Intraplate Strike-Sli p Deformatio n Belts
Geological Societ y Specia l Publication s Society Book Editors R. J . PANKHURS T (CHIE F EDITOR ) P. DOYL E F. J . GREGOR Y J. S . GRIFFITH S A. J . HARTLE Y R. E . HOLDSWORT H
A. C . MORTO N N. S . ROBIN S M. S . STOKE R J. P . TURNE R
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GEOLOGICAL SOCIET Y SPECIA L PUBLICATIO N NO. 21 0
Intraplate Strike-Sli p Deformatio n Belt s EDITED BY F. STORT I Universita degl i Stud i "Rom a Tre" , Rome, Ital y
R. E . HOLDSWORT H Durham University , Durham, U K
F. SALVIN I
Universita degl i Stud i "Rom a Tre" , Rome, Ital y
2003 Published b y The Geologica l Societ y London
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Contents
Preface vii Acknowledgements viii STORTI, F. , HOLDSWORTH , R . E . & SALVINI , F . Intraplat e stike-sli p deformatio n belt s 1 VAUCHEZ, A . & TOMMASI , A . Wrenc h fault s dow n t o th e asthenosphere : Geologica l an d geophysical evidenc e an d themo-mechanica l effect s 1
5
SCHREURS, G . Faul t developmen t an d interactio n i n distribute d strike-sli p shea r zones : a n experimental approac h 3
5
BUSLOV, M . M. , KLERKX , J. , ABDRAKHMATOV , K., DELVAUX , D. , BATALEV , V . Yu , KUCHAI, O . A. , DEHANDSCHUTTER , B . & MURALIEV , A . Recen t strike-sli p deformatio n o f the Norther n Tie n Sha n 5
3
CUNNINGHAM, D. , DUKSTRA , A. , HOWARD , J. , QUARLES , A . & BADARCH , G . Activ e intraplate strike-sli p faultin g an d transpressiona l uplif t i n th e Mongolia n Alta i 6
5
UTTAMO, W. , ELDERS , C . & NICHOLS , G . Relationship s betwee n Cenozoi c strike-sli p faultin g and basi n openin g i n norther n Thailan d 8
9
FERRACCIOLI, F . & Bozzo, E . Cenozoi c strike-sli p faultin g fro m th e easter n margi n o f th e Wilkes Subglacia l Basi n t o th e wester n margi n o f th e Ros s Se a Rift : a n aeromagneti c connection 10
9
PERRITT, S . H . & WATKEYS , M . K . Implication s o f lat e Pan-Africa n shearin g i n wester n Dronning Maud Land , Antarctic a 13
5
ROCCHI, S. , STORTI , F. , D i VINCENZO , G . & ROSSETTI , F . Intraplat e strike-sli p tectonic s a s an alternativ e t o mantl e plum e activit y fo r th e Cenozoi c rif t magmatis m i n th e Ros s Se a region, Antartic a 14
5
MARSHAK, S. , NELSON , W . J . & MCBRIDE , J . H . Phanerozoi c strike-sli p faultin g i n th e continental interio r platfor m o f the Unite d States : example s fro m th e Laramid e Orogen , Midcontinent, an d ancestra l Rock y Mountain s 15
9
MURPHY, J . B . Lat e Palaeozoi c formatio n an d developmen t o f th e S t Mary s Basin , mainlan d Nova Scotia , Canada : a prolonged recor d o f intracontinental strike-sli p deformatio n durin g the assembl y o f Pangae a 18
5
REIJS, J . & McCLAY , K . Th e Salin a de l Frail e pull-apart basin , northwest Argentin a 19
7
MOHRIAK, W . U . & ROSENDAHL , B . R . Transfor m zone s i n th e Sout h Atlanti c rifte d continental margin s 21
1
Index 229
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Preface Intraplate strike-sli p deformatio n belts are common tectonic features , particularl y a t convergen t plat e boundaries, wher e the y ar e produce d b y bot h oblique convergenc e an d continenta l indentation . These lithosphere-scal e structures , tha t als o occu r in othe r geodynami c environments such as passive margins, ar e characterise d b y comple x structura l architectures, b y th e occurrenc e o f larg e earth quakes, an d by th e fas t uplif t and/o r subsidenc e of localised crusta l sectors . Intraplat e strike-sli p belt s can als o contro l th e ascen t an d emplacemen t o f deeply-sourced magma . I n som e cases , intraplat e strike-slip belt s lin k wit h oceani c fractur e zone s and transfor m faults , transferrin g transfor m shea r from th e ridge s t o th e interio r o f th e plates . Thi s evidence ha s a n importan t impac t o n th e classica l concept o f transfor m faulting . Thi s volum e con tains a selectio n o f paper s describin g th e tectoni c architecture o f intraplat e strike-sli p deformatio n belts an d relate d structures . Th e volum e contain s 13 papers, includin g experimental an d case studie s from a global se t of contributors. The opening contribution by Storti et al. is an overview of the basic features o f intraplat e strike-sli p deformatio n belt s aimed a t settin g th e scen e fo r th e mor e detaile d papers t o follow . Th e firs t pape r b y Vauche z & Tommasi, discusse s th e geologica l an d geophysi cal evidenc e supportin g an astenospheri c dept h of major intraplate strike-sli p belts. The second paper, by Schreurs , describe s laborator y experiment s o n
the structura l architectur e o f faultin g produce d b y distributed shear . Th e remaining papers ar e organised following a geographic criterion. W e start with three contribution s dealin g wit h intraplat e strike slip belt s locate d i n th e India-Asi a collisiona l region (Buslo v et«/., Cunningha m et al., Uttamo et a/.) ; the n mov e t o Antarctic a (Ferracciol i & Bozzo, Perrit t & Watkeys, Rocch i e t al.), Nort h America (Marsha k e t al., Murphy ) an d Sout h America (Reij s & McClay). Th e las t contributio n (Mohriak & Rosendahl) deal s wit h the evolutio n of riftin g i n th e Sout h Atlantic . The editor s woul d lik e t o than k al l th e parti cipants t o Symposiu m LS04, hel d a t th e BU G X I Congress (Strasbourg , Apri l 2001) , wh o provide d the impetu s fo r thi s volume . Man y thank s t o th e Geological Society , London , for encouraging u s to edit the volume and for their constan t support during th e editoria l process . Th e interes t o f F . Stort i and F . Salvin i o n th e interna l architecture , evol ution an d geodynami c setting s o f intraplat e strike slip deformation belts was driven by their involve ment i n the stud y of th e Cenozoi c geodynamic s a t the northeastern edge of the Antarctic plate, funde d by the Italian National Antarctic Program (PNRA). Fabrizio Storti , Rom a Tre , Ital y Bob Holdsworth , Durham , UK Francesco Salvini , Roma Tre , Ital y
Acknowledgements The editor s than k th e followin g colleague s an d friend s wh o kindl y helpe d wit h th e reviewin g o f th e papers submitte d fo r thi s volume : John C . Behrendt, Boulder , US A Marco Bonini , Florence , Ital y Luigi Burlini , Zurich , Switzerlan d Dickson Cunningham , Leicester, U K Mike Curtis , BAS , Cambridg e Nicola d'Agostino , Roma , Italy Giorgio Vittori o Dal Piaz , Padova , Ital y Tim Dooley, London , U K Ian Fitzsimmons , Curtin , Australi a Mario Grasso , Catania , Ital y Stephane Guillot , Lyon , Franc e Martin Insley , Infoterra , Barwell , UK James Jackson , Cambridge , U K Laurent Jolivet , Paris , Franc e
Robin Lacassin , Paris , Franc e Emanuele Lodolo , Trieste , Ital y Colin Macpherson , Durham , UK Massimo Mattei , Roma , Italy Brendan Murphy , St. Francis Xavier , Canad a Claude Rangin , Paris, Franc e Jean Francoi s Ritz , Montpellier, Franc e Yann Rolland , Grenoble , Franc e Mike Searle , Oxford , UK Robin Strachan , Oxfor d Brookes , U K Christian Teyssier , Minnesota , US A Bruno Vendeville , Austin , Texa s John Waldron , Calgary , Canad a One anonymou s reviewer
Intraplate strike-sli p deformatio n belt s F. STORTI 1, R . E . HOLDSWORTH 2 & F. SALVINI 1 l
Dipartimento di Science Geologiche, Universita "Roma Tre", Largo S. L. Murialdo 1, I00146 Roma, Italy. ^Reactivation Research Group, Department of Geological Sciences, Durham University, Durham DH1 3LE, UK Abstract: Intraplat e strike-sli p deformatio n belt s ar e typicall y steeply-dippin g structure s tha t develop i n bot h oceani c an d continenta l lithospher e wher e the y for m som e o f th e larges t an d most spectacular discontinuities found o n Earth. In both modern and ancient continental settings , intraplate strik e sli p deformatio n belt s ar e o f majo r importanc e i n accommodatin g horizonta l displacements wher e they additionally for m ver y persistent zone s of weakness tha t substantially influence th e rheological behaviou r o f th e lithospher e ove r ver y long tim e period s (u p to 1 Ga or more) . Thes e deformatio n zone s provid e a fundamenta l geometric , kinemati c an d dynami c link between th e more rigi d plate-dominated tectonic s o f the oceans and the non-rigid, comple x behaviour of the continents. During convergence, the y help to transfer major displacement s dee p into the plate interiors . Durin g divergence, the y act as transfer zones tha t segmen t rifts , passiv e continental margin s and, ultimately, oceanic spreadin g ridges. Suc h belts are also of great econ omic importance, controllin g the location o f many destructive earthquakes, offshore an d onshore hydrocarbon deposit s an d metalliferous or e deposits . I n the oceans , intraplat e strike-sli p move ments ar e relativel y mino r alon g transform-relate d fractur e zones , bu t ther e ar e a n increasin g number o f documente d example s tha t ma y reflec t spatia l an d tempora l variation s i n spreadin g rate alon g individua l activ e ridg e segments .
Strike-slip deformatio n belt s ar e region s i n whic h tectonic displacement s occu r predominantl y paral lel t o th e strik e o f th e zon e (fo r a general review , see Woodcock & Schubert 1994) . Th e recognitio n of strike-slip-dominate d plat e boundarie s o r transform faults (Wilso n 1965) , togethe r wit h their geo metric linkag e to , an d kinemati c interactio n with , constructive an d destructiv e plat e margin s wer e central t o th e emergenc e o f plat e tectonic s (e.g . McKenzie & Parke r 1967 ; Morga n 1968 ; Co x 1973). Plat e tectonic theor y assume s that the lithosphere i n th e plat e interior s is , t o a firs t approxi mation, rigi d an d tha t most deformatio n relate d t o plate interaction s wil l be concentrate d int o narrow belts alon g th e plat e margins . Thi s mode l work s reasonably wel l i n region s underlai n b y oceani c lithosphere, wit h th e resul t tha t muc h o f th e seis micity alon g transfor m fault s occur s onl y wher e they for m activ e plat e boundaries . A s the y pas s into th e plat e interior , transfor m fault s becom e relatively tectonicall y quiescen t feature s known as oceanic fractur e zone s whic h for m som e o f th e largest topographi c structure s o n th e Earths ' sur face (e.g . Whit e & William s 198 6 an d reference s therein). Man y o f thes e fractur e zone s segmen t o r even boun d majo r sedimentar y basin s wher e the y impinge upo n th e continenta l margin . Example s
include th e souther n Atlanti c margin s (Francheteau & L e Picho n 1972 ) an d th e wester n margin o f Australi a (Son g e t al. 2001). In continenta l regions , intraplat e structure s an d their relationship t o plate tectonic s ar e complicate d by th e non-rigi d behaviou r o f substantia l region s of continenta l lithospher e (e.g . Molna r 1988) . Thi s behaviour i s well-illustrate d b y th e broad , diffus e zones o f seismicit y observe d i n man y continenta l regions - notabl y Centra l Asi a fro m Tibe t north wards - extendin g dee p int o th e plat e interiors . Geologically, thes e continenta l deformatio n zone s may compris e interlinke d system s o f fault - an d shear zone-bounded blocks an d flakes that partition strains and other geological processes int o comple x regions o f displacement , interna l distortio n an d rotation o n various scale s (e.g . Dewe y e t al. 1986 ; Foster & Gleadow 1992 ; Park & Jaroszewski 1994 ; Tommasi e t al. 1995 ; Butle r et al 1997 ; Salvin i et al. 1997 ; Marshak et al. 2000). It may be useful t o view suc h regions o f non-rigid behaviour as broad, diffuse plat e boundaries (e.g . se e Gordon 199 8 an d references therein) , bu t i n th e presen t pape r w e shall refe r t o al l region s locate d awa y fro m th e major plat e boundarie s a s 'intraplate' . The non-rigi d behaviou r o f continenta l litho sphere probably arise s from the presence of a weak
From: STORTI , F. , HOLDSWORTH , R . E . & SALVINI , F . (eds ) Intraplate Strike-Slip Deformation Belts. Geologica l Society, London , Specia l Publications , 210 , 1-14 , 0305-8719/037 $ 15 © Th e Geologica l Societ y o f Londo n 2003 .
2
F. STORTI, R. E . HOLDSWORTH & F. SALVINI
quartzofeldspathic crusta l laye r an d fro m pre existing mechanica l anisotropies . Suc h aniso tropies, primaril y ol d fault s an d shea r zones , ma y undergo reactivation in preference to the formation of ne w tectoni c structure s durin g regiona l defor mation episode s (e.g . Thatche r 1995 ; Holdswort h et al 1997 , 2001b) . Th e buoyanc y o f continenta l crust mean s tha t i t an d it s underlyin g lithospheri c mantle ar e no t normall y subducted . A s a result , zones o f pre-existin g weaknes s ar e effectivel y 'locked-in' t o the continents and can potentially b e reactivated many times during successive phases of continental deformatio n an d accretion . Thi s long lived architectur e o f inheritanc e i s no t generall y found in oceanic lithosphere (e.g. Sutto n & Watson 1986). Crustal-scale reactivated fault system s in the upper crus t broaden wit h dept h int o ductil e shea r zones o f regiona l extent , i n whic h substantial volumes o f lowe r crusta l an d uppe r mantl e rock s experience episode s o f reworkin g (Holdswort h e t al. 2001a) . In intraplat e region s o f th e continents , deep seated fault s o r shea r zone s ofte n manifes t them selves a t the surfac e by th e developmen t o f linea r zones o f geological , geophysica l o r topographi c features know n as lineaments (e.g . Sutto n & Watson 198 6 an d reference s therein) . Significantly , a majority o f these lineaments seem to coincide with large, steeply-incline d strike-sli p deformatio n zones (e.g . O'Driscol l 1986 ; Dal y e t a l 1989) . Seismological an d geodeti c studie s o f neotectoni c intraplate deformatio n (e.g. Molna r & Tapponnier 1975) sugges t tha t horizonta l movement s ar e pre dominantly accommodate d b y strike-sli p faulting . Thus intraplat e strike-sli p deformatio n belt s ar e particularly importan t i n determinin g th e defor mation response o f continental lithosphere i n plate interiors ove r lon g tim e scales . Interestingly , th e longevity of intraplate continental strike-slip deformation zones and their deeply penetrating natur e is central t o man y o f th e globa l tectoni c hypothese s that hav e bee n propose d prio r t o plat e tectonic s (e.g. rhegmatic tectonics; Vening Meinesz 1947 ) or as alternativ e paradigms . I t i s probabl y significan t that th e mos t prominen t o f thes e - th e Sovie t endogenous regim e mode l (Belousso v 1978 ; Pav lenkova 1995 ) - wa s developed by scientists based in an intraplate region almost entirely underlai n by continental lithosphere . Long-lived strike-sli p deformatio n zone s - o r structures that reactivate them - ar e of considerable economic significance and represent important geological hazards . Although subordinate t o the grea t concentrations o f earthquake s aroun d th e plat e margins, ther e ar e man y region s o f frequen t an d sometimes highl y destructiv e seismicit y focuse d along strike-sli p deformatio n zone s i n intraplat e regions. Example s includ e th e N an d E Anatolian
faults i n Turke y (e.g . Jackso n & McKenzie 1988 ) and th e Ne w Madri d Seismi c Zon e i n th e US A (e.g. Johnsto n & Shedloc k 1992 ; Marsha k e t al . this volume). Intraplate strike-slip fault s ofte n hav e a profoun d influenc e o n th e location , architectur e and subsidenc e histor y o f associate d sedimentar y basins, man y o f whic h ar e ric h i n hydrocarbons, forming importan t tectoni c an d palaeogeographi c boundaries. Goo d example s ar e th e man y long lived intraplat e strike-sli p fault s tha t hav e repeat edly influence d th e evolutio n o f th e ric h hydrocarbon-bearing basin s of S E Asia (e.g . Mor ley 200 2 an d references therein). Finally, there ar e numerous intraplat e strike-sli p deformatio n belt s that have acted a s channels for the flow of magma and hydrotherma l fluids , leadin g t o th e accumu lation of economically significan t or e deposits (e.g. the Easter n Goldfield s Provinc e i n th e Yilgar n Block, wester n Australia; Cox 1999) .
Size an d mechanica l significanc e o f intraplate strike-sli p deformatio n belt s In all plate tectonic settings, strike-slip deformation belts a t th e surfac e ar e characterise d b y steeply dipping anastomosin g arrays o f faults , ofte n wit h many bends an d offset s i n individua l fault strand s (Fig. la ; e.g . Sylveste r 1988 ; Woodcock & Schubert 1994) . Regionall y significan t faul t zone s ar e typically a few tens of kilometres wide and several hundred kilometre s long . B y definition , plate boundary transfor m fault s cu t throug h th e whol e lithosphere i n al l settings . Th e dee p geometr y o f intraplate faults i s less straightforward, particularly as i t i s difficul t t o imag e stee p structure s at depth . However, ou r improve d understandin g o f th e relationships betwee n faul t dimension s an d dis placement (e.g. Walsh & Watterson 1988, Cowie & Scholz 1992 ) suggest s tha t sub-vertica l strike-sli p fault zone s wit h strike-length s an d offset s greate r than 30 0 k m an d 3 0 km , respectively , ar e ver y likely t o b e o f a size sufficien t t o cu t much, if no t all, o f th e lithospher e (Fig . la) . A growin g body of geological , geochemica l an d geophysical obser vations sugges t a direct link between the mantle at depth and regional-scale strike-slip faults an d shear zones i n th e crust . I n summary , thi s evidenc e includes th e followin g (see Vauche z & Tommasi , this volume, and references therein) : i) Geologica l observation s i n ancien t exhumed mid- an d lowe r crusta l rocks preserv e many examples o f verticall y cross-cuttin g strike slip shea r zones , wit h littl e evidenc e o f detachment alon g sub-horizonta l surfaces , even i n partiall y molte n rock s (e.g . Borbor ema Province , Brazil : Vauchez et al . 1995) . ii) Chemica l an d stabl e isotop e studie s o f
INTRODUCTION
3
Fig. 1 . (a ) Cartoo n showin g ho w majo r continenta l intraplat e strike-sli p deformatio n belt s ma y ultimatel y roo t int o the asthenospher e (afte r Teyssie r & Tikof f 199 8 an d Vauche z e t al. 1998) . Strik e sli p fault s i n th e uppe r crus t pas s down int o increasingl y broa d shea r zone s (fabri c trace s show n schematically ) i n th e lowe r crus t an d lithospheri c mantle. Dashe d line s i n expose d faul t & fabri c surfac e neare r t o viewe r ar e transport-paralle l lineations . Schemati c strength v s dept h profil e fo r continenta l lithospher e show n to th e left , (b ) O n th e left , sketc h cross-sectiona l vie w of a load-bearin g laye r (suc h a s th e crus t o r lithosphere ) o f thicknes s t cu t b y a principa l displacemen t zon e (PDZ ) o f length L dippin g a t a n angl e 5° . Th e relativ e siz e o f th e PD Z i s give n b y L/t . Grap h o n th e righ t show s ho w th e relative siz e o f a PD Z rapidl y decrease s a s th e di p increases .
magmas an d hydrotherma l fluid s channelle d along larg e strike-sli p fault s an d shea r zone s suggest CO 2-rich, mantle origin s (e.g . Madagascar: Pil i e t al 1997) . iii) Man y continental-scal e shea r zone s an d reactivated fault s overli e large-scal e aniso tropies suc h a s low-velocit y zone s i n th e upper mantl e (e.g . Housema n & Molna r 2001) and/o r positive gravity anomalies asso ciated wit h localise d uplif t o f th e crust mantle boundar y (e.g . Pil i e t al. 1997) . iv) Shea r wav e splittin g measurement s
v)
(reviewed i n Silve r 1996 ) collecte d alon g several intraplat e strike-sli p fault s an d shea r zones sugges t tha t fabric s relate d t o thes e structures ar e develope d a t al l level s i n th e lithosphere, includin g th e uppe r mantl e (e.g . Fig. 1 ; Tommas i e t al . 199 6 an d reference s therein). Thes e result s ar e broadly supporte d by magnetotelluri c an d electrica l anisotrop y measurements o f dee p mantl e fabric s (e.g . Pous e t al . 1995 ; Senecha l e t al 1996) . Deep seismi c reflectio n profiling studie s have imaged numerou s examples o f regional-scal e
4
F. STORTI , R . E. HOLDSWORT H & F. SALVIN I faults tha t cu t th e entir e crus t an d tha t the y penetrate dee p int o th e mantl e (e.g . Grea t Glen Fault : McGeary 1989) .
The large-scal e mechanica l behaviou r o f the litho sphere ha s bee n investigate d widel y throug h th e application o f experimentall y derive d strengt h v s depth profiles (e.g . Goetze & Evans 1979 ; Brac e & Kohlstedt 1980 ; Kirb y 1983) . Typically , thes e assume a simpl e horizontall y layere d lithosphere , with eac h laye r havin g a unifor m composition , a limited numbe r o f competin g deformatio n mech anisms (usuall y brittl e failur e an d dislocatio n creep) an d specifie d environmenta l (P , T , strai n rate etc ) an d lithologica l (composition , grai n size , crustal thickness ) conditions . Thes e diagram s ar e gross simplification s o f th e likel y rheologica l behaviour (se e fo r exampl e Paterso n 1978 ; Schmid & Handy 1991 ) but a s first order approxi mations the y provid e usefu l informatio n concern ing the vertical distributio n o f strength in the litho sphere. I t i s generall y agree d tha t th e mechanica l properties o f th e stronges t layer(s ) wil l determin e the overal l behaviou r o f th e lithospher e (e.g . England 1983) . I n mos t continenta l settings , th e extrapolations o f experimental dat a suggest that the main load-bearin g regio n i n the lithosphere shoul d lie i n th e uppe r mantle , wit h a secondar y stron g region i n the mid-crust (Fig . la ; Molna r 199 2 and references therein) . Thi s vie w has been questione d recently b y Maggi el al. (2000a, b ) who argue that the distribution of earthquake focal depth s suggest s that th e mai n load-bearin g regio n lie s i n th e crus t and tha t th e aseismi c uppe r mantl e i s weak . Thi s conclusion i s base d o n th e premis e tha t th e pres ence o f seismicit y i s indicativ e o f strength , bu t i t remains a distinct possibility tha t the upper mantle may b e bot h aseismi c an d strong , eve n ove r lon g time scales . Mechanically wea k tectoni c discontinuitie s wil l be most significant whe n they cut through the loadbearing region s o f th e lithospher e and , fro m th e foregoing discussion , i t i s clea r tha t thi s i s parti cularly likel y fo r steeply-incline d t o sub-vertical , regional-scale strike-sli p fault s an d shea r zone s (§.g. Fig ia} , FfSff l a §iffigi § §§§ffl§iri 2 viswgsiiu , the steeper a fault, the smaller it has to be (in terms of length , are a o r displacement ) i n orde r t o cu t through a horizonta l load-bearin g laye r o f thick ness t (Fig . Ib) . Thi s ma y b e on e reaso n wh y strike-slip fault s an d shea r zone s ar e particularl y prone t o reactivatio n i n continenta l settings . Field studie s o f regional-scal e reactivate d fault s suggest that profound weakening can occur following textura l an d retrograd e metamorphi c modifi cation o f faul t rock s unde r mid-crusta l an d uppe r mantle conditions (e.g . Vissers e t al. 1995 ; Stewart et al . 2000 ; Imbe r e t a l 2001 ; Holdswort h e t al .
200Ib). Thes e processe s see m t o b e particularl y effective i n regions where a large influx o f H2O- or CO2-rich flui d ha s occurre d durin g shearing . Sub vertical strike-sli p belt s wil l focu s th e weakenin g effects o f fault-relate d processe s particularl y strongly a s al l faul t strand s ar e verticall y aligned . Thus, thei r persistenc e ove r lon g tim e scale s an d apparent importanc e i n intraplat e deformatio n regimes i s perhaps no t surprising .
Origins of intraplate strike-sli p deformation belt s The generatio n o f intraplat e strike-sli p belt s i s particularly favoure d whe n one or more of the following occurs : (i ) collisio n o f irregularl y shape d continental margin s an d indentors , a proces s tha t often lead s to lateral escape (e.g . India-Eurasia col lision; Tapponnie r e t al. 1986 ; Arabi a - Eurasi a collision generatin g th e Anatolia n faul t block ; McKenzie 1972 ; Dewey 1977) ; (ii ) deformation of lithosphere i n whic h marke d latera l variation s i n rheological strengt h occu r du e t o rift-relate d changes in crustal thickness o r geothermal gradient (e.g. Borborem a Province , Brazil ; Tommas i & Vauchez 1997 ; Vauche z e t al. 1998) ; (iii ) conver gence continue s afte r initia l continenta l collisio n (e.g. India-Eurasia collision ; Molnar & Tapponnier 1975); (iv ) relative motions amon g adjacen t plate s are governed by differen t Euleria n poles (e.g . Australia-East Antarctica-Ne w Zealand; Stoc k & Molnar 1982) ; (v ) differential rotation s occu r within a major plate (e.g. the Cenozoic motion between East and West Antarctica; Cande et al. 2000); (vi ) kinematic strai n partitionin g o f a regiona l intraplat e transpressional o r transtensiona l deformatio n (e.g . The Main Recent Fault, NW Iran; Talebian & Jackson 2002 ; se e als o Jackso n 1992) . In continenta l regions , man y intraplat e strike slip deformatio n belt s ar e reactivate d structure s that formed initially at continental plate boundarie s as transfor m faults , or , a s i n th e cas e o f trench linked an d indent-linke d strike-sli p faults , du e t o the operatio n o f plate-boundar y processe s (e.g . Woodcock 1986) . Other s hav e initiate d a s majo r dig-siig algssfliiimiik s §uy h a s §2§ani 2 §mu£§g , thrusts or rift-bounding faults. Reactivate d oceani c transforms ar e restricte d t o ophiolite s i n ancien t settings an d see m t o b e relativel y uncommon . I n all othe r cases , th e discontinuitie s hav e becom e intraplate feature s followin g continenta l collisio n and ma y hav e undergon e steepenin g int o a sub vertical attitud e tha t i s particularl y favourabl e t o reactivation. Som e intraplat e strike-sli p fault s ma y form a s ne w structure s i f n o favourabl y oriente d zones o f pre-existin g weaknes s ar e present , Once presen t i n th e continenta l lithosphere , strike-slip deformatio n zone s clearl y influenc e th e
INTRODUCTION
segmentation o f rift s an d th e resultin g location o f salient-re-entrant feature s i n passiv e continenta l margins durin g break-u p (e.g . Daly e t al. 1989) . As first recognised b y Wilson (1965) , th e resulting irregularities in the continental margin significantl y determine th e locatio n an d developmen t o f trans form fault s i n th e evolvin g spreadin g ridg e an d their associate d intraplat e fractur e zones . Signifi cantly, man y regions o f enhanced seismicit y occur along pre-existin g strike-sli p deformatio n belt s adjacent t o and continuous with the terminations of transform-related fractur e zone s i n passiv e conti nental margin s (e.g . Sykes 1978) . Thes e obser vations sugges t a direc t lin k betwee n intraplat e faulting i n continental an d oceanic lithosphere an d illustrate tha t th e structura l inheritanc e locked-u p in the continents ultimately plays an important role in controllin g th e geometri c an d kinemati c evol ution o f oceani c plates .
Termination zone s Two mai n classe s o f intraplat e strike-sli p defor mation zones are recognised based on the nature of their termination s (Fig . 2) . Transfer intraplat e strike-slip fault s occu r whe n displacemen t i s accommodated a t a plat e boundary , eithe r b y th e extrusion of a single, rigi d block (rigi d escape), o r by extrudin g a numbe r o f linke d block s wit h a rotational componen t (rotationa l escape). Confined intraplate strike-sli p faultin g occur s whe n th e displacement decreases an d is fully accommodate d by strain withi n th e plat e interior . Deformatio n patterns a t thes e faul t termination s fal l int o fou r end member types : extensional, contractional, strikeslip o r rotational (Fig. 2). In some cases, more than
Fig. 2 . Highl y conceptua l sketc h showin g the tw o main classes of intraplate strike-slip deformation belts and their different mode s o f termination .
5
one type may occur associated wit h individual terminations (see below). These second-orde r accom modation structure s form a t an angle to the maste r strike-slip fault . Th e dominan t type(s) formed will probably depen d o n th e interactio n o f th e strai n fields relate d t o faul t motio n an d shap e (loca l bends, offsets , tips ) with the mechanical propertie s of th e adjacent hos t rocks , particularl y th e orien tation o f pre-existing anisotropie s i n th e basemen t (e.g. se e Sylveste r 1988 ; Woodcock & Schuber t 1994 an d reference s therein) . I n cas e o f bloc k rotation, th e angl e betwee n th e block-boundar y faults an d the master strike-slip belt changes markedly throug h time (Scott i e t al. 1991) . Once formed , secondar y accommodatio n struc tures provid e weaknesse s int o whic h par t o f th e residual strike-sli p displacement ca n be transferred from th e maste r strike-sli p bel t (e.g . Storti e t al . 2001). Repeate d propagatio n o f th e maste r strike slip fault syste m into the plate interior ca n produce a sequence of accommodation structures becoming younger toward s th e faul t ti p (a s see n i n small scale faults: Willemse & Pollard 1998) . Thus, complex an d superimpose d structure s ca n develo p i n the terminatio n regio n o f intraplat e strike-sli p deformation belts . Examples o f terminatio n structure s i n th e ti p region o f intraplat e strike-sli p deformatio n belt s include the strike-slip faults in the northern Aegean Sea, whic h en d i n a regio n o f norma l faultin g i n central Greec e (Tayma z e t al . 1991) , an d th e Priestley Fault , a Cenozoi c intraplat e right-latera l fault syste m i n nort h Victori a Land , Antarctic a which terminate s o n its souther n sid e int o a series of extensiona l an d transtensiona l fault s includin g the Terro r Rif t (Fig . 3, Salvin i e t al . 1997 , 1998 ; Storti e t al . 2001) . Th e norther n sid e o f th e faul t termination is characterised by ESE-WNW striking strike-slip an d transpressional splay faults illustrat ing tha t bot h contractiona l an d extensiona l struc tures can form o n opposite side s of a single termination. Th e geometri c arrangemen t o f thes e termination structure s is a majo r clu e t o th e sens e of faul t movemen t (e.g . see Fig . 3 inset) . A s ye t there ar e n o palaeomagneti c dat a t o constrai n th e amount o f bloc k rotatio n abou t vertica l axe s tha t may hav e occurred , bu t ther e i s n o independen t geological evidenc e t o sugges t tha t thi s i s signifi cant (Stort i e t al. 2001) . A rotationa l an d contractiona l structura l archi tecture is developed a t the termination o f the rightlateral Sa n Gregorio-Sur-Sa n Simeon-Hosgr i faul t system, i n Souther n Californi a (Sorlie n e t al . 1999). Th e souther n Hosgr i Faul t comprise s tw o main strand s bounding compressional fold s tha t lie at hig h angl e t o th e fault s (Fig . 4). Right-latera l shear acros s th e souther n Hosgr i Fault i s absorbe d mainly b y clockwis e vertical-axi s rotatio n o f th e
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F. STORTI , R . E. HOLDSWORT H & F. SALVIN I
Fig. 3 . Structura l architectur e a t the termination o f the Priestley Fault , nort h Victori a Land , Antarctic a (se e Fig . 9 for location). Th e inse t show s ho w extensional , contractional , an d strike-sli p deformatio n accommodate s th e residua l horizontal displacement s a t the ti p o f th e faul t syste m (afte r Stort i e t al. 2001).
Fig. 4 . Cartoo n showin g th e tip of the Hosgri Fault , Cali fornia, wher e contractiona l an d strike-sli p deformation , together wit h bloc k rotatio n abou t vertica l axe s accom modate horizonta l displacement s (afte r Sorlie n e t al . 1999).
elongated block s between the fault strands , as well as b y foldin g an d thrustin g (Sorlie n el al . 1999) . Other example s o f rotationa l termination s o f intraplate strike-slip belt s include the Marlborough
fault syste m o f Ne w Zealan d (Littl e & Robert s 1997), the Sa n Jacinto fault syste m of southeastern California (Armbruste r et al. 1998 ) and the Whittier faul t syste m i n th e Lo s Angele s Basi n (Wright 1991) . Many intraplat e strike-sli p belt s en d i n area s of distributed thrusting , like thos e i n th e easter n an d northern Tibet (Molnar & Lyon-Caen 1989; Meyer et a l 1998) . Bayasgala n e t al . (1999 ) describe d field example s o f contractiona l terminatio n o f intraplate strike-slip belts in Mongolia. At both the eastern end of the Artz Bogd fault syste m (Fig. 5a) and o f fault s i n th e Toromho n regio n (Fig . 5b) , thrust faults develope d a t high angles to the strike slip faul t systems . Thrus t displacemen t decrease s progressively away from th e strike-slip faults , suggesting th e relativ e rotatio n o f th e thrus t footwall and hangin g wall block s abou t vertica l axe s (Fig . 5c; Bayasgalan et al. 1999) .
Bends an d stepover s The fault system s associated with strike-slip deformation zone s ar e rarel y perfectl y straigh t a s th e host rock s ar e invariabl y mechanically anisotropi c
INTRODUCTION
7
Fig. 5 . Simplifie d sketc h ma p o f th e structura l architecture a t th e terminatio n o f th e Art z Bog d Faul t (a ) an d Bog d Fault (b) , Mongolia, showin g th e dominan t role o f contractiona l deformation s tha t accommodat e residua l horizonta l displacement a t faul t tips . Th e progressiv e decreas e o f thrus t displacemen t awa y fro m th e maste r strike-sli p faul t suggests th e occurrenc e o f bloc k rotatio n abou t vertica l axe s (c ) (afte r Bayasgala n e t al. 1999) .
and th e fault s typicall y gro w b y th e linkag e o f second-order non-paralle l faul t segment s (e.g. Wilcox e t al . 1973) . Irregularitie s ca n b e subdivide d into tw o end-membe r types: bends wher e the faul t trace i s continuou s an d stepovers o r jogs wher e a discontinuity occurs in the fault trace (Fig 6a; Sylvester 1988 ; Woodcoc k & Schubert 199 4 and refer ences therein) . I n man y cases , stepove r zone s developed i n sedimentar y cove r sequence s nea r t o the surfac e ma y pas s downward s wit h dept h into bends i n th e faul t wher e i t cut s th e basement . Bends an d stepover s for m loca l zone s o f trans pressional (restraining ) or transtensional (releasing ) deformation dependin g o n the sens e o f overstep o r bending relativ e t o th e overal l sens e o f movemen t along th e principa l displacemen t zon e (PDZ) . A t the surface , restrainin g bends o r offset s produc e localised region s o f uplif t referre d t o a s push-ups whilst releasin g bend s o r offset s ar e associate d with the development o f pull-apart basins. In crosssections derived from seismi c reflectio n dat a across
many strike-sli p deformatio n belts, upward diverging fault patterns are commonly imaged originatin g from a singl e sub-vertica l discontinuit y a t dept h (e.g. Hardin g 1985) . Thes e ar e know n a s flowe r structures an d they ar e particularly commo n i n the region o f faul t bend s an d stopovers . A good example of the effects o f fault bends and offsets - an d what happens when the sense of shear is reverse d durin g successiv e reactivatio n epi sodes - i s provided b y the Late Archaea n t o the Late Proterozoi c Carajas-Cinzent o strike-sli p faul t systems i n th e Amazonia n Crato n o f Brazi l (Fig. 6b; Pinheir o & Holdswort h 1997 a & b ; Hold sworth & Pinheir o 2000) . Lat e Archaea n brittl e dextral movement s alon g E- W trending, sub vertical faul t zone s reactivate d pre-existin g base ment fabric s i n th e underlyin g Itacaiuna s shea r zone, down-faultin g cover sequence s o f lo w grad e and unmetamorphosed rocks into a series of releasing bend s an d offset s (Fig . 6b). Later faul t reacti vation and partial inversio n o f the cover sequence s
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F. STORTI , R . E . HOLDSWORT H & F. SALVIN I
Fig. 6 . (a ) Ma p view s o f stepove r an d ben d geometrie s found alon g strike-sli p fault s (afte r Woodcoc k & Schubert 1994) . (b) Simplified map of the structural architecture along th e Carajas-Cinzent o strike-sli p faul t systems , i n the Amazo n Crato n o f Brazi l (afte r Holdswort h & Pinheiro 2000) . Shadin g highlight s th e locatio n o f th e Archean cover rocks and the Itacaiunas shear zone in the older, underlyin g basement rocks . Lat e Archaea n fault s include the Carajas Fault Zone (CFZ), the Carajas strikeslip syste m (CaSSS ) an d th e Cinzent o strike-sli p system (CzSSS) .
Contractional-, extensional-, and strike-slip-relate d structures characteristicall y alternat e alon g thes e impressively lon g deformatio n belts, i n whic h th e internal architectur e is thought to be influence d b y inherited crusta l fabrics (e.g. Vauche z e t al. 1995; Rossetti e t al. 2002) . There i s a n ongoing debate concernin g the relative importanc e an d role s o f crusta l thickening , extensional collaps e an d strike-sli p faultin g i n bringing abou t lateral extrusion in the Tibetan Plateau an d Asia n regions t o th e N an d th e relation ship between these processes an d the collision and indentation o f Indi a (e.g . Tapponnie r e t al . 1982; Davy & Cobbol d 1988 ; Englan d & Molnar 1990; Shen et al. 2001). Irrespective of the relative merits of the various competing models, there is clear evidence in much of Asia that significant deformation and displacemen t hav e occurre d alon g a serie s o f very larg e intraplat e strike-sli p deformatio n belt s during th e Cenozoic . The simplifie d tectoni c sketc h map o f Asia published b y Jolive t e t a l (1999 ) (Fig . 7) illustrate s the tectoni c architectur e o f majo r intraplat e deformation belts , thei r impressiv e lengt h an d complexity. I n thi s interpretation , th e Pamir-Baikal Okhotsk shea r zon e comprise s interlinke d exten sional rifts , suc h a s th e Baika l basin , an d strike slip faul t segments . Th e deformatio n bel t appear s to exten d fro m th e collisio n zon e t o th e Berin g Strait, separating the stable Eurasian block from the
occurred during a subsequent sinistral shearing episode. This le d to the formatio n of complex assem blages o f folds, thrusts, oblique slip and strike-sli p faults whic h wer e preferentiall y develope d i n th e cover rock s clos e t o th e pre-existin g faul t trace s in bend s an d offset s tha t ha d becom e restrainin g features du e t o th e reversa l i n th e sens e o f shea r (Pinheiro & Holdswort h 1997a ; Holdswort h & Pinheiro 2000) . Th e adjacen t basemen t gneisse s remained comparativel y undeforme d durin g thes e later episodes , undergoing regiona l uplif t an d exhumation that stripped awa y the cover sequences everywhere except where they were initially down= faulted i n bend s an d offset s durin g dextral move ments.
Intraplate strike-sli p belt s an d plate convergence Intraplate strike-sli p belt s hav e bee n extensivel y studied i n convergen t setting s (e.g . Vauche z e t al . 1998). Thes e belt s o f localise d intracontinenta l deformation ar e typically severa l tens o f km wid e and man y hundred s o f k m lon g (e.g . Molnar & Fig. 7 . Highl y simplifie d tectoni c sketc h ma p o f Asi a Tapponnier 1975 ; Pil i e t al. 1997 ; Ludma n 1998) . based o n the interpretatio n of Jolive t e t al . (1999) .
INTRODUCTION
deformed part of the Asian plate (Davy & Cobbold 1988). Thus, th e Pamir-Baikal-Okhotsk shea r zon e represents a possibl e exampl e o f a transfe r intra plate strike-sli p deformatio n belt, sinc e it connects the northwes t corne r o f th e India n indenter , th e western Himalaya n syntaxis , t o th e boundar y region o f th e Pacifi c Plate . Anothe r exampl e o f a transfer intraplat e strike-sli p deformation belt ma y be provided by the roughly N-S envelop e o f rightlateral strike-sli p faul t system s and extensional basins (bot h pull-apar t an d back-arc ) tha t develope d along th e easter n borde r o f Asi a (Fig . 7) . Thi s right-lateral intraplat e deformatio n bel t connect s the northeast corner of the Tibetan Plateau with the Pacific plate boundary region where it abuts a complex arra y o f left-latera l strike-sli p faul t system s (e.g. Jolive t e t al. 1999) . Examples of confined intraplat e strike-slip fault s include th e Re d Rive r Faul t an d th e Alty n Tag h Fault (Molna r & Tapponnie r 1975 ; Lelou p e t al . 2001) (Fig . 7) . The Red River Fault is a left-lateral intraplate strike-sli p deformatio n bel t whic h bounds th e Indonesia n bloc k t o th e north an d ter minates i n th e extensiona l domai n o f th e Sout h China Se a (e.g . Morle y 2002) . Despit e it s interna l complexity, th e Re d Rive r Faul t ca n b e broadl y described a s havin g a n extensiona l termination . The Alty n Tag h Faul t i s a EN E t o E- W strikin g left-lateral strike-sli p deformatio n bel t boundin g the Tibetan Platea u t o the north. At the point where the faul t trajector y start s bendin g clockwise , i t shows a compressiona l componen t (Fig . 7) . Th e Altyn Tagh Fault terminates agains t the NNE-SSW thrust syste m tha t bound s th e Tibeta n Platea u t o the E an d ca n thu s be describe d a s havin g a con tractional termination .
Intraplate strike-sli p belts and plate divergence The occurrenc e o f strike-sli p belt s tha t ar e signifi cantly active in intraplate regions past or present is uncommon i n divergen t plat e boundarie s tha t ar e more generall y dominate d b y se a floo r spreadin g and passiv e margi n development . Substantia l strike-slip movement s d o no t occu r alon g oceani c fracture zone s onc e the y pas s outboar d o f thei r associated ridg e segment s an d away from th e plat e boundary (Fig . 8a) . A good example of intraplate strike-sli p faultin g in a divergent setting comes from the Cenozoic tectonic evolutio n a t the eastern edge o f the Antarcti c Plate, whic h include s th e Souther n Ocea n eas t o f 139°E, nort h Victoria Land, and the Ross Sea (Fig. 9). Interpretatio n o f seismi c reflectio n profile s i n the Ros s Se a an d correlatio n o f th e offshor e tec tonic fabri c wit h th e onshor e majo r structura l lin eaments allow s reconstruction o f a tectoni c archi -
9
Fig. 8 . Conceptua l cartoon showing the possible relation ships betwee n transfor m faultin g an d spreadin g rate s a t mid oceani c ridges , (a ) "conventional " geodynami c framework wit h constan t spreadin g rat e an d transfor m faulting confine d betwee n ridg e segments . Out-of-ridg e transform segment s ar e inactive (fracture zones) , (b ) Differential spreadin g rates at the plate boundary cause plate segmentation by active intraplate strike-sli p faul t system s that includ e bot h "classical " transfor m fault s an d thei r associated fractur e zone s alon g strike .
tecture dominate d b y NW-SE-striking right-latera l strike-slip faul t system s i n nort h Victori a Land , which t o transfe r thei r horizonta l displacemen t i n to the N-S trending basins of the Ross Sea (Salvini et al. 1997) . The continuity o f the NW-SE strikin g right-lateral strike-sli p deformatio n belt s con necting the Ross Sea into the impressive, co-linea r fracture zone s o f th e Souther n Ocea n i s demon strated b y th e developmen t o f prominen t recen t positive flowe r structure s i n reflection seismi c pro files recorded acros s the seismicall y activ e Balleny Fracture Zon e adjacen t t o th e continenta l shel f (Spezie e t al . 1993) . Thi s evidenc e suggest s tha t major fracture zones in the Southern Ocean, east of 139°E, ar e tectonically activ e an d that right-latera l partitioned transtensio n i n th e wester n Ros s Se a (Wilson 1995 ; Rossetti e t al. 2000) accommodate s transform shea r i n th e Souther n Ocea n (Salvin i e t al. 1997) . Suc h a n excess shear appea r to be transmitted fro m th e oceani c ridge s t o th e Ros s Se a through a networ k o f long , intraplat e strike-sli p deformation belt s cuttin g acros s bot h oceani c an d continental lithospher e (Fig . 9) . Simila r processe s might als o explai n wh y som e o f th e larges t intra -
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F. STORTI , R. E . HOLDSWORTH & F. SALVINI
Fig. 9. Cenozoi c geodynamic framework a t the northeastern edge of the Antarctic Plate showing the intraplate termination of transform shear by transtensional faulting the western Ross Sea. (after Salvin i et al. 1997). Earthquake location is fro m th e Harvar d CM T catalog.
plate shock s in the continents ar e located alon g preexisting fault s locate d inlan d fro m th e en d o f oce anic transfor m faul t fractur e zone s (e.g . se e Sykes 1978) . One possibl e explanatio n fo r th e occurrenc e o f intraplate strike-sli p deformatio n belt s i n passiv e margin setting s ma y relat e t o change s i n th e spreading rat e alon g mid-oceani c ridges . Plat e tec -
tonic theor y generall y assume s rigidit y s o that th e rate o f spreadin g is constant and is proportional t o the distanc e fro m th e Euleria n pole . I f th e rigidit y constraint i s relaxe d (Gordo n 1998) , however , intraplate strike-sli p movement s alon g transfor m fracture zone s an d a t thei r termination s ar e poss ible. I n particular, differences in the spreadin g rate at th e mid-oceani c ridg e i n adjacen t transfor m
INTRODUCTION fault-bounded compartment s coul d lea d t o strike slip shea r alon g th e intraplat e fractur e zone s (Fig . 8b). Th e sens e o f shea r i n th e intraplat e segment s is towards the ridge in the low-spreading plate sec tors an d awa y from th e ridg e i n th e fas t spreadin g sectors. Th e exces s shea r alon g th e intraplat e strike-slip belt s ca n terminat e i n th e oceani c plat e interior or in the continental passive margin following on e o r mor e o f th e terminatio n mechanism s described earlier .
Conclusions Intraplate strike-sli p deformatio n belt s for m som e of th e mos t prominen t tectoni c an d topographi c features o n bot h th e Eart h and , possibly , othe r planets (e.g . Grumpie r e t al. 1986) . A majorit y o f these structure s appear t o originate i n plate boundary deformatio n zones an d in the continent s where the lithospher e i s no t subducted , the y becom e incorporated int o the plate interior by the processes of collisio n an d accretion . Onc e establishe d the y actively transfe r displacements fro m plat e margin s into the interior regions, fundamentally influencing the location an d evolution of a broad range of geological features , including sedimentary basins, orogenic belts, active sesimicity , hydrothermal activity and magmatism . I n th e continent s especially , the y form majo r persisten t zone s o f apparen t weakness whose influenc e ma y b e fel t ove r man y hundreds or eve n thousand s o f millio n years . I t therefor e seems likel y tha t intraplat e strike-sli p deformatio n belts for m on e o f th e most significan t source s o f long-term mechanical anisotrop y in the lithosphere. Financial suppor t for this work wa s provided by the Ital ian Programm a Nazional e d i Ricerch e i n Antartid e (PNRA; grant s t o F . Salvini) . Nige l Woodcock , Mar k Allen an d Jonatha n Turne r ar e thanke d fo r detaile d an d thoughtful reviews .
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Wrench fault s dow n to the asthenosphere : geological an d geophysical evidenc e an d thermomechanical effect s A. VAUCHE Z & A. TOMMAS I Laboratoire de Tectonophysique, Universite de Montpellier II et CNRS, PL Eugene Bataillon, F-34095 Montpellier cedex 5, France (e-mail:
[email protected]) Abstract: W e revie w a se t of geologica l an d geophysica l observation s tha t strongl y suppor t a coherent deformation o f the entir e lithosphere in major intracontinenta l wrench faults. Tectoni c studies o f wrench fault s erode d dow n t o th e middl e to lowe r crust sho w that , eve n i n case s in which th e lowe r t o middl e crus t i s partiall y melted , strai n remain s localize d (althoug h les s efficiently) i n transcurren t shea r zones . Seismi c profilin g a s wel l a s seismi c tomograph y an d magnetotelluric soundings provide strong argument in favour o f major wrenc h faults crosscuttin g the Moh o an d deformin g th e uppe r mantle . P n velocit y anisotropy , shear-wav e splittin g an d electric conductivit y anisotrop y measurement s ove r majo r wrenc h fault s an d i n transpressiona l domains suppor t that a wrench fault fabri c exist s over most or even the entire lithosphere thickness. Thes e seismi c an d electrica l anisotropie s ar e generate d b y a crystallographi c preferre d orientation o f olivine an d pyroxenes develope d i n the mantl e durin g th e faul t activity , which is frozen i n the lithospheric mantle when th e deformation stops . The preservation of such a 'wrench fault type ' fabri c withi n the upper mantle may have major effect s o n the subsequent tectonothermal behaviour of continents, because olivine is mechanically an d thermally anisotropic . Indeed, the associatio n o f numerical model s an d laboratory dat a on textured mantl e rocks strongl y sug gests tha t th e orogeni c continenta l lithospher e i s a n anisotropi c mediu m wit h regard s t o it s stiffness an d t o hea t diffusion . Thi s anisotrop y ma y explai n th e frequen t reactivation , a t th e continents scale, of ancient lithospheric-scale wrench fault s an d transpressional belts during sub sequent tectoni c events .
Introduction Assumin
g tha t major , i.e . continental-scale , strike-slip fault s observed toda y at the surface conHorizontal displacements in transcurrent faults rep- tinu e dow n to th e bas e o f th e lithospher e implie s resent on e of the fundamental mode s of accommo- a stron g mechanica l couplin g between th e various dation o f deformation i n the crust. It is quite obvi- rheologica l layer s of the lithosphere. Thi s raises the ous tha t transcurren t fault s generate d a t transform questio n o f th e mechanica l propertie s o f th e ho t plate boundaries, like the San Andreas Fault in Cal- middl e t o lowe r crust . Strai n localizatio n shoul d ifornia o r the Alpine Fault in New Zealand, cross - remai n efficien t enoug h t o allo w th e developmen t cut th e entire lithosphere . I t is, however, less clea r o f strike-slip faults zone s at this level. I n addition, whether intracontinenta l strike-sli p faul t system s rheologica l contrast s between th e lowe r crus t and generated i n activ e margin s o r i n collisiona l th e uppe r mantl e shoul d remai n moderate ; other domains are only crustal structures or are rooted in wis e the lower crus t would behave a s a horizontal the uppe r mantle . Th e penetratio n o f a 'wrenc h decouplin g leve l i n whic h uppe r crusta l wrenc h fault type' tectonic fabric (i.e. a vertical flow plane fault s woul d root. Thes e issue s hav e been alread y associated wit h a horizonta l flo w direction ) dee p addresse d in a large number of studies on the rheolinto the upper mantle ma y have major geodynami c ogica l stratificatio n o f th e continenta l lithospher e implications, sinc e it would generate an anisotropy (e.g . Ranall i & Murphy 1987 ; Molna r 1988 ; Vauof th e mechanica l an d therma l propertie s o f th e che z et al. 1998; Meissner et al. 2002), but experilithospheric mantl e and , hence , modif y th e large - menta l dat a o n th e rheolog y o f lowe r crusta l scale rheologica l behaviou r o f continenta l plate s material s ar e s o limite d tha t thes e studie s ar e no t during subsequen t tectonic event s (Tommasi e t al. conclusive . 2001; Tommasi & Vauchez 2001). I n thi s paper , i n orde r t o evaluat e ho w dee p a From: STORTI , F. , HOLDSWORTH , R . E . & SALVINI , F . (eds ) Intraplate Strike-Slip Deformation Belts. Geologica l Society, London , Specia l Publications , 210 , 15-34 , 0305-8719/037 $ 15 © Th e Geologica l Societ y o f Londo n 2003 .
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coherent 'transcurren t fabric ' ma y penetrate , w e analyse direc t observation s fro m surfac e geology , which ar e o f cours e restricte d t o th e crust , an d indirect informatio n fro m geophysic s an d geo chemistry tha t give s a hin t o n th e crust/mantl e coupling. W e conside r evidenc e fro m activ e an d fossil tectoni c domain s an d discus s observation s from bot h individua l shea r zone s an d broa d trans pressive domains . The review of this broad dataset suggests tha t majo r wrenc h fault s d o crosscu t th e entire lithosphere . Thi s lead s u s t o discus s th e effect o f thes e lithospheric-scal e wrenc h fault s o n the thermo-mechanica l evolutio n o f continenta l plates.
Transcurrent shear zone s and strain localization i n a hot middle to lower crust If majo r transcurren t fault s wer e roote d int o th e crust, th e wrenc h deformatio n i n th e uppe r crus t must be decouple d fro m th e mantle flow. Decoupling betwee n crusta l an d mantl e deformation s i s supposed t o b e favoure d i n th e middl e t o lowe r crust (especiall y i n region s displayin g hig h geo thermal gradients ) b y th e lo w stiffnes s o f crusta l material a t hig h homologou s temperatur e (T/Tm , with T m = meltin g temperature) . I t woul d b e marked b y rooting o f the strike-sli p fault s into this low-stiffness layer , and therefore by a listric shape
of the fault i n order t o accommodate th e transitio n from a vertical to a horizontal flow plane. In this section, w e examine a set of continental scale transcurren t faults erode d t o increasingl y deeper level s fro m th e middl e t o th e lowe r crust . In al l these cases , durin g transcurrent deformation, the crustal level s exposed toda y wer e submitte d to high temperature s an d even partia l melting . Thes e levels represen t forme r low-viscosit y layer s int o which crustal-scal e strike-sli p fault s migh t hav e rooted. In northeastern Brazil , the Neoproterozoic province o f Borborema display s a complex networ k of wrench faults (Fig . 1 ) that are several hundred kilometres long and up to 30 km wide (e.g. Vauchez et aL 1995) . Satellit e images highlight a clear textura l contrast betwee n th e shea r zone s an d th e country rock. Thi s contras t i s mostl y du e t o th e transition from a predominan t low-angl e metamorphi c foli ation outsid e the shea r zone s t o a steepl y dippin g mylonitic foliatio n withi n th e shea r zones . A t th e satellite imag e scale , th e boundarie s o f th e faul t zones appea r usuall y rather sharp , althoug h in th e field a continuous transition from th e external flatlying foliatio n t o th e interna l steepl y dippin g foli ation (half 'flower-structure') i s observed where no subsequent reactivatio n conceale d th e origina l relationships. Mylonite s outcroppin g i n th e shea r zones were formed at depths of 16-18 km (P = 500
Fig. 1 . Th e high-temperatur e Borborem a shea r zon e syste m of northeastern Brazil (Vauche z et al. 1995) . (a ) Sketc h map showing the complex patter n of transcurrent faults formed during the Neoproterozoic orogeny : (1 ) Neoproterozoic granitoids, (2 ) Mid - an d Lat e Proterozoi c sedimentar y basins , (3 ) Mesozoi c sedimentar y basins , (4 ) Neoproterozoi c high-temperature shear zones, and (5) Neoproterozoic low-temperatur e shear zones, (b) and (c) are two Landsat images showing segment s o f tw o majo r high-temperatur e wrenc h faults : th e Pato s an d th e Wes t Pernambuc o shea r zones , respectively. Gre y line s i n (b ) mar k th e shea r zon e limits .
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emplaced a s syn - o r late-kinemati c dyke s (Fig . 4 ) and/or elongate d pluton s withi n th e shea r zone s (Vauchez e t al 1995 ; Neve s e t al 2000) ; thi s strongly suggest s tha t the faults were connecte d t o a partiall y melted uppe r mantle .
Fig. 2 . High-temperatur e vertica l foliatio n (S ) an d hori zontal mineral stretchin g lineatio n (L ) in a mylonite fro m the Borborem a shea r zon e system . Deformatio n i n thi s felsic mylonit e occurre d a t T > 600°C . Location o n Fig. la. Scal e ba r i s 0.5 m.
MPa) an d a t hig h temperatur e (>65 0 °C). Unde r these conditions , th e protolith s o f th e mylonite s (metasediments, pre-kinemati c intrusives , felsi c gneisses fro m th e basement) wer e partiall y melte d and th e resultin g roc k i s indee d a migmatiti c mylonite. A t thes e temperatur e conditions , felsi c rocks ar e expected t o displa y lo w viscosity, which will be further decrease d by partial melting. Nevertheless, eve n whe n th e degre e o f melting i s rathe r high, the foliatio n i n the shea r zone s remains con sistently steepl y dippin g an d bear s a shallow dipping stretchin g lineatio n (Fig . 2) . Shear-sens e indicators develope d in the partially melted mylonites consistently suppor t dextral wrenching (Fig. 3) . Evidence o f downwar d decreas e o f th e foliatio n dip, suggesting rooting o f the faults, has never been reported. O n th e contrary , a larg e volum e o f mantle-derived magmas , especiall y diorites , wa s
Fig. 3 . Migmatiti c mylonit e fro m th e Wes t Pernambuc o shear zon e (se e locatio n o n Fig . la) . Downwar d view . Intense shearin g occurred alon g a subvertical foliation in a partially melte d crust . White layers ar e leucocratic neosome. Arrow s indicate dextra l shear .
Fig. 4 . Diorit e dyke s injected in a porphyritic granodior ite emplace d i n th e Pernambuc o shea r zon e (locatio n o n Fig. la) . Dyke s wer e emplace d withi n th e transcurren t shear zon e an d deforme d before complet e solidification . No evidenc e o f solid-stat e deformatio n ha s bee n observed.
The Neoproterozoic Mozambiqu e belt i n Madagascar an d East Afric a is also characterize d by the development o f a larg e networ k o f wrenc h fault s (Fig. 5 ) a t c . 530-50 0 M a (Martela t e t al . 2000) . The present-day level of exposure shows rocks that were 20 to 30 km deep during the deformation [0.5 to 1. 1 GPa; Martelat e t al. 2000; Pili et al. 1997a] . At these depths, deformation took place at temperatures >750°C . Th e majo r shea r zone s i n thi s domain ar e typicall y severa l hundre d kilometre s long an d up to 40 km wide . Numerous minor ductile wrenc h fault s forme d unde r simila r P- T con ditions ar e also documented. The tectonic fabri c in the shea r zone s i s typica l o f ductil e strike-sli p faults: th e foliation is steeply dipping, the mineral stretching lineation i s subhorizonta l an d consistent shear-sense criteria ar e observed. Outside the shear zones, the granulites that form the country rock display a low-angl e foliatio n an d th e fabri c i s meta morphic-migmatitic rather than mylonitic. According t o Martela t e t al . (2000) , th e deformatio n regime in the southern Mozambique belt was transpressional an d th e deformatio n wa s partitioned ; transcurrent shearin g wa s localized within th e ver tical shear zones and large-scale folding accommodated transvers e shortening . Throug h a regional scale investigation of the C- and O-isotope compositions of carbonates from marbles and metabasites, Pili e t al . (1991b) hav e show n tha t CO 2 i n th e major wrenc h fault s o f th e networ k ha s a mantl e origin. Thi s suggest s tha t thes e majo r fault s wer e connected t o th e mantle . O n th e othe r hand , i n
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Fig. 5 . Th e Neoproterozoi c wrenc h faul t syste m o f Madagascar , (a ) Sketc h ma p an d simplifie d cros s sectio n D-D ' (Martelat et al. 2000): (1 ) post-Cambrian sediments, (2) granulites, (3) anorthosites, (4) granitoids, (5 ) and (6) foliation trends outsid e an d withi n shea r zones , respectively 3 an d (7 ) majo r brittl e faults , (fe ) gf| 4 (g ) LSfidBa l IrM f §S 6nSWin £ two majo r shear zonee: the Ampanihy and BOFagot a gHga f g§fi§g , f§Bg§§iiY§ly . Trie WRif e 'inclusiona ' ii i ffio Arnpamhy shear zone (b ) ar e anorthosite massifs aroun d wnic n th e steepl y dippin g myloniti c foliatio n i s deflected .
minor shea r zone s an d i n metamorphi c rock s out side the shear zones, CO 2 has a crustal isotopic sig nature. In the same region, Pili et al. (1991 a) documented a systemati c associatio n o f a short wavelength positiv e gravit y anomal y t o majo r strike-slip shea r zone s tha t als o support s a dee p rooting of th e majo r wrench fault s of the Mozam bique belt . Thi s anomal y wa s interprete d a s du e to a shallowe r crust-mantl e boundar y beneat h th e
faults. Suc h a n upwar d deflectio n o f th e Moh o might result from thinnin g of the crus t in response to th e intens e stretchin g associate d wit h simpl e shear i n th e faul t zone s (Pil i e t al . 1997 a). In northeastern Brazil, as well as in the Madagascar Neoproterozoi c belts , strai n localizatio n i n transcurrent shear zones is observed eve n at crustal levels wher e synkinematic temperature s were high enough to induce partial melting . The width of the
WRENCH FAULT S DOW N T O THE ASTHENOSPHER E
fault zone s i s extremel y larg e (severa l ten s o f kilometres) compare d t o typica l width s o f shea r zones develope d unde r lowe r temperatur e con ditions (centimetres to hundred metres). This points out that , a t these high temperatures , strai n localiz ation wa s les s efficien t an d strai n wa s distribute d over a large r volum e o f rock s tha n i s usuall y observed i n uppe r crusta l shea r zones . Rock s within th e shea r zone s displa y a high-temperature mylonitic fabri c largel y du e t o dislocatio n cree p assisted b y ver y effectiv e diffusiona l processe s (i n particular grai n boundar y migration) , an d consist ent shear criteria. In addition, petrological and geochemical observation s strongl y sugges t tha t fluids percolated fro m th e mantl e int o th e crus t alon g these majo r shea r zones , an d therefor e tha t th e faults wer e continuou s through the uppe r mantle. 4
Moho' fault s versu s lithospheric fault s
The observations presented above strongl y suppor t that majo r transcurren t fault s d o no t roo t i n som e intracrustal decouplin g level , bu t rathe r crosscu t the entire crust an d are, in some way, connected t o the upper mantle. These observations are, however, not sufficien t t o evaluat e whethe r thos e fault s ar e rooted a t th e crust-mantl e interfac e o r penetrat e deeply int o th e uppe r mantle . Clea r evidenc e sup porting that major wrench faults crosscut the Moho and penetrat e deepl y int o th e uppe r mantl e i s nevertheless obtaine d b y combinin g variou s tech niques o f geophysica l exploratio n o f th e litho sphere. Evidence may be subdivided in two groups. Seismic profiling , magnetotelluri c soundings , an d seismic tomograph y hav e image d 'Moh o faults ' (Diaconescu e t al. 1997) , i.e. discontinuities crosscutting th e Moh o beneat h severa l wrenc h fault s observed a t the surface. On the other hand, electric conductivity anisotrop y evidence d i n magnetotel luric soundings , azimutha l anisotrop y o f P n velo cities, an d S-wave s splittin g ar e directly related t o the tectonic fabric o f the upper mantle an d suppor t that the lithospheric mantl e was deformed in majo r wrench faults . Electric conductivit y anisotrop y i n th e uppe r mantle i s interprete d a s du e t o a preferre d orien tation o f graphit e film s elongate d alon g th e foli ation (Marescha l e t al . 1995 ) o r t o a n anisotropi c electrical conductivit y in a 'wet ' mantl e due to the anisotropy o f H + diffusio n i n th e olivin e crysta l (Mackwell & Kohlsted t 1990 ; Simpso n 2001) . I n both cases, a 'wrenc h fault type ' fabri c (i.e . a stee ply dippin g flo w plane , o r foliation , containin g a subhorizontal flo w direction , o r lineation ) withi n the mantl e woul d generat e a highe r conductivity parallel t o th e trac e o f th e wrenc h faul t observe d at th e surface . Seismic anisotrop y i n th e uppe r mantle , whic h
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may b e characterize d b y measuremen t o f a n azi muthal anisotrop y o f Pn velocities or by the split ting of teleseismic S-waves, results from th e lattice preferred orientatio n (LPO ) o f rock-formin g min erals durin g high-temperatur e deformatio n b y dis location creep . Wrenc h faultin g withi n th e lithospheric mantl e woul d generate a LP O o f oli vine, th e dominan t minera l phas e i n mantl e peri dotites, characterize d b y a concentratio n o f [100 ] axes clos e to the lineatio n (i.e . subhorizontal ) an d of [010 ] axe s normal to the foliation plane [Fig . 6; Tommasi e t al 1999] . Olivin e i s elastically aniso tropic. Thu s i f deformatio n produce s coheren t oli vine LPOs a t the scal e o f tens of kilometres i n th e upper mantle , it als o result s i n anisotropi c seismi c properties (Nicola s & Christense n 1987 ; Main price & Silve r 1993 ; Silve r e t al . 1999) . P-wave s that propagat e eithe r paralle l t o th e maximu m of [100] o r [010 ] axe s o f olivin e i n th e mantl e ar e respectively th e fastes t an d th e slowest . O n th e other hand , S-wave s propagatin g throug h a deformed uppe r mantl e spli t int o tw o quasi-Swaves polarize d i n orthogona l planes ; th e fastes t one i s polarize d i n a plan e containin g bot h th e maximum concentratio n o f olivin e [100 ] axi s an d the propagation direction . The delay tim e betwee n the arrival s o f th e tw o spli t wave s i s proportiona l to both the length of wave propagation path within the deforme d laye r an d th e propagatio n directio n relative to the structural fabric; the largest S-waves splitting i s observe d fo r wave s tha t propagat e a t low angle s t o th e maximu m o f [001 ] axis . A wrench faul t fabri c i n th e mantl e woul d therefor e be evidence d (Fig . 6 ) b y a fas t propagatio n o f P waves (i n particular , horizontall y propagatin g Pn waves) paralle l t o th e faul t an d a polarizatio n o f the fast spli t S-wave in a plane containin g bot h the direction o f propagatio n o f th e wav e an d th e lin eation, i.e . paralle l t o the faul t directio n fo r waves having a n almos t vertica l incidenc e (suc h as SKS, SKKS, PKS...) . I t i s als o i n thi s cas e tha t th e birefringence wil l b e th e largest , leadin g t o rela tively larg e tim e lag s betwee n th e arrival s o f th e fast an d slo w spli t S-waves . Indeed, SK S splittin g data abov e transfor m boundaries , suc h a s the Car ibbean or the Alpine fault in New Zealand, systematically displa y fas t shea r wave s polarized paralle l to th e transfor m directio n an d dela y time s signifi cantly large r tha n 1 s, whic h imply tha t th e entir e lithosphere deformed i n a strike-slip regime (Russo et al . 1996 ; Klosk o e t al. 1999) . These techniques 'probe ' the upper mantle fabric with differen t spatia l resolution s an d dept h sensi tivities. Magnetotelluri c (MT ) sounding s usin g a large spectru m o f measuremen t frequencie s allo w an evaluatio n o f th e electrica l conductivit y ani sotropy fro m th e crus t t o th e asthenospheri c mantle. However , M T dat a depen d o n bot h ani -
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Fig. 6. Cartoo n illustratin g th e concept o f lithospheric fault, i n which crustal fault zone s broaden downwar d and tend to coalesc e formin g a broad shea r zon e that cuts acros s th e entir e lithospheri c mantle . It display s the tectoni c fabri c associated wit h th e faul t withi n the crus t an d th e mantle , the crystallographi c fabri c of olivin e expected t o develo p in th e mantl e sectio n o f suc h a faul t zon e (oriente d i n th e structura l framework of th e fault : X = lineatio n an d Z = normal t o th e foliation) , an d th e splittin g o f a polarized incomin g shea r wav e tha t propagates acros s a lithospheri c mantle displayin g a 'wrenc h faul t type ' fabric . A seismi c statio n locate d abov e suc h a lithospheri c shea r zon e will record a fas t shea r wav e polarized paralle l t o th e shea r zon e tren d ( X direction) an d stron g delay time s (> 1 s) .
sotropy an d heterogeneit y o f electrica l conduc tivity, an d reliabl e anisotrop y determination s ma y only b e obtaine d whe n high-quality , long-perio d MT transfer functions ar e available and lateral conductivity gradient s ar e smal l (Simpso n 2001) . Pn waves sampl e th e uppermos t mantl e (3- 5 k m beneath th e Moho) , bu t th e measure d velocitie s depend o n both th e anisotrop y an d th e heterogen eity (i n temperatur e an d composition ) alon g th e wave path. Teleseismic S-wave s splitting provide s reliable evidence o f seismic anisotropy wit h a very good spatia l resolutio n (c. 5 0 km) , bu t thes e measurements integrat e al l anisotropi c contri butions along the wave path (which is roughly vertical from th e core-mantle boundary to the surface for th e mos t commonl y use d SKS-waves) . Th e association o f thes e technique s shoul d therefor e allow u s to better constrai n the structura l fabric of the uppe r mantle . Indeed , compariso n o f electri c conductivity anisotropy determined b y magnetotelluric sounding s an d S-wave s splittin g measure ments show s tha t th e directio n o f larges t conduc -
tivity an d th e fas t spli t S-wav e polarizatio n plan e are often almos t parallel (Wannamake r et al. 1996; Barruol e t al . 1997£ ; Simpso n 2001 ) o r mak e a slight, but consistent angle (Mareschal et al. 1995) . Ji e t al , (1996 ) interprete d thi s sligh t obliquit y a s representing the obliquity between the foliation and the shea r plan e in shea r zones . To investigat e how dee p a 'wrenc h faul t fabric ' may penetrat e int o th e uppe r mantle , w e analys e geophysical dat a fo r severa l ancien t o r activ e wrench faults an d transpressiona l belts . I n eac h case, transcurren t displacement , eithe r i n a singl e fault o r i n a broade r domai n o f transpressiona l deformation, i s supporte d by surfac e geology.
Transcurrent shear zones Recently, Pollit z e t al. (2000 , 2001) , usin g a combination o f GP S an d syntheti c apertur e rada r (InSAR) data , have show n that the deformatio n in the year s followin g th e 1992-Lander s an d 1999 Hector Min e majo r earthquake s i n th e Mojav e
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Fig. 7 . (a ) Structura l sketc h displayin g the activ e fault s i n California , (b ) Shear-wav e splittin g i n wester n Californi a from Harto g & Schwartz (2001) . Anisotropy beneat h th e westernmost stations , i.e . thos e abov e th e San Andreas faul t system, results from th e superposition of two anisotropic layers. The upper layer, which corresponds t o the lithospheric mantle, i s characterize d b y a polarization o f th e fas t shea r wav e (blac k bars ) i n a plane paralle l t o th e Sa n Andrea s fault syste m an d a delay tim e clos e t o o r even highe r thanls . Th e easternmos t station s displa y a simple r anisotrop y pattern (gre y bars ) tha t ma y b e accounte d fo r b y a singl e anisotropi c laye r wit h a roughl y E- W flo w direction . A similar flow direction i s inferred for the lower anisotropic laye r (grey bars) in the westernmost California, (c ) Horizonta l velocity fiel d showin g th e contemporar y interseismi c deformatio n acros s souther n Californi a (relativ e t o a grou p o f GPS and VLBI station s on the stable North American Plate) . Geodeti c dat a include Global Positionin g Syste m (GPS), Very Long Baseline Interferometry (VLBI) , and Electro-optical Distance Measurement (EDM ) obtained by the Crusta l Deformation Working Group of the Southern California Earthquake Center during the past three decades. Error ellipse s are region s o f 95 % confidence . Release 2 , 1998 , availabl e a t http://www.scecdc.scec.org:3128/group_e/release.v2 .
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desert (California , USA ) wa s abou t thre e time s greater tha n before th e earthquakes. Thi s interseismic velocity fiel d support s a right-lateral displace ment paralle l t o th e Sa n Andrea s transfor m faul t system (Fig . 7) . Accordin g t o thes e authors , th e visco-elastic relaxatio n o f th e lowe r crus t an d upper mantl e wa s th e dominan t post-seismic pro cess; thi s require s tha t th e lowe r crus t acte d a s a coherent stres s guide coupling the upper crust with the uppe r mantl e (Pollit z e t al 2001) . Thes e con clusions ar e consisten t wit h thos e draw n fro m th e analysis o f the seismi c anisotrop y measured acros s the Sa n Andreas faul t syste m slightly nort h o f th e Landers an d Hecto r Min e earthquake s are a (Silver & Savage 1994 ; Ozalaybey & Savage 1995 ; Hartog & Schwart z 2001) . Th e shea r wave s split ting parameter s retrieve d fro m a larg e numbe r of records consistentl y sugges t tw o layer s o f ani sotropy within the upper mantle (Fig. 7). The upper layer, whic h correspond s t o th e lithospheri c mantle, is characterized by a polarization o f the fas t split shea r wav e parallel t o th e Sa n Andrea s faul t system. Thi s suggest s tha t th e lithospheri c mantl e has a tectonic fabric consistent with the crustal fabric, i.e . a steepl y dippin g foliatio n bearin g a sub horizontal lineation. Both geodetic and seismologi c observations therefor e converg e toward s a coheren t deformation o f th e entir e lithosphere . The Himalayan orogen provides some of the best examples o f activ e wrenc h fault s i n a n intraconti nental setting . Thes e fault s hav e accommodate d large latera l displacement s associate d wit h th e India-Asia collisio n (e.g . Tapponnie r e t al. 1986) . The mai n fault s o f th e syste m hav e bee n mappe d over hundreds o f kilometre s an d ar e commonl y several kilometre s wide . Th e Re d Rive r fault , fo r instance, wa s recognized ove r 100 0 k m from Tibe t to th e Gul f o f Tonkin . Pha m e t al . (1995 ) hav e performed a 70-km-lon g magnetotelluri c profil e across the Red River fault syste m in North Vietnam (Yen Ba i region) . In thi s area , th e Red Rive r sys tem i s formed by thre e paralle l transcurren t faults , a fe w ten s o f kilometre s apar t (Tapponnie r e t al . 1990). Thi s M T surve y (Fig . 8 ) show s that : (1 ) each faul t i s characterize d b y a hig h conductivit y zone dow n t o th e uppermos t mantle , (2 ) th e Sm g Hing fault, th e mai n branch o f the Red Rive r faul t system, separate s tw o lithospheri c domain s presenting contraste d electrica l properties , an d (3 ) a large conductivity anisotrop y i s observed i n both the crus t an d th e uppermos t mantle ; th e directio n of highes t conductivit y i s consistentl y paralle l t o the strike of the faults. This anisotropy is consistent with a steepl y dippin g foliatio n withi n th e upper most mantl e a s wel l a s i n th e entir e crust. In Tibet , seismi c anisotrop y measurement s have been performe d abov e an d i n th e vicinit y o f tw o other well-know n majo r wrenc h faults , th e Alty n
Fig. 8. Magnetotelluri c sounding s fro m Pha m e t aL (1995) acros s th e Re d Rive r faul t system , (a ) M T geoelectrical sectio n obtaine d b y 2 D numerica l modellin g showing marke d resistivit y contrast s betwee n domain s separated b y th e faults . Eac h blo c i s characterize d b y it s longitudinal (i.e . parallel t o th e strik e o f th e faults ) an d transverse (i n brackets ) resistivitie s (i n ftm) . Low resistivity domain s beneat h eac h branc h o f th e faul t ar e displayed i n ligh t grey . Conductiv e zone s i n th e lowe r crust and uppermost mantle are displayed i n medium and dark grey, respectively, (b) MT sounding curves showing a pronounced variation in apparent resistivity between the transverse (norma l t o th e strik e o f th e faults ) an d longi tudinal directio n i n bot h th e crus t an d th e uppermos t mantle. Th e highes t conductivit y is paralle l t o th e strik e of th e faults , a result i n goo d agreemen t wit h a 'wrenc h fault type ' fabri c i n th e uppermos t mantle . Tagh an d th e Kunlu n faults . Thes e faults , severa l hundreds o f kilometre s lon g (180 0 k m fo r th e Altyn Tag h fault) , hav e accommodate d severa l hundred kilometre s o f latera l escap e durin g th e India-Eurasia collisio n (e.g . Tapponnie r e t al . 1986). Wittlinge r e t a l (1998 ) hav e performe d a seismic tomograph y stud y o f a n are a wher e th e Altyn Tagh fault juxtapose s Precambrian basement with th e Qaila m sedimentar y basin. This tomogra phy show s a southeastern domain characterized by low-velocity perturbations i n contrast with a northwestern domai n wher e high-velocity perturbations
WRENCH FAULT S DOW N T O TH E ASTHENOSPHER E
dominate. Th e limi t betwee n thes e domain s i s marked b y a low-velocit y anomal y locate d jus t beneath th e Altyn Tagh fault (Fig . 9a). Fro m thes e results Wittlinger e t al. (1998) have suggeste d that the Altyn Tagh fault in the mantle i s c. 40 km wide and i s continuou s down t o a dept h o f 14 0 k m a t least. I n addition , shear-wav e splittin g measure ments abov e th e Alty n Tag h faul t (Herque l e t al . 1999) sho w fas t spli t shea r wave s polarize d i n a plane paralle l t o th e tren d o f th e faul t an d dela y times betwee n th e fas t an d slo w S-wave s arrival s of c . 1 s. Suc h dela y time s requir e a thicknes s o f anisotropic mantle of c. 10 0 km, in agreement wit h the value s o f faul t penetratio n inferre d fro m seis mic tomograph y (Fig . 9). Shear-wav e splittin g measurement abov e an d acros s th e Kunlu n faul t (McNamara et al 1994 ; Herque l e t al. 1999 ) hav e reached simila r results . Approachin g th e Kunlu n fault zon e the orientatio n o f the fas t S-wav e polarization plan e progressivel y rotate s into parallelis m
Fig. 9 . Mantl e structur e beneat h th e Alty n Tag h an d Kunlun active faults i n Tibet, (a) Cross section displaying the mai n geologica l structure s an d th e P-wav e velocit y structure acros s th e Alty n Tag h faul t syste m (Wittlinge r et al. 1996) . Light grey and dark grey colours correspond to th e crus t an d mantle , respectively . Lighte r shade s i n both layer s indicat e domains o f lower P-wav e velocities. (b) Compilatio n o f shear-wav e splittin g measurement s across th e Kunlu n an d Alty n Tag h fault s fro m Herque l et al. (1999) . Both faults ar e characterized b y a fast spli t shear wav e polarize d paralle l t o th e tren d o f th e fault , contrasting significantl y with the anisotropy patter n away from th e faults .
23
with the trend of the fault, suggesting a shear strai n gradient an d a n upper mantle fabric similar t o that in the crust. The 2 s of delay time measured above the Kunlun fault requires a thickness o f anisotropic material >20 0 km , assumin g a steepl y dippin g flow plane and a subhorizontal flow direction, thus larger than the lithosphere thickness. This suggests that the asthenosphere fabric also contributes to the recorded anisotrop y an d deforms somewha t coher ently wit h the lithosphere . Similar observation s als o characteriz e ancien t wrench fault s whos e fabri c wa s froze n int o th e lithospheric mantl e a t the end of the orogenic evol ution. Th e Grea t Glen-Wall s Boundar y faul t (GGWBF) is a major wrenc h fault that belongs to a more complex fault array developed i n the northern segment o f th e Caledonia n bel t betwee n 42 8 an d 390 M a (e.g . Stewart e t al . 1999) . Tw o segment s of the initial fault ar e exposed: the Great Glen faul t in Scotlan d an d th e Wall s Boundar y faul t i n th e Shetland Islands . Palaeomagneti c reconstruction s suggest that several hundred kilometres o f sinistral strike-slip displacemen t hav e bee n accommodate d along thi s fault . Shear-wav e splittin g ha s bee n measured (Helffric h 1995 ) a t station s clos e t o th e GGWBF i n Scotlan d (Fig . 10; statio n MCD ) an d in th e Shetlan d Island s (Fig. 10; station LRW) . I n both stations , th e fas t spli t shea r wav e is polarized in a plan e paralle l t o th e trac e o f th e faul t an d a delay tim e of 0.94 an d 0.53 s is observed between
Fig. 10 . Shear-wav e splittin g i n th e norther n Unite d Kingdom fro m Helffric h (1995) . Initial s (e.g . MCD , LRW...) represen t th e nam e o f the stations . AP M i s th e Absolute Plat e Motio n i n th e hot-spo t framewor k calcu lated usin g Morgan an d Morgan's mode l (see Barruol et al. 1997a) . Thick gre y line north of the Shetlan d Islands marks th e locatio n o f th e UNS T dee p seismi c reflection profile displaye d i n Figur e 11 .
24
A. VAUCHEZ & A. TOMMASI
the arrival s o f th e tw o SKS-wave s fo r MC D an d LRW, respectively . Th e fas t S-wav e polarizatio n direction clos e t o th e faul t is significantl y obliqu e to the fas t polarizatio n directio n measure d a t other stations i n th e Britis h Caledonide s (Barruo l e t al. 1997'a). Interestingly , severa l seismi c profile s performed acros s th e GGWBF , i n mainlan d Scotlan d as wel l a s i n th e Shetlan d Island s (e.g . McGear y 1989; Klempere r & Hobbs 1991 ; Klempere r e t al . 1991), show a topography and a change in the seismic expression o f the Moho tightly associate d with the trac e o f th e GGWB F a t th e surfac e (Fig . 11) . These feature s have been interprete d a s du e to th e fault crosscuttin g th e Moh o an d boundin g tw o initially remot e domains tha t sho w contrasted seis mic responses. This interpretation i s in good agree ment wit h shear-wav e splittin g measurements . Altogether thes e result s strongl y sugges t tha t th e GGWBF, rathe r tha n being roote d in som e crusta l decoupling level (McBride 1995) , is a lithospheric fault that crosscuts the Moho and penetrates deeply into th e upper mantle . The well-know n Sout h Armorica n Shea r Zon e (SASZ) i n Brittany , France , i s a majo r intraconti nental transcurrent fault forme d during the Hercynian orogeny . Surfac e geolog y evidenc e o f strai n localization an d strike-sli p displacemen t ha s bee n reported i n a large numbe r o f paper s (e.g . Berth e et al . 1979 ; Jegouz o 1980) . Th e faul t i s locate d north o f th e high-pressur e domai n tha t mark s th e trace of the suture between two collided continents. A seismi c velocit y mode l o f th e structur e o f th e lithosphere dow n t o 20 0 k m beneat h Brittan y has been obtained throug h a recent passive seismolog y experiment (Grane t e t al . 2000 ; Judenher c 2000) . P-wave velocit y perturbatio n model s sho w a marked contras t betwee n tw o domain s (Fig . 12a) : the northeastern domain is characterized b y a positive velocit y anomaly , wherea s th e southwester n domain display s negativ e anomalies . Th e limi t between thes e tw o domain s coincide s wit h th e
Fig. 11. Dee p seismic reflection profil e acros s the Shetland platfor m (McGear y 1989) . Ml , M2 , M 3 indicate Moho reflectors. D refer s t o diffractio n hyperbolae .
