless, i t i s wort h whil e examinin g som e alternative use s t o determin e whethe r an y consensus ca n b e draw n an d a s a backgroun d for th e papers that follow .

Uplift Uplift i s a term in common usag e and appeals t o standardization ar e likel y t o fail . I t is , however , useful t o examin e som e way s i n whic h th e concept i s formall y used . Englan d & Molna r (1990), Summerfiel d (1991 ) an d Rii s & Jense n (1992) all pointed to the existence of two types of uplift. Thes e ar e surface uplift, referrin g t o th e upward movemen t o f th e Earth' s surfac e (th e land o r se a bottom ) wit h respec t t o a specifi c datum, usuall y mean se a level o r the geoid, an d crustal uplift, referrin g t o upwar d movemen t o f the rock column with respect t o a similar datum. The relationshi p betwee n surfac e uplif t an d crustal uplif t depend s upo n th e amoun t o f denudation o r deposition . I f there i s n o denudation or deposition, the n surface uplift an d crustal uplift wil l b e equal . If , a s i s likely , denudation occurs the n crusta l uplif t wil l b e greate r tha n surface uplift , an d i f denudatio n i s greate r tha n crustal uplif t surfac e elevatio n wil l b e reduced . Surface uplif t i s mostl y relate d t o activ e tectonics, bu t crusta l uplif t ca n occu r simpl y as the isostatic response t o denudation. Th e term net uplift ha s been used by several of the 'Norwegia n school' o f worker s t o indicat e th e presen t elevation o f a marke r be d abov e it s maximu m burial dept h (Nylan d et al. 1992 ; Rii s & Jensen 1992; Dore & Jensen 1996) . It is a measure of the vertical distance a rock has been uplifted through a therma l fram e o f reference , an d i s thu s particularly usefu l i n studie s o f diagenesi s an d source roc k maturation.

Erosion Ring et al. (1999) defined erosion as 'the surficial removal o f mas s a t a spatia l poin t i n th e landscape b y bot h mechanica l an d chemica l processes'. Biologica l processe s coul d als o b e added to this definition. Leeder (1999 ) provided a similar definition , but pointe d ou t tha t erosio n must b e define d wit h referenc e t o co-ordinate s fixed beneath the surface, as the surface potential elevation ma y itself chang e becaus e o f tectonics. As indicate d b y Rii s & Jense n (1992) , erosio n may b e subaeria l o r submarine , thu s als o emphasizing th e nee d fo r referenc e t o fixe d subsurface coordinates . Erosion rates ma y refe r to loca l measure d absolut e values or to regional values integrate d ove r a wide r area . Th e difference betwee n erosio n an d denudatio n i s considered below . Denudation Denudation has been defined by Leeder (1999 ) as the los s o f materia l fro m bot h surfac e an d subsurface part s o f a drainage basi n o r regiona l landscape b y al l type s o f weathering , physica l and chemical. Leeder indicate d that, like erosion, denudation shoul d b e define d wit h referenc e t o fixed interna l co-ordinates . So , accordin g t o Leeder, the important difference between erosion and denudation is that erosion is measured a s an effect o n th e surface , wherea s denudatio n ca n include subsurfac e processe s suc h a s chemica l dissolution, an d i s no t alway s accompanie d b y erosion. I t shoul d b e note d tha t i f th e Leede r definition i s used , i t follow s tha t denudatio n should b e characterize d a s los s o f mas s rathe r than thickness . Rin g e t al . (1999 ) provide d a n important modifie r t o thes e definition s o f denudation b y includin g the los s o f materia l by tectonic processe s (e.g . thin-skinned extensional faulting i n core complexes) . We advocat e thi s consensual view o f erosio n and denudation , althoug h los s o f materia l b y tectonic unloadin g doe s no t pla y a n importan t part in this volume. It is, however, worth pointing out tha t man y othe r usage s exist . Fo r example , Summerfield (1991 ) fro m a geomorphologica l perspective, preferre d t o defin e erosio n a s mechanical denudation o r th e remova l o f soli d particles, as opposed to chemical denudation (i.e. the remova l o f dissolve d material) . A ver y different emphasi s wa s provide d b y Brow n (1991) an d Rii s & Jense n (1992) , arguin g fro m a subsurfac e petroleum exploratio n perspective . They define d denudatio n a s erosio n (i n thei r terms, decreasin g thicknes s of overburden) seen in a thermal frame o f reference and quantified b y

INTRODUCTION

such measures as vitrinite reflectance and apatite fission-track trend s (se e als o 'ne t uplift') . Thi s definition is, however, specialized compare d with most others in the literature. It is encumbered by the potential for thermal regimes to vary through time, thereb y givin g false value s for denudation or erosio n i f a fixed thermal regim e i s assume d (see discussio n give n i n thi s volum e b y Gree n et aL). Exhumation Exhumation i s th e mos t loosel y define d o f th e terms used in this volume, but precisely becaus e of its descriptiv e natur e it is useful a s an overal l shorthand to describe th e removal of material by any means from a basin or other terrane such that previously burie d rock s ar e exposed . I t ca n b e characterized a s a process o r a history (thus, this definition fro m Ring et aL (1999): 'the unroofing history o f a rock , a s cause d b y tectoni c and/o r surficial processes'). It is seldom characterized as a measure , unlik e erosio n o r denudation , which are ofte n describe d i n terms o f loss o f thickness mass or volume. Inversion Compressive reactivation with reversal of slip on formerly extensiona l faul t system s i s terme d basin inversio n and , ultimately , i t lead s t o folding, thrustin g an d expulsio n o f th e synrif t fill (see, for example, Coope r & Williams (1989 ) and paper s therein , an d Buchana n & Buchanan (1995) an d paper s therein) . Becaus e inversio n often cause s th e processe s describe d abov e (uplift, erosion , denudation , exhumation ) i t i s frequently use d interchangeabl y wit h thes e terms, especially i n NW Europe where compres sive reactivatio n an d genera l regiona l exhuma tion ofte n coexist . Thi s usag e is misleading an d should be avoided unless the underlying cause of the regiona l exhumatio n i s know n t o b e compression. Towards an understanding of North Atlantic exhumation The studies presented her e are based on a variety of techniques that have been employed to address the main concerns o f North Atlantic exhumation history, includin g timing , mechanism s an d th e sedimentary respons e o f the continenta l margin . The 2 4 paper s presente d i n thi s volum e hav e accordingly bee n arrange d i n fou r section s t o reflect the highly varied approac h to this subject, and th e commercia l implications . Par t 1 i s

5

mainly concerned wit h exhumation mechanisms; parts 2 an d 3 presen t curren t researc h o n th e continental margi n record o f offshore Scandina via an d Britain , Irelan d an d th e Faeroes , respectively; par t 4 cover s th e implication s o f exhumation for hydrocarbon-bearing basins . Key aspects o f thi s interdisciplinar y approac h ar e summarized below . Th e geographica l sprea d o f the papers i s shown in Fig. 1 .

Techniques All o f th e mai n technique s fo r determinin g amount an d timin g o f uplift , erosio n an d denudation ar e represente d i n thi s selectio n o f studies. They ar e summarize d i n Table 1 , which also indicate s th e advantage s an d disadvantage s of eac h metho d an d where it is discussed i n this volume. Th e extrem e end s o f th e spectru m ar e those studie s tha t infe r exhumatio n chronolog y from eithe r exclusivel y onshore o r offshor e studies: th e forme r utilizin g moder n technique s of geomorphologica l analysi s (e.g . Lidmar Bergstrom & Naslund , Hal l & Bishop) , th e latter focusing on detailed seismic analysis of the offshore erosio n product s (e.g . Evan s et aL, Faleide et aL, Stoker). There i s no w ful l awarenes s o f th e nee d t o integrate onshore and offshore studie s to provide a mor e complet e pictur e o f uplift , erosio n an d sedimentary response . Availabl e approache s include simpl e graphica l reconstruction , tha t is, the projectio n o f offshor e stratigraphi c break s into onshor e surface s (e.g . Graversen , Japse n et aL} o r analysi s o f th e mas s balanc e betwee n basins an d hinterland s (Jone s et aL, Anderse n et a/.) . Thes e methods , b y thei r nature , provide only a coarse chronolog y an d are most effectiv e if combine d wit h physico-chemica l measure ment technique s fo r reconstructin g exhumatio n history. The apatite fission-track technique is by far the main 'growt h area ' an d i s represente d i n thi s volume b y severa l regiona l studie s (Hendrick s & Andriessen , Cederbom , Gree n et aL, Alle n et aL} Fission-trac k wor k i s provin g highl y effective i n establishing th e coolin g histor y of a rock, bu t wit h bot h thi s an d th e vitrinit e reflectance metho d (e.g . Japse n et aL, Gree n et aL} th e potentia l exist s fo r interpretin g anomalously hig h palaeo-hea t flow s a s highe r burial depth s i n th e past , an d vic e versa . A s indicated b y severa l workers , fission-trac k dat a are unreliable a t low temperatures (<60°C ) and can produc e spuriou s result s fo r geologicall y recent exhumation . Compaction studie s (Japse n et aL, War e & Turner ) benefi t fro m plentifu l

Table 1 . Summary o f techniques for measuring amount and timing o f uplift an d erosion-denudation, with attractions, limitations and where they ar e represented i n this volume Paper in this volume

Measurement technique

Attractions

Problems, limitation s

Geomorphological analysis

Field-based, plentifu l data , relate s landfor m developmen t t o geological histor y

Difficult t o obtain quantitativ e measures o f erosion -denudation, and to constrain timing ; most effectiv e when use d togethe r wit h offshore analysi s

6,9,10,14

Graphical reconstruction

Simple technique to estimate erode d thickness, correlate surface s and deriv e onshor e -off shore relationship s

Relies o n long-distanc e extrapolatio n an d assumptio n of thickness

4, 6 , 1 1

Offshore sedimentary response

Provides indicato r of denudation chronolog y o f sourc e areas ; seismic dat a allo w ful l sedimentar y sectio n t o be observe d

Erosion an d redepositio n o f offshore succession s complicat e correlation wit h onshore denudation chronology ; mos t effectiv e when use d togethe r wit h onshore analysi s

8, 12 , 13 , 16

Mass balanc e

Directly an d quantitatively correlates denudation wit h offshore sedimentation

Loss of mas s i n solution ; difficult y i n assignin g sedimen t t o correct catchmen t are a

1, 1 5

Vitrinite Preserve reflectance fissio

Apatite fissio track onshor

n Ca

Cosmogenic Use nuclide developmen

n trac

s recor d o f higher temperatures-burial depth s than k

n establis h detailed exhumatio n and buria l chronology bot h e an d offshor e d t o obtain detail on geologicall y recen t landform t not obtainabl e fro m fissio n trac k

Compaction, sonic Plentifu l sourc e o f wel l data (soni c logs) ; easier t o distinguish velocity change s i n basal hea t flo w fro m exhumatio n compared wit h thermal method s For th e numberin g o f th e paper s se e Fig . 1 .

Vitrinite absent fro m man y sediments and al l basement; vitrinit e 1 1 , 1 may b e misidentified ; difficul t t o separat e change s i n basal an d transient hea t flo w fro m exhumation ; reworked an d oxidized vitrinite a problem; onl y a crude timin g indicator Unreliable i n establishing recent exhumatio n events (coolin g 7 below annealin g temperature); difficul t t o separat e change s i n 1 basal an d transien t heat flo w fro m exhumatio n

7

, 10 , 17 , 9

Onshore only , mainly limited to chronology o f presen t lan d 9 surface Unreliability o f baseline compaction trend s fo r a basin o r a 1 1 , 1 lithology; difficult y i n identifyin g 'typical ' lithologie s for analysis; mechanica l compactio n retarde d b y overpressure , leading t o erosion underestimates ; only a crude timin g indicator

8

INTRODUCTION

data and , give n th e righ t stratigraphica l infor mation an d lithologies , ca n fill in som e o f thes e gaps. Th e mor e recen t onshor e denudatio n chronology is partially catered for by the rapidly developing cosmogeni c nuclid e techniqu e (Stroeven et a/.) , althoug h thi s metho d i s no t capable o f resolvin g larg e (kilometre-scale ) amounts of denudation. The essential conclusion, clear fro m Tabl e 1 , is tha t th e mor e technique s that can be brought to bear, the fuller th e picture that is likely t o emerge. Mechanisms Views o n th e natur e an d caus e o f th e Nort h Atlantic Cenozoi c exhumatio n hav e tende d t o depend o n wher e th e studie s hav e bee n carrie d out. Thu s th e olde r 'Scandinavia n school' , impressed b y th e evidenc e fo r rapi d Neogen e outbuilding an d th e concentri c natur e o f th e Cenozoic outcrops around the coasts, has tended to emphasiz e Neogen e effect s includin g glacio isostasy. Th e 'Britis h school', on the other hand, presented wit h evidenc e fo r hig h Paleocen e sedimentation rate s an d contemporaneou s vol canism aroun d th e islands , ha s mainl y stresse d Paleogene hotspo t model s t o explai n th e uplift , although George (1966) is a notable exception to this i n favourin g a substantia l amoun t o f Neogene reshapin g o f th e Britis h an d Iris h 'Massifs'. Thi s volum e i s a n initia l attemp t a t bringing thes e school s together , a s a mean s o f examining whethe r relationship s exis t an d ho w the differen t mechanism s ma y var y i n import ance i n tim e an d space . A summar y o f th e postulated mechanisms , togethe r wit h thei r attractions and limitations, is provided in Table 2. Part 1 o f th e boo k focuse s primaril y o n mechanisms related to the Iceland Plume. Jones et al. provid e mas s balanc e evidenc e tha t th e Iceland Plum e cause d significan t exhumatio n in Early Cenozoi c time , an d equall y impressiv e gravimetric evidenc e tha t th e plum e cause s a significant deflectio n o f th e geoi d ove r a ver y wide area today . It is widely believed tha t much of th e Paleogen e emergenc e ca n be attribute d to underplating o f th e lithospher e fro m th e plum e (Brodie & Whit e 1995) , bu t ther e i s stil l a requirement t o explai n th e apparen t absenc e o f (or lac k o f evidenc e for ) underplatin g i n som e areas, an d th e puzzlin g absenc e o f igneou s activity in classic uplifted area s such as Norway. Nevertheless, ther e i s soli d evidenc e fro m Bouguer gravit y an d tomograph y tha t eve n th e most recen t uplif t o f Norwa y occurre d wit h mantle involvement , perhap s upwellin g o r asthenospheric flo w withou t th e direc t impingement of a hotspot (Rohrma n et al.).

7

A variatio n o n thi s them e b y Nielse n et al. suggests tha t th e impingemen t o f th e Icelan d Plume hea d cause d delaminatio n o f th e lowe r lithosphere i n areas wit h deep crustal roots suc h as mainland Norway, leading to isostatic uplift. A central tene t o f th e Nielsen et al. mode l i s that, after initia l Paleocen e plume-relate d uplift , n o further activ e tectonic mechanis m i s required t o explain eithe r th e denudatio n chronology o r the present-day elevation of Norway. Post-Paleocene pulses o f erosio n an d depositio n ar e explaine d solely i n term s o f eustati c base-leve l fall , glaciation an d isostatic response t o these events . This view is emphasized in a supporting paper on the offshor e sedimentar y respons e (Huuse ) bu t rejected b y Japse n et al., wh o firmly believe i n the necessity fo r a Neogene tectonic uplift event . Detailed seismi c analysi s o f th e Cenozoi c sequences i n the norther n North Se a by Faleide et al. als o seem s t o suppor t a tectonic caus e fo r the late Neogene exhumation. Resolution of this fascinating debat e require s considerabl y mor e modelling an d researc h o n detaile d chronolog y and mas s balance . Graverse n provide s a controversial vie w (agai n involvin g mantl e upwelling) speculativel y linkin g domin g i n southern Scandinavi a wit h earlie r (Mesozoic ) doming in the central Nort h Sea . To complet e th e sectio n o n mechanisms , Blundell describe s th e rol e o f Alpin e stres s i n the Cenozoi c inversio n an d uplif t o f souther n Britain, supporte d i n thi s pape r b y thermo mechanical modellin g o f th e lithosphere . Blundell remarks , however , tha t apparen t lat e Neogene uplif t i n wester n part s o f th e are a cannot b e attribute d t o inversion . War e & T\irner, workin g i n th e Eas t Iris h Se a Basin , show how localization o f inversion-related strain superimposed o n a 'background ' epeirogeni c erosion signatur e cause d rapi d change s i n th e degree o f exhumation within a single basin. Continental margin record The paper s presente d i n part s 2 (Scandinavia ) and 3 (the Faeroes, th e British Isles an d Ireland) illustrate th e communicatio n tha t i s no w occurring betwee n the two regiona l researc h schools an d th e acknowledgemen t o f a multi phase Cenozoi c denudatio n chronology fo r both areas. Potentia l no w exist s fo r a compariso n between thes e area s t o determin e whic h event s are regionally correlated, an d which events are of local significanc e only . Fo r example , th e majo r geomorphological review s o f Scandinavi a (Lidmar-Bergstro m & Naslund) an d northern Scotland (Hal l & Bishop ) appea r t o detai l a similar event chronology, although late Neogene

Table 2 . Summary o f potential mechanisms for exhumation o f th e North Atlantic margin, with attractions, limitations an d where they are represented i n this volume Paper i n this volume

General grouping and mechanism

Attractions

Problems limitation s

Associated with mantle plume Dynamic uplif t Underplating

Regionally applicable ; supporte d b y geomorphological , palaeogeographical, mas s balance an d regional gravit y studies

Distribution o f underplatin g not full y understood ; absenc e o f igneous activit y in som e uplifte d areas , e.g . Fennoscandi a

1,3,12,13

Supported by Bougue r gravity and tomography , mainly southern Norway Supported by Bougue r gravity, mainly Norway

Causal agenc y poorl y understoo d

2,4

New mode l require s mor e testing ; connection betwee n plum e and delaminatio n beneath craton s no t clea r

3

Inversion tectonics widesprea d o n Atlantic margin; can explain rapid loca l variatio n i n exhumation

Difficult t o separate local from mor e regiona l backgroun d mechanisms; significan t shortenin g occur s before 'classic ' inversion geometrie s develop , henc e difficul t t o quantify bul k strain; cannot explain uplift o f craton s

5, 15 , 1 8

Strong correlation i n some area s between glaciatio n an d offshore sedimen t flux Could eliminat e need fo r enigmatic Neogene tectoni c uplif t mechanism

Emphasizes pre-existin g topography , bu t does not explain i t

12, 13 , 1 6

More studie s correlating denudation , sediment flux and glacio eustatic lowstand s required to remove necessit y fo r Neogen e tectonic uplif t

3, 1 2

Mantle upwelling without magmatism Ray leigh- Taylor instability Compressive Inversion Intraplate stres s Technically 'passive ' Glacio-isostatic Base-level change

For th e numberin g o f th e paper s se e Fig . 1 .

INTRODUCTION

doming i s considere d mor e prominen t i n th e former, wherea s th e latte r emphasize s th e particular importanc e o f Paleocene uplift . The fission-trac k coolin g chronologie s estab lished fo r Irelan d (Alle n et a/.), th e UK (Gree n et a/.) , Swede n (Cederbom ) an d norther n Norway (Hendrick s & Andriessen ) als o bea r comparison. Alle n e t a/.'s useful compilatio n o f fission-track dat a show s that , althoug h for any given tim e perio d Irelan d wa s a patchwor k o f differing exhumatio n profiles , som e genera l conclusions o n intensit y throug h tim e ca n b e drawn. Cederbom' s wor k o n souther n Swede n reaches th e sam e conclusion . Followin g thi s logic, a synthesis of the latest fission-trac k wor k over al l o f N W Europ e would provid e valuabl e insights int o th e regionall y significan t exhuma tion signatures , especiall y i f integrate d wit h methods capabl e o f resolvin g th e mos t recen t events suc h a s (U-Th)/H e thermochronolog y and perhaps even in situ and detrital cosmogeni c nuclide techniques. The integration of these three techniques is the key aim of the recently initiated CRUST projec t (Constrainin g Regiona l Uplift , Sedimentation & Thermochronology) , a colla borative projec t betwee n th e Universitie s o f Glasgow, Edinburg h an d Aberdeen , an d th e Scottish Universities ' Environmenta l Researc h Centre, Eas t Kilbride . As a final example, a similarly instructive view of regionally important events would be obtained by correlatin g th e Cenozoi c seismi c sequenc e chronologies reporte d fro m th e Mid-Norwegia n shelf (Evan s et al.\ Nort h Se a (Faleide et a/.), Faeroes shel f (Anderse n et al.) an d UK Atlantic margin (Stoker) . To this end, a unified, regiona l Neogene (Miocene-Holocene ) stratigraph y ha s recently bee n establishe d fo r th e entir e Atlanti c continental margi n between th e Lofote n Island s (off North Norway) and the Porcupine region (off southern Ireland), which includes adjacent deep water basin s suc h a s th e Norwegia n Basin , Faeroe-Shetland Channel , Rockal l Troug h an d Porcupine Basin (Stratagem Partners 2002). This study demonstrate s tha t th e entir e N E Atlanti c margin covere d b y thi s volum e (Fig . 1 ) wa s affected b y a n early-'mid'-Pliocen e even t tha t resulted, mos t significantly , i n th e initiatio n o f the Plio-Pleistocene prograding wedges . Implications for petroleum exploration Exhumation lead s t o coolin g an d lithostati c pressure decrease, an d these factors add a degree of difficult y t o petroleu m exploration . Th e following effect s ar e among the most important : (1) sealin g horizon s ar e remove d and/o r thei r effectiveness i s severel y reduced ; (2 ) fault s ar e

9

often reactivated , causin g the m t o becom e conduits fo r hydrocarbon leakag e to the surface ; (3) sourc e rock s wil l b e a t a highe r degre e o f maturation tha n expecte d fro m thei r presen t depth and will cease generation upon cooling; (4) potentially attractiv e reservoirs may , likewise, b e overcompacted an d downgraded ; (5 ) pressur e reduction during exhumation causes oil accumulations an d formatio n wate r t o exsolv e gas , causing ga s flushin g an d th e spillag e o f oi l accumulations; (6 ) regiona l tiltin g durin g uplif t results in changes to trap configurations and fluid migration directions . In spit e o f this , man y N W Europea n sedimentary basin s tha t underwen t sever e lat e Mesozoic an d Cenozoi c exhumatio n hav e retained prospectivity. These includ e the Barents Sea of f northern Norwa y and th e Eas t Iris h Se a Basin, whil e i n th e Wesse x basi n o f souther n England, Europe' s larges t onshor e oilfiel d a t Wytch Farm is located i n the footwall to a major compressionally reactivate d faul t zone . In par t 4 o f th e volume , five paper s describ e the significan t change s t o th e hydrocarbo n systems tha t occu r i n exhume d basins . Dor e et al. catalogu e these effects i n term s o f two of the standar d procedure s carrie d ou t i n th e oi l industry, namely , petroleum resourc e evaluatio n and ris k analysis . Thi s pape r document s bot h positive and negative implications, including the tendency toward s gas-dominate d systems , and references thes e t o a selectio n o f exhume d basins o n th e Nort h Atlanti c seaboard . Pric e considers th e sam e phenomen a i n som e o f th e well-documented uplifte d basin s i n wester n North America . Thi s pape r i s presente d i n abstract for m onl y becaus e o f th e untimel y death o f th e autho r durin g th e preparatio n o f these proceedings , a n even t tha t robbe d u s o f an importan t an d entertainin g contributo r to th e uplift debate . The three final papers consider specific aspect s of exhume d hydrocarbo n systems . A revie w b y Parnell examine s th e migratio n o f fluids , bot h hydrocarbons an d formatio n water , an d th e diagenetic change s tha t occu r durin g uplift o f a basin. Cramer & Poelchau draw attention to the potential fo r liberatio n o f hug e amount s o f methane fro m formatio n wate r durin g pressur e and temperatur e decrease , a proces s tha t ha s contributed significantly t o the giant gas fields of Western Siberia , an d b y implicatio n t o man y other gas accumulations worldwide. Complete or partial failur e o f seal s a s a result o f brittleness , stress-induced fracturin g and hydrofracturin g is shown b y Corcora n & Dor e t o b e a frequen t outcome o f exhumation , leadin g t o th e predic tion o f underfille d trap s an d near-hydrostati c

10

A. G . DORE ETAL.

gradients. Thes e inference s ar e agai n illustrate d by cas e studie s o n the North Atlantic margin. Concluding remarks The Nort h Atlanti c margi n describe d i n thi s volume is , o f course , par t o f a mor e globa l system o f margina l uplift s borderin g man y oceanic basins . Thes e includ e th e almos t classical are a fo r geomorphologist s o f th e southern Africa n margina l escarpments , alon g with th e margina l uplifte d massif s o f Easter n Australia an d Antarctica . Althoug h thes e othe r marginal uplift s wer e beyon d th e scop e o f thi s volume, man y o f th e technique s develope d an d refined fo r th e Nort h Atlanti c margin s ar e applicable mor e generally , an d ma y hel p t o broaden an d deepe n th e investigatio n o f thi s global tectoni c process . Perhaps th e ke y t o reall y advancin g ou r understanding o f th e processe s involve d i n oceanic margi n uplift s lie s o n a globa l scale , that is, to identify thei r spatial distribution within a high-resolutio n chronostratigraphi c frame work. Ther e i s thu s a prim e nee d fo r bette r resolution o f th e timin g an d magnitud e o f al l these margina l uplifts , a s par t o f an y attemp t t o synthesize their distributio n globally. For th e Nort h Atlanti c margins , significan t progress ha s bee n mad e alon g thes e lines , a s shown b y man y contribution s i n thi s volume . Additional effort i s required to fully integrat e the work of the 'Scandinavian ' an d 'British' schools, and als o t o forg e close r link s acros s disciplines . The sequenc e stratigrapher s workin g o n th e offshore seismi c recor d nee d greate r awarenes s of th e man y contribution s mad e b y onshor e geomorphologists an d vic e versa . Bette r resol ution of climate variation through Cenozoic tim e is o f paramoun t importanc e i n constrainin g th e long- an d short-ter m rate s o f erosio n an d sediment transport . I n thi s way , depositiona l systems analysi s offshor e ca n b e linke d mor e effectively t o surfac e uplif t onshore , an d lag s between uplif t an d sedimen t flu x fro m th e uplifted region s ca n b e identified . Thi s researc h problem i s trul y demandin g an d multidisciplinary, an d althoug h much is still to be done , much has alread y bee n achieve d toward s a bette r process understanding. References BEETE-JUKES, J . 1872 . Th e Student's Manual o f Geology. 3r d Edition. A . & C. Black, Edinburgh . BRODIE, 1 & WHITE , N.J . 1995 . Th e lin k betwee n sedimentary basi n inversio n an d igneou s under plating. In: BUCHANAN , J.G. & BUCHANAN , P.O.

(eds) Basin Inversion. Geological Society , London , Special Publications, 88 , 21-38. BROWN, R.W. 1991 . Backstacking apatite fission-trac k 'stratigraphy': a method fo r resolving the erosional and isostatic rebound components o f tectonic uplif t histories. Geology, 19 , 74-77. BUCHANAN, J.G. & BUCHANAN, P.G. (eds) 1995 . Basin Inversion. Geologica l Society , London , Specia l Publications, 88 . CHALMERS, J.A . & CLOETINGH , S . (eds ) 2000 . Neogene Uplift and Tectonics around the North Atlantic. Global and Planetary Change, Special Issue, 34 (3-4). COOPER, M.A . & WILLIAMS , G.D . (eds ) 1989 . Inversion Tectonics. Geologica l Society . London , Special Publications , 44. COWAN, G. , BURLEY , S.D. , HOEY , A.N . & 5 OTHER S 1999. Oi l an d ga s migratio n i n th e Sherwoo d Sandstone o f th e Eas t Iris h Se a Basin . In : Fleet , A.J. & Boldy , S.A.R . (eds ) Petroleum Geology o f Northwest Europe: Proceedings of the 5th Conference. Geologica l Society , London , 1383-1398. DORE, A.G . & JENSEN , L.N. 1996 . Th e impac t o f lat e Cenozoic uplif t an d erosio n o n hydrocarbo n exploration: offshor e Norwa y an d som e othe r uplifted basins . Global an d Planetary Change. 12 , 415-436. DORE, A.G . & LUNDIN , E.R . 1996 . Cenozoi c compressional structure s o n th e N E Atlanti c margin: nature , origi n an d potentia l significanc e for hydrocarbo n exploration . Petroleum Geoscience, 2, 299-311. ELDHOLM, O. , THEIDE , J . & TAYLOR , E . 1989 . Evolution o f th e V0rin g volcani c margin . In : ELDHOLM, O. , THEIDE , J . & TAYLOR . E . (eds ) Proceedings of the Ocean Drilling Program, Scientific Results, 104. Ocea n Drillin g Program , College Station , TX, 1033-1065 . ENGLAND, P. & MOLNAR . P. 1990. Surface uplift, uplif t of rocks , an d exhumatio n o f rocks . Geology. 18 . 1173-1177. GABRIELSEN, R.H . & DORE , A.G . 1995 . Histor y o f tectonic model s o n th e Norwegia n continenta l shelf. In : HANSLIEN , S . (ed. ) Petroleum Exploration an d Exploitation i n Nonvay. Norwegia n Petroleum Societ y (NPF ) Specia l Publication , 4 . 333-368. GEORGE, T.N . 1966 . Geomorphi c evolutio n i n Hebridean Scotland . Scottish Journal o f Geology. 2, 1-34 . GHAZI, S.A . 1992 . Cenozoi c uplif t i n th e Stor d Basi n area an d it s consequence s fo r exploration . In : JENSEN, L.N . & Rus , F . (eds ) Post-Cretaceous Uplift and Sedimentation along the Western Fennoscandian Shield. Norsk Geologisk Tidsskrift. vo!72, 285-290. GJESSING, J.P . 1967 . Norway' s palei c surface . Norsk Geografisk Tidsskrift. 21 , 69-132. GODARD, A . 1962 . Essai s d e correlatio n entr e Taltitudes de s relief s e t le s caractere s petro graphique de s roche s dan s le s socle s d e TEcoss e du nord . Comptes Rendus d e rAcademic de s Sciences, 255. 139-141 .

INTRODUCTION GREGORY, J.W. 1913 . Th e Nature and Origin of Fjords. John Murray, London . HALL, A.M. 1991 . Pre-Quaternary landscape evolutio n in th e Scottis h Highlands . Transactions o f th e Royal Society o f Edinburgh: Earth Sciences, 82 , 1-26. HARLAND, W.B . 1969 . Mantl e change s beneat h th e Barents Shelf . Transactions o f th e Ne w York Academy o f Sciences, Series 2 , 31, 25-41. HILLIS, R.R. , THOMSON, K . & UNDERBILL , J.R. 1994 . Quantification o f Tertiar y erosio n i n th e Inne r Moray Firt h usin g soni c velocit y dat a fro m th e Chalk an d th e Kimmeridge Clay . Marine an d Petroleum Geology, 11 , 283-293. HOLTEDAHL, O . 1953 . O n th e obliqu e uplif t o f som e northern lands . Norsk Geografisk Tidsskrift, 14 , 132-139. JAPSEN, P . & CHALMERS , J.A . 2000 . Neogen e uplif t and tectonic s aroun d th e Nort h Atlantic : over view. In : Chalmers , J.A . & Cloetingh , S . (eds ) Neogene Uplift and Tectonics around the North Atlantic. Global an d Planetary Change, 2 4 (3-4), 165-174 . JENSEN, L.N . & SCHMIDT , BJ . 1993 . Neogen e uplif t and erosion offshore sout h Norway: magnitude and consequences fo r hydrocarbo n exploratio n i n th e Farsund Basin . In : SPENCER , A.M . (ed. ) Generation, Accumulation and Production of Europe's Hydrocarbons, HI . Specia l Publicatio n o f th e European Associatio n o f Petroleu m Geoscientists , 3, 79-88. JENSEN, L.N. , Rns , F . & BOYD , R . (eds ) 1992 . Post Cretaceous Uplif t an d Sedimentatio n alon g th e Western Fennoscandia n Shield . Norsk Geologisk Tidsskrift, Symposium Issue, 3 , 72 . JOHNSON, C . & GALLAGHER , K . 2000 . A preliminar y Mesozoic an d Cenozoi c denudatio n histor y o f th e North East Greenlan d onshor e margin . Global and Planetary Change, 24 , 261-274. LEEDER, M.R . 1999 . Sedimentology an d Sedimentary Basins. Blackwell Science , Oxford . LYELL, C . 1830-1875 . Principles o f Geology, 1 2 editions. Joh n Murray, London. MATHIESEN, A. 1998 . Modelling o f Uplift History from Maturity and Fission-track Data, Nuussauq, West Greenland. Danmark s o g Gr0nland s Geologisk e Unders0gelse Rapport , 87 . MURDOCH, L.M. , MUSGROVE , F.W . & PERRY , J.S . 1995. Tertiar y uplif t an d inversio n histor y i n the Nort h Celti c Se a Basi n an d it s influenc e o n source roc k maturity . In : CROKER , PF . & SHANNON, P.M . (eds ) Th e Petroleum Geology of Ireland's Offshore Basins. Geologica l Society, London , Specia l Publications , 93 , 297-319. NANSEN, F . 1904 . Th e bathymetrica l feature s o f th e North Pola r seas , wit h discussio n o f th e continental shelve s an d previou s oscillation s o f the shoreline . In : NANSEN , F . (ed. ) Th e Norwegian North Polar Expedition 1893-1896, Scientific Results. Jaco b Bydwad , Christiani a (Oslo). NAYLOR, D . 1992 . Th e post-Varisca n histor y o f Ireland. In: PARNELL, J. (ed.) Basins on the Atlantic

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Seaboard: Petroleum Geology, Sedimentology and Basin Evolution. Geologica l Society , London , Special Publications, 62, 255-275. NYLAND, B. , JENSEN , L.N. , SKAGEN , J. , SKARPNES , O. & VORREN , T . 1992 . Tertiar y uplif t an d erosion i n th e Barent s Sea ; magnitude , timin g and consequences . In: LARSEN , R.M. , BREKKE , H., LARSEN , B.T . & TALLERAAS , E . (eds ) Structural and Tectonic Modelling and its Application t o Petroleum Geology. Norwegia n Petroleum Societ y (NPF ) Specia l Publication , 1 , 153-162. PARKER, J.R. 1975 . Lowe r Tertiar y san d developmen t in th e centra l Nort h Sea . In : WOODLAND , A.W . (ed.) Petroleum and the Continental Shelf o f NorthWest Europe. Applie d Science , Barking , UK , 447-454. PARNELL, J., CAREY , P.P. , GREEN , PF . & DUNCAN , W . 1999. Hydrocarbo n migratio n history , wes t o f Shetland: integrate d flui d inclusio n an d fissio n track studies . In : FLEET , A.J . & BOLDY , S.A.R . (eds) Petroleum Geology o f Northwest Europe: Proceedings o f th e 5t h Conference. Geologica l Society, London , 613-626 . REUSCH, H . 1901 . Nogl e bidra g til l forstaaelse n a f hvorledes Norge s dal e o g fjeld e e r blevn e til . Norges Geologiske Unders0gelse, Aarbog, 14 , 96-102. Rns, F . & JENSEN, L.N. 1992. Introduction: measuring uplift an d erosion—proposal for a terminology. In: Jensen, L.N. & Riis, F. (eds) Post-Cretaceous Uplift and Sedimentation along the Western Fennoscandian Shield. Norsk Geologisk Tidsskrift, 72 , 223-228. RING, U. , BRANDON , M.T. , LISTER , G.S . & WILLETT, S.D . 1999 . Exhumatio n processes . In : RING, U. , BRANDON , M.T. , LISTER , G.S . & WILLETT, S.D . (eds ) Exhumation Processes: Normal Faulting, Ductile Flow and Erosion. Geological Society , London , Specia l Publications , 154, 1-27 . ROWLEY, E . & WHITE , N . 1998 . Invers e modellin g o f extension and denudation in the East Irish Sea and surrounding areas . Earth an d Planetary Science Letters, 161 , 57-71. SCOTCHMAN, I.C . & THOMAS , J.R.W . 1995 . Tertiar y uplift an d inversion history in the North Celtic Se a Basin and its influence o n source rock maturity. In: CROKER, PF . & SHANNON , P.M . (eds ) Th e Petroleum Geology of Ireland's Offshore Basins. Geological Society , London, Specia l Publications , 93,385-411. SOLHEIM, A. , Rns , E , ELVERHOI , A. , FALEIDE , J.I. , JENSEN, L.N . & CLOETINGH , S . 1996 . Impac t o f glaciations o n basi n evolution : dat a an d model s from th e Norwegia n margi n an d adjacen t areas — introduction an d summary . Global an d Planetary Change, 12 , 1-9 . STEUVOLD, L.M . & ELDHOLM , O . 1996 . Cenozoi c uplift o f Fennoscandia inferre d from a study of th e mid-Norwegian margin . Global an d Planetary Change, 1 2 (1-4), 359-386. STRATAGEM PARTNERS , 2002. The Neogen e stratigra phy o f th e Europea n margi n between Lofote n an d

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. G. DORE ETAL.

Porcupine. Stoker , M.S . (compiler) . A produc t o f WHITE , R.S . 1988 . A hot-spot model for Early Tertiary the EC-funde d STRATAGE M project . Online . volcanis m in the North Atlantic. In: MORTON, A.C. Available a t http://www.stratagem-europe.org. & PARSONS, L.M. (eds) Early Tertiary Volcanism in SUMMERFIELD, M.A . 1991 . Global Geomorphology. th e North Atlantic. Geologica l Society , London , Longman, Harlow , UK. Specia l Publications , 39, 241-252.

Present an d past influence of the Iceland Plum e o n sedimentatio n STEPHEN M . JONES 1, NICKY WHITE 1, BENJAMIN J. CLARKE 1'2, ELEANOR ROWLEY 1'3 & KERRY GALLAGHER 1'4 l Bullard Laboratories, Madingley Rise, Madingley Road, Cambridge CB3 OEZ, UK (e-mail: jones@ esc.cam.ac.uk) 2 Present address: Department of Geology and Geophysics, University of Edinburgh, King's Building, West Mains Road, Edinburgh EH9 3JW, UK 3 Present address: Shell Eygpt, 6 Hassan El-Sherley Street, PO Box 2681 El Horreya, Heliopolis, Cairo, Eygpt 4 TH Huxley School of Environment, Earth Science, and Engineering, Imperial College of Science, Technology and Medicine, South Kensington, London SW7 2AS, UK Abstract: Th e Cenozoic developmen t o f the North Atlantic provinc e ha s been dramaticall y influenced b y the behaviour of the Iceland Plume , whose striking dominance i s manifest by long-wavelength free-air gravity anomalies and by oceanic bathymetri c anomalies. Here, we use these anomalies to estimate the amplitude an d wavelength o f present-day dynami c uplif t associated wit h this plume. Maximum dynamic support in the North Atlantic is 1.5-2 km at Iceland itself . Mos t o f Greenlan d i s currently experiencing dynami c suppor t o f 0.5-1 km, whereas th e N W Europea n shel f i s generall y supporte d b y < 0.5 km. Th e proto-Icelan d Plume had a n equally dramatic effec t o n the Early Cenozoi c palaeogeograph y o f the North Atlantic margins , a s w e illustrat e wit h a stud y o f plume-relate d uplift , denudatio n an d sedimentation o n th e continenta l shel f encompassin g Britai n an d Ireland . W e infe r tha t during Paleocen e tim e a hot subvertica l shee t o f asthenospher e welle d u p beneath a n axi s running from the Faroes through the Irish Sea towards Lundy, generating a welt of magmatic underplating of the crust which is known to exist beneath thi s axis. Transient an d permanen t uplift associate d wit h this magmatic injection caused regional denudation, and consequently large amount s of clastic sedimen t hav e been she d into surrounding basins during Cenozoi c time. Mass balance calculations indicat e agreemen t betwee n the volume of denuded materia l and the volume of Cenozoic sediment s deposited offshor e in the northern Nort h Se a Basin and th e Rockal l Trough . Th e volum e o f materia l denude d fro m Britai n an d Irelan d i s probably insufficien t to account fo r the sediment i n the Faroe-Shetland Basin and an excess of sediment has been supplied to the Porcupine Basin. We emphasize the value of combining observations fro m bot h oceani c an d continenta l realm s t o elucidat e th e evolutio n o f th e Iceland Plume throug h spac e an d time.

The free-ai r gravit y fiel d ove r th e norther n estimate s of dynami c support derived from th e hemisphere i s dominate d by a long-wavelengt h bathymetr y an d gravit y fiel d o f th e Nort h high centre d o n Iceland , stretchin g fro m th e Atlanti c oceanic real m t o constrai n the present Azores t o Siberi a an d fro m Baffi n Islan d t o magnitud e of dynami c suppor t of bot h oceanic Denmark (Fig . 1) . Bot h theoretica l consider - crus t and the adjacent continenta l margins, ations and observations from th e world's oceans Next , we consider the region that was affecte d suggest that long-wavelength ( > 1000km ) posi- b y th e plum e in th e past . Temporal and spatial tive anomalie s ar e generall y associate d wit h variatio n of plume-relate d uplift ha s playe d an mantle upwellin g an d dynami c uplif t (Sclate r importan t role in the evolutio n of both margins, et al 1975 ; McKenzie 1994). If these inferences especiall y i n controllin g th e generatio n an d also hold in the North Atlantic province, the areal distributio n o f clasti c sediments . Here , w e extent o f th e gravit y hig h suggest s tha t th e conside r the specifi c exampl e of sedimen t mass continental margin s of N W Europ e and easter n balanc e around Britain and Ireland. Geochemical Greenland are dynamically supported at present, evidenc e has proved that this region experienced In th e firs t par t o f thi s paper , w e compar e magmati c underplatin g o f th e crus t durin g From: DORE , A.G., CARTWRIGHT, J.A. , STOKER , M.S. , TURNER, J.P . & WHITE , N. 2002 . Exhumation of th e North Atlantic Margin: Timing, Mechanisms an d Implications for Petroleum Exploration. Geologica l Society , London, Special Publications, 196, 13-25 . 0305-8719/02/$15.00 © The Geological Society of London 2002.

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Fig. 1 . Free-air gravity anomaly ove r part of the northern hemisphere , displaye d using a Lambert azimutha l equal area projectio n centre d o n Iceland . Th e long-wavelengt h gravit y hig h centre d o n Iceland , whic h extend s fro m south o f the Azore s t o Spitzberge n an d fro m Baffi n Islan d t o Scandinavia , shoul d b e noted .

Paleocene time , leadin g t o permanen t uplift , which drov e denudatio n (Thompso n 1974 ; Brodie & Whit e 1995) . Mas s balanc e provide s a mean s o f comparin g an d verifyin g comp lementary measure s o f Earl y Cenozoi c denuda tion, estimate d fro m th e onshor e recor d o f denudation an d th e offshor e record o f sedimen t accumulation. Two forms of surface uplift are associated wit h mantle plumes . Dynami c suppor t alway s results when abnormall y ho t mantl e i s emplace d beneath th e lithosphere. Thi s support i s transient and disappear s whe n th e therma l anomalie s i n the asthenospher e an d lithospher e dissipat e b y convection an d conduction . Th e Nort h Atlanti c region i s dynamicall y supporte d toda y an d i t must als o hav e bee n dynamicall y supporte d during Earl y Cenozoi c time , a s th e volcani c record attest s t o abnormall y ho t mantl e a t tha t time. Permanen t uplif t ma y als o occu r i f mantle thermal anomalies induce melting and the melt is injected int o or just beneath the crust. Permanen t uplift affecte d Britai n an d Irelan d durin g

Paleocene time , an d th e region s flankin g th e line o f continental separation a t the Paleocene Eocene boundary. Comparison o f the two themes we presen t i n thi s paper therefor e demonstrate s the relativ e spatia l exten t an d magnitud e o f transient an d permanen t uplift . Th e ter m epeirogenic uplif t refer s t o uplif t tha t coul d b e permanent and/o r transient . I n general , epeiro genic uplif t refer s t o uplif t tha t is no t generate d by horizonta l plat e motions , an d th e ter m nee d not impl y an y particula r mechanism . However , the presen t an d pas t epeirogeni c uplif t o f th e North Atlanti c region w e discuss in this study is generated b y the Iceland Plume. Present-day dynamic support The Nort h Atlanti c Ocea n i s anomalousl y shallow i n th e vicinit y o f Iceland . Anomalou s topography culminate s a t Icelan d itself , whic h rises t o c . 2 km abov e sea-level , o r c . 4. 5 km above the average depth of the global mid-ocea n ridge system . Tw o methods ca n b e employe d t o

ICELAND PLUM E PAST AND PRESEN T

investigate dynami c suppor t o f th e regio n o f oceanic crus t roun d Iceland . Th e firs t metho d exploits th e fac t tha t th e dept h o f oceani c crus t away from th e influence of mantle plumes varies with ag e i n a well-understoo d manner , b y comparing th e present-da y bathymetr y aroun d Iceland wit h thi s referenc e depth . Th e secon d method involve s establishin g a lin k betwee n dynamic suppor t an d th e long-wavelengt h free air gravit y anomaly . Analysi s o f gravit y anomalies should always be treated with caution, as an y gravit y fiel d ca n b e explaine d b y a n infinite numbe r o f densit y distributions , eac h with differen t implication s for dynamic support. In this section, we first establish that estimates of dynamic suppor t derive d fro m bathymetr y an d gravity ar e i n genera l agreemen t ove r oceani c crust aroun d Iceland . Thi s resul t the n give s u s confidence i n estimatin g dynami c suppor t o f the adjacen t continenta l margin s usin g gravity alone.

15

Estimates from bathymetry Figure 2 is a plot of anomalous topography in the North Atlantic , calculate d b y subtractin g th e well-known age-dept h mode l o f Parson s & Sclater (1977 ) fro m th e bathymetry . Unfortu nately, th e anomalou s topograph y i n Fig . 2 cannot be interpreted solel y i n terms o f present day dynami c suppor t because i t als o include s a component o f permanent topograph y caused b y spatial variation s i n th e thicknes s o f oceani c crust. However, the magnitude of this permanent topography ca n b e estimate d give n determi nations o f oceani c crusta l thicknes s fro m wide angle seismi c experiments . I n th e ocean s surrounding Icelan d bu t awa y fro m th e con tinental margin s an d the Greenland-Iceland Faroes Ridge , anomalousl y ho t mantl e ha s generated crus t 7-10k m thic k (Whit e 1997) . Isostatic balancing shows that this variation of up to 3k m greate r tha n th e thicknes s o f standar d

Fig. 2 . Anomalou s topograph y o f th e Nort h Atlanti c Ocean , calculate d b y subtractin g th e age-dept h coolin g relationship of Parsons & Sclater (1977) from th e ETOPO5 bathymetr y grid. The age of oceanic lithosphere was taken from Miiller et al (1997) ; the Greenland-Iceland-Faroes Ridge was excluded from the calculation because its age is not well known. Anomalous topography is corrected for sediment loading of oceanic basement using the method of Le Douaran & Parsons (1982) and the sediment thickness map of Laske & Masters (1997). It should be noted that the anomalous topography displayed here contains both a component of present-day dynamic support and a permanent component cause d by crustal thickness variations.

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oceanic crus t (7 km, White et al. 1992 ) generates permanent topograph y o f 0-0.5 km. Thi s valu e of 0. 5 km i s smalle r tha n th e tota l variatio n i n anomalous topograph y acros s thes e regions , suggesting tha t most o f th e anomalou s topogra phy i s generate d b y present-da y dynami c support. However , clos e t o th e continenta l margins an d th e Greenland-Iceland-Faroe s Ridge, th e thicknes s o f oceani c crus t i s 10-15 km, varyin g ove r distance s o f <50k m (Barton & White 1997; Wei r et al. 2001). In these regions, permanen t topograph y probabl y accounts fo r mos t o f th e anomalou s topography in Fig. 2 . To summariz e th e discussio n above , anom alous topography o f those regions away from the continental margin s an d th e Greenland Iceland-Faroes Ridg e ca n b e interprete d a s a n estimate o f dynamic suppor t with an error rang e of 0.5 km. However, short-wavelengt h variation s in anomalou s topograph y associate d wit h con tinental margin s an d th e Greenland-Iceland Faroes Ridge are most likely to reflect permanent topography associate d wit h crusta l thicknes s variations. Bearin g i n min d thes e caveats , thre e important result s concernin g present-da y dynamic suppor t ca n b e gleane d fro m Fig . 2 . First, the amplitude o f dynamic suppor t clos e to Iceland itsel f i s 1.5- 2 km. Secondly , th e amplitude o f dynami c suppor t i s partiall y controlled b y the active mid-ocean ridg e system . This fac t i s clearl y see n bot h t o th e nort h o f Iceland, wher e th e activ e Kolbeinse y Ridg e i s more anomalousl y elevate d tha n th e extinc t Aegir Ridge, and to the south of Iceland, where a tongue o f anomalou s uplif t extend s alon g th e Reykjanes Ridge . Thirdly , th e continenta l margins o f easter n Greenlan d an d N W Europ e are currentl y experiencin g a t leas t 0.5k m o f dynamic support , implyin g tha t th e adjacen t continental shelve s ar e als o experiencin g significant dynami c suppor t a t present. Estimates from gravity The free-air gravity field over a convecting layer results fro m tw o competin g effects : densit y variations withi n th e laye r an d surfac e defor mation induced by the convective circulation . In the cas e o f upwellin g plumes , ho t mantl e ha s a lower density than the surrounding mantle, which acts to reduce the gravity. However, a n upwelling plume deform s th e Earth' s surfac e upwards , which acts to increase the gravity. The sign of the total free-ai r gravit y anomal y depend s o n th e relative magnitude of thes e effects . A t Rayleig h numbers appropriat e t o Earth , th e effec t o f surface deformatio n outweigh s the effec t o f th e

reduction i n density, so upwelling regions in the mantle ar e characterize d b y positiv e free-ai r gravity anomalie s a t the surface. The transfe r functio n betwee n th e free-ai r gravity anomaly g and topography h is called the admittance an d i s given by Admittance i s a functio n o f wavenumbe r k . A t wavelengths shorte r tha n c . 500km , admittance is controlled b y the mechanical propertie s o f the lithosphere. Admittanc e decrease s systemati cally with increasing wavelengt h (i.e . decreasing wavenumber), an d th e rat e o f decreas e i s dependent upo n th e effectiv e elasti c thicknes s of th e lithosphere . McKenzi e (1994 ) an d McKenzie & Fairhea d (1997 ) exploite d thi s behaviour t o estimat e th e effectiv e elasti c thickness o f th e lithospher e i n th e Pacifi c an d Indian Oceans fro m th e short-wavelengt h part of the gravit y an d topograph y fields . A t wave lengths abov e c . 500k m th e admittanc e calcu lated fro m th e observe d gravit y and topography diverges fro m tha t calculate d fro m theoretica l models assumin g a n elasti c plate . A t thes e lon g wavelengths, th e flexura l strengt h o f th e litho sphere play s n o part i n supportin g topography . Instead, topograph y i s supporte d b y stresse s exerted o n th e bas e o f th e lithospher e b y th e convecting mantl e an d b y long-wavelengt h variations i n th e densit y structur e o f th e lithosphere, which are isostatically compensated. Thus, dynami c suppor t produce s long-wave length anomalou s topography A/z conv tha t corre lates wit h th e long-wavelengt h free-ai r gravit y anomaly. Isostaticall y compensate d topography, such a s th e permanen t anomalou s topograph y that arises from crustal thickness variations in the North Atlantic , is no t correlate d wit h a measurable gravit y anomaly. The behaviou r o f th e admittanc e functio n within th e wavelengt h ban d 500-3000k m suggests a linea r relationshi p betwee n gravit y and topography . I f thi s simpl e relationship , observed i n th e Pacifi c an d India n Oceans, als o holds i n th e Nort h Atlanti c the n anomalou s topography cause d b y dynami c convectiv e support ca n b e calculate d fro m th e long wavelength free-ai r gravit y field using The value of Z to be used can b e constrained by observations. It has lon g been recognize d tha t Z ~35mGalkm~ 1 i s appropriat e fo r Earth' s oceans (Sclate r e t al . 1975) . Anothe r measur e of th e admittanc e t o b e use d t o calculat e dynamic support can be obtained from numerical

ICELAND PLUM E PAST AND PRESEN T

convection experiments . McKenzi e (1994) sum marized severa l numerica l model s o f axisym metric plumes, includin g the model of Watson & McKenzie (1991 ) tha t successfull y matched th e observed gravity , topograph y an d mel t pro duction o f th e Hawaiia n Plume . Thes e convec tion model s ar e characterized b y admittance s i n the rang e 34. 4 ± 2. 2 mGal km"l . Hence , observed an d theoretica l value s fo r th e admit tance betwee n topograph y an d gravit y o f th e oceans a t lon g wavelength s ar e i n goo d agreement. The free-ai r gravit y datase t use d her e i s a compilation o f poin t measurement s ove r land , together wit h th e satellit e gravit y datase t o f Sandwell & Smit h (1997 ) ove r th e oceans , a s described b y McKenzie & Fairhead (1997) . Th e short-wavelength part o f the gravit y field that is influenced b y th e flexura l strengt h o f th e lithosphere was removed usin g a low-pass filter. The regio n o f interes t ha s dimension s compar able with the radius of Earth, so low-pass filtering was carried out using a spherical harmonic model of th e gravit y dataset . Th e long-wavelengt h gravity mode l was generate d usin g spherica l harmonic coefficient s o f degrees 0 < / = m ^ 53 (equivalent to a low-pass filter of c . 750km ) with a suitabl e tape r a t th e uppe r cut-of f t o prevent ringing.

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Figure 3 show s th e dynami c componen t o f anomalous topography A/iconv estimated from the free-air gravity field using two different values of Z. These estimates are simply a scaled version of the long-wavelengt h gravit y fiel d an d resembl e the original gravit y field fairly closel y (compar e Figs 1 and 3). In the region surroundin g Iceland, estimates o f convective suppor t from th e gravity field agre e reasonabl y wel l wit h independen t estimates of convective support from topography (compare Fig s 2 an d 3) . I n particular , th e dynamic suppor t estimate s fro m gravit y reinforce th e thre e observation s derive d fro m Fig. 2 an d note d i n th e previou s section . First , peak dynami c suppor t i s c . 1.8k m a t Iceland . Secondly, th e activ e spreadin g axi s exert s a n important control on the long-wavelength gravity field an d o n th e magnitud e of dynami c support . Dynamic suppor t is centred o n the Mid-Atlantic Ridge, an d t o th e nort h o f Icelan d th e activ e Kolbeinsey Ridge is experiencing greater support than th e extinc t Aegi r Ridge . Thirdly , th e continental margin s ar e currentl y experiencin g significant dynami c suppor t of 0.5-1 km. It i s importan t t o not e tha t estimate s o f present-day dynami c suppor t calculate d directl y from long-wavelengt h gravity anomalies ar e not always i n agreemen t wit h estimate s calculate d from th e bathymetry . Figur e 3 suggest s tha t

Fig. 3 . Estimates of present-day dynami c suppor t in the North Atlanti c region , calculate d b y dividing the longwavelength free-air gravity field by a constant admittance, as discussed in the text. Bold continuous lines indicate continent-ocean boundaries , (a ) Dynami c suppor t predicte d usin g a n admittanc e o f Z = 3 5 mGal km"1 (appropriate for subaqueous regions); (b) dynamic support predicted usin g an admittance of Z = 5 0 mGal km"1 (equivalent value for subaeria l regions). K , Kangerlussuaq; S, Scoresby Sund.

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significant dynami c suppor t occur s from Iceland to th e Azores . However , Fig . 2 indicate s n o measurable dynami c suppor t jus t sout h o f th e Charlie Gibb s fractur e zone. Thus, the amplitud e of dynamic suppor t calculate d fro m gravity dat a is unlikel y t o be correc t i n detai l an d shoul d b e treated wit h som e caution . Nevertheless , th e agreement betwee n dynami c suppor t estimate s based o n bathymetr y an d gravit y i n th e vicinity of Icelan d suggest s tha t Fig . 3 provide s a reasonable estimat e o f dynami c suppor t o f th e Greenland-Iceland-Faroes Ridg e an d th e adjacent continenta l shelves . Free-air gravit y anomalie s ma y b e use d t o estimate dynamic suppor t of the continents in the same wa y a s fo r th e oceans . Wherea s i t i s appropriate t o emplo y a n admittanc e o f Z = 35mGalkm~ 1 to estimate dynami c suppor t of region s covere d b y water , a n equivalen t air loaded valu e o f Z = 50mGalkm~ 1 shoul d b e used fo r subaeria l regions . Dynami c suppor t estimates calculate d usin g bot h value s o f admittance ar e show n i n Fig . 3 . Greenlan d i s currently experiencin g dynami c suppor t o f 0.5-1km. Dynami c suppor t i s greates t o n th e east coast , notabl y i n th e regio n betwee n Kangerlussuaq an d Scoresb y Sund , adjacen t t o the Greenland-Iceland-Faroe s Ridge . I n con trast, the magnitud e of dynamic suppor t beneat h the N W Europea n shel f seldo m exceed s 0. 5 km. Dynamic suppor t increase s westward s fro m Scandinavia acros s th e Norwegia n continenta l shelf. Th e Nort h Se a an d souther n Englan d d o not appear to be dynamically supported . Cenozoic denudation of Britain and Ireland In thi s section , w e focu s o n on e aspec t o f th e problem o f determinin g th e Earl y Cenozoi c shape o f th e Icelan d Plume , namel y th e relationship between Cenozoic uplift , denudation and sedimentatio n o n th e continenta l shel f surrounding Britai n an d Ireland . Ou r long-ter m goal i s t o us e mas s balanc e calculation s i n conjunction wit h estimate s o f permanen t uplif t caused b y magmati c underplatin g t o determin e the behaviou r o f th e Icelan d Plum e throughou t Cenozoic time . We hav e chose n t o carr y ou t mas s balanc e calculations fo r th e Britis h Isle s fo r thre e important reasons . First , geochemica l evidenc e collected fro m rock s o f th e Britis h Cenozoi c Igneous Provinc e prove s tha t th e crus t beneat h much o f Britai n an d Irelan d ha s bee n thickene d by a substantia l amoun t o f igneou s materia l a s we explai n below , implyin g significan t perma nent uplif t durin g Paleocen e time . Secondly , a substantial bod y o f informatio n abou t th e

denudation o f Britai n an d Irelan d i s availabl e based o n modellin g o f subsidence , vitrinit e reflectance, apatit e fission-trac k an d soni c velocity datasets . Thirdly , th e product s o f Cenozoic denudatio n hav e bee n carefull y mapped i n al l o f th e surroundin g sedimentar y basins. Although detailed wor k ha s been carrie d out previousl y o n bot h onshor e denudatio n an d offshore deposition , n o attempt s hav e ye t bee n made t o check these estimate s b y constructing a mass balanc e o n a regional scale . The evidence for magmatic underplating of the crust beneat h Britai n an d Irelan d i s no t widel y recognized, despit e th e fac t tha t th e conse quences of suc h an igneous addition, in terms of uplift, denudatio n and sedimentation, can explain many feature s o f th e surfac e geolog y an d thu s have obviou s implication s fo r th e hydrocarbo n industry. For decades i t has been recognized that the compositio n o f floo d basalt s fro m th e Hebrides can be explaine d onl y by fractiona l crystallization o f u p t o 70 % o f thei r origina l liquid mas s (Thompso n 1974) . The crystallize d residuum must therefore remain at depth. From a surface processe s poin t o f vie w th e argumen t ends here , becaus e i f suc h materia l i s adde d anywhere within the upper half of the lithosphere its densit y wil l b e les s tha n tha t o f th e asthenosphere i t displaces , an d isostati c balan cing shows that permanent uplift wil l result. The depth a t whic h crystallizatio n occurred, an d a t which the crystallized residuu m remains , ca n be established usin g geobarometr y techniques , which determin e th e pressur e a t whic h th e major-element composition s o f bot h floo d basalts an d th e phenocryst s the y contai n ar e i n equilibrium. Pressures o f c . I GP a are estimated, equivalent t o a dept h o f aroun d 30km , i.e . th e depth o f th e Moh o (Thompso n 1974 ; Brodi e & White 1995) . This result is not surprising , as the fact tha t basalti c mel t i s les s dens e tha n th e mantle lithospher e but roughly the sam e density as the lower crust means that the melt should rise through th e mantl e an d pon d a t th e bas e o f th e crust, givin g ris e t o th e concep t o f magmati c underplating. Wide-angl e seismi c experiment s across Nort h Atlanti c volcani c continenta l margins hav e image d high-velocit y bodie s a t the bas e o f th e continenta l crust , whic h ar e usually interprete d a s pod s o f igneou s materia l underplated beneat h th e crus t a t th e tim e o f continental separatio n (Barto n & Whit e 1997) . Preliminary result s fro m modellin g o f a wide angle seismi c lin e spannin g th e Iris h Se a suggest tha t a high-velocit y zon e exist s nea r the Moho , whic h probabl y represent s th e igneous materia l tha t w e expec t t o se e underplated beneat h th e onshor e par t o f th e

ICELAND PLUM E PAS T AND PRESEN T British Cenozoi c Igneou s Provinc e (S . Al-Kindi, pers . comm.) . However , a s thi s discussion implies , seismi c evidenc e alon e ca n never directl y revea l th e ag e o r natur e o f thes e high-velocity layers , s o it is always necessary t o interpret seismi c evidenc e i n conjunctio n wit h geochemical evidence . T o conclude, i t should b e noted that crustal magmatic underplating is not a peculiar featur e o f th e Nort h Atlanti c bu t i s common to all continental flood basalt province s (Cox 1993) .

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Mass balance calculations The mas s balanc e calculatio n ha s thre e parts: th e exten t an d amoun t o f denudatio n of th e sedimen t sourc e are a o r catchmen t (Fig. 4) ; th e mas s o f sedimen t accumulate d through tim e i n th e sedimentar y basin s immediately adjacen t t o th e Britis h Isle s (Fig. 5) ; an d th e mas s o f materia l los t fro m the syste m b y solutio n an d b y escap e t o th e deep ocean .

Fig. 4. Estimate s o f Cenozoi c denudatio n o f Britain an d Ireland base d o n modelling subsidenc e historie s an d apatite fission-trac k lengt h distributions. +, locations of well sections in extensional sedimentary basins used for subsidence modelling. The amount of missing post-rift basin fill was predicted by fitting a theoretical subsidenc e curve to the remnant synrif t stratigraphy, assumin g the standard lithospheric stretching model. Thi s technique has been describe d full y b y Rowle y & White (1998) , an d results hav e been tabulate d by Hal l (1995 ) an d Rowley (1998). O , location s o f apatit e samples . Apatit e fission-trac k lengt h distribution s wer e modelle d t o fin d Mesozoic-Cenozoic therma l historie s b y K . Gallaghe r usin g th e Laslet t e t al. (1987 ) annealin g mode l fo r Durango apatite and the method described by Gallagher (1995); th e results were reported by Rowley (1998). The amount of Cenozoic coolin g wa s converted t o a range of denudation estimate s b y Monte Carlo modellin g usin g the range of geothermal gradients observed in the North Sea today (Rowley 1998). Data covering Ireland are from Allen e t al . (2002) . (a ) Minimu m denudation estimat e foun d b y contourin g result s derive d fro m subsidenc e analysis, th e lowe r bound s o f the Rowle y (1998 ) denudatio n estimate s derive d fro m modellin g apatit e fission track length distributions, and the minimum denudation estimate fo r Ireland o f Allen et al. (2002, fig . lOb) . (b ) Maximum denudation estimate foun d b y contouring the modes of the Rowley (1998) denudation estimate s an d the maximum denudation estimat e for Ireland o f Allen et al (2002 , fig. lOa). These contou r plots were generate d by taking the mean of all estimates within blocks of dimension 1° longitude X 307 latitude (c. 50 km X 50 km) an d gridding the resulting values using the continuous curvature spline method of Smith & Wessel (1990). It should be noted that a further se t of Cenozoic denudation estimates based on modelling of vitrinite reflectance profiles fro m a subset of the wells used for subsidence analysis yields a denudation estimate that lies between the minimum and maximum estimates illustrate d her e (Rowle y 1998) .

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Fig. 5 . Map of the NW European continenta l shel f showin g the Cenozoic soli d sedimen t thickness. This isopac h map wa s constructe d fro m a databas e o f 2 D an d 3 D seismi c reflectio n survey s calibrate d wit h well-lo g information. Soli d thicknes s wa s calculated fro m th e observe d thicknes s b y subtractin g the pore-space volum e predicted b y th e standar d exponentia l relationshi p betwee n fractiona l porosit y =
The best-constraine d elemen t i n th e mas s balance proble m i s th e mas s o f Cenozoi c sediment i n th e offshor e basins . Here , th e volume of soli d sedimen t i s used i n place o f the mass o f sediment . Th e soli d volum e o f th e offshore sedimen t pil e is calculated b y removin g the volum e accounte d fo r b y porosity , predicte d by the standard exponentia l porosit y model . The principal sourc e o f error i n calculatin g the soli d volume i s th e dept h conversio n procedure . However, the error introduced by this uncertainty can be realistically quantifie d by using a range of velocity-depth functions, and the resulting erro r estimates ar e illustrate d in Fig. 6 . Parameteriza tion o f th e porosity-dept h relationshi p use d i n the compactio n calculatio n i s relativel y wel l determined fo r th e basin s surroundin g Britai n and Ireland . Fo r example , usin g parameter s fo r the end-membe r lithologie s o f shal e an d san d determined by Sclate r & Christie (1980 ) for the North Se a yields a variation in solid volume that

is only 14 % of the error range associated wit h the depth conversio n calculatio n i n tha t region . Figure 5 show s th e soli d sedimen t thicknes s accumulated aroun d Britai n an d Irelan d durin g Cenozoic time . Durin g thi s tim e th e mai n sediment sink s wer e th e Porcupin e Basin , th e Rockall Trough , th e Faroe-Shetlan d Basi n an d the Nort h Se a Basin . Th e soli d volum e o f Cenozoic deposit s i n souther n Englan d an d offshore souther n Ireland i s negligible. The tota l Cenozoi c denudatio n i s show n i n Fig. 4 , base d o n thre e independen t line s o f evidence. Th e firs t lin e o f evidenc e depend s o n our knowledg e o f extensiona l sedimentar y basins. Man y extensiona l basin s aroun d Britai n and Irelan d contai n a fault-controlle d syn-rif t stratigraphy bu t th e anticipate d post-rif t strati graphy i s partiall y o r entirel y absent (Brodi e & White 1995) . Th e amoun t o f missin g post-rif t stratigraphy ca n b e predicted , base d o n ou r knowledge o f th e kinematic s o f extensiona l

ICELAND PLUME PAST AND PRESEN T

Fig. 6. Summary of mass balance calculations for NW European continenta l shelf . Ope n bar s represen t th e amount o f soli d sedimen t accumulate d offshore , calculated b y integratin g Fig . 5 ove r eac h offshor e depocentre. Error range s reflec t uncertaintie s i n dept h conversion. The soli d sedimen t volum e for th e North Sea plotted here has been halved to account for the fact that Scandinavia ha s als o supplie d sedimen t t o th e North Sea . Fille d bar s represen t volum e o f roc k denuded fro m onshor e catchments , calculate d b y integrating th e minimu m an d maximu m estimate s given in Fig. 4 over the regions marked in Fig. 5. The Porcupine sedimen t sin k i s supplie d b y tw o catch ments: SI , souther n Irelan d an d th e Iris h Sea ; WI, western Ireland . Th e othe r sedimen t sink s hav e on e catchment each .

basins (Rowle y & White 1998) . Th e othe r tw o methods o f estimatin g denudatio n rel y o n thermal indicator s withi n th e sedimen t pil e tha t retain a memor y o f thei r buria l history . Reflectance o f vitrinit e increase s wit h risin g temperature b y mean s o f a non-reversibl e reaction, s o vitrinit e retain s a memor y o f th e highest temperatur e i t has experienced. Denuda tion estimate s ca n b e obtaine d b y comparin g vitrinite reflectanc e profile s wit h a globa l reflectance datase t fro m non-inverte d basin s (Rowley 1998) . Analysi s o f fissio n track s i n apatite als o provide s temperatur e historie s tha t can be interpreted in terms of denudation through time. The most prominent featur e of Fig. 4 is the peak in denudation centred on NW England. This region suffere d denudatio n o f 1-2. 5 km during Cenozoic time . A peak in denudation centre d o n

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NW Englan d ha s als o bee n suggeste d b y previous studie s base d o n apatit e fission-trac k data alone (Lewis et al. 1992). The magnitude of Cenozoic denudatio n generall y decrease s towards th e presen t offshor e areas . Anothe r important messag e fro m Fig . 4 i s tha t th e erro r range on the denudatio n estimat e for any particular region is c. 1 km. It is difficult t o quantify los s of mass from th e system. Loss by solution depends strongl y on the rock typ e being eroded . I t woul d b e difficul t t o include th e amoun t o f los s b y solutio n i n th e mass balanc e calculation s becaus e o f th e uncertainty ove r th e origina l lithologie s bein g eroded an d the variety of lithologies withi n each catchment. Her e w e appl y n o correction s t o account for loss o f mass from th e system . When loss o f mas s fro m th e syste m i s neglected , th e solid volum e o f sedimen t measure d i n offshor e basins provides a lower bound on the denudation. If th e soli d sedimen t volum e measure d offshor e is foun d t o be greater tha n the onshor e estimat e of denudatio n the n a n additional, unidentifie d source of sediment to the offshore basin would be implied. O n the other hand , if the offshor e soli d sediment volume is found to be significantly les s than the onshore estimate o f denudation then the difference betwee n th e tw o estimate s ma y provide a n estimat e o f th e amoun t o f materia l lost from th e system. Mass balance results Figure 6 summarizes the result of balancing th e volume o f soli d sedimen t accumulate d offshor e with direc t estimate s o f denudatio n mad e onshore fo r th e Porcupine , Rockall , Faroe Shetland an d norther n Nort h Se a systems . Th e drainage catchment matche d wit h each offshor e sediment sink is shown in Fig. 5. The boundaries of these catchments are based on the present-day topography an d bedloa d transpor t patterns . Palaeotopographic reconstruction s o f Britai n and Irelan d durin g earlies t Eocen e time , a t th e time o f maximu m dynami c suppor t b y th e Iceland Plum e (se e discussio n later) , sugges t that th e north-sout h drainag e divid e runnin g through Scotlan d an d Englan d ha s remaine d relatively stati c sinc e Paleocen e tim e (Jone s 2000). Th e position s o f th e west-east-oriente d drainage divide s ar e more likel y t o have altere d position durin g Cenozoi c time , an d we conside r the effec t o f suc h migratio n i n th e followin g discussion. In the Rockall Troug h and northern North Sea systems, th e volum e o f Cenozoi c sedimen t accumulated offshor e i s th e sam e a s th e denudation measure d onshore , withi n error .

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However, i n th e Faroe-Shetlan d system , th e volume o f sedimen t accumulate d offshor e i s greater than the denudation measured onshore by at leas t 2 x 10 4km3. Thi s discrepanc y implie s that th e Faroe-Shetlan d Basi n ha s anothe r source o f Cenozoi c sedimen t i n additio n t o th e catchment i n th e regio n o f N W Scotland . Th e additional sedimen t sourc e ma y b e a catchmen t in th e regio n o f th e Faroe s o r Greenland . Alternatively, th e boundarie s o f th e drainag e catchment show n i n Fig . 5 coverin g N W Scotland ma y b e incorrect . Th e souther n boundary o f tha t catchmen t i s no t wel l constrained an d it i s possibl e tha t sedimen t wa s also source d fro m wester n Scotlan d an d chan nelled betwee n th e Oute r Hebride s an d th e Scottish mainlan d toward s th e Faroe-Shetlan d Basin. A thir d possibilit y i s tha t additiona l sediment wa s transporte d int o th e Faroe-Shet land Basi n b y current s runnin g alon g th e basi n axis. Stron g botto m current s flowin g south westwards alon g th e Faroe-Shetlan d Troug h were initiate d i n Oligocen e tim e an d persis t t o the present da y (Davie s e t al. 2001). Lack o f a significan t differenc e betwee n th e two estimate s o f denudatio n i n th e Rockal l an d northern Nort h Se a system s suggest s tha t littl e mass has been lost from thos e systems. However, the tota l sedimen t supplie d t o th e Porcupin e Basin b y bot h th e catchmen t coverin g wester n Ireland an d th e catchmen t coverin g souther n Ireland an d th e Iris h Se a i s greate r tha n th e material accumulate d offshor e b y a t leas t 4 x l 0 4 k m 3 . Thus , a significan t mas s o f sediment ha s bee n los t fro m thi s system . Th e magnitude o f this discrepancy show n in Fig. 6 is a lower bound because no account has been taken of sedimen t supplie d t o th e wester n sid e o f th e Porcupine Basi n b y th e Porcupin e Ban k an d Ridge. Thre e effect s hav e probabl y contribute d to th e los s o f mas s fro m th e Porcupin e system . First, Carboniferou s limeston e crop s ou t ove r a large area of the Irish catchments , an d erosion of this limeston e produce s negligibl e clasti c sedi ment. Secondly , i t i s likel y tha t mos t o f th e sediment supplie d t o th e souther n en d o f th e Porcupine Basin has escaped directly to the deepsea sediment fa n observed on the North Atlanti c oceanic abyssa l plai n vi a th e canyo n syste m i n the S E o f th e Porcupin e Basi n (Fig . 5). Thi s canyon syste m ha s bee n activ e sinc e Earl y Oligocene tim e (Jone s 2000) . Thirdly , a variety of evidenc e fro m bot h th e continenta l shel f surrounding Britai n an d Irelan d an d adjacen t oceanic crus t suggest s tha t th e hea d o f th e Iceland Plum e expande d rapidl y durin g earlies t Eocene time, causing dynamic uplift centred N W of Britai n an d Irelan d (se e discussion below) .

The resultin g southeasterl y til t o f Britai n an d Ireland probabl y cause d mos t o f th e materia l eroded fro m souther n Irelan d an d th e Iris h Se a during Eocen e tim e t o b e she d southward s into the Bay of Biscay, rather than westwards into the Porcupine Basin.

Cenozoic permanen t an d transient uplift In this section, we discuss evidence fo r tempora l and spatia l variatio n i n th e Icelan d Plum e throughout Cenozoi c time . Paleocene permanent uplift The histor y o f sedimen t flu x int o th e basin s surrounding Britai n an d Irelan d durin g Paleo cene tim e add s detai l t o th e denudatio n history established usin g th e mas s balance . Th e rat e o f sediment flux into an offshore basi n is related t o the siz e an d th e rat e o f denudatio n o f th e corresponding drainag e catchment. The lag time between a n increas e i n denudatio n rate an d th e corresponding increas e i n sediment flux offshore is likely t o be < lOOk a (Readin g 1991 ; Burgess & Hoviu s 1998) . Therefore , sedimen t flu x histories i n th e basin s surroundin g Britai n an d Ireland ca n b e directl y relate d t o uplif t o f thei r sediment sourc e regions . Calculatio n o f th e volumes o f Paleocen e an d Eocen e sedimen t sequences i n the northern North Se a and FaroeShetland Basin s ha s show n tha t sedimen t flu x into these basins grew through Paleocene tim e to a maximum at 59-58 Ma and then decreased into Eocene tim e (Reynold s 1994 ; Clark e 2002) . Thus w e ma y infe r tha t epeirogeni c uplif t o f Scotland an d norther n Englan d was initiate d i n Early Paleocen e tim e an d th e rat e o f uplif t peaked durin g mid-Lat e Paleocen e time . I n contrast, sedimen t flux into the Porcupine Basin remained lo w throughou t Paleocen e time , suggesting tha t uplif t rate s wer e lowe r awa y from th e regio n o f Scotlan d an d norther n England a t this time (Jones 2000) . The denudatio n histor y o f Scotlan d inferre d from th e histor y o f sedimen t flu x aroun d Scotland correlate s wit h th e histor y o f onshor e igneous activity, which also peaked a t 59-58 Ma (White & Lovel l 1997) . A s w e hav e seen , th e sediment flu x dat a sugges t tha t Paleocen e epeirogenic uplif t wa s confine d t o th e regio n close t o th e present-da y surfac e expressio n o f onshore igneou s activity . W e therefor e sugges t that th e Paleocen e igneou s activity , epeirogeni c uplift an d denudatio n wer e initiate d whe n a subvertical shee t o f unusually hot asthenospher e was injecte d beneat h th e presen t Faroes-Iris h Sea-Lundy axis . Th e surfac e expressio n o f

ICELAND PLUME PAST AND PRESENT

onshore igneou s activit y i s concentrate d i n western Scotlan d an d Ulster , bu t thi s igneou s activity i s offse t fro m th e locu s o f maximu m denudation, whic h i s centre d o n norther n England an d th e Iris h Sea . I f th e denudatio n shown i n Fig . 4 i s drive n mainl y b y permanen t uplift resultin g from magmati c underplating, the greatest amount of melt must have been added to the crus t beneat h thi s region . Preliminar y modelling o f a wide-angle seismi c lin e crossing Ireland, th e Iris h Se a an d norther n England has imaged a high-velocit y po d nea r th e Moho , which is thickest beneath the centre of the profile and thin s toward s wester n Irelan d an d towards the Nort h Se a (S . Al-Kindi, pers . comm.) . Thi s pod ma y represen t th e laye r o f magmati c underplating beneat h th e crus t tha t i s predicte d by bot h petrologica l an d sedimentologica l evidence. Paleocene-Eocene dynamic support The sedimentar y record s i n al l basins surrounding th e denude d are a o f Britai n an d Irelan d indicate a major regression-transgression cycle , with maximu m regressio n correspondin g t o th e upper Flett Formation and its lateral equivalents, which wer e deposite d durin g earlies t Eocen e time (e.g . Milton e t al 1990 ; Ebdo n et al 1995) . It is now generally believed that this regressiontransgression cycl e i s relate d t o a phas e o f transient dynami c uplif t tha t peake d in earlies t Eocene tim e (Nadi n e t al . 1997 ; Jone s e t al . 2001). The magnitude of dynamic support can be quantified withi n extensiona l sedimentar y basins, where we can isolate epeirogenic vertica l motions fro m tectoni c vertica l motions , whic h are ultimatel y cause d b y horizonta l plat e motions. Given the synrif t subsidenc e history of an extensiona l basin , th e post-rif t subsidenc e history ca n b e calculate d usin g th e well established lithospheri c stretchin g model . Post rift marke r horizon s wit h sedimentologicall y well-constrained water depths are then compared with the anticipated post-rif t subsidence curv e to reveal the magnitude of dynamic support through time. Th e mos t importan t o f thes e marke r horizons aroun d Britai n an d Irelan d ar e th e earliest Eocene delta-top coal s of the upper Flett Formation an d its lateral equivalents. The results of thi s subsidenc e analysi s sho w tha t pea k dynamic support was c. 0.5 km in the Porcupine and Faroe-Shetlan d Basins , an d dynami c sup port decreased in a southeasterly direction to zero across souther n Englan d (Nadi n e t al . 1997 ; Jones 2000; Jones e t al 2001) . Peak dynami c suppor t a t th e Paleocen e Eocene boundar y wa s coeva l wit h voluminous

23

intrusive an d extrusive igneou s activit y offshor e NW o f Britai n an d Irelan d tha t wa s associate d with break-u p o f Europ e an d Greenlan d abov e unusually ho t asthenosphere . Whit e (1997 ) collated oceani c crustal thickness measurement s and showe d tha t th e ho t asthenospher e o f th e Iceland Plum e hea d extende d a t leas t 1000k m along th e continent-ocea n boundarie s t o th e north an d sout h o f th e presen t Greenland Iceland-Faroes Ridge . Dynami c suppor t esti mates fro m subsidenc e analyse s aroun d Britain and Ireland suggest that the plume head extended a simila r distanc e inboar d o f the N W Europea n continental margin. We migh t expec t tha t a s maximu m transient dynamic suppor t o f th e basin s surroundin g Britain an d Irelan d occurre d durin g earlies t Eocene time , th e acm e o f denudatio n an d offshore sedimen t flux should also have occurred at thi s time . However , a s w e discusse d above , maximum sedimen t flu x int o th e Faroe-Shet land an d Nort h Se a Basin s actuall y occurre d 3-4 Ma previously, during Late Paleocene time . Only in the Porcupine Basin, west of Ireland, was the pea k sedimentatio n rat e coeva l wit h pea k dynamic suppor t i n earlies t Eocen e tim e (Jone s 2000). Thi s discrepanc y i n timin g o f severa l million years between the two different measures of epeirogenic uplif t i s an important observatio n that ha s ye t t o b e explained . On e possibl e explanation i s tha t ther e wer e tw o separat e phases o f mantl e plum e activity , th e firs t a n upwelling hot vertical shee t that led to magmatic underplating o f th e crus t beneat h th e Faroes Irish Sea-Lundy axis, and the second the growth of a mushroom-shape d plum e hea d beneat h a region ove r 1000k m in radius. Oligocene-Recent dynamic support Variations i n crusta l thicknes s an d structur e around Iceland record changes in the siz e of the thermal hea d o f the Icelan d Plume , measure d at the Mid-Atlantic Ridge , through Cenozoic time . As we have alread y discussed , th e radius o f the plume hea d wa s > 1000 km a t th e tim e o f continental separatio n betwee n Europ e an d Greenland i n earlies t Eocen e time . However , the distributio n o f normal-thicknes s oceani c crust sout h of Iceland suggest s that during Late Eocene times, the part of the plume head beneath the Mid-Atlantic Ridge extended < 300 km from the presen t centr e of Iceland (Whit e 1997) . Th e portion of the plume head beneath the ridge then increased t o it s presen t radiu s o f c . 1000k m between Oligocen e tim e an d the present (Fig s 2 and 3). Studies of plate motion with respect to the hotspot reference frame suggest that the centre of

24

S. M. JONES ETAL.

the Iceland Plum e lay beneath Greenlan d durin g mid-Cenozoic time an d ha s effectivel y move d eastwards toward s Europ e throug h tim e (Lawve r & Mulle r 1994) . W e sugges t tha t thi s relativ e motion betwee n th e Mid-Atlanti c Ridg e an d a plume o f relativel y constan t mas s flu x ma y account fo r th e apparen t increas e i n th e siz e o f the head of the Iceland Plum e between Oligocen e time and the present. Thi s history of motion als o agrees wit h ou r observatio n tha t Greenlan d i s currently experiencin g greate r dynami c suppor t than N W Europ e (Fig . 3) . Th e reaso n i s tha t a greater volum e o f ho t plume-hea d materia l accumulates beneat h th e plat e tha t th e plum e stem i s movin g awa y fro m tha n accumulate s beneath th e plat e tha t th e plum e ste m i s approaching (Rib e & Delattr e 1998) . A s Rohrman & van der Beek (1996 ) have suggested , relative movement o f the Iceland Plum e toward s Europe ma y als o provid e a n explanation fo r th e Miocene-Recent epeirogeni c uplif t an d conse quent increas e i n denudatio n tha t i s know n t o have affecte d Scandinavia .

Conclusions The Cenozoi c evolutio n o f th e Nort h Atlanti c province ha s bee n dominate d b y interactio n between th e Icelan d Plum e convectiv e syste m and sea-floo r spreadin g betwee n Europ e and Greenland o n a hierarchy o f spatial an d tempora l scales. Th e continenta l sedimentar y recor d seems to be influenced by the relative importance of dynami c support , whic h ha s varie d throug h through time , an d permanen t uplift , whic h wa s driven b y magmati c underplatin g o f th e crust . Further progres s i n understandin g thi s relation ship will depend upon measuring the distribution of Paleogen e magmati c underplatin g an d upo n an improve d quantitativ e understandin g o f Cenozoic denudation . Our conclusions concernin g th e present shap e and siz e o f th e Icelan d Plum e an d concernin g Cenozoic mas s balanc e are : (1) maximu m dynami c suppor t i n th e Nort h Atlantic is 1.5-2 km at Iceland itself . (2) Th e magnitud e o f dynamic suppor t i n the North Atlantic is influenced by the location of the active spreadin g centres . (3) Th e continent-ocea n boundar y o f N W Europe i s currently experiencing dynami c uplif t of c . 0.5km , whic h decrease s t o zer o acros s Scandinavia and the North Sea . Th e continent ocean boundary of eastern Greenland is currently experiencing dynami c uplif t o f c. 1 km. (4) Cenozoi c mas s balanc e validate s onshor e denudation estimate s b y showin g the y ar e

compatible wit h th e soli d volum e o f sedimen t accumulated offshore . S.MJ. wa s supporte d b y a n NER C studentshi p an d B.J.C. wa s supporte d b y a B P studentship . W e ar e indebted t o B . Mitchene r an d J . Perr y o f B P fo r providing dat a an d support . A . Carte r an d T . Hurfor d provided th e fission-track dat a tha t wer e modelle d t o produce Fig . 4 , an d P . Allen allowed u s t o us e results reported elsewher e i n thi s volume i n th e sam e figure . We thank H. Walford for help in producing Fig. 3 , and M. Shaw-Champion for help in producing Fig. 5. A.G. Dore, J.-I . Faleide , B . Lovel l an d M . Rohrma n provided helpfu l reviews . Thi s pape r i s Departmen t of Eart h Science s Contributio n ES.6677 .

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Timing and mechanisms o f North Atlantic Cenozoi c uplift : evidence fo r mantle upwellin g MAX ROHRMAN 1'2, PETER A . VAN DER BEEK 3, ROB D. VAN DER HILST 4 & PAUL REEMST 5 1 Landmark Graphics, Stavanger, Norway 2 Present address: Shell UK Exploration & Production, 1 Aliens Farm Rd, Nigg, Aberdeen AB12 3FY, UK (e-mail: max. rohrman @expro. shell, co. uk) 3 Laboratoire de Geodynamique des Chaines Alpines, Universite Joseph Fourier, Grenoble, France 4 Department of Earth, Atmospheric and Planetary Sciences, MIT, Cambridge, MA 02139, USA 5 Geologica AS, Stavanger, Norway (present address: NAM, Assen, The Netherlands) Abstract: Postrif t doma l uplif t pattern s ar e a distinc t featur e o f norther n Nort h Atlanti c margins. O n th e basi s o f apatit e fission-trac k data , offshor e seismi c stratigraphy , geomorphology, gravit y an d seismi c tomography , w e argu e tha t souther n Norwa y i s characterized by predominantly Neogen e domal uplift. Th e uplift is technically driven an d estimated at around 1. 5 km. Low flexural rigidity (c . 10 22 N m) and corresponding equivalen t elastic thickness T e (c. 15km ) value s fo r the souther n Norwegia n lithospher e indicate tha t the lithospher e i s relatively weak . Additionally , hig h temperatur e estimate s derive d fro m low-velocity mantle P - and S-wave seismi c tomography belo w th e dome sugges t a therma l anomaly a t depth. Therefore, th e observed topography i s most plausibly explaine d by mantle upwelling. Thi s woul d superced e othe r previousl y propose d primar y mechanism s suc h a s eustasy, isostati c readjustmen t t o glacia l erosion , magmati c underplatin g an d intraplat e compression. Currentl y availabl e dat a sugges t simila r processes fo r othe r uplifte d region s such a s Spitsbergen, norther n Norway , th e Britis h Isle s and parts o f East Greenland .

During recen t decade s i t ha s becom e apparen t Neogen e phase (Rohrman & van der Beek 1996 ; that th e Nort h Atlanti c margin s experience d Japse n & Chalmer s 2000) , whic h ha s a mor e substantial vertical movements in Cenozoic time enigmati c cause. Thi s even t i s characterize d b y (e.g. Whit e & Lovel l 1997 ; Dor e e t al 1999 ; substantia l uplif t o f Britain , part s o f Norway , Japsen & Chalmer s 2000 ) characterize d b y th e Spitsbergen , th e Faeroe s an d Greenland (Japsen coupled emergenc e o f rif t margin s an d anom - & Chalmers 2000) an d by associated subsidenc e alous subsidenc e o f th e nearb y basin s (e.g . o f nearb y basin s (e.g . Nort h Sea , M0r e Basin , Cloetingh et al. 1990) . However , the mechanism offshor e Greenland , S0rvestnaget Basin, Rockall as well a s its temporal and spatial resolution has Trough , Porcupin e Basin) . Becaus e o f th e been a matter of debate. Recently, some progress associatio n o f uplif t an d subsidence , th e has bee n mad e wit h respect t o timing , an d tw o mechanis m drivin g thes e processe s i s probabl y phases tha t hav e influence d th e whol e Nort h tectoni c (e.g . Dor e e t al . 1999 ; Japse n & Atlantic regio n hav e bee n identifie d (e.g . Rii s Chalmer s 2000) . Variou s mechanism s hav e 1996; Martinse n e t al . 1999) . Th e firs t i s a bee n propose d includin g intraplate compression Paleogene phas e primaril y associate d wit h (Cloeting h et al. 1990 ; Dore et al. 1999) , mantle Eocene riftin g an d contemporaneou s volcanis m phas e change s (Rii s & Fjeldskaa r 1992) , generated by the Iceland mantle plume (e.g. Dore magmati c underplatin g (Co x 1993 ; Brodi e & et al. 1999) . Ther e i s considerable evidenc e tha t Whit e 1995 ) an d small-scal e asthenospheri c this even t affecte d al l region s aroun d the Nort h convectio n (Vagne s & Amundse n 1993 ; Rohr Atlantic (Britain , Norwa y an d Greenland) , ma n & van der Beek 1996 ; Stuevol d & Eldholm although it s magnitud e differed fro m regio n t o 1996) , amon g others. region (e.g . Gree n e t al . 1993 ; Rii s 1996 ; Da m I n thi s paper , w e briefl y revie w propose d et al . 1998 ; Dor e e t al . 1999) . Th e secon d i s a mechanism s for uplift aroun d the North Atlantic. From: DORE , A.G., CARTWRIGHT , J.A., STOKER , M.S., TURNER , J.P . & WHITE , N . 2002 . Exhumation of th e North Atlantic Margin: Timing, Mechanisms and Implications for Petroleum Exploration. Geological Society , London, Special Publications , 196 , 27^3. 0305-8719/027$ 15.00 © The Geological Societ y of London 2002 .

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Subsequently, we focus on southern Norway, and review the evidence for the timing and magnitude of Neogen e uplif t an d denudation . W e calculate tectonic uplif t fro m th e elevatio n o f plateau surface remnant s an d a n estimat e o f flexura l rigidity fro m th e coherenc e betwee n Bougue r gravity an d topography . Thes e result s ar e the n used t o differentiat e betwee n mechanism s proposed. Additionally , w e presen t seismi c tomographic image s o f P-wav e velocitie s i n th e upper mantl e aroun d th e Nort h Atlanti c an d correlate thes e wit h othe r data , t o arriv e a t th e most plausibl e mechanis m operatin g durin g Neogene time .

Mechanisms proposed Early studie s wer e mainl y focuse d o n Norwa y and primaril y base d o n geomorphologica l observations (e.g . Holtedah l 1953 ; Torsk e 1972). Th e rugge d mountain s o f souther n Nor way, spectacula r fjord s an d preservatio n o f plateau surface s a t hig h altitud e (Gjessin g 1967; Peulvas t 1985 ; Riis 1996 ) strongly suggest a recen t upwar p o f th e souther n Norwegia n landmass. Intensiv e hydrocarbo n exploratio n o f offshore Norwa y ha s provide d furthe r evidenc e from observe d structura l basinwar d di p o f pre Neogene strat a an d build-u p o f larg e clasti c wedges (Jord t e t al. 1995) . Palaeogeographica l reconstructions hav e adde d additiona l evidenc e for a lat e emergenc e (Dor e 1992a , 1992b) . Apatite fission-trac k (AFT ) dat a constrai n denudation t o b e a mainl y Neogen e event , starting a t c . 3 0 Ma. Denudatio n occurre d i n a dome-like pattern wit h the amplitude of denudation decreasin g radiall y outwar d fro m a maxi mum o f 2. 0 ± 0. 5 km a t th e centr e (Rohrma n et al . 1995) . Th e timin g o f th e onse t o f denudation i s consisten t wit h offshor e strati graphic evidenc e (Rundber g & Smalle y 1989 ; Jordt e t al. 1995) . Any mechanis m proposed , t o b e successful , has to explain this timing of events (van der Beek & Rohrma n 1997) . Som e worker s favou r a Paleogene onse t o f uplif t fo r souther n Norway , but mos t availabl e evidenc e strongl y suggest s a primarily Neogen e even t (Rundber g & Smalle y 1989; Jense n & Schmidt 1993 ; Jord t et al. 1995 ; Rohrman e t al. 1995 ; Rii s 1996 ; Martinse n et a l 1999). Early interpretation s suggeste d tha t uplif t o f southern Norwa y an d othe r margin s aroun d th e North Atlanti c wa s associate d wit h PaleoceneEocene break-up an d plume activity (e.g. Torsk e 1972; Co x 1993) . However , timin g o f doma l uplift a t c . 30Ma , i.e . 25-30M a afte r onse t of Eocen e volcanis m an d rifting , preclude s

significant synrif t uplift . Moreover , dynami c plume-generated uplif t i s transient , an d shoul d reverse int o subsidenc e afte r break-up . Perma nent uplif t ca n b e generate d b y magmati c underplating, but in the case of southern Norway there i s n o sig n of onshor e Cenozoi c intrusions. Eocene magmati c activit y too k plac e 300 400 km offshore . Riis & Fjeldskaa r (1992 ) propose d tha t mos t of the Norwegian uplift wa s caused by PliocenePleistocene isostati c readjustmen t t o glacia l erosion. However , thei r stud y showe d tha t additional tectoni c uplif t i s require d t o explai n the present-da y elevatio n o f morphologica l surfaces. The y explaine d thi s b y mantl e phas e transitions as a result of erosional unloading, but the dynamic s o f mantl e phas e change s driving this tectoni c uplif t componen t ar e a t presen t poorly understood . Non-tectoni c mechanism s that hav e been suggeste d (Eyle s 1996 ) focu s o n climatic deterioratio n an d sea-leve l changes . However, the kilometre-scale magnitude of uplif t is not readily explaine d b y eustasy. Others hav e propose d tha t Neogen e uplif t o f western Fennoscandi a i s a resul t o f therma l instability cause d b y larg e horizonta l tempera ture gradients between the Fennoscandian mainland and the Norwegian-Greenland Sea (Theilen & Meissne r 1979 ; Peulvas t 1985 ; Vagne s & Amundsen 1993) . Thi s woul d se t u p secondar y convection i n th e sub-lithospheri c mantle , causing th e ho t asthenospher e t o ris e an d sub sequently generat e tectoni c uplift. Thi s could be a viabl e model , an d wil l b e discusse d i n th e following sections . Finally, variou s workers have drawn attention to th e synchronou s timing o f margi n uplif t an d anomalous basin subsidence around Norway and the whol e Nort h Atlantic , suggestin g tha t coupled uplif t an d subsidenc e i s flexura l i n nature an d induce d b y intraplat e stres s fluctu ations (Cloetingh et al. 1990) . However, flexureinduced uplift i s not consistent with the observed correlation betwee n topograph y an d Bougue r gravity anomalie s (u p t o — 80 mgal) belo w th e uplifted region s (Fig . 1) . Althoug h th e gravit y is largel y isostaticall y compensate d (Ballin g 1980), ther e i s a mas s defici t belo w th e regions o f highes t elevatio n tha t canno t b e explained b y topograph y an d difference s i n crustal thickness .

Timing of denudation on the eastern Atlantic margin There are two principal types of constraint on the timing o f Norwegia n denudation : th e structural

TIMING AN D MECHANISMS O F NORTH ATLANTIC CENOZOIC UPLIF T

29

Fig. 1 . Bouguer gravity anomaly map for southern Norway, data from Sverige s Geologiske Undersokels e (1985) .

and stratigraphi c relationshi p o f Cenozoic strat a offshore (Jord t e t al. 1995 ) an d planatio n surfaces onshore , an d th e AF T analysis o f th e Norwegian basemen t (Rohrma n e t al . 1995) . AFT dat a fro m souther n Norwa y defin e a structural dom e wit h the youngest age s increas ing radiall y fro m c . 10 0 Ma a t se a leve l i n th e inner fjords , t o c . 17 0 Ma a t th e to p o f th e Jotunheimen peak s an d aroun d 20 0 Ma a t elevations les s tha n 500 m nea r th e shoreline s (Fig. 2) . Mea n trac k lengt h distribution s ar e more variable , bu t th e younge r age s generall y

correspond t o lo w mea n trac k length s (c. 11.6jjLm ) an d a lac k o f lon g (recent ) track s (Fig. 2) . Th e latte r indicat e fas t coolin g fro m temperatures a t th e lowe r en d o f th e partia l annealing zon e (c . 60-7 0 °C) t o surfac e tem peratures. Althoug h ou r younges t AF T sample s yield mixed ages, it is possible t o extract therma l history informatio n b y usin g state-of-the-ar t modelling techniques . Th e younges t AF T samples (AF T age c. 100 Ma, mean track lengths 11.6(xm) sugges t a predominantl y Neogen e onset of denudation (Rohrman et al. 1995) .

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M. ROHRMAN E T AL.

Northern Norwa y show s simila r AF T base ment age s althoug h the y ten d t o b e somewha t older (aroun d 16 0 Ma) wit h distribution s show ing longer track lengths (Hendriks & Andriesse n 2002). Meanwhile , Neogen e clasti c wedge s offshore fro m Lofote n indicat e mor e recen t erosion (Mokhtar i & Pegru m 1992) . A recen t AFT stud y o f Spitsberge n b y Blyth e & Kleinspehn (1998 ) place d significan t denudatio n around 3 6 Ma, whic h wa s mainl y attribute d b y those worker s t o rif t flan k uplift , followe d b y Pliocene-Holocene glacia l erosion . Th e dat a pattern yield s strongl y varyin g AF T age s (c. 27-5 6 Ma), sometime s withi n th e sam e locality. Thi s patter n suggest s tha t th e sample s experienced othe r processe s tha n simpl e uplif t and erosion, possibl y indicatin g migration o f hot fluids. Thi s interpretatio n i s supporte d b y th e resetting of zircon fission-track ages and vitrinite reflection data , i n th e proximit y o f unrese t samples a t simila r elevations . A possibl e explanation fo r these pattern s migh t b e volcanic and hydrotherma l activit y durin g Miocen e an d

Quaternary tim e (e.g . Vagne s & Amundse n 1993). Simila r fission-trac k pattern s hav e bee n observed i n th e Permia n Osl o Rift , wher e igneous activit y i s wel l documente d (e.g . Rohrman e l al . 1994) . However , a t se a leve l there seem s t o b e a sligh t tren d fro m wes t t o east alon g th e Longyearbye n fjor d (Centra l Spitsbergen), whic h suggest s a decrease i n AF T ages fro m aroun d 5 0 Ma nea r th e basemen t o n the wes t coas t t o 3 8 Ma i n th e Eocen e deltai c strata o f th e Centra l Basin . Thi s concur s with earlier studie s (N0ttvedt el al. 1992) , suggesting Neogene doma l uplif t base d o n geologica l observations, vitrinit e reflectanc e an d seismi c velocity data . Evidently , more AF T sample s o f east Spitsberge n ar e neede d t o verif y o r refut e this assumption. Much of the British Isles lacks the high rugged topography o f souther n Norway . Elevation s ar e more subdue d an d d o no t excee d 1 km i n larg e parts o f England , Wale s an d Ireland . Onl y th e Scottish highland s reach highe r elevations, up to 1343m. Publishe d AF T dat a fro m Scotlan d

Fig. 2 . Mea n elevatio n an d AFT isochrons o f souther n Norwa y draw n a t se a level , definin g a structura l dom e concordant wit h topography, fro m Rohrma n e t al. (1995). Typica l track length distributions for selected samples are show n o n the right .

31

TIMING AN D MECHANISMS O F NORTH ATLANTIC CENOZOI C UPLIF T

(Lewis e t al 1992 ) sugges t predominantl y Triassic-Jurassic ages . Onl y nea r th e Eocen e intrusions are younger ages found. However, data are too sparse to infer any details for this region. AFT dat a fro m norther n Englan d an d th e Iris h Sea are a sho w AF T age s betwee n 4 5 Ma an d > 400 Ma (Gree n e t al . 1993 , 1997 , 2001) , suggesting significant exhumation around 60 Ma. This uplif t i s possibl y centre d i n th e Eas t Iris h Sea an d decreasin g t o th e SE , concordan t wit h the Mesozoi c sedimentar y outcro p patter n o f southeastern England . Recently , Japse n (1997 ) suggested evidenc e fo r a Neogen e denudatio n phase base d o n AF T dat a an d compactio n studies fro m easter n Englan d an d th e wester n North Sea . H e estimate d tha t Paleogen e an d Neogene denudatio n wer e equa l i n magnitud e (around 1 km each) . However , th e spatia l pattern o f bot h Neogen e an d Paleogen e denudation fo r th e entir e Unite d Kingdo m requires furthe r study . Moreover, the area of highest denudation (East Irish Sea) is at present below sea level, in contrast to th e dome-shape d topograph y o f th e othe r regions (souther n Norway, northern Norway and Spitsbergen).

Quantification of Neogene uplif t and denudation i n southern Norway Regional change s i n surfac e elevatio n ar e th e combined resul t o f tectoni c uplift , erosio n an d the isostati c respons e t o erosion . T o us e th e present-day elevatio n o f a n uplifte d regio n t o constrain tectoni c processes , th e component s o f elevation chang e tha t resul t fro m erosio n an d isostatic reboun d must be quantified (Englan d & Molnar 1990 ; Gilchris t et al. 1994) . The tectonic uplift (W T) is related t o the present-day elevatio n

(Ho) an d th e amoun t of denudation (A£) by

where H { i s the initial elevatio n and / i s isostatic rebound:

where p c i s th e densit y o f th e erode d crusta l section, pa is sublithospheric mantle density, kE is the spatial wavenumber of erosional unloading, g is acceleratio n o f gravit y an d D i s flexura l rigidity (va n de r Bee k e t al. 1994 ; se e Table 1) . As D — > 0 , equatio n (2 ) simplifie s t o / = AEp c/pa, th e loca l isostati c solution . A s D — > o o (fo r a n infinitel y stron g lithosphere) , I — * 0. A regiona l analysi s o f tectoni c uplif t therefore require s a n estimat e o f H^ an d D , a s well as the ability to map out spatial variations in A£ (Abbot t e t al . 1997 ; Smal l & Anderso n 1998). Whereas th e AF T thermochronologica l dat a discussed abov e giv e u s a hig h tempora l resolution t o decipher the denudatio n histor y o f southern Norway , th e spatia l resolutio n o f ou r data i s rathe r coarse . W e therefor e us e th e elevation o f preserve d remnant s o f a platea u surface (th e Talaei c surface' ) t o spatiall y constrain th e amount s o f denudation , isostati c rebound, and tectonic uplif t i n southern Norway. The correlation o f plateau remnants and their use in reconstructin g landscap e developmen t i s a relatively hazardou s undertakin g becaus e o f th e general lack of temporal constraints (Brown et al. 1999; Summerfiel d 1999) . I n Norway , a s else where, ther e i s n o consensu s o n th e ag e o f th e surface remnants , whic h hav e bee n varyingl y interpreted as being of Jurassic to Paleogene age , nor o n thei r correlatio n (Gjessin g 1967 ; Torsk e

Table 1 . Parameter values employed Symbol PC Pa g

D E v f k a L

Description

Value

Crustal density Asthenospheric density Gravitational acceleratio n Flexural rigidit y o f the lithospher e Young's modulus Poisson rati o Ratio o f surface to base loadin g o f lithospher e Thermal conductivity Thermal diffusivit y Lithospheric thickness afte r thinning

2600 kg nT3 3250 kg m"3 9.8 m s ~2 8.9 X 1021 to 7. 2 X l 0 2 2 N m 10 n Nm~ 2 0.25 1 0.0006 cal (c m s 0.01 cm 2 s"1 80-110km

°cr '

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M. ROHRMAN ETAL.

Fig. 3 . Maps o f mean elevatio n o f souther n Norwa y (fro m ETOPO-5 global topograph y data) , elevation of the Palaeic surfac e (afte r Riis & Fjeldskaar 1992 ) an d amoun t of erosion, calculate d b y subtractin g mean elevation from th e elevatio n o f the Palaeic surface .