Fig. 12. Dee p lithospheri c structure beneat h th e Sout h and Nort h Armorica n shea r zone s (SAS Z an d NASZ , respectively) i n Brittany , wes t France , (a ) P-velocit y model fro m Judenher c e t al . (i n press ) showin g tha t th e SASZ separates a northern domain characterized by high seismic velocitie s fro m a souther n domain , wher e lo w velocities dominate . Hig h P-wav e velocitie s below th e lithosphere (below 90 km) are interpreted as representing a fossi l slab , (b ) Shear-wav e splittin g measurements . Approaching the SASZ, the fast spli t shear wave polarization turn s parallel to the trend of the fault , suggestin g a coherent tectonic fabric i n both th e crust an d the mantle . In contrast, shear-wav e splitting measurements above the NASZ do not show fast shea r waves polarized parallel to the fault trend , suggesting that this latter is a crustal fault .
trace of the SASZ and is observed down to the base of th e lithosphere. In addition, th e direction o f fas t propagation o f Pn-wave s an d th e directio n o f th e polarization plan e o f th e fas t spli t shea r wav e ar e consistently parallel t o the trend of the SASZ (Fig . 12b). Th e dela y tim e betwee n th e fas t an d slo w split shea r wave s a t station s clos e t o th e SAS Z i s consistently large r tha n 1 s , als o suggestin g tha t
WRENCH FAULT S DOW N T O TH E ASTHENOSPHER E
the entire lithosphere display s a 'wrench fault type' fabric (Judenher c 2000) . Thes e result s ar e ver y consistent an d altogethe r sugges t tha t th e Sout h Armorican Shea r Zon e crosscut s th e entir e litho sphere. Combined M T an d seismi c anisotrop y measure ments (Fig . 13 ) hav e bee n recentl y performe d i n the vicinity o f th e Proterozoi c Grea t Slav e Lak e shear zon e (GSLSZ) , i n northwestern Canada (Wu et al. 2002). Thi s NE-SW-trending dextral wrench fault is 25 km wide and its magnetic expression can be correlate d ove r 130 0 km. Thi s stud y provide d interesting insight s o n th e lithospheri c structure s associated wit h thi s majo r wrenc h fault : (1 ) th e fault i s associate d wit h a crustal-scal e resistiv e zone whic h is coinciden t wit h a magnetic low, (2) the resistivity structur e in the lowe r crus t to uppe r mantle i s approximatel y 2 D wit h a geoelectri c strike N60°E parallel to the large-scale trend of the GSLSZ, an d (3 ) ther e i s a clos e parallelis m between the orientatio n o f the fast spli t shea r wave polarization plan e an d th e geoelectri c strik e retrieved fro m long-perio d M T measurements. This similarity o f seismi c an d electri c conductivit y anisotropies suggest s that they both have a n origin related t o the wrench fault fabri c of the lithospheri c mantle beneat h th e GSLSZ .
Transpressional orogenic domains Often, orogeni c domain s as a whole have been submitted t o a transpressiona l deformatio n charac terized b y the association of thrusting normal to the belt an d lateral escap e accommodate d b y transcur-
Fig. 13. Compariso n o f magneti c fiel d data , M T high conductivity strike s fo r the period ban d o f 20-500 s , and SKS fas t direction s fo r th e Grea t Slav e shea r zon e (W u et al . 2002) . H an d L refer t o magneti c high s an d lows , respectively. Dela y time s betwee n th e arrival s of the two split SKS-wave s ar e o f 1.1-1. 5 s.
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rent faulting parallel t o the belt. Recently, Meissner et al . (2002 ) usin g P n anisotrop y measurement s have show n tha t i n suc h domain s th e uppermos t (sub-Moho) mantl e i s characterize d b y a fas t propagation o f P-waves parallel t o the trend o f the belt, pointin g t o a flo w fabri c i n th e uppermos t mantle dominate d b y the lateral escape of lithosph eric blocks . Shear-wav e splittin g measurement s i n active and fossil orogeni c areas als o record orogen parallel flo w direction s i n th e uppe r mantl e (e.g. Vauchez & Nicola s 1991 ; Savage 1999 ; Silver e t al. 1999) . Fas t shea r wave s ar e polarize d paralle l to th e tren d o f th e transpressiona l belts , eve n i n domains wher e crusta l deformatio n i s essentiall y accommodated b y thrusting , an d dela y time s frequently attai n 1 s indicat e tha t thi s 'wrenc h faul t type' flo w fabri c affect s th e entir e lithospheri c mantle. Taiwan i s currently deformin g in response to the oblique convergenc e betwee n th e Philippine s an d the Eurasia n plates. A s a result, th e crus t display s evidence o f a transpressive deformatio n an d strai n is partitione d betwee n thrustin g and wrenc h fault ing norma l an d paralle l t o th e belt , respectivel y (e.g. Huan g e t al . 2000 ; Lalleman d e t a l 2001) . Shear-wave splittin g measurement s b y Ra u e t al . (2000) display , nevertheless , a coheren t patter n over th e entir e Taiwa n Islan d (Fig . 14). S-waves generated in the Benioff Zon e by local earthquakes that prob e th e mantl e abov e th e subductio n zon e are split . Th e fas t shea r wav e i s polarized paralle l to the tectonic grai n an d delay time s ar e up to 2 s. These observation s sugges t tha t th e uppe r mantl e beneath Taiwa n has a homogeneou s transcurrent/transpression fabri c du e t o northwar d tectonic escape , i.e. a transport directio n parallel to the activ e orogen . The Neoproterozoi c Ribeir a orogeni c bel t o f southeastern Brazil formed durin g the final amalgamation o f Gondwan a betwee n 58 0 an d 54 0 M a (Egydio-Silva e t al 2002) . Th e souther n an d central domain s o f th e bel t wer e subjecte d t o a n oblique convergenc e betwee n th e Sout h America n and Africa n protocontinent s (Fig . 15a) . Thi s resulted i n developmen t o f numerou s dextra l wrench faults , hundred s of kilometres lon g an d u p to 1 0 kilometres wide , oriented paralle l o r slightl y oblique to the belt. In the central domain , th e current leve l o f erosio n (17-2 0 km ) show s mylonite s that forme d a t high temperatur e ( T > 800°C ) an d continued t o defor m durin g a slo w coolin g dow n to c . 740°C . Southward , the erosio n leve l i s mor e superficial an d th e shea r zone s ar e marke d b y mylonites forme d unde r amphibolit e facie s meta morphic condition s (Vauche z e t a l 1994) . Th e wrench fault s reworke d a slightl y olde r low-angl e foliation du e to thrusting towards the South American protocontinent . Durin g th e lat e orogeni c
26
A. VAUCHE Z & A. TOMMAS I
Fig. 14. Dee p structur e beneath th e activ e Taiwa n orogen . (a ) Simplifie d ma p showin g the geodynami c situatio n of the Taiwa n oroge n (afte r Lalleman d e t al. 2001) . (b ) Shear-wav e splittin g measurement s (Ra u e t al . 2000 ) usin g S waves fro m loca l earthquake s an d teleseismi c ScS .
stages, bot h orogen-norma l thrustin g an d orogen parallel wrenc h faultin g occurred . A s a whole, th e southern-central Ribeir a bel t represent s a trans pressional orogeni c segmen t abou t 10 0 k m wide and almost 100 0 km long (Trompette 1994) . Shear wave splittin g measurement s performe d ove r th e southern branc h o f th e Ribeir a bel t (Heint z e t al . 2000) have yielded a coherent patter n characterize d by a polarization o f th e fas t S-wav e in a directio n parallel to the orogenic grain (Fig. 15b) , suggestin g that th e bul k volum e o f lithospher e i n th e trans pressional domain ha s a 'wrenc h fault type' fabric . Larger delay times between the fast an d slow shear
waves arrival s (u p t o 2. 5 s ) hav e usuall y bee n retrieved fro m dat a recorded abov e o r close t o th e main shea r zones , suggestin g tha t strai n wa s no t homogeneously accommodate d bu t wa s somewhat localized i n th e mai n shea r zones . The Pyrenee s i n Wester n Europ e (Fig . 16 ) formed durin g th e Mesozoi c du e t o displacemen t o f Iberia relativ e t o Eurasia . Thi s motion , generate d by th e openin g o f th e Atlanti c Ocea n betwee n North Americ a an d Iberia , wa s mainl y accommo dated alon g th e Nort h Pyrenea n faul t (e.g . Chou kroune 1992) . At first, the deformation regim e wa s transtensive an d severa l pull-apar t basin s formed .
WRENCH FAULT S DOW N TO TH E ASTHENOSPHER E
Fig. 15 . Lithospheri c structur e o f th e Neoproterozoi c Ribeira transpressiv e belt , (a ) Cartoo n showin g the geo dynamic situatio n o f th e Ribeira-Aracuai-Wes t Cong o orogen (ligh t grey ) a t th e en d o f Gondwan a assembl y (580-540 Ma): (1) Archean and Mid-Proterozoic cratonic domains, (2) Neoproterozoic belts , (3) main wrench faults in th e Ribeir a belt , an d (4 ) large-scal e kinematic s a t th e end of the Gondwana assembly. Shaded areas mark continental domain s stabilize d before 60 0 Ma. (b ) Cor e shea r waves splittin g measurement s i n th e central-souther n Ribeira bel t an d the souther n Brasili a bel t (Heint z e t al. 2000).
Then, durin g th e fina l stage s o f th e evolutio n i t became transpressiv e an d finall y compressive . Indeed, th e Nort h Pyrenea n fault , i.e . th e ruptur e between Iberi a an d Eurasia , reactivate d a n older , pervasive transpressiv e fabri c forme d durin g th e late stage s o f th e Hercynia n orogen y (e.g . Bou chez & Gleize s 1995 ; Vauche z & Barruo l 1996) . Shear-wave splittin g measurement s performe d across th e Pyrenee s an d adjacen t area s reveale d a
27
Fig. 16 . Shear-wav e splittin g i n th e Pyrenee s an d adjac ent areas, (a) Sketch map of the main Hercynian structural directions i n th e Pyrenee s an d adjacen t regions . NP F i s for th e Nort h Pyrenea n Faul t an d SAS Z fo r th e Sout h Armorican Shear Zone (see Fig. 12) . The relative position of Iberi a relativ e t o Europ e i s th e curren t position , (b ) Shear-wave splittin g measurement s i n th e Pyrenee s (Barruol e t al . 1998) . A t eac h location , th e siz e o f th e circle i s proportional t o the delay tim e tha t is usually > 1 s an d th e lin e indicate s th e polarizatio n o f th e fas t spli t shear wave .
very consistent pattern of anisotropy (Barruo l et al. 1998). Th e fas t shea r wav e polarizatio n plan e i s usually oriente d paralle l t o th e belt, an d th e dela y between the fas t an d slow S-wav e arrivals is larger than 1 s, even beyond the Mesozoic Pyrenee s belt . Pn anisotrop y measurement s (Judenher c e t al . 1999) ar e in good agreemen t wit h S-wav e splitting measurements; the fas t propagatio n directio n o f Pn is als o parallel t o the Hercynian/Pyrenea n tectoni c fabric, suggestin g tha t th e entir e lithospher e beneath th e probe d are a ha s a coheren t 'wrenc h fault type ' fabric . The analysi s o f th e seismi c anisotrop y dat a fo r the activ e oroge n o f Taiwan , th e Neoproterozoi c Ribeira bel t an d th e Hercynian/Alpin e Pyrenea n belt lead s t o simila r conclusions . S-wave s splittin g
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A. VAUCHE Z & A. TOMMAS I
results ar e consistent wit h seismic anisotropy models i n whic h th e lithospheri c mantl e deform s b y homogeneous transpression , instea d o f th e par titioned mode displayed by the crust. However, the tectonic fabri c o f th e mantl e doe s no t correspon d to th e classica l transpressio n a s define d b y San derson an d Marchin i (1984 ; i.e . wit h a vertica l stretching), but rather to lengthening-thinning shear (i.e. plan e transpression ; Tikof f & Fosse n 1999 ; Tommasi e t al. 1999) . Thi s deformatio n regim e involves simultaneou s shortenin g norma l an d stretching paralle l t o th e tren d o f th e bel t an d results i n a latera l escap e o f th e lithospheri c mantle. Thi s ma y explai n wh y observatio n o f a seismic anisotrop y coheren t wit h orogen-norma l thrusting at the scal e of the lithosphere is so scarce (e.g. Silve r 1996) .
Lithospheric wrench faults : thermo mechanical effects The variou s example s presente d abov e converg e towards a mode l o f majo r wrenc h fault s deepl y rooted int o the uppe r mantle. Seismi c tomography and shear-wav e splittin g observation s especiall y support that the fault fabri c affects th e entire lithosphere thickness . Th e widt h o f th e domai n presenting a 'wrenc h faul t type ' fabri c probabl y ranges between several tens of kilometres for a single faul t t o severa l hundred s o f kilometre s fo r a transpressional domain involving various transcurrent and thrust faults. Moreover, seismic anisotropy observations usin g long-perio d dat a suc h a s SKSwaves imply that the olivine lattice preferred orientation associated wit h this 'wrenc h faul t type ' fabric, characterized b y horizontal [100] axes and vertical (010 ) planes , bot h parallel t o th e faul t trace , is coheren t a t scale s large r tha n 50 km. On th e othe r hand , th e olivin e crysta l doe s no t only displa y a n anisotropi c elasticity , whic h leads to th e observe d seismi c anisotropy . Th e plasti c deformation an d therma l diffusivitie s o f olivin e also ar e highly anisotropic (Kobayash i 1974 ; Durham & Goetz e 1977 ; Ba i e t al . 1991 ; Cha i e t al . 1996). Thu s i f majo r wrenc h fault s ar e charac terized by a coherent olivine lattice preferred orientation tha t affect s th e entir e lithospher e ove r domains several hundreds (or thousands in the case of a transpressiona l belt ) o f kilometre s lon g an d tens (o r hundreds ) o f kilometre s wide , thes e domains migh t als o b e th e sourc e o f a large-scal e mechanical an d thermal anisotropy within the continental lithosphere tha t may influence th e thermomechanical behaviou r o f th e plat e durin g sub sequent tectonic events .
Strain-induced mechanical anisotropy of the continental lithosphere Experimental deformation of olivine single crystals under different orientations relativ e to its crystallographic lattic e show s tha t olivin e ha s onl y thre e independent sli p system s an d tha t thes e system s display significantl y differen t strengt h o r critica l resolved shea r stres s (CRSS ) value s (Durha m & Goetze 1977 ; Ba i e t al . 1991) . Unde r high temperature conditions , th e (010)[100] slip syste m displays th e lowes t critica l resolve d shea r stress ; this means that, compared t o the other possible slip systems, fo r a give n stres s i t i s abl e to accommodate th e larges t sli p rate , or , conversely , tha t i t requires the lowest built-up resolved shear stress to accommodate a give n strai n rate . I n othe r words , for deformatio n i n th e dislocatio n cree p regime , which i s expecte d t o prevai l i n th e lithospheri c mantle i n activ e areas , olivin e display s a n aniso tropic viscosity . In a lithospheri c wrenc h fault , th e weakes t (010) [100] sli p syste m i s oriente d paralle l t o th e fault, i.e . th e olivine crystals are preferentially oriented with the (010) plane sub vertical and the [100] axis horizontal, parallel t o the shea r direction. The question i s whethe r th e anisotropi c mechanica l behaviour o f th e olivin e singl e crysta l combine d with such an LPO coherent ove r large scale s i n the lithospheric mantl e may result , a t th e scal e o f th e lithospheric mantle , i n a n anisotrop y o f viscosit y large enoug h t o influenc e th e deformatio n o f th e lithosphere durin g subsequen t tectoni c solici tations. Tommasi and Vauchez (2001) used a poly crystal plasticity mode l to investigat e the effec t o f a pervasive 'wrenc h faul t type ' fabri c froze n i n th e lithospheric mantl e o n th e continenta l break-u p process. In this work, the deformation of an anisotropic continental lithosphere in response to an axisymmetric tensiona l stres s fiel d produce d b y a n upwelling mantle plume was evaluated by calculating the deformation of textured olivine polycrystals representative o f th e lithospheri c mantl e a t differ ent position s abov e a plume head (Fig . 17) . These models show that an LPO-induced mechanical anisotropy o f th e lithospheri c mantl e ma y resul t i n directional softening , leadin g t o heterogeneou s deformation. Reactivatio n o f th e inherite d crystal lographic fabric , whic h i s favoure d b y tensiona l stresses obliqu e t o it s trend , i s characterize d b y higher strai n rates than other deformation regimes. The reactivatio n o f th e pre-existin g fabri c als o results i n highe r strai n rates tha n those accommo dated b y an isotropic mantle i n similar conditions . During continenta l rifting , thi s mechanica l ani sotropy ma y thu s induc e strai n localizatio n i n domains wher e extensiona l stres s i s obliqu e (30 -
WRENCH FAULT S DOW N T O THE ASTHENOSPHER E
Fig. 17 . Predicte d deformatio n o f a lithospher e dis playing a wrench fault typ e fabric abov e a mantle plume (Tommasi & Vauchez 2001). (a ) Strai n rate (Vo n Mise s equivalent strain rate, normalized relative t o the isotropi c behaviour) a s a functio n o f th e orientatio n o f th e radia l tensional stres s relativ e t o th e [100 ] axi s maximu m of the pre-existin g LP O fo r point s abov e th e plum e hea d periphery for three models with different initia l LPOs. (b ) Normal an d shea r component s o f th e strai n rat e tenso r (normalized b y th e Von Mise s equivalen t strain rate dis played by an isotropic polycrystal) for the model in which the initia l LP O i s th e mode l aggregate . Th e referenc e frame i s defined relative to the pre-existing mantle fabric: X i s paralle l t o th e [100 ] axi s maximum , i.e . paralle l t o the pre-existin g structura l trend, Y i s norma l t o th e pre existing shea r plane , an d Z i s vertical . Positiv e norma l strain rate s denot e extension an d negative ones, shorten ing. Gre y regio n mark s orientation s tha t ma y trigge r strain localization .
29
60°) t o th e pre-existin g mantl e fabric . Th e direc tional softening associated wit h olivine LPO froze n in th e lithospheri c mantl e ma y als o guid e th e propagation of the initial instability that will follo w the pre-existin g structura l trend . Th e inherite d mantle fabric also controls the deformation regime , imposing a stron g strike-sli p shea r componen t t o the deformation. An LPO-induced mechanica l ani sotropy ma y therefor e explai n bot h th e systemati c reactivation o f ancien t collisional belts durin g rift ing (structura l inheritance ) an d th e onse t o f trans tension withi n continental rifts . These results , obtaine d fo r a specifi c geodyn amic case, can be extended to a more genera l situation. I n majo r strike-sli p fault s an d transcurrent/transpressional orogeni c domains , th e inherited fabri c o f th e lithospheri c mantl e shoul d induce a directiona l softening , wit h th e conse quence tha t thi s fabri c shoul d b e preferentiall y reactivated. Development of new structures oblique to th e pre-existin g shea r zone s shoul d onl y b e observed whe n th e ne w tectoni c solicitation s (either distensive or compressive, Fig. 18 ) are normal o r parallel t o the inherite d foliation , i.e. whe n no shea r stresse s ar e applie d paralle l t o th e inherited fabric . I n mos t cases , reactivatio n wil l occur through transtension o r transpression, an d the relative proportio n o f simpl e an d pur e shea r depends o n th e obliquit y o f th e stres s axe s rela tively t o th e inherite d fabric . The crustal fabri c i n lithospheric-scal e shea r zones als o contribute s t o thi s mechanica l ani sotropy. Indeed , localize d deformatio n i n th e middle an d lower crus t gives ris e t o stron g LPOs . Crustal minerals , i n particula r mica s tha t ar e important phase s i n mylonites , displa y a stil l stronger mechanica l anisotrop y tha n olivine ; thei r layered structur e result s i n plasti c deformatio n accommodated by glide on the (001) plane only. In addition, strength variation in polymineralic crustal rocks ofte n give s ris e t o a millimetre - t o centimetre-scale compositiona l layerin g paralle l t o the shea r zon e that , a t a larger scale , als o contrib utes to a directional weakenin g and reactivation of the shea r zone . Finally , grain-siz e reductio n asso ciated wit h shearing i n the upper/middle crus t may result i n a n isotropi c strain-softenin g withi n th e shear zone; at these depths, the shear zone will thus act a s a plana r wea k heterogeneit y localizin g th e subsequent deformation. Repeated reactivation s o f majo r transcurren t shear zones or domains during long periods of time and th e necessit y fo r th e caus e o f thi s persistenc e to b e i n th e lithospheri c mantl e hav e bee n recog nized lon g ag o (e.g . Watterso n 1975) . Man y examples o f suc h reactivatio n i n variou s geodynamic environment s ar e availabl e i n th e literature . Tommasi an d Vauche z (2001 ) hav e alread y dis -
30
A. VAUCHE Z & A. TOMMAS I
Fig. 18 . Compressiona l deformatio n o f a lithospher e displayin g a wrenc h faul t typ e fabric . Calculate d strai n rate s (Von Mise s equivalen t strai n rate , normalize d relativ e t o th e isotropi c behaviour ) ar e displaye d a s a functio n o f th e orientation o f th e impose d shortenin g relativ e t o th e (010 ) plan e maximu m o f th e pre-existin g LPO .
cussed thos e relate d t o th e reactivatio n o f lithospheric-scale shea r zon e o r transpressiona l belts durin g continenta l rifting . S o w e wil l focu s on on e o f th e bes t illustration s o f th e reactivatio n of a collisional wrenc h fault a s a transform boundary: th e developmen t o f th e Newfoundland Azores-Gibraltar transfor m plat e boundar y a t th e northern edg e o f the central Atlanti c Ocea n durin g the Early Mesozoi c (Fig . 19) . The Newfoundland Azores-Gibraltar faul t zon e forme d a majo r Her cynian dextral strike-sli p faul t zon e that offset s th e Appalachians orogeni c fron t i n Newfoundlan d (Keppie 1989) . Durin g th e fina l stage s o f th e Appalachian-Variscan convergence , thi s faul t accommodated th e relativ e displacemen t betwee n the Iberia n an d Nort h Africa n blocks . Thi s faul t subsequently playe d a majo r rol e o n th e Centra l Atlantic initia l rifting , limitin g on e of the promontories of the North American stable margin. Indeed, the openin g o f th e centra l Atlanti c Ocea n too k place almos t simultaneousl y fro m Florid a t o th e Newfoundland-Azores-Gibraltar transfor m (th e first Centra l Atlanti c magneti c anomaly , M25 , i s identified alon g thi s entir e segmen t (Owe n 1983)) , but furthe r northwar d propagatio n o f th e Centra l Atlantic leadin g t o separation betwee n Eurasia and North America di d not occur until Late Cretaceou s
time. Fro m Mid-Jurassi c t o Lat e Cretaceou s time , the Newfoundland-Azores-Gibralta r transfor m connected th e Central Atlantic an d the Tethys oce anic basins, accommodating the differential motio n between Afric a an d Europe .
Thermal conductivity anisotropy Heat transfe r i s a ke y proces s controllin g th e Earth's dynamics , sinc e temperatur e i s a majo r parameter controllin g th e rheologica l behaviou r of both crusta l an d mantl e rocks . Therma l conduc tivity i n bot h mantl e an d crus t i s usuall y assume d to b e isotropic . Yet , experimenta l dat a sho w that , at ambient conditions, the dominant mineral phases in th e crus t an d uppe r mantl e displa y a larg e ani sotropy o f therma l diffusivity . I n olivine , fo r instance, heat conduction parallel to the [100] crys tallographic axi s is 1. 5 times faster tha n parallel t o the [010 ] axi s (Chai et al. 1996). Quart z and micas, the main constituents of crustal mylonites, also display a strongl y anisotropi c therma l conductivity , with th e highes t an d lowest conductivitie s paralle l to th e [0001 ] axi s an d withi n th e (001 ) plane , respectively (Clause ? & Huenges 1995) . This therma l anisotrop y i s als o observe d a t th e rock scale . Recent studies combining petrophysica l
WRENCH FAULT S DOW N T O THE ASTHENOSPHER E
Fig. 19. Fi t o f th e Centra l an d Nort h Atlanti c Ocea n showing tha t th e initia l rif t i n th e centra l domai n propa gated paralle l t o th e Hercynia n oroge n an d tha t th e Newfoundland-Azores-Gibraltar Hercynia n wrench faul t was reactivated in the Mesozoic as a transform fault transferring extensio n fro m th e Centra l Atlanti c basi n t o th e Tethys basin .
modelling an d thermal diffusivit y measurement s on upper mantl e rock s (Tommas i e t al. 2001 ) sho w that a deformation-induced olivine LPO may result in a significant thermal diffusivity anisotrop y i n the uppermost mantle: heat transport parallel to the olivine [100] axe s concentration (flo w direction ) is up to 30% faster tha n normal to the flow plane ([010 ] concentration). Moreover , i n th e studie d tempera ture range (30 0 t o 1250°K) , th e thermal diffusivit y anisotropy doe s no t depen d o n temperature , sug gesting i t migh t b e preserve d eve n a t highe r tem peratures correspondin g t o asthenospheri c con ditions. Seismi c anisotrop y data , lik e thos e presented i n th e previou s sections , indicat e tha t major wrench faults ar e characterized by a coherent olivine lattic e preferre d orientatio n tha t affect s th e entire lithospher e ove r domain s severa l hundred s (or thousands in the cas e o f a transpressional belt ) of kilometre s lon g an d tens (o r hundreds ) o f kilometres wide. This 'wrenc h fault type' fabric should therefore induc e a large-scal e therma l diffusivit y anisotropy in the lithospheric mantle, characterized by faster heat conduction within the shear zone parallel t o th e shea r directio n an d slowe r conductio n normal t o th e shea r zone .
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A simila r therma l anisotrop y shoul d b e presen t in th e crusta l sectio n o f a lithospheric shea r zone . Laboratory measurement s o f therma l conductivit y of gneisse s drille d i n th e KT B borehol e sho w u p to 40 % o f anisotrop y (Buntebart h 1991) . I n thes e samples, which display mineralogical composition s (quartz, micas , an d feldspars ) an d microstructure s similar t o thos e o f high-temperatur e mylonite s i n the Borborema , Ribeira , an d Madagasca r shea r zones, hea t conductio n paralle l t o th e foliatio n plane i s o n averag e 1. 2 time s faste r tha n norma l to it . A weake r anisotrop y i s observe d withi n th e foliation plane, with the highest conductivity measured parallel t o the lineation. Compariso n betwee n measured therma l conductivitie s an d thos e pre dicted b y petrophysica l modellin g suggest s that , similarly t o th e mechanica l anisotropy , th e majo r contributions t o th e gneisse s therma l conductivit y anisotropy stems from th e strong LPO of micas and quartz (Siegesmun d 1994) . Existence o f a large-scale , strain-induce d ther mal anisotropy i n the upper mantle implies that the temperature distribution , rheology, and , hence, th e upper mantl e dynamic s depen d o n its deformatio n history. Olivine orientations frozen i n the continental lithospher e ma y modif y plume-lithospher e interactions fo r instance . Enhance d therma l diffu sivity alon g lithospheric-scal e wrenc h zones , i.e . parallel t o th e olivin e [100 ] preferre d orientation , may lea d t o anisotropi c heatin g o f th e lithospher e above a mantl e plume , favourin g th e reactivatio n of thes e structure s durin g continenta l break-u p (Vauchez et al. 1997 ; Tommas i & Vauchez 2001) . Such a contro l o f th e pre-existin g lithospheri c structure o n the propagation o f a thermal anomal y may b e inferred , fo r instance , fro m tomographi c images o f th e Eas t Africa n rif t i n Keny a (Achauer & krisp-group 1994). In these images, the low-velocity seismi c anomalie s displa y tw o mai n trends: a N- S trend , paralle l t o th e surfac e expression o f the Eas t Africa n rift , an d a NW-SE trend followin g Neoproterozoi c structure s tha t were reactivate d durin g the Mesozoi c t o giv e ris e to th e Anz a rift .
Conclusion Geological an d geophysica l observation s i n activ e and fossi l orogeni c belt s converg e t o suppor t tha t major wrenc h fault s ar e roote d int o th e uppe r mantle. Hug e transcurren t shea r zone s (severa l hundreds of kilometres long and a few tens of kilometres wide ) in Brazi l an d Madagascar hav e been eroded dow n t o level s wher e deformatio n wa s accommodated unde r high-temperatur e condition s (650 t o >800°C ) i n partiall y melte d rocks . I t i s remarkable that unde r thes e high-temperatur e and , hence, low-viscosity conditions, which were highly
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A. VAUCHE Z & A. TOMMAS I
favourable t o developmen t o f a decouplin g level , no evidenc e o f rootin g o f thes e shea r zone s ha s been observed ; o n th e contrary , strai n wa s stil l localized i n wide transcurrent shear zones. Seismi c profiling, seismi c tomography , P n azimutha l ani sotropy and magnetotelluric sounding s also support that several major wrenc h faults crosscu t the Mono discontinuity an d penetrate th e uppermos t mantle . In addition, shear-wave splitting measurements and electric conductivity anisotropy above major strike slip fault s ar e i n agreemen t wit h a 'wrenc h faul t type' mantl e fabri c coheren t acros s mos t o r eve n the totalit y o f th e lithospher e thickness . Indeed , transform faul t boundarie s such as the San Andreas Fault, fo r whic h a connectio n wit h th e mantl e i s required, displa y geophysica l characteristic s simi lar t o thos e o f th e mai n intracontinenta l faults , either activ e o r fossil . A simila r conclusio n i s reached fo r transpressiona l orogeni c domain s deforming i n respons et o obliqu e convergence/collision. The existence of a 'wrench fault type ' fabri c into the continenta l mantle , beside s inducin g aniso tropic elasti c an d electrica l properties , ma y resul t in th e developmen t o f a directiona l softenin g an d an anisotropic conduction of heat in the continental mantle. Thes e anisotropi c propertie s probabl y influence th e large-scal e tectoni c behaviou r of th e continents. Reactivatio n o f th e inherite d mantl e fabric represent s i n most cases th e most economi c behaviour in terms of energy. Only in very specifi c situations (solicitation orthogonal o r parallel t o the ancient fabric) , wil l th e pre-existin g fabri c o f th e lithospheric mantl e no t be reactivated. Preferentia l propagation o f continenta l break-u p paralle l t o ancient orogeni c belt s a s wel l a s th e systemati c reactivation of major wrenc h faults probably resul t from bot h a directiona l softenin g an d a n aniso tropic hea t transfe r du e t o wrench-typ e olivine preferred orientation s froze n i n th e continenta l mantle. Finally, th e wor k by Pollit z e t al (2000 , 2001 ) that suggests that the mantle beneath active wrench faults deform s coherentl y wit h th e crus t and , i n some way , determine s th e interseismi c character istics o f th e faul t raise s th e questio n o f th e effec t of th e mechanica l anisotrop y o f th e lithospheri c mantle on the dynamics of active faults. Characteristics of the fault lik e the slip rate, the stress building rate and therefore the magnitude and the recurrence o f earthquake s coul d be affecte d b y a lower stiffness o f th e mantl e in a specifi c direction . J. M. Lardeaux and J. E. Martelat provided th e map and images of the Madagascar shea r zone s and M. Granet th e seismological result s o n the Armorican massif. We thank C. Teyssier an d L . Burlin i fo r constructiv e reviews .
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Fault developmen t an d interaction i n distributed strike-sli p shear zones : a n experimental approac h G. SCHREUR S Institute of Geological Sciences, University of Bern, Baltzerstrasse 1, CH-3012 Bern, Switzerland (e-mail:
[email protected]) Abstract: Analogu e model experiments using both brittle and viscous materials were performed to investigate the developmen t an d interaction o f strike-sli p fault s in zone s o f distribute d shea r deformation. A t lo w strain , bul k dextra l shea r deformatio n o f a n initia l rectangula r mode l i s dominantly accommodate d b y left-stepping, en echelon strike-sli p fault s (Riede l shears , R ) that form in response to the regional (bulk ) stress field. Push-up zones form in the area of interactio n between adjacen t left-stepping Riedel shears . I n cros s sections , fault s boundin g push-up zones have a n arcuat e shap e o r merg e a t depth . Adjacen t left-stepping R shear s merg e b y sideway s propagation o r lin k by shor t syntheti c shear s tha t strik e subparalle l t o the bulk shea r direction . Coalescence o f en echelon R shear s results in major, through-goin g faults zone s (master faults) . Several paralle l maste r fault s develo p du e t o th e distribute d natur e o f deformation . Spacin g between master faults i s related t o the thickness o f the brittle layer s overlying the basal viscous layer. Master faults contro l to a large extent the subsequent fault pattern . With increasing strain, relatively shor t antithetic an d synthetic faults develop mostl y between old, but still active maste r faults. Th e orientatio n an d evolutio n o f the ne w fault s indicat e local modification s of th e stres s field. In experiments lacking lateral borders, closely spaced parallel antithetic faults (cros s faults ) define block s that undergo clockwise rotatio n abou t a vertical axi s with continuing deformation . Fault developmen t an d faul t interactio n a t differen t stage s o f shea r strai n i n ou r model s sho w similarities wit h natural examples tha t have undergon e distributed shear .
Introduction 1990)
. Althoug h thes e zone s ar e dominate d b y major syntheti c strike-sli p fault s whic h ar e mutu Deformation o f continental lithospher e i s generally all y subparallel , smalle r antitheti c strike-sli p faults not confine d t o narro w linea r belt s bu t distribute d ofte n strikin g a t larg e angle s t o th e majo r fault s over broa d zone s u p t o severa l hundreds t o thou - occu r a s well. A serie s o f analogu e mode l experi sands kilometre s wid e (Molna r an d Tapponnie r ment s wa s designe d t o better understan d the com1975; McKenzi e an d Jackso n 1983) . Deformatio n ple x fault pattern i n zones o f distributed strike-sli p in th e uppe r continenta l crus t i s predominantl y shear , an d especiall y faul t developmen t an d inter accommodated b y brittl e faultin g an d i s assume d action . to be a t least partly controlle d b y distribute d flow Althoug h quite a number of experimental studie s of th e underlyin g ductil e part s o f th e lithospher e hav e investigated strike-slip faulting , mos t of them (England 1989) . At shallow depths, the presence of use d a singl e basemen t strike-sli p faul t (o r basa l a Theologicall y wea k laye r consistin g o f salt , eva - velocit y continuity ) t o induc e faultin g i n a n over porites, o r overpressure d shale s ma y als o caus e burde n consisting of sand or clay with or without a deformation i n th e overlyin g competen t sedimen - viscou s decollement (e.g. Cloos 1928 ; Riedel 1929 ; tary rock s t o be distributed . Emmon s 1969 ; Tchalenk o 1970 ; Wilco x e t al Major strike-sli p fault s occur in distributed shea r 1973 ; Nay lor e t al . 1986 ; Richard 1991 ; Richard zones, which ar e thousands of kilometres lon g an d e t al . 1995 ; Ueta e t al . 2000 ; Schopfe r & Steyre r up t o severa l hundre d kilometre s wide . Example s 2001) . I n thi s typ e o f experimen t (referre d t o a s a of such zones ar e the Proterozoic Najd fault system Riede l experiment ) fault s i n th e overburde n wer e in Saud i Arabia (Moor e 1979) , th e Dea d Se a faul t i n fac t secondar y structures generally directl y consystem (Quennell 1959) , an d the San Andreas fault necte d t o th e pre-existin g basemen t faul t an d system (e.g . Crowell 1962 ; Atwater 1970 ; Page restricte d t o it s immediat e vicinity . Th e widt h o f From: STORTI , F. , HOLDSWORTH , R . E . & SALVINI , F . (eds ) Intraplate Strike-Slip Deformation Belts. Geologica l Society, London , Specia l Publications, 210 , 35-52, 0305-8719/037 $ 15 © Th e Geologica l Societ y o f Londo n 2003 .
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the faul t zon e in map view depended o n the thick ness o f th e overburden . Experimenta l studie s o n zones o f distribute d strike-sli p shea r involve d a large variet y o f differen t experimenta l set-up s an d used mostl y cla y (Cloo s 1955 ; Hoeppene r e t al 1969; Freund 1974 ; An & Sammis 1996; An 1998) , fault goug e (A n & Sammi s 1996 ; A n 1998) , o r sand (Gapai s e t al. 1991 ; Richar d e t al 1995) . In these experiments , first-generatio n fault s generall y included both synthetic and antithetic faults ( R and R' shears , respectively) , bu t wit h increasin g bul k shear on e o f th e tw o set s starte d t o dominate . In contras t t o previou s experiments , w e use d both brittl e an d viscou s analogu e material s t o investigate faultin g i n zone s o f strike-sli p shea r driven b y basa l distribute d flow . I n ou r particula r experimental set-u p th e viscou s materia l impose s deformation to be distributed homogeneously at the base o f the mode l an d als o allow s partia l decoup ling du e t o it s contrastin g rheologica l behaviou r with respec t t o th e overlyin g brittl e materials . A t crustal scale , th e viscou s analogu e materia l represents a detachmen t leve l i n th e middl e t o lower crust , wherea s a t basi n scal e i t simulate s a weak sedimentar y laye r (e.g . evaporites) . Th e brittle analogu e materia l represent s uppe r crusta l
rocks (a t crusta l scale ) o r lithified , non-evaporiti c sediments (a t basin scale) . The aim s o f th e experimenta l programm e wer e to (1 ) stud y th e developmen t an d interactio n o f faults in zones of distributed strike-sli p shea r deformation; (2 ) investigat e th e influenc e o f varyin g boundary conditions; (3) compare results with previous studie s an d wit h natura l examples ; an d (4 ) propose criteria for identifying zone s of distributed strike-slip shea r i n nature.
Experimental apparatu s and procedure The experimental set-u p used to mode l distribute d strike-slip shea r i s show n in Figur e 1 . The experi mental apparatu s i s a slightl y modifie d versio n o f the on e use d b y Schreur s & Collett a (1998 ) an d included tw o basal plates : on e remained fixe d an d the other moved by a geared moto r drive. 50 plexiglass bars , 0. 5 c m wide , 1 cm high, an d 5 0 o r 7 0 cm long, were stacked lik e cards between tw o parallel woode n bar s attache d to th e overlyin g longi tudinal vertical walls. The plexiglass bars were laterally confine d by a thin wooden bar on either side. One end of each bar ( A in Fig. Ic , d) was attached to th e movin g bas e plate , wherea s th e othe r en d
Fig. 1 . Experimenta l set-up , (a ) Perspective view o f experimental apparatu s an d stratified model befor e deformation . Lateral boundarie s consisting o f rubber sheets i n 'confine d experiments ' ar e not shown , (b ) Vertica l sectio n through undeformed model , (c ) Bas e o f model a t initial state , (d ) Bas e o f mode l a t deforme d state .
ANALOGUE MODELLIN G O F STRIKE-SLIP TECTONIC S
was allowe d t o sli p alon g a smal l pi n ( B i n Fig . Ic, d ) attache d t o th e fixe d bas e plate . A s on e of the bas e plate s sli d th e confine d plexiglas s bar s slipped past on e another an d the initial rectangula r configuration change d int o a parallelogram , thu s simulating distribute d strike-sli p shear . Movemen t of the longitudinal sidewall and base plate occurred at a pre-set velocity applied by stepper motors with computer control . As analogu e material s w e use d quart z sand and glass powder having an average grain size of about 100 /im , an d a viscou s polyme r (polydimethyl siloxane, PDMS). San d and glass powder obey th e Mohr-Coulomb criterio n o f failur e an d the y ar e considered t o be good analogue materials for simulating brittl e deformatio n i n th e uppe r crus t (Horsfield 1977) . Thei r cohesio n i s lo w an d thei r angle o f interna l frictio n a s determine d b y shea r tests i s 36 ° fo r san d an d 37 ° fo r glas s powder . These value s ar e approximatel y simila r t o thos e determined experimentall y fo r uppe r crusta l rock s (Byerlee 1978) . PDM S ha s a densit y o f 0.96 5 g cm"3 and an average value of 5 X 10 4 Pa s for th e viscosity i n th e Newtonia n flo w regime , whic h occurs below a strain rate of 3 X 10~ 3 s^ 1 a t 24°C (Weijermars 1986) . I t i s a good analogu e materia l to simulat e viscou s flo w o f evaporite s o r rock s i n the lowe r crus t (Vendevill e 1987) . Models wer e scale d using methods discusse d b y Hubbert (1937 ) an d Ramber g (1981) . Calculate d scale ratio s ar e give n i n Tabl e 1 an d var y depending whether one intends to model (1) evaporites overlai n b y competen t sediment s (Tabl e la ) or (2) ductile lowe r crus t overlai n b y brittle uppe r crust (Tabl e Ib) . Severa l parameter s suc h a s tem -
37
perature increas e wit h depth , por e pressure , faul t zone width , grai n size , an d compactio n wer e no t incorporated i n th e mode l design . Despit e thes e limitations, partiall y scale d model s ca n generat e ideas about the origin and development o f geologi cal structures . O f specia l importanc e i s th e abilit y to monitor the evolution of the model through time, instead o f th e stati c pictur e obtaine d fro m fiel d observations o r seismi c interpretations . A layer of viscous PDMS was placed at the base of the model, directly overlying the plexiglass bars. Sand an d glass powder wer e alternatel y poure d on top t o produc e a stratifie d model . Passiv e squar e grids mad e o f coloure d san d wer e trace d o n th e upper free surfac e o f the model. The widt h of each model wa s 2 5 cm an d displacement o f the moving base plate occurred at 8 cm h-1, resulting in a shear strain rate of 9 X 10~ 5 s" 1. The array of plexiglas s bars wa s initiall y eithe r 5 0 X 25 c m o r 7 0 X 25 cm, wit h th e longes t dimensio n bein g paralle l t o the shea r direction . Th e applie d bulk shea r defor mation wa s arbitraril y chose n a s dextra l i n al l experiments. Severa l analogu e model s wer e ana lysed b y X-ra y computerize d tomograph y (CT) , a non-destructive technique whic h makes i t possibl e to visualiz e th e 3 D geometr y o f a mode l (Mand l 1988; Collett a e t al 1991) .
Experimental result s Two type s o f experiment s wer e performed : (1 ) confined experiments , i n whic h th e latera l bound aries wer e confine d b y rubbe r sheets , an d (2 ) unconfined experiment s havin g n o rubbe r sheet s along the lateral boundaries, thus allowing materia l
Table 1 . Scale ratios a t (a ) basin scale an d fb j crustal scale. Strain rate i n experiments wa s calculated from th e shear strain rate using methods discussed by Ramsay (1967) and Ramsay & Graham (1970) a.
Length
Time
Velocity
Density (g air3)
Viscosity (Pas)
Strain rat e (s-1)
Model
1 cm
1.5 hours
4.8 X 10" 5
500m 2 X 10~ 5
270000 years 6.25 X 10- 10
0.965 (PDMS) 2.5 0.4
5 X 10 4
Nature Ratio: model/nature
8 cm rr1 (basal plate ) 2.2 cm a" 1 3.2 X 10 4
b.
Length
Time
Velocity
Density (g cm- 3)
Viscosity (Pas)
Strain rat e (s-1)
Model
1 cm
1.5 hours
4.8 x icr 5
5000 m 2 X 10- 6
2700000 year s 6.25 X 10" 11
0.965 (PDMS) 2.5 0.4
5 X 10 4
Nature Ratio: model/nature
8 cm hr1 (basal plate ) 2.2 cm a"1 3.2 X 10 4
1021 5 X 10~ 17
3 X 10~ 15 1.6 X 10 10
1019 5 X 10~
15
3 X 10~ 14 1.6 X 10 9
G. SCHREUR S
38
to mov e freel y sideways . Tw o experiment s wer e repeated t o check fo r reproducibility. Faul t pattern and faul t orientation s wer e nearl y identica l i n sur face vie w a t simila r stage s o f deformation , thu s demonstrating tha t results wer e reproducible . Sev eral experimenta l parameter s varie d betwee n experiments i n orde r t o asses s thei r influenc e o n the resultin g structure s (Tabl e 2) . Th e mos t influ ential parameters were thickness of the brittle cover and th e leve l o f confinemen t alon g th e latera l boundaries. We first describe an d illustrate in detail the structura l evolutio n o f tw o confine d experi ments having identical viscous layer thickness, but different brittl e cove r thickness . Subsequently , w e will describ e th e faul t evolutio n i n a n unconfine d experiment.
Confined experiment 1638: 0.5 cm viscous PDMS and 1.5 cm brittle layers During th e initia l stage s o f bul k dextra l shear , deformation i n th e brittle layer s occur s b y distrib uted grai n flow . Wit h increasin g shea r strain , dis crete faulting become s th e dominant mechanism of strain accommodation (Fig . 2) . After a shear strain of abou t y = 0.1 0 dextra l strike-sli p fault s (synthetic Riede l shears ; R i n Fig . 2a ) develop . They ar e e n echelo n an d left-stepping , an d thei r traces strik e between 1 7 and 24° from th e impose d bulk shear direction . Almost simultaneously , sinis tral strike-sli p fault s appea r strikin g a t 7 2 t o 78 ° (antithetic Riede l shears ; R ' i n Fig . 2a , b) . R ' shears ar e restricte d t o th e acut e corner s o f th e model an d ar e considere d t o b e a n edg e effec t related directl y t o th e scisso r effec t o f th e deforming model . Wit h increasin g shear , domain s with a slight vertica l relief (push-u p zones) appea r in the are a comprised betwee n two left-stepping R shears (Fig . 2b , c) . Th e lon g axi s o f th e push-u p zone i s paralle l t o th e strik e o f th e R shears . A s
individual R shear s propagat e alon g strike , the y overlap wit h adjacen t left-steppin g R shears , an d the propagating fault segments acquire gentler dips. The di p directio n o f individua l R shear s change s along strik e and the footwall becomes th e hanging wall wit h a smal l revers e offse t a t eac h faul t tip . Coalescence o f R shear s ma y occu r i n tw o ways : (1) individua l faul t segment s o f closel y adjacen t left-stepping R shear s propagat e alon g strik e and , as the y overlap , thei r surfac e strik e decrease s and they merge with an adjacent R shear, (2) short dextral strike-sli p faults form i n th e overla p are a between two adjacent left-stepping R shears, whose traces strik e a t a n angl e (lowe r angl e syntheti c shear, R L in Fig. 2c, d) with respect to the impose d shear direction that is lower than the angl e of older R shears . Coalescence o f e n echelo n R shear s result s i n the formation of a slightly anastomosing shea r zone that strike s a t a n overall angl e of abou t 15 ° an d t o which w e refe r a s maste r faul t (Fig . 2d) . I n ou r distributed shear experiments, severa l master fault s form subparalle l t o on e another . Thes e long-live d master fault s accommodat e mos t o f th e displace ment. With additional shear two new types of faults form, mostl y confine d betwee n maste r faults : (1 ) sinistral strike-sli p fault s strikin g a t angle s lowe r than R ' shear s (lowe r angl e antitheti c shear s o r cross faults , R' L i n Fig . 2d , e ) an d (2 ) dextra l strike-slip faults (lowe r angl e syntheti c shears , R L in Fig . 2d , e ) tha t strik e a t angle s lowe r tha n th e older R shear s Wit h increasin g shear , secondar y faults generall y strike progressively at lower angles with respect t o the shear direction. During the final stage o f th e experimen t ne w cros s fault s strik e a t angles o f les s tha n 50° , wherea s new lowe r angl e synthetic fault s ar e subparalle l t o th e shea r direc tion o r eve n strik e a t a smal l angl e counter clockwise wit h respec t t o th e impose d shea r direction.
Table 2. Parameters and boundary conditions used i n analogue models. Bold experiments ar e discussed i n detail i n the text; Experiment 1959A was not analysed by X-ray computerized tomography
Experiment number
Nature of transverse borders
Initial dimensions of stratifie d model (cm)
Initial thickness of viscous laye r (cm)
Initial thickness of granular material s (cm)
Maximum shear strai n
1553 1625 1638 1666 1959 1959A
Unconfined Confined Confined Confined
50X25 50X25 50X25 70 X 25 70 x 25 70 X 25
0.5 0.5 0.5 0.5 0.5 0.5
3.0 3.0 1.5 3.0 3.0 3.0
0.57 0.57 0.62 0.56 0.37 0.33
Unconfined
Unconfined
ANALOGUE MODELLIN G O F STRIKE-SLI P TECTONIC S
Experiment 1638
39
Fault geometr y i n vertica l section s i s visualized using transvers e computerize d tomograph y (CT) scans, which show vertical or slightly arcuate faults that exten d dow n t o th e bas e o f th e brittl e layer s (Fig. 3 a, b) . Usin g closel y space d sequentia l C T scans of the final stage of the experiment, compute r visualization softwar e allowed u s to generat e hori zontal slice s (Fig . 3d) an d 3 D perspectiv e view s (Fig. 3e , f) . Th e horizonta l slic e nea r th e bas e o f the mode l clearl y show s anastomosin g syntheti c master fault s (R ) an d bot h secondar y cros s fault s (R'L) an d lowe r angl e syntheti c fault s (R L) confined in betwee n maste r fault s (Fig. 3d). The perspective view s illustrat e push-u p zone s i n area s where e n echelon R shear s overla p (arrow s in Fig. 3e). Th e faul t plane s boundin g th e push-u p zones typically steepe n downwar d an d hav e a smal l reverse componen t o f slip . (Fig . 3f). Th e push-u p zone indicate d b y the righ t whit e arro w i n Fig. 3e has late r bee n transecte d b y a younger R L shear .
Confined experiment 1666: 0.5 cm viscous PDMS and 3 cm brittle layers Left-stepping e n echelo n R shear s for m initially , striking a t 17-23 ° wit h respect t o th e shea r direc tion, wherea s a fe w antitheti c strike-sli p fault s striking a t 71-80° develo p nea r th e acut e border s of the model (Fig. 4a). In vertical cros s section s en echelon an d overlappin g R shear s creat e push-u p zones (labelle d '+ ' i n Fig. 4a). R shea r plane s ar e vertical o r slightly arcuat e an d may merge a t depth (Fig. 4a) . With increasin g shear , R shea r plane s coalesc e to for m majo r through-goin g maste r fault s (Fig . 4b). Th e spacin g betwee n maste r fault s i s large r than i n th e previou s experimen t havin g a thinne r brittle layer . Onc e th e maste r fault s form , second ary cros s fault s (R' L ) an d lowe r angl e syntheti c faults (R L) develo p in between . Th e surfac e strik e of thes e newl y forme d fault s decrease s wit h additional shear . Durin g th e fina l stage s o f defor mation (Fig. 4c), a new generation of synthetic and antithetic fault s form s locally , clos e t o th e are a where maste r fault s an d cros s fault s intersec t (e.g. faults A , B an d C i n Fig. 4c). The coalescenc e o f master faults an d younger cross faults an d RL faults
Fig. 2 . Sequentia l development of faulting in experimen t 1638. Overhea d photograph s wit h superpose d lin e drawings fo r fiv e successiv e stage s o f distribute d strike-sli p shear. Thi n line s represen t passiv e marker s o n th e san d layer's uppe r surfac e (initiall y squar e grid) ; thic k line s represent trace s o f visibl e faults . R = syntheti c Riede l shear, R' = antithetic Riedel shear , R L = lower angle synthetic fault , R/ L= lower angle antithetic fault (cros s fault) .
40
G. SCHREUR S
Fill ?• Flai l IrcBMon i n eK5crimcrtf 1538 : la} Vortica l acction a a t 7 = 0.19; orientatio n o f section! indicated i n nig. 2rL (ft ) Vertical section s a t 7 - 0.37 ; oncntatio n o f section s indicate d i n Fig . 2d . (c ) Line drawin g fro m overhea d photograph showin g faul t patter n a t y = 0.60. (d ) Horizonta l slic e 7 mm above base o f model a t y = 0.60. Locatio n of sectio n i s show n in Fig . 3c . (e ) 3 D perspectiv e vie w o f mode l a t y = 0.60. Not e th e push-u p zones indicate d b y white arrows , (f ) 3 D perspective cut-ou t a t y = 0. 60 . Fo r notatio n se e Figur e 2 .
at the surfac e an d a t depth i s illustrated b y th e 3 D block diagrams in Figure 5. The evolutio n o f faul t geometr y i n vertica l sec tion is shown in Figure 6 . For a low amount of bulk strain, subvertica l fault s correspon d t o R shear s in
surface view . Closel y adjacen t R shear s (left stepping i n surfac e view ) converg e downwar d and delineate a push-up zone (Fig. 6), marked by slight vertical relief. Lower angle synthetic faults, linkin g overlapping e n echelo n R shear s i n surfac e view ,
ANALOGUE MODELLIN G OF STRIKE-SLI P TECTONIC S
41
Fig. 4. Sequentia l developmen t of faulting i n map view and cross section for experiment 1666 . Overhead photographs with superpose d lin e drawing s of visibl e fault s an d vertica l section s (C T images) ar e show n for successiv e stage s of distributed strike-sli p shear , (a ) y = 0.19; push-up zones labelled b y '+' . (b ) y = 0.37. (c) y = 0.56. The area covere d by th e C T scanne r wa s slightl y smalle r tha n th e widt h o f model , an d therefor e a smal l par t o f th e left-han d sid e o f the sectio n wa s no t considere d i n imag e computing . Notations a s i n Figur e 2 .
extend dow n t o th e bas e o f th e brittl e layer s o r merge a t dept h wit h R shears . Th e mor e diffus e fault zone s correspond t o cros s fault s tha t intersec t the CT acquisition plane a t a low angle thus reduc-
ing th e resolution . Th e sligh t shif t i n positio n o f faults i n successiv e section s i s due to th e displace ment o f fault s wit h respec t t o th e fixe d sectio n orientation durin g progressive bul k shear .
42
G. SCHREUR S
Fig. 5 . Bloc k diagram s o f faul t patter n a t fina l stag e o f distributed shear deformation i n experiment 1666 . (a ) 3D perspective view , (b) 3D perspective cut-out. For notation see Figure 2 .
Unconfined experiment 1553: 0.5 cm viscous PDMS and 3 cm brittle layers The absence of transverse rubber sheets in laterally unconfined experiment s allow s materia l t o mov e sideways durin g shea r an d result s i n a faul t evol ution (Fig . 7 ) markedl y differen t fro m tha t i n lat erally confine d experiments . Dextra l strike-sli p faults for m a t a shea r strai n o f abou t 0.09 . Thes e synthetic fault s ( R shears ) nucleat e a t th e uncon fined lateral boundarie s and propagate toward s the central par t o f th e model . Thei r trace s strik e between 2 8 an d 35 ° fro m th e directio n o f applie d bulk shear. Sinistra l strike-sli p fault s appear almos t at the sam e time an d strike a t about 70° (antithetic Riedel shears; R') . With increasin g bul k shear , ol d R shear s remai n active , an d som e o f the m propa gate alon g th e entir e lengt h o f th e model . A t th e same tim e ne w fault s form . The y ar e mostl y restricted t o area s locate d betwee n subparallel oriented major R shears (maste r faults) an d include evenly space d R' L shear s (cros s faults ) strikin g a t 60-65° an d R L shears . Cros s fault s rotat e wit h increasing strain , propagat e sideways , an d acquir e a sigmoida l Z shap e i n pla n view . The y hav e a small dip-slip component an d the sense o f fault di p changes alon g strike . Strike-sli p displacemen t along cros s fault s i s mino r compare d wit h tha t along maste r faults . Cros s fault s usuall y merg e
Fig. 6 . Vertica l section s fo r successiv e stage s o f distrib uted strike-sli p shea r i n experimen t 1666 . Heigh t o f brittle-viscous mode l i s abou t 3. 5 cm . Fo r notatio n se e Figure 2 .
with o r terminat e agains t maste r faults . Transten sional graben s develop mostl y nea r the unconfined lateral borders . A s shea r increases , th e array s o f cross fault s an d intervenin g unfaulte d domain s undergo significant clockwis e rotatio n about a vertical axis. At the end of deformation ( y = 0.57) th e central segment s of sigmoida l cross fault s strik e a t right angle s an d rotation amount s t o about 30°. 3 D views an d horizonta l slice s illustrat e ho w sig moidal cros s fault s coalesc e wit h maste r fault s (Fig. 8) , th e latte r one s strikin g a t abou t 25° wit h respect t o the bul k shea r direction .
Discussion Model results The presenc e o f a thi n basa l laye r o f viscou s material i s sufficien t t o allo w for distribute d shea r in th e brittl e cove r ove r th e entir e mode l width . After initia l distribute d grai n flow , th e san d an d glass powder layers defor m according t o the MohrCoulomb sli p criteri a an d distribute d shea r i s
ANALOGUE MODELLIN G O F STRIKE-SLI P TECTONIC S
43
Fig. 7 . Faul t evolutio n o f experimen t 155 3 fo r successiv e stage s o f distribute d strike-sli p shear . Lin e drawing s o f visible faults ar e superpose d o n photographs. Ticked line s in (e) and (f ) indicat e fault s wit h important dip-slip compo nent. Note how cross faults between R shears rotate with time and acquire a sigmoidal shape. For notation see Figure 2.
Fig. 8 . Detai l o f sigmoidal cros s fault s betwee n maste r faults , (a ) Surfac e view at y = 0.39, (b ) 3 D perspective vie w at y = 0.57 . Fo r notatio n se e Figure 2 .
44
G. SCHREURS
accommodated b y subvertica l strike-slip fault s tha t extend acros s th e entir e thicknes s o f th e brittl e cover. Accordin g t o th e Mohr-Coulom b sli p criteria, failur e i n a materia l tha t ha s no t ye t bee n faulted occur s at angles of ± (45 ° - <£/2 ) (<£ > being the angl e o f interna l friction ) wit h respec t t o th e direction o f maximu m compressiv e stres s (o^) . Thus, th e initia l orientatio n o f ne w fault s (Fig . 9) can be used to determine change s in orientation of cTj throug h time . Th e first-generatio n R an d R ' shears i n confine d experiment s strik e a t about 16 20° an d 72-80° , respectively , an d th e regiona l
Fig. 9. Diagram s showing the surface strike of new faults with increasin g shea r strain . Strik e is give n wit h respec t to the direction of bulk shear. Experiments 163 8 and 1666 are confine d models ; experimen t 155 3 is a n unconfme d model. R ' = antitheti c Riede l shear , R L — lower angl e synthetic fault , R' L= lowe r angl e antitheti c faul t (cros s fault). Note that the surface strik e of new faults decrease s with increasin g shea r strain .
maximum principal stress direction , o^ , lies within a horizonta l plan e a t abou t 45 ° t o th e impose d shear direction . Figure 1 0 illustrate s ho w a se t o f R shear s evolves into a master fault. Th e early, left-stepping R shear s for m i n respons e t o th e regiona l stres s field i n whic h o ^ lie s i n a horizonta l plane a t 45 ° with respect to the bulk shear direction (upper inset in Fig . 10) . Horizonta l displacemen t alon g left stepping R shear s propagatin g alon g strik e cause s shortening o f th e brittl e layer s i n th e intervenin g area. This cause s a local increas e in a l an d a local counterclockwise rotation of al towards the surface strike of the bounding R shears in the overlap area. Initially push-u p zones form tha t may sho w adjac ent slightl y arcuate R shears in cross section . Once
Fig. 10 . Developmen t of a master fault from initially left stepping R shears . Uppe r inse t show s th e initia l orien tation o f th e principa l compressiv e stres s (o-j) , wherea s the lowe r inse t show s th e loca l reorientatio n o f
ANALOGUE MODELLIN G O F STRIKE-SLIP TECTONIC S
the deviatori c stres s i s sufficientl y large , shor t R L shears form in the overlap area, cut across push-up zones, and connect pairs of adjacent left-stepping R shears (Fig . 10) . Th e R L shear s for m a t a n angl e of approximatel y 45 ° — (j>/2 wit h respec t t o th e locally rotate d cr l directio n (lowe r inset in Fig. 10) . Coalescence o f RL shears and older R shears results in a narro w zon e o f slightl y anastomosin g shear s (master fault ) tha t ma y cros s th e entir e lengt h o f the model . Du e t o th e distribute d natur e o f shea r deformation severa l paralle l maste r fault s form , their overal l strik e a t about 15 ° with respect t o the shear direction . Spacin g betwee n maste r fault s i s proportional t o th e thicknes s o f th e entir e brittl e layer; a thickness increas e results i n a larger spac ing (cf . Fig . 3 f an d Fig . 5b ; widt h of mode l i s i n both case s 2 5 cm) . The orientatio n an d distribution o f master fault s determine to a large extent the subsequent fault pattern (Fig. 11) . Cross faults (R'L) and RL faults form mostly betwee n master faults (Fig. 1 1 a). Although cross fault s an d maste r fault s ar e activ e coevally , they do not operate a s a true conjugate set. As bulk shear increases , th e angl e betwee n th e trace s o f both ne w R' L an d R L fault s an d the applie d shea r direction decrease s (Fig . 9) , indicatin g tha t thes e faults for m i n respons e t o a chang e i n stres s fiel d due t o th e presenc e o f paralle l maste r faults . Between tw o maste r faults , th e directio n o f o ^ rotates progressivel y counterclockwis e towards the surface strik e of previously formed, but stil l active master faults (Fig. lib) . Cross faults usually merge into or terminate against master faults. Where cross faults but t at master faults, they sometime s initiat e a cros s faul t o n th e opposit e sid e o f th e maste r fault. Continue d displacement o n th e maste r fault , however, immediatel y offset s th e cros s fault . At advance d stage s o f bul k shea r i n confine d experiments, displacemen t alon g R ' shear s an d master faults leads to local wedging of material and local failur e nea r thei r intersectio n (Fig . lie) . I n cases where displacement alon g an R'L shear dominates, cT j is locally reoriented subparallel to the surface strik e o f th e R' L shea r an d ne w antitheti c faults ma y develo p tha t strik e a t a highe r angl e (approximately perpendicular to the imposed shea r direction). I n case s wher e continuin g deformation is preferentiall y accommodate d b y movemen t along th e maste r fault , a l remain s approximatel y subparallel t o th e strik e o f thes e fault s an d ne w synthetic faults , strikin g subparalle l t o th e shea r direction, ma y develo p nea r th e intersectio n wit h R' L shears . The nature of the lateral boundaries of the initial rectangular mode l has a large influence on the fault evolution. Th e mai n difference s between confine d and unconfine d experiment s are : (1 ) surfac e strik e of earl y R shear s i n unconfine d model s (28-35° )
45
Fig. 11. Interpretatio n o f faul t patter n i n distribute d shear, (a) Synoptic map view. First-order R and R' shear s are the result of the regional (bulk ) stress field. Note that initial surfac e strike of early R' shear s is a t lower angles than shown ; R' shear s have rotated clockwise because of the scissorin g effec t i n th e acut e corner s o f th e model , (b) Lowe r angl e R' L shear s (cros s faults ) an d R L shear s confined betwee n maste r fault s ar e th e result o f counterclockwise rotation s o f th e maximu m compressiv e stres s parallel t o earlie r formed , bu t stil l activ e maste r fault s (M). (c ) Local wedgin g of material ma y cause short synthetic o r antitheti c fault s nea r th e intersectio n o f cros s fault an d maste r faul t (lat e shor t fault s ar e no t show n i n Fig. lla) .
is at somewhat larger angles than in confined mod els (15-25°) ; (2 ) well-develope d push-u p zone s develop onl y i n confine d experiments ; (3 ) i n ma p view cross faults (R'L) are sigmoidal in unconfined and approximatel y rectilinea r i n confine d experi ments; (4 ) closel y space d array s o f cros s fault s between maste r fault s onl y form i n unconfine d experiments; (5) considerable rotatio n of R'L fault s and block s betwee n maste r fault s take s plac e only in unconfine d models .
46
G. SCHREUR S
Rotation o f cross faults between maste r faults in unconfined experiment s i s clearl y relate d t o side ways displacement o f material paralle l t o the shea r zone. The sens e of rotation of cross faults i s clock wise for bulk dextral shear. The sigmoidal Z shap e of cros s fault s i n dextra l bul k shea r i s considere d to reflect tw o effects : (1 ) cross faults propagat e laterally an d new segment s star t t o form at angles of about 50-60 ° wit h respec t t o th e impose d bul k shear directio n (i n respons e t o th e chang e i n th e stress field) , wherea s th e olde r faul t block s rotat e passively; (2 ) simultaneou s movement alon g cros s faults an d maste r fault s cause s loca l wedgin g o f material nea r thei r intersectio n i n a directio n opposite t o tha t o f faul t rotation . Rotatio n abou t vertical axe s o f antitheti c fault s an d unfaulte d blocks i n betwee n create s spac e problem s tha t ar e accommodated locall y b y relativ e vertica l move ments. Indee d th e sigmoida l cros s fault s sho w a component o f dip-sli p whic h become s mor e pronounced wit h increasin g shear . Th e di p directio n changes alon g strik e o f th e faul t an d i s diametri cally opposit e a t either ti p o f the fault . I n th e central segmen t o f the faul t th e dip-sli p componen t i s zero. From the experimental evidence it is inferred that th e dip-sli p componen t an d the chang e i n di p direction alon g strik e ar e du e t o th e rotatio n o f cross faults , i n combinatio n wit h the possibility o f material t o b e displace d sideway s becaus e o f th e unconfined transvers e borders .
Comparison with theoretical and previous experimental studies The analogu e model s sho w that during the earlies t stages o f distribute d dextra l shea r strai n non overlapping, left-steppin g strike-sli p fault s form . With increasin g bul k shear , thes e fault s generall y propagate along strike until some degree of overla p has been reached. Segal l & Pollard (1980 ) investi gated theoreticall y th e orientatio n an d distributio n of secondar y structure s associate d wit h non-over lapping strike-sli p faults . Thei r approach , using the elastic interactio n method , indicate s tha t fo r left stepping fault s undergoin g dextra l bul k shea r th e shear-failure zon e i s restricte d t o th e neighbour hood o f th e faul t tips , an d - a s i n th e analogu e models - shea r fracture s migh t for m ahea d o f the existing faul t segments . However , a s pointe d ou t by Hempto n & Nehe r (1986) , th e result s o f Segall & Pollard (1980 ) ar e strictl y vali d onl y fo r the initia l stage s o f displacement an d not fo r finit e deformations wit h man y episode s o f slip , an d hence their applicatio n ma y be limited. I n the analogue models , th e overla p betwee n e n echelo n strike-slip fault s i s generall y large r a s th e spacin g of th e neighbourin g fault s increases . A simila r relation ha s bee n foun d b y Aydi n & Schlut z
(1990), who used a numerical mode l based on displacement discontinuit y to investigate th e overlap ping geometr y o f e n echelo n strike-sli p faults . Their result s indicat e tha t faul t interactio n enhances th e growt h o f e n echelo n fault s a s th e inner tips pass each other and impedes their growth after som e degre e o f overlap . Aydi n & Schult z (1990) als o quantifie d th e geometr y o f strike-sli p faults i n nature and their data suggest that the overlap increase s proportionall y wit h spacing , u p t o a limiting value. The classica l Riede l shea r experimen t ha s bee n widely use d t o stud y structure s tha t for m i n a n overburden i n response t o horizontal displacemen t along tw o underlying , paralle l basemen t block s (e.g. Riede l 1929 ; Tchalenko 1970 ; Wilcox e t al 1973; Naylo r et al 1986 ; Richard & Krantz 1991; Richard e t al 1995 ; Ueta e t al 2000 ; Schopfe r & Steyrer 2001). In Riedel experiment s with an overburden consistin g o f granula r materials, th e initia l fault patter n is quite similar to the on e observed i n the distribute d shea r experiment s o f this paper . I n both type s o f experiments , e n echelo n R shear s dominate initia l shea r deformatio n an d ar e subsequently linke d b y secondary structure s (syntheti c shears) i n the are a o f overla p (Naylo r et a l 1986 ; Richard & Krantz 1991; Richard etal. 1995). However, majo r difference s i n faul t evolutio n becom e prominent wit h increasing shear . I n Riede l experi ments, th e orientatio n o f th e fina l anastomosing , synthetic fault zone is parallel to the basement faul t and th e activ e faul t zon e narrow s wit h increasin g shear, wit h displacemen t concentrate d o n the central through-going faults (Naylo r et al 1986 ; Richard & Krant z 1991 ; Richard e t a l 1995) . Thi s i s in contras t to the distribute d shea r experiments , i n which anastomosing, synthetic fault zone s strike at an angl e o f abou t 15 ° t o th e bul k shea r direction , and the zone of active faulting widens with increasing shear. R' L shears are typically lacking in Riedel experiments a t advance d stage s o f shea r defor mation. Thei r absenc e ca n b e explaine d b y th e presence o f th e pre-existin g basemen t 'fault ' tha t is approximatel y paralle l t o th e potentia l orien tation o f lower angl e R L shears . Onc e overlappin g R shears have developed in Riedel experiments, the development o f Y shear s paralle l t o th e pre existing basement 'fault ' i s therefore favoured ove r the potentia l orientatio n o f th e R' L shear . I n contrast, bot h R L an d R' L shear s develo p a t advance d stages i n distribute d shea r experiments , althoug h the latte r ar e inefficien t a t accommodatin g signifi cant strain . R L shear s for m a t progressively lowe r angles wit h increasin g deformation , bu t d o no t necessarily strik e paralle l t o th e orientatio n o f Y shears in Riedel experiments . The fundamental differences i n faul t patter n betwee n Riede l experi ments an d distribute d shea r experiment s ar e attri -
ANALOGUE MODELLIN G O F STRIKE-SLI P TECTONIC S
buted to the distinct boundary conditions. In Riedel experiments, th e structure s i n th e overburde n ar e secondary wit h respec t t o th e pre-existin g base ment discontinuit y an d th e through-goin g anas tomosing shea r zon e doe s no t develo p indepen dently. Henc e th e applicabilit y o f th e Riede l experiment ma y b e restricte d t o studyin g th e effects o f strike-slip reactivatio n of a vertical basement faul t o n a n initiall y unfaulte d overburden . The basal viscous layer in the models presente d in this pape r distribute s th e deformatio n ove r th e entire width of the model an d these models may be more appropriate for studying the fault evolution in broad shea r zones . In previous distributed shea r experiments, initia l distributed fault s consiste d o f syntheti c R shear s and antithetic R' shear s (Cloos 1955 ; Hoeppene r et al 1969 ; Freun d 1974 ; Gapai s e t al 1991 ; A n & Sammis 1996 ; A n 1998) . Wit h increasin g defor mation one of the shears generally starte d to dominate. In these experiments fault s wer e mostly regularly space d an d initiall y straight . With increasin g distributed deformation , early shear s becam e non linear as they developed further b y in-plane growt h and out-of-plan e coalescenc e (e.g . Freun d 1974 ; An & Sammi s 1996 ; A n 1998) . Faul t coalescenc e subsequently resulte d i n majo r through-goin g fault zones . The distribute d shea r experiment s presente d i n this pape r confir m tha t straigh t R and R ' shear s take up initial deformation. In our particular experimental set-up , R ' shear s occu r onl y i n th e acut e corners o f th e mode l an d ar e considere d a n edg e effect. Th e R shear s becom e th e dominan t struc tures i n a distribute d faul t pattern . The y als o became non-linea r wit h increasin g shea r du e t o fault interactio n an d coalescence . A s i n previou s distributed shea r experiments , faul t coalescenc e resulted i n major through-goin g master faults. Th e angle o f thes e majo r fault s wit h th e bulk shea r direction depend s o n th e mechanica l propertie s o f the analogu e materia l use d (A n & Sammi s 1996) . Contrary t o th e experiment s presente d her e i n which a set of parallel maste r faults accommodate s most of the distribute d shear deformation of a rectangular model, parallel R ' shear s (striking at about 75° t o th e shea r direction ) dominate d durin g th e early stage s o f deformation i n confined distribute d simple shea r experiment s b y Gapai s e t al . (1991) . The difference is attributed to the specific boundary conditions use d i n thei r experiments , wher e th e sandpack had an initially square shape and the load was imposed b y moving the walls along the latera l boundaries. However, similaritie s exis t despit e th e difference i n the initial faul t pattern . I n both type s of experiments, early faults ar e long-lived and control th e formatio n o f younge r faults. Onc e maste r faults hav e formed, whether o r no t the y consis t o f
47
synthetic (ou r experiments ) o r antitheti c shear s (Gapais e t a l 1991) , th e intervenin g block s ar e subjected t o change s i n principa l stres s orien tations, wit h (j j rotatin g int o a direction paralle l t o the direction o f the master faults. These stres s field modifications resul t in new syntheti c and antithetic faults tha t mostl y terminat e agains t th e olde r an d long-lived maste r faults .
Comparison with natural examples En echelo n faul t pattern s hav e bee n observe d i n nature, especiall y i n low-displacemen t strike-sli p fault zone s associate d wit h recen t earthquake s i n young sedimentar y deposit s an d suppor t ou r experimental result s durin g earl y stage s o f strike slip shear . A n exampl e o f right-steppin g R shear s along a segmen t o f th e sinistra l Dash-al-Baya z (Iran) earthquake of 31 August 196 8 (Tchalenk o & Ambraseys 1970 ) i s show n in Figur e 12a . A positive vertical relief ha s formed in the overlap are a locate d betwee n tw o left-steppin g faul t segments o f th e dextra l Coyot e Cree k faul t i n southern Californi a (Shar p & Clar k 1972 ; Fig . 12b). This is similar to the push-up zones that form between e n echelo n R shear s durin g th e earl y stages o f deformatio n i n ou r models . I n vertica l sections throug h push-u p zones , fault s hav e a slightly arcuat e shap e o r merg e a t depth . Figur e 12c shows the clos e resemblanc e betwee n vertica l sections throug h a push-u p zon e i n a n analogu e model experimen t an d push-u p zone s interprete d on seismi c section s throug h strike-slip faul t zone s in th e Mora y Firt h Basi n i n th e Nort h Se a (McQuillin et al 1982 ; Naylor et al 1986) . In both experiment an d natural example two closely adjac ent fault s merg e a t dept h an d layer s i n th e regio n bounded b y fault s ar e bent upward. Fault pattern s analogou s t o thos e a t mor e advanced stage s in our experiments have also been observed i n natur e (Fig . 13) , fo r exampl e i n th e San Andrea s fault syste m in wester n North America (e.g . Ro n e t al 1984) , th e Dea d Se a fault sys tem (Nu r e t a l 1989 ; Westawa y 1995) , an d i n southern Spain (Osete et al 1989) . These fault systems consis t o f prominent , mor e o r les s paralle l major strike-sli p fault s (syntheti c master faults) and less prominen t cros s fault s havin g th e opposit e sense of slip (Fig. 13) . As in the experiments, most cross faults ar e confined between parallel and overlapping master faults. In the experiments sli p along cross fault s take s plac e simultaneousl y wit h sli p along maste r faults . Seismicit y studie s alon g seg ments of the San Andreas fault syste m indicate that sinistral displacemen t alon g cros s fault s ha s trig gered dextra l movemen t alon g maste r fault s (Nicholson e t al 19860 , b\ Peterse n e t al 1991) . Palaeomagnetic, geodetic , seismic , and structural
48
G. SCHREUR S
ANALOGUE MODELLIN G O F STRIKE-SLIP TECTONIC S
49
Fig. 13. Natura l examples of distributed strike-slip shear zones, where block rotations have been inferred from palaeo magnetic, geodetic , o r structura l studies, (a ) Strike-sli p faul t patter n i n S W Washington (afte r Well s 1989) . (b ) Sa n Andreas faul t syste m i n souther n Californi a (afte r Nicholso n & Seebe r 1989 . (c) Fault syste m i n th e Subbeti c zon e of th e Beti c Cordiller a i n souther n Spai n (afte r Oset e e t al. 1989) . (d ) Small-scal e strike-sli p faul t zon e i n Lowe r Jurassic limeston e a t Kilve , U K (afte r Kell y 1996) . Blac k area s denot e infillin g calcite .
Fig. 12. Natura l examples o f early stage s in the development o f strike-sli p faults, (a ) Right-steppin g majo r fault s along a fault segment of the sinistral Dash-al-Bayaz earthquake o f 3 1 Augus t 196 8 (modified afte r Tchalenk o & Ambraseys 1970) . Onl y fault s showin g th e greates t amount o f relativ e displacemen t ar e shown , (b ) Push-up zone betwee n tw o left-steppin g dextra l strike-sli p faul t segments of the Coyote Creek fault i n southern California (after Shar p & Clar k 1972) . (c ) Seismi c sectio n throug h strike-slip faul t zon e i n th e Mora y Firt h Basi n i n th e North Se a (modifie d after Naylo r e t al . 1986 ) and cros s section throug h a n experimenta l push-u p zon e bounde d by R shears .
studies have provided evidence for rotation of cross faults an d block s abou t vertica l axe s withi n majo r strike-slip faul t system s (e.g . Bec k 1976 ; McKen zie & Jackso n 1983 ; Ro n e t al . 1984 ) simila r t o our experiments . Alon g part s o f th e Sa n Andrea s fault syste m sinistra l cros s fault s definin g smal l crustal block s hav e undergon e clockwise rotation s between majo r NW-trendin g dextra l strike-sli p faults (Fig . 13a , b ) i n respons e t o regiona l dextra l shear (e.g . Nicholso n e t al . 1986a , & ; Nicholso n and Seebe r 1989 ; Peterse n e t a l 1991) . Suc h rotations requir e th e presenc e o f a horizonta l decoupling surface or layer at depth (corresponding to th e viscou s analogu e material) , suc h a s detach ments o f loca l o r regiona l extent . Rotation s o f fault-bounded block s see m t o b e restricte d t o th e
50
G. SCHREUR S
upper portio n o f th e continenta l lithospher e i n which earthquake s occu r (uppe r 10-2 0 km , Chen & Molna r 1983) . Palaeomagneti c evidenc e for bloc k rotation s i n th e Subbeti c Zon e o f th e Betic Cordiller a i n souther n Spai n (Fig . 13c ) has been presented b y Osete et al (1989) , who suggest that clockwis e rotation s i n thi s zon e ar e possibl y related to intracontinental distribute d dextra l shea r at th e Iberian-Africa n plat e boundary . I n Lowe r Jurassic limestone s a t Kilv e (UK) , Kelly (1996 ) documented small-scal e example s o f counterclockwise block rotations o f about 30° between pairs of right-stepping sinistra l strike-slip faults (Fig. 13d). Block rotation s abou t vertical axe s hav e als o bee n identified i n oceani c crust . Allerto n (1989 ) an d Allerton & Vine (1992 ) used bot h palaeomagneti c and structura l studie s t o revea l bloc k rotation s about steeply incline d axe s in parts of the obducted Troodos ophiolit e o n Cyprus , wher e a regio n o f fault block s alon g th e insid e corne r o f a ridge transform intersectio n underwen t clockwis e rotation du e t o dextra l sli p o n th e Souther n Troodos transfor m fault .
Conclusions These experiment s investigat e faul t developmen t and interactio n i n zone s o f distribute d strike-sli p deformation. Subvertical , e n echelo n R shear s accommodate mos t o f th e initia l deformation . Push-up zones develop in the overlap area between two closel y adjacen t e n echelo n R shears . Wit h increasing shear , adjacen t R shear s merg e o r ar e connected b y lowe r angl e syntheti c fault s (R L shears) i n th e overla p area , resultin g in th e forma tion o f a major anastomosing , syntheti c faul t zon e (master fault) . Becaus e th e impose d shea r i s distributed, severa l paralle l maste r fault s for m tha t greatly control the subsequent fault evolution. Both synthetic (R L shears ) an d antitheti c faults (R' L shears, o r cros s faults ) develo p betwee n maste r faults. Cros s fault s an d maste r fault s ar e activ e coevally, bu t no t a s conjugat e pairs . Th e spacin g between master faults an d the length of cross fault s depends o n the thickness o f the brittle layers. Master fault s sto p o r eve n offse t propagatin g cros s faults, suggestin g that classical crosscutting criteria generally use d t o distinguis h successiv e stage s o f faulting shoul d b e use d wit h caution . Th e faul t evolution indicate s tha t the stres s fiel d i s modifie d at a small scale between left-stepping R shears during earl y stage s o f shea r deformatio n an d a t a larger scal e i n betwee n maste r fault s durin g late r stages. I n bot h case s <j l rotate s progressivel y counterclockwise withi n a horizontal plane towards the surface strik e of older, but still active syntheti c strike-slip faults . Analogue modellin g ca n b e use d a s a n ai d i n
the interpretatio n o f the faul t patter n i n distributed strike-slip shea r zones . Th e followin g structura l criteria ar e proposed fo r identifyin g suc h zones on the basis of our experimental results . (1 ) The presence o f several , paralle l an d overlappin g majo r strike-slip fault s (maste r faults) . (2 ) Younge r an d shorter strike-sli p fault s (includin g cros s faults ) generally form afte r an d between maste r faults . (3) The sens e o f movemen t alon g cros s fault s i s opposite t o displacemen t alon g maste r faults , i.e . cross fault s ar e antitheti c an d maste r fault s synthetic wit h respect t o th e bul k shea r direction . (4 ) Strike-slip displacement alon g cros s faults i s smal l compared t o displacemen t alon g maste r faults . (5 ) Master fault s ar e long-lived an d mostly halt or offset propagation o f cross faults . (6) The orientatio n of cross faults (i n map view) is generally not in the 'conjugate' positio n wit h respect t o syntheti c master faults . (6 ) Antitheti c an d syntheti c fault s ar e subvertical. (7) Local wedgin g of material near the intersection o f cros s fault s an d maste r fault s ma y induce shor t fault s (eithe r antitheti c o r synthetic ) near thei r mutua l intersection ; alternatively , loca l wedging ma y produc e area s o f positive an d nega tive relie f nea r th e intersectio n o f cros s fault s an d master faults. (8 ) The traces o f cross faults ma y b e straight o r sigmoidal ; sigmoida l cros s fault s ma y have a dip-sli p componen t an d a chang e i n di p direction alon g strike . (9 ) Th e presenc e o f sig moidal cros s fault s suggest s fault-bounde d bloc k rotations betwee n maste r faults ; th e curvatur e of sigmoidal cros s faults indicate s th e sens e o f rotation: Z fo r clockwise , an d S fo r counterclock wise rotation . Significan t rotatio n o f cros s fault s between maste r fault s occur s onl y i n experiment s with unconfine d transversa l borders . Thi s suggest s that faul t an d block rotations i n distribute d strike slip shea r zone s i n natur e migh t b e restricte d t o areas wher e displacemen t o f materia l paralle l t o master fault s i s possible. This researc h wa s supporte d b y a gran t fro m th e Swis s National Scienc e Foundation . Experiment s wer e conducted a t th e Institu t Francai s d u Petrole , Ruei l Malmaison , France. B. Colletta and J. Letouzey are thanked for stimulating discussions, J.-M. Mengus for technical assistance , and the scanne r group for it s help in obtaining the X-ray CT images . Specia l thank s t o reviewer s T . Dooley , F . Storti, an d B . Vendevill e fo r thei r valuabl e comment s and suggestions .
References ALLERTON, S . 1989 . Fault bloc k rotation s i n ophiolites : results o f paleomagneti c studie s i n th e Troodo s complex, Cyprus. In: KISSEL, C. & LAJ, C. (eds) Paleomagnetic Rotations an d Continental Deformation. Kluwe r Academic Publishers , Dordrecht , 393^-10 .
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Jacinto faul t zone and adjacent faults, southern Califor nia: implication s fo r seismi c hazard . Tectonics, 10 , 1187-1203. QUENNELL, A . M . 1959 . The structura l an d geomorphi c evolution of the Dead Sea Rift. Quarterly Journal Geological Society, London, 133 , 311-327 . RAMBERG, H. 1981 . Gravity, Deformation an d the Earth's Crust. Academic Press , New York . RAMSAY, J . G . 1967 . Folding an d Fracturing o f Rocks. McGraw-Hill Boo k Company , Ne w York . RAMSAY, J . G . & GRAHAM , R. H . 1970 . Strain variation s in shea r belts . Canadian Journal o f Earth Sciences, 7, 786-813. RICHARD, P. 1991 . Experiments on faulting in a two-layer cover sequenc e overlyin g a reactivated basemen t faul t with oblique-slip . Journal o f Structural Geology, 13 , 459-169. RICHARD, P . & KRANTZ , R . W . 1991 . Experiment s o n fault reactivatio n i n strike-sli p mode . Tectonophysics, 188, 117-131 . RICHARD, P. , NAYLOR , M . A . & KOOPMAN , A . 1995 . Experimental model s o f strike-sli p tectonics . Petroleum Geoscience, 1 , 71-80. RIEDEL, W . 1929 . Zur Mechani k geologische r Brucher scheinungen. Zentralblatt fiir Mineralogie, Geologie und Palaontologie, 1929B , 354-368 . RON, H. , FREUND , R . & GARFUNKEL , Z . 1984 . Block rotation b y strike-sli p faulting : structura l an d paleo magnetic evidence . Journal o f Geophysical Research, 89, 6256-6270. SCHOPFER, M. P. J. & STEYRER, H. P. 2001. Experimental modeling o f strike-sli p fault s an d th e self-simila r behavior. In: KOYI , H. A. & MANCKTELOW, N. S. (eds) Tectonic Modeling: A Volume in Honor of Hans Ramberg. Geologica l Societ y o f Americ a Memoir , 193 , 21-27. SCHREURS, G . & COLLETTA , B . 1998 . Analogue model ling o f faultin g i n zone s o f continenta l transpressio n and transtension . In : HOLDSWORTH , R . E. , STRACHAN ,
R. A . & DEWEY , J . F . (eds ) Continental Transpressional an d Transtensional Tectonics. Geologica l Society, London, Specia l Publication , 135 , 59-79 . SEGALL, P . & POLLARD , D. D . 1980 . Mechanics o f discontinuous faults . Journal o f Geophysical Research, 85(B8), 4337-4350. SHARP, R . V . & CLARK , M. M. 1972 . Geologic evidenc e of previou s faultin g nea r th e 196 8 ruptur e o n th e Coyote Cree k fault . United States Geological Survey Professional Paper, 787, 131-140 . TCHALENKO, J. S. 1970 . Similarities betwee n shear zones of different magnitudes. Geological Society of America Bulletin, 81 , 1625-1640. TCHALENKO, J. S . & AMBRASEYS , N . N . 1970 . Structural analysis o f th e Dasht- e Baya z (Iran ) earthquak e frac tures. Geological Society o f America Bulletin, 81 , 41-60. UETA, K. , TANI , K . & KATO , T . 2000 . Computerize d X ray tomograph y analysi s o f three-dimensiona l faul t geometries i n basement-induce d wrenc h faulting . Engineering Geology, 56, 197-210 . VENDEVILLE, B . 1987 . Champs d e failles e t tectonique en extension: modelisation experimental. These Seme Cycle, Universite de Rennes. WEIJERMARS, R . 1986 . Flo w behaviou r an d physica l chemistry o f bounding putties and related polymer s i n view o f tectonic laborator y applications . Tectonophysics, 124 , 325-358 . WELLS, R . E . 1989 . Mechanisms o f Cenozoi c tectoni c rotation, Pacifi c Northwest convergen t margin , U.S.A . In: KISSEL , C . & LAJ, C . (eds) Paleomagnetic Rotations and Continental Deformation. Kluwe r Academi c Publishers, Dordrecht , 313-325. WEST AWAY, R . 1995 . Deformation aroun d stepover s i n strike-slip faul t zones . Journal o f Structural Geology, 17, 831-846. WILCOX, R . E. , HARDING , T . P . & SEELY , D . R . 1973. Basic wrenc h tectonics . American Association o f Petroleum Geologists Bulletin, 57 , 74-96 .
Recent strike-sli p deformatio n o f the norther n Tien Sha n M. M . BUSLOV 1, J . KLERKX 2, K . ABDRAKHMATOV 3, D . DELVAUX 4, V. Yu . BATALEV 5, O . A . KUCHAI 1, B . DEHANDSCHUTTER 6 & A. MURALIEV 3 1
United Institute of Geology, Geophysics and Mineralogy SB RAS, Koptyuga ave. 3, Novosibirsk-90, 630090, Russia (e-mail:
[email protected]) 2 International Bureau for Environmental Studies, Brussels, Belgium ^Institute of Seismology, Kyrgyzstan National Academy of Science, Bishkek, Kyrgyzstan 4 ISES Research Fellow, Earth Sciences, Vrije Universiteit Amsterdam, The Netherlands 5 United Institute of High Temperatures (IVTAN) RAS, Bishkek, Kyrgyzstan ^Catholic University of Leuven, Leuven, Belguim
Abstract: Th e paper present s a geodynamic interpretatio n o f the deep structure an d active tec tonics of the northern Tien Shan, with particular emphasis on strike-slip motions, which produced a pull-apart i n the centre o f the Issyk-Kul basin. The stud y is based o n a detailed interpretatio n of satellit e imagery , faul t plan e solution s of earthquakes , seismic , an d geodeti c data . Seismic an d magnetotelluric studies show tectonic layering of the Tien Sha n lithosphere, with several nearl y horizonta l viscoelasti c layer s an d th e lowe r laye r underthrus t northwar d i n th e northern Tien Shan . This activ e process may be responsible for the intricate present-day tectoni c framework o f th e norther n Tie n Shan . The recent tectonics o f the norther n Tien Sha n inherits the earlie r structure : The lens-shape d Issyk-Kul microcontinent comprisin g Precambrian-Palaeozoic metamorphic and magmatic rocks is surrounde d b y thic k shea r zone s whic h have been involved in th e activit y ove r mos t o f th e Cenozoic. In the Quaternary the strain propagated a s far as the central part of the Issyk-Kul basin.
The Tie n Sha n i s a uniqu e exampl e o f a n activ e fault , a majo r strike-sli p zon e o f th e region . Th e intracontinental mountai n bel t whos e origi n an d cumulativ e right-latera l offse t alon g i t sinc e th e evolution hav e bee n interprete d a s response s t o Lat e Miocen e approache s 10 0 km, wit h a 1 0 mm processes a t remot e plat e boundaries . Th e mech - a" 1 of average sli p rate (Burtma n e t al. 1996) , but anisms invoke d t o explai n th e formation o f th e GP S data obtained since 199 2 do not show ongoing present-day Tie n Sha n alternat e betwee n ductil e motion s (Abdrakhmato v e t al. 1996 , 2001 ; Mead e coupling o f a tectonically layere d lithospher e ove r an d Hager 2001 ; Zubovich et al. 2001). The higha mantl e diapi r (Gubi n 1986 ; Bakiro v e t al. 1996 ; es t sli p rat e i n th e souther n Tie n Shan , nea r th e Dobretsov e t al . 1996 ) an d lithospheri c foldin g Tari m plate , wa s 2- 3 m m a" 1 (Mead e an d Hage r (Burov e t al . 1993) . 2001) . Th e velocitie s an d direction s o f motions of The present-day Tien Sha n is a system of several crusta l blocks in the northern Tien Sha n have been ranges trending roughly west-east an d almost par- greatl y variabl e throug h it s history . Th e present allel sedimentar y basin s separate d wit h activ e da y left-latera l strike-sli p reache s 8 m m a" 1 faults. Th e intracontinenta l tectonic s o f th e Tie n (Mead e an d Hager 2001) . Shan i s apparentl y a consequenc e o f th e W e studie d th e norther n par t o f th e Tie n Sha n India/Eurasia collisio n (Fig . 1) , an d th e recen t range , a majo r activ e shea r zon e a t th e transitio n activity i s assume d t o b e controlle d b y th e pen - betwee n the Tarim plate and the stabl e Kazakhstan etration o f th e rigi d indente r o f Indi a int o Asi a platfor m (Figs 1 & 2), wit h a special focu s o n the (Molnar an d Tapponnier 1975 ; Cobbol d an d Davy processe s tha t contro l th e Issyk-Ku l pull-apar t 1988; Molna r e t al . 1993) . basi n near the norther n limits o f the mountain belt The activ e tectonic s o f th e Tie n Sha n ha s bee n (Fig . 2). The structural pattern of the northern Tien best documented in the region of the Talas-Fergana Sha n (Kyrgyzia ) an d th e Issyk-Ku l basi n recor d From: STORTI , F. , HOLDSWORTH , R . E . & SALVINI , F . (eds ) Intraplate Strike-Slip Deformation Belts. Geologica l Society, London , Specia l Publications , 210 , 53-64 , 0305-8719/037 $ 15 © Th e Geologica l Societ y o f London 2003 .
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M. M. BUSLOV£TAL.
Fig. 1 . Generalize d tectonics o f Central Asi a (Tarim , Turfan , and Junggar basins an d surrounding mountains). Modi fied after Cobbol d an d Dav y (1988) .
complex Cenozoi c deformatio n associate d wit h the India/Eurasia collisio n an d relate d strai n propa gation inward s int o th e continent , crusta l shorten ing an d thickening , an d mountai n building . Th e present-day N- S crusta l shortenin g i n the souther n and norther n Tie n Sha n occur s i n differen t direc tions an d at different rate s (10-1 5 mm a~ l an d up to 2-6 m m a"1, respectively), possibly because the northern Tie n Sha n enclose s a Precambria n microcontinent whic h ma y contro l th e Neogene Quaternary evolutio n o f Cenozoi c basin s an d active strike-sli p deformatio n processes . The geodynamic s o f th e norther n Tie n Sha n i s investigated usin g publishe d an d origina l geologi cal an d geophysica l data . Dee p structur e i s described fro m seismi c an d magnetotelluri c evi dence; the Cenozoic structura l evolutio n i s inferred from interpretatio n o f satellit e imager y an d fiel d structural and geomorphological studies; stres s and strain distribution in the crust is jeconstructed fro m fault plan e solution s o f earthquakes ; recen t crusta l movements ar e revealed b y geodetic survey s (GP S for horizonta l movement s an d repeate d levellin g for lan d uplift) .
Geological an d tectonic backgroun d As wa s firs t show n b y Molna r an d Tapponnie r (1975, 1978) , the Cenozoi c tectonic s ove r a broad territory o f Asi a ca n b e explaine d i n term s o f th e continuing convergence between India and Eurasia. After th e initia l collisio n betwee n 6 0 an d 3 5 M a (Mercier e t al 1987 ; L e Pichon e t al 1992 ) Indi a has continue d it s northwar d motio n a t reduce d velocity and acts as a rigid indenter penetrating (for -2000 km ) int o Asi a t o caus e post-collisiona l underplating (India) and uplift (Tibet ) (Molna r and Tapponnier 1978 ; Cobbol d an d Davy 1988) . Orogeny i n th e Pami r an d souther n Tie n Sha n regions starte d late r tha n th e Tibe t uplif t an d wa s accompanied i n th e Lat e Oligocen e b y depositio n of coarse-clasti c red-colou r continenta l molasse . The Miocen e landscap e o f th e Pamir s an d th e southern Tie n Sha n wa s mad e u p o f 3 km uplift s and depression s betwee n them . I n Pliocen e time , red molass e gav e wa y t o gre y one s a s a resul t of climatic cooling , an d th e uplift s reache d 4- 5 k m high. Furthe r Quaternar y uplif t o f th e Pamir s pro duced the typical glacial landscap e (Chedija 1986) .
STRIKE-SLIP TECTONICS I N TIEN SHA N
55
Fig. 2 . Generalize d tectonic s o f Tien Shan.
The norther n Tie n Sha n becam e involve d i n orogeny in the Late Miocene under tangential compression whic h cause d linea r folding , revers e an d thrust faulting , seismi c activity , an d crusta l thick ening u p t o 6 0 k m i n th e nort h an d 7 5 k m i n th e south (agains t 40-4 5 k m i n th e Kazakhsta n platform) (e.g . Gubin 1986 ; Sadybakaso v 1990; Burov e t al. 1993) . Th e growt h o f mountains wa s likewise accompanie d by molasse depositio n in the Issyk-Kul an d othe r basin s (Chediy a 1986 ; Sad ybakasov 1990) . Abdrakhmatov e t a l (1996 ) estimate d tha t th e evolution o f the norther n Tie n Sha n belt laste d 1 0 Ma, o n th e basi s o f 20 0 ± 5 0 k m o f tota l crusta l shortening an d it s interpolate d present-da y rat e o f 20 m m a" 1. The y attribute d th e origi n o f th e bel t to a notable increas e i n horizontal compressio n i n response t o th e abrup t ris e o f th e Tibeta n platea u reported b y Englan d an d Housema n (1989 ) an d Molnar e t al . (1993) . Apatit e fissio n trac k therm ochronology, structura l modellin g an d magnetos tratigraphy i n the Kyrgy z rang e an d the Ch u basin (Burbank and Bullen 1999) further showe d that the uplift an d strain rate in the northern Tien Sha n first increased 10-1 1 Ma ago, and then afte r 3 Ma. The latter episod e i s expressed i n th e stratigraph y by a marked chang e in Late Pliocene-Earl y Pleistocen e deposition environments , thick sequences of coarse
conglomerates, sedimentar y gaps , an d tectoni c unconformities (Abdrakhmato v 1993) .
Tectonic layering of the lithospher e of northern Tie n Sha n The followin g implications fo r tectonic layerin g i n the upper lithosphere of the Tien Sha n are based on geological, seismic , an d magnetotelluri c evidence , including published P- and S-wave seismic dat a for the Tie n Sha n an d neighbouring territorie s (Sabitova 1989 ; Roecker e t al . 1993 ; Sabitov a e t al. 1995 ; Roecker 2001 ; Sabitov a an d Adamov a 2001), dat a fro m 2 5 magnetotelluri c profile s recorded b y th e IVTA N statio n (Bishkek ) sinc e 1983 and processed by a method reported by Trapeznikov e t al. (1997) , a s wel l a s syntheti c seismi c and magnetotelluric data correlated o n two profile s from th e Kazakhsta n platfor m t o th e Pamir s (Profile I-I) and Tarim (profil e II-II) (Figs 2 & 3). P- an d S-wav e seismi c profile s (cros s sectio n I— I in Fig. 3) image 10-20 km thick nearly horizontal waveguides i n th e lowe r (35-5 0 km ) an d uppe r (10-20 km) crust throughout the region excep t fo r the Fergan a basi n an d the neighbourin g fla t areas . The uppe r crus t waveguid e i s a t depth s o f 10-20 km north of the Pamirs; the lower crust waveguide abruptly rise s t o 15-2 0 km beneat h th e Talas -
56
M. M. BUSLOVCTAL.
Fig. 3. Cros s section of crust an d upper mantle beneath Tien Shan, fro m P (Vp) and S (Vs) seismic velocities, alon g profiles Aktyuz-Torugar t (I ) an d Kindiktas-Karaku l (II ) (see Figs 1 & 2 fo r location) . Modifie d afte r Bakiro v e t al (1996) .
Fergana faul t an d is as deep as 20-40 km northeast of it and near the Tarim plate (profile s I-I and II II). Nea r th e souther n margi n o f th e Issyk-Ku l microcontinent, th e norther n en d of the waveguid e is 1 5 km deepe r tha n the souther n one (profil e II II in Fig. 3) . Magnetotelluric dat a (Trapezniko v e t al . 1997 ; Rybin e t al. 2001) indicate th e presenc e o f crustal conductors nearly at the same depths as the waveguides (se e Profile s I-I an d II-II i n Fig. 3). A 15 25 km thick conductor is located a t a depth of 35 50 k m nort h o f th e Issyk-Ku l microcontinent an d at 20-35 km south of it. A sloping conducto r mark s the souther n borde r o f th e Issyk-Ku l microconti nent. Southwards , th e conducto r i s locate d a t th e
same depth as the basement of the Tarim microcontinent (20-5 0 km). Therefore, geophysica l data provide evidence for the tectoni c layerin g o f th e present-da y crus t beneath th e Tie n Shan , possibl y associate d wit h high-temperature metamorphis m an d migmatiz ation: th e lowe r waveguid e is attribute d to partia l melting i n th e amphibolite-facie s P- T condition s (Bakirov e t al . 1996) . Th e intermediat e an d upper waveguides may correspond to magmatic chambers with plasti c migmatite s squeeze d ou t toward s th e surface. The occurrenc e o f a n asthenospheri c upwar p beneath the Tien Sha n is marked by abnormal heat flux (twic e tha t beneat h Kazakhstan ) (Yudakhi n
STRIKE-SLIP TECTONIC S I N TIE N SHA N
1983; Yudakhi n an d Belonovic h 1989 ) an d lo w seismic velocitie s (1-3 % AVp) . Hig h AV p wa s recorded northeas t o f the Talas-Fergana fault, wit h the maximu m o f 3 % sout h o f Lak e Issyk-Ku l (Roecker e t al 1993) . The hot anomalous mantle hypothesized beneat h the norther n Tie n Shan , northeas t o f th e Talas Fergana fault , ma y be responsible fo r magma gen eration an d lithospheri c layering . Magm a layer s serve a s a lubrican t facilitatin g movement s o f upper crus t blocks ove r them . The viscous plasti c layers i n the crust ma y con strain th e dept h o f faulting , an d consequentl y th e origin dept h o f mos t earthquakes . Takin g int o account tha t tectoni c strai n canno t exis t i n a vis cous medium , thi s ma y explai n wh y th e hypo centres o f al l Tie n Sha n earthquake s ar e restricte d to < 10-20 km . Thus, th e recen t orogeni c stag e ha s acte d upo n a thinned o r weak lithosphere. Stron g compressio n from Tari m an d Pami r i s accommodate d i n hori zontal movement s facilitate d b y th e presenc e o f subhorizontal zones o f weakness in the crust where the hypocentre s o f almos t al l regional earthquake s cluster (Gubi n 1986 ; Abdrakhmato v 1993) . The tectoni c layerin g o f th e Tie n Sha n litho sphere an d th e heterogeneou s compositio n o f th e crust (granite-metamorphi c microcontinents , accretionary complexes , shea r zones ) ma y b e responsible fo r the discordant behaviour of its constituent block s unde r genera l W- E compressio n (Figs 2 &3). Th e directio n o f block motion s prim -
57
arily depend s o n the geometry o f the border faults, which ar e mos t ofte n Lat e Palaeozoi c fault s reactivated i n th e Cenozoi c (Bakiro v an d Maksu mova 2001 ; Maksumov a e t al . 2001) . Recen t tec tonic movements follow the earlier Cenozoi c structure (Tapponnie r an d Molna r 1979 ; Mikolaichu k 2000), an d th e geometr y o f fault s i s controlle d b y the position o f their planes relative t o the directio n of compression .
The basemen t structur e o f the norther n Tien Shan The basemen t o f th e Kyrgy z Tie n Sha n include s the Issyk-Kul and Aktyuz-Boordin microcontinents composed o f Archea n high-grad e metamorphic s and Early Proterozoic quart z schists and carbonates overlain b y Lat e Riphea n an d Cambrian Ordovician volcanosedimentar y rocks . These , together wit h Lat e Riphean-Middl e Palaeozoi c intrusions, are covered b y Devonian-Carboniferou s volcanosedimentary an d sedimentar y deposits . The Issyk-Ku l microcontinen t wit h th e Issyk Kul basi n in it s centr e i s a miniature model o f the Tarim an d Jungga r microcontinent s tha t contai n basins of the same name reduced through the Cenozoic. I t i s a W-E-striking southwar d conve x len s c. 11 0 km wide and 350 km long (Fig. 4) bounded in th e sout h an d i n th e nort h b y th e Kyrgyz Terskey an d Chon-Kemi n shea r zones , respect ively. According t o geophysica l data , th e microconti -
Fig. 4 . Activ e tectonic s o f the norther n Tie n Sha n aroun d the Issyk-Ku l basin .
58
M. M. BUSLOV ETAL.
nent is located i n the upper crust abov e th e plasti c layer (Fig . 3) forme d unde r th e pressur e fro m th e Tarim microcontinen t thrustin g unde r th e Tie n Shan. Th e Issyk-Ku l microcontinen t ha s behave d as a rigid block during the Cenozoic and has played an importan t rol e i n th e regiona l distributio n o f strain induce d b y th e India/Eurasi a collisio n an d transmitted t o inne r Asi a alon g ol d faults.
The Cenozoic structure of the Issyk-Ku l region In Palaeogene-Miocen e time , th e Issyk-Ku l microcontinent remaine d mor e o r les s stabl e an d was a n area o f continuous lacustrin e depositio n i n the Palaeogene . Neogen e an d Quaternar y clasti c sediments recor d th e onse t o f tectoni c activity , especially clasti c transport from the growing southern and, possibly, northern Tien Shan. The tectonic activity i n th e norther n Tie n Sha n culminate d i n the Pliocene-Earl y Quaternar y a s compressio n changed t o N- S fro m N W i n th e Lat e Miocene . This episod e strongl y deforme d th e Issyk-Ku l microcontinent an d it s Cenozoi c cove r an d produced th e present-da y surfac e topography . Th e southern an d norther n margin s o f th e microconti nent wer e graduall y involve d int o uplifting , th e area o f th e Issyk-Ku l basi n strongl y reduced , an d ramp structure s forme d i n it s wester n an d easter n margins. Thrustin g o f mountain s ove r th e basi n was accompanie d b y molass e deposition . A s a result o f underthrustin g alon g th e Kyrgyz-Terske y zone, th e souther n margi n o f th e microcontinen t split int o severa l block s separate d b y obliqu e thrusts an d strike-sli p faults . Strike-sli p motion s along the Chon-Kemi n faul t induce d thrustin g and reverse faultin g alon g th e norther n margi n o f th e microcontinent (Fig . 4). In the Holocene, the deformation reache d th e centr e o f th e microcontinen t and th e Issyk-Kul basin (Buslo v e t al 2001) . The Issyk-Ku l basi n ha s bee n studie d i n som e detail (Chedij a 1986; Sadybakaso v 1990; Trofimov 1990; Mikolaichu k 2000 ) bu t it s recen t tectonic s and evolution remai n a subject of discussions. Th e basin forme d i n th e Earl y an d Middle Pleistocen e along W- E an d transvers e norma l fault s an d ha s accumulated abou t 4 km thick basi n fill. Its origi n was interprete d i n term s o f a pull-apar t structur e formed i n response t o right-latera l strike-sli p fault ing unde r N- S compressio n (Klerk x e t a l 1999) . At it s earl y evolutio n stag e th e basi n wa s large r than now , especially i n th e east . No w i t ha s a trapezium-shaped geometr y an d is bounded by W — E, NE , an d N W fault s (Fig . 4). Trofimo v (1990 ) interpreted th e evolution o f the basin a s successiv e centreward collaps e o f block s from th e wes t an d from th e eas t whil e th e fault s in th e sout h an d i n the north remained stable . I n Early and Middle Ple-
istocene tim e the basin floor subsided for 30-50 m along th e borde r faults . Reactivatio n o f norma l faults i n the Late Pleistocene produce d an offset o f 50-100 m. In the Middle Holocene th e central part of th e lak e underwen t a catastrophi c collaps e an d subsided fo r 20 0 m t o form a n ove r 70 0 m dee p lake i n 1 0 000 years. Th e collaps e ma y have been responsible fo r th e Middl e Holocen e regressio n and a 10 0 m fal l o f wate r leve l (Trofimo v 1990) . The western , central , an d easter n part s o f th e basin have different structura l styles (Fig. 4). In the western par t (Fig . 5, profil e I-II) , th e basi n ha s a well-defined ram p structur e whic h grade s int o a half-ramp eastwar d wher e th e Kunge y rang e i s thrust over th e basin alon g th e Toru-Aigyr faul t i n the north , an d sediment s ar e conformabl e t o th e basement i n th e sout h (Korzhenko v 2000 ; Miko laichuk 2000) . I n th e easter n par t o f th e basi n (profile V-V I i n Fig . 5), Cenozoi c sediment s ar e involved i n arched-up and linear fold s and are separated fro m th e basemen t b y thrust s an d revers e faults i n th e nort h an d i n the south . The particular structural style of the basin's cen-
Fig. 5. Geologica l cross sections of different segment s of northern Tie n Shan aroun d Issyk-Ku l basin (fo r location of profile s se e Fig . 4). Profil e I-I I (wester n Issyk-Kul microcontinent): Cenozoic sediments overthrust by basement rock s i n a ram p structure . Profil e III—I V (centra l Issyk-Kul microcontinen t and Lak e Issyk-Kul) : revers e faulting a t souther n extremit y of microcontinent . Profile V-VI (easter n Issyk-Ku l microcontinent) : basin fil l i s more strongl y deforme d i n a half-ramp structure.
STRIKE-SLIP TECTONICS IN TIEN SHA N
tral part is controlled by the Chon-Kemin and ForeTerskey maste r faults , th e Fore-Kunge y obliqu e thrust joining th e Taldy-Su thrust, North and South Issyk-Kul faults , an d th e transversa l Tang a an d Ottuk faults (Fig. 4) . The NE-striking Chon-Kemi n fault bordering the Issyk-Kul microcontinent on the north wa s interprete d a s a revers e faul t (Chedij a 1986) befor e a significan t left-latera l strike-sli p component wa s recentl y reveale d (Delvau x e t al . 2000). Th e Fore-Terske y faul t tha t border s th e Issyk-Kul microcontinen t o n th e sout h i s spli t b y the Tanga and Ottuk faults into three segments with different kinematic s o f faul t planes . Th e segmen t east of the Tanga fault i s a thrust in which the Terskey rang e i s thrus t ove r Cenozoi c sediments , th e western segmen t i s als o a thrust, an d the segmen t between th e Tang a an d Ottu k fault s i s a n underthrust. Cenozoi c sediment s an d Quaternar y terraces sout h o f th e lak e ar e folde d int o linea r folds o r E-W-striking flexures . Man y flexures an d folds hav e gently slopin g norther n limb s an d stee p and shor t souther n limbs , ofte n cu t wit h revers e faults. The change s i n kinematic s alon g th e basin axi s are variou s expression s o f th e impac t o f th e N- S compression o n the Issyk-Kul microcontinent. Th e underthrusting o f th e Terske y rang e apparentl y caused uplift o f the southern flank of the microcon tinent an d subsidenc e o f it s centra l part , whic h i s confirmed b y northwar d dippin g erosio n surface s of th e rang e an d th e Cenozoi c sediments . Th e boundary betwee n th e uplifte d an d subside d part s
59
of the microcontinent (wester n Terskey range ) runs along th e souther n shorelin e o f the lake .
Active crusta l movements Seismicity The seismicit y o f th e norther n Tie n Sha n an d neighbouring area s i s show n i n th e ma p o f M> 5 epicentres i n Fig . 6 (Vvedenskay a 1964-1973 ; Gorbunova e t al. 1975-1981 ; Kondorskay a 1982 1994; Starovoi t 1998-2001) . I n th e perio d o f his toric seismicit y (c. 12 0 years) , th e regio n wa s shocked b y a serie s o f grea t earthquake s whic h started with the 188 7 M s = 7.3 Yemen earthquak e in the vicinity of Alma-Aty; then followed the 188 9 Ms = 8. 3 Chili k even t an d th e M s = 8. 2 Kemi n (Kebin) earthquake s i n 1911 , whic h wer e amon g the stronges t histori c catastrophe s i n th e norther n Tien Shan . Th e 193 8 M s = 6.9 Kemin-Ch u earth quake wa s obviousl y the fina l event of this series . The earthquake s recor d westwar d stres s releas e along th e Kemin-Chili k faul t (Fig . 6) . The y pro duced a n intricat e patter n o f surfac e ruptur e an d numerous landslides an d rock avalanches withi n an area o f 1 0 00 0 squar e kilometre s (20 0 k m fro m east t o wes t along th e faul t an d 7 0 k m fro m nort h to south ) (Delvau x et al 2001) . The epicentre s o f al l grea t earthquake s i n th e northern Tie n Sha n ar e locate d withi n a narro w strip betwee n th e Aktyuz-Boordi n an d Issyk-Ku l microcontinents. Othe r M> 6 epicentre s mar k th e
Fig. 6 . Seismicit y an d tectonic s o f norther n Tien Sha n and neighbouring territories.
60
M. M. BUSLOV ETAL.
northern an d souther n margin s o f th e Issyk-Ku l microcontinent (Fig. 6 ) indicating the effect o f crustal heterogeneit y o n th e activ e structur e o f th e northern Tie n Shan .
Seismotectonics The distribution o f stress and strai n i n the crus t of the norther n Tie n Sha n wa s inferre d fro m faul t plane solution s o f 78 0 M = 3- 7 earthquake s between 196 2 and 199 5 (Vvedenskay a 1964-1973; Gorbunova e t al 1975-1981 ; Kostrov 1975 ; Kon dorskaya 1982-1994 ; Yung a 1990 ; Starovoi t 1998-2001), usin g onl y firs t motion s o f P-wave s (Vvedenskaya 1969 ) an d focal mechanisms withi n 0.1° fro m faul t planes . Strain estimates wer e obtained by the method of Riznichenko (1977 , 1985) , whic h implie s sli p measurements alon g seismi c rupture s o f differen t orientations an d calculatio n o f tenso r component s by summatio n o f contribution s fro m al l earth quakes. Th e tenso r component s wer e the n use d t o estimate th e amoun t an d directio n o f principa l stress fo r eac h 0. 4 X 0.4 ° averagin g cel l b y for mulas o f elasticity . Thi s averagin g cel l siz e wa s chosen becaus e o f uneve n aeria l distributio n o f seismicity. Depth s o f al l event s wer e assume d t o be i n th e uppe r 2 0 km o f the crust. Our implication s procee d fro m th e basi c assumptions that movements in earthquake source s and o n th e causativ e long-livin g fault s resul t fro m the same tectonic forces and that coseismic slip follows the orientatio n o f the faul t plan e (e.g . sli p on a left-latera l strike-sli p faul t mus t b e left-latera l as well) . Figure 7 shows the obtained distributio n o f stress in the crust of the northern Tien Sha n divided into
the northern, central, and southern blocks with different tectoni c styles, especially contrastin g around the Issyk-Kul basin. The northern block is bounded in th e sout h by the Chon-Kemin Fault , th e centra l block is located west of the North and South IssykKul faults , and the souther n bloc k i s east o f them . The norther n bloc k evolve s mostl y unde r N- S compression. I n th e centra l block , th e strai n pat terns indicat e N E extensio n i n it s centr e (Lak e Issyk-Kul), NW extension in the west, and roughly N-S compressio n in the northern and eastern parts. The southern block experience d roughl y N-S com pression i n it s wester n par t an d mostl y N E exten sion i n th e east . The direction s o f principa l horizonta l axe s ar e well define d agains t th e mosai c backgroun d an d reflect a regional-scale N- S compression . Th e vertical axe s sho w positive motions , i.e . relativ e sur face uplif t throughou t th e norther n Tie n Sha n bu t with mino r subsidenc e southeas t an d northeas t o f the Issyk-Kul basin . The obtaine d seismotectoni c dat a wer e use d t o analyse th e kinematic s o f the Chon-Kemin , Nort h and Sout h Issyk-Kul, and Fore-Terskey fault s (th e available solution s ar e insufficien t fo r reliabl e simulation o f othe r faults) , usin g th e metho d o f Kuchai (1978 , 1990) . The wester n and easter n segment s o f the Chon Kemin fault wer e considered separately . The western segmen t show s bot h left - an d right-latera l strike-slip faultin g an d th e easter n segmen t i s dominated b y left-latera l motions , whic h confirm s the geologica l evidenc e (Delvau x e t al . 2000) . Sixty mechanism s alon g th e Nort h Issyk-Ku l an d South Issyk-Ku l fault s sugges t sinistra l move ments. Th e Fore-Terske y faul t show s a n intricat e slip pattern , wit h left-lateral strike-sli p i n th e eas t (from 3 1 mechanisms) an d right-latera l sli p in th e west (fro m 2 3 mechanisms) .
Geodetic data
Fig. 7. Principa l stresses in northern Tien Shan, from tensor analysi s (se e text for explanation) .
The availabl e geodeti c dat a o n horizonta l crusta l movements wer e obtained fro m a network of GPS stations ru n betwee n 199 2 an d 199 9 (Abdrakhmatov e t al 1996 ; Mead e an d Hage r 2001; Zubovic h e t al . 2001) . High-precisio n measurements of crustal movements have been carried ou t a t 9 permanen t an d ove r 30 0 temporar y stations (statio n POL/ 2 belong s t o th e worl d GPS network) by a joint Russia-Kyrgyzstan-KazakhstanUSA project. The data are processed in the Institute of Hig h Temperatures , Russia n Academ y o f Science (forme r IVTA N RAS) , usin g th e GAMIT/GLOBK software designed a t the Technological Institut e of Massachusetts. Th e uncertainty of th e measurement s i s withi n < 2 m m a" 1 (Zubovich e t al 2001) .
STRIKE-SLIP TECTONICS I N TIEN SHA N
The GPS data suppor t th e divisio n o f the north ern Tien Shan into the northern, central, and southern block s (Fig . 8) with differen t rate s an d direc tions o f movements . Th e souther n block move s a t the highest rate (6-10 mm a"1), in the NNE direction i n it s centra l part , N i n th e wester n part , an d NE i n the east. Th e central bloc k experience s 4- 6 mm a"1 active horizontal movements in N and NE directions. Th e northern block move s a t 4-10 mm a"1, predominately i n the souther n direction . Recent vertica l crusta l movement s ar e inferre d from annua l repeate d geodeti c levellin g betwee n 1937 an d 198 0 (Gubi n 1986) . Thes e data , with the rms erro r n o mor e tha n 0. 2 m m a" 1, sho w 3 mm a"1 subsidenc e eas t (Rybachje ) an d wes t (Przhevalsk) of Lake Issyk-Kul, whereas the southern shor e i s subject to uplift a t 2 mm a"1 (Fig. 8) . The GPS data and land uplift calculate d relativ e to th e Azo r mar k acros s th e Kazakhsta n platform (Abdrakhmatov et al. 1996 ) likewise indicat e a N S compression an d recent strike-sli p movements in the northern Tien Shan .
Discussion The faul t plan e solution s o f earthquake s an d th e geodetic (GP S an d repeated levelling ) dat a made a basis for two schemes o f the active tectonics of the northern Tie n Sha n (Fig s 7 & 8) . Th e scheme s show th e crus t o f th e regio n divide d int o severa l blocks wit h differen t tectoni c styles . Th e wester n and easter n part s o f the territory experienc e nearl y horizontal N- S compressio n an d nearl y vertica l
61
extension. I n th e centra l par t o f th e Issyk-Ku l basin, extensio n i s nearl y horizonta l an d compression i s nearl y vertical . Th e mountai n range s around th e Issyk-Ku l basin ar e compresse d i n th e N-S direction . Th e strai n axe s ar e oriente d north ward i n th e wester n part an d northeastwar d i n th e central an d easter n part s o f th e region . Thus seismotectoni c an d geodeti c analysi s shows tha t th e Issyk-Ku l microcontinen t i s currently subjec t t o left-latera l strike-sli p motions , well pronounce d aroun d th e microcontinen t (eastern segment s o f th e Chon-Kemi n an d Pred Terskey faults ) an d i n it s centr e wher e left-latera l slip o n th e Nort h Issyk-Ku l an d Sout h Issyk-Ku l faults i s consisten t wit h th e formatio n o f a pull apart. The epicentre s o f grea t earthquake s an d relate d land slidin g ar e attribute d t o th e Chon-Kemi n and Chon-Aksu fault s separatin g Precambria n microcontinents (Delvau x e t al . 2001) . Almos t al l M>6 earthquake s i n th e norther n Tie n Sha n ar e spatially associate d wit h activ e tectoni c zone s around th e Issyk-Ku l microcontinen t (Fig . 6). As a result of recent crusta l movement s a t variable direction s an d rates , th e souther n shor e o f Lake Issyk-Ku l i s subjec t t o uplift , an d ther e ar e indications fo r subsidenc e i n the eastern an d western shores , wit h a ris k o f collapse . Activit y of th e mountains aroun d th e Issyk-Ku l microcontinen t and reactivatio n o f faul t border s o f th e Aktyuz Boordin microcontinen t i s expecte d t o continu e in the future . W e sugges t tha t reactivatio n o f fault s and th e relate d seismi c an d geologica l hazar d can
Fig. 8 . Activ e strike-sli p movement s in northern Tie n Sha n (afte r geodeti c surveys) .
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M. M. BUSLOV ETAL.
be predicted fro m change s i n directio n an d rate of block movements .
faulting ma y be responsible fo r th e formatio n of a pull-apart structur e i n the central par t o f the basin .
Conclusions
We ar e gratefu l fo r th e constructiv e criticism s an d suggestions from th e reviewers. Especially we would like to expres s our cordia l thanks t o F . Stort i fo r hi s critical reading and valuable comments. Our thanks are extended to I . Safonov a an d T . Perepelov a fro m th e Institut e o f Geology for their help with the preparation of the English version o f th e manuscrip t an d t o L . Smirnov a fro m th e same institute for her assistance with figure drawing . The work wa s supporte d by grant s INCO-COPERNIKUS N ° PL 96-321 2 and fro m th e Russia n Foundatio n fo r Basic Research N ° 02-05-64627.
We investigate d th e relationship s betwee n th e present-day structure , reactivation o f ancient faults, and interactio n o f old granite-metamorphic block s (microcontinents) withi n relativel y mobil e oro genic belts in the region o f Tien Shan , on the basis of geological information , detailed interpretation of satellite imagery , analysi s o f seismicit y an d faul t plane solutions , an d geodeti c measurements . The tectonic s o f th e Tien-Sha n evolve s i n response t o th e convergenc e betwee n Indi a an d Eurasia since their collisio n i n the Eocene (Molna r and Tapponnie r 1975 ; Tapponnie r an d Molna r 1979, etc.), a s India continues its northward motion at 5 0 m m a" 1 (Avoua c an d Tapponnie r 1993 ; Avouac e t aL 1993) . Th e propagatio n an d distri bution o f strai n induce d b y th e collisio n i s con trolled by the complicated structur e of the crust and lithosphere. Geophysica l dat a indicate tectonic layering o f th e lithospher e beneat h th e norther n Tie n Shan. Th e presenc e o f horizonta l viscoelasti c lay ers ma y influence the rotatio n an d underplating of the Tarim plate and indentation of its basement into the middle crust of the Tien Shan . The thrusting of the Tari m plat e unde r th e norther n Tie n Sha n has caused th e shortenin g o f th e uppe r crus t a t a rat e of <10-1 5 m m a"1 an d related rapi d movement s and failur e t o depth s o f 20-3 0 km . A par t o f th e strain ma y have been accommodate d b y the visco elastic laye r i n th e middl e crus t of Tie n Shan . The Issyk-Ku l microcontinen t play s a n important rol e i n redistributio n o f uppe r crusta l stress an d strai n i n th e norther n Tie n Shan . Th e lens-shaped microcontinen t is surrounde d b y thic k shear zone s whic h hav e bee n involve d i n th e activity over most of the Cenozoic. I n the Quaternary th e strai n propagated a s fa r a s th e centra l part of the Issyk-Kul basin. It has produced a piano-key fault patter n i n it s inne r par t an d favour s furthe r subsidence o f the lak e bottom . The movements in the upper crust are best illus trated b y th e Fore-Terske y activ e faul t boundin g the Issyk-Ku l microcontinen t i n th e south , wit h south-dipping reverse fault s and thrusts in it s eastern segmen t an d a serie s o f underthrust s i n th e western segmen t tha t border s Lak e Issyk-Kul . Al l these fault s hav e a strike-sli p component . Th e underthrusting of the Kunge y range resulted i n the uplift o f the souther n part the Issyk-Kul microcon tinent an d subsidenc e o f it s centra l part . Th e present-day activit y i n th e regio n o f Lak e Issyk Kul i s dominate d b y left-lateral strike-sli p faulting along th e Chon-Kemin, Chon-Aksu, Fore-Kungey , and Nort h an d Sout h Issyk-Ku l faults . Strike-sli p
References ABDRAKHMATOV, K. E. (ed.) 1993. Detailed Seismic Surveys i n the Issyk-Kul Basin. Him, Bishkek (in Russian). ABDRAKHMATOV, K . E., ALDAZHANOV e t al 1996 . Relatively recen t constructio n o f th e Tie n Sha n inferre d from GP S measurement s o f present-da y crusta l defor mation rates . Nature, 384 , 450-453 . ABDRAKHMATOV, K . E. , WELDON , R. , THOMPSON , S. , BURBANK, D. , RUBIN , CH. , MILLER, M . & MOLNAR , P. 2001 . Origin , direction , and rat e o f moder n com pression o f th e centra l Tie n Sha n (Kyrgyzstan) . Russian Geology an d Geophysics, 42(10) , 1585-1609 . AVOUAC, J . P. & TAPPONNIER, P . 1993 . Kinematic model of activ e deformatio n i n Centra l Asia . Geophysical Research Letters, 20(10) , 895-898. AVOUAC, J . P. , TAPPONNIER , P. , BAI , M. , You , H . & WANG, G. 1993 . Active thrusting and folding along the northern Tien Shan, and Late Cenozoic rotation of the Tarim relativ e to Dzungari a an d Kazakhstan . Journal of Geophysical Research, 98, 6755-6804 . BAKIROV, A . B . & MAKSUMOVA , R . A . 2001 . Geodyn amic evolutio n o f th e Tie n Sha n lithosphere . Russian Geology an d Geophysics, 42(10) , 435-443. BAKIROV, A . B. , LESIK , O . M. , LOBANCHENKO , A . P . & SABITOVA, T. M. 1996 . Indications of the modern deep magmatism i n Tie n Shan . Russian Geology an d Geophysics, 37 , 42-53. BURBANK, D . W. & BULLEN, M. E. 1999 . Late Cenozoic rates o f rock and surfac e uplif t i n th e Centra l Kyrgy z Range, Norther n Tie n Shan . Supplement t o EOS, Transactions, AGU, 80(46) , F1035. BURTMAN, V . S. , SKOBELEV , S . F . & MOLNAR , P . 1996 . Late Cenozoic slip on the Talas-Fergana fault, the Tien Shan, Central Asia. Geological Society American Bulletin, 108 , 1004-1021 . BUROV, E . V., LOBKOVSKY, L . L, CLOETINGH, S. & NIKISHIN, A . M . 1993 . Continenta l lithospher e folding i n Central Asia . Tectonophysics, 226 , 73-87. BUSLOV, M. , ABRAKHMATOV , K. , D E BATIST , M. , DELVAUX, D. , DEHANDSCHUTTER , B . & KLERKX , J . 2001. A majo r stag e o f convergenc e in th e Issyk-Ku l Basin (Northern Tien-Shan) at the end of the Neogene. Reports i n BUG XI, Strasbur g 2001 , 8-12 April , 336. CHEDIJA, O. K. 1986 . Morphostructures an d Recent Tectonism i n th e Tien Shan. Him , Frunz e (i n Russian) .
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GAILHARDIS, E . & HAN , T . L . 1987 . Change fro m lat e Tertiary compression t o Quaternary extension in southern Tibe t durin g India-Asi a collision . Tectonics, 6 , 275-304. MIKOLAICHUK, A . V . 2000 . Th e structura l positio n o f thrusts i n th e recen t oroge n o f th e centra l Tie n Shan . Russian Geology an d Geophysics, 41(7), 929-939. MOLNAR, P . & TAPPONNIER, P . 1975 . Cenozoic tectonic s of Asia: effects o f a continental collision. Science, 189, 419-426. MOLNAR, P . & TAPPONNIER, P . 1978 . Active tectonics o f Tibet. Journal o f Geophysical Research, 83 , 5361 5375. MOLNAR, P. , ENGLAND , P . & MARTINOD, J . 1993 . Mantle dynamics, th e uplif t o f th e Tibeta n Plateau , an d th e Indian monsoon . Reviews in Geophysics, 31, 357-396 . RIZNICHENKO, Yu . V . 1977 . Calculation o f deformatio n rates a t a seismi c flowin g o f rocks, Izvestija Academii Nauk SSSR, ser . Fizika Zemli , 10 , 34-47 (i n Russian, English abstract) . RIZNICHENKO, Yu . V . 1985 . 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Magnetotelluri c an d Brest Ma y 1999 , Abstract Book , T36. magnetovariational studie s o f th e Kyrgy z Tien Shan . KONDORSKAYA, N . V . (ed. ) 1982-1994. Earthquakes i n Russian Geology and Geophysics, 42(10), 1566-1573 . the USSR i n 1978-1991, Nauka , Moscow . KORZHENKOV, A . M . 2000. Cenozoic tectonic s an d seis SABITOVA, T . M . 1989 . Crustal Structure i n th e Kyrgyz Tien Shan, from Seismological Data. Him , Frunz e (i n micity o f th e northwester n Issyk-Ku l basi n (Tie n Russian). Shan). Russian Geology an d Geophysics, 41(7), 940 SABITOVA, T . M. & ADAMOVA, A . A. 2001. Seismic tom950. KOSTROV, B. V. 1975. Focal Mechanisms of Earthquakes. ography stud y o f th e Tie n Sha n crust : (results , prob Nauka, Mosco w (i n Russian) . lems, an d prospects) . Russian Geology and GeophysKUCHAI, O . A . 1978 . Focal mechanism s o f earthquake s ics, 42(10) , 1543-1553 . and crusta l tectoni c movement s i n th e Pamir s an d SABITOVA, T . M. , LESIK , O . M. , ADAMOVA , A . A . & southern Tadji k depression . In : GORSHKO V G . P. , MAMATKANOVA, R . O . 1995 . Seismotomograph y POLSHKOV, M . K. & SHCHUKIN Yu . K . (eds) Results of model o f the Tien Sha n eart h crus t i n connection wit h Comprehensive Geophysical Studies in Active Seismic seismicity. In: European Geophysical Society, Annales Geophysical, Part 1. Katlenburg-Lindau, FRG, 55. Zones. Nauka , Moscow , 159-18 0 (in Russian) . KUCHAI, O . A . 1990 . Deformation an d displacemen t o f SADYBAKASOV, I . 1990 . Neotectonics o f High Asia. crustal block s o f intracontinenta l orogens : evidenc e Nauka, Mosco w (i n Russian) . from foca l mechanisms o f earthquakes . In : LOGACHE V STAROVOIT, O . E . (ed. ) 1998-2001 . Earthquakes i n N. A. (ed.) Geodynamics of Intracontinental MountainNorthern Eurasia i n 1992-1995. Priroda , Mosco w (in Russian). ous Terrains. Nauka , Novosibirsk , 242-24 6 (i n Russian). TAPPONNIER, P . & MOLNAR, P . 1979 . Active faulting and LE PICHON , X. , FOURNIER , M . & JOLIVET , L . 1992 . Kin Cenozoic tectonic s o n th e Tie n Shan , Mongoli a an d ematics, topography , shortenin g an d extrusio n i n th e Baikal regions . Journal o f Geophysical Research, 84 , India-Eurasia collision . Tectonics, 11(6), 1085-1098 . 3425-3459. MAKSUMOVA, R . A. , DZHENCHURAEVA , A . V . & BERE TRAPEZNIKOV, Yu . A. , ANDREEVA , E . V . e t al . 1997 . ZANSKII A . V . 2001 . Structur e an d evolutio n o f th e Magnetotelluric sounding s i n th e Kyrgy z Tie n Sha n Tien Sha n fold-thrus t belt. Russian Geology and GeoMts. Izvestija Academii Nauk SSSR, ser. Fizika Zemli , 1, 3-20 (i n Russian, Englis h abstract) . physics, 42(10) , 1444-1452 . MEADE, B . J . & HAGER , B . H . 2001 . The curren t distri TROFIMOV, A . K. 1990 . Quaternary deposits o f the Issykbution o f deformatio n i n th e Wester n Tie n Sha n fro m Kul basin and their relation to tectonics. Izvetija Acadeblock model s constraine d b y geodeti c data . Russian mii Nauk Kyrgyzskoy SSR, 1 , 87-9 5 (i n Russian , English abstract) . Geology an d Geophysics, 42(10), 1622-1633 . VVEDENSKAYA, A . V . 1969 . Studies of Stress and Strain MERCIER, J . L. , ARMIJO , R. , TAPPONNIER , P. , CAREY -
COBBOLD, P . R . & DAVY , P . 1988 . Indentation tectonic s in natur e an d experiment . 2 . Centra l Asia . Bulletin Geological Institute Uppsala, NS , 14 , 143-162 . DELVAUX, D. , ABDRAKHMATOV , K., STROM , A. , HAVEN ITH, H . -B . & TREFOIS , P . 2000 . Active sinistra l transpression alon g th e Cho n Kemin-Chili k faul t zon e o n North Tie n Shan : palaeoseismi c an d neotectoni c significance. In: Geodynamics o f Tien Shan, Int . Workshop (abstract s an d papers) , Bishkek , Kyrgyzstan , 19. DELVAUX, D. , ABDRAKHMATOV , K., LEMZIN , I . N . & STROM, A . L . 2001 . Landslides an d surfac e breaks o f the 191 1 Ms = 8.2 Kemin earthquake, Kyrgyzstan. Russian Geology and Geophysics, 42(10), 1667-1677 . DOBRETSOV, N . L. , BUSLOV , M . M. , DELVAUX , D. , BERZIN, N . A . & ERMIKOV , V . D . 1996 . Meso- an d Cenozoic tectonic s of the Central Asian mountain belt: effects o f lithospheri c plat e interactio n an d mantl e plumes. International Geology Review, 38, 430-466. ENGLAND, P . & HOUSEMAN , G . 1989 . Extension durin g continental convergence , wit h applicatio n t o th e Tibetan Plateau . Journal o f Geophysical Research, 94(B12), 17561-17579 . GUBIN, I. E. (ed.) 1986. Lithosphere of Tien Shan. Nauka, Moscow (i n Russian) .
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1964-1973. Earthquakes i n th e USSR i n 1962-1977. Nauka, Mosco w (i n Russian) . YUDAKHIN, F . N . 1983 . Geophysical fields , deep structure an d seismicity o f th e Tian Shan, Him , Frun z [i n Russian]. YUDAKHIN, F . N. & BELENOVICH, T. YA. 1989. Contemporary crustal dynamics of Tian Shan and physical
processes i n earthquake sources. Izvestiy a Academi i Nauk Kyrgyzskoy Respubliki, 1, 101-103 [in Russian]. YUNGA, S . L . 1990 . Studies o f Seismotectonic Deformations: Methods an d Results. Nauka , Mosco w [i n Russian]. ZUBOVICH, A. V., TRAPEZNIKOV, Yu. A. , BRAGIN, V. D. , MOSIENKO, O . L , SHCHELOCHKOV , G . G. , RYBIN , A . K. & BATALEV, V. Yu. 2001. Strain field, crustal structure, an d seismicity patter n i n th e Tie n Shan . Russian Geology an d Geophysics, 42(10) , 1634-1640 .
Active intraplat e strike-sli p faultin g an d transpressional uplif t i n the Mongolian Alta i D. CUNNINGHAM 1, A . DIJKSTRA 1, J . HOWARD 1, A . QUARLES 2 & G. BADARCH 3 l
Orogenie Processes Group, Department of Geology, University of Leicester, Leicester, UK (e-mail:
[email protected]) 2 2304 Forest Hills Rd, Grapevine, Texas, USA ^Institute of Geology and Mineral Resources, Mongolian Academy of Sciences, Ulaan Baatar, Mongolia Abstract: Th e Mongolian Altai is a Late Cenozoic intraplat e strike-slip deformatio n belt which formed a s a distant strai n response t o the Indo-Eurasian collision ove r 2000 km to the south. We report results from 5 weeks of detailed fieldwork carried out during summer 2000 in northwestern Mongolia investigatin g th e crustal architectur e o f the Altai a t latitude 48°N . Th e region ca n be divided int o discrete Cenozoi c structura l domains each dominate d b y a major dextral strike-sli p fault syste m o r range-bounding thrus t fault . Gentle bend s alon g th e majo r strike-sli p faults ar e marked b y transpressiona l uplift s includin g asymmetri c thrus t ridges , restrainin g bends , an d triangular thrust-bounde d massifs . Thes e transpressiona l uplift s (Tsambagara v Massif , Altu n Huhey Uul , Sai r Uul , Ho h Serhiy n Nuruu , Omn o Hayrha n Uula, Mengildy k Nuruu ) compris e the highest mountain s i n the Mongolian Alta i an d are structural an d metamorphic culmination s exposing polydeforme d greenschist-amphibolit e grad e basemen t recordin g a t leas t tw o phase s of Palaeozoi c ductil e deformatio n overprinte d b y Cenozoi c brittl e structures . Cenozoi c thrus t faults with the greatest amounts of displacement bound the W and SW sides of ranges throughout the region an d consistently verg e WSW. Eac h majo r range i s essentially a NE-tilted block an d this i s reflecte d b y asymmetri c interna l drainag e patterns . Man y fault s ar e considere d activ e because the y defor m surficial deposits , for m prominent scarps , an d define range fronts with low sinuosity wher e activ e alluvia l fa n depositio n take s place . Reactivatio n o f th e prevailin g NW striking, NE-dippin g Palaeozoi c basemen t anisotrop y i s a regionall y importan t contro l o n th e orientation an d kinematic s o f Cenozoi c faults . A t firs t order , th e Alta i i s spatiall y partitione d into a low-angle thrust belt tha t overthrusts th e Junggar Basin o n the Chines e sid e an d a highangle SW-vergen t dextra l transpressiona l bel t o n th e Mongolia n side . Th e mechanicall y rigi d Hangay crato n an d Junggar basement bloc k whic h bound the Alta i o n either sid e hav e playe d a major role in focusing Late Cenozoic deformation along their boundaries and within the Altai. The geometric relationship between rigid block boundaries, Palaeozoic basemen t structura l anisotropy, an d th e dominantl y NE SHma x (derive d fro m India' s continue d N E indentation ) ha s dictated the kinematics of Late Cenozoic deformation in the Altai, Gobi Altai, and Sayan regions.
The Alta i i s on e o f th e grea t intraplat e mountai n (Tapponnie r & Molna r 1979 ; Baljinnya m e t al ranges o f centra l Asi a extendin g ove r 170 0 k m 1993 ; Cunningha m et a l 1996) . Compare d t o th e from Siberi a to the Gobi Desert (Fig. 1) . The range Himalayas , Karakorum , Pamirs , Tibet , an d Tie n is tectonicall y activ e an d cu t b y regional-scal e Shan , th e Alta i ha s receive d muc h les s attentio n strike-slip fault s characterize d b y dextra l trans - fro m worker s investigating th e effects o f the Indopressional deformatio n (Cunningha m et a l 1996 ; Eurasi a collision. Thi s i s presumabl y because th e Cunningham 1998) . Thes e fault s hav e produce d Alta i i s mor e distan t an d ha s accommodate d les s several larg e historical earthquake s (M = 7+) with tota l Cenozoi c strain . Nevertheless , th e Alta i i s impressive groun d rupturing (Khil'k o e t al 1985 ; particularl y interestin g t o structura l geologist s Trifonov 1988 ; Baljinnyam et al 1993 ; Bayasgalan intereste d i n transpressiona l deformatio n becaus e et a l 1999) . Lat e Cenozoi c faultin g an d uplif t o f th e range is less overprinted by contractional struc the Altai is believed to be ultimately driven by NE- ture s than major range s to the south , and the link directed compressiona l stresse s derive d fro m th e ag e betwee n strike-sli p faults an d thrus t an d Indo-Eurasian collisio n ove r 200 0 k m to the sout h oblique-sli p fault s i s spectacularl y exposed . Thu s From: STORTI , F. , HOLDSWORTH , R . E . & SALVINI , F . (eds ) Intraplate Strike-Slip Deformation Belts. Geologica l Society, London , Specia l Publications , 210 , 65-87, 0305-8719/03/ $ 15 © Th e Geologica l Societ y o f London 2003 .
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Fig. 1 . Digita l topographi c ma p of Altai region an d area investigated i n thi s study . Locatio n o f Landsa t imag e shown i n Figur e 3 is als o indicated .
the Alta i i s on e o f the bes t place s in th e worl d t o study th e evolutionar y developmen t an d interna l architecture of an active intraplate strike-slip defor mation belt . We carried out 5 weeks of fieldwork in the Mongolian Alta i regio n durin g th e summe r o f 200 0 (Fig. 1 ) investigating th e cross-strik e structur e of the range from east to west in order to better understand overal l processe s o f Cenozoic intracontinen tal mountain building an d the distant effect s o f the Indo-Eurasia collision . Fro m a moder n structura l geology standpoint , th e Alta i remain s on e o f th e least studie d majo r mountain ranges o n Eart h an d the first-orde r crusta l architectur e o f the range ha s never befor e bee n documented . Ou r stud y comp lements a detaile d transec t complete d i n 199 4 further sout h by Cunningham et al. (1996) and was chosen t o lin k wit h a structura l sectio n complete d by Qu and Zhang (1994) through the Chinese Alta i (Fig. 1) . In keeping wit h the theme of this volume , we focu s her e specificall y o n th e activ e trans pressional deformatio n associate d wit h Cenozoi c intraplate strike-sli p faultin g i n th e Alta i regio n (Figs 1,2) . Detailed information on the Palaeozoi c evolution o f th e basemen t rock s i s beyon d th e scope o f this paper an d is being prepared a s a separate manuscrip t fo r submissio n elsewhere .
The physiograph y o f th e Alta i reflect s tectoni c and climati c controls . Th e wester n an d northwestern Alta i region i n China , Russia , and Kazakhstan receives th e most precipitation, i s more incise d b y rivers, an d i s covere d b y forest . T o th e eas t an d south i n Mongolia , th e rang e i s i n a rain shado w and is more arid with less river incision, better preserved summi t peneplain s an d les s permanen t snow. For this reason, space views of the Altai give the impression tha t the Chinese Altai is topographically highes t becaus e i t contain s mor e rugge d snow-covered mountains. However, the Mongolian Altai has less snow , but higher mountains and better roc k exposur e (Fig . 2) . The Russia n an d Mongolia n Alta i contai n numerous peaks over 4000 m and permanent snow and glaciers startin g at approximately 3600 m . The Altai widen s to th e N W an d joins wit h the Sayan Mountains in Russia, whereas at its S E end it narrows an d i s topographicall y joine d t o th e Gob i Altai (Fig . 1) . The Altai is bordered on the east by the wide Valley of Lakes, whic h receives sedimen t transported b y river s an d win d fro m th e Alta i t o the west . Ther e i s n o evidenc e tha t th e Valle y o f Lakes i s downdroppe d (Cenozoi c norma l fault s have no t bee n foun d an d peopl e hav e looke d fo r them), rathe r th e Alta i i s simpl y uplifte d relativ e to the Valley of Lakes. Likewise, the Junggar Basin borders th e Alta i to th e wes t o n the Chines e side , but has not been downdropped, rather the Altai has been uplifte d relativ e to it. The Mongolian Altai is dominated by discret e bloc k uplift s o r topographi c culminations whic h ar e generall y asymmetri c an d elongate t o th e NW. The Mongolia n Alta i i s technicall y activ e with well-documented histori c earthquake s an d associated groun d rupturing (see revie w i n Baljinnya m e t al 1993 ; Bayasgalan e t al. 1999) . The major faults in the region are NW-trending throughout the range and accommodat e dextra l strike-slip , oblique-slip , and thrus t motions . Othe r indicator s o f moder n fault activit y includ e sharpl y define d mountai n fronts wit h low sinuosity , active alluvial fan depo sition alon g steep rang e fronts , displace d Quatern ary alluviu m an d glacia l deposits , offse t streams , asymmetric drainage patterns suggestin g block tilting, stee p canyon s wit h activ e rive r incision , an d preserved peneplaine d summit s indicatin g recen t uplift o f mountai n blocks . Satellit e image s o f th e region sho w prominen t NW-strikin g fault s whic h variably link and diverge an d in some cases can be traced fo r hundreds of kilometres (Fig s 3 , 4). GP S studies o f activ e deformatio n i n th e Alta i regio n indicate tha t crustal motion is currentl y northward directed a t approximatel y 1 cm a" 1 relativ e t o a fixed Siberia (J . Deverchere, E . Calais, pers. comm. 2001). Th e maximu m horizonta l stres s i n th e
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Fig. 2. Obliqu e shuttl e photograph #STS74-713-005 looking NW of Chinese and Mongolian Alta i region. Are a investigated an d individua l bloc k uplift s describe d i n tex t ar e indicated . JB : Jungga r Basin ; VL : Valle y o f Lakes ; MN : Mengildyk Nuruu ; OH: Omn o Hayrha n Uula ; TS : Tsambagara v Massif ; AH : Altu n Huhe y Uul ; HS: Ho h Serhiy n Nuruu; SU : Suta i Uul; JN : Jargalan t Nuruu . Locatio n o f Figure 6 a als o shown.
region i s approximatel y NE-S W base d o n earth quake dat a fZobac k 1992) .
Regional geology Basement rock s throughou t th e Mongolia n Alta i are generall y greenschis t grad e metasedimentar y and metavolcanic rock s date d a s Lower Palaeozoic (Fig. 5; Zaitsev 1978) . Precambrian rocks have not been identifie d i n th e regio n o f thi s study . Precambrian continenta l sliver s ar e reported fro m th e Chinese Alta i (Q u & Chon g 1991 ; Q u & Zhan g 1994) an d southernmos t Mongolia n Alta i (Mongolian National Atlas 1990) , but there are few reliable publishe d ages . Th e Lowe r Palaeozoi c basement rocks comprise monotonous greenschists, phyllites, an d slate s whic h generally di p EN E and strike NW ; thi s prevailin g basemen t grai n domi nates th e structur e o f th e Altai . Thes e rock s ar e metapelitic and metabasic an d are internally folde d and foliate d displayin g at least tw o phases o f NE SW-directed contractiona l ductil e deformation . Cutting th e basemen t ar e numerou s granitoid s o f early-late Palaeozoi c age . Unconformabl y over -
lying th e metamorphi c basemen t ar e Devonia n sedimentary rock s dominate d b y re d continenta l clastic sediment s an d shallo w marin e turbidites . Interpreting th e tectoni c settin g o f th e basemen t rocks i s controversial ; however , th e grea t thick nesses o f metasedimentary an d metavolcanic rock s suggest tha t the y wer e deposite d i n a subductio n complex o r arc-proximal settin g an d were contrac tionally deforme d durin g th e Cambrian-Siluria n (Sengor e t al 1993 ; Sengor & Natal'in 1996) . Mesozoic rock s ar e rare i n the Altai, but Meso zoic coars e clasti c sediment s includin g coal bearing conglomerates ar e found i n flanking basins to th e eas t suggestin g a n erodin g sourc e are a existed i n th e Alta i regio n durin g Jurassic-Cre taceous tim e (Sjostro m 1997 ; Howard e t al . i n press). N o evidenc e fo r Mesozoi c contractiona l deformation ha s yet been found i n the Altai, unlik e the Tie n Sha n t o th e sout h (Hendri x e t al . 1992) . Jurassic-Cretaceous extensiona l tectonis m wa s widespread i n the Gobi Altai region an d may have affected th e Alta i regio n als o (Trayno r & Slade n 1995; Webb et al 1999) . Cenozoic rock s consis t o f continenta l clasti c
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Fig. 3 . Landsa t MSS image o f part of Mongolian Alta i investigate d i n this study . Location o f image show n in Figure 1. Notice discrete mountain block s an d numerous sharpl y define d NW-trending faults . Image interpretation i s show n in Figur e 4 .
sequences tha t fill basins i n the adjacen t Valle y of Lakes t o th e eas t o f th e Altai , an d locall y derive d alluvial, fluvial , an d glacia l sequence s tha t fil l small intramontan e basin s withi n th e Altai . Th e sedimentary recor d indicate s a rapi d increas e i n erosion an d coarse locally derived clastic sedimen tation beginnin g in the Miocene an d continuing to the presen t (Devyatki n 1974) . Thi s recor d i s th e best indicato r o f th e timin g o f Cenozoi c reacti vation an d uplif t o f the Alta i i n Mongolia . Field result s
The stud y are a ca n b e subdivide d int o seve n dis crete structura l block s o r domain s whic h ar e characterized b y a major rang e bounding fault an d
adjacent bloc k uplif t (Fig s 3 , 4). Each domain wil l be describe d proceedin g fro m N E t o SW . Altun Huhey range Altun Huhe y Uu l i s a majo r bloc k uplif t o n th e eastern edg e o f th e Alta i borderin g th e Valle y of Lakes to the east (Fig. 2). The range is topographi cally asymmetri c with a sharply defined stee p S W mountain fron t i n contras t t o a gentl y slopin g E and NE slope (Figs 3 & 6). The SW mountain front is marke d b y fres h alluvia l fans , stee p relief , an d short stee p canyon s cuttin g int o th e range . A degraded faul t scar p cut s on e of the fan s (Fig . 7a , b) suggestin g Quaternar y activity . Thi s scar p ha s up to 1 8 m of measured vertical displacement sug-
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Fig. 4. Interpretatio n o f Figur e 3 showin g major Cenozoi c fault s an d sedimentar y basins , northwester n Mongolian Altai. Locations o f subsequen t figures shown. Cross sectio n (scale d down ) shows interpreted block geometry . gesting i t record s multipl e earthquak e events . Sense o f movement i s NE-side-up . At th e mountai n fron t itself , a SW-directe d thrust fault i s well exposed in a dry strea m canyon (Fig. 7c ) a t 48°43.08'N , 91°32.49'E . Thi s faul t
places biotit e quartzofeldspathi c gneisse s ove r quartzofeldspathic myloniti c gneisses . Th e faul t zone i s brecciate d ove r a thicknes s o f a t leas t 5 0 m and contains many slickensided goug e zones that define discret e plane o f brittle shearing . Th e majo r
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Fig. 5 . Genera l lithologica l ma p o f th e wester n Mongolia n Altai . Ma p are a i s simila r t o regio n show n i n Figure s 3 and 4. Basement rocks are dominantly Lower Palaeozoic metasedimentar y and metavolcanic rocks intruded by younger Palaeozoic granitoids. Map adapte d fro m Zaitse v (1978). V: Vendian; Cm : Cambrian; O : Ordovician; S : Silurian; D : Devonian; P ; Permian .
fault plan e strike s 321 7 dip s 22°NE 7 an d contain s slickensides plungin g 18°, 032, indicating a dextral component t o th e thrusting . Th e kinematic s ar e clear du e to the presence o f 0.5-1.0 m asymmetri c folds, dragge d layers , an d thrus t repeate d layers . The brittl e thrus t fabri c i s locall y paralle l t o th e gneissic fabri c suggestin g possibl e reactivatio n o f the basemen t anisotropy . Elsewhere , th e faul t truncates th e olde r basemen t structures . These tw o fault zones ar e responsible fo r Ceno zoic uplift an d NE tilting of the Altun Huhey range. Reconnaissance wor k elsewher e i n th e rang e an d examination o f satellite imagery (Fig . 3 ) and aeria l
photographs faile d t o reveal othe r majo r Cenozoi c thrust faults . Th e frontal thrust s lie near the northern en d o f th e Hov d dextra l strike-sli p faul t (Fig s 2, 6a ) whic h passes int o an d possibly throug h th e range a t it s easter n en d (Fig . 6a , b) . Th e fronta l thrust fault s ca n b e trace d toward s th e SE , wher e they probabl y lin k wit h th e Hov d Fault , althoug h the faul t junction is no t expose d (Fig . 6b) .
Tsambagarav Massif The Tsambagara v Massi f i s a topographi c an d structural culminatio n alon g th e tren d o f th e
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Fig. 6 . (a ) Kosmo s image of Hovd dextral strike-slip fault . Se e Figure 2 for location o f image. Note drag of sedimentary layers on fault's eas t side indicating dextral sense o f motion. Fault disappears without surface rupture under Hovd River Basi n t o north, but appears t o re-emerge along strik e i n eastern Altun Huhey Uu l (Fig. 6b) . (b ) Kosmos imag e of Altun Huhey (also see Landsat view, Fig. 3 ) showing sharply defined S W mountain front, fres h alluvia l fans, Hovd River canyo n and suggeste d along-strik e continuatio n o f Hov d Fault . Altu n Huhey Uul fronta l thrus t i s believed t o accommodate som e o f Hovd Fault's dextra l motion . Locatio n o f Figure 7 a shown .
regionally extensiv e A r Hoto l dextra l strike-sli p fault syste m (Fig s 3 , 4). Th e rang e i s topographi cally asymmetri c wit h a stee p S W mountai n fron t and a regionall y peneplaine d summi t tha t i s ice covered an d gentl y N E tilte d (Fig . 9a) . Th e range has a peculiar triangula r shape wit h high relief o n all side s excep t aroun d the N E corner . The rang e i s dominantl y compose d o f intrusiv e rocks an d paragneissi c greenschist-amphibolit e grade basemen t rocks . Th e intrusiv e rock s rang e from gabbr o t o granit e i n composition . The S W front o f the range is marked by a major active fault syste m that is clearly visible on satellit e and aerial photographs (Figs 3, 8). This fault is part of th e regional A r Hotol syste m (cf. Baljinnyam el al 1993 ) and in this study is referred to as the Main Tsambagarav Dextral Reverse Fault. This fault system was examined in detail in various places along the rang e fron t wher e evidenc e fo r recen t activit y includes fres h scarp s (Fig . 9b , c) , e n echelo n ten sion gashe s (Fig s 8a , 9e) , a sa g pon d (Fig s 8 a & 9d), and offset moraina l deposits (Fig. 8a). The offset morainal deposits an d orientation of en echelo n fissures indicat e dextra l displacemen t (Fig . 8a) . The mai n fronta l faul t scar p i s remarkabl y linea r on aeria l photograph s an d can be see n to dip stee ply N E wher e i t cut s acros s valley s (Fig . 8b ) Th e scarp vertically offset s recent alluvial deposits (Fig . 9c) by up to 6-7 m , indicating NE-side-up revers e displacement i n addition to the dextral history. The scarp i s generall y a compoun d featur e wit h pre -
served vertica l separation s a s high a s 32 m due t o multiple surfac e rupture events . The Mai n Tsambagara v Dextra l Revers e Faul t is beautifull y expose d wher e a majo r canyo n cut s through it at the NW end of the Tsambagarav Massif a t 48°38.488' , 90°43.360' , 288 0 m elevatio n (Figs 9f-h) . A t thi s location , th e faul t zon e i s u p to 30 0 m wid e an d contain s many semi-paralle l fracture surface s strikin g 32 0 ± 20 ° an d dippin g vertically t o steepl y N E wit h widesprea d brecci ation betwee n fractur e surfaces . Much of the zon e is iron-staine d an d clay-altered . Th e centr e o f th e fault zon e i s a n impressiv e 8-1 0 m wid e zon e of soft greenis h gouge (Fig. 9h). This is a rare outcrop of the core zone of a regional-scale strike-sli p fault . The goug e i s literall y oozin g ou t o f th e clif f an d spreading downward as soft mudflow s an d individual debri s flow s (Fig . 9h) . At thi s canyo n location , sens e o f movemen t o n the fault i s interpreted t o be dextra l reverse fo r the following reasons : the strea m i s diverted NW as it crosses th e fault (Fig . 9f), fracture s within the faul t zone an d th e faul t itsel f clearl y di p steepl y N E (Figs 8b , 9g) , an d th e loca l relie f acros s th e faul t indicates NE-side-u p displacements . Slickenside s are rare within the faul t zon e an d where measure d trend betwee n 31 2 an d 047° , whic h i s consisten t with both dextra l an d thrust displacements. Upper plate rock s consis t o f mylonitize d biotite muscovite-quartzofeldspathic gneiss . Th e mai n mylonitic fabri c strikes , 32 7 an d dip s 78°N E wit h
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Fig. 7 . (a ) Aerial photo of SW front of Altun Huhey Uul showing degraded thrust scarp which cuts Quaternary alluvial fan (als o se e Fig . 7b ) an d location o f fronta l thrus t show n in Fig. 6c . (b ) Vie w NE o f stee p relie f alon g S W front o f Altun Huhe y Uu l an d faulte d alluvia l fa n (arrows) , (c ) Vie w ES E o f fronta l thrus t ('uppe r thrust ' i n Fig . 7a ) whic h bounds SW front o f Altun Huhey Uul (48°43.08', 91°32.49')- Inset show s equal area, lowe r hemisphere stereographi c projection o f brittle faul t plan e an d tren d an d plung e o f slickenlines .
a well-develope d subhorizonta l quart z an d mic a lineation. Goo d S- C fabrics an d asymmetric quart z vein boudin s indicat e left-latera l shea r sense . Thi s fabric i s approximatel y paralle l t o th e brittl e frac tures and main fault trace of the Main Tsambagarav
Dextral Revers e Faul t a t th e mountai n fron t sug gesting tha t i t ha s reactivate d th e olde r ductil e mylonitic fabric . SE o f th e Mai n Tsambagara v Dextra l Revers e Fault a t a topographicall y lowe r level , anothe r
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Fig. 8 . (a ) Aeria l phot o o f active fault s whic h bound SW fron t o f Tsambagarav Massif . Not e dextrall y offse t glacia l deposits (arrows ) and en echelon tensio n gashes suggestin g dextral offse t (48°32.612' , 90°50.314'). Location o f photo shown i n Figur e 4 . (b ) Aeria l phot o o f sharpl y define d Mai n Tsambagarav Dextra l Revers e Fault , N W en d o f S W mountain fron t (48°38.295'N , 90°43.735'E) . major faul t wa s identified whic h is visible on satellite and aerial photos as a major step in the topography ('Lowe r Thrust ' o n Fig . 8a) . Thi s faul t als o appears t o hav e a n important thrus t history whic h has uplifted biotite-muscovite-garne t schist s i n its upper plate. Steeply incise d canyons cu t the uppe r plate suggestin g tha t stream s hav e been force d t o incise downward s as the block wa s being thraste d upwards (Fig . 8a) . Thi s faul t als o cut s alluvia l deposits a t th e mountai n fron t suggestin g recen t activity (Fig . lOa) . Th e scar p ha s u p t o 1 7 m o f
vertical displacemen t measure d acros s i t presum ably du e to multipl e surfac e rupture events . The schists at the mountain front ar e ultramylonitic with a strongly develope d quart z stretching lineation (Fig . lOb) . Th e schist s strik e 30 8 an d di p 20°NE wit h lineation s plungin g 14° , 016 . Awa y from th e front, th e schists are not mylonitic. Brittle gouge zone s tha t ar e fabri c paralle l ar e abundan t within th e mylonit e directl y a t the front . Thu s the modern faul t ha s reactivate d olde r ductil e fabrics. From a regiona l perspective , th e Tsambagara v
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Fig. 9 . (a ) Distan t vie w N E o f Tsambagara v Massif . Fla t summit s preserv e Cretaceous-Palaeogen e peneplai n no w capped by glaciers, (b) View NE of Main Tsambagarav Dextral Reverse Fault along SW front o f Tsambagarav Massif. Scarp ha s linea r Holocen e ruptur e trace an d i s locall y marke d b y spring s an d gree n gras s (48°32.612' , 90°50.314') (c) Surfac e ruptur e o f Mai n Tsambagara v Dextra l Revers e Faul t alon g S W mountai n front , Tsambagara v Massi f (48°34.323', 90°48.166') . (d ) Smal l sa g pond alon g trac e o f Main Tsambagara v Dextra l Revers e Fault , S W front o f Tsambagarav Massi f (se e Fig . 8 a fo r location) , (e ) Ope n tensio n crack s alon g trac e o f Mai n Tsambagara v Dextra l Reverse Faul t a t 48°38.295'N , 90°43.735'E . Crack s ar e lef t steppin g an d e n echelo n suggestin g dextra l strike-sli p movement. Perso n i n centr e middl e distanc e fo r scale , (f ) Vie w N W o f unname d canyon , NW en d o f S W fron t o f Tsambagarav Massi f (48°38.295'N , 90°43.735'E). Main Tsambagara v Dextra l Revers e Faul t i s nicely expose d alon g canyon wall s (Fig . 9g , h). Note stee p mountai n front an d deflectio n of drainag e t o N W a t canyon outlet wher e rive r crosses fault , (g ) Phot o lookin g S E o f brittle faul t plan e withi n Main Tsambagara v Dextra l Revers e Faul t a t canyon outlet (locatio n show n i n Fig . 9f) . Inse t show s equal area , lowe r hemispher e stereographi c projectio n o f brittle faul t planes withi n fault zone. Arrows indicate trend and plunge of fault plane slickenlines. (h) Photo lookin g S E of impress ive 8 m wid e goug e zon e o f Main Tsambagara v Dextra l Revers e Faul t expose d i n canyo n walls . Thi s i s interprete d as th e cor e displacemen t zon e fo r th e Mai n Tsambagara v Dextra l Revers e Faul t (48°38.488' , 90°43.360') .
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75
Fig. 10 . (a ) Vie w N E o f Holocen e surfac e rupture of lowe r Tsambagara v thrus t fault (48°32.703' , 90°43.618') . Se e Figure 8a for aerial photo perspective, (b) View NW of lower Tsambagarav thrust front wher e ductile mylonitic fabrics are reactivate d b y brittl e thrus t faultin g (48°34.323' , 90°48.166') . (c ) Vie w S W o f myloniti c quartzite s an d mic a schists whic h hav e bee n brittlel y reactivate d b y lowe r Tsambagara v thrus t (sam e locatio n a s Fig . lOb) . (d ) Brittl e SW-directed thrus t (348°, 28NE) cutting lower greenschist grad e phyllites approximately 1 0 km SW of lower Tsambagarav thrus t alon g Hacha t Go l Valle y (48°23.965 , 90°49.905') . Fo r locatio n se e Figure s 4 , 11 .
range forms a n uplifted triangula r block at a gentle restraining ben d alon g th e A r Hotol/Chikhtei n Fault (Fig s 3 , 4) . Activ e transpressiona l defor mation i s expressed b y revers e motio n an d dextral strike-slip displacement s tha t clearl y offse t Quat ernary sedimentar y deposits . Pre-existin g ductil e mylonite zone s helpe d contro l th e locatio n o f th e Cenozoic fault s whic h clearl y reactivat e olde r structural anisotrop y alon g som e section s o f th e range front . S W o f th e Tsambagara v rang e alon g the Hachat Gol canyon (Fig. 11) , we observed several othe r NE-striking , SW-directe d thrus t fault s which brittlely cut the basement phyllites and slates (Fig. lOd) . These fault s ar e not expressed o n satellite imager y an d thei r ag e i s uncertain.
Sair Uul Sair Uu l is a major structura l and topographic cul mination that has formed between divergent splays of th e regionall y extensiv e Ho h Serk h dextra l strike-slip faul t (Fig s 3 , 4, 11) . The easter n sid e of the range is bounded by a linear faul t tha t appear s
on satellit e imager y t o lin k northward s wit h th e Chikhtein Faul t (Fig . 3) . Thi s faul t cut s throug h glacial deposit s suggestin g Quaternar y movemen t and i s interprete d t o hav e a reverse componen t of motion responsible fo r uplif t o f th e easter n sid e of Sair Uul . The wester n sid e o f Sai r Uul i s bounded by several thrust faults which have also uplifted the range (Fig . 11) . Th e rang e i s therefor e interprete d to b e a structura l pop-u p wit h a n asymmetri c flower structur e geometry. The range was not studied in detail; however, fault scarp s cutting alluvium were observe d fro m a distanc e alon g th e range' s NW mountai n fron t suggestin g Quaternar y move ment. Major glacia l valleys an d streams within the range drai n eastward s suggestin g bul k eastwar d tilting du e t o thrustin g on th e wes t sid e (Fig . 3) .
Hoh Serhiyn Nuruu This range is bounded on the NE by the dextral Ar Hotol Fault and on the SW by the Hoh Serkh Fault (Figs 3 , 4) . Th e Ho h Serk h Faul t wa s previousl y referred t o a s th e Tolb o Nuu r Faul t (Baljinnya m
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Fig. 11 . Kosmo s imag e of Sair Uul and Asgat Nuru u region and active deformatio n alon g lengt h o f Hoh Serk h Fault . See Figur e 4 fo r exac t locatio n o f image . Inse t ma p show s interpretatio n o f majo r Cenozoi c fault s an d importan t geomorphologieal featuree . Locatio n o f Figur e 1 1 shown.
el al 1993 ) an d Hovd-Olgiy Faul t (Tapponnie r & Molnar 1979) ; however , the faul t canno t be trace d continuously t o Tolb o Nuu r (Fig s 3 , 4) , no r doe s it pas s clos e t o the town s o f Hov d o r Olgi y (Fig s 4, 5) and so these names are considered misleading . The Hoh Serkh Fault is a regionally importan t dextral strike-sli p faul t wit h a revers e componen t o f motion. Th e dextral componen t is clearly observe d on satellit e imager y nea r Mara a Uu l wher e si x
large stream s ar e consistentl y offse t i n a dextra l sense (Figs 3, 11, Tapponnier & Molnar 1979). The reverse componen t is inferred by the steep relief a t the mountain fron t an d th e asymmetri c til t o f th e Hoh Serhiy n rang e whic h ha s it s drainag e divid e close t o th e S W mountain front (Fig . 3) . The Ho h Serkh Faul t forms a n impressiv e linea r valle y below Maraa Uul and Asgat Nuruu (Fig. 11) ; however, alon g thi s trace , n o fres h scarp s wer e found .
STRIKE-SLIP FAULTIN G I N MONGOLI A
Where th e fault enter s th e Doroo Nuu r Basin (Fig . 11), there i s a very degraded trace of a surface rupture tha t i s onl y locall y discernible . Below Asga t Nuru u an d S W o f th e Ho h Serk h Fault, a separat e thrus t faul t wa s identifie d whic h is responsibl e fo r upliftin g th e smal l rang e wher e the prominen t dextrall y offse t stream s ar e foun d (Fig. 11) . W e nam e thi s faul t th e Tsagaa n Sala a Fault afte r th e canyo n wher e i t i s bes t exposed . This fault is similar to the lower thrust at the Tsambagarav Massi f i n tha t it : (1 ) appear s t o spla y of f of a majo r dextra l strike-sli p faul t (th e Hoh Serk h Fault) a t it s S E end , bu t die s ou t t o th e NW , (2 ) uplifts metasedimentar y schists in its hanging wall, and (3 ) forms a prominent ste p i n th e topography . It also form s a major scar p visibl e fro m a distanc e (Fig. 12a ; GPS : 48°03.239'N , 90°45.994'E ) an d excavations along the scarp indicate that it contains a well-develope d brittl e faul t fabri c strikin g 334 , 36°NE (Fig . 12b , c) . Slickenline s plung e moder ately NE , bu t wer e poorl y preserve d o r generall y absent. Th e fabri c i n th e basemen t schist s i s S W
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dipping at the mountain front an d therefore the NEdipping brittl e thrus t fabri c cut s acros s th e base ment structural anisotropy instea d of reactivating it . Above the fault, th e Tsagaan Salaa canyon is deeply incised an d there are small waterfalls at the fron tal scar p (Fig . 12b ) suggestin g recen t uplift . Upstream, ther e i s a prominent terrace perche d 2-4 m abov e th e moder n strea m suggestin g force d river incision du e to thrustin g at the mountain front . Doroo Nuu r i s a lak e tha t fill s a smal l basi n along th e norther n exten t o f th e Ho h Serk h Faul t where th e faul t divide s int o tw o branche s tha t bound th e Sai r Uu l Bloc k (Fig . 11) . Th e lak e appears to have once been linked to the SW branch of the Hachat Gol (Fig. 11) , which is a major valley with a dry stream bed. The depth of valley incisio n and siz e o f th e strea m be d sugges t that th e valle y once containe d a majo r stream . Th e valle y i s unusual i n th e regio n becaus e othe r simila r size d valleys contai n perennia l streams . There i s onl y a smal l topographi c ris e betwee n the Doro o Nuu r and th e S W branch of the Hacha t
Fig. 12 . (a ) Distan t vie w eastward s o f Tsagaa n Sala a thrus t fault , Asga t Nuruu . Se e Figur e 1 1 fo r exac t location . Boxed are a i s show n i n Figur e 12b . (b ) Vie w S E a t thrus t fron t showin g strea m cu t wher e faul t i s exposed . Not e waterfall a t thrus t fron t an d incise d terrace . Boxe d are a i s show n i n Figur e 12c . (c ) Vie w S E o f boxe d are a show n in Figur e 12 b where brittl e thrus t fabric s ar e expose d withi n Tsagaa n Sala a thrus t zone , Asga t Nuru u (48°03.239' , 90°45.994'). Main fault strike s 334 ° an d dips 36°N E as shown by equal area, lower hemisphere stereographi c projection (inset) .