1972; Peulvas t 1985 ; Dor e 1992a ; Rii s & Fjeldskaar 1992 ; Riis 1996) . We follow the mos t recent correlatio n o f Riis & Fjeldskaa r (1992 ) an d Rii s (1996) , wh o suggested the Palaeic surfac e to be of Paleogen e age becaus e (1 ) ou r fission-trac k dat a sugges t

that samples from clos e to the Palaeic surfac e in the Hardangervidd a are a reache d surfac e tem peratures i n Paleogen e time s (Rohrma n e t al. 1995) and (2) the elevation of the Palaeic surface in th e Jotunheime n area i s consisten t wit h th e amount o f Neogen e denudatio n recorde d b y

TIMING AN D MECHANISMS O F NORTH ATLANTIC CENOZOI C UPLIF T

33

Fig. 4 . Correlatio n o f amoun t o f Neogen e denudation , calculate d b y subtractin g present-day topograph y fro m elevation of Palaeic surface (see Fig. 3) with apatite fission-track ages (from Rohrman etal. 1994,1995). Neogene denudation fo r fission-trac k sample s i s corrected fo r sampl e elevation wit h respect t o mea n elevatio n o f the 5 ' resolution topographic grid .

fission-track thermochronolog y o f sample s fro m close t o se a leve l (2. 0 ± 0. 5 km; va n de r Bee k 1995). Figure 3 show s th e present-da y elevatio n o f southern Norwa y a s well a s the elevatio n o f th e Palaeic surface. The latter was digitized fro m th e map of Riis & Fjeldskaar (1992) and interpolated using a continuous curvatur e algorithm (Smit h & Wessel 1990) . Th e amoun t o f denudatio n sinc e the formation of the surface (i.e. since the end of Paleogene time) can be calculated by subtracting the present-da y elevatio n H Q fro m th e elevatio n of th e Palaei c surface . The resul t show s a mean amount of denudation o f c. 400m, with maxim a of > 1000m i n th e inne r fjord s an d alon g th e northwestern coast . Th e patter n o f denudatio n that i s calculate d i n thi s manne r i s largel y consistent with the pattern deduced from fissiontrack thermochronology . Figur e 4 show s th e correlation betwee n th e amoun t o f denudatio n calculated using this approach an d the AFT ages from Rohrma n e t al. (1995) . Althoug h th e fission-track age s ar e mixe d ages , wit h minim a around 10 0 Ma, a clea r tren d emerge s wit h samples wit h th e younges t fission-trac k age s encountered in the regions of maximum Neogene denudation. Late Paleocen e an d earl y Eocen e marin e diatoms ar e encountere d o n planatio n surface s in Swede n an d Finlan d (Fenne r 1988) , whic h may b e correlate d wit h th e Palaei c surfac e i n

Norway. O n th e basi s o f thi s observation , Rii s (1996) suggested that th e surfac e wa s at or nea r sea level in early Cenozoic time. This assertion is consistent wit h th e offshor e sedimentatio n dat a (Jordt e t al . 1995) , whic h sugges t ver y lo w sediment inpu t fro m Fennoscandi a befor e lat e Oligocene time . W e therefor e assum e tha t th e Palaeic surface can be used a s a marker no t only of Neogen e denudation , but als o of uplift . If we suppose Hi — 0 in equation (1), then the present-day elevatio n o f th e Palaei c surfac e represents HQ + A E — MT + /• I n th e cas e o f local isostas y ( D — 0), th e amoun t o f isostati c rebound reduces to 7 = AEp c/pa (c. 0.8AE1 for the values in Table 1 ) and can be calculated directl y from th e patter n o f denudatio n (Fig . 3) . Th e tectonic uplif t U T the n equal s th e present-da y elevation o f th e Palaei c surfac e minu s th e isostatic uplif t / . Figur e 5 show s calculate d amounts of Neogene tectoni c uplif t an d isostati c rebound fo r th e loca l isostati c case . Tectoni c uplift i n this case has the sam e domal pattern as the mea n elevatio n an d th e elevatio n o f th e Palaeic surface , reachin g maximu m value s o f c. 1500 m fo r th e hig h mountai n area s (wher e Neogene denudatio n i s negligible) , wherea s isostatic reboun d reache s a maximu m o f c. 750 m i n th e inne r fjord s an d o n th e N W coast. However, because the isostatic response to denudation is generally regional instea d of local, the pattern and the amount of tectonic uplif t wil l

34

M. ROHRMAN ETAL.

Fig. 5 . Calculated tectoni c uplif t an d isostatic reboun d fo r a model o f local isostatic response t o erosion. I n this case isostatic reboun d equal s 0. 8 times th e amount of erosion; tectonic uplif t wa s calculated by subtractin g this amount fro m th e elevatio n o f the Palaei c surfac e (see text).

also be a function o f the flexural rigidity D of the southern Norwegia n lithosphere . Estimating flexural rigidity To make a realistic assessmen t o f the amoun t of tectonic uplift , th e flexura l rigidity , o r corre sponding equivalent elastic thickness T e, must be estimated. Flexura l rigidit y ma y b e readil y estimated b y a n analysi s o f th e coherenc e o f topography an d Bougue r gravit y anomalies . Gravity an d topograph y wil l b e coheren t a t long wavelength s bu t no t a t shorte r ones, th e wavelength a t whic h coherenc e break s dow n being dependen t o n D. The observe d coherenc e 7§ is define d a s (Forsyt h 1985)

where E 0(k} i s the average power of topography for a discret e wavenumber , Ei (k) i s th e averag e power of gravity for that wavenumber, C(k} is the

cross-spectral powe r o f gravit y an d topograph y and th e overba r indicate s averagin g ove r a wavenumber band . Fo r a mechanicall y aniso tropic lithospher e a simila r concept ca n b e use d (Simons e t al. 2000) , bu t i n thi s analysi s w e assume isotropy. The theoretical coherenc e fo r a plate loade d bot h at its surface and base i s given by (Forsyt h 1985)

where H T i s th e amplitud e o f surfac e deflection as a resul t o f surfac e loading , W T i s th e amplitude o f deflectio n o f th e bas e o f th e plat e (i.e. th e Moho ) a s a resul t o f surfac e loading , //B th e amplitud e of surfac e deflection resultin g from loadin g a t th e bas e o f th e plat e an d W B the amplitud e o f Moh o deflectio n resultin g from bas e loading . // T, // B, W T and W E depen d on th e wavenumbe r k an d flexura l rigidit y D

TIMING AND MECHANISMS O F NORTH ATLANTIC CENOZOI C UPLIFT

via

with / th e rati o o f surfac e t o bas e loading ; Ap = (p a - p c); ( f > = \ + (D/c 4/pcg) an d
35

& Cathle s 1991) . I t is , however , consisten t with regiona l T & estimate s fro m th e post glacial til t o f palaeoshoreline s (Fjeldskaa r 1997), whic h suggeste d T e ^ 20km, a s wel l as wit h a n independen t coherenc e stud y b y Poudjom Djoman i e t al (1999) , wh o foun d 8 < 7 e < 18k m fo r souther n Norway. We hav e calculate d th e isostati c respons e t o denudation adopting constant T e values of 10 and 20 km, using equation (2). The predicted isostatic rebound ha s a muc h smoothe r patter n tha n th e local isostati c case , reachin g a maximu m o f c. 700 m nea r th e N W coastlin e (Fig . 7) . Th e magnitude o f isostatic reboun d is , however , no t much smaller than for the local isostati c case, as the inferre d flexura l rigidit y i s no t ver y large . Resulting tectonic uplift pattern s are also shown in Fig . 7 ; th e model s incorporatin g flexura l rigidity sho w a simila r dome-shape d uplif t pattern a s th e loca l isostati c case . Uplif t i s centred o n th e region s o f highes t present-da y elevation, reachin g a maximu m o f c . 1500m . This surprisingl y hig h valu e i s relativel y insensitive t o th e adopte d flexura l rigidit y because, a s D increases , th e amoun t of isostati c rebound decreases but also spreads mor e toward the region s of highest present-day elevation and contributes t o th e uplif t o f thes e regions . Th e critical assumptio n i n thi s analysi s i s tha t th e Palaeic surfac e was a t se a level befor e Neogene uplift; i f i t wa s a t som e initia l elevatio n H[ th e inferred Neogen e uplift wil l be overestimated by the sam e amoun t (se e equatio n (1)) .

where £ i s Young' s modulu s an d ^ i s th e Poisson rati o (se e Tabl e 1) . The gravity data we used for this analysis are from th e Sverige s Geologisk e Undersokels e 1985; Fig . 1 ) and topography fro m th e ETOPO5 database . Gravit y an d topograph y dat a wer e projected ont o a 50 0 km X 720 km UT M grid . Data were clipped in the deep offshore area s and tapered towards the mean value along the sides of the grid , befor e bein g transforme d int o the wavenumber domai n usin g a fas t Fourie r transform algorith m (se e va n der Beek 1995) . Results of the coherence analysis are shown in Fig. 6 . Although a best-fittin g / an d T e ca n be estimated independentl y fro m th e dat a usin g a least-squares criterio n (Forsyt h 1985) , w e fee l that thi s ma y pu t to o muc h emphasi s o n a best-fitting numbe r fo r th e presen t dat a quality. Visual inspectio n indicate s a best-fi 21t T e T , „ between 1 0 an d 20k m ( D = 8. 9 X 10 t o Souther n Norway: mechanisms of uplif t 7.1 X 1022 Nm), assumin g / = 1 . This valu e is The relativel y lo w flexura l rigidit y o f th e at th e lo w en d o f estimate s o f flexura l rigidit y lithosphere (c. 10 22Nm) and high tectonic uplif t for Fennoscandi a fro m glacia l reboun d studies , strongly sugges t a n endogenou s caus e fo r th e which constrai n T e t o b e <50k m (Fjeldskaa r uplift. Thi s leave s u s wit h essentiall y tw o possible solutions : intraplat e compressio n an d dynamic mantl e u p welling. Intraplat e processe s require positiv e gravit y anomalie s belo w th e dome regions , i n contras t t o th e observations . Furthermore, intraplat e compression canno t generate th e amoun t o f uplif t required , a s i t i s an orde r o f magnitud e to o lo w (va n de r Bee k 1995). Mantle upwellin g ha s bee n propose d b y various worker s fo r th e Nort h Atlanti c margin s (e.g. Vagne s & Amundse n 1993 ; Stuevol d & Eldholm 1996 ; Rohrma n & van der Beek 1996) . The upwellin g mode l seem s plausibl e fo r Spitsbergen, wher e hig h hea t flo w 2 Fig. 6 . Result s o f coherenc e analysi s fo r souther n (c. 130mWm~ ) prevail s an d mantl e xenoliths are presen t i n Quaternar y alkal i basalts . Thi s Norway. Dots indicate observed coherence for average wavenumber bands, bars denote Ic r errors. Continuous model seem s less obvious fo r souther n Norway, lines are the various equivalent elastic thicknesses T c. where lo w surfac e hea t flo w (c . 40mW m )

36

M. ROHRMAN ETAL.

Fig. 7 . Calculated tectoni c uplif t an d isostatic reboun d fo r a model o f regional isostati c respons e t o denudation and differen t flexura l rigidities , correspondin g t o T e = 10k m ( D = 8. 9 X 1021 Nm; top ) an d T t — 20km (D = 7. 1 X 1022 Nm; bottom) . (See text for discussion.)

TIMING AND MECHANISMS OF NORTH ATLANTI C CENOZOI C UPLIFT

(Cermak 1979 ; Ballin g 1995 ) an d lac k o f volcanism ar e not readily consisten t wit h mantle up welling. However, a recent study by Goes et al (2000) proposed high sub-Moho temperatures in excess o f 1000° C base d o n P - an d S-wav e tomography belo w souther n Norwa y (Fig . 8) . These temperature s ar e simila r t o thos e i n regions suc h a s th e Massi f Centra l (Sobole v et al . 1996) , wher e a mantl e plum e seem s evident. I f thes e temperatur e estimate s ar e correct, th e discrepanc y betwee n surfac e hea t flow and elevate d temperature s a t dept h ca n b e explained by a time lag between equilibration of high lithospheric heat flow and surface heat flow (McGuire & Bohanno n 1989) . Assumin g con ductive heat loss, the change in surface heat flow Ag0 i s related t o a change i n temperature a t the base of the thinned lithosphere A7"i, the thickness of th e lithospher e afte r thinnin g L , an d th e diffusion time f , accordin g t o

where k i s therma l conductivit y an d a th e thermal diffusivit y (Tabl e 1) . For L between 11 0 and 80 km and AT^ from 400 °C to 500 °C, it takes around 6 0 Ma to observ e a c. 10mWm~ 2 heatflow change fo r souther n Norway . Upwelling materia l i s mos t probabl y associ ated wit h advection , therefor e w e ca n us e a 10mWm~ 2 heat-flo w chang e a s a conservativ e estimate. As no regional heat-flow anomalie s are observed i n th e souther n Norwegia n data , w e assume that any dynamic mantle upwelling must be younger than 60 Ma. Along with the evidence

37

of reduce d P an d S wave s belo w souther n Norway (Banniste r e t al 1991 ; se e also below) , this strongl y suggest s tha t th e souther n Norwegian structura l dom e wa s generate d b y activ e mantle upwelling or diapirism i n Neogene time. Seismic tomography of the North Atlantic One of the major advances i n our understanding of the Earth has been the advent and subsequent development o f seismi c tomography , a clas s o f imaging tha t now provides increasingl y detaile d 3D maps o f seismi c velocit y variations that can be related t o thermal an d chemical variation s i n the mantle . Althoug h i t ha s prove n relativel y easy t o imag e downwelling s (i.e . subductin g slabs), i t seem s muc h mor e difficul t t o imag e mantle upwellings (e.g. Grand et al. 1997) . This is becaus e upwelling s ar e likel y t o occu r i n aseismic region s and are not as well sampled by seismic data , especiall y i n th e shallo w mantle . Furthermore, us e o f firs t arrival s o f seismi c waves cause s a natura l bia s towar d fas t anomalies, becaus e annealin g o f wavefront s creates a tendenc y t o underestimat e slo w anomaly amplitudes . Another important issue is that resolution depends on data coverage, which is uneve n owin g t o th e spars e distributio n o f sources (i.e . earthquakes ) an d receiver s (i.e . seismological stations ) an d th e 3 D geometr y of the ra y path s alon g whic h th e seismi c wave s propagate. Mantl e structur e has remaine d unre solved beneat h larg e region s o f th e Nort h Atlantic because of absence o f recording station s and low levels of seismi c activity. With this in mind, we present P-wave velocity maps o f th e Nort h Atlanti c a t variou s depth s (Fig. 9), which provide a snapshot of the mantle

Fig. 8 . Temperature-depth plot of three geotherms belo w souther n Norway . The 40 mW m 2 geotherm (broke n line) is based on surface heat-flo w data . T P and T s ar e geotherms derived from inversion o f P-wave an d S-wav e seismic velocities , respectivel y (Goe s e t al 2000 ; Goe s pers . comm.) . Th e relativ e convergenc e o f T P an d T s should b e noted. VBS, volatile-bearin g peridotite solidus; AS , anhydrous peridotit e solidus; AD , adiabat.

38

M. ROHRMAN ETAL.

Fig. 9 . P-wav e seismi c tomograph y map s fo r th e Nort h Atlanti c a t variou s depths . Fo r th e uppe r mantl e low velocities (red ) correspond t o high temperatures, wherea s hig h velocities (blue) correspond t o low temperatures. EG, East Greenland ; 1C, Iceland; SN , southern Norway ; NN, northern Norway; IS, Irish Sea ; SC , Scotland .

as it is today. Th e image (Fig . 9 ) depicts P-wav e utio n tes t fo r th e velocit y ma p a t 150km , velocity map s throug h th e globa l mode l o f usin g a chequerboar d model . Resolutio n i s Karason & va n de r Hils t (2000 ) fo r th e Nort h rathe r poo r belo w Eas t Greenlan d (EG ) an d Atlantic region . Figur e 1 0 show s a resol - mos t o f th e Nort h Atlanti c Ocean . A bette r

TIMING AN D MECHANISMS O F NORTH ATLANTIC CENOZOIC UPLIF T

39

Fig. 10 . (a ) Latera l variation s o f P-wav e spee d a t 150k m dept h beneat h th e norther n Atlantic , (b ) Inpu t 'chequerboard' mode l fo r resolution test, (c) Result of 'chequerboard ' resolutio n test; as expected, ou r ability to image structur e i n th e uppe r mantl e beneat h th e oceani c region s i s very poo r owin g t o absenc e o f earthquak e sources an d seismologica l stations , bu t beneat h souther n Norwa y th e latera l resolutio n i s reasonable. Vertica l resolution i s poor because o f the smal l incidence angles of the seismi c wave s used for imaging in this region.

defined low-velocit y anomal y i s detected belo w Iceland (1C). This anomaly extends from near the surface to c. 400km, but a deeper structure could have been overlooke d owin g to insufficient dat a coverage (e.g . Kelle r e t al 2000) . Belo w southern Norwa y (SN ) a low-velocit y anomal y is observe d tha t extends fro m > 50 km dept h to > 250 km. The vertical resolutio n i s poor i n this part o f the model , bu t th e slo w anomal y agree s well wit h previous P- an d S-wav e tomographic

studies (Banniste r et al 1991 ; Zielhui s & Nolet 1994a, 1994b ; Marquerin g & Sniede r 1996 ; Bijwaard e t al 1998) . Although this is not likely to be well resolved, the model suggests that a thin high-velocity laye r overlie s a low-velocit y anomaly belo w souther n Norway , whic h i s consistent wit h ou r estimate s of T e and suggests a thinned and relatively weak lithosphere. Farther eastward we encounter the Baltic shield, evident as a thic k (>250km ) high-velocit y area . Th e

40

M. ROHRMAN ETAL.

change fro m a low-velocit y zon e i n th e uppe r mantle o f souther n Norwa y (SN ) t o th e hig h velocities o f th e Balti c shiel d i s dramati c an d suggests som e fundamenta l proces s tha t ha s s o far bee n neglecte d i n ou r curren t vie w o f plat e tectonics. W e remar k tha t simila r feature s hav e been observe d i n southeaster n Australi a (Zielhuis & va n de r Hils t 1996) . Here , a low velocity upper-mantl e anomal y is depicted belo w the mountain s (u p t o 2230m ) o f Easter n Australia, wherea s th e low-altitud e (<200m ) shield toward s th e west show s higher velocities . Regional tomographi c studie s (Bannister et al 1991) indicat e a low-velocit y anomal y belo w northern Norwa y (NN) , but i n ou r global mode l resolution i s too low to infe r an y details fo r thi s region. Simila r resolutio n problem s ar e observed belo w th e Britis h Isles , wher e fain t low-velocity anomalie s ar e presen t belo w th e Irish Se a (IS ) a t c . 200k m depth . Lo w velocities belo w th e Irish Se a were also observe d in previous models (Marquerin g & Snieder 1996 ; Bijwaard e t al 1998) . Scotlan d (SC ) also suffer s from th e aforementione d resolutio n problems , but generall y suggest s highe r velocities .

Discussion Our result s fro m integratin g geological , AFT , geomorphological, Bougue r gravit y an d seismi c tomography dat a strongl y sugges t tha t mantl e upwelling i s presen t belo w souther n Norway . Furthermore, i t seems eviden t that this upwelling was mos t activ e i n Neogen e tim e an d directl y responsible fo r generatin g mos t o f th e present day topography . Whethe r othe r area s experi enced simila r upwelling s i s stil l uncertain , mainly becaus e o f lac k o f sufficien t data . However, there are strong indications that similar mechanisms generate d th e domal topograph y o f Spitsbergen, norther n Norwa y an d th e Eas t Greenland coas t (Rohrma n & va n de r Bee k 1996). There are also some structura l difference s among th e domes . Wherea s Spitsberge n record s Neogene volcanism , souther n Norwa y i s com pletely devoi d o f an y Cenozoi c magmati c activity. Thi s i s probabl y simpl y constraine d b y the amoun t o f activ e lithospher e thinnin g b y the upwelling. Spitsbergen's lithospheri c thickness is estimated t o b e c . 50k m (Vagne s & Amundsen 1993), whereas th e sout h Norwegia n lithospher e is muc h thicker , probabl y c . 80-10 0 km. Th e Norwegian dom e thu s possibl y represent s a n early stag e of upwelling. Additionally, the P- and S-wave temperature s o f Goe s e t al . (2000 ) fo r Europe sugges t tha t divergenc e o f T P (P-wav e temperature) an d T s (S-wav e temperature ) a t depths < 100k m coul d indicat e th e presenc e o f

melt, a s show n fo r th e Massi f Centra l area . Southern Norwa y show s relativ e convergenc e between T P an d T s (Fig . 8) , possibly suggestin g absence o f melt an d therefor e n o volcanism. Apart fro m explainin g observe d uplif t pat terns, th e mantle upwellings also offe r a solution for offshor e anomalou s basi n subsidence , where asthenospheric materia l flow s fro m belo w th e basins (Nort h Sea , M0r e Basin ) towar d th e upwelling (souther n Norway) . Furthermore , i n this interpretatio n th e timin g o f upwellin g becomes les s o f a n issue , a s th e availabl e evidence suggest s tha t boundar y condition s ar e generated b y riftin g ove r a n anomalousl y ho t asthenosphere. Subsequen t uplift pulse s might be inferred b y change s i n upwelling flu x (Rohrma n & va n der Bee k 1996 ; Whit e & Lovell 1997) . Some concern s remain ; mantl e upwelling s seem t o follo w a distinc t sequenc e o f events , starting wit h surfac e uplift , followe d by basalti c volcanism an d finally therma l subsidence . Alternatively, ou r result s migh t sugges t tha t no t al l upwellings caus e surfac e volcanism , a s upwel ling temperatur e an d velocit y coul d b e to o low . Another questio n i s th e relationshi p betwee n plate movemen t an d upwelling . Traditiona l theory suggest s tha t upwelling s remai n station ary wit h respec t t o eac h other , bu t ou r result s suggest tha t this might not alway s be th e case . We than k Shel l U K Exploratio n & Productio n fo r sponsoring th e colo r figure s i n thi s articl e an d Sask i Goes fo r providin g th e temperatur e dat a belo w southern Norway

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RASMUSSEN, E . 1992 . Hydrocarbo n potentia l o f the Centra l Spitsberge n basin . In : VORREN , T.O. , BERGSAKER, E. , DAHL-STAMNES , 0.A. , HOLTER , E., JOHANSEN , B. , LIE , E . & LUND , T.B . (eds ) Arctic Geology an d Petroleum Potential. Nor wegian Petroleu m Societ y Specia l Publication , 2 , 333-361. PEULVAST, J.-P . 1985 . Post-orogeni c morphotectoni c evolution o f th e Scandinavia n Caledonide s durin g the Mesozoi c an d Cenozoic . In : GEE , D.G . & STURT, B.A . (eds ) Th e Caledonide Orogen — Scandinavia an d Related Areas. Wiley, Chichester , 979-995. POUDJOM DJOMANI, Y.H. , FAIRHEAD , J.D. & GRIFFIN ,

W.L. 1999 . Th e flexura l rigidit y o f Fennoscandia : reflection o f th e tectonotherma l ag e o f th e lithospheric mantle . Earth an d Planetary Science Letters, 174 , 139-154 . Rus, R 1996 . Quantificatio n o f Cenozoi c vertica l movements o f Scandinavi a b y correlatio n o f morphological surface s wit h offshore data . Global and Planetary Change, 12 , 331-358.

Rus, R & FJELDSKAAR, W. 1992. O n the magnitude of the Lat e Tertiar y an d Quaternar y erosio n an d it s significance fo r th e uplif t o f Scandinavi a an d th e Barents Sea . In : LARSEN , R.M. . BREKKE , H. , LARSEN, B.T . & TALLERAAS , E . (eds ) Structural and Tectonic Modelling and its Application to Petroleum Geology. Norwegia n Petroleu m Societ y Special Publication , 1, 163-185. ROHRMAN, M . & VA N DER BEEK, P . 1996 . Cenozoi c postrift doma l uplif t o f Nort h Atlantic margins: a n asthenospheric diapiris m model . Geology. 24, 901-904. ROHRMAN, M. , VA N DER BEEK , P . & ANDRIESSEN , P.A.M. 1994 . Syn-rif t therma l structur e an d post rift evolutio n of th e Osl o Rif t (S E Norway) : new constraints fro m fissio n trac k thermochronology . Earth and Planetary- Science Letters, 127, 39-54. ROHRMAN, M. , VA N DER BEEK , P.A. , ANDRIESSEN , P.A.M. & CLOETINGH , S . 1995 . Meso-Cenozoi c morphotectonic evolutio n o f Souther n Norway : Neogene doma l uplif t inferre d fro m apatit e fission trac k thermochronology . Tectonics, 14 , 704-718. RUNDBERG, Y. & SMALLEY, PC. 1989 . High-resolutio n dating o f Cenozoi c sediment s fro m th e norther n North Se a usin g 87 Sr/86Sr stratigraphy . AAPG Bulletin, 73, 298-308. SIMONS, F.J. , ZUBER , M.T . & KORENAGA , J . 2000 . Isostatic respons e o f th e Australia n lithosphere : estimation o f effectiv e elasti c thicknes s an d anisotropy usin g multitape r spectra l analysis . Journal o f Geophysical Research. 105 . 19163-19184. SMALL, E . & ANDERSON , R.S. 1998 . Pleistocen e relief production i n Laramid e mountai n ranges , wester n Unites States . Geology, 26, 123-126 . SMITH, W.H. R & WESSEL , P . 1990 . Griddin g with continuou s curvatur e spline s i n tension . Geophysics, 55 , 293-305. SOBOLEV, S.V. , ZEYEN , H. , STOLL , G. , WERLING , R . ALTHERR, R . & FUCHS , K . 1996 . Uppe r mantl e temperature fro m teleseismi c tomograph y o f French Massi f Centra l includin g effect s o f composition, minera l reactions , anharmonicity . anelasticity an d partia l melt . Earth an d PlanetaryScience Letters, 139, 147-163 . STUEVOLD, L.M . & ELDHOLM , O . 1996 . Cenozoi c uplift o f Fennoscandia inferred from a study of th e mid-Norwegian margin . Global an d PlanetaryChange, 12 , 359-386. SUMMERFIELD, M.A . 1999 . Geomorpholog y an d global tectonics : introduction . In: SUMMERFIELD , M.A. (ed. ) Geomorphology an d Global Tectonics. Wiley, Chichester, 3-11. " Sveriges Geologisk e Undersokelse , 1985 . Scandina vian Caledonide s gravit y anomal y map . In : GEE, D.G. & STURT , B.A . (eds ) Th e Caledonide Orogen—Scandinavia an d Related Areas. Wiley , Chichester. THEILEN, R & MEISSNER , R . 1979 . A compariso n o f crustal an d uppe r mantl e feature s i n Fennoscandia and th e Rhenis h Shield , tw o area s o f recent uplift . Tectonophysics, 61 , 227-242.

TIMING AND MECHANISMS O F NORTH ATLANTI C CENOZOI C UPLIFT TORSKE, T . 1972 . Tertiar y obliqu e uplif t o f wester n Fennoscandia; crusta l warpin g i n connectio n wit h rifting an d break-u p o f th e Laurasia n continent . Norges Geologiske Unders0kelse, 273 , 43-48 . VAGNES, E . & AMUNDSEN , H.E.F . 1993 . Lat e Cenozoic uplif t an d volcanis m o n Spitsbergen : caused b y mantl e convection ? Geology, 21 , 251-254. VAN DE R BEEK , P.A . 1995 . Tectonic evolution o f continental rifts: inferences from numerical modelling an d fissio n track thermochronology. Ph D thesis, Vrije Universitei t Amsterdam. VAN DE R BEEK, P.A . & ROHRMAN , M . 1997 . Passiv e margin uplift aroun d the North Atlantic region and its rol e i n Norther n Hemispher e lat e Cenozoi c glaciation: Comment . Geology, 25, 282 . VAN DE R BEEK , P.A. , CLOETINGH , S . & ANDRIES SEN, P.A.M . 1994 . Mechanism s o f extensiona l

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basin formatio n an d vertica l motion s a t rif t flanks: constraint s fro m tectoni c modellin g an d fission-track thermochronology . Earth an d Planetary Science Letters, 121 , 417-433 . WHITE, N. & LOVELL, B. 1997 . Measuring the pulse of a plume with the sedimentary record . Nature, 387 , 888-891. ZIELHUIS, A . & NOLET , G . 19940 . Dee p seismi c expression of an ancient plat e boundary in Europe. Science, 265 , 79-81 . ZIELHUIS, A. & NOLET, G . 19946 . Shear-wave velocity variations in the uppe r mantl e beneath centra l Europe. Geophysical Journal International, 111, 695-715. ZIELHUIS, A . & VA N DER HILST, R.D . 1996 . Mantl e structure beneat h th e easter n Australia n regio n from partitione d wavefor m inversion. Geophysical Journal International, 127 , 1-16 .

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Paleocene initiation of Cenozoic uplift i n Norway S. B. NIELSEN 1, G. E. PAULSEN 1, D. L. HANSEN 1, L. GEMMER 1, O . R. CLAUSEN 1, B. H. JACOBSEN 1, N. BALLING 1, M. HUUSE 2 & K. GALLAGHER 3 1

Department of Earth Sciences, Aarhus University, Finlandsgade 8, DK-8200 Aarhus N, Denmark (e-mail: sbn@ geo.aau.dk)

Department of Geology & Petroleum Geology, University of Aberdeen, King's College, Meston Building, Aberdeen AB24 3UE, UK ^Imperial College of Science, Technology & Medicine, South Kensington, London SW7 2AS, UK Abstract: Th e timin g o f Cenozoic surfac e uplif t i n N W Europ e relie s o n th e assumptio n that the sedimentary response in basins is synchronous with tectonic processe s i n the source areas. However , man y of the phenomena commonl y use d to infer recent uplif t may as well be a consequence of climate change and sea-level fall. The timing of surface uplift therefor e remains unconstraine d fro m th e sedimentar y recor d alone , an d i t become s necessar y t o consider th e constraint s impose d b y physicall y an d geologicall y plausibl e tectoni c mechanisms, whic h have a causal relation to an initiating agent. The gradual reversal of the regional stres s fiel d followin g th e break-u p produce d mino r perturbation s t o th e therma l subsidence o n the Norwegian Shel f an d in the North Sea. Pulses o f increased compressio n cannot b e th e caus e o f Cenozoi c lan d surfac e uplif t an d accelerate d Neogen e basi n subsidence.Virtually deformation-free regional vertica l movements could have been caused by change s i n th e densit y colum n o f th e lithospher e an d asthenospher e followin g th e emplacement of the Iceland plume. A transient uplift componen t was produced as the plume displaced dense r asthenospher e a t the base o f the lithosphere . Thi s componen t decaye d a s the plume material cooled . Permanen t uplif t a s a result of igneous underplating occurred in areas o f a thin lithosphere (som e Palaeozoi c an d Mesozoic basins ) or for lithosphere under extension a t the time of plume emplacement (th e ocean-continent boundary). In areas of a thicker lithospher e (Eas t Greenland, Scotlan d an d Norway) plume emplacement ma y have triggered a Rayleigh-Taylo r instability , causin g partia l lithospheri c delaminatio n an d associated transient surface uplift a t a decreasing rat e throughout Cenozoic time . A possible uplift histor y fo r the adjacen t land areas henc e reads: initia l transient surfac e uplift aroun d the break-u p time a t 5 3 Ma cause d b y plum e emplacement , an d permanent tectoni c uplif t caused b y lithospheri c delaminatio n an d associate d lithospheri c heating . Th e permanen t tectonic uplif t increase d throug h Cenozoic tim e a t a decreasing rate. Denudation acted o n this evolvin g topograph y an d reduce d th e averag e surfac e elevation , bu t significantl y increased th e elevation o f the summi t envelope. The marke d variation s in the sedimentar y response i n the basins were caused b y climatic variation s and the generally fallin g eustatic level. Thi s scenari o bridges th e ga p betwee n th e idea s o f Paleocene-Eocene uplif t versu s repeated Cenozoi c tectoni c activity : th e tectoni c uplif t histor y wa s initiate d b y th e emplacement o f th e Icelan d plume , bu t continue d throughou t Cenozoi c tim e a s a consequence of early plume emplacement, with climatic and eustatic control on denudation. The mechanism is consistent with topography, heat flow, crustal structure, and the Bouguer gravity o f Norway, and may be applicabl e als o to East Greenland .

Geomorphological evidenc e an d th e ag e an d e t al. 1990 , 1992) , throughou t Cenozoi c time , structure of sediments poin t to the occurrence o f Contemporaneously , adjacen t continenta l area s large-scale Cenozoi c vertica l movements i n NW an d thei r inne r shelve s suc h a s th e Britis h Isle s Europe (Rii s & Fjeldskaa r 1992 ; Rii s 1996 ; (Japse n & Chalmer s 2000) , norther n an d Stuevold & Eldhol m 1996) . Th e Norwegia n souther n Norwa y (Rii s 1996 ; Lidmar-Bergstro m continental shelf , th e Vikin g Grabe n an d th e e t al. 2000), southern Sweden (Lidmar-Bergstrom central Nort h Se a al l experience d subsidence , 1991) , as well as the Barents Sea and the margins and apparently at an accelerating rat e (Cloetingh o f th e Nort h Se a Basi n (Dor e 1992 ; Jense n & From: DORE , A.G., CARTWRIGHT, J.A., STOKER, M.S. , TURNER, J.P. & WHITE , N. 2002 . Exhumation o f th e North Atlantic Margin: Timing, Mechanisms an d Implications for Petroleum Exploration. Geologica l Society , London, Special Publications, 196, 45-65. 0305-8719/027$ 15.00 © The Geological Society o f London 2002.

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Schmidt 1992 ; Japse n 1998 ) experience d uplif t and denudation . A simila r patter n o f vertica l movements i s observed a t the western margi n of the Nort h Atlanti c (Larse n 1990 ; Japse n & Chalmers 2000 ; Johnson & Gallagher 2000) . Continued Cenozoi c basi n subsidenc e ca n generally be understood in terms of a continuation of Palaeozoic-Mesozoi c mainl y rift-initiate d basin subsidenc e (McKenzi e 1978) . However , much controvers y exist s abou t th e causa l relationships regardin g th e simultaneou s Ceno zoic uplif t an d denudatio n o f th e adjacen t lan d areas. Wa s ther e a n initia l Paleocene-Eocen e surface uplif t relate d t o th e magmaticall y dominated openin g o f th e Nort h Atlantic , followed b y a climatically controlle d denudatio n response (Rii s & Fjeldskaa r 1992) , o r hav e a number o f tectoni c pulses , particularl y i n Paleocene an d (mainly ) i n Neogen e time s (Rii s 1996; Stuevol d & Eldhol m 1996) , bee n activ e throughout Cenozoi c time ? The role of climate and eustasy The argument s fo r th e timin g o f Cenozoi c tectonics ar e predicate d o n th e assumptio n tha t the sedimentar y respons e i n basin s reflect s contemporaneous tectoni c processe s i n th e source areas . However , denudatio n depend s

strongly o n climat e (whic h o n a regiona l leve l controls precipitatio n an d vegetatio n cover) , o n the erosiona l bas e leve l define d b y th e eustati c sea level , an d th e connectivit y o f th e drainag e basin t o th e regiona l bas e level . The questio n is then t o wha t exten t climat e change s ma y hav e influenced th e denudatio n proces s i n th e Nort h Atlantic region . Among th e climat e variations , obviousl y th e Quaternary glaciation s hav e ha d a majo r denudational impac t i n N W Europ e an d i n Greenland. Othe r climat e variation s wer e mor e subtle; however, i t i s wel l documented tha t pos t Late Eocen e tim e wa s characterize d b y a significant globa l climat e coolin g an d a fallin g eustatic level of about 200-250 m (e.g. Haq el al 1987; Mille r et al 1998) . These change s can be correlated wit h a world-wid e chang e i n sedi mentation styl e fro m carbonate-dominate d environments t o siliciclastic-dominate d systems with hig h rate s o f sedimentation . Mor e so , th e mid-Miocene climat e an d eustati c change s signalled a furthe r world-wid e acceleratio n i n the siliciclastic flux (Donnelly 1982; Bartek el al. 1991). Th e widesprea d distributio n o f thes e contemporaneous phenomen a point s to a global scale cause, such as climate change and sea-level fall. Cenozoi c climat e changes i n th e Nort h Se a region have been addressed i n moderate detail by

Fig. 1 . Averaged Fennoscandia n Bougue r gravit y field. Redraw n afte r Ballin g (1984) .

PALEOCENE INITIATION O F CENOZOIC UPLIFT IN NORWAY

Buchardt (1978) , wh o derive d a 6 18 O recor d based o n macrofossils. His results are in general agreement wit h thos e derive d fro m calcareous nannoplankton studie s (Mille r e t al. 1998 ) an d show th e onse t o f Cenozoi c coolin g i n lat e Eocene-Oligocene times (se e Huus e 2002b) . Apart fro m blurrin g th e causa l relatio n between tectonic s i n th e sourc e are a an d sediment stratigraphy , th e dependenc e o f denudation o n climate ca n effec t a coupling betwee n climatic deterioration an d the uplif t o f mountain ranges. Molnar & England (1990) argued that the transition t o a coole r an d mor e erosiv e climat e enhances relie f i n mountai n range s an d raise s mountain crest s i n isostati c respons e t o th e removal o f load . Thi s i s i n contras t t o Whippl e et al . (1999) , wh o claime d tha t erosiona l processes i n genera l reduc e relie f an d tha t th e isostatic peak uplift i s negligible. However , if the Norwegian Caledonide s wer e indee d pene plained at the onset of the North Atlantic opening it is clear that significant relief has been produced in th e mea n tim e (Fig . 2 , below) . W e therefor e believe tha t the arguments of Molnar & England (1990) mus t als o b e considere d i n th e Nort h

47

Atlantic region . A furthe r discussio n ha s bee n given by Huuse (2002b) . Tectonics Irrespective o f th e denudationa l respons e i n basins, primary surfac e uplift mus t be caused by physically plausible tectonic mechanisms, which lend themselves to a quantitative description and hypothesis testing. During mid-lat e Cenozoi c time , mid-ocea n ridge compressiona l stres s produce d larg e offshore dome s o n th e Norwegia n margi n (e.g. th e Hellan d Hanse n an d Orme n Lang e Domes), an d ha s bee n propose d t o b e a n agent i n Neogen e surfac e uplif t o f Norwa y b y either (1 ) th e bucklin g o f a n elasti c plat e (e.g . Cloetingh e t a l 1992 ; Dor e 1992 ) o r (2 ) thickening o f th e continenta l lithospher e b y shortening. (1) Elasti c buckling would for the wavelength (200km) and amplitude (1 km) require forces far beyond the compressional strength of continental lithosphere. Bucklin g o f a n elasti c plat e i s therefore no t a possibl e mechanism . Lambec k

Fig. 2 . Topographic relief o f southern Norwa y wit h locations of topographic profiles .

48

S. B. NIELSEN ETAL.

(1983) argue d tha t a viscoelasti c plat e wit h erosion an d depositio n produce s long-wave length large-amplitud e deflection s that resembl e buckling fo r stresse s tha t are much les s tha n th e elastic bucklin g stresses . However , thi s mode l fails the gravity and structural tests in Norway, as ridges an d highland s woul d becom e associate d with Bougue r gravit y maxim a becaus e o f a n elevated Mono , wherea s basin s woul d becom e associated wit h Bouguer gravity minima becaus e of a depressed Moho . A s pointed ou t by Ballin g (1984), th e Bougue r gravit y o f th e Norwegia n highlands (Fig . 1 ) show s significan t minim a (deeper tha n — 80 mgal) wit h a n axi s followin g closely th e axi s o f the topography . (2) Th e uplif t o f Scotland , th e Norwegia n Caledonides an d Eas t Greenlan d i s virtuall y deformation fre e an d canno t b e cause d b y shortening o f th e continenta l lithosphere , a s th e following simpl e argument demonstrates (Brodi e & Whit e 1994) . Th e exces s crusta l thickness , x, necessary t o suppor t topograph y o f heigh t h i s given by x = /zp m/Ap, where p m is the density of the uppe r mantl e an d A p is th e densit y contras t between th e uppe r mantl e an d th e crusta l root . For pm = 3300k g m~ 3 and Ap = 400k g m" 3, x

becomes 8.2 5 km per km of topography h. For an initial crusta l thickness o f 3 5 km, th e horizonta l shortening neede d t o produc e 1 km o f topo graphy amount s t o 24% , an d th e fina l dept h t o Moho become s 44.2 5 km, which is similar to the present-day Moh o dept h unde r th e Norwegia n Caledonides. However , 24 % o f horizonta l shortening woul d hav e produce d significan t compression structures , which are no t observed . The reversal of the regional stress field that the Norwegian Shel f an d th e Nort h Se a mus t hav e experienced doe s hav e consequence s fo r th e subsidence pattern , althoug h no t t o th e exten t previously assumed . Belo w w e discus s thi s i n more detail . Vertical movement s o n a regiona l scal e occur a s a consequenc e o f change s i n th e density colum n o f th e lithospher e an d astheno sphere an d th e associate d isostati c adjustments , or a s so-calle d dynami c topograph y supporte d by vertica l stresse s a t th e bas e o f th e lithosphere, whic h ar e generate d b y flo w i n the mantle . Th e detectio n o f dynami c topo graphy, particularl y ove r th e oceans , remain s controversial (Lithgow-Bertellon i & Silve r 1998). I n thi s pape r w e limi t ou r scop e t o

Fig. 3 . (a) Present-day topography (60°N) , summi t envelop e (dashed line) , and initial surface befor e erosion ; (b ) initial an d present (dashed ) Moh o position; (c ) calculated Bougue r gravity .

PALEOCENE INITIATION OF CENOZOIC UPLIFT IN NORWAY

49

Fig. 4 . (a) Present-day topography (61°N), summit envelope (dashed line), and initial surface befor e erosion ; (b) initial an d present (dashed ) Moho position; (c ) calculated Bougue r gravity.

isostatically supporte d vertica l movements . Virtually deformation-free , isostaticall y sup ported an d positive vertica l movement s ma y b e caused b y (1 ) magmati c underplatin g a t crusta l levels (McKenzi e 1984) , (2 ) lithospheri c delamination (Bir d 1979 ; Housema n e t al. 1981), o r (3 ) emplacemen t o f a ho t plum e a t the bas e o f th e lithospher e (Skogsei d e t al . 2000). I n vie w o f th e magmatic-dominate d opening o f th e Nort h Atlanti c thes e litho spheric -asthenospheric mechanism s therefor e must b e considere d t o b e th e candidate s t o produce th e Cenozoi c long-wavelengt h lan d surface uplift . In th e followin g sectio n w e produc e a brie f catalogue o f mechanism s o f passiv e an d activ e surface uplif t an d presen t ne w result s fo r late r reference i n the discussion. Mechanisms of surface uplif t Passive mechanisms Isostatic adjustments caused by erosion of existing topography. Erosio n o f existin g topo graphy result s i n lowerin g o f th e averag e

topography but , a s propose d b y Molna r & England (1990) , Rii s & Fjeldskaa r (1992 ) an d Gilchrest e t al . (1994) , th e summi t envelop e height ma y increas e becaus e o f th e isostati c uplift following the removal o f mass between th e peaks o r interfluves . I n thi s sectio n thi s mechanism i s discusse d withi n th e framewor k of unloadin g o f a n elasti c lithospher e b y erosion s o a s t o asses s th e amoun t o f tectoni c uplift require d t o produc e th e topograph y o f southern Norway . Th e approac h i s simila r t o that of Riis & Fjeldskaar (1992) excep t that, for a give n amoun t o f erosion , w e determin e th e initial topograph y require d to reproduc e the present topography . Furthermore , w e calculat e the associate d Bougue r gravit y anomaly . Th e timing o f th e erosio n i s no t addresse d b y thi s procedure. Three profile s a t 60 , 6 1 an d 61.7° N (Fig . 2 ) were extracte d fro m th e 1 km X 1 km digita l topography databas e GLOB E (GLOB E Tas k Team et al 1999) . The summit envelope of each profile wa s constructe d b y passin g a smoot h curve through the summits contained in a belt of 40km widt h surroundin g eac h profile . Th e amount erode d i s th e differenc e betwee n th e

50

S. B. NIELSEN ETAL.

Fig. 5 . (a) Present-day topography (61.7°N) , summit envelop e (dashed line) , and initial surface before erosion ; (b) initial an d presen t (dashed ) Moh o position; (c ) calculated Bougue r gravity .

Fig. 6 . The correlation between Bougue r gravit y an d average topographi c heigh t for the three profiles : (a) befor e erosion; (b ) afte r erosion . Continuous line is the correlation derived by Balling (1984) .

summit envelop e an d th e presen t topography . Next th e pre-erosio n Moh o dept h an d topography i s determine d s o tha t afte r erosio n of th e specifie d amoun t an d isostati c com pensation, th e mode l topograph y agree s wit h the presen t topography . I t i s assume d tha t

isostatic compensatio n take s plac e a t th e crust-mantle interfac e wit h a densit y contras t of 400k g m~ 3 , an d tha t th e uppe r mantl e reacts b y flexur e t o loadin g wit h a n elasti c thickness o f 15km . Th e averag e crusta l density i s 2800k g irT 3.

PALEOCENE INITIATION OF CENOZOIC UPLIF T IN NORWAY

Figures 3-5 show the results of applying this procedure. I t appears tha t the process o f erosio n has raised th e summit s a maximum of c. 800 m. The amoun t i s directl y visibl e a s th e difference betwee n initia l an d presen t Moh o depths. Th e maximu m initia l topograph y required t o reproduc e th e presen t topograph y i s located close to the axis of maximum present-day topography an d i s i n th e rang e 1.3-1. 5 km. T o the west and east the initial topography gradually approaches zero . Th e initia l topograph y i s identical t o th e tectoni c uplif t tha t mus t hav e been produce d b y th e mechanis m responsibl e for th e Cenozoi c uplif t o f th e Norwegia n Caledonides, sav e fo r th e possibl e existenc e o f topography befor e th e actio n o f th e mechan ism. Fo r a prio r topograph y o f c . 300m , corresponding approximatel y t o th e topography that woul d emerg e afte r th e post-mid-Cretac eous sea-leve l fall , i f topograph y the n ha d been peneplaine d t o se a level , th e maximu m necessary amoun t o f tectoni c uplif t i s i n th e range 1-1. 2 km. I f positiv e topograph y i s allowed fo r durin g th e tim e o f maximu m se a level th e necessar y tectoni c uplif t become s correspondingly less . Figure 6 show s th e correlatio n betwee n topography an d th e Bougue r gravit y anomal y before (a) and after (b) erosion. The correlation is negative becaus e topograph y is compensated b y a crusta l root , a fac t tha t i s no t modifie d b y a relief-producing denudationa l process. The modelled correlatio n i s i n agreemen t wit h th e correlation determine d b y Ballin g (1984 ) fo r the wester n Fennoscandia n Shiel d (Norway) . The mode l Moh o depth s beneat h souther n Norway ar e qualitativel y i n agreemen t wit h th e depths derived b y Kink et al (1993) , but exhibit some deviations to the east in the Fennoscandian

51

Shield, probabl y becaus e o f breakdow n o f th e assumption of a laterally homogeneou s crust . hostatic and erosional response of basin margins to sea-level fall. I t i s generall y acknowl edged tha t th e eustati c leve l decrease d fro m mid-Cretaceous tim e t o th e presen t da y b y 200-250 m (Haq et al. 1987; Miller et al. 1998) , although smalle r amplitude s hav e bee n suggested (Komin z e t al . 1998) . Thi s eustati c fall expose d th e margin s o f sedimentar y basins, which may have been established under a former higher eustati c level , an d cause d erosiona l unloading an d passiv e isostati c uplif t (i.e . erosional rebound) . The subsidenc e diagra m o f Fig . 7 demon strates thi s mechanis m i n actio n o n a basi n margin. Th e initia l eustati c leve l a t 10 0 Ma i s 250 m, which is also the water depth at that time. Sedimentation ha s fille d th e basin to the brim at the onse t (6 0 Ma) o f a gradua l eustati c fal l o f 250m. A maximu m o f 750 m o f sedimen t i s accommodated b y th e initia l 250 m o f accom modation space because o f sediment loading and compaction o f the alread y existing sediment. As the eustatic fall exposes the sediment surface it is eroded. A t th e presen t da y a tota l o f 500 m o f sediment ha s bee n eroded , an d th e sedimen t a t the surfac e an d a t an y dept h belo w therefor e shows a n overburia l (sensu Japse n 1988 ) of 500m. Figure 8a- c show s th e result s o f combinin g the abov e principl e wit h therma l subsidence , lateral sedimen t transport, and a zone of tectonic inversion in a 1300k m profile. Th e profile coul d simulate an east-west North Sea profile at 56°N. It represent s th e post-mid-Cretaceou s fillin g from th e side s o f a n intra-cratoni c basi n

Fig. 7 . Subsidence diagram showing the effect o f falling se a level (dashed line) on the erosion of a basin margin.

52

S. B. NIELSEN ETAL.

dominated b y therma l subsidenc e i n th e centra l Th e subsidenc e diagra m (Fig . 8a) shows quiet parts, falling eustatic level, and localized tectoni c sedimentatio n an d subsidenc e unti l th e clino inversion (e.g. the Sorgenfrei-Tornquist Zone) at form s migrating from the basin margins reach the the easter n margin . basi n centre . Fo r late r referenc e i t i s note d tha t

Fig. 8 . Subsidenc e diagra m (a) , sedimen t structur e a t OM a (b) , an d overburia l (c ) fo r a 1300k m profil e representing a n intra-cratoni c basi n wit h thermal subsidenc e i n th e centre , fallin g eustati c level , an d sedimen t infilling fro m th e sides . Als o show n is a zone of structura l inversion.

PALEOCENE INITIATIO N OF CENOZOIC UPLIF T I N NORWAY

passive infillin g o f existin g accommodatio n space, quie t therma l subsidenc e an d sedimen t loading lea d t o a n apparentl y acceleratin g Neogene subsidence . Th e present-da y structur e of the sediments (Fig. 8b) shows truncation of the clinoforms toward s th e margin s a s wel l a s th e inversion structur e an d its marginal troughs . The present-day buria l anomal y alon g th e profil e (Fig. 8c ) varie s fro m O m i n th e basi n centre , where therma l subsidenc e an d sedimen t loadin g and compactio n outpace d th e reductio n i n accommodation spac e produce d b y th e fallin g sea level and sedimentation, to more than 1 km in the inversio n zone . Th e backgroun d valu e o f overburial on the basin margins is caused by the mechanism o f Fig. 7. The abov e principles , includin g tempora l variations i n sedimen t sourc e area s reflectin g the clockwise rotation of source areas throughout Cenozoic time, have been built into a large-scale post-mid-Cretaceous North Se a model. B y trialand-error runs , i n whic h th e initia l therma l anomaly an d th e sedimen t feedin g wa s varied , this model was brought into good agreement with the present-da y thicknes s distributio n of the Upper Cretaceou s an d Cenozoic unit s (Gemme r et al 2002 ; Nielse n 2002) . Here we show two of the principal predictions of this model, which can be compared wit h observations: chal k overburia l (Fig. 9 ) an d th e pre-Quaternar y geologica l ma p (Fig. 10).

53

The regional pattern of chalk overburial in the North Se a Basi n was quantifie d b y Japse n 1998 by comparing measured compressional velocitie s of chalk with a reference depth-velocity model . He foun d tha t th e overburia l increase s toward s the easter n an d wester n Nort h Se a margins i n a symmetrical patter n aroun d a centra l zon e aligned alon g th e Central Graben area , i n which chalk is at its maximum depth at present day. The overburial predictio n o f Fig . 6 reproduce s th e general patter n o f overburia l inferre d b y Japse n (1998). However , th e predicte d amplitude s ar e smaller tha n th e amplitude s o f Japse n (1998) , except i n th e inversion zones , wher e ver y larg e values of overburial can occur also in the model. The predicte d pre-Quaternar y ma p o f Fig. 10 shows marked similaritie s wit h the pre-Quaternary onshor e Denmar k (Sorgenfre i & Berthelse n 1954) an d with the compilation of Japsen (1998). The area l exten t o f th e Oligocen e outcro p o f onshore Denmark , however , i s to o larg e a s a consequence o f erosion t o sea level in the model of al l topograph y in th e basin . The curvatur e of the Cenozoi c laye r boundarie s adjacen t t o th e Sorgenfrei-Tornquist Zon e agree s wit h obser vations and is caused by the filling of the flexural foredeep an d the later erosional truncation. The conclusio n of this sectio n i s that much of the know n post-mid-Cretaceou s sedimentar y structure o f th e Nort h Se a Basi n ca n b e reproduced b y a mode l tha t include s passiv e

Fig. 9 . Predicte d overburia l o f chal k (o f lat e Cretaceou s an d Dania n age) . In th e centra l Nort h Se a thermal subsidence an d compaction of previous sediments keep pace with the reduction of accommodation spac e caused by falling se a level an d sedimentation. Fou r zone s of structural inversion have been included.

54

S. B. NIELSEN ETAL.

Fig. 10 . Predicted pre-Quaternar y geologica l map .

isostatic effect s i n respons e t o therma l cooling , sea-level fall , erosion , an d changin g sedimen t source areas . Th e questio n tha t remain s i s whether th e discrepanc y o f chal k buria l ampli tudes towards the margins is a consequence o f an unknown mechanis m o f activ e surfac e uplift , which is not part of this numerical model , o r can be explaine d b y th e referenc e chal k velocit y model (Huus e 2002a, 2002b) . Active mechanisms Changing in-plane stress. It is well known that fluctuating in-plan e stres s cause s fluctuation s o f relative wate r dept h o n passiv e margin s an d i n intra-cratonic basin s becaus e o f th e induce d vertical deflection s of the lithosphere (Cloeting h etal. 1985 , 1990 , 1992 ; Karner 1989) . However , it i s perhap s les s wel l establishe d whethe r a n increase i n compressio n cause s a depressio n o r an elevatio n o f th e basi n floor . Th e principa l reason for this controversy is that the thin elasti c plate model s applie d i n th e majorit y o f studie s require preloading in order to respond b y flexure to horizonta l stres s changes . Thi s preloadin g produces a n initia l deflection , the amplitud e o f which is enhanced i n compression o r reduced i n extension, a s th e produc t o f th e in-plan e forc e and initial plate curvature acts in the same way as a vertica l load . Th e resul t o f a thi n plat e compression o r extensio n experimen t i s henc e given a priori b y the modeller's preferenc e about initial loading .

An indication of what might happen when the tectonic stres s o n a continenta l margi n change s from extensio n to compression wa s presented by Braun & Beaumon t (1989) , wh o simulate d passive margi n formatio n b y a numerica l thermomechanical model . The y foun d tha t a sudden relie f o f th e extensiona l tectoni c stres s causes basin floor rise and rift flank subsidence a s a ne w balanc e betwee n in-plan e stres s an d buoyancy force s is established. Here w e pursu e thi s a littl e furthe r b y simulating th e formatio n o f a generi c passiv e margin b y lithospheri c extensio n i n Mesozoi c time, and reversal of the stress field at the time of opening o f th e Nort h Atlanti c (Paulse n e t al 2001). W e conside r th e inherite d strai n history, state of stress, and sediment and thermal loading of th e margi n a t the tim e of stres s reversal. Th e results also apply to rifts and intra-continental rift basins such as the Viking Graben and the central North Sea . The numerica l mode l i s a dynamica l large strain Lagrangia n finit e elemen t mode l wit h a n elasto-viscoplastic rheology . Plasticit y i s mod elled usin g non-associate d plasti c flo w wit h a Driicker-Prager yield criterion. Th e viscoelastic deformation i s modelle d b y a non-linea r Maxwell rheolog y wit h viscosit y dependin g o n temperature, materia l type , and deviatoric stress . In general, the type of deformation occurring at a point i n th e mode l depend s o n temperature , deviatoric stress, confining pressur e and material type. Figur e 1 1 show s th e initia l undeforme d

PALEOCENE INITIATION O F CENOZOIC UPLIF T IN NORWAY

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Fig. 11 . Finite elemen t mode l o f passive margin-rift formation . The model includes uppe r and lower crus t and mantle lithosphere. The boundary conditions are 0 °C at the surface and 1300 °C at a fixed depth. There is no flow of hea t acros s th e vertical boundaries . Loadin g b y sedimentatio n an d erosion occu r a t the surface . The botto m boundary condition is a hydrostatic pressure. A kinematic boundary condition causes extension or compression of the profile. Mode l parameter s follo w Paulse n et al (2001) .

Fig. 12 . Cross-section s o f margi n evolutio n wit h positions o f tracke d basemen t point s indicate d b y arrow s a t locations 0 , 50 , 9 0 an d 160km : OMa , initia l sedimen t structure ; 1 5 Ma, en d o f rifting ; 10 5 Ma, onse t o f compression; 15 8 Ma, final sediment structure . The deposition o f syncompressional sediment s has been retarde d between 2 0 and 120km .

56

S. B. NIELSEN ETAL.

finite elemen t gri d an d th e therma l an d mechanical boundar y conditions . Localizatio n of margi n formatio n i s achieve d b y assumin g a weaker (wet ) rheolog y i n th e regio n tha t i s t o become extended . Therma l weakenin g woul d yield simila r results , bu t th e exac t mod e o f weakening i s not the subjec t here . The margi n i s created by extensio n fo r 3 2 Ma by movin g th e righ t mode l boundar y a t a constant velocity t o the right (Fig. 12) . A smaller rate of extension woul d result in more synrif t an d less postrif t therma l subsidence , an d vic e versa . However, th e duratio n o f extensio n i s no t important t o th e principa l results . Afte r exten sion, therma l subsidenc e follow s fo r a period o f 85 Ma. Subsequently , th e stres s fiel d i s reverse d by moving th e right boundary t o the left . Thi s i s equivalent t o applyin g compressiv e stresse s associated wit h th e openin g o f th e Nort h Atlantic.

Figure 13 a shows th e forc e histor y applie d t o the profile . Th e forc e i s obtaine d a s th e depth averaged squar e roo t o f th e secon d invarian t of the deviatoric stres s a t the right model boundary multiplied b y th e lithospher e thickness . Thi s force measur e i s alway s positive , althoug h th e sign o f th e principa l horizonta l tectoni c stres s changes. Th e initia l puls e o f force produce s th e initial extension. Afte r cessatio n o f strainin g the force relaxes by viscous dissipation of deviatoric stresses i n th e ductil e part s o f th e lithosphere . The gentl e reversa l yield s a slow increase i n the force. A t 1 2 Ma a shor t duratio n pulse simulates the effect s o f reorganizatio n o f Nort h Atlanti c spreading. Th e tota l compressiv e forc e end s a t c. 1. 7 X 1012N m" 1 , whic h ca n b e compare d with a n estimate d magnitud e o f c . 3. 9 X 10 12 N m" 1 o f th e ridge-pus h forc e i n a n oceani c lithosphere o f 10 0 Ma ag e (Turcott e & Schuber t 1982).

Fig. 13 . (a) The forc e histor y applie d t o the profile; (b ) subsidenc e histor y without compression; (c ) subsidence history wit h compression; (d ) difference between (c ) and (b) .

PALEOCENE INITIATION O F CENOZOIC UPLIF T IN NORWAY

Figure 13 b an d c show s th e vertica l move ments o f fixed basement point s along th e profil e in th e case s withou t (Fig . 13b ) an d wit h stres s reversal (Fig . 13c) . The location of the basemen t points an d th e evolutio n o f cross-section s ar e shown in Fig. 1 2 at ag e OMa . The initia l margi n formatio n result s i n basi n subsidence a s tracke d b y basi n point s Okm , 300km an d 400km . Poin t 500k m show s th e formation o f th e rif t flan k durin g rifting . Poin t 725km register s th e far-fiel d vertica l movements. Following extensio n ther e i s therma l subsidence o n th e passiv e margin , enhance d b y sediment loading . Fro m compariso n o f Fig. 13 b and c i t become s apparen t tha t thi s genera l picture continue s throug h th e gradua l stres s reversal a t 5 3 Ma. Th e smal l subsidenc e pertur bations produce d b y th e stres s reversa l ar e enhanced i n Fig. 13d , which shows the difference between the subsidence curves of Fig. 13 b and c. The subsidenc e rate s o f point s 300k m an d 400km increas e slightl y i n th e initia l phas e o f stress reversa l becaus e o f th e smal l elasti c volume reduction associated wit h stress reversal and becaus e o f th e continuou s adjustment s to a new equilibriu m betwee n in-plan e force ,

57

buoyancy force s an d evolvin g stresses . Fo r continuing compressio n th e subsidenc e o f th e basin point s becomes retarded as compared with the cas e o f pur e therma l subsidence . Th e rif t flank poin t (500km ) subside s a t a n increasin g rate in the initial phase of compression, an d later becomes slightl y uplifted . The respons e t o th e compressiona l puls e a t 12 Ma depends o n the position alon g th e profile . Generally, ther e is a small elevation o f the basin floor, which , however , i n th e cas e o f poin t 300km i s followe d b y a mino r increas e i n subsidence. A larger compressiona l puls e would produce larger-amplitude deflections of the same shape. Our mode l applie s t o a passiv e margi n wit h laterally smoothl y changin g rheological proper ties. I n reality , th e strai n followin g stres s reversal wil l b e accommodate d b y th e reactivation o f majo r basemen t faults , resultin g in localize d shortenin g an d th e developmen t o f inversion structure s o r dome s suc h a s th e Helland Hanse n Dom e an d Orme n Lang e o n the Norwegia n continenta l margin . Margina l troughs wit h loca l subsidenc e ma y develo p i n connection wit h suc h inversio n zone s (Nielse n & Hanse n 2000) .

Fig. 14. Thermal and topographic response of lithospheric column to 10km intrusion: (a) temperature profiles; (b) topography; (c ) surfac e heat flux.

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Magmatic underplating. I t i s wel l know n tha t magmatic underplatin g provide s a mechanism o f immediate epeirogeni c uplif t (McKenzi e 1984 ; Brodie & Whit e 1994) . Figur e 1 4 show s th e thermal an d topographi c respons e o f a I D lithospheric colum n t o th e introductio n o f a magma a t th e bas e o f th e crust . Th e intrusio n causes immediat e surfac e uplif t followe d b y a minor therma l subsidenc e a s the intrusion cools . The uplift , /i , a s a resul t o f underplatin g o f thickness x * s given b y h = ( 1 — px/pa)x, wher e px is the density of the underplating materia l an d pa is the density o f the asthenosphere. Erosion of the surfac e i s no t considered . Th e hea t flow , initially a t 5 0 mW m~ 2 , increase s i n les s tha n 10Ma t o 6 2 mW m~ 2 . A t 5 5 Ma afte r th e intrusion the surface heat flow is essentially bac k to the initial value . The underplatin g mechanis m produce s uplif t in direct response t o the underplating history. It is followed b y mino r therma l subsidence . Depend ing o n th e densit y contras t betwee n th e under plating magm a an d th e mantle , c . 10k m o f underplating i s require d t o produc e a n initia l uplift o f 1 km. Lithospheric delamination. Delaminatio n o f the lithosphere , o r bas e lithospher e erosion ,

means los s i n some wa y o f a fraction o f the base of th e mantl e lithosphere . Thi s mechanis m ha s been applie d t o explainin g th e sudde n uplift s o f the Colorad o Platea u (Bir d 1979 ) an d th e Himalayas (Housema n et al 1981) . I n thes e cases delamination happen s a s a consequence o f thickening an d destabilizin g o f th e mantl e lithosphere i n collision . I n th e Nort h Atlanti c case delaminatio n mus t b e relate d someho w t o the interactio n o f th e Icelan d plum e wit h th e lithosphere. W e sugges t tha t a gravitationa l (Rayleigh-Taylor) instability (Conrad & Molnar 1997; Housema n & Molna r 1997 ) triggere d b y the arriva l o f th e low-viscosit y an d low-densit y Iceland plum e ma y hav e bee n responsibl e fo r convective remova l o f a fractio n o f th e mantl e lithosphere. Fo r later reference we present here a ID mode l o f th e therma l an d topographi c consequences o f this process . It i s assume d tha t th e lithospher e remain s a t the reduce d thicknes s afte r delamination . An y recovery o f the lithosphere thickness reduces th e topographic respons e o f the partial delamination mechanism. Fo r a ful l recover y o f lithospher e thickness th e topograph y return s t o it s initia l position, unles s perhap s th e delaminate d lithosphere wa s chemicall y buoyan t an d distinct from th e remainin g lithosphere . Th e presen t scenario henc e assume s tha t th e lithospher e i s

Fig. 15 . Therma l an d topographi c respons e o f lithospher e sufferin g 3 5 km o f delamination : (a ) temperatur e profiles (8 5 km plume); (b ) topograph y i n various cases; (c) surfac e hea t flux (85 km plume).

PALEOCENE INITIATION OF CENOZOIC UPLIFT IN NORWAY

delaminated afte r destabilizatio n b y plum e emplacement t o a ne w an d thinne r equilibriu m thickness, which may be true if the lithosphere is initially thicke r tha n th e presen t equilibriu m thickness. Figure 1 5 shows the therma l an d topographi c response t o delaminatio n ove r 5 Ma o f a I D lithospheric column . Air y isostas y i s assumed , and th e initia l topograph y i s zero . A t OM a th e plume arrive s a t th e bas e o f th e lithosphere , initially 16 5 km thick. Over a time spa n of 5 Ma the plum e grow s i n thicknes s t o 4 5 o r 85k m simultaneously with erosion of 35 km of the base of the lithosphere. Other growth times could have been chosen but do not significantly influenc e the principal results . Th e plum e ha s a n exces s temperature of 100°C , which decays after plum e emplacement wit h a tentativ e tim e constan t o f 40 Ma. Complet e modellin g o f th e deca y o f th e excess plume temperatur e involves modelling of whole-mantle convection , whic h i s beyon d th e scope o f thi s paper . Th e case s o f delaminatio n without plume thickening, but in the presence of the basal temperatur e anomaly, and the effec t o f the basa l temperatur e anomal y alon e ar e shown for comparison . The lithospheri c temperatur e profile s (Fig. 15a ) ar e fo r th e cas e o f 85k m o f plume . The tectoni c uplif t historie s (Fig . 15b ) sho w a

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phase o f rapi d initia l uplif t a s plum e materia l replaces dense r asthenospher e an d 3 5 km o f th e lower lithosphere , followe d b y increasin g tec tonic uplif t cause d b y heatin g o f th e remainin g lithosphere t o a ne w therma l equilibriu m situation. Th e magnitud e o f th e initia l uplif t depends strongl y on the plume thickness and the amount o f lithospheri c delamination . However , the plum e contributio n decay s ove r tim e a s th e plume materia l cools . Th e resultin g permanen t tectonic uplif t depend s onl y on the amoun t of lithospheric delamination. The uplift rat e following plum e emplacemen t an d delaminatio n depends o n the amoun t of delaminatio n an d th e initial plum e thickness . Fo r give n delaminatio n thickness th e rat e increase s wit h decreasin g plume thickness. The surface heat flux (Fig. 15c ) shows a minor slow increase. Unlike lithospheric stretching, whic h als o thin s th e lithosphere , delamination cause s n o rapi d squeezin g o f th e isotherms an d therefor e show s a muc h quiete r thermal response . Fo r late r referenc e i t i s note d that the initial, possibly rapid, disturbance of the lithosphere-asthenosphere system s cause s tec tonic surface uplift t o evolve with a time constant of c . 60 Ma. A Paleocene-Eocene delamination hence produce s increasin g cumulativ e tectoni c uplift a t a decreasin g rat e throughou t Cenozoic time.

Fig. 16 . Th e topographi c response of a lithospher e t o delamination . Curv e paramete r i s initia l lithospher e thickness.

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Figure 1 6 summarize s th e abilit y o f th e delamination mechanis m t o produc e permanen t tectonic uplift . Fo r simplicity , th e transien t components cause d b y plum e emplacemen t an d the exces s plum e temperatur e ar e lef t out ; they disappear anyway . Th e uplif t a t th e en d o f delamination represents , therefore , th e pur e initial delaminatio n uplif t fo r th e give n litho sphere thickness. Th e dependence o f initial uplif t on lithospher e thicknes s i s cause d b y th e dependence o f th e lithospheri c geother m o n lithospheric thickness . Fo r a thick lithosphere th e delaminated material has an average temperature that i s close r t o th e asthenospheri c temperatur e than i t i s i n th e cas e o f a thi n lithosphere . Replacing i t with asthenosphere therefore yields less excess buoyancy. The present-day values are the tectoni c surfac e uplif t afte r therma l relax ation fo r 5 0 Ma followin g delamination. At that time, a thi n lithospher e i s almos t i n therma l equilibrium, wherea s a thick lithosphere stil l has not adjuste d t o th e ne w therma l equilibriu m situation. Th e limitin g valu e o f surfac e uplif t after infinit e relaxation tim e depend s linearl y on the delamination thickness . Discussion Following Dor e et al (1999) , the North Atlantic continental break-u p a t 53 Ma was the culmina tion o f a c . 35 0 Ma perio d o f Permo-Triassic , (mainly late ) Jurassic , Early Cretaceous , 'mid' Cretaceous, and latest Cretaceous-Early Eocen e rifting in the area of the Caledonian orogen. After these event s the topograph y of N W Europ e wa s low an d separate d b y shallo w sea s overlyin g deep Palaeozoi c an d Mesozoi c sedimentar y basins. The fina l stag e o f lates t Cretaceous-Earl y Eocene riftin g le d t o break-u p an d becam e associated wit h intense magmatic activity as the Iceland plum e arrive d a t th e bas e o f th e Nort h Atlantic lithospher e a fe w millio n year s befor e break-up (Skogseid et al. 2000). The plume fron t may have extended mor e tha n 1500k m from th e plume centre , encirclin g th e Nort h Atlanti c Volcanic Province, including the British Tertiary Volcanic Province , th e Labrado r Se a volcani c provinces, an d th e Greenland , Scottis h an d Norwegian Caledonides (Skogsei d e t al. 2000) . In th e Britis h Isles , wher e age s o f volcani c rocks i n th e Britis h Tertiar y Volcani c Provinc e and larg e offshor e thicknesse s o f Paleocene an d Eocene sequence s bea r direc t witnes s t o th e timing o f th e uplift , ther e i s a goo d correlatio n between plum e emplacemen t an d clasti c sediment influx , whic h ha s bee n use d t o infe r surface uplif t variation s (Whit e & Lovel l

1997). However , regardin g th e surfac e uplif t of th e Norwegia n (and Scottish ) Caledonides , the direc t rol e o f th e plum e a s a n initiatin g agent a t th e tim e o f emplacemen t ha s no t bee n widely considered . O n th e Norwegia n con tinental shel f an d i n larg e part s o f th e Nort h Sea th e direc t associatio n betwee n sedimen t patterns an d tectoni c activit y i n th e sourc e areas yield s mino r Paleocene-Eocen e tectoni c activity wit h th e majo r phas e startin g i n Oligocene tim e (se e Huus e 2002b ) an d accelerating throug h Neogen e times . Thi s leaves a tim e ga p o f c . 3 0 Ma betwee n plume emplacemen t an d th e majo r phas e o f surface uplift , an d produce s majo r problem s explaining th e mos t recen t uplif t mechanism : why di d th e plume , whic h wa s mos t vigorou s and ho t durin g th e shor t tim e o f emplacement , after whic h i t starte d t o cool , tak e 3 0 Ma t o produce significan t surfac e uplift ? Sediment structure and fission tracks We believe that the direct association o f intensity of sedimentar y respons e wit h contemporaneous tectonic activit y i n th e sourc e area s i s proble matic. Molnar & England (1990) argued that the phenomena commonly used to infer recent uplif t may as well be a consequence of climate change. On th e basi s o f example s fro m th e Alps , Pyrenees and the Rocky Mountains of Colorado, Wyoming an d Utah , the y argue d tha t th e existence o f geomorphologicall y youn g land scapes wit h deeply incised valley s and a jagged relief, whic h usually i s taken as a n indication of recent tectoni c uplift , i s a consequenc e o f a change i n th e proces s o f erosion , fo r exampl e caused by climate change, and in the extreme by glaciers. Th e denudatio n proces s produce s apparent uplif t b y raisin g th e summi t envelope by passive isostatic compensation. The North Sea area di d feel the global climat e change from lat e Eocene int o Oligocene time s and did experience deep glacia l erosion an d a general sea-level fal l (Riis & Fjeldskaar 1992 ; Huus e 2002b). There fore, i n ou r opinion , th e timin g o f a n uplif t history based on sediment ages must be tempered with th e influenc e of climate an d erosional bas e level o n denudation rates. The influenc e o f climat e an d eustas y i s no t known in grea t detail , but the coolin g histor y associated wit h denudatio n ma y b e inferre d directly fro m apatit e fission tracks (AFTs). This technique therefore has been extensively applied in Norway (Rohrman et al. 1995 ) to infer recent cooling an d ha s contribute d t o th e concep t o f accelerating Neogen e uplif t o f th e souther n and northern Norwegia n domes. However, publishe d

PALEOCENE INITIATION O F CENOZOIC UPLIFT IN NORWAY

models fo r annealin g o f fissio n track s sho w a marked lac k of sensitivity to temperatures below 60 °C, althoug h i t i s clea r tha t som e annealin g does occur at lower temperatures ove r geologica l time scales . Whe n modellin g rea l data , thi s can often lea d t o th e spuriou s inferenc e o f recen t cooling fro m aroun d 60 °C (Gallagher & Brown 1999). Th e uncertaint y inheren t i n constraining denudation (an d an y associate d uplift ) become s most acut e i n recently glaciate d areas , o r where only 1 km o r les s o f materia l ma y hav e bee n removed. Thi s i s a proble m i n man y part s o f southern Scandinavia . Thi s recentl y recognize d issue ha s no t bee n addresse d i n AF T studie s of the area and means that the inferred temperature histories d o no t properl y represen t th e tru e cooling histor y produce d b y denudation . Th e recently revive d (U-Th)/H e technique , applie d to apatite , whic h ha s a lowe r temperatur e sensitivity tha n fissio n track s (e.g . Hous e e t al 1998; Wol f e t al . 1998) , ma y provid e th e ke y timing information in this region.

Mechanism of surface uplift In th e absenc e o f direc t constraint s fro m sediment age s an d coolin g histories , th e discussion o f th e timin g o f activ e Cenozoi c surfac e uplift aroun d th e Nort h Atlanti c an d th e simultaneous basi n subsidenc e become s mos t meaningful whe n the requirement of a physically plausible mechanis m i s considered . Compressional stres s is not a plausible agent . Our mode l exampl e show s tha t th e gradua l change from extensional to compressional stress , which th e peri-Nort h Atlanti c region mus t have experienced followin g the break-up, has had only minor influenc e o n th e subsidenc e o f th e basin s and uplif t o f th e lan d areas . Fo r physicall y reasonable stres s levels , stres s reversa l cause s a small enhancemen t o f th e rat e o f therma l subsidence, an d a sudde n compressiona l puls e caused by, for example, spreading reorganization causes a n elevatio n o f th e basi n floor . W e therefore think that the accelerating late Neogene tectonic subsidenc e derive d b y Cloetingh e t al . (1990, 1992 ) for the North Atlantic margins and the North Sea area is not related to compressional pulses from spreadin g reorganization. The spac e needed t o accommodate th e Neogene sediment s is muc h to o larg e t o b e explaine d b y compres sional forces. Rather, we suggest that the derived tectonic subsidence may be a consequence of the use o f to o shallo w palaeo-wate r depth s i n th e tectonic subsidenc e calculation . Palaeo-wate r depths ar e notoriousl y difficul t t o assess , ye t have significan t influenc e o n th e result s o f

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tectonic subsidenc e calculations . I n our opinion, the apparentl y acceleratin g lat e Neogen e sub sidence i s simpl y therefor e passiv e fillin g o f already existing accommodation spac e enhance d by loading-induced subsidence (see Fig. 8a) . We no w tur n ou r attentio n t o th e rol e o f th e Iceland plum e i n explainin g th e Cenozoi c surface uplift . W e adopt the plume emplacemen t scenario o f Skogsei d e t al . (2000) : a flattene d plume head with an excess temperature of 100 °C (density contrast of c. — 11 kg m~ 3 ) arrives a few million years before break-up time at the base of the Nort h Atlanti c lithosphere , whic h show s marked thicknes s variation s followin g th e Palaeozoic-Mesozoic rifting period . As describe d b y Skogsei d e t al. (2000) , area s of relativel y thi n lithospher e becam e associate d with th e thickes t plum e colum n an d therefor e experienced th e larges t initia l isostati c uplift . These ar e als o th e area s tha t ar e pron e t o decompression meltin g a s th e plum e materia l rises higher , wit h th e possibilit y o f magmati c activity i n th e for m o f floo d basalts , magmati c underplating o f th e crust , an d intrusions . Th e Norwegian continenta l shel f an d th e norther n North Sea , wit h a dens e dat a coverage , provid e suitable area s fo r quantificatio n o f th e sudde n isostatic uplif t inflicte d b y plume emplacement . In th e neighbourin g continenta l area s o f thicker lithospher e suc h a s wester n Fennoscan dia, Scotlan d an d Eas t Greenland , plum e emplacement cause d a transien t isostati c uplif t component a s ho t plum e materia l displace d denser asthenosphere . Furthermore , i f partia l delamination o f th e lithospher e too k place , permanent surfac e uplift woul d have resulted. It is therefor e importan t t o conside r th e physica l reason for delamination, and we propose that the Rayleigh-Taylor instabilit y i s a candidat e fo r explaining the process o f partial delamination . Rohrman & van der Bee k (1996 ) applie d this principle t o explainin g th e peri-Nort h Atlanti c domal uplif t pattern . Thei r varian t o f th e Ray leigh-Taylor instabilit y focuse s o n th e generation o f diapir s i n th e buoyan t lowe r layer, whic h ris e int o th e rigi d li d a t selecte d locations an d produc e doma l uplift . Thi s application utilize s localize d upwellin g and is analogous t o th e generatio n o f sal t diapir s o r thunderstorms. Th e timin g o f thei r scenari o requires tha t plum e emplacemen t beneat h th e Norwegian an d Scottis h Caledonide s too k plac e around 40-30Ma , whic h i s i n contras t t o th e early plum e emplacemen t scenari o o f Skogsei d et al. (2000). Our applicatio n o f th e Ra y leigh-Taylor instability t o th e lithosphere-asthenospher e boundary follow s th e original concept.

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Conrad & Molna r (1997 ) an d Housema n & Molnar (1997 ) hav e show n i n a linearize d analysis ho w disturbances a t th e lithospher e asthenosphere boundar y wit h a wavelengt h between 10 0 and 200k m ca n becom e unstable , resulting i n th e bas e o f th e lithospher e bein g swept into drips o r blobs o f descending material . This ultimatel y result s i n thinning of the mantl e lithosphere a s th e dro p detache s an d sink s through th e low-viscosit y asthenosphere , carry ing mantl e lithospher e away . Th e resul t i s localized downwelling , whic h feed s o n litho sphere materia l fro m th e sides . Ther e ar e n o localized asthenospheri c diapirs , bu t a rathe r broad asthenospher e upwellin g t o fil l th e spac e vacated b y th e los t lithosphere . Th e possibl e growth rat e o f the instabilit y ranges fro m 10" 14 s"1 t o higher , correspondin g t o evolutio n time s of 3 Ma or longer. Depending on the timing of the process i t is possible tha t decompression meltin g may take place in the upwelling asthenosphere. It may therefor e b e theoreticall y possibl e tha t magmatic underplatin g als o occurre d unde r th e adjacent lan d areas, explainin g part o f the uplift . Whether thi s has happened ma y be tested b y the teleseismic receive r functio n experiment s tha t are in progress i n souther n Norway. The above-quote d analysi s wa s base d o n continental lithospher e destabilize d b y com pressional thickening. However , w e suggest tha t the arriva l o f a hot , low-viscosit y an d buoyan t plume a t th e bas e o f th e Nort h Atlanti c lithosphere coul d hav e triggere d th e Rayleigh Taylor instabilit y i n location s o f relativel y dee p lithospheric roots or in areas of a steep gradien t of the lithosphere-asthenosphere boundary. Th e northern an d souther n Norwegia n domes , th e Scottish Caledonides an d East Greenland, i n this

model, woul d have undergone particularly severe delamination, perhap s becaus e th e lithospher e under these areas afte r the Palaeozoic-Mesozoic rifting perio d wa s lef t relativel y thick and with a steep gradien t toward s th e basi n areas . A t present, th e lithospher e i s know n t o b e thic k beneath th e Fennoscandia n Shiel d t o th e east . Therefore, a transitio n t o th e norma l thicknes s lithosphere on the Norwegian margin must occur somewhere unde r western Fennoscandia . Assuming tha t delaminatio n i s possible , w e now combine th e possible tectoni c uplif t histor y inflicted b y th e Icelan d plum e o n th e adjacen t land areas with the possibility of increasing rates of denudation induced by the changing Cenozoic climate an d the fallin g erosiona l bas e level . Figure 1 7 show s th e topographi c respons e t o 35 km of lithospheric delaminatio n an d 45 km of plume emplacemen t aroun d th e tim e o f Nort h Atlantic break-up. The initial topography is set to 200m, approximatel y correspondin g t o th e topography tha t woul d emerg e i f th e surfac e of the lithospher e wa s a t se a leve l durin g th e Cretaceous culminatio n of globa l se a level . It i s assumed tha t th e rat e o f denudatio n increase d through Cenozoic time , simulating the effect s o f climate cooling . Change s o f denudatio n rate i n this simplified mode l ar e located a t the EoceneOligocene transition , at mid-Miocen e time s and at the onse t o f glaciations . The heigh t o f th e averag e surfac e increase s rapidly durin g the phas e o f plume emplacemen t and delamination , an d continue s t o increas e a s the remainin g lithospher e i s heated. A s denudation start s t o remov e materia l an d th e rat e o f lithospheric heating decreases, the rate of surface uplift decrease s an d eventuall y starts t o declin e with th e progressio n o f denudation.

Fig. 17 . Topographi c respons e t o 35k m o f delaminatio n an d 45k m o f plum e emplacement , wit h varyin g denudation rat e included .

PALEOCENE INITIATIO N O F CENOZOIC UPLIF T IN NORWAY

The differenc e betwee n th e summi t heigh t envelope (Molna r & Englan d 1990 ) and th e average surfac e represents the average thicknes s of materia l remove d b y denudation . Th e rathe r limited reductio n i n average surfac e heigh t i s caused b y th e larg e isostati c compensatio n o f removed mas s an d th e continuou s tectoni c surface uplif t throug h Cenozoic tim e (Fig. 15b). In reality , erosio n dept h show s larg e lateral variations from almos t uneroded peaks to deeply incised valley s wit h bottom s fa r belo w th e average surfac e height . Th e summi t heigh t envelope therefor e show s the elevatio n a t which uneroded isolate d peak s could , i n principle , occur, if indeed the y exist. The summi t heigh t envelop e (Fig . 17 ) increases throughou t Cenozoic tim e wit h a rate that primarily reflects th e changes in the average denudation rate. In the case of Fig. 17 it reaches a maximum valu e o f c . 1700 m fo r a n averag e surface o f c . 700m . Othe r example s wit h different ratio s o f summi t envelope an d averag e surface heigh t ca n b e produce d t o fi t loca l features o f th e Norwegian , Scottish , o r Eas t Greenland topograph y b y varyin g th e initia l topography an d lithosphere thickness , delamina tion thickness , duratio n o f th e delaminatio n process, an d denudation rates. Gravity The relatively dee p an d narrow Bouguer gravity anomaly (Fig . 1) indicate s tha t topograph y i n western Scandinavi a i s compensate d mainl y a t Moho depth . I n souther n Norwa y i n particular , there i s a goo d correlatio n betwee n averag e surface topography and Moho depth. In northern Norway, topograph y compensatio n b y latera l density variation s o f th e crus t seem s t o pla y a more significan t role . A n outstandin g question, not addresse d i n detail i n this paper, i s how this gravity anomal y i s relate d t o th e possibl e mechanism of land surfac e uplift . The emergenc e o f th e clos e correlatio n between topograph y an d Bougue r gravit y (Balling 1984 ) is mos t easil y understoo d i f magmatic underplatin g a t basa l crusta l level s supports th e topography . However , fo r th e amount o f underplatin g neede d th e absenc e o f dykes an d volcani c rock s o f th e relevan t ag e a t the surfac e is curious. The ne t gravitationa l effec t o f partial delami nation of the lithosphere is a significant negativ e Bouguer anomaly . Superimposin g thi s anomal y on th e gravitationa l fiel d inherite d fro m th e Mesozoic riftin g phas e produce s a negativ e Bouguer fiel d correlate d wit h topography . However, becaus e o f th e dept h o f origin , th e

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anomaly become s lon g wavelength , unlik e th e relatively narro w anomal y observed . A preliminary forwar d calculation o f the gravity respons e shows tha t th e widt h o f th e gravit y anomal y associated wit h delaminatio n i s significantl y reduced whe n th e initiall y doma l topograph y is eroded t o increasin g dept h wit h increasin g distance fro m th e centr e o f th e uplift . Thi s denudation pattern brings the Moho closer to the surface b y isostati c compensatio n an d add s a positive gravity contribution, which narrows the originally wid e gravit y anomal y s o tha t i t becomes consisten t wit h observations . Th e model require s o f th e orde r o f 1.0-2. 0 km o f Cenozoic denudatio n i n th e coasta l area s o f Norway, i n concordanc e wit h the result s of Riis & Fjeldskaar (1992) an d Riis (1996) . Heat flow The therma l effect s associate d wit h magmati c underplating (Fig . 14c ) an d delaminatio n (Fig. 15c ) are insignifican t a t th e presen t day. We therefore should expect the surfac e heat flux in Norway and on the Norwegian passive margin to b e clos e t o th e value s o f a standar d passiv e margin. Thi s i s supporte d b y th e evidenc e presented b y Ballin g (1993) , wh o foun d hea t flux in Norway to be 50-60 mW m~2 , similar to the flu x i n th e res t o f Scandinavia . Th e well defined heat flux-oceani c ag e relationship foun d by Sundvor et al. (2000) only supports the picture of a generall y norma l passiv e margi n therma l regime. Conclusions In this paper w e have outlined a scenario for the Cenozoic surfac e uplif t histor y o f wester n Scandinavia, whic h i s directl y linked t o plum e emplacement an d is consistent with topography, heat flow , crusta l structure , an d th e Bougue r gravity o f western Scandinavia . Th e scenari o may als o be applicable t o East Greenland. The fundamenta l surface uplift mechanis m i s furnished b y partial lithospheric delamination by the Rayleigh-Taylo r instability , whic h wa s triggered b y th e arriva l o f th e Icelan d plum e a t the bas e o f th e Nort h Atlanti c lithosphere . Delamination preferentiall y occur s i n area s o f deep lithospheri c root s o r a t stee p slope s o f th e lithosphere-asthenosphere boundary . Th e dela mination mechanis m allow s fo r a dela y o f surface uplif t accordin g th e finit e tim e scal e fo r evolution o f th e instability . Transien t surfac e uplift continue d afte r cessatio n o f delaminatio n because o f lithospheri c heatin g t o a warme r equilibrium state.

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This scenari o call s fo r th e occurrenc e o f (1 ) significant climat e an d erosiona l base-leve l control o n denudation , fo r th e denudatio n response i n th e basin s t o becom e consisten t with th e observe d sedimen t pattern , an d (2 ) differential Cenozoi c denudation , wit h denuda tion depth s increasin g awa y fro m th e centra l highland areas towards the west coast of Norway, for th e Bougue r gravit y fiel d associated wit h lithospheric delaminatio n t o becom e consisten t with observations . Continuing 8 18O profiling in the PaleoceneOligocene interva l in the Danish sector, receive r function experiment s an d join t apatit e fission track an d (U-Th)/H e analysi s i n souther n Norway, an d numerica l modellin g o f th e geodynamic processe s an d Bougue r gravit y ar e expected t o contribut e t o resolvin g thes e remaining questions.

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A structural transec t betwee n the central Nort h Sea Dome and the South Swedish Dome: Middle Jurassic-Quaternary uplift-subsidence reversal an d exhumation acros s th e eastern Nort h Sea Basin OLE GRAVERSE N Geological Institute, University of Copenhagen, 0ster Voldgade 10, DK-1350 Copenhagen K, Denmark (e-mail: oleg@ geo.geol.ku.dk) Abstract: Th e Jurassic-Cenozoi c structura l evolutio n o f th e easter n Nort h Se a Basi n i s influenced b y the central North Se a Dome, the Danish Megablock , the Tornquist Zone and the South Swedish Dome. The central North Se a Dome is a composite dome comprising the Triple Junctio n Dome , th e Centra l Grabe n Dom e an d th e Frieslan d Dome . Th e Danis h Megablock, newl y recognize d here , i s a first-orde r tectoni c elemen t betwee n th e Centra l Graben and the Tornquist Zone . I n Jurassic-Cretaceous time it was tilted toward s th e east during uplif t o f the Central Graben Dome, whereas the movemen t wa s reversed during th e Cenozoic post-rif t subsidence . Contemporaneou s wit h th e westwar d tiltin g o f th e Danis h Megablock, th e Sout h Swedis h Dom e wa s uplifte d t o th e east . Th e uplift-subsidenc e reversal acros s the eastern North Se a Basin link s th e collapse of the Central Graben Dome and the til t reversa l of the Danis h Megabloc k wit h the uplif t of the Sout h Swedis h Dome . The uplif t followe d b y subsidenc e probabl y involved mas s flo w i n th e asthenospher e t o account fo r th e observe d balanc e betwee n post-rif t subsidenc e an d margina l uplift . Th e model explain s th e uplif t o f bot h th e Sout h Swedis h Dom e an d souther n Englan d a s th e result o f Cenozoic post-rif t subsidenc e o f the Mesozoic Centra l Grabe n Dome.

Uplift an d erosion ca n only be inferred from the rock record , a s materia l i s remove d durin g exhumation. A s a result , studie s o f sedimen t distribution in adjoining basins and geomorphological studie s o f uplifte d area s giv e a n indication o f th e balanc e betwee n uplif t an d subsidence (e.g . Rii s 1996) . Structura l analysi s may hel p t o identif y crusta l movemen t an d processes. Petrographi c an d geophysica l methods (vitrinite reflectance, analysis of density and soni c velocit y trends ) allo w estimate s o f maximum buria l t o b e mad e an d fission-trac k analysis point s t o th e subsidence-uplif t histor y (see Japse n & Chalmer s 2000 , an d reference s therein). A primary objective of this paper is to analyse the Cenozoi c uplif t an d erosio n o f th e easter n North Sea Basin and adjoining parts of the Baltic Shield i n souther n Sweden . Thi s require s analysis o f Mesozoi c riftin g i n th e centra l North Sea , whic h wa s th e precurso r t o th e Cenozoic post-rif t sag basin. The paper presents a structura l analysi s o f first-orde r tectoni c elements, namel y the centra l Nort h Sea Dom e (Ziegler 1990 ; Underbill & Partington 1993), the South Swedis h Dome (Lidmar-Bergstro m 1988 ,

1991, 1993) , an d the Danis h Megablock , whic h is establishe d her e a s a ne w tectoni c elemen t (Fig. 1) . Th e backbon e o f th e pape r i s a geological cross-sectio n tha t combine s a cross section base d o n a regiona l seismi c lin e (RTD 81-22) acros s th e centra l an d easter n Nort h Se a Basin wit h a cross-sectio n acros s souther n Sweden (Fig s 1 and 2) . Fro m th e cross-sectio n and regiona l map s i t i s possibl e t o demonstrat e basin evolutio n fro m initia l Jurassi c rifting , accompanied b y volcanicit y an d uplif t o f th e central Nort h Se a Dome , t o Cenozoi c basi n subsidence an d Neogen e uplif t o f th e Sout h Swedish Dome . Upper-crustal configuration o f the eastern North Sea Basin The presen t structur e o f th e easter n Nort h Se a Basin i s outline d b y th e depth-structur e o f th e uppermost pre-Zechstei n surfac e an d th e con figuration o f overlyin g sedimentar y basin s (Ziegler 1990 ; Vejbae k & Britz e 1994) . Majo r tectonic element s ar e th e Centra l Graben , th e Danish Megablock , th e Tornquis t Zone an d th e

From: DORE , A.G., CARTWRIGHT , J.A. , STOKER , M.S., TURNER , J.P . & WHITE , N . 2002 . Exhumation o f th e North Atlantic Margin: Timing, Mechanisms and Implications for Petroleum Exploration. Geologica l Society, London, Special Publications, 196, 67-83 . 0305-8719/027$ 15.00 © The Geologica l Society of London 2002 .

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Fig. 1 . Jurassic-Quaternar y tectoni c element s of th e easter n Nort h Se a Basi n an d adjoinin g borde r o f th e Fennoscandian High . Line indicates position of cross-section in Fig. 2. The Tripl e Junction Dome , the Centra l Graben Dom e and th e Friesland Dome are dome centres withi n th e composit e centra l Nort h Se a Dome. MFG, Moray Firth Graben.

South Swedis h Dom e (Balti c Shield ) rimmed by the Skagerrak- Kattegat Platform (Fig s 1 and 2). The crystallin e basemen t o f th e Sout h Swedis h Dome extend s westward s int o th e Tornquis t Zone an d th e Danis h Megablock , wher e i t subcrops the Mesozoic and Palaeozoic sediment s (EUGENO-S Workin g Grou p 1988 ; Michelse n & Nielse n 1993 ; Vejbae k & Britz e 1994) . Th e Danish Megabloc k i s thu s separate d fro m th e South Swedis h Dom e b y th e fault-bounde d Tornquist Zone . The post-Palaeozoi c sedimentar y cove r o f the Danish Megabloc k ca n b e divide d int o a Mesozoic serie s thinnin g toward s th e wes t an d an overlying Cenozoic interva l that thins towards the eas t (Fig s 2 an d 3) . Th e oppose d thinnin g directions relat e t o Jurassic-Cretaceou s domal uplift alon g th e Centra l Grabe n an d eastwar d tilting of the Danish Megablock. Thi s movemen t was reversed in Cenozoic time , when a westward tilting brough t th e uppermos t pre-Zechstei n surface bac k dow n t o subhorizonta l level . Th e uplift followe d b y subsidenc e demonstrate s th e impact o f th e centra l Nort h Se a Dom e (Ziegle r 1990) o n th e Danis h Megabloc k tha t occupie d the easter n par t o f th e composit e dome . Th e central Nort h Se a Dome, th e Danish Megabloc k and th e Sout h Swedis h Dom e ar e thu s majo r tectonic constituent s i n th e evolutio n o f th e central an d easter n Nort h Se a Basi n sinc e Jurassic time .

The central North Sea Dome The central North Sea Dome i s a Jurassic arch or composite palaeo-dom e situate d i n th e centra l North Sea. The subaerial part of the dome can be identified b y a regiona l earl y Middl e Jurassi c erosional unconformit y (th e 'Mid-Cimmerian ' unconformity) (Ziegle r 1990 ; Underbil l & Partington 1993) . The dome i s cut by the Central Graben, th e sout h Viking Graben an d the Mora y Firth Basin , meetin g i n a tripl e junction . Th e uplift o f th e Jurassi c dom e wa s initiate d during the Early-Middl e Jurassi c transitio n (lat e Toarcian-early Aalenia n time) , an d th e larges t area uplifte d abov e th e erosiona l bas e wa s apparently reache d durin g Aalenian-earl y Bajocian tim e (Ziegle r 1990 ; Underbil l & Partington 1993 ) persistin g unti l lat e Jurassi c time. The centre of the dome wa s situated above the grabe n tripl e junctio n an d th e Bajocian Bathonian volcani c centr e t o th e nort h (Fig . 4) . Whereas th e Tripl e Junctio n Dome wa s (partly) flooded durin g Lat e Jurassi c tim e (Underbil l & Partington 1993) , Palaeozoi c sediment s an d basement subcro p the base Cretaceous unconformity alon g th e margin s o f th e souther n Centra l Graben an d the Friesland Hig h (Fig. 4). Subcro p patterns o n th e pre-Cretaceou s geologica l ma p can b e use d t o identif y tw o Lat e Jurassi c dom e centres t o th e south : on e alon g th e Danis h Central Grabe n rif t axis , th e Centra l Grabe n

Fig. 2 . Geosection across the eastern North Sea Basin and the South Swedish Dome on the Baltic Shield. The offshore interva l is based on seismic lin e RTD-81-22 adopted from Vejbae k (1997); the onshor e Sout h Swedish Dome i s adopted fro m Lidmar-Bergstro m (1988) . Positio n o f sectio n lin e is indicated i n Fig. 1.

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Fig. 3 . Jurassic-Cenozoi c rotatio n o f th e Danis h Megabloc k i n th e easter n Nort h Se a Basin . Th e Mesozoi c movement wa s dominate d b y greate r subsidenc e i n th e eas t wit h uplif t an d earl y erosio n i n th e west . A t th e Mesozoic-Cenozoic transitio n th e movemen t wa s reversed ; th e bas e Mesozoi c surfac e subside d i n th e west , whereas uplif t an d erosio n dominate d i n the east .

Fig. 4 . Pre-Cretaceou s geologica l ma p o f th e Nort h Se a Basi n (Ziegle r 1990 ) with positio n o f th e Middl e Jurassic-Cretaceous dom e centre s o f th e centra l Nort h Se a Dome . (Se e text fo r furthe r explanation. ) A-B, Altmark-Brandenburg Basin; CG, Central Graben; FB, Farsund Basin; LS, Lower Saxon y Basin; MFC, Moray Firth Graben ; VG, Viking Graben .

UPLIFT-SUBSIDENCE REVERSAL, EASTER N NORT H SE A

Dome, an d one along the axial depression of the Friesland High , th e Friesland Dome (Fig . 4) . On th e basi s o f th e abov e observation s i t i s concluded tha t the central North Sea Dome was a composite dom e forme d b y th e Tripl e Junctio n Dome t o th e north , th e Centra l Grabe n Dome , and the Friesland Dome to the south (Fig. 4). The long axis of the oval-shaped dome s changes with the orientatio n o f th e Centra l Grabe n axis . Th e Triple Junctio n Dom e an d th e Frieslan d Dom e trend NW-SE, and are connected by the Central Graben Dome , whic h show s a n overal l north south tren d wit h th e Danis h Megabloc k o n it s eastern flank . Th e ris e o f th e composit e dom e was initiate d t o th e nort h i n th e tripl e junctio n area (earl y Middl e Jurassi c time) . Th e zon e o f uplift the n graduall y move d southwar d t o th e Central Graben Dome (earl y Late Jurassic time) and the Friesland Dome (late Late Jurassic time ) whereas th e Tripl e Junctio n Dom e t o th e nort h suffered deflation . The Danish Megablock The Danis h Megabloc k i s a crusta l bloc k extending fro m th e Centra l Grabe n i n th e wes t to the Tornquist Zone i n the east (Fig s 1 and 2). To the nort h i t is bounded b y the Farsund Basi n (Fig. 4) . Th e souther n boundary i s les s distinc t and it s position i s suggeste d toward s th e Lowe r Saxony an d Altmark-Brandenburg basins. During earl y Palaeozoi c time , th e are a tha t became th e Danis h Megabloc k forme d a n integral par t o f th e Baltic a Platfor m (EUGENO-S Workin g Grou p 1988 ; Berthelse n et al 1992) . Decouplin g o f th e Danis h Mega block fro m th e Baltic a Platform-Balti c Shiel d took place i n Triassic time . In Middle JurassicCretaceous time , the Danis h Megablock forme d the eastern block of the Central Graben Dome as part o f th e centra l Nort h Se a dom e comple x (Fig. 1) . The Jurassic-Cenozoi c evolutio n o f th e Danish Megabloc k document s th e evolutio n t o the eas t of the rif t axi s of the Centra l Grabe n Dome. Th e mai n trend s o f th e uplif t o r subsidence ca n b e evaluate d fro m regiona l erosional unconformitie s an d isopac h thick nesses. Analysi s o f th e Danis h Megabloc k ha s also included the pre-Cretaceous geologica l map (Fig. 4), as well as the Berriasian palaeotectoni c map an d isopac h map s o f Triassic , Lowe r Jurassic, an d Lowe r an d Uppe r Cretaceou s rocks (Ziegle r 1990) . The restored interval s (b y simple horizon flattening) illustrate the uplift and erosion of the Triassic deposit s along the Central Graben rift , an d sho w that the Jurassi c intervals only partl y covere d th e Centra l Grabe n Dom e

71

(Fig. 2) . Th e Centra l Grabe n Dom e wa s gradually floode d durin g Earl y Cretaceou s tim e and remained submerged during Late Cretaceou s time (Ziegle r 1990) . Th e westwar d thinnin g o f the Cretaceou s deposit s (Fig . 2 ) indicate s continued eastwar d tiltin g o f th e Danis h Mega block durin g Cretaceou s time . A schemati c model o f th e rotatio n o f th e Danis h Megabloc k is illustrated in Fig. 3 . In Cenozoic time the subsidence of the Danish Megablock shifte d t o the west along the Central Graben post-rif t therma l sa g basin (Fig . 2) . Th e onset o f th e Cenozoi c perio d thu s mark s a til t reversal o f th e Danis h Megabloc k toward s th e west, i n contras t t o th e eastwar d tiltin g i n Mesozoic tim e (Fig . 3) . Progressively olde r preQuaternary sediment s ar e encountere d toward s the N E unti l th e Precambria n crystallin e base ment o f th e Balti c Shiel d i s reache d (Jense n & Michelsen 1992 ; Japsen 1998). This suggests that the regional uplif t an d erosion alon g the easter n margin o f th e Nort h Se a Basi n ma y hav e corresponded in par t to the subsidenc e of the central North Sea . The South Swedish Dome The South Swedish Dom e i s a broad, dome-lik e structure identifie d b y th e topograph y o f th e present-day Precambria n surfac e i n souther n Sweden (Lidmar-Bergstro m 1988 , 1991 , 1993 , 1999). Th e highes t elevatio n o f c . 400m abov e sea leve l i s encountere d aroun d th e souther n Vattern Graben. Lidmar-Bergstrom (1988, 1991 , 1993) ha s distinguishe d three group s of palaeo surfaces: a sub-Cambria n peneplain , a pre Cretaceous surfac e o f suppose d Permo-Triassi c age that has 'sub-Mesozoi c hill y relief, an d the Paleogene Sout h Smalan d Peneplai n (Fig . 5a) . Segments of the sub-Cambrian peneplain and the South Smalan d Peneplai n radiat e ou t fro m th e centre o f the dom e an d ar e cu t ou t b y th e mor e steeply dippin g pre-Cretaceou s surfac e encoun tered alon g th e margin s (Fig . 5 a an d d) . A n exception to this general pattern is formed by an oval-shaped, block-faulted depression situated to the S E o f th e Vatter n Graben . Th e structur e is interpreted a s a cresta l collaps e structur e (Fig. 5a) . Lowe r Palaeozoi c cove r rock s ar e found i n a continuou s bel t t o th e eas t an d a s outliers i n th e centra l an d norther n par t o f th e dome. Th e steepe r inclinatio n o f th e pre Cretaceous surfac e relativ e to the sub-Cambrian peneplain indicate s tha t uplif t o f th e Sout h Swedish Dome postdates the development of the pre-Cretaceous surface . I t i s believe d tha t th e Late Cretaceous transgression may have resulted in a cover of chalk being deposited over southern

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Fig. 5 . The South Swedish Dome , (a) Morphological an d structural elements based on Lidmar-Bergstrom (1994). Position o f cross-sectio n show n i n (c ) i s indicated . SSP , South Smalan d Peneplain ; VG , Vatter n Graben . (b ) Structure of the pre-Cretaceous surfac e (reconstruction) . Elevation o f structure contours above present sea level, (c) East-wes t cross-sectio n o f th e Sout h Swedis h Dome . Reconstructio n o f th e block-faulte d sub-Cambria n peneplain cu t out by the pre-Cretaceous surfac e to the west. (Interpretation based on data from Lidmar-Bergstro m 1988). (d ) Interpretatio n o f th e pre-Cretaceou s surfac e befor e exhumatio n o f th e sub-Cambria n peneplain , (e ) Reconstruction o f the pre-Cretaceous surfac e and the sub-Cambrian peneplain before uplift of the South Swedish Dome.

UPLIFT-SUBSIDENCE REVERSAL, EASTERN NORTH SEA

Sweden befor e th e uplif t o f th e dom e (Norlin g 1994). The sub-Cretaceou s hill y relie f alon g th e dome margi n i s eroded dow n int o th e basemen t below th e sub-Cambria n peneplai n an d ha s a kaolinitic surfac e cause d b y dee p Mesozoi c weathering. B y contrast , th e sub-Cambria n peneplain i n th e centr e o f th e dom e doe s no t have thes e features . Thi s indicate s tha t th e basement i n th e centr e o f th e dom e mus t hav e been covere d b y Palaeozoi c sediment s wit h a thickness exceedin g th e maximu m weatherin g depth o f 20 0 m durin g Mesozoi c time (Lidmar Bergstrom 1993 , 1995) . Extrapolatio n o f th e more steepl y incline d pre-Cretaceou s surfac e across th e dome , i n combinatio n wit h th e weathering dept h an d th e thicknes s o f th e Lower Palaeozoi c cove r t o th e east , suggest s that th e pre-Cretaceou s surfac e i n th e centr e o f the dom e ma y hav e bee n a t leas t 100-200 m above th e sub-Cambria n peneplai n befor e th e post-Cretaceous erosio n (Fig . 5d). This valu e is in agreemen t wit h the altitud e o f the Palaeozoi c outliers i n th e wester n par t o f th e dome , whic h rise u p t o 200 m abov e th e sub-Cambria n peneplain. Th e uplif t o f th e pre-Cretaceou s surface a t th e centr e o f th e Sout h Swedis h Dome ma y thus have been o f the order o f 500 -

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600m abov e presen t se a leve l judge d fro m present-day elevatio n an d estimate d erosion . A reconstruction o f th e uppermos t pre-Cretaceou s surface o f th e Sout h Swedis h Dom e befor e erosion i s illustrated in both map view and crosssection (Fig . 5b and d). A restoration o f the preCretaceous surfac e befor e domin g i s shown i n Fig. 5e. The occurrenc e o f gravell y saprolite s i n th e Precambrian crystallin e basemen t i s confined to the cresta l collaps e structur e i n the easter n par t of th e Sout h Swedish Dom e (Lidmar-Bergstro m 1997; Lidmar-Bergstro m e t al 1997) . Th e development o f thes e saprolite s i s ascribe d t o weathering i n a coo l t o col d climat e i n Plio Pleistocene tim e (Lidmar-Bergstro m e t al . 1997, 1999) . Thi s illustrate s lat e uplif t an d fracturing o f th e dom e accompanie d b y renewed weathering , par t o f th e 720- 3 Ma denudation episod e o f Lidmar-Bergstro m (1996), resultin g i n exhumatio n o f th e sub Cambrian peneplai n an d th e sub-Cretaceou s hilly relief . Th e propose d timin g i s i n accordance wit h fission-trac k analysis , whic h indicates a lat e uplif t an d exhumatio n durin g the las t 2 5 Ma (Cederbo m e t a l 2000) . In the marine platform area in the Hano Bay to the sout h o f th e dome , th e thicknes s o f th e

Fig. 6. Neogene uplift o f the eastern North Sea Basin and the South Swedish Dome. Uplift in the Danish Basin is based on sonic velocities and vitrinite reflection (Jensen & Michelsen 1992; Japsen 1998; Graversen 1999). Uplift of the South Swedish Dome is based on uplift of the pre-Cretaceous surface modelled in this paper (Fig. 5b) added to c. 1 km uplift established at the eastern border of the Danish Basin and AFTA from Ivo (Cederbom 2002). (See text for discussion and references.)

Fig. 7 . Uplift-subsidenc e reversal across th e eastern Nort h Se a Basi n an d the Sout h Swedis h Dome . Th e Centra l Grabe n Dom e wa s uplifte d an d erode d durin g Jurassic time. Th e dom e wa s raise d abov e se a leve l t o a maximu m i n Lat e Jurassi c time an d wa s the n graduall y flooded durin g Early Cretaceou s time . Th e Cenozoi c post-rif t subsidence wa s centred abov e th e Central Grabe n wherea s th e Sout h Swedis h Dom e wa s uplifte d an d erode d t o th e east .