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D. CUNNINGHAM E T AL .
Gol. Thi s topographi c boundar y i s fronte d b y th e Hoh Serk h faul t zon e whic h furthe r sout h clearl y has a revers e componen t o f motio n whic h ha s uplifted th e Ho h Serhiy n rang e an d tilte d i t east wards. It is suggested tha t the continued activit y of this faul t ha s le d t o uplif t o n th e eas t sid e o f th e fault whic h ha s damme d an d beheade d th e S W branch of the Hachat Gol and removed muc h of its catchment area . Doroo Nuur now receives the runoff tha t previousl y flowe d dow n th e S W Hacha t Gol valle y (Fig . 11) . The tectonic geomorpholog y along the length of the Ho h Serk h Faul t indicate s tha t th e kinematic s of activ e deformatio n chang e alon g strike . Whe n viewed from th e SE, the east side of the Hoh Serkh Fault i s uplifte d a s fa r nort h a s Doro o Nuur . Further north, th e west sid e of the fault is uplifte d where it bounds Sair Uul's steep eastern front . Thi s suggests tha t th e faul t segmen t wit h th e leas t amount o f revers e displacemen t i s adjacen t t o Doroo Nuur where the fault bends to a more northerly trend (Fig . 11) . Oblique kink bands with combined norma l an d dextra l sense s o f motio n wer e
observed i n phyllite s directl y S E o f Doro o Nuu r along th e trac e o f the Ho h Serk h Faul t suggestin g that, i f the y ar e Cenozoi c structures , a possibl e transtensional componen t of motion may have also contributed t o th e developmen t o f th e Doro o Nuur Basin .
Omno Hayrhan Uula This rang e forms a major bloc k uplif t N W o f Sair Uul an d ha s larg e activel y formin g alluvia l fan s along it s S W margin (Fig . 3) . These fan s ar e shed into an intermontane basin where they bound Tolbo Nuur (Fig . 13a) . Th e S W fron t o f Omn o Hayrhan Uula is a major thrus t zone that locally cut s Quaternary alluviu m indicatin g recen t activit y (Fig . 13b, c) . Degrade d scarp s u p t o 1 0 m hig h occu r along the front. At the SW end of the range, several step-like scarp s ar e present whic h represen t thrus t faults tha t have elevated the region where Sair Uul and Omn o Hayrha n Uula join (Fig s 3 , 13a) . The S W Omno Hayrhan Uula mountain front i s particularly interestin g a t th e entranc e t o th e Bor t
Fig. 13. (a ) Kosmo s imag e o f Tolb o Nuu r and Omn o Hayrhan Uul a region, Mongolia n Altai . Omn o Hayrhan Uula is boun d b y SW-directe d thrus t fault s o n it s S W side . Fres h alluvia l fan s alon g th e mountai n fron t hel p defin e th e limits o f Tolbo Nuur. Note dextral offsets o n Bort River canyon and the major canyo n NW of Bort River. For location of imag e se e Figur e 4 . (b ) Vie w S E o f S W fronta l thrus t zon e a t entranc e t o Bor t Rive r canyon , Omn o Hayrha n Uula. Sai r Uu l pop-up i n distance. Faul t i s a Palaeozoic shea r zon e containin g Fe-Ti mineralizatio n which has been reactivated by brittle Cenozoic thrusting , (c ) View NNE of surface rupture of thrust fault, S W front o f Omno Hayrhan Uula (48°02.899' , 90°17.054') . Scar p ha s approximatel y 5 m of vertica l offset , (d ) Vie w N E o f brittle fronta l thrus t zone cutting Palaeozoic minera l deposit, entrance to Bort River canyon, SW front o f Omno Hayrhan Uula (48°29.947', 90°15.252'). Inse t show s equal area , lowe r hemisphere stereographi c projectio n o f faul t plane .
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River canyon (48°29.947', 90°15.252', 2280 m). In the fron t range , th e Bor t Rive r cut s throug h a sequence o f lowe r greenschis t grad e metamor phosed volcaniclasti c sediments , an d volcani c breccias. However , a t th e mountai n front , thi s sequence form s th e uppe r plat e o f a brittl e thrust zone whic h contain s visuall y impressiv e Fe-T i mineralization, serpentinit e alteration , an d metasomatic carbonate developmen t (Fig . 13d) . The mineral deposi t onl y occur s withi n th e thrus t zon e which strike s 315° , dip s 24NE, an d contains slickenlines plungin g 24 , 050° . Thi s mineralizatio n i s believed t o b e hydrotherma l i n origin , perhap s related t o metamorphism i n a marine volcani c and volcaniclastic sequence . Thus the mineralization i s believed t o b e Palaeozoi c i n ag e whe n Alta i regional metamorphis m las t occurred . Therefore , modern Cenozoi c thrustin g a t th e mountai n fron t has reactivate d a n olde r mineralize d Palaeozoi c fault zone . Another majo r Cenozoi c faul t zon e form s a prominent linea r valle y withi n Omn o Hayrha n Uula approximately 5 km up the Bort River canyon from th e mountai n fron t (Fig s 3 , 13a) . Thi s faul t zone dextrall y offset s tw o majo r rive r system s which drai n th e rang e (Fig . 13a ) an d th e faul t i s
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well exposed alon g the cliffs o f the Bort River canyon. At the fault zone (48°32.117'N, 90°18.233'E), brittle thrust s diverg e fro m it s cor e i n flower-lik e geometry over a width of approximately 8 0 m. The main faul t stran d i s a vertical , 325°-striking , 2 m thick cemented breccia which cuts through sheare d phyllites compose d o f metasediments an d metavolcaniclastic rocks . Rock s t o th e S W an d N E ar e increasingly unmetamorphose d i n appearanc e within onl y a fe w hundre d metre s suggestin g tha t the faul t zon e i s a loca l pea k metamorphi c zon e perhaps du e t o concentrate d flui d flo w alon g it . Some o f th e faul t rock s ar e best describe d a s faul t schists an d contain strongly developed S- C fabrics indicating SW-directe d thrustin g withi n th e S W part o f th e faul t zone . Investigations i n th e remainde r o f Omn o Hayr han Uul a wer e no t carrie d out ; however , examin ation o f satellit e imager y suggest s that onl y the S , SW, an d W side s o f th e rang e hav e experience d Cenozoic fault activity . Further to the NW, Baljinnyam e t al. (1993) reported surfac e ruptures alon g the mountain front directl y NE of Tolbo Nuur. Satellite imager y indicate s tha t th e range' s drainag e systems ar e markedly asymmetri c with most rivers running E or NE awa y from th e activ e W an d SW
Fig. 14 . (a ) Kosmo s image o f Tal Nuur region, Mongolian Altai . Se e Figure 4 for location. Ta l Nuur fills depression bounded by Mengildy k Nuru u t o N E whic h is uplifte d alon g th e SW-directe d Ta l Nuu r thrust fault, (b ) Vie w SE of Tal Nuu r thrust whic h i s marke d b y brea k i n slop e alon g S W fron t o f Mengildy k Nuruu (48°02.899 r, 90°13.735') . Inset shows equal area, lower hemisphere stereographic projection of brittle fault plane, (c) View north towards Mengildyk Nuruu and clearly define d Ta l Nuur thrust, (d) View NW of summit ridge of Mengildyk Nuruu showing preserved Cretaceous-Palaeogene peneplai n a t 3450 m elevation (48 ° 11.102'. 90°08.007').
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mountain fronts. Thus the Omno Hayrhan range is regarded a s an asymmetricall y NE-tilte d Cenozoi c block uplift.
Mengildyk Nuruu Mengildyk Nuru u i s a flat-summitted , NW trending rang e whic h i s bounde d o n it s S W sid e by the Tal Nuur Basin (Fig s 3 , 14) . Th e S W range front i s abrupt and marked by the Ta l Nuur thrust, a majo r Cenozoi c thrus t faul t whic h i s largel y responsible fo r uplif t o f th e rang e (Fig . 14a-c) . Directly eas t o f Tal Nuur , this faul t place s granit e and gree n metasedimentar y phyllite s ove r tilted , and locall y overturne d an d folded , re d Devonia n clastic sediments . Th e faul t forms a scar p tha t i s locally u p t o 4 0 m high . Th e granit e a t th e scar p itself i s pervasivel y fracture d with a stron g brittl e
fabric strikin g 330° , dippin g 42NE, and containin g slickenlines plungin g 24 , 002° . A t th e S E en d o f Mengildyk Nuruu , the Ta l Nuu r thrust appear s t o change its polarity because the SW side of the faul t is uplifted along two en echelon segment s (Fig. 3) .
Hayrt Nuruu and the Chinese Altai Our investigations in Hayrt Nuruu and the Chines e border range s indicat e tha t severa l brittl e SW directed thrust s are found S W and S E of Tal Nuur bounding smal l uplifte d ridge s (Fig . 14a) . Thes e are th e onl y clea r Cenozoi c structure s identified. Qu an d Zhan g (1994 ) presente d a cross sectio n from nea r th e Mongolian-Chines e borde r t o th e Junggar Basi n (Fig. 1 ) which links with our study. They identifie d a t leas t si x majo r SW-directe d brittle thrus t faults i n the Chinese Alta i which they
Fig. 15 . Rive r patterns fo r major block uplifts, northwestern Altai. Area shown i s same as Figures 3, 4. Dextral offset s of Hov d Rive r alon g Chikhtei n Faul t ar e indicated an d visibl e i n Figur e 3 .
STRIKE-SLIP FAULTIN G I N MONGOLIA
argue ar e Palaeozoi c i n origin , bu t whic h wer e reactivated in the Cenozoic. Unfortunately, detailed evidence fo r Cenozoi c reactivatio n wa s no t pro vided. W e ar e unawar e o f an y moder n studie s investigating the Cenozoic constructio n of the Chinese Altai .
Discussion In al l area s studied , majo r Cenozoi c thrus t fault s are SW-directe d an d bound the S W sides of majo r block uplifts . Thes e fault s ar e commonl y trans pressional segment s o f majo r regiona l dextra l strike-slip fault s o r are commonly thrusts linked to them. Bloc k uplift s hav e markedl y asymmetri c
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drainage patterns consisten t wit h NE tilting due to SW-directed thrustin g along SW range fronts (Fig s 15, 16) . Area s betwee n activ e fault s ar e geomor phologically matur e an d lac k evidenc e fo r Ceno zoic tectoni c activit y (Fig s 3 , 11) . Thu s Cenozoi c deformation i n the Altai is limited to discrete zones of Cenozoic reactivation (Fig . 17) . The cross-strik e Cenozoic architectur e o f th e rang e a t thi s latitud e is relatively simpl e in that each range is essentially a larg e NE-tilted thrus t block . The Palaeozoi c structura l grai n o f basemen t metamorphic rocks in the region i s generally NW striking an d NE-dipping . I n numerou s cases , w e observed evidence for brittle reactivation o f ductile shear zone s an d metamorphi c fabric . I t appear s
Fig. 16. Direction s o f bulk tilting fo r different Cenozoi c uplifte d blocks, northwester n Altai. Are a show n is sam e as Figure 4. Directions o f tilting ar e interpreted fro m drainag e patterns (Fig . 15 ) and geometry of SW-directed Cenozoi c thrust faults whic h bound major range s in region. N E tilts are predominant. AH : Altun Huhey Uul; TM: Tsambagarav Massif; OHU : Omn o Hayrha n Uula; HS : Ho h Serhiy n Nuruu; MN : Mengildy k Nuruu ; HN : Hayr t Nuruu .
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Fig. 17. Interprete d Cenozoic block structure of Mongolian Altai from Valle y of Lakes SW to Chinese border. Attitude of basemen t fabric s i s fro m unpublishe d fiel d dat a b y authors . AH : Altu n Huhe y Uul ; TS : Tsambagara v Uul ; OH : Omno Hayrha n Uula; MN : Mengildy k Nuruu.
likely tha t a t firs t order , Cenozoi c dextra l trans pressional reactivatio n o f th e Alta i wa s controlle d by this fundamental basement anisotropy . This has also bee n suggeste d fo r th e Chines e Alta i (Q u & Zhang 1994) . If th e two-dimensiona l cross-strik e architectur e of the Altai appears relatively simpl e at the latitude of thi s study , th e three-dimensiona l architectur e i s not. Eac h majo r rang e i s a transpressiona l culmi nation wit h differen t strike-slip , oblique-slip , an d thrust faul t linkage s (Fig . 18) . Sair Uu l is a flower structure pop-u p betwee n divergen t splay s o f th e Hoh Serk h Fault (Fig s 3 , 11) . Altun Huhey Uul i s a partial restraining bend that appears to accommodate mos t o f th e strike-sli p motio n o f th e Hov d
Fault b y thrustin g alon g th e S W mountai n front ; however, ther e i s evidenc e tha t th e faul t als o bypasses th e rang e an d continue s towards the N E (Fig. 6b) . Th e Jargalan t range , whic h i s th e east ernmost range of the Altai, SE of Altun Huhey Uul (Figs 2 , 18) , appear s t o b e a mor e complet e ter minal restrainin g ben d (cf . Cunningha m e t al. 1996). Th e Tsambagara v rang e i s locate d a t a gentle ben d alon g th e A r Hotol/Chikhtei n Faul t where ther e i s a major componen t o f compressio n and reverse displacement (Figs 3, 4). The triangular shape t o th e rang e i s define d b y boundin g thrust faults an d ma y b e th e resul t o f southwar d propa gation of the Chikhtein Fault and northward propagation of the Ar Hotol Fault with the Tsambagarav
Fig. 18. Faul t geometrie s o f activ e transpressiona l culmination s i n Mongolia n Alta i includin g termina l restrainin g bends (Jargalan t Nuruu) , semi-termina l restrainin g bend s (Altu n Huhe y Uul) , triangula r til t block s (Tsambagara v Massif), asymmetri c flowe r structure s betwee n divergen t splay s (Sai r Uul) , an d oblique-sli p ridge s (Ho h Serhiy n Nuruu).
STRIKE-SLIP FAULTIN G I N MONGOLIA
range forming in the overlap/accommodatio n zon e for th e tw o faul t system s (Fig . 18) . Th e Ho h Serhiyn rang e i s a n elongate transpressiona l rang e with dextra l an d thrus t component s o f displace ment alon g bot h N E an d S W sides . Asymmetri c drainage pattern s an d overal l topograph y sugges t that th e N W en d o f th e rang e ha s greate r thrus t displacements alon g it s S W margin , wherea s th e SE end of the range has experience d greate r thrust displacements alon g it s N E margi n (Fig . 18) .
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The manner in which dextral strike-slip displace ments ar e transferre d t o termina l thrust s an d oblique-slip fault s i s comple x an d varie s betwee n individual ranges. I n most ranges (Sai r Uul , Altu n Huhey Uul , Tsambagara v range) , thrus t fault s di e out awa y fro m linkin g strike-sli p fault s indicatin g a decreas e i n thrus t displacemen t awa y fro m th e fault junction . Thi s suggest s tha t th e upthruste d block rotate d aroun d a vertica l axi s durin g dis placement transfe r (Bayasgala n e t al 1999) .
Fig. 19. Mechanis m for block rotations throug h time (t= l t o t=3) at the termination zones for dextral strike-slip fault s in the Mongolian Altai. Where the terminal thrust fault moves in the direction of strike-slip displacement (top example), the upthrusted block rotate s i n a counterclockwise sens e wit h thrust displacement decreasin g awa y from th e junction between th e thrus t faul t an d strike-sli p fault . Thi s i s interprete d t o hav e occurre d i n th e Tsambagara v rang e when viewed fro m N W to S E and may explai n the triangular shape o f the rang e (Fig s 3 , 4, 18) . Where th e termina l thrust fault move s in a direction opposit e t o the strike-sli p sense , th e upthrusted block rotate s i n a clockwise sens e an d the thrust fault propagate s dow n the length of the strike-sli p faul t overridin g it. This is interpreted to have occurred along the S W fron t o f th e Mengildy k Nuru u rang e (Ta l Nuu r thrust ) when viewed from th e S E (Fig s 3 , 4) .
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Rotation i s presumabl y greates t wher e th e faul t intersection angl e i s highes t an d th e thrus t offse t is largest . However , th e sens e o f block rotatio n i s dependent on whether the linking thrust moves forward in the same direction a s the strike-slip motio n or whether it moves in an opposite sense (Fig . 19) . For example , thrust s a t th e souther n en d o f th e Tsambagarav rang e hav e move d S E indicatin g counterclockwise rotatio n throug h tim e (Fig s 18 , 19). Conversely , th e Tal Nuur thrust is interprete d to hav e rotate d i n a clockwis e sens e wit h tim e t o the poin t wher e th e linkin g strike-sli p faul t a t th e southern en d o f th e Mengildy k Nuru u rang e i s almost completel y overridde n (Fig . 19) . A block rotation mechanis m for accommodatio n of Lat e Cenozoi c strike-sli p faul t displacement s i s documented by palaeomagnetic dat a in the Russian Altai, 40 0 k m nort h o f th e regio n studie d her e (Thomas e t al 1998 ; Bayasgala n e t al. 1999) . Unfortunately, to our knowledge, there ar e no published palaeomagneti c dat a fo r anywher e i n th e Mongolian Altai , s o the amoun t o f bloc k rotatio n within th e Mongolia n Alta i i s unconstrained . W e think i t i s als o likel y tha t th e regiona l strike-sli p faults within the Mongolian Altai have rotated with the block s the y bound . Counterclockwis e rotatio n of originall y mor e N-S-trendin g strike-sli p faults to mor e NW-trendin g orientation s woul d hav e reoriented th e fault s mor e favourabl y fo r contrac tional deformatio n an d migh t explai n wh y fault s like th e Ho h Serk h Faul t hav e importan t revers e components o f motion an d have nucleated discret e thrust splay s alon g thei r length s suc h a s th e Tsa -
gaan Sala a thrus t (Fig s 11 , 18) . I n summary , w e believe that the Mongolian Alta i is a superb natura l laboratory fo r studyin g a diverse rang e o f activel y forming an d interlinkin g transpressiona l faul t sys tems wit h associate d bloc k rotations . Structural evidenc e fro m th e Chines e Alta i adjacent t o th e are a studie d her e indicate s tha t the range is dominated by a NW-striking moderate- t o low-angle thrus t bel t tha t overthrust s th e Jungga r Basin to the SW (Qu & Zhang 1994) . These fault s are reported to be Palaeozoic thrusts that have been reactivated i n the Late Cenozoic. We see little evidence o n satellite image s o f the Chines e Altai that the thrust faults ar e linked wit h regional strike-sli p faults a s i s foun d i n th e Mongolia n Altai . Thu s i t appears tha t a t th e latitud e studie d here , th e Alta i is fundamentally partitione d into a low-angle thrust belt tha t overthrusts th e Junggar Basin o n the Chi nese side , an d a high-angle dextra l transpressiona l belt o n th e Mongolia n sid e (Fig . 20) . Structura l vergence is consistently towards the SW across the entire range . Elsewhere alon g strike , Cenozoi c structura l vergence ma y no t b e entirel y t o th e SW . Fo r example, Cunningha m e t a l (1996 ) reporte d coalesced parallel flower structures in the southernmost Alta i whic h contai n Cenozoi c thrus t fault s with bot h S W an d N E vergence . Ou r recen t fiel d results from area s 10 0 km south of the region studied her e als o indicat e tha t som e majo r Cenozoi c thrusts ar e N E directed , toward s th e Valle y o f Lakes. Clearly , mor e wor k i s neede d throughou t the range to better understand along-strike changes
Fig. 20. Simplifie d sectio n acros s Alta i incorporatin g dat a fro m thi s stud y an d informatio n i n Q u & Zhan g (1994) . Mongolian Alta i is dominated b y dextral transpressional block uplifts an d nigh-angle faults wherea s the Chinese Alta i is characterized b y a low-angle thrus t belt that overthrusts th e Junggar Basin. Cenozoi c structura l vergenc e alon g this line i s dominantl y toward s th e SW.
STRIKE-SLIP FAULTIN G I N MONGOLI A
in Cenozoi c faul t geometr y an d crusta l architec ture. There appea r t o b e thre e fundamenta l control s on wh y Cenozoi c mountai n buildin g i n th e Alta i region i s dominate d b y dextra l transpression : (1 ) NE-SW-directed SHma x du e to India's continue d NE indentatio n (Zobac k 1992) , (2 ) pre-existin g NW-SE crusta l anisotrop y i n mechanicall y wea k metasedimentary an d metavolcani c basemen t rocks, an d (3 ) the presenc e o f rigi d crusta l block s underlying th e Hanga y an d Junggar Basin region s
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which have resisted internal deformation , bu t have focused deformatio n in th e Alta i betwee n th e tw o blocks. Th e Hanga y regio n i s believe d t o b e underlain b y a mechanically rigi d Precambria n eratonic bloc k (Cunningha m 2001). Thi s i s base d o n Archaean-Neoproterozoic zirco n ages on basement rocks an d middle-late Precambrian Sm-N d mode l ages fo r granite s throughou t a wid e are a (Kovalenko et al 1996) . The outline of the Hangay Block i s wel l define d o n aeromagneti c map s o f Mongolia (Fig. 21). We view the Hangay Block as
Fig. 21. Unpublishe d aeromagneti c ma p o f Mongoli a courtes y o f Mongolia n Academ y o f Sciences , Institut e o f Geology an d Minera l Resources . Despit e area s o f missin g data , regiona l structura l trend s an d outlin e o f Hanga y Precambrian bloc k ar e wel l defined . Acronym s ar e i n Russia n o n origina l ma p (DZ H = Dzhida Bel t i n norther n Mongolia, OZ R = Valle y o f Lakes , KH A = Hanga y Block , HA N = Ha n Tayshi r ophiolit e occurrenc e i n souther n Valley o f Lakes) .
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Fig. 22. Cartoo n depictin g Alta i as weak Palaeozoic accretionar y belt sandwiched between more rigid Jungga r Bloc k and Hanga y Precambria n cratoni c block . Continue d Tertiar y indentatio n o f Indi a drive s Jungga r Bloc k northwards . Hangay crato n act s a s passiv e indento r an d Alta i bel t i s reactivate d i n dextra l transpressio n betwee n rigi d blocks . Counterclockwise rotatio n o f northern an d souther n end s o f Alta i i s drive n b y NE-directe d SHma x an d geometr y o f rigid bloc k boundaries .
a passive indentor which has focused late Cenozoic deformation aroun d it s W , S W an d S margin s in the Altai an d Gobi Altai (Cunningha m 2001). The ag e an d compositio n o f th e Jungga r Basi n basement ar e unknown. The crus t i s mechanicall y strong with an unusually high elastic thickness (T e -35 km; Maggi e t al 2000) . The high strengt h of the Jungga r Basi n crus t ma y b e du e t o a n earlier history of Permian rifting which resulted in basaltic intrusion an d underplatin g (Alle n e t a l 1991 ; M . Allen, pers . comm . 2001) . Wha t i s clea r i s tha t along the souther n and NE margins of the Junggar Basin Block , Cenozoi c mountai n buildin g i s actively occurring , wherea s th e basi n interio r ha s resisted deformation . Figur e 2 2 i s a simplifie d model showin g ho w dextra l transpressio n i n th e Altai regio n i s linke d t o th e large r deformatio n field o f Centra l Asia . Th e mode l suggest s tha t a s the Jungga r Basi n Bloc k i s displace d northward s with respec t t o th e relativel y fixe d Siberia n and Hangay cratons , th e Alta i regio n basemen t i s reactivated in dextral transpressional mode parallel to th e pre-existin g basemen t grain . Th e angula r relationship betwee n th e rigi d bloc k boundarie s and SHma x determine s th e kinematic s of defor mation. Wit h continue d northward s displacemen t (indentation) o f th e Jungga r Block , counterclock wise rotation o f the norther n an d souther n ends of the Altai occurs and is expressed in the gentle oroclines that ar e found a t opposite ends o f the rang e (Fig. 1) . We ar e gratefu l to Britis h Petroleum fo r providing satel lite imagery. Thank s t o Tsogo , Zoloo , Otgo n an d Tunga for assistanc e i n th e field . Thi s projec t wa s funde d b y National Geographic Societ y Committee for Research and Exploration Gran t 6308-9 8 t o W . D . Cunningham . A .
Dijkstra was supported by a European Commission Mari e Curie Fellowship .
References ALLEN, M . B. , WINDLEY , B . F. , Cm , Z. , ZHONG-YAN , Z. & GUANG-REI, W. 1991. Basin evolution within and adjacent t o th e Tie n Sha n Range , N W China . Journal of th e Geological Society o f London, 148, 369-378. BALJINNYAM, L , BAYASGALAN , A. e t al . 1993 . Ruptures of Major Earthquakes and Active Deformation in Mongolia an d it s Surroundings. Geologica l Societ y o f America Memoir , 181 . BAYASGALAN, A., JACKSON, J., RITZ , J. -F. & CARRETIER , S. 1999. Field examples of strike-slip fault termination s in Mongoli a an d their tectonic significance . Tectonics, 18(3), 394-411 . CUNNINGHAM, W . D . 1998 . Lithospheric controls on late Cenozoic constructio n of the Mongolian Altai. Tectonics, 17(6) , 891-902 . CUNNINGHAM, W . D . 2001 . Cenozoic norma l faultin g and regiona l domin g i n th e souther n Hanga y region , Central Mongolia : implication s fo r th e origi n o f th e Baikal rif t province . Tectonophysics> 331 , 389-411. CUNNINGHAM, W. D., WINDLEY, B . F., DORJNAMJAA, D., BADAMGAROV, G . & SAANDAR , M . 1996 . Structural transect acros s th e Mongolia n Altai : activ e trans pressional mountain building in central Asia. Tectonics, 15(1), 142-156 . DEVYATKIN, E. V. 1974 . Structure s and formational com plexes o f th e Cenozoi c activate d stage . In : Tectonics of th e Mongolian People's Republic. Moscow, Nauka , 182-195 (i n Russian). HENDRIX, M . S., GRAHAM. S. A., CARROLL, A . R., SOBEL , E. R. , MCKNIGHT , C . L. , SCHULEIN , B . J . & WANG , Z. 1992 . Sedimentar y recor d an d climatic implication s of recurrent deformatio n i n th e Tie n Shan : evidenc e from Mesozoi c strat a of th e nort h Tarim , sout h Jung-
STRIKE-SLIP FAULTIN G I N MONGOLI A gar, an d Turpa n basins , northwes t China . Geological Society o f America Bulletin, 104 , 53-79. HOWARD, J . P. , CUNNINGHAM , W . D. , DAVIES , S . J. , DUKSTRA, A. H . & BADARCH, G . (i n press). Th e strati graphic an d structura l evolutio n o f th e Dzere g Basin , Western Mongolia : clasti c sedimentation , faul t inver sion an d basin destruction i n a n intracontinental transpressional setting . Basin Research. KHIL'KO, S . D. , KURUSHIN , R . A. , KOCHETKOV , V . M. , BALJINNYAM, I . & MONKOO , D . 1985 . Earthquakes and th e Bases for Seismic Zoning o f Mongolia. Transactions 41, The Joint Soviet-Mongolian Scientific Geological Researc h Expedition , Moscow , Nauka . KOVALENKO, V . I. , YARMOLYUK , V. V. , KOVACH , V . P. , KOTOV, A . B. , KOZAKOV , I . K . & SALNIKOVA , E . B . 1996. Source s o f Phanerozoi c granitoid s i n Centra l Asia: Sm-Nd isotope data. Geochemistry International, 34, 628-640 . MAGGI, A. , JACKSON, J. A., MCKENZIE, D. & PRIESTLEY , K. 2000 . Earthquak e foca l depths , effectiv e elasti c thickness, an d th e strengt h o f th e continenta l litho sphere. Geology, 28(6), 495^98. Mongolian National Atlas 1990 . Ulaan Baatar , Moscow . Qu GUOSHEN G & CHON G MEIYIN G 1991 . Lea d isotop e geology an d it s tectoni c implication s i n th e Altaides , China. Geoscience, 5, 100-110 . Qu GUOSHEN G & ZHAN G JINJIAN G 1994 . Oblique thrust systems i n th e Alta y orogen , China . Journal o f Southeast Asian Earth Sciences, 9(3), 277-287. SENGOR, A . M. C. & NATAL'IN, B . A. 1996 . Paleotectonics o f Asia : fragment s o f a synthesis . In : YIN , A. & HARRISON, T. M . (eds) The Tectonic Evolution of Asia. Cambridge University Press, 486-640 . SENGOR, A . M . C. , NATAL'IN , B . A . & BURTMAN , V . S. 1993 . Evolution o f th e Altai d tectoni c collag e an d Palaeozoic crusta l growt h i n Eurasia . Nature, 364 , 299-307.
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SJOSTROM, D . J . 1997 . Lower-Middle Jurassic through Lower Cretaceous sedimentology, stratigraphy and tectonics of Western Mongolia. MSc thesis , University of Montana . TAPPONNIER, P . & MOLNAR, P . 1979 . Active faulting and Cenozoic tectonic s o f th e Tie n Shan , Mongolia , an d Baykal regions . Journal o f Geophysical Research, 84 , 3425-3459. THOMAS, J . C. , LANZA , R . KAZANSKY , A. , ZYKIN , C . V . S., SEMAKOV , N. , MITROKHIN , D . & DELVAUX , D . 1998. Paleomagneti c stud y o f Cenozoi c sediment s i n the Siberia n Alta i an d th e Zaisa n Basin , centra l Asia : evidence fo r heterogeneous vertica l axi s rotations during th e Tertiar y (abstract) . In: DOBRETSOV , N . L. , KLERKX, J. & LOGATCHEV , N . L. (eds ) Active Tectonic Continental Basins. Universit y Gent , Belgium , 67. TRAYNOR, J . J . & SLADEN , C . 1995 . Tectonic an d strati graphic evolution of the Mongolian People's Republic and its influence on hydrocarbon geology and potential. Marine an d Petroleum Geology, 12 , 35-52 . TRIFONOV, V . G . 1988 . Mongolia - a n intracontinental region o f predominantl y recen t strike-sli p displace ment: active faults. In: Neotectonics and Contemporary Geodynamics o f Mobile Belts. Moscow , Nauka , 239 272 (i n Russian). WEBB, L. E., GRAHAM , S. A., JOHNSON, C . L., BADARCH , G. & HENDRIX , M . S . 1999 . Occurrences, age , and implications o f the Yagan-Onch Hayrhan metamorphic core complex , souther n Mongolia . Geology, 27, 143 146. ZAITSEV, N . S . 1978 . Geological Map o f th e Mongolian Altai, 1:500,000 Scale. Acad . Sci . USSR, Acad . Sci. Mongolian People's Republic, Comb. Sov.-Mongolian Sci. Res . Geol. Exped . Nauka , Moscow (i n Russian). ZOBACK, M . L . 1992 . First- an d second-orde r pattern s of stress in the lithosphere: the worl d stres s ma p project . Journal o f Geophysical Research, 97(B8) , 11703 — 11728.
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Relationships betwee n Cenozoi c strike-sli p faulting an d basin openin g i n norther n Thailand WUTTI UTTAMO 1, CHRI S ELDERS 2 & GARY NICHOLS 2 ^Department of Geological Sciences, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand (e-mail:
[email protected]) 2 SE Asia Research Group, Department of Geology, Royal Holloway, University of London, Egham, Surrey TW20 OEX, UK Abstract: Norther n Thailan d i s locate d i n a structurall y comple x are a betwee n thre e majo r tectonic regimes , a region o f extensional tectonics t o the sout h and two majo r strike-sli p zones , the Sagain g faul t zon e t o th e wes t an d th e Re d Rive r faul t zon e t o th e northeast . Cenozoi c tectonics i n northern Thailan d resulted from th e collisio n betwee n the Indian plate an d Eurasia. The continued indentation o f the Indian plate into Eurasia caused polyphase extrusion of Sundaland an d the movement of major strike-sli p faults. Th e movement of these fault s accompanyin g the regional east-west extension during Late Oligocen e to Early Miocene initiate d the formation of th e Tertiar y basins . Thirty-six majo r fault s an d forty-tw o intra-cratonic depositiona l basin s i n norther n Thailand have been recognize d an d delineated usin g Landsat TM images. More than 70% of these basins are related t o strike-sli p tectonics. Five basin type s hav e bee n recognized o n the basi s o f geo metric an d kinemati c considerations . Thes e ar e fault-ti p basins, pull-apar t basins , fault-wedg e basins, faul t zon e basins, an d extensional basins. The openin g and development o f these basin s was influence d b y th e movement o f NW-trending dextra l faults an d NE-trending sinistral fault s associated wit h north-sout h shortenin g an d east-west extension .
In Thailand most indigenou s fossi l fuels hav e been i n a broad 'S ' shape . Th e average elevatio n o f the exploited from Tertiar y basins (Fig. 1) . Natural gas mountain s is abou t 160 0 m abov e mea n se a level , fields foun d i n these basins ar e located i n the Gul f Th e many Cenozoi c intermontan e basin s ar e vari of Thailand . Oi l fields are found i n onshor e basins ou s size s an d thei r floo r elevation s var y betwee n located i n the Centra l Plain an d northern Thailand. 25 0 an d 80 0 m abov e se a level . Most o f th e coa l an d oi l shal e deposit s ar e foun d Th e mai n data for studyin g Cenozoic fault s sys in smal l basin s locate d i n norther n an d wester n tern s in northern Thailand ar e Landsat TM satellit e Thailand. I n norther n Thailan d ther e ar e ove r 4 0 images . Thes e images, whic h cove r all of norther n Tertiary basin s rangin g fro m 10 0 t o 200 0 squar e Thailand , wer e provide d b y th e Southeas t Asi a kilometres i n area . Th e basin s ar e mainl y N-S - Researc h Group , Universit y o f London . A six trending, fault-bounde d half-graben s an d graben s scen e Landsa t T M imag e mosai c wa s produce d (Chaodumrong e t al. 1983 ) wit h sedimentar y fill s usin g ER-Mappe r imag e processin g software . The ranging i n thicknes s fro m 50 0 t o 300 0 m dat a hav e 3 0 m groun d resolutio n allowin g clea r (Pradidtan 1989) . The basins are separated by older definitio n o f most geographical, geomorphological , mountain range s tha t li e approximatel y betwee n an d large-scal e geologica l features . Mai n rivers , latitudes 17°30 ' an d 20°30'N an d between longi - mountains , highways , cities , an d town s coul d b e tudes 97°30 ' an d 101°10'E , coverin g a n are a o f identifie d an d relate d t o th e topographi c map s o f 180,000 squar e kilometre s (Fig . 1) . Th e are a i s th e sam e area . Basi n area s ca n b e delineate d bounded to the north and west by Myanmar and on directl y o n th e Landsa t images . Fault s an d larg e the eas t by Laos. The mountain ranges of northern fractur e zone s ar e als o delineated . Som e fault s Thailand occu r betwee n th e Sa l ween Rive r i n th e sho w relative sens e of movement where they offse t west an d th e Mekon g Rive r i n th e east . The y ar e geomorphologica l an d geological features, e.g. rivoriented mor e or less N-S t o NNE-SSW and trend ers , streams , folds , an d geological units . From: STORTI , F. , HOLDSWORTH , R . E . & SALVINI , F . (eds ) Intraplate Strike-Slip Deformation Belts. Geologica l Society, London , Specia l Publications , 210 , 89-108 , 0305-8719/037 $ 15 © Th e Geologica l Societ y o f Londo n 2003 .
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Fig. 1 . Tertiar y basin s i n Thailand an d locatio n o f stud y area .
Pre-Tertiary tectonic s Southeast Asi a comprise s a comple x assembl y o f continental terrane s (Fig . 2 ) tha t ar e bounde d b y suture zones, narro w mobil e belts , an d major faul t zones (Lacassin e t al. 1997). They were all derive d directly o r indirectl y fro m Gondwaualan d
(Metcalfe 1996) . Th e amalgamatio n o f th e Sout h China an d Indochin a terrane s alon g th e Son g Ma/Song Da zone (Fig. 2) occurred during the Late Devonian-Early Carboniferou s t o for m Cathaysia land (Metcalfe 1996) . The Shan-Thai terran e rifted from northern Gondwanaland in Late Carboniferous Early Permia n time , drifte d northward , an d col -
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Fig. 2 . Th e distributio n o f continenta l bloc k an d fragment s (terranes ) an d principa l sutur e zone s o f Southeas t Asi a (after Metcalf e 1996) . Thick black lines are terrane boundaries, dashed lines are inferred terrane boundaries, alignmen t of x' s ar e th e sutur e zones, an d C- M i s Changning-Menglia n suture .
lided wit h Cathaysialan d durin g Lat e Permian Triassic time. This collisio n resulte d i n the closur e of Palaeo-Tethy s (Metcalf e 1996) , an d i s widel y known a s the Indosinia n orogeny . The continental terrane s withi n which the northern Thailan d are a i s locate d ar e Shan-Tha i an d Indochina. Thes e terrane s ar e separate d b y th e Nan-Uttaradit suture (Fig. 2). After th e collision o f the Shan-Thai and Indochina plates alon g the NanUttaradit suture, the Khorat Plateau i n northeastern Thailand an d other continenta l sedimentar y basin s in th e regio n ma y hav e forme d a s forelan d basin s as a resul t o f flexura l subsidenc e i n fron t o f th e orogenic belt s create d b y th e Nan-Uttaradi t suture (Lovatt Smit h e t al 1996) . The wes t Burm a terran e (Fig . 2 ) rifte d fro m northwestern Gondwanalan d i n th e Lat e Triassi c and drifte d northwar d durin g th e Jurassi c a s th e Ceno-Tethys opene d an d th e Meso-Tethy s wa s destroyed by subduction beneath Eurasia (Metcalfe 1996). Accretion an d collision o f this terrane to the Shan-Thai terran e occurre d i n th e Cretaceou s (Metcalfe 1996) . Th e onse t o f structurin g an d inversion o f th e Khora t Grou p i n th e Khora t Pla -
teau durin g the Mid-Cretaceou s ma y b e th e resul t of thi s continenta l collisio n (Lovat t Smit h e t al . 1996). Upto n et al. (1997) studie d the Tertiary tec tonic denudation in northwestern Thailand by using an apatit e fissio n trac k method . The y foun d a gentle inversio n durin g the Lat e Cretaceous-Earl y Tertiary prio r t o a phas e o f rapi d coolin g i n a north-south bel t o f gneissi c an d plutonic rock s i n northern Thailan d durin g th e Lat e Oligocene Early Miocene .
Cenozoic tectonic s The tectonic evolution of Southeast Asia during the Cenozoic wa s partiall y influence d b y th e subduc tion, collision , an d indentatio n o f th e India n plat e into Eurasia . Th e duratio n o f thi s collisio n wa s about 5 0 millio n year s (Packha m 1996) . Hal l (1996) presente d th e reconstructio n o f Southeas t Asia a t 5-million-yea r interval s fo r th e pas t 5 0 million years . I n hi s work , a t 3 0 millio n years , (Mid-Oligocene) th e Indochin a bloc k wa s separ ated from souther n Thailand and peninsular Malaysia o n a lin e representin g th e Thre e Pagoda s an d
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Wang Chao (Ma e Ping) faults. Th e block als o wa s separated fro m th e Eurasia n plat e o n a lin e rep resenting th e Re d Rive r faul t zon e (Fig . 3) . Post Cretaceous clockwis e rotation s ar e recorde d i n Thailand, wherea s counterclockwis e rotation s ar e reported fro m Tertiar y an d olde r rock s furthe r south (Hall 1996) . Th e clockwise rotation s in Thailand coul d be explaine d b y extrusio n of Indochina (Hall 1996) . I n th e Andama n Se a are a (Fig . 2) , before 1 0 million year s ther e wa s a smal l amoun t of orthogona l extension . Afte r 1 0 millio n years , extension wa s greate r bu t highl y oblique . Thi s i s broadly consisten t wit h th e ag e o f th e oldes t oce anic crus t i n th e Andama n Sea , 1 1 millio n years , and th e recen t patter n o f openin g (Curra y e t aL 1979; Hal l 1996) . The central and eastern part of northern Thailand (Fig. 4 ) i s situate d withi n th e Sukhotha i fol d belt . This fol d bel t probabl y extende d fro m th e Sima o basin (Lacassi n e t al. 1997 ) (Fig . 4) . Th e folding in th e Sima o basi n i s du e t o Tertiar y ENE-WS W compression (Tapponnie r e t a L 1990) . I n th e cen tral par t o f norther n Thailand , th e N-S-trendin g
fold axe s are bent into an 'S' shap e along the Mae Chan strike-sli p fault (Fig . 4). The folds ar e generally upright , with som e fold s vergin g t o th e eas t and southeas t (Hah n e t al . 1986) . Th e Cenozoi c basins that have N-S-bounding norma l faults occur along syncline s o f thes e fold s (Lacassi n e t al . 1997). Thi s structura l styl e i s comparabl e t o tha t of th e Sima o basin , wher e E- W extensio n over prints th e earlie r Tertiar y ENE-WS W com pressional structur e (Tapponnier e t al. 1990) .
Regional strike-sli p fault s Major strike-sli p fault s develope d surroundin g the northern Thailan d regio n (Fig . 4) . I n th e fa r west , the Sagain g fault zon e in Myanmar developed a s a NNW-trending dextra l strike-sli p faul t (Maun g 1987). Thi s faul t separate s th e Sha n Platea u fro m the Centra l Lowland s o f Myanma r (Fig . 4) . Th e drag produce d by th e northwar d movement o f th e Indian plat e cause d th e Burm a plat e (Fig . 2 ) t o decouple fro m th e Easter n Highlan d o f Burm a (Shan Plateau ) alon g th e Sagain g faul t (Maun g
Fig. 3 . Th e reconstructio n of Southeas t Asia at 3 0 Ma (redraw n fro m Hal l 1996) .
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Fig. 4. Geologica l structur e o f mainlan d Southeas t Asi a showin g majo r strike-sli p faults , norma l faults , folds , an d Cenozoic basin s (modifie d afte r Lacassi n e t al. 1997) .
1987). Northwar d movemen t o f th e Burm a plat e has resulte d i n th e openin g o f th e Andama n Se a rift syste m sinc e th e mid-Miocen e (Maun g 1987) . In th e southwes t part o f norther n Thailand , th e Wang Chao fault zon e (also referre d to as the Mae Ping faul t zone ) occurre d i n a NW-S E directio n (Fig. 4) . Th e movemen t o f thi s strike-sli p faul t i s believed t o b e complex . Durin g Earl y Oligocen e time, th e faul t move d i n a left-latera l direction . During Early Miocene to Quaternary time it moved in a right-latera l directio n (Lacassi n e t al . 1997) . The total left-lateral offset o f about 300 km on this
fault zon e wa s interprete d a s resultin g fro m th e Tertiary indentatio n of India within Asia (Lacassi n et al . 1997) . Tulyati d (1997 ) studie d an d inter preted aeromagneti c an d radiometri c dat a i n thi s area and found that the Wang Chao fault had a left lateral offse t base d o n th e displacemen t o f highl y radioactive granit e unit s b y 8 0 t o 10 0 km . Fro m the stud y o f outcro p alon g th e Wan g Cha o faul t zone i n th e La n San g area , Ta k Province , Elder s et al . (2000 ) identifie d tw o stage s o f deformation : sinistral ductile shear an d dextral-slip brittle faults . In th e southeas t par t o f norther n Thailand , th e
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Uttaradit fault zone (Fig. 4) developed in a NE-SW direction with sinistral strike-slip movemen t (Bal et al. 1992; Tulyatid 1997) . This fault has a minimum displacement o f 25-3 0 k m base d o n th e offse t o f high magnetic anomalie s acros s th e faul t (Tulyati d 1997). Th e Uttaradi t faul t continue s towar d th e southwest unde r th e Cha o Pray a plai n an d form s the north flank of the Phitsanulok basi n (Ba l et al. 1992). The Uttaradit fault zon e occurred parallel to the Nan River sutur e zone (Fig. 2), which consist s of a belt of ophiolitic, mafic-ultramafic rock s (Fig. 4). I n th e norther n par t o f norther n Thailan d th e Mae Cha n faul t (Fig . 4) trend s ENE-WS W an d shows sinistra l strike-sli p movemen t (Fento n e t al. 1997). There i s stron g geomorphologica l evidenc e for lat e Quaternar y left-latera l displacemen t alon g this faul t an d recent trenchin g investigation s hav e confirmed Lat e Quaternar y faultin g (Fento n e t al . 1997).
Landsat TM images of northern Thailand The Landsa t satellit e imag e mosai c o f norther n Thailand (Fig . 5) covers a n area of 136,00 0 square kilometres. A Cenozoic basins and faults map (Fig. 6) has been derived from th e Landsat image mosaic (Fig. 5) . In the norther n area , Cenozoi c basin s ar e situated betwee n tw o larg e rivers , th e Salwee n River to the west and the Mekong River to the east (Figs 5 & 6). The Salween Rive r flows from nort h to sout h an d changes t o southwes t nea r th e ThaiMyanmar border . Th e Mekon g Rive r flow s fro m north t o sout h an d has bee n left-laterall y offse t b y the Nam Ma fault (NM F in Fig. 6) (Lacassin e t al. 1998). Th e Mekon g Rive r the n flow s i n a SS W direction t o th e Ma e Sa i basi n an d the n change s direction t o th e east . Thi s changin g tren d ma y be caused b y th e left-lateral , strike-slip , Ma e Cha n fault (MC F i n Fig. 6) (Bot t e t al. 1997 ; Fenton e t al. 1997 ; Kosuwan et al. 1998) . After crossin g th e fault, th e Mekong Rive r flows to the southeast and it form s th e border betwee n Thailan d an d Laos . The wester n are a o f norther n Thailan d consist s of hig h mountai n range s wit h smal l elongate , N— S-trending, Cenozoi c basins . Thes e basin s ar e th e Mae Sarieng, Mae Chaem , Pai, Wiang Haeng, and Chiang Da o basins (Fig s 5 & 6). North of the Chiang Dao basin, there is a narrow, N-trending elongate valley that extends more than 9 0 km across the border int o Myanmar. The Doi Inthanon mountain (Fig. 5) is the highest mountain in Thailand at 2590 m abov e mea n se a level . O n th e eas t sid e o f Do i Inthanon, the image show s a belt o f variably size d basins. Th e larges t i s th e Chian g Ma i basi n (Fig s 5 & 6) , whic h i n isolatio n display s a curve d 'Z 7shape. Nort h o f Chian g Ma i i s th e smal l N-S trending Phra o basin . Thi s basi n show s som e characteristics o f half-grabe n structure , wit h th e
main border faul t locate d o n the wes t margin. The Fang basin located near the border is a NE-trending basin wit h a curved, western margin. T o the south of th e Chian g Mai basin , th e Li basin formed i n a NW-SE orientation . In the centra l an d eastern are a o f northern Thailand, ther e ar e severa l Cenozoi c basin s o f varying sizes. The small basins, such as Chiang Muan, Ban Luang, Na Noi, Pua, and Chiang Khong , display a half-graben geometry , with the major borde r fault s on thei r easter n margin s (Fig s 5 & 6). The trend s of thes e smal l basins var y from NN W to NNE . In contrast, the large basins, such as Lampang, Phrae, Phayao and Chiang Rai (Figs 5 & 6) show no evidence of half-graben structures. The trends of these large basins vary from NN W to NE. The Mae Moh basin i s locate d i n th e centra l are a (Fig . 5). North of thi s basi n there i s a NE-SW-trending synclina l structure develope d withi n marin e Triassi c rocks . The Phaya o basi n ha s a freshwate r lak e i n it s southern part . Nort h of this lak e i s the N-S-trending synclina l structur e o f Jurassic-Cretaceou s rocks (Fig . 5). In th e southwes t o f the region , th e mos t promi nent featur e i s th e NW-strikin g Ma e Pin g faul t zone (MPF Z i n Fig . 6), whic h i s als o calle d th e Wang Chao fault zone . This fault zon e is displaye d on th e Landsa t image s a s lon g NW-trendin g lineaments (Fig . 5). Nort h o f th e faul t ther e i s onl y one small , NNW-trending , half-grabe n basin , th e Mae Tuen basin. In contrast, larger basins occur on the sout h sid e o f th e faul t zone . Th e mos t promi nent larg e basi n i s th e Mae Sot-Mae Ramat basi n (Fig. 5) . Thi s basi n show s a N-S-trending , half graben structur e wit h th e mai n W-dippin g borde r fault o n the east margin of the basin (Stokes 1988) . In the southeast of the region, the largest Cenozoi c basin i s th e N-S-trendin g Phitsanulo k basi n (Fig . 6). Nort h o f the Phitsanulo k basi n i s th e Uttaradi t sinistral strike-sli p faul t zon e (UDF Z in Fig. 6).
Relations between major strike-slip fault s and sedimentary basins Examination of the Landsat TM images of northern Thailand (Fig . 5) has allowe d identificatio n of 86 0 major lineaments . W e identifie d thes e lineament s based o n thei r morphologica l features , th e corre lation wit h tectoni c elements , an d wit h geologica l maps. Thes e lineament s includ e majo r an d mino r faults. Th e sens e o f movemen t o f thes e fault s i s inferred fro m existin g data , geologica l maps , an d geomorphological features . Th e majo r fault s ar e illustrated in Figure 6. We measured the orientation (degree azimut h of strikes) of these lineaments and present th e dat a in rose diagram s (Fig. 7). Six set s of faul t an d lineamen t patterns ca n be observe d i n descending orde r o f importance ; NNW-trending ,
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Fig. 5 . Landsa t T M image mosai c o f northern Thailand . Th e mosaic include s si x Landsat image s an d covers a n are a of 136,00 0 squar e kilometres .
N-trending, NE-trending , ENE-trending , NNE trending, an d NW-trendin g (Fig . 7e) . Mos t o f th e NNW-striking fault s appea r i n southwester n are a of northern Thailand (Fig . 7c ) while the N-striking faults ar e in northwester n are a (Fig . 7a) . Th e NE trending faults are common i n the southeastern are a
(Fig. 7d ) whil e mos t o f th e ENE-trendin g fault s occurred i n th e northeaster n are a (Fig . 7b) . T o understand th e relation s betwee n thes e fault s an d sedimentary basin s in northern Thailand, five areas of majo r fault s wer e selecte d fo r detaile d analysi s (Fig. 8) .
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Fig. 6 . Present-da y structura l relationship betwee n strike-sli p faults , norma l faults , an d th e Cenozoi c basins . SWF : Salween fault , NMF : Na m M a fault , MCF : Ma e Cha n fault , CKF : Chian g Khong fault , WHF : Wian g Haen g fault , PYF: Phaya o fault, NGF : Ngao fault, LIF : L i fault , MMF : Ma e Mo h fault , TNF : Thoe n fault , UDFZ : Uttaradi t faul t Zone, MPFZ : Ma e Ping faul t zone .
Area 1: Mae Chan fault (Fig. 9 ) 9)
. In this area several strike-slip faults hav e been observed. Man y o f thes e fault s tren d EN E an d The Mae Cha n faul t i s a sinistra l strike-sli p faul t s how left-latera l displacement . Th e Mekon g (Fenton e t al. 1997 ; Tulyati d e t al. 1999 ) locate d Rive r, whic h flow s fro m nort h t o south , ha s been in the northern part of northern Thailand (Figs 8 & O ffset b y the Nam Ma fault (Lacassi n e t al. 1998 )
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Fig. 7 . Ros e diagrams showing the orientatio n of faul t strike s in differen t part s o f northern Thailand (a , b , c , d) an d whole are a o f norther n Thailand (e) .
and the Mae Chan fault (Fig . 9) . These tw o fault s terminate a t th e norther n tip o f th e Ma e Sa i an d Fang basins , respectively . Nort h o f th e Na m M a fault, there is a sinistral strike-slip faul t which displays a releasin g offse t (Woodcoc k & Fische r 1986) associate d wit h developmen t o f a smal l pull-apart basi n (Woodcoc k & Fische r 1986 ;
Sylvester 1988) . Thi s faul t continue s t o th e southwest an d terminates near th e Salwee n River (Fig. 9) . Sout h o f th e Ma e Cha n fault , ther e i s another serie s o f strike-sli p faults , althoug h th e traces ar e relativel y small . Thes e fault s see m t o terminate a t the northern tip o f the Chian g Khong basin. Severa l N-trending , W-dipping e n echelo n
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Fig. 8 . Fiv e area s o f majo r fault s whic h hav e bee n selecte d fo r detaile d structura l analysis. These includ e Are a 1 : Mae Cha n fault , Are a 2 : Ma e Th a fault , Are a 3 : Ma e Pin g fault , Are a 4: Thoen-Phra e fault , an d Are a 5 : PhayaoNan area .
normal faults hav e been foun d a t the eastern mar- identified . To the wes t of the Fang basin, ther e i s gin o f thi s basi n (Fig . 9). I n th e Fan g basin , NE - a zone of NNW-trending lineaments. On e of these trending normal faults occur in the northwest mar- lineament s cut s throug h th e souther n par t o f th e gin of the basin. In the eastern margin, small seg- Fan g basin. Some small, N-trending normal faults mented, NNE-trendin g norma l fault s hav e bee n hav e been observe d withi n this zone .
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Fig. 9 . Th e Mae Chan faul t zon e an d the relationship between Tertiar y basin s an d strike-slip faults an d normal fault s in norther n Thailand.
Area 2: Mae Tha fault (Fig. 10)
Area 3: Mae Ping fault (Fig. 11)
The Mae Tha fault i s located t o the east of the Chiang Mai basin (Figs 8 &10). It s trend changes fro m NW i n th e nort h t o N E i n th e south . O n Landsa t images, the Mae Kuang River show s a right-lateral offset b y th e Ma e Th a fault . Th e Ma e Th a faul t continues t o th e northwes t an d i s probabl y dis sected b y th e E- W extensiona i faul t syste m of the Phrao basin . I n th e sout h i t splay s int o tw o seg ments, th e N- S an d NE-SW-trendin g faults . Th e Mae Kuang fault (Fig . 10 ) is NE-trending and continues to the northeast int o the Wiang Papao basin. In the norther n par t o f the Chian g Ma i basin, sev eral NW-trending , extensiona i fault s hav e bee n observed. Thes e fault s occu r as the bounding faults of th e Chian g Ma i basi n an d create d th e half graben structures in this area. These faults probabl y continue to the north and link with the extensiona i faults o f th e Chian g Da o basin . Thre e smal l N trending extensiona i basins , Chian g Dao , Phrao , and Wian g Papao , ar e locate d i n th e nort h an d a N-trending elongat e basin , th e Muan g Pa n basin , is locate d t o th e southeas t (Fig . 10) . Thes e basin s have boundin g norma l fault s an d displa y half graben structures .
The Ma e Pin g faul t zon e (Wan g Cha o faul t zone ) is a large , NW-SE-trending , strike-sli p faul t (Lacassin et al. 1997). Withi n this fault zone , there are several small , segmented fault s (Fig . 11) . In the northwest of the area, the NNW-trending Mae Sarieng faul t i s obliquel y linke d wit h th e Ma e Pin g fault t o th e south . I n th e centra l area , th e NNW trending extensiona i faul t a t the easter n margi n o f Mae Tua n basi n (Fig . 11 ) graduall y change s t o a strike-slip faul t linke d to the Mae Ping fault. I n the southeast area , th e Ma e Pin g faul t split s int o sev eral smaller strike-slip and dip-slip faults . Som e of these fault s curv e t o th e SS E an d transfor m into extensionai boundin g fault s o f th e Ma e Ramat Mae So t basin an d Mae Rama o basin (Fig . 11) . In th e Myanma r area , a larg e Cenozoi c basi n developed with NNW-trending, W-dipping, en echelon, extensio n fault s o n it s easter n margi n (Fig . 11). Severa l smal l NN W an d N-trendin g lin eaments develope d within an d aroun d thi s basin . The Mae i River , whic h i s locate d withi n th e faul t zone, flows from th e Mae Sot basin to the Salween River i n the northwest . Northeas t o f the Ma e Pin g fault zone , severa l smal l Cenozoi c basin s occur .
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Fig. 10 . Th e Mae Tha fault , Chian g Mai basin, and four smal l extensional basin s (Chiang Dao, Phrao, Wian g Papao , and Muan g Pan) . Thes e smal l basin s ar e oriente d N- S an d som e basin s displa y characteristic s o f half-grabens. Th e NE-trending Ma e Kuan g fault i s a conjugat e fault o f th e Ma e Th a fault .
These are the Li, Thoen , an d Doi Tao basins (Fig . 11). Thes e basin s hav e small , segmented , exten sional boundin g faults . Som e o f thes e fault s developed e n echelo n patterns .
Area 4: Thoen-Phrae fault (Fig. 12) This are a i s i n th e mid-souther n par t o f norther n Thailand (Fig . 8) . The majo r strike-sli p faults, nor -
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Fig. 11. Th e Ma e Pin g (Wan g Chao ) faul t zone , Tertiar y basins , an d th e associate d structure s i n th e southwester n part o f norther n Thailand .
mal faults , an d lineament s mainly have a NE-SW trend. The main fault in this are a is the Thoen faul t (Fig. 12) . I t i s a NE-trendin g sinistra l strike-sli p fault (Chantarame e & Wongyai 1991 ; Fenton e t al 1997). It links the Phrae basin in the northeast with the Thoe n basi n i n th e southwes t (Fig . 12) . Quaternary basal t erupte d an d flowe d withi n thi s faul t zone near Den Cha i tow n (Fig. 12) . Severa l smal l faults an d lineament s ar e subparalle l t o thi s faul t and som e o f thes e hav e offse t th e Yo m River . Three smaller NE-trendin g faults occur north of the Thoen fault . Thes e ar e the Ron g Kwan g fault, th e Mae Moh fault, and the Ngao fault (Fig . 12) , which links th e Nga o basi n i n th e nort h (Fig . 6 ) an d th e Mae Mo h basi n i n th e south . The Phra e faul t for med a s a NE-trending , norma l faul t boundin g th e eastern margi n o f th e Phra e basi n (Fig . 12) . Eas t of th e Wan g River , mos t o f th e smal l fault s an d lineaments have NE or NNE trends, whereas to the west o f th e river , th e lineamen t patter n i s domi nantly NN W an d NN E (Fig . 12) .
Area 5: Phayao-Nan area (Fig. 13) Three pattern s o f fault s hav e bee n recognize d i n this area : (1 ) NE-trendin g strike-sli p faults , (2 )
NNW-trending strike-sli p fault s an d (3 ) N-S trending normal faults. Tw o NE-trending fault s ar e prominent in the middle area. The first fault occur s north o f th e Pon g basi n (Fig . 13) . Thi s faul t con tinues southwest , passes th e western margin o f the Pong basin , an d continues to th e southeas t margin of the Ngao basin an d extends t o the northern par t of th e Ma e Mo h basi n (Fig . 12) . The secon d faul t is a NE-trending sinistra l strike-sli p fault that link s the Ba n Luan g basi n an d th e Ma e Tee p basi n i n the sout h (Fig . 13) . Two large NW-trending faults occur. The larger one occurs in the west margin of the Phayao basin (Fig. 13) . I t continue s to th e southeas t an d forms the main bounding normal fault of the Ngao basin. This faul t terminate s befor e reachin g th e Yo m River (Fig . 13) . Th e secon d faul t occur s nea r th e southeast corne r o f the Pon g basi n an d continue s to th e southeast , to the nort h o f the Na n basin . In the centra l an d easter n par t o f th e area , mos t o f the N-S-trendin g norma l fault s mainl y occu r a s W-dipping boundin g fault s a t th e easter n margi n of th e Cenozoi c Pua , Ba n Luang , Chian g Muan , Phayao, an d Ma e Tee p basin s (Fig . 13) . I n th e western part , thes e norma l fault s develope d a s NNW-trending boundin g fault s o f th e Win g
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Fig. 12. Th e relationship between strike-slip faults, norma l faults, an d Tertiary basins in an area between the Lampang and Phra e basins, northern Thailand.
Papao, Wan g Nua , Ngao, an d Maun g Pa n basin s (Fig. 13) .
Interpretation of strike-slip faulting and the opening of sedimentary basins Forty-two Cenozoi c basin s hav e bee n interprete d and delineated from th e Landsat T M image mosaic (Fig. 5 ) and most of these ar e related t o major and minor strike-sli p faults an d normal fault s (Fig . 6) . From th e stud y of structura l relationships betwee n major fault s an d sedimentar y basin s i n th e are a 1 to 5 (Figs 9 to 13) , th e Tertiary basin s i n northern Thailand ca n be interpreted an d classified into five basin type s (Uttamo 2000). These are : (I ) fault-tip basins, (II ) pull-apart basins, (III) fault-wedge basins, (IV ) fault-zon e basins , an d (V ) extensiona l basins. Figure 1 4 shows a proposed tectoni c mode l of the opening o f Tertiary sedimentar y basin s asso ciated wit h the movemen t alon g strike-sli p faults .
Type I: Fault-tip basin The fault-ti p basins (als o calle d faul t ben d basins) are develope d a t th e ti p o f large-scal e strike-sli p
faults (Elder s e t al. 2000). A good exampl e o f this type i s th e Fan g basi n (Fig s 9 & 14) . Th e Fan g basin occurre d a t the ti p o f the sinistra l strike-sli p Mae Cha n fault. The geometr y an d relationship of the Mae Chan fault an d the Fang basin is comparable t o th e relationshi p betwee n th e Priestle y faul t and the Terror Rif t i n northern Victoria Land , Antarctica (Stort i e t al. 2001). Th e Priestley faul t i s a major Cenozoic right-lateral strike-slip faul t system in th e Ros s Se a region , Antarctic a (Salvin i e t al . 1997). Thi s faul t syste m include s a principa l dis placement zone , wher e mos t horizonta l displace ment i s accounte d for , an d a transtensiona l spla y zone i n th e souther n sid e consistin g o f a majo r basin a t th e faul t tip , th e Terro r Rif t (Stort i e t al . 2001). I n thi s system , th e strike-sli p displacemen t is transferre d b y faul t splayin g fro m th e principa l displacement zon e t o th e basin-boundar y faults in the transtensiona l spla y zon e (Stort i e t al . 2001) . In norther n Thailand , th e Ma e Cha n faul t ca n b e interpreted a s a principal displacemen t zon e whil e the normal faults a t the southwest tip of Mae Chan fault ma y b e th e basin-boundar y faults o f a transtensional spla y zon e of the strike-sli p syste m (Fig . 9). Therefore the Fang basin is a fault-tip basin that
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Fig. 13. Th e relationshi p betwee n strike-sli p faults , norma l faults , an d Tertiar y basin s i n Phayao-Na n area , north ern Thailand .
developed within a transtensional spla y zone of the sinistral strike-sli p Ma e Cha n faul t syste m (Fig . 14). Thi s interpretatio n i s simila r t o th e simpl e shear model o f Sylvester (1988) . H e stated that the splay fault s for m a t the tip s o f R shear s an d curve toward parallelis m wit h the extensio n fracture s s o that a n R shea r wil l b e a strike-sli p faul t i n th e central par t of the deformatio n zone, but it will be a norma l faul t a t its extremitie s (Sylveste r 1988) . Eight Tertiar y basin s hav e bee n identifie d a s fault-tip basin s i n northern Thailand. Th e Li basin is interprete d a s a fault-ti p basi n a t th e ti p o f th e dextral strike-sli p L i fault (Fig . 14) . The Phrae and Thoen basin s ar e interprete d a s fault-ti p basin s a t the opposite tips of the Thoen fault (Fig s 1 2 & 14) . The Chian g Ra i an d Chian g Khon g basin s i n th e northern par t o f norther n Thailan d ma y b e inter preted a s fault-ti p basin s a t th e tip s o f sinistra l strike-slip fault s i n this are a (Fig s 9 & 14) .
Type II: Pull-apart basin Pull-apart basin s ar e a n integra l par t o f intraplat e and interplat e strike-sli p faul t zone s (Sylveste r 1988). Bend s o r sidestep s i n th e mai n strike-sli p fault syste m produc e eithe r zone s o f extensio n (pull-apart basins ) a t releasin g bend s o r sidestep s or, alternatively, region s o f compression (uplift s o r pop-up structures ) a t restrainin g bend s o r
restraining sidestep s (Doole y & McClay 1997) . I n northern Thailand , seve n basin s hav e bee n inter preted a s pull-apart basin s (Fig . 14) . Th e sinistra l movement o f th e Nga o and Ma e Mo h fault s (Fig s 12 & 14 ) created th e extensio n pull-apar t zon e a t releasing sidestep s o f thes e faults , an d produce d the rhombic-shaped , pull-apar t Ma e Mo h basin . The Ma e Sa i basi n (Fig s 9 & 14 ) was develope d as a pull-apar t basi n betwee n th e sinistra l move ment of the Nam M a (NMF ) an d Mae Cha n fault s (MCF). The other three smal l pull-apart basins are located in the north and west of the study area (Fig. 14). Th e Myanma r 1 basi n wa s develope d a s a small grabe n betwee n tw o NNE-trendin g sinistral strike-slip faults . Th e Myanma r 2 basi n i s a pullapart basin associate d wit h the sinistra l movemen t of th e Salwee n faul t (SWF ) wherea s th e Khu n Yuam basi n wa s develope d b y th e dextra l move ment o f th e Ma e Hon g So n (MHF)-Ma e Sarian g faults (MSF) . These pull-apart basins appear on the Landsat imager y a s rhombic-shape d depression s (Figs 5 & 6). The Ma e So t basin i s interprete d a s a pull-apart offse t whic h was generated b y dextra l movement of the Mae Ping fault zon e (MPFZ) and Three Pagoda s faul t zon e to the sout h of the stud y area (Fig s 4 & 14) . Th e structura l relationshi p between th e Mae Ping faul t zon e an d the Mae Sot basin i n norther n Thailan d (Fig . 6 ) i s comparabl e to th e relationshi p o f th e Dungu n strike-sli p faul t
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Fig. 14 . A proposed tectoni c mode l o f Tertiary basi n formatio n in northern Thailand. I n this mode l the movement of major strike-sli p fault s an d the regional E- W extension initiate d the opening o f five basin types: fault-ti p basin , pull apart basin , fault-wedg e basin , fault-zon e basin , an d extensiona l basin . SWF : Salwee n fault , NMF : Na m M a fault , MCF: Ma e Cha n fault , CKF : Chian g Khon g fault , WHF : Wian g Haen g fault , PYF : Phaya o fault , NGF : Nga o fault , LIF: L i fault , MMF : Ma e Mo h fault , TNF : Thoe n fault , UDFZ : Uttaradi t fault zone , MPFZ : Ma e Pin g faul t zone .
zone an d Dungun pull-apart basin i n offshore Pen insular Malaysi a (Doole y & McClay 1997) .
Type III: Fault-wedge basin The Nga o basi n (Fig . 13 ) is interprete d a s a fault wedge basin (Fig. 14 ) since it is located a t a wedge
zone betwee n tw o strike-sli p faults , th e Phaya o (PYF) an d Nga o (NGF ) faults . Whe n thes e tw o faults moved , th e are a t o th e wes t o f thes e fault s was extruded , o r escape d t o wes t an d create d a n extensional basi n a t the wedge zone between thes e two faults . Th e Phitsanulo k basi n (Fig s 6 & 14 ) may hav e forme d a s a fault-wedg e basin becaus e
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it i s locate d a t th e extensiona l zon e o f th e wedg e area betwee n th e Uttaradi t faul t zon e (UDFZ ) an d the Ma e Pin g faul t zon e (MPFZ) . Th e nam e o f fault-wedge basi n was initially define d b y Crowel l (1974) whe n he studie d Cenozoi c basin s i n southern California. I n northern China, Vincent & Allen (1999) recognize d thi s basi n type . The y suggeste d that th e Mesozoi c Bayanhaot e an d Wuwe i basin s occurred a s th e extrusio n fault-wedg e basins an d association wit h dextra l motio n alon g th e basin bounding Chaha n fault s (Vincen t & Allen 1999) .
Type IV: Fault-zone basin Ten small basins in northern Thailand occu r within large faul t zone s an d they ar e interprete d a s fault zone typ e basin s (Fig . 14) . Som e smal l basin s within the Mae Chaem fault zon e (MACF) contain coal-bearing strata . Som e o f thes e fault s probabl y developed a s Riedel shea r fault s a t th e sam e tim e as th e movemen t o f large-scal e strike-sli p faults . The movemen t of these shea r faults create d exten sion zone s an d graben s tha t develope d int o sedi mentary basins. Within the Mae Chaem fault zone , the surfac e elevatio n o f the present-day basin floor is abou t 80 0 m abov e se a level , thi s bein g abou t 500 m higher than the other basin floors to the east. The uplift o f the metamorphic core complex during the Lat e Miocen e (Upto n e t al. 1997 ) cause d th e exhumation an d erosion o f the Tertiary strat a in the Mae Chae m faul t zon e basins .
Type V: Extensional basin From the stud y o f structura l feature s o f the Phra o (Fig. 10 ) an d Chian g Mua n basin s (Fig . 13) , Uttamo (2000) foun d tha t these basins have a halfgraben structure , with one main N-S-trending normal faul t a t eithe r th e wester n o r easter n margin . The subsurface structur e of the Chiang Muan basin confirms a half-grabe n geometr y (Uttam o 2000) . This N-S-trendin g half-grabe n is interprete d a s an extensional basin in this study. Sixteen basins have been identifie d a s extensiona l basin s (Fig . 14) . Many o f thes e ar e locate d i n th e easter n par t o f northern Thailand . I n thi s area , th e smal l an d medium-sized extensiona l basin s hav e a half graben form, wit h the main border extension fault s on thei r easter n margi n (Fig . 14) . Thi s contrast s with the half-graben s i n the wester n par t i n whic h the mai n borde r fault s ar e o n th e wester n margi n (Fig. 14) . The norther n section s o f th e Chian g Ma i basi n and Phra o basi n (Fig s 6 & 10) , whic h hav e bee n interpreted a s extensional basins (Fig. 14) , are also influenced o r controlle d b y sinistra l strike-slip . These basins , whe n observe d wit h th e Fan g basi n to the north, for m a N-S-trending, 'S'-shape d belt
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(Fig. 6 ) tha t i s consisten t wit h the sinistra l move ment o f the Mae Chan fault (MCF) . This evidenc e may impl y tha t th e E-W extensio n wa s relate d t o sinistral strike-sli p tectonics . I n th e easter n par t of northern Thailand , mos t o f th e extensiona l basin s developed withi n th e Chian g Rai-Phra e fol d bel t (Uttamo 2000 ) an d ther e ar e numerou s N- S an d NNE-trending folds . Mos t o f thes e fold s ar e i n Upper Palaeozoic t o Cretaceous strat a (Fig. 4). The orientation o f th e extensiona l basin s i n thi s fol d belt conform s t o th e tren d o f folds . Som e basin s are well developed within the synclinal structur e of these fold s (Fig s 4 & 5).
Discussion Northern Thailan d i s located in a structurally com plex are a betwee n thre e majo r tectoni c regimes . These three regimes are a region of extensional tectonics t o th e south , th e Centra l Plai n an d Gul f o f Thailand, an d tw o majo r strike-sli p zones , th e Sagaing faul t zon e t o th e wes t an d th e Re d Rive r fault zon e to the northeast (Fig. 4). During the Cretaceous, th e wester n Burm a micro-plat e (Fig . 2 ) was subducte d underneat h th e wester n margi n o f Southeast Asia . Thi s subductio n create d th e Cre taceous granite belt (Fig . 4 ) along the western border of Thailand an d eastern Myanmar (Charusiri et al. 1993) . Th e collisio n betwee n th e wester n Burma micro-plat e an d th e Southeas t Asi a main land occurred durin g the Late Cretaceous (Mitchel l 1981). Thi s collisio n produce d a n E- W com pression in northern Thailand. This stress field may have initiated th e lateral movemen t of the NW and NE-trending fractur e zone s an d sutur e zones . Therefore, thes e tw o wea k zone s becam e sinistra l strike-slip fault s an d dextra l strike-sli p faults , respectively. Thi s compressiona l stres s probabl y produced th e foldin g o f th e Mesozoi c strat a tha t form the N-S fol d belt in the eastern par t of northern Thailan d (Fig . 4) . Th e E- W compressio n i n northern Thailan d durin g the Late Cretaceou s als o caused th e right-latera l movemen t o f the Uttaradit fault (Fig . 4 ) an d th e inversio n i n Khora t basi n (Sattayarak e t al 1989) . The collisio n o f th e wester n Burm a micro-plat e during the Late Cretaceous als o created large-scal e strike-slip fault s i n norther n Thailan d (Fig . 4) . These fault s ar e th e NW-trendin g Ma e Pin g fault , or Wan g Cha o fault , i n th e southwest , an d th e ENE-trending Ma e Cha n faul t i n th e north . Th e sinistral movemen t o f th e Ma e Pin g faul t resulte d in th e left-latera l offse t o f th e metamorphi c cor e complex bel t (Fig. 4). The dextral movemen t of the Mae Cha n faul t resulte d i n th e right-latera l offse t of this belt an d Triassic granite in the northern part of th e stud y are a (Fig . 4) . The collision betwee n the Indian plate and Eura-
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sia initiated in the Mid-Eocene (50-4 0 Ma) created the crustal thickening under the Tibet Plateau . The continued intrusion of the Indian plate into Eurasia caused th e ESE polyphase extrusio n o f Sundalan d and th e sinistra l movemen t alon g th e Thre e Pagodas, Ma e Ping , an d Re d Rive r faul t system s (Tapponier et al. 1986) . Durin g the Early Miocen e the Ma e Pin g faul t move d i n dextra l motion (Fig. 6). This changin g of movement was caused by the changing o f th e stres s patter n aroun d th e Hima layan syntaxe s (Hucho n e t al . 1994) . Thi s stres s field als o change d th e movemen t o f majo r strike slip faults in northern Thailand. The NNE, NE, and ENE-trending fault s hav e a sinistra l movemen t while th e N W an d NNW-trendin g fault s hav e a dextral movemen t (Fig s 6 & 14) . The movemen t of thes e fault s accompanyin g th e regiona l E- W extension durin g th e Lat e Oligocen e t o Earl y Miocene initiate d th e formatio n o f Tertiary basin s in norther n Thailan d (Fig s 6 & 14) . The Thre e Pagodas an d Ma e Pin g fault s ca n b e trace d int o Myanmar an d ar e juxtapose d t o th e large-scale , right-lateral Sagain g faul t (Fig . 4). Th e movement of th e Sagain g faul t durin g th e Mid-Miocen e caused the opening of the Andaman Sea rift system (Fig. 2 ) (Maung 1987; Metcalfe 1996) . The movement of this faul t probabl y cause d th e reactivation of th e norther n Thailan d strike-sli p faul t syste m during the Lat e Neogene . Interpretation o f geometr y an d structura l relationships betwee n majo r strike-sli p fault s an d sedimentary basin s i n norther n Thailan d indicate s that the initiation and development of the sedimentary basin s i n norther n Thailan d wa s cause d b y regional E- W extensio n an d movemen t alon g major strike-sli p faults . Thirty-si x major fault s ar e identified i n northern Thailan d (Fig . 6). Thes e ar e strike-slip an d norma l faults . Th e sens e o f move ment o f these faults i s inferre d from existin g data, geological maps , an d geomorphologica l feature s identified fro m Landsa t TM imagery. The NW and NNW-trending faults have a dextral movement and the NE and ENE faults have a sinistral movement . The N-S-trending fault s ar e locate d a t basi n margins an d sho w dip-slip o r oblique-sli p movement . Forty-two Cenozoi c basin s have been delineate d from Landsa t imager y (Fig . 6). Fiv e basi n type s (fault-tip basin, pull-apart basin, fault-wedge basin, fault-zone basin , an d extensional basin ) hav e bee n interpreted base d o n thei r geometry , orientation , and relation s wit h th e majo r fault s (Fig . 14). The fault-tip basin s occurre d a t th e ti p o f large-scal e strike-slip faults . Th e pull-apar t basin s develope d in the extension zon e of two en echelon strike-sli p faults. Th e fault-wedg e basin s ar e locate d a t a wedge zon e betwee n tw o strike-sli p faults . Th e movement o f thes e fault s caused extensio n i n th e wedge zone. The fault-zone basins occurred within
large faults . Thes e ar e smal l i n siz e an d oriente d parallel t o th e tren d o f th e mai n faults (Fig . 14). Many o f th e extensiona l basin s develope d i n th e central an d eastern part of the study area. They are both larg e an d smal l i n size , an d thei r geometr y shows a half-grabe n structure . N-S-trendin g nor mal fault s ar e th e mai n boundar y fault s o f thes e basins. Some importan t conclusion s ca n b e mad e fro m this study . Th e movemen t o n majo r strike-sli p faults i s consisten t wit h th e formatio n o f Tertiar y basins (Fig . 14). Those fault s oriente d N W an d NNW trendin g ar e associate d wit h dextra l strike slip an d basi n formation . Thos e fault s oriente d ENE an d N E ar e associate d wit h sinistra l strike slip and basin formation. Some NE-trending fault s cut throug h o r lin k th e Tertiar y basin s together . The N-S-trending faults sho w normal fault dip-sli p movement an d form th e mai n borde r fault s o f extensional basins . Som e fault s appea r a s lin k faults betwee n basins. However, from th e satellit e image w e canno t specif y th e actua l kinematic s o f strike-slip faults and basin-boundary faults but only infer the m fro m indirec t criteria . Thi s i s a limi tation o f this stud y in tha t we stil l d o not know if these basin s formed in pur e extension o r i n transtension, an d som e basin s probabl y ar e reactivated in transtensio n t o accommodat e strike-sli p dis placement fro m th e majo r strike-sli p faults . Th e detailed structura l analysi s o f thes e strike-sli p faults an d basin-boundar y fault s a t outcro p i n th e future migh t help u s t o validat e the propose d tectonic model of Tertiary basin formation in northern Thailand (Fig . 14). We woul d lik e t o than k th e Chian g Ma i Universit y an d the Ministr y o f Universit y Affairs , Roya l Tha i Govern ment, fo r providing th e financial support fo r this research and th e Southeas t Asi a Researc h Group , Universit y o f London, fo r providin g th e Landsa t T M digita l data . Th e first autho r i s ver y gratefu l t o R . Hal l fo r allowin g hi m to work withi n the Research Group, an d for his generous support an d discussion throughou t the project. W e would like to thank our colleagues at the Department o f Geological Sciences, Chiang Mai University, for their help during the fieldwor k i n norther n Thailan d an d fo r thei r suppor t and discussion ,
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STRIKE-SLIP FAULTIN G I N THAILAND seismicity i n norther n Thailand an d its tectonic impli cations. In : Proceedings o f th e International Conference on Stratigraphy and Tectonic Evolution of Southeast Asia an d th e South Pacific. Departmen t o f Mineral Resources , Bangkok , 453-464. CHANTARAMEE, S . & WONGYAI , A . 1991 . Structura l geology o f Ba n Th a Makwen , Ampho r Thoen , Lampang. Journal o f Thai Geosciences, 2, 75-82 . CHAODUMRONG, P. , UK-KAKIMAPAN , Y. , SNANSIENG , S., JANMAHA, S. , PRADIDTAN , S . & SA E LEOW, N . 1983 . A revie w o f th e Tertiar y sedimentar y rock s o f Thai land. In : NUTALAYA , P. (ed. ) Proceedings of a Workshop on Stratigraphic Correlation of Thailand and Malaysia. Geologica l Societ y o f Thailand , Bangkok/Geological Society of Malaysia, Kuala Lumpur, 159-187 . CHARUSIRI, P. , CLARKE , A. H. , FARRAR , E., ARCHIBALD , D. & CHARUSIRI , B . 1993 . Granite belt s i n Thailand : evidence from the Ar/Ar geochronological and geological syntheses . Journal o f Southeast Asian Earth Sciences, 8 , 127-136 . CROWELL, J . C . 1974 . Origin o f lat e Cenozoi c basin s in southern California. In: Dickinson, W . R. (ed.) Tectonics and Sedimentation. Society of Economic Paleontol ogists an d Mineralogist s Specia l Publication , 22 , 190-204. CURRAY, J . R. , MOORE . D . G. , LAWYER , L . A. , EMMEL , F. J. & RAITT, R. W. 1979 . Tectonics o f the Andaman Sea an d Burma . In: WATKINS , J. , MONTADART , L. & DICKENSON, P . W . (eds ) Geological and Geophysical Investigations o f Continental Margins. America n Association o f Petroleu m Geologists , Memoir , 29 , 189-198. DOOLEY, T . & McCLAY , K . 1997 . Analog modelin g o f pull-apart basins . American Association o f Petroleum Geologists Bulletin, 81(11), 1804-1826. ELDERS, C. , UTTAMO , W. , NICHOLS , G. , CHANTRAPRASERT, S. , SRISUWON , P. & AL-BARWANI, B. 2000. Tertiary strike-sli p basi n formatio n i n a n extrude d conti nental wedge , norther n Thailand . In : Th e Symposium on the Intracontinental Effects of the Indo-Eurasian Collision. Th e Geoscienc e 200 0 Meeting , Manches ter, UK . FENTON, C. H., CHARUSIRI, P., HINTHONG , C., LUMJUAN, A. & MANGKONKARN , B. 1997 . Late Quaternary fault ing i n norther n Thailand. In: Proceedings of th e International Conference on Stratigraphy and Tectonic Evolution of Southeast Asia and South Pacific. Depart ment o f Mineral Resources, Bangkok , Thailand , 436 464. HAHN, L . H. , KOCH , K . E. , WITTEKINDT , H. , ADEL HARDT, W . & HESS , D . 1986 . Outline o f th e geolog y and th e minera l potentia l o f Thailand . Geologisches Jahrbuch Reihe B, 59 , 3-49. HALL, R . 1996 . Reconstructing Cenozoi c S E Asia . In : HALL, R . & BLUNDELL, D. J. (eds) Tectonic Evolution of Southeast Asia. Geological Society , London, Special Publication, 106 , 153-184. HUCHON, P., LE PICHON, X. & RANGIN, C. 1994 . Indochina Peninsul a an d th e collisio n o f Indi a an d Eurasia . Geology, 22 , 27-30 . KOSUWAN, S. , CHARUSIRI , P. , TAKASHIMA , I. & LUMCHUAN, A . 1998 . Active tectonic s o f th e Ma e Cha n fault, Northern Thailand. In: Programme and Abstracts
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of the Regional Congress on Geology, Mineral and Energy Resources o f Southeast Asia. Geologica l Society o f Malaysia, 234-235 . LACASSIN, R. , MALUSKI , P . e t al 1997 . Tertiary dia chronic extrusio n an d deformatio n o f wester n Indoch ina: Structural and 40 Ar/39Ar evidence fro m N W Thailand. Journal o f Geophysical Research, 102 , 10013 — 10037. LACASSIN, R. , REPLUMAZ , A . & LELOUP , P . H . 1998 . Hairpin rive r loop s an d slip-sens e inversio n o n Southeast Asia n strike-sli p faults. Geology, 26 , 703-706. LOVATT SMITH , P . P. , STOKES , R . B. , BRISTOW , C . & CARTER, A . 1996 . Mid-Cretaceou s inversio n i n th e North Khora t Platea u o f La o PD R an d Thailand . In : HALL, R. & BLUNDELL, D . J. (eds) Tectonic Evolution of Southeast Asia. Geological Society , London, Specia l Publication, 106 , 233-246. MAUNG, H . 1987 . Transcurren t movement s i n th e Burma - Andama n Sea region. Geology, 15, 911-912. METCALFE, I . 1996 . Pre-Cretaceou s evolutio n o f S E Asian terranes. In: HALL , R. & BLUNDELL , D . J . (eds) Tectonic Evolution o f Southeast Asia. Geologica l Society, London , Specia l Publication , 106 , 97-122. MICHELL, A . H . G . 1981 . Phanerozoic plat e boundarie s in mainland S.E. Asia, the Himalaya and Tibet. Journal of th e Geological Society, London, 138, 109-122. PACKHAM, G. 1996. Cenozoic S E Asia: reconstructing, its aggregation an d reorganization. In: HALL, R . & BLUNDELL, D. J. (eds) Tectonic Evolution of Southeast Asia. Geological Society , London , Special Publication , 106, 123-152. PRADIDTAN, S. 1989. Characteristic an d controls of lacustrine deposit s o f som e Tertiar y basin s i n Thailand. In: THANASUTHIPITAK, T . & OUNCHANUM , P. (eds ) Proceedings of the International Symposium on Intermontane Basins: Geology an d Resources. Departmen t o f Geological Sciences , Chiang Mai University, Thailand, 133-145. SALVINI, F. , BRANCOLINI , G. , BUSETTI , M. , STORTI , F. , MAZZARINI, F . & COREN , F . 1997 . Cenozoic geodynamics of the Ross Sea region, Antarctica: crustal extension, intraplate strike-slip faulting , and tectonic inherit ance. Journal o f Geophysical Research, 102 , 2466924696. SATTAYARAK, N. , SRILUWONG , S . & PUM-IM , S . 1989. Petroleum potentia l o f th e Triassi c pre-Khora t inter montane basin in Northeastern Thailand . In : THANASUTHIPITAK, T . & OUNCHANUM , P. (eds ) Proceedingsof the International Symposium on Intermontane Basin: Geology & Resources. Departmen t o f Geologica l Sciences, Chian g Ma i University, Thailand, 43-58. STOKES, R . B . 1988 . Structural control o f Neogene sedi mentation i n th e Ma e So t Basi n (Thai-Burmes e border): implication s fo r oil-shal e reserves . Journal of Petroleum Geology, 11, 341-346. STORTI, F. , ROSSETTI , F . & SALVINI , F. 2001 . Structural architecture an d displacemen t accommodatio n mech anisms at the termination of the Priesley Fault, northern Victoria Land , Antarctica . Tectonophysics, 341 , 141161. SYLVESTER, A . G . 1988 . Strike-slip faults . Geological Society o f America Bulletin, 100, 1666-1703. TAPPONNIER, P. , LACASSIN , R . e t al . 1990 . Th e Aila o Shan-Red River metamorphic belt: Tertiary left-lateral
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Cenozoic strike-sli p faultin g fro m th e easter n margin o f the Wilke s Subglacia l Basi n t o the western margi n o f the Ros s Se a Rift : an aeromagnetic connectio n FAUSTO FERRACCIOLI 1'2 & EMANUELE BOZZO 1 l
Dipartimento per lo Studio del Territorio e delle sue Risorse, Univ. Genova, Viale Benedetto XV, 5 16132 Genova, Italy (e-mail:
[email protected]) ^British Antartic Survey, High Cross, Madingley Road, Cambridge CB3 OET, UK Abstract: Tectoni c modellin g o f regiona l aeromagneti c anomal y pattern s suggest s Cenozoi c right-lateral strike-sli p faultin g alon g a n inherite d faul t syste m of th e Transantarcti c Mountains and adjacent hinterland. We name it here the Prince Alber t Fault System . The Reeves Faul t and David Faul t ar e Cenozoi c right-latera l strike-sli p fault s an d for m par t o f th e NW-SE-strikin g segment o f this complex faul t system , extending to the easter n margi n of the Wilkes Subglacia l Basin. Our aeromagnetic interpretatio n suggest s therefore that the Wilkes Subglacia l Basi n may be connecte d t o the Cenozoi c strike-sli p kinematic framewor k o f the Transantarctic Mountain s and western Ross Sea Rift. Th e southernmost segment of the Prince Albert Fault System parallels the N-S-strikin g McMurd o Soun d Faul t Zon e and , togethe r wit h it , define s a transtensiona l western Ros s Se a Rif t margin . High-resolutio n aeromagneti c image s defin e th e Cap e Robert s pull-apart basin and suggest that Cenozoic magmatis m may have focused along the transtensional western Ros s Se a Rif t margi n itself .