UPLIFT-SUBSIDENCE REVERSAL, EASTER N NORT H SE A

Cretaceous sediment s range s u p t o c . 650 m (Norling & Bergstro m 1987 ) (Fig . 6) . Thi s should b e regarde d a s a maximu m value , a s supposed Paleogen e (Paleocene? ) sediment s ar e encountered abov e (Kumpa s 1980 ; Norlin g & Bergstrom 1987 ; Svirido v e t al. 1995) . Also , erratics o f Paleocene-Eocene marine sediment s have bee n foun d i n easter n Scani a (Norlin g & Bergstrom 1987) , an d a marin e diatomit e o f Eocene ag e ha s bee n reporte d fro m Finlan d (Fenner 1988) , suggestin g tha t Paleocene Eocene sediment s wer e als o deposite d i n southern Sweden . A n estimat e o f th e thicknes s may b e obtaine d fro m th e Viborg- 1 an d Linde 1 well s i n th e Danis h Basi n t o th e west , wher e 200m an d 250m, respectively , o f Paleocen e Eocene sediment s hav e bee n drille d (Dinese n et a l 1977 ; Heilmann-Clause n 1995 ) (Fig . 6) . The Cretaceou s an d Paleogen e sediment s ad d up t o a total thicknes s o f u p t o 90 0 m tha t ma y have bee n deposite d abov e th e pre-Cretaceou s surface. A n evaluatio n o f th e uplif t shoul d als o take pre-compactio n thicknesse s an d wate r depth int o accoun t an d thi s ma y ad d anothe r 100-200 m. A Cenozoi c uplif t u p t o abou t 1 km ma y therefor e b e adde d t o th e pre Cretaceous surfac e o f th e Sout h Swedis h Dome. Thi s valu e shoul d b e regarde d a s a n absolute maximum , an d a realisti c figur e ma y be considerabl y less . Rii s (1996 ) estimate d Plio-Pleistocene erosio n i n southwester n Swe den o f th e orde r o f 500-1000m . Analysis o f reflectance data fro m th e Jurassic Fjerritslev Formatio n (Jense n & Michelsen 1992 ) and soni c velocitie s fro m th e Fjerritsle v For mation and the Cretaceous Chalk Group from the Danish Basi n an d th e Skagerrak-Kattega t Platform indicate s a n uplift o f the orde r o f 1 km or more (Japsen 1992, 1998; Jensen & Michelsen 1992; Michelse n & Nielse n 1993 ; Japse n & Bidstrup 1999) . Apatit e fission-trac k analysi s (AFTA) indicate s tha t Cretaceous-Paleogen e sediments amountin g to c. 650 m (Goteborg) and c. 1 km (Ivo) have been eroded fro m th e western and southern border o f the South Swedish Dome (Cederbom 2002) . On th e basi s o f th e structura l analysis , th e uplift i n th e centr e o f th e dom e shoul d b e expected t o excee d th e borde r uplif t b y 500-600 m. Although there are some differences between th e value s obtaine d fro m th e variou s methods, th e presen t morphologica l dom e i n southern Swede n rise s t o nearl y 400m an d thi s should b e regarde d a s a minimu m uplif t o f th e dome centr e abov e th e margina l areas . Th e uplift o f th e dom e centr e i n relation th e presen t sea level is estimated to be 500-600 m (Fig. 5b), and th e tota l uplift , whe n addin g th e missin g

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section alon g th e borde r o f th e dome , ma y ad d another 1000 m when the sonic an d fission-track analyses ar e take n int o account . Figur e 6 combines estimate s o f th e uplif t (missin g section) fro m soni c velocit y analysi s an d AFTA wit h th e reconstructe d structur e o f th e pre-Cretaceous surfac e o f th e Sout h Swedis h Dome (Fig . 5b) . Jurassic -Cenozoic uplift - subsidence reversal across the eastern North Sea Basin The structural evolution o f the eastern North Sea Basin, i.e . th e Danis h secto r an d souther n Sweden, i s dominate d b y th e Middl e Jurassic Cretaceous uplif t o f th e Centra l Grabe n Dom e followed by Cenozoic subsidence i n the west and uplift o f th e Sout h Swedis h Dom e i n th e east . This i s summarized in Fig. 7 . Mesozoic structure and evolution of the Central Graben Dome In Early-Middl e Jurassi c time , riftin g an d subsidence was concentrated along the Tornquist Zone in the east, whereas only minor subsidenc e and depositio n too k plac e i n the Danish Centra l Graben (Fig . 7) . Th e emergen t Centra l Grabe n Dome expande d i n Lat e Jurassi c tim e an d th e main depocentr e shifte d t o th e Centra l Grabe n whereas subsidenc e diminishe d i n the Tornquist Zone. Renewe d differentia l subsidenc e too k place i n the Tornquist Zone i n Early Cretaceou s time; th e Centra l Grabe n Dom e wa s graduall y flooded, an d subsidenc e i n th e Centra l Grabe n slowed down . Throug h Lat e Cretaceou s time , subsidence an d cresta l collaps e alon g wit h structural inversio n o f th e grabe n sediment s characterized th e Centra l Grabe n Dome . East ward tiltin g o f th e Danis h Megabloc k wa s stil l active an d structura l inversion wa s als o encountered t o th e eas t i n th e Tornquis t Zone . I n Jurassic-Cretaceous time, th e intensit y of basin subsidence shifte d severa l time s acros s th e Danish Megabloc k betwee n th e Centra l Grabe n and th e Tornquis t Zone . Thi s ma y indicat e a n active function o f the Danis h Megablock durin g tilting tha t wa s possibl y relate d t o a n episodi c rise o f th e Centra l Grabe n Dome . I n Mesozoi c time the Danish Megablock an d the Baltic Shield were decouple d alon g th e Tornquis t faul t zone . During th e Cenozoi c subsidence , however , th e Tornquist Zon e i n Skagerra k an d th e norther n part o f Kattega t wa s n o longe r active , an d th e Danish Megabloc k an d th e Balti c Shiel d ma y have acted together a s a single mega-unit.

76

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Fig. 8 . Pre-Quaternar y geolog y o f th e centra l an d souther n Nort h Se a Basi n an d margina l areas . Ther e i s a conspicuous symmetr y along the Cenozoic basi n axis situated above the Mesozoic Centra l and Viking grabens. Compiled fro m Ziegle r (1990 ) an d Japsen (1998) . S P FZ, Sol e Pit faul t zone ; TZ, Tornquis t Zone . Cenozoic structure and evolution of the North Sea Basin The Cenozoi c post-rif t sediment s for m a gentl e syncline wit h th e basi n axi s situate d abov e th e aborted Mesozoi c rift s (Fig . 8). Th e geologica l map o f th e Uppe r Cretaceous-Neogen e sequences exhibit s a symmetrica l patter n centred aroun d th e Neogen e interva l alon g the basi n axis . Pre-Mesozoi c basemen t crop s out alon g th e basi n margin s i n Norwa y an d Sweden, an d i n th e Britis h Isles . Th e basi n margins wer e uplifte d an d erode d wherea s th e basin centr e continue d t o subsid e (Japse n 1998).

In th e centra l Nort h Sea , Middle Miocen e t o Recent sediment s represen t abou t hal f o f th e Cenozoic sedimen t infil l (Nielse n e t al 1986 ; Michelsen e t al . 1998) , an d stron g tectoni c subsidence i s observe d i n Mid-Lat e Miocen e and Quaternar y time s (Koo i e t al . 1991 ; Clausen e t al . 1999) . Accumulatio n i n th e basin centr e (Fig . 9) i s base d o n lithologica l thicknesses i n well s (Nielse n & Japse n 1991) and isopac h map s (Nielse n et al. 1986 ) along the modelle d seismi c profil e (RTD-81-22 , Fig. 2) ; th e genera l tren d demonstrate s increasing accumulatio n an d sedimentatio n rates throug h Cenozoic tim e wit h a culmination in Pliocene-Quaternar y time . Th e Cenozoi c

UPLIFT-SUBSIDENCE REVERSAL, EASTER N NORT H SE A

77

Fig. 9 . Cenozoi c accumulation , erosio n an d averag e sedimentatio n rate s i n th e centra l Nort h Se a Basin , th e Skagerrak-Kattegat Platform, th e East Midlands Shel f an d southern Englan d (se e Fig. 8) . Hatched are a indicate s average Tertiar y sedimentatio n rate s i n the Danish an d Norwegian Centra l Graben . Erosio n i n southern Englan d assumes gradua l Paleogen e uplift. Compile d fro m Nielse n e t al (1986) , Nielsen & Japsen 1991 , Japsen (1997 ) and Japsen & Bidstrup (1999).

erosion (missin g overburden ) o f th e basi n flanks befor e Quaternar y tim e ha s bee n modelled b y Japse n (1997 , 1998 , 2000 ) an d Japsen & Bidstru p (1999) . Erosio n increase d towards th e basi n flanks , wher e i t exceed s 1 km, wherea s maximu m buria l o f th e Chal k Group stil l occur s i n th e centr e o f th e basin . I n the Sout h Swedis h Dom e an d onshor e area s i n southern England , th e tota l uplif t amount s t o 1.5-2km (Fig s 6 an d 9 ) (Japse n 1997) . The timin g o f erosio n ha s bee n modelle d b y Japsen (1997 ) an d Japsen & Bidstrup (1999). A t the S W borde r o f th e basin , maximu m buria l occurred o n th e Eas t Midland s Shel f i n mid Miocene tim e (Fig . 9 ) (Japse n 1997) . I n th e Danish area , th e erosio n wa s o f Miocen e Pliocene age , an d i n th e Skagerrak-Kattega t Platform o n th e N E borde r o f th e basin , th e erosion i s possibl y no t muc h earlie r tha n lat e Miocene tim e (Japsen & Bidstrup 1999) (Fig . 9). In the South Swedish Dome , th e late NeogeneQuaternary gravell y saprolite s ar e weatherin g

products o f th e crystallin e basemen t forme d during lat e uplif t an d exhumatio n i n a col d climate (Lidmar-Bergstro m e t al . 1997) . I n southern Englan d tiltin g an d uplif t i n Plio Pleistocene tim e i s recorde d (Watt s e t al . 2000; Westawa y e t a l 2002) . Lat e NeogeneQuaternary subsidenc e i n th e centra l Nort h Sea i s thu s contemporaneou s with , an d counterbalanced by , uplift o f the Sout h Swedis h Dome an d southern England (Fig. 9). Later uplif t an d erosio n ha s remove d direc t evidence fo r th e uplif t o f th e Balti c Shiel d an d evolution of the Sout h Swedish Dome. Mappin g of th e Cenozoi c sequence s i n th e easter n Nort h Sea Basi n indicate s tha t sediment s fro m th e Baltic Shield were routed into the basin from th e north and NE in Paleogene tim e (Danielsen et al. 1997; Michelse n e t a l 1998 ; Clause n e t a l 2000). I n Neogene time, the main transport route was fro m th e NE i n Miocene tim e an d fro m th e east i n Pliocen e time (S0rense n e t a l 1997 ; Clausen e t a l 1999) . Th e clockwis e rotatio n of

78

O. GRAVERSE N

the infil l direction s durin g Paleogene-Neogen e time i s believed t o indicat e tha t Cenozoi c uplif t of th e Balti c Shiel d wa s initiate d i n souther n Norway during Paleogene time and then changed to th e Sout h Swedis h Dom e i n Neogen e tim e (Riis 1996 ; Japse n 2000) . Lithosphere models of the Jurassic Cenozoic North Sea Basin The Mesozoic central Nort h Se a domes wer e rif t dominated, accompanie d b y volcani c activit y and underlai n b y a thin crus t (Ziegle r 1990) . In contrast, th e souther n Scandinavia n Cenozoi c domes are non-volcanic, developed alon g the SW rifted margi n o f th e uplifte d Precambrian Palaeozoic crystallin e basemen t wher e crusta l thickness reache s 50k m (Thyb o 1997 , 2000) . During Mesozoi c time , th e Centra l Grabe n Dome an d th e Danis h Megabloc k wer e decoupled fro m th e Balti c Shield-Platfor m along th e Tornquis t Zon e (Fig . 7). Afte r th e Late Cretaceous-Dania n inversio n differentia l subsidence-uplift i n the northern Tornquist Zone stopped, an d th e Cenozoi c subsidenc e o f th e Central Graben Dome was accompanied by uplif t of the South Swedish Dome. Thi s may indicate a deep-seated subcrusta l reorganizatio n an d a possible mutua l relationship between subsidenc e and uplift, a s both th e Central Grabe n Dom e an d the Sout h Swedis h Dom e ar e interprete d a s crustal structure s involvin g th e mantl e litho sphere and/o r th e asthenosphere . The Jurassic-Cretaceou s uplif t o f the Centra l Graben Dom e and the Cenozoic subsidenc e bot h amount t o c . 2k m alon g th e analyse d sectio n (Fig. 2) . Thi s valu e i s simila r t o th e propose d 1.5km uplif t o f th e Sout h Swedis h Dome . I t i s therefore suggeste d tha t ther e ma y b e a lin k between th e subsidenc e o f th e Centra l Grabe n Dome an d the uplift o f the South Swedish Dom e to the ENE . Models of synrift and post-rift basin evolution Existing model s o f synrif t an d post-rif t basi n evolution ar e base d o n th e McKenzi e (1978 ) concept o n sedimentar y basi n formation . Th e structure an d stratigraph y o f th e post-rif t basi n depends o n th e rheologica l propertie s o f th e lithosphere. Watt s e t al (1982 ) modelle d th e shape an d syntheti c stratigraph y o f post-rif t basins develope d o n eithe r a n elasti c o r a viscoelastic plat e overlyin g a wea k substratum . In the elastic model , th e post-rift basin develop s as a transgressiv e basin , wit h a so-calle d steershead geometr y i n cross-sectiona l view . By

contrast, i n th e cas e o f a viscoelasti c plate , th e post-rift basi n develop s a s a regressiv e basi n (Watts e t a l 1982) . Althoug h th e modelle d maximum subsidence is of the same order in both settings, the viscoelastic mode l result s in a postrift subsidenc e c . 20 % greate r tha n th e elasti c model. Th e effectiv e elasti c thicknes s o f th e lithosphere determines whether the sediments are locally supported , i.e . short-wavelength , Airy type isostati c compensatio n o f a viscoelasti c plate, o r supporte d b y longer-distance , latera l strength of the plate, i.e. long-wavelength, elastic compensation (Watts etal 1982 ; Barton & Wood 1984). Cloetingh e t a l (1985 ) an d Cloeting h (1986 ) introduced intraplat e stresse s a s a tectoni c mechanism i n continenta l basi n development . The mode l superimpose s fluctuatin g latera l stresses o n existin g basins situate d on a n elastic lithosphere a s a mechanism to explai n observe d vertical plat e movements . Th e intraplat e stres s model ma y b e viewe d a s a lat e short-ter m tectonic overprin t o n th e long-ter m McKenzi e (1978) concep t o f basi n evolutio n an d litho spheric flexur e modelle d b y Watt s e t a l (1982 ) (Kooi & Cloetingh 1989 ; Cloeting h e t al 1990 ; Cloetingh & Kooi 1992) . Previous models of Mesozoic-Cenozoic evolution of the North Sea Basin Previous models of synrift an d post-rift evolution of th e Nort h Se a are a wer e base d o n th e McKenzie (1978 ) concep t o f lithospher e exten sion applie d to the Mesozoic Centra l Graben rif t basin, supersede d b y th e Cenozoi c post-rif t thermal sa g basi n (Sclate r & Christi e 1980 ; Watts e t a l 1982 ; Barton & Woo d 1984 ; Cloetingh e t al 1985) . Bot h Watt s et al (1982 ) and, fo r example , Cloeting h e t a l (1990 ) hav e modelled th e post-rif t Nort h Se a Basi n t o hav e developed o n a n elasti c plat e exhibitin g a steershead geometr y fo r th e post-rif t sediments . Watts et al (1982 ) estimated the effective elasti c thickness a s th e dept h t o the 450° C isotherm ; i n the case of the North Sea Basin this is of the order of 30-35 km. On the basis o f gravity modelling, however, Barto n & Woo d (1984 ) arrive d a t th e conclusion that th e effectiv e elasti c thicknes s o f the lithospher e i s only o f th e orde r o f 5 km, an d that th e sedimentar y loa d wa s compensate d b y local, Airy-type isostasy, i.e. a viscoelastic plate. Intraplate stresse s hav e als o bee n propose d t o be involve d i n post-rif t basi n subsidenc e an d uplift o f th e margins . Cloeting h e t a l (1990 , 1992), Ziegle r (1990) , Koo i e t a l (1991) , Cloetingh & Koo i (1992 ) an d Dor e (1992 )

UPLIFT-SUBSIDENCE REVERSAL, EASTER N NORT H SEA

described th e gentle warping of the Base Tertiary surface during Cenozoic subsidenc e o f the North Sea Basin alon g wit h uplift o f southern Norway and th e Britis h Isle s a s initiate d b y sedimen t loading couple d wit h uplif t an d erosio n o f th e basin margins ; th e vertica l deflectio n wa s interpreted t o have bee n amplifie d b y intraplat e compression. The geometr y o f th e Cenozoi c post-rif t sediments ma y giv e th e impressio n tha t i t possess a steershea d geometr y (Fig . 2) . How ever, th e thic k interva l abov e th e Centra l Graben i s du e t o a combinatio n o f post-rif t compaction o f th e synrif t sediment s i n th e underlying graben , an d th e thic k Plio-Pleisto cene sectio n i n th e centr e o f th e Nort h Se a Basin (Japse n 1998) ; th e Tertiar y "steershorn " extending t o th e EN E (Fig . 2 ) i s mainl y related to erosio n durin g uplif t o f th e Sout h Swedis h Dome. Furthermore , i n th e cas e o f a steershea d geometry, th e syntheti c stratigraph y shoul d exhibit a n onlapping , transgressiv e basin , whereas observation s outlin e a n overal l regres sive basin-fil l progradin g fro m th e basi n margins (S0rense n e t al 1997 ; Michelse n et a l 1998 ; Clause n e t al 1999) . The Cenozoi c subsidence rat e i n th e centr e o f th e souther n North Se a Basi n increase d throughou t Neogene time (Fig . 9) (Barton & Wood 1984 ; Koo i et a l 1991). Thi s observatio n contrast s wit h mos t models o f post-rif t therma l subsidenc e tha t imply a decreasin g subsidenc e wit h age .

New model: Cenozoic subsidence linked with uplift of marginal domes The collaps e o f th e Centra l Grabe n Dom e caused reversa l o f th e til t directio n o f th e Danish Megablock . Wherea s th e Mesozoi c period witnesse d a n increasin g til t toward s th e ENE, th e til t wa s reverse d i n Cenozoi c time : the uppermos t pre-Zechstein-lowermos t Trias sic leve l alon g th e analyse d sectio n i s no w almost bac k t o subhorizonta l (Fig s 2 an d 3) . During Mesozoi c tilting , th e Danis h Mega block wa s disconnecte d wit h th e Balti c Shiel d along th e Tornquis t Zon e (Fig . 7) . I n Cenozoi c time, however , th e discontinue d faultin g alon g the Tornquis t Zon e indicate s tha t th e mega block an d th e shiel d acte d togethe r a s a singl e unit. Both pre-Quaternary geolog y an d the amount of erosio n outlin e a symmetrica l arrangemen t across the souther n North Sea area (Fig . 8) . The geometry illustrate s Cenozoi c subsidenc e alon g the basi n axi s an d uplif t o f th e margins .

79

Subsidence an d depositio n i n th e centr e (2km ) was counterbalanced b y uplift an d erosion in the South Swedis h Dom e an d souther n Englan d (1.5-2km) (Fig . 9) . Deposition alon g th e basin axis increase d ove r time , an d th e curv e o f deposition is mirrored by the curve of uplift. Th e Skagerrak-Kattegat Platform an d the East Mid lands Shel f alon g th e easter n an d wester n margins o f the Nort h Se a Basin both underwent uplift and/o r erosio n durin g Miocene-Quaternary time ; th e gradient s and magnitude s of uplif t correspond t o depositio n i n th e basi n centr e (Fig. 9) . Th e observatio n tha t depositio n wa s counterbalanced b y (pene)-contemporaneou s uplift and/o r erosio n i s interprete d t o indicat e that Cenozoi c post-rif t subsidenc e wa s linke d with marginal uplift . In a cas e wher e post-rif t therma l subsidenc e has induce d a flexura l bulg e i n margina l areas , uplift woul d equal only a small fraction (6-7% ) of the subsidence (Turcotte & Schubert 1982) . If this was the situation, uplift of the southern North Sea margins should only amount to c. 150m . In addition, Cenozoic post-rif t subsidenc e increased (Fig. 9) , wherea s therma l modellin g invoke s decreasing subsidenc e ove r tim e (McKenzi e 1978; Watt s e t a l 1982) . Thes e observation s indicate that thermal subsidence alone is not able to accoun t fo r th e observe d balanc e betwee n subsidence an d uplif t (Fig . 9) . I t i s suggested instead that the subsidence o f the Central Grabe n Dome, accompanie d b y th e backward tilt of the Danish Megablock , wa s associate d wit h a reorganized flo w patter n i n th e asthenospher e below the subsiding North Sea Basin (Fig. 10). In this case , mas s movements directed toward s the marginal dome s ma y accoun t fo r th e observe d uplift of the South Swedish Dome (Fig. lOa) . It is difficult t o evaluat e th e strengt h o f th e litho sphere during the collapse of the Central Graben Dome, a s vertical movement s of the lithosphere may be in interaction with, and supported by, the asthenosphere. The Sout h Swedis h Dom e an d th e Souther n Scandes Dom e i n Norwa y ar e situate d alon g individual segment s o f th e rifte d margi n o n the S W corne r o f th e emergen t Precambrian Palaeozoic crus t (Fig . 1) . Uplif t o f souther n Norway probabl y als o ha s a deep-seate d origin i n th e uppe r mantl e an d th e astheno sphere (Rohrma n & va n de r Bee k 1996) . Th e suggested interrelationshi p modelle d fo r th e South Swedis h Dom e an d th e Centra l Grabe n Dome may als o be propose d for the souther n Norway uplif t an d th e Tripl e Junctio n Dome . Initial uplif t i n souther n Norwa y too k plac e i n Paleogene tim e wherea s th e uplif t o f th e South Swedis h Dom e peake d i n Neogene -

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Quaternary tim e (Rii s 1996 ; Japse n 2000) . The differenc e i n timin g o f initia l Cenozoi c uplift ma y b e relate d t o th e migratio n o f earl y uplift an d subsidenc e o f th e Tripl e Junctio n Dome relativ e t o th e Centra l Grabe n Dom e t o the SE .

Discussion and conclusions The propose d mode l o f linke d subsidenc e an d uplift show s tha t a bette r understandin g o f Cenozoic uplif t i n th e Nort h Se a Basi n ma y b e obtained whe n a lin k i s mad e t o th e Mesozoi c evolution. Analysi s o f th e centra l Nort h Se a Dome has demonstrated th e composite characte r and differentia l uplif t an d subsidenc e o f th e dome. Riftin g an d uplif t wa s initiated i n Middle Jurassic tim e i n the nort h i n the Tripl e Junctio n Dome. Thi s dom e wa s floode d durin g Lat e Jurassic time as rifting an d uplift move d sout h to the Central Grabe n Dom e (i n early Late Jurassic time) an d then th e Friesland Dom e (i n late Lat e Jurassic time) . Th e Centra l Grabe n Dom e wa s flooded durin g Early Cretaceou s time . The large-scal e evolutio n o f th e Jurassic Cretaceous an d Cenozoi c sedimentar y basin s in the easter n Nort h Se a show s tha t th e Mesozoi c depocentres were situate d i n the Central Grabe n and alon g th e Tornquis t Zon e wherea s th e Cenozoic depocentre s wer e concentrate d i n th e central Nort h Se a abov e th e aborte d Mesozoi c rifts (Fig s 7 an d 8) . Th e change d subsidenc e pattern aroun d th e Mesozoic-Cenozoi c tran sition indicate s a n uplift-subsidenc e reversal . The change of the overall basin morphology wa s governed by the first-order tectonic elements, i.e. the Centra l Grabe n Dom e an d th e Danis h Megablock, th e Tornquis t Zone , an d th e Sout h Swedish Dome . Thes e structure s ar e severa l hundred kilometre s acros s an d involv e vertica l movements o f betwee n 1 an d 2km . Th e magnitude o f th e structure s an d th e movemen t suggest causa l mechanism s i n th e mantl e involving th e entir e overlyin g crust . Jurassi c doming wa s characterize d b y thinnin g o f th e crust an d th e developmen t o f rif t basin s wit h volcanic activit y (Ziegler 1990) , bu t by contras t the Cenozoi c dome s develope d o n a thic k Caledonian an d Precambria n crystallin e crust , and ther e ar e n o sign s o f volcani c activit y an d only limite d faultin g is observed (i.e . th e crestal collapse structur e o f the Sout h Swedis h Dome) . The Cenozoic domes all have a marginal position along th e wester n faulte d margi n o f th e Fennoscandian High . Th e domes appea r t o have been stationar y throughou t thei r existence , an d

this suggest s tha t th e crusta l structur e wa s th e locating factor . I f th e dome s ha d develope d above mantl e plume s ascendin g fro m th e dee p mantle, the y woul d hav e a mor e rando m distribution. The subdivision of the central North Sea Dome into individual dome centres connects the Danish Megablock wit h th e Centra l Grabe n Dom e (Fig. 1) . The til t reversa l o f th e Danis h Mega block a t the Mesozoic-Cenozoic transitio n thus suggests a mutua l linkag e betwee n th e sub sidence o f th e Centra l Grabe n Dom e an d th e uplift o f souther n Swede n (Fig . 7) . Th e accelerated subsidenc e of th e centra l Nort h Se a Basin durin g lat e Neogene-Quaternar y tim e (Fig. 9) (Nielsen etal 1986 ; Ziegler 1990 ) is thus contemporaneous wit h th e uplif t o f th e Sout h Swedish Dom e an d th e exhumatio n o f th e sub Cambrian peneplai n establishe d b y Lidmar Bergstrom (1995) . Fission-trac k thermochronol ogy supports a model of accelerated uplif t durin g Neogene tim e (Cederbom et al 2000 ; Cederbo m 2002). The Cenozoi c post-rif t subsidenc e i n th e central Nort h Se a Basi n doe s no t follo w th e McKenzie (1978 ) model . Instea d o f th e modelled decreasin g subsidenc e ove r time , th e observations outlin e increasin g subsidenc e (Fig. 9) . Th e Mesozoic-Cenozoi c evolutio n of the Nort h Se a are a involve d uplif t o r sub sidence of the orde r of 2k m or mor e of larg e crustal plates ; fo r example , th e Danis h Mega block coverin g th e easter n hal f o f th e souther n North Sea . Th e mode l propose d i n th e presen t paper attribute s uplif t o f th e Sout h Swedis h Dome t o subsidenc e o f th e easter n Nort h Se a Basin durin g collaps e o f th e Centra l Grabe n Dome. Th e lithospher e carryin g th e Danis h Megablock an d adjoinin g Balti c Shiel d wer e linked i n a commo n movemen t tha t involve d a reorganized flo w patter n i n th e asthenospher e (Fig. 10) . The uplif t followe d b y subsidenc e probabl y involved mas s flo w i n th e asthenospher e t o account fo r th e observe d balanc e betwee n post rift subsidenc e an d margina l uplift . I n thi s scenario th e impac t o f therma l subsidenc e may be suppresse d an d not easy t o model . The regressive character of the Cenozoic basin (Michelsen et al 1998 ) and limited occurrence of major fault s (Ziegle r 1990 ) suggest s tha t th e Cenozoic packag e was deposited o n a viscoelastic plate (Watts et al. 1982) . This conclusion is in accordance wit h th e modellin g o f Barto n & Wood (1984) , who suggeste d an effective elasti c thickness o f th e orde r o f 5km . Th e Cenozoi c basin show s increasin g subsidenc e rate s ove r time (Fig . 9) , an d hig h subsidenc e rate s i n lat e

UPLIFT-SUBSIDENCE REVERSAL, EASTER N NORT H SEA

Fig. 10 . Lithospher e mode l o f crusta l uplift-subsidenc e reversa l acros s th e souther n Nort h Se a Basin . CG , Central Graben ; N-DB , Norwegian-Danish Basin ; NSB , North Se a Basin ; S P FZ , Sol e Pi t faul t zone ; TZ , Tornquist Zone, (a) During the Cenozoic post-rift subsidence, faulting along the Tornquist and Sole Pit fault zones was discontinued. The subsidence was accompanied by reverse tilting of the flanks, whereas southern Sweden and southern Englan d wer e raise d alon g th e borders . I t i s suggeste d tha t Cenozoi c subsidenc e o f th e lithospher e resulted in a revised flow pattern in the asthenosphere directe d toward s southern Sweden and southern England, (b) Mesozoi c riftin g an d uplif t o f th e Centra l Grabe n Dom e wa s accompanie d b y crusta l thinnin g an d asthenosphere risin g belo w th e rifte d crust . Th e uplif t o f th e Centra l Grabe n Dom e wa s decouple d fro m th e adjoining area s alon g the Tornquist and Sole Pi t faul t zones . Neogene-Quaternary time ma y b e viewe d a s a natural par t o f th e observe d tren d a s a n alternative t o intraplate deformation . The bimoda l symmetr y o f th e Nort h Se a Basin alon g th e centra l Mesozoi c rift s an d Cenozoic basi n axi s implie s tha t th e propose d model o f th e Cenozoi c uplif t i n th e eas t alon g the borde r o f th e Fennoscandia n Hig h ma y b e mirrored i n th e wes t i n th e Cenozoi c evolutio n of Britai n (Fig s 8 an d 10) . Wherea s th e uplif t along th e easter n margi n o f th e Nort h Se a Basin ma y b e relativel y simple , th e uplif t i n Britain ma y interfer e wit h deformation s o f th e fault-bounded shel f basin s t o th e sout h an d west, a s wel l a s wit h th e effect s o f th e opening of the Atlantic . However , Neogene Quaternary uplif t o f th e Sout h Swedis h Dom e corresponds t o uplif t i n souther n Englan d (Fig. 9) , an d Paleogen e uplif t o f Scotlan d based o n geomorphologica l evidenc e (Hal l 1991) ma y counterbalanc e th e uplif t i n southern Norway .

I than k K. Lidmar-Bergstro m (Universit y o f Stock holm), who introduced m e to the South Swedish Dom e in Ma y 199 9 durin g a Nordi c researc h excursio n through souther n Sweden ; I als o than k T . Dore , R . Gatliff, E . Hakansso n an d a n anonymous reviewer for constructive review s an d languag e correction s o f a n early draf t o f the manuscript .

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Cenozoic inversio n an d uplift of southern Britai n DEREK J. BLUNDELL Department of Geology, Royal Holloway, University of London, Egham TW20 OEX, UK (e-mail: d. blundell @gl. rhul ac. uk) Abstract: Wherea s significant exhumatio n o f northern Britai n too k place during Paleocen e time, probabl y a s a consequenc e o f uplif t cause d b y a mantl e plume , Paleogen e basi n inversion an d uplif t i n souther n Britai n appear s t o b e a consequence o f Alpine tectonism. Recent publications demonstrat e that inversio n o f Mesozoic basins across souther n Britai n was accompanie d b y subsidenc e o f flankin g basin s i n areas that had previousl y remaine d stable. Structures observe d on seismic sections across the Weald Basi n in SE England revea l that inversion occurred locally by north-directed reverse movements on pre-existing normal faults that cu t down a t a low angle dee p into the basement. The overall effect o f inversion o f the Weal d Basin, however , i s a bulk deformatio n tha t produced a domal uplift, flanke d b y subsidence o f th e Londo n an d Hampshire-Diepp e basins . A 2 D finit e elemen t thermo mechanical model of continental lithospher e containing a region of reduced strengt h i n the crust simulates Jurassic-Early Cretaceous extension to form th e Weald Basin, followed by compression during th e Tertiary to produce it s inversion an d the flanking basins. Th e timin g of tectonic events across souther n Britain correlates with times when Alpine stresses were transmitted int o th e forelan d t o th e nort h sufficientl y wel l t o lin k them . Throug h mos t o f Tertiary time , th e landscap e o f souther n England wa s o f relatively lo w elevation an d low energy surfac e processes . However , lat e Neogen e uplift , generall y greate r i n th e west , appears t o hav e bee n par t o f a larger-scal e uplif t o f lan d area s wit h har d roc k a t surface , which ha s no obvious tectoni c explanation .

In simpl e terms , th e surfac e geolog y outcro p pattern o f th e Britis h Isle s show s th e younges t rocks i n th e S E an d th e oldes t i n th e nort h an d west. Th e leve l o f erosio n i s suc h tha t th e Phanerozoic successio n i s full y expose d a t surface, alon g wit h Precambria n unit s datin g back t o Earl y Proterozoi c time . Moreover , th e extent o f Tertiary sediment s expose d onshor e i s limited to southern England and that of Neogene sediments i s restricted t o eastern Eas t Anglia . In similar fashion , topographic elevatio n an d relief are generall y greate r i n the nort h an d west , and least in the sout h and east. Thi s rang e of surfac e geology an d topograph y contrast s wit h othe r regions o f N W Europ e wit h similar , 30k m crustal thickness , suc h a s th e Netherlands , northern Germany , Denmark and Poland. Brodie & White (1994 ) explained th e denudation of northern Britain as a consequence of 8 km of magmati c underplatin g relate d t o a mantl e plume beneat h N W Scotland , recognize d a t surface a s the Tertiary Volcanic Province. White & Lovel l (1997 ) correlated th e associate d uplif t and erosion from a succession of volcanic pulses with the depositio n o f large quantitie s of clastic sediment a s submarin e fan s i n neighbourin g basins (Nort h Sea , Wes t Shetlands ) durin g th e

period 62-54 Ma. Their estimate s of up to 3 km of exhumation, base d o n vitrinite reflectance and fission-track data, were independently confirme d by a careful stud y of denudation of the Iris h Se a Basin b y Rowle y & White (1998) . In this , the y estimated th e amount of denudation by calculat ing the stretching and thermal subsidence histor y of th e basi n t o determin e ho w muc h post-rif t sediment thicknes s i s missing . Th e mismatc h between th e present-day an d the predicted dept h to basemen t represent s th e amoun t o f denuda tion. They obtained value s for the East Irish Sea Basin rangin g betwee n 0. 4 an d 2.6km , wit h values u p t o 1.5k m an d 1.7k m fo r th e neighbouring Wes t Lancashir e an d Cheshir e basins, respectively . Doubt s remai n abou t th e extent of uplif t modellin g based o n fission-trac k and vitrinit e reflectanc e data , whic h depend s o n the value used for the geothermal gradien t at the time o f uplift , an d coul d b e halve d t o aroun d 1.5km. None th e less, there i s a strong cas e fo r explaining the exhumation of northern Britain as a result of an Early Tertiary plume, which moved away aroun d 5 4 Ma whe n Nort h Atlanti c sea floor spreadin g wa s initiated , leavin g th e are a above se a level t o the present day . However, th e influence o f th e plum e di d no t exten d acros s

From: DORE , A.G., CARTWRIGHT , J.A. , STOKER , M.S. , TURNER , J.P . & WHITE , N . 2002 . Exhumation o f th e North Atlantic Margin: Timing, Mechanisms an d Implications for Petroleum Exploration. Geological Society , London, Specia l Publications , 196 , 85-101 . 0305-8719/02/$15.00 © The Geological Societ y o f London 2002 .

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southern Britain , s o tha t othe r explanation s ar e needed t o accoun t fo r th e basi n inversio n an d uplift tha t i s observed . Thes e ar e generall y ascribed (e.g . Chadwick 1993 ) to th e deforma tional response t o lithospheric stresses generate d during the Alpine Orogeny that were transmitted laterally northwards. The purpose of this paper is to examin e th e natur e an d timin g o f basi n inversion across souther n Britain, and the Weald in particular , t o asses s whethe r the y ca n b e correlated wit h Alpin e event s an d wh y th e deformation i s apparentl y mor e pronounce d across souther n Britai n tha n elsewher e i n N W Europe. To understand the Cenozoic inversion of sedimentary basin s acros s souther n Britai n i t i s necessary firs t t o appreciat e thei r earlie r evol ution a s extensional basins.

1988) an d b y pur e shea r i n th e lowe r crust . Lateral ramp s an d strike-sli p escap e structure s created NE-SW-trendin g fault s t o th e wes t and NW-SE-trending fault s t o th e eas t (Fig . 2). I n the collaps e o f th e Varisca n Orogeny , sedimen tary basin s wer e initiate d during Permia n time , filled wit h terrestria l red-be d sequence s i n th e hanging wall s o f forme r thrusts , reactivate d a s low-angle norma l faults . Thes e wer e usuall y bounded b y steeper , short-cu t norma l faults , mainly downthrow n t o th e south . Extensiona l basins develope d throug h Triassi c tim e and , in the Wessex an d Central English Channel basins, were compartmentalize d b y NW-SE-trendin g faults inherite d from Varisca n structures. Further extension an d subsidenc e during Jurassic-Early Cretaceous tim e resulte d i n th e depositio n o f shallow marin e sequence s tha t includ e lime stones, sandstone s an d organic-ric h claystones . At th e sam e time , changin g stres s condition s Evolution of sedimentary basin s throug h resulted in the eastward migration of depocentre s the Mesozoic era within sub-basin s o f th e Wesse x an d Centra l The Celtic Sea, Bristol Channel, Central English English Channe l basins and th e developmen t of Channel, Wesse x and Weal d basin s (se e Fig. 1 new e n echelo n faul t set s an d transtensiona l for locations ) wer e buil t upo n a Varisca n movements o f th e earlie r norma l east-wes t structural framewor k (Fig . 2). Th e northwar d faults. Continuin g subsidenc e durin g Earl y movement o f th e Armorica n Bloc k toward s th e Cretaceous time resulted in the deposition locally Midland Crato n and London Platform resulted in of fluvial-lacustrin e deposits . The structura l and compression o f th e intervenin g crust . Thi s stratigraphic evolutio n of th e Wesse x Basi n ha s deformed dominantl y b y north-vergent , east - been describe d by a number of workers, notably west-trending thrust s an d fold s tha t develope d Chadwick (1986), Lake & Karner (1987), Butler (1998) an d Hawke s e l a l (1998) , an d ha s bee n above a mid-crusta l decollemen t (BIRP S & well summarized by Underhill & Stonely (1998), ECORS 1986 ; Chadwick 1986 ; Brooks e t al

Fig. 1 . Simplifie d geologica l ma p o f souther n Englan d showin g mai n fault s an d sedimentar y basins . SF , Sticklepath Fault; SH , Sout h Hewit t Fault; WF , Watchet-Cothelstone Fault ; PW, Purbeck-Wight Disturbance; CCH, Centra l Channel High. 3 , line o f section show n in Fig. 3; 4, SWT4 profile, bol d sectio n show n in Fig. 4; 6, line of BGS sectio n show n in Fig. 6. Inset: seismi c line s C78-02, C78-0 3 an d C78-04 ; B ? Brightlin g well.

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Fig. 2 . (a) Map showing the Variscan structural framework of southern Britain (after Lefor t & Max 1992) . VTF , Variscan Thrust Front , (b) Summar y of post-Variscan tectonic event s affecting souther n Britain.

who regarde d th e Permian-Lowe r Cretaceou s succession a s a singl e megasequence . It s uppe r bound i s marke d b y a regiona l unconformit y around Aptia n tim e (12 4 Ma), whic h truncate s successively olde r strat a toward s th e west , cutting dow n t o th e Permo-Tria s deposits . Th e Permian-Early Cretaceou s structura l and strati graphic evolutio n o f the Nort h an d Sout h Celti c Sea basin s ha s bee n describe d b y Petri e e t al.

(1989), tha t o f th e Bristo l Channe l Basi n b y Brooks e t a l (1988 ) an d tha t o f th e Centra l English Channel Basi n by Hamblin e t al (1992) . These basins all have similar histories. Through out thi s perio d th e neighbourin g stabl e blocks , such as the London Platform , Cornubia n Massi f and Armorica n Block , ha d remaine d a s positiv e regions wit h n o histor y o f subsidence . Durin g Late Cretaceous time , a broadl y westwar d

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progressing marin e transgressio n le d t o th e deposition o f sandston e an d clayston e followe d by th e Chal k Group , whic h extende d acros s the whole regio n an d wel l beyond , includin g th e former lan d areas . Thi s i s well illustrate d i n th e palaeogeographical atla s o f Ziegle r (1990) . Deposition o f Chalk i s known to have continued through t o Maastrichtia n tim e i n th e souther n North Se a an d th e Englis h Channe l (Rawso n 1992) bu t in most area s the youngest portion ha s been remove d b y erosion . A regional disconfor mity represent s a perio d o f uplif t an d erosio n between 7 4 an d 6 0 Ma, whe n th e earlies t recorded Tertiar y sediment s were deposited . Cenozoic inversion Extensive erosio n o f Chal k acros s souther n England le d t o th e developmen t o f a sub Paleogene surface , in some areas covered b y thin lateritic soi l deposit s (Gree n 1985) . Durin g Paleogene time , basi n inversio n i s eviden t i n various ways. In some areas it is concentrated on particular faults , wherea s elsewher e i t i s characterized a s a broad domal uplift (Chadwic k 1993), wit h periphera l basin s develope d acros s areas tha t had remained a s stable positive block s during Permian-Late Cretaceous times . Fault inversion The mos t spectacula r faul t inversio n i s th e Purbeck-Wight Disturbance , see n alon g th e Dorset coas t t o th e Isl e o f Wight . Underbil l & Paterson (1998) described th e structures in detail and demonstrated th e effect o f inversion upon the east-west-trending segmente d faul t system , linked b y rela y ramps , extendin g betwee n Abbotsbury in th e wes t and Whiteclif f Bay, Isle of Wight, in the eas t (Fig. 1) . Using information from seismi c sections , well s and surface geology , they showe d tha t th e faul t syste m acte d a s a normal growt h fault , downthrow n t o th e south , from Permia n t o Early Cretaceous time , but was inverted b y revers e movemen t durin g th e Tertiary. Althoug h th e faul t syste m remain s i n net extension for the Permian-Lower Cretaceous succession, invers e movemen t i s greates t wher e formerly extensio n was greatest. Deformation of incompetent hanging-wall strata against Chalk in the footwall, which acted as a semi-rigid buttress, implies tha t inversio n resulte d fro m northward directed stress . Movemen t o f hanging-wal l sediments relative to footwall Chalk and footwall deformation o f the Chalk can be used to estimat e the amoun t o f inversion , uplif t an d latera l shortening. Fo r example, fro m a seismic sectio n across th e Abbotsbury-Ridgewa y Faul t (Butle r

1998, Fig . 8 ) north of Weymouth, at least 350m Tertiary displacemen t ca n b e estimated . O n th e Isle o f Wight, uplif t o f a t least 1500 m has bee n recorded (Butle r 1998 , Fig . 13) . I n additio n t o localized uplift on inverted faults, there is a broad upwarp o f th e Wesse x Basi n o f betwee n 25 0 m (Chadwick 1993 ) an d 500m (La w 1998) . Some , at least, of the variation in structural style can be attributed t o th e presence , o r otherwise , o f sal t (Harvey & Stewart 1998) . Inversion i n th e Centra l Englis h Channe l Basin, to the south, is concentrated on the Central Channel Hig h (Fig . 1) , wher e La w (1998 ) ha s estimated ove r 1000 m o f uplif t fro m soni c velocity measurement s i n boreholes , supporte d by fission-trac k an d vitrinit e reflectanc e data . The structura l evolutio n o f th e Centra l Channe l High wa s investigate d b y Beele y & Norto n (1998) usin g tw o north-sout h high-resolutio n seismic section s acros s it , wit h wel l tie s fo r stratigraphic correlatio n an d dept h conversion . Sequential balance d sectio n restoratio n o f th e two true-scal e dept h section s resulte d i n estimates o f 1100-1500 m uplif t o f th e hang ing-wall sediment s ove r th e Centra l Channe l High relative to the footwal l to the sout h during Tertiary inversion . The fault geometr y require s a northerly di p o f 55 ° decreasin g t o 30 ° a t abou t 12 km depth. From this, and the deep structur e of the Wesse x Basi n identified byChadwic k (1986, 1993), Beele y an d Norto n propose d tha t th e inversion developed upon a linked set of crustalscale fault s tha t initiated as par t o f th e Varisca n thrust system, evolve d durin g basin extension as low-angle norma l fault s an d reactivate d agai n during Tertiar y tim e t o produc e th e fault controlled inversion . Thei r schemati c cross section i s reproduced i n Fig . 3 . The implication from thi s is a north-south shortenin g by c. 4km of som e 20 0 km o f crust. There ha s been considerabl e debat e abou t the timing o f inversion. The sub-Paleogen e regional unconformity, wit h differentia l erosio n o f th e Chalk, indicate s uplift acros s souther n England, but a s th e Uppe r Paleocene-Eocen e successio n within th e Hampshire-Diepp e Basi n i s itsel f involved i n th e faul t inversion , for exampl e o n the Isl e o f Wight , a t leas t par t o f th e inversio n must b e younger . Chadwic k (1993 ) argue d strongly i n suppor t o f a Miocen e ag e fo r th e main inversio n episode , 'correspondin g t o th e main Alpin e deformatio n event s a s continental collision occurre d betwee n Afric a an d Europe' . A successio n o f Tertiar y sediment s o f 650 m thickness i n th e Hampshire-Diepp e Basi n i s well exposed in Alum Bay at the western end and in Whitecliff Bay at the eastern end of the Isle of Wight. Restin g unconformabl y upo n a n erode d

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Fig. 3 . Schematic crusta l section by Beeley & Norton (1998) (locatio n show n in Fig. 1) , showing basin inversion on linke d crustal-scal e faults . CCH , Centra l Channe l High ; PW , Purbeck-Wight Disturbance ; WP , WardourPortdown structure , P, Vale of Pewse y Fault; VTF, Varisca n Thrust Front .

Chalk surfac e ar e re d mottle d soil s (Readin g Beds, c. 58 Ma) of Late Paleocene age . These ar e followed by an Eocene successio n comprising , in turn, th e marin e Londo n Clay , shallo w marin e and lagoona l sand s an d clay s wit h occasiona l coals o f th e Bracklesha m Group , an d marin e clays an d sand s o f the Barto n Group . Thes e are followed b y th e Solen t Grou p o f nea r shore an d freshwater silt s an d clays , an d a freshwate r limestone. Th e younges t o f thes e i s o f Earl y Oligocene age , c . 30 Ma. A s all these sediment s were deposited at or near sea level, sedimentatio n rates mus t hav e kep t pac e wit h subsidence , t o fill th e accommodatio n spac e withi n th e Hampshire-Dieppe Basin. A carefu l stud y b y Gal e e t al (1999 ) o f th e Eocene successio n expose d i n Whiteclif f Ba y established tha t clasts derive d fro m successivel y older horizon s occurre d i n successively younger strata, a s th e hanging-wal l sediment s o f th e Sandown Pericline , just a few hundred metres to the sout h (th e easter n segmen t o f th e inverte d fault syste m describe d b y Underhill & Paterso n (1998)), wer e bein g uplifte d an d eroded . Gal e et al (1999 ) foun d tha t uplif t occurre d i n tw o main phases . Uplif t o f 200-30 0 m occurre d between 47. 5 an d 4 4 Ma, wit h rates a t time s of 0.1 mma"1. A furthe r 200-300 m uplif t an d erosion occurre d durin g th e perio d 42-3 6 Ma, probably i n shor t pulse s o f les s than 1 Ma duration. Th e presenc e o f derive d fossil s an d clasts withi n th e uppermos t bed s i n th e succes sion suggest s tha t uplif t persiste d int o Earl y Oligocene time . Th e tota l uplif t durin g th e Eocene faul t inversio n o f som e 500 m i s a substantial part o f the displacemen t recorded o n faults withi n th e Wesse x Basin . However , th e Chalk an d overlyin g Reading Bed s an d Londo n Clay ar e vertica l i n bot h Alu m Ba y an d Whitecliff Bay , th e Bracklesha m Grou p ar e steeply dippin g to the north, but the dip reduces to a lo w angl e o f aroun d 5 ° through th e Barto n and Solen t Groups , som e 80 0 m fro m th e Chal k

outcrop. Clearly, this deformation, which has upended th e Paleogen e sequence , i s younge r than the 3 0 Ma sediments involved . Linked t o the east-west-trending fault system of th e Purbeck-Wigh t Disturbanc e vi a th e Lytton Cheyne y Faul t i s th e NW-SE-trendin g Watchet-Cothelstone Faul t (Fig . 1) . Reviewin g evidence o f faul t movements , Miliorizo s & Ruff ell (1998 ) conclude d tha t i t originate d a s a Variscan structure and has acted at various times subsequently as a strike-slip faul t i n both dextral and sinistral senses. East-west horsetail splays at either en d sugges t tha t th e mos t recen t move ments hav e bee n sinistral , bu t thei r timin g i s constrained only to post-Jurassic time. Sixty-fiv e kilometres t o th e west , th e NW-SE-trendin g Sticklepath Faul t (Fig . 1 ) appears t o hav e had a similar histor y o f strike-sli p movements . Seg mented alon g strike , relay s hav e acte d a s releasing bend s t o produc e pull-apar t basins , the Lundy, Petrockstow an d Bovey basins, whic h are fille d wit h terrestrial sediment s o f Eocenemid-Oligocene age . Thes e includ e fluvia l an d lacustrine gravels , sand s an d clay s wit h thi n lignite bands . Althoug h th e Bove y Basi n i s a t least 1000 m deep, an d the oldest strat a have not been sampled , th e natur e o f th e sediment s implies tha t th e tectonicall y drive n subsidenc e was matched by the rate of sedimentation so that accommodation spac e wa s filled . Thu s th e sediments dat e th e faul t movement . Hollowa y & Chadwic k (1986 ) argue d tha t thi s movemen t was sinistral, with 6 km horizontal displacement. However, compariso n wit h a scale d analogu e model (Doole y & McCla y 1997 ) suggest s tha t the sinistra l displacemen t wa s n o mor e tha n 2km. Th e age s o f sediment s i n th e smalle r Petrockstow an d Lund y basin s ar e simila r t o those i n th e Bove y Basi n an d impl y tha t movements o f th e Sticklepat h Faul t continue d to mid-Oligocene time , around 28 Ma. This may also hav e bee n th e cas e fo r th e Watchet Cothelstone Fault .

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Inversion by bulk deformation and uplift Although th e near-surfac e mechanis m o f basi n inversion ma y b e throug h invers e displacement s on individual faults, the bulk effect i s the uplift of a basin to a domal structure . This i s observed i n the North Celti c Se a Basin an d the Weald Basin . The SWA T 4 dee p seismi c sectio n (BIRP S & ECORS 1986), Fig. 4 illustrates the overall effec t of inversio n o f th e Nort h Celti c Se a Basin . Th e basin rest s o n th e hangin g wal l o f a low-angle , south-dipping, plana r faul t tha t ca n b e trace d from surfac e t o 15k m depth , wher e i t merge s with th e to p o f a highl y reflectiv e zon e o f subhorizontal reflectio n segment s tha t extend s down to the Moho. This fault can be correlated a t surface i n souther n Irelan d wit h th e Varisca n Thrust Front , bu t i t ha s clearl y bee n reactivate d in extension a s a low-angle normal fault t o allow space fo r th e formatio n o f th e Nort h Celti c Se a Basin a s a half-graben. Th e basin is filled with a thick Permo-Triassi c terrestria l red-be d succes sion an d a Jurassic-Cretaceou s marin e succes sion simila r t o tha t i n th e Wesse x Basi n (Petri e el al. 1989) . Late r compressiv e reactivatio n o f the majo r faul t ha s resulte d i n th e inversio n of faults withi n th e Nort h Celti c Se a Basin , producing localize d anticline s an d positiv e flower structures , togethe r wit h a broa d doma l uplift o f 1-2 km (Roberts 1989) . Robert s argue d that although som e inversio n may have occurre d

during Paleocen e time , th e majo r par t i s o f Oligocene age . Erosio n ha s resulte d i n th e outcrop of Jurassic strata at the sea bed. Flanking the inverte d Nort h Celti c Se a Basi n ar e sa g basins fille d wit h Paleogen e sediments , on e o f which i s show n i n Fig . 1 . It appear s fro m thei r association tha t thei r subsidenc e accompanie d the uplift o f the North Celtic Sea Basin. A similar configuration i s evident with the domal inversion of th e Weal d Basi n an d th e subsidenc e o f th e flanking London and Hampshire-Dieppe basins, which ar e no w examine d in more detail. Inversion of the Weal d Basin The Weal d Basi n initiate d a s a n easterl y prolongation o f th e Wesse x Basi n a t th e beginning o f Jurassi c tim e (20 8 Ma). I t devel oped as an extensional basin, subsiding by means of norma l growt h fault s o f mainl y east-wes t trend, th e mos t activ e of which were clos e t o its northern margi n agains t th e Londo n Platform , which ha d acte d a s th e undeforme d forelan d t o the Varisca n orogen . Thu s th e Weal d Basi n developed durin g Jurassi c tim e mainl y a s a n asymmetric basi n wit h stron g down-to-sout h normal faults alon g its northern margin, founded upon reactivated Varisca n thrusts, which acted as low-angle extensiona l detachments. An element of transtensiona l faul t movement s durin g thi s

Fig. 4 . Deep seismic profile SWAT 4 (BIRPS & ECORS 1986 ) (locatio n show n in Fig. 1) , interpreted t o show the Variscan Thrus t Fron t (VTF ) reactivate d a s a low-angl e norma l faul t t o provid e spac e fo r th e Mesozoi c Nort h Celtic Sea Basin i n its hanging wall , then inverted a s indicated b y structures within the basin. The broad uplif t o f the basin an d the flanking Tertiary basi n t o the sout h shoul d b e noted .

CENOZOIC INVERSION AN D UPLIFT OF SOUTHERN BRITAI N

period i s marked b y th e e n echelon geometr y of the normal faults in plan view and the presence of associated WNW-ESE-trendin g fault s (Fig . 1) . Within th e basin , lowe r Lia s marin e claystone s and limestone s res t directl y upo n a Devonian Carboniferous basement . Th e Jurassi c succes sion, althoug h simila r t o tha t i n th e Wesse x Basin, i s dominate d b y carbonat e rock s lai d down i n a shallo w marin e low-energy , wave dominated environment , indicativ e o f clea r se a and a limite d suppl y o f clasti c materia l fro m nearby land, particularly the London Platform to the north . Depositio n throug h Earl y Cretaceou s time o f marin e sandstones , mudstone s an d sabkha-type evaporite s testifie d t o continuin g subsidence, a t a rate greate r tha n i n th e Wesse x Basin, an d depositio n o f clasti c sediment s i n a nearshore environment . Sedimentation thu s kept pace wit h subsidence . A mid-Cretaceou s (124 Ma) unconformity marking th e terminatio n of thi s megasequenc e (compar e th e Wesse x Basin) represents a relatively small time gap and sedimentation resumed , i n continuit y wit h th e Wessex Basin , wit h marin e sandstone s an d claystones succeede d b y th e Chalk . Althoug h the younges t survivin g Chal k i s o f lat e Campanian ag e (8 3 Ma), i t i s likel y tha t deposition continue d throug h Maastrichtia n time t o aroun d 74 Ma, yielding a total thicknes s of Chal k o f abou t 500m . Durin g th e followin g 16 Ma, regiona l uplif t an d erosio n le d t o th e removal o f a substantia l portio n o f th e Chalk , ranging, throug h differentia l erosion , betwee n 100 an d 350m . A sub-Paleogen e surfac e developed, upo n whic h th e commencemen t o f subsidence o f th e Londo n Basi n t o th e nort h of the Weald, an d the Hampshire-Dieppe Basin to the south , i s recorde d i n th e depositio n o f terrestrial, intertida l an d shallo w marin e sedi ments of late Paleocene ag e (58 Ma). These wer e succeeded b y th e Londo n Clay , representin g marine conditions with no evidence o f shorelin e facies, whic h ma y hav e inundate d muc h o f southern Englan d betwee n 5 4 an d 5 0 Ma. Interbedded shallo w marin e sand s an d clay s were deposite d i n successio n throug h Eocen e time, th e younges t preserve d bein g o f lat e Eocene ag e (c. 30 Ma). The maximu m thickness of Tertiary deposits in the London Basin is 300 m and in the Hampshire Basin it is 650 m. Evidence that subsidenc e o f th e Londo n Basi n accompanied uplif t o f th e Weal d i s les s convincing tha n o n th e Isl e o f Wight. However , Gale et al. (1999 ) pointe d out tha t cher t clast s identified a s bein g derive d fro m th e Lowe r Cretaceous Hyth e Bed s (c . 14 0 Ma) foun d i n Lutetian ag e pebbl e bed s i n th e Londo n Basi n imply tha t th e Weal d wa s bein g uplifte d a t th e

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same tim e a s th e firs t phas e o f inversio n o f th e Isle of Wight (47.5-4 4 Ma). Inversion o f the Weald ca n be observed o n oil industry seismi c section s no w availabl e fo r research us e throug h th e U K Onshor e Geophy sical Library . Figur e 5 present s thre e north south true-scal e dept h converte d section s acros s the Weal d showin g th e principa l Jurassi c horizons interprete d fro m th e seismi c data : lin e locations ar e show n i n Fig . 1 . Faults hav e bee n migrated withi n th e vertica l plan e o f section . Line 78-0 2 wa s sho t wit h explosives , givin g greater dept h penetratio n tha n th e vibrosei s source use d fo r th e othe r lines . O n thi s line , reflections from low-angl e faults can be traced to 8km depth , wel l int o th e crystallin e basement . Similar south-dippin g fault s observe d o n th e adjacent line s cannot be traced a s far down. The low di p angle s o f thes e fault s sugges t tha t the y originated a s thrusts (probably Variscan) , which then acte d i n extensio n durin g Jurassi c tim e a s the Weal d Basi n subsided . The y wer e late r inverted during Tertiary time, as is evident on the fault-related uplif t structure s withi n th e Juras sic-Lower Cretaceou s succession . Th e sens e of movement o f inversio n i s t o th e north . Th e Brightling well , offse t 4k m eas t o f lin e 78-02 , provides stratigraphi c correlatio n wit h seismi c reflectors and , in addition , records a duplication by reverse faulting of the Lower Lias succession. Assuming th e faul t dip s a t 45 ° t o th e south , consistent wit h fault s observe d o n th e seismi c section, implies a 350m northward displacement as a result of Tertiary inversion, comparable with that on the Abbotsbury-Ridge way Fault. Further examples o f faul t inversio n i n th e Weal d wer e presented b y Butle r & Pulle n (1990) , wh o als o found NW-SE-trendin g fault s tha t ha d under gone significan t transpression . Although there is no direc t evidenc e o n th e timin g o f inversion , Butler an d Pulle n reporte d th e presenc e o f ferroan calcit e precipitate d o n th e crest s o f Tertiary uplif t structure s tha t hav e S r isotop e ratios correspondin g t o a Late Oligocene-Earl y Miocene ag e (c. 24 Ma). However, such evidence is highly suspect.

Inverse model of the Weald Although th e seismi c dat a indicat e tha t th e mechanism o f inversion i s based o n reactivatio n of low-angle , south-dippin g fault s tha t cu t dee p into th e crust , th e overal l effec t o f the inversio n of th e Weal d i s a broa d doma l uplif t o f som e 1500m, togethe r wit h th e downwarpin g o f th e flanking London and Hampshire-Dieppe basins, seen in Fig. 1 . Figure 6a is a simplified version of

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Fig. 5 . True-dept h interpretation s o f thre e north-sout h seismi c section s acros s th e Weal d (locatio n show n i n Fig. 1) . Formation tops : K , Kimmeridge; C, Corallian; O , Oxfordian; L, Lias; D , Devonian.

the cross-sectio n produce d b y th e Britis h Geological Surve y (BGS ) wit h thei r 1:25 0 00 0 series soli d geolog y maps , sheet s 50NO O an d 51NOO(British Geologica l Surve y 1988 , 1989) . The line of section i s located i n Fig. 1 . In Fig. 6a, individual faults shown on the BGS section have been remove d an d th e cross-sectio n ha s bee n smoothed accordingl y t o focu s o n th e effec t o f distributed bul k pur e shea r deformation , a s a basis fo r preparin g a n invers e model . Criticall y for th e modelling , th e deep , crustal-scal e fault s provide a zon e o f lo w strengt h i n th e crust , laterally confine d below th e Weal d Basin , with relatively stron g crus t beneat h th e flankin g basins. A 10: 1 vertical exaggeratio n emphasize s the doma l uplift . Isochron s hav e bee n adde d t o provide tim e marker s tha t monito r th e basi n evolution. Usin g a knowledg e o f Tertiar y stratigraphy an d landscape evolutio n base d o n a reconstructed cross-sectio n o f th e Weal d b y Jones (1999 , Fig . 3) , the cross-sectio n ha s bee n reconstructed bac k t o mid-Eocen e tim e (4 0 Ma, Fig. 6b) , a t whic h tim e th e flankin g basin s ha d subsided t o a maximu m extent . Becaus e strati graphic evidenc e indicate s shallo w seas , th e

reconstruction assume s th e wate r dept h i s negligible. No accoun t is take n of the lan d surface topograph y acros s th e centra l Weald , where faul t inversio n woul d hav e create d localized uplift . Reconstructio n bac k t o lat e Paleocene tim e (6 0 Ma, Fig . 6c ) whe n th e flanking basin s bega n t o subside , togethe r with the broa d doma l uplif t o f th e Weald , wa s accomplished b y strippin g off the younger strata and adjustin g th e geometr y o f th e underlyin g strata b y vertica l movement . N o accoun t wa s taken o f latera l compressio n fro m basi n inver sion. Th e uppe r surfac e o f th e cross-sectio n represents the sub-Paleogene surface. The cross section at latest Cretaceous time (68 Ma, Fig. 6d) represents the time when deposition of the Chalk had bee n complete d an d indicate s a unifor m thickness of Chalk across the section. The extent of uplif t and erosio n of Chal k betwee n 68 and 60 Ma (Fig. 6c) is based on the estimates of Jones (1999). Using the work of Butler & Pullen (1990) for th e Jurassic-Cretaceou s succession , th e reconstruction continue d to th e mid-Cretaceou s unconformity a t 12 4 Ma (Fig. 6e) and back to the initiation o f th e Weal d Basi n a t th e beginnin g

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Fig. 6 . Inverse model of Weald (location show n i n Fig. 1). Isochrons (Ma ) are show n in italics.

of Jurassi c tim e (20 8 Ma). I n thi s reconstruc tion, accoun t wa s take n o f th e effect s o f compaction an d decompaction o f the Jurassic Lower Cretaceou s successio n durin g subsi dence an d subsequen t uplift , bu t thi s wa s regarded a s unwarrante d fo r th e Uppe r

Cretaceous an d Tertiar y deposits . N o accoun t was take n o f an y deepe r crusta l thermo tectonic effects , a s th e purpos e o f th e invers e model wa s t o reconstruc t th e basi n geometr y in cross-section , whic h coul d ac t a s a templat e for forwar d modelling .

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Forward model The purpos e o f forwar d modellin g wa s t o lear n what cause d th e subsidenc e o f th e Londo n an d Hampshire-Dieppe basin s a t th e sam e tim e a s the Weald Basin was being inverted and uplifted . A 2 D thermo-mechanica l forwar d modellin g scheme develope d b y Nielsen & Hansen (2000) , based o n th e mode l o f Brau n & Beaumon t (1987), provide s a physica l basi s o n whic h t o simulate th e subsidenc e o f the Weal d Basi n an d its subsequen t inversion . Detail s o f th e math ematical expression s use d fo r th e mode l ca n b e found i n thes e publications . Figur e 7 show s schematically ho w the model works . The uppercrust, lower-crus t an d upper-mantl e lithospher e are define d by thei r physica l properties , suc h as density, Young' s modulus , Poisson' s ratio , compressional strength , cree p parameters , specific heat , hea t productio n an d therma l conductivity, appropriat e fo r quartz-rich , feld spar-rich an d olivine-ric h minera l rheologies , respectively, a s show n i n Tabl e 1 . Thei r behaviour i s dictated b y th e prevailing tempera ture, pressure an d strai n rate a t each poin t i n the model a s i t evolves , accordin g t o function s expressing th e response of the materia l t o stres s in a viscoelasti c o r plasti c manner . A constan t heat flux of 35 mW m~2 is applied t o the base of the mode l upwar d fro m th e underlyin g mantle , which result s i n a surfac e hea t flo w o f 60-70 mWm~2 , dependin g o n the thicknes s o f the crust. Buoyancy forces operate throughou t to maintain isostati c equilibrium . Th e mode l consists o f a finit e elemen t mes h tha t i s closel y

spaced i n th e uppe r crust , intermediat e i n th e lower crus t an d mor e ope n i n th e uppe r mantle. Time-variable condition s ar e impose d o n th e model, includin g hea t flo w upwar d fro m th e mantle, surfac e rate s o f erosio n an d sedimen t deposition, an d kinemati c boundar y conditions. These las t condition s impos e extensio n o r compression fro m on e end , whereas th e othe r end remain s pinned . Eustati c sea-leve l change s (Haq e t al. 1987 ) are als o incorporate d int o th e model. I n th e mode l fo r th e Weald , a centra l region o f 100k m widt h i s presen t i n whic h th e compressional strengt h o f th e uppe r crus t i s reduced t o 40% of the values on either side, given in Table 1 , and the creep parameter B of the lower crust i s reduce d t o 85% , asymmetrically northward, o f th e value s o n eithe r sid e (Fig . 7) . Whereas the reduction of compressional strength ensures plastic yieldin g at lower stres s levels, the reduction i n B lead s t o accelerate d cree p a s a result o f lowe r viscosities . The mode l wa s firs t subjected t o latera l extensio n a t 0. 1 mm a"1 between 21 0 an d 17 0 Ma, risin g t o 0. 5 mm a"1 between 16 5 and 15 5 Ma before reducing to zero by 14 5 Ma. Thi s simulate d th e Jurassic-Earl y Cretaceous extensio n require d t o creat e th e Weald Basin , amountin g t o 13km . Therma l relaxation followed for 8 5 Ma unti l compression began a t 6 0 Ma, risin g t o a rat e o f 0.2 5 mm a"1 between 5 0 an d 2 0 Ma, reducin g t o 0. 1 mm a * by 1 0 Ma, and continuing to the present, a total of 11.5km. Because th e model ignores faul t move ments, th e amount s o f extensio n an d com pression ar e overestimate d by a factor o f two s o that th e value s given above shoul d be halve d to

Fig. 7. Forward modelling schem e (Nielse n & Hansen 2000).

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Table 1 . Parameters for th e thermo-mechanical model o f th e Weald Parameter

Sediments

Upper crus t

Lower crus t

Mantle

Surface porosit y c/> 0 Density (k g m~ 3 ) Young's modulu s (Pa ) Poisson's rati o Creep paramete r n Creep paramete r B (MP a s 1/n) Creep activation energ y (k J mol"1) Compressional strengt h (MPa ) Thermal conductivit y (W m"1 K" 1) Specific hea t ( J kg" r KT 1) Heat productio n rat e (jx W m~ 3 )

0.60 2700 1011 0.25 3.10 208 135 17.5 2 1000 1.3

2700 1011 0.25 3.10 208 135 26.2 3 850 1.3

2900 1011 0.25 3.20 12.28 239 26.2 2.3 900 0.3

3300 1011 0.25 4.48 0.2628 498 52.4 4 1000 0.01

Sediment porosit y 0 varies wit h dept h z (m ) accordin g t o 4> = 0oexp(—z/2000) . Cree p activatio n energ y an d parameters n and B relate t o expressions fo r viscous deformation (Brau n & Beaumont 1987) .

be realistic . Th e progres s o f th e mode l throug h this deformationa l histor y wa s monitore d a t 1 Ma intervals . Sedimentatio n rate s o f u p t o 0.1 mm a~~ 1 ensured that sedimentation kept pace with subsidence . Accommodatio n spac e wa s largely fille d an d wate r depth s wer e small . Similarly, erosio n rate s wer e suc h tha t erosio n kept pac e wit h uplif t t o kee p surfac e elevatio n and relie f low . Th e maximu m erosio n rat e wa s O.lSmma^ 1 during the rapid fall o f se a level in Tertiary time . Figur e 8 present s th e forwar d model a t 124 , 68 , 60 , 4 0 an d OM a fo r direc t comparison wit h the inverse model, Fig . 6. Because the mode l i s an oversimplification of reality ther e i s a limi t t o th e exten t t o whic h i t should attemp t t o replicat e th e detail s o f th e inverse model. However, the main features of the model offe r ne w insight s int o th e histor y o f subsidence an d inversio n o f th e Weal d an d th e reasons wh y th e latte r wa s accompanie d b y th e formation of the London and Hampshire-Dieppe basins. The critica l factor i s the lo w strengt h of the crus t across th e Weald, sandwiche d between the high strengt h o f the Londo n Platfor m to th e north and the relatively high strength of the crust to th e south , i n th e easter n par t o f th e Englis h Channel an d norther n France . Thi s strengt h pattern wa s undoubtedl y inherite d fro m th e Variscan Orogeny and the area of weak crust was exploited durin g Mesozoi c tim e t o contro l th e locations of extensiona l basin s acros s souther n Britain. Th e model shows how the high strengt h of th e crus t i n th e area s flankin g th e Weal d requires th e developmen t o f compressiona l basins a s a necessar y accompanimen t t o th e updoming o f th e Weal d durin g inversion . Extension o f th e uppe r crust resulte d i n a 2 km rise o f th e Mon o beneat h th e Weal d Basin . Subsequent inversion thickened th e crust beneath

the basin so that the end result is a deepening of the Moh o t o 35k m dept h belo w th e zon e o f weakness. Th e Moh o beneat h th e flank s remained a t 34k m throughout . Thi s mode l contrasts wit h one proposed b y Cloeting h e t al (1990) in which lateral compressio n impose d on a rifte d basi n i n lithospher e tha t ha s vertically variable but laterally uniform rheolog y results in a deepening of the basin an d uplift o f the flanks, the opposit e o f what is observed i n the Weald . The mode l simulate s the subsidenc e histor y and geometr y o f th e Weal d Basi n durin g Jurassic-Early Cretaceou s time , whic h i s indi cated b y th e isochron s show n a t 12 4 Ma i n Fig. 8e . I t als o simulate s th e unconformit y a t 124 Ma a s a respons e t o th e cessatio n o f extension an d th e dominanc e o f therma l relaxation. Th e mode l the n simulate s erosion of the Chal k betwee n 6 8 an d 6 0 Ma, bu t fail s t o include the commencement of uplift of the Weald at this time. Between 60 and 40 Ma, uplift o f the Weald i s accompanie d b y subsidenc e o f th e London an d Hampshire-Diepp e basins . Between 4 0 Ma an d th e present , th e Weal d i s further uplifte d an d erode d dow n t o uppermos t Jurassic level , th e Weal d dom e become s mor e pronounced an d th e flankin g basin s ar e furthe r compressed an d partially eroded. Th e latter is in part due to the eustatic fall in sea level of around 100m sinc e Miocene tim e (Ha q et al. 1987) . Correlation of events in southern Britain with Alpine tectonics A very extensive literature chronicles the tectonic evolution o f th e Alp s a s a consequenc e o f th e convergence of Africa wit h Europe over the past 120 Ma (Dewe y e t a l 1989 ; Pfiffne r 1992) .

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Fig. 8 . Forward model of the Weald Basin, its inversion and the subsidence of the London and Hampshire-Dieppe basins produce d b y Hansen , based o n th e modellin g schem e show n in Fig . 7 . This ma y b e compare d wit h th e inverse model, Fig . 6 . Isochrons (Ma ) are show n in italics.

Ziegler e t al. (1995 ) examine d i n detai l th e effects o f the Alpine collisional history upon the foreland t o th e north . The y pointe d ou t tha t 'forces relate d t o collisiona l plat e interactio n appear t o b e responsibl e fo r th e mos t importan t intra-plate compressiona l deformations ' an d concluded tha t 'th e bul k o f Lat e Cretaceou s

and younge r intra-plate compressiona l defor mations observed i n western an d central Europe developed i n response t o stresse s tha t developed as a consequenc e o f collisiona l couplin g of th e Alpine an d Pyrenea n orogen s wit h thei r fore lands'. Th e transmissio n o f stresse s int o th e foreland thu s took place whenever they could not

CENOZOIC INVERSIO N AN D UPLIFT O F SOUTHERN BRITAI N

be accommodate d withi n th e oroge n o r th e foreland basin . Th e evidenc e fo r th e timin g o f such event s ca n b e foun d withi n th e forelan d basin, i n particula r th e Nort h Alpin e Forelan d Basin, wit h regar d t o north-directe d stresse s that coul d hav e affecte d souther n Britain . Figure 9 summarize s th e correlation , base d o n work b y For d e t al. (1999 ) an d For d (pers . comm.) o n th e stratigraphi c an d structura l evolution o f th e Nort h Alpin e Forelan d Basin . She ha s recognize d fou r stage s o f progressiv e deformation: (1) a Mid-Lat e Eocen e stag e (46-3 6 Ma), when the convergence o f the Apulian Plate with Europe, a t a rat e o f 15mma ~ , resulted i n th e migration of the flexura l basi n and the fron t o f a low-angle external orogenic wedge, allowing the northward propagation of stress into the foreland; (2) a n Early-Lat e Oligocen e stag e (33 23 Ma), whe n th e migratio n o f th e flexura l basin an d wedg e fron t slowe d significantly ; n o growth structure s developed an d shortenin g was accommodated withi n th e thicknes s o f th e orogenic wedge ; n o stres s woul d hav e propa gated int o the foreland; (3) a n Early-Mid-Miocene stage (16-11 Ma), when th e syste m remaine d i n muc h th e sam e state as in the previou s stage , wit h the wedg e

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front stationary , s o that littl e o r n o stres s would have propagated int o the foreland; (4) a Lat e Miocene-Pliocen e stag e (11 3 Ma): aroun d 1 1 Ma th e oute r orogeni c wedg e effectively collapse d an d compressiona l defor mation concentrated on the Jura fold belt , which detached o n high-leve l Triassi c evaporites , t o accommodate som e 30k m o f N W shortening ; beneath th e Jur a decollement , stres s withi n the lithosphere is likel y to have propagated int o the foreland. In additio n to Alpine tectonics, extensio n and subsidence withi n th e Europea n Rif t System , involving th e Lowe r Rhenis h Basin , th e Rhine , Rhone an d Limogne Graben , occurre d mainl y in Oligocene time through to Miocene tim e (Meie r & Eisbache r 1991) . A s th e grabe n ar e oriente d north-south, riftin g i s inferre d t o hav e resulted from east-wes t tension. As show n i n Fig . 9 , inversio n event s i n southern Britai n ca n b e correlate d i n tim e with events i n th e Alp s t o explain : (1 ) th e uplif t an d erosion o f th e Chal k betwee n 6 8 an d 6 0 Ma a s due to stres s generated b y convergenc e between Africa an d Europe ; (2 ) th e uplif t o f th e Weal d and subsidenc e o f th e Londo n Basi n an d th e uplift of the Sandown Pericline and subsidence of the Hampshire-Dieppe Basin in a succession of

Fig. 9. Correlation of Tertiary tectonic events between southern Britain and the Alpine Foreland. Isle of Wight

uplift phase s 1 and 2 identified by Gale e t al. (1999) .

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short pulse s o f deformation a s a consequence o f Alpine stresse s tha t could no t be accommodate d within the North Alpin e Forelan d Basin, stag e 1 ; (3) furthe r uplif t o f th e Weald , generatio n o f thrusts extendin g int o the Londo n Basi n cappe d by thrus t anticline s (e.g . Windsor), inversio n o n faults i n th e Wesse x Basi n an d tightenin g o f folds, foldin g o f Eocen e strat a (e.g . Isl e o f Wight), associate d wit h th e compressio n o f th e Jura fol d bel t an d northwar d propagatio n o f stress, stag e 4. The timin g o f strike-sli p faul t movement s o n the Sticklepath Fault (and probably the WatchetCothelstone Fault ) coul d correlat e wit h th e generation o f th e Europea n Rif t Syste m an d it s associated stres s field . Alternatively , bot h the y and the inversion of the Celtic Se a basins may be associated with changes i n spreading rate s in the North Atlantic . Nort h o f th e Varisca n Thrus t Front, th e S t George s Channe l Basi n i s no t inverted bu t is instead th e site of a Tertiary basi n flanking th e Nort h Celti c Se a Basin . Furthe r north, Robert s e t al (1999 ) identifie d majo r compression event s o f Eocen e t o Oligicen e ag e along th e Nort h Atlanti c margi n betwee n th e West Shetland s an d V0rin g tha t correspon d t o changes in spreadin g rate s an d plate boundarie s in th e Norwegian-Greenland Sea. Inversion event s i n th e souther n Nort h Sea, separated fro m souther n Britai n b y th e Londo n Platform, appea r t o hav e acte d independently . For example, Badley et al. (1989) interpreted the stratigraphic an d structural evidenc e depicte d o n seismic section s t o demonstrat e tha t a Lat e Cretaceous (8 8 Ma) compressiona l even t inverted th e NW-SE-trendin g Sout h Hewit t Fault (Fig . 1) , resultin g i n typica l harpoo n structures, an d a secon d compressiona l even t in Miocene time superimpose d a further inversion . Tertiary landscape evolutio n across southern Britain In hi s revie w o f Tertiar y landscap e evolution , Jones (1999 ) dre w attentio n to th e developmen t of a sub-Paleogen e surfac e (68-6 5 Ma) upo n which ther e develope d a residua l soi l cove r indicative o f tropica l conditions . Thi s i s pre served acros s th e Haldon Hills , near Exeter, an d plateau gravel s ar e preserve d i n Dorse t an d across Salisbur y Plai n (Gree n 1985) . Jone s agrees wit h Green tha t the preservatio n o f thes e soils an d th e lac k o f terrigenou s sedimen t offshore mean s that , throughou t Paleogen e time, souther n Englan d forme d a lan d are a o f low relie f an d lo w elevatio n (Green , pers . comm.), excep t i n localize d area s o f tectoni c

uplift. I n Neogen e time , thi s low-lyin g ground was floode d fro m th e eas t b y a marin e transgression a t 2. 6 Ma i n whic h th e Re d Cra g was deposited , succeede d b y th e Norwich Crag. These deposits have been subsequently deformed into basins and swells so that the Red Crag is now at — 50 m Ordnanc e datu m (OD) in Eas t Anglia but ove r H - 10 0 m O D a t th e wester n en d o f it s outcrop, 110k m t o th e west , nea r Bishop' s Stortford. Thi s i s jus t par t o f mor e extensiv e evidence o f Lat e Pliocene-Pleistocen e uplift , tilting an d deformatio n o f souther n England , which ha s place d th e lan d surfac e o f th e Chal k Downs o f S E Englan d a t 200 m OD , risin g t o surface elevation s o f 450 m O D i n th e wes t o f England. Uplif t o f Thame s terrace s (Madd y 1997) i s consistent with this general rise , which from thei r datin g indicate s a n uplif t rat e o f 0.07 mm a"1 durin g the past 2 Ma i n the middle Thames Valle y area . Walsh et al (1999 ) reviewed the evidence for a set of planation surfaces at elevations between 50 and 150 m O D developed acros s wester n Britain and Ireland , an d linke d the m wit h a numbe r o f early Neogen e sedimen t outlier s an d saprolit e bodies. Fro m this , they identified a sub-Neogene surface drape d abov e th e presen t topograph y o f western Britai n and Ireland that they regard a s a land surface with subdued topography of vertical relief measure d i n ten s o f metres . Thi s i s supported by a map o f Neogene an d Quaternary uplift o f norther n Englan d prepare d b y Frase r et al . (1990) . Th e pictur e thu s emerge s o f a British landscap e o f lo w elevatio n an d subdue d topography throug h Neogen e tim e unti l abou t 2.6 Ma whe n a broad-scal e uplif t occurred , culminating acros s th e wester n sid e o f England and Scotland . The exten t o f thi s uplif t ha s bee n quantified b y Clayto n & Shamoo n (1999) , fro m an analysis of the topography of Britain based on a 1 km squar e grid , a s a regional uplif t o f u p t o 300-400 m acros s Wales , th e Pennines , Lak e District an d Souther n Upland s of Scotland , an d up t o 500-600 m across th e Scottis h Highlands. In particular , the y calculate d tha t th e uplif t o f mountain peak s wa s amplifie d b y th e isostati c response t o denudationa l offloading, create d b y the dee p dissection o f topograph y relate d t o th e most resistan t rocks . Thei r calculatio n o f denudational isostati c uplif t account s fo r approximately hal f th e actua l mea n elevation , leaving the remaining uplift t o be explained. The late Neogene uplift an d deformatio n of southern England appear s t o hav e bee n par t o f a larger scale uplif t tha t affecte d al l th e lan d are a o f Britain. Clearly, there are exceptions, highlighted by Battiau-Quenc y (1999) , suc h a s th e 500-800 m contrast in relief between Anglesey,

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where elevation s ar e clos e t o se a level , an d th e mountains o f Snowdonia, whic h ar e separated by a major fault along the Menai Straits. Differential uplift ca n b e relate d t o boundin g fault s that ar e seismically active , indicativ e o f moder n differ ential movements . The mountain s o f Norwa y resulte d mainl y from lat e Plio-Pleistocen e uplif t an d ar e deepl y incised, wit h denudatio n accompanie d b y hig h sedimentation rate s i n offshor e basins , wher e deposits ar e u p t o 1500 m thic k (Rii s & Fjeldskaar, 1992) . Befor e 2. 5 Ma, th e lan d surface was one of low relief. Riis and Fjeldskaar calculated th e amoun t o f isostati c uplif t t o b e expected fro m denudationa l offloading , usin g flexural rigidit y wit h a n effectiv e elasti c thick ness of 22 km. They also allowed for the effect of phase change s a t dept h resultin g fro m pressur e changes owin g t o loadin g an d offloadin g a t surface. Thei r calculate d uplif t surfac e account s for les s tha n 70% o f the observe d elevation . In northern France, a general uplift is observed from th e Armorica n bloc k t o th e Rhin e Grabe n (Guillocheau et al 2000) . Differentia l uplif t i s responsible fo r large-scale relie f developmen t of the Armorica n massi f durin g Quaternar y tim e (Bonnet e t al 2000) . Lagarde e t al. (2000) hav e demonstrated ho w small-scal e deformatio n structures confir m tha t differentia l uplif t i s controlled b y faul t movements . The land areas of Britain, northern France an d Norway hav e rise n durin g th e pas t 2. 5 Ma t o create th e present-da y uplands . Fo r a lon g tim e before tha t th e landscap e wa s generall y o f lo w relief and low elevation. The uplands are all areas where har d rock s existe d a t surfac e befor e th e uplift 2. 5 Ma ago . Climat e chang e t o glacia l conditions increase d geomorphologica l energy , erosion an d dissectio n o f th e landscap e i n har d rock areas . Isostati c reboun d fro m denudationa l offloading enhance d topographi c rang e an d raised th e summi t level , bu t ca n accoun t fo r only 50-70 % o f th e observe d uplift . Uplif t i s differential an d faul t controlled . Presen t seismi city indicates that a number of the faults involved are currentl y activ e a s norma l faults . Th e caus e of th e uplif t i s unclear . I t i s difficul t t o explai n simply a s continenta l margi n uplif t originatin g from bod y force s a t th e continent-ocea n crus t boundary, a s thi s forme d wes t o f Britai n 3 0 Ma ago. No r ca n i t b e attribute d easil y t o plat e margin stresse s fro m th e Alp s o r Mid-Atlanti c Ridge, transmitte d laterally , whic h wer e muc h greater earlie r durin g Tertiar y tim e tha n i n th e past 2. 5 Ma. Differentia l erosio n betwee n har d and sof t rock , creatin g a differenc e i n surfac e elevation o f 400 m betwee n uplan d an d troug h from a n initial flat surface, produces a deviatoric

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stress fro m differentia l offloadin g o f lOMPa . This appear s t o b e sufficien t t o induc e faul t movements (Hardebec k & Hauksson 2001 ) and , hence, differentia l uplift . Thi s effec t i s on e o f strain relief, in addition t o isostatic rebound , and is aki n t o roc k burst s i n mines , bu t o n a large r scale. I t i s climaticall y induced , resultin g fro m the much increased rate s o f erosion fro m glacia l processes. Bu t it remains to be seen whether this is sufficien t t o accoun t fo r th e hithert o unex plained differenc e between th e observe d surfac e uplift an d th e amoun t calculate d a s du e t o isostatic rebound. The author is deeply indebte d to D. L. Hansen for using the forwar d modellin g progra m whic h h e an d S. Nielsen ha d develope d t o simulat e th e evolutio n of the Weald , Londo n an d Hampshire-Diepp e basins , and t o S . Nielse n fo r invitin g hi m t o a worksho p a t Aarhus i n Jun e 1999 , whic h provide d th e impetu s fo r much o f th e wor k i n thi s paper . H e ha s enjoye d an d benefited greatl y fro m stimulatin g discussion s wit h colleagues J . Rose, C . Green an d K. McClay, an d from helpful advic e fro m M . Ford, D. Roberts, S . Egan an d M. Butler . H e i s ver y gratefu l t o th e U K Onshor e Geophysical Librar y fo r th e provisio n o f seismi c sections across th e Weald, and to the Leverhulme Trus t for th e awar d o f a n Emeritus Fellowship , whic h mad e this research possible .

References BADLEY, M.E., PRICE , J.D . & BACKSHALL , L.C. 1989 . Inversion, reactivate d fault s and related structures : seismic example s fro m the souther n Nort h Sea . In: COOPER, M.A . & WILLIAMS , G.D . (eds ) Inversion Tectonics. Geologica l Society , London , Specia l Publications, 44 , 201-219. BATTIAU-QUENCY, Y . 1999 . Crusta l anisotrop y an d differential uplift : thei r role in long-term landfor m development. In : SMITH , B.J. , WHALLEY , W.B . & WARKE, PA . (eds ) Uplift, Erosion an d Stability: Perspectives on Long-term Landscape Development. Geologica l Society , London , Specia l Publications, 162 , 65-74 . BEELEY, H.S . & NORTON , M.G . 1998 . Th e structura l development o f th e Centra l Channe l High : constraints fro m sectio n restoration . In : UNDERBILL, J.R . (ed. ) Development, Evolution and Petroleum Geology of the Wessex Basin. Geological Society , London , Specia l Publications , 133, 283-298 . BIRPS & ECORS, 1986. Dee p seismi c reflectio n profiling between England , Franc e an d Ireland . Journal o f the Geological Society, London, 143 , 45-52 . BONNET, S. , GUILLOCHEAU , F. , BRUN , J.-P . & VA N DEN DRIESSCHE , J . 2000 . Large-scal e relie f development relate d t o Quaternar y tectoni c uplif t of a Proterozoic-Palaeozoic basement: the Armor ican Massif , N W France . Journal o f Geophysical Research, 105, 19273-19288 .

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CENOZOIC INVERSIO N AN D UPLIFT OF SOUTHERN BRITAI N UNDERBILL, J.R . (ed. ) Development, Evolution and Petroleum Geology of the Wessex Basin. Geological Society , London , Specia l Publications , 133, 187-197 . LEFORT, J.P . & MAX , M.D. 1992 . Structur e o f th e Variscan bel t beneat h th e Britis h an d Armorica n overstep sequences . Geology, 20 , 979-982. MADDY, D . 1997 . Uplift-driven valle y incisio n an d river terrac e formatio n i n souther n England . Journal o f Quaternary Science, 12 , 539-545. MEIER, L . & EISBACHER , G.H . 1991 . Crusta l kinematics an d dee p structur e o f th e Norther n Rhine Graben. Tectonics, 10, 621-630. MILIORIZOS, M . & RUFFELL, A . 1998 . Kinematics and geometry o f th e Watchet-Cothelstone-Hatc h Fault System: implications for the structural history of th e Wesse x Basi n an d adjacen t areas . In : UNDERHILL, J.R . (ed. ) Development, Evolution and Petroleum Geology of the Wessex Basin. Geological Society , London , Specia l Publications , 133,311-330. NIELSEN, S.B . & HANSEN , D.L . 2000 . Physica l explanation o f th e formatio n an d evolutio n o f inversion zone s and marginal basins. Geology, 28, 875-878. PETRIE, S.H., BROWN , J.R., GRANGER, P.J. & LOVELL , J.P.B. 1989 . Mesozoic histor y o f th e Celti c Se a Basins. In: TANKARD , A.J . & BALKWILL , H.R. (eds) Extensional Tectonics and Stratigraphy of th e North Atlantic Margins. Memoirs , America n Association of Petroleum Geologists, 46,433-444. PFIFFNER, A . 1992 . Alpine orogeny . In : BLUNDELL , D.J., FREEMAN , R . & MUELLER , S . (eds ) A Continent Revealed: the European Geotraverse. Cambridge Universit y Press , Cambridge , 180-190. RAWSON, P.P . 1992 . Th e Cretaceous . In : DUFF , P.McL.D. & SMITH , A.J. (eds) Geology of England and Wales. Geologica l Society , London , 355-388 . Rus, F. & FJELDSKAAR, W. 1992. O n the magnitude of the Lat e Tertiar y an d Quaternar y erosio n an d it s significance fo r th e uplif t o f Scandinavi a an d th e Barents Sea . In : LARSEN , R.M. , BREKKE , H. , LARSEN, B.T . & TALLERAAS , E . (eds) Structural and Tectonic Modelling and its Application to Petroleum Geology. Norwegian Petroleum Societ y (NPF) Specia l Publication , 1, 163-185.

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ROBERTS, D.G . 1989. Basin inversio n i n an d aroun d the Britis h Isles . In : COOPER , M.A . & WILLIAMS , G.D. (eds) Inversion Tectonics. Geological Society , London, Specia l Publications, 44 , 131-150. ROBERTS, D.G. , THOMPSON , M. , MITCHENER , B. , HOSSACK, J. , CARMICHAEL , S . & BJORNSETH , H.-M. 1999 . Palaeozoic t o Tertiar y rif t an d basi n dynamics: mid-Norwa y t o th e Ba y o f Biscay— a new contex t fo r hydrocarbo n prospectivit y i n th e deep wate r frontier . In : FLEET , A.J. & BOLDY , S.A.R. (eds ) Petroleum Geology o f Northwest Europe: Proceedings of the 5th Conference. Geological Society , London, 7-40 . ROWLEY, E . & WHITE , N. 1998 . Inverse modellin g o f extension an d denudatio n i n th e Iris h Se a an d surrounding areas . Earth an d Planetary Science Letters, 161 , 57-71 . UNDERBILL, J.R . & PATERSON , S . 1998 . Genesis o f tectonic inversio n structures : seismi c evidenc e fo r the developmen t o f ke y structure s alon g th e Purbeck-Isle o f Wigh t disturbance . Journal o f the Geological Society, London, 155 , 975-992. UNDERHILL, J.R. &STONELY , R. 1998 . Introduction to the development, evolution and petroleum geology of the Wessex Basin. In: UNDERHILL, J.R. (ed.) The Development, Evolution and Petroleum Geology of the Wessex Basin. Geologica l Society , London , Special Publications , 133, 1-18 . WALSH, P. , BOULTER , M . & MORAWIECKA , I . 1999. Chattian an d Miocen e element s i n th e moder n landscape o f wester n Britai n an d Ireland . In : SMITH, B.J., WHALLEY , W.B. & WARKE , PA. (eds) Uplift, Erosion and Stability: Perspectives on Long-term Landscape Development. Geologica l Society, London , Special Publications, 162,45-63 . WHITE, N. & LOVELL, B. 1997. Measuring the pulse of a plume with the sedimentary record. Nature, 387, 888-891. ZIEGLER, PA . 1990 . Geological Atlas o f Western and Central Europe. 2n d edition ; Shel l Internationale Petroleu m Maatschappij , Th e Hague. ZIEGLER, P.A. , CLOETINGH , S . & VA N WEES, J.-D. 1995. Dynamic s o f intra-plat e compressiona l deformation: th e Alpin e forelan d an d othe r examples. Tectonophysics, 252, 7-59.

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Landforms an d uplift in Scandinavia K. LIDMAR-BERGSTROM & J. O. NASLUND Department of Physical Geography and Quaternary Geology, Stockholm University, SE-106 91 Stockholm, Sweden (e-mail: karna@ natgeo.su.se) Abstract: Th e relatio n betwee n Scandinavia n landform s an d Cenozoi c uplif t event s i s examined by analysis of digital elevation data in a regional geological context as well as in a geomorphological proces s perspective . Re-expose d fla t sub-Cambria n an d sub-Mesozoi c hilly relief aids in deciphering uplift and erosional events. The highly dissecte d mountains of the Norther n Scande s (NS ) rise maximall y 1500 m abov e a slightl y tilte d lowes t leve l continuing i n the Muddus plain s eastwards at 300-550 m above sea level (a.s.L). This level is correlated with the lowest, slightly warpe d leve l of the Palaeic relief at 1000-1300 m a.s.L of th e Souther n Scande s (SS) , over whic h mountain s o f simila r heigh t rise . Thi s lowes t surface i s though t t o b e th e en d resul t o f Paleogen e erosio n t o th e genera l bas e level . Northern Scandinavi a wit h th e N S an d th e Muddu s plain s acte d a s a bloc k tha t wa s progressively tilte d t o th e SE , wherea s th e Souther n Scande s experience d continuou s doming, wit h a majo r uplif t even t o f abou t 1000 m i n Neogen e tim e causin g dee p valle y incision i n the uplifted plateau . Th e Sout h Swedis h Dome emerged from it s Palaeozoic and Mesozoic cover in Neogene time an d still retains well-preserve d re-expose d palaeosurfaces.

Uplift alon g continenta l margin s ha s latel y become a topi c o f commo n interes t fo r geologists, geochronologist s an d geomorpholo gists (Japse n & Chalmer s 2000 ; Summerfiel d 2000). Scandinavi a i s locate d clos e t o th e Atlantic margin . Its large-scal e relie f i s charac terized b y thre e domes , th e Norther n Scande s (NS) reachin g abou t 2000 m abov e se a leve l (a.s.L), th e Souther n Scande s (SS ) reaching 2500 m in south Norway, and the South Swedis h Dome (SSD ) reachin g 375 m i n sout h Swede n (Fig. 1) . Th e uplif t o f th e Scande s ha s bee n discussed sinc e th e beginnin g o f th e centur y (Reusch 1901 ; Ahlman n 1919 ) and Neogen e uplift o f th e S S ha s latel y bee n supporte d b y a fission-track stud y (Rohrma n et al 1995) . It has been suggeste d o n differen t ground s tha t th e domes have different uplif t historie s with a main uplift i n Paleogen e tim e i n th e nort h an d i n Neogene tim e i n th e sout h (Rii s 1996 ; LidmarBergstrom 1999) . I n thi s paper w e examine and compare th e topograph y o f th e dome s i n mor e detail an d correlate surface s between th e dome s to reveal areas of Neogene uplift and subsidence. Methods The relie f wa s examine d b y analysin g heigh t layer maps, slop e map s and topographic profiles . All map s an d profile s wer e constructe d fro m a digital elevatio n mode l (DEM ) o f Scandinavia .

For Sweden the elevation data have a true spatial resolution o f 50 0 m X 500 m (Nationa l Lan d Survey o f Sweden) , wherea s origina l elevatio n data fo r surroundin g area s ha d a resolutio n o f 1000m X 1000m (Staten s Kartver k i n Norway, and ETOPO5). Th e latter data were subsequently resampled to 500 m X 500 m. Both resolutions are suitable for the study of large-scale morphology. The major relief features are described with the aid o f a heigh t laye r map . Local topograph y i s analysed fro m slop e map s an d evaluate d i n a regional geologica l contex t a s wel l a s i n a geomorphological proces s perspectiv e following recent advances in knowledge on the effect of deep weathering in the shaping of relief (Thomas 1994). Three majo r palaeosurface s ar e identified an d correlated between the domes: the sub-Cambrian peneplain, a Mesozoi c surfac e and a Tertiar y surface. Th e profile s ar e located t o elucidate the suggested correlation s o f palaeosurface betwee n domes an d surroundin g terrain . Further , th e different degree s o f valle y dissectio n o f th e domes ar e examined. Analysis of maps Major shape of the Northern and Southern Scandes The Norther n Scande s for m an elongated dome , 1000 km long and 270 km or 165 km wide (at the

From: DORE , A.G., CARTWRIGHT, J.A, STOKER , M.S. , TURNER , J.P. & WHITE , N . 2002 . Exhumation o f th e North Atlantic Margin: Timing, Mechanisms an d Implications for Petroleum Exploration. Geologica l Society , London, Special Publications, 196, 103-116. 0305-8719/027$ 15.00 © The Geological Societ y of London 2002.

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Fig. 1. relief of Scandinavia. The following features, expressed in the landscape as a result of etching of geological strutures, should be noted: the Dellen meteorite impact (D), the Silijan ring meteorite impact (S), and

the plutoni c rocks o f th e Osl o rift . Th e location s o f Fig s 3- 6 an d 8 are indicate d by squares . MTFZ, M0re Tr0ndelag Faul t Zone; H , Hardangeroidda; O , Otta Valley; G, Gud brandsdalen valley.

LANDFORMS AN D UPLIFT IN SCANDINAVIA

600m o r 1000 m levels ) (Fig . 1) . The Souther n Scandes for m a mor e ova l an d somewha t ben t dome, 68 0 km lon g an d 400 o r 265 km wid e (a t the 600 m o r 1000 m levels) . Thu s th e S S ar e about 100k m wider and 300km shorte r than the NS. The N S ar e cu t b y valley s i n a NW-S E direction, leaving intact interfluves above 1000m a.s.l. wit h a maximu m widt h o f 20km . Th e

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maximum length of the interfluves is 40 km. This is in contrast to the SS, which have an area with 430km length from sout h to north above 1000 m a.s.l. Thi s elevated are a i s cut only by two major valleys, the Gudbrandsdalen valley and its majo r tributary valley , the Ott a valle y (Fig . 1) . Here, i t is possible to walk on the so-called Palaei c relief for ove r 300 km without descending into a valley below th e 1000 m level . I n th e east-wes t t o

Fig. 2 . Domes an d generalized palaeosurfaces of Scandinavia. The Palaeic relief i s located withi n the SS above about 1000 m a.s.l . (se e Fig . 1) . Locatio n fo r profile s 1 , 2 an d 3 i n Fig . 7 ar e indicated . Modifie d fro m Lidmar-Bergstrom (1999) .

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SE-NW directions , th e interfluve s ar e her e unbroken fo r u p t o 130km . Th e origi n o f th e Palaeic relief is discussed below . Thus th e dome s o f th e Norther n Scande s an d the Souther n Scande s sho w characteristicall y different shape s i n term s o f width , lengt h an d valley incision . I n addition , th e mai n valley s i n both area s are widened an d deepened t o varying degrees b y glacia l erosion . Scandinavian palaeosurfaces formed by etching and planation Four characteristi c landscap e types , forme d b y etching and planation, were studie d i n the maps : (1) sub-Cambrian peneplain; (2) undulating hilly relief (etc h surfaces) ; (3 ) plain s wit h residua l hills (Muddu s plains) ; (4 ) th e Palaei c relie f o f southern Norwa y (Fig . 2). Sub-Cambrian peneplain. Withi n th e Precam brian par t o f Fennoscandi a th e bedroc k wa s

denuded t o a n almos t leve l plai n a t th e en d o f Proterozoic tim e (Hogbo m 1910) . Vendian , Cambrian an d Ordovicia n strat a wer e succes sively deposite d directl y o n th e fla t basement . The surfac e wa s t o som e exten t overridde n b y Caledonian nappe s i n the west an d buried below thick cover s o f sedimentar y strat a i n th e eas t during lon g period s o f tim e (Koar k e t al. 1978 ; Zeck e t al . 1988 ; Lidmar-Bergstrom 1995; Cederbom e t al . 2000) . Thi s surfac e ha s bee n re-exposed an d ove r larg e area s i t ha s bee n totally obliterate d b y subsequen t denudation. In other areas it is still well preserved, and is called the sub-Cambria n peneplain . I t i s encountere d more o r les s intac t i n easter n an d south-centra l Sweden fro m se a leve l t o ove r 300 m a.s.l . (Lidmar-Bergstrom 1988 , 1996) and als o i n contact wit h Cambria n cove r rock s o n Hardangervidda, sout h Norway , a t abou t 1100-1350m a.s.l . (Schipul l 1974) . Th e subCambrian peneplain is met with along the eastern part o f th e NS , i n th e sout h at abou t 300m a.s.l and i n th e nort h a t ove r 1000 m a.s.l . (see

Fig. 3 . Slope an d height layer map of the SSD in combination with surrounding cover rocks . SSP . South Smaland Peneplain. A generalize d pictur e o f th e exten t o f th e sub-Cambria n peneplai n is show n b y th e blac k line . I t i s almost intac t in the S E up to 300m a.s.l . I n the norther n par t o f the ma p i t reaches fro m belo w Cambria n cove r rocks at 200m a.s.l. to summits furthe r sout h a t 350m a.s.l. Smal l area s i n the west have lo w relief an d Cambria n fissure fillings indicating long-lastin g Cambria n cover . Exhume d sub-Cretaceou s hilly relief extend s from belo w Cretaceous cove r rock s i n th e sout h an d west . Coordinate s ar e fro m th e nationa l grid o f Sweden .

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Fig. 4 . Slope ma p o f central Swede n an d southeastern Norway. Features t o be noted ar e the overall undulating , hilly relie f o f etc h character , th e Silja n Rin g (meteorit e impact) , th e volcani c rock s o f th e Osl o rift , an d th e exhumed sub-Cambrian peneplain with distinc t faults i n the southeaster n corner.

Ljungner 1950) . Th e presen t lan d surfac e i n Precambrian rock s i n Swede n coincide s wit h or is a s muc h a s 600 m belo w thi s surface , whic h can b e looke d upo n a s th e primar y peneplai n (Lidmar-Bergstrom 1995 , 1996) . Where th e subCambrian peneplai n i s wel l preserve d th e land scape show s a n extremel y fla t topograph y without residual hill s (Fig s 3 and 4). Undulating hilly relief. Alon g the eastern flan k of th e Souther n Scande s th e topograph y i s hilly and th e difference s i n bedrock compositio n an d structure ar e well expresse d i n the relie f (Fig s 1 and 4) , jus t a s withi n th e re-expose d sub Cretaceous relie f i n southernmos t Swede n (Fig. 3) . Th e latte r landscap e wa s forme d i n Late Mesozoi c tim e b y dee p weatherin g (etch ing) an d subsequen t strippin g o f th e weathering mantle (Lidmar-Bergstro m 1989) . Eas t o f th e Southern Scande s th e resul t o f denudatio n o f meteoric impact s such o as th e Lat e Palaeozoi c (Bottomley etal 1978;AberginWickman n 1988 ) Dellen structur e (Fig . 1 ) an d th e Cretaceou s (Deutsch et ai 1992 ) Silja n ring (Figs 1 and 4) is

clearly see n i n th e topography . Th e resistan t plutonic rock s i n th e Osl o fiel d giv e ris e t o massive hill s surrounde d b y lo w area s wit h Palaeozoic sedimentar y rocks (Figs 1 and 4). The relief a t th e souther n ti p o f Norwa y i s ofte n interpreted t o hav e a rathe r well-preserve d subCambrian surfac e o n interfluve s betwee n a fe w major joint-aligned valley s (e.g . Rii s 1996) . Th e peneplain i s no t intac t an d alon g th e coas t th e relief i s o f undulating character (Rudber g 1960 ) and classifie d a s a n etc h surfac e (Lidmar Bergstrom e t al. 2000) . Alon g th e wes t coas t of south Norwa y th e geologica l structure s ar e well expressed i n the relief, and this is mainly the case along th e entir e coas t o f wester n Norway . Mos t of th e easter n flan k o f th e Souther n Scande s i s located i n Swede n an d thi s par t contain s on e of Sweden's or e provinces . Thes e ore s ar e calle d soft ore s becaus e o f th e dee p weatherin g the y have experience d (Vivallo & Broma n 1993) . Other claye y weatherin g residue s ar e know n from thi s area , a s wel l a s alon g th e coas t o f Norway (Lidmar-Bergstro m e t al . 1999) . Dee p weathering an d subsequen t strippin g o f sapro lites ar e a majo r caus e fo r th e expressio n o f

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geological structures in the relief (Thoma s 1994) . The undulating hilly relief o n the eastern flank of the SS is thus interpreted t o be of etch character , maybe o f Mesozoic ag e (Reusc h 1903 ; Lidmar Bergstrom 1995 ; Cederbo m e t al 2000) . Th e structurally controlled relie f alon g the west coast of Norway is also thought to have an etch origin, but wit h furthe r incisio n alon g th e etche d structures afte r uplift . Th e Mesozoi c ag e o f th e etching eas t o f th e S S i s no t confirme d bu t possible (Cederbo m e t al 2000) . Along the coast of S W Swede n th e basemen t surfac e emerge s from belo w Mesozoi c strata , whic h dat e th e etching here . Grus saprolites an d landforms. Beside s th e remnants of kaolinitic saprolite s associate d wit h the re-exposed Mesozoi c undulatin g hilly relief, gravelly saprolite s (grus ) ar e o f commo n occurrence withi n man y part s o f Fennoscandi a (Lidmar-Bergstrom e t al 1999) . The y ar e interpreted t o hav e develope d mainl y i n Plio Pleistocene tim e bu t ma y dat e bac k t o Miocen e time. Withi n sout h Swede n the y ar e ofte n associated wit h a n etche d landscap e forme d a t the expens e o f th e re-expose d sub-Cambria n peneplain (Lidmar-Bergsto m e t al. 1997) .

Plains with residual hills. I n contras t t o th e dome flanks in the south, the relief east of the NS is characterized b y plains with residual hills, the so-called Muddu s plains , situate d mainl y a t 300-550 m a.s.l. (Fig. 5) (Wrak 1908) . Th e hills rise t o 300 m abov e th e plains . Th e plain s ar e interpreted t o hav e develope d fro m undulatin g hilly relie f (etc h topography ) b y pedimentatio n processes durin g mor e ari d period s o f Tertiar y time (Lidmar-Bergstro m 1995) . Th e plain s have acted a s base level s for the valleys that penetrate the mountain s i n th e wes t (Fig . 5). Individual plains ar e separate d b y lo w step s int o a number of separat e level s (Rudber g 1954 ; Lidmar Bergstrom 1996) . Simila r plain s (th e Sout h Smaland Peneplain , SSP ) wit h relativel y fe w and lo w residua l hills occu r at th e southwestern flank o f th e SS D abov e th e exhume d subCretaceous hill y relie f (Fig . 3). Th e plain s here cut of f th e re-expose d sub-Cretaceou s etche d relief an d ar e thu s o f Tertiar y ag e (Lidmar Bergstrom 1982) . Palaeic surface (or relief) of southern Norway. Traveller s i n souther n Norwa y ca n observe tha t mos t o f th e highe r groun d abov e about 1000 m a.s.l . is occupied by a high plateau

Fig. 5 . Slop e ma p o f region i n northern Swede n showin g th e Paleogene Muddu s plains in yellow (300-700 m a.s.l.) i n th e eas t an d thei r continuatio n a s valley s int o th e mountain s in th e west . Th e lin e mark s th e borde r between Precambria n basement an d Caledonian nappe s (se e Fig. 6).

LANDFORMS AND UPLIFT IN SCANDINAVI A

(Fig. 6). From the west deep valleys cut far inland into thi s plateau , name d th e Palaei c surfac e b y Reusch (1901) . The difference betwee n th e high plateau an d th e deepl y incise d valley s wa s interpreted t o reflec t a late uplif t (Reusc h 1901 ; Ahlmann 1919 ; Peulvas t 1978 , 1985) . Th e Palaeic surfac e i s separate d int o differen t level s thought to be induced by renewed valley incision as a resul t o f continue d warpin g durin g severa l Mesozoic-Paleogene uplif t event s (Lidmar Bergstrom e t al. 2000) . Th e Palaei c surfac e i s composed o f severa l surface s separate d b y distinct step s an d i s bette r referre d t o a s th e Palaeic relief. Th e lowes t leve l o f th e Palaei c relief, situated at about 1000-1200 m a.s.L, has a vast extent on the eastern side of the high dome in southern Norway . Thi s leve l ha s acte d a s bas e level for the river systems penetrating the higher ground (Fig s 1 an d 6 ) an d ca n b e followe d westwards alon g th e majo r rive r systems . Th e 1000m leve l occur s als o alon g the western sid e of sout h Norwa y (Lidmar-Bergstro m e t al . 2000). In detail, the Palaeic relief is characterized by slightl y undulatin g plains wit h residua l hill s with surroundin g pediment s (Fig . 6) . Dee p weathering an d pedimentatio n processe s ar e thought to have been importan t in the formation of thi s relief (Gjessin g 1967) .

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Zone of incised valleys Around the plateau wit h Palaeic relief i n the SS there is a zone with deeply incised valleys (Figs 1 and 6) . Th e valley s ar e deepe r o n th e wester n side, wher e the y exten d belo w se a level . A difference i n relief of up to 800 m between valley bottoms o f incise d valley s an d adjacen t lowes t shallow valle y leve l o f th e Palaei c relie f i s common. Th e valley s are glacially deepene d b y more tha n 1000 m i n th e Sognefjor d o n th e western, Atlanti c side , an d b y c . 250 m o n th e eastern side , e.g . i n th e Mjos a are a (Ahlman n 1919).

Comparison between the lowest level of the Palaeic relief and the Muddus plains The lowest level of the Palaeic relief constitutes a plateau strikingl y simila r t o th e Muddu s plain s but a t a highe r leve l (Fig s 5 an d 6) . The y bot h have acte d a s bas e level s fo r rive r system s penetrating westwards . I n detail , ther e ar e differences betwee n th e two , whic h ca n b e explained by differences i n bedrock composition. In th e Palaei c relie f depicte d i n Fig . 6 , th e

Fig. 6 . Slope and height laye r map showin g the lowes t leve l o f the Palaeic relief o f southern Norwa y i n yellow (900-1200m a.s.l.) . Th e lin e mark s th e borde r betwee n Precambria n basemen t an d Caledonia n nappe s (se e Fig. 5) .

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southern par t ha s develope d b y erosio n o f th e Caledonian nappe s an d now is mainly shape d i n Precambrian rocks . Th e associate d residua l hill s often includ e remnant s o f th e Caledonia n cover . Further t o th e N E (Fig . 6 ) the Palaei c relie f ha s formed i n highl y variabl e Caledonia n rocks , which hav e resulte d i n a somewha t differen t morphology wit h some resistant rocks givin g rise to hig h summits . I n contrast , th e correspondin g Muddus plain s in northern Swede n (Fig . 5 ) were formed o n Precambria n rocks , whic h ha d lon g since los t their Palaeozoic cover . In addition, the different appearanc e o f the relief i n Figs 5 and 6 is als o t o a minor exten t th e resul t o f th e us e of elevation dat a wit h differen t resolution s i n th e construction o f th e maps . However , mos t o f the differenc e i n appearanc e i s cause d b y th e difference i n geology . It i s likel y tha t th e Muddu s plain s o f northern Swede n an d th e lowes t leve l o f th e Palaeic relie f o f souther n Norwa y develope d at th e sam e leve l an d a t th e sam e time . Th e individual form s ar e th e en d resul t o f etching , stripping an d pedimentatio n i n war m climates . This unifor m lan d surfac e wa s subsequentl y deformed b y differentia l uplift . This , includin g the timin g o f events , i s furthe r discusse d below. Analysis of profile s The location s o f th e profile s (Fig. 2 ) ar e chose n to illustrat e th e correlatio n an d deformatio n o f the thre e palaeosurfaces , namely , th e sub Cambrian peneplain , th e Mesozoi c undulatin g hilly relief , an d the Tertiary plain s wit h residua l hills (correspondin g t o th e lowes t leve l o f th e Palaeic relief) . Th e vertica l positio n o f th e palaeosurfaces i s illustrate d i n th e profile s an d used fo r a discussio n o n uplift . Profile s 1 and 2 are use d t o discus s th e relationshi p betwee n th e SSD and the SS, as well as the deformation of the sub-Cambrian peneplain , th e inferre d Mesozoi c surfaces, an d the lowest Palaei c level . Profile 3 is used fo r discussing th e N S an d it s eastern flank . Profile 1 (Figs 2 and 7) The sub-Cambrian peneplai n extend s from below Cambrian cove r rock s i n th e SE . The peneplai n forms a low dome, constituting the SSD, which is interrupted b y the Vattern Graben , fille d wit h up to 1000 m o f Late Proterozoi c sedimentar y roc k (Axberg & Wadstei n 1980) , an d th e Hokensa s Horst. Palaeozoi c remnant s occu r o n th e slope s down t o Lak e Vaner n bu t d o no t occu r o n th e bottom o f th e lake . Furthe r t o th e N W th e peneplain i s dow n warped an d form s th e Vane r

Basin. Fro m Lak e Vaner n th e sub-Cambria n peneplain rises to the NW and can be followed in the summit s fo r som e distance . Profil e 1 the n follows a mor e westerl y directio n (Fig . 2) . Th e sub-Cambrian peneplain is met with again below Palaeozoic strat a i n th e Osl o rif t an d the n encountered a t abou t 1250-1300 m a.s.l . o n th e Hardangervidda. Here i t is identified wit h the aid of Palaeozoi c outliers . Th e peneplai n rise s towards the NW and thereafter it is downfaulted, where Caledonia n rock s mee t th e Precambria n basement along a steeply dipping front a t H in the profile (Fig . 7) . Northwest o f Lak e Vaner n th e sub-Cambria n peneplain i s replace d b y a hill y relie f (a n etc h surface), whic h i s tentativel y interprete d a s a Mesozoic surfac e (see above). This hilly relief is correlated wit h a n inferre d surfac e alon g th e highest summit s furthe r t o th e west . Th e weathering-resistant plutoni c rock s o f th e Osl o rift exten d above thi s surface. The present lowest level of the Palaeic relief at Hardangervidda partl y coincide s wit h bu t i s mainly slightl y belo w th e sub-Cambria n pene plain. I n th e westernmos t part s shallo w valley s are incise d i n th e lowes t level , her e a t abou t 1100m a.s.L , t o slightl y belo w 1000 m a.s.l . Structurally controlle d dee p valley s o f th e Hardangerfjord syste m penetrat e th e Palaei c relief i n th e NW . Dee p valleys , Tinnsj o an d Numedalen, ar e incise d alon g th e southeaster n flank. Profile 2 (Fig. 7) Profile 2 is identical to profile 1 in the SE but then continues straight towards the NW (Fig. 2). In the area o f hilly relief, NW of Lake Vanern , the sub Cambrian peneplai n ha s disappeare d an d i s me t with agai n belo w th e Palaeozoi c strat a i n th e Oslo rift. Thereafter, it directly disappears belo w the Caledonian rocks and has no further influenc e on the presen t topograph y alon g the profile. In this profile a correlation has also been mad e between th e undulatin g hill y relie f an d th e highest summit s o f th e SS . Th e inferre d Mesozoic surfac e ha s experience d domin g an d been uplifte d abou t 2000 m i n th e NW . Subsequently, i t has bee n successivel y dissected by major valleys, which has resulted in a stepped pattern o f th e presen t summi t surfaces (Lidmar Bergstrom e t al. 2000) , indicate d b y th e thre e summit level s in th e profile . A slightl y warpe d surfac e i s see n a t abou t 1000-1200m a.s.l. , dissecte d b y dee p valley s (Eikesdalen drainin g westwards ; Gudbrandsda len an d othe r valley s drainin g southeastwards). This i s th e lowes t leve l o f th e Palaei c relief ,

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Fig. 7 . Topographic profiles showin g Scandinavia n dome s and palaeosurfaces. Locatio n of profiles i s shown i n Fig. 2 . Profile 1 crosses th e souther n par t o f the SS , Vaner Basin an d SSD . B , Bergen ; H , Hardangerfjord ; Ha , Hardangervidda. Profil e 2 crosse s th e norther n par t o f SS , Vane r Basi n an d SSD . E , Eikesdalen ; G , Gudbrandsdalen. Profil e 3 crosses the NS and the Muddus plains in the east. O, Ofotfjorden; N , Norddalen; Cal , Caledonian.

comparable wit h th e lowes t leve l o f Hard angervidda in profile 1 . Above this lowest level, mountains rise to heights o f 1500-200 0 m a.s.l . The highest peaks of south Norway reach 150 0 m above thi s lowes t level , an d ar e indicate d b y a point showin g thei r vertica l positio n i n th e profile. Befor e the profile reache s th e Atlantic it crosses th e M0re-Tr0ndela g Faul t Zon e (MTFZ). Profile 3 (Fig. 7) The exhumed sub-Cambrian peneplain is seen at the easter n coas t an d ca n b e followe d i n som e

summits toward s th e west . Th e inferre d sub Cambrian peneplai n i s ben t down , belo w th e Caledonian rocks . The eastern parts of the NS are here formed in Precambrian basement rock with the Caledonian nappes followin g westwards. The highest mountains ar e forme d o f rock s relativel y resistan t t o deep weatherin g an d their summi t surfaces may date bac k t o Mesozoic tim e (Lidmar-Bergstro m 1996). Mesozoi c surface s hav e tentativel y been placed across the highest peaks along the profile. A lowes t Mesozoic(?) leve l i s tentatively shown as a surface inclined t o the east alon g the whole profile (se e Wra k 1908) .

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A fe w step s ar e incise d belo w th e sub Cambrian surfac e clos e to the coas t in the eas t and the n follo w westwar d th e mai n plain s wit h residual hill s (th e Muddus plains), which exten d to th e Scandes . Withi n th e mountain s thi s leve l can b e followe d a s th e bas e fo r valle y incision . Mountains ris e t o 1500 m abov e thi s level . Comparison and correlation of the Scandinavian domes from the profile data Profile 3 (Fig . 7 ) show s th e highl y dissecte d mountains o f the NS rising fro m a slightly tilted lowest level , continuin g i n th e Muddu s plain s eastwards. I n th e ma p analysi s (Fig . 5) , th e Muddus plain s wer e correlate d wit h th e lowes t level o f the Palaei c relie f i n the S S (Fig. 6) , an d they ar e show n a s the sam e surfac e i n profiles 1 and 2 (Fig . 7) . Th e correlatio n betwee n th e Muddus plains and the lowest level of the Palaei c relief i s strongl y supporte d b y th e fac t tha t th e vertical distanc e fro m thes e level s t o the highes t summits o f th e Scande s ar e 1500 m i n bot h regions (Fig . 7 , profile s 2 an d 3) . Thi s suggest s that th e tw o dome s wer e o f th e sam e heigh t before th e majo r uplift o f the SS . Discussion Uplift of the Scandes Riis (1996 ) suggeste d tha t th e uplif t alon g th e western margi n o f Fennoscandi a di d no t occu r simultaneously in the north and the south. On the basis o f offshor e geology an d identificatio n of a Paleogene surfac e it was suggested tha t the main uplift i n th e nort h occurre d i n Paleogen e time , whereas i t too k plac e i n Neogen e tim e i n th e south. Lidmar-Bergstrom (1999 ) agreed with this interpretation, a s a resul t o f analysi s o f th e relation between the mountains and the two types of relief tha t occur along the eastern flanks of the NS an d the SS . The Northern Scandes and their eastern flank. Th e shap e o f th e N S an d th e steppe d morphology o f th e Muddu s plain s indicate s tilting toward s th e east . Th e mai n relie f withi n the mountain s i s th e resul t o f valle y incisio n since th e initiatio n o f uplift , wherea s th e successively widene d oute r valley s forme d i n a tectonically relativel y stabl e environmen t clos e to a genera l bas e leve l i n th e eas t wit h th e Muddus plains as the end result. It is possible that the Precambria n surface , wher e no t i n contac t with th e sub-Cambria n peneplain , wa s expose d during Mesozoic tim e and that the Muddus plains

ultimately wer e forme d i n Paleogen e time , a s indicated by finds of redeposited marin e Eocen e diatomaceans (Cleve-Eule r 1941) . Continuin g apatite fission-trac k analysi s (AFTA) in the are a will shed mor e ligh t on this. So far, these studies suggest a til t o f th e are a alon g th e profil e beginning i n Cretaceou s tim e an d acceleratin g in Paleogen e tim e (Hendrik s & Andriesse n 2001). A ver y hig h relativ e relie f occur s wes t of th e Ofotfjorde n faul t line , and , i n detail , the relie f i s highl y irregular . I t i s likel y tha t this wa s originall y a n etc h surfac e o f Mesozoic (o r mayb e older ) age , a s Mesozoi c (or older? ) rock s ar e stil l preserve d i n a downfaulted basi n o n a kaolinitize d basemen t surface (Stur t e t al 1979) . Uplif t ha s cause d incison o f th e etche d structures . Analysi s o f fission-track dat a indicate s severa l phase s o f vertical movemen t i n thi s regio n (Hendrik s & Andriessen 2002) . The Southern Scandes. Th e surfac e form s o f the S S ar e cu t acros s bot h Caledonia n an d Precambrian basement . I n th e souther n par t o f the dome the present surface is relatively close to the sub-Cambria n peneplain , wherea s i n th e north th e sub-Cambria n peneplai n disappear s below th e Caledonia n nappes . Th e Mesozoi c surface inferre d in th e profile s indicate s that th e line throug h th e highes t summit s represent s a warped surfac e o f thi s age . I n th e Osl o rif t an d southeastwards i t become s mor e likel y tha t th e present relie f i s par t o f a re-expose d sub Cretaceous etc h surface . Apatit e fission-trac k modelling show s that this interpretation is viable (Cederbom e t al . 2000) . Profil e 2 crosse s th e MTFZ. Thi s i s a n ol d Caledonia n structur e that has subsequentl y bee n reactivate d (Dor e e t al . 1999). I t i s possibl e tha t vertica l movement s along th e MTF Z hav e affecte d th e Mesozoi c surface an d contributed to a slight asymmetry of the SS. The lowest level of the Palaeic relief has a position a t abou t 1000-1200 m a.s.l . compare d with th e tilte d plai n fro m whic h th e N S ris e a t 300-700 m a.s.l . I t i s a slightl y domed surface , which support s the ide a o f a continuation of th e Mesozoic doming . Th e Neogen e uplif t o f th e SS ca n thu s b e estimate d t o b e abou t 1000 m (Fig. 7 an d Lidmar-Bergstro m e t al . 2000) . Th e valleys alon g th e coas t ar e mainl y structurally controlled an d i t i s suggeste d tha t th e dee p incision followe d structure s etche d ou t durin g Mesozoic time . A major hinge line across central Scandinavia? A lin e fro m th e MTF Z toward s th e

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113

Fig. 8 . Slope map of centra l Scandinavia. The M0re-Tr0ndelag Fault Zone (MTFZ) an d the fault line s at the east coast of Sweden sugges t a hinge line in a SW-NE direction. This lin e marks the border betwee n th e undulating hilly relie f t o the sout h an d the Muddus plains t o the north.

WSW-ENE-trending faul t line s o f th e sub Cambrian peneplai n i n nort h Swede n approxi mately coincide s wit h th e border betwee n th e plains with residual hills (the Muddus plains) and the undulatin g hilly relief (Fig . 8) . In the region where thi s lin e crosse s th e mountai n chai n th e Precambrian basement has a low position (300 m a.s.l.)- Thi s i s i n contras t to bot h i n th e N S an d SS, wher e th e basemen t reache s abov e 1000 m a.s.l. Thi s lin e ma y approximatel y mar k th e border betwee n area s wit h differen t uplif t histories.

the cove r i s therefor e suppose d t o hav e bee n eroded befor e th e downwarp . A Neogen e ris e of th e SS D a t th e Oligocene-Miocen e bound ary wit h subsequen t erosio n o f it s cove r i s supported b y a fission-trac k stud y o f sout h Sweden (Cederbo m 2002 ) an d th e observatio n of a large amoun t o f erosio n o f Cretaceou s an d Paleogene strat a fro m nearb y Jyllan d an d th e Skagerrak-Kattegat Platfor m (Japse n & Bidstrup 1999) .

Summary of uplift an d relief development South Swedish Dome and Vdner Basin The SS D developed it s present shap e in Tertiary time, whe n th e basemen t successivel y wa s exposed afte r erosio n o f Palaeozoic cove r in th e north an d eas t an d Mesozoi c cove r i n th e sout h and wes t (Lidmar-Bergstro m 1991 , 1993) . W e suggest tha t i n connectio n wit h th e Neogen e uplift o f th e SS , th e Vane r Basi n wa s slightl y depressed an d th e SS D uplifted . N o Palaeozoi c rocks occu r o n th e botto m o f Lak e Yaner n an d

The Precambrian shiel d of Scandinavia has been covered by Palaeozoic rocks . In some parts they were erode d durin g Mesozoi c tim e an d etche d surfaces with undulating hilly relief were formed in th e basement . Mesozoi c denudatio n als o caused relie f developmen t acros s Caledonia n rocks an d th e Palaeozoi c cover . Jurassi c an d (probably mainly ) Cretaceou s strat a wer e the n deposited ove r the area to an unknown extent. In some case s the y wer e deposite d directl y on the etched basemen t an d protecte d it s Mesozoi c

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relief fo r a lon g time . Larg e part s o f th e Caledonian area s experience d continuou s relie f development withou t an y Mesozoi c temporar y cover. A t the end of the Paleogene period, plains with residua l hill s ha d forme d ove r th e easter n parts o f Scandinavi a wit h valle y system s penetrating mountainou s area s i n th e west . Th e mountains reache d abou t 1500 m abov e thi s lowest leve l o f th e Palaei c surface . I n souther n Norway th e sub-Cambria n peneplai n wa s successively re-expose d an d th e Precambria n basement her e experience d furthe r relie f development. The tectoni c uplif t differe d betwee n th e N S and th e SS . Th e N S mainl y acte d a s a block , which experience d simpl e tilt , wherea s th e S S were characterize d b y doming . Neogen e uplif t caused a sligh t furthe r til t i n th e nort h an d a major uplif t wit h continue d domin g i n th e S S with re-exposur e o f sub-Mesozoi c relie f alon g their flanks . Th e deepl y incise d valleys of the SS were mainl y forme d i n Neogen e tim e a s a consequence o f uplift . Subsequent , Lat e Cenozoic ic e shee t erosio n ha s furthe r widene d and deepene d thes e valleys. Neogene uplif t wit h a centr e sout h o f Lak e Vattern cause d th e formatio n o f th e SSD . Th e buried sub-Cambrian peneplai n was successively re-exposed. Th e re-exposure o f this surface at the top o f th e dom e cause d reactivatio n o f th e weathering system s alon g th e fractur e systems , which wer e successivel y expresse d i n th e topography a s joint aligne d valleys . Re-exposure of etched Mesozoi c relie f i n the S W caused total stripping o f th e remain s o f th e kaoliniti c saprolites b y th e SW-flowin g drainag e system , down t o c . 125 m a.s.l . Her e th e SS P gradually formed, governe d b y se a level a s the bas e fo r it s erosion. It s detailed form s probably developed i n semiarid climate s tha t promote d pedimentatio n (Lidmar-Bergstrom 1988) . Neogen e gru s sapro lites ar e stil l o f commo n occurrenc e i n th e eastern par t o f th e dom e abov e thi s leve l an d testify t o continue d etching. The lates t rise , probabl y i n Pliocen e time , caused th e re-exposur e o f th e sub-Cretaceou s etch surface s below 12 5 m a.s.l . in the sout h and west, an d successiv e re-exposur e o f th e extre mely flat sub-Cambrian rock surfaces in the north and east . Conclusions (1) Th e main uplift o f the Northern Scande s is older tha n th e uplif t o f the Souther n Scandes , a s the NS are considerably more dissected by deep valleys. A s a resul t o f th e Mesozoi c Paleogene uplift an d til t o f th e NS , th e Muddu s plain s

formed an d the y acte d a s bas e level s fo r th e development o f the deep valley s of the mountain range. I n Neogen e tim e th e are a experience d additional mino r uplif t an d tilting. (2) Th e mai n uplif t o f th e Souther n Scande s took place i n Neogene time , the uplift amounting to c . 1000m . Th e Neogen e uplif t wa s a continuation o f Mesozoic-Paleogene doming . (3) Th e Muddu s plains and their continuation in the valleys of the NS are tentatively correlated with th e lowes t leve l o f th e Palaei c relie f o f th e SS, because of strong similarities in morphology. (4) A hing e fro m th e MTF Z t o N E Swede n separates th e N S wit h th e Muddu s plain s fro m the S S an d th e undulatin g hill y relie f o n it s eastern flank. (5) I n Neogene tim e the South Swedish Dom e was elevated , wit h subsequen t developmen t o f the Sout h Smalan d Peneplain . A t th e sam e tim e the Va'ne r Basi n was downwarped . (6) Geneticall y interprete d landform s ar e important dataset s i n morphotectoni c analyses , complementary t o studie s o f th e sedimentar y records an d thermotectoni c evolutio n o f th e bedrock. The stud y was supporte d b y a grant fro m th e Swedis h Natural Scienc e Researc h Council . W e als o wan t t o thank P . Japsen, S . A. Cloeting h an d P . Andreissen fo r encouraging discussion s an d input . Elevatio n mode l data ove r Swede n courtes y o f Swedis h Nationa l Lan d Survey 2000 . Excerp t fro m GSD-elevatio n database , case no . L2000/646 . Elevatio n mode l dat a ove r Norway courtes y o f Staten s Kartverk . 3504 H0nefoss . Norway.

References AHLMANN, H.W . 1919 . Geomorphologica l studie s i n Norway. Geografiska Annaler. I. 1-20 . AXBERG, S . & WALDSTEIN . P . 1980. Distributio n of th e sedimentary bedroc k i n Lak e Vattern . souther n Sweden. Stockholm Contributions i n Geology. 3 4 (2). 15-25 . BOTTOMLEY, R.J. . YORK , D . & GRIEVE . R.A.F . 1978 . 40 Ar- 38 Ar ages of Scandinavian impact structures: I Mien and Siljan. Contributions to Mineralogy an d Petrology, 68 , 79-84. CEDERBOM, C. 2002. The thermotectonic development of southern Sweden durin g Mesozoic an d Cenozoi c time. In: DORE , A.G., CARTWRIGHT , J.A., STOKER , M.S., TURNE R & J.P., WHITE, N. (eds ) Exhumation of the North Atlantic Margin: Timing, Mechanisms and Implications for Petroleum Exploration. Geological Society . London . Specia l Publications . 196, 169-182 . CEDERBOM, C. , LARSSON , S.E., TULLBORG , E.-L . & STIBERG, J.-P . 2000 . Fissio n trac k thermochronol ogy applie d t o Phanerozoi c thermotectoni c events

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LIDMAR-BERGSTROM, K . 1999 . Uplif t historie s revealed b y landform s o f th e Scandinavia n domes. In : SMITH , B.J. , WHALLEY , W.B . & WARKE, PA . (eds ) Uplift, Erosion an d Stability: Perspectives on Long-term Landscape Development. Geologica l Society , London , Specia l Publications, 162 , 85-91 . LIDMAR-BERGSTROM, K. , OLLIER , C.D. & SULEBAK , J.R. 2000 . Landforms and uplift history of southern Norway. Global an d Planetary Change, 24 , 211-231. LIDMAR-BERGSTROM, K. , OLSSON , S . & OLVMO , M . 1997. Palaeosurface s an d relate d saprolite s i n southern Fennoscandia . WIDDOWSON , M . (ed. ) Palaeosurfaces: Recognition, Reconstruction and Palaeoenvironmental Interpretation. Geologica l Society, London , Specia l Publications . In : vo l 120,95-123. LIDMAR-BERGSTROM, K., OLSSON, S. & ROALDSET, E . 1999. Relie f feature s an d palaeoweatherin g remnants i n formerl y glaciate d Scandinavia n baemen t areas. In : THIRY , M . & SiMON-CoiNgoN , R . (eds) Palaeoweathering, Palaeosurfaces and Related Continental Deposits. Internationa l Associatio n o f Sedimentologists, Specia l Publications , 27 , 275-301. LJUNGNER, E. 1950 . Urbergsytan s for m vid fjall randen. Geologiska Foreningens i Stockholm Forhandlingar, 72, 269-300. PEULVAST, J.P . 1978. Le bourrele t Scandinav e e t le s Caledonides: u n essa i d e reconstitutio n de s modalites d e l a morphogenes e e n Norvege . Geographic Physique Quaterniaire, 32, 295-320. PEULVAST, J.P . 1985 . Postorogeni c morphotectoni c evolution o f the Scandinavia n Caledonide s durin g the Mesozoi c an d Cenozoic . In : GEE , D.G . & STURT, B.A . (eds ) Th e Caledonide Orogen — Scandinavia an d Related Areas. Wiley, Chichester, 979-995. REUSCH, H . 1901 . Nogle bidra g til l forstaaelse n a f hvorledes Norge s dal e o g fjeld e e r blevn e til . Norges Geologiske Unders0gelse, Aarbog (1900), 32, 124-263. REUSCH, H . 1903 . Glommen s bojnin g ve d Kongs vinger. Norges Geografiske Selskab, Aarbog, 14 , 96-102. Rus, F . 1996 . Quantificatio n o f Cenozoi c vertica l movements o f Scandinavi a b y correlatio n o f morphological surface s wit h offshore data . Global and Planetary Change, 12 , 331-357. ROHRMAN, M. , VA N DER BEEK , P. , ANDRIESSEN , P .

& CLOETINGH , S . 1995 . Meso-Cenozoi c morphotectonic evolutio n o f souther n Norway : Neogene doma l uplif t infere d fro m apatit e fission trac k thermochronology . Tectonics, 14 , 704-718. RUDBERG, S . 1954 . Vdsterbottens berggrundsmorfologi. Geographica , 25 . RUDBERG, S . 1960 . Geology an d geomorphology . In : S0MME, A . (ed. ) A Geography o f Norden. J . W . Cappelens, Oslo , 27-40 . SCHIPULL, K . 1974 . Geomorphologische Studien i n zentral Siidnorwegen mit Beitrdgen uber Regelungs- und Stuerungssysteme in der Geo-

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morphologic. Hamburge r Geographische r Studien , 31. STURT, B. , DALLAND , A . & MITCHELL , J . 1979 . The ag e o f th e Sub-Mid-Jurassi c tropica l weathering profil e o f And0ya , norther n Norway , and th e implication s fo r th e Lat e Palaeozoi c palaeogeography i n th e Nort h Atlanti c region . Geologische Rundschau, 68 , 523-542 . SUMMERFIELD, M . 2000 . Geomorphology an d Global Tectonics. Wiley , Chichester . THOMAS, M.F . 1994 . Geomorphology in th e Tropics. Wiley, Chichester . VIVALLO, W. & BROMAN , C. 1993 . Genesi s of the earthy ore s a t Garpenberg , sout h central Sweden .

Geologiska Foreningens i Stockholm forhandlingar, 115 , 209-214. WICKMANN, F.E . 1988 . Possibl e impac t structure s in Sweden. In : BODEN , A . & ERIKSSON , K.G . (eds ) Deep Drilling i n Crystalline Bedrock. Springer , Berlin, 1 , 299-327. WRIK, W . 1908 . Bidra g til l Skandinavien s reliefkro nologi. Ymer,28, 141-191 . ZECK, H.P. , ANDRIESSEN , P.A.M. , HANSEN , K. , JENSEN, P.K. & RASMUSSEN, B.L . 1988 . Palaeozoi c palaeo-cover o f th e souther n par t o f th e Fenno scandian shield—fissio n trac k constraints . Tectonophysics, 149 , 61-66.

Pattern an d timing of the post-Caledonian denudatio n o f norther n Scandinavia constrained b y apatite fission-trac k thermochronolog y BART W . H. HENDRIKS & PAUL A . M. ANDRIESSEN Department of Isotope Geochemistry, Faculty of Earth Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HVAmsterdam, The Netherlands (e-mail: henb@ geo.vu.nl) Abstract: Apatit e fission-trac k thermochronolog y ha s bee n use d t o stud y th e post Caledonian denudatio n histor y o f norther n Scandinavia . Post-orogeni c denudatio n progressively shifte d fro m th e interior of the continent towards the North Atlantic margin . The present-da y are a o f maximu m elevatio n i n th e Northern Scande s mountai n rang e ha s experienced continuou s denudatio n a t leas t sinc e Jurassi c time . I n Jurassic-Cretaceou s time, the area north an d east of this region experienced either no denudation a t all or some denudation followed b y a transient thermal event wit h a peak temperature in late Cretaceous time. Final denudation of the area to the east of the Northern Scandes probably starte d in late Cretaceous-Paleogene tim e an d possibl y accelerate d i n Neogen e time . Th e denudatio n history o f northern Scandinavi a ca n b e explaine d by scar p retreat o f a n uplifte d rif t flank . The patter n an d timin g o f denudation o f the Northern Scande s is different fro m tha t o f th e Southern Scandes , whic h experience d domal-style, late-stag e postrif t uplif t i n Neogen e time. Geomorphologica l observations , offshor e dat a fro m th e Atlanti c an d Barent s Se a margins, and scarce stratigraphical information fro m th e mainland are in general agreement with th e new thermochronological data .

Because o f the almost complet e absenc e o f postCaledonian sediment s o n th e Scandinavia n mainland, denudatio n ('uplif t o f rock s relativ e to th e surface ' accordin g t o th e definitio n o f Summerfield & Brow n (1998) ) o f Scandinavi a has bee n studie d mainl y b y analysi s o f geomorphology (Lidmar-Bergstro m 1993 , 1999) an d it s correlatio n wit h th e offshor e geology (Rii s 1996) . Low-temperatur e thermo chronology ha s bee n applie d successfull y i n southern Scandinavi a (Lehtovaar a 1976 ; Andriessen & Bo s 1986 ; Zec k e t al. 1988 ; Rohrman 1995 ; Hansen et al. 1996 ; Larson et a l 1999; Cederbo m e t a l 2000) , bu t unti l now n o comparable stud y has been undertake n i n north ern Scandinavia . The geomorpholog y o f Scandinavi a ca n b e characterized a s a plateau wit h two large dome s in th e wes t an d nort h an d a smalle r dom e i n southern Swede n (Fig . 1) . Th e sout h Swedis h dome reaches 37 7 m above sea level and is partly covered b y Cambrian an d Mesozoic cove r rocks . The souther n Norwegian dome, usually referred to a s th e Souther n Scandes , reache s 2469 m above se a leve l o n Jotunheimen . Th e mor e elongated norther n Scandinavia n dome , usuall y referred to as the Northern Scandes an d the study area here, has a maximum elevation of 2113 m on Kebnekaise.

Riis (1996 ) an d Lidmar-Bergstro m (1999 ) both conclude d tha t th e denudatio n histor y o f the Souther n Scande s i s differen t fro m tha t o f the Norther n Scande s mountai n range . Accord ing t o both studies , th e lates t uplif t phas e i n th e north wa s earlie r (i n lat e Cretaceou s t o Paleogene time ) tha n i n th e sout h (i n Neogen e time). Thi s i s in agreemen t wit h apatit e fission track dat a fro m Rohrma n (1995 ) an d fro m th e present study . Accordin g t o Rohrma n (1995) , the centra l Souther n Scande s experience d Triassic-Jurassic erosio n o f 2. 4 ± l.lk m an d a Neogen e denudatio n o f 2. 0 ± 0. 5 km, decreasing radiall y outwar d t o les s tha n 0.5km nea r th e coastline . West of the Northern Scandes mountain range, the Lofoten and Vesteralen island groups (Fig. 2) are part o f a hors t an d grabe n syste m tha t i s structurally very complex and poorly understood. According t o Riis (1996) , final uplif t o f this are a occurred in Neogen e time , wit h a stron g Plio Pleistocene component. Invers e modelling o f the small amoun t o f apatit e fission-trac k dat a w e have fro m Lofote n an d Vesterale n a t present , indicates considerabl e Neogen e denudatio n a s well. A detaile d stud y applyin g apatit e fission track analysi s and (U-Th)/He thermochronometry t o thi s region , aimin g a t unravellin g th e denudation histor y o f th e variou s structura l

From: DORE , A.G., CARTWRIGHT , J.A, STOKER , M.S. , TURNER , J.P . & WHITE , N. 2002 . Exhumation o f th e North Atlantic Margin: Timing, Mechanisms and Implications for Petroleum Exploration. Geological Society , London, Special Publications, 196, 117-137 . 0305-8719/027$ 15.00 © The Geological Societ y of London 2002.

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Fig. 1 . Map o f Scandinavi a an d surroundin g areas , showin g th e locatio n o f th e stud y are a an d indicatin g the Southern Scandes , Norther n Scande s and Sout h Swedish Dome .

blocks an d datin g faul t movements , i s i n progress.

Geological settin g During th e Caledonia n orogen y th e Fennoscan dian shiel d wa s overthrus t fro m th e (north)wes t (Roberts & Ge e 1985) . A t present , severa l windows i n th e Caledonia n cove r rock s expos e parts o f th e Fennoscandia n shiel d t o th e wes t of the Caledonia n fron t o n Lofoten , Vesteralen , Senja an d t o th e S W an d S E o f Narvi k (Geological Survey s o f Finland , Norwa y an d Sweden 1987) . In th e immediat e post-orogeni c period , i n Devonian time , fault-bounde d molass e basin s developed i n th e precurso r Norwegian-Green land rif t syste m an d continenta l molass e continued t o accumulate i n half-grabens border ing the remnant Caledonian mountains until midPermian tim e (Dor e & Gag e 1987) . Crusta l extension i n th e Norwegian-Greenlan d rif t system accelerate d durin g lat e Permia n an d Triassic time and resulted in low-relief doming of the flankin g areas , particularl y durin g lat e Triassic tim e (Ziegle r 1987) . Jurassi c regiona l uplift o n th e norther n margins of Laurenti a and

Baltica an d mid-Jurassi c reactivatio n o f th e margins an d othe r sourc e area s withi n th e Atlantic Rif t domai n she d thic k coars e clasti c deposits ont o surroundin g area s (Dor e 1991) . Major riftin g affecte d th e entir e Norther n Atlantic domai n a t th e Jurassic-Cretaceou s transition (Faleide et al. 1993). In late Cretaceous time, th e Barent s Se a domai n wa s tectonicall y decoupled alon g th e Senja-Hornsun d faul t system fro m th e Atlanti c Rif t domai n wher e downwarping continue d (Dore 1991) . Following break-up in the Norwegian-Greenland Sea at the Paleocene-Eocene transitio n (Srivastav a & Tapscott 1986) , th e western Barent s Se a margi n developed as a shear margin (Faleide et al. 1993) . According t o Rii s (1996) , lat e Cretaceous early Tertiar y uplif t o f norther n an d wester n Fennoscandia cause d dee p erosion , an d Paleo gene uplif t reache d a maximum value of almos t 1500m i n norther n Scandinavia . Neogen e tectonic uplif t cause d domin g i n souther n Norway and was of the order of 1 -1.5 km in the area of highest topography (Rohrman 1995) . Lofoten and Vesterale n experience d tw o phase s o f Tertiary uplif t (Stuevol d & Eldhol m 1996) , with th e Neogen e uplif t componen t bein g o f the orde r o f 1000 m (Rii s 1996) .

DENUDATION HISTORY O F NORTHERN SCANDINAVIA

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Fig. 2 . Map of the study area showing topographic elevation , locations of samples and transects, an d other features referred to in this paper. Locations of faults and basins after Faleide et al. (1993), Olesen et al. (1997) and Brekke (2000).

Plio-Pleistocene glacia l erosio n cause d unloading an d uplif t o f Fennoscandi a an d modified th e olde r fluvia l drainag e syste m t o a considerable extent, but did not totally obliterat e the landform s tha t ar e typica l o f uplifte d landmasses alon g passiv e continenta l margin s (Lidmar-Bergstrom et al. 2000). Glacia l erosio n also affecte d Svalbar d (Elyth e & Kleinspeh n 1998) an d th e wester n Barent s Sea , whic h wa s subaerial i n preglacia l times . Tectoni c uplif t played an important role in this region before and possibly als o durin g th e glaciation s (Dimaki s et al . 1998) . Postglacia l crusta l domin g i s estimated t o hav e reache d a maximu m uplif t value o f 850 m i n th e centr e o f Fennoscandi a (Gudmundsson 1999) .

Apatite fission-track thermochronology Annealing concept Fission track s are damag e zone s in the crysta l lattice that are formed by the spontaneous fission of 238U (Wagner 1968; Fleischer et al 1975) . The density o f spontaneou s fissio n track s i s pro portional t o th e elapse d tim e an d th e uraniu m content. Except for rapidly cooled rocks, fissiontrack age s ar e systematicall y younger compared with other radiogenic age determinations. This is a result of the instability o f fission tracks at high temperatures. Within a mineral-specific temperature range , calle d th e partia l annealin g zon e (PAZ), th e track s begi n t o annea l until they are

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completely erase d a t th e uppe r temperatur e boundary. Fo r apatite, th e mos t commonl y use d mineral, th e PA Z i s usuall y define d a s a temperature rang e betwee n c . 6 0 °C an d c. 130° C (Gleado w & Dudd y 1981 ; Naese r 1981; Wagner & Van den Haute 1992) , but even at roo m temperatur e low-rat e annealin g take s place (Donelic k e t al 1990) . Th e temperatur e range o f th e PA Z wil l b e differen t fo r apatite s with differen t chemica l composition s (e.g . Carlson e t al . 1999) . Fo r extrem e composition s the uppe r boundar y o f th e PAZ , meanin g tota l annealing, ma y b e a s low a s 9 0 °C or a s high as 200 °C (Ketcha m e t a l 1999) . Fissio n track s formed i n th e PAZ , an d track s tha t hav e bee n heated int o th e PA Z afte r formatio n a t lowe r temperatures, hav e reduce d lengths , compare d with thei r initia l lengt h / 0 o f betwee n 15. 8 an d 16.6 |xm. Track lengt h reduction withi n the PAZ results i n a mea n lengt h shorte r tha n / 0 an d a negatively skewe d distributio n toward s th e smaller tracks . Th e annealin g characteristic s o f fission tracks thus allow the reconstruction of the thermal history of a sample from th e track length distribution (Gleado w e t a l 1986a , 1986b) . Reheating o f a sampl e b y a transien t therma l event ca n b e inferred , bu t i t i s difficult , o n th e basis o f fission-trac k dat a alone , t o differentiate this fro m a purel y coolin g history . Thi s i s because th e fission-trac k syste m i s essentiall y one tha t records cooling , an d eve n i n a thermal history that has included some reheating, most of the record wil l come fro m th e cooling segment s (Gleadow & Brown 2000). Analytical procedure After minera l separation , apatit e grain s wer e mounted i n epoxy , polishe d an d etche d i n 7 % HNO3 a t 2 0 °C t o revea l spontaneou s fissio n tracks. Etc h tim e varie d betwee n 2 5 an d 40s , depending o n the etching rat e o f the apatite . Fo r all sample s th e externa l detecto r metho d wa s applied usin g micas tha t were heate d fo r 4 8 h at 600 °C to eras e an y existin g tracks . Al l mounts were irradiate d togethe r wit h dosimete r glas s CN5 in the LFR facility of the ECN at Petten, the Netherlands. Micas coverin g th e apatite s wer e subsequently etche d i n 48 % H F a t 2 0 °C fo r 12 min. Micas coverin g th e dosimeter glass wer e etched i n 48 % H F a t 2 0 °C fo r 3 0 min. Th e counting o f fissio n track s i n th e apatite s an d external detector s an d th e measuremen t o f confined fission-trac k length s wer e performe d at 100 0 X magnificatio n usin g 10 0 X dr y objective wit h a numerica l apertur e o f 0.90 . Only apatites with sharp polishing scratches were used fo r counting and measuring.

Age and track length statistics The fission-track ages were calculated using the £ age calibration method (Hurfor d & Green 1983) . A Rvalu e o f 35 5 ± 1 5 (la) wa s obtaine d fro m Mt. Dromedary , Fis h Canyo n an d Durang o apatite standard s usin g CN 5 dosimete r glass . All sample s displaye d P(x*) > 5% , indicating that th e dat a ar e consisten t wit h a singl e population o f age s i n eac h sampl e (Galbrait h 1981). Therefor e al l fission-trac k age s i n thi s paper ar e reporte d a s poole d ages , wit h a l a error. Initia l fission-track length was determined on confine d induce d track s o f Fis h Canyo n apatite, givin g a mea n valu e of 16. 3 (Jim wit h a standard erro r o f 0. 1 fjim an d a l a valu e o f 0.49 [Jim . Inverse modelling of fission-track data Inverse modellin g o f th e ag e an d lengt h measurement result s wa s carrie d ou t usin g th e AFTSoLVE 1.2.l a progra m (Ketcha m e t a l 2000). Compare d wit h AFTSoLVE , man y other fission-track modellin g program s (Corriga n 1991; Lut z & Oma r 1991 ; Gallaghe r 1995 ) tend t o generat e simple r historie s becaus e the y implicitly o r explicitl y favou r paths wit h fairl y few degree s o f freedo m (Ketcha m e t a l 2000) . AFTSoLVE yields a much wider array of possible modelling results . Th e annealin g mode l o f Crowley e t a l (1991 ) fo r F-apatite s an d th e annealing model of Laslett et al (1987) , which is based o n laborator y studie s o f Durang o apatite , have been used for inverse modelling. Compared with th e annealin g mode l o f Crowle y e t a l (1991), th e annealin g mode l o f Laslet t e t a l (1987) i s les s sensitiv e t o low-temperatur e annealing an d overemphasize s th e importanc e of recen t coolin g (va n de r Bee k 1995) . Th e annealing model of Laslett et al (1987 ) therefore often reveal s therma l historie s wit h a rapi d lat e cooling event , whic h i n man y case s i s a modelling artefact . However , th e par t o f th e thermal histor y belo w c . 6 0 °C i s onl y loosel y constrained b y an y annealin g mode l an d on e should alway s b e ver y critica l abou t an y interpretations based on this part of the modelled thermal history . A t hig h temperature s th e annealing mode l o f Crowle y e t a l (1991 ) i s much more retentive than the annealing model of Laslett e t a l (1987) . Extrapolatio n o f th e annealing mode l o f Crowle y e t a l (1991 ) t o geological tim e scale s predict s F-apatit e t o b e more resistant to annealing than the more Cl-rich Durango apatite , i n contras t t o geologica l observations (Gallagher et al 1998) . The Laslett et al (1987 ) annealing model is the most widely

DENUDATION HISTOR Y O F NORTHERN SCANDINAVI A

used annealin g model , bu t th e Crowle y e t al. (1991) mode l fo r F-apatite s ha s bee n use d i n many fission-trac k studie s i n Scandinavi a (Rohrman 1995 ; Larso n e t al . 1999 ; Cederbo m et al. 2000). To compare ou r results with studies using eithe r th e Laslet t e t al . (1987 ) o r th e Crowley e t al. (1991 ) annealin g model, al l dat a have been modelled wit h both annealing models. In addition, i t is possible t o test the sensitivity of the therma l historie s obtaine d fro m invers e modelling t o the annealin g model applied . Modelling strategies for AFTSoLVE have been outlined b y Ketcha m e t al . (2000) . Initia l trac k length was fixed at 16.3 jjurn . No other constraints were used than a present-day surface temperature of 0-10 °C. For samples S21 and P6, which were collected i n th e LKA B Kirun a an d Malmberge t mines a t 765 m an d 815 m belo w th e surface , respectively, a present-da y temperatur e o f 10-25°C wa s used . Th e oute r envelop e o f th e thermal historie s presente d i n thi s pape r repre sents therma l historie s tha t giv e a valu e large r than 0.05 fo r both the age goodness-of-fit an d the Kolmogorov-Smirnov tes t fo r th e lengt h distribution. Thi s typ e o f history cannot be rule d ou t by th e dat a (Ketcha m e t al . 2000) . Th e inne r envelope represent s therma l historie s wit h a value larger tha n 0.5 for both th e ag e goodness of-fit an d th e Kolmogorov-Smirno v tes t fo r th e length distribution . Thi s typ e o f histor y i s supported b y th e data . Also , th e best-fittin g thermal histor y i s depicte d fo r al l model s presented i n thi s paper . Durin g th e firs t invers e modelling run s fo r eac h sample , wit h th e Crowley e t al . (1991 ) annealin g mode l a s wel l as with the Laslett et al. (1987) annealing model, AFTSoLVE wa s restricte d t o produc e purel y cooling historie s alone . I f modellin g wit h thi s restriction yielde d onl y therma l historie s tha t cannot b e rule d ou t b y th e data , o r very fe w thermal historie s tha t ar e supporte d b y th e data , many differen t tim e interval s o f reheatin g wer e tested fo r eac h sample . Specia l attentio n wa s given t o mak e sur e tha t th e boundarie s o f th e time interva l did not forc e the mode l resul t in a certain direction , whic h coul d exclud e possibl e thermal histories .

Sampling Most sample s ar e concentrate d i n tw o transect s through th e Norther n Scande s mountai n range . Transect A-A' (Fig. 2) runs from Andenes on the northernmost ti p o f th e Vesterale n island s towards th e Gul f o f Bothnia . Thi s transec t includes tw o vertica l profiles . On e i s o n Kebnekaise, rangin g fro m 57 5 to 1530 m abov e sea level. The four samples in this vertical profil e

121

are all within 8 km in the horizontal direction. No post-Caledonian o r neotectonic faul t activit y has been reporte d fo r thi s area . Th e secon d vertica l profile i n transect A-A 7 is in the LKAB mine in Kiruna. Th e thre e lowermos t sample s i n thi s profile, fro m —344 m t o 196 m abov e se a level, are almost perfectly vertically aligned insid e the mine itself . Th e to p tw o samples , a t 50 2 an d 712m above sea level, are about 3 km to the NE of th e min e o n a nearb y hill , Luossavaara . Neotectonic activit y ha s bee n reporte d fo r th e Kiruna are a (Dehl s e t al . 2000) , bu t agai n n o other post-Caledonia n faul t activit y ha s bee n reported fo r this area to our knowledge. Another subsurface sampl e i n this transect wa s collecte d inside the LKAB min e in Malmberget, a t 815m below th e surface , a t 191 m belo w se a level . Transect B-B 7 (Fig . 2 ) run s betwee n Troms0 , Norway, an d Muonio , Finland . Thi s transec t includes a vertica l profil e o n Tromsdalstinden , SE o f Troms0 , fro m se a leve l t o 1238 m elevation. Thi s vertica l profil e include s fiv e samples, an d except for the sampl e a t se a level , which is about 8 km to the west of the summit of Tromsdalstinden, al l sample s ar e n o mor e tha n 3 km from eac h other in the horizontal direction. Two SW-NE-trending faults of unknown age lie within 5-1 0 km t o th e S E an d N W o f Tromsdalstinden (Zwaa n e t al . 1998) . Th e on e to th e N W o f Tromsdalstinde n exhibit s neotec tonic activit y (Dehl s e t al . 2000) . However , w e do no t kno w o f an y post-Caledonia n fault s tha t would cut an d might distur b the vertical profile . Block rotatio n betwee n th e tw o faults, however , may hav e cause d rotatio n o f th e vertica l profil e through time . In addition to the samples of the two transects, many surfac e sample s wer e collecte d acros s th e study area . Som e o f thes e sample s wer e take n inside fjords, whic h can be up to 2km deep. The fjords ar e glacially overdeepened valley s and the original fluvial incision may be much older than the Plio-Pleistocen e glaciation s (Lidmar-Berg strom e t al . 2000) . Becaus e o f th e perturbatio n effect o f eroding topography on isotherms i n th e crust (Stiiw e e t al . 1994) , th e result s fro m samples that wer e collecte d inside fjord s canno t be immediatel y interprete d i n th e sam e wa y a s those fro m 'normal ' surfac e samples . Several sample s wer e collecte d especiall y t o investigate th e effect s o f a pronounced negativ e gravity anomal y centre d aroun d Sulitjelma , i n the S W o f th e stud y are a (Olese n e t al . 1997) . In contrast , th e regio n encompassin g R0st , Vaer0y an d Lofote n i s characterized b y a strong positive gravit y anomaly . Sample s hav e bee n collected fro m thi s region , bu t result s ar e no t available yet .

Table 1 . Fission-track results Mean Number trac k of lengt h grains (|xm )

SE S D MTL MT L ((Jim) (jxm )

Number of lengths Roc

13.1 11.6 13.3 12.3 12.9 13.7 12.9 12.8 13.0 13.5 12.9 13.0 12.8 13.0 13.4 13.3 12.6

0.2 1. 0.2 2. 0.2 1. 0.2 2. 0.2 0.2 0.3 0.3 0.2 0.3 0.3 0.3 0.2 0.1 0.1 0.2

2 .3 .6

86 175 51 101 100 58 23 36 100 20 23 17 127 109 139 100 100

Gneiss Gneiss Granite Schist Granite Granite Schist Schist Gneiss Dolerite Volcanite Volcanite Iron or e Iron or e Iron or e Gneiss Iron or e Granodiorite Granite

46 51 53 40 47 49 54 27

13.7 13.7 13.3 13.2 13.4 12.8 13.2 14.0

0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1

.0 .1 .0 .2 .3 .3 .3 .1

28 40 101 100 183 204 204 150

Gneiss Gneiss Gneiss Gneiss Gabbro Schist Gneiss Granite

32 34 34 53 34 41

13.8 _ 12.9 14.3 13.0 13.0

0.1 0. 0. 1 1 O.I 1. 0.1 1. 0. 1 1.

8

18 107 200 100 100

Granitoid Volcanite Granite Gneiss Phyllite Gneiss

Latitude (N)

Pooled F T Longitude ag e ± S E P()C) (E) (Ma ) (% )

PS (MJ (106 tracks cm^ 2 )

Pi (N\) 6

(10 tracks cm~ 2 )

tracks cm )

10 360 5 380 12 1530 1070 815 575 450 712 502 196 -29 -344 418 -191 430 40

69.31 69.28 68.56 68.52 68.42 67.93 67.88 67.87 67.84 67.99 67.88 67.87 67.84 67.84 67.84 67.65 67.18 67.13 66.31

16.08 16.01 16.44 17.89 17.78 18.54 18.54 18.57 18.75 19.93 20.23 20.21 20.18 20.18 20.18 21.00 20.67 20.57 22.82

128 ± 1 4 142 ± 1 4 180 ± 2 1 134 ± 1 6 124 ± 1 2 220 ± 2 5 205 ± 2 4 157 ± 2 1 113 ± 1 4 243 ± 2 3 303 ± 3 6 268 ± 2 9 225 ± 2 2 251 ± 3 0 212 ± 2 4 322 ± 3 6 264 ± 2 5 318 ± 2 9 268 ± 2 3

23 83 28 76 9 30 36 91 30 32 26 94 80 100 45 100 17 8 79

0.368 (396) 0.767(553) 0.330(180) 0.393 (240) 0.802(615) 0.339(395) 0.133(349) 0.155(182) 0.263 (207) 1.937(1375) 0.773 (404) 0.346(508) 1.019(784) 0.270(359) 0.503 (428) 0.627 (536) 1.570(1449) 3.196(2220) 0.847(508)

0.491(528) 0.924(666) 0.308(168) 0.501 (306) 1.104(846) 0.271(316) 0.110(288) 0.167(196) 0.396(312) 1.352(960) 0.427 (223) 0.227(333) 0.763 (587) 0.176(234) 0.400 (340) 0.317(271) 1.045(965) 1.695(1177) 0.510(306)

0.957(12033) 0.957(12033) 0.957(12033) 0.957(12033) 0.957(12033) 0.994(12054) 0.951(11870) 0.951(11870) 0.951 (11870) 0.957(12033) 0.951(11870) 0.994(12054) 0.951(11870) 0.927(14206) 0.951(11870) 0.927(14206) 0.994(12054) 0.957(12033) 0.927(12904)

30 30 25 22 28 30 36 22 21 55 22 52 26 37 23 28 31 33 20

Transect B-B1 1238 Tl 753 T2 530 T3 262 T4 5 T9 15 Fl 465 F51 F48 300

69.61 69.61 69.60 69.61 69.63 69.26 68.84 68.35

19.15 19.11 19.11 19.07 18.95 19.92 21.17 22.90

230 ± 241 ± 189 ± 203 ± 216 ± 170 ± 247 ± 365 ±

26 26 1 9 20 22 1 5 22 34

84 12 81 75 96 60 53 24

0.216(442) 0.139(522) 0.492(783) 0.714(864) 0.273(655) 2.509(3235) 0.667(2408) 1.416(1956)

0.159(324) 0.097 (364) 0.460(732) 0.622(753) 0.213(511) 2.492(3213) 0.454(1639) 0.647(894)

0.951(11870) 0.951(11870) 0.994(12054) 0.994(12054) 0.951(11870) 0.951(11870) 0.951(11870) 0.95 1 ( 11 870)

Other F6 Fll F16 F28 F34 F37

69.79 70.03 70.7 1 69.47 70.86 70.01

20.94 23.07 24.59 25.85 29.11 29.17

214 ± 2 4 268 ± 3 6 205 ± 1 9 297 ± 2 6 269 ± 2 7 306 ± 2 8

50 98 61 15 33 84

0.478 (384) 0.35 1 (229) 0.573(1617) 1.111(3415) 1 . 1 7 1 (774) 1.044(2418)

0.393(316) 0.230(150) 0.472(1332) 0.628(1930) 0.766(506) 0.597(1382)

0.994 ( 1 2054) 0.994(12054) 0.95 1 ( 11 870) 0.951 (11870) 0.994(12054) 0.994(12054)

Sample name

Elevation (m a.s.l. )

Transect A N33 N34 N39 N2 N5 K9613 Kl K2 K3 S13 S23 S24 LI S21 L2 S16 P6 S18 GRM3

-A '

1 40 5 140 10 15

2

0.2 :

8 2 1 0 .6 .2 .3 .9 .7 .4 .7 .0 .8

.4 2 1 1

k type

DENUDATION HISTORY OF NORTHERN SCANDINAVI A

123

Results Apatite fission-track analysis has been performed on 4 3 samples . Despit e th e generall y lo w uranium concentratio n i n th e samples , reflecte d by low pi values (Table 1) , counting statistics fo r all sample s ar e a t a n acceptabl e level . Al l samples displaye d P(x 1) > 5% , indicatin g tha t the data are consistent with a single population of ages i n eac h sampl e (Galbrait h 1981) . Th e standard erro r o f th e poole d ag e wa s generall y lower tha n 10 % and neve r mor e tha n 13% , an d for al l sample s mor e tha n 2 0 grain s coul d b e counted. Th e lo w uraniu m concentratio n did , however, mak e i t difficul t t o obtai n enoug h length measurement s o f horizonta l confine d tracks for severa l samples . One hundre d length measurements are considered necessary to obtain the statisticall y reliabl e lengt h distributio n needed fo r confidenc e in th e invers e modellin g of th e fission-trac k dat a (Gris t & Ravenhurs t 1992). Si x sample s containe d onl y betwee n 5 0 and 10 0 confined track s (N33 , N39 , Kl , N13 , N15 and N49) but nevertheless inverse modelling was als o undertake n fo r thes e samples . Th e results ar e ver y simila r t o thos e fro m sample s nearby tha t containe d 10 0 o r mor e confine d tracks. Therefor e thes e model s ar e include d i n this paper , bu t shoul d b e considere d supportin g evidence for the nearby samples and conclusions should no t b e base d o n thes e sample s individu ally. Fo r sample s tha t containe d fewe r tha n 5 0 confined tracks , onl y the mea n trac k length can be use d becaus e invers e modellin g woul d result in larg e time-temperatur e field s rathe r tha n time-temperature paths . Sample s tha t d o no t have any track length information in Table 1 did not contain enough apatite to prepare a mount for the length measurements . The oldes t fission-trac k age s (>250Ma ) ar e from sample s mos t distant from th e Atlantic and southwestern Barent s Se a margins . Th e fission track ages in transect A-Ar range from 11 3 ± 1 4 to 32 2 ±36Ma (Fig . 3a) . I n transec t B-B ' (Fig. 3b ) th e fission-trac k age s rang e fro m 170 ± 1 5 t o 36 5 ± 3 4 Ma. Th e along-transec t variation o f th e fission-trac k age s i n transec t A-A/ an d transec t B-B 7 clearl y indicate s a decrease o f the fission-trac k age s from S E to NW. Closer t o bot h margin s th e fission-trac k age s decrease, bu t mor e s o towards th e Atlanti c margin than towards the SW Barents Sea margin. As a resul t o f complicate d therma l histories , reflected b y comple x confine d trac k lengt h distributions, ther e i s n o obviou s patter n fo r th e mean track length in the stud y area. The highest mean trac k length s wer e obtaine d fro m sample s F28 an d F4 8 (14. 3 an d 14. 0 |xm, respectively )

124

B. W. H. HENDRIK S & P. A. M . ANDRIESSE N

Fig. 3. (a) Fission-track ages for samples in transect A-A 7 , (b) Fission-track ages for samples in transect B—B' .

and these two samples ar e also among th e oldes t samples in the stud y area . Figure 4 plot s fission-trac k age s v . elevatio n for th e vertica l profile s o f Tromsdalstinde n (Fig. 4a) , Kebnekais e (Fig . 4b) an d Kirun a (Fig. 4c) . All fission-trac k age s i n Fig . 4 ar e displayed wit h 2c r errors . Ther e i s n o obviou s trend fo r the fission-trac k age s v . elevation i n the vertical profil e o f Tromsdalstinde n (Fig . 4a) . Near-invariant apatit e fission-trac k age s ove r elevation ranges of 1 -2 k m have been interprete d as th e resul t o f ver y hig h erosio n rates , bu t ca n also resul t fro m coolin g o f th e footwal l durin g normal faultin g (Gallagher e t al. 1998) . Becaus e the ag e o f mos t brittl e fault s i n th e are a o f Tromsdalstinden i s unknow n (Zwaa n e t al . 1998), i t i s difficul t t o determin e wha t mechan ism i s responsibl e fo r th e near-invarian t fission track age s o f th e Tromsdalstinde n vertica l profile. Th e fission-trac k ag e v . elevatio n plot s

for th e vertica l profile s o f Kebnekais e (Fig . 4b ) and Kirun a (Fig . 4c) sho w a n increas e o f th e apatite fission-trac k age s wit h highe r elevation . Unfortunately, man y o f th e sample s fro m th e vertical profile s of Tromsdalstinden, Kebnekais e and Kiruna yielded only small amounts of apatite that generally also were of poor quality and had a low concentratio n o f uraniu m (reflecte d by lo w values fo r p { i n Tabl e 1) . Therefor e th e trac k length informatio n fro m thes e sample s i s ver y limited, whic h severel y reduce s th e amoun t o f information tha t ca n b e extracte d fro m th e vertical profiles , suc h a s a n estimat e o f th e (palaeo)geothermal gradient . Interpretation of fission-trac k dat a and inverse modelling Thermal historie s obtaine d fro m invers e modelling ar e non-uniqu e an d th e uncertainties

Fig. 4 . Fission-track ages (2cr error) v. elevation for the vertical profile s o f (a) Tromsdalstinden , (b ) Kebnekais e and (c ) Kiruna .

DENUDATION HISTOR Y O F NORTHERN SCANDINAVI A

in the solution s for individual samples ar e large. However, th e combinatio n o f modelle d therma l histories o f samples fro m differen t location s an d different elevation s make s i t possibl e t o recon struct a regiona l denudatio n histor y wit h confidence. Becaus e th e annealin g model s use d for invers e modellin g o f apatit e fission-trac k data, i n thi s stud y th e Laslet t e t al (1987 ) annealing mode l an d th e Crowle y e t al . (1991 ) annealing model , d o no t accuratel y mimi c th e behaviour of fission tracks at temperatures below 60 °C, this part of the modelled therma l history is not considered i n the interpretation . Post-orogenic cooling Inverse modellin g o f sample s fro m transect s A-A' and B-B/ indicates post-orogeni c cooling progressively shiftin g toward s th e wes t (Fig s 5 and 6) . Thermal historie s fro m th e Laslett e t al. (1987) annealin g mode l (Fig s 5 b an d 6b ) ten d toward slightl y lowe r temperature s an d indicat e cooling t o commenc e somewha t earlie r tha n i n the therma l historie s fro m th e Crowle y e t al . (1991) annealing model (Fig s 5a and 6a). Taking into account , fro m bot h annealin g models , th e thermal histories that are supported by the data, it is clea r tha t the easternmos t sample s in bot h transects sho w coolin g fro m Devonia n tim e onwards. The westernmost samples di d not coo l to temperatures within the PAZ before Triassicearly Jurassi c time . Thi s Triassic-earl y Jurassic timing probabl y i s relate d t o domin g o f th e flanking area s o f the Norwegian-Greenland rif t system (Ziegle r 1987) . Th e hig h value s fo r th e mean track length of samples F28 and F48 (14. 3 and 14.0jjim , respectively ) reflec t tha t the y cooled rapidly through the PAZ in Carboniferous and Devonia n time , respectively , an d thereafte r have no t bee n reheate d int o th e PAZ . Thi s indicates rapi d post-orogeni c downwearin g o f the Caledonides , followe d b y a lon g perio d o f relative tectoni c stabilit y o f th e interio r o f th e continent. Jurassic-Cretaceous denudation of the Northern Scandes Samples fro m hig h elevation s i n th e Norther n Scandes mountai n range (K9613 , Kl ) recorde d continuous coolin g i n Jurassi c an d Cretaceou s times (Fig. 5). The Laslett et al. (1987) annealin g model (Fig . 5b ) agai n tend s toward s lowe r temperatures tha n th e Crowle y e t al . (1991 ) annealing mode l (Fig . 5a) , bu t bot h annealin g models indicat e a ver y simila r purel y coolin g trend. Although sample Kl i s from a lower level

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in the vertical profile on Kebnekaise than sample K9613, the thermal history solutions presented in Fig. 5 show that the temperature of Kl ha s been higher than that of sample K961 3 for most of its thermal history . Thi s does not make much sense , and shows that the modelled therma l histories of samples from which only such a small number of track length measurements could be obtained (58 confined track lengths for Kl), have to be treated with caution . Beside s sample s K l an d K9613 , samples N5 , N48 , N4 9 an d N6 0 als o recorde d cooling i n Cretaceous tim e (Fig s 5 and 7). They probably experience d Jurassi c coolin g too , bu t except for sampl e N60 , i n that period the y were still at temperatures too high to be constrained by apatite fission-trac k thermochronology . Whe n samples N48 , N49 and N60 were modelled wit h the Crowle y e t al . (1991 ) annealin g mode l an d only coolin g wa s allowed , fe w thermal historie s that ar e supporte d b y th e dat a wer e foun d fo r each o f thes e samples . Withou t thi s restriction , the mode l indicate d risin g temperature s fo r lat e Cretaceous-Paleogene tim e fo r thes e sample s (Fig. 7 a). With the Laslett et al. (1987) annealin g model, however , man y purel y coolin g historie s supported b y th e dat a wer e foun d (Fig . 7b) . Although the annealin g models do not agre e on the lat e Cretaceous-Paleogen e temperatur e history, bot h predic t coolin g durin g mos t o f Cretaceous time for samples N48, N49 and N60. For man y o f th e sample s t o th e eas t o f th e Northern Scandes , invers e modellin g run s wit h cooling only , usin g th e Crowle y e t al . (1991 ) annealing mode l a s wel l a s th e Laslet t e t al . (1987) annealin g model, generall y resulted only in therma l historie s tha t canno t b e rule d ou t b y the data . Ver y fe w therma l historie s tha t ar e supported by th e data were obtained in this way and fo r mos t o f thes e sample s non e a t all . Therefore reheatin g wa s allowe d durin g late r runs. However , i t i s importan t t o kee p i n min d that th e fission-trac k syste m i s essentiall y on e that record s cooling , an d tha t i t i s difficul t t o differentiate a thermal histor y tha t ha s included some reheatin g fro m a purel y coolin g histor y (Gleadow & Brown 2000) . Thermal historie s fro m th e Crowle y e t al . (1991) annealin g mode l indicat e tha t samples t o the eas t o f th e Norther n Scande s (GRM3 , S16 , P6, S21 and S13 in Fig. 5a ; F51 in Fig. 6a ; P2 in Fig. 7a) , ma y hav e experience d heatin g durin g Jurassic-Cretaceous time . Also , mos t o f thes e samples probably have experienced temperature s lower tha n 6 0 °C i n thi s period . Fo r sample s GRM3 an d SI3, th e Laslett e t al (1987 ) mode l also show s Jurassic-Cretaceou s reheating, wit h a lat e Cretaceou s pea k temperatur e somewha t higher tha n 6 0 °C (Fig . 5b) . Thi s mode l als o

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Fig. 5 . Transec t A-A 7. Result s o f invers e modellin g o f fission-trac k dat a wit h (a ) th e Crowle y e t al. (1991 ) annealing mode l an d (b ) th e Laslet t e t al . (1987 ) annealin g model , togethe r wit h resulting fit for trac k lengt h distributions. Dashed lin e in diagram of thermal histories defines envelope fo r thermal histories with acceptable fit; grey field shows envelope fo r thermal histories wit h good fit; continuous line inside grey field indicates bestfitting therma l history . Diagra m fo r trac k lengt h distributio n show s histogra m o f trac k lengt h measurement s (dotted line) and resulting fit from the best-fitting thermal history (continuous line). AGE, pooled fission-track age with 1 crerror; MTL, mean track length (in |xm); SD, standard deviation of mean track length (in jjim); N, number of track length measurements; F, fault. Frequency : relative frequency o f track lengths in 1 jxm bins. Locations of samples have been projected ont o transect line.

DENUDATION HISTORY OF NORTHERN SCANDINAVIA 125

Fig. 5. continued

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Fig. 6 . Transec t B-B'. Result s o f invers e modellin g of fission-trac k dat a with (a ) th e Crowle y e t al . (1991 ) annealing mode l and (b ) th e Laslet t et al . (1987 ) annealin g model , together wit h resultin g fi t fo r trac k lengt h distributions. (For other details, see caption of Fig. 5.)

indicates tha t all other sample s t o the east of the Northern Scande s wer e belo w 6 0 °C during th e last 15 0 Ma. Thus together, the annealing models indicate that , durin g Jurassic-Cretaceou s time , most o f the sample s t o th e eas t o f th e Norther n Scandes experience d temperature s o f aroun d 60 °C or less. Sample s GRM 3 and S1 3 probably were still inside the PAZ in late Cretaceous time . This mean s they may have experienced temperatures belo w 6 0 °C durin g Jurassic-Cretaceou s time an d a late Cretaceous therma l even t wit h a peak temperature somewhat higher than 60 °C, or that the y experience d a mor e o r les s stabl e temperature als o somewha t highe r tha n 6 0 °C during al l of Jurassic-Cretaceous time.

Also fo r sample s T3 , T4 , T 9 an d F l (Fig . 6 ) and fo r sample s F1 6 an d F3 7 (Fig . 7) , th e Crowley et al. (1991) and the Laslett etal. (1987) annealing model s indicat e that during Jurassic Cretaceous tim e the y wer e a t temperature s o f around 60 °C or less. For these samples, again, it was almos t impossible , wit h bot h annealin g models, t o obtai n purel y coolin g therma l histories tha t ar e supporte d b y th e data . Fo r samples F34 and F40, i n the northeastern corner of th e stud y area , i t wa s possibl e t o obtai n thermal historie s tha t ar e supporte d b y th e dat a with th e Crowle y e t al (1991 ) annealin g model when only cooling wa s allowed (Fig. 7a). But for these sample s i t wa s necessar y t o allo w fo r

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Fig. 6. continued

reheating t o obtai n therma l historie s tha t ar e supported b y th e dat a wit h th e Laslet t e t al. (1987) annealing model (Fig . 7b). However, both annealing model s indicat e tha t samples F3 4 and F40 experience d a temperatur e o f aroun d 6 0 °C or less , wit h mayb e som e coolin g o r reheating , during Jurassic-Cretaceou s time . Fo r sample s SE4 an d N13 , also , i t wa s difficul t t o obtai n thermal historie s tha t are supporte d by the dat a when onl y coolin g wa s allowed . Modelle d thermal historie s o f sample s SE 4 an d N1 3 (Fig. 7) , t o th e S W o f Troms0 , ar e simila r t o those o f sample s fro m th e vertica l profil e o n Tromsdalstinden (Fig . 6). The main difference i s that sample s SE 4 an d N1 3 generall y hav e a temperature somewha t highe r tha n tha t o f samples T3 , T 4 an d T9 . Fo r sample s SE 4 an d

N13 bot h annealin g model s indicat e tempera tures withi n th e PA Z i n lat e Cretaceous Paleogene time . Thi s mean s tha t sample s SE 4 and N1 3 experienced eithe r temperature s belo w 60 °C during Jurassic-Cretaceous time and a late Cretaceous-Paleogene therma l even t wit h a peak temperatur e insid e th e PAZ , o r a mor e o r less stabl e temperatur e als o withi n th e PA Z during al l of Jurassic-Cretaceous time. Samples fro m hig h elevation s in the Norther n Scandes mountai n rang e (K l an d K9613 ) an d samples from th e Atlantic margin (N5, N48, N49 and N60) , recorde d Jurassic-Cretaceou s cool ing. Thi s coul d b e th e resul t o f lowerin g o f th e geothermal gradient . Unfortunately , w e canno t obtain an y informatio n o n th e geotherma l gradient directl y fro m ou r fission-trac k data .

Fig. 7. Samples outside transects. Rresult ofinverse modelling of fission-track data with (a) the Crowley et al. (1991) annealing model and (b) the Laslett et al. (1987)

annealing model. togeter with resulting fit for track length distributions. (For other details, see caption of Fig. 5.)

Fig. 7. continued

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There is extensio n an d crusta l thinnin g offshor e (Brekke 2000) , bu t ther e i s n o indicatio n o f extension i n th e are a wher e thes e sample s wer e collected, an d therefor e on e woul d no t expec t a significant lowerin g o f th e geother m t o occur . This mean s i t i s unlikel y tha t lowerin g o f th e geotherm is the cause o f the observed cooling of samples Kl , K9613 , N5 , N48 , N49 an d N60. Generally, subsurfac e therma l effect s o f exten sion ar e largel y restricte d t o th e regio n under going extension , crusta l thinnin g and subsidenc e (Gallagher e t al. 1998) . Awa y fro m thi s region , the low-temperatur e therma l histor y o f rock s i s primarily controlle d b y denudation (Gallagher & Brown 1997 ; Summerfield & Brow n 1998) . I f cooling i s primaril y th e resul t o f denudation , i t follows tha t th e present-da y are a o f maximu m elevation an d par t o f th e Atlanti c margi n experienced Jurassic-Cretaceou s denudation . Most o f th e sample s fro m th e res t o f th e mainland wer e around o r below 6 0 °C during this period, and, at least in late Cretaceous time , som e probably experience d temperature s withi n th e PAZ. Thi s mean s tha t i n Jurassic-Cretaceou s time th e rest o f the mainlan d di d not experienc e denudation, or that following some denudation, it was affecte d b y a transien t therma l even t tha t reached it s pea k temperatur e i n lat e Cretaceou s time. A possible transien t thermal even t could be the deposition o f a sedimentary cover . This cover subsequently mus t hav e bee n removed , becaus e there ar e n o Jurassic-Cretaceou s sediment s o n the northern Scandinavia n mainland at present. A small amoun t o f Jurassic-Cretaceou s sedimen t is preserved, however , on And0ya, in the north of the Vesterale n island grou p (Dallan d 1980) . The amoun t o f denudatio n o f th e present-da y area o f maximu m elevatio n ca n b e calculate d from th e thermal histor y o f sampl e K9613 . Thi s sample staye d withi n th e PA Z fo r almos t th e entire Jurassic-Cretaceou s period , an d th e thermal historie s fro m th e Crowle y e t al . (1991) annealin g mode l an d th e Laslet t e t al . (1987) annealin g mode l ar e very simila r fo r this sample. Fo r samples N5 , N48, N4 9 an d N60 the differences betwee n th e therma l historie s fro m the Crowle y e t a l (1991 ) annealin g mode l an d the Laslet t e t al . (1987 ) annealin g mode l ar e significant. Fo r sampl e K l onl y 5 8 containe d track length s coul d b e measured . Therefor e th e amount o f denudatio n ha s no t bee n calculate d from thes e fiv e samples . Accordin g t o th e best fitting therma l histor y fro m th e Laslet t e t al . (1987) annealin g model , sampl e K961 3 experi enced 20-2 5 °C of Jurassic-Cretaceous coolin g (Fig. 5b) . The best-fittin g therma l histor y fro m the Crowle y e t al . (1991 ) annealin g mode l indicates c . 3 0 °C o f Jurassic-Cretaceou s

cooling (Fig . 5a). To calculat e th e amoun t o f denudation, the (palaeo)geotherma l gradien t ha s to b e known . Becaus e w e canno t calculat e th e (palaeo)geotherm fro m ou r vertica l profiles , w e used th e Mesozoi c geother m o f 2 8 ± 8 °C that Rohrman (1995 ) calculate d fro m fission-trac k data fro m a vertica l profil e o n Jotunheimen , southern Norway . Usin g thi s geother m t o calculate th e amoun t o f denudatio n o f th e present-day are a o f maximu m elevatio n i n ou r study area , w e arriv e a t a n estimat e o f 0.6-1.3 km wit h th e Laslet t e t al . (1987 ) annealing model , an d 0.8-1. 5 km wit h th e Crowley e t al . (1991 ) annealin g model . Takin g into accoun t the uncertaintie s from bot h annealing models, th e Jurassic-Cretaceous denudation of the present-day are a o f maximum elevation is estimated t o be 1 ±0.5 km. Tertiary denudation To obtai n detaile d informatio n abou t th e post Cretaceous therma l histor y o f ou r sample s fro m inverse modelling , th e time-ste p siz e fo r th e Cenozoic perio d wa s alway s kep t smal l compared wit h that for th e olde r par t o f th e therma l history. This wa s done by increasing the number of nodal points for this time interval. In this way, there ar e fewe r restrictions in term s o f wher e i n time a lat e coolin g even t ca n occu r (Ketcha m et al. 2000) . Inverse modellin g wit h th e Crowle y e t al . (1991) annealing model indicates that by the time of th e Cretaceous-Tertiar y transition , al l samples t o th e eas t o f th e Norther n Scande s were coolin g insid e th e PA Z o r wer e alread y below 60°C (GRM3 , SI8 , SI6, P6, S21 and S13 in Fig. 5a; F48 and F51 in Fig. 6a; P2 in Fig. 7a). The Laslet t e t al . (1987 ) mode l indicate s tha t only samples S13 and GRM3 were still inside the PAZ a t tha t time , an d tha t the y wer e coolin g (Fig. 5b) . Becaus e th e Laslet t e t al . (1987 ) annealing mode l i s renowne d fo r producin g anomalous lat e coolin g event s (va n der Bee k 1995), man y scenario s for the Cenozoi c therma l history o f eac h sampl e hav e bee n tested . I t wa s found tha t because, according t o this model, only samples S13 and GRM3 were still inside the PAZ at th e en d o f Cretaceou s time , thi s mattere d fo r the interpretatio n o f thes e tw o sample s only . When th e time-ste p siz e wa s decrease d onl y fo r Neogene tim e an d no t fo r Paleogen e time , th e Laslett et al. (1987) annealing model found many thermal historie s tha t ar e supporte d b y th e dat a that indicat e a Neogen e onse t fo r th e fina l cooling of samples S13 and GRM3. But when the time-step siz e was decrease d fo r all of Cenozoi c time, th e mode l n o longe r indicate d a Neogen e

DENUDATION HISTOR Y O F NORTHERN SCANDINAVI A

onset for the final cooling of the two samples, but tended more towards a Paleogene onset . Clearly , with th e Laslet t e t al (1987 ) annealin g model , scenarios with a Paleogene or a Neogene onset of the fina l coolin g phas e bot h produc e therma l histories that are supported by the data. However, one would certainly expec t that the Laslett et al (1987) annealin g mode l woul d indicat e a Neogene onse t o f th e fina l coolin g phas e fo r a sample that really experienced Neogen e cooling, even whe n the time-ste p siz e wa s decrease d fo r all o f Cenozoi c time . Thi s i s no t th e case , an d therefore i t i s considere d likel y tha t th e fina l cooling of samples S13 and GRM3 started before Neogene time . Thi s the n means tha t denudation of th e are a t o th e eas t o f th e Norther n Scande s probably resumed in late Cretaceous-Paleogene time. Thi s leave s ope n th e possibilit y o f a Neogene accelerate d cooling . For mos t o f th e sample s clos e t o the Atlantic margin (N 5 in Fig. 5; N48, N49, N13 and SE4 in Fig. 7 ) a Neogen e onse t fo r th e fina l coolin g phase i s predicte d b y bot h annealin g models , although th e Crowle y e t al . (1991 ) mode l als o allows for an earlier cooling of samples N13 and SE4. For sample N60 (Fig. 7a) the Crowley et al. (1991) mode l als o predict s a Neogen e onse t of cooling, wherea s th e Laslett e t al. (1987 ) mode l indicates a sub-PAZ temperature for this sampl e already i n Paleogene time . For sample s clos e t o the Barent s Se a margi n (Fl , T3 , T 4 an d T 9 i n Fig. 6 ; F16 , F34, F37 an d F4 0 i n Fig . 7 ) th e Laslett e t a l (1987 ) mode l indicate s tha t thes e samples wer e belo w 6 0 °C already i n Paleogen e time, o r tha t thei r fina l coolin g ou t o f th e PA Z started i n Neogen e time . Th e Crowle y e t al . (1991) annealing model predicts a Neogene onset of cooling for samples Fl (Fig . 6a ) and F34, F37 and F40 (Fig. 7a) as well. For samples T3, T4 and T9 (Fig. 6a) the Crowley et al. (1991 ) annealing model seem s t o indicat e a Paleogen e onse t o f cooling, bu t a Neogen e onse t o f coolin g i s certainly possibl e too , within th e limit s se t b y thermal historie s tha t are supported b y the data. Therefore fo r sample s o n the Atlanti c margin as well as on the Barents Sea margin, the Laslett et al. (1987) annealing model predicts a Neogene onset of the final cooling phase, or indicates that they wer e alread y belo w 6 0 °C. Th e Crowle y et al . (1991 ) annealin g mode l indicate s a Neogene phas e o f fina l coolin g fo r th e Atlanti c margin and for the part of the Barents Sea margin in th e northeaster n corne r o f th e stud y are a a s well. Although it seem s t o indicate a Paleogen e onset o f coolin g fo r th e par t o f th e mainlan d adjacent to wher e the Atlanti c and Barent s Sea margins com e together , i t certainl y doe s no t exclude the possibility of a Neogene onset of the

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final cooling phas e for this area. Assuming onc e again tha t coolin g i s th e resul t o f denudatio n (Summerfield & Brown 1998), it follows that the Atlantic an d th e Barent s Se a margin s probabl y experienced significan t Neogen e denudation . In th e par t o f th e mainlan d adjacen t t o wher e the Atlanti c an d Barent s Se a margin s meet , denudation ma y hav e starte d earlier . It should be noted that almost all samples from the margins are taken inside fjords, whic h may be incised a s deep a s 2 km. Fjords are probably not simply glacial features, but remnants of an older fluvial drainage system . Consecutiv e glaciation s preferentially exploite d th e pre-existing valley s (Lidmar-Bergstrom et al. 2000). It is not immediately clea r wha t th e coasta l sample s actuall y record: regiona l denudation , o r rapi d fluvia l incision an d glacia l erosio n tha t coul d locall y disturb the thermal structure of the crus t (Stiiwe etal 1994) . Lofoten and Vesteralen Samples from th e Lofoten and Vesteralen islands have thermal historie s ver y differen t fro m thos e of most other samples (Figs 5 and 7). Structurally this are a is ver y complex , wit h man y undate d faults an d probably many more brittle structure s than are currently known. These islands therefore are no w bein g studie d i n a separat e project , which aim s a t resolvin g th e timin g o f faultin g and denudation of the various structural blocks in this area with fission-track analysis and (U-Th)/ He thermochronometry . However , th e fe w available fission-trac k dat a (sample s N15 , N23, N33, N3 4 an d N39 ) sugges t tha t thi s are a experienced considerabl e Neogen e denudation , supporting the observations of Riis (1996) . Pattern of denudation Samples i n th e northeaster n corne r o f th e stud y area, close to the southern margin of the Barent s Sea, hav e fission-trac k age s i n th e rang e o f 200-300 Ma. These ages correspond t o those of samples fro m Lapland , eas t o f th e Norther n Scandes range, rather tha n to ages fro m sample s close t o th e Atlanti c margin , whic h ar e o f th e order of 90-150 Ma. The inverse models and the pattern of fission-trac k age s (Fig. 3) indicate that the greatest Mesozoi c and Cenozoic coolin g was experienced b y the part of the stud y area closes t to th e Atlanti c margin . I t i s eviden t tha t th e denudation o f th e Norther n Scande s mountai n range did not produce a domal pattern simila r to the one recognized i n the topography an d apatite fission-track dat a o f th e Souther n Scande s (Rohrman 1995) . Instead , th e Atlanti c margi n

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has experience d muc h mor e Mesozoi c an d Cenozoic denudatio n tha n th e present-da y are a of maximu m elevation , whic h in tur n was mor e affected tha n the interio r o f the continent .

erosion playe d in this region by removing earlie r Tertiary deposits , bu t make s i t ver y difficul t t o study the correlation wit h the Tertiary geologica l evolution onshore .

Correlation with offshore geology

Discussion

Because provenanc e i s unclea r fo r mos t o f th e sediments in basins surrounding the study area, it is difficul t t o stud y the relationshi p betwee n th e evolution o f th e onshor e par t o f th e stud y are a and th e adjacen t offshor e area . Th e comple x structural evolutio n o f th e offshor e area s als o obscures th e direc t connectio n betwee n on - and offshore. A furthe r complicatin g facto r i s th e limited time-resolutio n o f th e invers e model s from apatit e fission-trac k data , compare d wit h the detaile d stratigraphica l informatio n tha t i s available for mos t basins . However , despit e all this, ther e is a clea r lin k in spac e and tim e between th e off - an d onshor e geologica l evolution. Rapi d Triassic-Jurassi c denudatio n of samples in the Troms0 regio n (T3, T4, T9 and Fl) wa s contemporaneou s wit h depositio n o f Triassic-Jurassic sequences (Faleid e et al 1993 ; Reemst 1995 ) i n th e nearb y Troms 0 an d Hammerfest basin s (Fig . 2) . Th e Cretaceou s sequences i n these basins covered a t least part of the Finnmar k platfor m as wel l (Faleid e et al. 1993). Cretaceous deposits are now missing fro m the Finnmar k platfor m an d th e mainland , an d have bee n tilte d awa y fro m th e mainlan d i n th e basins furthe r nort h an d west . Th e inferre d Cenozoic denudatio n o f sample s fro m th e Troms0 regio n the n implie s a hing e zon e clos e to th e present-da y coastline . Th e Vestfjorden , Ribban, R0s t an d Harsta d basin s ar e locate d offshore th e Atlanti c margi n o f th e stud y are a (Fig. 2). All four basins contain thick Cretaceous deposits tha t ar e underlai n b y thinne r Jurassi c sediments (Faleid e e t a l 1993 ; Olese n e t a l 1997; Brekke 2000). Late Jurassic-early Cretaceous sediment s ar e als o preserve d o n And0ya , Vesteralen (Dalland 1980) . These sediment s thus were deposite d a t a tim e o f inferre d stron g denudation o f the present-da y are a o f maximum elevation an d th e Atlanti c margin , whic h indicates the y ma y hav e bee n derive d fro m thi s region o f Jurassic-Cretaceou s denudation . Except for the Harstad basin , the basins offshore the Atlanti c margi n o f th e stud y are a contai n relatively smal l amount s o f Tertiar y sediment s compared wit h the thic k Cretaceou s sequences . Thin Paleocen e sequence s ar e presen t i n th e Vestfjorden, Ribba n an d R0s t basins , whic h ar e unconformably overlai n b y Plio-Pleistocen e sediments (Brekk e 2000) . Thi s reflect s th e important rol e Neogen e denudatio n an d glacia l

Rohrman & va n de r Bee k (1996 ) explaine d th e domal styl e o f late-stag e postrif t uplif t o f th e Southern Scande s wit h an asthenospheri c diapi r model wherei n a ho t Icelandic ' asthenospher e layer meet s col d cratoni c lithosphere . Thi s mechanism woul d explai n mor e o r les s simul taneous domal-styl e uplif t o f th e Souther n Scandes, Norther n Scande s an d als o Svalbard . However, a comparison of the denudation history reconstructed on the basis of apatite fission-trac k data fo r th e Souther n Scande s (Rohrma n 1995 ) range wit h tha t o f th e Norther n Scande s (thi s study) make s clea r tha t ther e ar e importan t differences betwee n th e tw o i n bot h th e pattern and timin g of denudation . Inverse modellin g o f apatite fission-trac k dat a fro m th e Souther n Scandes suggest s two distinct phases of denudation (Rohrma n 1995) . Triassic-Jurassic denudation of c. 1.3-3.5 km, probably as a consequence of base-leve l lowerin g an d rif t flan k uplift , wa s followed by much slower denudation rates during Cretaceous-Paleogene time . Rapi d denudatio n caused b y tectoni c uplif t starte d fro m c . 3 0 Ma onward an d produce d th e doma l patter n recog nized i n the topography an d apatite fission-trac k ages. Therefore, there are similarities in timing of denudation fo r th e Souther n an d Norther n Scandes until late Cretaceous time, but certainly the patter n o f denudatio n an d probabl y als o th e timing o f denudatio n i n Cenozoi c tim e ar e different fo r the tw o ranges. Although denudatio n i s no t necessaril y con nected t o an y technicall y drive n surfac e uplif t (Summerfield & Brow n 1998) , i t i s temptin g to explain th e result s of th e fission-trac k dat a fro m the stud y area b y passive margin uplift followe d by scar p retreat . This type of model predict s the maximum amount of denudation near the margin and moderat e t o lo w amount s o f denudatio n i n the interior ; this characteristic produce s a strong gradient i n apatit e fission-trac k age , wit h th e oldest age s occurrin g i n th e interio r an d decreasing toward s th e coas t (Gallaghe r & Brown 1997 ; Gallaghe r e t a l 1998) . Rif t shoulders ar e thu s ofte n characterize d b y a marked large-scal e asymmetr y (Beaumont et a l 2000). Th e stud y area clearl y display s all thes e characteristics. Another important prediction of a scarp retrea t mode l i s tha t th e timin g o f maximum denudatio n decreases inlan d fro m th e coast towar d the final position of the escarpment

DENUDATION HISTORY OF NORTHERN SCANDINAVIA (Gallagher & Brow n 1999) . Nowher e o n th e Atlantic margin i n the study area is the samplin g density hig h enoug h t o convincingl y prov e tha t the margi n display s thi s characteristic . Sampl e N57 i s fro m c . 25k m furthe r fro m th e margin , and als o fro m 360 m highe r than sampl e N60 . Indeed, th e fission-trac k ag e o f sampl e N5 7 i s 30 Ma younge r tha n tha t o f sampl e N60 . Bu t o f course thes e ar e onl y tw o samples , an d thi s i s also th e only plac e o n the margin wher e w e can test thi s predictio n o f th e scar p retrea t model . Passive margin s ar e als o generall y associate d with thic k synrif t sediment s an d a postrif t unconformity (Brau n & Beaumon t 1989) . Although the Norwegian shel f is a very complex system o f basin s an d high s an d thi s complexit y makes i t difficul t t o recogniz e som e o f thes e characteristics readily , i t clearl y display s thes e features (Brekk e & Rii s 1987 ; Reems t 1995 ; Brekke 2000). I t appears therefor e that rift flan k uplift resultin g from passiv e margin development followed b y scar p retrea t ca n explai n th e reconstructed denudatio n history of the Northern Scandes range . This certainl y i s not the case fo r the peculia r denudatio n histor y o f th e Souther n Scandes (Rohrma n 1995) . The atypica l thermotectoni c developmen t o f the Souther n Scande s ha s implication s fo r th e accuracy o f th e estimat e o f th e amoun t o f Jurassic-Cretaceous denudatio n o f th e North ern Scandes . Th e Mesozoi c geother m o f 2 8 ± 8 ° C k m ~ 1 (Rohrma n 1995 ) fro m th e Southern Scande s regio n ha s bee n use d i n thi s calculation an d seems to be high compare d wit h the present-day geotherm of less than 20 °C km~l on th e Scandinavia n mainlan d (Ballin g 1990) . The use of too high a value for the geotherm wil l lead t o underestimatio n o f th e amoun t o f denudation. Th e estimat e give n in this paper fo r the amoun t o f Jurassic-Cretaceou s denudatio n of th e Norther n Scande s coul d therefor e b e to o low.

Summary and conclusions Post-Caledonian coolin g an d denudatio n i n northern Scandinavi a progressivel y shifte d fro m the interio r o f th e continen t toward s th e Nort h Atlantic margin . I n th e Norther n Scande s mountain range , th e present-da y are a o f maxi mum elevatio n ha s experience d continuou s cooling an d denudatio n a t leas t sinc e Jurassi c time. Th e combine d Jurassic-Cretaceou s denu dation of this region wa s 1 ± 0. 5 km. Except for most o f the North Atlanti c margin , th e norther n Scandinavian mainlan d eithe r wa s no t affecte d by denudatio n i n Jurassic-Cretaceou s time , o r experienced som e denudatio n followe d b y a

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transient therma l even t tha t reache d it s pea k temperature i n lat e Cretaceou s time . Thi s coul d indicate the occurrence o f a Jurassic-Cretaceous sedimentary cove r o n th e mainland . I t sub sequently mus t hav e bee n removed , becaus e there ar e n o Jurassic-Cretaceou s sediment s preserved o n th e norther n Scandinavia n main land. Final denudation of the eastern flank of the Northern Scandes had probably already started in late Cretaceous-Paleogen e tim e an d possibl y accelerated i n Neogen e time . Th e fission-trac k record o f th e Atlanti c an d Barent s Se a margin s indicates a Neogene onse t fo r the final phase o f cooling an d denudation , althoug h denudatio n may already have affected th e area where the two margins mee t befor e tha t time . Th e combine d Mesozoic-Cenozoic denudatio n wa s stronges t on th e Atlanti c margin . Th e interio r o f th e continent experience d th e leas t Mesozoic Cenozoic denudation . The patter n an d timing of denudation o f th e Norther n Scande s mountai n range i s differen t fro m tha t o f th e Souther n Scandes range , whic h experience d domal-styl e late-stage postrif t uplif t i n Neogen e time . Th e reconstructed denudatio n histor y o f norther n Scandinavia can be explained b y scarp retreat of an uplifte d rif t flank . This researc h i s funde d b y Nors k Hydro , Statoi l an d the Norwegian Petroleum Directorate. I t is part o f the NSG an d ISE S researc h school s an d ha s bee n performed i n the Department of Isotope Geochemistry of the Vrije Universiteit Amsterdam. We thank P. Green and P. Bishop for comments an d critical revie w o f the manuscript. We acknowledge the EC N a t Petten, Th e Netherlands, fo r irradiatin g the fission-trac k samples . We ar e gratefu l t o LKA B Kirun a an d LKA B Malmberget fo r providin g dee p sample s fro m thei r mines, an d t o O . Svenningse n for providin g us with samples fro m Kebnekaise . Thi s pape r i s Publicatio n 20010501 o f th e Netherland s Researc h Schoo l o f Sedimentary Geology .

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Along-slope variation i n the late Neogen e evolutio n of the mid-Norwegian margi n i n response to uplift and tectonis m 1

D. EVANS 1, S . McGIVERON2, Z. HARRISON 2, P. BRYN3 & K. BERG 3 British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 SLA, UK 2 Svitzer Ltd, Morton Peto Road, Great Yarmouth NR31 OLT, UK 3 Norsk Hydro ASA, PO Box 200, N-1321, Stabekk, Norway Abstract: A s par t o f th e Norwegia n Dee p Wate r Programme , a regional geologica l an d geophysical interpretation of the mid-Norwegian margin resulted in the establishment of a late Paleogene to Holocene stratigraphi c framewor k fo r the margin , and identification an d mapping o f a range o f possible geohazards, includin g slides . At the V0rin g margi n in th e north, ther e ha s bee n th e build-ou t o f a hug e progradin g wedg e o f sedimen t i n Plio Pleistocene times. The sediments of the wedge are assigned to the Naust Formation , whic h has been subdivide d int o eigh t unit s (A-H) , an d includes onl y a few palaeoslides. To the south o f the V0ring margin lies the Storegga Slide Complex and the North Se a Fan, an d the whole of this souther n regio n shows evidenc e of several major palaeoslides . The sediments are also referred t o the Naust Formation , which her e are subdivided into nin e units (O-W ) that hav e been partly correlate d wit h equivalen t Naus t unit s t o the north. The oldest Naus t unit in the south, Naus t Unit W, is largely made up of slide deposits that provide evidence for the earlies t identifie d large-scal e slop e instabilit y i n th e Storegg a Slid e Complex . Thi s instability wa s penecontemporaneou s wit h th e initiatio n o f th e progradin g wedg e t o th e north, an d both feature s ar e postulated to be the result o f uplift of the Norwegian mainland . In th e cas e o f th e Storegg a Slid e Complex , whic h lie s clos e t o th e mai n are a o f uplift , oversteepening o f th e margin , togethe r wit h seismicit y associate d wit h th e Ja n Maye n Fracture Zon e an d M0re-Tr0nderla g Faul t Complex , ma y hav e initiate d slidin g tha t ha s since occurre d intermittentl y u p t o Holocen e times , ove r a tim e interva l whe n ther e ha s additionally bee n muc h glacio-isostati c movement .

In th e south , th e shel f of f wester n Norwa y i s Britis h Geologica l Surve y acting a s consultants, narrow, but it widens considerable t o the north in A n understandin g o f margi n developmen t an d the Haltenbanke n an d beyon d th e Traenadjupe t slop e stability in the area is very important to the (Fig. 1). In the south the limit of the shelf north of hydrocarbo n industr y as deep-wate r exploratio n the Nort h Se a Fa n i s marke d b y th e scarp , o f continue s an d developmen t o f majo r ne w field s 290 km length (Bugge etal 1987) , of the Storegga begins . Slide, wherea s i n th e nort h a more gentl e slop e Th e summarize d result s o f Phas e I I o f th e leads down to the V0ring Basin, beyond which it projec t were published by McNeill e t al. (1998), rises slightl y at the V0ring Plateau. wh o described th e Cenozoic stratigraphic frame The mid-Norwegia n margi n becam e a n wor k and discussed geohazards relevant to the oil important are a fo r hydrocarbo n exploratio n industry . Bry n e t al . (1998 ) concentrate d o n following th e deep-wate r 15t h Norwegia n issue s o f slop e stability . Thi s assessmen t wa s round o f licensin g i n 1996 . T o gai n a bette r update d during Phase III of the project, for which understanding o f the shallo w geolog y an d slope man y additiona l dat a wer e available . Th e stability o f th e area , severa l companie s jointl y complet e databas e include d 1 0 000km o f ne w formed th e Seabe d Project , whic h wa s a an d reprocesse d seismi c an d high-resolutio n component o f th e Norwegia n Dee p Wate r seismic , mini-airgun , deep-to w boomer , 3 D Programme (Bry n e t al. 1998) . This projec t was seismi c dat a i n licenc e block s (Fig . 1) , towe d managed by Norsk Hydro, and one aspect of the ocea n botto m instrument (TOBI) sidesca n sona r project was geological and geophysical interpret- data , thre e Ocea n Drillin g Progra m (ODP ) o r ation, which was carried out in three phases. The Dee p Se a Drillin g Projec t (DSDP ) sites , thre e second an d thir d phase s wer e contracte d t o exploratio n well s an d fou r geotechnica l Svitzer Lt d betwee n 199 7 an d 1999 , wit h th e boreholes . From: DORE , A.G., CARTWRIGHT , J.A. , STOKER , M.S. , TURNER , J.P . & WHITE , N . 2002 . Exhumation o f th e North Atlantic Margin: Timing, Mechanisms and Implications for Petroleum Exploration. Geologica l Society , London, Special Publications, 196, 139-151 . 0305-8719/027$ 15.00 © The Geological Societ y of London 2002 .

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Fig. 1 . The bathymetry (i n metres) an d location o f the stud y area in relation t o the mid-Norwegian shel f an d the Storegga Slide . Als o shown ar e the 15t h Roun d licenc e block s (se e als o Fig. 6) , the location o f seismic sections , and th e landwar d limi t o f the slid e deposit s tha t for m par t o f Naus t Uni t W.

This wa s a regiona l interpretatio n i n whic h only th e outermos t portio n o f th e shel f wa s included i n th e are a o f study . The are a o f study covers ove r 100 000km2, and extends alon g the continental slop e fro m 62°15' N t o 68°N . Th e westward exten t o f th e are a i s approximatel y a t the V0rin g Escarpmen t i n th e nort h i n wate r depth o f abou t 1200-1300m , wherea s i n th e south i t extend s t o depth s o f 250 0 m withi n the Storegga Slide . It is important t o note that in this paper the term 'Storegg a Slide' refers only to the most recen t movemen t o n the slid e a s describe d by Bugge (1983 ) an d Bugge et al (1987 , 1988) , whereas th e ter m 'Storegg a Slid e Complex ' i s used to describe an area of longer-term instability centred around the Storegga Slide. It must also be emphasized tha t this paper is based on the results of th e Seabe d Projec t u p t o 1999 ; man y

commercial dat a hav e bee n collecte d sinc e that time, includin g 3D seismi c data , bu t th e newe r data ar e no t used here. Although th e projec t considere d th e whol e post-Paleocene succession , thi s paper wil l focu s on th e Plio-Pleistocen e history , wit h particula r emphasis o n th e lat e Pliocen e t o earl y Pleisto cene sedimentar y response. This includes , in the northern part of the area, the build-out of a major prograding wedge , whic h i s comparabl e wit h similar feature s observe d o n glaciate d margin s world-wide (e.g. Larter & Barker 1991 ; Clausen 1998; Kristofferso n e t al . 2000 ) an d ha s commonly bee n described a s a response t o uplif t of Scandinavi a (Pool e & Vorre n 1993 ; Hendriksen & Vorren 1996 ; Rii s 1996 ; Stuevold & Eldhol m 1996) . However , a marke d along slope contrast between the north and south of the

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study are a wil l b e described , an d som e possibl e reasons fo r these difference s discussed .

Stratigraphic framework The Stratigraphi c framework established fo r th e area i s base d o n seismi c interpretation , ag e information fro m th e projec t database , an d published information . Th e framewor k illus trated i n Fig . 2 i s adapte d fro m McNeil l e t al. (1998) following work in Phase III of the project. The formatio n names are those o f Dalland e t al. (1988), althoug h the age-rang e o f their Pliocen e Naust Formatio n ha s bee n extende d t o includ e the Pleistocene deposits . It ca n b e see n tha t th e Eocen e t o Oligocen e Brygge Formation extend s the length of the area . This formatio n was folded during late Eocene t o Oligocene compressio n an d inversio n (Dor e & Lundin 1996 ; Swiecick i et al. 1998) , an d Vagnes et al . (1998 ) hav e argue d tha t inversio n i s continuing t o th e present . However , younge r deposits d o not have this continuity. Kai units B and C ar e though t o n th e basi s o f similarit y of

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seismic characte r t o hav e bee n originall y continuous beneat h th e Storegg a Slide , bu t Kai Uni t A is a t present foun d onl y t o the nort h of Storegga . Th e shale-pron e Brygg e an d Ka i formations i n th e stud y are a wer e bot h deposited i n relativel y deep-wate r basins , probably largel y beyon d th e contemporar y shelf (Gradstei n & Backstro m 1996 ; Eidvi n et al . 1998) . I t ca n b e see n fro m Fig . 3 tha t these formation s ar e commonl y thi n o r absen t on th e shelf , althoug h the y typicall y hav e a combined thicknes s o f ove r 1000 m i n th e V0ring an d M0r e basins . Above the Kai Formation , ther e is a marke d change i n th e styl e o f sedimentation , fo r i n lat e Pliocene time s (Eidvi n e t al . 2000 ) th e larg e prograding wedge of the Lower Naust and Naust formations buil t ou t t o for m th e present-da y shelf. Thi s wedg e ha s resulte d i n th e advance ment o f the shel f by u p to 100k m locally i n th e north (Hendrikse n & Vorre n 1996) ; th e forme r extent in the south is unknown because of erosion at th e Storegg a Slid e Complex . Ther e i s als o a significantly differen t Stratigraphi c breakdow n

Fig. 2 . The seismostratigraphic framework establishe d for the area, illustrating th e time range of the Brygge, Kai , Naust and Lower Naust formations. This is a modification of the stratigraphy proposed b y McNeill e t al. (1998).

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Fig. 3 . Cross-section illustrating the general relationships of the Neogene stratigraphic unit s in the study area, with schematic extensio n acros s th e shel f t o th e S E based o n Rokoenge n e t al. (1995) . Th e locatio n o f th e portio n within th e stud y are a is close to tha t o f Fig. 4 , wit h th e schemati c extensio n toward s the Norwegia n coast .

to th e nort h an d sout h o f th e norther n flan k o f the Storegg a Slid e (Fig . 2) , wit h onl y a limite d seismic correlatio n establishe d betwee n th e tw o successions. North of the Storegga Slide Complex In the north, the basal componen t abov e the Kai Formation is the informal Lower Naust formation, a uni t tha t withi n the stud y are a form s th e thin ,

distal facie s o f th e earl y build-ou t o f th e prograding wedg e (Fig . 3). The formatio n is, however, locall y thicke r i n mounde d section s (Fig. 4), probably as a result of preferential finegrained sedimentatio n unde r th e influenc e o f contour currents . Th e Lowe r Naus t formatio n unconformably overlie s olde r deposits , an d i s itself unconformabl y overlai n b y th e down lapping unit s o f th e Naus t Formatio n o n th e slope, wherea s i n th e basi n it i s overlai n by th e

Fig. 4 . A seismic profile through the northern succession of the Naust Formation, indicating it s relationships wit h underlying formations . (Fo r location , se e Fig. 1. )

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distal facie s o f th e Naus t Formation. Th e Naust Formation ha s been subdivide d int o units A-H, and the discrete, westerly downlappin g package s of unit s B-H for m th e diachronou s bas e t o the formation. Thei r landwar d extent s are truncated at a glacia l unconformit y a t th e bas e o f Naus t Unit A , althoug h glacia l erosio n o f th e shel f probably pre-date s Naus t Uni t A (Haflidaso n et al. 1991) . Thi s glacia l erosio n surfac e wa s termed th e UR U (uppe r regiona l unconformity) by Hendriksen & Vorren (1996) . The Naus t Formatio n i n th e nort h i s u p t o 1500m thick , an d i t ha s bee n estimate d tha t its total volum e i n th e nort h i s 8 0 000km 3 (Evan s et al 2000) . A typical seismi c sectio n presented in Fig . 4 (se e als o McNeil l e t al 1998 ; Evan s et al. 2000) illustrate s the downlapping character of the units, and shows that each unit is made up of on e o r mor e package s an d i s separate d b y well-defined reflectors . Thes e package s ma y show bedding , bu t ar e generall y acousticall y structureless, althoug h adjacen t t o th e Storegg a Slide Complex they are locally of an acoustically well-bedded facies (Fig . 5) . From th e dat a availabl e t o th e study , i t i s apparent tha t there is evidence fo r only a limited number of major palaeoslide s withi n this sector ,

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and no evidence of such erosion i n the older par t of the succession. The Traenadjupet and NE Nyk slides ar e found in the far north of the study are a (Laberg et al 1999) , but these lie where the slope extends down to the Lofoten Basin to the north of the V0rin g margi n sector . A relativel y smal l palaeoslide at the northern flank of the Storegg a Slide has been describe d b y Evans et al. (1996) , and a part o f Palaeoslide-2 has been mappe d on the flan k o f th e Storegg a Slid e Comple x an d i n the far west of the study area. The onl y example on th e V0rin g slop e durin g thi s projec t i s th e Traenabanken Slid e t o th e nort h o f th e Hellan d Hansen licence area , a slide that took place afte r Naust Unit B times and commonly employed the top of Naust Unit C as a glide plane. The location of th e hea d wall o f this slid e i s evident i n Fig. 6 from th e remova l o f mid-Pleistocen e sediment s from the middle of the main northern depocentre. South of the Storegga Slide Complex The lates t movement s o f th e Storegg a Slid e a s described b y Bugge et al. 198 7 wer e Slides I , II and III, and these formed the present topograph y of the region. Slide I was described a s pre-dating the las t glaciation , wit h Slide s I I an d II I

Fig. 5 . A seismic sectio n acros s th e northern flank of the Storegga Slid e illustratin g th e largely acousticall y well bedded natur e of the Naust Formation immediatel y nort h of the sidewall. Palaeoslide s ca n be seen at depth belo w the Storegg a Slide . (Fo r location , se e Fig. 1. )

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Fig. 6 . Map showin g the thickness of middle Pleistocene sediment s (Naust units B and C) in the study area. The sediments have been wholly or partly removed fro m th e Storegga Slid e Complex region, and partially removed by the Traenabanken Slide . Also shown is the 2D seismic grid available to the study, and the 15t h Round deep-water licence blocks .

occurring abou t 7000-720 0 BP (Svendse n & Bondevik 1995 ; Bondevik ef a/. 1997) . However, the deposit s o f Slid e I ar e no t blankete d b y a well-defined sedimentar y cove r (Fig. 5) as would be expecte d i f tha t slid e pre-date d th e las t glaciation, an d it is now considered tha t all three

movements post-dat e th e las t glaciatio n an d probably al l occurre d a t aroun d th e sam e time . Haflidason e t al (2000 ) suggested that they took place between 6000 and 8000 BP. This suggestion is in accordance with an alternative interpretation originally propose d b y Bugge (1983) .