The Transantarcti c Mountain s ar e th e highes t an d longest non-contractiona l mountai n bel t i n th e world (te n Brin k e t al 1997) . Th e rang e reache s elevations ove r 4000 m and extends fo r ove r 350 0 km acros s Antarctic a (inse t i n Fig . 1) . The rifted nature of the adjacen t continental crus t of the Ros s Sea Rift , par t o f th e Wes t Antarcti c Rif t System , is wel l documente d fro m seismic , gravity , an d magnetic evidenc e (Coope r e t al . 1987 ; Treh u e t al 1993 ; Brancolin i e t a l 1997 ; Behrend t 1999 ; Trey e t al 1999) . Crustal architecture of the Transantarctic Mountains is less well constrained but has been investigate d wit h gravit y (e.g . Reitmay r 1997), aeromagneti c (e.g . Ferracciol i & Bozz o 1999), an d large offse t seismi c data (O'Connel l & Stepp 1993 ; Delia Vedova et al 1997) . The Wilke s Subglacial Basi n (Drewr y 1976 ) lie s i n the remot e and entirely ice covered hinterlan d of the Transantarctic Mountains (inset in Fig. 1) . Hence, its crustal structure an d tectoni c origi n i s presentl y ver y poorly constrained . Early structura l model s envisage d th e Trans antarctic Mountains as the upper plate o f an asymmetric extensiona l oroge n (Fitzgeral d e t a l 1986 ) or a s a flexura l uplifte d footwal l o f a half-graben -
type extensiona l basi n locate d i n th e Ros s Se a (Stern & te n Brin k 1989) . Th e Wilke s Subglacia l Basin ha s been modelle d either as a component o f the flexura l tria d includin g th e Ros s Se a Rift , th e Transantarctic Mountains, an d the basin itsel f (te n Brink & Stern 1992 ; te n Brink et al 1997) , o r previously a s a rif t basi n withi n Eas t Antarctic a (Drewry 1976 ; Stee d 1983) . Extensive geophysica l investigations are lacking to validate either o f these models. New gravity an d magnetic model s along a single travers e appea r t o favou r broa d crustal extension (Ferracciol i e t a l 2001 ) rathe r tha n lithospheric flexur e acros s th e Wilke s Subglacia l Basin (Ster n & ten Brin k 1989 ; te n Brin k & Stern 1992; te n Brink e t al 1997) . The Cenozoi c tectoni c settin g o f th e Ros s Se a Rift an d Transantarcti c Mountain s ma y b e mor e complex tha t previousl y modelled . Th e regiona l tectonic framewor k ma y relat e t o reactivatio n o f Palaeozoic-age faults a s majo r right-latera l strike slip fault s (Salvin i e t a l 1997 a, b\ Salvin i e t a l 1998; Ferracciol i & Bozzo 1999 ; Salvin i & Stort i 1999; Ferracciol i e t al. 2000). Cenozoi c strike-sli p faults ma y cu t acros s bot h th e thicke r continenta l lithosphere o f the Transantarctic Mountain s and the
From: STORTI , F. , HOLDSWORTH , R . E . & SALVINI , F . (eds ) Intraplate Strike-Slip Deformation Belts. Geologica l Society, London , Specia l Publications , 210 , 109-133 , 0305-8719/037 $ 15 © Th e Geologica l Societ y o f Londo n 2003 .
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Fig. 1 . Aeromagneti c stud y are a ove r th e Transantarcti c Mountains , easter n margi n o f th e Wilke s Subglacia l Basi n and Ross Sea. Bold dashed line indicates the western edge of aeromagnetic surve y coverage. Th e inset depicts relevant continental-scale Antarcti c tectoni c structures .
thinner Ross Sea Rift lithospher e an d link to southwestern Pacifi c Ocea n transfor m faults (Salvin i e t al 1991 a, b ) (Fig . 2) . Cenozoi c intraplat e strike slip faultin g ma y hav e induce d reactivation o f th e Mesozoic Ros s Se a Rif t b y narro w mod e riftin g localized i n th e wester n Ros s Se a an d promote d renewed Transantarcti c Mountain s uplif t (Wilso n 1995; Salvin i et al 1991'a, b; ten Brink et al 1997) . Recent geologica l fiel d investigation s hav e delin eated intens e brittl e deformation s alon g th e south ern terminatio n o f the Priestle y Faul t (Stort i e t a l 2001), i n th e are a betwee n Reeve s Glacie r an d Mawson Glacie r (Rossett i e t a l 2000 ) an d alon g the Lanterman Fault (Cappon i et al 1999 ; Rossett i et a l 2002 ) (Fig s 1 & 2) . Th e observe d defor mation pattern s ar e kinematically compatibl e wit h
major right-latera l strike-sli p belt s of Cenozoic age over thi s par t o f th e Transantarcti c Mountain s (Salvini e t al I991a, b) . We interpre t enhance d aeromagneti c image s t o provide a new window over some Cenozoic strike slip fault s o f th e Transantarcti c Mountains-Ros s Sea Rif t region . I n particula r w e focu s upo n a mostly ic e covere d right-latera l strike-sli p faul t system of the Transantarcti c Mountains and adjac ent hinterland . W e name it the Prince Alber t Faul t System. This faul t syste m i s composed o f discret e fault zones , som e o f whic h matc h recentl y pro posed strike-sli p fault s (Salvin i & Stort i 1999) . This faul t syste m ha s significan t implication s fo r the Transantarcti c Mountain s an d mos t importan t for th e easter n margi n o f th e Wilke s Subglacia l
STRIKE-SLIP FAULTIN G I N TH E WILKES BASI N
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Fig. 2 . Tectoni c sketc h ma p over majo r Cenozoi c right-latera l strike-sli p faults o f the Transantarcti c Mountain s an d adjacent Ros s Se a Rift , modifie d fro m Salvin i e t al . (\991a). Dashe d lin e indicate s aeromagneti c stud y area . Majo r inherited thrus t faults fro m Flottman n & Kleinschmidt (1991) . The newly proposed Princ e Albert Fault System , along the eastern margi n of the Wilkes Subglacia l Basin , is show n with a grey shade . Note it s co-linearit y wit h the Reeve s Fault an d th e Davi d Faul t fro m Salvin i an d Stort i (1999 ) (se e tex t fo r explanation) . Als o not e th e locatio n o f Cap e Roberts alon g th e wester n margi n o f th e Ros s Se a Rift . Th e inse t show s a satellite-derive d free-ai r gravit y anomal y map modified from Sandwel l & Smith (1997). It highlights co-linearity between oceanic transform faults and continental fault s o f th e Transantarcti c Mountains . Abbreviations : SCB : Souther n Cros s Block , DFR : Dee p Freez e Rang e block, PAB : Princ e Alber t Bloc k (Ferracciol i & Bozzo 1999) ; LR : Lanterman Range .
Basin. W e als o focu s o n Cap e Roberts, along the western margin of the Ross Sea Rift (Fig s 1 & 2). At this location, drilling information, multi-channel seismic reflection , an d structura l results (Wilso n 1995; Cape Roberts Science Team 2000; Hamilton et al . 2001 ) ar e combine d wit h aeromagneti c images. This leads to the suggestion that Cenozoic transtension ma y contro l Cenozoi c magm a emplacement along the western margin of the Ross Sea Rift . Thi s suggestio n is als o consisten t with
observations furthe r nort h ove r th e Transantarctic Mountains (Salvin i & Stort i 1999 ; Rossett i e t a l 2000).
Structural framewor k Northern Victori a Lan d include s th e Robertso n Bay Terrane , th e Bowers Terrane, an d the Wilson Terrane (Bradshaw 1989) (Fig. 2). During the Ross Orogen metamorphi c rocks o f the Wilson Terrane
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were intrude d b y Cambro-Ordovicia n magmati c arc rocks, th e Granit e Harbou r Intrusive s (Ghezz o et al 1989) . The Ross Orogen relates to subduction of the palaeo-Pacific plate beneath the East Antarc tic Crato n (Kleinschmid t & Tessensoh n 1987 ; Ricci e t a l 1997) , a s indicate d als o b y aeromag netic signature s (Fin n e t al 19990 ; Ferracciol i e t al 2002) . NW-SE-strikin g fault s (Fig . 2 ) developed durin g th e Ros s Oroge n an d includ e major thrus t faults suc h as the Leap Year Fault, the Lanterman Fault, the Wilson Thrust, and the Exiles Thrust (Flottman n & Kleinschmid t 1991) . I n con trast, Musumec i & Pertusat i (2000 ) argu e tha t i n the Deep Freeze Range-Eisenhower Range (Fig. 1 ) transpressional an d transtensiona l dextra l strike slip shea r zone s forme d durin g th e Ros s Orogen , coeval wit h th e emplacemen t o f magmati c intrusions. These divers e tectoni c model s hav e considere d the structura l framework of the region mostl y a s a fossil earl y Palaeozoi c picture , i.e . the y hav e neglected significan t reactivatio n o f inherite d struc tures by Cenozoic strike-sli p faultin g (Salvin i et a l 1997(2; Ferraccioli & Bozzo 1999) . Deformation of Triassic Beaco n an d Jurassi c Ferra r rock s i n th e Lanterman Rang e (Grindle y & Olive r 1983) , requires post-Jurassi c reactivatio n o f th e inherite d Lanterman faul t zon e (Rolan d & Tessensoh n 1987). I n th e Lanterma n Rang e Cambro Ordovician Granit e Harbou r Intrusive s ar e thrus t onto Ferra r rock s (L R i n Fig . 2) . Thes e feature s may b e explaine d b y strike-slip-induce d trans pression alon g th e Lanterman Faul t (Rossett i e t a l 2002). Timing o f reactivation i s hard to determin e because o f th e lac k o f post-Jurassi c rock s a t thi s location. A Cenozoi c ag e of the right-latera l Lant erman Fault is, however, suggested by the intimate link wit h Cenozoi c magmatis m alon g it s souther n onshore terminatio n (Rossett i e t a l 2002) . Th e Priestley Faul t i s also a major Cenozoi c strike-sli p fault zon e including a transpressional principa l dis placement zon e an d a transtensiona l spla y zon e (Storti e t al 2001) . The geodynamic model by Salvini et al (1997a) predicts that Cenozoic strike-sli p faultin g ma y also have controlled emplacement o f Cenozoic alkalin e magmatism (LeMasurier & Thomson 1990 ; Tonarini et al 1997) . Indeed Mount Melbourne quiescent volcano (Fig s 1 & 2) is locate d a t the intersectio n between th e Campbel l strike-sli p faul t an d th e northern terminatio n o f th e Cenozoi c Terro r Rif t (Salvini etal 1997<2 ; Salvini & Storti 1999 ; Ferraccioli e t al 2000) . Transtensiona l t o strike-slip tec tonics hav e bee n prove d t o contro l Cenozoi c magma emplacement furthe r sout h between Reeves and Mawson Glacier (Rossett i e t al 2000 ) (Fig. 1) . Near Cap e Robert s faul t kinemati c solution s als o indicate a Cenozoi c dextra l transtensiona l regim e
along th e Ros s Se a Rif t margi n (Wilso n 1995) . Transtension ma y hav e cause d pre-existin g fault s to becom e leaky , therefor e favourin g Cenozoi c magma ascen t (Wilso n 1995) . Offshore, withi n the Ross Se a Rift , Lat e Cenozoic(? ) magmatis m i s concentrated alon g Mesozoi c trend s (Behrend t e t al 1996 ; Behrend t 1999) , likel y reactivate d i n response to Cenozoic strike-slip faultin g (Salvin i et al 1997a , b) . Within the Ross Sea Rift, right-latera l strike-sli p faulting i s seismicall y constraine d t o b e post RSU6, i.e post Eocene in age (Busetti 1994 ; Salvin i et al 1991 a). The RSU6 seismic uni t may correlate with an angular unconformity drilled at Cape Rob erts, providin g a n ag e o f 2 4 M a (Dave y e t a l 2002). Negativ e an d positive flowe r structure s and Cenozoic faul t inversion s ar e evident fro m seismi c profiles (Salvin i e t a l 1998) . Al l thes e structura l features ar e consistent with post-RSU6 right-latera l strike-slip faultin g (De l Ben et al 1993 ; Salvin i et al 1997 'a). The Mesozoi c Ros s Se a Rif t basin s ma y hav e formed firs t i n response t o broad Basin and Range type extension , whic h cease d a t abou t 8 5 M a (Lawver & Gahagan 1994 ; Trey et al 1999) . Reac tivation of the rift basi n faults relates to later Cenozoic strike-sli p faultin g (Salvin i e t a l 1991'a), which affected mainl y the western part of the Ross Sea Rift (Dave y & Brancolini 1995). The Cenozoic strike-slip phas e may have triggered bot h Cenozoi c alkaline magmati c activit y o f th e McMurd o Vol canic Grou p (LeMasurie r & Thompson 1990 ) an d induced openin g o f the narro w Terror Rift (Fig . 2 ) (see als o Salvin i e t al 19976 , p. 588 , fig. 2). At plate tectoni c scale , strike-sli p kinematic s of the Transantarcti c Mountain s an d Ros s Se a Rif t may relat e t o Cenozoic motion s along the Tasman and Balleny Transforms in the southwestern Pacific Ocean (Fig . 2 ) (Salvin i e t al 19970 , b) . A majo r plate boundar y reorganizatio n i s eviden t i n th e westernmost Pacific-Antarcti c Ridg e durin g th e Late Miocen e (Lodol o & Coren 1997) . Moreover , Eocene an d Oligocen e motio n betwee n Eas t an d West Antarctic a ma y explai n misfit s i n magneti c anomalies east of the Balleny Transform (Cande et al 2000) .
Aeromagnetic dat a All availabl e aeromagneti c dat a collected ove r th e Ross Sea-Transantarcti c Mountain s regio n hav e recently bee n compile d (Chiappin i e t a l 2002 ) within the framework o f the continental-scale Ant arctic Digita l Magneti c Anomal y Projec t (Golynsky e t al 2001) . Thi s regiona l compilatio n provides a ne w too l fo r tectoni c studie s ove r thi s part of Antarctica. Only part of the extensive aero magnetic datase t i s use d her e t o focu s mainl y o n
STRIKE-SLIP FAULTIN G I N TH E WILKES BASIN
the tectonics of the Prince Albert Mountain s region within th e Transantarcti c Mountain s and upo n th e Cape Roberts area , locate d alon g the western Ross Sea Rift margi n (Fig s 1 & 2). The data used in the first par t o f ou r stud y wer e acquire d alon g reconnaissance surve y grids with a 4.4 an d 2.2 km line spacin g an d wit h a 2 2 k m tie-lin e interval . Flight altitude s range fro m 366 0 and 2700 m ove r the Transantarcti c Mountain s t o 61 0 m ove r th e Ross Se a Rif t (Bosu m e t al 1989 ; Bozzo e t al 1991 a). Profile line s are oriented WNW-ESE, with perpendicular ti e lines . A singl e drape d survey , flown in th e Cap e Roberts-Ros s Islan d are a (Fig. 1), at 30 5 m above terrain an d with a line spacing of 2 km, is also included. Detail s regarding instru mentation, individual surve y layouts , dat a pro cessing, an d surve y mergin g technique s ar e presented b y Chiappini e t al. (2002 and references therein). I n th e secon d par t o f thi s stud y w e us e high-resolution aeromagneti c dat a acquire d offshore Cap e Robert s (Bozz o et al I991b, c), flown along E- W fligh t lines , 500 m apart, an d at 12 5 m above se a leve l (Fig . 1) . Al l aeromagneti c dat a used i n ou r stud y wer e microlevelle d t o reduc e flight-line relate d corrugatio n (Ferracciol i e t al . 1998).
Aeromagnetic interpretation techniques We us e analyse s o f amplitud e an d wavelengt h of aeromagnetic anomalie s an d apparent anomaly offsets observed across magnetic lineament s a s a tool to identif y mostl y burie d faul t system s ove r thi s part o f th e Transantarcti c Mountain s and adjacen t Ross Se a Rift . Magneti c lineament s ca n b e enhanced in shaded relief presentations, such as the one displayed in Figure 3. Recognition of magnetic lineaments base d o n thi s typ e o f presentation ma y be subjective , sinc e i t ca n depen d upo n th e direc tion o f illuminatio n selecte d fo r shading . Fo r example, in Figure 3, a NW-SE-trending structura l fabric i s ver y eviden t becaus e shadin g i s applie d from th e NE . T o delineat e magneti c lineament s more accuratel y an d objectivel y w e therefor e als o applied boundar y analysis techniques , suc h a s th e maximum horizonta l gradien t o f pseudo-gravit y (Cordell & Grauc h 1985 ; Blakel y & Simpso n 1986) an d 3 D Eule r Deconvolutio n (Rei d e t al . 1990). The pseudo-gravity map displayed in Figure 4 is obtained b y applyin g a transformation , involving reduction t o th e pol e an d vertica l integratio n (Baranov 1957) . Th e ter m pseudo-gravit y take s into account the fact tha t the magnetic distribution is no t necessaril y relate d t o a densit y distribution. The horizontal derivatives act as a high-pass filter, which enhance s shallowe r anomal y sources , suc h as volcanics. Peaks in the horizontal derivatives are
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picked wit h a n automate d metho d t o locat e loca l maxima (Blakel y & Simpso n 1986) . In 3 D Eule r Deconvolution , boundarie s an d depths-to-source o f magneti c anomalie s ar e analysed, using the total magnetic field and its gradient components. Th e metho d require s inpu t of a parameter know n a s th e 'structura l index ' (SI) , which for fault s i s equa l t o 0. 5 o r 1 (Reid e t al . 1990) . The mos t appropriat e SI , whic h i s essentiall y a measure of anomaly fall-off rat e with distance, can be interactivel y selecte d b y pickin g Eule r Decon volution solutions which exhibit the tightest spatia l clustering. An example of the output of this method is displaye d i n Figur e lOb .
Aeromagnetic signature s Once aeromagnetic lineament s ar e identified, thes e are correlate d wit h mappe d o r inferre d faults , where possible. T o accomplish a tectonic interpret ation, aeromagneti c anomalie s ar e correlate d wit h geological map s and published structura l interpretations (e.g . Skinner & Ricker 1968 ; Warren 1969; Carmignani e t al . 1989 ; Kleinschmidt & Matze r 1992; Molzah n e t al . 1996 ; Salvini e t a l I991a; Salvini & Storti 1999; Rossetti e t al 2000) . Anomalies ar e also interpreted togethe r with results fro m rock magneti c property studie s over Victoria Land (Bozzo e t al . 1992 ; Lanz a & Zanell a 1993 ; Lanza & Tonarini 1998) . Aeromagnetic anomal y studie s combine d wit h susceptibility constraints and geology over Victoria Land identify typica l magnetic anomal y signatures . High-frequency magneti c anomalie s wit h wavelengths o f about 5-15 k m ar e correlated wit h Cenozoic alkaline volcanic rocks and with Jurassic Ferrar dolerit e sill s (Bosu m et a l 1989 ; Behrendt et a l 1996 ; Ferraccioli & Bozzo 1999) . Anomaly amplitudes ar e generall y highe r ove r magnetite rich Cenozoi c volcani c rock s (100-100 0 nT ) than over thi n Jurassi c dolerit e sill s (50-10 0 nT ) (Behrendt e t a l 1991) . Higher-amplitud e anomal ies (over 500 nT) mark Jurassic Kirkpatrick Basal t rocks. The highest observed amplitudes (500-2000 nT) refe r t o near-circular anomalies , detecte d ove r Cenozoic alkalin e intrusive s (Bosu m e t a l 1989 ; Tonarini e t a l 1997 ; Ferraccioli & Bozz o 1999) . Typically, they exhibit a 30 km wavelength . Similar wavelength , bu t lowe r amplitud e anomalie s (100-500 nT) mar k som e Granit e Harbou r Intrus ives o f Ros s ag e (Ferracciol i & Bozzo 1999 ; Finn et a l I999a\ Ferracciol i e t al. 2002) .
Crustal block s and previously delineate d strike-slip fault s o f th e Transantarcti c Mountains Crustal blocks bordered by fault system s may exhibit contrastin g aeromagneti c patterns , whic h ca n
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Fig. 3 . Tota l fiel d aeromagneti c anomal y ma p ove r th e Transantarcti c Mountains-Ros s Se a Rif t region . Re d line s indicate faults fro m Salvin i et al. (1991 a) an d Salvin i & Storti (1999). Note the locatio n of th e eastern margi n of th e Wilkes Subglacia l Basi n wit h respect t o th e Centra l Victori a Lan d Boundary , a prominen t aeromagnetic lineamen t system (Ferracciol i & Bozzo 1999) .
contribute t o thei r identificatio n (Fig. 3) . Tectoni c blocks identifie d fro m aeromagneti c analysi s ove r the Transantarctic Mountain s include the Southern Cross Mountain s Block , th e Dee p Freez e Rang e block, an d the Prince Alber t Bloc k (Ferracciol i & Bozzo 1999 ) (Figs 2, 3 & 4). We propose here that the tectonic block boundaries interpreted fro m aero magnetics may correlate with strike-slip faults delin eated i n th e Salvin i e t al. (1997 'a) tectoni c map. The Souther n Cros s Mountain s Bloc k (Fig . 3 ) features high-amplitud e anomalie s relate d t o exposed an d burie d Cenozoi c alkalin e intrusion s and volcanics , ofte n spatiall y associate d wit h Cenozoic strike-sli p faults , suc h a s th e Aviato r Fault (Salvin i e t al . 1997a ; Tonarin i e t a l 1997) .
These Cenozoic plutons terminate to the southwest against th e prominen t Campbel l Fault . Thi s i s a Cenozoic strike-sli p fault , probabl y controllin g magma ascen t i n th e are a o f quiescen t M t Mel bourne volcano (Salvini et al. 1997a ; Ferraccioli e t al 2000) . Th e Campbel l an d Priestle y fault s ar e Cenozoic strike-sli p fault s whic h converg e a t th e Ross Se a (Salvin i e t a l 1991 b; Salvin i & Stort i 1999; Stort i e t al 2001) . Thes e fault s ar e marked by coinciden t magneti c lineament s borderin g th e Deep Freez e Rang e block . Withi n thi s tectoni c block magneti c anomalies relate t o Jurassic tholeiites (Ferracciol i & Bozzo 1999) , includin g Ferra r sills an d locall y Kirkpatric k Basal t (Rolan d & Worner 1996) .
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Fig. 4 . Maximu m horizontal gradient of pseudo-gravity map revealing unprecedented structura l detail over the Prince Albert Bloc k an d adjacen t Ros s Se a Rift .
The Southern Cross Mountains and Deep Freez e Range tectoni c block s featur e a broa d magneti c minima, consisten t wit h virtuall y non-magneti c basement. Low-susceptibilit y metamorphi c an d Granite Harbou r basemen t rock s dominat e thes e blocks (Bozz o et al 1992) . In contrast, th e Princ e Albert Mountains Block is marked by a broad, over 300 km long, magnetic high. The magnetic high is flanked by the prominent NW-SE-trending Centra l Victoria Lan d Boundar y (Fig . 3) . Th e source s o f the long-wavelength aeromagneti c hig h are mostly buried Granit e Harbou r Intrusive s (Ferracciol i & Bozzo 1999) , rathe r tha n Precambria n rock s (Bosum e t al . 1989) . Hig h susceptibilitie s hav e been measure d i n the Reeves Glacie r (Fig . 1 ) over a magnetite-ric h Granit e Harbou r Intrusiv e (Lanza & Tonarini 1998) , corroboratin g thi s aero -
magnetic interpretation . Thes e magneti c ar c rock s have bee n inferre d t o relate to the buried souther n prosecution o f th e Ross-ag e Exile s Thrus t syste m (Ferraccioli & Bozzo 1999) . Thi s i s a speculativ e inference, sinc e th e Exile s Thrus t i s locate d ove r 300 k m furthe r north , a t th e Pacifi c Coas t (Flottmann & Kleinschmid t 1991) . Th e distan t location o f th e Exile s Thrust , wit h respec t t o ou r study area , i s displaye d i n Figure 2 . The speculat ive aeromagneti c inferenc e is , however, consisten t with hig h susceptibilitie s measure d a t th e Pacifi c Coast ove r Granit e Harbou r Intrusives (Talaric o et al. 2001 ) emplace d alon g th e Exile s Thrus t itsel f (Roland & Olesch 1997) . Ferracciol i e t al (2002 ) further propose d tha t different magneti c signature s of ar c rock s acros s th e Centra l Victori a Lan d Boundary might reflect differentia l uplift . Shallow -
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level, magnetite-ric h ar c intrusion s migh t hav e been emplaced directl y alon g this fault zone , while deeper ar c segments to the east of the fault contai n ilmenite-rich rocks only. A magnetic configuration relating t o differentia l uplif t i n th e Transantarcti c Mountains would match the one typically observe d over magmatic arc s in California, Chile, and Japan (Gastil 1990) . In the following section w e discuss whethe r th e prominent Central Victori a Land magnetic Boundary simpl y reflect s a n inherite d earl y Palaeozoi c fault an d related magmatic ar c rocks, as previously proposed, o r if i t als o mark s a more recen t strike slip faul t o f the Transantarcti c Mountains.
Tectonic modelling of the Prince Albert Fault System Tectonic model 1 The maximu m horizonta l gradien t o f pseudo gravity map suggests a considerably mor e complex structure o f th e Centra l Victori a Lan d Boundar y than previousl y recognize d (Fig . 4) . On e peculia r feature of this aeromagnetic lineament system is its apparent right-lateral step-lik e geometr y northwest of Reeves Glacier . I n tectonic model 1 (Fig. 5 ) we propose tha t suc h a configuratio n reflect s th e inherited earl y Palaeozoi c geometr y o f th e Exile s Fault(?) system . Suc h a step-lik e geometr y i s no t likely to stem from thrusting alone, contrary to previous interpretation s (Ferracciol i & Bozz o 1999) . A step-lik e geometr y o f suc h a fault syste m could instead relate to strike-slip stepovers , either in form of a releasin g stepove r (pull-apar t structure ) o r a restraining stepove r (uplif t o r pop-u p structure ) (Aydin & Nur 1985) . I n tectonic model 1 we infe r
Fig. 5 . Tectoni c mode l 1 assume s a n inherite d Earl y Palaeozoic step-lik e geometr y o f th e Centra l Victori a Land Boundary.
that magneti c pluton s alon g th e Centra l Victori a Land Boundar y might represen t shallow-leve l ar c segments emplaced in a coeval transtensional shear zone. I n contras t th e magneti c minim a ove r th e Larsen Glacie r are a (Fig. 1 ) might indicate ilmenite rich arc plutons emplaced a t a deeper crusta l level, in a coeva l transpressiona l shea r zone . Tectoni c model 1 may b e attractiv e becaus e o f it s kinshi p with Lat e Cambrian-Earl y Ordovicia n transpressional an d transtensiona l tectoni c an d magmatic features , recognizable ove r th e adjacen t Eisenhower Rang e an d Dee p Freez e Rang e (Musumeci & Pertusati 2000) .
Tectonic model 2 Tectonic mode l 1 fails t o consider tha t there is not only a long-wavelengt h magneti c anomal y brea k across th e Centra l Victori a Lan d Boundary , bu t also a brea k i n high-frequenc y magneti c pattern , which i s addresse d i n tectoni c mode l 2 (Fig . 6a) . The high-frequenc y magnetic anomalie s dominat e the entir e are a wes t o f th e Centra l Victori a Lan d boundary. They ar e related to Jurassic tholeiite s of the Ferra r Supergrou p includin g Ferra r sill s an d dykes, an d Kirkpatrick Basalt rocks (Ferracciol i & Bozzo 1999) . Thi s interpretatio n i s strongl y sug gested b y spatia l correspondenc e o f som e o f th e anomalies with outcrop and by rock magnetic data, which indicat e hig h susceptibilitie s rangin g between 0.00 1 an d 0.05 3 S I an d Koenigsberge r ratios betwee n 1 and 5.5 (Lanz a & Zanella 1993) . Moreover magnetic modelling alon g a seismic lin e across th e Princ e Alber t Mountain s reveals Ferra r Supergroup rock s unde r ic e cove r (Ferracciol i & Bozzo 1999) . Thi s i s consisten t wit h seismi c an d radio-echo soundin g evidence fo r subglacia l mes a topography, related to Beacon Supergroup intruded by Ferrar rocks and locally overlain by Kirkpatrick Basalt (Delisl e 1994 ; Deli a Vedov a e t al 1997 ; Ferraccioli et al 1997) . Preferential focusin g o f Jurassi c tholeiiti c mag matism alon g th e inherite d faul t zon e migh t hav e occurred, explainin g th e observe d differenc e i n high-frequency magneti c anomal y patterns . Alter natively, mor e recen t reactivatio n o f th e faul t sys tem ma y involv e significan t differentia l uplif t across th e faul t system . A relativel y downthrow n Prince Alber t Bloc k woul d impl y les s erosio n o f the Ferrar Province with respect to the more highly uplifted Dee p Freez e Rang e bloc k (Ferracciol i & Bozzo 1999 , p . 25314 , fig . 9) . Differentia l uplif t provides a n explanation for the considerably lower elevations an d mino r til t o f th e Kukr i Peneplain , separating basement from Beaco n Supergroup over the Prince Albert Mountains compared to northern Victoria Lan d tectoni c block s (Mazzarin i e t al . 1997). The differential uplif t hypothesi s might also
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Fig. 6 . (a ) Tectonic model 2 assumes NE-SW-directed extension i n the Larsen Basin; (b) tectonic model configuration as woul d be obtaine d b y complet e closur e o f th e Larse n Basin ; (c ) tectoni c mode l 3 assume s left-latera l strike-sli p faulting alon g th e easter n margi n o f th e Wilke s Subglacia l Basin ; (d ) tectoni c mode l configuratio n prio r t o strike slip faulting .
be consistent with apatite fission track data indicat ing a smalle r amoun t o f exhumatio n an d henc e a shallower crustal block in the Prince Albert Mountains region (Fitzgeral d 1992) . I t ma y als o explai n the differen t magneti c signatur e o f Earl y Palaeo zoic ar c rocks ove r the Prince Albert Block . I n tec tonic model 2 , we propose tha t the Central Victori a Land Boundar y may therefor e hav e bee n reactiv ated a s a majo r norma l faul t system . Pertusat i e t al. (1999 ) presente d a geologica l ma p indicatin g an inferre d norma l fault , terme d th e Davi d Fault , coinciding wit h a segmen t o f th e Centra l Victori a Land Boundary . In this framework we predict that the magnetic minim a ove r the Larse n Glacie r are a might relate to syn-Ferrar or later crustal extension , resulting in downmrown magnetic basement at this location. Assumin g that th e directio n o f extensio n
was NE-S W directed , the n a possibl e pre-exten sion end-membe r configuratio n woul d involv e a n original clos e t o linea r geometr y o f th e magneti c arc batholith (Fig . 6b) .
Tectonic model 3 In tectoni c mode l 3 (Fig . 6c) , w e conside r a n additional facto r wit h respec t t o mode l 2 . Thi s important facto r i s th e spatia l associatio n o f th e northwestern par t o f th e aeromagneti c lineamen t system wit h a segmen t o f th e easter n margi n o f Wilkes Subglacia l Basi n (Drewr y 1976 ; Stee d 1983). Recentl y th e Wilke s Subglacia l Basi n ha s been interprete d a s relatin g t o crusta l extensio n (Ferraccioli et al. 2001), rathe r than to lithospheric flexure (Stern & ten Brink 1989 ; ten Brink & Stern
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1992; te n Brin k e t al 1997) . Consequentl y i n model 3 we propose the existence o f a major fault , lying o n strik e wit h th e easter n margi n o f th e Wilkes Subglacia l Basi n itself . High-amplitud e magnetic anomalie s ar e locate d withi n the Wilke s Subglacial Basi n a t the margi n o f the surve y area . Previously thes e anomalie s wer e interprete d a s reflecting mafi c middl e to late Proterozoic igneou s rocks o f th e Eas t Antarcti c Craton , i n analog y t o similar anomalies observed over the Gawler Craton in Australia (Finn e t al I999b). However , w e put forward a different interpretation, i.e . thick Jurassi c Kirkpatrick Basal t preserve d fro m erosio n i n a down-faulted Wilke s Subglacia l Basin . Th e interpretation tha t Ferrar Supergroup rocks, including Kirkpatrick Basalt, may continue into the basin is consistent wit h new magnetic models acros s the Wilkes Subglacia l Basi n (Ferracciol i e t al. 2001) . Tectonic mode l 3 predict s left-latera l strike-sli p displacement o f earl y Palaeozoi c magneti c ar c crust along a single fault zone, lying on strike with the eastern margin of the Wilkes Subglacial Basin. A hypothetica l pre-strike-sli p configuratio n would lead t o a broad, ove r 10 0 km wide region o f magnetic ar c crust. Following thi s model i t might also be tempting to correlate inferre d left-latera l strike slip faulting along the eastern margin of the Wilkes Subglacial Basi n wit h regiona l Cretaceou s left lateral shearin g i n norther n Victori a Land , speculatively inferre d b y Tessensoh n (1994) . Indeed, a previous upward continued aeromagnetic anomaly ma p suggeste d tha t th e Centra l Victori a Land Boundar y wa s truncate d b y Cenozoi c fault s of th e Victori a Lan d Basi n (Ferracciol i & Bozzo 1999, p. 25306, plate 2). This observation could be consistent wit h a Cretaceous rathe r tha n Cenozoi c age of reactivation of the fault zone . The new horizontal gradien t ma p suggests , however , tha t thi s previously observe d crosscuttin g relationshi p may not hold true, eliminating the requirement for a preCenozoic ag e o f reactivatio n o f th e faul t zon e (Fig. 4) .
Tectonic model 4 Tectonic mode l 4 i s ou r preferred mode l (Fig . 7) . It relies upon analysis of many more features than considered in model 3 or in any other previous tec tonic mode l o f th e area . I t take s int o accoun t (a ) the magneti c minim a ove r th e Larse n Glacie r region; (b ) high-frequenc y anomalie s alon g th e western margin of the Victoria Land Basin, within the Ross Sea Rift; (c ) high-amplitude anomalies at the eastern margi n of the Winces Sublgacial Basin; and (d ) anomaly patterns withi n the Prince Alber t Block. Mos t importantly , th e tectoni c mode l als o incorporates recen t geologica l constraint s (Salvini & Storti 1999 ; Rossetti et al 2000) . Model
4 predicts tha t the Central Victoria Land magnetic boundary images a complex right-lateral strike-slip fault system , whic h w e nam e th e Princ e Alber t Fault System, of likely Cenozoic age (Fig. 8) . This name is proposed first following it s location in the Prince Alber t Mountains . Second , thi s nomencla ture is introduce d to better relate thi s faul t syste m to th e previousl y propose d Princ e Alber t Moun tains Tectoni c Bloc k (Ferracciol i & Bozzo 1999) . The NW-SE-strikin g segmen t o f th e faul t syste m includes th e Reeve s Faul t an d th e Davi d Faul t described b y Salvin i & Stort i (1999 ) a s Cenozoi c right-lateral strike-sli p faul t systems . I n th e Larsen/Reeves Glacie r are a th e geometr y o f th e fault syste m ma y resembl e a sinuou s right-latera l pull-apart o r releas e ben d structure , whic h w e name the Larsen pull-apart. This structure connects segments o f th e right-latera l Reeve s Faul t an d David Fault (Salvini & Storti 1999) . If this hypothesis hold s true , magneti c basemen t ma y b e downthrown i n a pull-apart-lik e structure , a s a result o f NW-SE-directe d transtensio n oblique t o the Ros s Sea Rift basin s (Figs 7 & 8). Clockwise bendin g o f th e NW-S E strike-sli p fault syste m t o a NNW-SS E orientatio n occur s between David Glacier and Mawson Glacier. South of Mawson Glacier it rotates t o a N-S orientation . This faul t segmen t is subparallel , bu t distinc t with respect to the western margin of the Ross Sea Rift , where right-latera l strike-sli p faultin g is distinctl y imaged in seismic profiles (De l Ben et al 1993 , p. 82, fig. 68; Salvini et al 1997a , p. 24,672, fig. 3), The southern segment o f the Ross Se a Rift margi n has been referred to as the McMurdo Sound Fault Zone (Hamilto n e t a l 2001) . Thi s faul t zon e i s clearly punctuate d b y high-frequenc y magneti c anomalies (Fig. 3), related to preferential clustering of Cenozoi c magmati c rocks alon g th e faul t zon e itself (Behrend t e t a l 1996 ; Ferracciol i & Bozz o 1999). Transtension may develop along the coastal region o f th e Princ e Alber t Mountain s bloc k because o f a right-latera l simpl e shea r couple t between th e Princ e Alber t an d McMurd o Sound Fault Zones . Thi s interpretatio n i s consisten t with tension gash-lik e arrangement s o f Cenozoic dyke s within th e maste r fault s an d left-steppin g dyk e arrays i n th e intrafaul t zone s between Reeves an d Mawson Glaciers (Rossett i e t al 2000) . However, the regiona l characte r of th e aeromagneti c dataset makes i t impossibl e t o imag e th e Cenozoi c dyk e systems at this locatio n (Fig . 8) . Other element s spea k i n favou r o f a tectoni c model predicting tha t the Prince Albert Faul t System is a major post-Jurassi c right-lateral strike-slip fault system . High-amplitude (over 500 nT) anomalies reveal Jurassic Kirkpatrick Basalt rocks in the region (Ferracciol i e t a l 1997) . Th e horizonta l gradient map suggests that Kirkpatrick Basalt rocks
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Fig. 7 . Tectoni c mode l 4 i s ou r preferre d model . I t involve s comple x right-latera l NW-SE-trendin g transtensiona l faulting i n the Prince Albert Block region. White arrows indicate interpreted extensio n directions oblique to the Ross Sea Rif t basins , suc h a s th e Victori a Lan d Basi n (VLB) .
are confine d t o lozenge-shape d right-latera l pull apart-structures alon g th e wester n margi n o f th e Prince Alber t Bloc k (Fig . 4) . Th e alignmen t o f Kirkpatrick Basal t rock s o f th e Princ e Alber t Mountains lie s o n strik e wit h th e easter n margi n of th e Wilke s Subglacia l Basin . Sinc e Kirkpatrick Basalt rock s outcro p a t Brimston e Pea k (Gun n & Warren 1962 ) w e hav e terme d thi s composit e structural featur e a s the Brimstone pull-apar t (Fig . 8). Moreover a high-amplitude anomal y a t Martin Nunataks relates to Kirkpatrick Basalt (Carmignani et al 1989) . Kirkpatric k Basal t withi n thi s bloc k may relat e t o a down-faulte d sigmoidal-shape d tension-gash-like structur e (Fig . 8) . However , th e WNW orientatio n o f thi s featur e differ s fro m th e one typicall y expecte d fo r a tensio n gas h forme d in a right-lateral strike-slip faul t zone .
Also particularl y noteworth y ar e th e high frequency magneti c trends markin g Ferrar Supergroup rocks . Thes e ar e generall y obliqu e t o th e Ross Se a Rif t basins . Ther e i s a n indicatio n o f clockwise rotatio n o f thes e magneti c trend s fro m NNE to NE and finally to ENE proceeding southwards. Suc h magneti c trend s migh t b e imagin g oblique-slip faul t array s relatin g t o post-Ferra r oblique extension , induce d b y strike-sli p faul t motion within the Prince Albert Faul t System . At least on e majo r NN E magneti c trend , withi n th e Ferrar-related pattern, i s clearly associate d wit h a major post-Jurassi c faul t zone , th e Rollin g worth Fault, presentl y o f unknow n kinematic characte r (Pertusati e t a l 1999) . Ther e ma y a relatio n between NN E t o ENE-trendin g aeromagneti c lineaments an d normal-obliqu e Cenozoi c fault s
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Fig. 8. Tectoni c sketch ma p constructed fro m ne w aeromagnetic interpretatio n o f the Prince Albert Faul t Syste m an d adjacent NW-S E strike-sli p faul t belt s (Salvin i & Stort i 1999 ; Ferraccioli e t al 2000 ; Stort i el al 2001) . Th e twostage Cenozoi c transtensiona l evolutio n alon g th e wester n margi n o f th e Ross Se a Rif t involve s a late-stage increas e in the strike-slip component, a s interpreted fro m Cenozoic dyk e and fault arrays by Rossetti e t al (2000) . This structura l interpretation mayb e consisten t wit h th e shea r couple t alon g th e Princ e Alber t Faul t Syste m an d McMurd o Soun d Fault Zone , a s interprete d her e fro m aeromagneti c pattern s (se e lower lef t inset) . Ou r aeromagneti c interpretatio n implies a strike-slip-induced couplin g betwee n th e eastern margi n o f the Wilkes Subglacia l Basin , th e Transantarcti c Mountains, an d th e wester n margi n o f th e Ros s Se a Rift .
observed i n th e Cap e Robert s are a (Wilso n 1995).
Aeromagnetic anomalie s and faults in the Cape Robert s Rift regio n The Cap e Robert s regio n i s a key are a t o addres s in furthe r detai l th e geometr y an d kinematic s o f strike-slip faultin g an d it s contro l o n Cenozoi c
magmatism a t th e boundar y betwee n th e Trans antarctic Mountain s an d the Ros s Se a Rift. A t thi s location high-resolutio n aeromagneti c dat a wer e acquired a s par t o f th e sit e survey fo r drillin g within th e Cap e Robert s Drillin g Projec t (Barret t et al . 1995 ; Bozz o et al 1997*) . Th e drillin g pro ject ha s no w bee n complete d providin g ne w con straints fo r tectoni c interpretatio n (Cap e Robert s Science Team, 2000). Also new multi-channel seis-
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mic reflection result s are available (Hamilton et al. 1998, 2001) allowing for a more detailed structural analysis o f aeromagneti c pattern s tha n previousl y possible (Bozz o et al 1997c) . To addres s correlatio n betwee n magneti c lin eaments and faults the tectonic sketch map of Hamilton e t al . (2001 ) constructe d fro m seismi c interpretation i s superimpose d upo n a se t o f enhanced aeromagneti c anomal y image s fo r th e Cape Robert s regio n (Fig s 9 & 10) . Th e tectoni c sketch ma p depict s fault s o n to p o f seismi c uni t V4b, which has been drilled yielding an age of 23.7 to 24.1 Ma (Hamilton et al 2001) . A major aerom agnetic lineamen t (LI ) i s eviden t i n th e tota l field shaded relie f ma p (Fig . 9a) . I t form s th e wester n flank o f th e Cap e Robert s anomaly , originall y called 'Anomal y A ' alon g multi-channe l seismi c line USGS-40 3 (Behrend t e t a l 1987) . Thi s anomaly ha s bee n interprete d a s arisin g fro m a submarine volcano , th e 'Barret t volcano' , wit h an associated shallow-leve l intrusio n (Behrendt, p. 89 of LeMasurie r & Thomso n 1990) . Magneti c lin eament L I coincide s wit h faul t zon e B , whic h forms th e wester n flan k o f th e Cap e Robert s Rif t Basin. Diffraction s an d change s i n amplitud e an d shape o f seismi c trace s ar e consisten t wit h high angle basemen t faultin g an d associate d magmati c intrusions (Hamilto n e t a l 1998) . Th e L I lin eament is offset b y NE to ENE-trending transverse features ( T and T'). Thes e transverse magnetic features li e subparalle l t o EN E faul t system s delin eated fro m seismi c dat a i n th e Robert s Ridg e region (e.g . J in Fig. 9a) . These fault s hav e an estimated vertica l separatio n of as much as 800 m and exhibit a left-obliqu e componen t (Hamilto n e t a l 1998). A horizonta l derivativ e aeromagneti c ma p (Fig . 9b) enhance s lineamen t L I an d highlight s lin eament syste m L 2 alon g th e easter n flan k o f th e Cape Roberts Rif t Basin . Lineament L2b lies a t an angle wit h respec t t o NNE-trendin g faul t zone s D and E but is subparallel to fault zon e G. Lineament L2a lie s o n strik e an d clos e t o faul t zon e H . Thi s map als o delineate s high-frequenc y lineamen t arrays bot h i n th e Macka y Se a Valle y regio n an d over Roberts Ridge. In the Mackay Sea Valley area NE an d N- S trend s ar e easil y recognized . N E trends als o occur close t o Cape Roberts . Th e Roberts Ridg e are a features , i n contrast , dominan t NNW trends . The maximu m horizonta l gradien t o f pseudo gravity map confirms th e right-stepping e n echelon geometry of LI (Fig . lOa) . A subparallel trend (LO) is als o detecte d an d exhibit s a n opposit e left stepping e n echelo n geometr y wit h respec t t o LI . Notably, L O also appear s t o crosscu t tren d M o f the Mackay Sea Valley. There i s an apparent rightlateral displacemen t o f M across LO . The ma p als o
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indicates a horsetail-like spla y (HT) of L2 into two distinct lineament s L2 b an d L2a . Th e latte r tren d exhibits a sinuou s anastomosin g geometr y wit h opposite curvatur e t o faul t H . NN W trend s ar e located a t th e ti p o f faul t H i n th e Robert s Ridg e region. Thes e trend s appea r t o b e truncate d b y a NE tren d a t the southwester n edg e o f surve y area . The 3D Euler Deconvolution map, calculated for structural inde x 1.0 , i s use d fo r furthe r lineamen t location an d fo r a crud e dept h estimat e o f sourc e bodies (Fig . lOb) . Thi s ma p clearl y delineate s th e ENE-trending transverse structur e T truncating the Cape Robert s Rif t Basi n t o th e north . Alon g th e western flan k o f th e basi n magneti c sourc e dept h ranges from abou t 50 0 m belo w se a level , corre sponding t o se a floor , t o abou t 2 k m belo w se a level. Thi s i s i n genera l agreemen t wit h result s obtained fro m 2 an d I dimensiona l modellin g o f the Cap e Robert s igneou s comple x (Bozz o e t a l 1991 b, p . 1131 , fig . 2) . Som e deepe r solution s (more tha t 3 k m belo w se a level ) ar e apparentl y located wes t o f faul t zon e B withi n crystallin e basement. I n pla n vie w th e overal l S-shape d geometry o f th e Cap e Robert s igneou s comple x closely resemble s a sigmoida l tension-gash-lik e feature. In Figure ll a we display th e total field magnetic anomaly an d th e maximu m horizontal gradien t o f pseudo-gravity profil e alon g seismi c lin e NBP9 7 (see Fig . 9 a fo r location) . Th e non-migrate d seis mic sectio n i s take n fro m Hamilto n e t a l (2001) . Here correlatio n o f L I wit h th e McMurd o Soun d Fault zone, i.e with the master fault zon e along the western sid e o f th e Ros s Se a Rift , i s evident . Th e double pea k anomal y ove r th e Cap e Robert s Rif t has bee n previousl y modelle d alon g seismi c lin e USGS 40 3 b y introducin g a non-magneti c bod y placed directl y belo w th e centr e o f th e anomaly , flanked b y magneti c bodie s o n bot h side s (Behrendt e t a l 1987 , p . 163, fig . 6) . Th e non magnetic body was furthe r interprete d a s a molten magma chambe r o r a recentl y solidifie d magm a chamber, i.e . roc k abov e th e Curi e temperature . However, seismicall y image d faultin g withi n th e Cape Robert s Rif t (Hamilto n e t a l 2001 ) ca n explain th e observe d magneti c patter n withou t requiring such a recent hot body. Also faulting provides a n explanatio n fo r th e apparen t east-dipping attitude o f th e magneti c intrusiv e a s modelle d b y Bozzo e t a l (1991 b). Thi s magneti c intrusiv e has an apparen t susceptibilit y o f 0.029 SI units, which is within the range of values measured ove r Ceno zoic lava s an d flow s o f th e McMurd o volcani c group, an d i s highe r tha n typicall y measure d ove r Jurassic Ferra r dolerite s (0.012 ) (Behrend t e t a l 1987; Bozz o et al 1992) . Ther e i s a spatial corre lation o f lineament L2a with negative flower structure H a s interprete d b y Hamilto n e t a l (2001 )
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Fig. 9 . (a ) Total field shaded relief aeromagneti c anomal y map in the area of the Cape Roberts Rift Basin ; (b) horizontal derivativ e map . Lettering A- H refer s to fault s a t top o f seismic uni t V4 b a s proposed b y Hamilton e t al. (2001) . The locatio n o f selecte d seismi c lines an d o f drill-sites (CRP-1 , CR P 2/2A , and CR P 3 ) is als o reported .
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Fig. 10. (a ) Maximu m horizontal gradien t of pseudo-gravity map i n the are a o f the Cap e Robert s Rif t Basin ; (b) 3 D Euler Deconvolutio n map.
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beneath th e Robert s Ridg e region . Th e observe d magnetic signatur e migh t ste m from Cenozoi c vol canic rock emplaced i n intimate spatia l associatio n with the inferred right-lateral strike-slip faul t zone . A blow-u p o f flowe r structure s H an d G , a s inter preted fro m seismi c profiles , i s displaye d i n fig. 5 by Hamilto n e t al (2001) . Another importan t observatio n i s th e regiona l eastward magneti c gradien t ove r th e Robert s Ridge. This may be explained by eastward thickening o f Cenozoic volcani c roc k i n the uppe r part of the sedimentar y sequenc e (Behrend t e t a l 1987 ) or, a t depth, b y Jurassic Ferra r sills within Beaco n Supergroup, formin g eastward-dippin g magneti c basement, a s modelled by Bozzo e t al. (1991 b). A further explanatio n might be magnetic arc(? ) basement a t eve n greate r dept h dippin g t o th e wes t towards th e Cap e Robert s Rift . I f true , thi s westward-dipping geometr y o f dee p magneti c basement coul d speculativel y b e relate d t o a pro posed low-angl e detachmen t faul t marke d b y th e V7b acousti c basemen t reflectio n (Hamilto n e t al . 2001). In Figure li b we display the total field magnetic anomaly an d th e maximu m horizontal gradien t of pseudo-gravity profil e alon g seismi c lin e NBP8 9 (see Fig. 9a for location). A simplified line-drawing and th e projecte d locatio n o f th e Cap e Robert s drillholes ar e als o reporte d (Cap e Robert s Scienc e Team 2000 ; Hamilto n e t al . 2001) . Her e w e observe tha t lineamen t L2 b ma y correspon d t o flower structur e G , whic h displace s seismi c uni t V4b (Hamilto n e t a l 2001) . Lineamen t L2 a matches agai n flowe r structur e H , whic h ca n b e traced als o withi n 21-1 7 M a strata (V3) . CRP-2/2A recovere d volcani c as h layer s date d between a t 21.4 4 ± 0.00 5 M a an d 24.2 2 ± 0.0 6 Ma b y 40 Ar/39Ar (Fieldin g & Thomso n 1999) . These as h layer s hav e bee n interprete d t o requir e a fairl y proxima l forme r volcani c centre . W e pro pose tha t th e abou t 2 km thic k intrusiv e complex ,
Fig. 11. (a ) Profile view of total field and maximum horizontal gradien t o f pseudo-gravit y alon g seismi c lin e NBP97. Non-migrate d seismi c sectio n modifie d fro m Hamilton e t a l (2001) . Not e th e Cenozoic(? ) intrusiv e body beneat h th e Cap e Robert s Rif t an d correspondenc e between magnetic lineament LI an d the McMurdo Sound Fault Zone . Lineamen t L2 a lie s jus t eas t o f negativ e flower structur e H ; arro w tips sho w dextral sens e of displacement, (b ) Detai l ove r th e Cap e Robert s drill-site s in th e Robert s Ridg e regio n alon g seismi c lin e NBP8 9 (modified fro m Cap e Robert s Scienc e Tea m 200 0 an d Hamilton e t a l 2001) . Lineamen t L2 b i s marke d b y a broad maximu m horizonta l pea k ove r faul t zon e F an d negative flower structure G, while L2b is a shorter-wavelength peak closely matching negative flower structure H; arrows tip s a s per uppe r panel.
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modelled b y Bozz o e t a l (1991 b) a s causin g th e Cape Robert s Rif t magneti c anomaly , migh t b e related t o thi s forme r Cenozoi c volcani c centre , consistent wit h Hamilto n e t al (2001 ) hypothesis . A thinner sheet-like bod y underlying V5 was modelled b y Bozz o e t a l (1991 b) t o th e eas t o f th e Cape Roberts Rift , beneat h the Roberts Ridge. This body exhibit s a n apparen t susceptibilit y o f 0.01 2 SI units, i.e. simila r t o mean measured values over Ferrar sills. Indeed the Cape Roberts Science Tea m (2000) drille d a n igneou s bod y intrudin g Beaco n strata at CRP-3 between 901 and 918 m below se a level. The drilled bod y coul d b e a thin sill , related to th e approximatel y 50 0 m thic k magneti c sill , which ma y explai n th e aeromagneti c hig h eas t o f the Cape Roberts Rift (Bozz o et al 1991 b,p. 1131 , fig. 2). This drilled bod y may represent an anomalous phas e o f Jurassic Ferra r dolerit e o r a hithert o unrecognized subalkalin e episod e o f earl y rift related magmatism , older tha n 3 4 Ma (Cap e Roberts Scienc e Tea m 2000) .
Discussion Cenozoic transtension in the Cape Roberts Rift region A new tectonic sketc h ma p was compiled b y com bining enhanced aeromagnetic imaging , multichannel seismi c results , an d brittl e faul t analyse s onshore (Wilso n 1995 ) ove r th e Cap e Robert s region (Fig . 12a) . W e propos e tha t th e newl y recognized Cap e Robert s Rif t Basi n (Hamilto n e t al 2001 ) i s a pull-apar t o r releas e ben d structur e which accommodate s right-latera l transtensiona l faulting alon g th e wester n Ros s Se a Rif t margin . To th e eas t o f th e aeromagneti c surve y are a loca l transpressive deformatio n an d a positiv e flowe r structure i s consisten t wit h regiona l right-latera l strike-slip faultin g (De l Ben et al 1993 ; p. 82 , fig. 68). Thi s positiv e flowe r structur e ma y be counte r to th e negativ e flowe r structure s identifie d alon g the southeaster n margi n o f th e right-latera l Cap e Roberts pull-apart (Fig . 1 1 a, b an d Hamilton e t a l 2001). Cenozoi c igneou s bodie s wer e likel y emplaced durin g right-latera l transtensio n i n th e Cape Robert s pull-apar t basin . Th e overal l sig moidal shap e o f the mai n Cenozoic intrusiv e complex i s simila r t o a tension-gash-structur e forme d in a right-latera l NW-SE-trending faul t zone . Th e ENE t o N E fault s probabl y lin k e n echelo n faul t segments o f th e NNW-trendin g maste r faul t sys tem, marke d b y th e McMurd o Soun d Faul t Zon e (Hamilton e t a l 2001) . Th e EN E fault s ma y rep resent left-obliqu e faul t system s whic h accommo dated differentia l right-latera l strike-sli p motio n along the NNW fault segment s or maybe conjugate left-lateral strike-sli p faults. At least one NNE faul t
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Fig. 12 . (a ) Tectoni c sketc h ma p constructe d fro m aeromagnetic , seismic , an d brittl e faul t interpretation , suggesting that the Cape Roberts pull-apart and associated magmatism stems from Cenozoic transtension along the western margin of th e Ros s Se a Rift . Lowe r lef t inse t depict s Cenozoi c brittl e faul t orientation s ove r th e Transantarcti c Mountain s (Wilson 1995) . Right inset show s two-stage faul t kinematic s in the Cenozoic, a s interpreted b y Wilson (1995) . In the early stage fault s exhibit normal-oblique displacements, extensio n i s subhorizontal and NW-SE directed (bold arrow), and contractio n i s subvertica l (soli d circle) ; in th e followin g stag e fault s featur e low-angle oblique o r strike-sli p dis placements, th e extensio n directio n i s NNW-SS E an d th e contractio n i s subhorizonta l an d NE-S W directe d (se e Wilson 199 5 fo r furthe r explanation) , (fe ) Regiona l aeromagneti e ma p an d interprete d fault s a t the transition betwee n the Ros s Se a Rif t an d the Transantarcti c Mountains ; Abbreviations: CR : Cap e Roberts ; GH : Granit e Harbour .
STRIKE-SLIP FAULTIN G I N TH E WILKE S BASIN
exhibits apparen t dextra l offse t o f Cenozoi c igneous bodies. Hamilto n e t al. (2001 ) argue d that the NNE faults are dominantly normal fault s whic h may hav e accommodate d a minor clockwis e verti cal axi s rotation , i n analog y t o structure s detecte d over othe r dextra l obliqu e rift s (Luyendyk 1991) . The rhomboi d geometr y o f th e Cap e Robert s pull-apart basin, is broadly similar to that predicted in analogu e modellin g o f pull-apart basin s fo r 90 ° releasing offse t sidestep s (Doole y & McCla y 1997). I n particular , th e apparen t V shap e define d in pla n vie w b y fault s withi n th e souther n par t of the basi n (Hamilto n e t al . 2001 ) ma y relat e t o cross-basin faul t systems , whic h cu t th e floo r o f the pull-apart and tend to link the stepped principa l displacement zone s (Doole y & McCla y 1997) . When compare d t o other natura l examples o f pullapart basins, suc h as the sinistra l Dead Se a Basin, which i s abou t 13 2 km lon g an d 7-1 6 k m wid e (ten Brin k & Ben-Avraham 1989; te n Brink et a l 1993), th e newly proposed Cap e Roberts pull-apart basin i s a relatively smal l feature , being abou t 2 0 km long an d 6-8 k m wide. Its plan dimensions ar e more comparabl e t o (a ) th e Mesquit e Basin , developed a t a releasing sideste p o f the BrawleyImperial dextra l strike-sli p faul t zone , i n souther n California (Doole y & McClay 1997 , p . 1820 , fig . 14) and (b) a pull-apart structur e located alon g the dextral Atho s faul t zon e i n th e Nort h Aegea n Trough (Doole y & McClay 1997 , p . 1823 , fig 18). We propos e tha t Cenozoi c dyk e array s wit h a NE trend may have formed t o the west of the NNW master faul t boundin g the Cape Roberts pull-apart , in th e Macka y Valle y regio n an d offshor e Cap e Roberts (Fig . 12a) . I n analog y t o dyk e emplace ment mechanism s observe d b y Rossett i e t a l (2000) further north , between Reeve s an d Mawson Glaciers (Fig . 8) , dyke s i n th e Cap e Robert s are a may have been emplaced durin g non-coaxial Ceno zoic transtension along the rift margin . These interpreted NE-trendin g Cenozoi c dyk e array s ar e no t imaged t o th e eas t o f the Cap e Robert s pull-apart, possibly indicatin g decouplin g acros s th e McMurdo Soun d Fault Zone . Onshore, brittl e faul t array s ove r th e Transant arctic Mountain s includ e NNE , NE , an d EN E oblique-slip fault s wit h a consistent obliqu e orien tation to the Ros s Se a Rift margi n (lower lef t inse t in Fig. 12 a from Wilson 1995) . The integrated geo physical datase t an d drillin g result s strongl y cor roborate a Cenozoic ag e for these oblique-slip faul t arrays, which lie subparallel to those identified offshore alon g th e Ros s Se a Rif t margin . Kinemati c analysis o f thes e Cenozoi c faul t array s ove r th e Transantarctic Mountain s suggest s a n obliqu e extensional o r transtensiona l tectoni c regim e fol lowed b y a dominantl y transcurren t regim e (righ t inset i n Fig . 12 a from Wilso n 1995) . W e propos e
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that the Cape Roberts pull-apart basin relates to this Cenozoic transtensiona l tectoni c settin g alon g th e Ross Se a Rif t margin .
Strike-slip faulting and regional oblique rifting By combinin g Cap e Robert s high-resolutio n dat a with reconnaissanc e aeromagneti c anomal y dat a i t is possible t o further discus s relationships betwee n strike-slip faultin g i n th e Ros s Se a Rif t an d ove r the Transantarctic Mountains (Fig. 12b) . The rightstepping geometr y o f th e McMurd o Soun d Faul t Zone may continue also to the sout h of Cape Roberts, thoug h it s definitio n i s les s secur e owin g t o the regional characte r o f the aeromagnetic data . To the north , th e McMurd o Soun d Faul t Zon e ma y parallel th e Kirkwoo d Faul t o f th e Transantarcti c Mountains (Gun n & Warre n 1962 ; Ferracciol i & Bozzo 1999) . We propose tha t the Kirkwood Fault may i n turn represent a transtensional spla y o f th e major right-latera l Princ e Alber t Faul t Syste m identified t o th e nort h ove r th e Transantarcti c Mountains (Fig. 8). Apparently the Kirkwood Fault or some associate d fault zon e continues als o t o the south i n th e Cap e Robert s region , wher e dextra l transtension i s prove n b y structura l dat a (Wilso n 1995). Hamilton e t a l (2001 ) note d tha t obliqu e faul t array geometrie s an d associate d displacement s i n the Cape Roberts region (Wilso n 1995 ) matc h typical pattern s observe d i n analogu e faul t model s o f oblique rif t system s (Clifto n e t al 2000 ) an d ove r the obliqu e extensiona l Malaw i Rif t i n Afric a (Chorowicz & Sorlei n 1992) . W e propos e tha t magnetic basement presentl y flooring the Ross Sea Rift (Behrend t et al 1996 , p. 672, plate 4; Ferraccioli & Bozz o 1999 , p . 25306 , plat e 2 ) ma y hav e been, in a pre-rift configuration , adjacent to simila r magnetic basement detecte d ove r the Prince Alber t Block (Fig . 8) . Restoration o f these two basemen t blocks i n a n originall y adjacen t positio n woul d clearly requir e significant NW-SE-directed oblique extension i n th e Transantarcti c Mountains-Ros s Sea Rif t region . Non-coaxia l NW-SE-directe d Cenozoic transtension , a s interprete d betwee n Reeves an d Mawso n Glacier s (Rossett i e t a l 2000), ma y relat e t o strike-sli p faultin g alon g th e Prince Alber t Faul t Syste m an d McMurd o Faul t Zone. A t a more regiona l scale , obliqu e extensio n could b e relate d t o differentia l NW-SE-directe d intraplate strike-sli p displacements , accommodat ing transfor m faul t shearin g i n th e southwester n Pacific Ocean , a s describe d i n th e Salvin i e t a l (19970) geodynamical model (Fig. 2) . Bosum et al (1989) als o propose d a simila r geodynamica l model, base d upo n kinematic interpretatio n o f aeromagnetic pattern s ove r th e Ros s Se a Rif t an d
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Transantarctic Mountains , predictin g tha t regiona l NW-SE-oriented obliqu e extensio n ma y relat e t o transform faul t motio n i n th e southwester n Pacifi c Ocean (Bosu m et al 1989 , p . 225 and 227, figs 46 and 47). Sutherland (1999 ) showe d tha t Cenozoi c and possibly Lat e Cretaceous obliqu e riftin g i n the Ross Se a is require d fo r closur e o f th e Australia New Zealand-Antarctic a plat e tectoni c circui t a t chron 3 3 (74Ma) .
Strike-slip faulting and uplift of the Transantarctic Mountains Structural an d aeromagneti c evidenc e fo r obliqu e extension induce d by strike-sli p faulting i s hard to reconcile with rift flank models of the Transantarctic Mountains , whic h rel y upo n perpendicula r extension i n th e Ros s Se a Rif t (e.g . Bott & Ster n 1989; va n de r Bee k e t a l 1994) . Wilso n (1995 ) noted tha t onl y smal l isostati c uplif t o f th e Trans antarctic Mountains would be induced by Cenozoic transtension along the Ross Sea Rift margin , unless the proportion of divergence and thus of orthogonal unloading wer e dominant . Salvin i e t al . (1991 a) proposed tha t uplif t o f th e Transantarcti c Moun tains wa s relate d t o isostati c reboun d o f th e Ros s Sea Rif t shoulde r wit h virtuall y n o contributio n from Cenozoi c strike-sli p tectonics . Hamilto n et al. (2001) interprete d initia l uplif t o f the Transantarc tic Mountains to have occurred between 55(?) an d 34 Ma ago. This uplif t phas e coul d relat e t o E-W extension and low-angle detachment faulting along the eas t sid e o f th e Victori a Lan d Basi n an d extending westwar d beneat h th e Transantarcti c Mountains. Betwee n 3 4 an d 1 7 M a ago , when strike-slip faultin g an d NW-SE-directe d obliqu e extension became important, uplift o f the Transantarctic Mountain s decreased (Hamilto n e t al 2001 , p. 339 , fig. 8). In contrast , refine d flexura l modellin g indicate s that Cenozoi c transtensio n ma y hav e broke n th e once continuou s Antarcti c lithospher e alon g th e Transantarctic Mountain s front , changin g bot h elevation an d surfac e slop e o f th e range , an d tha t these effect s accelerate d denudatio n (te n Brink e t dl. 19973 . Th e presenc e o f a majo r lithosphcri c break alon g the Ross Se a Rift margi n i s consisten t with th e prominen t aeromagneti c signatur e o f th e McMurdo Soun d Faul t Zon e an d Princ e Alber t Fault Syste m (Fig. 3). Most important , steep gravity gradient s acros s th e rif t margi n als o requir e a contrast between highly extended crus t beneath the Ross Se a Rif t an d thicke r crus t beneat h th e rang e (Reitmayr 1997 ; Trey el al 1999 ; Ferraccioli el al 2001). Such a crustal thickness contras t is apparent also from seismi c data further north over the Transantarctic Mountain s (O'Connel l & Step p 1993 ; Delia Vedov a e t al 1997) . A s note d b y te n Brin k
et al. (1997), stee p Moho dips have been observe d across transfor m plat e boundaries . Thi s an d a variety o f othe r line s o f evidenc e hav e bee n use d t o support th e ide a tha t uplif t o f th e Transantarcti c Mountains ma y represent a transform flank, rather than a rif t flan k phenomeno n (te n Brink e t al . 1997). W e infer tha t such a model ma y well apply to uplift o f the Prince Albert Block of the Transantarctic Mountains .
Strike-slip faulting along the eastern margin of the Wilkes Subglacial Basin None o f the regiona l geodynami c model s o r uplif t models o f th e Transantarcti c Mountain s pu t forward s o fa r ha s considere d th e possibilit y tha t Cenozoic strike-sli p faulting ma y extend further t o the wes t toward s th e Wilke s Subglacia l Basi n region. W e have presented a tectonic mode l base d upon aeromagneti c pattern s an d structura l data , suggesting that Cenozoic strike-sli p faulting, ident ified alon g th e Princ e Albei t Faul t System , ma y continue alon g th e easter n margi n o f th e Wilke s Subglacial Basin (Figs 7 & 8). This new inference, if correct, may contrast with models predicting that the Wilke s Subglacia l Basi n simpl y represent s a broad flexura l depressio n induce d b y uplif t o f th e Transantarctic Mountains (Stern & ten Brink 1989; ten Brin k & Ster n 1992 ; ten Brin k e t a l 1997) . This hypothesis may also be a t odds with a simple continental rif t mode l fo r th e Wilke s Subglacia l Basin (Drewr y 1976 ; Steed 1983) . Finally , i t ma y also appea r t o contradic t a mor e recen t mode l depicting th e Wilke s Subglacia l Basi n a s a broa d extended terran e (Ferracciol i e t al 2001) . Following th e geodynami c mode l o f Salvin i e t al (1991 a) fo r th e adjacen t Transantarcti c Moun tains region , w e speculat e tha t strike-sli p faultin g may extend to the Wilkes Subglacia l Basin eastern margin a s a resul t o f shearin g alon g th e Tasma n transform, whic h is co-linear with the basin margin itself (inse t i n Fig. 2). Continenta l overla p i n Lat e Cretaceous Australia-Antarctic a se a floo r recon structions may be reduced b y assumin g that conti ncntnl cxtcmion occurre d in the Wilkes Subglacia l
Basin regio n (Tikk u & Cand e 1999) . W e pu t forward the hypothesis that the assumed crustal extension ma y a t leas t i n par t relat e t o inferre d strike slip faulting . I n analog y t o bette r constraine d observations alon g th e Ros s Se a Rif t margin , w e infer tha t strike-sli p faultin g ma y hav e reactivate d the easter n margi n o f the Wilkes Subglacia l Basi n in a dominantly transtensional tectonic setting. Our aeromagnetic connectio n show s tha t strike-sli p faulting alon g par t o f th e easter n margi n o f th e Wilkes Subglacia l Basi n ma y continu e alon g th e Prince Alber t Bloc k an d link to strike-sli p faultin g
STRIKE-SLIP FAULTIN G I N THE WILKE S BASIN
along th e wester n margi n o f th e Ros s Se a Rif t (Fig. 8) . New aeromagneti c dat a ar e clearl y neede d between th e Pacifi c Coas t an d th e Princ e Alber t Mountains regio n t o tes t ou r suggestio n tha t Cenozoic (?) strike-sli p faultin g ma y characteriz e the easter n margi n o f the Wilkes Subglacia l Basi n (Fig. 2) . New radar dat a could refin e th e geometr y of th e easter n margi n o f th e Wilke s Subglacia l Basin and analyse possible bedroc k faulting . Aero gravity dat a coul d bette r constrai n whethe r ther e is crustal thinnin g beneat h Wilke s Subglacia l Basin, as proposed i n a recent gravit y mode l (Ferracciol i et al. 2001) , o r conversel y i f ther e i s thic k crus t beneath th e basi n (Ster n & te n Brin k 1989 ; ten Brink & Ster n 1992 ; te n Brin k e t a l 1997) . Oversnow seismi c experiment s coul d the n b e targeted t o definin g crusta l structur e an d tectonic s across th e newl y suggeste d strike-sli p faul t zon e along th e easter n margi n of th e Wilke s Subglacia l Basin. Seismi c dat a shoul d b e acquire d als o offshore, a t th e Pacifi c Coast , t o tes t th e speculativ e link betwee n th e easter n margi n o f th e Wilke s Subglacial Basi n an d th e Tasma n transform .
Conclusions The utilit y o f aeromagnetic s fo r intraplat e strike slip faul t identificatio n an d tectoni c analysi s i s demonstrated b y ou r study . This i s particularly the case o f th e Transantarcti c Mountains-Ros s Se a Rift regio n o f Antarctica, where right-lateral Ceno zoic strike-sli p belt s reactivat e th e inherite d Earl y Palaeozoic structura l architecture. Ou r main results are a s follows : 1. Majo r aeromagneti c lineament s identif y th e right-lateral Prince Albert Fault System, which is impose d upo n a n inherite d faul t zone . Th e right-lateral Princ e Alber t Faul t Syste m continues alon g th e easter n margi n o f th e Wilke s Subglacial Basin , whic h ma y therefor e b e coupled to the strike-slip, dominantl y transtensional, tectonic framewor k of the Transantarctic Mountain s and Ros s Se a Rift . 2. Th e NW-SE-strikin g segmen t o f th e Princ e Albert Faul t Syste m include s tw o Cenozoi c right-lateral strike-sli p fault s o f th e Transant arctic Mountains, namely the Reeves Fault and David Fault . Th e N-S-striking segmen t o f the Prince Alber t Faul t Syste m continue s south wards, wher e it parallels th e McMurd o Sound Fault Zone , markin g th e Cenozoi c wester n margin o f th e Ros s Se a Rift . 3. Th e Cenozoi c shea r couple t betwee n th e Prince Alber t Faul t Syste m an d the McMurdo Sound Faul t Zon e induce d NW-SE-oriente d oblique extension alon g th e easter n margi n of
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the Princ e Alber t Block . NW-SE-directe d transtension explains geometrie s o f pull-aparts or release bends in the Prince Albert Fault System. I t als o provide s a suitabl e kinemati c framework t o restor e magneti c basemen t presently underlyin g the Ross Se a Rif t basins , against th e Prince Alber t Block . 4. Structura l an d aeromagneti c evidenc e fo r oblique extensio n suggest s tha t th e Princ e Albert Bloc k o f th e Transantarcti c Mountain s cannot be regarded a s a simple rift flank uplift. 5. Aeromagneti c image s offshor e Cap e Robert s indicate tha t transtensio n alon g th e wester n Ross Se a Rif t margi n ma y hav e controlle d emplacement o f a Cenozoic intrusion , associa ted wit h th e Cap e Robert s pull-apar t basin . This structure developed at a releasing sideste p in th e McMurd o Soun d Fault Zone . This researc h i s a contributio n t o tw o PNR A projects : Mesozoic and Cenozoic evolution in the Ross Sea area, co-ordinated b y F . Salvin i (Dip. Sci. Geol., Univ . Roma Tre) an d WIBEM: Wilkes Subglacial Basin Eastern Margin, co-ordinate d b y E . Bozz o (DIP.TE.RIS . Univ . Genova). E . Lodol o an d J . C . Behrend t accomplishe d helpful an d constructiv e reviews . F . Stort i provide d important suggestions , whic h clarified interpretatio n an d also improve d upon text organization .
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Implications of late Pan-African shearin g in western Dronning Maud Land, Antarctica S. H. PERRITT 1 & M. K . WATKEYS 2 1
Council for Geoscience, c/o University of Natal, PO Box 18091, Dalbridge 4014, South Africa (e-mail:
[email protected]) 2 School of Geological and Computer Sciences, University of Natal, Durban 4041, South Africa (e-mail: "
[email protected]) Abstract: Th e Mesoproterozoic cove r rocks o f the Archaean Grunehogn a Provinc e i n western Dronning Mau d Land , Antarctica , ar e gentl y folde d an d intensel y fracture d b y a progressiv e ductile to brittle deformatio n event. The cause was dominantly NNW-SS E transpression related to a regionally extensive intracontinental sinistral strike-slip event at c. 520 Ma. This reactivated the boundar y betwee n th e Archaea n Grunehogn a Provinc e an d the hig h grad e Mesoproterozi c Maudheim Province , an d resulte d i n th e formatio n an d deformatio n o f th e Cambria n Urfjel l pull-apart basin . I n a Gondwan a refit, thi s strike-sli p syste m ca n b e linke d t o simila r zone s i n southeast Afric a whic h post-dat e th e 750-65 0 Ma earl y Pan-Africa n collisiona l event s o f that region. Th e NNW movement o f the Maudheim Province alon g this strike-sli p zon e wa s respon sible fo r th e developmen t o f thrust s i n th e Luri o Bel t o f norther n Mozambique . Th e drivin g force fo r thi s movemen t i s considere d t o b e relate d t o th e lat e Pan-Africa n collisiona l event s elsewhere i n th e growin g supercontinent .
Introduction Western Dronnin g Maud Lan d (WDML) , Antarc tica, consist s o f tw o distinc t geologica l terrane s (Fig. 1) : an Archaean cratonic fragment, termed th e Grunehogna Provinc e b y Krynau w (1996) , an d an adjacent deformed , high-grad e metamorphi c belt , the Maudheim Province (Groenewal d e t al 1991) . The basement granite s o f the Grunehogna Provinc e are overlain b y a n extensive Mesoproterozoic vol cano-sedimentary cove r sequence , the Ritscherfly a Supergroup (Wolmaran s & Ken t 1982) . Thi s sequence accumulate d i n a periphera l forelan d basin develope d i n respons e t o th e accretio n o f a Mesoproterozoic volcani c ar c an d back-ar c basi n terrain (th e Sverdrupfjell a Group , Maudhei m Province) onto the margin of the Grunehogna Province, durin g the period 1. 1 to 0.9 Ga (Moyes e t a l 1995; Moye s & Harri s 1996) . Th e boundar y between thes e tw o geologicall y distinc t terrain s (presently maske d b y th e Pencksokke t an d Jutul straumen glaciers) i s often regarded a s having been inactive sinc e th e Mesoproterozoi c collisiona l event wit h the exceptio n o f possible riftin g durin g the fragmentatio n of Gondwana (e.g. Grantha m & Hunter 1991) . Recen t investigation s fro m region s in th e Maudheim Provinc e hav e indicate d tha t the
Fig. 1 . Localit y an d regional geolog y o f wester n Dron ning Maud Land, Antarctica. WDML is dominated by two distinct provinces : th e Grunehogn a Province t o th e north and west of the Jutulstraumen-Pencksokket glacial divide , and the linear Maudheim Province exposed across the Heimefrontfjella, Kirwanvegge n an d H . U . Sverdrupfjell a t o the sout h and eas t (modifie d from Jackso n 1997) .
Urfjell Grou p wa s deposite d i n a pull-apar t basi n that forme d an d deforme d durin g th e Cambria n (Croaker et al. 1999) . This paper provides evidence that this strike-slip even t is far more extensive tha n
From: STORTI , F. , HOLDSWORTH , R . E . & SALVINI , F . (eds ) Intraplate Strike-Slip Deformation Belts. Geologica l Society, London , Specia l Publications , 210 , 135-143 , 0305-8719/037$ 15 © The Geologica l Societ y o f London 2003 .
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previously suspecte d an d provide s a mode l t o explain it s development .