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To th e sout h o f th e norther n flan k o f th e Storegga Slide , Ka i Unit A is absent an d there is no direc t equivalen t o f th e Lowe r Naus t formation. A correlatio n ha s bee n establishe d between th e top of Naust Unit W in the souther n succession an d th e to p o f Naus t Uni t F i n th e north (Fig . 2) , a stratigraphi c leve l tha t fro m limited well evidence may approximate to the top of the Pliocen e sequence , but is though t mor e likely t o b e o f earl y Pleistocen e age . Figur e 5 shows the equivalence of the top of Naust Unit W (the oldest Naus t unit of the southern successio n in this region) with the top of Naust Unit F at the junction o f th e tw o succession s beneat h th e northern sid e wall of the Storegg a Slide . The characte r o f Naus t Uni t W i s i n marke d contrast to that of its coeval unit s (Naust units F, G and H) in the north. Th e base of the unit is in many place s severel y erosional ; th e exampl e i n Fig. 7 shows that one of the erosional scarp s at its base exceed s 250m s (c . 250m ) i n height . Although the scal e o f erosion i s small compare d with th e downcuttin g associate d wit h th e Holocene movement s o f th e Storegg a Slide , i t none the less represents erosion o n a large scale . Elsewhere th e Uni t W sediment s hav e th e

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slightly mounde d an d acousticall y opaqu e internal characte r tha t is commonl y associate d with slide deposits, although not all profiles show evidence o f a slide-related origin . The exten t of the slidin g i n Naust Unit W ha s been mappe d an d i s show n i n Fig . 1 , wherea s Fig. 7 shows the development of a headwall. The poorly defined trac e of the headwall lies seaward of th e present-da y Storegg a Slid e scarp , an d i s seen t o b e o f comparabl e length , indicatin g th e magnitude o f th e erosio n a t tha t time, althoug h the mass-movemen t ma y represen t man y separ ate events. It is considered that Naust Unit W was formed b y more than one process, but that majo r slide erosio n wa s a significan t component . Importantly, th e slidin g associate d wit h Naus t Unit W is the oldest so far clearly identified in the study area , an d ha s a clos e geographica l association wit h th e present-da y bathymetr y o f the lates t movement s o n th e Storegg a Slid e a s documented b y Bugg e e t al (1987 , 1988) . Bearing i n min d th e absenc e o f Ka i Uni t A an d the Lowe r Naus t formation , i t i s possibl e tha t earlier instabilit y may hav e caused th e irregula rities see n locall y beneath th e bas e o f the Naust Formation i n Fig. 7 , but this remains unclear.

Fig. 7 . Seismi c profil e fro m th e Storegg a Slid e Complex , illustratin g th e locall y erosiv e natur e o f the base of Naust Uni t W, as well a s deep erosion to the base of the sediment s deposite d during Holocen e movement o n the slide. (Fo r location , se e Fig. 1. )

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Fig. 8 . Th e relationshi p o f identifie d slide s an d palaeoslide s t o th e stratigraphi c subdivision s o f th e Naus t Formation. TJ , Tranadjupe t Slide ; NEN , N E Ny k Slide ; TB , Traenabanke n Slide ; TAM , Tampe n Slide ; FSE , Faeroe-Shetland Escarpment Slide ; 1 , 2, 3 and 4 are Palaeoslides-1, -2, -3 and -4. The Vigra and North Sea Fan Slide-1 were no t specificall y identified during thi s study , but wer e recognize d b y Evan s et al. (1996 ) an d King et al. (1996) within Naust units V-S.

The souther n successio n i n the North Se a Fan region t o th e sout h o f th e Storegg a Slid e i s o f comparable thicknes s t o the northern succession , but ther e ar e pronounce d difference s i n it s seismic character . Th e fa n becam e increasingl y important a s a depocentr e i n mid - t o lat e Pleistocene time a s vas t quantitie s o f sedimen t were carrie d t o th e shel f brea k alon g th e Norwegian Channe l (Sejru p e t al . 1996) . Th e units sho w a variet y o f acousti c characteristic s ranging fro m opaqu e t o wel l bedded , bu t a key characteristic o f the succession i s that it includes evidence fo r a numbe r o f majo r translationa l slides. Evans et al (1996 ) and King et al (1996 ) have described Nort h Se a Fan Slide-1, the Vigra Slide, th e M0r e Slid e an d th e Tampe n Slid e o n the fan. The presen t stud y ha s identifie d th e slidin g associated wit h Naus t Uni t W , th e Faero e Shetland Escarpmen t Slid e tha t lie s clos e t o th e eponymous escarpment , an d Palaeoslides-1 , -2 ,

-3 an d - 4 (Fig . 8) . I n th e presen t seismi c correlation, Palaeoslide- 2 is equivalent in age to the Traenabanken and M0re slides , and has been traced alon g th e norther n flan k o f th e Storegg a Slide a s wel l a s i n th e sout h (Fig . 6) . Isolate d thick remnants of mid-Pleistocene deposit s see n in Fig. 6 adjacent to the area of complete removal testify t o thei r probabl e forme r widesprea d presence befor e thei r remova l b y Palaeoslide- 2 and contemporaneous events. The precise exten t of these slides is unknown, largely becaus e significan t part s o f the m ma y have bee n remove d b y late r slid e erosion , bu t some wer e undoubtedl y ver y large , eve n compared wit h th e lates t Storegg a Slid e move ments, which are the most recent major erosional events in this M0re Basin region. All these slides are locate d aroun d th e Storegg a Slid e a s described byBugg e e t a l (1987) , an d i t i s clea r that the term Storegga Slide does not adequately describe th e lon g histor y o f movements . Th e

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term Storegg a Slid e Comple x i s therefor e proposed, wit h th e ter m Storegg a Slid e use d t o define onl y th e post-glacia l movement s an d consequent bathymetri c expressio n o f thos e events.

Discussion The abov e description s sho w tha t the histor y o f Plio-Pleistocene margi n developmen t ha s bee n significantly differen t eithe r sid e o f a line that is approximately equivalent to the northern flank of the Storegg a Slide . Thi s lin e i s coinciden t wit h the location of the Jan Mayen Lineament , whic h has controlle d th e tectoni c developmen t o f th e area sinc e Cretaceou s time s (Brekk e 2000) . I n particular, ther e was a marked differenc e durin g late Pliocene o r early Pleistocene time , when the prograding wedg e wa s initiate d i n th e nort h during th e depositio n o f th e Lowe r Naus t formation an d its mor e proxima l equivalent s o n the inner shelf, and Naust units F, G and H on the slope. However, a significant degree of progradation ma y als o hav e occurre d i n th e Storegg a region, an d ma y hav e bee n subsequentl y removed durin g Unit W or later erosion .

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The initiatio n o f th e wedg e represente d a fundamental chang e i n sedimentar y architectur e as the depocentre move d t o the inne r shel f fro m the earlie r deep-wate r basi n o f th e Ka i an d Brygge formations (Fig . 3) . Workers such as Riis (1996) an d Stuevol d & Eldhol m (1996 ) hav e related thi s chang e t o a perio d o f uplif t o f mainland Norway that led to increased erosio n of the mountainou s region s an d a n enhance d sediment suppl y to th e shel f an d th e generatio n of accommodation spac e seaward of a hinge line. Climatic deterioration , wit h mor e effectiv e fluvial erosion and the onset of upland glaciation, probably als o playe d it s par t i n th e increase d rate o f erosion, an d it is difficult t o separat e th e two force s (Lidmar-Bergstro m e t al . 2000) . Any mechanis m propose d fo r vertica l move ments i n th e mid-Norwegia n are a (e.g . Cloetingh e t al . 1990 ; Rii s & Feldskaa r 1992 ) needs t o b e consisten t wit h observation s indicating lat e Neogen e uplif t i n th e broade r North Atlanti c regio n (Stoke r 1995 , 2002 ; Andersen e t al. 2000; Chalmers 2000 ; Japse n & Chalmers 2000) . The V0rin g margi n woul d hav e bee n fe d b y sediments derive d fro m centra l Norway , which, according t o Rii s (1996 ) wa s a n are a o f lesse r

Fig. 9 . Ma p showin g th e genera l relationship s o f majo r slide s an d palaeoslides wit h structura l features , Plio Pleistocene uplif t (Rii s 1996 ) an d a zon e o f maximu m post-glacia l compressiv e stres s (Gudmundsso n 1999) . Surface slide s i n the nort h ar e fro m Laber g e t al. (2000) . MTFC, M0re-Tr0ndela g Faul t Comple x (Gabrielse n et al . 1999) , whic h i s thought t o be a particularly significan t zon e o f seismicity .

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uplift tha n southern an d northern Norwa y a t that time (Fig. 9) . Although evidenc e o f progradatio n in th e south i s lacking , i t i s clea r tha t a vas t quantity o f sedimen t wa s delivere d t o th e sea , and certainly le d to the pronounced progradatio n and seawar d advancemen t o f th e shel f brea k i n the north . At th e sam e tim e a s th e wedg e wa s bein g initiated, there was both sedimentatio n an d major sliding i n the M0r e Basi n t o the south , althoug h the Unit W sliding could have occurred a s late as Unit F times. Give n tha t the souther n part o f the area lies close t o the region o f maximum onshor e uplift i t i s likel y tha t ther e wa s a n abundan t sediment suppl y i n lat e Pliocen e t o earlies t Pleistocene times. Indeed , rapi d sedimen t suppl y may hav e bee n a facto r i n triggerin g th e slide s that characterize Naus t Unit W, as may have been the increas e i n slop e tha t i s likel y t o hav e bee n experienced a t the continental margi n a s a result of uplif t o f th e lan d an d subsidenc e offshore . Present evidenc e point s to late Pliocen e uplif t a s being a factor in the initiation of the slides at that time, a s wel l a s i n thei r continue d occurrenc e throughout Quaternar y time , bu t wha t wa s th e trigger fo r the slides ? Earthquakes ar e a commonl y quote d trigge r for slide s i n thi s regio n (e.g.Bugg e e t al. 1987 ; King e t a l 1996 ; Laber g e t al . 1999) . Figur e 9 shows that the Storegga Slid e Complex lie s along the Ja n Maye n Lineamen t (Blysta d e t al . 1995 ) and th e Ja n Maye n Fractur e Zone , a s ha s previously bee n note d b y Bugg e e t al . (1987) . Although thi s fracture zon e i s not a strong focu s of modern earthquake s (Dehl s et al. 2000), som e seismic event s hav e bee n recorde d alon g i t offshore, an d are particularly common wher e the lineament meet s th e Norwegia n coas t (Bungu m et al . 1991) . Brekk e (2000 ) consider s i t t o hav e acted a s a transfe r zon e durin g Cenozoi c time , when i t controlle d th e occurrenc e o f compres sional tectonics . Anothe r facto r i s tha t th e headwall (Fig . 9 ) i s broadl y coinciden t wit h th e still seismicall y activ e M0re-Tr0ndela g Faul t Complex (Gabrielse n e t al . 1999) . Ther e i s a clear relationshi p betwee n th e locatio n o f majo r structures or fracture zon e an d the position of the Storegga Slide Complex, and it is therefore likel y that ther e ha s bee n lat e Pliocen e o r earl y Pleistocene tectoni c movemen t i n th e vicinit y of th e slid e comple x tha t coul d hav e create d significant seismi c events . Althoug h th e Ja n Mayen Fractur e Zon e ma y n o longe r b e active , the lineamen t ha s continue d t o exer t a contro l over th e locatio n o f majo r slide s durin g Quaternary time . This indicatio n i s strengthene d b y a stud y o f the distribution of similar feature s on the V0rin g

margin an d farther north around Lofoten . Figur e 9 show s tha t ther e ar e n o crusta l lineament s o r fracture zone s cuttin g the V0ring margin , whic h lies adjacen t t o a zon e o f lesse r onshor e uplift . This i s th e regio n wit h littl e evidenc e o f palaeoslides; th e only significant mappe d featur e south of Traenadjupet is the Traenabanken Slide , which interestingl y lies o n th e projecte d lin e of the Gleipn e Fractur e Zone . To the north of the V0ring margin off Lofoten, the zon e o f majo r uplif t lie s clos e t o th e shel f break, an d thi s i s a margi n tha t display s ampl e evidence o f downslop e movemen t (Dowdeswel l & Kenyon 1997 ; Taylor et al. 2000). In particular there ar e two majo r slides , the Traenadjupet an d Andoya slide s (Laber g e t al . 1999 , 2000) , tha t respectively lie along the Bivrost Fracture Zone / Bivrost Lineamen t an d th e Senj a Factur e Zon e (Blystad e t al . 1995) . Th e ful l histor y o f thes e slides i s no t known , an d although , a s wit h th e Storegga Slid e Complex , thei r mos t recen t movements wer e durin g Holocen e time , olde r movements probably did occur but have yet to be fully documented . Nevertheless , the y d o con tribute t o a well-defined correlatio n betwee n th e location o f majo r crusta l structure s an d th e distribution o f larg e slide s alon g th e Norwegia n margin. In mid - t o lat e Pleistocen e times , glaciation s became stronge r and ice sheets extended onto the shelf wit h greate r frequenc y (William s e t al . 1988; Mangeru d e t al . 1996 ; Vale n e t al . 1996 ; Vorren & Laber g 1996) . Durin g thes e times , movements alon g th e Norwegia n margi n ar e likely t o hav e bee n increasingl y affecte d b y isostatic movement s relate d t o th e onse t an d removal o f th e ic e cove r fro m th e shelf . Thes e events increas e th e ris k o f seismicity , especially when superimpose d o n pre-existin g crusta l stresses relate d t o plat e tectonic s (Talbo t & Slunga 1988) . Although they noted that observed stresses i n Norwa y ar e consisten t wit h uplif t o f Fennoscandia, Fejersko v & Lindhol m (2000 ) considered tha t ridge-pus h associate d wit h sea floor spreading i s the primary caus e o f compres sional stress . Figur e 9 show s th e present-da y zone o f maximu m compressiv e stres s aroun d Norway a s a resul t o f post-glacia l uplif t a s calculated b y Gudmundsso n (1999) . Thi s zone , which i s likel y t o hav e bee n th e locu s o f earthquakes tha t coul d trigge r slides , cover s th e headwall o f al l thre e majo r Holocen e slides . I t has been estimate d that seismi c event s as stron g as M w 7. 9 ma y hav e occurre d i n Scandinavi a during the last ice retreat (Muir Wood 1988) , and given th e occurrenc e o f weak layer s i n the Plio Pleistocene sedimentar y column , slide s coul d well hav e bee n generate d b y thi s mechanism .

LATE NEOCENE EVOLUTION O F MID-NORWEGIAN MARGI N

Other mechanism s ma y als o hav e generate d slides, o r facilitated thei r initiation ; suc h factors include th e presenc e o f ga s o r ga s hydrate s (Bugge e t al 1987 ; Henrie t & Miener t 1998 ; Bouriak et al. 2000), although these factors may be o f mor e loca l importanc e tha n th e regiona l view taken in this paper.

Conclusions Late Neogen e uplif t o f Norwa y ha d a pro nounced influenc e o n margi n sedimentation , changing the pattern from one of slow depositio n in deep-water basins to more rapid sedimentatio n on th e inne r shel f a s a progradin g wedg e wa s initiated i n respons e t o increase d sedimen t supply an d th e generatio n o f accommodatio n space o n the shelf . Stratigraphic analysi s show s tha t ther e ha s been a significan t along-slop e differenc e i n th e latest Cenozoi c histor y o f margi n developmen t between th e V0rin g an d M0r e margins . Th e former i s characterize d b y th e depositio n o f a vast progradin g wedg e wit h littl e evidenc e o f major instability , wherea s th e latte r ha s a lon g history of sliding, which has largely removed any evidence o f wedg e development , an d ma y hav e received les s sedimen t late r in Quaternary time . The oldest identified sliding in the M0re Basin was penecontemporaneous wit h the earl y stage s of developmen t o f the progradin g wedg e in lat e Pliocene an d earliest Pleistocen e times, and both occurrences are considered t o be related to uplif t of th e mainland a t this time. Uplif t wa s greates t in the south adjacent t o the M0re margin , which has experience d a long history o f instability. As isostatic uplif t i s continuin g today, as suggeste d by Rii s (1996) , thi s ma y hav e implication s fo r present-day slop e stability . Three large slides on the Norwegian margin lie at the junctions of oceanic fracture zones with the continental crust , or alon g crusta l lineament s or major faul t zones . The y als o li e adjacen t t o th e zones of maximum onshore uplift. Thi s suggests strong structural control on the location o f slide s on this margin, although there is little evidence of modern seismicit y alon g the fractur e zones , and it ma y b e tha t th e M0re-Tr0ndela g Faul t Complex i s a particularly significan t structure . The extent of structural control is emphasized by the observation that the largest slide area with the longes t histor y o f movement , th e Storegg a Slide Complex , lie s a t th e conjunctio n o f th e largest oceani c fractur e zone , th e zon e o f maximum Plio-Pleistocen e uplift , a majo r faul t zone, an d th e zon e o f maximu m post-glacia l compressive stress .

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The author s woul d lik e t o than k th e oi l companie s involved in the Seabed Project (BP Norge, Esso Norge, Mobil, Norsk e Conoco , Nors k Hydro , Shel l an d Statoil) fo r the opportunity t o participate in the project and for their permission t o publish the data included in this paper . W e ar e als o gratefu l t o E . Gillespi e fo r producing th e diagrams . Th e pape r benefite d signifi cantly bot h fro m th e earl y comment s o f M.S . Stoke r and D. Long a s well a s those o f referees T . Eidvin an d A.G. Dore . The contributio n o f D.E. i s made wit h the permission o f th e Directo r o f th e Britis h Geologica l Survey (NERC) .

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Reconstructing the erosion history of glaciated passive margins: applications of in situ produce d cosmogenic nuclide techniques ARJEN P . STROEVEN1, DEREK FABEL 2, JON HARBOR 3, CLA S HATTESTRAND 1 & JOHAN KLEMAN 1 1

Department of Physical Geography and Quaternary Geology, Stockholm University, S-106 91 Stockholm, Sweden (e-mail: arjen@

geo.su.se)

2

School of Earth Sciences, University of Melbourne, Parkville, Vic. 3052, Australia

^Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN 47907-1397, USA Abstract: Offshor e sedimen t accumulation s provid e a n intriguin g recor d o f th e ne t sediment outpu t resultin g fro m geomorphologica l evolutio n o f th e circum-Atlanti c continental margi n sinc e the commencement o f Neogene glaciation . However, th e onshor e record of the timing, pattern and amount of bedrock erosion that produced these sediments is comparatively poorl y constraine d an d understood, although there ar e good genera l model s of glaciation history. The geomorphology of circum-Atlantic continental margin mountains, as assessed fro m remot e sensin g data and field observations, includes palimpsest landform s and landscapes that reflect a complex pattern of spatial and temporal variations in the impact of glacial , fluvia l an d periglacial processes. Perhaps mos t surprisin g is that, despite having been repeatedl y overridde n b y larg e ic e sheets , part s o f th e landscap e appea r t o be relict , with nonglacia l morphology . Thi s ha s importan t implication s bot h fo r glaciologica l conditions unde r ice sheets , an d for sedimen t sourc e area s an d erosion rates . Conventional dating an d analysi s hav e provide d a n excellent wa y t o begi n unravellin g th e timin g an d pattern o f erosion , landfor m development , an d possibl e landfor m preservatio n unde r ice. However, testin g hypothese s develope d fro m curren t models , an d addressin g critica l unresolved questions, requires additional approaches. The use of in situ cosmogenic nuclide production i n bedroc k i s a ne w approac h fo r investigatin g landscap e evolutio n i n mountainous areas . Wit h carefu l interpretatio n o f geomorphologica l settings , cosmogeni c nuclides can be used to determine apparent surface exposure age and landscape preservation, and constrain erosion depths and duration of burial by ice. Here we provide a framework for the interpretatio n o f cosmogeni c nuclid e concentrations i n bedrock surface s of landscape s affected b y glacial , fluvia l an d periglacia l processes , illustrate d wit h example s fro m th e northern Swedis h mountains . Thi s demonstrate s potentia l use s o f cosmogeni c nuclid e techniques, an d provide s a foundatio n for attempt s t o improv e geomorphologicall y base d reconstructions o f relict landscapes , t o reconstruct an d analys e the dynamic s of landscap e change in glacial times, and to define the consequences of different proces s regimes i n terms of erosio n patterns , sedimen t transport , an d th e suppl y o f sediment s tha t ar e deposite d offshore.

The evolutio n o f the circum-Atlantic continental 1996) . I n particular , curren t technique s o f margin sinc e th e commencemen t o f Neogen e estimatin g onshor e exhumatio n pattern s an d glaciation i s reflected in large offshor e sedimen t rate s d o no t resolv e margi n exhumatio n durin g accumulations (e.g . Solhei m e t al. 1996) . Th e lat e Cenozoi c tim e (Hendrik s & Andriesse n thickness o f offshor e sediments , importan t 2002) . quantities o f whic h wer e generate d b y glacia l Th e broad-scal e glaciatio n histor y o f th e processes, i s on e ke y ingredien t considere d i n circum-Atlanti c continenta l margi n i s wel l hydrocarbon exploration . However , th e onshor e understood i n genera l term s (e.g . Shackleto n record o f th e amount , timin g an d patter n o f e t al . 1984 ; Janse n & Sj0hol m 1991 ; Holeman n bedrock erosio n tha t wa s th e sourc e o f thes e & Henric h 1994 ; Mangeru d e t al . 1996 ; Janse n offshore sediment s i s no t wel l constrained , e t al . 2000) . I n Scandinavia , fo r example , ic e except at the mos t generalize d leve l (e.g . Rii s sheet s centre d wes t of the mountai n elevatio n From: DORE , A.G., CARTWRIGHT, J.A., STOKER , M.S. , TURNER, J.P . & WHITE , N . 2002 . Exhumation o f th e North Atlantic Margin: Timing, Mechanisms an d Implications for Petroleum Exploration. Geologica l Society, London, Special Publications, 196, 153-168. 0305-8719/027$ 15.00 © The Geological Society of London 2002.

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axis wer e th e dominan t for m o f glaciatio n form patter n that is both areally and altitudinally between 2. 0 an d 0. 7 Ma ag o (Ljungne r 1949 ; variable. The mos t prominen t valley s o f th e preglacia l Kleman 1992) . A n importan t threshol d wa s passed a t c . 0.9-0. 7 Ma (e.g . Porte r 1989 ; landscape wer e presumabl y exploite d b y earl y Raymo e t al 1989 ; Imbrie e t al 1992 , 1993 ; ice sheet s a s primary route s of ice drainage, an d Clark & Pollar d 1998) , afte r whic h ic e sheet s hence became primar y location s fo r subglacia l grew large r an d becam e centre d eas t o f th e bedrock erosio n an d valle y deepening . mountain elevatio n axi s (Klema n & Stroeve n Subsequent glaciation s would the n hav e prefer1997). However , wha t i s no t wel l establishe d i s entially exploite d thes e deepene d valleys , the timin g an d patter n o f erosio n an d landscap e because the y woul d hav e bee n mor e efficien t development associate d wit h thi s glacia l for ic e drainag e wit h eac h successiv e glaciatio n cycle (Sugde n & John 1976) . Th e implicatio n of chronology. It ha s lon g bee n recognize d tha t th e typica l this mode l i s tha t intervenin g highland s wer e geomorphology o f circum-Atlanti c continenta l covered b y relativel y stagnan t ice , ensuring tha t margin mountain s i s a patchwor k o f glaciate d subglacial froze n conditions , an d henc e land terrain and relict upland surfaces (Gjessing 1967 ; scape preservation , prevailed . Th e mos t visibl e alteration o f relic t uplan d surface s sinc e th e Sugden 1968 ; Sugden & Watt s 1977 ; Hall & commencement o f glaciatio n i n th e circum Sugden 1987 ; Ballantyn e 1994 ; Klema n & Stroeven 1997) . Fo r example , remnan t surface s Atlantic regio n ha s bee n th e deepenin g o f were recognize d i n th e Norwegia n an d Swedis h V-shaped valley s terminating in glacia l troughs. mountains earl y i n th e twentiet h centur y b y The deepenin g o f thes e V-shape d valley s Reusch (1901) , Wra k (1908 ) an d Ahlman n presumably happene d i n interglacia l times , a s (1919). Glacia l terrai n i s characterize d b y th e these river s adjuste d thei r profile s b y vertica l presence of U-shaped valleys , cirques, horn s and erosion toward s th e ne w base-leve l condition s arretes, valle y an d mountai n truncation , moun - produced b y troug h deepenin g durin g previous tain asymmetry , an d th e ubiquitou s presenc e of glaciation(s) (Ahlman n 1919 ; Rudberg 1992) . lakes (e.g . Sugden & Joh n 1976) . Relic t (o r The implication of this geomorphological history remnant) upland surfaces, on the other hand, are for interpretin g circum-Atlanti c continenta l characterized b y windin g V-shape d valleys , margin sediment s durin g Neogen e glaciatio n i s mountain symmetry , tors , weatherin g mantle s that offshor e sediment s wer e derive d primaril y and a n absenc e o f (water-filled ) roc k basin s from areall y an d altitudinall y restricte d sourc e (Figs 1 an d 2) . A patchwor k occurrenc e o f areas, rather than from equal erosion across much glacially scoure d an d relic t surface s indicate s of th e landscape . that lat e Cenozoi c subglacia l erosio n mus t have This proposed mode l of the geomorphologica l been areall y variabl e (Sugde n 1968 , 1974). This history i s base d o n fiel d an d remot e sensing patchwork patter n o f glacia l erosio n an d based mapping and interpretation of geomorpho preservation coul d eithe r resul t fro m restricte d logical feature s (e.g . Klema n & Stroeven 1997) . ice extents (glaciers limited to major valleys) or, This interpretation lead s to the following testabl e where we know that large ic e sheets covere d th e implications: (1 ) large-scal e relic t bedroc k mountains, fro m comple x pattern s o f ic e shee t morphology shoul d hav e surfac e exposur e age s much olde r tha n adjacen t glaciall y cu t surface s basal therma l regimes . On the basis of observations o f a patchwork of (potential ag e differences o f 10 5-106 years); (2) glacially scoure d an d relic t surfaces , typica l fo r bedrock erosio n o n glaciall y cu t surface s ha s glaciated passiv e margi n mountains , th e sub- been order s o f magnitud e highe r tha n o n relic t glacial therma l regime of average Quaternary ic e surfaces (rangin g fro m abou t 1 0 t o 10 3m); (3 ) sheets (Porte r 1989 ) was froze n o n th e upland s many bedroc k surface s hav e undergon e a and meltin g i n th e mai n valleys , wher e outle t complex histor y of multipl e burials (underneath glaciers an d ice-stream s forme d (Sugde n 1968, non-erosive ice ) and re-exposures . Thes e impli 1974). Relic t surface s ar e bes t preserve d a t cations, an d thu s th e large r mode l fo r th e intermediate elevations, lo w enough no t to hav e geomorphological history o f the circum-Atlanti c been covere d b y cirqu e glaciers , an d apparentl y mountains, can now be tested usin g an approach high enough not to have experienced melted-be d based o n measurin g multiple in situ cosmogenic conditions an d subglacia l erosio n durin g ic e nuclides in exposed bedrock . sheet overridin g event s (Klema n & Stroeve n This paper serve s to establish a framework for 1997). Hence , th e morphologica l erosiona l the interpretatio n o f i n situ cosmogeni c nuclid e impact o f glacier s an d ic e sheet s overridin g an d concentrations i n term s o f th e timing , pattern s expanding throug h thes e circum-Atlanti c con- and magnitud e of bedrock erosio n o f landscape s tinental margi n mountain s lef t a distinc t land - affected b y glacial , fluvia l an d periglacia l

IN SITU COSMOGENI C NUCLIDE S AN D PASSIV E MARGIN S

process systems . Formulating suc h a framework in advance minimizes the use of special pleading to interpre t forthcomin g dat a fro m glaciate d passive margi n mountains . The practica l appli cation o f thi s framewor k t o th e geologica l an d geomorphological 'traces ' o f th e last glacia l cycle establishes a set of attributes (typically, the timing, amoun t and patterns o f erosion) tha t can also serv e a s a basi s fo r interpretin g geo morphological evidenc e fro m prio r glacia l cycles. First , w e provid e a n overvie w o f cosmogenic nuclid e technique s an d theoretica l considerations. W e the n discus s th e geomor phological aspect s o f landscap e surfac e recon struction o n glaciate d passiv e margins . Finally , we illustrat e potentia l use s o f cosmogeni c nuclide technique s i n testin g erosio n historie s for glaciate d passiv e margins .

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Cosmogenic nuclid e productio n rate s var y with latitud e an d altitud e becaus e o f th e dissipation of cosmic radiatio n within the Earth's atmosphere an d th e dependenc e o f th e cosmic ray flu x o n the strengt h o f the Earth' s magneti c field (La i 1991 ; Robinso n e t a l 1995 ; Dunai 2000). Othe r variable s influencin g cosmogeni c nuclide productio n rate s ar e shielding o f incom ing cosmi c ray s b y surroundin g terrain , th e geometry of the sample surface, and erosion and/ or buria l o f th e sampl e sit e (La i 1987 , 1991). Provided th e geomorphologica l context o f th e sample is understood, appropriat e correction s fo r each of these variables ar e available (Nishiizumi etal 1989 ; Lai 1991; Dunne etal 1999 ; Fabel & Harbor 1999) . Surface exposure dating and erosion rates

With prolonge d exposure , cosmogeni c nuclide s accumulate withi n rock a s a function o f time and depth below th e surface . The time elapse d sinc e initial exposur e o f th e roc k surfac e ca n b e The cosmogenic radionuclides 10 Be and 26A1 and calculated fro m cosmogeni c nuclid e concen the cosmogenic stabl e nuclide 21Ne are produced trations i n th e rock , usin g know n rate s o f in rock s nea r th e groun d surfac e b y reaction s production. I f th e surfac e undergoe s erosion , with secondar y an d tertiary cosmic-ray neutrons depth profiles of nuclide concentrations, ratios of and muon s (Lai & Peters 1967) . Thes e nuclide s different nuclide s i n th e rock , an d nuclid e are commonl y use d i n studie s o f landscap e concentrations i n sediment s ca n al l provid e evolution (reviewe d b y Nishiizum i e t al. 1993; measures o f th e erosio n rat e (Cerlin g & Crai g Bierman 1994 ; Cerlin g & Crai g 1994 ; Fabe l & 1994; Grange r e t al . 1996) . Th e compariso n o f Harbor 1999) . Thi s i s because al l three isotope s concentrations of stable and radioactive isotope s are produced withi n a few metres o f the Earth' s can eve n facilitat e th e unravellin g o f comple x surface i n quartz, a ubiquitous mineral i n crustal histories o f exposure an d burial suc h as occurre d rocks and sediments, which has a simple 16 O and on glaciated passiv e margins. 28 Si target chemistry and a tight crystal structure Erosion o f a rock surfac e leads t o removal o f that minimizes diffusio n an d contamination . accumulated cosmogeni c nuclide s an d henc e a Applications of in situ produced cosmogenic 10 Be, 26 A1 and 21Ne

Fig. 1 . Landscap e typica l fo r glacia l erosion . Cirqu e glaciers hav e deepene d an d widene d bedroc k depressions i n the Part e Massif, Sare k Nationa l Park , northern Sweden , a mountai n massi f tha t wa s als o a prominent par t o f th e preglacia l landscape . However , the glacie r forefield , althoug h riddle d wit h lakes , ha s not bee n significantl y erode d belo w it s preglacia l elevation.

Fig. 2 . Landscap e shape d b y non-glacia l processes . Upland surfaces , Tarrekais e Massif, norther n Sweden , are though t t o retai n relic t morphology . Th e fluvia l valley, wit h interlockin g spurs , i s considere d t o b e younger in age and cut in response to (local) base-leve l lowering b y glacie r erosion . Thes e interlockin g spur s also show that glacial erosio n playe d no significant rol e in the evolutio n o f valley pattern .

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reduction i n nuclid e concentratio n (Fig . 3) . Because productio n decrease s roughl y exponen tially wit h dept h belo w th e surface , th e accumulated cosmogeni c radionuclid e concen tration in a mineral grai n record s the speed with which tha t grai n ha s bee n uncovered ; slowe r erosion rate s impl y longe r exposur e time s nea r the surface , an d thu s higher concentration s (Lai 1991). Therefore, t o calculate th e exposure tim e from a measure d cosmogeni c radionuclid e concentration i n a sampl e require s tha t th e erosion rate is known. If independent erosion rate evidence i s no t available , ther e i s n o uniqu e solution t o th e exposur e ag e calculatio n wit h a single radionuclide . However , a maximu m steady-state erosio n rat e ca n b e calculated . Th e cosmogenic radionuclid e concentratio n ca n b e used to calculate th e steady-state erosion rat e for the surfac e i f th e exposur e tim e ca n b e independently constrained , o r vic e vers a (se e Lai (1991 ) fo r equations). The measuremen t o f 10 Be an d 26 A1 concen trations in the sam e sampl e ca n provide a means of estimatin g bot h th e exposur e tim e an d th e steady-state erosio n rat e o f th e sampl e becaus e the ratio of the 26A1 and 10 Be concentrations i n an

eroding horizon changes sensitively with the rate of erosio n (La i & Arnold 1985) . Th e z °Al/luBe production rat e rati o i n quart z i s 6. 0 ±0.3 (Nishiizumi et al 1989) , regardles s o f th e absolute productio n rate . Becaus e 26 A1 decay s more rapidl y tha n 10 Be, th e 26 Al/10Be rati o decreases wit h increasin g exposur e time . Fo r a continuously expose d sampl e wit h n o erosion , the 26 Al/10Be rati o wil l follo w a smoothl y varying trajector y reachin g a n en d poin t where productio n an d radioactiv e deca y ar e balanced fo r bot h isotope s (Fig . 4 ; constan t exposure curve) . I f th e sam e sampl e i s subjec t to steady-stat e erosio n i t i s losin g mas s fro m the surfac e an d th e 26 Al/10Be rati o shoul d li e on th e steady-erosio n curv e a t a poin t determined b y th e erosio n rat e (Fig . 4) . Provided th e cosmic-ra y intensit y ha s remaine d constant th e 26 Al/10Be rati o wil l plo t betwee n these curve s (steady-stat e erosio n island ) fo r any simpl e exposure history under conditions of steady-state erosion . I t i s therefor e possibl e t o determine th e steady-stat e erosio n rat e an d exposure tim e o f a sampl e b y measurin g tw o cosmogenic radionuclide s wit h differen t half-lives i n the sam e sample .

Fig. 3. 10Be concentration v. depth for three differen t exposur e times and three different steady-stat e erosion rates. The first number i n the ratios is the exposure time (ka ) and the second i s the steady-state erosio n rate (cmka" 1). The curve s include 10 Be production by muon s and were calculate d accordin g to Granger & Smit h (2000) fo r a 10 Be productio n rat e o f 5. 1 atoms g"1 (SiO 2) yr" 1 an d a densit y o f 2.7gcirT 3. I f a glacia l erosio n even t removes 100c m o f bedrock (horizonta l dashe d line ) fro m th e surfac e afte r 10 , 10 0 and lOOOk a exposur e wit h zero steady-stat e erosio n (continuou s curves) , th e inherite d 10 Be concentration s in th e resultin g 'new ' surfac e are equivalen t t o apparen t 10 Be age s o f c . 2ka , c . 18k a an d c . 155ka , respectively.

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Fig. 4 . 26 Al/10Be ratio plotted agains t 10 Be concentration . 26 A1 (half-life 705 ka) decay i s relatively rapi d wit h respect to 10Be (half-life 150 0 ka), forcing the 26Al/10Be ratio of a sample to decrease exponentially over time (Lai & Arnold 1985 ; Klei n et al 1986) . If there is no erosion, 26 Al/10Be ratios will fall somewher e alon g the blue line, yielding an apparent exposure age. If there is steady-state erosion (measure d here in metres per million years), the ratio lies on the red steady-stat e erosion curv e a t a point determine d b y the steady-stat e erosion rate. The area between th e two curves is called th e steady-state erosio n islan d (La i 1991) . I f ratios plo t below tha t island, the n burial is inferred. Upon burial, ratios wil l fall i n the direction o f the dashed blac k arrow s an d burial tim e can be inferred whe n measure d ratio s are related to the 10 Be concentration in the sample (green lines) .

Erosion depths from cosmogenic radionuclide inheritance Passive continenta l margin s i n glaciate d regions, suc h as those i n Scandinavia , Scotland, Greenland, Canad a an d Antarctica , have appar ently repeatedly experienced hundred s of metres of erosio n underneat h outle t glacier s wherea s adjacent upland s remaine d unmodifie d (Sugde n 1968, 1974 ; Hall & Sugden 1987; Glasser & Hall 1997; Kleman & Stroeven 1997) . Hence, there is a remarkabl e spatia l variatio n i n subglacia l erosion tha t ma y b e reflecte d i n cosmogeni c radionuclide inheritance. Inheritance refers to the remnant cosmogeni c radionuclid e concentration from a prio r exposur e history . Thu s fa r ou r discussion ha s largel y deal t wit h cosmogeni c nuclide accumulation in the absence of, or under conditions o f steady-stat e erosion . Glacia l erosion i s a non-steady-stat e event . Whe n ic e overrides a rock surface it effectively shield s that surface fro m cosmi c ray s an d an y furthe r cosmogenic nuclid e accumulation . I f th e over riding even t i s erosive , cosmogeni c nuclide s accumulated befor e th e glaciatio n wil l b e removed. Dependin g o n th e dept h o f glacia l erosion, thi s cosmogeni c nuclid e remova l ma y

not b e complete. I n this case the surfac e retain s some o f th e cosmogeni c nuclide s and henc e a n inheritance fro m th e previou s exposur e even t (Fig. 3). The depth of erosion required to remove the entir e cosmogeni c nuclid e inventor y o f a previously exposed rock depends on the duration and erosio n rat e o f th e prio r exposur e histor y (Fig. 3) but is of the order of up to several metres. In case s wher e th e timin g o f glaciation s i s independently constraine d cosmogeni c nuclid e inheritance ca n b e used t o calculate the amount of roc k remove d b y th e glacia l even t (Brine r & Swanson 1998 ; Fabel & Harbor 1999) . Complex exposure and shielding histories Another challeng e i n datin g landscap e surface s on glaciate d passiv e continenta l margins is that the effects o f burial by ice in a complex exposure and burial history for long-lived surfaces need to be addressed . Man y bedroc k outcrop s o n glaciated passiv e margi n mountain s underwent a complex histor y of burials (typical duration of c. lOOka ) an d re-exposure s (typica l duratio n of c. 10 ka) (e.g. Kleman & Stroeven 1997) . We can approach unravelling complex burial histories by comparing multipl e cosmogeni c radionuclide s

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can be quantified. A comparison wit h PliocenePleistocene offshor e deposit s require s tha t th e amount, pattern and timing of this glacial erosio n is bette r understood . A s a firs t ste p w e nee d t o consider th e principle s fo r reconstructin g th e reference surface across areas that were dissected by glacial erosion . On th e larges t landscap e scal e tha t w e consider, tha t o f individua l mountai n blocks , glacial valley s an d th e mountai n foreland , reconstructing th e elevatio n an d shap e o f th e reference surfac e involve s considerations o f (1 ) the configuratio n o f th e preglacia l fluvia l drainage system , (2 ) fluvia l incisio n durin g interglacial periods, (3 ) glacial erosion of valleys by selective linea r erosion, (4 ) glacial erosion b y areal scouring , (5 ) localize d subglacia l modifi cation o f preglacia l uplan d surfaces , an d (6 ) lowering o f preglacia l uplan d surface s b y nonglacial processes . W e wil l firs t conside r th e significance o f eac h o f thes e erosiv e phase s fo r preglacial landscap e modification , an d the n consider th e applicatio n o f i n situ produce d cosmogenic nuclid e techniques to illuminate the magnitude, patter n an d timin g o f thes e erosio n phases.

with differen t productio n rate s an d half-live s (Bierman et al 1999 ; Fabe l & Harbor 1999) . The cosmogeni c nuclide concentratio n an d 26 Al/10Be rati o i n quart z a t th e groun d surfac e should plo t somewher e withi n th e steady-stat e erosion islan d (Fig. 4). If the surface is suddenly shielded fro m cosmi c radiatio n by ice, then 10 Be and 26 A1 production ceases . Radioactiv e deca y then lower s th e inherite d 26 Al/10Be rati o ove r time alon g th e dashe d blac k arrow s (Fig . 4 ) because 2e >Al decays faster than 10 Be (Lai 1991) . Burial tim e (gree n curve s i n Fig . 4 ) ca n b e inferred whe n measured ratio s ar e related t o the 10 Be concentratio n i n th e sampl e (Grange r & Muzikar 2001) . A s i n th e cas e o f obtainin g steady-state erosio n rates an d exposure duration , burial datin g requires steady-stat e conditions . A sample experiencin g a non-steady-stat e erosio n event afte r prolonge d exposur e a t lo w steady state erosion rates will also plot below the steadystate erosion island, but in this case burial cannot be inferred. This serve s t o illustrate the need fo r careful geomorphologica l interpretatio n whe n selecting sampl e sites . Although surfac e exposur e ages , erosio n rates , and buria l age s ca n b e obtaine d fo r well constrained situations , comple x surfac e exposur e histories ar e far more challenging . Interpretatio n of cosmogeni c radionuclid e concentration s i s much mor e complicate d i f a surfac e ha s experienced multipl e period s o f exposure , ero sion and burial, each of variable lengt h (Bierman etal. 1999). Stable nuclides such as N e provide an importan t additiona l too l t o assis t i n investigating comple x exposur e histories . Because 21 Ne i s stable , ther e i s n o reduction i n concentration durin g periods o f burial b y ice , i n contrast t o the radionuclides. Thus measurement s of 21 Ne provid e a n indicatio n o f tota l exposur e time, with loss owing to erosion, bu t not to burial. Measurement o f ! Ne permit s studie s o f exposure historie s beyon d th e limi t o f a fe w million year s imposed b y the l °Be and 26A1 halflives, an d compariso n wit h 10 Be an d 26 A1 concentrations shoul d provid e furthe r insigh t into buria l history . Thus , cosmogeni c 21 Ne ca n provide additiona l glaciologica l informatio n o f pertinence t o landscap e histor y an d landscap e surface reconstructio n (Niederman n et al. 1993) .

Configuration of the preglacial fluvial drainage system The preglacia l evolutio n o f th e fluvia l drainag e pattern o f th e passive-margi n mountain range in northern Scandinavi a ha s bee n controlle d pri marily b y uplift-induce d base-leve l change s i n the Balti c depressio n (Wra k 1908 ; Rudber g 1954; Lidmar-Bergstro m & Naslund 2002) . Th e eastern mountai n foreland i s characterize d b y a stepped 'plain s wit h residua l hills ' morpholog y generally a t 300-400 and 400-550 m above sea level (e.g . Frede n 1994 , pp . 50-51), referre d to as the Muddus Plains (Wrak 1908) . The Muddus Plains are the youngest preglacial fluvial surfaces of th e region , th e younges t o f whic h wa s th e last bas e leve l fo r th e evolutio n o f th e preglacial fluvia l drainag e syste m i n th e mountain rang e an d th e on e tha t mus t b e use d in an y reconstructio n o f upstrea m mountai n geomorphology.

Geomorphological principles for landscape surface reconstruction: examples from northern Sweden The presenc e o f relic t surfac e remnant s i n th e northern Swedish mountains provides a reference surface agains t whic h subsequen t glacia l erosio n

Fluvial incision during interglacial periods Fluvial incisio n o f th e preglacia l mountai n morphology continue d afte r th e Neogen e onse t of glaciation . Fo r example , Kleman & Stroeve n 1997, Figs 1 0 and 11 ) showed the distributio n of what ar e presumabl y Plio-Pleistocen e fluvia l valleys i n th e northwester n Swedis h mountains .

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These valley s ar e al l containe d withi n relic t th e size , dept h an d gradien t o f thes e fluvia l surface remnants , ar e lowere d relativ e t o thi s valleys reflec t th e amoun t o f tim e tha t wa s surface by < 300 m, and join prominent glaciall y available for cuttin g them . River s probabl y cut modified U-shape d valley s (Fig . 5) . Potentially , these valley s during interglacia l time s an d i n

Fig. 5 . (a) Stereogram o f the largest fluvial valley in the northern Swedish mountain range, Atnajakka, joining a glacial valley, the Teusajaure valley. The V-shaped valley has been lowered by 300 m below the valley bench at its mouth (panel b). The river, of 9 km length, has a concave valley profile and is graded to a level that is 110 m above the Teusajaur e valle y floo r (Klema n & Stroeve n 1997) . Abov e th e 'old ' valle y benc h (pane l c ) th e preglacia l surface extends towards the summits of the Karsatjakka mountain block. The Teusajaure valley, on the other hand, has been lowered by glacial erosion , initiatin g the accelerated downcuttin g of Atnajakka, leaving a typical valley floor (ice-moulde d bedroc k surface s an d elongate d lake s occupyin g overdeepenings ) an d valle y wal l glacia l morphology. Th e occurrenc e o f preglacial surfac e remnants helps i n reconstructing th e preglacial morphology . Cosmogenic nuclid e concentration s o f bedrock surface s i n eac h o f these thre e setting s (preglacial , glacia l an d fluvial) would potentiall y yiel d widel y differen t results . Thi s is because, on the basis of the geomorphologica l interpretation of Kleman & Stroeven (1997), (1) preglacial surfaces were preserved underneath cold-based ice and would tend to yield high concentrations of isotopes an d low surface erosion rates, (2 ) glacial surface s underwent intense modificatio n a t som e point durin g glaciation , resettin g th e cosmogeni c isotop e clock , an d (3 ) fluvia l surfaces have recorded maximu m surface lowering during interglacial period s and would tend to yield the lowest cosmogenic isotop e concentration s an d highes t surfac e erosio n rate s (Copyright : Nationa l Lan d Surve y o f Sweden 2002). (b ) Topographical ma p with profiles (Courtesy of the National Land Survey , 2002. Excerp t fro m GSD-elevation data , cas e numbe r L2000/646) . (c ) Topographica l profile s illustratin g potentia l cosmogeni c nuclide samplin g site s (dots , expecte d relativ e age s indicated ) an d possibl e preglacia l valle y reconstruction s (dashed). Fo r Profile s A-A ' an d B-B' , tw o alternative s hav e bee n given , reflectin g a n uncertaint y i n th e geomorphological interpretatio n o f the exten t of the preglacial surface .

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A. P. STROEVEN ETAL.

response t o base-leve l lowerin g a s a resul t o f glacial erosio n (Ahlman n 1919 ; Rudber g 1992) . Kleman & Stroeven (1997 ) hypothesized that the largest fluvia l valley , Atnajakk a (Fig . 5) , i s th e oldest valley , whic h implie s tha t th e U-shape d

valley whic h Atnajakk a joins , th e Teusajaur e valley, should be one of the oldest glacial valleys in th e mountai n range . Extendin g thi s logi c t o conclude tha t othe r glacia l valley s (som e o f which ar e much larger) must be younge r in age,

Fig. 6. (a ) Stereogra m o f the junction between a smal l glacia l valley , Vealevuomus , in the lowe r centre , an d a much larger glacia l valley , Rautas, in the upper centre (water-filled) . The floor of Vealevuomus is hanging 180 m above the floor of Rautas valley (panel b). Hanging valleys are typical features of glaciated landscapes (Sugden & John 1976 ) and ofte n a consequenc e o f selectiv e linea r erosion . Althoug h ice modifie d bot h thes e preglacia l valleys, it did s o more severel y i n Rautas valley, presumably because i t was deeper, wide r and better aligned fo r ice flow . Preglacia l surfac e remnant s occu r o n highe r surface s surroundin g thi s junctio n an d ca n ai d i n reconstructing th e preglacial valley topography (pane l c). Cosmogenic nuclid e concentrations o f bedrock surfaces in thes e valley s coul d potentiall y yiel d differen t results . This i s because , give n this differenc e i n topography , Rautas valle y ma y hav e bee n significantl y modifie d b y subglacia l erosion , whic h woul d rese t th e cosmogeni c isotope clock , wherea s basa l ic e i n Vealevuomu s ma y hav e remaine d belo w th e pressur e meltin g poin t an d inhibited erosion . Henc e cosmogeni c isotop e concentration s on multipl e isotopes coul d addres s difference s i n glacial erosio n rate s (Copyright : Nationa l Land Surve y o f Swede n 2002). (b ) Topographica l ma p wit h profile s (Courtesy o f the Nationa l Lan d Survey , 2002. Excerp t fro m GSD-elevatio n data , cas e numbe r L2000/646). (c ) Topographical profile s illustratin g potentia l cosmogeni c nuclid e samplin g site s (dots , expecte d relativ e age s indicated) an d possible preglacia l valle y reconstructions (dashed).

IN SITU COSMOGENI C NUCLIDES AND PASSIV E MARGINS

on th e basi s o f th e siz e o f th e fluvia l valley s draining int o them , represent s on e possibl e approach. However , thes e fluvia l valley s ma y be th e upper-reac h remnant s o f fluvia l valley s that wer e onc e muc h larger , bu t fo r whic h th e lower reaches have been truncated as subsequent glaciers widene d thei r valleys . A detaile d geomorphological stud y o f th e valle y benche s may revea l th e dept h t o which th e preglacia l valley wa s cu t befor e glaciall y induce d accelerated erosio n commenced . Glacial erosion by selective linear erosion Glacial modificatio n o f landscap e surface s occurs i n tw o distinctiv e patterns , b y selectiv e linear erosio n o f pre-existin g depression s o r areas les s resistan t t o erosion , an d b y area l scouring o f topographically les s variabl e terrai n (Sugden 1968 ; Sugde n & Joh n 1976 ; Harbo r 1995). Selective linear erosion occurs when there are extreme spatia l variation s in rates of erosio n under a n ic e sheet . Becaus e pressure-meltin g conditions a t th e bas e o f a n ic e shee t ar e necessary fo r bot h basa l slidin g an d extensiv e glacial erosion , an d thes e condition s ar e mos t common unde r thicke r ice , pre-existin g depressions an d the deepest o f preglacial valley s are lowere d preferentiall y b y glacia l erosion . This enhances relief an d has a positive-feedback effect o n subglacia l temperatur e an d pressure melting patterns (Oerlemans 1984 ; Maz o 1991) . Thus pre-existin g depression s an d valley s aligned wit h the ic e flow direction ar e preferentially eroded, an d other areas are subject to much less o r no erosion . Two feature s typica l o f passiv e margi n land scapes that can be explained in terms of selectiv e linear erosio n ar e shar p glacia l sur f ace-preglacial surfac e boundarie s (Fig . 5 ) an d hangin g valleys (Fig . 6) . The tota l relie f i n the norther n Swedish mountain s i s > 1 km bu t relie f withi n existing upland remnants is c. 600 m (Kleman & Stroeven 1997) . Hence , ther e wa s ampl e preglacial relie f t o establis h stron g subglacia l temperature gradient s favourin g preferentia l deepening o f pre-existin g fluvia l valley s o r preglacial depressions . I t is possible t o constrain the origina l dept h o f th e fluvia l valley s o r preglacial depression s base d o n (1 ) th e inferre d amount o f glacia l lowerin g fro m th e dept h o f 'glacial-age' fluvial valleys joining them, (2) the shape and gradient o f bordering relict slopes, and (3) fluvia l valle y gradient s betwee n rar e pre served 'upland ' preglacial-ag e valle y floors and the youngest lowland Muddus Plain. On the basis of these line s of reasoning i t is apparent tha t the larger glacial valley s have not been deepened b y

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more tha n c . 400 m (Fig . 5 b an d c) , althoug h some valley s appea r t o hav e experience d mor e erosion locall y (Klema n & Stroeven 1997) . Glacial erosion by areal scouring On a n ice-shee t scale , zone s o f area l scourin g occur wher e th e subglacia l therma l effec t o f topographical convergenc e an d divergenc e i s of insufficient magnitud e t o counterac t th e overal l subglacial meltin g regime . Thes e condition s occur wher e topograph y i s no t pronounce d compared wit h th e thicknes s o f th e ic e sheet , and where the heat of ice deformation suffices t o initiate basa l melting . Thes e condition s wer e apparently presen t wes t o f th e elevation axi s i n the norther n Swedis h mountains , probabl y because ic e accelerate d toward s th e fjorda l coast o f Norwa y (Klema n & Stroeve n 1997) . West o f thi s regio n o f area l scour , ic e flo w crossed a threshold i n basal topograph y an d ic e thickness, leadin g t o condition s o f selectiv e linear erosio n an d th e preservatio n o f high elevation perche d relic t surface s alon g th e Norwegian coas t (Wra k 1908 ; Dah l 1966 ; Peulvast 1985) . Reconstructing th e origina l elevatio n an d shape of landscapes that subsequently underwent areal scourin g i s especially challengin g becaus e there are few, if any, undisturbed surfaces against which t o compar e modifie d surfaces . On e approach is to view the areally scoured landscape as, at minimum, the result of stripping of material produced b y preglacia l chemica l weathering . Hence, th e minimu m amoun t o f erosio n wa s approximately equa l t o th e thicknes s o f th e weathering mantle s tha t wer e remove d (Glasse r & Hal l 1997) . Limite d observation s i n th e northern Swedis h mountain s indicat e a weath ering mantle thickness of < 10 m (e.g. Lundqvist 1985; Peulvas t 1985 ; Hirva s e t al 1988 ; Olse n et al. 1996 ; Re a et al 1996) . I n addition, glacial erosion ma y hav e extende d wel l belo w th e preglacial weatherin g base. Carefu l examinatio n of overridde n features , suc h a s degrade d cirqu e headwalls (Kleman & Stroeven 1997) , may yield an additiona l estimat e o f th e amoun t o f soli d bedrock removed . Finally , convergenc e mus t be sought betwee n thes e forme r estimate s o f weathering mantl e an d bedroc k erosio n an d a n interpolated surfac e betwee n th e westernmos t occurrences o f preglacia l remnant s i n Swede n and the easternmost outlier s in Norway. Localized subglacial modification of preglacial upland surfaces Detailed studie s o f surfac e morpholog y (e.g . Kleman 1992 ; Clarhall & Kleman 1999 ) indicate

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that subglacia l reorganizatio n o f som e area s within relic t uplan d surface s ha s occurred . Typically, thi s involve s sporadi c mino r erosion , transport an d depositio n o f th e weatherin g mantle (lee-sid e scarps , stoss-sid e moraines) , reshaping o f a weathering mantl e or til l int o till lineations (fluting) , erosio n o f bedroc k (striae) , and depositio n o f till . Withi n relic t area s evidence fo r subglacia l reworkin g i s generall y confined t o shallo w depression s (Klema n e t al. 1999). Boundarie s betwee n nonglacia l an d glacial surface s ar e sometime s extremel y shar p and hav e bee n interprete d a s longitudina l an d transverse subglacia l slidin g boundaries (Klema n & Borgstro m 1990 ; Klema n 1992) . I n the latte r case, erosio n occurre d o n th e le e sid e o f relic t hills (creatin g a scarp ) wherea s zone s o f deposition (o f th e erode d weatherin g mantle ) occurred o n the stos s side of relict hills (Kleman et a l 1999) . Currently , th e timin g o f landscap e modification ca n onl y be hypothesize d fro m th e association o f glacia l landform s (fo r example , the transvers e lee-sid e scarps ) wit h th e inferre d ice flow direction (and its age relative to other ice flow events).

processes o n slope s ar e ubiquitous , a s i s mountain-top detritu s o n uplan d flat s (e.g . Re a et a l 1996) . Th e presenc e o f preserve d periglacial phenomen a withi n uplan d relic t landscapes (e.g . Clarhal l & Klema n 1999 ) a s well a s activ e form s i n lowlan d location s indicates tha t landscape lowerin g b y periglacia l processes dominate s an d i s probabl y o f inter stadial age. Tors are also conspicuous features of periglacial landscapes. Generally, it is considered that tor s for m a s dee p chemica l weatherin g features (a n uneve n weathering surface) , whic h are subsequentl y uncovere d b y strippin g pro cesses (e.g. Linton 1955) . Suc h tors exhibit clear sheeting and rounded edges an d are found i n the northern Swedis h lowlands, where they survived despite ic e overridin g (e.g . Hattestran d & Stroeven 2002 ; Stroeve n et a l 2002) . However , tors i n th e norther n Swedis h mountain s exhibit an angula r frost-shattere d appearance , an d thi s type o f to r ha s previousl y bee n considere d t o have forme d o r t o hav e bee n uncovere d i n a periglacial environmen t (Palme r & Radle y 1961).

Lowering of preglacial upland surfaces by nonglacial processes

Application of in situ cosmogeni c nuclide techniques to passive margin erosion history

The lowerin g o f preglacia l uplan d surface s b y nonglacial processe s i s probabl y mainl y b y periglacial activit y a s indicate d b y (1 ) th e absence o f linea r feature s o f fluvia l origin , (2 ) the ubiquitou s presenc e o f gentl e convex concave slop e profiles , (3 ) th e presenc e o f sporadic permafrost , especiall y a t highe r elevations, whic h promote s periglacia l surfac e activity, (4) the ubiquitous presence o f active and relict periglacia l phenomen a acros s uplands , and (5) th e presenc e o f tor s o n interfluves . Th e absence o f fluvia l erosiv e feature s o n relic t upland surfaces , suc h a s dendritic rive r pattern s with interlockin g spurs , indicate s tha t fluvia l processes lef t n o significan t geomorphologica l imprint on the uplan d preglacial landscap e unti l accelerated fluvia l incisio n occurre d durin g interglacial periods . However , i n conjunctio n with th e sporadi c presenc e o f permafros t (Lundqvist 1962;Solli d e t a l 2000) , transpor t of solutes in (melt)water through the active layer probably promote d widesprea d bu t lo w rate s of long-term landscap e lowering . The mos t importan t processe s o f landscap e denudation i n thes e relic t areas , i t appears , ar e frost weatherin g an d periglacial slope processes, such a s solifluction . Fo r example , ston e stripe s and other sorted phenomena resulting from cree p

Measurements o f multipl e cosmogeni c nuclid e concentrations o f bedroc k surface s ca n poten tially be used to reconstruct th e timing and rates of landscap e chang e o f formerl y glaciate d passive continenta l margins . I n particular , the y can be used to (1) distinguish between surfaces of different age , (2 ) distinguis h betwee n surface s that experience d differen t erosio n histories , an d (3) addres s comple x exposur e an d shieldin g histories. O f these , calculation s o f long-ter m surface erosio n rate s ar e particularl y usefu l i n aiding reconstruction s o f th e origina l surfac e morphology acros s area s o f glacial erosion . The mos t fundamenta l postulat e tha t ca n b e tested usin g cosmogenic isotop e concentration s in bedroc k i s tha t 'preglacia l surfaces ' wer e preserved underneat h ic e sheet s an d thus can be used a s a referenc e horizo n agains t whic h t o measure th e magnitud e an d patter n o f glacia l erosion. I f these surface s wer e preserve d under neath cold-base d ice , the n th e cosmogeni c nuclide implication is that the difference betwee n the relativ e concentration s o f stabl e 21 Ne an d radioactive IO Be (i.e . th e 21 Ne-10Be contrast ) approaches th e maximu m possibl e give n th e glaciation histor y o f the passiv e margi n (Fig . 7 ) and that 26Al/10Be ratios are lower than expected for the other surfaces. If these surfaces are in fac t

IN SITU COSMOGENI C NUCLIDE S AN D PASSIV E MARGIN S

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Fig. 7 . Plot of the difference (%) between 21 Ne and 10 Be apparent exposure ages for a surface that has undergone a complex burial and exposure history imposed by the 8 18O record fro m DSD P Sit e 607 (top). The two curves are based o n the assumptio n tha t Mountain Ice Sheet s (MIS ) covered th e northern Swedis h mountain s when 8 18O was between 3.7 and 4.5%c, and Fennoscandian Ice Sheets (FIS) covered mountains and lowlands when 8 18O was >4.5%o. The grey shaded areas for both curves indicate 1 (Terrors if we assume that analytical precision fo r 10 Be and 21 Ne measurements ar e 5% and 20%, respectively .

not relic t a t all , an d wer e forme d eithe r i n postglacial time s o r b y significan t glacia l erosion, the n ther e shoul d b e n o relativ e difference betwee n thes e thre e isotopes ; tha t is , they should give the same apparent exposure age. There ar e thre e fundamentall y differen t bed rock surface s tha t coul d b e contraste d b y thei r cosmogenic nuclid e concentration s (Fig . 5) : relict surface s (unmodified , slightl y modified) , glacial surface s (selectiv e linea r erosion , area l scouring), an d fluvia l surface s (preglacia l age , glacial age). One would predict the following: (1) the unmodifie d relic t surface s woul d hav e th e highest concentration s o f isotopes , th e lowes t 26 Al/10Be ratios , th e highes t 2l Ne-10Be con trasts, an d th e lowes t surfac e erosio n rates ; (2 ) glacial surface s o f selectiv e linea r erosio n (U-shaped valleys ) woul d yiel d deglaciatio n ages o r olde r (dependin g o n th e amoun t o f glacial erosio n and , therefore , th e inheritanc e

signal), have 26Al/10Be ratios close to or equal to six, an d smal l 21 Ne-10Be contrasts ; (3 ) fluvial surfaces, if activel y forme d durin g interglacia l periods (Rudber g 1992 ; Klema n & Stroeve n 1997), includin g the presen t interglacial , woul d have th e lowes t cosmogeni c isotop e concen trations, 26Al/10Be ratios equal to six, 21Ne-10Be contrasts of zero, and the highest current surface erosion rates. If the stability of upland surfaces is supported b y th e isotopi c data , additiona l geomorphological interpretation s o f landscap e change ca n b e tested , an d preglacia l landscap e reconstructions based o n geomorphology ca n be strengthened. In th e cas e o f hangin g U-shape d valleys , cosmogenic nuclid e approache s provid e potential fo r differentiatin g between (1 ) glacia l valleys of different age , where the hanging valley is lef t 'hig h an d dry ' preserve d underneat h erosionally ineffectiv e ice , an d (2 ) valley s tha t

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are contemporaneou s bu t experience d differen t erosion rate s a s a resul t o f differin g subglacia l conditions (th e amoun t o f tim e availabl e fo r erosion and/or the rate o f erosion). I n the case of the Vealevuomu s an d Rauta s valley s (Fig . 6), if both valley s underwen t significan t erosio n during th e sam e glacia l even t (i.e . (2 ) above) , then bedroc k sample s fro m th e tw o location s should yiel d simila r exposur e ages , 26 Al/10Be ratios equa l t o six , and ^e-^B e contrast s of zero. However , i f basa l ic e i n Vealevuomu s remained belo w th e pressur e meltin g poin t an d inhibited erosio n (i.e . (1) above) , the n bedroc k samples i n thi s valle y shoul d hav e a n inheri tance signa l an d yiel d olde r apparen t exposur e ages, lowe r 26 Al/10Be ratios , an d large r 2 ^v[e-10Be contrast s tha n sample s fro m th e Rautas valley . Finally, cosmogeni c radionuclid e measure ments on bedrock samples ca n also contribute to resolving issue s o f glacia l modificatio n an d nonglacial lowerin g o f preglacia l uplan d sur faces. Fo r example , tor s ca n b e use d t o infe r minimum rates of preglacial landscap e lowering . Cosmogenic nuclid e concentration s o f multipl e isotopes from tor summit flats should indicate the timing o f to r exposur e an d long-ter m surfac e erosion rates . Thes e long-ter m surfac e erosio n rates on tors provide minimum long-term erosio n rates fo r th e surroundin g summi t flats . Thi s i s because th e erosio n rate s o f tors, assumin g their formation or uncovering occurred i n a periglacial environment, hav e t o be lowe r tha n those o f the surrounding summi t flats , otherwis e th e tor s would no t exist . A cosmogeni c radionuclid e study o f upland terrai n i n the Rock y Mountains, USA, yielde d a long-ter m surfac e lowerin g rat e of 10 m Ma" 1 fo r summi t flat s (Smal l e t al. 1997). Thi s valu e appear s reasonabl e fo r north ern Swedish condition s whe n compared wit h the height o f individual tors abov e thei r surrounding (generally < 10m ) and the amount of erosion by fluvial and glacial processes on adjacent surfaces (an orde r o f magnitude higher) . A framewor k fo r interpretin g cosmogeni c isotope concentration s i n term s o f patterns , rates an d timin g o f landscap e change , a s i s presented here , an d develope d befor e interpret ation o f actua l data , give s maximu m credibilit y to dat a interpretatio n an d help s i n effort s t o resolve difference s i n regional dataset s t o arriv e at a commo n interpretation . Th e practica l application o f thi s framewor k t o th e geologica l and geomorphologica l trace s o f th e las t glacia l cycle, wil l hel p establis h a se t o f attribute s (typically, erosio n rate s an d patterns ) tha t serv e as a workin g mode l fo r interpretation s furthe r back i n time .

Concluding remarks : uneve n sedimen t production o n glaciated passive margin s The glacia l chronologie s o f severa l passiv e margin mountain s ar e wel l understoo d a t a general leve l (e.g . Mangeru d e t al . (1996) , Kleman e l al . (1997 ) an d Klema n & Stroeve n (1997) fo r Scandinavia) . A patchwor k o f glaciated terrai n an d preglacia l relic t uplan d surfaces i n thes e mountain s reflects (1 ) the tota l erosive impac t of the lat e Cenozoic glacier s an d ice sheet s tha t covere d th e mountains , an d (2 ) that thi s subglacia l erosio n mus t hav e bee n areally restricted . What is not well established is the timin g an d patter n o f erosio n an d landscap e development associate d wit h thi s glacia l chron ology, a n issu e o f critica l importanc e whe n attempting a comparison o f th e onshor e erosio n histories an d offshor e sedimen t accumulation s (e.g. Glasse r & Hall 1997) . Conventional datin g an d analysi s hav e provided a n excellen t wa y t o begi n unravellin g the timing an d patter n o f erosion , landfor m devel opment, an d possibl e landfor m preservatio n under ice . However, attempt s t o utiliz e geomor phology an d remot e sensin g technique s hav e often bee n frustrate d b y th e limitation s o f conventional datin g techniques . W e describ e a new approac h fo r investigatin g landscape evol ution in mountainous areas, the in situ production of cosmogeni c nuclide s i n bedroc k surface s o f landscapes affecte d b y glacial, fluvial (preglacial or interglacial ) an d periglacia l (interstadial ) process system s (Cerling 1990 ; Nishiizumi et al. 1993; Bierma n 1994 ; Cerlin g & Crai g 1994 ; Small & Anderson 1995 ; Fabel & Harbor 1999) . Cosmogenic nuclide s produced i n rocks nea r the ground surface by reactions with cosmic rays can be use d t o determin e apparen t surfac e exposur e age an d landscap e preservation , an d constrai n erosion depth s an d duratio n of burial b y ice. This pape r present s a coherent framewor k for interpreting measurement s o f i n situ produce d cosmogenic nuclide s i n bedroc k i n term s o f timing, rates and patterns of landscape change on glaciated passiv e margins . Th e applicatio n o f cosmogenic nuclid e techniques to the reconstruction o f preglacia l surface s provide s ne w infor mation t o complemen t tha t obtaine d usin g traditional geomorphologica l approache s (Andersen & Nesj e 1992 ; Nesj e & Whillan s 1994; Rii s 1996 ; Glasse r & Hal l 1997) . Th e cosmogenic nuclid e techniqu e ha s previousl y been use d t o investigat e aspect s o f landscap e change i n glaciate d region s suc h a s th e ag e o f specific glacia l event s (e.g . Phillips e t al . 1990 , 2000; Broo k e t al. 1995 ; Ston e e t al 1996 ; Ivy Ochs e t al . 1997 ; Bierman e t al . 1999 ; Jackson

7W SITU COSMOGENI C NUCLIDE S AN D PASSIV E MARGINS et al 1999 ; Schafe r e t al 1999) , erosio n rate s over glacia l surface s (Nishiizum i e t a l 1989 ; Brook e t a l 1995 ; Brine r & Swanso n 1998) , landscape denudatio n rate s (Smal l e t a l 1997 ; Summerfield e t a l 1999) , an d th e elevatio n o f former ice sheet surface profiles (see Brook et al 1996; Ston e e t a l 1998 ; Acker t e t a l 1999 ; Kaplan e t a l 2001) . However , compare d wit h these othe r studies , a strengt h o f th e structur e presented her e i s tha t i t i s develope d before , rather tha n as a response to , dat a interpretation . We no w hav e a framewor k withi n whic h t o interpret nuclid e concentration s i n term s o f apparent exposur e ag e an d surfac e erosio n rates, an d whic h provide s a n overvie w o f sampling strategie s t o determin e th e timing , rate and pattern of landscape change on glaciated passive margins. This is particularly important as complex burial-shielding and exposure histories have probabl y affecte d al l sample s wher e substantial preservation length has been inferred from geomorphologica l evidence . Given the number of glacial cycles (c. 40) that have affected th e landscape in the last 2.7 Ma, we cannot expect cosmogeni c nuclid e techniques to resolve individua l events beyond the last glacia l cycle. However , throug h cosmogeni c datin g o f key landscap e element s w e ca n establis h th e pattern an d selectivit y o f erosio n b y th e las t ic e sheet, thereb y establishin g a mode l fo r th e erosional functionalit y o f ic e sheet s i n rugge d terrain. Onc e suc h a model i s erected , i t ca n b e used to provide insight into process pattern s and interrelationships acros s th e glacia l an d inter glacial cycle s o f th e Quaternar y period . Thi s provides a soli d foundatio n fo r attempt s t o improve geomorphologicall y base d reconstruc tions o f preglacia l surfaces , reconstruc t an d analyse th e dynamic s o f landscap e chang e i n glacial time , an d defin e th e consequence s o f different proces s regime s i n term s o f erosio n patterns, sedimen t transpor t an d th e suppl y o f sediments tha t ar e deposite d offshore . Th e thickness o f offshor e sediment s i s on e ke y ingredient considere d i n hydrocarbo n explora tion. However , despit e th e fac t tha t importan t quantities of sediment were generated by glacia l processes, curren t technique s o f estimatin g onshore exhumatio n pattern s an d rate s d o no t resolve margin exhumation during late Cenozoi c time (e.g.Lidmar-Bergstrom & Naslund 2001). It is thes e offshor e sediments , ofte n use d i n generalized recontruction s o f passiv e margi n development (e.g . Riis 1996 ) that , in th e future , will provide the link between offshore patterns of passive margi n sedimentatio n an d onshor e reconstructions o f passiv e margi n evolution . Furthermore, studie s o f th e erosiona l histor y o f

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upland landscape s wil l als o ai d i n resolvin g climate debate s wher e the sourc e are a o f erode d sediments i s o f ke y importance , suc h a s th e mechanisms leadin g to , an d th e implication s o f Heinrich event s fo r circum-Atlanti c climat e change an d landform development .

This paper has benefited fro m a detailed formal review by P . Bishop , an d fro m comment s b y K . Lidmar Bergstrom (informa l review) and A.G. Dore. Funding for par t of the research reporte d her e was provided by the Swedish Natural Science Research Counci l (NFR), Grants G-AA/G U 12034-30 0 an d G-AA/G U 12034 301, an d b y th e U S Nationa l Scienc e Foundation , Grant OP P 9818162. This manuscrip t was complete d while J.H. was supported by the New Zealand-United States Educationa l Foundatio n a s a Fulbrigh t Senio r Scholar.

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The thermotectonic developmen t o f southern Sweden durin g Mesozoic an d Cenozoic tim e CHARLOTTE CEDERBOM The CRUST Consortium, Department of Geology and Geophysics, Edinburgh University, Grant Institute, King's Buildings, Edinburgh EH9 3JW, UK (e-mail: cederbom@ glg.ed.ac.uk) Abstract: Lat e Carboniferous-Earl y Mesozoi c exhumatio n o f souther n Swede n ha s previously bee n trace d usin g apatit e fission-trac k thermochronology . I n addition , th e morphotectonic development of the region has been studied usin g geomorphology . The aim of thi s stud y i s t o attai n furthe r knowledg e o f th e Mesozoi c an d Cenozoi c thermotectoni c development o f souther n Swede n b y integratin g result s fro m thes e methods. Well-dated re-exposed palaeosurface s an d sedimentar y record s in the surroundin g area s were used a s constraints i n the modelling of apatite fission-track data from th e Precambrian basement. Th e obtaine d modelle d therma l historie s sugges t tha t souther n Swede n ca n b e divided into three main tectoni c areas associated with differen t cooling histories. In Triassic and Jurassic time, lo w to moderate exhumation i n the central part wa s accompanied by more rapid exhumatio n i n the S E and NW. Additionally, individua l block movements may hav e occurred i n th e NW . I t ha s als o bee n possibl e t o estimat e th e heatin g effec t o f renewe d Cretaceous-Paleogene burial t o 20-35 °C on the west an d SE coasts. Final Cenozoic unroofing o f the basement is indicated b y the modelled thermal histories. Areas aroun d th e souther n ti p o f Lake Vatter n togethe r wit h th e S E coast experience d th e most pronounce d exhumatio n compare d wit h th e surroundin g parts .

Southern Swede n i s characterize d b y a Precambrian basement , whic h emerge s a s a dome-shaped structur e fro m belo w lowe r Palaeozoic cove r rock s i n th e nort h an d eas t and Mesozoi c cove r rock s i n th e sout h an d west. Large-scal e Phanerozoi c event s i n th e area ca n therefor e b e identifie d onl y by therma l and relie f studie s of the basement . Palaeozoi c heating o f centra l an d souther n Swede n a s a result o f th e developmen t o f a Caledonia n foreland basi n ha s bee n establishe d (Larson et al 1999 ; Cederbo m e t al. 2000 ; Cederbo m 2001). I n addition , large-scal e Palaeozoi c t o Early Mesozoi c exhumation , perhap s accompanied b y tectonism , ha s bee n demon strated base d o n apatit e fission-trac k (AFT ) data fro m souther n Swede n (Cederbo m 2001) . From anothe r direction , Mesozoi c an d Tertiar y morphotectonic event s have been inferre d base d on studie s o f th e relie f an d it s relatio n t o remnants o f the cove r rock s (Lidmar-Bergstro m 1994, 1996) . I n thi s study , result s fro m AF T thermochronology ar e integrate d wit h geo morphological dat a t o attai n furthe r insigh t into th e Mesozoic an d Cenozoic thermotectoni c development o f souther n Sweden .

Palaeosurfaces The palaeosurface s o f souther n Sweden (Fig . 1 ) have been analyse d and roughly dated by mean s of their relative position, remnants of Palaeozoi c and Mesozoi c cove r rocks , an d saprolit e occurrences (e.g . Lidmar-Bergstro m 1982 , 1988, 1995) . I n addition , a mode l fo r th e long term morphotectoni c evolutio n o f souther n Sweden ha s bee n suggeste d (Lidmar-Bergstro m 1994, 1996) . Th e mai n type s o f palaeosurfaces, defined an d interprete d b y Lidmar-Bergstrom , are presente d in Fig . 1 and summarize d belo w (see Lidmar-Bergstro m 1996 , Fig . 2) . In th e nort h an d east , Cambria n strat a res t unconformably o n Precambrian basemen t rocks . The relie f o n this basement surfac e is extremely flat. I t i s possibl e t o trac e thi s exhume d Sub Cambrian Peneplai n (SCP ) fro m belo w lowe r Palaeozoic deposit s in the Lake Vattern and Lake Vanern are a an d alon g th e eas t coast , t o th e summits i n th e centra l par t o f souther n Swede n (Fig. 1 ; Lidmar-Bergstrom 1988) . A hill y etc h surfac e emerge s fro m belo w Upper Cretaceous cove r rocks in the SE and SW (Fig. 1) . The hill y relief i s associated wit h thick

From: DORE , A.G. , CARTWRIGHT , J.A. , STOKER , M.S., TURNER , J.P . & WHITE , N . 2002 . Exhumation o f th e North Atlantic Margin: Timing, Mechanisms an d Implications for Petroleum Exploration. Geologica l Society , London, Special Publications, 196, 169-182 . 0305-8719/027$ 15.00 © The Geological Societ y of London 2002 .

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Fig. 1. Simplified map of palaeosurfaces an d Phanerozoic sediments in southern Sweden (modified fro m LidmarBergstrom 1996) . Sample localities for apatite samples modelled in this study (•) ar e shown together with sample localities fo r additiona l apatit e sample s no t use d i n thi s stud y (O) (Larso n e t al. 1999 ; Cederbom el al. 2000 ; Cederbom 2001) . A, B and C indicat e the three mai n tectonic area s discusse d in the tex t (borders marked with bold dashed lines) . The elevations fo r the two highest points in the area are given in metres above sea level (a.s.L). CDF, Caledonia n Deformatio n Front ; FBZ, Fennoscandian Border Zone; SKP , Skagerrak-Kattega t Platform.

kaolinitic saprolit e remnants , consisten t wit h deep weatherin g durin g war m an d humi d conditions (Lidmar-Bergstro m 1995) . Thi s typ e of relief continue s alon g the entire west coast and is interpreted a s a re-expose d sub-Mesozoi c palaeosurface (Lidmar-Bergstro m 1994) .

A disintegrated part of the SCP is documented at c . 200 m elevatio n i n th e centra l par t o f southern Swede n (Fig . 1) . Farthe r south , a n almost horizonta l plain with low relief i s define d as th e Sout h Smalan d Peneplai n (SSP) . Thi s surface truncate s the sub-Mesozoi c etc h surfac e

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Fig. 2. Extensive lineaments (a), open linear structures (b) and sharp and straight linea r structures , shar p line s (c), i n souther n Swede n (modifie d fro m Tire n & Beckholmen 1992) .

(Lidmar-Bergstrom 1982 ) at abou t 100-125 m above se a leve l (a.s.l. ) an d i s therefor e inter preted a s a younge r surface . Frequent remnant s of gravelly saprolites, consistent with weathering during cold and humid conditions, are associated with thes e surface s (Lidmar-Bergstro m el ai 1997). Th e ag e o f th e gravell y saprolite s i s difficult t o constrain, but they are certainly olde r than Late Weichselian time , and their position in areas protecte d fro m glacia l erosio n give s support fo r a t leas t a Plio-Pleistocen e age . Possibly, the y ha d forme d alread y i n Miocen e time (Lidmar-Bergstro m e t al 1997) . Th e SS P and th e disintegrate d par t o f th e SC P ar e bot h interpreted a s Tertiary surface s that were formed by stepwis e exhumatio n an d exposur e o f th e basement durin g war m an d dr y condition s

(Lidmar-Bergstrom 1991) . Furthermore , th e vast exten t o f th e SS P indicate s a considerabl e time fo r it s formation. Faults and lineaments The regiona l occurrenc e an d exten t o f Phaner ozoic fault s i n souther n Swede n i s no t wel l documented. Ahli n (1987 ) presente d a stud y of Phanerozoic fault s i n th e northwester n part o f southern Sweden , base d o n fiel d wor k an d th e reconstruction o f th e SCP . Frequent NE-SW trending Phanerozoi c fault s wit h a n offse t o f <100m hav e bee n observed , an d i n Lak e Vanern, a n offse t o f 'som e 10 0 metres' i s recorded (Ahli n 1987) . I n addition , large-scal e north-south-trending fractur e zone s appea r

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along the eastern sid e of Lake Vatter n (Persson & Wikman 1986) . Th e lak e i s propose d t o b e a graben structur e (e.g . Lin d 1972 ; Persso n e t al 1985), an d i t contain s a c . 1000 m thicknes s o f Late Proterozoi c sediment s (e.g . Vida l 1984) . However, i n most area s of southern Swede n only lineaments an d fractur e zones , whic h appea r either o n aeromagneti c map s and/o r i n the field, have been mapped . I t is unknown whether som e of the m constitut e fault s o r not . The y ar e presented i n a fe w 1:5000 0 bedroc k map s (covering c . 20 % o f th e investigatio n area ) an d six 1:25000 0 temporar y genera l bedroc k map s published b y th e Swedis h Geologica l Survey , most o f which ar e not digitized . Phanerozoic fault s and fractures have also been interpreted fro m th e Relativ e Relie f Ma p o f Sweden (Lantmateriverke t 1986) , contou r map s and vertica l topographi c profile s (Elvhag e & Lidmar-Bergstrom 1987; Lidmar-Bergstro m 1991, 1996) . Relativ e relief maps are constructed by computerize d hil l shading , i.e . obliqu e illumination o f a terrai n mode l tha t i s base d o n topographic map s (Elvhage & Lidmar-Bergstrom 1987). Fro m th e Relativ e Relie f Map , th e topographically well-expresse d fractur e lines , rather tha n th e complet e fractur e pattern , wer e identified (Elvhag e & Lidmar-Bergstro m 1987 , p. 347) . I n comparison, o n the basi s o f the sam e Relative Relie f Map , Tire n & Beckholme n (1992) presente d a n analysi s o f th e tectoni c rock bloc k patter n i n souther n Swede n togethe r with a detaile d analysi s o f th e variou s fractur e sets. T o date , thi s i s th e onl y publishe d compilation o f linea r structure s for th e whol e of southern Sweden . In Fig . 2 , thre e type s o f structure s ar e illustrated (extensiv e lineaments , ope n linea r structures, shar p an d straigh t linea r structures) , which were presented an d interpreted by Tiren & Beckholmen (1992) . Th e uncertaint y of individual structure s i s high , bu t som e genera l trend s can be discerned fro m th e lineament pattern. The plot o f extensiv e lineament s (Fig . 2a ) illustrates that a bel t o f c . 50k m width , includin g Lak e Vattern, run s i n a N20 E direction , dividin g southern Swede n int o tw o halve s (Tire n & Beckholmen 1992) . Southward s th e bel t widen s and deflect s westwards , ou t int o th e sea . T o th e south, th e orientation o f lineaments is influenced by th e NW-SE-oriented Fennoscandia n Borde r Zone (Fig . 1) . Open structure s (Fig . 2b) , whic h represent flexures , th e terminatio n o f th e SC P and wide , trough-lik e valleys , ar e scattere d throughout souther n Sweden . I n contrast , shar p and straigh t linea r structure s (Fig . 2c ) ar e recognized mainl y i n th e northwester n par t o f southern Sweden. The surface nature of the sharp

lineaments i s no t known , bu t the y ar e probabl y the younges t structure s detecte d fro m th e Relative Relie f Ma p (Tire n & Beckholme n 1992). Modelling of AFT data The dating method Fission track s ar e create d b y th e spontaneou s fission o f 238 U an d ar e forme d continousl y through time . The y becom e instantaneousl y annealed a t hig h temperatures , an d th e numbe r of track s (i.e . th e trac k density ) i s therefor e proportional t o the time that has passed sinc e the grain coole d belo w a certai n temperatur e an d track retention started. For apatite, this temperature varie s b y severa l ten s o f degree s aroun d 100°C depending on the duration of heating and the minera l compositio n (Gleado w & Dudd y 1981; Naese r 1981 ; Carlson e t al 1999) . Furthermore, i t i s possibl e t o investigat e the thermal histor y i n apatit e b y studyin g th e distribution o f trac k lengths . Relativel y slowly cooled basemen t sample s ar e characterize d b y wide, ofte n negativel y skewe d trac k lengt h distributions wit h shorte r mea n trac k length s and highe r standar d deviation s compare d with , for example , volcani c sample s tha t hav e coole d very rapidly. Mixed track length distributions are formed whe n track s fro m earlie r event s hav e survived partia l reheatin g and a ne w population of tracks from th e las t period o f cooling is added to the older generation of tracks (Gleado w et a l 1986a, 1986b) . Forward modelling of AFT data Forward modellin g of AF T dat a i s the testin g of potential therma l historie s b y comparin g mod elled an d observe d AF T results . Th e annealin g model fo r apatit e presente d b y Ketcha m e t a l (1999) an d th e AFTSolv e modellin g progra m constructed b y Ketcha m et al (2000 ) wer e use d in thi s study . I n contras t t o earlie r publishe d apatite annealing models (i.e. Laslett et al 1987 ; Carlson 1990 ; Crowle y e t a l 1991 ; Laslet t & Galbraith 1996) , the Ketcham et al. (1999) model attempts to account for the variation in annealing behaviour tha t exist s betwee n differen t apatite s as a resul t o f compositiona l variations . Whe n comparing th e Ketcha m e t a l (1999 ) annealing model wit h the precursor models, it turned out to be mor e sensitiv e t o bot h low-temperatur e an d high-temperature annealin g tha n it s precursors . Generally, significantl y lowe r Lat e Palaeozoi c Early Jurassi c an d Paleogen e temperature s were obtained whe n usin g th e Ketcha m e t a l (1999 )

Table 1. Modelling results

Sample number Area A B14 S9819a S9819b P9910 S9620 S9621 P9909a P9909b S9622 S9623 9901 AreaB S9624a S9624b P9906a P9906b P9906c S9626 9802 9807 9808 Area C P9904 9905 9903a 9903b SA9629a SA9629b 9902 9805

Modelled MTL (ixm)

No. of grains

Observed FTA (Ma)

Modelled FTA (Ma)

GOF

No. of lengths

20 27 27 20 23 20 20 20 10 20 20

313 ± 25 208 ± 10 208 ± 10 161 ± 9 313 ± 17 220 ± 14 149 ± 8 149 ± 8 211 ± 16 188 ± 10 175 ± 12

322 209 214 160 313 226 150 149 218 188 176

0.72 0.92 0.55 0.94 1.0 0.68 0.86 0.98 0.65 0.99 0.90

50 100 100 100 100 110 59 59 100 100 90

13.6 12.7 12.7 12.7 13.1 12.7 13.1 13.1 12.4 12.8 13.6

± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.1

13.7 12.7 12.6 12.7 13.2 12.6 13.1 13.1 12.5 12.8 13.6

± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.2

0.58 0.31 0.30 0.26 0.50 0.51 0.71 0.74 0.64 1.0 0.39

11 11 20 20 20 20 21 20 20

261 ± 261 ± 213 ± 213 ± 213 ± 282 ± 231 ± 240 ± 231 ±

17 17 14 14 14 15 12 12 11

264 262 215 212 216 283 230 240 232

0.86 0.97 0.87 0.91 0.81 0.93 0.90 1.0 0.96

92 92 100 100 100 100 97 100 100

13.0 13.0 12.4 12.4 12.4 13.3 13.4 13.1 13.6

± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.1 ± 0.2 ± 0.1

13.0 13.1 12.6 12.5 12.5 13.4 13.4 13.1 13.5

± 0.2 ± 0.2 ± 0.3 ± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.2

0.89 0.97 0.11 0.70 0.81 0.45 0.38 0.51 0.27

20 14 20 20 21 21 20 23

196 ± 12 188 ± 15 165 ± 10 165 ± 10 172 ± 9 172 ± 9 160 ± 9 189 ± 9

198 188 165 169 174 171 161 188

0.84 0.96 0.99 0.63 0.86 0.93 0.90 0.90

80 22 52 52 166 166 100 100

13.0 12.8 12.8 12.8 13.5 13.5 12.3 12.9

± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.1 ± 0.1 ± 0.2 ± 0.1

13.0 12.9 12.8 13.1 13.5 13.4 12.4 12.9

± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.2

0.81 0.76 0.77 0.29 0.60 0.51 0.62 0.44

Observed MTL

K-S test

No. of grains, number of grains dated for each sample ; FTA, apatite fission-trac k age ; GOF, goodness o f fit; no. of lengths, number of track length measurements in eac h track lengt h distribution; MTL, mea n track length ; K- S test , Kolmogorov-Smirno v test . Th e datin g results were originall y publishe d b y Cederbo m e t al (2000 ) (fo r SA9629) and by Cederbom (2001) .

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C. CEDERBOM

Fig. 3. Modelled thermal histories for apatite samples fro m souther n Swede n (fo r sample localities , se e Fig. 1). Only th e 'discernibl e part' o f the modelle d Phanerozoi c history i s illustrate d fo r eac h sample .

annealing model compared with when the Laslett et al (1987 ) o r th e Crowle y e t al (1991 ) annealing mode l wa s adopted . A detaile d comparison betwee n th e annealin g model s wa s presented b y Ketcha m e t al. (1999) . The datase t use d i n thi s stud y doe s no t contain an y kineti c parameter s (i.e . trac k length orientation s o r etc h pi t widths) , an d th e

initial trac k lengt h wa s therefor e se t t o a fixe d value o f 16. 2 (jum. I t i s importan t t o hav e i n mind tha t th e therma l historie s presente d i n this stud y ar e no t unique . I n forwar d modelling, a s wel l a s i n invers e modelling , somewhat differen t therma l historie s tha t hav e not bee n trie d ma y als o fi t th e observe d AF T results.

THERMOTECTONIC DEVELOPMEN T O F SOUTHERN SWEDEN

Fig. 3. Continued

175

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C. CEDERBOM

In general, 2 0 grains should be dated to obtain a statisticall y reliabl e AF T ag e and , similarly, 100 track length s should be measured. Modelle d thermal histories for samples having fewer single grain ag e determination s and/o r trac k lengt h measurements (se e Table 1 ) must be interprete d with caution. Two statistical methods are used to gauge how well th e dat a an d th e mode l result s fit . Th e Kolmogorov-Smirnov test (K-S test) is used to compare th e measure d an d th e predicte d trac k length distributions and the GOF is the 'goodness of fit' between the age data and the age predicted by th e model . Fo r bot h statistics , a probability value >0. 5 implie s tha t th e modelle d therma l history i s 'supported ' b y th e dat a an d a probability value between 0. 5 and 0.05 indicates that th e modelle d therma l histor y i s 'no t rule d out' b y th e dat a (Ketcha m e t al 2000) . There fore, a value of 0.05 is regarded a s the lower limit for acceptance, although a value > 0.5 was aimed for when modelling. A detailed description of the statistical method s ha s bee n give n b y Ketcha m et al (2000) . Geological and geomorphological modelling constraints In modelling , severa l constraint s wer e set . Because al l sample s wer e collecte d a t o r clos e to the SCP , they ar e known to have experience d near-surface temperature s durin g Earl y Cambrian time . B y th e en d o f th e Caledonia n orogeny, souther n an d centra l Swede n wer e covered b y thick foreland basin deposits (Larso n et a l 1999 ; Cederbom e t a l 2000 ; Cederbo m 2001), resultin g i n tota l annealin g o f al l AF T samples i n souther n Sweden . Thi s heatin g wa s not se t a s a constraint whe n modelling, bu t wa s necessary t o obtai n a fi t betwee n observe d an d calculated AF T data . Additionally , heating t o at least 9 0 °C was required . Documented Cretaceou s remnants , lyin g directly o n basemen t o n th e wes t an d S E coas t of souther n Sweden , i n combination wit h a subMesozoic relief , revea l tha t thes e area s experi enced surface-leve l temperature s a t c . 14 0 Ma. The Cretaceou s palaeosurfac e temperatur e wa s estimated t o hav e bee n c . 3 0 °C. Surfac e leve l conditions continue d unti l Lat e Cretaceou s sedimentation starte d a t c . 8 5 Ma. I n severa l cases, the final cooling event was assigned an age of a t leas t 1 5 Ma, becaus e thi s i s th e expecte d minimum tim e fo r th e Tertiar y etc h surface s t o have develope d (Lidmar-Bergstro m 1991) . Samples fro m area s wher e th e SC P is preserve d probably experience d fina l exhumatio n i n

Neogene time . Th e recen t (OMa ) surface-leve l temperature was set to c. 2 0 °C. Hig h surface level temperature s (3 0 °C an d 2 0 °C, respect ively) were deliberately set to attai n a minimum value o f th e modelle d Cretaceous-Paleogen e reheating. The AFT dataset Modelled therma l historie s fo r 2 1 apatit e samples wer e achieve d usin g AF T dat a fro m Cederbom e t a l (2000 ) an d Cederbo m (2001) . The samplin g localitie s ar e presente d i n Fig . 1. All samples were collected fro m th e outcropping Precambrian basement (granitoids and gneisses), and the y al l experience d tota l annealin g during Late Palaeozoi c and/o r Earl y Mesozoi c tim e (Cederbom e t a l 2000 ; Cederbo m 2001) . Th e AFT ages fo r the 2 1 apatite samples used in this study al l passe d th e ^ tes t (Cederbo m e t a l 2000; Cederbom 2001), indicating that the grains in each sampl e belong t o a single population. There ar e mino r difference s i n AF T result s between th e sample s tha t ma y deriv e fro m kinetic variations among the apatites. The apatite composition ha s no t bee n examined , s o compo sitional variation s canno t b e excluded . Never theless, al l sample s passe d th e ^ tes t an d no major differenc e i n etch pit widt h wa s observe d when analysin g the grains. There i s no correlation between elevation and AFT age in southern Sweden. Instead, three areas characterized b y differen t trend s i n th e AF T results ca n b e discerne d (Cederbo m 2001) . Th e southeastern par t o f souther n Swede n (are a C ) is characterize d b y AF T age s younge r tha n 200 Ma. A SW-NE-trendin g bel t (are a B ) includes sample s wit h age s betwee n 23 1 an d 282 Ma, wherea s are a A i n th e northwester n part o f souther n Swede n represent s sample s with age s rangin g fro m 14 9 t o 31 5 Ma (se e Fig. 2 ; Cederbo m 2001) . Non e o f th e sample s have mixe d trac k lengt h distributions . T o summarize, area s A , B an d C ar e define d based o n difference s i n th e AF T result s that ar e too larg e t o b e cause d solel y b y compositiona l variations. Modelling results The modelling result s for the 2 1 apatite samples are shown in Fig. 3 and Table 1 . The Phanerozoi c thermal histor y wa s modelle d fo r al l samples. However, only the discernible part of the thermal history i s presente d i n Fig . 3. Whe n modellin g samples P9904 and 9905, for example, heating to at least 90 °C for the 200-300 Ma time interval is required for both samples (see Fig. 3). However,

THERMOTECTONIC DEVELOPMENT OF SOUTHERN SWEDEN heating abov e 9 0 °C is no t accepte d fo r sampl e P9904 during this time interval, wherea s heatin g above 9 0 °C make s n o differenc e i n th e calculated AF T results o f sample 9905 . There ar e mino r difference s betwee n th e modelled therma l historie s tha t ma y deriv e from kineti c variation s amon g th e apatites . However, severa l significan t observation s ca n be made . First, th e modellin g result s ca n b e separate d into thre e group s supportin g th e existenc e o f areas A, B and C in Fig. 1 (see Cederbom 2001) . The samples from are a C have relatively uniform cooling histories . The y al l cooled belo w c . 90 °C during Earl y Jurassi c tim e an d the y ar e characterized b y coolin g rate s o f c . 15° C pe r 10 Ma. Are a B is characterized b y sample s wit h older AF T age s tha n th e sample s i n are a C. Likewise , modelle d therma l historie s for the samples i n are a B indicate muc h earlie r coolin g and muc h lowe r coolin g rates . Coolin g belo w 90 °C had occurre d b y Earl y Triassic time i n the SW, and a s earl y a s Late Carboniferou s time i n the NE . Wit h tw o exception s th e coolin g rate s decrease fro m c . 5° C pe r 1 0 Ma i n th e S W t o c. 3 °C per 1 0 Ma in the NE. Samples S962 6 and 9808 a t th e margin s o f are a B sho w slightl y higher cooling rates, 7 °C per 1 0 Ma and 8 °C per 10 Ma, respectively . Are a A include s sample s showing a remarkabl e heterogeneity . Th e poin t of tim e whe n th e sample s coole d belo w 9 0 °C ranges fro m Lat e Carboniferous t o Late Jurassi c time. Additionally , th e coolin g rate s var y between c . 4 0 °C pe r 1 0 Ma an d c . 3° C pe r 10 Ma. Second, i t i s possibl e tha t al l sample s wer e reheated durin g Late Cretaceou s time . However, such reheatin g ca n b e establishe d onl y fo r samples collecte d fro m th e sub-Mesozoi c palaeosurface, whic h ar e know n t o hav e experienced surfac e condition s befor e and/o r during Cretaceou s time . Th e modellin g result s for sampl e 990 2 an d 980 5 indicat e c . 35 ° reheating i n S E Sweden , wherea s heatin g of th e order of c. 20 °C is indicated for the samples fro m the west coast (i.e . sample B14 , 9901 and 9808 ) (Figs 1 an d 3) . Modellin g o f th e remainin g samples show s tha t Cretaceou s reheatin g o f southern Swede n a s a whol e i s possible , bu t cannot b e take n a s certain . Alternativ e therma l histories are presented fo r a few samples (P9909 , S9624, P9906 , 9903 , SA9629 ) i n Fig . 3 an d Table 1 . Third, Oligocene-Miocen e fina l coolin g i s possible i n al l thre e area s accordin g t o th e modelling. Fo r occasional samples , Lat e Eocen e final cooling is also allowed, althoug h it is never required.

177

Discussion The forwar d modellin g o f AF T result s fro m southern Swede n confirm s th e conclusion s o f Cederbom (2001) , i.e . tha t souther n Swede n experienced differentiate d coolin g afte r th e Caledonian orogen y an d th e relate d forelan d basin formation. Furthermore, a pattern illustrating loca l variation s in both onse t o f cooling an d cooling rat e appear s an d is discussed below . The Palaeozoi c an d Mesozoi c coolin g wa s probably a resul t o f erosio n o f cove r rock s (unroofing), bu t change s i n th e geotherma l gradient ma y hav e als o ha d a n influence . I t i s likely that the thermal conductivity and heat flow of th e crystallin e basemen t an d sedimentar y cover have differed, bot h regionally an d through time during the Palaeozoic an d Mesozoic eras . It can be noted that Triassic and Jurassic sediments deposited i n shallow-wate r an d fresh-wate r environments ar e recorde d withi n th e Fennos candian Borde r Zon e i n southernmos t Swede n (Norling & Bergstro m 1987 ; Guy-Ohlso n & Norling 1988) , supportin g th e ide a o f a n Earl y Mesozoic cove r i n souther n Sweden . Th e observed Cretaceou s reheatin g wa s probabl y caused b y sedimen t deposition , an d th e fina l cooling even t i s explaine d b y Cenozoi c fina l exhumation. The onse t of unroofin g is not necessaril y the same a s th e poin t i n tim e whe n th e sample s cooled belo w 9 0 °C. I f a highe r temperatur e regime originall y prevailed , exhumatio n ma y have starte d muc h earlie r tha n whe n fissio n tracks started to accumulate. According to earlier published conodon t alteratio n an d organi c maturation studies of Lower Palaeozoic remnants (Bergstrom 1980 ; Buchard t e t al 1997) , however, a temperatur e regim e abov e 9 0 °C i s not likely fo r the area betwee n Lak e Vaner n and Lake Vattern . Triassic and Jurassic exhumation After th e Caledonia n orogen y an d the formation of a forelan d basin , unroofin g started . Th e basement, wit h it s heav y pil e o f sedimentar y strata, ma y no t hav e acte d a s a singl e entit y i n southern Sweden. The first recorded unroofin g i s indicated i n areas A and B. Within area A , Lat e Carboniferous, Earl y Jurassi c an d Lat e Jurassi c first recorded exhumatio n timing s ar e obtained . In area B the recorded exhumatio n started in Late Carboniferous t o Earl y Triassi c time , wit h a younging tren d toward s th e SW . Early Jurassi c first recorded exhumatio n is obtained i n are a C . A compilation of modelled coolin g rates for three points in time is presented in Fig. 4 together with

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C. CEDERBO M

Fig. 4 . A speculativ e reconstruction o f the Mesozoic an d Cenozoic tectoni c development i n southern Sweden. A heterogeneity i n Triassic-Jurassic exhumatio n betwee n th e thre e mai n tectoni c area s o f souther n Sweden , and between individua l samples i n the northwestern par t o f southern Swede n ca n be discerned. Oligocene-Miocen e final exhumation was mos t pronounce d aroun d th e souther n tip o f Lak e Vatter n an d o n th e S E coast.

THERMOTECTONIC DEVELOPMENT OF SOUTHERN SWEDEN the probabl e exten t o f Uppe r Siluria n t o Uppe r Jurassic sediment s (i.e . forelan d basi n relate d sediments). A s mentione d above , mino r differ ences betwee n th e modelle d therma l historie s may b e du e t o compositiona l variation s amon g the apatites . Nevertheless , Fig . 4 illustrates tha t there is a heterogeneity i n Mesozoic exhumation among the thre e area s of souther n Sweden . In the beginning of Triassic time (Fig. 4a), it is clear tha t mos t o f area s A an d B wer e stil l covered by thick, foreland basin related deposits . Slow to moderate exhumatio n rates are recorde d in area s A an d B , wit h a n increasin g tren d towards th e SW . A t th e sam e time , th e Skagerrak-Kattegat Platfor m (SKP ) (Fig . 1 ) basement wa s exposed; seismi c offshor e record s (e.g. Vejbae k 1997 ) revea l that the basement wa s exposed befor e Triassi c sedimentatio n started . The AF T dat a fo r sampl e B1 4 als o suppor t surface temperatur e conditions . I n contrast , th e AFT results for sample 9808 reveal temperatures of 9 0 °C, indicatin g thic k coverin g o f th e basement i n th e S W i n Earl y Triassi c time . T o solve thi s contradiction , additiona l studie s o f both th e sedimentar y recor d an d tectonics clos e to shor e o n th e SK P ar e needed , togethe r wit h denser AF T sampling. It is probable, but not established, that area C and centra l Swede n wer e covere d b y sediment s in Earl y Triassi c time . I n Earl y Jurassi c tim e (Fig. 4b) , however, i t is evident that are a C was covered b y thic k piles o f sediment , a s tempera tures > 90 °C are recorded b y the AFT data and exhumation is recorded i n the area. Cooling as a result of exhumation is also detected i n all thre e areas for Late Jurassi c time (Fig . 4c) . It i s interestin g t o not e tha t sample s fro m th e NE-SW-trending area B are consistent with slow to moderat e coolin g rate s throughou t Triassi c and Jurassi c time . Meanwhile , area s B an d C behaved i n a differen t manner , an d larg e variations ar e recorde d withi n are a A . I n are a A, bloc k movement s ma y hav e cause d uneve n erosion an d redeposition o n downfaulted blocks. It i s possibl e tha t are a C acte d a s a temporar y store for reworked sediment s from th e NW . Th e unroofing seem s t o hav e accelerate d i n earlies t Jurassic time , whe n are a C experience d rapi d exhumation, followed by block uplift s i n area A. Before o r durin g Earl y Cretaceou s time , a NW-SE-trending axi s o f elevation developed , resulting i n exposure o f the west an d S E coasts . The Palaeozoi c cove r ha d bee n remove d i n th e Bastad an d Kristiansta d area s o n the wes t coas t and th e S E coas t whe n Earl y Cretaceou s sedimentation starte d (Lidmar-Bergstro m 1982). I n Fig . 4d , area s wit h expose d basemen t are illustrate d together wit h th e probabl e extent

179

of Phanerozoic , pre-Cretaceou s sediment s b y that time ; erosio n o f the Uppe r Siluria n t o Lat e Jurassic sediments may have caused exposure of lower Palaeozoi c sediments . However , mos t parts o f th e sout h Swedis h basemen t surfac e were stil l protecte d fro m weatherin g (Lidmar Bergstrom 1991) . The Cretaceous and Paleogene cover Late Cretaceous an d Paleogene deposit s covere d the souther n par t o f th e Fennoscandia n Shield , but th e exten t of these sediment s t o the nort h is not established. The modelling results reveal that the west and SE coasts of southern Swede n wer e buried, and they illustrate that the central part of southern Swede n ma y als o hav e bee n covere d (Fig. 4e) . A n estimat e o f th e thicknes s o f thes e deposits o n the west an d SE coast ca n be made . The modelled therma l histories indicat e that the Cretaceous-Paleogene cover was thicker o n the SE coast (sample s 9902 , 9805) tha n on the west coast (9808 , 9901) , a s c . 3 5 °C an d c . 2 0 °C temperature increase s ar e recorde d fo r th e S E and west coasts, respectively. A n estimate of the thickness o f thes e sediment s involve s a n unconstrained estimat e o f th e geotherma l gra dient. I f the geotherma l gradien t durin g Cretac eous tim e wa s 3 0 °C km" 1, a sedimentar y thickness o f c. 650m in the west an d > 1000 m in the S E is indicated. In comparison, soni c data and basi n model s fo r dat a fro m Danis h well s support th e idea tha t the Cretaceous-Paleogene cover o n th e SK P exceede d 1000 m thicknes s before Neogen e uplif t an d erosio n (Japse n & Bidstrup 1999) . Cenozoic exhumation Where th e SC P is well preserved i t cannot have been re-exposed until Late Tertiary time , but it is uncertain whe n th e basemen t wa s re-expose d within th e highes t part s t o th e sout h an d S E of Lake Vattern. According to the modelled thermal histories, th e basement , wit h o r withou t a n additional Cretaceou s cove r o n to p o f th e remnant Palaeozoi c cover , di d no t reappea r a t the surfac e unti l a t leas t Oligocen e o r Miocen e time. Areas with a well-preserved SCP were still covered b y sediment s whe n th e SS P wa s developed (Fig . 4f) . I n Fig . 4f , th e relativ e amount o f fina l exhumatio n indicate d b y th e modelling result s i s als o shown . Thi s i s a ver y speculative picture , a s severa l alternativ e mod elled historie s matc h th e AF T data . Larg e exhumation i s indicate d fo r th e souther n ti p o f Lake Vattern , wher e th e highes t altitude s ar e found today , and fo r the coast i n the SE .

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Cenozoic uplif t an d domin g o f souther n Sweden ha s bee n supporte d b y Lidmar-Berg strom (1996 , 1999 ) an d Japse n & Bidstru p (1999). Cenozoi c uplif t o f souther n Swede n ha s also bee n discusse d i n a broade r perspective , together wit h Cenozoic uplif t an d doming o f the southern Scande s i n souther n Norwa y (e.g . Rohrman e t al 1995 ; Rii s 1996 ; Lidmar Bergstrom 1999) . Th e modelle d AF T dat a indicate tha t large-scal e Cenozoi c exhumatio n of souther n Swede n ha s occurred , whic h i s consistent wit h Cenozoi c uplift . However , th e speculative exhumatio n patter n illustrate d i n Fig. 4 f doe s no t suppor t symmetrica l domin g of the basement i n southern Sweden, as not only the central part o f souther n Sweden , but als o th e SE coast, experience d a larg e amoun t o f lat e exhumation. Large-scale Phanerozoic tectonism The mai n difference s i n th e AF T dat a an d th e modelled therma l historie s fo r sample s fro m areas A , B an d C ar e to o larg e t o b e explaine d solely by compositional variation s in the apatites. Neither i s long-term partia l annealin g a probabl e explanation fo r the differences , accordin g t o the observed trac k lengt h distributions . I f th e base ment in souther n Swede n coole d as an entit y during Lat e Palaeozoi c t o Lat e Jurassi c time , large-scale Cretaceou s an d Cenozoi c tectoni c movements mus t hav e occurred . Thi s seem s unlikely considering th e present-day topography . Alternatively, souther n Swede n experience d differentiated exhumatio n accompanie d b y tectonism durin g Lat e Palaeozoi c t o Lat e Jurassi c time. I n addition , larg e difference s i n th e AF T results an d th e modelle d therma l historie s recorded fo r sample s withi n are a A indicat e individual block movements i n the northwester n part o f souther n Sweden . The proposed extent of the three main tectoni c areas A , B an d C i s base d o n a coars e gri d o f apatite sample s (se e Fig . 1) , an d additiona l sampling is required before the existence of these areas ca n b e verified . A comprehensiv e investi gation o f th e occurrenc e an d exten t o f Phaner ozoic fault s i n souther n Sweden ha s no t bee n published. However , the study area where severa l Phanerozoic fault s wer e observe d b y Ahli n (1987) i s situate d withi n are a A . I n addition , two fracture zone s ar e mapped alon g th e easter n side o f Lake Vattern (Persso n & Wikman 1986) . One o f the m i s situate d betwee n th e sampl e points S9623 and S9624. Block movements hav e been observe d alon g th e easter n sid e o f the lake (Persson & Wikman , 1986) , bu t individua l blocks hav e not been identified.

It is also interestin g t o compare th e suggeste d extent of the three areas, which is based solel y on AFT data , wit h th e genera l trend s i n th e lineament pattern presented by Tiren & Beckholmen (1992) (Fig . 2). The shar p lines , interpreted as the younges t features (Fig. 2c) ar e foun d i n a restricted are a south of Lake Vanern, corresponding mor e o r les s t o are a A . Furthermore , th e NW-SE-trending bel t o f extensiv e lineament s that divide s souther n Swede n int o tw o halve s (Fig. 2a) may have a correspondence i n the areal extent o f are a B . The existenc e o f thre e mai n tectoni c unit s corresponding to areas A, B and C and individual block movement s withi n a t leas t northwester n Sweden i s supporte d b y th e studie s o f faults , fracture zone s an d linea r structures published to date. However , large-scal e Phanerozoi c tectoni c block movement s o f the orde r o f > 100 m hav e previously been suggeste d only for Lake Vanern (Ahlin 1987) .

Conclusions On their own, published AFT data have revealed a genera l pictur e o f the thermotectonic develop ment i n souther n Swede n fo r Palaeozoi c an d Early Mesozoi c time . However , b y forwar d modelling th e AF T dat a i n combinatio n wit h studies o f palaeosurface s an d relief , furthe r information o n exhumatio n rates , sedimentar y thicknesses an d th e Mesozoi c t o Cenozoi c thermotectonic histor y has been derived . Southern Swede n ca n b e divide d int o thre e main tectonic areas, which were characterized by different onse t o f recorde d unroofin g an d different exhumatio n rate s durin g Lat e Palaeo zoic t o Lat e Jurassi c time . Individua l bloc k movements i n on e o f thes e areas , i.e . th e northwestern par t o f souther n Sweden , ar e suggested fo r this time interval. There i s a contradictio n betwee n th e thermo tectonic developmen t i n S W Swede n an d o n the SKP furthe r west . Thic k deposit s covere d th e basement i n SW Sweden during earliest Triassic time, wherea s th e basemen t o n th e SK P wa s exposed. T o solv e thi s contradiction , additiona l studies of offshore sedimentar y records and nearshore tectonics , an d dense r AF T samplin g ar e needed. Southern Swede n experience d reburia l during Late Cretaceou s an d Paleogen e time . A temperature ris e o f c . 3 5 °C an d c . 2 0 °C ha s bee n detected fo r th e wes t coas t an d th e S E coast , respectively. Th e temperatur e differenc e indi cates tha t th e sedimentar y cove r wa s thicke r o n the west coast tha n o n the S E coast.

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fission track s i n fluorapatite . Geochimica e t Cosmochimica Acta, 55, 1449-1465. ELVHAGE, C . & LIDMAR-BERGSTROM , K . 1987 . Some working hypothese s o n th e geomorpholog y o f Sweden i n th e ligh t o f a ne w relie f map . Geografiska Annaler, 69A , 343-358. GLEADOW, A.J.W . & DUDDY , I.R . 1981 . A natura l long-term trac k annealin g experimen t fo r apatite . Nuclear Tracks, 5 , 169-174. GLEADOW, A.J.W. , DUDDY , I.R. , GREEN , P.F . & HEGARTY, K.A . 19860 . Fissio n trac k length s i n the apatite annealin g zone and the interpretation of mixed ages . Earth an d Planetary Science Letters, 78, 245-254. GLEADOW, A.J.W. , DUDDY , I.R. , GREEN , P.F . & LOVERING, J.F . 19866 . Confine d fissio n trac k lengths i n apatite : a diagnosti c too l fo r therma l history analysis . Contributions t o Mineralogy an d Petrology, 94, 405-415. GUY-OHLSON, D . & NORLING , E . 1988 . Upper Jurassic Litho- and Bio stratigraphy ofNW Scania, Sweden. Sverige s Undersokning Serie Ca, 72. JAPSEN, P. & BIDSTRUP , T. 1999. Quantification o f late Cenozoic erosio n i n Denmark based o n soni c dat a and basi n modelling . Bulletin o f th e Geological K. Lidmar-Bergstro m i s gratefull y acknowledge d fo r Society o f Denmark, 46 , 79-99. fruitful criticism . Th e manuscrip t wa s improve d b y KETCHAM, R.A. , DONELICK , R.A . & CARLSON , W.D. comments fro m K . Gallagher, A.G . Dore, H . Sinclai r 1999. Variabilit y o f apatit e fission-trac k annealin g and a n anonymou s reviewer . Thi s projec t ha s bee n kinetics: III . Extrapolatio n t o geologica l tim e financially supporte d b y th e Nuclea r Wast e an d scales. American Mineralogist, 84 , 1235-1255. Management Co . (SKB ) an d th e Roya l Swedis h KETCHAM, R.A. , DONELICK , R.A. & DONELICK , M.B. Academy o f Sciences . 2000. AFTSolve : a progra m fo r multi-kineti c modeling o f apatit e fission-trac k data . Geological Material Research, 2 (1), 1-32. References LARSON, S.A. , TULLBORG , E.-L. , CEDERBOM , C.E . & STIBERG, J.-P . 1999. Sveconorwegian an d Caledo AHLIN, S . 1987 . Phanerozoic fault s i n th e Vastergot nian forelan d basins i n th e Balti c Shiel d reveale d land basin area , S W Sweden. Geologiska Foreninby fission-track thermochronology . Terra Nova, 11, gen i Stockholms Forhandlingar, 109 , 221-227. 210-215. BERGSTROM, S . 1980 . Conodonts a s paleotemperatur e tools i n Ordovicia n rock s o f th e Caledonide s an d LASLETT, G.M . & GALBRAITH , R.F . 1996. Statistical modelling o f thermal annealing o f fission tracks i n adjacent area s i n Scandinavi a an d th e Britis h apatite. Geochimica e t Cosmochimica Acta, 60 , Isles. Geologiska Foreningen i Stockholms 5117-5131. Forhandlingar, 102 , 377-392 . BUCHARDT, B. , NIELSEN , A.T . & SCHOVSBO , N.H. LASLETT, G.M. , GREEN , P.P. , DUDDY , I.R . & GLEADOW, A.J.W . 1987 . Thermal annealin g o f 1997. Alu n Skifere n i Skandinavien . Geologisk fission tracks i n apatit e 2 . A quantitativ e analysis. Tidsskrift, 3 , 1-30. Chemical Geology, 65, 1 — 13. CARLSON, W.D . 1990. Mechanisms an d kinetics o f LIDMAR-BERGSTROM, K . 1982 . P re-Quaternary apatite fission-trac k annealing . American Geomorphological Evolution in Southern Mineralogist, 75 , 1120-1139. Fennoscandia. Sverige s Undersoknin g Seri e C , CARLSON, W.D. , DONELICK, R.A . & KETCHAM , R.A. 785. 1999. Variabilit y of apatit e fission-trac k annealin g LIDMAR-BERGSTROM, K . 1988 . Denudation surface s kinetics: I. Experimental results . American Minerof a shiel d are a i n sout h Sweden . Geografiska alogist, 84 , 1213-1223 . Annaler, 70 A (4), 337-350. CEDERBOM, C.E . 2001. Phanerozoic, pre-Cretaceou s thermotectonic events in southern Sweden reveale d LIDMAR-BERGSTROM, K . 1991 . Phanerozoic tectonic s in southern Sweden. Zeitschiftfiir Geomorphologie, by fissio n trac k thermochronology . Earth an d Neue Folge Supplement, 82 , 1-16. Planetary Science Letters, 188 , 199-209 . CEDERBOM, C.E., LARSON , S.-A. , TULLBORG, E.-L. & LIDMAR-BERGSTROM, K . 1994 . Morphology o f th e bedrock surface . In : FREDEN , C . (ed. ) Geology. STIBERG, J.- P 2000. Fissio n trac k thermochronol National Atlas o f Sweden. SN A Publishing , ogy applie d t o Phanerozoi c thermotectoni c event s Stockholm, 44-54. in centra l an d souther n Sweden . Tectonophysics, LIDMAR-BERGSTROM, K . 1995 . Relief an d saprolite s 316, 153-167 . through time on the Baltic Shield. Geomorphology, CROWLEY, K.D. , CAMERON, M . & SCHAEFER , R.L . 1991. Experimental studie s of annealing of etche d 12,45-61.

Final exhumatio n o f souther n Swede n durin g Cenozoic tim e i s supporte d b y th e modellin g results. This is consistent with previous studies of Cenozoic uplif t o f souther n Sweden (Rii s 1996 ; Lidmar-Bergstrom 1999 ; Japse n & Bidstru p 1999). However, the pattern of final exhumation obtained fro m th e modellin g o f AF T dat a doe s not suppor t a symmetrica l domin g o f th e basement i n southern Sweden . Finally, large-scal e Phanerozoi c tectoni c movements i n souther n Swede n canno t b e rejected base d on faul t an d fracture zon e studies published t o date . Additionally , ther e ar e similarities betwee n majo r feature s observe d i n the lineament pattern of southern Sweden and the suggested areal extent of the three main tectonic areas. Nevertheless , a mor e detaile d an d bette r constrained patter n o f th e tectoni c development in souther n Swede n demand s dense r apatit e sampling an d a regional study o f the occurrence of Phanerozoic faults .

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LIDMAR-BERGSTROM, K. 1996 . Long ter m morpho tectonic evolutio n in Sweden. Geomorphology, 16 , 33-59. LIDMAR-BERGSTROM, K . 1999 . Uplif t historie s revealed b y landform s o f th e Scandinavia n domes. In : SMITH , B.J. , WHALLEY , W.B . & WARKE, P.A . (eds) Uplift, Erosion an d Stability: Perspectives on Long-term Landscape Development. Geologica l Society , London , Specia l Publi cations, 162 , 85-91. LIDMAR-BERGSTROM, K. , OLSSON , S . & OLVMO , M . 1997. Palaeosurface s an d associate d saprolite s i n southern Sweden . In : WIDDOWSON , M . (ed. ) Palaeosurfaces: Recognition, Reconstruction and Palaeoenvironmental Interpretation. Geologica l Society, London, Special Publications, 120,95-124. LIND, G. 1972 . The gravity and geology o f the Vattern area, souther n Sweden . Geologiska Foreningen i Stockholms Forhandlingar, 94, 245-257. LANTMATERIVERKET. 1986 . Sveriges Relief. Lantmateriverket, Gavle. NAESER, C.W. 1981. The fading of fission tracks in the geologic environment—data from deep drill holes . Nuclear Tracks, 5, 248-250. NORLING, E . & BERGSTROM , J . 1987 . Mesozoic an d Cenozoic tectoni c evolutio n o f Scania , souther n Sweden. Tectonophysics, 137 , 7-19 .

PERSSON, L . & WIKMAN , H . 1986 . Provisoriska Oversiktliga Berggrundskartan Jonkoping, Map Sheet and Description. Swedis h Geological Survey Af39. PERSSON, L. , BRUUN , A . & VIDAL , G . 1985 . Berggrundskartan Hjo SO, Map Sheet and Description. Swedis h Geologica l Surve y A f 134. Rus, F . 1996 . Quantificatio n o f Cenozoi c vertica l movements o f Scandinavi a b y correlatio n o f morphological surface s with offshore data. Global and Planetary Change, 12 , 331-357. ROHRMAN, M. , VA N DER BEEK , PA. , ANDRIESSEN ,

P.A.M. & CLOETHING , S . 1995 . Meso-Cenozoi c morphotectonic evolutio n o f souther n Norway : Neogene doma l uplif t inferre d fro m fissio n trac k thermochronology. Tectonics, 14, 704-718. TIREN, S.A . & BECKHOLMEN , M . 1992 . Rock bloc k map analysi s o f souther n Sweden . Geologiska Foreningen i Stockholms Forhandlingar, 114 , 253-269. VEJB^EK, O.V . 1997. Dybe strukturer i sedimentcere bassiner. Geologisk Tidsskrift , 4 . VIDAL, G. 1984 . Lake Vattern. Geologiska Foreningen i Stockholms Forhandlingar, 106, 397 .

Neogene uplif t an d erosion of southern Scandinavia induced by the rise of the South Swedish Dome PETER JAPSEN 1, TORBEN BIDSTRUP 1 & KARNA LIDMAR-BERGSTROM 2 1 Geological Survey o f Denmark an d Greenland (GEUS), 0ster Voldgade 10 , DK-135 K0benhavn K, Denmark (e-mail: pj@ geus.dk) 2 Department of Physical Geography and Quaternary Geology, Stockholm University, SE-10691 Stockholm, Sweden Abstract: Basi n modelling an d compaction studie s based o n sonic data from the Mesozoi c succession i n 68 Danish well s wer e use d t o estimate th e amoun t of section missin g du e to late Cenozoi c erosion . Th e missin g sectio n increase s graduall y toward s th e coast s o f Norway and Sweden from zero in the North Sea to c. 500 m in most of the Danish Basin, but over a narro w zon e i t reache s c . 1000 m o n th e Skagerrak-Kattega t Platfor m i n northernmost Denmark . The increasing amount of erosion matche s the increase in the hiatus at th e bas e o f th e Quaternary , wher e Neogen e an d olde r strata ar e truncated , an d th e Mesozoic successio n i s thus found to have been mor e deepl y burie d by c. 500 PaleoceneMiocene sediment s i n large part s o f the area . Thes e observation s sugges t tha t th e onse t of erosion occurre d durin g th e Neogene , an d tha t th e Skagerrak-Kattega t Platfor m wa s affected b y tectonic movement s prio r to glacial erosion . I n southern Swede n just east of the Kattegat, th e expose d basemen t o f th e Sout h Swedis h Dom e attain s altitude s o f almos t 400 m. Th e formatio n o f th e Dom e starte d i n th e Lat e Palaeozoic , bu t geomorphologica l investigations hav e le d t o th e conclusio n tha t a ris e o f th e Dom e occurre d durin g th e Cenozoic. W e fin d tha t th e patter n o f lat e Cenozoi c erosio n i n Denmar k agree s wit h a Neogene uplif t o f the Sout h Swedis h Dom e an d o f the Souther n Scande s i n Norway. Thi s suggestion i s consisten t wit h majo r shift s i n sedimen t transpor t direction s durin g th e lat e Cenozoic observe d i n the eastern Nort h Sea, an d with formation of a new erosion surfac e as well as re-exposure o f sub-Cambrian an d sub-Cretaceous surface s in southern Sweden . Th e Neogene uplif t an d erosio n o f souther n Scandinavi a appear s t o hav e bee n initiate d i n tw o phases, a n early phase of ?Miocene ag e and a better-constrained late r phase that began in the Pliocene. Neogen e uplif t o f the Sout h Swedis h Dom e wit h adjoining areas i n Denmark fit s into a pattern o f late Cenozoi c vertica l movement s aroun d th e North Atlantic .

Recognition of the Neogene uplift an d erosion of 2k m i n severa l wells (Jensen & Schmid t 1992, Denmark an d Swede n is difficul t becaus e of it s 1993 ; Japsen 1993 , 1998 ; Michelsen & Nielsen regional extent , an d becaus e th e effect s ar e 1993) . This paper reports the results of a study of overprinted by the erosion during the subsequent th e lat e Cenozoi c erosio n o f cove r rock s i n Quaternary glaciation s (Fig . 1) . Consequently , Danis h well s locate d outsid e th e lat e Cenozoi c only few relevant observations were presented in depocentr e in the centra l North Sea an d outside the literatur e befor e th e 1990s . Studie s o f th e th e Bornhol m are a i n th e souther n Baltic Se a Miocene Vejl e Fjor d Formatio n le d Larse n & (Fig . 2) (Japsen & Bidstrup 1999) . Estimate s of Dinesen (1959 ) t o conclud e tha t considerabl e erosio n have been based on basin modelling and parts o f Fennoscandia , includin g no t onl y soni c dat a fro m severa l stratigraphi c units , basement bu t als o sedimentar y formations , resultin g i n maximu m value s o f c . 100 0 m, were erode d i n Neogen e time . Spjeldnae s whic h are considerably lower than those reported (1975) foun d tha t uplif t o f th e Fennoscandia n i n the above earlier studies . Shield in late Oligocene-Miocene time resulted Al l studie s find erosion t o increas e fro m th e in a significan t chang e i n th e sedimentar y easter n North Sea towards the coasts of Norway environment an d i n th e drif t o f th e coastlin e an d Sweden . This increasing amount of erosio n towards the SW . matche s the increase in the hiatu s a t the base of The effec t o f lat e Cenozoi c erosio n i n th e Quaternar y succession, where Neogene and Denmark ha s bee n quantifie d b y a numbe r of olde r strat a ar e truncated . Thes e observations workers, wh o estimate d erosion to be fro m 1 to sugges t that the onset of erosion occurred during From: DORE , A.G., CARTWRIGHT, J.A. , STOKER, M.S., TURNER, J. R & WHITE , N. 2002 . Exhumation of th e North Atlantic Margin: Timing, Mechanisms and Implications for Petroleum Exploration. Geologica l Society, London, Special Publications, 196, 183-207 . 0305-8719/02/$15.00 © The Geological Society of London 2002.

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Fig. 1. Pre-Quatemary geology of sourthern Scandinavia and landforms of the bedrock across the South swedish

Dome. Th e hiatu s at the base o f the Quaternary (c . 2.4 Ma, Zagwijn 1989 ) and the change i n sediment transport direction fro m Oligocene t o Pliocene tim e agrees wit h Neogene uplif t o f the South Swedish Dome. Compar e th e increasing ag e o f th e Quaternar y subcro p toward s th e expose d basemen t i n Norwa y an d Swede n wit h th e corresponding deepening o f the estimated erosio n (Fig. 13) . The mountains of the Southern Scandes constitut e the main part of southern Norway. Modified after Freden (1994), Vejbae k & Britze (1994), Lidmar-Bergstrom (1996), Japsen (1998 ) an d Clausen e t al. (1999).

Neogene time , an d indicat e tha t uplif t an d erosion hav e affecte d no t onl y Norwa y an d Denmark, bu t als o souther n Swede n (Japse n 1993). The geological recor d o f south Scandina via i s thus of grea t importanc e t o understandin g the Neogen e developmen t o f th e whol e o f Scandinavia an d th e Atlanti c margin s a s such . In thi s are a i t i s easie r tha n i n mos t place s t o

compare dat a fro m th e Cenozoi c sedimentar y cover (partl y onshore ) wit h observation s fro m exposed basemen t wher e pre-glacia l landform s are well preserved. Distance s are small, and data as well a s geoscientific studie s are abundant. The present study concludes that the Mesozoic succession has been c. 500 m more deeply buried than toda y i n mos t o f th e area , wher e th e

Fig. 2 . Locatio n maps , (a ) Plac e name s an d profile s ABC D (Fig . 15 ) and EF (Fig . 14) . (b) Locatio n o f th e 6 8 Danish and three Norwegian wells used in the study, (c) Structural elements. Basement highs indicated with grey. See als o wel l locatio n ma p o f Nielsen & Japsen (1991) .

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succession generall y i s overlai n b y Paleogen e strata, an d tha t a sectio n o f mainl y Paleocene Miocene ag e must have been remove d fro m thi s area durin g lat e Cenozoi c time . W e combine th e model o f late Cenozoic erosio n i n Denmark wit h observations fro m th e expose d basemen t i n Sweden, jus t eas t o f th e Kattegat . Here , geomorphological investigation s hav e le d t o th e conclusion that a rise of the South Swedish Dom e (the upland s o f Smalan d wit h altitude s u p t o 400 m) occurred durin g Cenozoi c tim e (Lidmar Bergstrom 1995 , 1996) . W e therefor e fin d tha t Neogene uplif t o f souther n Scandinavia , centre d around th e Sout h Swedis h Dome , coul d explai n both th e geophysica l an d th e geomorphologica l observations.

Erosion estimated fro m soni c dat a Derivation of normal velocity-depth trends A norma l velocity-dept h tren d (velocit y base line), VN(Z) , describe s i n a functiona l for m ho w the soni c velocit y o f a relativel y homogeneou s sedimentary formatio n saturate d wit h brin e increases wit h dept h whe n porosit y i s reduce d during norma l compaction . Th e pressur e o f th e formation i s hydrostatic durin g norma l compac tion, an d th e formatio n i s a t maximu m buria l depth; i.e. the thickness of the overburden has not been reduce d b y erosio n (e.g . Bula t & Stoke r

1987; Hilli s 1995 , Japse n 1998 , 2000) . Simpl e boundary condition s fo r suc h trend s ar e that th e normal velocit y at the surface equals the velocity of th e sedimen t whe n i t wa s firs t deposited , an d that velocit y a t infinit e dept h approache s th e matrix velocity of the rock whereas the velocitydepth gradien t approache s zero . The derivatio n o f a norma l velocity-dept h trend involve s thre e step s o f generalization : (1 ) identification o f a relativel y homogeneou s lithological unit ; (2 ) selectio n o f dat a point s representing norma l compaction ; (3 ) assignment of a functional expressio n t o the velocity-dept h trend. Velocity baselines ma y thu s be difficul t t o establish, and different trend s have been assigned to identica l unit s b y differen t worker s (compar e Bulat & Stoke r (1987 ) an d Japsen (2000)) . (1) 'Relativel y homogeneous ' refer s t o thos e properties tha t are important for the macroscopi c acoustic behaviou r o f th e unit . However , w e d o not always know if data from a well represent the typical developmen t o f th e uni t o r whic h mineralogical difference s may b e o f importanc e for its acoustic behaviour, e.g. the clay content in sandstones o r chalks. (2) 'Norma l compaction ' ma y b e a difficul t condition t o prove , becaus e w e d o no t alway s know if formation pressure is hydrostatic or if the formation ha s been burie d deepe r befor e erosio n (Fig. 3) . Lat e Cenozoi c erosio n alon g th e margins o f th e Nort h Se a Basi n an d over -

Fig. 3 . Buria l anomaly , dZ B(ra), relativ e t o a norma l velocity-dept h trend , V N. Uplif t an d erosio n reduc e th e overburden thicknes s and result i n overcompaction expressed as anomalously hig h velocitie s relative t o present day depth (negative dZB). However, post-exhumational burial, BE, will mask th e magnitude o f the missing section , Azmiss (Eq . 2) . Undercompactio n a s a resul t o f rapi d buria l an d lo w permeabilit y cause s overpressure , AP comp (MPa), an d lo w velocitie s relative t o dept h (positiv e dZ B). Modifie d afte r Japse n (1998) .

NEOCENE UPLIFT AND EROSION O F SOUTHERN SCANDINAVI A

Fig. 4 . Outlin e o f th e derivatio n o f th e norma l velocity-depth tren d fo r th e Nort h Se a Chal k (V£ h, Eq. 3 i n Appendix) . Th e shallo w par t o f th e tren d i s constrained b y soni c dat a fro m pelagi c carbonat e deposits of Recent age (a). The deeper part of the trend is defined by the upper boun d for interval velocit y dat a from well s wher e th e Chalk i s at maximum buria l an d at hydrostatic pressure (b). (a ) Sonic logs from pelagic carbonate deposit s o f Eocene to Pleistocene ag e drilled in hol e 807 , Ocea n Drillin g Progra m (ODP ) Le g 13 0 (Shipboard Scientifi c Part y 1991) , an d th e Chal k Group i n th e Danis h Stenlille- 6 (locatio n show n i n Fig. 2 ) an d th e Karl- 1 well s (centra l Nort h Sea ; onl y the uppe r thir d o f th e lo g i s shown) , (b ) Interva l velocity v . mid-poin t dept h fo r th e Chal k Grou p fo r wells wher e thic k Quaternar y an d Neogene sediment s are present in areas with limited or no overpressure an d for well s wit h maximu m velocit y fo r z > 2000 m (5 5 out of 845 wells in Chalk velocity database). In (a), th e

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pressuring a s a resul t o f rapid , lat e Cenozoi c burial i n th e basi n centr e hav e resulte d i n a systematic variatio n o f buria l anomalie s (se e below). Thes e anomalie s ar e withi n ± 1 km for the Uppe r Cretaceous-Dania n Chal k an d lower Cenozoic sediment s whereas the upper Cenozoic sediments ar e clos e t o norma l compactio n (th e anomalies fo r th e Cenozoi c sediment s ar e calculated relativ e t o a baselin e fo r marin e shale; se e Eq. 4 in the Appendix) (Japse n 1998 , 1999). Thu s a regression lin e fitte d t o velocity depth data may not represent a physical model of the subsurfac e if suc h anomalie s ar e no t take n into account. (3) 'Assignmen t of a functional expression ' to an observed velocity-depth trend is not straightforward becaus e o f th e limite d rang e o f an y dataset. A n observe d tren d may , however , b e extrapolated t o range from th e surfac e to infinit e depth provide d tha t th e extrapolate d tren d complies wit h th e abov e boundar y conditions . Assignment o f a n arbitrar y mathematica l vel ocity-depth relation fo r a formation may lead t o identification o f a n erroneou s baseline , fo r example, th e frequentl y applie d formul a fo r shale trends , t t = l/V = aexp(—biz), tha t pre dicts bot h velocit y an d velocit y gradien t t o increase toward s infinit y wit h depth (t t is transit time (sm" 1), an d a (sm" 1 ) an d b (m ) ar e parameters). Erosion may thus be underestimated if suc h a trend is applied t o singl e data points at great depth. Formulation of velocit y baseline s is thu s not an arbitrar y choic e o f mathematica l function s and regressio n parameters , bu t shoul d b e considered a s settin g u p a physical mode l fo r a given lithology . First , baseline s shoul d b e established fo r formation s tha t ar e relativel y homogeneous wit h regar d t o macroscopi c acoustic properties , e.g . chal k o r marin e shale dominated b y smectite-illite . Second , baseline s should reflec t norma l compaction , an d buria l anomalies relativ e t o th e tren d shoul d b e i n agreement wit h othe r estimate s o f erosio n an d overpressure. Consequently , a baselin e fo r a

depth-shift shoul d b e note d betwee n th e thre e soni c logs tha t al l represen t pelagi c carbonate s o f ver y uniform composition . Th e shif t i s suggeste d t o b e caused b y overcompactio n a s a resul t o f remova l o f overburden alon g th e margi n o f th e Nort h Se a Basi n during lat e Cenozoi c tim e (Stenlille-6 ) an d b y undercompaction a s a resul t o f rapi d buria l i n th e central Nort h Se a durin g lat e Cenozoi c tim e (Karl-1 ; Chalk formatio n overpressur e i s 15MP a i n a nearb y well). (Compar e Fig . 3. ) Modified afte r Japsen (1998 , 2000).

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Fig. 5 . Interva l velocit y v . mid-poin t dept h fo r th e Chalk Grou p i n th e well s studied , an d th e revise d normal velocity-depth trend for the Chalk, V^h (Eq . 3 in Appendix ) (Japsen 2000). Mos t dat a points reveal high velocities relative to the normal trend, and this is suggested generally to be due to overburden reduction. Estimates o f erosio n base d o n chal k soni c dat a correlate well with estimates based on basin modelling (Fig. 12a) . The dashe d lin e indicate s th e origina l baseline of Japsen (1998).

given litholog y shoul d b e constraine d a t th e surface b y th e velocit y o f recen t deposit s o f th e sediment, a t shallo w depth s b y th e lowe r boun d for velocity-dept h dat a fo r whic h th e effec t o f overcompaction a s a resul t o f erosio n i s minimum, an d at greater depths along th e uppe r bound fo r dat a fo r whic h th e effec t o f under compaction a s a resul t o f overpressurin g i s minimum. Third , th e mathematica l formulatio n of baseline s shoul d b e constraine d b y simpl e boundary condition s a t the surface and at infinit e depth. The shap e o f baseline s fo r unifor m sedi mentary formation s reflect s th e fac t tha t th e compaction processe s depen d o n th e minera logical compositio n o f th e formation s (th e derivation o f norma l velocity-dept h trend s fo r three unifor m formation s i s outline d i n th e Appendix; se e Fig. 4). (1) Th e chal k baselin e reveal s a moderat e velocity increas e fo r depth s les s tha n 1 km, whereas the velocity gradien t increase s a t greater depths unti l i t i s graduall y reduce d a t depth s below 1.5k m (Fig . 5; Eq . 3 i n th e Appendix) . This variatio n i s i n agreemen t wit h th e preservation o f chal k porositie s o f c . 40 % t o depths of 1 km during normal compactio n befor e the onse t o f calcit e cementatio n an d th e consequent increas e o f velocit y (Borr e & Fabricius 1998 ; Japsen 1998) .

Fig. 6 . Interva l velocit y v . mid-poin t dept h fo r th e Lower Jurassi c F- I Membe r in th e well s studied , an d the norma l velocity—dept h tren d fo r Lowe r Jurassi c shale, V^ h, suggeste d t o b e characteristi c for marine shale dominate d b y smectite-illit e (Eq . 4 i n Appendix). Al l dat a point s revea l hig h velocitie s relative to the normal trend, and this is suggested to be due t o overburde n reductio n an d t o a hig h conten t o f sand or kaolin in shale deposited close to the exposed basement o f th e Scandinavia n Shiel d durin g earlies t Jurassic time. Estimates of erosion based on sonic data for th e Lower Jurassic shal e are overestimated relative to estimate s base d o n chal k dat a i n N E Denmar k (Fig. 12b).

(2) Th e moderat e velocit y gradien t o f th e baseline fo r marin e shal e dominate d b y smectite-illite ma y also b e relate d t o miner alogical compositio n (Fig . 6; Eq . 4) . Smectite illite particle s ar e separate d b y wate r molecule s (Bailey 1980) , an d thi s interlaye r wate r i s adsorbed t o the particles eve n during deep buria l (van Olphe n 1966) . Japse n (1999 , 2000 ) argue d that th e wate r adsorbe d o n th e smectite-illit e particles coul d lea d t o wea k mechanica l grai n contacts, an d thu s t o th e lo w soni c velocit y observed fo r th e marin e shal e a t depth . (3) Th e baselin e fo r th e continenta l Bunte r Shale (Lowe r Triassic ) i s foun d t o b e simila r t o the normal trend of the Bunter Sandstone (Fig. 7 : Eq. 5 in the Appendix). Japsen (2000 ) suggeste d that thi s similarit y coul d b e relate d t o th e hig h kaolin conten t o f th e Bunte r Shale . Kaoli n ha s little adsorbed wate r and it builds up thick flake s that ar e u p t o a thousan d time s large r tha n smectite-illite particle s tha t ar e separate d b y water molecule s (Baile y 1980 ; Lindgreen, pers .

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189

estimating erosion , and that the trend fo r marin e shale give n b y Eq . 4 i s i n agreemen t wit h th e suggestions o f Scherbau m (1982 ) an d wit h tha t of Hanse n (1996 ) fo r th e uppe r c . 2k m o f th e shale trend . Suggeste d baseline s fo r th e Bunte r Shale diffe r significantly , bu t th e tren d given b y Eq. 5 agrees with the results of Marie (1975 ) an d Bulat & Stoke r (1987 ) fo r th e velocit y interva l from whic h mos t dat a ar e availabl e (se e discussion b y Japsen (2000)) . Burial anomaly and missing section Velocity-depth studie s hav e prove n usefu l because the y ar e base d o n easil y accessibl e data wit h a wide area l coverage, an d thus allo w for settin g up simple constraints on both physical and geologica l parameters . Over - an d under compaction relativ e t o a baseline may be studie d by computin g buria l anomalies , dZ B (m) , fo r a formation a s th e differenc e betwee n presen t depths an d depth s correspondin g t o norma l compaction fo r th e measure d velocit y ( z an d Z N (m), respectively , V i s soni c velocit y (ms" 1 ); Fig. 3 ) (Japsen 1998 ) (Eq . 1) : Fig. 7 . Interva l velocit y v . mid-poin t dept h fo r th e Bunter Sandston e an d Bunte r Shal e i n th e well s studied, an d th e Bunte r Shal e trend , V^ sh, whic h i s suggested t o b e characteristi c fo r lithologie s domi nated b y quart z and/o r kaoli n (Eq . 5 i n Appendix) . Considerable lithologica l variation s ar e likel y withi n these formations, bu t the plot show s a number o f dat a points clos e t o th e norma l tren d an d other s tha t plo t above th e tren d generall y a s a resul t o f overburde n reduction. Estimate s o f erosion base d o n Bunter Shal e and Sandstone dat a correlate wel l with estimates base d on Chalk data (Fig. 12c) . The clear distinction betwee n the trend for the continental Bunter Shale an d the trend for marin e Lowe r Jurassi c shal e i n Fig . 6 shoul d b e noted.

comm.). A shal e dominate d b y well-packe d flakes of kaolin coul d thus acquire rock physica l properties simila r t o thos e o f a consolidate d sandstone. A n alternativ e explanatio n could , however, b e relate d t o dominanc e o f quart z i n both the Bunter Sandstone an d the Bunter Shale. The shar p increas e o f velocit y aroun d 2 km fo r these formations could thus be related t o onset of quartz cementation , whic h ha s bee n reporte d t o develop a t depths belo w 2. 5 km in , for example , the Nort h Se a (Bj0rlykk e & Egeber g 1993) . Further studie s ar e require d t o clarif y thes e issues. Finally, i t shoul d b e note d tha t th e chal k baseline give n by Eq. 3 is in agreement wit h that of Hilli s (1995 ) fo r th e depth s relevan t fo r

A baselin e ma y b e give n a s a linea r trend , V — VQ + k-z, wher e V Q i s velocit y a t th e surface an d & (ms"1 m"1, or s"" 1) is the velocity gradient (se e Eq . 3) . Th e buria l anomal y thu s becomes (Japse n 1993 , 1998) :

where A z i s laye r thickness , A 7 (s ) two-wa y traveltime thickness an d z t i s depth t o the top of the layer . A baselin e ma y als o b e formulate d a s a constrained, exponentia l transi t time-dept h trend, t t = (tt 0 - rr 00)e~z/fc2 + #«> , wher e t t = l/V ( s m"1) is transit time, # 0 and ttoo are transit time a t th e surfac e an d a t infinit e depth , respectively an d b 2 (m ) i s a n exponentia l constant (se e Eq . 4) , I n thi s cas e th e buria l anomaly may be approximate d b y the following expression i f laye r thicknes s an d velocit y gradient ar e moderate (Japse n 1999) :

Low velocitie s relativ e t o dept h giv e positiv e burial anomalies , whic h ma y indicat e under compaction a s a resul t o f overpressur e (se e Japsen 1998) . Hig h velocitie s relativ e t o dept h give negativ e buria l anomalies , whic h ma y b e caused b y a reductio n i n overburde n thicknes s

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P. JAPSEN ETAL.

('apparent uplift' , Bula t & Stoke r 1987 ; 'ne t uplift an d erosion' , Rii s & Jense n 1992 ; 'apparent exhumation' , Hilli s 1995) . I t must , however, b e note d tha t an y post-exhumationa l burial, B E (m) , wil l mas k th e magnitud e o f th e missing overburde n section , Az miss (m) , an d w e obtain where th e minu s indicate s tha t erosio n reduce s depth (Hilli s 1995 ; Japse n 1998) . Whether a buria l anomal y i s a measur e o f erosion o r i s cause d b y othe r factor s (e.g . lithological changes ) i s subjec t t o a n integrate d evaluation o f th e are a i n question . Apar t fro m being i n agreemen t wit h othe r estimate s o f erosion, th e buria l anomalie s shoul d als o correspond geographicall y t o th e exten t o f a section missin g fro m th e stratigraphi c record . Erosion ma y b e underestimate d i f th e compac tion proces s is reversibl e whe n the loa d of the overburden i s reduced . However , w e fin d tha t previous estimates of erosion based on sonic data from th e stud y are a ar e exaggerate d b y u p t o 1000m, mainly becaus e o f the above-mentione d problems wit h identifyin g vali d baseline s fo r homogeneous formation s (se e sectio n 'Compari son wit h othe r studies') . Overestimatio n o f erosion fro m soni c dat a thu s seem s t o b e a typical proble m i n suc h studies , an d under estimation becaus e o f th e reversibilit y o f th e compaction proces s seem s t o b e onl y o f mino r importance. Summing up , a larg e area l an d stratigraphi c data coverag e i s o f crucia l importanc e fo r evaluating th e validit y o f estimate s o f erosio n from soni c data . Moreover , bot h velocit y base lines an d estimate s o f erosio n shoul d b e i n agreement wit h stratigraph y i n th e area , for mation overpressur e i n adjacen t basin s an d estimates o f erosio n base d o n independen t methods. I t i s thu s a n importan t tes t o f th e validity o f th e derive d baseline s tha t th e know n overpressure i n th e centra l Nort h Se a ma y b e predicted fro m buria l anomalie s relativ e t o th e trends fo r chal k an d fo r marin e shal e (Japse n 1998, 1999) . Sonic data Velocity-depth data fro m 6 0 Danish well s form part o f th e databas e fo r thi s stud y (Fig . 2) . Th e data were presented by Nielsen & Japsen (1991), apart fro m th e Ida- 1 an d Jelling- 1 wells . Fifty two o f th e well s have interva l velocitie s fo r th e Chalk Group , 3 1 fo r th e F- I Membe r o f th e Lower Jurassi c Fjerritsle v Formation (Michelse n

1989), an d 2 2 fo r th e Bunte r Sandston e o r th e Bunter Shal e (Bertelse n 1980) ; 4 2 well s hav e data fro m th e Chal k a s wel l a s fro m th e pre Chalk interva l (Fig s 5-7) . Dat a fro m thre e Norwegian well s wit h Chal k velocit y dat a ar e included t o support the contouring. Chalk buria l anomalie s wer e calculate d relative t o th e revise d norma l velocity-dept h trend fo r th e Chal k developed b y Japse n (1998 , 2000) (Eq . 3) . Anomalie s fo r th e pre-Chal k formations wer e calculate d relativ e t o baselines suggested by Japsen (2000). Burial anomalies for the Lowe r Jurassi c F- I Membe r ar e calculate d relative t o th e shal e tren d give n b y Eq . 4 , an d those fo r th e Bunte r Sandston e an d th e Bunte r Shale relative to the Bunter Shale trend are given by Eq . 5 . A singl e burial anomaly as a result of late Cenozoic erosion was estimated on the basis of th e availabl e soni c dat a fo r eac h well , an d corrected fo r the Quaternary reburial to obtain an estimate o f th e missin g sectio n (Eq . 2 ) (se e Japsen & Bistrup (1999) for details).

Erosion estimated from basin modelling Model description and input data The basin development of the study area has been modelled fro m 3 5 wells wit h a commercial, I D forward modellin g progra m (Yiikle r 1978 ; Iliff e & Dawson 1996) . The program starts simulation of th e geologica l developmen t fro m th e bas e o f the sedimentar y sectio n an d perform s a calcu lation of parameters such as formation thickness, pressure, temperatur e and vitrinite reflectance as a functio n o f time. From th e geologica l inpu t a t the wel l locatio n (thicknes s an d age s o f sediments, estimate d magnitud e an d timin g o f erosion an d estimate d heat-flo w history ) th e program calculate s vitrinit e reflectance , tem perature an d pressur e a s a function o f time. The calculated value s o f thes e parameter s fo r th e present-day situatio n ar e the n compare d wit h data that can be divided int o two groups: (1) data that constrai n th e therma l history : present-da y temperature (BHT , botto m hol e temperatures ) and thermal maturity indicators (mainly vitrinite reflectance values in the study area); (2) data that constrain th e compactio n o f th e sediments : pressure an d porosit y (n o overpressur e i s encountered withi n the stud y area). If th e measure d an d calculate d values do no t match, the input parameters ar e changed and the program is run again, until a satisfactory match is obtained. A genera l matc h t o man y dat a point s and a laterall y consisten t heat-flo w an d erosio n model wa s preferre d becaus e o f th e regiona l

NEOCENE UPLIFT AND EROSION O F SOUTHERN SCANDINAVI A

character o f th e study , th e varyin g dat a qualit y and the sometimes conflicting values . Chronostratigraphic event definition. Th e pro gram quantifie s al l importan t processe s a s a function o f time , an d th e basi n developmen t i s thus defined in terms of chronostratigraphic unit s valid for the entire area (mode l layer s or events). The numbe r o f event s i n th e geologica l model , including period s o f deposition , non-depositio n and erosion, must be chosen in such a way that all major change s ca n b e describe d an d relate d t o existing stratigraph y whil e avoidin g excessiv e calculation times . A total of 45 events was chosen to describe th e geological developmen t withi n th e stud y are a from Cambrian time until the present (a time step of 250 ka and a depth step of 25 m). The duration of the events is shorter in the Cenozoic interval to describe th e rapi d change s i n thi s period . Th e lithologies use d i n th e modellin g wer e kep t constant fo r th e sam e even t i f n o geologica l information dictate d otherwise . Th e mai n litho types for the events are: Cenozoic event s 30-45 (excluding Dania n time ; 60- 0 Ma): san d an d shale wit h occasiona l coals, mor e shal y toward s the bas e o f th e succession ; Lat e Cretaceous Danian events 26-29 (96-60Ma): Chalk; Early Cretaceous event s 24-25 (129-96 Ma): mixture of silt, marl and shale; Late Jurassic events 20-23 (152-129 Ma): shal e and siltstone; Mid-Jurassic events 17-1 9 (178-152Ma) : mixe d sandstone , siltstone and shale; Earl y Jurassi c event s 13-1 6 (210-178 Ma): shale,siltand sandy shale ;Triassic events 10-1 2 (250-21 0 Ma): sandstone , sand y shale t o shal e wit h carbonate ; locall y salt ; Lat e Permian even t 9 (256-250 Ma): clean sal t if the layer i s thick , otherwis e a mixtur e o f shale , anhydrites an d salt ; Cambrian-Earl y Permia n events 1-8 (570-256 Ma): shale and sandstone. Sparse temperatur e an d vitrinit e reflectanc e data ar e availabl e belo w uppermos t Triassi c level, and even fewe r below lowermos t Permia n level. Th e model fo r each wel l was extrapolate d to basement b y the use of seismic dat a where no well dat a wer e available . Palaeo-surf ace-temperatures. Th e palaeo-sur face-temperature is the averag e temperatur e at the sediment-water interface during a particular period. Estimate s o f palaeo-surf ace-temperature were modifie d fro m Buchardt (1978 ) (Fig. 8) . Heat-flow model. Th e heat-flo w histor y wa s constrained by two datasets only: (1) the presentday temperatur e i n th e wells , whic h combine d with the given lithologies (thermal conductivities)

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defines th e present-da y hea t flow ; (2 ) value s of vitrinite reflectance, whic h define the heat flow at the tim e o f maximu m buria l fo r a give n formation. A simpl e heat-flo w mode l tha t honour s thes e values i s estimated fo r eac h wel l base d o n a n assumption o f a constan t hea t flo w o f 1 Hea t Flow Uni t (HFU) fro m Cambria n time (even t 1) until the beginning o f Oligocene time (even t 34 ) (1 HFU = 42mWm~ 2 ). This valu e is typical for the constrained par t o f the heat-flo w history , but the exact choice i s of minor importanc e becaus e only the heat flow at maximum burial affects th e vitrinite reflectanc e an d mos t o f th e stud y are a has bee n affecte d b y erosion . Th e heat-flo w model fo r th e Oligocene-Recen t tim e interva l was modifie d t o matc h present-da y temperatur e and vitrinite reflectance data, but also to result in smooth heat-flo w variation s i n tim e an d spac e (Fig. 8) . The hea t flo w use d i n th e mode l i s th e hea t flow at the base of the sedimentary succession. In contrast t o th e backgroun d hea t flo w fro m th e upper mantle , th e hea t flo w a t thi s leve l i s affected b y transien t effect s fro m sedimentatio n and erosio n (Vi k & Hermanru d 1993 ) an d b y local variation s in the geometry an d lithology of the sedimentary units (e.g. sal t domes). Transient

Fig. 8 . Plots o f palaeo-surface-temperatures an d heatflow histor y use d i n th e modellin g o f Hyllebjerg- 1 well. A hea t flo w o f 1 HFU ha s bee n use d unti l Oligocene time , afte r whic h heat flow was allowed t o change smoothl y t o matc h calibratio n data . Th e palaeo-surface-temperatures ar e th e sam e fo r al l wells i n th e stud y (Buchardt , 1978) . (Se e th e corresponding calibratio n plot i n Fig. 9. )

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P. JAPSEN ETAL.

NEOGENE UPLIF T AND EROSION O F SOUTHERN SCANDINAVI A

heat-flow variatio n fro m 0. 9 t o 1. 1 HFU i n th e sedimentary sectio n wa s modelle d b y assumin g deposition ( 1 km pe r 5 Ma) followe d b y erosio n (also 1 km pe r 5 Ma) o n to p o f a sedimentar y succession o f 5 km an d assumin g constan t hea t flow o f 1HF U a t th e base o f a 50k m thic k basement sectio n (usin g th e PetroMo d 2 D software, versio n 6.1) . Erosion model. Vitrinit e reflectanc e i s mor e sensitive t o temperatur e tha n t o time , an d therefore depends mainly on maximum tempera ture an d no t o n th e timin g o f maximu m temperature. In this study , therefore, assessmen t of the timing of erosion was based on the existing stratigraphy and on general geologica l consider ations becaus e n o fission-trac k o r (U-Th)/H e data were available . A model o f erosion startin g in lat e Miocen e tim e an d continuin g unti l lat e Quaternary time was used in the basin modelling throughout th e are a (se e th e discussio n i n th e section 'Timin g o f maximu m buria l an d subsequent erosion') . Model calibration and results Basin modellin g wa s performe d fo r 3 5 well s where the heat-flow an d erosion mode l coul d be calibrated agains t temperature an d vitrinit e data (Figs 2 an d 9) . Th e estimate d heat-flo w mode l varies smoothl y wit h tim e fo r individua l well s (e.g. Fig . 8) , an d i n spac e fo r differen t tim e intervals (Fig . 10) . T o matc h th e stee p vitrinit e reflectance gradient s observe d i n thi s are a (Fig. 9) , lo w heat-flo w value s hav e bee n use d for central and northern Jylland during maximum burial (partl y a s a resul t o f transien t effect s caused b y sedimentatio n befor e maximu m burial). T o matc h present-da y temperatures , high heat-flo w value s hav e bee n use d fo r th e most recen t developmen t i n norther n Jyllan d (partly because of transient effects reflectin g late erosion). Calibration o f th e mode l assume d th e depo sition o f overburde n sediment s an d thei r subsequent remova l i n al l well s studied , apar t from th e L- l an d S- l well s (Fig . 11) . Th e thickness o f th e missin g sectio n ca n onl y b e estimated withi n a rang e o f possibl e solutions , and this range is constrained by th e data quality (e.g. vitrinite data), by th e latera l consistenc y of

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the mode l o f erosio n an d hea t flow, and b y th e general geological understanding of the area. The uncertainties o n th e estimate s o f erosio n ar e o f the orde r o f 100-200m . Quantification o f late Cenozoic erosion in Denmark Estimates of missing section based on different data Estimates of the missing sectio n base d o n Chal k sonic data and on basin modelling are similar; the correlation coefficien t i s 0.8 1 fo r 2 4 well s i n common an d th e mea n differenc e betwee n th e estimates i s 30m , standar d deviatio n 130 m (Fig. 12a) . The estimate s o f th e missin g sectio n based o n Chal k an d Triassi c soni c dat a ar e als o rather similar , bu t th e scatte r i s greater ; th e correlation coefficien t i s 0.72 fo r th e 1 9 wells in common an d th e mea n differenc e betwee n th e estimates i s 10m , standar d deviatio n 210 m (Fig. 12c) . Th e estimate s of the missin g sectio n based o n Chalk soni c dat a ar e generally smalle r than thos e base d o n Lowe r Jurassi c soni c data ; the correlation coefficien t i s 0.71 for the 27 wells in common an d the mean difference betwee n the estimates i s 150m , standar d deviatio n 270 m (Fig. 12b) . This difference i s suggested to be due to lithologica l variation s withi n th e Lowe r Jurassic sequenc e a s discussed below. Only i n N E Denmar k doe s th e magnitud e o f the buria l anomal y base d o n soni c dat a fo r th e pre-Chalk section generall y excee d the estimate s from Chal k sonic data and from basin modelling; e.g. difference s o f 500-1000 m betwee n anomalies base d o n soni c dat a fo r th e Chal k and th e Lowe r Jurassi c units . A possibl e interpretation o f thi s differenc e coul d b e tha t the Lower Jurassic units in that area experience d maximum buria l befor e th e depositio n o f th e Chalk (se e Japse n 2000) . I t i s not , however , possible t o identif y a majo r hiatu s i n th e stratigraphic recor d correspondin g t o a dee p erosional event tha t could explai n th e difference between th e Chal k an d th e pre-Chal k buria l anomalies. Maximu m buria l o f th e Mesozoi c succession i s thu s suggeste d t o hav e occurre d during Cenozoic tim e in all wells studied. Lateral lithological variation within the Lower Jurassic sequenc e i s a likel y caus e fo r th e hig h

Fig. 9 . Plot s o f calculate d an d measure d value s o f (a ) vitrinit e reflectanc e an d (b ) temperatur e fo r th e well s B0rglum-l, F-l, Hyllebjerg-1, 0rslev-l, S-l an d T0nder-2. The slow increase of vitrinite reflectanc e wit h dept h for, fo r example, th e B0rglum- l wel l shoul d be noted . Thi s is suggeste d t o be partly du e to a transient therma l effect relate d to deposition followed b y erosion during Cenozoi c time.

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Fig. 10. Maps of heat flow at three times during the late Cenozoic perio d resultin g fro m th e basi n modelling . The hea t flo w i s assume d t o b e constan t unti l Oligocene tim e ( 1 HFU), bu t throughou t Oligocene Recent tim e th e heat flow is allowed t o vary smoothly to matc h calibratio n data , (a ) Presen t day ; (b ) 2 Ma before present ; (c ) 1 0 Ma before present .

Fig. 11 . Maps o f erosion a t three interval s during late Cenozoic tim e resulting from th e basin modelling. The existing stratigraph y constrains the tempora l development of the erosion, (a ) Quaternary time ; (b) Pliocen e time; (c ) lat e Miocen e time . A mode l o f erosio n starting in late Miocene time has been used in the basin modelling throughou t the area .

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estimates o f erosio n base d o n soni c dat a fro m these sediment s compare d wit h estimate s fro m both basin modelling an d from Chal k soni c data. During earlies t Jurassi c tim e a shallo w marin e environment prevaile d clos e t o th e expose d basement o f th e Scandinavia n Shield , an d correspondingly a hig h conten t o f san d an d kaolin ha s bee n reporte d fo r the Lowe r Jurassic shale i n thi s are a (Pederse n 1985 ; Lindgree n 1991). The relatively high velocity of the Lowe r Jurassic sequenc e in NE Denmark coul d thus be due t o th e hig h conten t o f both san d an d kaoli n (see the section 'Derivatio n of normal velocit y depth trends ' an d Appendix) . Furthermore , th e general agreemen t betwee n th e erosiona l esti mates based on Chalk sonic data and those based on basin modelling i n northern Jylland, as in the rest of Denmark, suggests that estimates based on Chalk dat a ar e to be preferred t o those based o n Lower Jurassic data. The hig h velocitie s o f th e F- I Membe r hav e previously bee n suggeste d t o reflec t maximu m burial before Lat e Cretaceous-Paleogen e inversion withi n th e Sorgenfrei-Tornquis t Zon e an d the subsequent removal of 1 km of Chalk (Fig. 1 ) (Japsen 1993 ; Michelse n & Nielse n 1993) . Removal o f a thic k Chal k sectio n durin g th e inversion is , however , unlikel y becaus e seismi c reflectors withi n th e Chal k sectio n onla p a n anticlinal structur e alon g th e Sorgenfrei-Torn quist Zon e i n th e wester n part s o f th e Kattega t (Liboriussen e t al, 1987) . Syndepositiona l growth o f th e inversio n structure , wit h pea k movements durin g mid-Cretaceou s times , implies tha t onl y a thi n Chal k sectio n wa s deposited i n the inversion zone . Magnitude of late Cenozoic erosion The sectio n remove d b y lat e Cenozoi c erosio n has bee n estimate d fo r 6 8 Danis h well s o n th e

Fig. 12 . Correlatio n betwee n estimate s o f missin g section, (a ) Estimate s base d o n Chal k soni c data ,

AZ^SS, v . estimates fro m basi n modelling , AZ^ SS. (b ) Estimates based on Chalk sonic data, AZj^ss, v. estimates based o n soni c dat a fo r Lowe r Jurassi c F- I Member , AZJ^. (c ) Estimate s base d o n Chal k soni c data , AZ^SS, v . estimate s base d o n soni c dat a fo r Lowe r Triassic Bunte r Sandston e an d Bunte r Shale , AZ^ riss. The goo d correlatio n betwee n estimate s base d o n Chalk velocitie s an d o n basi n modellin g shoul d b e noted. Estimate s base d o n dat a fo r Lowe r Jurassi c shale ar e overestimate d relativ e t o estimate s fro m Chalk dat a i n N E Denmar k becaus e o f lithologica l variations i n th e shale . Th e line s illustratin g th e 1: 1 relationship betwee n th e estimate s ar e shown . Wel l names ar e give n fo r well s wit h result s o f basi n modelling show n i n Fig . 9 .

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Fig. 13 . Map of the section missing as a result of late Cenozoic erosion based on estimates from basi n modelling and sonic data from 6 8 Danish and three Norwegian wells. A succession of about 500 m of post-Chalk sediments is missing from larg e parts of the area. Towards the NE, c. 1000 m are missing where the Chalk is absent or deeply eroded o n an d alon g th e Skagerrak-Kattega t Platform.

basis o f a combinatio n o f result s fro m basi n modelling i n 3 5 well s wit h compactio n studie s based on velocity-depth data in 60 wells (Figs 13 and 14 ) (Japsen & Bidstrup 1999) . (1) Th e estimate d missin g sectio n reache s 1000m i n a numbe r o f well s i n N E Denmark , on o r clos e t o th e Skagerrak-Kattega t Plat form, wher e th e Chal k i s deepl y truncate d o r absent (Fig . 1) . Fo r th e Frederikshavn- 1 well , both method s indicat e a missin g sectio n o f c. 1000m , wherea s th e maximu m base d o n basin modellin g alon e i s 1200 m i n th e Hans- 1 well, an d th e maximu m fro m soni c dat a alon e is 1300 m i n th e Saeby- 1 well . Th e latte r valu e is, however , base d o n dat a fro m th e Lowe r Jurassic sequence , an d appear s t o b e over estimated whe n compare d wit h result s fro m neighbouring wells . (2) Th e missin g sectio n i s foun d t o b e jus t over 500 m i n man y Danis h well s locate d i n a broad ban d fro m N W t o S E wher e th e Uppe r Cretaceous-Danian Chal k i s preserved . (3) Missin g section s betwee n 25 0 an d 500 m are mappe d i n th e wester n an d souther n par t o f the stud y area. Erosio n estimate s o f onl y 100-250m ar e foun d i n sout h westernmost Jylland. Estimate s o f 250 m ar e clos e t o th e accuracy o f th e methods , an d th e Mesozoi c succession i s thu s probabl y clos e t o norma l compaction i n thi s area . Th e tw o easternmos t

Danish well s with Mesozoic sediment s found to be at maximum burial are the L-l an d S-l wells . The absenc e o f upper Pliocen e sediment s i n th e L-l indicate s non-depositio n rathe r tha n a n episode o f erosion (Laurse n 1992) . The magnitud e o f erosio n increase s clearl y across th e N E sid e o f th e Sorgenfrei-Tornquis t Zone, wher e estimate s o f erosio n ar e a s hig h as on the Skagerrak-Kattegat Platform to the north, c. 1000m . I n th e souther n par t o f th e inversion zone, estimate s o f erosio n ar e simila r to values found sout h of the zone, c. 500 m. This differenc e indicates stronge r tectoni c movement s o n th e Skagerrak-Kattegat Platfor m tha n in the are a t o the south. Along the S Wedge o f the Skagerrak-Kattegat Platform, th e missin g sectio n o f 1000 m i s suggested t o be c. 50 0 m of Cenozoic sediment s as i n the Danis h Basin to th e sout h plus a Chalk section o f c . 500m . Farthe r nort h o n th e Platform, th e missin g Cenozoi c sectio n i s suggested t o b e c . 250m , an d th e missin g Chalk sectio n c . 750m . O n th e basi s o f thes e assumptions a profil e alon g Jyllan d ha s bee n reconstructed t o th e situatio n befor e Neogen e uplift an d erosio n (Fig . 14) . Th e reconstructe d profile reveal s th e know n Chal k depocentr e (thickness <2km ) sout h o f th e Sorgenfrei Tornquist Zon e a s wel l a s on e N E o f th e Zon e (reconstructed thicknes s < 1 km) correspondin g

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to a predictio n base d o n thermo-mechanica l modelling (Gemme r 2002) . We have estimated th e thickness of sediment s deposited withi n th e stud y are a durin g th e Cenozoic epoch s fro m th e know n stratigraphy , the estimatio n o f th e tota l missin g sectio n an d from th e abov e assumption s abou t remove d chalk cover : n o Pliocene sediment s ar e found t o have been deposite d withi n the stud y area apar t from th e S W par t wher e thicknesse s ar e estimated t o hav e reache d 300m ; a Miocen e depocentre < 110 0 m thick is found t o have been located wes t o f Jyllan d wit h thicknesse s gradually decreasin g t o 100 m toward s th e N E and east and 300 m towards the SE; an Oligocene depocentre < 600 m thick i s found t o have bee n located N W o f Jylland ; i n th e res t o f th e are a thicknesses rang e fro m 5 0 t o 100m ; Eocen e deposits ar e foun d t o hav e covere d th e entir e area, excep t fo r th e S W par t o f th e Nort h Sea , with thicknesse s rangin g fro m 5 0 to 150m ; Paleocene (excludin g th e Danian ) deposit s ar e found t o hav e covere d th e entir e are a wit h thicknesses o f 50-100m, locally 150m .

Timing of maximum burial and subsequent erosion Along th e margin s o f th e Nort h Se a Basin , th e Chalk wa s a t maximu m buria l befor e Neogen e erosion (Japsen 1998). This conclusion applies to the majorit y o f wells i n the presen t stud y wher e Chalk i s overlai n b y Paleogen e sediments , an d where soni c dat a fo r th e Chal k indicat e a previous greater dept h of burial. Thus maximum burial o f th e Chal k mus t hav e occurre d durin g Cenozoic tim e afte r earl y Cenozoic burial. (1) Th e timin g o f erosio n ca n b e furthe r detailed i n souther n an d centra l Jyllan d an d offshore west of Jylland. Here erosion must postdate th e depositio n o f offshor e t o shorefac e sediments o f th e uppe r Miocen e Gra m For mation; fo r example, durin g Plio-Pleistocen e time (Rasmusse n 1961 ; Rasmussen , pers . comm.). Thi s erosiona l even t thu s matche s th e basin-wide hiatu s a t th e bas e o f th e Plio Pleistocene deposit s tha t ar e younge r tha n 2.4 Ma (the late Cenozoic successio n is complete only i n a narrow zone i n th e centra l North Sea ) (e.g. Zagwijn 1989 ; se e Japsen 1998) . Fig. 14 . (a ) Profil e o f th e post-Triassi c successio n i n Jylland, and reconstructions at (b) 2 Ma and (c) 1 0 Ma before presen t (befor e uplif t and erosion increasing to 1 km towards the NE). The (now partl y eroded) Chalk depocentre o n th e Skagerrak-Kattega t Platfor m

should b e noted . DB , Danis h Basin ; RFH , Ring k0bing-Fyn High ; SKP , Skagerrak-Kattega t Plat form; STZ , Sorgenfrei-Tornquis t Zone . Locatio n shown i n Fig. 2 . (Compare Japsen (1993). )

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(2) Th e timin g o f erosio n i s les s constraine d on an d alon g th e Skagerrak - Kattegat Platform , where th e Chal k i s deepl y erode d o r absen t (Fig. 1) . Th e pronounce d movement s o f th e Platform documente d her e may, however, lead to the suggestio n tha t erosio n o n the Platfor m wa s initiated earlie r tha n i n th e res t o f th e area ; fo r example, b y uplif t durin g mid-Miocen e time . This suggestio n i s further supported b y th e shif t in sedimen t transpor t directio n durin g mid Miocene tim e observe d i n th e northeaster n North Se a Basi n a s discusse d belo w (Fig . 1 ) (Clausen et al 1999) . The sedimentar y deposit s i n th e stud y are a were thu s erode d durin g lat e Cenozoi c tim e subsequent t o thei r maximu m burial , whic h generally mus t have taken place durin g Neogene time. The erosion i s likely to have been initiated in tw o phases , a n earl y phas e o f ?Miocen e ag e and a better constrained late r phase that began in the Pliocene . Th e lat e even t ha s affecte d a vast area tha t reaches far into the North Sea , wherea s the extent of the early phase must be restricted t o areas close to Scandinavi a where n o Miocene i s present, e.g . th e Skagerrak-Kattega t Platfor m and souther n Sweden . The lat e Cenozoi c erosio n wa s followe d b y Quaternary reburial , whic h i n onshore Denmar k occurred late r tha n c . 0. 3 Ma, th e ag e o f oldes t Quaternary sediment s (Knudse n 1995) . Th e ag e of the oldest Quaternary deposits is progressively older farthe r int o th e Nort h Sea , an d i n th e westernmost par t o f the Danis h sector, sedimen tation wa s almos t continuou s durin g lat e Cenozoic tim e (se e Konrad i 1995 ; Japse n 1998) .

Comparison with other studies Three studie s fro m th e earl y 1990 s foun d th e missing sectio n t o be substantiall y greater alon g the Sorgenfrei-Tornquis t Zon e tha n suggeste d here (Jense n & Schmid t 1992 ; Japse n 1993 ; Michelsen & Nielse n 1993) . Th e studie s b y Japsen (1993 ) an d b y Michelse n & Nielse n (1993) wer e base d o n dat a fro m th e Lowe r Jurassic F-I Member, which , as suggeste d i n th e previous section , ma y lea d t o overestimate d erosion toward s th e N E becaus e o f lithologica l variations withi n th e unit . Furthermore , th e difficulty i n definin g th e absolut e leve l o f baselines fo r differen t lithologie s contribute s t o the uncertainty of the estimated burial anomalies. Jensen & Schmidt (1992, 1993 ) overestimate d erosion b y a n average o f c. 450 m relative to th e results presente d her e fo r th e 1 4 Danis h well s in common ; th e maximu m overestimat e wa s 1000m. Th e buria l anomalie s o f Jense n &

Schmidt (1992 , 1993 ) wer e estimate d fro m vitrinite reflectance, density an d soni c data . Japsen (1993 ) overestimate d erosio n b y a n average o f c . 250 m relativ e t o th e result s presented her e fo r th e 3 1 well s i n common ; th e maximum overestimat e wa s 1000m . Th e esti mates of erosion wer e based o n the same data for the F-I Member as used here, and were calculated relative t o th e linea r velocity-dept h tren d fo r Lower Jurassi c shal e i n th e N W Germa n Basi n determined b y Scherbau m (1982) . Thi s tren d deviates les s tha n 100 m fro m th e shal e tren d applied her e fo r th e relevan t velocit y interva l (Eq. 4) . Michelsen & Nielse n (1993 ) overestimate d erosion b y c . 650 m relativ e t o th e result s presented her e fo r th e seve n well s i n common ; the maximu m overestimat e wa s 1300m . Th e burial anomalie s wer e calculate d fo r th e F- I Member relativ e t o a simpl e exponentia l transi t time-depth tren d establishe d b y thos e workers . This tren d result s in overestimate s o f erosio n o f up t o 300 m fo r th e relevan t velocit y interva l relative to the trend applied here (Eq . 4). Japsen (1998 ) overestimate d erosio n b y a n average o f c . 200 m relative to th e presen t study for th e 5 1 well s i n common ; th e maximu m overestimate wa s 500m . I n th e presen t study , Chalk burial anomalies are calculated relative to the revised Chalk baseline (Eq. 3), which, for the main velocit y interval , i s shifte d c . 200 m towards mor e shallo w depth s relativ e t o th e Chalk tren d applied by Japsen (1998) . Huuse et al. (2001) found erosion to be several hundred metres less whe n estimated from strata l geometries alon g a north-south seismi c sectio n west o f Jyllan d tha n whe n estimate d fro m th e maximum buria l studie s o f Jense n & Schmid t (1993) an d Japse n (1998) . Thi s conclusio n regarding the S-l wel l to the south of the section is in agreement with the present study, where the drilled sectio n i n thi s wel l i s foun d t o b e a t maximum buria l today . Thi s interpretation , which i s supporte d b y vitrinit e data , i s als o a consequence of the revision of the Chalk baseline as discusse d i n th e Appendi x (Fig . 4) . W e find , however, tha t a missin g sectio n o f c . 500 m o f Miocene sediment s i s compatibl e wit h th e stratigraphy aroun d th e norther n F- l well , where n o Miocene sediment s are present today. Neogene uplift of southern Scandinavia and of the South Swedish Dome Lidmar-Bergstrom (1999) discussed the geomorphological evidence concerning the uplift history of th e thre e surfac e domes i n Scandinavia . The

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Northern an d Souther n Scande s ar e th e mos t prominent o f th e domes . Th e Souther n Scande s constitute the main part of southern Norway and have a maximum elevation o f 2540 m above se a level (a.s.l.) . Ther e i s a genera l consensu s tha t major uplif t o f the Southern Scandes occurred i n Neogene time (Peulvast 1985; Jensen & Schmidt 1992; Rohrma n e t al 1995 ; Riis 1996) . Thi s timing of uplift i s in accordance with a Neogene onset o f erosio n i n norther n Denmark . Th e Southern Scande s wer e als o affecte d b y a Paleogene uplif t phas e (Clause n et al. 2000 ; Lidmar-Bergstrom e t al. 2000). Denudation surfaces of the South Swedish Dome The Sout h Swedish Dome, whic h is the smallest of th e Scandinavia n domes , culminate s N E o f Kattegat, jus t sout h o f lak e Yattern , an d ha s a maximum elevation of 380 m a.s.l . (Fig s 1 and 15) (Lidmar-Bergstro m 1988 , 1996) . O n it s northern an d easter n flanks , th e sub-Cambria n peneplain ca n b e trace d u p t o it s summit s (see

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Fig. 16) . This surfac e i s extremely fla t an d wa s formed durin g a period o f major denudation over all o f th e Balti c Shiel d i n lat e Proterozoi c tim e (e.g. Hogbo m 1910 ; Hogbo m & Ahlstro m 1924). I t i s stil l wel l preserve d i n part s o f eastern an d south-centra l Swede n (Rudber g 1954; Lidmar-Bergstro m 1996) . I n contrast , the sub-Cambrian peneplai n ha s bee n destroye d o n the souther n an d wester n flank s o f th e Sout h Swedish Dome, wher e th e Precambria n base ment was re-exposed during the warm and humid Mesozoic climat e (Lidmar-Bergstro m 1989 , 1995). Dee p weatherin g too k plac e an d thic k kaolinitic saprolite s wer e formed , and , afte r partial erosio n o f th e saprolites , a n undulating, hilly relief developed. This sub-Cretaceou s hill y relief ca n b e see n u p t o 125 m a.s.l . toda y a s a result o f preservatio n unde r a long-lastin g Cretaceous cove r (Lidmar-Bergstro m 1982 , 1996). Wher e th e sub-Cambria n peneplai n i s well preserved , i t mus t hav e bee n protecte d during Mesozoi c tim e b y a Palaeozoi c cover . Consequently, th e lates t uplif t o f th e Sout h Swedish Dom e wit h re-exposur e o f th e subCambrian peneplai n di d no t occu r unti l som e

Fig. 15 . Profile o f cover rocks an d basement topograph y an d across souther n Swede n fro m the Kattega t t o th e Baltic. A first phase o f Neogene uplif t an d erosio n le d t o the formatio n o f the Sout h Smalan d Peneplai n an d a second phas e t o the re-exposure o f the sub-Cretaceou s hill y relief (i.e . during mid-Miocene an d Pliocene time) . Location show n i n Fig. 2. SCP, Sub-Cambrian Peneplain . Modified fro m Kornfal t & Larsson (1987) , Britze & Japsen (1991) , Japse n & Langtoft e (1991a , 1991b) , Lykke-Anderse n (1991) , Lidmar-Bergstro m (1995 ) and Vejbaek (1997) .

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Fig. 16. Block diagrams illustrating the development of bedrock relief in southern Sweden between the Kattegat and the Baltic (see Figs 2 and 15) . Neogene uplift an d erosion is assumed to have been initiated i n two phase s (?mid-Miocene, Pliocene) . The fina l diagra m illustrates ho w th e Sout h Swedis h Dom e is composed of surfac e facets fro m widel y differen t periods . Surface feature s ar e exaggerate d an d Quaternar y deposit s are no t show n in th e fina l diagram . SCP, Sub-Cambria n Peneplain; SCr, Sub-Cretaceou s hilly relief ; SSP , Sout h Smalan d Peneplain. Modified fro m Nielse n & Japsen (1991), Frede n (1994) , Buchard t e t al. (1997) and Vejbae k (1997).

time durin g th e Cenozoi c perio d (Lidmar Bergstrom 1991) . The Sout h Smalan d Peneplai n i s a n almos t horizontal an d ver y fla t erosio n surfac e wit h only few, low , residua l hills , whic h extend s SW o f th e cresta l par t o f th e Sout h Swedis h Dome (Fig . 1 ) (Lidmar-Bergstro m 1988 , 1996). Th e peneplai n cut s of f th e incline d sub-Cretaceous hill y relief , an d therefor e developed durin g Cenozoi c tim e b y erosio n

by river s flowin g toward s th e sout h an d wes t down t o a bas e leve l tha t correspond s t o th e present leve l o f abou t 125 m a.s.l . (Figs 1 5 and 16) (Lidmar-Bergstro m 1982) . Th e formatio n of th e Sout h Smalan d Peneplai n destroye d th e sub-Cretaceous hill y relief . Th e peneplai n mus t have continue d acros s Cretaceou s rock s i n th e west wher e th e Kattega t i s foun d today . Towards th e NE , th e Sout h Smalan d Peneplai n was cu t int o th e sub-Cambria n peneplain ,

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Fig 16. - contiunued

leaving south - an d west-facin g erosiona l scarp s and hig h residua l hill s (150m ) i n th e basemen t surface. In th e cresta l par t o f th e Sout h Swedis h Dome, severa l occurrence s o f weatherin g products suggeste d a s being o f Plio-Pleistocen e age hav e bee n describe d (Lidmar-Bergstro m et al 1997) . Thes e gravell y saprolite s diffe r

markedly fro m th e kaoliniti c saprolite s associ ated wit h th e pre-Cenozoi c weathering . Th e stripping o f thes e weatherin g mantle s appear s to b e responsibl e fo r muc h o f th e hill y relie f incised i n th e uplifte d sub-Cambria n peneplain . Thus th e erosio n o f th e las t remnant s o f Palaeozoic rock s ma y no t hav e occurre d unti l Neogene time .

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Timing of the Cenozoic uplift of the South Swedish Dome We propos e a ne w theor y fo r th e Cenozoi c development o f southern Scandinavi a base d o n a combination o f thes e observation s fro m th e exposed basemen t i n souther n Swede n an d th e geological evidence from th e sedimentar y cove r in th e adjacen t area s i n easter n Denmark , e.g . Sjaslland, wher e Paleocen e sediment s ar e preserved an d a Cenozoic cove r o f c . 500 m ha s been remove d (Figs 1 and 13) . Neogene uplif t of the whol e o f souther n Scandinavia , centre d around th e Sout h Swedis h Dome , ma y accoun t for th e erosio n o f the missin g Cenozoi c cove r i n eastern Denmark , an d b e i n agreemen t wit h th e geomorphological evidenc e i n Sweden . The formatio n o f th e Sout h Swedis h Dom e started i n Late Palaeozoic tim e with continuation in Mesozoic tim e (Cederbom 2001 ; Fig. 16) . The present for m aros e after the Neogene uplif t wit h subsequent formatio n o f a ne w erosio n surfac e (the Sout h Smalan d Peneplain ) an d re-exposur e of sub-Cambrian an d sub-Cretaceous surfaces . In Denmark, th e Neogen e uplif t o f th e Sout h Swedish Dom e le d t o remova l o f Cenozoi c cover rock s i n large part s o f the territory , wher e the Skagerrak-Kattega t Platfor m i n particula r suffered dee p erosio n (Fig s 1 and 13) . A Neogene uplif t o f the Sout h Swedis h Dom e is i n agreemen t wit h interpretatio n o f fission track data, whic h indicat e th e remova l o f abou t 650m o f Uppe r Cretaceous-Paleogen e sedi ments fro m S W Sweden an d of about 1000 m of such sediments from SE Sweden during Neogene time (Cederbo m 2002) . A n Uppe r Cretaceous Palaeogene cove r ove r larg e part s o f souther n Sweden i s in agreement wit h evidence fo r a vast Cretaceous cove r i n sout h Swede n (Lidmar Bergstrom 1982 , 1995 ) an d wit h th e repor t o f redeposited Eocen e diatom s i n lakes i n Smaland at abou t 200 m a.s.l . (Cleve-Eule r 1941) . Th e suggested Plio-Pleistocen e age of the weath ering product s foun d o n th e cresta l par t o f th e South Swedis h Dom e is consistent wit h Neogen e uplift o f th e Dom e (Lidmar-Bergstro m e l al 1997). The re-exposur e o f th e sub-Cretaceou s hill y relief a t altitude s belo w 125 m a.s.l . occurre d after th e formatio n o f th e Sout h Smalan d Peneplain, whe n th e remainin g Cretaceou s cover wa s eroded . Consequently , th e Neogen e rise o f th e Sout h Swedis h Dom e mus t hav e taken plac e i n tw o phases : a ?Miocen e an d Pliocene phas e woul d b e i n agreemen t wit h the erosiona l patter n i n Denmar k (se e th e section 'Timin g o f maximu m buria l an d subsequent erosion' .

A ris e o f th e Sout h Swedis h Dom e durin g Neogene tim e i s als o consisten t wit h th e clockwise shif t i n sedimen t transpor t direction s that i s observed i n the NE Nort h Se a Basin. The transport direction s change d fro m southward s in Oligocene-Early Miocen e tim e t o southwest wards in mid- an d lat e Miocen e tim e an d finally westwards i n Pliocen e tim e (Fig . 1 ) (Clause n et al . 1999) . Th e Oligocen e transpor t directio n from th e nort h i n th e easter n Nort h Se a Basi n corresponds t o a Lat e Paleogen e uplif t phas e o f the Souther n Scande s (Lidmar-Bergstro m e t al . 1997, 2000 ; Clause n et al. 2000) . The lat e Cenozoi c sediment s foun d i n S W Denmark ar e thus expected t o reflect th e erosio n of Palaeozoic-Paleogene cover rocks in Sweden as well as the re-exposure o f a Mesozoic surfac e on Precambria n rock s an d th e subsequen t formation o f th e Sout h Smalan d Peneplain . Several observation s agre e wit h thes e sugges tions, fo r example , th e occurrenc e o f silicifie d Lower Palaeozoi c fossil s i n Miocen e san d i n Jylland (Spjeldnae s 1975) , an d th e occurrenc e o f gibbsite in the Gram Formation, whic h indicates erosion o f re-expose d tropica l soi l durin g lat e Miocene tim e an d a shor t fluvia l transpor t (Rasmussen & Larsen 1989) . Larse n & Dinesen (1959) foun d a n increasing content of amphibole in Miocen e sediment s i n th e Danis h area , an d suggested tha t th e considerabl e amoun t o f immature weatherin g material wa s derive d fro m exposed basemen t an d cove r rock s o n th e Scandinavian Shield. Discussion and conclusions Using a combinatio n o f basi n modellin g an d analysis of sonic data from differen t stratigraphi c levels to estimate erosion allow s identification of anomalous values and extends the areal coverage relative t o tha t achieve d b y th e applicatio n o f a single method. We estimate erosion in the eastern North Se a Basi n t o b e o n averag e 200-60 0 m lower tha n suggeste d i n previou s studies . I n particular, w e fin d erosio n estimate s base d o n Chalk velocitie s t o b e i n goo d agreemen t wit h estimates base d o n basi n modelling , an d thi s i s interpreted a s bein g du e t o th e homogeneou s composition o f th e Chalk , the larg e thicknes s of the Chalk section over which the mean velocity is calculated i n mos t wells , an d th e stres s dependence o f Chal k compaction . Basi n model ling predict s lo w hea t flo w durin g Oligocene Pliocene time , t o accoun t fo r observe d stee p vitrinite reflectanc e gradient s an d present-da y temperatures i n central an d northern Jylland . We suggest thi s to be partly due t o transient thermal effects induce d b y depositio n followe d b y

NEOCENE UPLIFT AND EROSION OF SOUTHERN SCANDINAVI A

erosion, bu t furthe r studie s o f thi s matte r ar e needed. Comparison o f results fro m differen t method s indicates tha t erosio n i s overestimate d whe n based o n sonic data from Lowe r Jurassic shal e in NE Denmark ; thi s coul d b e du e t o variations i n lithology, bu t furthe r studie s ar e neede d t o full y understand thes e variations . I t i s conclude d tha t maximum buria l of the Mesozoi c successio n throughout th e regio n occurre d befor e Neogen e erosion wher e Paleogene-Neogen e strat a ar e preserved in large parts. A previous suggestion of deep erosio n in the Sorgenfrei-Tornquis t Zon e during the Late Cretaceous-Paleogene inversion is rejected. The thickness o f the missin g sectio n remove d by lat e Cenozoi c erosio n increase s fro m th e eastern Nort h Se a toward s th e Norwegia n an d Swedish coasts . W e estimat e th e erosio n t o b e c. 500 m i n a broa d zon e acros s Denmar k from N W t o SE . Thi s zon e largel y conform s to th e are a wher e Paleocene-Miocen e deposit s subcrop th e Quaternary , an d th e erode d sedi ments mus t thu s hav e bee n o f Paleocen e Miocene age . Erosio n decrease s toward s zero in the wester n an d souther n par t o f th e Danis h North Sea . Th e ag e o f th e remove d sediment s must be progressivel y younge r in this direction, as for th e ag e o f the Quaternar y subcrop. To the north, th e missin g uppe r Cretaceous-Dania n section reache s c . 1000 m o n an d alon g th e Skagerrak-Kattegat Platform . Th e deepe r ero sion o n an d alon g th e Skagerrak-Kattega t Platform relativ e t o th e Danis h Basin cannot be explained b y glacia l erosio n o r by a drop i n sea level, an d thi s provide s a furthe r argumen t fo r tectonic uplif t o f th e Platfor m durin g Neogen e time. Finally , w e hav e demonstrate d tha t th e predicted thicknes s o f th e missin g strat a corresponds t o a likel y Cenozoi c depositiona l history. The patter n o f lat e Cenozoi c erosio n i n Denmark agree s wit h a Neogene uplif t o f south Norway centre d aroun d th e Souther n Scande s and o f souther n Scandinavi a centred aroun d th e South Swedis h Dome . Th e formatio n o f th e South Swedis h Dom e starte d in Late Palaeozoi c time, bu t th e presen t for m aros e afte r Neogen e uplift wit h subsequen t formatio n o f th e Sout h Smaland Peneplai n an d re-exposur e o f sub Cambrian an d sub-Cretaceou s surfaces . The re exposure o f th e sub-Cretaceou s hill y relie f a t altitudes belo w 125 m a.s.l . occurre d afte r th e formation of the South Smaland Peneplain, when the remainin g Cretaceou s cove r wa s eroded . Consequently, th e Neogen e ris e o f th e Sout h Swedish Dom e mus t hav e take n plac e i n tw o phases. We suggest that these phases correspon d

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to the erosiona l episode s o f ?Miocen e an d PlioPleistocene ag e tha t affecte d th e sedimentar y cover i n Denmark . Th e area l exten t o f th e ?Miocene episod e appear s restricte d t o southern Sweden an d th e Skagerrak-Kattega t Platform , whereas th e effect s o f th e late r episod e reac h from th e Balti c an d fa r int o th e Nort h Sea . A Neogene ris e o f the Dome i s consistent wit h the clockwise shif t i n sedimen t transpor t direction s observed i n th e N E Nort h Se a Basin , changin g from southward s i n Oligocene-Earl y Miocen e time to southwestwards in mid- and late Miocen e time an d finall y westward s i n Pliocen e tim e (Clausen e t al. 1999) . Th e Oligocen e transpor t direction fro m th e nort h correspond s t o a Lat e Paleogene uplif t phas e o f the Souther n Scandes . Apart fro m erosio n induce d b y tectoni c uplift , glacial erosio n wa s also important, a s witnessed by th e larg e volume o f Pleistocene sediment s in the central North Sea Basin (e.g. Japsen 1998) . A drop in sea level durin g late Cenozoic tim e ma y also hav e increase d erosio n (e.g . Nielse n e t al . 2001). The Neogen e uplif t alon g th e easter n margi n of the North Sea Basin and of the South Swedish Dome fit s int o th e genera l patter n o f lat e Cenozoic vertica l movement s aroun d th e Nort h Atlantic (Japse n & Chalmer s 2000) . A mode l explaining these phenomena must be constrained by observation s o f th e magnitud e and timin g of uplift an d erosion based on independent methods as w e hav e aime d a t demonstratin g here, an d i t must thus separate the effects o f Paleogene uplif t of plat e boundarie s fro m thos e o f Neogen e intraplate uplift . Appendix: Normal velocity-depth trends for homogeneou s formation s A revised normal velocity-depth trend for the North Sea Chalk Japsen (1998 ) publishe d a norma l velocity depth tren d fo r th e Chal k Grou p base d o n a n analysis o f dat a fro m 84 5 well s throughou t the North Se a Basi n an d OD P data . Fo r th e shallowest part of the trend, no data representing normal compactio n wer e foun d fo r th e Chal k of the Nort h Se a Basin , s o soni c lo g dat a fro m Eocene to Recent ooze and chalk deposits from a stable platfor m wer e use d t o guid e th e tren d (Urmos e t al . 1993) . A t intermediat e depths , Japsen (1998 ) applie d qualitativ e argument s t o identify Nort h Se a dat a representin g norma l compaction along the lower bound for velocity depth dat a fo r whic h th e effec t o f overcompac tion a s a result of erosion i s minimal. At greate r

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depths, dat a representin g norma l compactio n were identifie d along th e upper bound wher e th e effect o f undercompactio n a s a resul t o f overpressuring i s a minimum. Later, however , Japse n (2000 ) foun d additional geologica l constraint s t o refin e th e identification o f referenc e dat a a t intermediat e depths wher e th e influenc e o f erosio n an d overpressuring i s difficul t t o ascertain . Becaus e the soni c metho d identifie s deviation s fro m maximum burial , post-erosiona l reburia l o f a formation wil l reduc e it s observabl e buria l anomaly, e.g . a pre-Quaternar y erosio n o f 500m wil l b e maske d b y a subsequen t Quaternary reburia l o f 500 m (Eq . 2) . Thi s implies that , wher e th e Quaternar y sequenc e i s thick, eve n mino r deviation s fro m maximu m burial a s a resul t o f Neogen e erosio n ma y correspond to a substantial missing section. Deep erosion is , however , no t likel y wher e th e base Quaternary hiatu s i s minor , fo r example , wher e the Quaternar y sequenc e i s underlai n b y Neogene sediments . Normally compacte d Chal k is thus likely to be found i n areas wher e th e Quaternary sequenc e i s thick, Neogene deposits are present, and pressure is hydrostatic . Consequently , th e norma l velocity-depth tren d fo r th e Nort h Se a Chal k should follow the upper bound for data from such areas, wherea s dat a representin g undercompac tion a s a resul t o f overpressurin g shoul d plo t below th e tren d (Fig . 4) . A revised baselin e wa s thus define d b y Japse n (2000 ) usin g suc h maximum velocit y dat a fo r 90 0 < z < 1700m. Th e revise d tren d line s u p wit h th e maximum velocit y dat a use d t o defin e th e original tren d fo r z > 200 0 m, an d wit h a velocity a t the surfac e o f 1550ms" 1 :

The fourt h o f th e abov e segment s i s unchanged fro m th e origina l trend , i n whic h the uppe r thre e segment s wer e expresse d a s 1600 + z , 50 0 + 2z , an d 937. 5 + I.15z (Japse n 1998). Th e revise d tren d i s shifte d toward s shallower depth s b y a mea n o f 160 m fo r th e velocity interva l affecte d b y th e revisio n an d where Nort h Se a data ar e found , 210 0 < V < 4875ms" 1 ; th e maximu m shif t i s 210 m fo r 2920 < V< 3920ms" 1 .

The shif t toward s highe r velocitie s fo r th e revised baselin e result s i n a reductio n i n estimates o f erosio n b y u p t o 210m , an d a n increase i n estimate s o f overpressur e b y u p t o 2 MPa fo r dat a point s tha t plo t abov e an d below th e line , respectivel y (overcompactio n as a resul t o f overpressure : dZ B/100 = 210/100MPa-2MPa; se e Japse n 1998) . Th e increased overpressur e tha t th e revise d mode l predicts i s a n improvemen t relativ e t o th e original model , whic h explaine d onl y 80 % o f the observe d overpressur e i n th e Chal k fo r 5 2 wells in the central North Sea located away from diapirs an d wher e th e overpressur e exceede d 4 MPa (Japsen 1998) . The corresponding percentage base d o n th e revise d baselin e is 91% . Thi s improvement i s particularl y clear fo r dat a fro m relatively shallo w dept h o r moderat e over pressure, e.g . th e Danis h Da n fiel d wher e overpressure i s 7. 3 MPa, an d fo r whic h th e overpressure predictio n fro m velocit y dat a ha s been increase d fro m 4. 3 t o 6.5 MPa. A baseline for marine shale of the Lower Jurassic F-I Member Japsen (2000) formulated a constrained baseline, V^h, fo r marin e shal e dominate d b y smectite illite based on velocity-depth data for the Lower Jurassic F- I Membe r fro m 3 1 Danis h well s o f which 2 8 have data for th e Chal k (Fig. 6):

The baselin e wa s reconstructe d b y correctin g present formatio n depth s fo r th e effec t o f lat e Cenozoic erosio n a s estimated from th e velocity of th e overlyin g Chalk i n thes e well s relative to the revise d Chal k tren d (Eq . 3) . Th e correcte d depths correspond t o th e buria l o f th e formation before erosio n whe n th e sediment s wer e a t maximum buria l a t mor e location s tha n today . The baselin e can thu s be traced mor e easil y i n a plot o f velocit y versus the correcte d depths , and is wel l define d a t grea t dept h wher e velocity depth dat a fo r normall y compacte d shal e a t maximum buria l ca n b e difficul t t o identif y (2.1 < z < 3.8km) . Thi s formulatio n i s a constrained, exponentia l transi t time-dept h model tha t fulfil s reasonabl e boundar y con ditions a t th e surfac e an d a t infinit e depth : V0 = 1550ms" 1 an d V^ = 5405ms" 1 ; maxi mum velocity-dept h gradien t 0.6ms"1 m"1 for z = 2.0km. Th e shal e tren d give n b y Eq . 4 corresponds closel y to baselines for marine shale found b y othe r worker s (Scherbau m 1982 ; Hansen 1996 ; se e discussion by Japse n 1999) .

NEOCENE UPLIFT AND EROSION O F SOUTHERN SCANDINAVI A

A baseline for the Lower Trias sic Bunter Shale Japsen (2000 ) formulate d a segmented , linea r baseline, V^ sh, fo r th e Lowe r Triassi c Bunte r Shale base d o n velocity-dept h dat a fro m 14 2 British an d Danis h well s o f whic h 9 1 hav e velocity-depth data for the Chalk (Fig . 7) :

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baselines fo r th e Bunte r Shal e an d th e Bunte r Sandstone ma y b e du e t o th e dominanc e o f quartz i n both formations, and , correspondingly , a hig h conten t o f quart z i n th e Fjerritsle v Formation clos e t o th e expose d Scandinavia n basement ma y explai n th e relativel y hig h velocity observe d i n well s i n tha t area . We wish to thank reviewers D. Issler an d J. Turner, as well a s U . Gregersen , A . Mathiesen , C . Pulvertaft , E. S . Rasmusse n an d O . Vejbae k (al l GEUS) fo r constructive comment s tha t improve d th e manuscrip t considerably.

References The trend indicates a pronounced variation of the velocity gradient with depth. The gradient is only 0.5 m s"1 m"1 in the upper part, and increases to 1.5 ms"1!!!"1 fo r depth s aroun d 2km , fro m where i t decrease s graduall y wit h dept h t o 0. 5 and the n 0.2 5 ms"1!!!"1. Th e declin e o f th e gradient wit h dept h reflect s tha t velocit y approaches a n upper limit . The Bunte r Shal e baselin e wa s reconstructe d by applying the same procedure as for the Lower Jurassic shal e b y correctin g presen t formatio n depths fo r th e effec t o f late Cenozoic erosio n a s estimated fro m Chal k velocities . Th e tren d wa s constructed t o predic t likel y value s nea r th e surface (V o — 1550ms"1), an d i s base d o n reference dat a wit h correcte d depth s fro m 160 0 to 5600m (Japse n 2000). Rather tha n proposin g a specifi c baselin e fo r the Lowe r Triassi c Bunte r Sandstone , Japse n (2000) found that the trend derived for the Bunter Shale wa s a reasonabl e approximatio n fo r a dataset fro m 13 3 Britis h an d Danis h well s o f which 87 have velocity-depth data for the Chalk (see Fig . 7) . Dat a fro m shal e ar e preferabl e t o those fro m sandston e i n studie s o f maximu m burial fo r severa l reasons : shal e porosit y i s les s affected b y diageneti c processes , shal e doe s no t act a s a n aquife r wit h th e consequen t porosit y variations, an d shale ma y be mor e unifor m with regard t o both grai n siz e an d mineralogy. Burial anomalies fo r the Bunte r Sandstone ca n thus be used t o plac e a n uppe r limi t o n estimate s o f erosion base d o n Bunter Shale data . The dominance of smectite-illite in the distal parts o f th e Fjerritsle v Formatio n (Lindgreen , pers. comm.) , and of kaoli n in the continenta l Bunter Shal e was suggeste d by Japsen (2000 ) t o be a possibl e explanatio n o f wh y baseline s fo r these tw o formations diverge, and why those fo r Bunter Shal e an d Bunte r Sandston e converg e a t depth. Alternatively , th e similarit y o f th e

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Cenozoic sequenc e developmen t i n th e easter n parts o f th e Nort h Sea . Journal o f th e Geological Society, London, 156 , 809-816 . CLAUSEN, O.R. , NIELSEN , O.B. , HUUSE , M . & MICHELSEN, O . 2000 . Geologica l indication s fo r Palaeogene uplif t i n th e easter n Nort h Se a Basin . Global an d Planetary Change, 24, 175-187 . CLEVE-EULER, A . 1941 . Alttertiar e Diatomee n un d Silicioglagellanten i m innere n Schwedens . Palaeontographica, 92A , 165-208 . FREDEN, C. 1994 . Geology. National Atlas of Sweden. SNA, Stockholm . GEMMER, L. , NIELSEN , S.B. , HUUSE , M . & LYKKE ANDERSEN, H . 2002 . Post-mid-Cretaceou s easter n North Se a evolutio n inferre d fro m 3- D dynami c modelling. Tectonophysics, 350 , 315-342 . HANSEN, S . 1996 . Quantificatio n o f ne t uplif t an d erosion o n the Norwegian Shel f south of 66°N fro m sonic transi t time s o f shale . Norsk Geologisk Tidsskrift, 76 , 245-252. HILLIS, R.R. 1995 . Quantificatio n of Tertiary exhumation i n th e Unite d Kingdo m souther n Nort h Se a using soni c velocit y data . AAPG Bulletin, 79 , 130-152. HOGBOM, A.G. 1910 . Precambrian geology of Sweden. Bulletin of the Geological Institution of the University o f Uppsala, 10 , 1-80 . HOGBOM, A.G . & AHLSTROM , N.G . 1924 . Ube r di e sub-kambrische Landflach e a m fuss e vo m Kinne kulle. Bulletin o f th e Geological Institution o f th e University o f Uppsala, 19 , 55-88. HUUSE, M., LYKKE-ANDERSEN , H . & MICHELSEN , O . 2001. Cenozoi c evolutio n o f th e easter n Danis h North Sea . Marine Geology, 111, 243-269. ILIFFE, I.E . & DAWSON , M.R . 1996 . Basi n modellin g history an d predictions . In : GLENNIE , K.W . & HURST, A . (eds ) A D 1995: NW Europe's Hydrocarbon Industry. Geologica l Society , London , 83-105. JAPSEN, P . 1993 . Influenc e o f litholog y an d Neogen e uplift o n seismi c velocitie s i n Denmark ; impli cations fo r dept h conversio n o f maps . AAPG Bulletin, 11, 194-211. JAPSEN, P . 1998 . Regiona l velocity-dept h anomalies . North Se a Chalk : a recor d o f overpressur e an d Neogene uplif t an d erosion . AAPG Bulletin, 82 , 2031-2074. JAPSEN, P . 1999 . Overpressure d Cenozoi c shal e mapped fro m velocit y anomalie s relativ e t o a baseline fo r marin e shale , Nort h Sea . Petroleum Geoscience, 5 , 321-336. JAPSEN, P . 2000. Investigatio n o f multi-phas e erosio n using reconstructe d shal e trend s base d o n soni c data, Sol e Pi t axis , Nort h Sea . Global an d Planetary Change, 24 , 189-210 . JAPSEN, P . & BIDSTRUP , T . 1999 . Quantificatio n of lat e Cenozoic erosio n i n Denmark base d o n soni c dat a and basi n modelling . Bulletin o f th e Geological Society o f Denmark, 46 , 79-99. JAPSEN, P . & CHALMERS , J.A . 2000 . Neogen e uplif t and tectonics aroun d the North Atlantic : overview. Global an d Planetary Change, 24 , 165-173 . JAPSEN, P . & LANGTOFTE , C . 199la . Geological Ma p of Denmark 1:400 000. The Danish Basin. 'Base

Chalk' and the Chalk Group, Two-way Traveltime and Depth, Thickness and Interval Velocity. Geological Surve y of Denmark Map Series , 29. JAPSEN, P . & LANGTOFTE , C . 1991b . Geological ma p of Denmark 1:400 000. The Danish Basin. 'Top Triassic' and the Jurassic-Lower Cretaceous, Two-way Traveltime and Depth, Thickness and Interval Velocity. Geologica l Surve y o f Denmar k Map Series , 30 . JENSEN, L.N . & SCHMIDT , B.J . 1992 . Lat e Tertiar y uplift an d erosion i n the Skagerrak area; magnitude and consequences . Norsk Geologisk Tidsskrift, 12 . 275-279. JENSEN, L.N . & SCHMIDT , B.J . 1993 . Neogen e uplif t and erosion offshore South Norway; magnitude and consequences fo r hydrocarbo n exploratio n i n th e Farsund Basin . In : SPENCER , A.M . (ed. ) Generation, Accumulation, and Production of Europe's Hydrocarbons; III. Springer , Berlin, 79-88. KNUDSEN, K.L . 1995 . Kvartaeret . In : NIELSEN , O.B . (ed.) Danmarks geologi fra Kridt ti l i dag. Aarhus Universitet, Aarhus, 247-269. KONRADI, P. 1995. Foraminiferal biostratigraphy of th e post mid-Miocen e in tw o borehole s i n th e Danis h North Sea. In: MICHELSEN , O . (ed.) Proceedings o f the 2nd Symposium o n Marine Geology. Danmarks Geologisice Unders0gels e Series C, 12 , 101-112 . KORNFALT, K.A . & LARSSON , K . 1987 . Geological Maps and Cross-sections of Southern Sweden. Svensk Karnbranslehanterin g (SKB ) Technica l Report, 87-24 . LARSEN, G . & DINESEN , A . 1959 . Vejle Fjord Formationen ved Brejning. Danmark s Geologisk e Unders0gelse II . Raekke , 82 . LAURSEN, G.V . 1992 . Foraminifer a o f th e easter n North Sea . In : LAURSEN , G.V. , HEILMANN CLAUSEN, C . & THOMSEN , E . (eds ) Cenozoic Biostratigraphy of the Eastern North Sea based on Foraminifera, Dinoflagellates, and Calcareous Nannofossils. Geologis k Institut , Aarhus . 1-68. LlBORIUSSEN, J. , ASHTON , P . & TYGESEN . T . 1987 .

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NEOCENE UPLIFT AND EROSION O F SOUTHERN SCANDINAVI A LIDMAR-BERGSTROM, K . 1999 . Uplif t historie s revealed b y landform s o f th e Scandinavia n domes. In : SMITH , B.J. , WHALLEY , W.B . & WARKE, P.A . (eds ) Uplift, Erosion an d Stability: Perspectives on Long-term Landscape Development. Geologica l Society , London , Specia l Publications, 162 , 85-91. LIDMAR-BERGSTROM, K. , OLLIER , C.D . & SULEBAK , J.C. 2000. Landforms and uplift history of southern Norway. Global an d Planetary Change, 24 , 211-231. LIDMAR-BERGSTROM, K. , OLSSON , S . & OLVMO , M . 1997. Palaeosurface s an d associate d saprolite s i n southern Sweden . In : WIDDOWSON , M . (ed. ) Palaeosurfaces; Recognition, Reconstruction and Palaeoenvironmental Interpretation. Geologica l Society, London , Specia l Publications , 120 , 95-124. LINDGREEN, H . 1991 . Elementa l an d structura l changes i n illite/smectit e mixed-laye r cla y minerals durin g diagenesi s i n Kimmeridgian-Volgia n (-Ryazanian) clay s i n th e Centra l Trough , Nort h Sea an d th e Norwegian-Danis h Basin . Bulletin o f the Geological Society o f Denmark, 39 , 1-82 . LYKKE-ANDERSEN, H . 1991 . Nogl e hovedetrae k a f Kattegats kvartaergeolgi—fore!0big e resultate r a f en seismis k undersjagels e 1988-1991 . Dansk Geologisk Forening, Arsskriftfor 1990-91, 57-65. MARIE, J.P.P . 1975 . Rotliegende s stratigraph y an d diagenesis. In : WOODLAND , A.W . (ed.) Petroleum and the Continental Shelf of North-west Europe. Applied Science , London , 205-211 . MICHELSEN, O . 1989 . Revision o f th e Jurassic Lithostratigraphy of the Danish Subbasin. Geological Surve y of Denmark Serie s A , 24. MICHELSEN, O . & NIELSEN , L.H . 1993 . Structura l development o f th e Fennoscandia n borde r zone , offshore Denmark . Marine and Petroleum Geology, 10, 124-134 . NIELSEN, L.H . & JAPSEN , P . 1991 . Deep Wells i n Denmark 1935-1990. Litho strati graphic Subdivision. Geologica l Surve y o f Denmar k Serie s A, 31. NIELSEN, S.B. , PAULSEN, G.E. , HANSEN, D.L . et al. 2002. Paleocen e initiatio n o f Cenozoi c uplif t i n Norway. In : DORE , A.G. , CARTWRIGHT , J.A. , STOKER, M.S. , TURNER, J.P . & WHITE , N . (eds) Exhumation of the North Sea Margin: Timing, Mechanisms and Implications for Petroleum Exploration. Geologica l Society , London , Specia l Publications, 196 , 45-65. PEDERSEN, G.K. 1985. Thin, fine-graine d stor m layers in a mudd y shel f sequence : a n exampl e fro m th e Lower Jurassi c i n th e Stenlill e 1 well, Denmark . Journal o f th e Geological Society, London, 142 , 357-374. PEULVAST, J.-P . 1985 . Post-orogenic morphotectoni c evolution o f th e Scandinavia n Caledonides durin g the Mesozoi c an d Cenozoic . In : GEE , D.G. & STURT, B.A . (eds ) Th e Caledonide Orogen — Scandinavia an d Related Areas. Wiley, Chichester, 979-996. RASMUSSEN, E.S. & LARSEN , O. 1989 . Mineralogi o g geokemi afdet 0vre Mioccene Gram ler. Danmarks Geologiske Unders0gelse , Seri e D, 7.

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CLOETINGH, S . 1995 . Meso-Cenozoi c morpho tectonic evolutio n o f souther n Norway : Neogen e domal uplif t inferre d fro m apatit e fissio n trac k thermochronology. Tectonics, 14, 700-714. RUDBERG, A . 1954 . Vasterbotten s berggrunds morfologi. Geographica, 25 , 1 -457. SCHERBAUM, F . 1982 . Seismic velocitie s i n sedimen tary rocks ; indicator s o f subsidenc e an d uplift . Geologische Rundschau, 71 , 519-536 . SHIPBOARD SCIENTIFI C PARTY , e t al . 1991 . Site 807. In: KROENKE , L.W. , BERGER, W.H . & JANACEK , T.R. (eds ) Proceedings o f th e Ocean Drilling Program, Initial Reports, 130. Ocea n Drillin g Program, Colleg e Station , TX, 369-493 . SPJELDN^S, N . 1975 . Palaeogeograph y an d facie s distribution i n th e Tertiar y o f Denmar k an d surrounding areas . Norges Geologiske Unders0gelse Bulletin, 316, 289-311 . URMOS, J. , WILKENS , R.H. , BASSINOT, F. , LYLE , M. , MARSTERS, J.C. , MAYER, L.A . & MOSHER , D.C. 1993. Laboratory and well-log velocity and density measurements fro m th e Onton g Java Plateau: ne w in-situ correction s t o laborator y dat a fo r pelagi c carbonates. In : BERGER , W.H. , KROENKE, L.W., MAYER, L.A . & JANECEK , T.R. (eds) Proceedings of the Ocean Drilling Program, Scientific Results, 130. Ocean Drillin g Program, College Station , TX, 607-622. VAN OLPHEN , H . 1966 . Collaps e o f potassiu m montmorillonite clay s upo n heating—'potassiu m fixation'. In : BAILEY , S.W . (ed.) Clays an d Clay Mineralogy. Pergamon , Oxford , 393-405 . VEJB^K, O.V . 1997 . Dyb e strukture r i dansk e sedimentaere bassiner. Geologisk Tidsskrift, 1997/4 , 1-31. VEJB^K, O.V. & BRITZE , P . 1994 . Geological Ma p o f Denmark 1:750 000 Top Pre-Zechstein (Two-way Traveltime an d Depth). Geologica l Surve y o f Denmark Map Series , 45 . VIK, E . & HERMANRUD , C . 1993 . Transient therma l effects o f rapi d subsidenc e i n th e Haltenbanke n area. In: DORE , A.G., AUGUSTSON, J.H., HERMAN RUD, C. , STEWART , D.J . & SYLTA , O . (eds ) Basin Modelling: Advances and Applications. Norwegia n Petroleum Societ y Specia l Publication , 3 , 107-117. YUKLER, M.A . 1978 . One-dimensiona l mode l t o simulate geologic , hydrodynami c an d thermo dynamic developmen t o f a sedimentar y basin . Geologische Rundschau, 67 , 960-979. ZAGWIJN, W.H . 1989 . Th e Netherland s durin g th e Tertiary and the Quaternary: a case story of coastal lowland erosion . Geologic e n Mijnbouw, 68 , 107-120.

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Cenozoic uplif t and denudation o f southern Norway : insights fro m the North Se a Basi n MADS HUUSE 1'2 Department of Earth Sciences, University of Aarhus, Aarhus, Denmark 2 Present address: Department of Earth Sciences, Cardiff University, Cardiff CF 10 3 YE, UK (e-mail: m.huuse@ abdn.ac.uk) 1

Abstract: Th e Cenozoi c evolutio n o f th e Nort h Se a Basi n i s described , drawin g o n subsurface dat a an d a serie s o f palaeogeographica l map s compile d fro m a variet y o f published studies , mainl y emphasizin g the developmen t o f the easter n par t o f the basin . A model that accounts for the sedimentation history o f the North Sea Basin an d the topograph y (including maximu m an d mea n surfac e elevation ) o f souther n Norwa y i s proposed . Th e model involve s regiona l plume-relate d uplif t o f a n initia l low-elevatio n peneplai n i n earl y Paleogene tim e followe d b y repeate d episode s o f climati c deterioratio n an d eustati c fall , most notably a t the Eocene-Oligocene transition, i n late Mid-Miocene time, and eventuall y culminating wit h th e developmen t o f ful l glacia l condition s i n souther n Norwa y i n Plio Pleistocene time. Thes e episodes correspond to periods of accelerated sediment suppl y fro m southern Norwa y tha t reflec t increased rates of incision (dissection ) o f the sourc e area. It is argued tha t th e present-da y elevatio n o f > 2 km o f mountai n peak s i n souther n Norwa y adjacent t o deep valleys an d fjords coul d hav e been cause d b y isostatic uplift in response to dissection o f a high-elevatio n peneplain . Henc e i t ma y no t b e necessar y t o invok e lat e Cenozoic tectonic uplift event s t o explain th e present-day topograph y o f souther n Norway .