Deformation of the Ritscherfly a Supergroup Post-Archaean deformatio n withi n the Grunehogn a Province i s generall y considere d relativel y mil d and i s typicall y attribute d t o a singl e episod e o f extensional bloc k faultin g wit h th e Mesozoi c deformation confine d t o minor dyking and faultin g (De Ridde r 1970 ; Ferreir a 1986 ; Grantha m & Hunter 1991 ; Krynau w e t al 1991 ; Moye s e t al 1993; Swanepoe l 1995 ; Krynau w 1996) . Difficult ies i n establishin g mor e precis e constraint s o n the nature o f th e deformatio n ar e generall y attribute d to the isolated nature of the outcrops an d the masking o f large-scal e structure s b y th e extensiv e ic e cover. However , clos e examinatio n o f th e struc tures i n th e Ritscherfly a Supergrou p reveal s a coherent pattern that can be related t o a progressive deformation even t (Perrit t 2001). The oldes t structure s ar e fold s an d associate d fractures o f th e Raudbergdale n an d Frostlende t areas of the Borgmassivet (Figs 1 & 2). The buckle folds occu r a s tw o ope n syncline s wit h uprigh t axial plane s an d subhorizonta l fol d axes . I n th e east, thes e ar e orientate d EN E bu t rotat e toward s the NE furthe r wes t (Fig. 2). The earlies t fracture s identified i n th e Ritscherfly a rocks o f thi s regio n are two set s o f extensional fractures an d a pair o f conjugate fractures . On e se t o f extensiona l frac tures (EF1 ) occur s a t righ t angle s t o th e strik e o f the axial plane, while another se t occurs parallel to the axia l plan e (EF2) . Th e pai r o f conjugate shea r fractures (SF 1 and SF2 ) ar e bisected by EF1 . All fou r set s o f fracture s sho w th e sam e sens e and amoun t o f rotatio n a s th e synclina l fol d axe s and th e axia l plana r cleavag e whe n movin g fro m east t o west . Furthermore , th e sam e directio n o f maximum compressio n stres s (a ^ i s indicate d fo r the folds , conjugat e shears , an d extensiona l frac ture EF1. Consequently, al l the structure s are considered to have formed in the same event, with EF2 being relate d t o th e axia l plan e (se e 3 D sketch , Fig. 2). Th e directio n o f G ! rotate s systematicall y across th e region , varyin g fro m NN W i n th e eas t to N W in th e west . These structure s ar e cu t an d offse t b y anothe r system o f fractures which is found in the Borgmas sivet an d furthe r nort h i n th e Ahlmannrygge n (Fig. 3). Thi s secon d syste m contai n set s o f frac tures which have consistent orientation s acros s the whole regio n tha t ar e consistent wit h the expecte d orientation of fractures in a sinistral strike-slip system. A tensiona l T fractur e strike s NW , bisectin g a syntheti c sinistra l R shea r an d antitheti c dextra l R' shear . A sinistra l secondar y syntheti c P shea r
forms a mirro r imag e t o th e R shea r acros s th e expected directio n o f the master Y shear and overall directio n o f th e principa l displacemen t zon e (PDZ). The maximu m compressiv e directio n obtaine d from thes e younge r fractures i s fro m th e NN W a s indicated b y th e pole s t o th e T fracture s (Fig . 3). This i s identica l t o th e directio n indicate d b y th e folds an d older fractures i n the east (Fig. 2), where the directio n of G! is effectively give n by the poles to EF1 . Th e observe d systemati c variatio n o f C ^ across th e area, a s indicated b y the fold s an d olde r fractures i n th e wes t area , i s considere d t o b e related t o th e anticlockwis e rotatio n o f th e struc tures by the sinistral movement of the NE-trending strike-slip zone . I f ther e wer e exposure s furthe r southwest, th e fol d axe s woul d b e expecte d t o rotate back to their original orientation on the other side o f th e shea r zone . This deflectio n o f th e axia l plan e ma y b e use d to obtain a n estimate o f the overall amount of simple shear strain (y) in the shear zone and it indicate s a maximu m y of abou t 3 , treatin g th e shea r zon e as a singl e entit y tha t underwen t a systemati c increase i n inhomogeneou s simpl e shea r fro m th e margins towards the centre of the shear zone. However, given the nature of the observed brittl e defor mation i n th e expose d rocks , i t i s likel y tha t thi s strain i s partitione d int o narro w discret e brittle ductile zones , whic h hav e bee n preferentiall y weathered t o for m valleys , separatin g lozenge shaped block s whic h ar e th e exposure s o f th e region. I n thes e intervenin g blocks , th e simpl e shear strai n woul d b e minimal , henc e th e lac k o f high strai n i n th e expose d rocks . Without seein g th e other sid e o f the shea r zone , it i s no t possibl e t o calculat e y acros s th e whol e shear an d obtai n a n accurat e valu e fo r th e tota l minimum displacemen t fo r th e shear . However , estimates ca n be made. If the main shearing in this region i s confine d t o th e Frostlende t are a (Fig . 2), then a displacemen t i n th e orde r o f 9 0 km i s expected, bu t i f i t i s acros s th e whol e Borgmassi vet-Ahlmannryggen regio n (Fig . 3), the n a mini mum o f 300 km displacement is not unreasonable . Such an offset coul d be responsible fo r the overal l curved patter n o f th e Maudhei m Province , whic h wraps aroun d the easter n an d souther n sid e o f th e Grunehogna Province . I t i s emphasize d tha t thes e estimates ar e minimu m value s an d d o not includ e movement alon g individua l fault s whic h ar e no t exposed i n the regio n probabl y du e to preferential erosion s o tha t the y exis t onl y unde r the glacier s between nunataks . Due to this ice cover, i t is not possible to directl y trace th e strike-sli p syste m t o th e Urfjel l regio n of the souther n Kirwanvegge n (Fig . 1) . Howeve r a n analysis o f th e faultin g o n th e expose d nunatak s
PAN-AFRICAN SHEARIN G I N ANTARCTIC A
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Fig. 2 . Summar y o f the initial folding and fracturing event of the Ritscherflya Supergroup in the Borgmassivet region , Grunehogna Province , cause d b y NN W to NW compression.
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Fig. 3 . Summar y of th e secon d fracturin g o f th e Ritscherfly a Supergrou p in th e Borgmassive t an d Ahlmannrygge n region, Grunehogna Province, caused by a NE-trending sinistral strike-slip event. Average angles between the different joint sets : X-R' : 47° , R'-R : 48° , R-T : 26° , R'-P: 77° .
indicates th e existenc e o f an extensive curvilinear, sinistral strike-sli p zone tha t coul d b e a t least 10 0 km wide (Croacker 1999 ) (Fig . 4). This has caused the developmen t o f localized zone s o f transtensio n (releasing bends) and the formation of a number of rhomb-shaped graben s i n th e Ahlmannrygge n and Borgmassivet regions o f the Grunehogna Province.
Age o f the sinistra l strike-sli p movemen t A Cambria n ag e i s indicate d fo r thi s deformatio n from tw o sources . I n the Straumsnutan e area , nor-
theast Grunehogn a Provinc e (Fig . 1) , shearin g i s characterized b y strongly orientated flake s o f white mica whic h yiel d K-A r age s o f 52 2 ± 1 1 Ma an d 526 ± UM a (Peter s e t al 1991) . Thes e age s ar e also simila r t o th e 55 0 t o 53 0 Ma ag e suggeste d for the Urfjell Grou p in the southern Kirwanveggen of th e Maudhei m Provinc e (Fig . 4) (Croake r e t al. 1999) . Croaker (1999 ) interpret s th e Cambria n Urfjel l Group a s bein g deposite d i n a pull-apar t basi n developed within a NE-trending sinistral strike-sli p system. I t wa s initiate d b y significan t an d rapi d
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different area s represent different uppe r crustal levels, wit h th e Kirwanvegge n bein g nea r surfac e as apparent b y th e preservatio n o f th e Cambria n Urfjell strike-sli p basins , an d th e Grunehogn a region being deeper. Thi s faulting can be attributed to the formation of a PDZ located alon g the Pencksokket (Fig s 1 & 4) , reactivatin g th e boundar y between th e Archaea n Grunehogn a Provinc e an d the Mesoproterozoic Maudhei m Province . Second ary splay s may follow other glacial divide s suc h as the Jutulstraumen . Th e existenc e o f suc h a zon e was postulated b y Allen (1991 ) base d o n structures exposed i n th e H.U. Sverdrupfjella .
Implications o f the shea r zon e fo r Gondwana refit s
Fig. 4 . Interpretatio n o f th e regiona l patter n o f th e NE trending sinistra l strike-sli p zon e separatin g th e Grune hogna Provinc e an d the Maudhei m Province .
uplift alon g a linke d syste m o f thrust s an d left lateral transpressiv e zone s a t approximatel y 55 0 Ma, durin g whic h tim e top-to-nort h thrustin g wa s experienced withi n th e souther n Kirwanvegge n (Helferich e t al. 1999) . Thi s overal l transpressiv e system was due to the fault syste m o f the souther n Kirwanveggen bein g slightl y eas t of northeast, giv ing rise to a restraining bend geometry. Within this transpressive framework , localize d tensio n wa s produced a t releasin g bend s alon g th e strike-sli p zones. A t th e surface , thes e zone s o f transtensio n developed into extensional pull-apart basins of limited extent , formin g small depositorie s whic h rapidly fille d wit h detritus . Continue d movemen t along the strike-slip system produced further transpression, resultin g i n th e subsequen t deformatio n of the Urfjel l Grou p within a multiphase strike-sli p system, a t c . 53 0 Ma (Moye s e t a l 1997) . Th e compressional directio n withi n thi s syste m varied from NNW-SS E to NNE-SSW. Close similaritie s exist between th e nature of the deformation observe d i n th e Borgmassive t an d Ahlmannryggen region s o f th e Grunehogn a Prov ince an d th e strike-sli p faultin g recorde d i n th e southern Kirwanveggen , suggestin g coeva l development withi n a regiona l NNW-SS E compressional regim e durin g th e Cambrian . Th e
The Pan-African belt s of east and southeast Africa , Madagascar, an d India all contain strike-sli p shear s (Drury e t al . 1984 ; Manhic a e t a l 2001 ; d e Wi t et al 2001) . The recognition o f this regiona l Pan African strike-sli p syste m i n WDM L alon g th e boundary betwee n a n Archaea n provinc e an d a Kibaran province raise s the question of how it links with th e shear s o f southeas t Afric a i n a Gond wana refit . Trying to link this shear with a shear in southeast Africa involve s th e trick y proble m o f th e uncer tainties wit h regards t o th e Africa-Antarctic a refit . In a simila r exercise , Reeve s an d d e Wi t (2000 ) use ocean floor features in order to reconnect continental shea r zones between parts of Antarctica, Sri Lanka, India , Madagascar , an d part s o f southeas t Africa bu t di d no t includ e th e regio n discusse d here. Ther e ar e three basi c fits that have been proposed fo r th e southeas t Africa-Dronnin g Mau d Land refit . • Th e loose fit: This fit involves restoring the two continents as close a s possible withou t removing th e Mozambique Ridge (e.g . d e Wit e t a l 1988) whic h consist s o f bot h continenta l an d oceanic materia l (Scrutto n 1976 ; Roese r e t a l 1996) (Fig . 5a). Th e Mozambiqu e Ridg e an d the continental margin of KwaZulu-Natal for m the two flanks of the Natal Valley which neatly accommodates th e Falklan d Platea u i n th e Africa-South Americ a refit once 400 km extension of the plateau is removed (Marshall 1994) . Thus, if the ridge is predominantly a fragmen t of continenta l crus t i n o r clos e t o it s origina l position, i t i s no t possible t o brin g Antarctic a much close r t o th e Lebomb o regio n o f sou theast Afric a than about 36°E becaus e th e are a west o f thi s i s underlai n b y continenta l crust . The 14 0 Ma fit of Reeves & de Wit (2000) cor responds t o this fit and this is about as far back as it is possible to constrain the fits using ocean
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• Th e uptight fit : Thi s fi t als o eliminate s th e Mozambique Ridge an d involves removin g th e presumed extensio n o f the continental margin s so that the current continenta l margin s overla p in the refit (Fig . 5B). Geological an d geophysi cal feature s o f southeas t Africa , suc h a s th e Lebombo, ar e use d t o positio n th e correlativ e geology o f Antarctica . (Co x 1992 ; Reeve s 2000).
Fig. 5. Compariso n o f th e 'loose ' (a ) an d 'uptight ' (b ) refits fo r southeas t Afric a an d wester n Dronnin g Maud Land, Antarctica.
floor anomalies . An y tighte r fit s require s us e of othe r data . The tight fi t I n order t o move from th e 'loose ' fit to the 'tight ' fit, the Mozambique Ridge has to b e eliminate d s o tha t th e tw o continenta l shelves ca n b e juxtaposed . Marti n an d Hart nady (1986 ) provide d a longitudinall y con strained bu t latitudinall y flexibl e fit , wit h th e continental margi n o f Dronnin g Mau d Land , Antarctica? juxtapose d wit h tha t o f souther n Mozambique an d norther n KwaZulu-Nata L They proposed that a possible solution to maintaining bot h a tight fi t an d a continental Moz ambique Ridg e i s t o hav e th e ridg e initiall y move southwar d a s part of the Antarctic Plate . However, thi s doe s not move the ridge entirel y OUt o f th e Wa y t « allQ W Antarctic a t o mov e westwards fo r a tighte r fit . I f th e rid p 1 8 a n OCeaniC feature, its elimination is an easier process. Despit e thi s problem , thi s fi t ha s gaine d favour ove r th e year s wit h variou s worker s (Lawver & Scotes e 1987 ; Groenewal d e t al. 1991; Roese r e t al 1996) .
Using the Pan-Africa n shea r of WDML a s an indication o f whic h fi t i s mos t suitable , th e 'tight ' fi t can b e eliminate d a s ther e i s n o correspondin g structure in southeas t Africa. I n fact, i n this fit, the WDML shea r i s a t right-angle s t o th e tren d o f th e structures i n th e opposin g Luri o Belt . O f th e tw o remaining situations , both the 'uptight ' an d 'loose ' fits result in the WDML shea r linkin g wit h a PanAfrican shea r i n norther n Mozambiqu e (Fig . 5). However, whic h fi t i s correc t canno t b e resolve d from thi s evidenc e alone . In the 'loose ' fit, the Umkondo and Ritscherfly a basins woul d hav e bee n abou t 700k m apar t whereas in the 'uptight ' fit, the Ritscherflya Supergroup restores t o a position abou t 200 km southeast of th e Umkond o Grou p o f easter n Zimbabw e an d western centra l Mozambiqu e (Fig . 5). Thes e tw o sequences hav e lon g bee n considere d correlative s (e.g. Groenewal d e t al. 1991 ) an d have very simila r tectonic position s bein g situate d o n th e easter n edge o f Archaean craton s wit h Kibaran belts lyin g to the east. Their lin k is further strengthene d b y the proposal tha t ther e i s a strike-sli p zon e t o the east of the Umkond o (Manhic a e t al. 2001). Suc h geo logical similarit y make s i t ver y temptin g t o mak e an 'ultra-tigh t fit ' placin g th e Ritscherfly a directly south o f th e Umkondo, s o that not onl y woul d the Archaean an d Kibara n province s lin e u p bu t s o would th e shear s o n th e eas t sid e o f both basins. However, a detailed investigatio n o f the Ritsch erflya Supergrou p (Perritt 2001 ) ha s demonstrate d that, although the two sequences ar e similar i n age, they are most unlikely to be part of the same basin. Furthermore ther e is a remarkable differenc e i n the provenance o f th e sediment s despit e bot h basin s deriving mSi F materia l fro m th e W5£t . I n th e cas e of the Umkondo the material wa s derived from th e Archaean Zimbabw e crato n an d Limpop o Belt , while the Ritscherflya sediments were derived fro m an active Kibaran arc as shown by the geochemical characteristics o f th e sediment s a s wel l a s zirco n dating. Thu s a n 'ultra-ti£ht ' fi t i s no t possibl e t>ecaUB§ onl y Archaea n ag e emi t woul d li e t o th e west o f the Ritchersflya an a i n an 'uptight ' fit , th e Ritchesflya an d Umkondo have to have been separ ate basins . Unfortunately th e dat a o f th e WDM L shea r d o not constrai n whethe r a 'loose ' o r 'uptight ' refi t i s
PAN-AFRICAN SHEARIN G I N ANTARCTIC A
correct because, i n both cases , its NNW projection extends int o a sinistral Pan-Africa n shea r zon e o n the western edg e o f the ENE-WSW-trending Luri o Belt o f norther n Mozambiqu e (Pinn a e t al. 1993 ) (Fig. 5). I n ligh t o f thi s match , thi s shea r i s regarded a s providin g a lin k betwee n th e Pan African event s o f southeas t Afric a wit h thos e i n WDML.
Tectonic consequence s fo r southeas t Africa While th e Pan-Africa n event s resulte d i n th e assembly o f Gondwana , ther e i s uncertaint y with regards t o th e exac t timin g o f collisio n i n eas t Africa an d environ s because, a s yet, no unequivocal sutur e ha s bee n identifie d (Shackleto n 1996) . Taken regionally , ther e appea r t o b e tw o distinc t Pan-African phases . Investigation s i n Kenya , Tanzania, an d Madagasca r revea l a n earl y collisio n event, between 65 0 Ma and 750 Ma (Mosley 1993 ; Muhongo 1997 ; Nedele c e t al . 2000) , althoug h some author s argu e fo r collisio n i n Madagascar a t 580-560 Ma (Markl et al 2000) . This earl y phas e involved islan d ar c accretion , ophiolit e obduction , and the development o f the extensive N-S-trending fold axe s an d thrust zones i n the region o f the Eas t African Oroge n (Ster n 1994 ; Shackleto n 1996) . I t is considere d t o b e th e resul t of th e closur e o f th e Mozambique Ocean an d suturing of East and West Gondwana prior t o 60 0 Ma (e.g . Shackleto n 1996 ; Nedelec e t al . 2000) . There i s als o th e secon d -55 0 M a even t (e.g . Shiraishi e t al. 1994) , whic h is mor e enigmatic . It involves NW-S E transpressio n whic h resulte d i n the developmen t o f extensiv e sinistra l strike-sli p faults an d shea r zone s in eas t Africa (e.g . Pinn a et al 1993) , Madagascar (e.g . Kriegsman 1995 ; Ned elec et al 2000) , India (e.g. Drury et al 1984) , and central DML (e.g. Jacobs et al 1998) . The WDML shear i s par t o f thi s secon d phas e o f Pan-Africa n deformation. In either the 'loose ' o r 'uptight' fits, the WDML shear link s u p wit h NNW-trendin g sinistra l shear s on the west side of the Lurio Belt in northern Moz ambique. Furthe r northwest , th e tren d o f thes e shear zones progressivel y chang e to an ENE trend, whereupon th e structure s develo p int o thrust s o f the Luri o Bel t (Pinn a e t a l 1993 ) (Fig . 5). Th e eastern sid e o f the Lurio Belt is once agai n marke d by th e developmen t o f extensiv e NNW-orientate d strike-slip shear s (Reeve s & d e Wi t 2001) . Whe n the WDM L dat a ar e included , i t i s apparen t tha t the geometry is that of a restraining bend of a sinistral strike-sli p system . A block , consistin g o f th e Maudheim Provinc e eas t o f the WDM L shea r an d the souther n par t o f th e Luri o Belt , mus t hav e moved NNW with respect to the Grunehogna Prov-
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ince and regions wes t of the Lurio Belt. This bloc k must hav e bee n uplifte d i n thi s even t a s indicate d by th e 490 Ma Natal Grou p expose d i n KwaZulu Natal, Sout h Afric a (Thoma s e t a l 1992) . Thi s group consist s o f immatur e continenta l sediment s transported sout h an d southwestward s b y fluvia l systems derive d fro m rapi d erosio n o f a n elevate d region. Th e locatio n o f thi s sourc e regio n corre sponds t o position o f th e bloc k propose d here . There are a number of reasons why such a strikeslip syste m shoul d develo p durin g thi s time , bu t the one that seems most attractive given the overall tectonics o f th e Pan-Africa n i s escap e tectonic s i n a compressiona l regim e whic h woul d hav e bot h moved an d elevate d th e escapin g block . Th e firs t phase o f Pan-Africa n geologica l histor y o f sou theast Africa an d environs up to this time basically involved th e earl y stage s o f th e growt h o f Gondwana s o b y thi s tim e thi s regio n wa s intra continental. Aroun d th e margin s o f thi s growin g supercontinent, th e Pan-Africa n event s continue d as dominantl y subduction-relate d event s wit h ter rane accretion, suc h as the Ross orogen y alon g th e palaeo-Pacific margi n t o th e sout h (Gun n & War ren 1962 ; Bor g & DePaol o 1991 ; Stum p 1995 ; Encarnacion & Gruno w 1996) , an d a s collisiona l events t o th e wes t (Grees e 1995 ) an d eas t (Fitzsimons 2000 ; Boge r e t a l 2001) . A Hima layan-type escape tectoni c event (Tapponnier et al 1986) migh t have been cause d by the latter . However, i t require s a suitabl e indento r an d currently there is no obvious candidate, although it might be hidden under the Antarctic ice cover. Alternatively escape tectonic s migh t have been b e due the complex framewor k o f th e assemblin g supercontinent being subjecte d t o changin g stres s regime s tha t necessitated som e interna l reorganizatio n du e t o events suc h as the additiona l amalgamatio n o f terranes an d th e gravitationa l collaps e o f th e firs t phase Pan-African belts (d e Wit e t al 2001) . Whichever wa s th e case , th e bloc k movemen t proposed her e would explain the Pan-African 'dou ble event ' proble m i n southeas t Afric a an d environs. Th e olde r collisiona l event s wer e reworked b y a n extensiv e networ k o f late r shear s related t o escape tectonics . Rathe r tha n th e escap ing block bein g a single entity , i t may rather hav e been a collage of terranes separated b y Pan-Africa n shears. Interna l differentia l movemen t withi n th e block woul d hav e cause d comple x movement s o f the shear s t o the northwest of this block bot h wit h respect t o timin g an d direction . Thi s coul d be th e explanation fo r th e lac k o f agreemen t concernin g the event s of tha t region . Financial an d logistica l suppor t provide d b y th e Depart ment o f Environmenta l Affairs an d th e suppor t provide d by the airforce personnel of 22 Squadron, SAAF, is grate-
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fully acknowledged . Th e assistanc e i n th e field provided by M. Croaker i s greatly appreciate d a s were the reviews by M . Curti s an d I. Fitzsimons .
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Intraplate strike-sli p tectonics a s an alternative to mantle plum e activit y for th e Cenozoi c rif t magmatism i n the Ross Sea region, Antarctica S. ROCCHI 1, F. STORTI 2, G . DI VINCENZO 3, F. ROSSETT P l
Dipartimento di Scienze della Terra, Universita di Pisa, Via S. Maria 53-56126 Pisa, Italy (e-mail:
[email protected]) 2 Dipartimento di Scienze Geologiche, Universita Roma Tre, Largo S. L. Murialdo 1-00146 Roma, Italy 3 Istituto di Geoscienze e Georisorse, CNR, Via Moruzzi 1 - 56124, Pisa, Italy Abstract: Th e Wes t Antarcti c Rif t Syste m is on e o f th e larges t area s o f crusta l extensio n i n the world . Curren t interpretations on it s drivin g mechanisms mostly rely o n th e occurrenc e of one o r mor e mantl e plumes , activ e durin g th e Cenozoi c o r th e Mesozoic . Recen t studie s o f structural-chronological relationship s betwee n emplacemen t o f plutons , dyke swarms , an d volcanic edifice s since middl e Eocen e in norther n Victori a Lan d impl y tha t magm a emplacemen t is guide d b y strike-sli p faul t system s tha t dissec t th e wester n rif t shoulde r i n Victori a Land . These studie s led t o a critical re-examination o f th e argument s used t o suppor t plume models. In Victori a Land , th e linea r geometr y o f th e uplif t an d th e relativ e chronolog y o f uplif t an d extension ar e inconsistent with the traditional concepts of lithospheric evolution above a mantle plume. The geochemical signature of the mafic rock s is equivocal, because both OIB and HIMU features canno t b e exclusivel y interprete d i n terms o f plum e activity . Fro m a thermal poin t of view, magma production rate s ar e low compared wit h the core part of plume-related provinces . Additionally, th e ho t mantl e belo w th e West Antarcti c Rif t Syste m i s no t documente d a s dee p as expecte d fo r mantl e plume s an d th e shap e o f therma l anomal y i s relate d t o lithospheri c geometry, being linear rather than having circular symmetry. The lack of'any decisiv e evidence for plum e activit y i s contraste d b y evidenc e tha t large-scal e tectoni c feature s guid e magm a emplacement: th e Cenozoi c faul t system s reactivate d inherite d Palaeozoi c tectoni c disconti nuities an d thei r activit y i s dynamicall y linke d t o th e Souther n Ocea n Fractur e Zones . A s a n alternative t o both active , plume-drive n rifting and passive rifting , we propose tha t lithospheri c strike-slip deformatio n coul d hav e promote d transtension-relate d decompressio n meltin g o f a subplate mantle already decompressed an d veined during the late Cretaceou s amagmatic extensional rif t phase . Magma ascent and emplacement occurred along the main strike-sli p faul t systems an d alon g the transtensiona l faul t array s departing from th e maste r faults .
Thinning o f th e lithospher e an d riftin g hav e lon g marke d b y a topographi c trough 75 0 t o 100 0 k m been considere d i n term s o f th e end-member s wid e and 3000 km long (LeMasurier & Rex 1991) , model o f active versu s passive rifting: namely, the runnin g fro m th e bas e o f th e Antarcti c Peninsul a forces leadin g t o rif t formatio n ma y b e relate d t o i n th e Weddel l Sea , t o th e Ros s Se a Embayment hot mantle upwelling from significan t depth or may norther n Victori a Lan d (Fig . 1) . Both flank s o f th e be drive n b y plat e dynamic s processes . Recen t rif t syste m hav e bee n affecte d b y lat e Oligocene multidisciplinary studie s carrie d ou t o n th e Wes t Miocen e t o Recen t volcani c activit y Antarctic Rif t Syste m (WARS ) she d ligh t o n th e (LeMasurie r & Thomso n 1990) . Investigation s o f relative role of different processe s on rift evolution, th e volcanis m i n Mari e Byr d Land le d t o th e pro Trie WAR S i s on e o f th e larges t area s o f crusta l posa l o f a genetic lin k betwee n th e WAR S an d an stretching in the world, being simila r i n siz e to the activ e plume centred below Marie Byrd Land. The East Africa n Rif t Syste m an d t o th e Basi n an d evidenc e cite d i n favou r o f thi s hypothesi s Range extensiona l provinc e o f th e wester n US A includes : (1 ) th e geochemica l similarit y betwee n (e.g. Tessensoh n & Worne r 1991) . Th e WAR S i s th e basalts from WAR S and basalts associated with From: STORTI , F. , HOLDSWORTH , R . E . & SALVINI , F . (eds ) Intraplate Strike-Slip Deformation Belts. Geologica l Society, London , Specia l Publications , 210 , 145-158 , 0305-8719/037 $ 15 © Th e Geological Societ y o f London 2003 .
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S. ROCCHI£rAL. with the main characteristics expected for a mantle plume dominate d tectonomagmatic scenario . Such a critica l compariso n le d u s t o propos e a n alternative t o bot h plume-relate d an d passive rift ing scenarios , wit h th e genesi s an d emplacemen t of Cenozoi c magma s triggere d b y deep-reachin g intraplate strike-sli p t o transtensiona l tectoni c dis continuities.
The West Antarctic Rift System : an overview
Fig. 1 . Locatio n ma p of the WARS. The box represent s the are a enlarge d i n Fig . 2, i.e . th e northeaster n portio n of th e Ros s Embayment . Abbreviations : BI : Ballen y Islands; PI : Pete r I 0y ; AI : Alexande r Island ; SN : Sea l Nunatak; JRI: Jame s Ross Island.
long-lived hot-spo t track s (Hol e & LeMasurie r 1994); (2 ) th e presenc e i n Mari e Byr d Lan d o f horst-graben sub-ic e topography producing a large uplifted dom e (LeMasurier & Landis 1996) ; (3) the modest Cenozoi c extensio n i n th e WARS , insuf ficient to generate the observed amoun t of magmatism; (4) the lack of significant plate tectonic events coeval to rifting and volcanism in West Antarctica (Hole & LeMasurier 1994) ; (5 ) the high heat flow in the Ross Sea area (Blackman et al. 1987; Storey et al . 1999) . Thi s mode l ha s bee n progressivel y extended to different par t of the rift system, mainly based o n th e geochemica l feature s o f magmas , leading t o th e hypothesi s o f riftin g linke d t o tw o plumes activ e belo w Mari e Byr d Lan d an d M t Erebus, respectivel y (Store y e t al. 1999) . Recent geological-geophysica l investigation s i n the Ross Sea region (namel y Victoria Land and the Ross Sea ) highlighte d a comple x Cenozoi c geo dynamic scenario , dominate d b y intraplat e right lateral strike-sli p tectonic s whic h induce d a significant oblique component in the rifting process (Salvini et al. 1997) . This, coupled with the spatial, structural, and chronological distribution of plutons and dyk e swarm s recentl y foun d o n th e wester n Ross Se a shoulder (e.g . Tonarini et al. 1997 ; Rossetti et al. 2000; Rocchi et al. 2002) casts doubts on the plume scenario and may support a transtensionrelated sourc e fo r th e Cenozoi c magmatis m in th e Ross Se a region. In thi s pape r w e revie w th e majo r structural , magmatic, an d chronologica l feature s o f th e WARS i n th e Ros s Sea region an d compar e them
The WARS is geometrically asymmetric . The eastern flan k i n Mari e Byr d Land i s characterize d b y a basin-and-range topography, with about 3 km of uplift i n the central part (LeMasurier & Rex 1989) . The opposit e flan k i n norther n Victori a Lan d (NVL) i s constitute d by th e Transantarcti c Mountains, whic h include s th e exhume d root s o f th e early Palaeozoi c Ros s orogeni c bel t (Stum p 199 5 and reference s therein ) wit h NW-S E t o NNW SSE-striking crusta l discontinuitie s (Gibso n & Wright 1985 ; Salvin i e t al. 1997 ; Finn et al. 1999 ) that wer e reactivate d durin g th e Cenozoic . Th e Ross Oroge n wa s erode d t o produc e a flat-lyin g erosional surfac e Devonia n t o Triassi c i n age , known a s th e Kukr i peneplain . Riftin g affecte d NVL an d Eas t Antarctic a in th e middl e Jurassic, causing the generation of the Ferrar Large Igneous Province (Store y & Alabaste r 1991 ; Ellio t 1999) . During th e lat e Cretaceous , th e Antarcti c plat e reached th e souther n pola r positio n an d ha s no t moved appreciabl y sinc e then . At that time, wide spread denudatio n occurre d i n NV L (Stum p & Fitzgerald 1992 ; Balestrier i e t al . 1994 ; Fitzger ald & Stum p 1997 ) an d the Transantarcti c Mountains starte d t o rise , relate d t o a majo r phas e o f amagmatic riftin g i n th e Ros s Embaymen t (e.g . Tessensohn & Worner 1991) . Crusta l thinning led to the formation of four main N-S elongate d basins in th e Ros s Se a (Victori a Lan d Basin , Norther n Basin, Centra l Trough , an d Easter n Basin ) separ ated b y basement highs (Coope r e t aL 1991) . The middle Eocene was characterized by a major change in the geodynami c scenari o o f the WARS, marked b y th e inceptio n o f intraplat e right-latera l strike-slip faultin g tha t cause d th e chang e fro m orthogonal t o obliqu e riftin g alon g th e wester n shoulder (Salvin i e t al . 1997) . A majo r phas e o f renewed uplif t an d denudation affecte d th e Trans antarctic Mountain s (Fitzgeral d & Stum p 1997 ) and deepl y source d magmati c activit y starte d i n Victoria Lan d an d wester n Ros s Se a (Tonarin i e t al. 1997) , continuin g unti l th e Recen t o n bot h flanks o f th e rift .
STRIKE-SLIP TECTONIC S VS . MANTLE PLUM E
Magnitude of extension in the WARS The amoun t o f extensio n i n th e WAR S i s uncer tain. The maximum lat e Cretaceou s t o Recent dis placement betwee n Eas t an d Wes t Antarctic a ha s been estimate d b y Fitzgerald e t al. (1986) as 255 350km, whil e Tre y e t al . (1999 ) propose d 480 500km, an d DiVener e e t al . (1994 ) suggeste d about 1000km . Fo r th e Cenozoi c magnitud e o f extension, a ver y lo w valu e (<5 0 km) i s propose d by Lawver & Gahagan (1994), while a higher value is regarde d a s possibl e b y Kam p & Fitzgeral d (1987). Cand e e t al. (2000) suggeste d tha t 180k m of separatio n i n th e Wester n Ros s Embaymen t occurred i n Eocene-Oligocene times .
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Cenozoic tectonics of the Ros s Se a region: a revie w Recent geological-geophysica l dat a o n Cenozoi c faulting i n the Ross Se a region strongl y impact on the previous interpretations tha t assign onl y a negligible rol e o f Eocen e t o Recen t deformation s i n the structura l architectur e o f th e whol e are a (Salvini et al. 1997 ; Salvini & Storti 1999 ; Storti et al. 2001 ; Rossett i e t al. 2002). Th e new Cenozoi c tectonic fabri c identified from th e multidisciplinar y integration o f offshore and onshore dat a has a complex 3 D distributio n o f faul t geometr y an d kin ematics (Fig . 2). Well-develope d NW-SE-strikin g
Fig. 2 . Tectoni c ma p o f Cenozoic faul t pattern s in the Ros s Se a region (Victori a Land an d Ross Sea) , redraw n afte r Salvini et al. (1997 ) an d Salvini & Storti (1999) . Th e area reported i n Fig. 3 is the central-northern portion o f coasta l Victoria Land .
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S. ROCCHI £TAL.
Geology and geochemistry of igneous rocks
time span of almost 50 Ma, from middl e Eocen e to the present. The plutons crop out in a 200-km-long section o f th e wester n rif t shoulder , o n th e Ros s Sea coas t o f NV L betwee n Campbel l Faul t an d Leap Yea r Fault , an d ar e usuall y associate d wit h strong positiv e magneti c anomalie s (Miille r e t al . 1991). Th e larges t intrusion s cove r individuall y about 70-8 0 km2 (Fig . 3). Al l th e intrusion s hav e isotropic interna l fabric , and the overall ma p shape in some case s is weakly elongated trendin g around N140E. Thes e intrusiv e masse s ar e mad e u p o f gabbroic an d syenitic portions, sometime s interlay ered, sometime s mingle d togethe r (Rocch i e t al . 2001). Gabbro s ar e mainl y ol-hy normativ e an d mildly alkaline , whil e syenite s ar e generall y Qnormative, sometimes peralkaline. The dykes occur over a 40 0 X 50 km are a a s widesprea d swarm s cutting either the Palaeozoic basement or the Cenozoic igneous complexes. Most hypabyssal rocks are basic (basanites , tephrites , alkali-basalts ) t o inter mediate i n composition , partl y bridgin g th e SiO 2 gap foun d fo r th e intrusiv e rock s (Fig . 4a). Th e dyke associatio n i s alkalin e t o strongl y alkaline , and is generally ne-normative. Among the volcanic features, large active volcanoes (e.g. Mt Erebus and Mt Melbourne) , smal l activ e edifice s (M t Rittmann), an d rathe r youn g monogeneti c scori a cones coexist wit h older volcanoes, eithe r wel l preserved (e.g. Mt Overlord, M t Discovery), o r largely dissected (M t Morning an d som e larg e edifice s i n the Daniel l an d Adare peninsulas) . The overal l compositiona l spectru m o f th e Meander Intrusiv e Grou p overlaps tha t of the geo graphically overlappin g Melbourn e Volcani c Prov ince, tha t howeve r als o contain s scarc e phonolite s (Fig. 4a). Th e overal l distributio n o f incompatibl e elements i s dominated by high ratios of Nb and Ta relative to LILE and Y-HREE (Fig. 4b) , a s typical of oceanic island basalts (OIB; Sun & McDonough 1989). Th e ubiquitou s prominent K an d P b nega tive anomalie s ar e noticeable . Intrusiv e rocks , dykes, an d younger volcanic rocks display comparable trac e elemen t distribution , includin g marked K an d Pb trough s (Fig. 4b). Th e 87 Sr/86Sr(t) ratio s for fresh , crustall y uncontaminate d sample s fro m plutons an d dyke s var y fro m 0.7029 9 to "0.70372 and 143 Nd/144Nd(t) i s betwee n 0.51283 9 an d 0.512941. Sr-N d isotop e composition s o f youn g lavas range acros s th e same interval, althoug h data for dyke s and plutons cluster towards slightly mor e enriched composition s wit h respec t t o younge r lavas (Fig . 5; Rocchi e t al. 2002) .
Plutons an d dyke swarms of the Meander Intrusiv e Group (Tonarini et al. 1997) , alon g with volcanoe s of th e McMurd o Volcani c Grou p (LeMasurie r 1990), ar e exposed in the coastal region of Victoria Land, i.e . th e wester n rif t shoulder , an d cove r a
Chronological dat a are available fo r igneous rocks from NV L nort h o f th e Priestle y Faul t (Fig . 6; (Mttller e t a l 1991 ; Tonarin i e t a l 1997 ; Arrni -
right-lateral strike-sli p faul t system s occur in NVL and i n th e norther n secto r o f th e Ros s Sea . The y include th e strike-sli p brittl e reactivatio n o f tw o major terran e boundar y faults , i.e . th e Lanterma n Fault an d the Leap Yea r Fault. Detaile d field work along th e Lanterma n Faul t (Rossett i e t al. 2002 ) and th e Priestle y Faul t (Stort i e t al . 2001 ) docu mented ver y comple x interna l structura l architec tures, whic h ar e typical o f intraplate faul t systems , characterized b y strike-sli p to transpressional con ditions alon g the master faul t segments . Approach ing thei r southeaster n terminations , th e NW-S E strike-slip faul t system s ar e abutte d b y th e N-S striking basin boundar y faul t system s of th e west ern Ros s Se a (Fig . 2). Interpretatio n o f reflectio n seismic profile s acros s thes e basin s basicall y showed a two-stage evolution: (1) early extensional deformations (Coope r e t al . 1991 ) overprinte d b y (2) transtensiona l deformation s with local positiv e inversion phenomen a (Salvin i e t al . 1997 , 1998) . Transtensional conditions have also been describe d along the western shoulder of the Ross Sea (Wilson 1995; Rossetti e t al. 2000). I n this frame, th e hori zontal slip component along the N-S-striking faults in th e wester n Ros s Se a i s relate d t o th e transfe r of th e residua l right-latera l strike-sli p shea r fro m the NW-SE-strikin g faul t system s int o th e basin s (Salvini e t al 1997 ; Stort i e t al. 2001). The availabilit y o f a good offshor e stratigraphi c record i n th e seismi c profile s provide s ag e con straints o n th e ag e o f faultin g i n th e Ros s Se a region (Coope r e t al. 1991 ; Brancolini e t al. 1997) . Early extensional deformation s in the basins, which were preserve d b y younge r reactivation , ar e sys tematically suture d b y th e RS-U6 unconformity , a major break-u p unconformit y i n th e Ros s Sea . Transtensionally reactivate d forme r extensiona l faults an d th e offshor e portio n o f th e NW-S E right-lateral strike-sli p faults cu t across RS-U6 and, in many cases, have a bathymetric expressio n suggesting ver y recen t activity . Th e passag e fro m extensional t o transtensiona l regim e i n WAR S evolution is chronologically documented by dating the RS-U6 unconformity. This ag e is stil l a matter of debat e an d ha s bee n tentativel y constraine d a t about 3 0 or 4 2 Ma by Busett i (1994) .
Cenozoic magmatism in the Ross Sea region: a review
Geochronology of igneous rocks
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Fig. 3 . Satellit e imag e o f central-norther n portio n o f coasta l Victori a Land , reporting : (1 ) outcro p contour s o f th e Cenozoic plutons , (2 ) locatio n o f th e mai n dyk e swarms (arro w points) , an d (3 ) rose diagram s fo r th e dyk e swarms . Base imag e i s a Landsa t satellit e imag e mosai c i n Lamber t conforma l coni c projection , courtes y o f Lucchitta e t al. (1987). The numbe r o f measure d dyke s i s 220 .
enti & Baroni 1999 ; Rocch i e t al. 2002). Th e gabbro-syenite pluton s hav e age s betwee n 4 8 an d 23 Ma, an d th e dyke s cove r th e tim e spa n 47 35 Ma, representin g th e earlies t recor d o f regionally extensiv e Cenozoic rift-relate d igneous activity in Antarctica. Th e dykes cutting the Cenozoic pluton s ar e generall y a fe w millio n year s younger than the host intrusion , and no significan t age difference ca n be detected between these dykes
and those cutting the Palaeozoic basement . The age ranges fo r NNW-SS E dyke s an d th e N- S dyke s (see below ) are indistinguishabl e within analytical errors. I n th e are a betwee n Campbel l Faul t an d Aviator Fault , th e igneou s activit y (pluton s an d dyke swarms ) is restricted t o the time interval 48 35 Ma, wherea s betwee n Aviato r Faul t an d Lea p Year Faul t th e magmatis m range s fro m 3 1 t o 18 Ma i n age .
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Fig. 4. (a ) TA S diagram s (L e Ba s e t a l 1986 ) fo r th e studied plutonic-subvolcani c association . Volcani c pro ducts from the Melbourne and Hallett Volcanic Province s (LeMasurier & Thomson 1990 ; Armienti et al 1991 ) an d the Erebu s Volcani c Provinc e (LeMasurie r & Thomso n 1990; Kyl e et al 1992 ) ar e reported fo r comparison, (b ) Primitive mantl e normalized (McDonoug h & Su n 1995 ) multielement spidergram s of average s o f volcani c provinces linke d t o th e WARS . Shade d fiel d represent s th e variability of the middle-late Eocen e dykes from northern Victoria Land . Averages ar e calculated fro m mafi c rock s with MgO>5wt% . Source s o f data : MIG-NVL : mafi c dykes fro m Rocch i e t a l (2002) ; MMVG-NVL : lavas from th e Melbourn e an d Hallet t Volcani c Province s (Worner et al. 1989 ; Rocholl et al 1995) ; MMVG-EVP: Erebus Volcani c Provinc e (Kyl e e t a l 1992) ; MBL : Marie Byrd Land (Hol e & LeMasurier 1994 ; Har t et a l 1997); Pete r I 0y (Prestvi k et al 1990 ; Har t et al 1995) ; Balleny Island s (Green 1992) .
Relations between faulting an d dyk e injection The orientatio n o f dyke s alon g th e wester n shoulder o f the Ross Se a is almos t bimodal i n th e area north of the Reeves Glacier and is unimodal in the southern secto r (Fig . 3) . In the northern sector, dykes strike NW-SE and almost N-S , i.e . paralle l to th e majo r right-latera l strike-sli p faul t system s and t o th e basi n boundar y faults , respectively . A t Terra Nova Bay Station, dyke arrangement in left stepping e n echelo n tensio n gas h array s wit h a
Fig. 5 . Multipl e plot summarizing the isotopic variations of mafi c product s acros s th e WARS, the adjoining , contemporaneously activ e volcani c provinces , an d the main OIB reservoirs . The arro w in th e middl e diagra m points to die high U3 Nd/1IulNd ratio for DMM-A: Soufg§ 8f ^SIS! MIG-NVL: mafi c dyke s fro m Rocch i ct a l 2002 ; MMVG-NVL: lavas from the Melbourne and Hallett Volcanic Provinces (Worner et al 1989 ; Rocholl et al 1995) ; MMVG-EVP: Erebu s Volcani c Provinc e (Kyl e e t a l 1992); MBL : Mari e Byr d Lan d (Hol e & LeMasurie r 1994; Hart et al 1997) ; Peter I 0y (Prestvi k et al 1990 ; Hart e t a l 1995) ; Ballen y Island s (Har t 1988 ; Gree n 1992); JRIVG : Jame s Ros s Islan d Volcani c Grou p (Antarctic Peninsula ; Hol e e t a l 1995 ; Lawve r e t a l 1995); SNVG : Seal Nunataks Volcanic Group (Antarcti c Peninsula; Hol e 1990 ; Hol e et al 1993) ; BSVG: Bellingshausen Se a (i.e . Alexande r Island ) Volcani c Grou p (Antarctic Peninsula ; Hol e 1988 ; Hol e e t a l 1993) ;
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Fig. 7 . Outcrop-scal e relation s betwee n right-latera l strike-slip faultin g an d dyk e injection , (a ) Left-steppin g en echelo n tensio n gas h array s of basalti c dyke s a t Terr a Nova Ba y Statio n afte r Stort i e t al . (2001) . (b ) Cartoo n showing th e tension gash geometr y o f dykes a t Starr Nunatak (afte r Rossetti e t al. 2000). For locations, se e Fig. 3.
genetic relationship s betwee n tectoni c an d mag matic activitie s constrain the ag e o f onshore fault ing, which had t o be activ e since Eocene times , at least nort h of Priestle y Faul t (Fig . 6).
Fig. 6 . Ag e dat a o f igneou s activit y i n Victori a Lan d north o f Campbel l Fault . 40 Ar-39Ar age s afte r Rocch i e t al. (2002) ; Rb-S r age s afte r Tonarin i e t al. (1997) , K-A r ages after Mtiller et al. (1991). Acquisition o f geochronological dat a fo r dyke s fro m sout h o f Campbel l Faul t i s in progress .
NW-SE envelop e tren d (Fig . 7a), support s thei r emplacement i n a NW-S E right-latera l strike-sli p shear zon e whic h constitute s a spla y faul t o f th e Priestley Faul t (Stort i e t al. 2001). I n the southern sector, dykes are arranged in left-stepping arrays at an angl e o f abou t 30 ° t o th e N- S transtensiona l master fault s an d in places sho w tension-gash-lik e relationships wit h these faults (Fig . 7b), indicatin g syntectonic dyk e emplacement i n a dextral regim e (Rossetti e t al. 2000). This field evidence indicates that dyke emplacement an d geometr y wer e initiate d an d drive n b y the ongoin g tectonics. In th e norther n sector , bot h the NW-S E right-latera l strike-sli p faul t system s and th e basi n boundar y faults alon g th e Ros s Se a shoulder induce d magm a emplacement . I n th e southern sector , th e emplacemen t o f dyke s wa s linked t o th e activit y o f transtensiona l faults . Th e PAVF: Pal i Aik e Volcani c Fiel d (souther n Patagonia ; D'Orazio et al. 2000). BSE (Bulk Silicat e Earth), DMM A (Deplete d MOR E Mantle-typ e A) , an d OIB-HIM U (Ocean Islan d Basalt s wit h hig h ^U/^P b ratio ; Zindler & Hart 1986) .
Discussion: a role fo r a mantle plume on WARS development ? Evidence from the regional tectonic framework Typical features o f a mantle plume dominated tec tonic scenario are the development of a low-amplitude, broad-wavelengt h uplifte d regio n wit h a roughly circula r symmetr y an d a n almos t radia l pattern o f extensiona l faul t system s (Olse n 1995) , particularly for the Antarctic plate, whic h has been almost stationar y sinc e Cretaceou s time . Th e present-day tectoni c an d morphologica l architec ture o f Mari e Byr d Lan d ha s bee n interprete d a s fitting these features (LeMasurier & Landis 1996) . Conversely, Victori a Lan d i s characterize d b y a n elevated linea r rif t shoulde r (th e Transantarcti c Mountains) developed by N-S extensiona l to transtensional faultin g an d transvers e faultin g (e.g . Cooper e t al . 1991 ; Behrend t et al . 1996 ; Wilso n 1999; Rossetti et al 2000 ) that, in the northern sector, abu t NW-SE-strikin g intraplat e right-latera l fault system s (Salvini et al. 1997 ) with no evidence for eithe r domin g or radial structures. The relativ e chronolog y o f uplif t an d extension also counter s th e traditiona l concept s o f litho spheric evolutio n above a mantle plume. The main extension episod e occurre d i n th e lat e Cretaceou s (e.g. Lawve r & Gahaga n 1994) , whil e th e mai n uplift episod e occurre d durin g th e Eocen e (Stump & Fitzgeral d 1992 ; Fitzgeral d & Stump 1997). A thermal source for the uplift o f the Transantarctic Mountain s has bee n suggeste d (Smit h &
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Drewry 1984 ; Berg et al 1989 ; Ster n & Ten Brink 1989), bu t alternativ e mechanism s have been pro posed suc h a s isostati c uplif t o f th e hangin g wal l of a major faul t cutting the lithosphere (Ster n et aL 1992) o r uplift linke d t o a shallow-dipping detach ment faul t i n a n asymmetri c passiv e rif t settin g (Fitzgerald e t al . 1986 ; Fitzgeral d & Baldwi n 1997). Finally , th e activit y o f a plum e carryin g higher-than-normal mantle temperatures is difficul t to reconcile with the prolonged subsidenc e o f Ross Sea basins durin g Cretaceou s an d Cenozoic time .
Evidence from geochemical data The pluton s an d dyk e swarm s o f NV L (Meande r Intrusive Group ) displa y th e typica l OI B geo chemical features . The geochemical characteristic s of th e whol e igneou s associatio n sho w restricte d variations ove r 5 0 myr an d acros s th e whol e are a (Rocchi e t al . 2002) . Additionally , a clos e simi larity i s observe d wit h the neighbourin g Cenozoi c magmatic provinces o f the Antarctic plate an d with magmatic province s fro m oceani c an d continenta l setting classicall y definin g th e HIM U affinity , i.e . characterized b y ^Pb/^P b i n exces s o f 20. 5 (Fig. 5; Hofman n 1997) . Thes e geochemica l fea tures represent on e of the classical argument s used to infe r th e activit y o f a dee p mantl e plum e fo r many volcanic provinces in both oceanic an d continental settings , base d o n th e ambiguou s relatio n between OIB-HIM U chemistr y ( a chemica l reservoir) an d mantle plume (a physical entity). On this ground, the wide diffusion o f Cenozoic volcanism wit h simila r geochemica l affinit y throughou t Antarctica, Souther n Ocea n islands , Tasmania , New Zealand, an d Campbell Plateau led Hart et al. (1997) t o hypothesiz e th e origi n o f magma s fro m a fossi l plum e hea d source , whic h impacte d th e Gondwana lithosphere befor e break-up in this area, i.e. befor e th e lat e Cretaceous . Th e momen t o f plume impingemen t i s controversia l an d coul d b e related t o eithe r th e middl e Jurassi c emplacemen t of th e Ferra r Larg e Igneou s Province (LIP ) o r th e mid-late Cretaceou s break-u p o f Ne w Zealan d from Wes t Antarctic a (Weave r e t al. 1994) . However, th e occurrenc e o f Ferra r basalt s exclusivel y along th e Transantarcti c Mountain s couple d wit h their absenc e i n Mari e Byr d Lan d an d th e othe r sites o f Cenozoi c OI B magmatis m i s evidenc e against a role for widespread sublithospheri c mas s contribution t o the sourc e of Cenozoic magmatis m by a Jurassi c plume . O n th e othe r hand , th e ide a of a late Cretaceous plume activity in West Antarctica is overruled by the lack of Cretaceous magmatism acros s th e whol e Victoria Lan d couple d with evidence fo r subsidenc e instea d o f buoyan t uplif t (LeMasurier & Landi s 1996) . Actually , th e geo chemically grounded clai m for a fossil plume hea d
source i s se t u p t o satisf y th e nee d fo r a shallow , weak, enriched layer, common to wide areas below the Souther n Ocea n an d th e adjoinin g continents . One o f th e mos t use d isotopi c issu e t o clai m fo r mantle plumes is the high 206pb/204Pb ratio, thought to b e derive d fro m dee p mantl e plume s tha t entrained sla b materia l recycle d int o th e dee p mantle ove r a lon g tim e perio d (10 9 years) . However, Halliday et al. (1995) showed that such a high 206 Pb/204Pb ratio can be also attained by the magma source a t rathe r shallo w depth , i n shorte r tim e interval (108 years, provided the source has a rather high U/P b ratio ) an d propose d a mode l fo r U/P b fractionation clos e t o mid-ocea n ridge s an d late r sampling o f suc h hig h U/P b sourc e b y oceani c islands magmatism . I t i s wort h notin g tha t Ceno zoic mafi c dyke s an d lava s fro m NV L hav e a n average U/Pb ratio of 0.44 ±0.1 1 and 0.66 ± 0.17 . This implie s a hig h U/P b rati o i n th e magm a source, whic h therefor e ha s bee n abl e t o produc e high 206 Pb/204Pb ratios i n a time spa n o f the orde r of 10 8 years . Therefore , w e propos e a model (se e further on ) i n whic h th e sourc e enrichmen t occurred i n th e lat e Cretaceou s som e ten s o f million year s before the magmatism.
Evidence from the thermal and magmatic regional framework Two classical piece s o f evidence for the activit y of mantle plume s ar e th e preservatio n o f hot-spo t tracks an d th e hig h volum e o f magma s produced. In th e WARS , chronological-area l progressio n o f magmatism i s lackin g an d the volum e o f magma s produced i s low . However , thes e fact s canno t b e unequivocally use d t o counte r th e plum e hypoth esis owing to the very low mobility of the Antarctic plate sinc e th e lat e Cretaceou s an d th e peculia r 'stationary' settin g o f th e Antarcti c plate , almos t completely encircle d b y mid-ocean ridge s (Hol e & LeMasurier 1994) . The presenc e o f seismicall y slo w (hot ) mantl e in th e WARS has been imaged fro m surfac e wave tomography (Danes i & Morell i 2000 , 2001) . Th e depth to which hot mantle extends cannot be safely modelled belo w 20 0 km, no t dee p enoug h t o sup port o r discar d th e occurrenc e o f a n active plume . Nevertheless, i t i s wort h notin g th e slo w mantl e does not have a circular symmetry, as expected for a plume : th e grou p velocit y map s o f Rayleig h waves (Danes i & Morell i 2000 ) sho w minimu m values arrange d o n a line corresponding t o the belt of transformatio n of the ridg e betwee n the Antarctic an d Australia n plate s (Fig . 8). Thi s indicate s that shallow hot mantle is related to a linear geodynamic featur e >400 0 km long , suc h a s th e bel t o f Southern Ocea n fractur e zones . Thes e large-scal e tectonic lineament s cros s th e continenta l litho -
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Fig. 8 . Souther n Ocea n Fracture Zones. Redrawn afte r Salvin i e t al. (1997), wit h mantle low-velocit y anomalie s afte r Danesi & Morelli (2000) .
sphere through NV L t o th e Ros s Embaymen t (Salvini e t al . 1997 ) an d coul d b e responsibl e fo r the transtensiona l Cenozoi c riftin g phas e wit h a rise o f mantl e geotherm s t o generat e mel t i n th e mantle an d strike-sli p t o transtensiona l tectoni c activity controllin g th e emplacemen t o f magma s within th e crus t o r a t the surface .
Intraplate strike-slip faulting: an alternative geodynamic scenario for magma genesis and emplacement The equivoca l geochemica l data , coupled wit h the lack o f diagnosti c tectonomagmati c an d geo morphic evidence , d o no t suppor t th e activit y o f a mantl e plum e a s th e drivin g mechanis m fo r th e generation an d emplacemen t o f Cenozoi c magma s in NVL . O n th e othe r hand , spatia l an d tempora l links betwee n th e Cenozoi c strike-sli p tectoni c regime an d th e igneou s activit y suppor t intraplat e right-lateral shea r a s a n effectiv e an d alternativ e geodynamic scenari o fo r magm a emplacement . According t o Salvin i e t al . (1997 ) th e magmati c activity i s focuse d i n a bel t alon g th e wester n shoulder o f th e Ros s Se a owin g to th e Theologica l zoning of the brittle crus t induced by th e eastward shallowing o f the Moho moving from the Transantarctic Mountain s to th e Easter n Basi n i n th e Ros s Sea (Coope r e t al . 1991) : crusta l thicknes s belo w the wester n Ross Se a shoulde r would be appropri ate fo r th e fracturing/permeabilit y condition s
required fo r magm a ascent . Additionally , thi s belt corresponds a t dept h t o a topographi c gradien t a t the base of the lithosphere, that could enhance convection-driven meltin g (Anderso n 1995) . The recen t dat a reviewe d i n thi s pape r o n (1 ) the structura l architecture of som e of the intraplate right-lateral faul t system s an d thei r relation s wit h dyke emplacement , (2 ) th e attitud e an d chemica l composition o f Cenozoic dyke s alon g a significan t segment o f th e wester n shoulde r o f th e Ros s Sea , and (3 ) th e geochronologica l constraint s suppor t the strike-slip-relate d mode l fo r magm a emplace ment. Th e space-tim e distributio n o f pluton s an d dykes i n NV L (Fig . 9), sugges t tha t igneou s activity ha s been activ e in different crustal sector s and/or alon g differen t boundar y faults in differen t times. The boundaries between these sectors are the major right-latera l faul t system s identifie d b y Sal vini e t al . (1997) . I n particular , th e crusta l secto r affected b y Cenozoi c pluto n emplacemen t i s bounded t o th e nort h b y th e Lea p Yea r Faul t an d to th e sout h by th e Campbel l Faul t (Fig . 9a). Th e three adjacen t sector s wit h differen t dyk e geo metries an d frequency ar e bounded, from th e north to th e south , b y th e Lea p Yea r Fault , th e Aviato r Fault, an d th e Priestle y Fault , respectivel y (Fig. 9b); the two adjacent sectors characterized by different timin g o f magm a emplacemen t ar e bounded by the Leap Year Fault, the Aviator Fault, and th e Priestle y Fault , respectivel y (Fig . 9c) . During th e las t 5 0 myr NV L ha s bee n affecte d
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Fig. 9. Space-tim e distributio n o f Cenozoic plutoni c an d subvolcani c igneou s product s i n northern Victori a Land .
by lithospheri c shea r processe s whic h divide d th e area into sectors characterized by different tectono magmatic history . Suc h a compartmentalizatio n may relat e t o th e crusta l fabri c inherite d fro m th e previous tectonic histor y o f the region, namel y the early Palaeozoi c Ros s Orogen y an d th e lat e Cre taceous Ros s Se a initia l opening . Differen t sli p rates alon g th e majo r intraplat e right-latera l faul t systems ma y hav e induce d a tempora l zonin g t o magma genesi s an d emplacement .
A general model for the tectonomagmatic history of the western Ross Embayment The alternativ e contex t propose d fo r magm a gen esis an d emplacemen t o n th e wester n shoulde r of the WAR S lead s t o a mode l fo r th e Mesozoic Cenozoic tectonomagmati c histor y o f th e wester n Ross Embaymen t (Fig . 10) . Durin g th e lat e Cre taceous, a n earl y rif t phas e occurre d wit h orthog onal extensio n tha t stretche d th e crus t an d th e underlying strong lithospheric mantle . Lithospheri c attenuation probabl y le d t o the productio n o f ver y small degre e partia l melts . Thes e wer e no t suf ficient t o giv e wa y t o surfac e magmatis m (amagmatic rift phase), but were essential i n distributing fertile , enriched , low-meltin g poin t veins/domains i n a wide zone of the Antarctic plate mantle. A t th e middl e Eocene , th e increas e o f differential velocit y alon g th e Souther n Ocea n Fracture Zones reactivate d th e Palaeozoi c tectoni c discontinuities in northern Victoria Land as intraplate dextral strike-slip fault systems . The activit y of
these lithospheric deformation belts promoted local decompression meltin g o f th e enriche d mantl e domains created during the late Cretaceous and isotopically mature d sinc e then (Fig . 11) . The magm a rose an d wa s emplace d alon g th e mai n NW-S E discontinuities an d alon g th e N- S transtensiona l faults array s departin g fro m th e maste r NW-S E systems (Fig . 10) . Thi s mode l relate s th e drivin g forces of events such as uplift, activ e faulting, magmatism, and seismicity to the dynamics of the Antarctic plat e rathe r tha n t o deep-sourc e force s suc h as mantl e plumes.
Conclusions The occurrenc e o f Cenozoi c magmatis m i n th e Ross Embayment has long been related to the presence of a mantle plume, associate d with the origi n and developmen t o f the whole West Antarctic Rif t System. Th e plum e hypothesi s wa s propose d o n the basi s o f geochemica l constraint s an d morpho logical evidenc e i n Mari e Byr d Land . Ou r revie w of the tectonomagmatic framewor k along the western shoulde r o f th e Ros s Se a cast s doub t o n th e mantle plume-related sourc e for magma generation and ascen t an d favour s intraplat e right-latera l strike-slip faultin g a s an alternative mechanism for magma genesis and emplacement. In particular, the tight lin k betwee n tectoni c activit y an d magm a emplacement suggest s tha t th e inherite d litho spheric fabri c o f northern Victoria Lan d led t o th e tectonomagmatic compartmentalizatio n o f th e whole lithosphere, wit h the boundaries between the
STRIKE-SLIP TECTONIC S VS . MANTLE PLUM E
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Fig. 10 . Genera l mode l fo r th e Meso-Cenozoi c tectonomagmati c histor y o f th e wester n Ros s Embayment .
sectors playin g a n activ e rol e i n bot h mel t gener ation an d emplacement . We are gratefu l t o P. Armienti, M. D'Orazio, F . Mazzarini, an d F . Salvin i fo r th e stimulatin g discussion s o n th e
geology o f th e Ros s Se a region . Thank s ar e du e t o B . Murphy an d C . Macpherso n fo r th e constructiv e an d sti mulating review that helped u s to improve and clarify th e paper. Th e whol e wor k i s par t o f th e Italia n Antarcti c Research Progra m (PNRA) .
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S. ROCCHIEJAL.
Fig. 11. P- T diagram , modifie d afte r Smit h & Lewi s (1999) . Geotherm s an d adiabati c decompressio n path s afte r McKenzie & Bickle (1988) .
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Phanerozoic strike-sli p faulting i n the continental interio r platfor m o f the Unite d States: example s fro m the Laramid e Orogen , Midcontinent, an d Ancestral Rock y Mountain s S. MARSHAK 1, W. J . NELSON 2 & J. H . McBRIDE 3 1
Department of Geology, University of Illinois, 1301 W Green Street, Urbana, Illinois 61801, USA ^Illinois State Geological Survey, 615 E Peabody Drive, Champaign, Illinois 61820, USA ^Department of Geology, Brigham Young University, P. O. Box 24606, Provo, Utah 84602, USA Abstract: Th e continental interior platform of the United States i s that part of the North Amer ican crato n wher e a thin venee r of Phanerozoic strat a cover s Precambrian crystallin e basement . N- to NE-trending and W- to NW-trending fault zones , formed initially by Proterozoic/Cambrian rifting, brea k th e crus t o f th e platfor m int o rectilinea r blocks . Thes e zone s wer e reactivate d during th e Phanerozoic , mos t notabl y i n th e lat e Palaeozoi c Ancestra l Rockie s even t an d th e Mesozoic-Cenozoic Laramid e orogen y - som e remai n activ e today . Dip-sli p reactivatio n can be readil y recognize d i n cros s sectio n b y offse t stratigraphi c horizon s an d monoclina l fault propagation folds . Strike-sli p displacemen t i s har d t o documen t becaus e o f poo r exposure . Though offset palaeochannels , horizontal slip lineations, and strain at fault bends locally demon strate strike-sli p offset , mos t report s o f strike-sli p movement s fo r interior-platfor m fault s ar e based o n occurrenc e o f map-vie w belt s o f e n echelo n fault s an d anticlines . Eac h bel t overlie s a basement-penetratin g maste r fault , whic h typically splay s upwards into a flowe r structure . In general, bot h strike-sli p an d dip-slip component s o f displacemen t occu r i n th e sam e faul t zone , so some belts o f en echelon structure s occur o n the flank s o f monoclinal folds . Thus, strike-sli p displacement represent s th e latera l componen t o f oblique faul t reactivation ; dip-sli p an d strike slip component s ar e th e sam e orde r o f magnitude (tens of metres t o ten s o f kilometres). Effec tively, faults with strike-slip components of displacement act as transfers accommodating jostling of rectilinea r crusta l blocks . I n thi s context , th e sens e o f sli p o n a n individua l strike-sli p faul t depends o n block geometry , not necessaril y o n the trajectory of regional or l. Strike-slip faultin g in the North American interior differ s markedl y from tha t of southern and central Eurasia , poss ibly because o f a contrast in lithosphere strength . Weak Eurasia strained significantl y during the Alpine-Himalayan collision , forcin g crusta l block s t o underg o significan t latera l escape . Th e strong North American craton straine d relatively littl e durin g collisional-convergent orogeny , so crustal block s underwen t relatively smal l displacements .
Introduction ('basement'
) overlai n b y a relatively thi n (0-7 km thick) venee r o f unmetamorphose d Phanerozoi c The continenta l interio r platfor m o f th e Unite d sedimentar y strat a ('cover') . Locally , Neoprotero States, broadl y defined , consist s o f a zoi c t o Cambria n faile d rifts , o r aulacogens , fille d 2000 X 150 0 km regio n bounde d b y th e Canadia n wit h many kilometres o f sedimentar y an d volcanic Shield o n th e north , th e Appalachia n thrus t fron t rocks , cu t the crust. The interio r platform, together on th e east , th e Ouachit a thrus t front o n th e south , wit h th e Canadia n Shield , compris e th e Nort h and th e Cordillera n thrus t fron t o n th e wes t America n craton , tha t portion o f th e continen t no t (Fig. 1) . I n thi s region , th e crus t consist s o f affecte d b y penetrativ e deformatio n an d regiona l Archean through Mesoproterozoic crystallin e rocks metamorphis m during the past 1 billion years. The From: STORTI , F. , HOLDSWORTH , R . E . & SALVINI , F . (eds ) Intraplate Strike-Slip Deformation Belts. Geologica l Society, London , Specia l Publications , 210, 159-184, 0305-8719/037$ 15 © Th e Geologica l Societ y o f Londo n 2003 .
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STRIKE-SLIP FAULTIN G I N THE US A CRATON
interior platfor m o f Nort h Americ a include s thre e physiographic provinces : (1 ) Th e Midcontinent , a region o f broad, temperate plains in which bedrock exposures ar e generall y poo r t o non-existent ; (2 ) The Colorado Plateau, a semi-arid region now at an average elevatio n o f about 1. 5 km, i n whic h cove r bedrock is well exposed; an d (3) The Rocky Mountains, a semi-ari d regio n o f tal l basement-core d uplifts, separate d fro m on e another by deep basins, in whic h bedrock is locall y wel l exposed . The interio r platfor m o f Nort h Americ a lie s i n the forelan d o f Phanerozoi c convergen t an d colli sional orogens . Thus , i n term s o f location , th e region resemble s th e continenta l interio r o f southern an d centra l Eurasia , a regio n tha t lie s i n th e foreland o f th e Cenozoi c Alpine-Himalaya n orogen. Collisiona l tectonis m along the margins of central/southern Eurasi a generate d significan t (o f the order of tens to perhaps hundreds of kilometres ) strike-slip displacement s o n majo r regional-scal e faults i n orde r t o accommodat e latera l escap e o f crustal block s (e.g . Molna r & Tapponie r 1975 ; Tapponier & Molnar 1976) . I t i s fai r t o ask : Have significant strike-sli p displacements occurre d in the interior platform of the United States in associatio n with Phanerozoi c continental-margi n tectonis m of North America ? In thi s paper , w e presen t a revie w o f evidenc e for Phanerozoi c strike-sli p displacement s o n interior-platform fault s o f th e Unite d States . Afte r briefly reviewin g th e geologica l settin g o f th e interior platform, we addres s th e challenge of how to identif y strike-sli p displacemen t o n faul t zone s of th e region . W e the n revie w cas e studie s o f strike-slip displacements , firs t fo r th e Palaeozoi c (primarily, th e Carboniferous-Permia n Ancestra l Rockies event) , the n th e Mesozoic-Cenozoi c (th e Laramide event), and finally for the Holocene. W e Fig. 1 . (a ) Map of the USA continental interior, illustrating the distributio n of Midcontinent fault-and-fold zones . Modified fro m Marsha k e t al. (2000) . Thi s ma p show s the limit s o f Nort h America's interio r cratoni c platform, the portio n o f th e platfor m tha t ha s develope d int o th e Cenozoic Rocky Mountains, and the portion that has been uplifted t o for m th e Colorad o Plateau . The Midcontinent proper lie s betwee n th e Rock y Mountai n fron t an d th e Appalachian front. Abbreviations : BE = Belt embayment; UT = Uinta trough; WB = Williston basin ; O A = Oklah oma aulacogen; NU = Nemaha uplift; MC R = Midcontinent rift ; O D - Ozark dome ; R R = Reelfoo t rift ; I B = Illinois basin; N M — New Madri d seismi c zone ; L D = LaSalle belt ; N D = Nashvill e dome ; M B = Michiga n basin; CA = Cambridge arch; BG = Bowling Green fault ; M-S = Mojave-Sonor a megashear . Th e darke r shade d area i s th e Rock y Mountain s province , an d th e lighte r shaded are a i s th e Colorad o Plateau , (b ) Locatio n ma p showing the locatio n o f other area s discusse d in the text. The number s refe r t o figur e numbers .
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conclude that , though evidenc e for strike-sli p on a given faul t zon e ca n b e circumstantial , sufficien t data exis t overal l t o demonstrat e tha t strike-sli p deformation doe s occu r i n th e interio r platform of the Unite d States . But , in contras t wit h the strike slip deformation of south/central Eurasia, the magnitude o f thi s strike-sli p deformatio n i s small . Further, the pattern o f faulting i n the United States is very different fro m tha t of south/central Eurasia. We suggest tha t this contrast reflect s differences in lithosphere strength .
Geological backgroun d Structural style of the interior platform As note d above , th e continenta l interio r platfor m of th e Unite d State s ca n b e divide d int o thre e physiographic provinces . Becaus e th e Colorad o Plateau and Rocky Mountains provinces ar e so dramatically affecte d b y th e Cenozoi c uplif t an d deformation, whil e th e Midcontinen t provinc e i s not, th e forme r two ma y als o b e considere d t o b e part o f th e Nort h America n Cordillera . However , we emphasize tha t all three provinces shar e simila r crustal structur e - namely , Precambria n basemen t overlain b y a venee r o f Phanerozoic cove r - an d all have responded t o deformation by the formation of a similar style of faults an d folds, s o we consider all thre e province s i n thi s paper . Specifically , deformation o f th e interio r platfor m cause s dis placement o n basement-penetratin g fault s tha t splay up-dip in cross sectio n forming a fan o f subsidiary faults that resembles a flower structure (e.g . Marshak & Paulsen 1997 ; cf. Lowell 1985 ; Wood cock & Fischer 1986 ; Sylvester 1988) . While som e faults hav e bee n exhume d an d reac h th e contem porary land surface, man y are blind and die out updip in monoclinal folds before reaching the surface. In cross section , normal-sens e offse t remain s a t the level of the basement/cover contact on some faults , even i f reverse-sens e offse t occur s neare r th e ground surfac e (Fig . 2). Thi s configuratio n sug gests tha t thes e fault s initiate d wit h a norma l component o f slip , bu t wer e late r reactivate d wit h a revers e component . Base d o n trend , faul t zone s in Nort h America' s interio r platfor m fal l into tw o sets, on e nort h t o northeast , an d th e othe r wes t to northwest (e.g . Marshak & Paulsen 1997 ; Marsha k et al. 2000; Timmons et al. 2001). Thus, faults divide th e crus t o f th e interio r platfor m int o roughly rectilinear blocks (e.g . Chamberli n 1945) . A variety o f names have been use d for th e styl e of faultin g an d folding characteristic o f the interio r platform. Commonly , suc h structure s ar e calle d 'Laramide-style structures' , becaus e structure s o f this style formed in the Rocky Mountains and Colorado Platea u durin g th e 80-4 0 Ma Laramid e
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Fig. 2. A schemati c cros s sectio n o f the Rough Cree k Grabe n i n wester n Kentuck y an d adjacen t Illinoi s (lin e YY'). Note how the border faults hav e reverse-sense slip near the surface , bu t residual normal sens e sli p at depth. Note the flower structure at shallowe r depth. Source : Nelson & Lumm 1987 , as simplifie d b y Marshak & Paulsen (1997) .
orogeny. However , sinc e th e sam e styl e o f struc tures als o forme d i n th e interio r platfor m durin g pre-Laramide events, and in regions not affected b y the Laramide orogeny , th e ter m ca n be confusing . Alternative adjective s use d fo r thes e structure s include 'thick-skinned' , t o contras t the m wit h 'thin-skinned' structure s tha t li e abov e a shallo w subhorizontal detachment , an d 'basement-cored' , to emphasiz e deformatio n involve s basement penetrating faults . Example s fro m th e Midcontin ent region have also been called 'plains-typ e structures' t o emphasiz e tha t the y occu r i n th e Grea t Plains regio n (Power s 1928) , o r 'Midcontinen t fault-and-fold zones ' (Marsha k & Paulse n 1996 ; Marshak e t al. 2000). Monoclina l fold s forme d i n response t o faultin g i n th e interio r platfor m hav e also been referre d to by a variety of names. Earlier literature refers t o them a s 'force d folds ' o r 'drap e folds', t o emphasiz e tha t the y forme d i n respons e to a pus h fro m below . I n mor e recen t literature , these fold s ar e calle d fault-propagatio n folds , fol lowing th e terminolog y o f Supp e an d Medwedeff (1990), an d the process o f forming these folds can be referre d t o a s 'trishea r fault-propagatio n fold ing', because the strained region above the fault tip can be viewed as a triangular zone of shear (Erslev 1991; Allmendinge r 1998) . So far , w e hav e emphasize d th e similarit y o f structural style s i n th e thre e physiographi c prov inces o f th e interio r platform . But , though lat e Palaeozoic Midcontinen t fault-and-fol d zone s ar e identical i n styl e t o Mesozoic-Cenozoic structure s formed durin g the Laramide orogen y in the Rocky Mountains an d Colorad o Plateau , th e thro w o n Midcontinent zone s i s generall y muc h less tha n is typical fo r Rock y Mountain s or Colorad o Platea u examples (Marsha k & va n de r Pluij m 1997 ; McBride 1997 ; McBrid e & Nelson 1999 ; Marshak et al. 2000) . Specifically , maximum throw reache s about 1 5 km i n th e Rock y Mountain s province. , about 1. 5 km i n th e Colorad o Plateau , an d gener ally n o mor e tha n 15 0 m i n th e Midcontinen t
(though a fe w Midcontinen t fault s hav e displace ments o f u p t o 1. 5 km). Marshak an d Paulsen (1996) and Marshak et al . (2000), amon g others , argu e tha t majo r fault s o f the continenta l interio r platfor m initiate d durin g unsuccessful riftin g event s i n th e Proterozoi c an d Early Palaeozoic . Onc e formed , th e fault s hav e remained a s long-live d weaknesse s i n th e crust , because the y hav e neve r bee n anneale d b y meta morphism, o r stitche d togethe r b y plutons . Whe n boundary loads on the continent change orientation or magnitude , th e fault s ar e susceptibl e t o reacti vation. Thus , Phanerozoi c movement s o n thes e faults represent s sli p o n pre-existin g fault s — an individual faul t ma y b e reactivate d severa l time s (Holdsworth e t al. 2001) . Fault zone s an d relate d fold s ar e no t th e onl y consequences o f tectonis m i n Nort h America' s interior platform. The region has undergone epeirogenic movement s (gradua l vertica l displacement s of broa d areas ) t o form regional-scal e basin s an d domes (Fig . 1) , whos e presenc e ha s profoundl y affected th e distributio n of sedimentar y facies an d the thickness o f formations (e.g. McBride 199 8 and references therein) . Also, strat a of the interior plat form record regionally consistent strain in the for m of twinning in carbonates and deformation bands in sandstones. Strain magnitude recorded by twinning decreases markedl y from orogeni c front s toward s the interior . Bu t eve n i n th e centr e o f th e Unite d States, twinnin g strain s o f 1-3 % ca n b e docu mented (Craddoc k e t al . 1993 ; va n de r Pluij m e t al 1997) .
Deformation events in the interior platform Faults o f th e interio r platfor m hav e bee n affecte d by severa l episode s o f deformation. Contemporary seismicity indicate s tha t movemen t happen s today in a few locations , mos t notabl y th e Ne w Madri d seismic zon e i n th e centra l Mississipp i Valle y (Fig. 1) , and t o a lesse r degre e alon g a portio n of
STRIKE-SLIP FAULTIN G I N TH E US A CRATO N
the Nemah a uplif t i n northeaster n Kansa s an d th e southern Oklahom a aulacogen . Structure s o f th e Rocky Mountain s an d th e Colorad o Platea u wer e active durin g th e Laramid e orogeny , a shortenin g event tha t occurre d betwee n 8 0 and 40 Ma. Coney and Reynolds (1977) , amon g others, argu e that this event happene d i n response t o shallo w subductio n along the west coast, though, more recently, Maxon and Tikoff (1996) suggest that it is due to the colli sion o f a n exoti c terran e wit h Nort h America . Reactivation in Jurassic-Cretaceous time , in associ ation wit h Nort h Atlanti c rifting , ma y hav e trig gered normal-sens e reactivatio n o f fault s i n th e eastern par t o f th e interio r platform . Evidence fo r th e timin g o f Palaeozoi c tectoni c activity i n th e interio r platfor m come s primaril y from stratigraphi c studies . Fo r example , localize d unconformities an d shoals , bordere d b y clasti c wedges, indicat e formatio n o f uplifts , wherea s anomalously thick sections o f clastic strata indicate formation o f basins. Stratigraphic data indicate that movement o n fault-and-fol d zone s occurre d i n pulses durin g th e Ordovician , Devonian , an d Carboniferous-Permian (e.g . Klut h & Coney 1981 ; Nelson & Marsha k 1996 ; McBrid e & Nelso n 1999). Th e Carboniferous-Permia n even t wa s th e most significan t Palaeozoi c event , in that its conse quences are more widespread an d of greater magnitude that those of other events. Melton (1925 ) used the phrase 'Ancestra l Rocky Mountains' t o identif y uplifts tha t formed during this event, because many of th e uplift s ar e i n th e sam e o r simila r position s to th e present-da y Rock y Mountain s (Fig . 3). Th e uplifts o f th e Ancestra l Rockie s ar e fault-bounded
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blocks, man y o f whic h borde r dee p basin s fille d with thousands of metres o f sediments. Ve r Wieb e (1930) recognize d tha t th e Ancestra l Rockie s tec tonic even t als o affecte d a large are a o f the south ern Grea t Plains , producin g a series o f basins and uplifts (e.g . Nemaha, Matador), som e of which host major petroleu m fields . McBrid e an d Nelso n (1999) extende d th e Ancestra l Rockie s origi n far ther east , recognizing tha t many structure s in Missouri and Illinois share d structura l timing and style with th e classi c Ancestra l Rockie s structure s further west . Here , w e poin t ou t tha t timin g o f movements o n fault s i n Ohi o i s roughl y the same , emphasizing tha t th e Ancestra l Rockie s even t affected th e entir e continenta l interio r platfor m (Fig. 4). The caus e o f th e Ancestra l Rockie s even t ha s been debate d fo r decades. Klut h and Coney (1981 ) and Kluth (1986) sugges t that it represents a continental interior response to loads applied to the eastern an d souther n margi n o f Nort h Americ a durin g collision with Gondwanaland during the AlleghanianOuachita orogen y (Fig . 5A). Thi s mode l implie s that th e faultin g i s analogou s t o faultin g i n th e interior o f Asi a resultin g fro m collisio n o f Indi a with Asi a during the Himalayan orogeny . Alterna tively, Ye et al. (1996) compar e th e classic Ances tral Rockie s an d th e Cenozoi c Laramid e orogeny , and suggest that the former, like the latter, resulte d from compressio n i n th e forelan d o f a n Andeantype convergen t boundar y tha t existe d alon g th e southwest margi n o f Nort h Americ a (Fig . 5B). Thus, Ye et al. (1996) imply that the loading which triggered faul t movemen t wa s du e t o shallo w sub duction. Y e e t a/.' s mechanis m canno t explai n structures i n th e Grea t Plain s an d easter n USA , leading u s t o conclud e tha t bot h Alleghanian Ouachita collisio n an d Cordillera n convergenc e may hav e contribute d t o generatin g Ancestra l Rockies structures .
Tools for identifyin g strike-slip displacements o n faults of the continenta l interior platform
Fig. 3 . Ma p showin g th e distributio n o f th e 'classic ' Ancestral Rockie s uplift s i n th e wester n Unite d States . Source: Klut h 1986 .
Subsurface dat a (seismic-reflectio n profiles ; wel l logs) allo w geologist s t o characteriz e vertica l movements o n continental interio r fault s relativel y easily - offset s o f stratigraphi c horizon s an d the shapes o f layer s giv e a clea r imag e o f thi s move ment. Bu t ho w ca n w e determin e i f ther e i s a strike-slip componen t o n thes e faults ? Geologist s analyse th e kinematic s o f well-expose d strike-sli p faults b y studyin g sli p lineations , offse t markers , mesoscopic folds , e n echelo n veins , th e geometr y of Riede l shear s (and , i f myloniti c rock s ar e present, C- S fabrics , rotate d porphyroclasts , mic a
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Fig. 4. Ma p of the conterminous United States, illustrating the continent-wide distribution of structures resulting fro m the Ancestra l Rockie s event , an d th e maximu m principa l stres s directio n calculated fro m calcit e twinnin g i n th e Midcontinent (van der Pluij m e t al. 1997) . (Modified fro m McBrid e & Nelson 1999 ; Marshak e t al. 2000). Abbreviations: U = Uncompahgre, M = Matador-Red River , A B = Arbuckle Mts ; OA = Oklahoma aulacogen, N = Nemaha, L = LaSalle belt, BG = Bowling Green fault, CS = Cambridge-Burning Springs fault, W = faults of southern Wisconsin, C = Commerce fault zone , RC = Rough Creek, CG = Cottage Grove. The thick dashed line is the edge of the AtlanticGulf Coas t coastal plain.
fish, and porphyroclast tails). While suc h structures can be found locall y i n fault zone s of the Colorad o Plateau an d Rock y Mountains , i t i s rar e fo r the m to be visible in association wit h continental interior fault zone s o f th e Midcontinent . Indeed , man y interior-platform fault-and-fold s ar e blin d and , even wher e emergen t o r exhumed , ar e weathere d or poorl y exposed . I n essence , definin g strike-sli p kinematics o n these faul t zone s is a bit lik e recon structing th e skeleto n o f a Palaeolithi c homini d from thre e teet h an d a to e bone . W e emphasiz e from th e outse t that 7 i n studie s o f continenta l interior faul t zones , i t i s simpl y no t possibl e t o obtain th e qualit y o f dat a tha t i s usuall y expecte d for kinemati c studies . Indeed , par t o f ou r purpos e in this paper i s to illustrate the challenges involve d in studyin g thes e structures . Features tha t hav e bee n use d t o indicat e th e occurrence o f strike-sli p displacemen t o n interior platform faul t zone s includ e th e following : (1) Displacement o f markers: I t i s locall y poss ible t o estimat e th e sens e an d amoun t o f strike-slip offse t o n continental interio r fault s
by examinin g offse t isopachs , facie s bound aries, offse t palaeochannels , an d potential field anomalies . Offse t anomalies , however , are no t particularl y reliabl e indicators , because it is hard to demonstrate that anomalies on opposite sides of a fault were once continuous. (2) Mesoscopic structural analysis'. Standar d methods of mesoscopic structura l analysis can be use d to identif y strike-sli p component s of displacement o n continenta l interio r faul t zones, where the zones ar e exposed. Feature s that constrai n kinematic s includ e sli p lin eations, mesoscopi c fold s withi n th e faul t zone, rip-ou t clasts , an d mesoscopi c e n ech elon extension-gas h arrays . (3) E n echelon faults an d folds i n surface an d subsurface ma p view: It i s wel l know n fro m model studie s an d studie s o f well-expose d examples o f strike-sli p fault s tha t en echelo n faults an d fold s develo p i n strike-sli p shea r zones (e.g . Na y lor e t al . 1986 ; Sylveste r 1988). Thes e ca n eithe r b e a consequence o f
STRIKE-SLIP FAULTIN G I N TH E US A CRATON
Fig. 5 . Tw o competin g hypothese s fo r th e origi n o f Ancestral Rockie s structures , (a ) Klut h (1986 ) mode l relating th e Ancestra l Rockie s t o th e Alleghanian Ouachita collision . Th e cross-hatche d are a i s th e Trans continental Arch, a relatively high region of the continent during the Palaeozoic, (b) Ye et al. (1996) mode l relating the Ancestral Rockie s to a subduction zone on the southwestern margi n o f Nort h America . Th e patterne d area s are Ancestra l Rockie s uplifts .
accommodation fo r shortenin g an d extensio n oblique t o th e strik e o f th e fault , assumin g a model i n which the zone accommodates sim ple shea r in ma p view , or can consis t o f Riedel shear s forme d earl y durin g the rupturing of th e strat a a s displacemen t o n th e underlying basement-penetratin g faul t progresse s (Fig. 6; Smit h 1965 ; Mand l 1988) . (4) Flower structures'. Flowe r structure s ar e defined b y a n upward fan o f faul t splay s that merge a t dept h wit h a steepl y dippin g faul t (Sylvester 1988 ; Hardin g 1990) . I f ther e i s a thrust componen t o n th e faul t splays , a positive flowe r results, whil e if ther e i s a normal component o n th e faul t splays , a negativ e flower results. Flowe r structur e can b e ident ified i n seismic-reflectio n profiles , an d the y have been documented along many strike-slip faults. Th e presenc e o f flowe r structure s alone, however , i s no t sufficien t evidenc e t o confirm strike-sli p displacement , becaus e similar fault geometrie s ca n also develop simply b y inversio n o f antitheti c an d syntheti c fault splay s formed in the hanging-wall block above a n originall y norma l fault . (5) Vertical displacement components a t fault bends: A s describe d b y Sylveste r (1988) ,
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Fig. 6 . Cla y mode l illustratin g th e geometr y o f Riede l shears an d norma l fault s (extensio n gashes ) forme d i n a weak cla y laye r ove r tw o stron g woode n blocks , (e.g . Mandl 1988) .
among other s (e.g . Hold s worth & Pinheir o 2000), restrainin g bend s an d releasin g bend s may develo p alon g a strike-sli p faul t system . Reverse faultin g an d uplif t occu r acros s th e former, yieldin g push-up ridges, whereas normal faultin g an d subsidenc e occu r acros s th e latter, commonl y formin g a pull-apar t basin . The presenc e o f suc h bends , an d th e strai n that occur s a t them , suggest s th e occurrenc e and sens e o f strike-slip . In terms of reliability, offset linea r markers and slip lineations provid e th e bes t indicato r o f strike-sli p components o f movement . Displacement s o n restraining bends and releasing bends, may also be definitive. Th e occurrenc e of e n echelon structures may b e reliable , i f th e natur e o f th e structure s (Riedel shear s vs . extensio n gashes ) ca n b e specified. Claim s o f strike-sli p offset s tha t rely o n apparent offse t o f magneti c anomalies , o r o n th e occurrence o f flowe r structures , ar e les s reliable , but, nevertheless , suc h feature s ma y provid e th e only hin t o f strike-sli p movements .
Case studie s o f Palaeozoic strike-sli p In thi s sectio n w e discus s representativ e example s of structure s forme d durin g th e lat e Palaeozoic . Most o f thes e represen t Carboniferous-Permia n deformation - th e Ancestral Rockies even t - in the
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continental interio r platfor m of North America. Fo r some o f th e case s provide d below , ther e i s evi dence fo r earlie r movement .
North-central New Mexico Based o n a stud y o f offse t magneti c anomalies , Woodward et al. (1999) summarized evidenc e indicating that four nearly N-S-trending dextral strike slip fault s cu t acros s centra l an d norther n Ne w Mexico. Specifically , the y argue d tha t a serie s o f distinct magneti c low s define s th e northwester n boundary o f th e Mazatza l provinc e ( a NE-SW trending belt of post-1.7 Ga Proterozoic basement) , and tha t thes e low s ar e offse t t o th e sout h o n th e east sid e o f eac h faul t (Fig . 7). The y suppor t th e strike-slip hypothesi s b y notin g tha t seismic reflection profilin g indicate s tha t fault s spla y up dip into flower structures. Woodward et al., following Ye et al. (1996) , sugges t tha t the faulting happened i n respons e t o subductio n alon g th e south western margi n o f Nort h America . Th e map s provided by Woodward e t al indicat e that the combined displacemen t acros s thes e faults is 14 5 km in a right-latera l sense . W e poin t ou t tha t thi s dis -
placement i s muc h larger tha n strike-sli p displace ment o n other Ancestra l Rockie s faults . Thus, part of th e displacemen t ma y reflect Laramid e reacti vation, a s describe d b y Karlstro m an d Danie l (1993).
Paradox basin and Uncompahgre uplift The NW-SE-trendin g Uncompahgr e uplift , whic h cuts diagonall y acros s southwester n Colorado , i s one o f th e larges t uplift s o f th e classi c Ancestra l Rockies. I t lies alon g strike o f the Oklahoma aulac ogen, though there i s a gap in faulting between th e two. Formatio n o f th e uplif t brough t Precambria n metamorphic rocks up , relative t o adjacent Palaeo zoic strata . Th e metamorphi c rock s serve d a s a source fo r coars e sediment s tha t collecte d i n th e adjacent Parado x basin, which subsided until about 7.6 km o f structura l relief ha d forme d betwee n th e top o f the Uncompahgr e uplif t an d th e floo r o f th e Paradox basin . Baars & Stevenson (1982 , 1984 ) an d Stevenso n and Baar s (1986 ) argu e that th e faults forming th e boundary between th e Uncompahgre uplif t an d the Paradox basi n ha d a right-latera l componen t o f
Fig. 7. (a ) Ancestral Rockies in the New Mexico/Colorado region (adapted from Pazzaglia e t al. 199 9 and references therein), showin g the locatio n of tw o o f the strike-sli p faults propose d by Woodward et al. (1999) . The dar k shaded areas ar e Ancestra l Rockies uplifts , whil e th e ligh t stipple d areas ar e basin s formed durin g th e Ancestra l Rockies event, (b ) Woodwar d et a/'s . interpretatio n - offse t o f th e shea r zone definin g th e norther n edg e o f the Mazatza l province. Th e shade d area i s underlai n by pre-1. 7 G a basement , while the whit e area i s underlai n by th e younge r Mazatzal province.
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strike-slip. Base d o n th e shap e o f th e Parado x basin, the y conclud e tha t th e Parado x basi n itsel f is a pull-apart basi n forme d i n respons e t o dextra l strike-slip. The principal evidence that they present for thi s mode l i s th e occurrenc e o f a n e n echelo n set of anticlines involvin g Pennsylvanian-age strata in th e centr e o f th e Parado x basin , alon g th e Colorado/Utah border (Fig. 8) . Here, isopach maps demonstrate tha t a n arra y o f NNW-trendin g anti clines occu r between NW-trending enveloping surfaces.
Matador uplift/Red River Arch (Texas) The Matador-Red River uplift consists of a narrow zone of fault-bounded uplifts an d troughs that trend
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E-W, fro m easter n Ne w Mexic o acros s th e Texa s Panhandle, t o th e Oklahom a border , a distanc e of more than 450 km (Fig. 4). Offset o f the basementcover contac t acros s th e zon e indicate s tha t ther e is a maximu m o f 1. 4 km o f dip-sli p offse t acros s the zone . Becaus e n o well s cu t acros s fault s eve n in densel y drille d areas , Rega n & Murphy (1986 ) concluded tha t fault s ar e essentiall y vertical . Notably, th e mai n fault s o f th e zon e chang e di p along strik e (i.e . the y ar e 'propelle r faults') , so , as a result , th e upthrow n sid e i s no t alway s o n th e same sid e o f a give n fault . Radica l difference s i n the thicknes s o f Pennsylvania n rock s occu r acros s the fault , indicatin g tha t displacemen t occurre d during the Ancestral Rockies event. Most faults die out up-di p within th e Earl y Permia n (Rega n & Murphy 1986) . Two source s o f evidenc e sugges t tha t a compo nent of strike-slip displacemen t accompanie d verti cal component s o f movemen t i n th e faul t zon e of the Matador-Re d Rive r uplift . First , NW-trendin g faults i n th e zon e defin e e n echelo n array s i n E W-trending envelopin g surface s - thi s geometr y hints a t a dextra l sens e o f shear . Second , pro prietary seismic-reflectio n profile s acros s a n uplifted segmen t revea l tha t i t i s underlai n b y a positive flowe r structure . A ne w stud y b y Briste r et al. (2002 ) reveal s a pull-apar t basi n alon g th e uplift. Th e geometr y o f thi s basi n suggest s a component o f left-latera l sli p alon g th e uplift .
Southern Oklahoma aulacogen
Fig. 8 . (a ) Locatio n ma p showin g th e Uncompahgr e uplift an d th e adjacen t Parado x basin , (b ) E n echelo n folds withi n th e Parado x basin , suggestiv e o f strike-sli p displacement accordin g t o Baar s & Stevenso n (1982) . Baars and Stevenson stat e that the fault i s dextral, though the arrangemen t o f anticline s look s lik e the y woul d b e associated wit h sinistral movement . Locatio n o f this ma p is shown in 'A'. (Adapted from Baar s & Stevenson 1982) .
The souther n Oklahom a aulacoge n originate d i n Cambrian tim e o r earlie r a s a failed rif t tha t filled with several kilometres of igneous rocks an d strata. Palaeozoic inversio n of the rif t bega n in Late Mis sissippian tim e an d continue d int o Earl y Permia n (i.e. durin g th e Ancestra l Rockie s event) . Thi s inversion yielde d a bel t o f WNW-trendin g uplift s and fault s tha t cu t acros s souther n Oklahom a an d the Texa s Panhandl e (Fig . 9). Locally , erosio n stripped th e cover t o expose underlyin g Precambrian crystalline rocks (Ham et al. 1964) . At the same time, flankin g basin s san k so , a s a result , vertica l relief between basin floors and crest of the adjacent uplifts i s a s grea t a s 1 4 km (Ha m 1978 ; Donova n 1986, 1995 ; Johnso n 1989 ; Perr y 19890) . Inversion o f th e aulacoge n clearl y involve d shortening oriente d roughly perpendicula r t o th e rift axis . Som e o f th e majo r rift-boundar y faults , originally basement-penetratin g norma l faults , became revers e faults , and , a s the y moved , fold s formed (Fig . 10A) . Bu t strike-sli p displacement s unquestionably als o occurre d durin g thi s event . Direct field evidence includes observations of horizontal an d obliqu e slickensides , e n echelo n fold s and shear zones, pull-apart grabens, and lateral off-
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Fig. 9 . Ma p o f the souther n Oklahoma aulacoge n an d adjacen t Ancestra l Rockies uplifts . Inse t shows the locatio n of this ma p area . (Adapte d fro m Budni k 1986) .
Fig. 10. (a ) Ma p o f a portion o f the souther n Oklahom a aulacogen, includin g th e Arbuckl e Mountain s an d Ard more basin (Hardin g 1985) . (b ) Classi c interpretatio n o f a seismi c lin e acros s nea r th e Ardmor e basi n (approximately lin e X—X' i n 'A' ) a s portrayed b y Hard ing (1985) .
sets o f fol d axes , formatio n contacts , an d isopac h lines (cf . Hardin g 1990) . Flowe r structure s ar e locally wel l develope d acros s thes e fault s (Fig. 10B) . Faults of the southern Oklahoma aulacogen stan d ou t amon g fault s o f th e interio r plat form becaus e their strike-slip displacements can be more clearly documented, and are an order of magnitude large r tha n those o f othe r examples . Both right-latera l an d left-latera l displacement s have been reported for the southern Oklahoma aulacogen, i n som e case s o n side-by-sid e fault s (Dunham 1955) . Estimate s o f th e left-latera l dis placement o n individua l fault s rang e fro m 4. 8 km (Perry 1989£ ) t o 6 4 km (Tanne r 1967) . Unfortu nately, a s Deniso n (1995 ) pointe d out , reliabl e piercing point s ar e difficul t t o com e by , an d esti mates of slip based on offset isopac h lines or facies trends of Palaeozoic units are subject to large errors due t o lac k o f wel l control , an d du e t o structura l complications. Nevertheless , withi n th e las t tw o decades, a consensu s favour s overal l left-latera l transpressive displacement, with strike-slip components o f offse t o n individual fault s in th e rang e of a few kilometres. Allowin g for the width and complexity of the fault zone , the overall lateral compo nent acros s th e whole zon e coul d be substantial up to tens of kilometres (McConnel l 1989; Denison 1995). Som e fault s i n th e Oklahom a aulacoge n appear to exhibit Holocene left-latera l obliqu e dis placements o f th e orde r o f 1 2 to 2 0 m (Cron e &
STRIKE-SLIP FAULTING IN THE US A CRATO N
Luza 1986 ; Madol e 1986 ; Ramell i & Slemmon s 1986).
Nemaha uplift (Kansas) and faults of northcentral Oklahoma The Nemah a uplif t trend s NN E fo r abou t 65 0 k m from Oklahom a Cit y t o Omaha , Nebrask a (Fig. 11) . I t i s a 10-8 0 k m wid e fault-bounde d uplift, containin g abundan t smal l horst s an d grab ens tha t overlie s th e souther n exten t o f th e 1. 1 Ga Midcontinent Rif t System . Thus, the Nemaha uplift formed b y inversio n o f rif t faults . Th e Nemah a structure was a high during most of the Palaeozoic , though occasionall y i t wa s submerge d (Berendsen & Blai r 1995) . Faul t movemen t occurred durin g the Early to Middle Pennsylvanian (i.e. durin g th e Ancestra l Rockie s event) . The main fracture zone that borders the east sid e of th e Nemah a uplif t i n Kansas i s called th e Hum boldt faul t zone . Seismic-reflectio n profile s indi cate tha t th e zon e include s high-angl e revers e faults. A s muc h a s 79 0 m o f cumulativ e dip-sli p displacement, dow n to the east, occurred across th e zone. Man y NW-trendin g transfe r fault s wit h
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throws a s grea t a s 450 m cros s th e Humbold t an d subdivide th e Nemah a uplif t (Berendse n & Blai r 1995). Based on the occurrenc e o f en echelon faul t patterns an d apparen t pull-apar t grabens , Berendsen an d Blai r inferre d tha t a left-latera l component o f strike-sli p displacemen t occurre d along th e Humbold t faul t zone , bu t th e tota l amount o f displacemen t remain s unknown . Seg ments of the Humboldt fault zone appear to be seismically activ e today , particularl y nea r Manhattan , Kansas (Burchet t e t al 1985) . Several belts of en echelon faults occur southeas t of th e Nemah a uplif t i n Oklahom a (Fig . 11) . Th e belts tren d N t o NNE , paralle l t o th e Nemah a uplift, an d ar e compose d o f fault s tha t strik e NW . The fault s tha t make u p the e n echelon belt s strik e N45-70°W an d di p 5 0 t o 65 ° eithe r northeas t o r southwest. Al l ar e norma l faults . Th e longes t i s about 5 km and the greatest throw about 40 m. The fault belt s paralle l th e strik e o f Upper Pennsylvan ian strat a i n thi s par t o f Oklahoma . Mappin g o f these fault s le d t o wha t may b e th e earlies t recog nition o f strike-sli p faultin g in th e America n Mid continent, by Path (1920 ) an d Foley (1926) . Usin g simple cla y model s fo r analogues , Pat h an d Fole y
Fig. 11. Faul t traces and en echelon fracture trace s from th e Nemaha uplift an d nearby fault zones . The shade d areas are th e area s represent the interior s of rifts . (Compile d from Fole y 1926 ; Berendsen & Blair 1995).
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proposed tha t the e n echelon zone s ar e the surface expression o f left-latera l movemen t o n fault s i n Precambrian basement .
Grays Point fault (Missouri) The Grays Point fault is part of the Commerce faul t system i n southeaster n Missouri . Thi s faul t paral lels th e northwester n margi n o f th e Reelfoo t rif t (Fig. 1) , whic h underlie s th e Mississipp i embay ment. Clendenin an d Diehl (1999) have interpreted the movemen t histor y o n th e Gray s Poin t fault , where it cuts exposures o f Ordovician an d Silurian strata expose d i n a quarry. By examinin g sidewal l rip-out clast s i n faul t veins , a s viewed i n thin sec tion, an d b y lookin g a t the geometr y o f subsidiar y faults whic h the y interpre t t o b e Riede l shears , Clendenin an d Dieh l argu e fo r a phas e o f lat e Palaeozoic dextra l strike-sli p o n th e Gray s Poin t fault. Th e detail s o f faul t geometr y ar e complex , and thus the interpretation o f this fault remain con troversial.
Plum River fault zone The Plu m Rive r faul t zone , whic h trend s N85° E across easter n Iow a an d northwester n Illinois , i s approximately 18 0 km lon g an d it s maximu m width i s abou t 1.2k m (Fig . 12) . Th e ne t vertica l
throw resultin g fro m movemen t i n th e zon e i s down t o th e north , an d range s fro m 3 0 t o 12 0 m. In bot h Illinoi s (Kolat a & Buschbac h 1976 ) an d Iowa (Bunke r e t al. 1985) , th e Plu m Rive r zon e consists o f nearl y vertica l fault s tha t border horst s and graben s i n Ordovicia n throug h Pennsylvanian sedimentary rocks . Fault s ar e marke d b y wid e zones o f silicifie d breccia , whic h contai n rotate d blocks o f dolomite . Th e zon e probabl y root s i n a Proterozoic basement-penetratin g faul t (Anderso n 1988). Stratigraphi c relationships suggest that most of th e movemen t o n it too k plac e betwee n Middle Devonian an d Middle Pennsylvania n - thu s the structure may have been activ e prior to the Ancestral Rockie s event . A strike-sli p componen t o f movemen t i n th e Plum Rive r faul t zon e i s indicate d b y th e occur rence o f a band o f N45° W t o N67° W e n echelo n faults borderin g the zone (Templeton 1951) . Thes e faults ar e vertica l t o steepl y dipping , displa y mostly normal offsets , an d contain vein-fille d breccias. Dee p graben s o f thi s orientatio n occu r i n Iowa. Th e occurrenc e o f horizontal sli p lineation s on smal l fault s tha t strik e N69° W t o N90° W i n a quarry just nort h o f the Plu m Rive r zon e support s this proposal . Mor e recen t mappin g (Fig . 13A) , indicates tha t th e zon e als o contain s NE-trendin g low-angle thrust faults. Because of the geometry of en echelo n fault s i n the Plu m River zone , we suggest tha t the zon e experienced a n episode o f right lateral oblique , down-to-the-north displacement. In this context , th e dee p graben s o f Iow a ar e smal l pull-apart basins . Ou r proposa l concur s wit h th e speculations o f Trap p an d Fenste r (1982 ) an d Heyl (1983) .
Sandwich fault zone
Fig. 12 . Sketc h ma p of the Illinois basin region, showing the locatio n o f principa l structura l features referred t o in the text . R R -Reelfoot rift; RC -Rough Cree k graben ; CG = Cottage Grov e fault ; C L = portion o f Commerc e geophysical lineament ;F A = Fluorspar Area ; LDB^LaSalle belt ; S F = Sandwich faul t = PR = Plum River fault ; W V = Wabash Valle y faul t zone .
The Sandwic h faul t zon e run s NW-S E approxi mately 13 5 km acros s norther n Illinoi s (Fig . 12) . As mappe d an d describe d b y Kolat a e t al. (1978) , the fault zon e is 1 to 3 km wide and contains vertical to steeply dipping normal and reverse fault seg ments tha t outlin e horst s an d grabens . Th e ne t throw alon g th e middl e portio n o f th e zon e i s a s much as 250 m down-to-the-northeast, but near the southeastern terminus of the zone the southwestern block i s downdropped . Stratigraphi c constraint s require onl y tha t movemen t occurre d betwee n th e Silurian an d the Pleistocene. Base d on exposures in quarries, Kolat a et al . (1978 ) an d Nelso n (19956 ) described a n arra y o f subparallel , NW-strikin g high-angle norma l fault s alon g wit h a fe w nearl y vertical revers e faults . I n a n unpublishe d manu script, Templeton (1951 ) presented details on structural feature s nea r the town o f Orego n (Fig . 13B) . Here, a narrow , NW-trending hors t o f th e Cambr ian Franconi a Sandstone , upthrow n b y 75-9 0 m
STRIKE-SLIP FAULTIN G I N THE USA CRATON
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Fig. 13. (a ) Sketc h ma p o f a portion o f th e Plu m River faul t zon e showin g en echelon grabens , indicativ e o f dextral displacement, (b ) Sketc h ma p o f a portion o f the Sandwic h fault zon e nea r th e tow n of Oregon , illustratin g graben s and horst s incline d t o th e mai n faul t zone .
and juxtapose d wit h th e Middl e Ordovicia n S t Peter Sandston e o n bot h sides , i s expose d alon g the eas t sid e o f th e Roc k River . Templeto n als o describes a se t o f WNW-strikin g fault s tha t for m an e n echelo n array . The structura l features described b y Templeton , Kolata e t al., an d Nelso n ar e evocativ e o f strike slip faultin g - e n echelon border-faul t arrays , and both horst s (th e slic e o f Franconia ) an d graben s occur alon g map-vie w bend s i n th e system . Notably, no t al l th e feature s indicat e th e sam e sense o f shear . Specifically , the orientatio n o f th e Franconia slic e suggests tha t it lies at a restrainin g bend, ye t thi s upthrow n slice ha s th e sam e orien tation a s th e releasin g ben d o r extensiona l duple x northwest o f Oregon . Perhap s th e Sandwic h faul t zone has ha d tw o (o r more) episode s o f tectonism with strike-sli p components , on e dextra l an d on e sinistral movement .
Southwestern Wisconsin Several strike-sli p fault s hav e been reporte d i n th e Upper Mississipp i Valle y Zinc-Lea d District . I n a highly detaile d treatis e o n th e district , Hey l e t al . (1959) cite d severa l example s o f strike-sli p fault s encountered withi n undergroun d mines . On e
WNW-trending fault, in the Liberty Mine of Lafayette County , Wisconsin , ha s horizonta l striation s and display s left-latera l offse t o f 7. 6 m. A nearb y fault strikin g N- S produce d a n apparen t right lateral offse t o f 60 m on an ore body between tw o adjacent mines . A NW-strikin g fault , th e Miffli n Fault o f Iow a County , Wisconsin , produce d approximately 30 0 m of right-lateral displacemen t on a n or e bod y an d tw o NE-trendin g fol d axes .
Cottage Grove fault system (Illinois) The Cottag e Grov e faul t zone , a n E-W-trendin g structure tha t ca n b e trace d fo r 115k m acros s southern Illinoi s (Fig . 12) , i s arguabl y th e best documented exampl e o f strike-sli p faultin g i n th e Midcontinent (Clark & Royds 1948 ; Heyl & Brock 1961; Hey l e t al . 1965 ; Wilco x e t al . 1973 ; Nel son & Krauss e 1981) . Althoug h thi s structur e ha s little surfac e expression, seismic-reflectio n profiles, mineral-exploration boreholes , an d exposure s o f the zone in coal mines provide abundant kinematic information. Seismic-reflectio n section s (Fig . 14) show tha t faul t displacement s affec t th e entir e Palaeozoic sectio n and disrupt the top of Precambrian basement 3. 0 km below th e surface (Duchek e t al. 2001) . Vertica l fault s i n th e lowe r Palaeozoi c
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S. MARSHAKCTAL .
Fig. 14 . Migrate d seismic-reflectio n profil e acros s th e Cottage Grov e faul t syste m illustratin g th e flowe r struc ture at shallow depths. K x = Knox Group; pC = top of the Precambrian. Line i s located ove r the easter n par t of th e fault (Fig . 15) .
section propagat e upward s int o flowe r structures . The timin g o f faultin g ca n b e constraine d i n par t by stratigraph y (th e younges t rock s displace d ar e early Lat e Pennsylvanian) , an d i n par t b y radio metric datin g o f ultramafi c dykes , whic h cu t th e faults (Earl y Permian). These observations indicat e that th e Cottag e Grov e faul t syste m i s Lat e Pennsylvanian to Early Permian i n origin, and thus moved durin g th e tim e o f th e Ancestra l Rockie s event (Nelso n & Lumm 1987) . Near the ground surface, the Cottage Grov e fault zone include s a sinuou s maste r faul t tha t locall y bifurcates, formin g tw o paralle l strands . A s exposed i n coa l mines , th e maste r faul t include s both high-angl e norma l an d revers e faults , wit h vertical throw s o f a s muc h a s 6 0 m. Notably , th e side o f th e faul t tha t ha s move d relativel y dow n reverses severa l time s alon g the length o f the faul t (i.e. th e faul t i s a scissor fault) . Hundred s o f sub sidiary NW-trendin g norma l an d oblique-sli p e n echelon faults , wit h maximu m vertica l separatio n of 18m , border the maste r faul t (Fig . 15) . Kinem atic indicator s o n these faults includ e horizontal o r obliquely plungin g sli p lineations , an d latera l off sets o f vertica l contact s expose d undergroun d mines (Nelso n & Krausse 1981) . Several anticline s have bee n mappe d alon g th e Cottag e Grov e faul t system, al l of which lie close to the master fault , or directly in line with the fault's westwar d extension
(Fig. 15) . Som e o f th e anticline s tren d ENE , for ming a n e n echelo n syste m (Nelso n & Krauss e 1981). Evidence fo r a strike-sli p componen t o f dis placement o n th e faul t zon e come s fro m a variety of sources . First , a s noted above , e n echelo n fold s and subsidiar y fault s occu r withi n th e zone . Th e geometry of these suggests that they formed during dextral shea r acros s th e zone . Second , i n under ground coa l mines , Nelso n an d Krauss e (1981 ) mapped many mesoscopic faults wit h horizontal or gently plungin g slickenside s an d mullion , an d lat eral offset s (u p t o a fe w metres ) o f stratigraphi c contacts o r o f othe r fault s provid e evidenc e fo r lateral motion . Third , sli p o n th e maste r faul t produced 0.6-1. 6 km o f dextra l offse t o f th e boundaries o f a Pennsylvania n palaeochanne l (Nelson & Krauss e 1981) . Finally , bot h seismic reflection profiles, an d cross sections prepared fro m coal min e data , sho w positive an d negative flower structures along the master fault, wit h the appropri ate orientations to be associated with dextral strikeslip displacement .
Bowling Green Fault (Ohio) The Bowling Gree n faul t zon e ha s been trace d fo r about 10 0 km, along a NNW trend in northwestern Ohio an d adjacen t Michiga n (Fig . 4). I t lie s ove r the Grenville Front , a major Lat e Proterozoic crus tal boundar y tha t separate s high-grad e metamor phic rock s o f th e 1. 1 Ga Grenvill e oroge n o n th e east fro m a n unmetamorphose d 1. 3 Ga granite rhyolite terrane on the west (Onasch & Kahle 1991; Wickstrom et al 1992 ; Root 1996). Though largely buried, th e zon e i s expose d i n a numbe r o f lime stone quarries , an d ha s als o bee n studie d throug h data fro m oi l an d ga s exploratio n hole s an d seis mic-reflection profiles . These dat a indicate tha t the zone consist s o f high-angl e revers e an d norma l faults tha t exten d fro m th e bedroc k surfac e down ward i n Precambria n basemen t (Wickstro m e t al . 1992), as well as low-angle thrust faults (Onasc h & Kahle 1991) . Overall displacement across the faul t dropped th e easter n sid e dow n by 15 0 m. Two recen t publication s presen t divergen t interpretations o f th e Bowlin g Gree n faul t zone , but bot h conclud e tha t a t times , left-latera l offse t developed acros s th e zone . Onasc h an d Kahl e (1991) inferre d si x episode s o f movement , begin ning in the Late Ordovician. Fiv e of these episode s involved dip-slip but the third produced left-lateral motion a s show n by nearly horizontal slickenside s on Siluria n dolomite . In contrast, Wickstro m e t al . (1992) suggest the zone had three major period s of activity; Precambrian , Lat e Ordovician , an d post Silurian. Thes e author s inferre d a significan t left lateral componen t o f displacemen t durin g th e
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Fig. 15 . Geologica l ma p o f th e Cottag e Grov e faul t syste m an d nearb y structure s o f souther n Illinois . Not e th e e n echelon fault s that border the master fault. Modified from Nelso n and Krausse (1981) an d Nelson (1995^) . Inset shows the strai n ellips e fo r dextra l strike-sli p faultin g (Nelso n & Krauss e 1981) . Se e Figur e 1 2 for location .
Ordovician - rock s eas t o f th e faul t movement northward. The y bas e thi s conclusio n o n th e geometry o f a restraining bend .
Burning Springs-Cambridge fault zone (Ohio and West Virginia) This zone can be traced for 350 km, trending NNW across Ohi o an d int o West Virginia , thoug h it ca n barely b e recognize d a t th e groun d surfac e (Roo t 1996). Th e zon e i s severa l kilometre s wide , an d contains severa l faul t strand s wit h a ne t displace ment measure d i n ten s o f metre s (Roo t & Onasc h 1999). A seismi c sectio n acros s th e zon e (Deylin g 1993, reproduced in Root 1996) , portrays a vertical fault i n Precambria n basemen t tha t branche s upward into a positive flower structur e that dies out up-dip i n a bo x anticline . Roo t (1996 ) propose d that th e zon e underwen t right-latera l movemen t during th e Alleghania n orogeny , base d o n recog nition o f flowe r structur e alon g th e fault , an d th e occurrence o f a left-steppin g restraining bend .
Rough Creek fault system (Kentucky and Illinois) The Roug h Cree k faul t syste m extend s E- W approximately 21 0 km acros s wester n Kentuck y and souther n Illinoi s (Fig s 2 & 12 ; Nelso n 1991 ; Nelson 1995a). I t i s on e o f th e larges t faul t zone s of th e Midcontinent , an d ha s undergon e severa l episodes o f displacemen t datin g bac k a t leas t t o Cambrian time . Th e zon e marks th e northern mar gin o f a Lat e Proterozoi c o r Cambria n faile d rift , the Roug h Cree k Grabe n (Soderber g & Kelle r 1981). A number of geologists , includin g Clar k &
Royds (1948) , Hey l & Broc k (1961) , Hey l e t al (1965), an d Heyl (1972 ) postulated tha t the Rough Creek ha s a left-latera l strike-sli p componen t o f displacement, becaus e th e faul t i s bordere d b y a belt o f NE-trending en echelo n faults , an d becaus e the master fault splay s up dip into a positive flower structures. Nelson and Lumm (1987) an d Lumm et al. (1991), however, examine d th e fault syste m and concluded tha t th e flowe r structur e o f th e bel t i s dominantly a consequenc e o f faul t inversio n o f a rift margin , no t o f strike-slip . I f a strike-sli p component o f displacemen t occurre d o n th e fault , then th e displacemen t wa s minimal , fo r no signifi cant pull-apart basin has developed a t the west end of th e faul t zone , wher e th e zon e makes a n abrupt 60° ben d t o th e south . Further , Pennsylvania n palaeochannels tha t cros s th e faul t syste m in Ken tucky ar e no t offse t laterall y (Davi s e t al . 1974) . In one place where the fault syste m exhibits 450 m of dip-sli p displacement , palaeochanne l mappin g limits possibl e strike-sli p t o les s tha n 30 0 m.
Case studie s o f Mesozoic-Cenozoic strike slip Structures that formed in the portion o f the Unite d States continenta l interio r platfor m tha t la y i n th e foreland o f the Cordilleran oroge n wer e active during th e Laramid e orogen y o f lat e Mesozoi c an d early Cenozoic time . Their developmen t resulte d in both th e monocline s o f th e Colorad o Platea u an d the towering basement-core d uplifts o f the present day Rock y Mountains . Numerou s author s hav e pointed out that strike-slip component s of displacement occur on some faults o f the region (e.g. Sale s 1968; Ston e 1969) .
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East Kaibab Monocline (Utah-Arizona) The Eas t Kaiba b Monoclin e i s on e o f th e large r Laramide monoclines of the Colorado Plateau, with vertical structura l relie f o f 1. 6 km. Th e monoclin e has a sinuou s trace , 18 0 km long , bu t generall y trends N- S wit h a n E-facing stee p lim b (Fig . 16) . Exposures i n th e Gran d Canyo n demonstrat e tha t the monoclin e i s a fault-propagatio n fol d forme d due to reverse-sense displacemen t o n a W-dipping basement-penetrating faul t (Huntoo n 1993 ; Hun toon et al. 1996). Exposures within the canyon also show that the fault originated as a normal fault during Proterozoi c tim e (boundin g a half-grabe n which fille d with strat a o f th e Gran d Canyo n Ser ies, the n wa s reactivate d a s a reverse faul t durin g the Laramid e orogen y (Walcot t 1890 ; Maxso n 1968; Beu s & Morale s 1990 ; Tindal l & Davi s 1999). Tindall an d Davi s (1999 ) provide d a detaile d structural analysi s o f th e norther n 50k m o f th e East Kaiba b Monocline , showin g that , i n additio n
to reverse-sens e componen t displacement , ther e i s a componen t o f right-latera l displacement . Thei r mapping reveal s tha t a multitud e o f e n echelo n faults, whic h bear shallowl y raking sli p lineations , cut th e stee p lim b o f th e monoclin e i n souther n Utah. I n detail , tw o e n echelo n faul t set s ar e present, on e trendin g N W an d th e othe r trendin g NE. Tindal l an d Davis (1999) conclud e that development o f th e monoclin e involve d approximatel y 1.6 k m o f revers e displacemen t an d a s muc h a s 8.0 k m o f dextra l strike-sli p displacement . Thus , movement o n th e faul t underlyin g th e monoclin e was oblique-slip . Th e sens e o f sli p i s compatibl e with regiona l NE-S W Laramid e shortening .
Owl Creek Mountains (Wyoming) Laramide structure s o f th e Rock y Mountain s i n Wyoming rang e i n tren d fro m NN W t o WN W (Fig. 17A) . Some author s have argued that the different trend s forme d i n respons e t o tw o separat e shortening event s wit h radicall y differen t orien -
Fig. 16. (a ) Sketc h map o f the Eas t Kaibab monoclin e i n Uta h an d Arizona , (b ) Detai l of e n echelon faulting alon g the trac e of th e Eas t Kaibab monocline, (c ) Cross sectio n of the Eas t Kaibab monocline. (Modified fro m Tindal l & Davis 1999 ; Huntoon e t al. 1996) .
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Fig. 17 . (a ) Regional map showing th e Laramide uplift s and structures o f the Colorado Plateau and the Rocky Moun tains, (b ) Ma p fro m Ston e (1969 ) showin g trace s o f fold s adjacen t t o fault s i n Wyoming , suggestiv e o f strike-slip . (c) Ma p fro m Paylo r an d Yi n (1993 ) illustratin g th e e n echelo n fold s alon g th e Nort h Ow l Cree k fault . Boxe s show locations .
tations (e.g . Grie s 1983) . However , a growin g consensus favour s a singl e NE-S W shortenin g direction, wit h a relativel y mino r amoun t o f rotation (e.g . Varg a 1993 ; Ersle v & Wiechelma n 1997). If there is a uniform regional NE-SW short ening direction, the n one might predict oblique-sli p to strike-sli p displacemen t o n fault s tha t ar e no t NW-trending. Stone (1969 ) applie d thi s concept t o the entire Rocky Mountai n region , an d interprete d numerous array s o f e n echelo n anticline s an d thrusts t o be indicative of strike-slip displacement s (Fig. 17B) . Brow n (1993 ) reviewe d th e evidenc e for strike-sli p displacements an d suggested severa l locations wher e it probably occurred . On e of thes e locations occur s alon g th e flan k o f th e Ow l Cree k Mountains. Paylor an d Yin (1993) investigated th e kinematics o f the North Ow l Creek faul t syste m in detail, an d demonstrate d tha t E-W-trendin g faul t segments d o displa y strike-sli p lineations , an d ar e bordered b y e n echelo n fold s an d thrust s tha t locally defin e a transpressiona l duple x (Fig . 17C) . They conclud e that the fault is , effectively, a sinis-
tral latera l ram p tha t act s t o transfe r displacemen t between tw o E-dippin g fronta l ramps .
Cat Creek anticline and Lake Basin fault zones (Montana) The Ca t Creek anticlin e i s a 10 0 km long by 8-1 9 km wid e fol d tha t trend s WN W acros s th e plain s of central Montan a (Nelso n 19930 , b, 1994, 1995Z? ; Fig. 18A , B) . It s northeas t lim b dip s steepl y (30 90°), wherea s it s southwes t lim b ha s a gentl e di p (1-6°). Borehol e penetration s an d seismi c dat a indicate tha t the stee p flan k o f the Ca t Cree k anti cline (Fig . 18B ) i s underlai n b y a SW-dippin g reverse fault , th e Ca t Cree k fault , whic h dip s 55 70° i n Mesozoic strata , flattenin g slightl y a t dept h to 45-60° in Palaeozoic strata . The Cat Creek faul t bifurcates upwar d an d die s ou t withi n th e Uppe r Cretaceous shal e sectio n befor e reachin g th e sur face, an d ha s undergon e a t leas t fiv e episode s o f displacement. I t began a s a normal fault durin g the Proterozoic (Sonnenber g 1956 ; Shepar d 1987) ,
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Fig. 18. (a ) Sketc h o f Montana , showin g th e locatio n o f Laramid e strike-sli p faults , includin g the Ca t Cree k fault . (b) Cros s sectio n o f th e Ca t Cree k fault , showin g th e associate d fold .
underwent revers e movemen t i n th e Cambria n (Nelson 1993a , Z>) , and again in the Devonian. Normal displacemen t occurre d betwee n Middl e Pennsylvania an d Middl e Jurassic , an d fina l reverse/left-lateral obliqu e displacemen t occurre d during th e Laramid e orogen y (Nelso n 1993a , b, 1994, 1995/7) . The left-latera l componen t o f displacemen t o n the Ca t Cree k faul t durin g th e Laramid e i s indi cated b y e n echelo n fold s an d faults . Specifically , nine domes are arrayed along and south of the crest Of th e Ca t Cree k anticlin e (Fig . 18C) . Thei r axe s are slightl y obliqu e t o tha t o f th e mai n anticline , so the y for m a left-hande d en echelo n set . A bel t of NE-trendin g e n echelo n norma l fault s follow s the cres t o f th e anticline . N o piercin g point s hav e been identified , s o th e exac t magnitud e o f left lateral displacemen t canno t b e measure d directly , However, th e sigmoida l bendin g o f NE-striking en echelon fault s adjacen t t o th e mai n faul t suggest s a maximu m latera l displacemen t o f abou t 1.4km , a valu e tha t i s slightl y greate r tha n th e maximum dip-slip componen t (Nelso n 1995&) . The Ca t Creek anticlin e i s not a n isolated struc ture. I n centra l an d easter n Montana , Laramid e faults strik e east ENE and ESE, an d are either left lateral strike-slip or are oblique transpressive , wit h reverse-left-lateral motio n (Fig . ISA) . Th e left lateral componen t o f thes e structure s i s indicate d by belts o f NE strik e en echelon fractures , a s illus-
trated b y the Lake Basi n fault, whic h trends abou t N80°W an d contain s a bel t o f e n echelo n norma l faults tha t strik e northeas t an d di p a t 3 0 60° (Chamberlin 1919 ; Hancoc k 1919 , 1920 ; Robin son & Barnum 1986 ; Lope z 2000) . Judging by the small magnitud e of surfac e displacements , and th e absence of a through-going master fault , th e latera l component o n th e Lak e Basi n faul t i s probabl y small ( 1 km or less).
Holocene examples Commerce fault zone Outcrops and borehole studie s indicate that Palaeo zoic strat a o f souther n Illinoi s an d Missour i ar e extensively cu t by NE-trending faults . This regio n of faultin g include s the Fluorspa r Are a faul t com plex, know n fo r economi c deposit s o f fluorspa r precipitated fro m fluid s passin g alon g th e faults , and th e Commerc e faul t zone , whic h lie s t o th e west of the Fluorspa r Area faul t comple x (Fig . 12 ; Nelson 1991 ; Nelso n e t al 1997 , 1999) . Th e Commerce fault zone has displaced Holocene sediment s (Harrison e t al . 1999) . I n southeaster n Missouri , the fault-zon e correspond s t o th e regionall y extensive Commerc e geophysica l lineament , tha t parallels th e Reelfoot rift and the trace of the New Madrid seismi c zon e (Harriso n & Schult z 1994 ; Hildenbrand & Rava t 1997 ; Langenhei m & Hild -
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enbrand 1997) . Langenhei m an d Hildenbran d argue tha t ther e ha s bee n left-latera l sli p o n th e fault, base d o n thei r interpretatio n o f offse t mag netic anomalies . However , exposure s o f th e faul t are decorated wit h slip lineations that indicate dextral strike-slip displacement (Harriso n e t al. 1999).
New Madrid seismic zone By fa r th e mos t activ e seismi c regio n i n th e Mid continent i s th e Ne w Madri d seismi c zon e (Johnston & Shedlock 1992) . The zone was the site of thre e o r fou r earthquake s tha t ha d body-wav e magnitude o f 7. 0 o r greate r i n 1811-181 2 (Fig. 19A) . Th e fault s o n whic h th e earthquake s occurred li e burie d beneat h gravel s o f th e Mississippi Valley , s o they cannot be examined in outcrop, bu t geophysica l studie s sho w tha t th e earthquakes occu r i n and along th e Reelfoot rift , a NNE-trending troug h that forme d in Lat e Protero zoic t o Earl y Cambria n time , an d wa s reactivate d in pulse s throug h th e Phanerozoi c (Ervi n & McGinnis 1975 ; Brail e e t al . 1986) . Seismi c activity concentrate s alon g tw o NE-trendin g belt s linked b y a shor t NW-trendin g belt . Foca l mech anisms indicat e tha t earthquake s o n th e northeas t segments o f Ne w Madri d seismi c zon e ar e du e t o right-lateral strike-slip , whil e movemen t o n th e NW-trending segment is due to thrust displacement (with th e hangin g wal l movin g northeast ; Staude r 1982; Prat t 1994 ; Va n Arsdale e t al 1998) . Thus, the northwes t segmen t behave s lik e a restrainin g bend linkin g tw o non-coplana r fault s (Fig . 19B). Notably, a smal l uplif t ha s develope d ove r th e thrust segment . Th e movemen t i s compatibl e wit h the contemporar y regiona l stres s fiel d o f easter n North America , whic h indicate s tha t maximu m compressive stres s trajectorie s tren d NE-S W (Zoback & Zoback 1980) .
Discussion an d conclusions The continenta l interio r platfor m o f th e Unite d States is the portion of the craton where a veneer of Phanerozoic sedimentar y strata covers Precambrian crystalline basement . I t ca n b e divide d int o thre e physiographic provinces: Rocky Mountains, Colorado Plateau , an d Midcontinent . Regional-scal e faults occu r i n al l thre e provinces , thoug h i n th e Midcontinent most fault s ar e not well exposed and thus ar e known primarily fro m subsurfac e studies . Overall, faults fal l int o two sets , based on trend: N to NE trending, an d W to NW trending. The fault s probably initiated in the Proterozoic, i n response to crustal extension , an d thu s thei r orientatio n doe s not reflec t Phanerozoi c stres s fields . Rather , Phanerozoic episode s o f slip on the faults represen t reactivation in response t o boundary loads applie d
Fig. 19. (a ) Ma p showin g th e schemati c locatio n o f earthquake epicentres i n the New Madrid seismic zone of southern Missouri, (b ) Interpretive sketch , illustrating the sense o f sli p o n faults , base d o n fault-plan e solution s (Chiu e t al . 1992) .
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to the continent by marginal convergent and/o r collisional orogeny . Becaus e th e faults ar e typicall y not paralle l t o th e tren d o f a principa l stress , sli p during their reactivation mus t be transpressional o r transtensional (i.e . oblique-slip ; Bot t 1959) . Vertical displacemen t component s o n interior platform fault s ca n b e identifie d relativel y easil y on seismic-reflectio n line s o r drillhol e controlle d cross sections , becaus e the y caus e distinc t strati graphic offsets . Bu t strike-sli p displacements , a s we hav e shown , ar e generall y har d t o identify , because the y ar e not obviou s in cros s section , an d rarely caus e obviou s latera l offset s a t th e surface . Specific clue s tha t hav e bee n use d t o strike-sli p include belts o f en echelon faults , e n echelon anti clines, offse t palaeochannels , occurrenc e o f sub horizontal lineation s o n faul t surfaces , an d displacements a t restrainin g an d releasin g bends . Our survey of studies claiming strike-sli p displace ments o n interior-platfor m fault s indicate s tha t most of these interpretations are based primarily on the occurrenc e o f e n echelo n structures . Researchers studyin g interior-platfor m faultin g cannot, in general, obtai n the qualit y of data available t o thos e studyin g fault s i n Phanerozoi c ero genic belts . Based o n th e example s describe d i n thi s paper , we conclude tha t 'typical ' strike-sli p displacemen t on fault zones of the interior platform of the United States i s expresse d a t the surfac e b y a belts o f e n echelon second-orde r faults . Suc h belt s ar e u p t o 100-600 km long, an d 2-20 k m wide. Most of the en echelo n fault s ar e norma l o r normal-obliqu e faults and , in cross section , hav e listric geometrie s so they cu t onl y th e uppe r part o f the sedimentar y rock column . In som e cases , however , th e e n ech elon fault s ar e strike-sli p o r oblique-slip . E n ech elon dome s o r anticline s als o for m alon g som e strike-slip faults. Where subsurfac e data ar e avail able, the y demonstrat e tha t through-goin g maste r faults underli e e n echelo n system . Suc h maste r faults ar e vertica l o r nearl y s o an d bifurcat e upward, producin g flowe r structures . Dip-sli p components o f motion s o n thes e fault s lea d t o development o f monoclina l uplift s (fault-propa gation folds) . Information o n the magnitud e o f offse t i s avail able for relatively fe w interior-platform fault zones. In th e cas e o f documente d examples , thes e zone s have bot h strike-sli p an d dip-slip component s tha t range from a few tens of metres to more than 2 km. Generally, th e overal l strike-sli p componen t i s comparable t o or less tha n the dip-sli p component , so mos t interior-platfor m faul t zone s ar e bes t described a s oblique slip-fault s - pur e strike-sli p faults ar e rare . Th e observatio n tha t continenta l interior-platform strike-sli p faul t zone s ar e mani fested nea r th e surfac e b y e n echelo n faul t belts ,
Fig. 20. Sketc h map showin g the sense-of-sli p o n selected strike-slip faults, and regional palaeostress trajectories (from va n der Pluij m e t al. 1997) . As can be seen , some known strike-slip senses on mapped faults match the 'predicted' sens e of strike-slip while others do not. The inset shows a 'conjugat e shear ' interpretatio n o f faulting .
and that basement-penetrating maste r faults d o not reach th e surfac e confirm s tha t interio r platfor m strike-slip fault s ar e small displacement faults. This style of deformation is typical of laboratory model
Fig. 21. Th e jostlin g bloc k mode l o f interior-platform faulting, (a ) I n thi s model , strike-slip is a component of oblique-slip faulting tha t occurs along the lateral edge of a block . (Modifie d fro m Ston e (1969). (b ) Th e sens e of slip o n a strike-sli p faul t depend s o n th e geometr y o f faulting, relativ e to regional strain.
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studies i n whic h a clay cak e ha s bee n place d ove r two woode n block s tha t begi n t o shea r pas t eac h other i n a strike-slip sens e b y only a slight amount (Mandl 1988) . I n thi s context , e n echelo n strike slip fault s ca n b e considere d t o b e Riede l shears , while e n echelo n norma l fault s ar e effectivel y extension gashe s resultin g fro m th e sligh t stretch ing that accompanie s simpl e shea r acros s a belt of finite widt h (Fig . 6) . Many i f no t mos t o f th e faul t zone s o f th e interior platfor m date to th e Proterozoic, an d have undergone multiple episodes of displacement under a variety of stres s fields . Strike-sli p components of displacement wer e imparte d durin g severa l event s that coincid e wit h margina l orogenie s o f Nort h America. Fo r example , som e displacemen t occurred durin g th e Ordovicia n (th e Taconi c event), the Devonian (th e Acadian event) , and dur-
179
ing th e lat e Palaeozoi c (th e Alleghenian-Ouachit a event). Th e las t o f thes e wa s th e mos t significant , causing faul t reactivatio n acros s th e entir e interio r platform. Thi s faul t reactivatio n i s th e Ancestra l Rockies event . Convergenc e alon g th e southwestern margi n o f th e continen t ma y hav e contributed to the Ancestral Rockies event. Faults in the Rocky Mountains and Colorado Plateau were also reactivated durin g th e Mesozoic-Cenozoi c Laramid e event. Som e fault s (e.g . fault s o f th e Ne w Madri d zone) remai n activ e today . I n mos t cases , th e strike-slip episod e wa s no t th e earlies t displace ment fo r th e faul t i n question . Developing a regiona l interpretatio n o f shea r sense o n th e interior-platfor m strike-sli p fault s o f the Unite d State s remain s problematic , fo r shea r sense dat a ar e incomplete . Severa l author s hav e assumed tha t the shear sens e o n a given faul t mus t
Fig. 22. Compariso n map s o f th e interio r platfor m o f th e Unite d States , a t th e tim e o f th e Alleghanian-Ouachit a orogeny, an d eastern Eurasia today, (a ) Th e interior platfor m o f the United States is a rigid craton , whose upper crust has bee n broke n int o a mosaic o f blocks b y faults . I n easter n Eurasia , the souther n margin o f th e continen t is a sof t Phanerozoic orogen. (b) During the Alleghanian-Ouachita collision, the craton strain s onl y slightly , s o crustal blocks move only slightly . In eastern Eurasia , crustal blocks undergo lateral escap e when the lithosphere strain s significantly . (c) Th e mosai c o f crusta l block s i n th e interio r platfor m o f th e Unite d State s contrast s wit h majo r regiona l fault s i n eastern Eurasia . (Eurasi a figure s modifie d fro m Tapponie r & Molnar 1976) .
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be compatibl e wit h th e prediction s o f a conjugate shear model, i n which the predicted shea r sense on a given faul t i s taken to be the shea r sens e that the fault would have if it were a member of a conjugate shear se t whos e acut e bisectri x i s th e maximu m principal compressiv e stress . As an example of this model, conside r th e stres s fiel d resultin g fro m th e late Palaeozoic Alleghanian-Ouachita orogeny (th e collision o f Sout h Americ a an d Afric a wit h North America). Studie s o f calcite twinning i n limestone units o f th e US A continenta l interio r sugges t tha t crl during this event trended roughly NW (Figs 4 & 20A). In the conjugate shear model, faults trendin g approximately N-S shoul d be sinistral, while those trending approximatel y E- W shoul d b e dextral . Taken a t fac e value , th e strike-sli p shea r sens e reported for faults activ e during the late Palaeozoi c in th e interio r platfor m d o no t al l fi t thi s mode l (Fig. 20B) . I n som e cases , paralle l fault s hav e opposite shea r senses . We propos e tha t a n alternativ e approac h t o understanding th e regiona l patter n o f shea r sens e on continenta l interio r fault s come s fro m examin ing ho w regiona l strai n ca n b e accommodate d i n the contex t o f th e 'jostlin g block ' mode l (e.g . Davis 1978 ; Tikoff & Wojtal 1999) . A s noted earlier, the two sets of faults o n the continental interio r platform divid e th e upper crus t into roughl y recti linear blocks. Movement on the faults occur s when these block s jostl e wit h respec t t o on e anothe r i n response t o a regiona l strai n o f th e interio r plat form - suc h strains result primarily fro m collisional and/or convergen t orogen y alon g th e continenta l margin. I n thi s model , whic h ha s bee n applie d t o individual examples previousl y (e.g . Paylo r & Yin 1993), strike-sli p o r oblique-sli p fault s ar e effec tively transfe r fault s accommodatin g th e dip-sli p displacement o n a frontal faul t alon g anothe r edge of a block. Thus , the sens e o f sli p simpl y depends on th e di p o f th e fronta l faul t t o whic h th e strike slip faul t link s (Fig . 21). If, for example, the transfer faul t link s t o a NW-dipping revers e fault , the n it will have a dextral sens e o f slip, whil e i f it link s to a SE-dipping revers e fault , the n it has a sinistral sense o f slip , Becaus e o f th e complexit y o f th e regional patter n o f faults , an d th e fac t tha t som e faults hav e a propeller shape , th e regiona l patter n of strike-sli p shea r sens e woul d be expecte d t o b e quite complex . The souther n Oklahom a aulacoge n stand s ou t among th e faul t zone s o f th e interio r platfor m i n hosting a n order of magnitude more sli p than othe r faults (Fig s 1 & 9) . Bot h it s vertica l an d latera l components o f displacement ar e significantly large r than o n othe r faults . Thi s contras t ma y reflec t th e fact that the fault zones of the aulacogen ar e longer, and i f linke d t o thos e o f th e Uncompahgr e uplift , effectively extende d to the late Palaeozoi c wester n
continental margin. Thus, during Ancestral Rockies strain, the continent north of this block wa s fre e t o translate westward s by ten s o f kilometres . I n thi s regard, th e faul t behave d lik e intracontinenta l transform, muc h like its neighbour to the south, the similarly trendin g Mojave-Sonor a megashea r (Fig. 1) , did durin g the Mesozoic . We conclude b y comparing th e nature of strikeslip faultin g i n th e interio r platfor m o f th e Unite d States t o th e strike-sli p faultin g o f centra l an d southern Eurasia (Fig. 22). Kluth and Coney (1981) suggested tha t ther e i s a n analog y betwee n deformation o f th e Nort h America n continenta l interio r during the Alleghanian orogeny-Ancestral Rockie s event, an d deformation of central Asi a in respons e to the collision o f India. While there is some merit to thi s concept , i n tha t far-fiel d erogeni c stresse s are drivin g intracontinenta l deformatio n i n bot h regions, w e emphasize tha t the interior platfor m of North Americ a contrast s significantl y wit h central/southern Eurasia , i n that i t strike-sli p faulting magnitudes are one to two orders of magnitude less tha n they ar e i n Eurasia. We sugges t that difference reflect s th e differenc e i n th e relativ e strengths o f th e lithospher e o f th e tw o continents . In Nort h America , th e interior-platfor m fault s cu t the crust of an essentially strong craton. The upper crust o f thi s crato n containe d a numbe r o f pre existing faults, breakin g it into a mosaic of blocks, and thes e blocks jostled wit h respect t o each othe r when th e regio n underwen t strain, bu t becaus e of the strength o f the lithosphere, eve n the great collision of the Alleghanian orogeny did not cause large strains. I n Asia, however, th e collision of souther n continents deforme d lithospher e tha t ha d bee n weakened during the heating that accompanied pre-
Fig. 23. Bloc k diagrams indicatin g ho w stronge r conti nental lithosphere strains by a smaller amount while weak lithosphere strains by a larger amount. Crusta l blocks of soft lithospher e undergo greate r displacemen t tha n thos e of stron g lithosphere .
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vious Phanerozoi c orogenie s (Sengo r & Nata l 1996). I n thi s wea k lithosphere , larg e fault s developed an d accommodate d significan t latera l escape (Fig . 23) . This research wa s supported, in part, by the US Geologi cal Surve y (USGS ) unde r USG S awar d numbe r 99HQGR0075 (Universit y o f Illinois) . Th e view s an d conclusions containe d i n thi s documen t ar e thos e o f th e authors an d shoul d no t b e interprete d a s necessarily representing the official policies , either expressed or implied , of th e U S Government . Thi s wor k wa s als o supporte d in par t b y th e Earthquak e Engineerin g Researc h Center s Program o f th e Nationa l Scienc e Foundatio n unde r Award Number EEC-9701785. We also acknowledge the support o f thi s researc h b y Landmar k Graphic s vi a th e Landmark University Grant Progra m a t the Universit y of Illinois a t Urbana-Champaign . Dat a processin g fo r thi s study wa s performed usin g Landmark's ProMA X 2-D™ . Finally, w e wish t o than k R . Hold s worth an d B . Tikoff , for ver y helpfu l reviews , an d th e editor s o f thi s volum e for thei r patience .
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Late Palaeozoic formatio n an d developmen t of the S t Marys Basin, mainland Nov a Scotia , Canada: a prolonged recor d o f intracontinenta l strike-slip deformatio n during th e assembl y o f Pangaea J. B . MURPHY Department of Geology, St Francis Xavier University, Antigonish, Nova Scotia, Canada, B2G 2W5 (e-mail: [email protected]) Abstract: Th e Lat e Palaeozoi c evolutio n o f th e S t Mary s Basin , mainlan d Nov a Scotia , preserves evidence of protracted dextral shear along an intracontinental fault zone during collisional orogenesis an d the assembl y o f Pangaea. The S t Marys Basin formed within the E-W-trendin g Minas Faul t Zon e (MFZ ) along th e boundar y between th e Avalo n an d Megum a terrane s an d contains latest Devonian-Tournaisian continental clastic rocks that are 3000-4000 m in thickness. The origi n an d evolutio n o f th e basi n i s attribute d t o eithe r discret e o r progressiv e dextra l strike-slip tectonic s alon g th e MF Z betwee n th e Lat e Devonia n an d Lat e Carboniferous . Evidence fo r th e Lat e Devonia n origi n o f th e basi n i s recorde d alon g it s souther n flan k b y th e fabrics o f the deformed c. 370 Ma granites, the overall sedimentar y facies distribution, and some syndepositional feature s within the clastic rocks . Th e most intense deformation within the basin is concentrated i n a relatively narro w ENE-trending zone , i n whic h predominantly fine-graine d clastic rock s ar e deformed into periclinal fold s and related revers e faults. The orientation o f this zone relativ e t o th e MF Z i s consisten t wit h dextra l shear . A t leas t som e o f thi s deformatio n occurred afte r th e depositio n o f the overlyin g Visean Windsor Group . The styl e o f deformation along th e presen t norther n margi n o f th e basi n (th e Chedabucto Fault ) i s als o consisten t wit h regional dextra l shear . The St Marys Basin is an example of basin development and evolution adjacent to an intracontinental fault zone associated wit h oblique convergence during orogenesis. It s evolution provides constraints on the potential relationship betwee n the termination of the mid-Palaeozoic Acadia n orogeny, subsequen t basin development , an d the ongoin g interaction s betwee n th e Avalo n and Meguma terranes, an d between Laurentia and Gondwana during the assembly of Pangaea. Mor e generally, because the relationship between fabri c developmen t and motion along intracontinental strike-slip fault s i n continental zones is difficult t o interpret, the sedimentology an d structural geology i n basin s develope d alon g thes e faul t zone s ma y preserv e a les s ambiguou s record o f the mai n tectoni c events .
Introduction mo
n manifestation s o f these faul t motion s an d the complex interpla y o f sedimentatio n an d defor In moder n continenta l collisio n zones , suc h as the matio n durin g basin origi n an d evolutio n ca n proAlps and the Himalayas, intracontinental strike-slip vid e a distinctive signa l fro m whic h the kinemati c faults focu s an d accommodat e long-ter m crusta l evolutio n o f th e faul t syste m ca n b e deduced , movements during regional plate convergence (e.g. I n th e norther n Appalachia n orogen , th e Lat e Teyssier e t al. 1995). Although their kinematic his- Palaeozoi c history of Atlantic Canada is dominated tories ca n provid e importan t constraint s o n devel - b y the development o f post-collisional basins, such opment of the orogen, late r movements along these a s th e Magdale n Basin , forme d followin g th e faults ten d t o overprin t an d obscur e th e evidenc e destructio n o f th e lapetu s Ocea n (William s 1979; of the earlier events makin g this history difficul t t o William s & Hatcher 1983 ; Keppie 1985) . The Late decipher. Basi n formatio n i s on e o f severa l com - Devonian-Earl y Carboniferou s Horto n Grou p an d
From: STORTI , F. , HOLDSWORTH , R . E . & SALVINI , F . (eds ) Intraplate Strike-Slip Deformation Belts. Geologica l Society, London , Specia l Publications , 210, 185-196, 0305-8719/037$ 15 © Th e Geologica l Societ y o f London 2003 .
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its correlatives wer e deposited in the earliest stage s of th e developmen t o f th e Lat e Devonian-Earl y Permian Magdale n Basi n (Fig . 1). Continenta l reconstructions (e.g . McKerrow & Scotes e 1990; Dalziel et al. 1994; Golonka et al 1995 ; Keppie el al. 1996 ) suggest tha t Late Devonia n to Carboniferous basi n developmen t als o wa s influence d b y clockwise rotatio n o f Laurenti a relativ e t o Gond wana prior to Late Carboniferous to Permian continental collisio n an d th e formatio n of Pangaea. The S t Mary s Basi n (8MB , Fig . 1 ) i s th e southernmost depocentr e o f th e composit e Magdalen Basin and consists almost entirely of the Late Devonian-Early Carboniferou s Horto n Grou p continental clasti c rock s (Fig s 1 & 2) . I n addition , i t occurs along the boundary between the Avalon and Meguma terranes , tw o terrane s tha t ha d collide d with Laurenti a befor e Horto n Grou p depositio n (Williams & Hatcher 1983; Keppie 1985 ; Stockmal et al . 1987 ; van Staa l 1994 ; Murphy et al . 1995; Keppie e t al . 1996 ; van Staa l e t al . 1998) . Th e basin configuratio n is though t t o hav e bee n influ enced b y episodi c dextra l motio n alon g th e Avalon-Meguma terran e boundar y (know n a s th e Minas Faul t Zone ) durin g the latest Devonia n and Carboniferous (Keppi e 1982 ; Murphy et al . 1995;
Gibbons e t al. 1996) , whic h has been attribute d to the clockwis e rotatio n o f Laurenti a relativ e t o Gondwana durin g tha t tim e (e.g . Keppi e 1982 ; Keppie e t al. 1996) .
Regional settin g The evolutio n o f th e Appalachia n oroge n consist s of Earl y Palaeozoi c telescopin g o f th e Laurentia n margin, the accretion o f suspect terranes a t various times i n th e Earl y t o Mid-Palaeozoic, followe d by Late Palaeozoic terminal collision between Laurentia an d Gondwan a an d th e fina l amalgamatio n o f Pangea (William s 1979 ; Williams & Hatcher 1983; Keppie 1985 ; van Staa l e t al . 1998) . Mainlan d Nova Scoti a expose s tw o accrete d terranes , th e Avalon an d Meguma terranes, separate d b y th e S t Marys Basin , whic h develope d alon g th e Mina s Fault Zon e (Murph y 2000) . Avalonia i n mainlan d Nov a Scoti a lie s t o th e north o f the S t Marys Basin. I t is characterized b y voluminous low-grad e lat e Neoproterozoi c (c . 630-570 Ma) arc-relate d volcani c an d cogeneti c plutonic rocks an d coeval sedimentar y succession s deposited i n a variet y o f arc-relate d basin s (e.g. Murphy & Keppi e 1987 , Pe-Piper & Pipe r 1987,
Fig. 1. Lat e Palaeozoi c reconstructio n o f th e circum-Atlanti c are a showin g th e distributio n o f Avaloni a an d related peri-Gondwanan terranes , the Meguma terran e (coarse-stippled) , and the Magdalen Basi n (lined) . Th e inset show s the southern portio n o f th e Magdale n Basin , an d th e S t Mary s Basi n (SMB ) and Mina s Faul t Zon e (MFZ ) along th e Avalon-Meguma terran e boundary .
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Fig. 2 . (a ) Summar y geologica l ma p o f th e S t Mary s Basi n showin g distributio n o f Horto n Grou p clasti c rocks , regional fol d hinges , an d representativ e stereoplo t domain s o f th e structura l data fro m variou s domain s i n th e basin . All fault s occu r withi n the Minas Fault Zone (Fig . 1) . Little Stewiack e River, LS, Barrens Hills, B , Lochiel, L , Cros s Brook, C, West River St Marys, WR, Graham Hill, GH. CHF -Country Harbour Fault. For location, see inset Figure 1 . The inse t show s typica l facie s (modifie d fro m Murph y & Ric e 1998) . Equa l are a stereoplot s sho w example s o f th e structural styl e i n variou s part s o f th e basin , (b ) Summar y geologica l ma p o f th e wester n portio n o f th e S t Mary s Basin. Note the continuity of fold structure s across the Horton-Windsor Group contact. Regional relationships indicat e that th e contac t i s a n angular unconformity . Not e th e variabl e plung e o f regional folds .
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1989; Murph y e t al. 1990 ) overlain b y a n Earl y Cambrian platformal successio n that contains diagnostic Acado-Balti c (Avalonian ) faun a (Theokritoff 1979 ; Keppie 1985) . To the south of the St Marys Basin, the Meguma terrane (Fig s 1 & 2 ) consist s o f a c . 1 0 km thic k sequence of Cambrian-Ordovician turbidites o f th e Meguma Group , whic h ar e interprete d a s submarine fa n deposit s (Schen k 1991) . Th e Megum a Group is unconformably overlain by a latest Ordo vician-Silurian successio n o f bimoda l within-plat e volcanic rocks , fine-graine d siliciclastics , an d minor limestones o f the White Rock and Torbroo k formations. Volcani c rocks in the White Rock Formation yiel d a U-P b zirco n ag e o f 44 0 Ma (Keppie & Krog h 2000 ) an d th e Torboo k Forma tion sedimentar y rock s contai n a Rhenish Bohemian fauna ranging in age from Early Gedinnian t o Earl y Emsia n (Bouco t 1960) . The Megum a terran e lie s t o th e sout h o f th e S t Marys Basin . I t wa s initiall y deforme d an d meta morphosed unde r mainl y greenschis t facie s conditions a t c. 415-390 Ma which produced regiona l folds an d a penetrativ e cleavag e (Keppi e & Dallmeyer 1995 ; Hicks e t al 1999) . Thi s wa s followed a t c . 380-37 0 Ma b y voluminou s granitoi d plutonism, relatively mino r mafic intrusions , penecontemporaneous mesotherma l gol d mineralization, low-pressure/high-temperatur e metamorphism, an d developmen t o f dextra l shea r zones (Keppie etal 1991 , 1998; Clarke et al. 1991 \ Kontak e t al. 1998 ; Murphy et al. 1999) . Although th e origina l relationshi p betwee n th e Meguma an d Avalo n terrane s i s controversia l (Keppie 1993 ; van Staal et al. 1998) , it is generally accepted that the onset of the Acadian orogeny at c. 415 Ma wa s associate d wit h obliqu e convergenc e between these terranes durin g or following the collision o f th e Megum a terran e wit h Laurenti a (Keppie & Dallmeye r 1995) . Thi s even t i s predominantly expresse d withi n Megum a terran e rocks by polyphase deformation and regional metamorphism. The styl e o f subsequen t Lat e Devonian Carboniferous tectonotherma l activit y varie s throughout th e oroge n dependin g o n th e relation ship betwee n convergenc e an d th e irregula r shap e of th e continenta l margi n (e.g . Stockma l e t al . 1987). Intens e telescoping , magmatism , an d fore land basi n developmen t associate d wit h approxi mately orthogona l convergenc e predominate s i n northeastern USA , whereas i n Maritim e Canad a i t is characterize d b y predominantl y strike-sli p tectonic activit y an d coeva l basi n developmen t asso ciated wit h highly obliqu e convergence (e.g . Kep pie & Dallmeyer 1995) . Th e basi n fil l consist s o f Late Devonian-Early Carboniferou s 3000-4000 m continental clasti c rock s o f th e Horto n Grou p that
were deposite d i n fluvia l an d lacustrin e environ ments after th e peak of the Acadian orogeny. These rocks exten d ove r muc h o f Maritim e Canad a an d overstep th e boundarie s betwee n th e previousl y accreted terranes . Thes e rock s ar e overlai n b y th e Visean Windso r Grou p (e.g . Fig. 2b), whic h consists o f a predominantly marine sequenc e of lime stones, evaporites, and clastic rocks that are in turn overlain by thic k accumulations (c. 1 0 km) o f Late Carboniferous-Early Permia n clasti c rock s (Durling & Marillie r 1993) . Th e contac t betwee n the Horton and Windsor groups varies in character from a sharp angular unconformity i n some regions to a concordan t contact i n other s (e.g . Boehner & Giles 1993 ; Boehne r 1994) . Fossi l evidenc e als o indicates th e presenc e o f a gap i n th e depositiona l record acros s th e contac t (e.g . Giles & Boehne r 1982).
Geology o f th e S t Marys Basi n The S t Mary s Basi n i s bounde d t o th e nort h an d south b y majo r E-W-trendin g fault s whic h belong to th e Mina s Faul t Zone , th e Chedabuct o Faul t t o the north and the West River St Marys Fault to the south (Fig. 2). The basin fill rocks are divided into six partiall y laterall y equivalen t formation s fo r a total thickness of about 4500 m that were deposited in a fluviatile to lacustrine environment (Murphy & Rice 1998) . Each formation contains latest Devonian (Famennian ) t o Tournaisia n fossil s (Benso n 1967, 1974 ; G. Dolby , pers . comm . 1994) , whic h indicates depositio n betwee n c . 36 5 Ma an d 350 Ma (e.g. Tucker et al. 1998) . Contact relationships betwee n thes e formation s ar e conformabl e and gradational with interlayering of the characteristic lithologies . Clas t compositio n (Murph y et al . 1994; Jenne x e t al . 2000) , lithogeochemistr y (Murphy 2000) , an d detrita l zirco n population s (Murphy & Hamilto n 2000 ) al l indicat e tha t th e clasts wer e predominantl y derive d fro m th e Meguma terrane . The Little Stewiacke River Formation comprise s the stratigraphicall y lowes t rock s an d i s predomi nantly expose d i n th e centra l par t o f th e basi n i n a serie s o f ENE-WSW e n echelo n anticlines . Th e formation predominantl y consists o f c. 800-950 m of interstratified , thinl y bedded , fine-graine d ligh t to dark grey and black clastic rocks that range from mudstone t o slat e dependin g o n th e developmen t of cleavage . Interbedde d siltston e an d relativel y minor light to dark grey sandstone commonly containing abundan t comminute d plan t debri s als o occur. Th e overlyin g Barren s Hill s Formatio n i s characterized b y c . 100 0 m of resistant intervals of fine t o ver y coars e graine d quartz-ric h sandston e that weather s ligh t gre y t o grey-white , monomic t or polymic t granuleston e an d conglomerat e inter -
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stratified wit h recessive interval s o f grey - t o dark grey-weathering shal e and/or siltstone. The Lochiel Formation is considered to be largely a lateral facies equivalen t o f th e Barren s Hill s Formatio n (se e Murphy e t al. 1994 ) an d is dominated b y c. 500 m of grey-green - t o grey-brown-weathering , feld spathic, micaceous , medium - t o ver y coarse grained sandstone interstratified wit h less abundant grey, micaceou s siltstone . The northwester n portio n o f the S t Marys Basin is dominated b y the Graham Hill Formation whic h consists of c. 122 0 m of red- and maroon-weathering finer-grained sandstone and siltstone with thick intervals o f grey-weathering interstratifie d coarser grained sandstone , pebbl y sandstone , an d granul e to pebbl e conglomerate . Clast s includ e quartz , mica, intraformationa l siltston e (0.5- 1 cm), an d rhyolite. Regional relationship s indicat e tha t th e Graha m Hill Formation is also a lateral facies equivalent of the Barren s Hill s Formatio n (Fig s 2 & 3) . I n separate stratigraphi c sections , bot h formation s
Fig. 3 . Schemati c depictio n o f the sedimentar y environ ment followin g th e depositio n o f th e earl y lacustrin e phase showing the depositional environment of each sample analysed . The Barren s Hills (B ) and Lochie l (L ) formations represen t th e axia l drainag e system ; th e Cros s Brook an d West Rive r S t Marys formations are, respect ively, deposited a s the distal and proximal portions of prograding alluvial fans alon g the souther n flank of the basin adjacent t o an active fault. The Graham Hill (GH) Formation is inferred to represent a mixture of influences including axial drainage and the distal portion o f an alluvial fan sourced in Avalonia. Note the position of the future (Lat e Carboniferous) Chedabuct o Faul t whic h define s th e present norther n limi t o f S t Mary s Basi n rocks . Th e present location of the St Marys Basin to the north of this fault i s unknown.
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conformably overli e th e Littl e Stewiack e Rive r Formation. I n th e easter n portio n o f th e basin , characteristic lithologie s o f th e Graha m Hil l For mation ar e foun d interstratifie d with both th e Bar rens Hill s an d Littl e Stewiack e Rive r formations . This styl e of interstratification is thought to characterize th e stratigraphi c relationship s betwee n for mations throughou t the basin . The Cros s Broo k Formatio n consist s o f c . 1000 m o f grey-green-weatherin g sandston e inter stratified wit h less abundant grey-green-weatherin g siltstone, shal e an d conglomerate , an d rar e lime stone. The sandstones and conglomerates contai n a wide variet y o f clasts , includin g psammite , pelite , micaceous granite , vei n quartz , an d detrita l mus covite. Th e Wes t Rive r S t Mary s Formatio n extends alon g th e souther n flan k o f th e basi n an d consists o f reddish-brown - t o grey-brown weathering orthoconglomerat e interstratifie d wit h grey-brown-weathering sandstone . Th e predomi nant clast lithologies includ e schist, phyllite, meta sandstone, an d micaceou s granite . Horton Grou p rock s probabl y accumulate d i n a longitudinal drainag e syste m (Murph y & Ric e 1998) in which the Little Stewiacke Formatio n wa s deposited i n a lacustrin e environmen t overlai n b y the Barren s Hill s an d Lochie l formations , whic h were lai d dow n i n braide d fluvia l environment s (Fig. 3). To the north, where the Barrens Hills For mation i s demonstrabl y a lateral equivalen t t o th e Graham Hil l Formation , th e latte r i s though t t o have been deposited i n a highly sinuous fluvial system. Towards th e souther n flan k o f th e basin , th e sequence i s exposed alon g a gently dippin g south facing monoclin e i n whic h th e bed s coarse n an d thicken upward . Alon g thi s flank , th e predomi nance o f clasti c lithologie s suc h a s coars e fanglo merates tha t fine to the north (Murphy et al. 1995 ) is typical of basin margin facies in which the rocks were deposite d i n alluvia l fan s (Wes t Rive r S t Marys an d Cross Broo k formations , Fig . 3).
Structural evolutio n o f th e S t Mary s Basin The styl e an d intensit y o f structure s varie s mark edly across the basin. Along the southern flank, the strata, dominate d b y conglomerate s an d coars e sandstones, ar e gentl y tilted . However , regiona l considerations, togethe r with sedimentologica l and textural features , indicat e depositio n i n a n activ e strike-slip regime, an d provide evidence o f the tectonic settin g i n the earl y stage s o f deposition. In th e centra l an d easter n par t o f th e basin , th e sandstone strata are generally folde d into open and upright fold s wit h a poorly develope d axia l plana r cleavage. However, the predominantly fine-grained
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Little Stewiack e River Formation has zones of relatively intense deformation characterized b y tight to isoclinal folds , revers e faults , an d a locall y developed penetrativ e axia l planar cleavage. Thes e structures ar e indicativ e o f motio n alon g basin bounding faults , an d therefor e pertai n t o th e rela tive motion betwee n th e Megum a and Avalon terranes. Alon g th e norther n margi n o f th e basin , these structure s ar e overprinte d b y th e late-stag e motion alon g the Chedabucto Fault , whic h record s the latest stage s of deformation in the basin. Taken together, thi s analysi s provide s constraint s o n th e tectonic environmen t durin g earl y basi n develop ment an d subsequen t deformation o f the basi n fill.
Deformation along the southern flank of the basin A stereoplot show s that the strata in this region di p very gently (e.g. Smithfield, Fig. 2). Although there are significan t variation s i n th e orientation s o f th e beds in this domain, no folds were identified i n the field, no regional patterns o f folds ar e discernible , and th e rock s ar e uncleaved . I n th e southernmos t part of this domain (i.e. adjacent to the contact with the Meguma Group) the rocks generally di p gently to the south or southwest, i.e. towards the fault contact. Despite th e lack o f structures in the clasti c sedi ments in this region, th e nature of tectonic activit y that accompanie d earl y basi n developmen t ca n b e deduced. Jus t sout h o f th e basin , granites , typica l of c. 370 Ma Meguma terrane granitoid rocks, have well-developed S- C fabric s an d subhorizonta l t o moderately plunging stretching lineations (Fig . 4 a, b). Th e lac k o f deformatio n i n th e Horto n Grou p rocks indicate s tha t thi s deformatio n occurre d between 37 0 an d 36 5 Ma , an d therefor e probabl y heralds basi n development . Bot h S an d C fabric s are verticall y dipping . Th e C fabri c strike s E- W and i s paralle l t o th e basi n margi n an d th e E—W trending Wes t Rive r S t Marys Fault. Th e S fabric strikes EN E to NE and is defined by elongate muscovite an d quart z ribbons . Take n together , thes e textural feature s an d th e relativ e orientation s o f fabrics ar e indicativ e o f obliqu e sli p wit h dextra l and down-to-the-nort h component s alon g th e S t Marys Faul t a t temperature s i n exces s o f 300° C (e.g. Kuzni r & Park 1982) . There ar e als o subtl e feature s withi n th e clasti c rocks tha t ar e indicativ e o f depositio n i n a tec tonically activ e regime . Th e overal l facie s distri bution, wit h coars e conglomerate s alon g th e basi n margin and lacustrine t o braided stream deposits in the centra l basin , i s typica l o f tectonicall y activ e basin margin s (e.g . Mial l 1996) . Moreover , alon g the souther n flan k o f th e basin , th e pebble s i n coarse conglomerate s contai n E-W-strikin g frac -
tures not present in the matrix (Fig. 4c). According to Eidelman an d Reches (1992) , thes e feature s are typical o f conglomerate s deposite d i n tectonicall y active regimes. The orientation of the fractures, an d quartz fibres in these fractures , ar e consisten t with down-to-the-north movement . Som e sedimentar y features i n uncleaved clastic rocks such as dewatering structure s an d anomalousl y high-angl e cross bedding (Fig . 4d, e ) are als o consisten t wit h deposition i n a tectonically activ e region .
Deformation within the basin In th e easter n portio n o f th e basi n (e.g . aroun d Lochiel, Fig . 2) severa l example s o f outcrop-scal e folds ar e found , an d th e strat a ar e deforme d int o gentle t o ope n fold s wit h stee p NE-strikin g axia l planes an d subhorizonta l t o gentl y NE-plungin g folds. The dominan t regional structur e is a broad NE plunging anticline tha t can be traced, albei t discon tinuously, fo r abou t 1 5 km wit h a wavelengt h o f about 4 km. Cleavag e i s ver y poorl y developed , but, wher e present , i s steepl y dippin g an d axia l planar. In contrast , i n th e centra l portio n o f th e basi n (Stewiacke Rive r region, Fig . 3), relatively intens e deformation occur s i n a narro w ENE-trendin g zone. On a regional scale, this domain is dominated by a series o f anticlinal structure s that extend in an en echelo n fashio n alon g strik e fo r almos t 3 0 km. The lowes t stratigraphi c uni t (Littl e Stewiack e River Formation) is exposed in the core of the anticline, an d o n th e souther n lim b th e stratigraph y faces mor e o r les s consistentl y t o th e sout h fo r about 2 5 km, t o th e souther n flank of th e basin . Deformation i n thi s regio n i s especiall y visibl e in th e incompeten t fine-graine d rock s o f th e Littl e Stewiacke Rive r Formation , whic h ar e locall y transformed int o slate s an d ar e characterize d b y outcrop-scale isoclina l folds , thrusts , an d revers e faults, locall y develope d penetrativ e cleavage , an d transposition o f bedding int o th e cleavage . Anticlinal an d synclina l structure s withi n thi s zone commonl y hav e contrastin g styles . Th e anti clinal hing e zone s ar e tigh t t o isoclinal , paralle l folds wit h a relatively well-develope d axia l planar cleavage. I n general, i t is difficul t t o trace individ ual layer s continuousl y acros s anticlina l hinges . Flexure i n th e hinge s o f thes e structure s i s com monly accommodate d b y minor offset s alon g frac tures o r b y loca l faultin g i n th e fol d hinge s (Fig. 5a). I n contrast , small-scal e synclina l struc tures ar e broa d t o open , hav e greate r half wavelengths, an d ar e rarel y cleave d o r fracture d (Fig. 5b) . Analysi s o f th e fiel d an d structura l dat a show tha t thi s zon e i s dominate d b y e n echelo n ENE-trending periclina l anticlina l structure s
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Fig. 4 . Fiel d photographs showin g structura l feature s alon g th e souther n flan k o f the basin, (a ) an d (b ) sho w defor mation in a Meguma granitoid rock alon g the contact of the basin with the Meguma terrane. (a ) C-S fabrics in granite showing evidenc e o f dextra l shear , (b ) moderately plungin g stretchin g lineations . (c) , (d) , an d (e ) sho w feature s tha t indicate depositio n o f strat a in a tectonically activ e environment , (c ) Fractures i n pebbles i n conglomerates alon g the southern flan k o f the basin d o no t penetrat e int o the matrix , (d ) dewatering structures , (e) high-angled cross-beddin g (the angl e is significantl y greate r tha n th e 20 ° angl e o f repos e o f th e sand).
(Fig. 2) wit h a moderat e t o ver y steepl y NW dipping axia l plana r cleavag e an d cleavage-bed ding intersectio n lineation s tha t ar e simila r t o th e orientations measure d fol d axes . Mos t o f thes e regional features , includin g plung e reversal s an d faulted anticlines , ca n als o b e see n o n a n outcro p
scale, and an excellent example o f the relationshi p between feature s i s preserve d i n a n outcro p nea r the mout h o f Littl e Stewiack e Rive r (Fig . 6) . Lin eations on the bedding surfaces approximately per pendicular t o fol d axi s orientation s an d sheare d hinge area s ar e common. Thes e structura l features
192
J. B . MURPH Y by NE-S W upright , gentl y t o moderatel y WSW plunging folds . Ther e ar e fe w outcrop-scal e fold s within this part of the basin and folds are identified by regional-scal e variation s i n th e orientatio n o f bedding an d younging directions . Thes e fold s ca n be mappe d int o th e adjacen t Windso r Grou p (Figs 2 & 7), wher e the y di e ou t westward s (e.g . Stevenson 1958) . Althoug h th e orientatio n o f th e axial plane s o f thes e folds i s simila r t o tha t o f th e central an d eastern portions of the basin, the overal l variations i n plung e sugges t th e developmen t o f large-scale periclina l structures .
Deformation along the northern margin of the basin Deformation o f th e basi n fil l lithologie s i s inti mately related to motion along the present northern margin o f th e basi n (Chedabuct o Fault) . ENE trending deformatio n fabric s withi n th e basi n ar e rotated clockwis e (i.e . i n a dextra l sense ) toward s parallelism wit h th e E- W Chedabuct o Faul t (e.g . East Rive r S t Marys , Fig . 2), indicatin g coeva l o r subsequent dextra l motio n alon g th e fault . Finer grained lithologie s contai n a ver y stron g E- W planar fabric that overprints the ENE fabric within about 1. 2 km o f th e fault . In contras t t o th e souther n margin, ther e ar e no facies variation s adjacent to the Chedabuct o Fault, and thi s faul t i s obliqu e t o formatio n boundarie s within th e basin . Thi s geometr y implie s tha t th e original configuratio n o f th e basi n ha s bee n dis membered b y dextra l faultin g suggestin g tha t motion alon g the Chedabucto Faul t has removed a portion o f the basin (Murph y et al. 1995 ; Webste r et al 1998) .
NW-trending structures
Fig. 5, Fiel d photographs showing examples of structural features i n th e relativel y intensel y deforme d ENE-trend ing zon e withi n th e basi n (se e Fig . 2). (a ) ShallOWl y plunging anticlinal structure with a faulted hinge (centre), (b) Shallowl y plungin g synclin e (right ) - anticlin e (left ) pair. The anticline has a faulted hinge , the syncline does not. (c ) Typica l cleavage-beddin g relationship s o n a fold limb . are typica l o f deformatio n o f multilayere d sedi mentary sequence s b y flexura l sli p (e.g . Tanne r 1989). The wester n par t o f th e S t Marys Basi n (Nort h Pembroke, Sout h Pembroke , Fig . 2) i s dominate d
The predominan t ENE-WS W structura l grai n i s locally rotate d counterclockwis e (i.e . i n a sinistra l sense) towar d a stee p N W orientation . Thes e narrow zone s ar e adjacen t t o fractur e system s identified o n Digita l Elevatio n Mode l image s b y Webster e t al. (1998) . Thes e fracture s transect th e St Marys Basin (Webste r e l aL 1998 ) an d are continuous wit h NW-strikin g fault s mappe d i n th e Meguma Grou p an d th e Visea n Shubenacadi e Basin t o th e sout h by Giles an d Boehner (1982) . This rotatio n i s mos t eviden t alon g th e easter n margin o f th e basin , wher e th e Wes t Rive r S t Marys Faul t ha s a pronounce d arcuat e curvatur e (Fig. 2). Th e orientatio n o f rock s o f th e Megum a terrane i s paralle l t o thi s curve d faul t boundary . This structur e has been interprete d a s a 'megakink' (Williams e t al . 1995 ) associate d wit h sinistra l motion alon g th e NW-strikin g shea r zon e know n as th e Countr y Harbou r Faul t (CHF , Fig . 2a) .