This pape r describe s th e Cenozoi c infil l histor y of th e Nort h Se a Basi n an d it s implication s fo r understanding ho w th e topograph y o f souther n Norway wa s created . A t present , th e Nort h Se a Basin i s fille d t o it s brim . I t comprise s th e shallow Nort h Sea , th e low-relie f area s o f Denmark, norther n Poland , norther n Germany , the Netherlands and SE England. This low-relief area i s surrounde d b y th e topographicall y hig h areas of southern Norway to the NE, Scotland to the N W and the Centra l Europea n Massi f t o the south (Fig. 1) . During Cenozoic time the margins of the North Sea Basin became exhumed whereas the centr e o f the basin subside d more tha n 3 km (Fig. 2) . Th e subsidenc e histor y o f th e basi n i s fairly wel l constraine d b y th e preserve d sedi mentary record , althoug h th e details ar e stil l the subject of debate (McKenzie 1978 ; Nielsen et al. 1986; Vinke n 1988 ; Cloeting h e t al 1990 , 1992 ; Ziegler 1990 ; Jo y 1992 ; Gallowa y e t a l 1993 ; Jordt e t al 1995 ; Liu & Galloway 1997 ; Japse n 1998; Michelse n e t a l 1998 ; Huus e 2002 ; Nielsen e t al 2002) . I n contrast, the magnitude, mechanisms an d exac t timin g o f uplif t an d subsequent denudatio n of the basin margins and the hinterland mountains has remained enigmatic to thi s da y (Torsk e 1972 ; Dor e 1992 ; Jense n & Michelsen 1992 ; Jense n & Schmid t 1993 ; Jord t

et al 1995 ; Rohrma n e t al 1995 ; Hanse n 1996 ; Riis 1996 ; Solhei m e t a l 1996 ; Japse n 1998 ; Michelsen et al. 1998 ; Dor e et al 1999 ; Lidmar Bergstrom 1999 ; Chalmer s & Cloeting h 2000 ; N0ttvedt 2000 ; Huuse , 2002). I t i s ofte n argue d that uplif t an d denudatio n occurre d i n tw o phases, on e synrif t an d on e post-rif t (e.g . Rii s & Fjeldskaa r 1992 ; Eyle s 1996 ; Rii s 1996 ; Rohrman & va n de r Bee k 1996 ; Lidmar Bergstrom et al 2000) , althoug h the two phases can b e difficul t t o separat e (Japse n & Chalmer s 2000). The magnitud e of uplif t an d denudatio n ma y be estimate d fro m proxie s suc h a s geothermo metry (fissio n tracks , vitrinit e reflectance , etc.), compaction estimates , structura l trends , sedi mentary geometrie s an d geomorphology . Thes e methods are , however , al l associated wit h rather large uncertainties , a s indicate d b y th e discre pancies observe d whe n comparin g uplif t esti mates derive d usin g different method s (compar e Jensen & Schmid t 1993 ; Japse n & Bidstru p 1999; Huuse 2002). It is even more speculative to assess th e mechanism s an d th e exac t timin g of uplift, an d th e amoun t and timin g of denudation that followed . The uplif t an d denudation history of souther n Norwa y i s a n intriguin g problem i n its own right, but is also o f significan t interes t to

From: DORE , A.G. , CARTWRIGHT , J.A. , STOKER , M.S., TURNER , J. R & WHITE , N . 2002 . Exhumation o f th e North Atlantic Margin: Timing, Mechanisms an d Implications for Petroleum Exploration. Geologica l Society , London, Special Publications, 196, 209-233 . 0305-8719/027$ 15.00 © The Geological Societ y of London 2002 .

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Fig. 1 . Present-day topography and bathymetry o f NW Europe. The study areas of Jordt el al. (1995) (long dash) , and o f Michelsen e t al. (1998 ) (shor t dash ) an d location s o f Figs 2 , 4. 5 and 9 are show n fo r reference .

the oi l industry , a s uplif t an d denudatio n ma y have bot h positiv e an d negativ e effect s o n th e hydrocarbon potentia l o f th e N W Europea n margin (e.g. Sales 1992 ; Jensen & Schmidt 1993 ; Dore & Jensen 1996 ; Dor e e t al. 1997 , 1999) . Uplift an d denudation Definition When discussin g the uplif t histor y of any area of the Eart h i t i s extremel y importan t t o properl y

define what is actually meant by the term 'uplift' , i.e. i s i t regiona l surfac e uplif t o r loca l uplif t o f mountain peak s ('uplif t o f rocks') . I t i s equally important t o properl y defin e th e referenc e level to which uplift i s measured. In this paper the term 'surface uplift ' i s use d t o describ e uplif t o f th e Earth's surfac e with respect t o th e geoi d (mea n global se a level ) average d ove r a n are a o f c. 10 4km2, a s thi s i s th e relevan t scal e fo r studying vertica l movement s o f th e lithospher e (England & Molnar 1990) . The local inversion of former norma l faults i s thus not considered here.

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Fig. 2. Depth to the Upper Cretaceous-Danian limeston e (Ziegle r 1990 ; Japsen 1998) . The stud y areas of Jordt et al (1995 ) (lon g dash) , o f Michelse n e t al (1998 ) (shor t dash ) an d o f Huus e (1999 ) (continuou s line) an d locations o f Figs 4 , 5, 6 and 8 are show n for reference.

Surface uplif t ove r larg e area s require s upwar d displacement o f rocks wit h respect t o the geoid , i.e. wor k agains t gravity , an d thu s require s a n active tectoni c mechanis m o f larg e magnitud e (England & Molnar 1990; Summerfield & Brown 1998; Nielse n e t al., 2002) . Whe n discussin g relatively smal l amount s (som e hundred s o f metres) o f surfac e uplift , i t i s importan t t o consider eustati c change s (Englan d & Molna r 1990; Huuse , 2002). I n particular, when discussing Cenozoic uplift events, the long-term eustatic fall sinc e mid-Eocene tim e of c. 200m (e.g. Haq et al. 1987 ) corresponds t o (nontectonic) surfac e uplift o f th e sam e magnitude . Finally , i t i s important t o not e tha t 'surfac e uplift' , 'uplif t o f rocks' an d 'denudation ' ar e relate d i n th e

following wa y (Englan d & Molna r 1990) : surface uplif t = uplif t o f rock s — denudation. Hence, th e implici t assumptio n ofte n mad e that roc k uplif t i s equa l t o surfac e uplif t i s true only o n th e rar e occasion s whe n denudatio n i s zero (Englan d & Molna r 1990) . Th e ter m 'denudation' i s use d her e instea d o f 'exhuma tion' as the latter is generally used to describe the re-exposure o f a burie d lan d surfac e (se e Summerfield & Brow n 1998) . I t shoul d b e noted als o tha t surfac e uplif t i n itsel f doe s no t cause increase d denudation , a s i t i s th e loca l relief rathe r tha n averag e elevatio n tha t govern s denudation rate s (Ahner t 1970 ; Summerfiel d 1991). Moreover , denudatio n rate s wil l usually be les s tha n rates o f surfac e uplift, a s otherwise

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there would b e no mountains. Indeed , it has been shown tha t i n som e case s th e denudationa l response ma y la g severa l ten s o f millio n year s behind a n uplif t even t (Summerfiel d & Brow n 1998). Tectonic mechanisms A larg e numbe r o f activ e tectoni c mechanism s have bee n propose d t o explai n th e denudatio n record an d present-da y topograph y o f th e Nort h Atlantic margins , i n particula r th e Norwegia n and Scottis h highland s an d th e eas t Greenlan d margin (e.g . Dor e 1992 ; Japse n & Chalmer s 2000). Crusta l shortenin g i s a well-know n mechanism fo r generatin g surfac e uplift , bu t the amoun t o f shortenin g observe d i n N W Europe i s nowher e nea r enoug h t o explai n th e amount o f exhumatio n inferre d fro m vitrinit e reflectance profile s an d fission-trac k analyse s (Brodie & Whit e 1995) . Crusta l shortenin g i s observed i n th e shap e o f elongat e dome s alon g the N W Europea n Atlanti c margi n (Dor e & Lundin 1996 ; Boldree l & Anderse n 1998 ; Dor e et al. 1999) , bu t thes e dome s ar e significantl y smaller than , for example , th e sout h Norwegia n dome, whic h must be explained b y other means . Other activ e tectoni c mechanism s capabl e o f generating regiona l surfac e uplif t includ e increased buoyanc y b y heate d lithospher e (White 1992) , mantl e upwellin g (Rohrma n & van der Beek 1996 ; Nadi n et al 1997) , magmati c underplating of the lower crust (McKenzie 1984 ; White & Lovel l 1997) , an d delaminatio n o f th e lower lithospher e (Bir d 1979 ; Molna r e t al . 1993). These mechanisms coul d all be related t o the impingemen t o f th e Icelan d Plum e ont o th e North Atlanti c an d subsequen t Nort h Atlanti c rifting a t th e Paleocene-Eocen e transitio n (c. 5 4 Ma). Thu s the y ar e al l plausibl e agent s o f surface uplif t (se e discussio n b y Nielse n e t al . 2002). However, i t has been argued tha t no singl e mechanism seem s capabl e o f explainin g al l o f the observe d (an d inferred ) 'up s an d downs ' around and within the North Atlantic (Dore et al. 1999). In lat e Cenozoi c tim e th e N W Europea n margin wa s fa r remove d fro m th e Icelan d Hotspot (Lawve r & Mulle r 1994 ; Dor e e t al . 1999). Becaus e o f th e increase d distanc e t o th e Iceland Plum e an d th e lac k o f extensiv e crusta l shortening, activ e tectoni c surfac e uplif t o f lat e Cenozoic ag e i s difficul t t o invok e withou t jumping t o rathe r exoti c explanations . Th e various tectoni c mechanism s capabl e o f causin g early Paleogen e surfac e uplif t aroun d th e Nort h Atlantic ar e discusse d i n a companio n pape r (Nielsen e t al . 2002) , whic h argue s tha t earl y

Paleogene delamination o f the lithospher e i s the most plausibl e mechanism . Thi s mechanis m causes severa l hundre d metre s o f almos t instantaneous uplif t followe d b y deceleratin g uplift rate s throug h th e remainde r o f th e Cenozoic. To kee p th e mode l develope d i n th e presen t paper a s simpl e a s possibl e an d independen t o f the mechanis m of the tectonic uplift, i t is simply assumed tha t significan t (500-1000m ) surfac e uplift occurre d i n relatio n t o th e arriva l o f th e plume an d riftin g o f th e Nort h Atlanti c in early Paleogene time . Also , i t i s assume d tha t fo r th e case o f souther n Norwa y thi s uplif t wa s permanent, as opposed t o transient, uplift cause d by increase d buoyanc y of heated lithosphere. Isostatic uplift response to denudation Gravity dat a indicat e tha t th e mea n surfac e topography o f souther n Norwa y i s isostaticall y compensated a t depth (Balling 1980; Rohrman & van de r Bee k 1996) , suggestin g tha t Cenozoi c denudation wa s compensate d b y regiona l iso static uplif t o f th e crust . I t ha s bee n argue d that intense localize d denudatio n (dissection ) o f a n initially fla t topograph y o f hig h elevatio n could theoretically caus e mountai n peak s t o reac h elevations o f twic e o r mor e th e heigh t o f th e initial topograph y (see Molnar & England 1990 ; Gilchrest e t al . 1994) . I t shoul d be note d tha t in this cas e uplif t i s a consequenc e o f denudation and not vice versa. This is possible because of the regional isostatic response to dissection, which is governed b y th e isostati c respons e functio n (/) , which equals pc/pm, where pc is the density of the material erode d fro m th e to p o f th e crust , p m i s the densit y o f th e mantl e a t th e dept h o f compensation (p m ~ 3.3gem~~) , an d / i s th e amount o f isostatic uplift pe r uni t mea n dept h of dissection (Gilchres t e t al . 1994) . I n th e centra l parts o f souther n Norway , wher e th e remove d material is likely to have been mainly crystalline rock (p c ~ 2.7-3.0gem" 3 ), the compensation is relatively hig h (approachin g 0.8-0.9) . I n mar ginal areas , wher e a greate r proportio n o f th e rocks remove d ar e likel y t o hav e bee n o f sedimentary origi n (p c ~ 2.3-2.7gcm~~ ) i t i s somewhat lowe r (c . 0.7-0.8). Climatic and eustatic change The denudatio n histor y of souther n Norway an d the infil l pattern s o f th e Nort h Se a Basi n wer e modulated b y climati c an d eustati c change s (Spjeldnaes 1975 ; Dor e 1992 ; Jord t e t a l 1995 ; Eyles 1996 ; Solhei m et al. 1996 ; Michelsen et al.

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Fig. 3 . Cenozoic stratigraphy of the eastern North Sea Basin. Time scale according to Berggren et al. (1995). The composite 6 18 O curve is compiled fro m Mille r et al. (1987 , 1998) . Th e Mille r e t al (1987 ) curv e was shifte d - 0.25% c at 1 0 Ma and +0.20%c at 37 Ma to match values of the Miller et al. (1998) curve. Correlation of North Sea sequence s wit h th e Berggre n e t al . (1995 ) tim e scal e an d th e 8 18O curv e i s base d o n th e calcareou s nannofossil zonatio n o f Martin i (1971) . Th e Nort h Se a 5 18O curve o f Buchard t (1978) , adjuste d t o fi t earl y Eocene an d mid-Miocene peaks on the Miller curve, is shown for reference. *Episodes of marked increase in the supply of coarse clastic sediment s to the North Sea Basin. 1 , North Atlantic rifting an d volcanism; 2, major icesheet expansion (Lear et al. 2000). The mid-Paleocene event roughly coincides wit h the commencement of rift related uplif t o f area s borderin g th e Nort h Atlanti c rift . Th e lates t Eocene-earlies t Oligocene , th e lat e MidMiocene and the Plio-Pleistocene events all coincide with major coolin g events and eustatic lowerings as a result of increased continental ic e volume (Lea r et al. 2000). Foraminifera: NSP , Nort h Se a Planktic; NSB , Nort h Sea Benthic; NSA, Nort h Sea Agglutinated.

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1998; Clause n e t al 1999 , 2000; Huus e 2002 ; Nielsen e t al 2002) . Most researcher s agre e tha t eustasy peaked i n response t o relativel y larg e spreadin g rate s a t mid-ocean ridge s and greenhouse climate durin g mid-Cretaceous time . Th e long-ter m trend s o f global se a level (eustasy) may be estimated fro m spreading rates o f mid-ocean ridge s (e.g . Pitman 1978; Komin z 1984) , seismi c stratigraph y (Vail et a l 1977 ; Ha q e t a l 1987) , flexura l back stripping (Peka r & Mille r 1996 ; Steckler e t a l 1999) an d from stable isotopes (Fig. 3; 8 18O, Sr, Mg/Ca; Matthews 1984; Millers al 1987 , 1996 , 1998; Abre u & Anderson 1998 ; Lear et al 2000) . From thes e studie s a genera l consensu s ha s emerged tha t long-ter m globa l se a leve l ha s fallen som e 150-25 0 m durin g Cenozoi c time . This, of course, corresponds to a surface uplift of the sam e magnitud e relativ e t o sea level. A long-ter m tren d o f climati c coolin g starte d in lat e Mid-Eocen e time , bot h globall y (Savi n 1977; Wolf e 1978 ; Mille r e t a l 1987 , 1998; Zachos e t al 1992 ; Lear e t al 2000 ) an d in the North Sea region (Buchardt 1978; Collinson et al 1981). I n associatio n wit h th e chang e toward s a colder climate , th e continenta l ic e shee t o n Antarctica expanded , causin g a pronounce d eustatic lowering , culminatin g a t th e Eocene Oligocene transitio n (c . 3 4 Ma; Mille r e t a l 1998; Lea r e t a l 2000) . A simila r episod e o f cooling an d eustati c lowerin g occurre d onc e again i n lat e Mid-Miocen e time , followin g a n early t o mid-Miocen e war m perio d (Molna r & England 1990 ; Mille r e t a l 1998 ; Lear e t a l 2000), an d i n Plio-Pleistocen e time , eventuall y causing ful l glacia l condition s i n N W Europ e (Eyles 1996 ; Solhei m e t a l 1996 ; Lear e t a l 2000). With the exception of the Plio-Pleistocen e glaciations, the role of climate change has largely been neglecte d i n previou s studie s o f th e Nort h Atlantic margins. Thi s ma y be due t o the notion that climate a s a mechanism act s on a time scal e an orde r o f magnitud e shorte r tha n tha t o f tectonics (see Vail et al 1991 ) or simply because of lack of attention to Cenozoic climat e changes . However, i t i s importan t t o bea r i n min d tha t a major long-ter m chang e i n climat e ma y signifi cantly affec t denudatio n rate s (Summerfiel d & Brown 1998) . Major episode s of climatic cooling during the Cenozoic generall y corresponde d t o majo r icesheet expansions on Antarctica that caused major eustatic lowering s (Lea r e t a l 2000) . Thu s i t is possible tha t th e effect s o f majo r eustati c fall s and stepwis e climati c deterioratio n coul d caus e effects simila r t o thos e widel y attribute d t o regional surfac e uplift , i.e . accelerated denuda tion o f topograph y an d increase d sedimen t

supply t o adjacen t basin s (Donnell y 1982 ; Molnar & England 1990 ; Huuse 2002). Rationale From the above discussion it should be clear that the following factors should be considered when attempting t o accoun t fo r th e observe d topogra phy o f souther n Norwa y an d th e sedimentar y record o f the North Sea Basin: (1) plume-related surface uplif t alon g Atlanti c margin s i n earl y Paleogene time ; (2 ) episodic inversio n tectonic s driven by Alpine compression an d Atlantic ridge push; (3 ) stepwis e climati c deterioratio n sinc e mid-Eocene tim e a s documente d b y stabl e isotope record s (Fig . 3); (4 ) eustati c lowerin g of c. 250 m since mid-Cretaceous time (c. 200 m since mid-Eocen e time) ; (5 ) passiv e (flexural? ) isostatic respons e t o denudatio n and deposition. These ar e al l relativel y well-documente d phenomena, althoug h ther e ma y b e som e uncertainty abou t th e exac t mechanism s o f early Paleogen e uplif t an d th e magnitud e an d effects o f climati c deterioratio n an d eustati c fall (see Dor e e t al 1999 ; Nielsen et al 2002) . Another facto r tha t mus t b e addresse d i n a n account of the genesis of present-day topography is palaeo-topography , i.e. the topograph y o f th e so-called 'pre-uplif t peneplain ' (Stuevol d & Eldholm 1996 ) or 'palaei c surface ' o f Norwa y (Gjessing 1967 ; Lidmar-Bergstrom e t al 2000) . It is assumed here tha t the present-day elevation of th e palaeo-peneplai n roughl y coincide s wit h the summit envelope (see Dore 1992) . The purity of th e Lat e Cretaceous-Dania n chalk s o f th e North Se a Basi n indicate s tha t an y siliciclasti c source are a mus t have been clos e t o a peneplain by lat e Cretaceou s tim e (Hancoc k 1975) . However, th e presenc e o f uppe r Cretaceou s siliciclastic wedge s alon g th e Atlanti c margi n (Knott e t a l 1993 ; Dore e t a l 1999 ) demonstrates tha t ther e mus t hav e bee n som e topography abov e se a level . Thi s topograph y probably coincide d wit h th e Shetlan d Platfor m and the present-day Norwegian mainland . Som e topography wa s probabl y als o generate d alon g the lat e Cretaceou s an d earl y Paleogen e inver sion zone s (Ziegle r 1990 ; Gemmer e t al 2002) . The elevatio n an d relie f o f th e topograph y o f southern Norwa y an d Shetlan d i s difficul t t o constrain becaus e o f th e absenc e o f lat e Mesozoic an d Cenozoi c sediments , bu t a maximum elevatio n o f th e orde r o f a fe w hundred metre s above (palaeo-) se a level, gently sloping toward s th e shorelin e wit h onl y mino r local relie f seem s plausible . Th e shorelin e wa s probably clos e t o o r slightl y inboar d o f th e present shoreline s o f souther n Norway, whereas

UPLIFT AND DENUDATION O F SOUTHER N NORWA Y

all o f Denmar k an d souther n Swede n wa s submerged (Spjeldnae s 1975 ; Ziegle r 1990 ; Stuevold & Eldhol m 1996) . Thi s combinatio n of maximu m elevatio n an d shorelin e position s would correspond t o an average surfac e slope of about 0.1°, which does not seem unrealistic for a peneplain acros s a former mountai n range . Against thi s background , i t wil l b e assesse d whether th e interactio n o f th e above-mentione d mechanisms ca n accoun t fo r th e present-da y

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topography o f souther n Norwa y an d th e strati graphic record o f the adjacent basins. If this were the case, the n there would be no need t o invoke late Cenozoic tectoni c uplif t events . Conversely, if th e combinatio n o f the abov e factor s doe s no t support th e hypothesis , on e ma y begi n t o speculate abou t lat e Cenozoi c tectoni c events . The scenari o give n her e i s base d mainl y o n qualitative evidenc e an d simpl e isostati c calcu lations, an d shoul d therefor e b e regarde d a s a

Fig. 4 . Post-Dama n depocentre s i n th e easter n Nort h Se a superimpose d o n dept h contour s o f th e Uppe r Cretaceous-Danian limestone . Depocentre s wer e mappe d b y Bidstru p (1995 ) and Michelsen e t al. (1998).

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somewhat subjectiv e perception o f how present day topograph y an d sedimentar y thicknesse s o f the Nort h Se a are a ma y b e accounte d for . Th e importance o f globa l (allocyclic ) mechanism s (climate, eustasy ) is stressed, a s opposed t o local or regiona l (autocyclic ) mechanism s (e.g . tec tonics). O n th e basi s o f geodynami c modelling , Nielsen et al (2002 ) have provided a quantitative account o f a similar scenario . The ultimate test of any qualitative scenario of the development o f source area s and their basins would be to carry ou t mass-balanced palaeogeo graphical reconstruction s suc h as that performed for th e Mississippi catchmen t and the associate d depocentre i n th e Gul f o f Mexic o (Ha y e t al . 1989). Suc h a n approac h require s a detaile d integration o f regiona l denudatio n estimate s from onshor e area s wit h th e extensiv e offshor e database o f seismi c an d wel l dat a an d thus calls for internationa l co-operatio n an d integratio n of large regiona l databases. Cenozoic evolution of the North Sea Basin Database The Cenozoi c evolutio n o f th e Nort h Se a Basin has bee n piece d togethe r fro m a larg e databas e that cover s variou s part s o f th e basin . Seismi c data, wel l data , outcrops , structur e an d isopac h maps (Fig. 4; Nielsen et al 1986 ; Bidstrup 1995 ;

Jordt e t a l 1995 ; Michelse n e t a l 1995 , 1998 ; Joy 1996 ; S0rense n etal 1997 ; Huus e & Clausen 2001), seismi c an d sedimentary facies maps (Joy 1996; Mudg e & Buja k 1996 ; Danielse n e t a l 1997; S0rense n e t a l 1997) , pattern s o f clino form breakpoin t migratio n (Fig s 5 an d 6 ; S0rensen e t a l 1997 ; Clause n e t al 1999) , an d palaeogeographical compilation s (Graman n & Kockel 1988 ; Kocke l 1988 ; Ziegle r 1990 ) hav e been compile d t o yiel d palaeogeographica l (palaeobathymetric) map s o f th e entir e Nort h Sea Basi n (Fig . 7) . Huuse , 2002 ha s provide d a complete lis t o f references for eac h o f th e map s (Fig. 7b-g) an d an account of their compilation. Palaeogeographical development It i s generall y accepte d tha t th e overal l infil l o f the Nort h Se a Basin was dominated b y westerly source area s durin g Paleocene an d Eocene time , whereas easterl y sourc e area s dominate d during the remainde r o f Cenozoic tim e (e.g . Jordt et a l 1995, 2000 ; Jo y 1996 ; Mudg e & Buja k 1996 ; Michelsen e t a l 1998) . However , thi s pictur e may be severely biased if one merely looks at the preserved sediments . Fo r example , regiona l cross-sections o f th e norther n Nort h Se a (e.g . Jordt e t a l 1995 , Fig . 3) , sho w a completel y preserved Paleocene-Eocen e successio n pro grading fro m th e Shetlan d Platform , wherea s similar age progradational deposits ar e truncated

Fig. 5 . Spatial and temporal migratio n o f clinoform breakpoint s i n the eastern Nort h Se a Basin superimposed o n depth contours of the Upper Cretaceous-Danian limestone (clinofor m breakpoint s afte r Funnel l 1996 ; Clause n et al 1999) . The clockwise infil l fro m th e Oligocene tim e onwards should be noted. This caused the northern part of the are a t o be fille d t o base leve l som e 15—2 0 Ma befor e th e souther n part .

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Fig. 6 . ENE-WSW-oriented seismic profil e (RTD81-22 ) showing large-scale depositiona l geometrie s alon g the Norwegian-Danish secto r boundary . (For location , se e Figs 2 an d 7. ) I n the Centra l Grabe n are a a condense d upper Paleocene t o lower Middle Eocene successio n is overlain by thick upper Middle-Upper Eocene smectiti c clays with chaotic internal structure onlapping towards the east. The overlying succession o f Oligocene silty clays interbedded wit h thic k san d unit s exhibit s markedl y southwestwar d progradationa l geometries . Th e Miocen e deposits ar e les s markedl y progradationa l alon g thi s profil e an d consis t mainl y o f silt y cla y wit h thi n san d stringers. High-angle progradational geometries ar e also observed i n the upper Pliocene successio n to the WSW. The final phase of infill in Pleistocene tim e is characterized b y regional onlap of mainly shallow-water sediments supplied fro m th e SSE . Th e height o f the Oligocene-Miocene clinoforms indicates that palaeo-water depth s in the Centra l Grabe n are a wer e substantia l (500-1000m).

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Fig. 7 . Palaeogeographical developmen t of the eastern Nort h Se a Basin (modified afte r Huus e 2002); a t (a) midPaleocene, (b ) late Eocene, (c ) late Oligocene, (d ) mid-Miocene (e ) late Miocene, (f ) lat e Pliocene, and (g) midPleistocene time . Th e map s ar e base d o n observation s fro m th e easter n Nort h Se a Basi n integrate d wit h a n extensive number of published data. Se e Huus e (2002) fo r complete listin g o f data and reference s used .

UPLIFT AND DENUDATION O F SOUTHERN NORWA Y

at a hig h angl e toward s th e Norwegia n coast . These progradationa l wedge s ar e probabl y th e remnants o f a large r syste m o f progradationa l lower Paleogen e deposit s originatin g fro m southern Norway . It is thus likely tha t significant amounts of sediment were supplied from easterl y source area s durin g Paleocen e an d Eocene tim e (Jordt et al. 2000). Thi s is also indicated by sandprone depocentre s o f lat e Paleocen e t o earl y Eocene ag e S W o f Norwa y (Fig . 4) . Possibl e causes fo r th e differentia l preservatio n o f th e Paleocene-Eocene successio n o n eithe r sid e o f the northern North Se a will be discussed later in this paper . The large middl e t o upper Eocene depocentr e in th e Centra l Troug h (uni t 3 , Fig. 4 ; Michelse n et al . 1998 ) consist s mainl y o f smectite dominated cla y (Thyber g e t a l 2000) . Th e deposits contai n little , i f any , evidenc e fo r palaeo-transport direction s an d compris e a n abnormally thic k successio n of hemipelagi c clays deposite d fa r awa y fro m potentia l sourc e areas. In th e easter n Nort h Sea , th e Eocene Oligocene transitio n i s characterize d b y a massive increas e i n the amoun t of coarse clasti c sediments supplie d fro m souther n Norway , resulting i n a n almos t 1 km thic k depocentr e o f markedly progradationa l sand-pron e deltai c sediments o f Oligocen e ag e i n th e Norwegian Danish Basi n (Figs 4-6) . Thi s shif t fro m hemi pelagic clay s an d marl s t o silt y an d sand y clay s was probabl y a n effec t o f th e lat e Eocen e climatic deterioratio n (Buchardt 1978; Collinson et al . 1981) , whic h le d t o increase d seasonalit y and eustati c lowerin g (Ivan y e t al . 2000 ; Lea r et al . 2000) . Th e lowe r temperature s an d increased seasonalit y probabl y increase d th e amount o f precipitatio n an d cause d significan t changes i n vegetatio n (Spjeldnae s 1975 ; Collin son et al., 1981), thus increasing th e erosivity o f the geomorphologica l syste m (se e Summerfiel d & Brown 1998) . The clockwis e rotatio n o f th e directio n o f progradation (Fig . 5 ; Clausen et al. 1999 ) show s how the basin was filled by sediments prograding from th e NE , east , S E an d finall y sout h durin g Oligocene t o Pleistocen e time . A s th e mos t proximal part s o f th e basi n wer e fille d durin g Oligocene tim e (Fig . 7c ; Danielsen e t al. 1997) , sediments simpl y bypasse d th e Oligocen e depocentre durin g Miocen e time , fillin g u p th e easternmost par t of the basin coinciding wit h the central part s o f Denmar k (Fig s 4- 6 an d 7d) . Following th e Hodde transgressio n (Koch 1989) , sediments started prograding from the east across the norther n par t o f German y int o the relatively deep waters of the German Bight of the southern

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North Se a Basi n (Fig s 5 an d 7e ; Kocke l 1988 ; Jiirgens 1996) . Th e distal toes o f these lat e Mid to Uppe r Miocen e delta s o f th e souther n Nort h Sea obliquel y onla p th e Oligocene-lowe r Mid Miocene depocentre s farthe r nort h (se e Fig s 6 and 8) . In Pliocen e tim e th e easter n part s o f th e basin ha d bee n fille d an d th e patter n o f progradation wa s eas t t o west along mos t o f th e North Se a Basi n (Fig s 5 an d 7f ; S0rense n e t al . 1997). Th e fina l phas e o f infil l durin g lates t Pliocene-mid-Pleistocene tim e wa s mainl y sourced fro m th e SS E b y th e ancestor s t o th e large N W Europea n river s o f th e present-da y landscape (Figs 5 and 7g; Gibbard 1988; Zagwijn 1989), whereas sediments from souther n Norway made thei r way int o the northern Nort h Se a and the North Atlanti c (Jord t e t al 1995 , 2000 ; Rii s 1996; Evan s et al. 2000) . The general picture of infill is thus comparable with tha t o f passiv e margins , whic h generall y show a n overal l basinwar d progradatio n o f successive sedimentar y wedges . However , th e last phase of infill i s remarkable i n that the lowe r to middl e Pleistocen e sediment s ar e regionall y very extensive compare d wit h previous units. In fact, the y attai n thicknesse s o f u p t o mor e tha n 500m i n area s wher e accommodatio n ha d previously bee n fille d b y th e Miocen e an d Pliocene delta s (se e wester n par t o f Fig . 6) , thus indicatin g tha t additiona l accommodatio n was bein g create d i n earl y t o mid-Pleistocen e time. Apar t fro m th e centra l part s o f th e basin , the lowe r t o middl e Pleistocene sediment s ar e generally o f relativel y shallow-wate r origin , suggesting tha t sedimentatio n rate s kep t u p with th e increase d subsidenc e rate s (Huus e 2002). Correlation with regional tectonic events Apart fro m regiona l plume-relate d uplif t i n Paleocene-early Eocen e tim e ther e i s onl y minor sig n of tectonic activit y in Cenozoic tim e in the North Sea Basin. This is mainly in the form of local inversion of old fault system s such as the Sorgenfrei-Tornquist Zon e an d th e Centra l Graben (Vejba? k & Andersen 1987 , 2002; Ziegler 1990). Th e inversio n o f thes e structure s wa s probably cause d b y th e combine d effect s o f Atlantic ridg e pus h an d Alpin e compressio n (e.g.Vejbaek & Anderse n 2002 ) rathe r tha n b y Alpine compression alone . The effect o f Atlantic ridge push is also see n a s large inversio n dome s along th e N W Atlantic margi n (Dor e & Lundin 1996; Boldree l & Anderse n 1998 ; Dor e e t a l 1999). It i s unlikel y tha t intra-plat e compressio n could caus e significan t uplif t o f cratoni c sourc e

UPLIFT AND DENUDATION O F SOUTHERN NORWA Y

areas such as southern Norway (Rohrma n & van der Bee k 1996 ; Dor e e t al 1999) . O n the othe r hand, i t ma y b e possibl e tha t intra-plat e compression coul d creat e enoug h disturbanc e along ol d faul t zone s t o cause avulsion of majo r rivers an d thu s resul t i n majo r change s o f sediment inpu t direction s suc h a s observe d i n mid-Miocene tim e (se e Figs 5 and 7d and e). It ha s bee n suggeste d tha t abnormall y rapi d early Pleistocen e subsidenc e coul d hav e bee n caused b y intra-plat e compressio n (e.g . Cloe tingh et al 1990 , 1992) . However, the absence of evidence fo r earl y Pleistocen e compressiona l faulting in the North Sea area makes it less likely that extensiv e compressio n wa s th e caus e o f rapid earl y Pleistocene subsidence . Another effect t o consider is the load-induce d subsidence cause d b y th e las t phas e o f infil l o f the North Sea Basin. By late Pliocene time only a narrow seawa y o f relativel y grea t wate r dept h existed i n th e centra l Nort h Sea . Th e infil l o f a narrow (sa y 400 m) deep basi n remainin g a t the beginning of Pleistocene tim e could cause som e degree o f flexura l dow n warping o f th e margins. The exten t t o whic h th e sediment-loade d subsidence woul d b e distribute d laterall y is , however, strongl y dependen t o n th e flexura l strength o f th e lithosphere , an d numerica l modelling o f th e loadin g effec t need s t o b e carried ou t t o quantif y thi s effect .

Correlation with global climate and sea level The suppl y o f coars e clasti c sedimen t t o th e North Sea Basin accelerated severa l times during Cenozoic time , mos t notabl y i n lat e Paleocen e time, at the Eocene-Oligocene transition, in late Mid-Miocene tim e an d in Plio-Pleistocene tim e (Fig. 3) . I t appear s straightforwar d tha t th e abruptly increase d suppl y o f siliciclasti c sedi ments in late Paleocene tim e was caused by uplift

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of sourc e area s i n relatio n t o th e arriva l o f th e plume and rifting of the North Atlantic. Because of th e substantia l tim e la g (>20Ma) , direc t plume-related effect s canno t satisfactoril y explain th e increases i n sedimen t suppl y a t th e Eocene-Oligocene transition , an d i n lat e Mid Miocene an d Plio-Pleistocen e times . Hence , i t should b e investigate d whethe r thes e increase s could have been caused by other well-documented phenomena such as major long-term climatic and eustati c changes . Althoug h plume-relate d effects canno t accoun t fo r abrup t change s i n sediment supply, the effect o f delamination of the lithosphere ma y b e par t o f th e explanatio n b y providing decelerating bu t continue d uplif t afte r the initial uplift puls e (Nielsen et al. 2002) . Previous studie s hav e note d a conspicuou s correlation betwee n majo r Nort h Se a sequenc e boundaries and major 8 8 O increases (Jordt et al. 1995; Huus e & Clause n 2001 ; Huus e 2002) . Huuse (2002 ) als o note d tha t large-scal e sedimentation pattern s o f th e easter n Nort h Se a Basin ar e comparabl e wit h sedimentatio n pat terns observe d alon g man y continenta l margins (see Donnelly 1982 ; Bartek et al. 1991 ; Cameron et al . 1993 ; Mille r e t al . 1998 ; Serann e 1999 ; Huuse & Clause n 2001) . Moreover , th e period s of increase d sedimen t suppl y a t th e Eocene Oligocene transition , an d i n lat e Mid-Miocen e and Plio-Pleistocene times roughly correlate with episodes o f climati c deterioratio n an d majo r increases in continental ice volume (see Buchard t 1978; Molnar & England 1990; Lear et al. 2000). Other evidenc e o f significan t increase s i n denudation come s fro m geomorphologica l studies, which have indicated that rates of stream incision accelerate d i n lat e Cenozoi c time , both in souther n Norwa y an d i n th e easter n US A (Lidmar-Bergstrom e t al . 2000 ; Mill s 2000) . Such virtuall y contemporaneou s increase s i n denudation indicate a global rather than regional or local caus e (Molna r & England 1990) .

Fig. 8 . North-south-oriente d seismi c profil e (DA94-04 ) showin g a conformabl e uppe r Paleocen e t o Eocen e succession consistin g o f hemipelagic cla y an d marl (unit s 1—3 ) overlying the Uppe r Cretaceous-Dania n Chal k Group. Th e condense d Paleocene-Eocen e successio n i s overlai n b y a thic k progradationa l successio n o f Oligocene t o mid-Miocen e ag e (unit s 4-6) . Th e post-middl e Miocen e sequenc e (uni t 7 ) is characterize d b y a thick shallowing-upward s aggradationa l successio n o f marin e cla y an d silt , onlappin g th e mid-Miocen e unconformity. P-wave velocities fro m fou r wells located alon g the profile (S-l, R-l, Inez-1, F-l) indicate that the velocity o f the post-Chalk Grou p is very clos e t o 2kms~ 1 (i.e . 1 s two-way trave l tim e (TWT ) ~ 1 km). Thi s relationship is used to directly compare geometrie s on the seismic wit h inferred amount s of 'missin g section' or 'Neogene uplift ' base d o n sonic-derive d compactio n trend s o f th e Chal k Grou p (Japse n 1998 ) an d o f Jurassi c shales (Jense n & Schmidt 1993) , respectively . Estimate s o f 'missin g section' an d 'missin g overburden' base d on integration o f chal k compactio n an d vitrinit e reflectanc e dat a (Japse n & Bidstru p 1999 ) ar e als o shown . Th e amount o f uplif t estimate d fro m th e seismi c geometrie s i s significantl y lowe r tha n tha t base d o n compactio n trends. Q , Quaternary valley . (Fo r location , se e Figs 2 and 7.)

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Cenozoic uplift an d denudation of southern Norway Constraints on magnitude Apatite fission-trac k thermochronolog y (AFTT ) has become a popular prox y fo r inferring uplift . However, AFTT relates to cooling histories of the rocks analysed , i.e . t o denudatio n an d no t directly t o uplif t (Brow n 1991) . Thi s i s important, a s denudatio n ma y la g uplif t b y several ten s o f millio n year s (Summerfiel d & Brown 1998) . Moreover, AFTT is sensitive onl y to temperature s o f minimu m c. 60° C (depth s >2-3 km) and is thus generally no t sensitive to late Cenozoi c effect s (Gunnel l 2000) , althoug h inversion o f th e AFT T dat a ma y hin t a t lat e Cenozoic coolin g historie s (e.g . Rohrma n e l al. 1995). The latter indicate a maximum denudation of 2 ± 0.5k m i n th e inne r fjord s o f souther n Norway decreasin g outwar d t o < 0.5 km a t the present-day coastline , wherea s th e amoun t o f denudation at the highest mountain peaks suc h as Jotunheimen i s belo w th e resolutio n o f AFT T inversion (Rohrma n e t al 1995 ; Fig . 8) . Th e rather large uncertainties of denudation estimates based o n th e AFT T metho d cal l fo r additiona l constraints o n denudatio n o f souther n Norway . The most powerful method fo r producing reliabl e denudation estimate s i s probabl y (U-Th)-H e thermochronometry, couple d wit h investigations of cosmogeni c isotop e studie s t o dat e expose d landforms, an d i t i s recommende d tha t suc h investigations be carrie d ou t i n the future . The denudatio n estimate s cite d abov e ar e almost opposit e t o th e estimate s o f Rii s & Fjeldskaar (1992) , whic h sho w greates t averag e amounts of denudation at the coastline (thei r fig. 11). Thi s stud y relie s o n geomorphologica l characteristics of the landscape an d extrapolation of well-date d offshor e surface s ove r lan d area s (Dore 1992 ; Rii s & Fjeldskaar 1992 ; Rii s 1996) . The use of this method i s somewhat problemati c when databl e rock s ar e absen t i n th e area s o f maximum uplif t an d th e ag e o f th e surfac e onshore i s thus poorly constrained . It is possible, however, t o defin e relativel y coheren t palaeo surfaces i n eve n severel y denude d area s usin g geomorphological criteria , althoug h th e ag e o f the surfac e wil l b e conjectura l (Gjessin g 1967 ; Riis & Fjeldskaa r 1992 ; Rii s 1996 ; Lidmar Bergstrom e t al . 2000) . Th e us e o f palaeosur faces thu s ma y yiel d som e ide a abou t th e magnitude o f uplift , bu t wit h rathe r poo r constraints o n timing , an d may , i n som e cases , agree poorly wit h result s o f AFTT. A widely used method o f estimating uplif t an d denudation i s t o compar e regiona l compactio n

trends o f shale s an d chalk s relativ e t o a define d 'normal trend' (Jense n & Schmidt 1993 ; Hansen 1996; Japsen 1998) . The estimates from compac tion-trend method s ar e highl y variable , eve n using the same wells (Fig. 8 ) and it would appea r that th e regiona l trends o f over - (an d under- ) compaction ar e mor e usefu l tha n th e absolut e values. Th e estimate s o f 'missin g section ' ma y be further constraine d by integrating compaction analyses with vitrinite reflectance studie s (Fig. 8; Japsen & Bidstrup 1999). However, although this integration yield s even lowe r estimate s of uplif t and denudation , the y ar e stil l o f th e orde r o f 300 m abov e thos e base d o n seismi c geometrie s (Fig. 8 : compare with 'buria l anomaly' o f Japsen & Bidstru p 1999) . Japse n & Bidstru p inferre d that lat e Neogen e erosio n i s responsibl e fo r th e 'burial anomaly ' show n i n Fig . 8 . I f thi s i s th e case, the n a 300-400 m thick wedge of Pliocene sediments must have been deposited between the (complete) Middle-Uppe r Miocen e successio n and the Lower-Middle Pleistocene unit in Fig. 8. However, i n Pliocen e tim e th e easter n Danis h North Se a wa s mainl y bypasse d b y sediments , which wer e deposite d i n larg e delta s 100k m farther to the west and SW (Fig. 7e and f) - Hence, the notio n of rapid lat e Neogen e depositio n an d erosion i n the eastern Danish North Sea inferre d from compaction-base d exhumatio n estimate s does no t agre e wit h th e palaeogeographica l evolution o f the area . As demonstrate d b y th e abov e example , i n areas wher e the bulk of the Cenozoic successio n is preserved , i t i s possibl e t o us e large-scal e depositional geometrie s observe d i n seismi c profiles t o estimat e th e amoun t o f tiltin g an d denudation tha t ha s occurre d withi n th e basi n during Cenozoic time . It should be noted that it is the amoun t o f til t o f previousl y horizonta l surfaces tha t indicate s differentia l uplif t o r subsidence; th e volum e o f sedimen t o f an y given age reflects only the amount of denudation. Geometrical denudation estimates and 'uplift ' and/or denudatio n estimate s base d o n compac tion trends al l show simila r trends of denudation increasing northwards , bu t th e amplitude s var y widely, wit h the geometrica l estimat e being th e lowest (Fig . 8) . I t i s beyon d th e scop e o f thi s study to scrutinize the methods and assumptions behind compactio n analyses , bu t i f th e seismi c geometries ar e t o b e relied upon , it appear s that compaction method s overestimate the amount of uplift an d denudatio n o f th e easter n Nort h Sea . Moreover, i t i s remarkabl e tha t th e geometrica l estimate i s i n accor d wit h a n estimat e o f 'overburial' o f th e Chal k Grou p base d o n 3 D basin modellin g (S.B . Nielse n pers . comm . 2000). Th e modellin g stud y incorporate s a

UPLIFT AND DENUDATION O F SOUTHERN NORWA Y

thermally subsidin g North Se a Basin, subjec t to long-term eustati c fall , varyin g sedimen t inpu t and sedimentar y loading , withou t th e influenc e of late Cenozoic tectonics . Hence, it appears that the large-scal e strata l geometrie s observe d o n seismic dat a (Fig s 6 and 8 ) reflect th e infil l o f a thermally subsidin g basi n durin g generall y falling se a level. Constraints on timing Paleogene an d Neogen e sediment s ar e almos t exclusively locate d aroun d the fringe s of the uplifted area s i n Scandinavi a an d Britai n an d there ar e n o sediment s preserve d i n th e mos t elevated parts . Hence , unti l high-resolutio n fission-track dat a becom e available , th e timin g of uplif t an d denudatio n has t o be inferre d fro m the sedimentar y recor d o f th e adjacen t basins . This task requires regional dat a coverage to filter out loca l variation s i n sedimen t supply , whic h could caus e potentiall y misleadin g sedimen tation patterns . Whe n tryin g t o establis h uplif t and denudatio n historie s fro m th e sedimentar y record i t i s extremely importan t to bear i n mind that althoug h a volum e o f sedimen t i s directl y related t o denudatio n i t tells u s ver y little abou t uplift. Also , i t i s important t o bea r i n min d that isopach map s sho w only the present distribution of th e erosiona l products . Hence , i t i s possibl e that sediment s recordin g earl y denudatio n hav e now bee n remove d a s a result o f late r uplif t (o r base-level fall ) an d associated erosion . Thus, the present distributio n o f sediment s ma y b e dominated b y th e mos t recen t episode s o f denudation and the associated isostati c response . This i s especiall y tru e fo r th e earlies t proxima l sediments deposite d alon g th e margin s o f th e source area . It seem s likel y tha t th e apparen t lac k o f proximal sediments of Paleocene an d Eocene ag e off Norwa y i s du e t o cannibalizatio n an d redeposition. Thi s i s indicated b y th e truncation of thick, highly progradational sedimen t wedge s off th e wes t coas t o f Norwa y (se e Jord t e t al. 1995; Fig. 3) . The less marked truncation around the Shetlan d Platfor m coul d b e du e t o th e narrowness o f th e Shetlan d topograph y (abou t one-third th e widt h o f th e sout h Norwegia n dome) causing less uplif t a s a result of erosiona l unloading. However, variations in thermal uplif t and subsidenc e (wit h th e Shetlan d Platfor m experiencing th e larges t o f both ) ma y als o hav e influenced thi s pattern. From th e sedimentar y recor d i t appear s tha t denudation accelerate d at least fou r times durin g Cenozoic time : i n lat e Paleocen e time , a t th e Eocene-Oligocene transition , i n lat e Mid -

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Miocene tim e an d in Plio-Pleistocene time . Th e first episode wa s probably a response to regional uplift associate d wit h plume activit y an d riftin g in th e Nort h Atlanti c region . Th e remainin g episodes appea r t o correlat e wit h simila r responses o n continenta l margin s fa r fro m th e North Atlanti c domain , thu s indicatin g th e influence o f globa l factor s suc h a s climat e an d eustasy. Hence , i t seem s tha t th e latte r thre e episodes o f accelerate d denudatio n ma y reflec t climatic and eustatic changes rather than tectonic events. Isostatic response to localized denudation (dissection) The width of the south Norwegian dome is of the order o f 300-40 0 km alon g an y cross-sectio n (Figs 1 and 9). The area of the dome i s therefore c. 10 5km2 and remova l of materia l fro m the surface o f th e dom e i s thu s likel y t o b e isostatically compensate d b y additio n o f mantl e material a t depth (England & Molnar 1990) . This is in agreement wit h gravity data (Balling 1980) , which indicat e tha t th e topograph y o f souther n Norway i s isostaticall y compensate d a t depth . Because th e lithospher e ha s a certai n flexura l strength ther e ar e likel y t o b e flexura l effects , seen a s subdue d uplif t (o r subsidence ) respons e to erosion (o r sedimentation), alon g th e margins of th e dome . A schemati c illustratio n o f th e formatio n o f dissected topograph y fro m a n initia l low elevation peneplai n i s show n i n Fig . 1 0 an d described below , leavin g ou t flexura l effect s a t the margins of the rock column. The dimension s of the rock column are comparable wit h those of the centra l part s o f th e sout h Norwegia n dome . Removal o f crusta l materia l (p c ~ 2.7gcm~ 3 ) from th e to p o f th e 200k m wid e column s i s isostatically compensate d b y additio n o f mantle material (p m ~ 3.3gcm~ 3 ) a t depth . Th e iso static compensation (/ = p c/pm) for denudation is thus c . 0.8 . Hence , 1 km (mean ) denudatio n would decreas e th e surfac e elevatio n b y onl y 0.2 km as a result of the isostatic response, which would caus e 0.8k m uplif t o f th e entir e roc k column. Regional studie s o f activ e orogen s (Europea n Alps, Andes, Himalayas) indicat e that dissection is rarely fully develope d i n their central parts and that onl y abou t hal f o f th e pea k heigh t a t th e centre o f orogen s ca n b e explaine d b y th e isostatic respons e t o denudation (Gilchrest e t a l 1994). Looking a t a regional topographi c cross section o f souther n Norwa y (Fig . 9 ) i t appear s that denudatio n i s unevenl y distributed , wit h

Fig. 9 . Topographic cross-section s o f th e sout h Norwegia n dome . Locatio n i s shown i n Fig. 1. The uppe r profil e shows a hypothetica l cross-sectio n o f th e dom e a t midEocene tim e (c. 40 Ma). The lower profile shows the present topography and summit envelope (afte r Torske 1972 ) and the mean surfac e elevation averaged ove r 50-100 km. The mea n surfac e elevation is a qualitative estimate o f the isostaticall y compensated topograph y envelope . I n mid-Eocen e tim e (c. 40 Ma) a hilly relie f ha d develope d i n response t o weathering of a Mesozoic peneplai n uplifted t o 1 -1.5 km elevatio n in earliest Eocen e time. A warm climate , dense vegetatio n an d a low-gradien t local relie f probably cause d lo w rates o f denudation . It should be note d tha t sea leve l wa s c . 200 m highe r at 40 Ma tha n a t present . Th e present-da y deepl y incise d relie f probabl y developed i n response t o repeated episode s o f climatic deterioration an d eustati c lowering during mid - to lat e Cenozoic time , culminating wit h full glacia l conditions and extreme rates of incision i n Plio-Pleistocene time. Deep dissection of the former high-elevation peneplain caused mountai n peak s to rise to approximately twice their initial elevation. As a result of the eustatic fall o f c. 20()m sinc e mid-Eocene time , the mea n surfac e elevation (with respec t t o se a level ) has remaine d a t roughly the sam e leve l throughout.

UPLIFT AND DENUDATIO N O F SOUTHERN NORWA Y

maximum denudatio n o f 1-1. 5 km i n a zon e stretching c . 100k m inboar d o f th e coastline . Local denudation is of the order of 0.7-0.8 km in the area of highest mean elevation an d decrease s to 0.5 km farther t o the SE . Average denudation (defined a s summit envelope minus mean surfac e elevation) i s highest (c . 1 km) abov e th e centra l and northwestern parts of the dome (Fig. 9). This picture is , o f course , strongl y dependen t o n th e length scal e ove r whic h denudatio n is average d and on the 3D distribution o f valley incision, bu t it seem s t o indicat e tha t th e centra l part s o f th e dome have been affected b y the combined effect s of loca l denudatio n (causin g surfac e lowering ) and isostatic uplift in response to deep incision of neighbouring areas .

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caused accelerate d rate s o f strea m incisio n an d thus increased denudatio n rates. Finally, in PlioPleistocene time , ful l glacia l condition s i n th e high-lying part s o f souther n Norwa y cause d extreme rates o f incision, eventually carving out the dissecte d topograph y w e observ e toda y (Fig. 9) . In th e scenari o describe d here , th e onl y tectonic uplif t o f th e surfac e (an d of th e roc k column) occurred i n early Paleogene time . Dee p dissection o f the uplifted peneplain an d resultant isostatic uplif t o f th e roc k colum n cause d th e remaining par t o f th e roc k uplift , wherea s the mea n surfac e wa s lowere d b y c . 20 % o f th e amount o f mea n denudation . I t shoul d b e note d that th e averag e denudatio n o f 1 km (surfac e lowering of 0.2 km) is compensated by a eustatic lowering o f the sam e magnitude , thu s maintain ing th e mea n surfac e elevatio n (wit h respec t t o Early Cenozoic uplift and late Cenozoic sea level) a t c. 1 km. denudation; a hypothetical model The landscape evolutio n as depicted i n Figs 9 A regional cross-sectio n o f the south Norwegian and 1 0 i s i n agreemen t wit h denudatio n a s dome is shown in Fig. 9. The upper panel shows a recorded b y th e offshor e stratigraphi c recor d hypothetical low-relie f landscap e afte r regiona l (Figs 4-7 ) an d wit h geomorphologica l studies , uplift o f a peneplain, befor e th e developmen t o f which indicat e accelerate d rate s o f incisio n i n the deepl y incise d valley s an d fjord s tha t late Cenozoi c tim e (Lidmar-Bergstro m et al. characterize th e present-da y topography , illus - 2000). A lat e Cenozoi c increas e i n th e rate s of stream incisio n wa s als o foun d i n a regiona l trated b y the lower profile . A hypothetica l mode l o f Cenozoi c uplif t an d study o f the easter n US A (Mill s 2000) , support denudation o f th e centra l part s o f souther n ing the notion of an allogenic control on incision Norway i s show n i n Fig . 10 . A t th e en d o f rates, i.e. climate and eustasy rather than regional Mesozoic time , souther n Norwa y wa s probabl y tectonics. Th e Plio-Pleistocen e increas e i n th e worn dow n t o a peneplai n o f som e mea n rates o f incisio n i s als o reflecte d b y th e elevation abov e se a level . Th e exac t elevatio n occurrence o f thic k Plio-Pleistocen e sedimen t of the peneplain i s poorly constraine d an d here it wedges al l alon g th e N W Europea n Atlanti c is assume d tha t th e mea n heigh t o f th e centra l margin (Rii s & Fjeldskaa r 1992 ; Riis 1996 ; parts wa s close t o 200 m elevation, althoug h the Evans e t a l 2000 ) an d i n th e centra l Nort h Se a highest peak s i n souther n Norwa y ma y b e (Figs 4 and 6). The concept of mountain building by isostati c respons e t o dissection o f an uplifte d remnants o f inherite d topograph y (Rii s & peneplain i s als o backe d b y th e apatit e fissionFjeldskaar 1992) . Late Paleocene-early Eocene plume- and rift- track analyse s o f Rohrma n e t al . (1995) . Thes e related tectonic s uplifte d the peneplain t o c. 1 km results indicate rapid late Cenozoic denudatio n at elevation (includin g pre-uplif t elevation) , bu t the bas e o f the deepes t valley s (fjords ) an d only incision was initially minimal in the central parts minor denudatio n (belo w th e detectio n of the dome, as a result of the low local relief, and threshold) o n th e mountai n peaks . However , a war m humi d climat e favourin g dens e veg- acquisition o f high-resolutio n fission-trac k dat a etation an d limited runoff . Th e bul k of sediment is neede d t o furthe r constrai n th e Cenozoi c supplied t o th e basi n a t thi s tim e (40-6 0 Ma) denudation history of souther n Norway. The model propose d here is not in agreemen t probably derived fro m the margins of the uplifted with th e interpretation s b y Rii s (1996) , wh o dome (not shown in Fig. 10) . In late Eocene-early Oligocene time , climatic found tha t easter n Denmar k ha s suffere d mor e deterioration (increase d seasonality ) and eustatic than 1 km o f Plio-Pleistocen e denudation . Thi s lowering cause d increase d rate s o f strea m estimate wa s probabl y drive n b y a n attemp t t o incision. Climat e recovere d durin g lat e Oligo - honour th e patter n o f lat e Neogen e erosio n cene-early Mid-Miocen e time , thu s stabilizin g inferred fro m compactio n analyses . A s argue d above, suc h estimates probabl y overestimat e th e incision rates . In lat e Mid-Miocen e tim e anothe r phas e o f amount o f exhumatio n o f th e easter n Nort h Se a climatic deterioratio n an d eustati c lowerin g Basin b y severa l hundre d metres. Moreover , th e

Fig. 10. Schematic illustration of uplift and bisection of an initial low-elevation peneplain. The dimensions are comparable with those of the contral parts of the south Nonwegian dome. (see text for discussion)

UPLIFT AND DENUDATIO N O F SOUTHERN NORWA Y

study relied on extrapolation o f key surfaces over several hundre d kilometres , obviousl y a some what tricky disciplin e whe n Cenozoic sediment s are absent over most of the area. In particular, the occurrence o f lat e Earl y Eocen e diatom s i n northern Finlan d (Tynn i 1982 ; Fenne r 1988 ) ha s been use d t o infe r a Paleogene episod e o f low elevation peneplanation an d submergence (Rohrman et al 1995 ; Rii s 1996) . The locations of the diatom find s are , however , al l o f relativel y lo w elevation (<300m ) an d coul d thu s hav e bee n submerged durin g the early Eocene highstan d of sea level (Fig . 3) without the need for Paleogene peneplanation o f souther n Norway . Th e sub sequent emergence o f the diatom locations could easily b e explaine d b y th e combine d effect s o f post mid-Eocen e eustati c lowerin g o f c. 200 m and flexura l uplif t a s a resul t o f denudatio n (dissection) an d isostati c uplif t (o f rocks) o f th e adjacent norther n Norwegian highland . Modelling studie s (e.g . Rii s & Fjeldskaa r 1992; Stuevol d & Eldhol m 1996 ) generall y fal l some hundre d metre s shor t o f explainin g th e present topograph y o f southern Norway, leading the researcher s t o sugges t tha t activ e tectoni c mechanisms suc h a s mantl e phas e change s o r palaeo-topographic relie f ar e required to explain present-day topograph y of souther n Norway . However, thes e studie s assume d a constan t global se a leve l throug h Cenozoi c tim e a s opposed t o th e 200 m eustati c lowerin g tha t occurred sinc e mid-Eocene time . Neglecting this fall woul d introduc e 200 m surfac e uplif t o f presumed tectoni c origi n int o th e modellin g results. Differences between southern Norway and the Shetland Platform Differential preservatio n o f upper Paleocene and Eocene sediment s acros s th e northern Nort h Sea could b e du e to a number of factors suc h as: (1 ) different amount s o f transien t uplif t cause d b y heating o f th e lithospher e beneat h th e Shetlan d Platform an d souther n Norway ; (2 ) differen t amounts o f permanen t uplif t cause d b y under plating o r delaminatio n o r othe r non-transien t mechanisms; (3) presence v. absence of extrusive volcanic rock s i n Scotlan d v . Norway ; (4 ) different (flexural ) isostati c respons e t o denudation; (5 ) palaeobathymetric variations ; (6) width of th e topograph y (ful l isostati c o r flexura l isostatic behaviour). Plate tectoni c reconstruction s (Skogsei d e t al. 2000) indicat e tha t th e Shetlan d Platfor m wa s closer t o the Iceland Plum e a t the time of riftin g (c. 54 Ma) and thus experienced a larger thermal

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(transient) uplift tha n southern Norway. It should thus be expected that the depocentres adjacent to the Shetlan d Platfor m subsequentl y experience d greater therma l subsidenc e an d thus had greate r preservation potentia l tha n thos e a t th e sam e latitude of f souther n Norway. Possible mechanism s responsibl e fo r non transient uplif t o f souther n Norwa y an d othe r uplifted area s wer e discusse d b y Nielse n e t al . (2002), wh o invoke d delaminatio n o f th e litho sphere a s th e uplif t mechanis m fo r souther n Norway. Other areas, e.g. Scotland, may not have been affecte d t o th e sam e degre e o r ma y eve n have experience d underplating , leadin g t o a different uplif t an d subsidenc e history . Early Paleogen e extrusiv e volcani c rock s ar e abundant i n wester n Scotland , bu t absen t i n Norway. Extrusiv e volcani c rock s ar e readil y erodible an d volcani c activit y contribute s t o a high loca l relief , bot h o f whic h ma y hav e contributed t o a large r sedimen t suppl y fro m Scotland in early Paleogene time (see Hall 1991) . The south Norwegian dome has a much large r areal exten t tha n Scotlan d an d th e Shetland s (Fig. 1) . I t i s possibl e tha t thi s coul d caus e differences i n th e amoun t o f (flexural ) isostatic response to denudation (and deposition) betwee n the tw o areas . Th e relativel y larg e exten t o f the south Norwegian dome probabl y cause s most of the loadin g and unloadin g of materia l acros s southern Norwa y t o b e isostaticall y compen sated, excep t aroun d the fringe s of the dome , where flexural strength may have an effect. I n the case o f Scotlan d an d Shetlands , th e narrownes s of th e topograph y ma y caus e a relativel y larg e part o f th e loadin g an d unloadin g o f materia l across th e platfor m t o b e supporte d b y th e flexural strengt h o f th e lithosphere . Hence , th e amount o f roc k uplif t a s a resul t o f denudatio n may b e significantl y les s fo r Scotlan d an d th e Shetlands than for souther n Norway. The effect s of loadin g b y deposition adjacen t t o the uplifte d area woul d als o hav e a greate r effec t o n th e Shetlands b y depressin g an y isostati c uplif t response t o denudation. Palaeogeographical reconstruction s (Fig . 7a ; Ziegler 1990 , fig . 54 ) indicat e tha t sediment s deposited eas t o f Scotlan d an d th e Shetland s were fe d acros s a narro w shel f int o relativel y deep wate r wherea s th e sediment s o n th e Norwegian sid e wer e deposited o n a wide shelf . Such differences i n palaeobathymetry could have caused th e greate r thicknes s an d bette r preser vation o f depocentre s observe d eas t o f Scotlan d and the Shetlands . The 'self-perpetuating ' mode l of uplif t and denudation o f souther n Norwa y invoke d her e (Fig. 10 ) works only until maximum equilibrium

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of valle y slope s i s reached ; the n mountai n top s will commenc e erodin g an d regiona l down wearing will prevail (Ahnert 1984; Gilchrest et al. 1994). Becaus e o f thei r narrowness , th e Shet lands migh t alread y hav e reache d thi s stag e i n Eocene time whereas i t has not yet been reache d in th e central part s o f souther n Norway (Fig . 9) . This could explain why sediment supply from th e northern par t o f the Britis h Isles slowed dow n in Neogene time . Conclusions The Cenozoi c evolutio n o f th e Nort h Se a Basin has been summarize d base d o n a compilation of detailed studie s from the eastern North Sea Basin integrated wit h published regiona l studies . The first pulses of coarse clastic sediment into the basin occurred i n response to regional surfac e uplift coincidin g wit h increase d plum e activit y and th e onse t o f Nort h Atlanti c riftin g i n lat e Paleocene an d earlies t Eocen e time . Th e preserved uppe r Paleocen e an d Eocen e sedi ments mainl y originat e fro m th e west , wherea s progradational siliciclasti c wedge s o f simila r age hav e bee n truncate d toward s th e sout h Norwegian dome . In early Oligocene time, sediment supply from Scotland an d the Shetlands diminished, probably as a resul t of therma l subsidenc e and regiona l down-wearing o f topography in Scotland an d the Shetlands. Th e suppl y o f coarse clasti c materia l from souther n Norwa y accelerate d a t th e Eocene-Oligocene transition , an d fro m Oligo cene tim e onward s th e central , easter n an d southern Nort h Se a Basi n wa s fille d b y larg e deltas prograding in a clockwise fashio n from th e NE (Oligocene-earl y Miocene) , eas t (lat e Miocene-Pliocene), an d finall y SS E durin g Pleistocene tim e (Fig s 5 and 7). It i s argue d her e tha t seismi c geometrie s ca n be use d wit h confidenc e t o infe r uplif t an d denudation o f area s wher e Cenozoi c sediment s are preserved . Suc h geometri c uplif t estimate s have bee n compare d wit h estimate s base d o n compaction trend s alon g a profile i n th e easter n Danish Nort h Sea . Al l method s sho w simila r trends o f increasin g uplif t toward s souther n Norway, bu t th e compariso n indicate s tha t compaction-based estimate s ma y overestimat e the amoun t o f uplif t b y severa l hundre d metres . Recent uplif t estimate s integratin g chal k com paction an d vitrinit e reflectanc e dat a (Japse n & Bidstrup 1999 ) ar e lower tha n previous compac tion-based estimates , bu t stil l c . 300 m to o high compared wit h th e geometri c estimates . Th e geometric estimate s ar e simila r t o estimate s o f overburial base d o n 3 D basi n modellin g o f a

passively subsiding basin filled during falling sea level (S.B. Nielsen pers. comm. 2000), indicating that lat e Cenozoi c tectoni c event s ma y no t b e required to explain the sedimentation and erosion patterns of th e Nort h Se a Basin. A mode l describin g th e developmen t o f topography i n souther n Norwa y i s develope d (Fig. 10 ) based o n th e evidenc e provide d b y th e Cenozoic successio n of the North Se a Basin and the present-da y topograph y o f th e adjacen t land areas. Th e mode l involve s lat e Paleocene earliest Eocen e uplif t o f a low-elevatio n pene plain develope d durin g late Mesozoi c an d earl y Paleocene tim e i n a n are a roughl y coincidin g with present-da y souther n Norway . Th e uplif t episode wa s followe d b y c . 1 5 Ma o f war m climatic conditions favouring a dense vegetation and lo w amount s of runoff . I n combination with low loca l relie f an d hig h se a level , thi s cause d relatively low rates of denudation during Eocene time. Subsequently, repeated episodes of climatic deterioration an d eustati c fal l cause d increase d rates of incision, most markedl y a t the EoceneOligocene transition , and i n lat e Mid-Miocen e and Plio-Pleistocen e times . Th e developmen t of full glacia l condition s i n souther n Norwa y i n Plio-Pleistocene tim e was probably instrumental for carvin g out the main parts of the deep valleys and fjords o f southern Norway, i.e. dissecting the initial high-elevatio n peneplai n an d causin g uplift o f th e adjacen t mountai n peak s t o twic e the initia l elevatio n o f th e uplifte d peneplain . This mode l i s i n agreemen t wit h th e large-scal e sedimentation pattern s o f th e Nort h Se a Basin , with geomorphologica l studie s (Lidmar-Berg strom e t al . 2000 ) an d wit h apatit e fission-trac k modelling (Rohrma n e t al . 1995) . I t als o accounts fo r th e occurrenc e o f thic k Plio Pleistocene wedge s alon g th e N W Europea n Atlantic margi n (Riis 1996 ; Evan s e t al. 2000) . The rise of the mountain peaks is facilitated by the regional isostatic response t o dissection of an initially flat surface of some elevation (Molnar & England 1990 ; Gilchres t et al. 1994) , which will cause an uplift of the rock column c. 0.8 times the amount o f mea n denudation . Th e amoun t o f denudation average d ove r th e sout h Norwegia n dome approximate s 1 km (Rii s & Fjeldskaa r 1992). Thi s woul d correspon d t o c . 200 m o f mean surface lowering. However, the eustatic fall of c . 200 m sinc e mid-Eocen e tim e ha s maintained the mean surfac e elevation of central southern Norwa y a t approximatel y th e leve l o f the uplifted peneplain . To test the model advocated here and to furthe r constrain th e uplif t an d denudatio n histor y o f southern Norway and other uplifted area s there is a nee d fo r a n integrate d approach , leadin g t o

UPLIFT AN D DENUDATION O F SOUTHERN NORWAY

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mass-balanced palaeogeographica l reconstruc - BIRD, P . 1979 . Continenta l delaminatio n an d th e Colorado Plateau . Journal o f Geophysical tions. Thi s require s integratio n o f well-date d Research, 884,7561-7571. sediment volume s (mainl y offshore ) wit h detailed denudation estimates from both onshore BOLDREEL, L.O . & ANDERSEN , M.S . 1998. Tertiar y compressional structure s o n th e Faroe-Rockal l and offshor e regions . In particular , the onshore Plateau i n relation t o northeast Atlantic ridge-pus h regions are in need of more detailed studies, e.g. and Alpin e forelan d stresses . Tectonophysics, 300, based o n apatite (U-Th)-He thermochronome13-28. try, t o resolve the Cenozoic denudation history in BRODIE, J . & WHITE , N . 1995 . Th e lin k betwee n sufficient detail . Ideally , suc h a n integrate d sedimentary basi n inversio n an d igneou s under plating. In : BUCHANAN , J.G. & BUCHANAN , P.G. database woul d b e fe d int o 3 D geodynami c (eds) Basin Inversion. Geologica l Society , London, models incorporatin g isostati c an d flexura l Special Publications , 88 , 21-38. effects o f loading and unloading of the crust . The content s o f thi s pape r deriv e fro m a Ph D stud y carried ou t a t th e Departmen t o f Eart h Sciences , University o f Aarhus , co-sponsore d b y th e Danis h Natural Scienc e Researc h Counci l (Grant s 940116 1 and 9502760) and the Faculty of Science, University of Aarhus. Th e pape r wa s writte n whil e i n receip t o f funding b y EFP-200 0 (Projec t ENS-1313/00-0001) . Discussions wit h N. Balling, O . R. Clausen, P. Japsen and S . B . Nielse n ar e greatl y appreciated . I than k A. Hurst, G. E. Paulsen and the referees for comments, which helped improv e th e manuscript.

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Tectonic impact on sedimentary processes during Cenozoic evolution of the northern North Sea and surrounding areas JAN INGE FALEIDE 1, RUN E KYRKJEB0 2, TOMAS KJENNERUD 3, ROY H. GABRIELSEN 2, HENRI K JORDT 1'4, STEIN FANAVOLL 3'5 & MORTEN D . BJERKE 1'6 1 Department of Geology, University of Oslo, P.O. Box 1047, Blindern, N-0316 Oslo, Norway (e-mail: j.i.faleide@ geologi.uio.no) 2 Geological Institute, University of Bergen, Allegaten 41, N-5007 Bergen, Norway 3 SINTEF Petroleum Research, N-7465 Trondheim, Norway 4 Present address: Aarhus Amt, Lyseng Alle 1, DK-8270, H0jbjerg, Denmark 5 Present address: IPRES International Ltd., Nedre Vollgt. 4, N-0158 Oslo, Norway 6 Present address: PGS, Strandveien 4, N-1366 Lysaker, Norway Abstract: Thi s pape r focuse s o n th e Cenozoi c evolutio n o f th e norther n Nort h Se a an d surrounding areas, with emphasis o n sediment distribution, composition an d provenance, a s well a s o n timing , amplitud e an d wavelengt h o f differentia l vertica l movements . Quantitative informatio n abou t palaeo-wate r dept h an d tectoni c vertica l movement s ha s been integrate d wit h a seismi c stratigraphi c framewor k t o bette r constrai n th e Cenozoi c evolution. Th e data an d modelling result s suppor t a probable tectoni c contro l o n sedimen t supply an d o n the formatio n o f regional unconformities . Th e sedimentar y architectur e an d breaks ar e relate d t o tectoni c uplif t o f surroundin g clasti c sourc e areas , thu s the offshor e sedimentary recor d provide s th e bes t ag e constraint s o n Cenozoi c exhumatio n o f th e adjacent onshor e areas . Tectoni c subsidenc e accelerate d i n Paleocen e tim e throughou t the basin, with uplifted area s to the east and west sourcing prograding wedges, which resulted in large depocentres clos e to the basin margins. Subsidence rates outpaced sedimentatio n rates along th e basi n axis , an d wate r depth s i n exces s o f 60 0 m ar e indicated . I n Eocen e time s progradation fro m th e Eas t Shetlan d Platfor m wa s dominan t an d majo r depocentre s wer e constructed in the Viking Graben area, with deep water along the basin axis. At the EoceneOligocene transition , souther n Norwa y an d th e easter n basi n flan k becam e uplifted . Th e uplift, i n combinatio n wit h progradin g unit s fro m bot h th e eas t an d west , gav e ris e t o a shallow threshold in the northern North Sea, separating deeper water s to the south and north. The uplift an d shallowing continued into Miocene tim e when a widespread hiatu s formed in the norther n Nort h Sea , as indicate d by biostratigraphi c data . The Pliocen e basi n configuration was dominated by outbuilding of thick clastic wedges from the east and south. Considerable lat e Cenozoi c uplif t o f th e easter n basi n flan k i s documente d b y th e stron g angular relationshi p an d tiltin g o f th e complet e Tertiar y packag e belo w th e Pleistocen e unconformity. Cenozoi c exhumatio n is documented o n both side s o f the North Sea , but the timing is not well constrained. Two major uplift phases in early Paleogene an d late Neogen e times ar e relate d t o rifting , magmatis m an d break-u p i n th e N E Atlanti c an d isostati c response t o glacia l erosion , respectively . Additional uplif t event s may be related t o mantle processes an d the episodi c behaviou r of the Icelan d plume.

The norther n North Se a rif t basi n (Fig . 1 ) has & va n Hoor n 1989 ; Gabrielse n e t al 1990 ; been affecte d b y tw o majo r episode s o f riftin g Robert s et al. 1990;Ziegler 1990, 1992; Yielding since Devonian time. The two events took place et al . 1992 ; Milto n 1993 ; Ratte y & Ha y ward in Permia n t o earlies t Triassi c an d lat e Mid - 1993 ; N0ttved t e t a l 1995 ; Faerset h 1996 ; Jurassic t o earlies t Cretaceou s times , eac h Faerset h e t al . 1997) . Mos t o f th e post-rif t followed b y period s o f post-rif t therma l relax - tectoni c subsidenc e related t o th e Lat e Jurassic ation and subsidence (Badley et al. 1988;Ziegle r rif t phas e had cease d a t th e en d o f Cretaceous From: DORE , A.G., CARTWRIGHT, J.A. , STOKER, M.S. , TURNER, J.P. & WHITE , N . 2002 . Exhumation o f th e North Atlantic Margin: Timing, Mechanisms and Implications for Petroleum Exploration. Geological Society, London, Special Publications, 196, 235-269 . 0305-8719/02/$15.0 0 © The Geological Society of London 2002.

J. I . FALEIDE ETAL.

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V

^

Fig. 1 . Regional settin g an d locatio n o f stud y are a in the norther n Nort h Sea .

time an d th e basi n ha d becom e a wid e are a o f deposition wit h low relief. Henc e i t is suggeste d that therma l equilibriu m wa s reache d i n lates t Cretaceous tim e (Gabrielse n e t al. 2001). The Cenozoic norther n Nort h Se a basin form s a wid e sagge d depocentr e containin g u p t o c. 2500 m o f sediments . Shift s i n depocentr e locations, outbuildin g direction s an d sedimen t composition hav e bee n relate d t o differentia l vertical movement s o f th e basi n floo r an d surrounding clasti c sourc e area s (Nielse n e t al . 1986; Rundber g 1989 ; Rundber g & Smalle y 1989; Gallowa y e t a l 1993 ; Jord t e t a l 1995 , 2000; Michelsen e t al 1995 , 1998 ; Clause n et al 1999b; Martinse n e t a l 1999 ; Thyber g e t a l 2000). Basi n modellin g (bot h backwar d an d forward) ha s bee n applie d t o quantif y th e syn and post-rift evolution o f the northern Nort h Se a basin (e.g . Jo y 1992 , 1993 ; Robert s e t al 1993 ; White & Latin 1993 ; Hal l & White 1994 ; Nadi n & Kusznir 1995, 1996) . However, an d as pointed out b y thes e workers , th e inpu t parameters , an d especially th e assesse d palaeo-wate r depth , ar e

associated wit h uncertaint y an d ma y hav e resulted i n erroneous subsidenc e histories . This pape r focuse s o n the Cenozoic evolutio n of the northern North Sea and surrounding areas, with emphasi s o n th e timing , amplitud e an d wavelength o f differentia l vertica l movements . Quantitative informatio n abou t palaeo-wate r depth an d tectoni c vertica l movement s ha s been integrated wit h a seismi c stratigraphi c frame work t o better constrai n the Cenozoic evolution. We will also briefly discuss the main mechanisms responsible for the tectonic evolution of the area. The main study area of the northern North Sea is restricte d t o 1°W-5° E an d 58-62° N (Fig s 1 and 2) . However , when discussin g th e Cenozoi c sediment distribution and provenance we have to include th e surroundin g lan d area s (Fig . 1) . Cenozoic exhumatio n i s documente d o n bot h sides of the North Sea, mainly from geomorpho logical an d apatit e fission-trac k studie s (Gree n 1986, 1989 ; Bra y et al 1992 ; Lewis et al 1992 ; Holliday 1993 ; Rohrma n et al 1995 ; Rii s 1996 ; Lidmar-Bergstrom e t a l 2000 . A t leas t tw o significant episode s o f Cenozoi c exhumatio n have bee n suggested , bu t th e timin g is no t wel l constrained an d i n man y place s i t i s difficul t t o separate th e tw o (Japse n 1997 ; Japse n & Chalmers 2000) . Th e Cenozoi c developmen t of the stud y area i s also linke d to the plate tectonic evolution o f th e Nort h Atlanti c (Talwan i & Eldholm 1977 ; Eldhol m e t al 1990 ; Dor e e t a l 1999). The wor k ha s bee n carrie d ou t a s par t o f th e project Tectoni c impac t o n sedimentar y pro cesses in the post-rift phase—improve d models' , which focuse d o n th e Cretaceous-Cenozoi c succession filling in the structural relief resulting from Lat e Jurassic-earliest Cretaceous riftin g i n the norther n Nort h Se a (Fig . 3) . Thi s pape r summarizes th e Cenozoi c par t o f thi s study , an d Gabrielsen e t al (2001 ) have given a summary of the Cretaceou s post-rif t developmen t i n th e northern Nort h Sea . Seismic mapping In thi s stud y w e hav e interprete d a gri d o f regional high-qualit y seismi c reflectio n profile s tied t o ke y wells . The mai n databas e comprise d eight regional seismi c reflectio n surveys , providing the best dat a coverage i n the Norwegian part of th e norther n Nort h Sea . I n addition , fou r regional dee p seismi c reflection lines were used . Regional crusta l transect s (Fig . 2 ) wer e con structed alon g thes e line s b y combinin g the m with conventiona l seismi c line s an d gravit y an d magnetic data . Th e fou r transect s wer e depth converted using velocity information from wells ,

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Fig. 2 . Structural map of stud y are a in the northern North Sea and location of regional transects and key wells. ESB, East Shetland Basin; ESP, Eas t Shetland Platform; HP , Horda Platform; LT , Lomre Terrace; MFB, Mal0 y Fault Blocks; MgB, Magnus Basin; MrB, Marulk Basin; SB, Stord Basin; SG, Sogn Graben; TS, Tampen Spur; UH, Utsira High; UT, Uer Terrace; VG, Viking Graben; WG, Witchground Grabe n ;AG,Asta Graben.

interval velocitie s fro m stackin g velocitie s an d velocities fro m dee p seismic refractio n data. The transects wer e used in the modelling . Twelve well s (Fig . 2 ) wer e selecte d t o b e principal sources for analysis on the basis of their location a t or close t o the regiona l transect s an d as representative s o f differen t structura l pos itions. The key well s wer e analyse d to obtai n information abou t ag e an d lithologica l compo sition o f th e mai n Cenozoi c sequences , t o

recognize stratigraphica l breaks , an d to estimate palaeo-water depths. In addition, biostratigraphic data and well logs from mor e than 60 wells were used to calibrate th e interpretation of the seismi c data. The Cenozoic seismic stratigraphi c framework is based o n the work byJordt et al (1995 , 2000) . The Cenozoi c successio n i s subdivide d int o 1 0 seismic sequence s (CSS- 1 t o CSS-10 ) (Fig . 4). The sequence s hav e bee n date d usin g biostrati-

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Fig. 3 . Regiona l seismi c lin e NVGTI-92-105 . (Se e Fig . 2 fo r lin e locatio n an d Fig . 4 fo r Cenozoi c seismi c stratigraphy.) BT , Base Tertiary; MC, mid-Cretaceous ; BC , Bas e Cretaceous ; twt . two-wa y trave l time .

Fig. 4 . Cenozoi c seismi c stratigraphi c framework , based o n Jordt e t al (1995 , 2000) .

graphic dat a fro m ke y well s publishe d b y Steurbaut e t a l (1991) , Eidvi n & Rii s (1992) , van Vee n e t al . (1994) , Gradstei n & Backstro m (1996), Martinsen e t al (1999 ) and Eidvin et a l (1999, 2000) , i n additio n t o th e 1 2 ke y well s o f the TecSe d projec t (Fig . 2) . Th e seismi c stratigraphic framewor k wa s relate d t o th e tim e scale o f Gradstei n & Og g (1996 ) an d forma l lithostratigraphy o f th e norther n Nort h Se a (Isaksen & Tonsta d 1989 ; Kno x & Hollowa y 1992). The Paleogen e successio n comprise s fou r seismic sequence s (CSS-1 , CSS-2 , CSS- 3 an d CSS-4). Th e CSS- 1 sequenc e i s o f Lat e Paleocene-earliest Eocen e ag e an d it s to p corresponds t o th e to p o f th e Balde r tuffs . Th e CSS-2 sequenc e i s o f Eocen e age . Th e seismi c boundary a t the top o f CSS-2 correlate s wit h the Eocene-Oligocene transition , whic h i s associ ated wit h a hiatus , i n particula r alon g th e basi n flanks. Th e CSS- 3 sequenc e cover s a narro w period i n Earl y Oligocen e time . A n upwar d change i n seismic signatur e from progradation t o marked aggradatio n an d onlap occurs agains t the top o f CSS- 3 i n the norther n Nort h Sea . A mid Early Oligocene ag e is inferred for this sequence boundary. The to p of sequence CSS- 4 i s of latest Oligocene age .

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Fig. 5 . Regional profiles 1- 5 across the northern North Sea. (See Fig. 2 for profile location s and Fig. 4 for colour codes of the Cenozoic seismi c sequence s CSS- 1 to CSS-10.)

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The Miocen e successio n i s divided int o thre e seismic sequence s (CSS-5 , CSS- 6 an d CSS-7) , and the boundaries separatin g thes e are dated i n the centra l Nort h Se a (Jord t e t al 1995; Michelsen e t al . 1995) . Miocen e sediment s ar e also presen t i n th e norther n Nort h Sea , bu t i t i s difficult t o mak e thi s subdivisio n her e becaus e some o f th e sequence s thi n belo w seismi c resolution. Mos t worker s agre e o n a prominen t mid-Miocene hiatus ; however , th e biostratigra phy is equivocal (Steurbaut et al. 1991 ; Eidvin & Riis 1992 ; Gradstein & Backstro m 1996; Martinsen e t al . 1999 ; Eidvi n e t al . 2000 ; se e Jordt e t al. 2000 for discussion) . The Plio-Pleistocen e sequence s (CSS-8 , CSS-9 an d CSS-10 ) consis t mostl y o f glacia l sediments. The CSS- 8 sequenc e is of Pliocen e age (Eidvin & Riis 1992 ; Gradstein & Backstrom 1996; Eidvi n et al. 2000) whereas the CSS-9 and CSS-10 sequence s ar e of Pleistocene age . The seismi c interpretatio n focuse d o n identi fication o f sequenc e geometries , locatio n o f

depocentres, outbuildin g directions , recognitio n of tectoni c influenc e an d establishmen t o f a general Cenozoic framewor k for further analyses and basi n modelling . Seismi c sequenc e geome tries an d outbuildin g direction s provid e infor mation abou t change s i n th e basi n topography , and the y ar e relate d t o underlyin g structures, shifts i n provenance area an d changes in relative sea leve l an d sedimen t accumulatio n rates. Fo r each seismi c sequenc e w e hav e constructe d time-thickness maps.

Palaeo-water depth Temporal an d spatia l variation s i n palaeo-wate r depth are crucial parameters i n basin analysis, as changes i n palaeobathymetr y detai l th e amoun t of sedimen t underfil l durin g basi n evolutio n (Gradstein & Backstro m 1996) . I n forwar d o r backward basin modelling, palaeo-water depth is an importan t inpu t paramete r tha t control s th e measured subsidenc e o r uplif t an d thu s th e

Fig. 6 . Palaeo-water depth summary for the Cenozoic sequence of well 30/10-6 (Viking Graben). Modified fro m Kyrkjeb0 e t al. (2001) . Dominant lithologies: shales-mudstone s i n green; sandstones i n yellow. Sedimentatio n rates in mm ka" 1 ar e also shown. (Se e Fig. 4 for Cenozoic seismic stratigraphy. ) K-6 , Maastrichtian.

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1

Fig. 7 . Cenozoi c palaeo-wate r dept h fo r selecte d Vikin g Graben wells , modifie d fro m Kyrkjeb 0 e t al . (2001) . Long-term eustati c curv e fro m Ha q e t al . (1987 , 1988 ) i s show n i n magent a fo r comparison . (Se e Fig . 2 fo r location o f wells.)

Fig. 8 . Tectoni c modelling and subsidenc e analysi s along Transec t 1 (see Figs 2 an d 5) . Tectoni c subsidenc e curves fo r selecte d position s ar e show n abov e th e profile . Relative vertical movement s indicated by arrows (red, uplift; blue , subsidence). MgB, Magnus Basin; ESB, East Shetland Basin; TS, Tampen Spur ; VG, Viking Graben; LT, Lomre Terrace ; HP , Horda Platform. Modified fro m Kyrkjeb 0 et til. (1999) .

CENOZOIC EVOLUTION OF THE NORTHERN NORTH SEA recognition of tectonic events. However, palaeowater depth is difficult t o determine, and depends largely o n the qualit y an d quantit y of data fro m boreholes an d seismi c dat a (Bertra m & Milto n 1989; Joy 1992 , 1993 , 1996 ; Hall & White 1994 ; Jones & Milto n 1994 ; Nadi n & Kuszni r 1995 , 1996). By carefull y integratin g seismic stratigraphi c observations wit h palaeo-wate r dept h estimate s from structura l restoratio n an d micropalaeonto logical data , change s i n accommodatio n spac e throughout Cretaceous-Tertiar y time s ca n b e documented o n a regiona l scal e i n th e norther n North Se a (Fig s 6 an d 7 ) (Kyrkjeb 0 e t al 2001). The palaeorelie f wa s restore d alon g th e regional transect s (Fig s 2 an d 5 ) usin g th e depositional geometries , indication s o f zer o o r near-zero wate r dept h (suc h a s subaeria l unconformities an d coals ) an d faul t restoratio n (Kjennerud e t al . 2001) . Th e metho d provide s estimates o f palaeorelie f alon g th e transect s a t the base of each seismic unit rather than absolute palaeo-water depth . I n mos t case s thi s basi n relief could be characterized as a minimum water depth. Subaeria l topograph y canno t b e deter mined b y thi s method . Progradin g sequences , which are characteristic for parts of the Cenozoic development i n th e norther n Nort h Sea , ar e associated wit h th e leas t uncertaint y i n th e restorations. The 1 2 key well s fro m th e Norwegia n part of the norther n Nort h Se a (Fig . 2 ) wer e studie d i n great detai l t o asses s palaeo-wate r depths . Th e micropalaeontological analysi s (Gillmor e e t al . 2001) gav e a rang e estimat e o f palaeo-wate r depth an d i n some case s additiona l maxima and minima ar e included (Fig . 6). Next, a most likely trend through time was determined for each well by integratin g th e result s fro m th e structura l restoration an d th e micropalaeontologica l anal ysis (Fig . 6) . A s i t i s no t possibl e t o determin e exactly the palaeo-water depth, we have focused on determinin g most likel y dept h intervals , and identifying th e principal shallowing and deepening trends. In the seismic interpretation i t was not possible t o differentiat e betwee n th e Miocen e seismic unit s CSS-5 , CSS- 6 an d CSS- 7 i n th e northern Nort h Sea . Therefore , palaeo-wate r depth estimate s wer e no t obtainabl e fo r eac h Miocene uni t by structura l restoration . Furthermore, onl y a few sample s o f Miocen e ag e have been availabl e fo r micropalaeontologica l anal ysis. The sensitivity for shallowing o r deepenin g trends i s considere d t o b e n o bette r tha n 100 m (Kyrkjeb0 e t al 2001) . The inferre d Cenozoi c deepenin g o r shallow ing trend s fro m th e investigate d well s ar e

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generally i n good agreemen t wit h each othe r on a regiona l scale , especiall y whe n th e tectoni c position withi n th e basi n i s take n int o accoun t (Fig. 7). The inferred general trends in Cenozoic time are (Kyrkjeb0 et al. 2001): (1) deepening in Early t o Lat e Paleocen e time ; (2 ) shallowin g from Lat e Eocen e t o Lat e Miocen e time ; (3 ) deepening fro m Lat e Miocen e to Early Pliocen e time; (4) shallowing during Pliocene time . Most worker s agre e on falling global eustati c sea leve l throughou t Cenozoi c time s (Pitma n 1978; Watt s & Steckle r 1979 ; Ha q et al . 1987 , 1988). However , th e suggeste d amplitude s vary between abou t 10 0 an d 300m , whic h ar e les s than th e suggeste d amplitude s of th e deepenin g or shallowing trend (Fig . 7). The Late Eocene to Late Miocen e shallowin g correlate s wit h th e long-term eustati c curv e o f Ha q e t al . (1987 , 1988), bu t th e genera l shallowin g tren d wa s probably amplifie d b y tectono-therma l effects . The deepening events in Early to Late Paleocen e and Lat e Miocen e t o Pliocen e time s canno t b e explained b y th e eustati c sea-leve l curve , an d must therefor e b e explaine d b y purel y tectono thermal event s (Kyrkjeb 0 e t a l 2001) . Fro m Mid-Eocene time , ther e i s a reasonabl e corre lation betwee n eustati c curve s derive d fro m sequence stratigraphi c studie s (e.g . Ha q e t a l 1987, 1988 ) an d th e composit e oxyge n isotop e record reflecting variations in global climate and glaciations (Abreu & Anderson 1998) . Modelling The Cenozoi c deepenin g o r shallowin g trend s summarized above clearly point towards tectonothermal event s affectin g Cenozoi c basi n evol ution. On e o f th e mai n objective s o f ou r stud y was to constrain the amplitude and wavelength of differential vertica l movements . Togethe r wit h timing, these are critical factors to a discussion of possible mechanism s fo r Cenozoi c vertica l movements. To asses s tectoni c subsidenc e an d uplif t i n a more quantitativ e way , w e hav e carrie d ou t modelling alon g th e fou r regiona l transect s (Figs 2 , 5 an d 8 ) (Kyrkjeb 0 e t a l 1999) . Conventional backstrippin g technique s (e.g . Steckler & Watts 1978 ; Watt s et al 1982 ; Allen & Allen 1990 ; Robert s e t al 1993 , 1998 ; Kuznir et a l 1995 ; Nadi n & Kusznir 1995, 1996 ) were applied to the transects after key parameters such as age , lithology , porosity-dept h an d palaeo water depth for each seismi c sequenc e had bee n constrained from analysi s of the seismic and well data. Th e lithospheri c respons e t o loadin g o r unloading of sediment and water was assumed to be compensate d fo r by local Air y isostasy .

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Vertical movement s differen t fro m thos e predicted b y therma l contractio n an d sedimen t and wate r loadin g wer e regarde d a s potentia l tectonic signals . Kyrkjeb 0 et al (1999 ) regarde d events to be truly tectonic whe n the amplitude of the vertical movement s exceed s th e uncertainties related t o changes in palaeo-water depth adjusted for eustati c sea-leve l change s accordin g t o Ha q et al. (1987, 1988) . Figur e 9 shows how change s in palaeo-water dept h ar e crucial for detection of tectonic event s othe r tha n thermal cooling . The tectoni c subsidenc e analysi s o f th e northern Nort h Se a point s t o th e followin g Cenozoic trend s (Kyrkjeb 0 e t al . 1999 ) (Fig s 8 and 9) : (1 ) accelerate d tectoni c subsidenc e i n Paleocene time; (2 ) standstill to uplift i n Eocen e time followe d b y uplif t i n Oligocen e time ; (3 ) tectonic subsidenc e durin g Miocen e tim e fol lowed b y uplift i n Pliocene time . Kyrkjeb 0 et a l (1999) emphasize d tha t these trend s ar e general , and that local variation s occur . The genera l lac k of palaeo-wate r dept h informatio n fo r th e Miocene perio d affect s th e resolutio n i n thes e

Fig. 9. Typica l palaeo-wate r dept h and tectoni c subsidence curve s i n the northern Nort h Se a compare d with standar d rif t mode l (post-rif t therma l cooling) .

results. On the basis of evidence discussed below , we believ e tha t th e chang e fro m uplif t t o subsidence occurre d i n Mid-Miocen e tim e an d the subsequen t change fro m subsidenc e to uplif t probably too k plac e i n Late Pliocen e time .

Cenozoic evolutio n Here w e integrat e th e result s o f th e studie s described abov e an d summariz e th e Cenozoi c evolution o f th e norther n Nort h Se a an d surrounding area s usin g seismic sections , time thickness maps an d palaeo-water dept h maps. Paleocene time The CSS- 1 sequenc e (Uppe r Paleocene-lower most Eocen e units ) i s typifie d b y progradin g wedges tha t buil t ou t fro m th e Eas t Shetlan d Platform an d from souther n Norway. Lowermos t CSS-1, markin g th e beginnin g o f th e Tertiar y sequence, was probably characterized by shallow water dept h an d littl e relie f (Fig . lOa) . Th e lowermost Paleocen e brea k (66-6 2 Ma) i s interpreted to represent both an erosional vacuit y and a hiatus . Par t o f th e brea k i s probabl y a marine condensatio n o f sedimentatio n befor e progradation o f th e Paleocen e depositiona l systems fro m th e eas t an d th e wes t (Martinse n et al. 1999) . The prominen t depocentre s alon g th e wester n basin margi n (Fig . lOc ) wer e mainl y source d from th e uplifted Eas t Shetlan d Platform an d the Scottish Highlands . Th e progradin g shelf-slop e system i s i n par t fairl y san d ric h an d severa l phases of sand deposition were related to tectonic uplift an d erosio n o f th e sourc e area . Th e sequence geometrie s reflec t tha t som e o f th e Mesozoic grabe n faults , particularl y alon g th e western margin of the East Shetlan d Basin, were reactivated an d that differentia l compactio n too k place ove r deeper-seate d Mesozoi c faul t blocks . The syndepositiona l faulting wa s not related to a new phas e o f rifting , bu t probabl y t o regiona l subsidence. Th e shap e o f th e depocentr e wa s defined b y a combinatio n o f increase d sub sidence towards the basin centre, and differentia l compaction o f th e Mesozoi c sediment s i n th e graben relativ e t o thos e o n th e platfor m area s (Milton e t al. 1990) . A depocentr e i n th e northeaster n par t o f th e North Sea (Fig. lOc ) was probably source d fro m mainland Norway . I t ha s a progradationa l stacking patter n tha t thicken s pronouncedl y eastward althoug h i t ha s bee n subjecte d t o late r erosion (Fig . 11) . Thi s uni t i s characterize d b y more fine-graine d lithologies . Th e cla y minera l distribution show s a n increasin g kaolinite -

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Fig. 10. (a) Palaeo-water depth map for earliest Paleocene time (lowermost CSS-1); (b) palaeo-water depth map for earlies t Eocene time (lowermos t CSS-2); (c ) time-thickness map fo r CSS-1 (Uppe r Paleocene-lowermost Eocene sequence) ; (d) time-thickness map for CSS-2 (Eocen e sequence).

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Fig. 11 . Seismic line NVGTI-92-105 showing CSS- 1 building ou t from the east i n northernmost Nort h Sea . (Se e Fig. lO c fo r line location and Fig . 4 for Cenozoi c seismi c stratigraphy. )

smectite t o illit e rati o toward s th e depocentr e (Rundberg 1989 ) indicatin g tha t these sediment s were derived fro m the east (Thyberg et al 2000) . A relativ e sea-leve l ris e i n latest Paleocen e earliest Eocen e tim e i s indicated b y aggradatio n and even sedimen t thicknes s i n th e uppermos t part o f CSS-1 , whic h include s th e Balde r Formation, comprisin g interbedde d shale s an d tuff layers . Explosiv e volcanis m i n th e Britis h and th e Faeroe-Greenlan d Tertiar y volcani c provinces has been regarded a s the source for the widespread volcani c ashe s foun d i n th e Nort h Sea (Mal m e t a l 1984 ; Kno x & Morto n 1988 ) and onshor e Denmar k (Spjeldnae s 1975 ; Nielsen & Heilmann-Clause n 1988) . The highe r smecti tic conten t i n th e uppe r par t o f th e CSS- 1 sequence indicate s weatherin g o f a basalti c source rock . Th e mos t likely provenanc e are a for thes e sediment s i s the Late Paleocene-Earl y Eocene basal t province alon g the North Atlantic rift zon e (Fig . 12 ) in additio n t o th e widesprea d tuff deposits . Uppermos t CSS- 1 probabl y indi cates a condense d sectio n relate d t o period s of low clasti c sedimen t suppl y an d a starve d depositional environment. The sequenc e geometrie s an d thicknes s distribution o f CSS- 1 indicat e tha t dee p marin e conditions existe d alon g th e Vikin g Grabe n an d

towards th e continenta l margi n i n th e north . A deepening during Paleocene time , reaching water depths of about 800 m in the deepest part s of the basin, ha s bee n inferre d fro m biostratigraphi c data (Gradstei n e t al . 1994 ; Gradstei n & Backstrom 1996 ; Gillmor e et al 2001 ; Kyrkjeb0 et al 2001 ) and structural restoration (Kjenneru d et a l 2001 ) (Fig s 7 an d lOb) . A s th e inherite d post-Cretaceous bathymetry along the North Sea basin margin areas was not significant (Fig . lOa) , the accommodation and development of the thick Upper Paleocen e wedges probably resulte d fro m subsidence o f th e basi n floo r combine d wit h increased sedimen t suppl y relate d t o tectoni c uplift and subsequent erosion o f the adjacent land areas. Shallo w marin e condition s probabl y prevailed alon g th e basi n margin s i n th e wes t and east. A delta succession characterized by the presence o f coa l an d i n situ lignites , a s wel l a s freshwater floras and faunas, has been reported in the Mora y Firt h (Andrew s et al 1990) . Uplift abov e se a leve l o f th e Hebrides Shetland axi s t o th e N W of the Nort h Se a basi n was accompanie d b y th e outbrea k o f extensiv e volcanicity i n th e area s surroundin g the presen t NE Atlanti c Ocean . Th e uplif t establishe d a wholly ne w geograph y (Fig . 12) . An easterl y to southeasterly flowin g drainag e syste m becam e

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Fig. 12. Regional setting in Late Paleocene-earliest Eocene time. 1 , early Tertiary flood basalt province; 2, areas of uplift; 3 , intrabasinal highs; 4, main depocentres; 5 , plate boundary-line o f breakup; 6, V0ring and FaeroeShetland escarpments; 7, outbuilding directions; 8, Sorgenfrei-Tornquist Zone-Fennoscandia n Border Zone.

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established o n th e Orkney-Shetlan d Platfor m and Scottis h Highlands , resultin g i n th e rework ing o f th e sedimentar y cove r ove r th e newl y emergent terrains , an d dispersa l o f thei r detritu s into th e Nort h Se a basi n (Johnso n e t al 1993 ; Jones & Milton 1994) . The northwester n corne r o f souther n Norwa y (Fig. 12) , o n tren d wit h th e Hebrides-Shetlan d axis, wa s als o uplifted . Th e uplifte d are a wa s bounded b y the 0ygarden an d M0re-Tr0ndelag fault complexe s toward s th e wes t an d NW , respectively. I t extended southward s to about the Sognefjorden area . Northeastward s th e uplifte d area wa s probabl y bounde d b y th e landwar d extension o f th e Ja n Maye n Lineament . Earl y Tertiary volcanis m (c . 5 6 Ma) i s documente d within thi s are a jus t of f th e coas t o f Norwa y (Vestbrona Formation; Bugg e et al. 1980 ; Torsk e & Prestvik 1991 ; Prestvik et al. 1999) . Clast s of Danian chal k hav e bee n foun d i n Pliocen e sediments buildin g out from Norway i n this are a (Thyberg e t al . 2000) , indicatin g that n o clasti c source are a existe d her e befor e th e Lat e Paleocene uplift . Most o f th e sediment s derive d fro m th e uplifted are a o n th e Norwegia n sid e wer e deposited i n th e northeaster n Nort h Se a an d southeastern margin of the M0re Basi n (Fig. 12) . However, som e sediment s ma y hav e bee n transported southwards , wher e onl y a smal l portion reached the North Sea in the NorwegianDanish Basin. Here, a Late Paleocene depocentr e (Fig. 12 ) reflects outbuilding fro m th e NE . Th e source are a fo r th e CSS- 1 sequenc e i n thi s are a may also have include d th e uplifted Sorgenfrei Tornquist Zon e (STZ)-Fennoscandia n Borde r Zone (FBZ) , whic h wa s inverte d i n respons e t o Alpine compression . Th e uplifte d STZ-FB Z must hav e bee n a barrie r fo r an y sediment s coming fro m souther n Scandinavia . Erosiona l valleys an d kars t topograph y sugges t tha t th e uppermost chal k wa s subaeriall y expose d i n the eastern Nort h Se a i n mid-Paleocen e time s (Huuse 1999 ; Clause n & Huus e 1999) . Th e fossil conten t i n th e Danis h Fu r Formatio n indicates tha t th e lowermos t Eocen e sediment s were deposite d relativel y clos e to a coastlin e i n the Norwegian-Danish Basin . Erosional product s (sands ) wer e also trans ported northwestwar d fro m th e uplifte d Heb rides-Shetland are a int o th e Faeroe-Shetlan d Basin (Fig . 12) . Renewe d tectoni c movemen t allowed depositio n adjacen t t o majo r faults , bu t Late Paleocen e an d Earl y Eocen e event s wer e dominated b y regiona l uplif t an d massiv e volcanism associated wit h North Atlantic rifting. Towards th e en d o f Paleocen e times , transfe r faulting ha d largely ceased, an d the northwestern

margin o f th e Faeroe-Shetlan d Basi n wa s inundated b y subaeriall y emplace d lava s o f th e Faeroes lowe r series , an d t o th e N E th e Erlen d and West Erlend volcanoes were erupting (Stoker etal 1993) . Late Paleocene-earlies t Eocen e depocentre s in th e wester n M0r e an d V0rin g basin s wer e sourced fro m a n uplifte d are a i n th e wes t alon g the incipien t plat e boundar y (Fig . 12) . O n th e M0re Margina l High , Paleocen e deposit s ar e thin, typicall y 200-350m , an d lithologie s consist mainl y o f lava s an d shallow-wate r sediments. A thic k wedg e i n th e M0r e Basi n was source d fro m th e west. Aggradation of sequence CSS-1.2 (Jordt et al. 1995) indicate s tha t sedimen t suppl y fro m Norway wa s significantl y reduce d i n earlies t Eocene time , and tha t th e lan d areas furthe r eas t were subjecte d t o a marin e transgression . Reduced runof f cause d b y climati c change , changes i n lan d vegetation , tectonic quiescence or erosiona l levellin g o f souther n Norway , ar e possible mechanisms that could explain a sudden reduction in sediment supply from th e east in the very earlies t Eocene tim e (Jordt et al 2000) . Eocene time There i s a composite brea k i n the Earl y Eocen e deposition correspondin g t o th e CSS-l-CSS- 2 boundary. Th e biostratigraph y indicates that the break spans the period 55-52 Ma. The lower part of the break (c. 55-54 Ma) is laterally extensive. The uppe r par t o f th e brea k (c . 54-5 2 Ma) seemingly ha s a limited latera l extent , and in the North Se a basi n i s accompanie d b y a shif t o f sedimentation i n th e basinward s direction , an d subsequent onla p ont o th e basi n margin . Thus , the brea k increase s i n exten t toward s th e east . This Earl y Eocen e brea k i s interprete d t o represent bot h a n erosiona l vacuit y (althoug h evidence is limited) and a hiatus (Martinsen et al. 1999). The Eocen e (CSS-2 ) depocentre s (Fig . lOd ) are mainl y locate d i n th e centra l an d wester n parts of the northern North Sea basin and indicate outbuilding int o the basin fro m the uplifted East Shetland Platform . CSS- 2 infill s an d drape s th e topography o f to p CSS- 1 an d th e depocentr e i s located basinward s o f th e forme r CSS- 1 shel f edge. Eocene depositio n was dominated by deepwater an d slop e processe s includin g turbidit y currents. Dee p water existe d throughou t mos t of Eocene time s (Fig s lO b an d 13a) . Water depth s of abou t 1000 m hav e bee n estimate d fo r th e Lower Eocen e Frig g Formation , whic h wa s deposited a s a submarin e fa n i n a deep-se a environment (Heritie r et al . 1979 , 1981) .

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Fig. 13 . (a) Palaeo-water dept h map for latest Eocene tim e (lowermos t CSS-3); (b) palaeo-water depth map for Early Oligocen e tim e (lowermos t CSS-4); (c) time-thickness map for CSS-3 (Lowe r Oligocene sequence) ; (d) time-thickness map for CSS-2 (Uppe r Oligocen e sequence) .

250

J. I. FALEIDE ETAL.

CSS-2 consist s o f stacked , thickl y bedde d immature sandstone s wit h interbeds o f clay- and siltstone a t th e margi n o f th e Eas t Shetlan d Platform an d i n th e Mora y Firt h Basi n an d generally of claystones in the Viking Grabe n an d Central Grabe n area s (Mudg e & Buja k 1994) . Lignite layer s penetrated i n well 8/27a-l poin t to the existenc e o f swamp y coasta l lake s landwar d of th e inne r shel f (Veeke n 1997) . The Eocen e successio n thin s eastwards an d is almost absen t i n som e section s clos e t o th e eastern basi n margin . However , a mino r Earl y Eocene (CSS-2.1 of Jordt et al 1995 ) depocentr e off Sognefjorden , i n th e sam e are a a s fo r th e underlying CSS- 1 sequence , comprise s a pro gradational uni t tha t downlap s ont o th e upper most Balde r surface . Th e outbuildin g o f th e lower part o f sequence CSS- 2 i n the northeastern North Se a an d depositio n o f san d alon g th e eastern basi n margi n indicat e renewe d tectoni c uplift an d sedimen t suppl y fro m th e eas t (Rundberg 1989 ; Jord t e t al 1995 , 2000) . From mid-Eocen e tim e onwards , Tertiar y sedimentation i n th e centra l area s o f th e Nort h Sea basi n wa s dominate d b y monotonou s sequences o f marin e mud s an d silts , althoug h sands wer e deposite d a t the periphery. Th e finegrained CSS- 2 sediment s of low rate s of non volcanic clasti c sedimen t suppl y indicat e a relatively dee p marin e facies . Mor e o r les s starved sedimentatio n conditions , as a result of a relative rise in sea level, favoured the enrichmen t in bot h volcaniclasti c an d organi c matte r (Thyberg e t al . 2000) . Th e unifor m litholog y and compositio n o f th e Eocen e sediment s (smectitic mudstones ) i n th e northeaster n Nort h Sea (Rundber g 1989 ; Thyber g e t al 2000) , and the velocit y distributio n o f sequenc e CSS- 2 (Jordt e t a l 2000) , suggest s a rathe r unifor m source an d sedimen t suppl y fro m a n are a dominated b y basalti c rocks , i.e . th e Nort h Atlantic basal t provinc e t o th e N W (Thyber g et al 2000) . The dominance of fine-grained Middle-Upper Eocene deposit s i n easter n Nort h Se a well s (Rundberg 1989 ; Steurbau t et al 1991 ; Mudge & Bujak 1994 ; Michelse n e t a l 1995) , an d th e regional thinnin g o f CSS-2. 2 o n th e Hord a Platform an d furthe r south , suggest s tha t th e Scandinavian topograph y wa s to o limite d t o supply sufficien t clasti c materia l an d t o allo w progradation fro m th e east, and that large parts of Scandinavia, eas t o f th e stud y area , ma y hav e been submerge d i n Mid-Lat e Eocen e time s (Jordt e t al 1995) . During Lat e Eocen e time , a lowerin g o f relative se a leve l (eustati c fal l and/o r uplift? ) promoted th e advanc e o f delt a system s int o th e

North Se a fro m th e west , wit h depositio n o f successions characterize d b y th e presenc e o f i n situ lignite s an d freshwate r flora s an d faunas . Biostratigraphical evidenc e indicate s tha t Uppe r Eocene strata ar e absent i n much of the norther n North Se a an d alon g th e basi n flank s furthe r south. In these areas, Middle Eocene deposit s are overlain unconformabl y b y Lowe r Oligocen e shales (e.g . Gradstei n et al 1994) . The boundary between th e Lowe r an d Middle Eocene sequence s i n th e Norwegian-Danis h Basin i s associate d wit h a chang e i n sedimen t outbuilding directio n fro m th e N E (Norway ) t o the wes t (Britis h Isle s an d Shetlan d Platform ) (Michelsen et al 1995) . A complete deep marine Eocene successio n i s preserve d i n th e Norwe gian-Danish Basi n an d i n th e Centra l Grabe n (Heilmann-Clausen e t a l 1985) . Here , th e Eocene successio n appear s unaffecte d b y erosion. In th e Faeroe-Shetlan d Basin , riftin g an d volcanism waned through Early Eocene tim e and was followe d b y subsidence . Middl e t o Uppe r Eocene sediment s of the Faeroe-Shetland Basin are generall y fine-graine d claystone s an d silt stones wit h occasiona l limestone s an d sand stones. Thes e reflec t th e retur n t o full y marin e conditions afte r a regressiv e phas e i n Lat e Paleocene an d Early Eocen e time s (Stoke r e t a l 1993). In th e M0r e Basin , subsidence durin g Eocen e time allowe d a thic k sequenc e t o accumulate . Wells drille d o n th e flan k o f th e Margarit a Spu r penetrated nearl y 1000 m o f Lowe r t o Middl e Eocene sediments , overlai n unconformabl y b y the Oligocene sequenc e (Stoke r et al 1993) . The thickness increas e o f CSS-2 toward s th e Atlantic continental margi n indicate s sedimen t suppl y from tha t direction . A t th e V0rin g margi n of f mid-Norway, uplif t create d th e mai n wester n source are a fo r th e Paleogen e V0rin g Basi n sediments (Skogsei d & Eldhol m 1989 : Stuevold etal 1992 ; Stuevold & Eldholm 1996 : Hjelstue n etal 1999) . Oligocene time Biostratigraphical dat a indicate a hiatus between the Eocen e (CSS-2 ) an d Oligocen e (CSS-3 CSS-4) sequence s (va n Vee n e t a l 1994 ; Gradstein & Backstro m 1996) . Martinse n e t a l (1999) place d a widesprea d composit e brea k i n Early Oligocen e tim e (33-2 7 Ma), o f variabl e extent dependin g o n location. During Oligocene time (CSS-3 and CSS-4) the basin configuratio n changed . Th e thickes t Oligocene strat a i n th e norther n Nort h Se a basin ar e locate d ove r th e Norwegia n secto r o f

CENOZOIC EVOLUTION OF THE NORTHERN NORTH SEA the Nort h Vikin g Graben , wher e u p t o 900 m accumulated. Isopach s o f Oligocen e deposit s indicate a genera l north-sout h trend , wit h thinning both t o the west an d east (Fig . 13 c and d). The concentratio n o f CSS-3 sediment s in the centre o f th e norther n Nort h Se a basi n an d contemporaneous erosio n o n th e flank s indicat e an increase d topographi c relief , whic h wa s probably cause d b y uplif t alon g th e easter n an d the wester n basi n margin s i n Earl y Oligocen e time. The sequenc e geometrie s reflec t uplif t o f th e eastern basin flank (Fig. 14) . Minor progradation occurred a s a consequenc e o f tectoni c uplif t activity i n th e eas t initiate d i n Earl y Oligocen e time. A n increase d rat e o f inpu t o f coarser grained materia l occurre d i n Earl y Oligocen e (CSS-3) time , probabl y i n response to uplift an d erosion o f soft Eocen e an d Paleocene sediments . However, th e volcani c inpu t i s stil l distinctiv e (Thyberg et al 2000) . Ne w ashe s fro m Icelan d and reworke d Eocen e sediment s fro m Norwa y may hav e contribute d t o th e hig h smectit e content. Parts o f the basi n wer e uplifted together wit h Norway s o tha t CSS- 4 (Uppe r Oligocen e sequence) locall y overlie s CSS- 2 (Eocen e sequence) (Fig s 13 b an d 14) . A mino r dro p i n sea level , leadin g t o truncatio n o f th e Eocene Lower Oligocene sequence(s) , was followed by a renewed ris e tha t resulte d i n onla p o f CSS- 4 deposits. Lat e Oligocen e depositio n wa s characterized b y basina l accumulatio n o f aggradin g silts an d clays . Locall y CSS- 4 build s ou t fro m uplifted sourc e area(s ) withi n th e basin . Th e CSS-4 depocentre s ar e mainly located alon g the eastern basi n margin (Fig . 13d) . Clausen et al. (1999a) studied intraformational faults withi n th e Uppe r Oligocen e sequenc e (CSS-4) i n the Trol l are a o n the norther n Hord a Platform. Th e fault s hav e a strongl y dominan t NW-SE tren d indicatin g SW-N E extension , and forme d abov e a n intra-Oligocen e unconformity tha t was establishe d a s a result o f uplif t o f Fennoscandia an d a sea-level fall . Outbuilding continued from th e East Shetland Platform durin g bot h CSS- 3 an d CSS- 4 times , resulting i n a thic k depocentr e i n th e Vikin g Graben. I n the transgressive phas e followin g th e intra-Oligocene lowstan d o f se a level , a majo r shelf-deltaic forese t uni t u p t o 500 m thic k prograded fro m th e Eas t Shetlan d Platform into the central par t o f the norther n Nort h Se a basin. The Earl y Oligocen e glacio-eustati c sea-leve l fall tha t was caused by expansion of ice sheets in Antarctica (Miller et al. 1987; Zachosef a/. 1992 ; Abreu & Anderso n 1998 ) wa s overprinte d b y local vertica l tectoni c movement s i n th e Nort h

251

Sea area . Palaeo-wate r dept h dat a indicat e shallowing (Fig . 7 ) an d a t th e en d o f Oligocen e time the northern North Sea formed a narrow and shallow basi n separatin g deepe r basin s t o th e south and north (Fig. 13b ) (Thyberg et al. 1999) . In th e Norwegian-Danis h Basin , th e marke d shift fro m Lat e Eocen e dista l deep-wate r sedimentation t o shor e progradatio n wit h sedi ment transpor t fro m nort h an d N E i n earlies t Oligocene tim e indicate s rapid tectoni c uplif t o f southern Norwa y (Fig . 15 ) (Jord t e t al . 1995 , 2000; Michelse n e t al . 1995 ; Danielse n e t al . 1997; Clause n e t al . 2000) . Th e geometr y an d thickness o f CSS-3 indicat e that water depths in the Central Nort h Sea may have exceeded 60 0 m in Earl y Oligocen e tim e (Jord t e t al . 1995 ; Michelsen et al. 1995). The presence of reworked Paleocene and Eocene nannofossils in Oligocene sediments is interprete d to be the resul t of denudation o f Paleocene-Eocen e sediment s exposed a t the basin margin to the east (Clausen et al. 2000) . The Lowe r Oligocen e sequenc e form s a n extensive depositiona l uni t that extend s paralle l to th e mid-Norwegia n coas t fro m M0r e t o Lofoten (Rokoenge n et al. 1995) . The sediments are interprete d as deltai c and coasta l deposits , probably forme d i n a wave-dominate d environ ment wit h extensiv e longshor e drift . Seismi c lines acros s thi s uni t show tha t i t i s represente d by an interval of steeply dipping reflectors, which are interpreted to be a set of prograding foresets. The bas e o f th e successio n i s seismicall y identified a s a downla p surface . Th e wester n boundary is a morphological ram p formed by the distal foresets (Eidvi n et al. 1998) . Outbuilding fro m Norwa y observe d fro m th e southeastern Nort h Se a t o Lofote n (Fig . 15 ) reflects th e onse t o f regiona l tectoni c uplif t o f Scandinavia at the Eocene-Oligocene transition. Southern Norwa y wa s uplifted , erosio n acceler ated an d sedimen t transpor t toward s th e wes t became mor e important. Early Oligocen e outbuildin g is also observe d at th e edg e o f th e Hebride s Shel f (Stoke r e t al . 1993). I n th e Faeroe-Shetlan d Channel , th e Oligocene sequenc e is thin. During Earl y Oligocen e time , th e Green land-Svalbard ga p bega n opening , allowin g greater communicatio n betwee n th e Atlanti c and Arcti c oceans , an d causin g a n influ x o f colder wate r int o the Nort h Sea . Thi s chang e i s evidenced b y th e replacemen t o f warm-wate r fish by borea l forms , an d a dro p o f 12° C i n th e North Se a botto m temperature s (Buchard t 1978). Thi s chang e wa s coinciden t wit h shallowing o f th e basi n a s a resul t o f sedimen t infilling an d tectoni c uplift .

252 SG8043-201

NE

1.5-

(a) 5 km

SG8043-303

CSS-3

(b) 25km

Fig. 14 . Seismic line s showing pinchout of CSS-3 towards area at the eastern basin flank uplifted a t the EoceneOligocene transition , (a) Seismic line SG8043-201; (b) seismic line SG8043-303. (Se e Fig. 13 b for line locations and Fig. 4 for Cenozoic seismi c stratigraphy.)

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Fig. 15 . Regional setting in Early Oligocene time . 1 , early Tertiary flood basalt province; 2, areas of regional uplif t (Rohrman et al 1995 ; Japsen 1997) ; 3, domes or anticlines (Dore & Lundin 1996); 4, main depocentre; 5 , extinct spreading axis ; 6, V0ring an d Faeroe-Shetland escarpments; 7 , outbuilding directions .

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J. I . FALEIDE ETAL.

Miocene time A prominen t hiatu s i s presen t i n th e Miocen e sequence bu t th e biostratigraph y i s equivoca l (Steurbaut e t al 1991 ; Eidvi n & Rii s 1992 ; Gradstein & Backstro m 1996 ; Martinse n e t al 1999; Eidvi n e t al . 2000) . Accordin g t o Martinsen e t al. (1999) , thi s break extend s fro m latest Oligocene (c . 25 Ma) to Late Miocene tim e (c. 8-9Ma), interrupted b y sedimentar y units of Late Oligocen e an d Earl y t o Mid-Miocen e age . The Miocen e brea k i s locall y characterize d b y erosion o f the underlying Oligocen e sequence . During Miocen e tim e th e norther n Nort h Se a constituted a shallo w marin e basin , whic h connected th e deepe r centra l Nort h Se a an d th e Norwegian-Greenland Sea (Fig. 16a) . The mai n Miocene depocentre s (CSS-5 , CSS-6 and CSS-7) are locate d sout h o f 60° N (Fig . 16b) . Farthe r north some o f the sequence s ar e thin or missing, so tha t i t i s no t possibl e t o ti e thei r boundarie s within the seismi c grid . The CSS- 5 sequenc e (Lowe r Miocen e sequence) buil t ou t int o th e norther n Nort h Se a from a basin margi n t o th e west . I t comprise s a seismically well-define d sequenc e reachin g about 200 m i n thicknes s i n th e basi n centre . The bas e i s marke d b y onla p ont o th e irregula r uppermost Oligocen e sequenc e boundary ,

whereas th e to p i s identifie d a s a prominen t reflector representin g th e bas e o f th e sand y Utsira Formation . T o th e north , th e Lowe r Miocene sequenc e pinche s ou t a t abou t 61°30'N (Eidvi n et al. 2000) . Depositio n during Early-Mid-Miocene tim e wa s dominate d b y low accumulatio n rates. The overlyin g sequenc e (CSS-6 ) thin s north ward an d ma y b e absen t i n larg e part s o f th e study are a nort h of 60°N. A mid-Miocene fall i n glacio-eustatic se a level (Miller et al. 1987 ; Haq et al . 1987 , 1988 ; Abre u & Anderso n 1998 ) i n combination wit h regiona l uplif t gav e ris e t o erosion an d formatio n of a prominent unconfor mity, in particular along the eastern basin margin. A Miocen e progradin g wedg e observe d i n th e M0re Basi n (Martinse n e t al . 1999 ) probabl y represents outbuildin g from uplifte d an d erode d areas in southern Norway and the northern North Sea (Fig . 17) . A Middl e Miocen e sequenc e boundary i n th e Norwegian-Danis h Basi n represents a shif t fro m a progradin g reflectio n pattern belo w t o a n aggradin g patter n abov e (Michelsen e t al 1995) . In quadran t 3 5 o f th e Norwegia n norther n North Se a ther e i s evidenc e o f incisio n int o Oligocene strata (Fig. 18 ) (Rundberg et al 1995 : Gregersen 1998 ; Martinse n e t a l 1999) . Th e formation o f th e incise d valley s (submarin e or

Fig. 16 . (a) Palaeo-water dept h ma p for Late Oligocene time (lowermost CSS-5); (b ) time-thickness map for the Miocene units (CSS- 5 to CSS-7).

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Fig. 17. Regional setting in mid-Miocene time reflecting a hiatus in the northern North Sea. 1 , early Tertiary flood basalt province ; 2, areas o f regional uplift (Rohrma n e t al. 1995 ; Japsen 1997) ; 3 , domes or anticlines (Dore & Lundin 1996) ; 4, mai n depocentres ; 5 , extinct spreadin g axis ; 6 , V0ring an d Faeroe-Shetland escarpments ; 7, outbuilding directions.

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J. I . FALEIDE ETAL.

Fig. 18 . Neogene incised valleys i n the northern Nort h Sea. (a) Seismic exampl e acros s incised valley ; (b) semi regional map showing distribution of incised valleys (modified fro m Martinse n et al. (1999); (c ) earliest Miocene time map, Block 34/ 6 (Martinsen et al. 1999) ; (d ) earliest Miocene tim e map, Bloc k 35/4 (Gregersen 1998) ; (e) earliest Miocen e tim e map, Block 35/8.

subaerial?) an d th e timin g o r ag e ar e critica l questions i n constrainin g th e histor y o f vertica l movements. Martinse n e t al . (1999 ) interprete d the incise d feature s a s subaerial , incise d valley s because o f th e morphologica l patterns . A

subaerial origi n fo r th e incise d feature s implies considerable tectoni c uplif t followe d b y Lat e Miocene-Pliocene subsidence . Th e incisio n probably too k plac e durin g formatio n o f th e mid-Miocene unconformity. However, according

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257

Fig. 19 . (a) Palaeo-water dept h map for earliest Pliocen e tim e (lowermos t CSS-8) ; (b ) time-thickness map for CSS-8; (c ) time-thickness map for CSS-9; (d ) time-thickness map for CSS-10.

to Eidvin et al. (2000) th e incision is Pliocene i n mid-Miocen e unconformit y i n th e norther n age (see discussion below). Nort h Sea . Th e formatio n accumulate d i n tw o The Utsir a Formatio n correspond s t o par t o f mai n depocentre s alon g th e basi n centre , sout h CSS-7 an d wa s deposite d o n to p o f th e an d nort h o f 60°N , respectively . Th e Utsir a

258

J. I . FALEIDE ETAL.

Formation i s interprete d a s a shallow-marin e deposit, she d mainl y fro m th e easter n basi n margin durin g lat e Mid-Lat e Miocen e risin g relative se a leve l (Isakse n & Tonsta d 1989 ; Rundberg 1989 ; Gallowa y et al 1993 ; Gregerse n et a l 1997 ; Martinse n e t a l 1999) . Th e Utsir a Formation i s san d rich , bu t toward s th e wester n basin margin it fines considerably. The sediment s were mainly derived from Scandinavia (Rundberg 1989), bu t sedimen t influ x fro m th e Shetlan d Platform o r Scotland i s also proposed (Gregerse n et al 1997) . Althoug h the Utsira sandstone s ar e likely t o hav e bee n supplie d fro m th e basi n margin, the y wer e probabl y significantl y reworked b y basina l current s (Gallowa y e t a l 1993). Th e basi n i s unlikely t o hav e bee n mor e than 150-200 m deep, s o that such currents could have existed . Th e wate r depth increase d an d culminated i n earlies t Pliocen e tim e wit h condensed deposit s o n whic h th e succeedin g Pliocene deposit s downlapped . Late Miocen e uplif t o f sout h centra l Norwa y promoted furthe r tiltin g o f th e Hord a Platfor m area, an d hence , erosio n o f Miocen e sediment s (Clausen e t a l 1999a) . Th e continue d uplif t o f Scandinavia force d th e Lat e Miocen e an d Pliocene deposit s t o prograd e westward , down lapping ont o the mid-Miocene unconformit y and the underlying Lower-Middle Miocene deposit s and fillin g i n th e mor e centra l part s o f th e northern Nort h Sea . Deeper-wate r starve d

conditions existe d i n th e Norwegian-Danis h Basin an d the M0re Basi n i n Late Miocen e tim e contemporaneous wit h depositio n o f th e Utsir a Formation i n the norther n Nort h Sea . The Mid-Miocen e an d younger sedimentation in the souther n Nort h Se a was dominated b y the expansion of massive delta systems on its eastern seaboard associate d with former Baltic rivers that drained fro m th e Fennoscandian Shield (Bijlsm a 1981; Gibbar d 1988 ; Camero n e t al 1993) . Plio-Pleistocene time A majo r depocentr e o f mainl y Uppe r Pliocen e (CSS-8) glacia l sediment s derived fro m uplifte d Norway is located i n the northernmost North Sea (Fig. 19) . Sequence geometrie s characterize d by westward progradin g clinoform s (Fig . 20 ) sho w that th e dominan t sedimen t transpor t direction s are from th e Norwegian shelf margin towards the west an d NW , bu t occasiona l input s fro m th e Shetland Platfor m ar e als o observed . Th e regional downla p surfac e a t th e bas e o f th e Pliocene sequenc e i s interprete d a s reflectin g starved sedimentatio n probabl y cause d b y a relative sea-leve l ris e (Gregerse n e t a l 1997) . The clinoform s withi n sequence CSS- 8 indicat e deep wate r in excess of 500m (Figs 19 a and 20). An ag e o f 2.75 Ma i s taken as the maximum age of the Upper Pliocene section , corresponding to a large increase in the supply of ice-rafted material

Fig. 20 . Seismi c lin e NVGTI-92-10 8 showin g Pliocen e progradatio n fro m th e eas t an d flat-lyin g Pleistocen e sediments abov e angula r unconformity . (Se e Fig . 19 b fo r lin e locatio n an d Fig . 4 fo r Cenozoi c seismi c stratigraphy.)

CENOZOIC EVOLUTION OF THE NORTHERN NORTH SEA related t o a marke d expansio n o f norther n European glacier s (Eidvin et al 2000) . A perio d o f transgressio n i n Earl y Pliocen e time resulte d i n strongl y reduce d rate s o f deposition, an d sediment s o f thi s ag e ar e preserved mainl y in the centra l Nort h Sea (Eidvin et al 2000) . Most of the Lower Pliocen e sediments, whic h ma y hav e existed , wer e probably erode d i n the subsequent period during an extensiv e relativ e fal l i n th e globa l se a leve l (4.1-2.9 Ma). A period o f regression i n earlies t Late Pliocen e tim e probably resulted i n erosio n of most of the Norwegia n continental shelf with the exceptio n o f th e deepe r area s o f th e Centra l and Vikin g grabens . I f th e incise d valley s described abov e (Fig . 18) wer e forme d a t thi s time (earlies t Lat e Pliocen e time) , an d the y formed abov e se a level , i t implie s extremel y rapid Lat e Pliocen e subsidenc e t o creat e th e water depth inferred from th e clinoform geometries within the CSS-8 sequenc e (>500m) . This period wa s immediatel y followed , i n th e late r part of Late Pliocene time, by rapid deposition of glacially derive d sediment s prograding along the entire shelf . In general , th e Pleistocen e developmen t i s a continuation o f th e Lat e Pliocen e evolution , but is marked by more extensive erosion of the inner shelf (Eidvi n et al. 2000). Flat-lyin g Pleistocene beds lie with an angular unconformity on more or less progradationa l Uppe r Pliocen e deposit s (Fig. 20) . The lowe r par t o f th e Pleistocen e sequence an d uppermos t par t o f th e Uppe r Pliocene sequence ar e eroded ove r large area s of the continental shelf. The base of the Pleistocene section i s date d t o 1. 2 Ma (Sejru p e t a l 1995) . This dating coincides wit h a marked intensifica tion of glacial activity, as is observed in the deepsea recor d (Ruddima n e t a l 1986 ; Berger & Jansen 1994) . Repeated glaciation s (Sejru p et al 1995) erode d th e margina l part s o f the norther n North Sea and the associated glacio-eustati c sea level falls an d the overall lowstand have resulted in majo r channe l cut s o r incise d valley s i n th e marginal part s o f th e Nort h Se a basi n (Sejru p etal 1991) . Differences i n Pliocen e an d Pleistocen e depositional pattern s ar e probabl y th e resul t o f changes i n glaciatio n cycle s tha t occurre d a t c. 1. 1 Ma. Durin g th e perio d befor e this , th e Fennoscandian ice cap probably extended only to the presen t coastlin e (Janse n & Sj0hol m 1991) . Subsequent t o c . 1. 1 Ma, glacier s periodicall y extended ove r th e continenta l shel f an d trans ported sediment s ove r greate r distance s (Sejru p et al 1995 , 2000 ; King et al 1996) . Large-scale Pliocen e progradatio n i s als o observed offshor e mid-Norwa y (Fig . 21 )

259

(Stuevold & Eldhol m 1996 ; Hjelstuen e t a l 1999; Eidvi n etal 2000) . The narrow-elongate d depocentr e alon g th e basin axis in the central North Sea shows that the basin flank s wer e uplifte d o r exhume d togethe r with mainlan d Norwa y an d th e Britis h Isle s (Fig. 21 ) (Hilli s 1995a , 1995b ; Hanse n 1996; Japsen 1998 , 1999 , 2000) . Th e Grea t Europea n delta, fe d mainl y fro m th e eas t an d south , continued to expand northwards into the southern and centra l North Se a (Zagwij n 1989 ; Camero n etal 1993;Funnel l 1996) . The regional uplif t o f Scandinavi a is believed to be the main control on sediment suppl y to the prograding Pliocen e sequences . Oscillation s o f the eustatic se a level punctuate d the tectonicall y controlled progradatio n and affected variation s in the accommodatio n space , an d thu s create d th e high-frequency sequence s (S0rensen etal 1997) . An upwar d increasin g numbe r o f Pliocen e sequences i n th e centra l Nort h Se a ha s bee n related t o a n increasin g Neogen e uplif t o f Scandinavia an d adjacen t areas , combine d wit h a general lowered glacio-eustati c se a level during Pliocene time . Biostratigraphi c studie s b y Seidenkrantz (1992 ) suppor t a shallowin g upward and gradually colder environment during Late Pliocen e time . Th e gradua l coolin g (Buchardt 1978 ) an d ice-ca p growt h cause d a general glacio-eustati c sea-leve l lowerin g (Ha q et a l 1988 ) through Pliocen e time , which , together wit h th e uplif t durin g Neogen e tim e (Ghazi 1992 ; Jensen & Schmid t 1992 , 1993 ; Hansen 1996) , led to erosion o f the exposed shel f areas an d increased sedimen t influx . Possible mechanisms causing vertical movements The main mechanisms suggeste d i n the literature to cause Cenozoic vertica l movement s alon g the NE Atlanti c margin an d adjacen t area s include: (1) arriva l o f th e Icelan d plum e an d th e subsequent latera l spreadin g of th e plum e head; (2) mantle processes and thermal regime, and the episodic behaviou r o f th e Icelan d plume ; (3 ) emplacement o f magma in and at the base of the crust (magmati c underplating ) leadin g t o isostatic uplift ; (4 ) metamorphi c phas e change s i n the mantle; (5) intra-plate stress related to ridgepush from the North Atlantic plate boundary and/ or t o Alpin e compression ; (6 ) erosio n an d isostatic rebound; (7) flexura l effects . It i s beyond th e scop e o f this paper to test all these mechanism s quantitatively . Here, som e o f them will be briefly discussed in light of our new constraints on timing, amplitude and wavelength

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Fig. 21. Regional settin g in Late Pliocene time . 1 , early Tertiary flood basalt province ; 2, areas of regional uplif t (Rohrman e t at. 1995 ; Japse n 1997) ; 4 , mai n depocentres ; 5 , extinc t spreadin g axis ; 6 , V0rin g an d Faeroe Shetland escarpments ; 7 , outbuilding directions; 8 , major rive r system (Gibbard 1988) .

CENOZOIC EVOLUTION OF THE NORTHERN NORTH SEA of Cenozoic differentia l vertica l movements . For each o f th e mai n regiona l event s w e wil l summarize th e mai n observation s o r fact s tha t any proposed mechanism ha s to explain .

Late Paleocene-Early Eocene vertical movements In simpl e terms , th e Lat e Paleocene-Earl y Eocene uplif t i s referre d t o a s 'margina l uplift ' related t o th e riftin g an d break-u p o f th e N E Atlantic. However , ther e ar e larg e variation s i n the wavelengt h an d amplitud e o f th e uplif t observed alon g th e N E Atlanti c margins , an d possibly als o i n timing. Important observation s tha t hav e t o b e explained include : (1 ) uplif t o f th e Hebrides Shetland axi s an d northwester n corne r o f southern Norway ; (2 ) uplif t o f th e are a alon g the incipien t plat e boundary, with subaeria l sea floor spreading; (3) accelerated subsidence in the North Sea . The accelerate d tectoni c subsidenc e i n th e North Se a basin i n Paleocene tim e an d uplif t o f surrounding area s wa s probabl y associate d wit h arrival o f th e Icelandi c plum e an d crusta l break-up in the North Atlantic . Ther e appea r to have bee n tw o majo r phase s o r pulse s o f Earl y Tertiary magmatis m withi n th e Nort h Atlanti c igneous province , wit h a pea k a t 5 9 Ma an d a t 55 Ma (Whit e & Lovel l 1997 ; Ritchi e e t al 1999). Several uplif t mechanism s ca n b e attribute d to thi s regiona l tectonomagmati c event : (1 ) transient therma l uplif t generate d b y buoyanc y of lithospher e heate d b y ho t plum e materia l (Sleep 1990 ; Clif t & Turne r 1998) ; (2 ) transient dynami c uplif t generate d b y mantl e fluid flo w drive n b y th e ascendin g ho t plum e material (Nadi n e t al . 1995 , 1997) ; (3 ) permanent, isostaticall y compensate d uplif t generated b y crusta l thickenin g cause d b y igneous underplatin g associate d wit h mantl e plume activit y (Brodi e & Whit e 1994 , 1995 ; White & Lovel l 1997 ; Clif t & Turne r 1998 ; Clift 1999) . It i s likel y tha t on e o r mor e o f thes e mechanisms contribute d t o th e regiona l uplif t of th e norther n Britis h Isle s an d adjacen t area s (surrounding th e Britis h Tertiar y igneou s pro vince). Th e mai n developmen t o f Paleocen e sandstone reservoir s alon g th e axi s o f th e Faeroe-Shetland Basi n appear s t o hav e bee n synchronous wit h th e phase s o f therma l uplif t along th e basi n margi n an d pulse d volcanis m (White & Lovel l 1997 ; Naylo r e t al 1999) .

261

A simila r relatio n betwee n outbuildin g o f sandy deposit s an d th e episodi c natur e o f th e plume-related uplif t ha s bee n suggeste d fo r th e southern Nort h Se a area (Kno x 1996) . However, i t i s mor e difficul t t o se e how the y can hav e contribute d t o th e formatio n o f th e regional dom e i n souther n Norway . We sugges t that th e uplifte d are a relate d t o th e Paleocene Early Eocen e phas e o n th e Norwegia n sid e i s much narrower, covering the northwestern corner of souther n Norway , includin g th e onl y are a where early Tertiary volcanism has been reported (Vestbrona Formation , Bugg e e t al . 1980 ; Prestvik e t al . 1999) . Magmati c underplatin g added to relatively thic k continental crust and/or buoyancy fro m heate d lithospher e ca n bot h explain the uplift . The Late Paleocene-Early Eocene accelerate d subsidence in large parts of the North Sea basin is also difficul t t o explain . Whit e & Lati n (1993 ) and Hall & White (1994) claimed that a period of crustal extension occurred i n early Tertiary time . Minor norma l faultin g is observable in the basin to support this hypothesis. Skogseid e t al . (2000 ) relate d Paleocen e vertical movement s i n th e Nort h Atlanti c region t o th e arriva l o f th e Icelan d mantl e plume t o lithospheri c level s an d th e subsequen t lateral spreadin g o f th e plum e head . Th e thickness o f th e plum e bod y i s bot h a functio n of th e distanc e fro m th e plum e centr e an d th e structure o f th e overlyin g lithosphere . Th e ho t plume materia l preferentiall y fill s trap s a t th e base o f th e lithospher e create d eithe r b y previous plat e deformation s o r b y continuin g lithospheric thinnin g (Slee p 1996 , 1997) . I n their dynami c mode l th e centra l rif t zon e wit h the thinnes t lithospher e an d th e thickes t plum e body i s uplifte d an d eroded . I n region s wher e the lithospher e i s thic k wit h respec t t o th e thickness o f th e plum e bod y (e.g . th e Nort h Sea) significan t an d rapi d subsidenc e i s expected durin g th e plum e emplacement . Thi s subsidence shoul d theoreticall y b e followe d b y a rapi d rebound , resultin g i n ne t uplif t a s th e plume movemen t cease s (Skogsei d e t al . 2000). Rapid Eocene subsidenc e has been related to a decrease i n dynamic uplift cause d by a reduction in plum e activit y (Nadi n e t al . 1997) . Rapi d decay o f uplif t i s attribute d t o a temperatur e decrease o f th e plum e i n Earl y Eocen e tim e (c. 5 5 Ma), resultin g i n a decreas e i n dynami c uplift. Thi s even t coincide d wit h a decreas e o f the plum e activit y i n th e Britis h Tertiar y an d Greenland igneou s provinces , an d th e initiation of sea-floo r spreadin g betwee n Greenlan d an d NW Europe .

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Oligocene-Miocene domal uplift of southern Norway Important observations tha t have to be explaine d include: (1) Early Oligocene onse t of outbuilding from Norway-Denmar k t o Lofoten ; (2 ) con temporaneos uplif t o f th e easter n basi n flan k (offshore areas) ; (3 ) a break i n sedimentatio n a t the Eocene-Oligocen e transition ; (4 ) low velocity o r hotte r uppe r mantl e beneat h th e domes i n souther n and northern Norway . The doma l uplif t o f th e Norwegia n mainland probably starte d i n Lat e Eocen e t o Earl y Oligocene time , an d als o contribute d t o uplif t of th e shel f areas . Th e uplif t an d shallowin g i n the norther n Nort h Se a basi n continue d int o Miocene time . Severa l mechanism s hav e bee n suggested t o explain th e Neogene doma l uplif t of southern Norwa y an d simila r uplifte d area s around th e North Atlantic. Rohrman & va n de r Bee k (1996 ) propose d a model based on simple fluid dynamics and linked this to the elevated Nort h Atlanti c upper-mantl e thermal regim e surroundin g the Iceland hotspot . In thei r model , a n anomalou s hot , buoyan t asthenospheric layer or lens is present a t the base of the lithosphere. O n interaction of the hot (lowviscosity) asthenospher e laye r wit h th e col d (higher-viscosity) shiel d lithosphere , diapir s wil l start to form as a result of convective instabilities. The diapiris m wa s probabl y triggere d b y litho spheric stres s pattern s tha t cause d th e plat e reorganization a t c . 3 0 Ma. Th e mode l implie s that uplif t i s transien t an d wil l chang e t o subsidence whe n th e therma l anomal y decays . However, diapir s penetratin g th e lithospher e could generat e partia l meltin g an d produc e underplating, hereb y generatin g permanen t uplift. Th e area s o f uplif t ar e associate d wit h strong negativ e Bougue r gravit y anomalie s an d reduced lithospheri c P - an d S-wav e velocities , suggesting a n anomalou s mantl e structur e an d temperature underneat h th e dome s (Banniste r etal. 1991) . Plio-Pleistocene glacial erosion and uplift Important observation s tha t have to be explaine d include: (1) Upper Pliocene and older sequences tilted awa y fro m th e dom e i n sout h Norway; (2) Pleistocene sediments, nearl y flat-lying above an angular unconformity ; (3 ) the angular unconfor mity separatin g Pleistocen e an d Pliocen e sedi ments becomin g les s pronounce d toward s bot h the central North Se a and the shelf offshore midNorway; (4 ) accelerate d subsidenc e o f basi n centres adjacen t t o the uplifted landmasses .

The seismi c line s sho w tha t th e post-Eocen e sequences see m to be uniformly tilted away fro m the Scandinavia n dome , indicatin g tha t a significant par t o f th e Neogen e uplif t o f sout h Norway occurre d i n lates t Pliocen e an d earlies t Pleistocene times . Isostati c response t o unloading cause d b y glacia l erosio n contribute d significantly t o th e Neogen e uplif t (Rii s & Fjeldskaar 1992) . However, unloading cannot be the onl y operatin g mechanis m fo r th e sout h Norway Neogen e uplif t (Rii s 1996) .

Intra-plate stress: compressional deformation Tertiary epeirogen y i s ofte n attribute d t o compression tha t i s assume d t o b e relate d i n a general sens e t o Alpin e mountai n building . However, t o remov e c . 3k m o f sedimentar y rock fro m a basi n c . 100k m wid e require s > 15k m o f shortenin g (Brodi e & White 1994) . Minor Tertiary compressio n i s observed al l ove r the continenta l shelf, but nowher e is it sufficien t to accoun t for th e require d amoun t of uplif t an d erosion. I n addition , exhumatio n dramaticall y increases fro m sout h t o north , wherea s th e observed compressio n decreases markedl y in the same direction. Compressional structure s o f Cenozoi c age , including simpl e dome s o r anticlines , revers e faults an d broad-scale inversion , are widesprea d in th e Nort h Se a an d alon g th e N E Atlanti c margin (Fig s 1 5 an d 17 ) (Dor e & Lundi n 1996). A multiphas e growt h histor y ha s bee n reported fo r som e o f th e structure s a t th e Norwegian margi n (Mid-Eocen e t o Earl y Oligocene an d Miocen e times ; Dor e & Lundin 1996) an d i n th e Faero e regio n (Lat e Eocen e t o Early Oligocen e an d mid-Miocen e times ; Boldreel & Anderse n 1998 ; Anderse n e t al 2000). Vagne s e t a l (1998) , o n th e othe r hand , reported a surprisingl y constan t growt h rat e fo r the Orme n Lang e Dom e fro m earlies t Eocen e time t o th e present . Thes e studie s al l relat e th e Cenozoic compressiona l deformatio n t o tw o main sources : (1 ) far-fiel d effect s reflectin g episodes o f deformatio n i n th e Alpin e Orogen y and (2 ) ridge-pus h fro m th e Nort h Atlanti c mid-ocean ridg e system . Late Miocen e an d Pliocen e differentia l vertical movement s i n th e Nort h Se a basi n (increased subsidenc e rate s a t basi n centr e an d relative uplif t alon g basi n edges ) hav e als o been relate d t o change s i n intra-plat e stres s correlated wit h plate tectoni c adjustments in the Alpine hinterlan d an d th e Atlanti c spreadin g system (Cloeting h e t al . 1990 ; Gallowa y e t al . 1993).

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Summary and conclusions

References

This pape r ha s focuse d o n th e Cenozoi c evolution o f th e norther n Nort h Se a an d surrounding areas , wit h emphasi s o n sedimen t distribution, composition an d provenance as well as o n timing , amplitud e an d wavelengt h o f differential vertica l movements . Th e dat a an d modelling result s suppor t a probabl e tectoni c control on sediment suppl y and on the formation of the regional unconformities . The sedimentar y architecture an d break s ar e relate d t o tectoni c uplift o f surroundin g clasti c sourc e areas ; thus , the offshore sedimentary recor d provide s the best age constraint s o n th e Cenozoi c exhumatio n of the adjacen t onshor e areas . However , climati c and glacio-eustati c change s als o playe d a rol e and fo r som e event s it is difficul t t o separat e th e effects o f tectonics an d eustasy . Cenozoic exhumatio n i s documente d o n bot h sides of the North Sea , but the timing is not well constrained. Two major uplif t phase s ar e clearly reflected b y th e availabl e data : (1 ) a n earl y Paleogene (Late Paleocene-Early Eocene) phase and (2 ) a late Neogene (Plio-Pleistocene ) phase. The firs t i s relate d t o rifting , magmatis m an d break-up i n th e N E Atlanti c associated wit h the arrival o f the Icelan d plum e an d th e subsequent lateral spreadin g of the plume head. The latter is related t o th e isostati c respons e t o unloadin g caused b y glacial erosio n durin g th e widesprea d Northern Hemispher e glaciations . However , other uplif t event s interpose d betwee n thes e episodes, i n particula r o n th e Scandinavia n side during Oligocen e an d Miocen e times . Som e o f these may also be related to mantle processes and the episodi c behaviou r o f th e Icelan d plume . Intra-plate stres s relate d t o ridge-pus h fro m th e North Atlanti c plat e boundar y and/o r t o Alpin e compression als o contribute d t o differentia l vertical movements , bu t canno t b e th e mai n uplift mechanis m for the long-wavelength domal uplifts.

ABREU, V.S . & ANDERSON , J.B. 1998. Glacial eustasy during th e Cenozoic ; sequenc e stratigraphi c implications. AAPG Bulletin, 82 , 1385-1400. ALLEN, PA . & ALLEN , J.R . 1990 . Basin Analysis; Principles an d Applications. Blackwel l Scientific , Oxford. ANDERSEN, M.S. , NIELSEN , T. , NIELSEN , T. ,

The pape r i s base d o n wor k withi n th e projec t 'Tectonic impact o n sedimentary processe s in the postrift phase—improve d models' . Th e projec t wa s supported b y the Research Counci l of Norway through grant 32842/211 . The author s would like to thank the companies participatin g in the project (Amoc o Norway Oil Company , de n Norsk e Stat s Oljeselska p (Statoil) , Mobil Exploratio n Norwa y Inc. , Nors k Agi p A/S , Norsk Hydr o ASA , Phillip s Petroleu m Compan y Norway, Sag a Petroleu m ASA) . Th e seismi c dat a were kindl y made availabl e by TGS-NOPEC . W e are also gratefu l t o D . Mudg e an d K . G . R0sslan d fo r reviewing the paper an d for suggesting improvements.

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Scotland's denudational history: an integrated view of erosion and sedimentation at an uplifted passive margin ADRIAN HALL 1'2 & PAUL BISHOP3 Department of Geography, University of Edinburgh, Drummond Street, Edinburgh EH8 9XP, UK 2 Fettes College, Carrington Road, Edinburgh EH4 1QX, UK (e-mail: am.hall@ fettes.com) 3 The CRUST Project, Department of Geography and Topographic Science, University of Glasgow, Glasgow G12 8QQ, UK 1

Abstract: Denudationa l histor y i s commonly reconstructe d fro m basi n sediment s derive d from th e denude d sourc e area , an d les s frequentl y fro m th e sourc e are a itself . Norther n Britain i s a n importan t sourc e are a fo r th e surroundin g sedimentar y basin s an d thi s pape r reviews the erosional histor y o f Scotland fro m Devonian time to the present using evidenc e both fro m onshor e geolog y an d geomorpholog y an d fro m pattern s o f sedimentatio n i n surrounding basins. Cover rocks were extensive in Scotland durin g late Palaeozoic tim e but the persistenc e of sedimen t sourc e area s withi n the uplan d area s of Scotlan d make s it unlikely tha t basemen t high s wer e eve r completel y buried , an d depth s o f post-Devonia n erosion of basement have been correspondingly modest (< 1 -2 km) . During Mesozoic time, Scotland experience d severa l majo r erosional cycles , beginnin g with uplift, reactivatio n of relief an d strippin g o f cove r rocks , followe d b y progressiv e reductio n o f relie f throug h etchplanation an d culminatin g i n extensiv e marin e transgression s i n Lat e Triassic , Lat e Jurassic an d Late Cretaceou s time . Mid-Paleocene pulses of coarse sedimen t t o the Mora y Firth Basi n coincide d wit h majo r uplift. Thi s uplif t wa s associate d wit h major differentia l tectonics withi n th e Highlands, wit h warping an d faulting alon g th e margins o f the Minc h and th e inne r Mora y Firt h Basins . Tectoni c activit y was renewe d o n a lesse r scal e i n lat e Oligocene tim e an d continued into Late Neogene time . Differential weathering an d erosion under the warm to temperate humid climates of Neogene time created th e major elements of the preglacial relief, wit h formation o f valleys, basins, scarps and inselbergs, feature s ofte n closely adjuste d to lithostructura l control s and , i n som e cases , wit h precursors tha t can b e traced bac k t o Devonia n time . Th e histor y tha t ca n b e 'read ' fro m th e onshor e regio n complements th e source area history interpreted from sedimentar y basins derived from thes e

areas

The nature and rate of deposition of sedimentary determin e the exten t t o whic h potential energy basin sequence s depen d o n man y factors , provide d by uplif t ca n b e converte d to kineti c including rate s o f basi n subsidence , sea-leve l energy , namely, th e geomorphologica l character history an d sourc e area characteristics , such a s o f th e sourc e area, an d th e exten t t o whic h th e climate history, lithology and uplift rates . Source sourc e are a 'knows ' abou t th e uplif t event . A area uplif t i s generall y interprete d t o b e high-elevation , upliftin g sourc e are a tha t i s associated wit h a n essentiall y instantaneou s efficientl y connecte d to base leve l will generate 'basin' signa l of high rates of flu x o f sediment s hig h volume s of sedimen t via a combination of that hav e experienced limited chemica l weath - rive r incision , mass movement from stee p valley ering. Althoug h i n som e cases suc h a respons e side s and efficien t evacuatio n o f sediment . The may be demonstrable (e.g. Copeland & Harrison southward s drainin g river s o f th e Himalaya s 1990), th e degre e t o whic h majo r uplif t i s offe r excellen t examples of suc h system s (e.g. signalled by a sedimentary pulse depends on the Burban k e t al 1996 ; Hancoc k e t al 1998) , extent t o whic h the potentia l energ y associated whic h contras t markedl y wit h th e system s with hig h elevation ca n b e converte d int o th e drainin g northwards fro m th e Himalaya s to th e kinetic energ y (and henc e th e strea m power ) high-elevation , bu t low-energ y an d internall y necessary t o detach , entrain and transpor t high drained , Tibeta n platea u (Summerfiel d & volumes o f sediment . Two interrelate d factors Brow n 1998) . From: DORE , A.G., CARTWRIGHT, J.A., STOKER, M.S. , TURNER, J.P . & WHITE , N . 2002 . Exhumation of th e North Atlantic Margin: Timing, Mechanisms and Implications for Petroleum Exploration. Geological Society, London, Special Publications, 196, 271-290. 0305-8719/027$ 15.00 © The Geological Society of London 2002.

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The mos t rapidl y upliftin g areas, suc h a s th e Southern Alp s (Hoviu s 2000 ; Tippett & Hoviu s 2000), Himalaya s (Fieldin g 2000) , Japa n (Ohmori 2000 ) an d Taiwa n (Li n 2000) , ar e associated wit h plate convergenc e zones . O n the other hand , passive margi n uplands , suc h a s the southern Africa n upland s (Brow n e t al 2000) , the Wester n Ghat s i n Indi a (Gunne l & Fleitou t 2000), th e S E Australia n highland s (Bisho p & Goldrick 2000) , an d th e Scottis h Highland s portion o f the western European Atlantic margin, are commonl y associate d wit h lo w t o ver y lo w rates o f denudatio n an d sedimen t flux . Flemin g et a l (1999 ) an d Cockbur n e t a l (2000) , fo r example, hav e reporte d ver y lo w rate s o f denudation fro m th e souther n Africa n passiv e margin highlands , o f th e sam e orde r o f magnitude a s thos e reporte d fro m th e S E Australian upland s from mas s balance , geomorphological an d thermochronologica l studie s (Bishop 1985 ; Bisho p & Goldrick 2000). Bishop & Goldrick (2000 ) also reported widesprea d and persistent disequilibri a i n S E Australia n rive r long profiles, reflecting the low gradients and low stream powe r o f this margin's drainag e systems . These long profil e disequilibria ca n be attribute d to passiv e denudationa l reboun d (Bisho p & Brown 1992 ; Bishop & Goldrick 2000) , wit h the margin evidentl y no t havin g experience d activ e tectonic uplif t durin g Cenozoi c tim e apar t fro m temporary uplif t event s relate d t o transien t thermal effect s a t th e centra l volcanoe s tha t mark easter n Australia' s Cenozoi c passag e ove r mantle hotspot s (Wellma n & McDougal l 1974 ; Wellman 1986 ; McDougall & Duncan 1988; Sun etal 1989) . The tectoni c characte r an d historie s o f mos t of th e passiv e margin s descibe d abov e hav e been reconstructe d largel y fro m subaeria l terrestrial data , wit h relativel y littl e relianc e on th e sedimentar y basi n record . Th e sedimen tary recor d ha s bee n use d mainl y fo r mas s balance studie s o f thes e margin s t o determin e rates o f sourc e are a subaeria l denudatio n (e.g . Bishop 1985 ) o r a s a guid e t o th e evolutio n o f the rive r system s (e.g . Rus t & Summerfiel d 1990). Rate s o f sourc e are a denudatio n ma y also b e determine d mor e directl y fro m th e source are a itsel f usin g geomorphologica l studies (e.g . Bisho p 1985 ; Not t e t a l 1996) , cosmogenic isotop e analysi s (Fleming e t a l 1999; Cockbur n e t a l 2000) , an d low temperature thermochronologica l techniques , such a s apatit e fission-trac k analysi s (Gleado w & Brow n 2000) . Conflicts , whic h ar e no t ye t fully resolved , ar e ofte n apparent , however , between geomorphologica l interpretation s o f source are a histor y an d thermochronologica l

approaches t o sourc e are a denudatio n (Koh n & Bishop 1999) . Reconstruction of the evolution of the Scottish Highlands (Fig . 1 ) o n th e Wester n Europea n continental margi n ha s relie d o n bot h offshor e and onshor e data , wit h ofte n muc h greate r emphasis o n th e offshor e record , n o doub t because o f th e wealt h an d qualit y of thes e dat a (see Jones e t al 2002) . Ther e i s a corresponding wealth o f dat a fro m onshor e areas , an d i n thi s paper w e re-examin e th e post-Palaeozoi c geo morphological histor y of th e Scottis h Highlands as a sourc e are a fo r surroundin g basins , especially th e mai n sedimen t receivin g area, th e North Se a Basin . W e hav e tw o aims : (1 ) t o summarize criticall y th e onshor e dat a o n th e evolution o f th e Scottis h Highlands , fo r a readership tha t migh t no t b e full y awar e o f th e literature on this topic; (2) to assess the extent to which sourc e are a uplift , denudatio n an d geomorphological development , an d landscap e antiquity, ca n b e 'read ' fro m th e sourc e are a itself, thereb y complementin g th e offshor e record.

An outline of the history of the Highlands source regio n from Palaeozoic time to the present The dispositio n an d provenanc e o f (ofte n thin ) remnants of Devonian sediments show that many key morphotectoni c element s o f th e curren t Highlands relie f wer e alread y establishe d b y the end of Devonian time (Fig. 2). These includ e the main Grampia n watershed, the linear depressio n of th e Grea t Glen , th e larg e basin s o f N E Scotland an d majo r valle y system s draining NE towards th e Mora y Firt h alon g th e Caledonia n fracture zones . Th e Caledonia n mountain s ha d been eroded , exposin g man y lat e Caledonia n Newer Granites , togethe r wit h som e olde r intrusions (Watso n 1985) . Th e Caledonia n granites wer e intrude d int o alread y stabilize d crust o r thei r intrusio n complete d th e stabiliz ation proces s (Leak e & Cobbin g 1993) . Recon struction o f th e sub-Devonia n relie f aroun d th e inner Moray Firth implies surfaces of high relief, with fault-bounde d half-basins an d fault-guided valleys partl y infille d wit h conglomerate s an d sandstones. Thes e Devonia n fill s wer e largel y removed betwee n lat e Palaeozoi c tim e an d th e present s o tha t th e curren t leve l o f erosio n lie s close t o tha t a t th e en d o f Devonia n tim e (se e Leake & Cobbin g 1993) . A simila r equivalenc e exists i n th e N W Highlands , wher e th e presen t terrain lie s a t th e sam e genera l elevatio n a s th e base of the Torridonian sequence (Watso n 1985) .

SCOTLAND'S DENUDATIONA L HISTOR Y

N

Fig. 1. Scotland, showing sites and features mentioned in the text.

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Fig. 2 . Indicators o f post-Devonian depth s o f denudation (after Hal l 1991) .

There i s little direc t evidenc e o f event s i n th e Highlands durin g th e Carboniferou s period , a s rocks o f thi s ag e ar e restricte d t o th e margina l basins o f th e Midlan d Valle y an d th e Mora y Firth. Th e considerabl e thicknesse s o f Carbon iferous sediments, wit h up to 4 km in the Midlan d Valley (Franci s 1991 ) an d 1.5k m i n th e oute r Moray Firt h (Andrew s et al. 1990) , wer e largel y sourced fro m th e Highlands . Th e Westphalia n outliers i n Morver n restin g o n th e Moin e sequence impl y a former cove r o f Carboniferous rocks i n part s o f th e S W Highland s (Franci s 1991). A reworke d Carboniferou s microflor a i s present i n Jurassi c rock s a s fa r wes t a s th e onshore outcrop s i n the inner Mora y Firt h Basi n (Andrews e f al . 1990) . The Highland s ar e generall y show n a s a n emergent an d exposed basemen t are a i n palaeogeographical map s o f th e Carboniferou s perio d (Guion e t al . 2000 ) bu t th e exten t an d dept h o f Carboniferous denudatio n i n th e Highland s remain unclear . Recen t apatit e fission-trac k studies impl y tha t a s muc h a s 3k m o f lat e

Palaeozoic cove r rocks hav e been remove d fro m the Highland s are a (Thomso n e t al . 1999 ) bu t deep Lat e Palaeozoi c erosio n appear s incompa tible with the widespread surviva l of near-surface volcanic an d intrusiv e rock s o f lat e Carboni ferous ag e (Watso n 1985 ; Hal l 1991) . Volcani c activity continue d throughout Permian tim e an d is represente d i n th e Highland s b y th e campto nite-monchiquite dyk e swarm s o f Orkne y an d the Wester n Highland s (Franci s 1991) . Associ ated uplif t wa s probabl y limite d a s th e tota l volume o f magm a wa s smal l (Watso n 1985) . This accord s wit h the surviva l of Carboniferous deep weatherin g mantle s i n norther n Scotlan d that acted a s a major source o f kaolinitic detritu s for Jurassi c sediment s i n th e inne r Mora y Firt h (Hurst 1985a) . Alternating period s o f moderat e uplift , reduction o f relie f an d marin e transgressio n affected th e Highland s durin g Mesozoi c tim e (Hall 1991) . Triassi c sediment s ar e u p to 500m thick agains t th e Grea t Gle n Faul t bu t thi n t o 150m around Elgin (Frostick et al. 1988) . By the

SCOTLAND'S DENUDATIONA L HISTOR Y

end o f th e Triassi c perio d uplif t ha d cease d an d relief wa s considerabl y reduced , an d th e oute r Moray Firt h forme d par t o f a n extensiv e continental plai n o f lo w relie f (Andrew s e t al 1990). Calcrete s an d silcrete s forme d an d th e Rhaetic Se a transgresse d clos e t o th e presen t margins o f th e inne r Mora y Firth . Continue d transgression i n Earl y Jurassi c tim e sa w th e deposition o f fluviatil e san d aroun d th e margin s of th e Mora y Firth . San d an d cla y mineralog y suggests derivatio n dominantl y fro m Devonia n and Carboniferou s cove r rock s t o th e nort h (Hurst 1985a ) an d fro m Moinia n chloriti c metasediments t o th e sout h (Hurs t 1985b) . Thermal domin g i n Mid-Jurassi c tim e i n th e Moray Firt h Basi n cause d dee p truncatio n o f Early Jurassi c and older sediment s an d sediment transfer t o th e Vikin g Grabe n an d inne r Mora y Firth. Crustal collapse i n the central North Sea in Callovian tim e wa s accompanie d b y rapi d sedimentation i n th e inne r Mora y Firt h an d synsedimentary movements along the Helmsdal e Fault (Anderto n e t al . 1979) . Margina l marin e sands oversteppe d the current basi n margin s an d may hav e covere d th e axi s o f th e Grea t Gle n (Hallam & Sellwood 1976 : Wignal l & Pickering 1993). Th e fault s controllin g sedimentatio n i n the inne r Mora y Fort h appea r t o have als o bee n active o n th e adjacen t lan d are a (Robert s & Holdsworth 1999) . Tectonic activit y was renewed a t the JurassicCretaceous boundar y i n th e Mora y Firt h Basin . Uplift o f th e Halibu t Hors t le d t o erosio n o f Carboniferous sandstones . Fault scarps alon g the northern margi n o f th e inne r Mora y Firt h generated coars e mas s flo w deposit s (Anderto n et al . 1979) . Earl y Cretaceou s sediment s late r overstepped th e Helmsdal e Faul t nort h o f Helmsdale t o overli e Jurassi c an d Devonia n sediments (Cheshe r & Lawso n 1983) . I n Morvern, Cretaceous greensand s rest on Moinian schists (Georg e 1966) . A smal l outlie r o f lat e Hauterivian-early Barremia n glauconiti c sand stone rest s o n Devonia n an d basemen t rock s i n eastern Bucha n (Hal l & Jarvis 1994) . By Lat e Cretaceou s tim e th e Highland s ha d been reduce d t o a n are a o f relativel y lo w relief . Cretaceous sequence s alon g th e easter n margi n of th e Hebride s basi n ar e thin, implyin g limite d sediment supply , an d li e clos e t o se a level , implying tectoni c stabilit y (Hancoc k 2000) . Terrigenous sedimentatio n cease d i n th e Mora y Firth wit h th e depositio n o f thic k chal k sequences. O n land , th e sub-Cenomania n sur face, before transgression, carried dee p kaolinitic weathering mantles , late r reworke d t o for m th e highly quartzose sands of Lochaline (Humphrie s 1961) an d th e kaoliniti c Paleocen e sand s an d

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muds of the inner Moray Firth (Carman & Young 1981). The extent of marine transgression i n Late Cretaceous tim e i s unclear bu t depositio n o f th e chalk i n depth s o f severa l hundre d metre s o f water (Hancoc k 1975 ) suggest s that only a small area o f th e Highland s ca n hav e escape d submergence. Around 6 0 Ma, th e passag e o f th e Icelan d plume wa s accompanie d b y majo r magmati c activity in western Scotland (Bell & Jolley 1997) . Magmatism involve d emplacemen t o f igneou s centres, extrusio n o f floo d basalt s wel l beyon d the presen t outcro p an d injectio n o f regiona l dyke form s t o for m th e Tertiar y Igneou s Province. Th e perio d o f magmatis m wa s brief , concentrated betwee n 6 1 an d 5 5 Ma (Jolle y 1997), an d in individual igneous centre s volcanism wa s largel y confine d t o singl e palaeomag netic polarit y interval s o f 0.4- 3 Ma (Musse t 1984). Accelerate d san d accumulatio n i n th e Moray Firth Basin (Liu & Galloway 1997 ) can be linked via sediment routeways and provenance to erosion o f uplifted sourc e area s o n the OrkneyShetland Platfor m an d in the Highlands (Jone s & Milton 1994) . Sediment flux reached a maximum in Late Paleocene tim e and declined int o Eocene time (Jo y 1993 ; Whit e & Lovell 1997) . Small outlier s o f thi n Cretaceou s sequence s occur o n bot h th e wester n (Hancoc k 2000 ) an d the easter n (Hal l & Jarvis 1994 ) margin s o f th e Highlands. A s an y emergen t area s o f th e Highlands ha d bee n reduce d t o lo w relie f b y the end o f Cretaceous tim e (Hal l 1991) , patterns of Tertiar y uplif t ca n b e reconstructe d usin g th e present summi t topograph y o f th e Highland s (Fig. 3) . Th e distributio n o f summit s abov e 800 m defines a zone of maximum uplift; terrai n that no w form s th e mai n watershed s o f th e N W Highlands an d o f th e Grampia n Mountains . I n Northern Scotland , hig h summit s overloo k th e sedimentary basin s o f Th e Minc h an d th e innermost Mora y Firth , basin s wit h margin s that retain attenuate d and localized sequence s of Mesozoic sediment s (Fig . 4) . Majo r differentia l tectonics i s implie d betwee n th e Highland s an d the surroundin g basins . Earl y Tertiar y reactiva tion o f th e Helmsdal e Faul t produce d a majo r fault scarp , no w marke d b y th e lin e o f hill s between Be n Wyvi s an d Helmsdale . Anothe r major escarpmen t existe d i n Early Tertiar y tim e on th e wes t coas t o f th e Norther n Highlands , stretching fro m th e Cuillins to Cape Wrath. Thi s escarpment i s no w dissecte d int o a chai n o f isolated hill s an d hil l groups , includin g th e inselbergs o f Suilve n an d Quinag . It s alignment runs parallel to the edge of the Minch Basin but it is not fault controlled , implyin g that uplift o f the NW Highland s wa s associated wit h significan t

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Fig. 3 . Patterns o f Tertiary uplif t implie d b y Highlan d summi t height s (i n hundreds of metres) . Fin e line s give locations o f diagrammatic cross-section s i n Fig. 4 (longer section lines ) and Fig. 5 (shorter sectio n lines).

warping o n it s wester n margin . Sout h o f th e Great Glen, ther e is also evidence o f differentia l uplift. I n Buchan the preservatio n o f Cretaceou s chalk flint s an d greensan d demonstrate s modes t Tertiary uplif t ye t the Cairngorms, onl y 50 km to the west , eve n toda y reac h 1300 m (Fig . 5) . Differential movement s ar e required , wit h possible downwarpin g toward s th e eas t i n th e Dalradian bel t betwee n Ballate r an d Keit h (Ringrose & Migo n 1997 ) an d dislocatio n a t the easter n edg e o f th e Mount h an d th e Hil l o f Fare (Hall 1987) . O n th e southwester n edg e o f the Wester n Grampian s lie s a zon e o f lowe r summits, centre d o n Cowal , wher e th e presenc e of vesicula r dyke s (Gun n e t al. 1897 ) suggest s relative proximit y t o th e Earl y Tertiar y lan d surface (Fig . 4) . Th e preservatio n o n th e Lom e Plateau o f a smal l Carboniferou s outlie r a t Bridge o f Awe , restin g o n Devonia n lava s (Johnstone 1966) , i s noteworthy , a s ar e th e fragments o f sub-Triassi c surface s (Godar d 1965) foun d i n Morvern . Thes e occurrence s together impl y tha t post-Caledonia n vertica l movements o f the Cowal peninsula and adjacent areas hav e bee n modes t whe n compare d wit h

those that have affected th e main area of the S W Grampians. In earl y Eocen e tim e th e Highlan d are a foundered a s i t move d awa y fro m th e Icelan d plume (Nadi n & Kuszni r 1995 ) an d sedimen tation rate s droppe d i n th e Nort h Se a (Li u & Galloway 1997) . This coincided with the onset of a period , c . 2 0 Ma i n duration , o f humi d an d initially subtropica l conditions , an d apparentl y limited uplift. Dee p kaolinitic weathering covers probably develope d widel y i n associatio n wit h extensive erosion surface s (Hall 1991) . Tectonic activity wa s resume d throughout NW Europ e i n Late Oligocen e time , wit h th e onse t o f majo r uplift o f Fennoscandia (Rohrman et al. 1995 ) and basin developmen t throughou t wester n Britain , including th e Hebridea n regio n (Fyf e e t al . 1993). Th e presenc e o f depositiona l hiatuse s west o f th e Shetland s (Rid d 1981) , deltai c an d lignitic sands east of the Shetlands (Johnson et al. 1993) an d unconformitie s i n th e centra l Nort h Sea (Gatliff e t al. 1994 ) indicate significant uplif t in th e Scottis h are a an d associate d erosio n an d enhanced sedimen t suppl y (Li u & Gallowa y 1997).

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Fig. 4 . Schematic cross-section s illustratin g major morphotectoni c units along two traverses across the Scottis h Highlands.

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Fig. 5 . Schemati c cross-section s illustratin g differentia l tectonic s an d deundatio n i n th e Tertiar y Igneou s Province: Sky e an d Mull (fro m variou s dat a sources) .

Early t o mid-Miocen e tim e wa s th e secon d long perio d o f relativ e tectoni c stabilit y i n th e Tertiary period, lasting for over 1 0 Ma (Le Coeur 1999). Kaoliniti c weatherin g resume d unde r humid, war m t o temperat e condition s (Bersta d & Dypvik 1982) and it is possible that this was an important period for the formation, extension and remodelling o f erosio n surface s b y etc h processes. Miocen e marin e clays , u p t o 8 m thick , together wit h Cretaceous sandstone , ar e reporte d from Leavad , Caithness, as part of a large glacial erratic transporte d fro m th e Mora y Firt h (Crampton & Carruthers 1914) . The occurrenc e appears t o demonstrat e Miocen e marin e sedimentation i n the inne r Moray Firth . A marke d chang e occur s i n th e cla y miner alogy o f Nort h Se a sediment s i n Lat e Miocen e time. A n increasin g conten t o f feldspar, chlorit e and illite (Karllson et al. 1979; Bersta d & Dypvik 1982) reflect s climatic coolin g an d th e inpu t of immature terrigenous material. This material was increasingly sourced first from Fennoscandi a an d later fro m th e Rhin e an d Balti c rive r systems , reflecting uplif t o f source areas. The significance of Neogen e uplif t i n th e morphogenesi s o f northern Britai n ha s lon g bee n recognize d (George 1966) , althoug h it s scal e an d patter n are a s ye t uncertain . Exhumatio n o f th e Chal k from beneat h c . 1 km i n th e inne r Mora y Firt h appears t o hav e bee n achieve d largel y i n Neogene time (Japsen 1997 ; Japsen & Chalmer s 2000) and implies contemporaneou s uplif t o f the Scottish Highlands . Geomorphological evidenc e for Lat e Tertiar y tectonic s i s provide d b y th e apparent warpin g o f mid-Tertiar y erosio n surfaces i n norther n Scotlan d (Godar d 1965) , th e uplift, warpin g an d dislocatio n o f Lat e Tertiar y

surfaces i n wester n Scotlan d (L e Coeu r 1988, 1999) an d th e widesprea d evidenc e o f valle y incision and deepening of topographic basins set into mid-Tertiary erosion surface s throughout the Highlands a t this time (Hall 1991) . The influ x o f ice-rafte d materia l t o th e Hebridean margi n a t 2. 5 Ma mark s th e onse t of mid-latitude climati c deterioration (Stoke r el al. 1994). Episodi c mountai n glaciation i s likel y t o have occurre d thereafte r (Clapperto n 1997 ) but the firs t ic e sheet s reache d th e Nort h Se a Basin only afte r 1 Ma (Andrew s et al. 1990) . Multiple glaciation o f th e Highland s an d th e adjacen t shelves during the Quaternary period brought the transfer o f sedimen t fro m curren t lan d an d nearshore are a to the axia l area of the North Se a Basin an d t o th e continenta l shelve s (Clayto n 1996). On land, each glaciation tended to remove the deposit s o f it s predecesso r s o tha t onl y th e deposits of the last (Late Devensian) ice sheet are usually preserved . The impac t o f th e Quaternar y glaciation s has varie d i n tim e an d space . Th e volum e o f sediment deposite d i n th e centra l Nort h Se a indicates tha t sedimentation , an d hence denuda tion o f th e adjacen t lan d masse s an d shelves , doubled betwee n th e dominantl y non-glacia l conditions of Pliocene and early Pleistocene time and th e condition s o f episodi c ic e shee t glaciation ove r the las t 1 Ma. Th e las t ic e sheet s during Lat e Quaternar y tim e wer e als o les s effective agent s o f erosio n an d transportatio n than those of mid-Quaternary time, reflecting th e earlier remova l o f pre-glacia l weathere d rock , the progressiv e adaptatio n o f th e glacie r be d t o the efficien t evacuatio n o f ice , and th e greate r thickness an d extent of the Elsterian an d Saalia n

SCOTLAND'S DENUDATIONAL HISTOR Y

ice sheet s (Glasse r & Hal l 1997) . Th e Scottis h Highlands als o exhibi t a wid e rang e o f glacia l landscapes, fro m th e deepl y dissecte d terrai n of the wester n Highland s t o th e zone s o f selectiv e linear erosio n o f the Cairngorms an d the limite d erosion o f th e Bucha n lowland s (Linto n 1959 ; Clayton 1974) . Th e averag e dept h o f glacia l erosion acros s Britai n is estimated a t 76 m, with 175m i n mountainou s zone s o f intens e erosio n and a s little a s 1 5 m in zones o f slow-movin g or cold-based ic e (Clayton 1996). This is equivalent to a volum e les s tha n th e tota l amoun t o f Quaternary sedimen t o n the shelve s surrounding Britain, implyin g tha t a significan t componen t has bee n derive d fro m th e dee p erosio n o f material fro m th e inne r shelve s (Clayto n 1996) , including the inner Moray Firth an d The Minch . Mass transfe r o n thi s scal e mus t hav e cause d isostatic uplift i n the glaciated mountai n areas of western and northern Britain. The depth of som e of th e wes t coas t fjord s ma y reflec t glacia l incision int o th e still-risin g edg e o f th e N W Highlands. Former cover rocks in the Scottish Highlands Palaeogeographical map s o f the Highland s hav e tended t o sho w th e are a a s a persisten t topographic hig h (e.g . Anderto n e t al. 1979 ; Ziegler 1981) , bu t th e exten t an d thicknes s o f former cove r rock s i n th e Scottis h Highland s remain controversial . Th e regio n i s routinel y seen a s a sourc e are a fo r sediment s tha t hav e accumulated i n th e surroundin g basins . Thi s seems consisten t wit h evidenc e o f relativel y modest depth s (
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Current model s o f Highlan d sourc e are a evolution base d o n AFT T (Thomso n e t al . 1999) sugges t that : (1 ) lat e Palaeozoi c cove r rocks u p t o 3k m thic k formerl y covere d th e basement rock s o f th e Highlands ; (2 ) Tertiar y erosion has removed 1- 2 km of rock from abov e the current topography. There i s littl e doub t tha t lat e Palaeozoi c rocks onc e covere d considerabl y large r area s of the Highland s massi f tha n a t present . Devonia n outliers occu r widel y aroun d th e coasta l ri m o f the Mora y Firt h an d reac h thicknesse s o f several hundre d metres i n fault-bounde d basins. Carboniferous rock s formerl y extende d acros s part o f th e S W Highland s (Georg e 1960 ) an d reach a thicknes s o f ove r 1 km i n th e inne r Moray Firt h (Thomso n e t a l 1999) . Ye t i t i s unlikely tha t lat e Palaeozoi c rock s onc e covered al l o f th e Highland s an d henc e pre Mesozoic overburde n thicknesse s o f 2- 3 km seem unreasonable . Th e Lowe r an d Middle Ol d Red Sandstone s aroun d th e Orcadia n Basi n were largel y derive d fro m mountain s site d i n the are a o f th e curren t Easter n Grampian s an d Western Highland s (Mykur a 1983 ) and ther e i s little obviou s sig n tha t thes e mountai n area s were eventuall y wor n down sufficientl y t o b e buried. Typically , a regiona l unconformit y separates th e Uppe r Ol d Re d Sandston e fro m older sediment s an d thi s i s ascribe d t o regiona l tectonics (Mykur a 1983) . I n Morayshire, o n th e margin o f th e Orcadia n Basi n wher e uplif t might be expecte d t o be limited , th e Uppe r Old Red Sandston e rest s i n place s directly o n th e Moine sequenc e (Hom e 1923) , indicatin g removal o f th e Lowe r an d Middl e Ol d Re d Sandstone before deposition . A simila r situation occurs i n th e Midlan d Valley , where th e Uppe r Old Re d Sandstone , includin g conglomerate s with pebble s o f metamorphi c rock s derive d from th e Souther n Highlands, rests with marked angular unconformit y o n th e Lowe r Ol d Re d Sandstone (Franci s et al 1970) . B y implication , substantial remova l o f th e Lowe r an d Middl e Old Re d Sandston e ha d bee n achieve d befor e the en d o f Devonia n time . The Highland s als o acte d a s a sourc e regio n for Carboniferou s sediment s i n th e Midlan d Valley (Franci s 1991 ; Guio n e t al, 2000 ) an d Namurian-Westphalian sandstone s i n th e Stir ling distric t contai n heav y mineral s ultimatel y derived from low-grade metamorphic rocks north of th e Highlan d Boundar y Faul t (Franci s e t a l 1970). Alon g th e easter n margi n o f th e Minc h Basin numerou s smal l outlier s o f Permo-Tria s occur. Only at Inninmore on the Sound of Mull is Carboniferous sedimen t foun d underlyin g Tria s units an d here th e Carboniferou s rock s ar e only

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100-160m thick . Elsewher e th e Permo-Tria s sequence rest s o n olde r rock s (Johnston e & Mykura 1989) . B y implication , th e Carbonifer ous rock s wer e remove d befor e th e star t o f th e Trias perio d o r wer e neve r lai d dow n t o an y significant thicknes s alon g th e wester n edg e o f the Northern Highlands . The thic k sequence s o f Mesozoi c clasti c sediments i n th e centra l Nort h Sea , whic h might b e take n t o indicat e dee p erosio n o f th e Highlands, deriv e onl y i n part fro m th e Scottis h area. A majo r contributio n o f materia l fro m Fennoscandia occurre d durin g Triassic an d early Jurassic time (Ziegle r 1981) . Erosio n o f intra basinal high s als o provide d sedimen t (Andrew s et al. 1990) . Ye t th e presenc e o f igneou s an d metamorphic debri s i n sediment s a t the margins of the Highland s implies continuin g erosion an d thus exposur e o f th e basemen t o f th e Highland s area (Hudso n 1964 ; Hurst 1985a , 1985b) . Th e volume o f debri s als o implie s a significant , but unknown depth of erosion. I n the Minch Basin, it is possibl e to quantif y depth s of Permia n to Jurassic erosio n o f th e terrai n surroundin g th e Sea of the Hebrides Trough . Preserve d sedimen t volumes ar e clos e t o th e originall y deposite d volumes (Stee l 1978 : Fyf e e t a l 1993 ) an d suggest tha t c. 280 m o f roc k wa s remove d i n Permo-Trias tim e an d c . 210 m i n Jurassi c tim e from th e mai n contributin g are a o f th e souther n Outer Hebride s Platform . Thes e relativel y lo w values ar e consisten t wit h th e remova l o f onl y thin cover from the Highlands . Depths o f Tertiar y erosio n fro m th e sourc e area o f th e Mora y Firt h Basi n i n th e Highland s can be quantified using sedimen t volume s in the North Sea . The Nort h Se a acte d a s a sedimen t trap throughou t Tertiar y tim e (Li u & Gallowa y 1997), wit h onl y a narro w an d shallo w connec tion t o th e Norwegian-Greenlan d Se a (Nielse n et al. 1986) . In early Tertiar y tim e the dominant sediment sourc e wa s the Scottis h Highlands an d the Orkney-Shetlan d Platfor m bu t throughou t Neogene tim e sediment was increasingly derived from Scandinavi a an d th e grea t rive r system s o f NW Europe . Roug h calculation s indicat e th e removal o f 600-800 m o f roc k fro m th e contributing are a eas t o f th e mai n Scottis h watershed durin g Tertiar y tim e (Hal l 1991) . A s only 25% of the Scottish lan d area now lies above 300m (Haynes 1983) , i t appears that the summit envelope surfac e o f th e Highland s a t 900 1200m (Fig . 3) lie s onl y a fe w hundre d metre s below th e uplifte d sub-Cretaceou s lan d surface . On these estimates, there appears to be a good fit between th e volume o f rock remove d b y erosio n from the Highlands and that received i n the North Sea Basin . I n th e inne r Mora y Firth , soni c

velocities i n th e Kimmeridg e Cla y indicat e removal o f aroun d 1 km o f overburde n (Hilli s et a l 1994 ; Thomson & Hilli s 1995 ) and thi s seems generally compatible with the existence of hills, suc h a s Be n Rinnes , a t u p t o 800 m elevation o n bot h th e norther n an d souther n margins o f the basin. In contrast , the remova l of over 1. 1 km o f roc k fro m abov e th e presen t terrain o f Caithnes s an d Sutherlan d (Thomso n et al 1999 ) seems excessive. On Morvern, AFTA data indicate c. 1.7k m of Tertiary erosio n o n the Strontian Granodiorit e (Thomso n e t a l 1999) , yet th e curren t lan d surfac e nearb y retain s thi n sequences o f Mesozoic sediment s buried beneath Early Tertiary lavas (Johnstone & Mykura 1989). Further judgement on thes e issue s must await continuing wor k usin g AFT T an d (U-Th)/H e thermochronology. Non e th e less , onshor e geological an d geomorphologica l dat a fro m th e passive margin s of souther n Afric a (BJ . Bluck , pers. comm) an d southeastern Australi a (Kohn & Bishop 1999 ; Bishop & Goldrick, 2000) see m to suggest tha t AFTT ma y overestimat e the depths of forme r cove r rock s an d o f denudation . Of course, th e regiona l patter n o f AFT T data , an d the denudatio n tha t the y imply , cannot b e take n to provid e a detaile d histor y o f an y particula r locality. Th e lava s o f 10 0 Ma ag e o n th e sout h coast o f Ne w Sout h Wales , fo r example , ar e inconsistent wit h dee p denudatio n havin g occurred alon g an d acros s th e whol e o f th e coastal stri p belo w th e S E Australia n escarp ment, a n interpretatio n tha t ha s bee n implici t i n much o f th e AFT T discussion . Likewise , ther e are man y area s throughou t th e S E Australia n highlands, a s i n th e Scottis h Highlands , wher e ancient landscap e element s (datin g from Meso zoic tim e i n th e S E Australia n case ) hav e bee n identified (Youn g 1981 ; Bir d & Chiva s 1989 : Twidale 1994 ; Twidale & Campbel l 1995 ; Hill 1999). Hil l (1999 ) ha s argue d tha t variation s in relief i n th e S E Australia n highlands and acros s the coasta l stri p belo w th e escarpment , an d differential preservatio n o f Mesozoi c landscap e elements, ma y explai n th e apparen t conundru m in S E Australia of kilometre-scale denudatio n in Late Mesozoi c tim e an d th e preservatio n o f Mesozoic landscap e elements . Wher e local , detailed AFTT data are available in SE Australia, particularly i n areas wher e the regional structur e is dominate d b y individua l faul t blocks , differ ential faul t bloc k movemen t i s clearl y indicated by th e AFTT dat a (Koh n & Bisho p 1999 ; Kohn et a l 1999) . Elevate d geotherma l gradient s would als o assis t i n minimizin g th e amount s o f denudation require d fo r Lat e Mesozoi c fission track age s t o cro p ou t a t th e presen t groun d surface i n S E Australia , but ther e i s currentl y a

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Fig. 6 . Early Tertiary differentia l tectonic s an d sediment transport (from variou s data sources).

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strong consensu s amon g AFT T researcher s tha t geothermal gradient s alon g th e S E Australia n continental margi n wer e probabl y no t signifi cantly greate r tha n 25-30°Ckm" 1 i n Lat e Mesozoic tim e (Dumitr u e t al. 1991 ; Brow n et aL 1994) . These matter s wil l almost certainl y emerge a s importan t fo r th e Scottis h Highland s as more detai l become s available .

Cenozoic uplif t and denudation o f the Scottish Highland s The timing , amoun t an d patter n o f uplif t i n th e Highlands throug h Cenozoi c tim e ar e graduall y becoming clearer . Uplif t appear s t o hav e bee n intermittent, wit h th e mai n phase s centre d o n Late Paleocen e an d Lat e Oligocen e tim e an d a later phas e o f continuin g uplif t startin g i n Lat e Miocene tim e (Hal l 1991 ; Li u & Gallowa y 1997). In terms o f sediment suppl y to the Mora y Firth, th e firs t o f thes e phase s i s th e mos t important (Nielse n e t al 1986 ; Li u & Gallowa y 1997), implyin g tha t thi s wa s th e perio d o f maximum Tertiar y uplif t i n th e Highlands . Th e timings of these vertica l movement s matc h thos e in wester n Norwa y (Rii s 1996) . Indeed, th e fou r major unconformitie s datin g fro m mid-Paleo cene, mid - t o lat e Oligocene , mid-Miocen e an d latest Pliocen e tim e ar e remarkably consisten t in development acros s th e centra l an d norther n North Sea and the Norwegian Shel f (Huuse et al., 2001). I t appear s therefor e tha t th e Scottish Norwegian sectio n of the NE Atlantic margin has reacted contemporaneousl y t o crustal break-up in Paleocene time , intraplat e deformatio n i n Neo gene tim e (Stuevol d & Eldholm 1996 ) an d to the onset o f regiona l glaciation . Th e magnitud e o f these events , however , ma y wel l hav e varie d spatially. Th e maximu m uplif t o f souther n Fennoscandia appear s t o hav e occurre d fro m late Oligocen e t o Pliocen e tim e (Dor e e t al . 1998; Japse n & Chalmer s 2000 ) an d thu s postdates th e mai n uplif t phas e i n th e Scottis h Highlands. Th e reconstructe d buria l histor y o f the Chalk in the western Nort h Sea (Japsen 1997 ) implies tha t part s o f easter n Scotlan d ma y hav e experienced maximu m Tertiar y uplif t i n Neo gene time . Major uplif t occurre d a t th e Paleocene Eocene boundar y vi a a combinatio n o f permanent an d transien t (dynamic ) uplif t (Jones e t al . 2002). Thi s uplif t even t i s consistent wit h th e evidenc e o f rapi d loca l denudation i n th e Hebridea n Igneou s Province . As muc h a s 2k m o f roo f an d cove r rock s ar e missing fro m abov e th e igneou s centre s o f Skye, Mul l an d Arra n (Georg e 1966) , bu t thi s unroofing i s likel y t o hav e bee n spatiall y

variable (Fig . 6 ) (se e foregoin g discussio n o f SE Australia) . Holness (1999) , fo r example , ha s argued tha t th e metamorphi c grad e o f th e Torridonian arkos e intrude d b y th e Rhu m Igneous Comple x implie s 500-55 0 m o f over burden, whic h i s les s tha n th e curren t relie f o n Rhum, an d 'point s t o a topograph y a t th e tim e of meltin g [i.e . metamorphism ] a s ver y simila r to tha t o f today ' (p . 538) . Thi s erosio n o f th e Hebridean Igneou s Provinc e too k plac e remark ably quickly . Som e 2 km o f basal t wa s remove d from Mul l betwee n 5 8 an d 5 6 Ma an d th e unroofing o f th e Wester n Granophyr e o f Rhu m took plac e withi n 3 Ma (Emeleu s 1983) . Sedimentary interbed s i n th e Rhu m Centra l Complex demonstrat e tha t th e comple x wa s already unroofe d an d wa s undergoin g activ e erosion a s th e lava s wer e bein g extrude d (Emeleus & Forste r 1979 ; Holnes s 1999) . Palynological studie s o f forme r interbasalti c vegetation sugges t tha t durin g th e 0.2 4 Ma o f existence o f th e Sky e Lav a Fiel d ther e wer e two majo r subsidenc e phase s an d on e uplif t phase. Therma l domin g relate d t o th e emplace ment o f th e Cuilli n centr e cause d elevatio n t o altitudes i n exces s o f 1200 m a.s.l . (abov e se a level; Jolle y 1997) . Lav a erupte d mainl y fro m fissures spread ou t over a n area o f c. 40 000km 2 stretching fro m Harri s t o th e Firt h o f Clyd e over th e axi s o f th e dyk e swarm s (Presto n 1982). B y th e en d o f th e magmati c phase many igneous centre s wer e reduce d t o clos e t o present levels , a s show n b y th e lat e lava s resting o n th e Wester n Granophyr e o f Rhu m (Emeleus 1983 ) an d th e deepl y denude d basalts lying beneat h th e Sgur r o f Eig g pitchston e (Dickin & Jone s 1983) . Thi s dee p erosio n implies tha t th e lav a field s wer e als o largel y stripped i n earl y Tertiar y time , togethe r wit h any underlyin g Palaeozoi c an d Mesozoi c sediments. A s Permia n sandstone s o n Lewi s and Jurassi c sandstone s o n Sky e ar e associate d with maximu m burial depth s o f onl y 1.5-2k m (Carter e t al . 1995) , i t i s likel y tha t th e Paleogene uplif t even t wa s a ke y phas e i n th e stripping o f Paleozoi c an d Mesozoi c cove r rocks throughou t th e Inne r Hebrides . The Leava d clay s probabl y relat e t o earl y Miocene marin e transgressio n (Crampto n & Carruthers 1914) , althoug h a moder n fauna l study o f thes e intriguin g deposits i s required . If correct, a n earl y Miocen e ag e implie s tha t dee p erosion o f th e Jurassi c an d lowe r Cretaceou s sequence i n th e inne r Mora y Firt h (Hilli s et al . 1994) wa s largel y complete d betwee n Lat e Paleocene an d Late Oligocene time . Jones e t a l (2002 ) interprete d gravit y dat a from th e N E Atlanti c i n terms o f Neogene uplif t

SCOTLAND'S DENUDATIONAL HISTOR Y

but because of the absence of later Tertiary rock s from muc h of the Highlands area it is not easy to assess amount s of subsequent uplift an d denudation. Th e smal l fault-bounde d Lat e Oligocen e basins o f The Minc h an d the NW Scottis h shel f provide important evidence of a phase of tectonic activity tha t ca n b e trace d throughou t western Britain an d ma y b e relate d t o mino r plat e reorganization (Evan s et al 1991 , 1997) . It may be significan t tha t the Late Oligocen e floodplai n and swamp deposits in these basins do not rest on Eocene sediment s bu t o n kaolinize d basement , implying prolonge d weatherin g unde r humi d conditions in Eocene time. O n the NW Scottish shelf, th e Oligocene sediment s ar e succeeded b y Miocene shallo w marin e sand s (Evan s e t al . 1997). The sequence implies that despite Eocene crustal collaps e Th e Minc h an d th e adjacen t shelf remaine d abov e se a leve l unti l marin e transgression i n Miocen e time . Crustal movemen t resume d i n Lat e Miocen e time (Stoke r e t al . 1994) . Japse n (1997 ) considered tha t th e amoun t o f exhumatio n during Neogen e uplif t wa s equivalen t i n Britai n to tha t achieve d i n Lat e Paleocene-Earl y Eocene tim e (se e als o Japse n & Chalmers , 2000). Thi s seem s unlikely , a s Neogen e sediments i n th e Nort h Se a ar e largel y source d from Scandinavi a an d N W Europe , rathe r tha n Britain (Jord t e t al . 1995) . Moreover , th e larg e volumes o f coars e clasti c sediment s o f Lat e Paleocene an d earl y Eocen e ag e i n th e Mora y Firth contras t markedly with the more restricte d sequences o f Neogen e mud s (Li u & Gallowa y 1997). Th e increasin g resolutio n o f th e Neogene successio n i n th e centra l an d norther n North Se a an d o n th e Norwegia n shel f help s t o constrain th e timin g o f thi s lat e uplift , wit h pulses o f terrigenou s sediment s eviden t i n Lat e Miocene tim e and , coinciden t wit h th e firs t Scandinavian ic e sheets , i n Lat e Pliocen e tim e (Eidvin e t a l 2000) . The thick Neogen e sequence s i n the Faeroe Shetland basi n syste m (Stoke r e t al . 1993 ) ar e more difficul t t o accoun t for . Likewise , Jone s et al . (2002 ) reporte d tha t th e volum e o f Paleocene sedimen t i n th e Faeroe-Shetlan d basins canno t b e accounte d fo r b y denudatio n of a source area in NW Scotland. They suggeste d that Faeroe-Shetland basin sediment s ma y also have been derive d fro m sourc e area s othe r tha n NW Scotland , such as the Faeroe Islands regio n or more widely in northern and western Scotland, and a similar explanation may have to be invoked to reconcil e th e thicknes s o f th e Neogen e sequence i n the Faeroe-Shetland basi n system s and th e apparentl y mino r Neogen e uplif t o f Scotland.

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The signatur e o f thes e Neogen e vertica l movements i s recorde d b y element s o f th e Highland topography. In the Northern Highlands, Godard (1965 ) recognize d thre e majo r erosio n surfaces: a surfac e betwee n 40 0 an d 550m , which cut s acros s part s o f th e Tertiar y igneou s centres an d s o i s post-Paleocen e an d possibl y Eocene i n age ; a n intermediat e an d extensiv e Scottish Surfac e a t c . 300 m o f possibl e Oligocene age ; th e lo w coasta l platea u o f th e Niveau Pliocene a t c. 100 m. Assuming that these erosion surface s originate d clos e t o se a level , i t has bee n estimate d tha t uplif t i n th e N W Highlands i s o f th e orde r o f 400 m i n th e las t 40Ma(LeCoeur 1999) . The long-ter m tren d ha s bee n toward s th e progressive tiltin g or downwarping o f the High lands toward s th e inne r Mora y Firt h Basin . Despite a degre e o f glacia l diversio n an d disruption th e mai n drainag e route s continu e to flow toward s th e Mora y Firth , jus t a s i n Earl y Tertiary an d Lat e Devonia n time . Th e mos t extensive erosion surfac e recognized south of the Moray Firt h i s th e Easter n Grampian s Surface , comprising th e ramp-lik e interfluve s o f th e Monadliath, th e Dee-Do n watershe d an d th e Mounth. Thes e surface s slop e fro m thei r inne r margins around the Cairngorms a t around 80 0 m towards the inner Moray Firth and the North Sea, dropping t o elevation s o f c . 500 m a t th e oute r margins (Hall 1991) . Recent morphometri c analysi s ha s confirme d that regiona l tiltin g wa s fa r fro m uniform . Erosion surface s between 20 0 and 600 m on the Monadliath are strongly influenced b y the Grea t Glen Faul t an d th e Ericht-Laido n Fault . Thi s may imply a significant tectoni c event in mid- to late Tertiary time in which an extensive mediumlevel erosio n surfac e wa s disrupte d b y bloc k movement (Ringros e & Migon 1997) . Ringros e & Migo n als o identifie d a possibl e zon e o f flexure located i n the Dalradian belt between the high tops of the Cairngorms an d the lowlands of Buchan. Lat e Neogen e faultin g ha s als o bee n proposed alon g the S E coasts o f Rhum an d Col l along th e Camasunary-Skerryvor e Faul t (L e Coeur 1988 ) an d i n th e Elgi n are a (Hal l 1991 ) and ma y represen t par t o f a continuing patterns of neotectoni c activit y (Muir Wood 1989) . Denudation and landscape evolution The mai n morphotectoni c unit s i n th e Scottis h area were alread y i n existence b y th e en d o f th e Palaeozoic era . The Orkney-Shetland Platform, the Highland s an d Souther n Upland s massif s have remained abov e se a level for mos t o f postPalaeozoic tim e an d hav e she d sedimen t t o

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surrounding basins . Th e persistenc e o f differential movement s betwee n basemen t massif s an d sedimentary basin s ha s allowe d conservation , during surfac e lowering , o f majo r topographi c features fro m a s lon g ag o a s Devonia n time , including watershe d zones , drainag e pattern s and, especially , th e scarp s a t th e margin s o f th e buoyant basement massifs . Th e Helmsdale Fault , for example , ha s bee n intermittentl y activ e throughout Mesozoi c an d Cenozoi c tim e (Andrews e t al 1990) . Superimposed o n thi s physiographi c frame work ar e th e mai n Tertiar y landforms . Th e magnitude o f Paleocen e uplif t an d denudatio n means tha t thes e Tertiar y landform s ar e o f mid to lat e Tertiar y age . Onl y i n area s o f minima l displacement, suc h a s Bucha n an d Caithness , i s the preservatio n o f extensiv e Mesozoi c land forms feasible . Exhumed terrain s ar e o f restricte d exten t i n Scotland. The y includ e th e rugge d sub Torridonian surfac e i n N W Scotlan d (Godar d 1957; Stewar t 1972 ) an d th e equall y irregula r sub-Devonian surfac e expose d beneat h th e out liers around the margins of the Moray Firt h Basi n (Godard 1965) . Asid e fro m area s suc h a s these , the oldes t landform s recognize d i n Scotlan d occur i n the lowland s o f Buchan. This i s an are a of long-ter m relativ e stabilit y wher e typica l Highland rock s ar e associate d wit h surface s o f low relie f (Clayto n & Shamoo n 1999) . I t is als o an are a o f ver y limite d glacia l erosion . Th e antiquity of the landscape i s demonstrated b y the Cretaceous residue s i n th e for m o f chal k flint s within th e quartzite - an d kaolin-ric h Bucha n Gravels (Hal l 1987 ) an d a n outlie r o f Lowe r Cretaceous greensan d (Hal l & Jarvi s 1994) . These residue s res t o n weathere d igneou s an d metamorphic rocks , know n i n borehole s t o extend t o depth s o f man y ten s o f metre s (Hal l 1986). Largely b y correlation wit h clay mineral s in Nort h Se a sediments , th e spatiall y restricte d kaolin-rich weatherin g profile s hav e bee n assigned t o pre-Pliocen e tim e an d th e les s mature, bu t stil l dee p sand y weatherin g profile s to Pliocene and Pleistocene time (Hall 1985 ; Hall et al . 1989) . Give n th e apparen t stabilit y o f th e area, i t is conceivable tha t som e o f the kaolinitic weathered material s ar e older , survivin g fro m Paleogene tim e o r exhume d fro m beneat h Lat e Cretaceous cover rock s (Hal l 1993) . Deep weatherin g i s o f fundamenta l import ance in understanding the nature and evolution of the pre-Quaternar y relie f throughou t Scotlan d and N W Europ e (Godar d 1965 ; Thoma s 1989 ; Migon & Lidmar-Bergstrom 2001) . Its presenc e has bee n use d widel y a s a n indicato r o f th e preservation o r limited modification of preglacial

forms (Linto n 1951 ; Godar d 1961 ; Hal l & Sugden 1987) . Ther e i s often a close correspon dence between pre-glacia l morphology, level s o f rock resistanc e t o chemica l weatherin g (Godar d 1962) an d deep weatherin g pattern s (Hal l 1986) . The Tertiar y perio d i n Scotlan d wa s a tim e o f warm to temperate humid climates when most, if not all , o f th e countr y stoo d abov e bas e level , conditions favourin g th e dee p penetratio n o f weathering. The sustaine d etching out o f contrasts i n roc k resistance b y chemica l processe s le d t o th e formation o f majo r landform s o f differentia l weathering an d erosion . Thes e includ e valle y systems, such as the pre-glacial headwaters of the Spey, Do n an d Dee , wher e ther e i s pervasiv e litho-structural control s o n valle y alignmen t (Threlfall 1981) . Dee p topographi c basins , some floore d b y Devonia n sediments , are strung out alon g man y o f th e valley s tha t draine d eas t from th e mai n watershed s i n th e N W Highland s and Grampia n Highland s toward s th e Mora y Firth (Fig . 4) . Th e larges t example s includ e the Rannoch, Atholl and Naver basins, with areas of more tha n 500km 2 (Linto n 1951) . I n N E Scotland, th e basi n floor s ar e preferentiall y located o n biotite-bearin g granit e an d gabbr o and borehole s sho w widesprea d dee p weath ering, reachin g depths o f a s muc h a s 5 0 m (Hall 1986, 1991) . Thes e susceptibl e rock s hav e provided th e foc i fo r weatherin g an d erosio n a s the surroundin g terrain wa s uplifted . In counter point stan d inselbergs . Thes e isolate d hill s include thos e o f resistance , notabl y quartzit e hills suc h a s Schiehallion . Inselbergs of position also occu r (Godar d 1965) , where the isolation of the hil l mas s appear s t o b e a resul t o f back wearing o f slopes . A fe w exhume d hill s occur , notably th e sub-Devonia n inselber g o f Scarabe n in Caithness (Crampton & Carruthers 1914) . The quartzite inselber g o f Mormon d Hil l i n Bucha n may b e a Mesozoic relic , as i t is associated with deep kaolinization and lies close to or at the level of th e sub-Cenomania n surfac e (Hal l 1987) . The landform s o f differentia l weatherin g an d erosion occu r a s mesoscal e feature s a s par t o f major erosio n surfaces . Th e Bucha n Surface , a t an elevatio n o f c . 100m , include s mos t o f th e lowlands of NE Scotland, apart from th e glacially modified coasta l strip. It is an etch surface, where subtle difference s i n roc k resistanc e giv e ris e t o hills an d depression s an d dee p weatherin g i s widespread. Th e origin s of the eastern part of the surface dat e back to late Mesozoic time , for Late Cretaceous greensan d an d chal k wer e deposite d on it s surface , bu t i t ha s a lon g histor y an d it s various element s ar e o f differen t age . Th e preservation o f unwor n flint s a t th e bas e o f th e

SCOTLAND'S DENUDATIONA L HISTORY

flint gravels in Buchan testifies t o th e proximity of the sub-Cenomania n surfac e (Bridgland et al. 1997; Merritt et al. 2002). O n the high ground of central Bucha n a t 100-150 m highl y kaoliniti c saprolites an d th e flin t gravel s themselve s hav e been ascribe d a pre-Pliocen e ag e (Hal l 1985) . The lowe r tier s o f th e terrai n suppor t dee p sandy weatherin g cover s an d appea r t o b e o f Plio-Pleistocene age . It is likely tha t high-level erosio n surface s ar e also polycycli c an d spatiall y diachronous . Th e surfaces a t 800 m i n th e Gaic k Fores t (Hal l & Mellor 1988 ) an d a t hig h elevation s i n th e Cairngorms (Hal l 1996 ) retai n pocket s o f dee p sandy weatherin g datin g fro m th e lates t Plio Pleistocene phas e o f etching . Withi n th e Cairngorms mountain s ar e a rang e o f majo r paleic form s whic h hav e a longe r history , including high-leve l basin s an d ope n valley s (Hall 1996 ) an d th e majo r depressio n o f th e Upper Avo n Embaymen t (Linto n 1950) . Th e precursors o f th e majo r Tertiar y river s o f N E Scotland, th e De e an d th e Don , wer e alread y established in Paleocene tim e and fed material to the Gannet Fan in the North Sea (Morton 1979) . The Cairngorms and the Eastern Grampians have been a n are a o f positive relie f sinc e a t least th e start of Tertiary time and these headwater erosion surfaces have evolved far above sea level. Phase s of valley incision, indicated b y benches o n valley and basin sides, ar e likely to have been driven by changes i n loca l bas e level s rathe r tha n i n response t o regional uplift .

Discussion an d conclusion s Variations in th e characte r and rate o f suppl y of sediments t o th e offshor e regio n ha s generall y been interprete d i n term s o f sourc e are a responses to Cenozoic regional tectonics. Indeed, Liu & Galloway (1997 ) wer e explici t abou t the link between uplif t an d erosion i n the North Se a Basin: Tectoni c uplift o f source area s exercise s the commandin g rol e i n modulatin g lon g ter m sediment suppl y to the North Sea' (p . 1506). The precise proces s linkage s betwee n uplif t an d enhanced denudatio n are , however , no t alway s as clea r a s i s implie d b y th e assume d lin k between uplif t an d enhance d sedimen t flux (see Introduction). White & Lovel l (1997 ) acknowledge d tha t a simple proces s lin k betwee n a n increas e i n offshore sedimentatio n rat e an d th e uplif t o f Scotland canno t be assumed , an d suggested tha t uplift ma y be linked closel y i n time to increase s in offshore sedimentation via an uplift-driven fall in relativ e se a leve l an d a n associate d mobiliz ation o f sediment s tha t wer e i n storag e i n th e

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nearshore are a an d o n th e continenta l shelf . I t seems clear, none the less, that parts of Scotland, for example , the Tertiary Igneous Province, have responded rapidly to presumed uplift events. This is probabl y becaus e thes e wester n area s ar e composed o f smal l catchment s tha t ar e wel l connected to base level and that, therefore, would be expected t o respond rapidly to a fall in relative sea level. The preservation o f ancient landscap e elements throughou t Scotlan d demonstrates , however, tha t suc h rapi d respons e canno t b e assumed fo r al l areas , an d tha t no t al l area s ar e equally sensitive to base-level changes. Variations in lithology will also be expected to be associated with variation s i n landscap e sensitivit y (se e Brunsden & Thorne s 1979) ; lithologica l vari ations in the respons e to Cenozoi c weatherin g regimes i n th e Scottis h Highland s poin t t o a variant o f suc h sensitivity . I n th e cas e o f th e Tertiary Igneou s Province, i t is also worth noting that constructio n o f th e Province' s centra l volcanic edifices and the extrusion of the regional lava fields must have caused a relative elevation of the lan d surface , i n tur n triggerin g incisio n an d enhanced denudation. The relative magnitudes of this latter effect an d uplift remain to be quantified. Many problems remain in understanding longterm landscape development in Scotland and yet there are many promising lines of enquiry. There is a n urgen t nee d fo r detaile d morphometri c analysis o f th e relie f comparabl e wit h tha t available i n souther n Fennoscandi a (e.g . Lid mar-Bergstrom e t al . 2000) . Th e scattere d outliers o f sedimentar y rock s o f Devonia n t o possible Miocen e ag e tha t occu r o n lan d i n Scotland ar e importan t archive s o f information regarding th e evolutio n o f th e terrain , bot h vi a provenance studies and by investigation of burial histories. Th e dyk e system s o f lat e Caledonian , Carboniferous, Permia n an d Paleogen e ag e that criss-cross muc h o f Scotlan d hav e ye t t o b e examined i n detai l wit h regar d t o depth s o f emplacement beneat h contemporar y lan d sur faces. Saprolite s occu r throughou t Scotland , including burie d an d formerl y burie d pre Cenozoic weatherin g mantle s an d Cenozoi c deep weatherin g profiles. Thes e saprolite s await detailed mineralogica l stud y an d datin g (Hal l 1993), using techniques such as K-Ar datin g of mica clays (Sturt et al. 1979 ) and D-H analysi s of kaolin s (Gil g 2000) . Finally , ther e ar e opportunities t o tie in the uplift an d denudational histories of regional sourc e areas within Scotland with sub-basin s an d fan s i n th e offshor e area , such as the Tertiary Barra Fan NW of The Minch (Stoker et al 1993 ) and the Early Tertiary Gannet Fan fed by rivers draining the Eastern Grampians (Gatliffetai 1994) .

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The preservatio n o f ancien t landscap e remnants o f varyin g spatia l exten t i n th e Scottish Highland s point s t o th e type s o f spatial variabilit y o f denudatio n tha t hav e been identifie d i n mor e detaile d studie s i n southeastern Australia . Jone s e t al (2002 ) ha s called fo r improve d quantitativ e understanding of th e Cenozoi c denudatio n of norther n Britain and i t i s therefor e timel y fo r thermochronolo gically base d studie s o f th e denudatio n o f Scotland: (1 ) t o mov e beyon d regiona l denudational studie s t o mor e focuse d AFT T studie s and (U-Th)/H e analysis in apatite (Zeitler et al. 1987) t o identif y spatia l variabilit y i n denuda tion; (2 ) t o us e AFT T an d (U-Th)/H e analysi s to asses s th e exten t o f differentia l movement s of crusta l blocks ; (3 ) t o us e th e (U-Th)/H e system t o asses s th e age(s ) o f majo r landscap e elements feature s (e.g . Hous e e t al . 1998) ; (4 ) to us e cosmogeni c isotop e analysi s t o asses s i n more detai l th e spatia l an d tempora l variation s in Quaternar y denudation . Despite th e adven t o f thes e variou s recentl y developed techniques , th e majo r challenge s o f dating landscape s an d landforms , an d o f determining th e timing o f uplift an d denudation, remain. S o far, the excellent tempora l resolutio n available i n th e offshor e recor d elude s u s i n studying the onshore, source area record, but this shortcoming doe s no t justify , fo r example , oversimplified assumption s concernin g th e relationships betwee n uplift an d denudation. A.M.H. thank s th e Carnegi e Trus t fo r th e Univer sities o f Scotlan d fo r financia l suppor t o f fieldwork . P.B. gratefull y acknowledge s th e suppor t o f th e Australian Researc h Council . Th e CRUS T projec t (Constraining Regiona l Uplift , Sedimentatio n an d Thermochronology) i s supporte d b y a Scottis h Higher Educatio n Counci l Researc h Developmen t Grant. W e thank K . Lidmar-Bergstrom, J . Cartwrigh t and th e Edito r fo r thei r review s an d suggestion s t o improve thi s contribution , an d M . Shan d fo r preparing th e figures . References ANDERTON, R. , BRIDGES , PH. , LEEDER , M.R . & SELLWOOD, B.W . 1979. A Dynamic Stratigraphy o f the British Isles. Alle n an d Unwin, London. ANDREWS, I.J., LONG, D., RICHARDS , PC., THOMSON , A.R., BROWN , S. , CHESHER , J.A . & MCCORMAC , M. 1990 . The Geology o f th e Moray Firth. Britis h Geological Survey , Keyworth . BELL, B.R . & JOLLEY , D.W . 1997 . Applicatio n o f palynological dat a t o th e chronolog y o f th e Palaeogene lav a field s o f th e Britis h Province : implications fo r magneti c stratigraphy . Journal o f the Geological Society, London, 154, 701-708.

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Cenozoic evolution of the Faroe Platform: comparing denudation and deposition MORTEN SPARR E ANDERSEN 1, AAGE BAC H S0RENSEN 1, LARS OL E BOLDREEL 2 & TOVE NIELSEN 1 1

Geological Survey of Denmark and Greenland (GEUS), Thoravej 8, DK-2400 Copenhagen NV, Denmark (e-mail: msa@ geus.dk)

University of Copenhagen, Department of Geology, 0ster Voldgade 10, DK-1350, Copenhagen K, Denmark Abstract: Throughou t Paleocen e an d Eocen e tim e th e Faroe-Shetlan d Channe l an d th e eastern par t o f the Faroe Platform wa s a subsiding marin e basin . I n Early Paleocen e time , basin-floor fan s o f a British provenance wer e deposite d i n the eastern par t o f the basin. I n Late Paleocen e time , c . 6 km o f basalt entered th e basi n fro m th e wes t and north , an d th e basin wa s constricte d b y th e larg e volume s o f basal t tha t entere d th e basin , creatin g th e Faroe-Shetland Escarpment . I n Eocen e tim e subsidenc e continue d i n th e basina l areas . Again, sediments o f a dominantly easter n provenanc e were deposited. Throughou t Eocen e time, erosio n product s fro m th e Faroe Platform wer e possibly deposite d i n the Faroe Ban k Channel an d the Norwegian Sea Basin, but only to a limited degre e i n the Faroe-Shetland Channel. The oldest sediment s of documented western provenance on the eastern margin of the Faroe Platfor m are of Early Oligocen e age . During a compressional phas e commencing in Mid-Late Miocene time some basinal areas emerged and erosion took place on the top of emerged anticlines . However , denudation throughout Late Miocene an d Early Pliocene time was apparentl y rather limited compared wit h a Late Pliocene phas e of denudation. During this phase of denudation, a large progradational wedge was deposited on the eastern margin of th e Faro e Platform . O n th e basi s o f a structura l analysi s o f th e Faro e Platform , th e amount o f basal t remove d fro m i t durin g Cenozoic tim e i s estimated t o be c . 46 000 km3 (131 100 X 1012kg). Usin g 2900kgnT ^ a s th e densit y o f basal t an d 2300kgnT 3 a s sediment densit y th e estimate d amount o f remove d basal t i s i n fai r agreemen t wit h th e estimate o f th e volum e o f sediment s derive d fro m th e platfor m (c . 56000km 3, 114800 X 1012kg). The greates t depositio n rate s o n the eastern Faro e Platform an d in th e Faroe-Shetland Channe l apparentl y occurre d afte r tw o distinc t inversion o r compressio n events in Mid-Eocene an d Mid-Late Miocene time . However, uplift o f the Faroe Platfor m could hav e been force d b y denudation rather tha n endogenous processes .

In this paper w e present a preliminary attemp t to documente d Mesozoi c riftin g i n N W Europ e quantify th e denudationa l an d depositiona l (Fig . 2). history o f th e Faro e regio n durin g Cenozoi c Th e mos t importan t Cenozoi c structura l time (Fig . 1) . The stratigraphi c subdivisio n an d element s withi n th e Faro e regio n ar e show n i n discussion o f th e depositiona l histor y is mainl y Fig . 1 . Most of the structural elements are known based o n publishe d wor k (Nielse n & Va n fro m previou s work (e.g . Boldree l & Andersen Weering 1998 ; Anderse n e t al 2000 ; Stoke r 1994 ; Lamer s & Carmichae l 1999) . Th e et al. 2002). Foinave n Basi n i s th e Paleocene-Eocene depo centre in the southern part of the Faroe-Shetland Channel. Geographicall y i t i s equivalen t to th e „, , j • i • Foinave n Sub-basi n o f Lamer s & Carmichae l Structural elements an d regional setting (1999 ) Th e Fugloy ^^intr oduced here , is a In a broader context the Faroes ar e located close broa d Eocene-Oligocen e depocentr e i n th e to th e lin e o f th e Lat e Paleocen e break-u p norther n par t o f th e Faroe-Shetlan d Channel , between Europ e an d Greenland, centrall y i n the Uppe r Eocene-Oligocen e sediments , whic h ar e area affecte d b y th e Icelandi c mantl e plum e thi n o r absen t i n th e Foinave n basin , thicke n during Paleocen e time , an d o n th e margi n o f northwar d i n th e Fuglo y Basi n (Fig . 3) . From: DORE , A.G., CARTWRIGHT, J.A. , STOKER , M.S. , TURNER, J.P. & WHITE , N . 2002 . Exhumation o f th e