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Fig. 6 . Sketc h an d stereoplo t o f structure s i n a n outcro p nea r th e mout h o f Littl e Stewiack e Rive r (modifie d fro m Benson 1967) . Th e sketc h show s tha t th e typica l relationshi p betwee n anticline s an d revers e faults . Not e als o th e evidence o f plung e reversals .
Fig. 7 . Fold s i n laminate d siltstone s o f th e Visea n Windsor Grou p showin g NE-trendin g fold s identica l i n orientation t o adjacen t Morto n Grou p lithologies .
Webster e t al. (1998) showe d tha t th e trace o f this fault extend s int o th e S t Marys Basin a s a fractur e with n o offset . Thi s suggest s tha t th e mos t important episod e o f motio n too k plac e alon g th e fault prio r t o depositio n o f the basin-fill rocks , an d that subsequent , post-Horto n Grou p rejuvenatio n reactivated th e structur e formin g a fractur e i n th e St Marys Basin. It is probable that these lineament s have bee n activ e a t variou s time s (e.g . William s et al . 1995 ; Webste r e t al . 1998 ) an d th e ag e o f deformation associate d wit h these structure s in the basin i s uncertain .
Synthesis Taken together , th e origi n an d evolutio n o f th e S t Marys Basi n is attribute d t o eithe r discret e o r pro gressive episode s o f dextra l strike-sli p tectonic s along the Minas Fault Zone in the Late Palaeozoic . Evidence fo r th e origi n o f th e basi n i n th e Lat e Devonian i s recorde d alon g it s souther n flan k i n
the fabric s o f th e c . 37 0 Ma granites , th e overal l facies distributio n an d som e o f the syndepositiona l features within the clastic rocks. The age and orientation o f th e C- S fabric s i n Megum a terran e gran ites indicat e tha t dextra l transpressio n occurre d between c . 37 0 an d 36 5 Ma an d thi s deformatio n is interprete d t o hav e accompanie d exhumatio n of the terran e an d ma y hav e influence d th e origi n o f the basi n itself . Thi s interpretatio n i s consisten t with studie s in the Meguma terran e whic h indicat e rapid uplif t (estimate d betwee n 5 an d 1 2 km) an d dextral translatio n relativ e t o the Avalon terrane in the Lat e Devonia n (e.g . Keppi e 1993) . Based o n structura l an d sedimentologica l analy sis, this margin of the St Marys Basin is interpreted to have undergone dextral transcurren t motio n an d uplift betwee n 37 0 and 365 Ma. This interpretatio n is indicate d b y th e presenc e o f 380-37 0 M a deformed granite s alon g th e southern basin margi n that display dextral C-S fabrics which were uplifte d and expose d prio r t o depositio n o f unconformably overlying Horto n Grou p rock s (Murph y e t al . 1994). 40 Ar/39Ar dat a fro m muscovite s i n dextra l shea r zones nea r th e contac t wit h th e S t Mary s Basi n indicate tha t coolin g throug h ~400-350° C con tinued unti l abou t 34 5 M a (Keppi e & Dallmeye r 1995). Thi s implie s tha t uplift an d regional dextra l shear continue d durin g S t Marys Basi n deposition . The clas t compositio n o f th e Horto n Grou p rock s (schist, phyllite, metasandstone, micaceou s granite , gold), togethe r wit h lithogeochemica l an d detrita l zircon data , indicat e derivatio n fro m th e Meguma terrane and imply exhumation of these source rocks by c . 36 5 M a (e.g . Murph y e t al . 1994 ; Jenne x e t al. 2000 ; Murph y 2000 ; Murph y & Hamilto n 2000). Deformation o f the basin-fil l rock s is mos t pro -
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nounced i n a narrow ( 2 km wide ) zon e dominate d by les s competen t fine-graine d clasti c rock s an d produced ENE-trendin g periclina l fold s an d asso ciated reverse faults. The orientation of these structures relativ e t o th e E-W-trendin g Mina s Faul t Zone i s consistent wit h thei r origi n b y coeval dex tral shea r alon g th e faul t zone . T o th e north , th e rotation o f these structures int o parallelism with the Chedabucto Fault could be attributed to a progressive continuation o f the sam e event, or could reflec t a late r phas e o f dextra l shear . The ag e o f thes e deformatio n event s i s poorl y constrained within the basin. Mapping of structures across th e Horto n Group-Windso r Grou p contac t suggests that a t least som e o f the deformatio n wa s post-Tournaisian i n age . Regiona l considerations , including th e 34 5 M a ag e o f mica s i n adjacen t shear zones , sugges t tha t th e dextra l strike-sli p deformation ma y hav e occurre d a t variou s time s during th e Carboniferou s alon g th e Mina s Faul t Zone. Stratigraphi c an d palaeontologica l studie s indicate a gap in the depositional record acros s this contact. Contac t relationship s (Fig . 8 ) var y fro m concordant in some sections to pronounced angular unconformity i n other s (e.g . Boehne r & Gile s 1993; Boehne r 1994) , indicatin g tha t foldin g occurred i n som e region s prio r t o Windso r Grou p deposition. Thes e relationships ar e compatible wit h the fault-relate d heterogeneou s styl e o f defor mation o f Horto n Grou p strat a dependin g o n faul t geometries an d proximity o f active faults . Angular unconformities betwee n Horto n an d Windso r groups ar e especiall y pronounce d nea r basi n mar gins, suggestin g a relationshi p betwee n pre -
Fig. 8. Schemati c diagra m showing the variation in style of th e Horto n Group-Windso r Grou p contact . I n som e sections, faul t geometr y ha s resulted i n localize d intens e deformation an d reverse faulting prio r to Windsor Group deposition, suc h a s i s eviden t i n th e S t Mary s Basi n (section a) . Nea r basi n margins , motio n alon g fault s deformed Horto n Grou p strat a prio r t o Windso r Grou p deposition, als o resultin g a n angula r unconformit y between th e Horto n an d Windso r group s (sectio n b) . I n Horton Grou p section s tha t wer e distan t fro m activ e faults, pre-Windso r deformatio n is minor, resulting in an approximately concordan t contac t (sectio n c) . Th e tim e gap implied by the fossil evidenc e indicates that this contact i s a disconformity .
Windsor deformatio n an d motio n alon g basin bounding fault s (Fig . 8). To the wes t of the S t Marys Basin, a knocker of mafic granulite-grad e mylonite expose d i n a megabreccia within the Minas Fault Zone records a similar histor y o f rapi d uplif t an d coeva l faulting . According to Gibbons et al. (1996), granulite-grad e metamorphism o f the mafi c protolit h too k place a t 370 M a (U-Pb , zircon ) a t lowe r crusta l pressure s of 7.5 to 9.5 kbar. Brittle fractures i n the granulite, at pressure s o f c . 2 kbar, contai n amphibol e date d at 33 5 Ma ( 40Ar/39Ar) which post-dates depositio n of the basal Windsor Group units. The megabrecci a itself contain s slab s o f th e Namuria n Mabo u Group, whic h implie s tha t a furthe r episod e o f faulting occurre d betwee n abou t 33 0 an d 32 0 Ma. In wester n mainlan d Nov a Scoti a an d souther n New Brunswick , dextra l motio n alon g th e Mina s Fault Zon e between abou t 320 and 300 Ma reactiv ated Acadian thrusts (Culshaw & Liesa 1997 ; Culshaw & Reynold s 1997 ) an d produce d positiv e flower structure s (Nanc e 1986 , 1987 ; Waldro n e t al 1989) .
Conclusions During continenta l collision , intracontinenta l faul t zones ar e commonl y characterize d b y comple x protracted historie s o f shear , an d cycle s o f fabri c development an d destruction . A s a result , th e relationship o f a given fabri c t o a specifi c tectoni c event i s difficul t t o decipher . Sedimentar y basin s developed withi n o r adjacen t t o thes e faul t zones , however, commonl y displa y a variet y o f sedi mentological an d structura l features tha t facilitat e regional tectoni c interpretations . Th e pronounce d facies variation s produced durin g basin depositio n provide competenc e contrast s tha t profoundl y influence th e styl e o f subsequen t deformation . I n the S t Mary s Basin , finer-graine d deposit s i n th e centre o f th e basi n focu s an d accommodat e strai n within th e basin-fil l lithologies , i n contras t t o th e relatively undeforme d basi n margi n facies . The S t Mary s Basi n i s a n excellen t exampl e o f basin developmen t an d evolutio n adjacen t t o a n intracontinental faul t zon e associate d wit h obliqu e convergence durin g orogenesis . It s evolutio n pro vides constraint s o n th e potentia l relationshi p between th e terminatio n o f th e Acadia n orogeny , subsequent basin development, th e ongoing dextral translation alon g th e Avalon-Megum a terran e boundary, an d th e relationshi p betwee n Laurenti a and Gondwan a durin g the assembl y o f Pangaea . I wish to acknowledge the continuing support of the Natural Sciences an d Engineering Researc h Counci l (NSERC ) Canada, a Gledden Senio r Fellowshi p awar d while I was on sabbatical leave at the University of Western Australia,
STRIKE-SLIP FAULTIN G AN D PANGAE A ASSEMBL Y a University Counci l fo r Researc h Gran t a t S t Franci s Xavier University , th e constructiv e review s o f R . Stra chan an d J . Waldron , th e logistica l an d fiel d suppor t of F. Chandler, an d discussions wit h L. Jennex , R. Rice, T . Stokes, an d T . Webster . Initia l fieldwor k wa s supporte d by th e Canada-Nova Scoti a Minera l Developmen t agree ment.
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MIALL, A . D . 1996 . The Geology o f Fluvial Deposits. Springer Verlag , Berlin . MURPHY, J. B. 2000. Tectonic influence on sedimentatio n along th e souther n flan k o f th e Lat e Paleozoi c Magdalen Basi n i n the Canadia n Appalachians: geochemi cal an d isotopi c constraint s o n th e Horto n Grou p i n the St Marys Basin , Nova Scotia. Bulletin, Geological Society o f America, 112, 997-1011 . MURPHY, J . B. & HAMILTON, M . A. 2000. U-Pb detrital zircon ag e constraint s o n evolutio n o f th e Lat e Paleo zoic S t Mary s Basin , centra l mainlan d Nov a Scotia . Journal o f Geology, 108 , 53-72. MURPHY, J . B. & KEPPIE, J. D. 1987. Late Devonian palinspastic reconstruction of the Avalon—Meguma terrane boundary: implication s for terran e accretio n an d basi n development i n the Appalachian orogen . Tectonophysics, 284, 221-231. MURPHY, J . B. & RICE, R. J. 1998. Stratigraphy an d depositional environmen t o f Horto n Grou p rocks i n th e S t Marys Basin , centra l mainlan d Nov a Scotia . Atlantic Geology, 35 , 1-26 . MURPHY, J . B. , KEPPIE , J . D. , DOSTAL , J . & HYNES , A . J. 1990 . Late Precambria n Georgevill e group : a n volcanic arc rift succession i n the Avalon Terrane o f Nova Scotia. Geological Society o f London Special Publication, 51, 383-393. MURPHY, J . B. , RICE , R . J., STOKES , T . R . & KEPPIE , D . F. 1995 . The S t Mary s Basin , central mainlan d Nova Scotia: Late Paleozoic basi n formation and deformation along th e Avalon-Megum a Terrane boundary , Canad ian Appalachians. In : HIBBARD, J., VAN STAAL, C. R. & CAWOOD, P . (eds ) New Perspectives i n th e Caledonian-Appalachian Orogen. Geologica l Associatio n o f Canada Specia l Paper , 41 , 409-420. MURPHY, J . B., STOKES , T . R., MEAGHER, C . & MOSHER , S. J. 1994 . The geology o f the eastern S t Marys Basin. Current Research, 1994-D, Geological Surve y of Canada, 95-102 . MURPHY, J . B., VA N STAAL, C . R . & KEPPIE , J . D . 1999. Is the mid to late Paleozoic Acadia n Orogeny a plumemodified Laramide-styl e orogeny ? Geology, 27, 653 656. NANCE, R . D. 1986 . Late Carboniferou s tectonostratigra phy in the Avalon terrane of southern New Brunswick. Maritime Sediments an d Atlantic Geology, 22 , 308 326. NANCE, R . D . 1987 . Dextral transpressio n an d lat e Carboniferous sedimentatio n i n the Fund y coastal zon e of southern New Brunswick . In'. BEAUMONT , C, & TANK ARD, A . J . (eds ) Sedimentary Basins an d Basin Forming Mechanisms. Canadia n Societ y o f Petroleu m Geologists Memoir , 12 , 363-377. PE-PiPER, G. & PIPER, D. J. W. 1987. The pre-Carbonifer ous rock s o f th e Cobequi d Hills , Avalo n Zone , Nov a Scotia. Maritime Sediments and Atlantic Geology, 23, 41-49. PE-PIPER, G. & PIPER, D . J. W. 1989 . The upper Hadrynian Jeffer s Group , Cobequi d Highlands , Nov a Scotia : a bac k ar c volcani c complex . Geological Society o f America Bulletin, 101, 364-376.
SCHENK, P . E . 1991 . Events an d se a leve l change s o n Gondwana's margin : th e Megum a Zon e (Cambrian Devonian) o f Nova Scotia, Canada. Geological Society America Bulletin, 103, 512-521. STEVENSON, I. M. 1958 . Truro Map-Area, Colchester and Hants Counties, Nova Scotia. Geologica l Surve y o f Canada Memoir , 297. STOCKMAL, G . S. , COLMAN-SADD , S . P. , KEEN , C . E. , O'BRIEN, S . J . & QUINLAN , G . 1987 . Collision alon g an irregular margin : a regional plate tectoni c interpret ation of the Canadian Appalachians. Canadian Journal of Earth Sciences, 24, 1098-1107 . TANNER, P . W . J . 1989 . The flexura l sli p mechanism . Journal o f Structural Geology, 11 , 635-655. TESSYIER, C., TIKOFF, B . & MARKLEY, M. 1995 . Oblique plate motio n an d continenta l tectonics . Geology, 23 , 447_450. THEOKRITOFF, G . 1979 . Early Cambria n provincialis m and biogeographi c boundarie s i n th e Nort h Atlanti c region. Lethaia, 12 , 281-295. TUCKER, R . D. , BRADLEY , D. C. , VE R STRAETEN, C . A. , HARRIS, A . G. , EBERT , J . R . & MCCUTCHEON , S . R . 1998. Ne w U-P b zirco n age s an d th e duratio n an d division o f Devonia n time . Earth an d Planetary Science Letters, 158 , 175-186 . VAN STAAL , C. R. 1994 . The Brunswic k subduction complex i n the Canadia n Appalachians : recor d o f the Lat e Ordovician to Late Silurian collision between Laurentia and th e Gande r margi n o f th e Avalon . Tectonics, 13, 946-962. VAN STAAL , C . R. , DEWEY , J . F. , MACNIOCAILL , C . & McKERROW, W . S . 1998 . The Cambrian-Siluria n tectonic evolutio n o f th e Appalachian s an d Britis h Cale donides: histor y o f a complex , wes t an d southwes t Pacific-type segment of lapetus. In: BLUNDELL, D . J. & SCOTT, A . C . (eds ) Lyell: Th e Past i s th e Ke y t o th e Present. Geological Societ y o f Londo n Specia l Publi cation, 143 , 199-242 . WALDRON, J. W. F., PIPER, D. J. W. & PE-PIPER, G. 1989. Deformation o f th e Cap e Chignect o Pluton , Cobequi d Highlands, Nov a Scotia : thrustin g a t th e Meguma Avalon boundary. Atlantic Geology, 25, 51-62 . WEBSTER, T . L. , MURPHY , J . B . & BARR , S . M . 1998 . Anatomy o f a terran e boundary : a n integrate d struc tural, GIS , an d remot e sensin g stud y o f th e Avalon Meguma terran e boundary , mainlan d Nov a Scotia , Canada. Canadian Journal o f Earth Sciences, 35 , 787-801. WILLIAMS, H . 1979. Appalachian orogen in Canada. Canadian Journal o f Earth Sciences, 16 , 792-798. WILLIAMS, H . & HATCHER, R. D. 1983 . Appalachian suspect terranes , /» : HATCHER , R , D, , WILLIAMS , H . & ZIETZ, I. (eds) Contributions to the Tectonics and Geophysics o f Mountain Chains. Geologica l Societ y o f America Memoir , 158 , 33-53. WILLIAMS, P . F., GOODWIN, L . B. & LAFRANCE, B. 1995. Brittle faultin g i n th e Canadia n Appalachian s an d th e interpretation o f reflectio n seismi c data . Journal o f Structural Geology, 17, 215-232.
The Salin a de l Frail e pull-apart basin , northwest Argentin a J. REIJS * & K. McCLA Y Fault Dynamics Research Group, Geology Department, Royal Holloway, University of London, Egham, Surrey TW20 OEX, UK ^Present address: Shell International Exploration and Production B.V., Postbus 60, 2280 AB Rijswijk, The Netherlands (e-mail: [email protected]) Abstract: Th e Salina del Fraile in northwest Argentina is a Pliocene t o Recent pull-apart basin developed a t a releasin g stepove r alon g th e NNE-SSW-trendin g E l Frail e sinistra l strike-sli p fault. Th e basi n i s 3 5 km lon g an d 1 2 km wid e wit h a characteristi c rhomboida l shap e an d i s starved o f synkinemati c sediment s thu s providing unique 3D exposures . Prominen t basi n side wall faul t system s for m scarps 70 0 m high an d larg e tilte d faul t block s for m a terraced syste m along th e southwes t basin sidewall . A short-cut , basin-floor fault transect s th e pull-apar t basin connecting th e northwes t stran d o f th e E l Frail e Faul t t o th e southeas t strand . A n anticlina l positive flowe r structur e i n th e northwes t o f th e basi n i s a relic t o f early , segmented , faul t growth typica l o f strike-sli p faul t evolution . Extensio n fault s i n th e basin floo r indicat e a NESW intrabasinal extensio n direction durin g the Pliocene t o Recent. The pull-apart basin accom modates a n estimate d 7. 7 km o f sinistra l displacemen t alon g th e E l Frail e faul t system . Th e morphology an d faul t architectur e o f th e Salin a de l Frail e pull-apar t ca n b e directl y compare d to scaled sandbox models of strike-slip pull-apar t basins. This first detailed analysi s of the Salina del Frail e pull-apar t basi n provide s a mode l fo r 3 D architectur e an d evolutio n of simila r pull apart basins , an d ma y serv e a s a templat e fo r th e interpretatio n o f othe r pull-apar t systems .
Introduction Salina de l Fraile is a classic rhomboida l pull-apar t basin develope d a t a releasin g stepove r i n th e NNE-SSW-striking sinistra l strike-sli p E l Frail e fault syste m o f th e southwes t Pun a Altiplano , northwest Argentin a (~26°S ; 68°W ; Fig . 1) . Th e Puna Altiplano in the N-trending Andean erogenic belt i s a plateau extendin g fro m Per u t o northwest Argentina with an average topographic elevation of 3500 m above se a level. The southwes t part of the Puna Altiplano consists of fault-bounded ranges of basement (Precambria n metamorphi c rocks , Ordo vician metasediments , Palaeozoi c granitoid s an d volcanic rocks), and intervening fault-bounded basins infilled wit h Cenozoic to Recent lake sediments and salt-pan s (i.e . 'salars' ; Fig . 1) . Cenozoi c t o Recent volcani c edifice s and flows generate a rugged topography on top of this basement terrane and some peak s reac h elevation s o f 650 0 m (e.g . Vol can Antofalla ; Fig s 1-3) . Thi s par t o f th e Pun a Altiplano ha s undergon e a lon g an d comple x polyphase deformatio n histor y involvin g Lat e Palaeozoic deformatio n an d metamorphism , Earl y Mesozoic extension , an d Mesozoic-Tertiar y con -
traction an d strike-sli p faultin g (e.g . Allmendinge r et al. 1982 , 1989 ; Coir a e t al. 1982 ; Allmendinge r 1986; Forsyth e 1986 ; Fieldin g e t a l 1987 ; Jor dan & Alons o 1987 ; Marret t e t al . 1994 ; Mo n & Salfity 1995) . The Salin a de l Frail e i s a 3 5 km long , 1 0 km wide, fault-bounde d depression forme d in Permia n to Pliocen e strat a (Fig . 1 ) i n a stepove r i n th e E l Fraile faul t system , west o f th e Sala r d e Antofalla (Fig. 1) . Th e basi n floo r i s a t a n elevatio n o f 3500 m an d th e basi n sidewal l fault s for m 70 0 m high scarps. The basin is essentially starve d of synkinematic sediment s wit h onl y thi n gravels , pla y a lake sediment s an d saltpan s develope d o n th e basin floor. There ar e relativel y fe w well-expose d natura l examples o f pull-apar t system s tha t ca n provid e good 3 D insights int o pull-apar t basin architectur e and evolutio n (e.g . Crowel l 1974 ; Noble t e t al . 1988; Wood et al. 1994) . The lack of synkinematic sediment infil l an d th e 3 D exposure s mak e th e Salina del Fraile an exceptional exampl e o f a pullapart basi n (man y pull-aparts ar e buried ) an d per mits a detailed structura l analysis of the fault archi tecture o f thi s basin .
From: STORTI , F. , HOLDSWORTH , R . E . & SALVINI , F . (eds ) Intraplate Strike-Slip Deformation Belts. Geologica l Society, London , Specia l Publications , 210 , 197-209 , 0305-8719/037 $ 15 © Th e Geologica l Societ y o f London 2003 .
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Fig. 1 . Schemati c geologica l ma p o f th e Altiplano-Pun a o f northwes t Argentin a showin g th e locatio n o f th e Salin a del Fraile pull-apart basin . Drafte d from Landsa t TM imagery an d published maps by Allmendinger e t al. (1982) and Mon an d Salfit y (1995) .
THE SAUN A DE L FRAILE PULL-APAR T BASI N
Fig. 2 . Landsa t TM image of the Salar de Antofalla basin formed alon g a dextra l releasin g bend . Not e th e spindl e geometry o f the basin and fault scar p development on the eastern basi n sidewall .
This pape r present s th e firs t structura l analysi s of th e Salin a de l Fraile pull-apart, integratin g fiel d mapping wit h aeria l photograp h an d Landsa t T M image interpretations . Solel y base d o n satellit e images, Forsyth e (1986 ) firs t mentione d th e exist ence of the Salina de l Fraile graben and McClay & Dooley (1995 ) first described i t as pull-apart basin. McClay & Doole y (1995 ) an d Doole y & McClay (1997) note d th e strikin g resemblanc e betwee n their analogu e model s o f fille d pull-apar t basin s and satellite image s of Salina del Fraile. The analyses performe d fo r thi s wor k provid e ne w insight s into th e geometrie s an d kinematic s o f pull-apar t basin evolutio n a s wel l a s ne w dat a o n th e timin g and style s o f deformatio n i n thi s par t o f th e Pun a in th e Centra l Andes .
Regional geolog y Most o f th e present-da y structur e i n th e souther n Puna Altiplan o ca n b e relate d t o a comple x
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sequence o f Cenozoi c deformatio n phase s (Table 1 ; Fig . 1) . Fault-controlle d range s separat e basins wit h lak e sediment s an d salt-pan s (i.e . salinas o r salars) an d upon these ar e superimpose d NW-trending Lat e Cenozoi c volcani c lineaments , probably relate d t o large-scal e faultin g (Fig . 2; Salfity e t al 1984 ; Viramont e et al 1984 ; Fieldin g et al . 1987 ; Allmendinge r e t al . 1989 ; Alons o e t al. 1991 ; Marret t e t al. 1994) . The basement consist s o f Precambrian metamor phic rocks , Ordovicia n metasediments , an d Lower Palaeozoic granitoid s an d lava s (Allmendinge r e t al. 1982 ; Breitkreutz & Zeil 1984) . During the Late Ordovician-Silurian, th e Ocloyi c deformatio n (associated wit h magmatis m o f th e Pun a arc ) folded an d metamorphose d th e sedimentar y an d volcanics rock s (Coir a e t al . 1982 ; Allmendinge r 1986). A t tha t time , a brea k i n sedimentatio n occurred ove r mos t o f th e presen t souther n Pun a (Coira e t al . 1982 ; Breitkreut z & Zeil 1984) . Th e Charlie orogen y i s recorde d b y Lat e Devonian Early Carboniferous folding i n the region (Coir a et al. 1982 ; Mo n & Salfit y 1995) . Carboniferous Early Permia n marin e sedimentatio n ende d wit h the Permo-Triassi c Saalie n deformatio n an d sub sequent Lat e Palaeozoic-Tria s sic plutonis m occurred togethe r wit h depositio n o f Triassi c ter restrial red-bed s an d conglomerates (Allmendinge r et a l 1982 ; Coir a e t al . 1982 ; Breitkreut z & Zei l 1984). From Early Cretaceous t o Eocene, 'Andean cycle' rift s develope d t o th e eas t an d north o f that part o f th e Pun a Altiplano show n in Figur e 1 , an d these wer e infille d wit h Salt a grou p (Cretaceous ) continental red-beds (Salfit y 1979 ; Allmendinger et al. 1982 ; Comfngue z & Ramo s 1995 ; Mo n & Salfity 1995) . The Eocene-Oligocen e Incai c deformatio n caused dextra l transpressio n alon g the Andes trend and magmatis m (Jorda n & Alons o 1987) , mos t likely induce d b y oblique subductio n of the Nazc a oceanic plat e a t th e activ e Sout h America n conti nental margin to the west (Reutter et al. 1996) . The Mid-Miocene t o Pliocen e Quechu a deformatio n caused NW-S E shortenin g associate d wit h thrust ing an d uplif t o f th e Pun a platea u (Allmendinge r 1986; Jorda n & Alonso 1987 ; Allmendinge r e t al . 1989; Dewe y & Lam b 1992) . Tertiar y volcanis m along NW-S E lineament s produce d aligned , hig h volcanic peaks (e.g . Volcan Antofalla; Fig s 1 & 2) and widesprea d ignimbrit e sheet s (Franci s e t al . 1978; Coir a et al. 1982 ; D e Silva & Francis 1991) . The Diaguit a deformatio n (lates t Pliocen e t o Quaternary) produce d dextra l an d sinistra l strike slip an d norma l faultin g i n th e area , wit h a n approximately NNW-SS E leas t horizonta l stres s direction, subparalle l t o th e tren d o f th e Ande s (Allmendinger e t al. 1989 ; Marret t e t al. 1994) . I n isolated, internall y draine d basin s (e.g . Sala r d e
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Fig. 3 . Stratigraph y of the Salina del Fraile area . Age boundaries from Coir a & Pezzutti (1976) and Voss et al. (1996) .
THE SALIN A DE L FRAIL E PULL-APAR T BASI N 20
1
Table 1. Summary tectonic history o f th e Argentine Puna Plateau TIME Quaternary Pleistocene Pliocene Miocene Oligocene
MAGMATISM
SEDIMENTATION
Basaltic volcanism
Ignimbrite Sheets
Evaporite & lake deposits in local depressions; deposition of sands, volcanics & conglomerates
Volcan Antofalla
Calcalkaline volcanism
TECTONICS
Quechua NW-SE shortening Phase thrusting & folding Deposition of Oligo-Miocene continental red-beds Phase
Eocene
References
Orogen-parallel sinistral strike-slip This paper Diaguita around Salina del Fraile Allmendinger etal., 1989 Phase switch Orogen-parallel dextral strike-slip Marrettetal., 1994 NNW-SSE extension Allmendinger et al., 1989 Jordan & Alonso, 1987
Localised shortening
Jordan & Alonso, 1987
Dextral transpression
Jordan & Alonso, 1987 Salfity, 1979
Paleocene Salta Group: red-Beds
Allmendinger et al., 1982
SET
™«ng
Cretaceous
Mon & Salfity, 1995 Cominguez & Ramos, 1995
Jurassic
Basaltic volcanism
Limestone deposition
Voss et al., 1996
Permo-Triassic
Plutonism
Continental sedimentation Saalien Phase
Coiraetal., 1982
Carboniferous
Marine sedimentation
Siluro-Devonian
Formation Puna arc Mostly no sedimentation
Ordovician
Granitoids & lavas
Precambrian
Chanic Orogeny
,dina Fo hoiain
Coira etal.. 1982 Mon & Salfity, 1995
Ocloyic Phase
Folding & mzetamorphosi
Allmendinger et al., 1982 Coiraetal., 1982
9
Marine sedimentation
Metamorphic basement
Antofalla; Fig s 1 & 2 ) ove r 400 0 m o f Lat e Miocene-Recent play a lak e sediment s an d larg e volumes o f evaporite s accumulate d (Jorda n & Alonso 1987 ; Alonso e t al . 1989 , 1991; Vandervoort e t al. 1995) . Durin g the Quaternary-Recent , basalts wer e erupte d (locall y < 1 0 000 a; D e Silva & Francis 1991 ) and a cover of alluvial gravels an d sand s wa s deposite d throughou t the area . The 140k m lon g an d 4- 6 km wid e fault bounded Sala r d e Antofall a strike-sli p basin , directly eas t o f Salin a de l Frail e (Fig s 1 & 2 ) formed alon g a gian t faul t ben d interprete d a s a dextral releasin g ben d i n thi s paper . A n 197 3 earthquake (magnitud e 5.8 ) just beneat h Sala r d e Antofalla a t 1 1 km dept h alon g a NNE-SS W t o NE-SW-striking foca l plan e indicate s dextra l transpression wit h a n E- W shortenin g axi s (Chinn & Isacks 1983 ; Allmendinger e t al. 1989) .
Geology o f Salin a de l Frail e The stratigraph y o f th e Salin a de l Frail e are a i s summarized in Figure 3. In the northwest corner of the pull-apart basin (Figs 4 & 5), in a remnant positive flowe r structure , 20 0 m o f Permia n red-bed s (277 ± 6 Ma an d 25 6 ± 5 Ma ; Vos s e t al . 1996) are exposed . Th e sequenc e consist s o f re d sand -
Allmendinger etal., 1 982
Pampean cycle
Coira et al., 1982
stones an d siltstone s overlai n b y cross-bedde d re d aeolian quart z sandstones . Thi s Permia n sequenc e is intrude d b y Lowe r Triassi c aci d dyke s (212 ± 5 Ma; Vos s e t al . 1996 ) and covere d b y Lower Jurassic limestone s an d basi c lav a (194 ± 6 Ma; Voss e t al. 1996) . The northeaster n basi n sidewall s o f th e rhom boidal Salin a de l Frail e pull-apar t basi n consis t mainly o f Oligo-Miocene (34. 0 ± 0. 9 Ma; Krame r et al . 1996 ) continental red-bed s (Fig s 3 & 5) . In the southwestern sidewall, red-beds ar e overlain by a variabl e sectio n o f sandstones , volcanoclastics , and conglomerate s tha t ar e probabl y Miocen e i n age. A stratigraphicall y younge r sequenc e o f re d sand an d siltstone s an d a growth-wedge sectio n of lake bed s ar e down-faulte d by th e basi n sidewal l faults. Th e sediment s o f thi s fault-bloc k sequenc e cannot b e directl y correlate d wit h neighbourin g succession becaus e the y appea r t o hav e bee n accumulated i n a n isolate d depressio n a s a resul t of loca l subsidence . Pliocene volcanoclastics , ignimbrites , an d basalts of the Puna volcanic system (Coira and Pezzutti 1976 ; Marrett . e t al . 1994 ) overli e th e Miocene successions with an angular unconformity and for m th e uppermos t part s o f th e sidewall s t o the Salin a de l Frail e pull-apar t basin . Correlatio n
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Fig. 4 . Landsa t T M imag e o f th e Salin a de l Frail e pullapart basin . Not e th e rhomboida l geometr y o f th e basi n and majo r scar p developmen t o n th e basi n sidewal l fault system .
with a simila r date d successio n describe d directl y northwest o f Salin a de l Fraile , give s ag e indi cations o f 3. 0 ± 2. 0 Ma an d 2. 0 ± 1. 0 Ma (Coir a and Pezzutt i 1976 ) fo r th e volcanoclastic s an d basalts, respectively. Th e youngest units in the area are Quaternar y gravels , evaporites , an d playa-lake silts an d mud s forme d o n th e floo r o f th e Salin a del Fraile itself. Dacite intrusion s along fault bend s and at fault intersection s i n the corners o f the basin are possibl y synkinemati c wit h Plio-Quaternar y pull-apart formation .
Structure o f the Salin a de l Fraile pull apart basin The rhomboidal Salin a de l Fraile basin i s bounded by arc-shaped sidewal l fault s i n plan view that link
two offse t segment s o f th e sinistra l NNE-trendin g El Frail e faul t syste m (Fig s 4, 5 & 6) . Th e basi n has a flat floor surrounded b y basi n sidewal l faul t scarps o f u p t o 70 0 m i n height . I t i s essentiall y starved, having only a thin surficial post-kinemati c fill. The terrace s tha t forme d alon g th e southwes t basin sidewal l faul t syste m ar e interprete d a s down-faulted an d rotate d faul t block s (Fig s 4 , 5 & 7a). A fault-bounde d anticlina l structur e alon g th e northwestern margi n o f th e basi n (Fig s 4 & 5 ) i s the surfac e expressio n o f a positiv e flowe r struc ture. The flower structure brings Permia n strat a up to the level o f the Eocene t o Oligocene continenta l red-beds giving a n estimate d minimu m uplif t o f 300 m. Th e anticlina l structur e ha s bee n cu t an d extended b y later NW-S E to E-W-oriented exten sional faults . Other WNW-ESE-strikin g extensiona l faul t scarps present on the basin floor (Fig. 7b) also indicate a n averag e NNE-SS W directe d extensio n within the basin. The topography from th e regional 1:250 000 map was digitized (Fig . 8) and the basin volume wa s calculated . B y assumin g tha t fault s converge int o on e ductil e shea r zon e a t 1 0 km depth (a t this depth most strike-slip releasin g over steps becom e aseismic ; cf . Parkinso n & Doole y 1996), th e minimu m amoun t o f pull-apar t exten sion required t o create the calculated basi n volume is calculate d t o b e a t leas t 7. 7 km (Fig s 5 & 8). A basin short-cut fault links the northwest strand with th e southeas t stran d o f th e E l Frail e faul t across th e basin (Fig s 5, 6 & 9a). Along the northernmost par t o f th e basi n short-cu t fault , gypsum dominated fault rock is exposed and shows sinistral shear indicator s (Fig . 9b). Durin g evolution , thi s fault probably became dominant in transferring displacement acros s the offset segment s o f the El Fraile faul t system . The southwes t margi n o f th e basi n i s charac terized b y a larg e SW-tilte d faul t bloc k alon g th e basin sidewal l faul t (Fig s 5, 6 , & 7a) . Thi s faul t block subside d 50 0 m along th e NE-dipping basi n sidewall fault . Th e tota l subsidenc e o f th e basi n floor northeast of the fault block has been estimate d at > 1200m. Locall y withi n thi s faul t block , be d rotations an d clearl y develope d sedimentar y growth-fault wedg e system s indicat e synsedimen tary extensio n (Fig . 9c), probably induce d by early strike-slip pull-apar t formation . This extensio n initiall y caused rotations toward s the northeas t (recorde d withi n th e fault-bloc k sequence describe d above ; Fig . 9c , d). Analysi s of these bed s sho w tha t th e tota l faul t bloc k rotatio n accumulated t o 64* . Subsequen t southwes t rotations ar e recorde d b y th e overlyin g 'growth wedge sequence ' withi n th e sam e faul t block . These late r rotation s resulte d a tota l o f 56 ° til t t o
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Fig. 5 . (a ) Geologica l ma p of the Salin a del Fraile pull-apart basin, (b ) Simplifie d fault ma p of the Salin a de l Fraile pull-apart basi n (PD Z = principal displacemen t zone) . (Se e Fig . 1 for location).
the SW . Sinc e the depositio n o f th e uppe r beds of the fault block , i t has been a rotated wit h a cumulative til t o f 30 ° toward s th e southwes t alon g th e present basi n sidewal l faul t system . NE-striking revers e an d norma l fault s northeas t and southwes t of th e basi n (Fig s 4 & 5 ) ar e post dated b y th e Salin a de l Fraile basin sidewal l faul t scarp formation (Fig. 9e). These faults ar e probably associated wit h the formation of the Salar de Antofalla, i n thi s pape r interprete d a s a Lat e Miocen e dextral releasin g bend , directl y wes t o f th e are a (Figs 1-2) .
Discussion Fault architecture of the Salina del Fraile pull-apart basin The Salin a de l Fraile basin exhibit s a classi c pull apart morphology and is interpreted to have formed by sinistra l displacemen t o n offset segment s of th e
El Frail e faul t syste m (Fig . 5). Basi n formation i s young, Pliocene t o Recent in age, because the pullapart fault s displac e 3- 2 M a (Coir a & Pezzutt i 1976) ol d volcani c an d volcanoclasti c rocks . Th e bulk displacemen t alon g th e sinistra l Salin a de l Fraile strike-slip fault syste m i s calculated t o be at least 7. 7 km, producin g a t leas t 120 0 m o f basi n floor subsidence. Basin sidewall faults lin k the northwest and southeast strand s of the El Fraile fault syste m and terraced basi n sidewall s ar e developed . Th e basi n interior consist s o f down-faulted , rotate d block s and ENE-WS W t o NE-SW-strikin g (oblique ) extensional faults. Thes e faul t orientation s indicat e a dominan t NNE-SSW-directe d extensio n withi n the basin . Early faul t bloc k rotation s ar e recorde d i n th e sidewall synkinemati c 'fault-bloc k sequence ' an d the overlyin g growth-wedge sequenc e (Fig . 9c, d). From thes e rotation s (whic h ar e i n opposit e directions) th e probabl e existenc e o f parallel ,
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Fig. 6 . Cros s section s throug h th e Salin a de l Frail e pull-apar t basin . Cros s section s ar e constructed perpendicula r t o the E l Fraile maste r faul t (Fig . 5a for locations ; Fig s 3 & 5a for key) .
opposite-dipping, curve d growth faults during early basin formation is inferred. Due to the high rotation angles (64 ° t o N E an d 54 ° t o SW ) thes e growt h faults ar e probably o f a curve d nature . Associate d small-scale syntheti c extensiona l fault s (Fig . 9c) also indicat e a probabl e curve d natur e o f thes e faults. Analogue model studies show that such fault curvature rapidl y change s alon g strik e (Doole y & McClay 1997) . Th e basi n sidewal l fault s chang e from curve d i n th e middl e t o stee p an d plana r toward th e corner s o f th e basi n wher e th e fault s are linke d wit h th e principa l displacemen t zone s (PDZs) o f the E l Fraile faul t system .
The fault-bounde d anticline wit h a t least 30 0 m uplifted Permia n t o Jurassic strat a in the northwest of Salina del Fraile is interpreted a s a remnant positive flowe r structure , formed a s the E l Frail e faul t system develope d fro m singl e offse t faul t seg ments. Suc h flowe r structur e developmen t i s a common characteristi c i n th e formatio n o f strike slip faul t system s an d doe s no t requir e restrainin g oversteps o r transpressiona l strai n (e.g . Doole y & McClay 1996) . A late basin shortcut fault transfers displacemen t between th e oversteppin g strand s o f th e E l Frail e fault syste m acros s th e basin . Thi s shortcu t faul t
THE SALIN A DEL FRAIL E PULL-APAR T BASIN
Fig. 7 . (a ) Rotate d downthrow n faul t bloc k o n th e southwest basi n sidewal l faul t system ; (b ) NNE-dippin g normal fault s i n th e basi n floo r o f th e Salin a de l Frail e pull-apart.
development i s typica l fo r pull-apar t basi n evol ution and extinction as it produces a smoother faul t system an d a singl e principa l displacemen t zon e (PDZ). Continued displacement in the shortcut system woul d eventually lead t o th e extinctio n o f th e basin (cf . Wesnousky 1988 ; Zhan g e t al 1989) .
Tectonic model An evolutionar y tectoni c mode l fo r th e Salin a de l Fraile pull-apar t basi n ca n b e erecte d (Fig . 10)
205
using the field data and comparative analogue modelling dat a (McCla y & Doole y 1995 ; Doole y & McClay 1997) . The remnan t flowe r structur e wit h uplifte d an d folded Permia n an d Jurassi c strat a i n th e wester n part of Salina del Fraile documents early evolution of the segmente d faul t syste m (Fig. lOa) . The early northern segmen t of the El Fraile fault syste m was growing fro m th e north and consisted o f fault seg ments an d splays alon g which fault blocks uplifted and folded . The flowe r structure is post-Oligocene because i t affect s Eocene-Oligocen e strata . Subsequently, th e E l Frail e faul t syste m gre w southwards toward s th e overstep . Thi s i s recorde d by th e formatio n o f th e curved , ~SW-dipping , (oblique) extensiona l faul t i n th e northeas t tha t caused early tilting towards the northeast, recorded in th e 'fault-bloc k sequence ' (Fig . lOb) . A secon d curved, (oblique) extensional fault develope d i n the southwest with a dip in opposite direction (NE) and involved synsedimentar y tiltin g t o th e southwest , recorded i n th e 'growth-wedg e sequence ' (Fig. lOb) . A t leas t 50 0 m o f subsidenc e accumu lated i n thi s earl y stage . The earl y (oblique ) extensiona l fault s lin k with the norther n an d souther n strand s o f th e E l Frail e faults syste m t o for m th e ful l faul t architectur e of the pull-apart basin. During this stage of main pullapart formation , large-scal e bloc k faultin g post dated th e 'faul t bloc k sequence ' an d 'growth wedge sequence ' an d th e presen t terrace d basi n sidewall syste m develope d (Fig . lOc) . A t th e southwest basi n margin , faul t block s subside d along th e curve d basi n sidewal l faul t wit h a 30 ° SW rotation . In th e lates t stag e o f evolution , th e basi n short cut faul t developed , transferrin g displacemen t across th e offse t strand s of th e E l Frail e fault sys tem. The sinistra l regim e o f pull-apar t formatio n along the Pliocene to Recent El Fraile fault syste m is somewha t anomalous in this part o f the Andean
Fig. 8 . Bloc k diagra m showin g a two times verticall y exaggerate d vie w of the 3 D basin topography . Note the rhomboidal geometr y an d th e southwes t basin sidewal l faul t block .
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Fig. 9 . (a ) Pop-u p anticlin e wit h Permia n an d Jurassi c sequence s i n th e northwes t o f th e Salin a d e Frail e basin, adjacent t o th e norther n en d o f th e basi n short-cu t fault , (b ) Gypsum-dominate d faul t roc k i n th e northwes t o f th e Salina d e Fraile basin, wit h sigma clasts indicating a left-lateral sens e o f shear, (c) Growth wedge showing bed rotating from 86° E t o 74° W (th e burie d listri c growt h faul t ha s bee n rotate d t o nea r horizontal) . Not e th e bed s pinchin g ou t towards th e top , an d the curve d small-scal e syntheti c extensiona l faults , (d ) Schemati c cartoo n illustratin g earl y nor theast rotatio n an d late r southwes t rotation s i n th e fault-bloc k an d growth-wedg e sequences , (e ) Spectacula r 70 0 m high faul t scar p o n th e NN W boundar y o f th e Salin a de l Frail e basi n show s a n unconformit y (dashe d whit e line ) between tilte d Tertiar y red-bed s an d horizontal Pliocen e volcanics an d a cross-sectional vie w of NE-SW reverse faults .
Cordillera, a s a predominance of dextral strike-sli p motion i s indicate d o n mos t o f th e larg e regiona l N-S to NW-SE faults in the area (cf. the Acazoque fault, Sala r d e Antofall a faul t system ; e.g. Marret t et al. 1994 ; Fig . 1) . Obliqu e subductio n a t th e active margi n apparentl y favour s orogen-paralle l dextral strike-sli p durin g Pliocen e t o Recen t (Pardo-Casas & Molnar 1987) . However, reversals from orogen-paralle l dextra l transpression to sinis-
tral transtensio n durin g th e Diaguit a deformation (±2 Ma) have also been documented by Kramer et al. (1996) , an d a t th e sam e latitud e i n Chil e b y Reutter e t al . (1996) . Sinistral , orogen-paralle l strike-slip a t thi s tim e ma y hav e resulte d fro m reduced rat e o f plat e convergenc e an d tectoni c coupling a t th e activ e subductio n margi n whil e angle o f obliquenes s o f th e plat e vecto r remained unchanged (Reutte r et al . 1996) .
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Fig. 10 . Evolutio n o f th e Salin a de l Frail e pull-apar t basi n i n thre e stages : (a ) pop-u p formatio n cause d b y dextra l movement o n the El Fraile fault system an d formation o f early NE-SW normal faults , (b ) Early pull-apar t developmen t due to sinistral displacement s along the El Fraile fault system, depositio n of the fault-block sequence , an d displacement and partia l inversio n o f th e NE-S W norma l faults , (c ) Formatio n o f th e presen t rhomboida l basi n architectur e an d further displacemen t an d partia l inversio n o f th e NE-S W norma l fault s (PD Z = principal displacemen t zone) .
Implications for the interpretation of other pull-apart basins The Salin a del Fraile basi n is a classic exampl e of rhomboidal pull-apar t basi n architecture . I t show s the basin architecture without sedimentary fill. The tectonic mode l fo r Salin a de l Fraile indicate s ho w pull-apart basins could possibly evolv e fro m inter acting segment s t o comple x system s o f rapidl y changing linked (alon g strike an d at depth), kinked and curve d faults . Growt h strat a an d rotate d faul t blocks demonstrat e a locall y curve d natur e of th e oblique extensional basin forming faults. Suc h curvature ca n rapidly chang e alon g strike . Anticlinal structures that can occur within a pullapart basi n a s remnan t positiv e flowe r structure s (or pop-ups ) ar e commo n alon g linea r strike-sli p faults (e.g . Confidenc e Hills , E California ; Dooley & McClay 1996) , in other pull-apart basins (e.g. Hanme r basin , Ne w Zealand ; Woo d e t al. 1994), an d i n analogu e model s o f strike-sli p faul t systems (McCla y an d Doole y 1995 ; Doole y & McClay 1997) . Th e structure s ca n for m alon g any strike-slip faul t syste m and do not imply th e exist ence o f a restraining stepove r o r transpression, bu t
may b e incorrectl y wrongl y interprete d a s restraining of transpressional structures (e.g. Wood et al . 1994) . Few pull-apart basins have revealed their full 3 D nature (e.g. Crowell 1974 ; Wood et al. 1994 ; Katzman et al. 1995) . Seismic imaging in 2D often fail s to captur e th e abrup t change s i n th e faul t geo metries alon g strik e (e.g . Hardin g 1990) . N o thor ough high-resolutio n 3 D seismi c studie s o f pull apart basins have been publishe d t o date, an d only a fe w example s ar e briefl y describe d (e.g . Richar d et al . 1995) . Therefore , th e 3 D exposur e o f th e basin architecture of the Salina del Fraile pull-apart basin ca n b e use d a s a typ e exampl e fo r th e interpretation o f buried pull-apart basins.
Conclusions The Salin a del Fraile basi n i s a classic exampl e of a sinistral , rhomboida l pull-apar t basi n forme d b y the offse t E l Frail e faul t system . Th e analysi s an d documentation o f the exceptionall y well-displaye d Salina del Fraile pull-apar t basin provide important constraints an d model s fo r th e interpretatio n o f other burie d pull-apar t structures.
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The Salin a de l Fraile pull-apart basi n develope d relatively quickl y afte r th e lates t Miocene , leavin g the origina l basi n morpholog y preserved , du e t o insufficient sedimen t supply . Th e basi n mor phology an d faul t architectur e closel y matche s th e geometries identifie d i n scale d sandbo x model s o f pull-apart systems . The pull-apart basin i s characterized b y terrace d basin sidewalls with fault blocks rotated and downfaulted alon g curve d basi n sid e wall faults . I n th e early histor y o f pull-apar t formatio n a t Salin a de l Fraile, localize d extensio n forme d i n a n isolate d depression wher e a 50 0 m thic k sedimentar y suc cession includin g synkinemati c growth-faul t wedges accumulate d (Fig . lOb) . Salin a de l Frail e shows tha t fault-paralle l anticlina l uplifte d struc tures occur in pull-apart basins as remnant positive flower structures , withou t th e requiremen t o f restraining overstep s o r a transpressional regime . The ne w dat a for El Frail e faul t syste m ar e evi dence fo r orogen-paralle l sinistra l strike-sli p i n latest Miocen e t o Recen t i n thi s par t o f th e Cen tral Andes . This researc h wa s supporte d by the Fault Dynamic s Project (sponsore d b y Arc o Britis h Limited , Petrobras , UK Ltd, BP Exploration, Conoco (UK ) Limited, Mobi l North Sea Limited , an d Su n Oi l Britain) . K . McCla y als o acknowledges fundin g fro m Arc o Britis h Limited . Th e authors express thei r gratitude to RTZ Mining and Exploration, fo r al l thei r financia l an d logisti c suppor t o f th e field expeditions in the Puna. In particular, K . Schroeder, D. Hopper , S . Gigola , R . an d V . Rojo , R . Gallo , an d P . Riveros-Zapata o f RTZ' s Salt a offic e mad e th e logis tically difficult expedition s possible . Th e manuscript benefited fro m discussion s wit h T . Doole y an d A . Younes , and review s b y F . Storti , M . Mattel , M . Bonini , P . Cobbold, an d C . Noblet . Thi s i s Faul t Dynamic s Publi cation 72 .
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THE SAUN A DE L FRAILE PULL-APAR T BASI N KRAMER, B. , Voss , R . & GORLER , K . 1996 . Beckenentwicklung, Tektonik un d Vulkanismu s in der siidlichen Puna/N W Argentinien . Terra Nostra, 8/96) , 76-77. MARRETT, R . A. , ALLMENDINGER , R . W. , ALONSO , R . N. & DRAKE , R . E . 1994 . Neotectonic deformatio n o f the souther n Pun a Plateau , north-wester n Argentina . Journal o f South American Earth Sciences, 7, 209-228. McCLAY, K . & DOOLEY , T . 1995 . Analogue model s o f pull-apart basins . Geology, 23 , 711-714 . MON, R . & SALFITY , J . A . 1995 . Tectonic evolutio n o f the Ande s o f Norther n Argentina . In : TANKARD , A. J. , SUAREZ SURUC O R. & WELSINK, H . J. (eds) Petroleum Basins o f South America. America n Association o f Petroleum Geologist s Memoir , 62 , 269-283. NOBLET, C, LAVENU , A . & SCHNEIDER , F . 1988 . Etude geodynamique d'u n basssi n intramontagneu x tertiar e sur decrochement s dan s le s Ande s d u su d d e 1'Equat eur: 1'example du bassin de Cuenca. Geodynamique, 3, 117-138. PARDO-CASAS, F . & MOLNAR , P . 1987 . Relative motio n of th e Nazc a (Farallon ) an d Sout h America n plate s since lat e Cretaceou s time . Tectonics, 6 , 233-248 . PARKINSON, C. & DOOLEY, T . 1996 . Basin formatio n and strain partitioning alon g strike-slip fault zones . Bulletin of th e Geological Survey o f Japan 47 , 427-436 . REUTTER, K., SCHREUBER , E . & CHONG , G . 1996 . Th e Precordilleran faul t syste m o f Chuquicamata, Norther n Chile: evidenc e fo r reversal s alon g arc-paralle l strike slip faults . Tectonophysics, 258 , 213-228. RICHARD, P . D. , NAYLOR , M. A . & KOOPMAN , A. 1995.
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Experimental model s o f strike-sli p tectonics . Petroleum Geoscience, 1(1) , 71-80. SALFITY, J . A . 1979 . Paleogeologi a d e l a cuenc a de l Grupo Salt a de l nort e d e Argentina . Cuarto Congreso Geologico Chileno, 2, 119-137 . SALFITY, J . A. , GORUSTOVICH , S . A. , MOYA , M . C . & AMENGAL, R. 1984 . Marco tectonic o de l a sedimenta cion y efusivida d Cenozoica s e n l a Pun a Argentina . Noveno Congreso Geologico Argentina, 1 , 539-554. VANDERVOORT, D . S. , JORDAN , T. E. , ZEITLER , P . K . & ALONSO, R . N . 1995 . Chronology o f interna l drainag e development an d uplift, souther n Puna plateau, Argentine centra l Andes . Geology, 23 , 145-148 . VIRAMONTE, J. , ALONSO , R. , GUTIERREZ , R . & ARANA RAZ, R. 1984. Genesis de l litio en los salares de la Puna Argentina. Noveno Congreso Geologico Argentina, 3, 471-481. Voss, R. , GORLER , K. , KRAMER , B . & VA N DEN BOGAARD, P. 1996 . Neue Daten zu r palaozoischen un d mesozoischen Palaogeographi e i n de r sdliche n Pun a (NW-Argentinien). Terra Nostra, 8/96 , 147. WESNOUSKY, S . G . 1988 . Seismological an d structura l evolution o f strike-sli p faults . Nature, 335 , 340-343 . WOOD, R . A. , PETTINGA , J . R. , BANNISTER , S., LAMAR CHE, G . & McMoRRAN , T . J . 1994 . The structur e o f the Hanmer strike-slip basin , Hope fault, New Zealand . Geological Society o f America Bulletin, 106 , 1459 1473. ZHANG, P. , BURCHFIEL , B . C. , CHEN , S . & DENG , Q . 1989. Extinctio n o f pull-apar t basins . Geology, 17 , 814-817.
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Transform zone s i n the Sout h Atlanti c rifte d continental margin s W. U . MOHRIAK 1 & B. R . ROSENDAHL 2 l
Petroleo Brasileiro S.A., E&P - GEREX/GESIP and UERJ, Avenida Chile 65-s. 1301, 20031 170 Rio de Janeiro, RJ, Brazil 2 Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149, USA Abstract: Integratio n of seismic, potentia l field, and borehole dat a from conjugat e basins along the Sout h Atlanti c continenta l margin, particularly the northeaster n Brazilia n an d northwestern African segments , indicate s that th e rif t architectur e is controlled by fractur e zone s that exten d from th e oceani c crus t an d penetrat e throug h th e continenta l crust , locall y correspondin g t o Precambrian structure s in cratonic regions. The fracture zone s may divide the continental margin into severa l compartment s with independen t sedimentar y depocentres , separat e crusta l domain s along oceani c transforms , and affec t th e rif t architectur e b y shearing . Oceani c transfor m zones may lea k igneou s rock s originate d fro m th e mantle . This work discusses conjugate sedimentary basins in the South Atlantic salt basins, particularly from Jacuip e t o Sergipe-Alagoa s o n th e Brazilia n side , an d fro m Gabo n t o Ri o Mun i o n th e African side . The following aspects are emphasized: (1 ) rift depocentres are controlled by border faults subparalle l to the margin and by transverse faults tha t may continue as transform fracture s in th e oceani c crust ; (2) the southernmos t segment o f the Sout h Atlantic continental margin s is characterized b y Early Cretaceous volcani c rocks that underlie continental lacustrine Neocomia n to Barremian syn-rif t sediments ; (3) the pre-rift sequence s (Mesozoi c an d Palaeozoic sediments ) that underli e th e syn-rif t depocentre s i n Gabo n an d Sergipe/Alagoa s ar e mainl y devoi d o f vol canics; (4 ) ther e i s seismi c evidenc e o f magmati c underplatin g i n th e deepe r portion s o f th e continental crust , whic h ar e expresse d b y antiforma l feature s locall y aligne d wit h transfor m fractures; (5 ) basement-involved extensiona l fault s an d volcanic activit y alon g leakin g transfor m faults ar e image d alon g severa l conjugat e segments o f the margin , particularly alon g the equa torial margi n (Romanch e fractur e zone) ; (6 ) i n som e segment s o f th e divergen t margin , th e transition fro m oute r rift block s t o oceanic crus t is characterized b y wedges of seaward-dippin g reflectors wit h a possible origi n associate d wit h emplacement o f oceanic ridges ; (7) locally, the outermost rif t block s nea r th e continental-oceani c crus t boundary see m t o be highly erode d b y post-rift uplif t cause d b y transfor m faul t shearin g o r b y magmati c underplating ; (8 ) tectono magmatic episode s climaxe d i n th e Lat e Cretaceous/Earl y Tertiar y i n northeaster n Brazi l an d extended t o th e Lat e Tertiar y o n th e Wes t Africa n margin , formin g larg e volcani c complexe s along transvers e lineament s tha t affec t bot h oceani c an d continenta l crust .
Introduction Ridge
. Th e inse t boxe s sho w th e locatio n o f th e Jacuipe-Sergipe/Alagoas an d Gabon-Ri o Mun i There ar e several difference s between passive con- conjugat e sedimentar y basin s alon g th e northeast tinental margins formed by divergent processes and er n Brazilia n margi n an d the northwester n Africa n sheared continenta l margin s associate d wit h trans - margin , th e mai n focu s o f th e presen t discussion , form fault s an d strike-sli p motion . Thes e end - Th e Aptia n sal t basi n extend s t o th e Santos member continental margin types are recognized i n Namib e region i n the south, and the equatorial segthe Sout h Atlantic an d ar e characterize d b y differ - men t o f th e Sout h Atlanti c i s characterize d b y ent geologica l an d geophysica l signature s becaus e majo r oceani c fractur e zone s suc h a s th e Roman the tectonic processes tha t created the m ar e funda- che , Chain, an d Charco t (Fig . 2). mentally different . Moder n altimetri c an d gravimetric imager y (e.g. Figure 1 shows the South American an d African Sandwel l & Smit h 1997 ; Smith & Sandwel l 1997; plates an d th e locatio n o f conjugat e sedimentar y Dickso n & Fairhea d 1998 ; Dickson 1999 ; Ode basins whic h wer e separate d b y th e Mid-Atlanti c gar d & Dickso n i n press ) show s tha t th e Sout h From: STORTI , F. , HOLDSWORTH , R . E . & SALVINI , F . (eds ) Intraplate Strike-Slip Deformation Belts. Geologica l Society, London , Specia l Publications , 210 , 211-228, 0305-8719/037$ 15 © Th e Geologica l Societ y o f Londo n 2003 .
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Fig. 1 . Sout h Atlanti c region . Left : Morphologica l ma p o f th e Sout h Atlanti c continenta l margin s an d sedimentary basins, wit h identification of the mid- Atlantic spreadin g ridge, th e Rio Grande Rise, th e Walvis Ridge, the Sa o Paulo Plateau, th e Abrolho s Volcani c Complex , an d th e Cameroo n Volcani c Line . Th e Sergipe-Alagoa s an d th e Gabo n basins ar e encompasse d withi n the are a marke d b y rectangles . Right : Palinspasti c reconstructio n showin g conjugate sedimentary basin s i n th e Sout h Atlantic, whic h i s divide d into thre e mai n segment s based o n transfor m zones.
Atlantic Ocea n opene d i n a zipper-lik e fashio n from sout h to north, progressively rupturin g Gondwana i n th e Lat e Jurassic-Earl y Cretaceou s (Rabinowitz & LaBrecqu e 1979 ; Cand e e t al. 1988; Muelle r e t al. 1997 ; Jackso n e t al 2000) . This process is commonly associated wit h extensional tectonic s durin g th e syn-rif t sequence , an d is characterize d b y basement-involve d norma l faulting alon g tilte d block s (wit h predominantl y dip-slip offsets ) representin g th e genera l patter n associated wit h continental rifting. Th e extensiona l faults ar e generall y subparalle l t o th e margi n an d control th e rif t depocentres , particularl y alon g th e border maste r faults . Simila r t o othe r rif t basins , transfer o r accommodatio n zone s ar e obliqu e t o these faults an d separate the basins into distinct rift sub-basins (Rosendah l 1987) . Th e depocentre s o f purely extensiona l rif t basin s ar e paralle l t o th e oceanic spreadin g centr e an d ar e locate d betwee n major transfor m offsets , Here , structura l develop ment is dominated by progressive stretchin g of the continental lithospher e unti l a locu s o f ruptur e eventually allow s a ne w spreadin g syste m t o b e developed. A goo d exampl e i s th e Gabo n margi n
of West Africa sout h of Port Gentile (Teisseren c & Villermin 1989 ) o r th e Campo s Basin , offshor e Brazil (Guardad o e t al. 1989) . In sheare d continenta l margin s (suc h a s th e equatorial Sout h Atlantic , Fig . 2) elevate d conti nental crust is juxtaposed against subsided oceani c crust acros s a transform fault, o r a serie s o f transform faul t strand s (Mascl e 1976) . Th e transfor m zone is often expresse d as a basement ridge, which may continu e toward s oceani c fractur e zones . A n end-member example occur s wher e the Romanche fracture zon e intersect s th e Ivor y coas t o f Wes t Africa (Benkheli l e t al . 1995) . Thes e transfor m fracture zone s ar e als o characterize d b y shearin g processes an d igneou s activit y i n oceani c crus t (Gasperini e t al. 2001) . The tectonic evolution of transform margin sedimentary basin s has been analyse d recently by sev eral authors , particularl y b y correlatin g potentia l field dat a wit h geologica l data . Seismi c interpret ation an d result s fro m explorator y an d scientifi c drilling indicate that the tectonic evolution of transform margi n sedimentar y basin s include s differen t phases. Followin g Bir d (2001) , thes e phase s
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inset) ha s bee n interprete d b y severa l author s t o include structura l elements typica l o f sheare d con tinental margin s (e.g . Lan a & Milan i 1986 ; Szat mari & Milani 1999) . In this work we show seismic evidence fo r shea r zone s affectin g previousl y deposited syn-rif t sequence s an d als o characteriz e magmatic activit y alon g leakin g transfor m zones , particularly i n th e Sergipe-Alagoa s an d Gabon Rio Mun i basins.
Geological an d geophysical characteristic s of sheare d margin s
Fig. 2 . Equatorial Sout h Atlantic - N W Africa - Ghana . Schematic tectoni c ma p showin g transform segmen t and fracture zone s (Romanche , Chain , Jea n Charcot) . Th e Romanche fractur e zon e limits th e dee p Ivoria n basin t o the north fro m th e Gul f of Guinea abyssal plain (oceani c crust) t o th e south .
involve (1) shearing of continental crust and formation o f pull-apar t basins ; (2 ) developmen t o f a n active spreading ridg e wit h transform fault s separ ating continenta l fro m oceani c crust ; (3) migration of th e spreadin g ridg e alon g th e pull-apar t basins ; and (4), development o f a passive margin sedimentary wedge along an inactive fracture zon e that also separates continental fro m oceani c crust. The pullapart basin s associate d wit h shearin g ar e charac terized by a complex spectru m of compressional to extensional structure s forme d durin g wrenchin g that in most cases is subparallel to the margin. The thermal effect s o f th e migratio n o f th e oceani c spreading ridge movin g along transfor m fault s ha s been analyse d b y severa l author s (e.g . Clif t e t al. 1997), an d seem s t o b e associate d wit h uplif t o f marginal ridges . Transform shearin g along the Atlantic equatorial margins ha s bee n recognize d i n severa l works , particularly alon g th e regio n betwee n Nigeri a an d Liberia i n Wes t Afric a (Mascl e & Blare z 1987 ; Basile e t a l 1993 ; Clif t e t a l 1997 ; Edward s & Whitmarsh 1997) . Shearin g alon g th e Brazilia n equatorial basins has also been recognized (Lan a & Milani 1986 ; Mato s 1992 , 1999 , 2000 ; Szatmar i & Milani 1999) . However, the relationship wit h frac ture zones whic h may control divers e rif t segment s along divergen t margin s ha s no t bee n analyse d in detail, particularl y alon g th e sal t basi n segmen t i n northeast Brazil an d West Africa . Th e northeaster n extremity of the South Atlantic rift syste m (Fig. 1 -
The Sout h Atlanti c ma y b e segmente d int o thre e different tectoni c provinces on the basis of fractur e zones an d magmati c hot-spo t trace s (Fig . 1) . Th e equatorial Sout h Atlantic , nort h o f th e S t Hele n hot-spot, include s th e Cameroo n Volcani c Lin e i n West Africa , an d this segmen t i s characterize d b y major transfor m zones , suc h a s th e Jea n Charcot , Chain, Fernand o d e Noronha, Romanche, an d Sa o Paulo fracture s i n the Brazilia n side . The Sout h Atlanti c sal t basi n alon g th e easter n Brazilian an d western Africa n margin s (Fig . 1 inset) is located southward s of the equatorial transform margin and is limited to the south by the Tristan hot-spo t (whic h i s relate d t o th e Walvi s Ridge/Rio Grande Rise an d the Florianopolis frac ture zon e (Cainell i & Mohria k 1999) . Th e southernmost Sout h Atlantic , sout h o f th e Ri o Grande Rise/Walvi s Ridge , include s th e Brazilia n Pelotas Basi n an d th e offshor e Argentin a basins , corresponding t o th e Sout h Africa-Namib e seg ment alon g th e Africa n margin . Most o f th e sal t basin s i n th e souther n Sout h Atlantic are characterized b y Early Cretaceous vol canic rock s underlyin g th e Neocomia n syn-rif t sediments (Cainell i & Mohriak 1999) . O n the other hand, pre-rift sequence s (Mesozoi c an d Palaeozoi c sediments) tha t underlie the syn-rif t depocentre s i n the northeaster n Brazilia n margi n (fro m Bahi a t o Sergipe-Alagoas) an d along the Gabon-Equatoria l Guinea margi n ar e mainl y devoi d o f volcanic s (Chang e t al 1992 ; Daill y 2000) . In the salt basin province, the Neocomian to Barremian rif t sequenc e i s overlain by Aptian evapor ites, then by Albian to Cenomanian carbonates, and finally b y Upper Cretaceous t o Tertiary siliciclasti c rocks (Guardad o e t a l 1989 ; Chan g e t a l 1992 ; Cainelli & Mohria k 1999) . Th e norther n limi t o f the Aptia n sal t basi n seem s t o correlat e wit h th e transform corridor interpreted nort h of the SergipeAlagoas Basin . The Cote d'lvoire-Ghan a region ma y constitut e a paradigm for the stud y of transform margin sedi mentary basins from Liberi a to Nigeria. These basins ar e related t o centra l Atlanti c Ocea n inceptio n around 11 8 Ma (Edward s & Whitmars h 1997 ;
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Edwards e t al. 1997 ; Mato s 2000 ; Bir d 2001) . Figure 2 show s th e schemati c tectoni c ma p of th e West Africa n transfor m margin , wit h th e Roman che oceanic transfor m zone outstandin g westwards of th e Nige r Delt a an d sout h o f th e onshor e Ket a Basin. Figur e 3 show s a lin e interpretatio n o f a seismic sectio n extendin g fro m th e margina l pla teau toward s th e abyssa l plain o f the Ghana-Ivor y Coast transfor m margin . The Romanch e fractur e zon e (Fig . 2) i s charac terized b y deforme d sedimentar y belt s an d b y igneous intrusion s alon g th e oceani c fractur e zon e (Gasperini e t al. 2001) . Thes e igneou s rock s hav e been dredged , an d predominantl y consis t o f gab bros an d peridotites fro m th e mantle , and , surpris ingly, ther e i s no evidenc e fo r basalt extrusion s i n the dredged are a along the fracture zon e (Gasperin i et al . 2001) . Thi s indicate s tha t som e tectoni c mechanism tha t als o involve d foldin g o f sedimen tary rock s wa s responsible fo r bringing th e mantl e rocks u p t o th e se a surfac e lik e diapiri c bodies . Although th e shape s o f volcanic an d igneous plug s are similar t o sal t diapirs , thei r highe r densit y prevents thei r ascensio n throug h th e crus t b y simpl e gravity buoyancy, thus a tectonic mechanis m mus t be invoked. I n some areas , structura l features simi lar to sal t diapir s an d igneous plugs occur near the crustal limit , particularl y alon g fractur e zone s (Mohriak 1995 ; Mohria k e t al. 20006) . This raise s th e problem o f identifying an d inter preting manifestation s o f oceani c transfor m zone s towards the continental crust . It has been observe d that severa l transfor m zone s continu e alon g th e continental margi n sedimentar y basin s a s lin eaments an d basement-involve d fault s (e.g . Asmus & Guazell i 1981).Th e Vaza-Barri s faul t system in the Sergipe-Alagoas basin (wes t of Ara-
caju i n th e Sergip e sub-basin , Fig . 4 - left ) may constitute on e exampl e o f a Precambria n zon e o f weakness tha t separate d tectoni c province s durin g the Brazilian Orogen y an d subsequently controlle d the depocentr e flip-flo p i n th e Tucan o Basi n (Magnavita & Cupertin o 1988 ; Mohria k e t al . 2000a). Thi s faul t zon e continue s towar d th e offshore Sergip e sub-basin , boundin g th e Mosqueir o Low (southeast of the city of Aracaju, Fig. 4 - left) , and probably continue s to the crustal limit, where it apparently connects with the Sergip e fractur e zon e (Mohriak e t a l 1998 ; Mohria k e t al . 2000
Fig. 3 . Equatoria l Sout h Atlantic margina l ridge - schemati c geologica l section. Regional transec t crossin g the Romanche fractur e zone , fro m th e mid-slop e basi n toward s th e abyssa l plain . Th e margina l ridg e i s a majo r featur e o f th e Ivorian Basi n transfor m margin . Th e sectio n identifie s stratigraphi c sequence s 2- 5 i n th e mid-slop e basi n extendin g from th e shelf-brea k toward s the marginal ridge scarp . Basinward s of the basement uplif t i n the margina l ridge a thin sequence o f abyssa l plai n sediment s overli e th e transitio n fro m continenta l t o oceani c basement .
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Fig. 4 . Northeaster n Brazi l sedimentar y basin s an d West Afric a continenta l margi n wit h PROB E an d SPOG seismi c lines. Left : Locatio n ma p o f the Reconcavo-Tucano-Jatob a an d Sergipe-Alagoa s basin s i n northeastern Brazil , with the locatio n o f mai n oceani c fractur e zones . Locatio n o f profile s in th e Sergip e an d Alagoa s basin s ar e identified a s D-l an d D-2 . Right : Locatio n ma p o f the Gabon-Ri o Mun i basins seismi c profiles , wit h identificatio n of th e mai n oceanic fractur e zone s an d volcanic lineaments .
The identificatio n o f equivalen t feature s i n th e salt basins sout h of the Niger Delt a (Fig . 4 - right ) is hampere d b y th e thic k evaporit e sequence s observed in the deep water region. However, som e regions sho w evidenc e fo r transform-relate d de formational mechanism s durin g par t o f thei r geo dynamic evolutio n (e.g . Turne r 1995 , 1999 ; Szatmari & Milan i 1999) . A palinspasti c recon struction o f th e northeas t Brazi l an d northwes t Africa margi n segments (base d on fitting continental-oceanic crus t boundaries , gravit y anomalies , and transfor m fractur e zones , se e Unterneh r e t al. 1988 fo r discussion ) show s man y fittin g problem s for adjustin g an d rotating the Neocomian rift basins without considerabl e gap s an d overlaps (Fig . 5). There i s mounting evidence that initial break-u p of th e Africa n an d Brazilia n margins i n thei r sub equatorial regio n canno t b e modelle d b y simpl e rotational openin g abou t th e post-11 7 Ma pol e a s suggested b y previou s work s (e.g . Rabinowit z & LaBrecque 1979) . Fo r example , Lan a an d Milan i (1986), Szatmari and Milani (1999) , an d Rosendahl et al. (in press) have suggested that break-up of the subequatorial Sout h Atlanti c involve d transfor m fault dislocatio n prio r t o final opening. The gravit y dat a an d model s produce d b y Geo physical Exploratio n Technolog y (GETECH ) per -
sonnel a s par t o f th e 'SAMBA ' Projec t (e.g . Dickson & Odegard 2000 ; Odegar d & Dickson i n press) were modelled b y Rosendahl et al. (in press) to indicat e a n openin g progressio n o f th e sub equatorial rif t basins , subsequentl y affecte d b y shearing. Th e mai n regio n wher e thi s proces s wa s operative extend s fro m th e Transvers e Zon e (Pernambuco-Parafba-Douala-Rio Mun i basins , see Fig . 1 ) towar d th e Eas t Brazilia n rif t syste m south o f th e Reconcav o basin . Th e mai n crustal types in this segment ar e shown on Figure 5 , which also indicate s a n abrup t chang e i n th e structura l trends o f th e Precambria n rock s nea r th e Ombou e Divide, whic h separate s th e Cong o ductil e sheet s in th e nort h fro m th e Sett e Cam a ductil e sheet s i n the sout h (Rosendahl e t al. in press). The Pernambuco-Paraiba an d th e Krib i fractur e zon e (Fig . 5) are the northern limit o f these sheare d segment s of the margin . Thi s regio n als o mark s th e transitio n from th e Aptian salt basins in the south to the nonevaporite basin s alon g th e equatoria l margi n (Matos 2000) .
Geological an d geophysical interpretatio n The GEOSA T dat a (Sandwel l & Smith 1997 ) ha s been intensively used in the West African an d east-
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Fig. 5 . Reconstructio n o f Jacuipe-Sergipe-Alagoas-Gabo n rif t syste m wit h locatio n o f seismi c lines . Palinspasti c map showin g locatio n o f tw o regiona l profile s acros s conjugat e basin s tha t exhibi t som e geologica l an d geophysica l characteristics o f divergen t segment s o f th e Sout h Atlanti c rif t system .
ern Brazilia n margin , an d derivativ e map s hav e been used to constrain the geological interpretatio n of th e mai n tectoni c element s o f conjugat e sedi mentary basin s (e.g . Dickso n & Odegar d 2000 ; Mohriak e t al. 2000a; Rosendahl e t al. in press) . The principa l seismi c dat a se t i n th e Wes t African margi n correspond s t o th e PROB E Stud y grid (Fig . 4) . The programme wa s acquire d b y the vessels GEC O Ta u an d Prob e Researche r usin g a semi-tuned Arric h airgu n sourc e arra y arrange d i n a chevro n configuration . Detail s o f th e seismi c acquisition and processing procedures ca n be found in Meyers et al. (1996*2, b) and Rosendahl and Groschel-Becker (1999 , 2000) . The PROB E dat a ar e supplemente d b y th e SPOG dataset fro m offshor e Gabo n (Wannesson et al. 1991 ) an d by a n MCS profile recentl y acquire d by Infreme r personne l a s a tes t fo r a subsequent project of f Angola , Integratio n o f potentia l fiel d (gravity and magnetic) an d seismic dat a resulted i n detailed mapping of the Gabon-Rio Muni sedimentary basin s an d th e transfor m zone s tha t contro l syn-rift depocentre s i n th e Wes t Africa n margi n (Fig- 4). Similar geological and geophysical datasets wer e analysed i n th e conjugat e margi n o n th e Brazilia n side, including the integration o f industry and deep seismic profile s i n the Sergipe-Alagoas Basin (see, for example , Mohriak e t al. 1995 , 1998 , 2000a , b) . Figure 5 show s a reconstructio n ma p wit h th e location o f two transects alon g conjugate basins of
the Brazilian an d West Africa n margin , selecte d to correspond t o predominantly extensiona l segment s of th e rif t syste m i n northeas t Brazil . Figure 6 show s a PROB E deep-imagin g profil e extending from th e platform toward the deep water region o f th e Gabo n Basin , sout h o f th e N' Kom i fracture zone . I t show s a thic k syn-rif t sequenc e that continue s basinward s o f th e shelf-brea k fo r tens of kilometres, an d which is overlain by a thick Aptian sal t diapi r provinc e (include d i n th e Mesozoic sedimentary sequence) that disappears toward s the oceanic crustal region (Wannesso n e t al. 1991 ; Rosendahl e t al. 1992 ; Meyer s e t al . 19960 , b) . Figure 7 show s a lin e interpretatio n (modifie d from Wannesso n e t al . 1991 ) o f a profile crossin g
Fig. 6 . (a ) PROB E LIN E 2 3 seismi c lin e uninterprete d and with a schematic line interpretation o f the depth-con verted profil e (b ) potentia l fiel d dat a (Bougue r and mag netic anomalies). The Gabon sedimentary basin is characterized b y a thic k wedg e o f syn-rif t Mesozoi c sediment s and Aptia n evaporites , an d by a gradual crusta l thinnin g from th e platfor m toward s th e crusta l limit . Th e Pre cambrian continenta l crus t i s characterize d b y seismi c reflectors tha t sho w a landward-dippin g tren d (Sett e Cama ductil e sheets) . Th e Mesozoic sediment s (pre - and syn-rift) abu t agains t Cretaceou s volcani c rock s o f th e Gabon proto-oceani c crust . Th e gravit y an d magneti c anomalies (bottom ) mar k th e continental-oceani c crus t boundary nea r th e wester n limi t o f th e Aptia n sal t diapir province .
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Fig. 7 . Detai l o f SPOG line 1 offshore Gabon crossin g th e NE-SW-trending N' Kom i fractur e zone. Th e rift block s are uplifte d an d eroded , an d th e basemen t an d bas e o f sal t reflector s are offse t alon g th e fractur e zone . Sal t diapir s are observe d toward s th e southeast , wherea s n o halokineti c feature s ar e observe d t o th e nort h o f th e fractur e zon e crossed b y th e profile.
the N ' Kom i fractur e zone , an d it s effect s o n th e deep wate r extensio n o f th e Gabo n sedimentar y basin. The N' Kom i fracture zon e limits th e southern extremit y o f th e Lambaren e Hig h onshor e (Wannesson e t al . 1991) , control s syn-rif t depo centres i n the sedimentar y basin, and also control s a majo r offse t i n th e CO B (continental-oceani c crustal boundary) . Th e seismi c interpretatio n (Fig. 7) show s severa l basement-involve d struc tures tha t ar e probabl y associate d wit h a pro longation o f th e oceani c transfor m faul t tectoni c manifestations i n dee p waters , impactin g an d deforming th e rifte d continenta l crust . Th e N ' Komi faul t zone , marke d b y larg e offset s i n th e basement an d bas e o f sal t reflectors , als o control s post-salt sedimentar y depocentres . Th e faul t zon e is associated wit h a major uplift an d erosion o f the rift sequences , formin g a hig h tha t wa s sub sequently onlappe d b y Cretaceou s an d Tertiar y sediments. Similar observation s ca n b e obtaine d fro m analysis o f th e seismi c profile s alon g extensiona l segments o f th e Brazilia n conjugat e margin . Figure 8 show s a regional dee p seismi c profil e i n the Sergip e sub-basin , wit h a stron g reflecto r extending fro m th e platfor m toward s th e dee p water region. This reflector has had several alterna tive interpretations: Pontes e t al. (1991) interpreted it to correspond t o the boundary between sediment s and Precambria n basement ; Mohria k e t al . (1995 ) suggested i t migh t correspon d t o a detachmen t zone, to p o f underplate d layer s i n th e crust , o r t o the transitio n fro m crus t t o uppe r mantl e rocks . Figure 9 show s a schemati c geologica l sectio n
based o n depth-convertin g th e seismi c lin e interpretation (Mohria k e t al. 1998 , 2000a) , showing the transition fro m continenta l to oceanic base ment marke d b y pinch-ou t of syn-rif t sedimentar y rocks an d by wedges o f seaward-dipping reflector s (Mohriak e t al. 20006) . Gravity modellin g an d seismi c interpretatio n o f the regiona l profile s acros s th e Brazilia n margi n indicate a rather abrupt crustal thinning basinwards of th e shelf-break , associate d wit h lithospheri c extension an d crustal thinnin g durin g th e syn-rif t phase (Mohria k e t al. 2000a). A major uplift o f the rift block s alon g the landwards prolongation of the Sergipe fractur e zon e is also show n near the shelf break. A stron g reflecto r i n th e middl e t o lowe r crust (from 7 to 9 s TWTT in the deep water region shown on Fig. 8 ) may correspond t o a detachmen t plane tha t merge s wit h th e seismi c Moh o toward the oceanic crus t (Mohriak e t al. 1998) . Th e schematic depth-converte d geoseismi c sectio n (Fig . 9) indicates tha t low-angl e basement-involve d rif t faults apparentl y detac h o n thi s reflector , whic h may correspond to the transition from th e brittle to a more ductile crust. The antiform structur e image d at th e bas e o f th e crust , nea r th e shelf-break , ma y be associated wit h underplated igneou s rocks or to a Moh o uplif t du e t o crusta l thinnin g (Mohria k e t al. 1995) . Another possibility i s that this feature i n the lower crus t ma y correspond t o a manifestatio n of transfor m fault s extendin g from oceani c t o con tinental crust . The transitio n fro m continenta l t o oceani c crus t (Figs 8 & 9) is marked b y seaward-dippin g reflec tors interprete d t o correspon d t o magmati c layer s
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Fig. 8 . Seismi c sectio n Sergip e Basin . Regional deep seismi c lin e i n the Sergip e Basin showing domal feature in the lower crust, pre- an d syn-rif t block s uplifte d an d eroded, an d an abrupt transition to oceanic crus t by seaward-dipping reflectors. A strong reflecto r is observed nea r the base of the crust, probably correspondin g t o top of underplated rock s or t o detachment s tha t toward s oceanic crus t merge wit h the seismi c Moho . Se e Figure 5 fo r lin e location .
that form a proto-oceanic crust . Igneous intrusions are identified along the transition to a pure oceanic crust, particularly alon g lineaments clearl y associa ted with fracture zones because of their linear magnetic anomalies (Mohriak et al. 1998). One of these volcanic plugs is located along the Sergipe fractur e zone, an d i s interprete d i n th e geoseismi c sectio n (Fig. 9) extendin g fro m 8 0 to 9 0 km fro m th e ori gin o f th e seismi c profile. We sugges t tha t th e prolongatio n o f oceani c fracture zone s towards the continental margin s may result i n arcuat e reflector s i n th e crust , formin g antiform structures , and may also locally cause offsets i n the reflection Moho. A n example o f a possible interpretatio n o f suc h structure s i n th e Wes t African conjugat e margi n i s show n o n PROB E Line 5 (Fig . 1 0 - Tabl e 1 provides a ke y t o the abbreviations o n thi s an d followin g figures) . Thi s line crosse s a numbe r o f NE-S W transfor m faul t zones betwee n th e souther n par t o f th e Doual a Basin an d th e norther n par t o f th e Gabo n Basi n (Fig. 4). Here , th e reflectio n Moh o (whic h ma y correspond t o th e to p o f underplate d layers ) i s characteristically arcuate , forming antiforms in th e transition from th e lower crust to the upper mantle. There i s a strikin g offse t betwee n th e to p o f thi s
strong reflecto r image d in the continental to protooceanic crus t an d th e equivalen t reflecto r i n th e oceanic crust (betwee n shotpoint s 200 0 and 3000), which i s attribute d t o transfor m faulting. Figure 1 1 show s a regiona l deep-imagin g lin e (PROBE Lin e 20 ) crossin g th e Bilab o Basi n an d the Ogoou e proto-oceani c crust , wit h th e gravit y and magneti c anomal y profile s superimpose d o n the seismi c data . A prominen t volcani c plu g (Tengue Arc h o r Loire t plug ) i s characterize d b y seismic an d potential field dat a nea r th e transitio n between continenta l an d proto-oceani c crust . Basinwards o f thi s feature , seaward-dippin g reflectors within th e Libbrevill e Uni t ma y correspon d eithe r to Neocomia n rif t strat a o r to volcani c wedges . Rosendahl e t al. (i n press) sugges t tha t approxi mately conjugat e profile s acros s th e northeaster n Brazilian an d Wes t Africa n margin s ma y exhibi t wedges o f seaward-dippin g reflectors . Thes e sea ward-dipping reflector s (SDR ) ar e interprete d i n the Brazilian basins by Mohriak et al (1995 , 1998 , 2000a) a s volcanic rock s extruded subaeriall y dur ing th e initia l stage s o f the oceani c crust , and thus are relate d t o earl y rupturin g of th e Sout h Amer ican an d Africa n plate s b y oceani c spreadin g centres tha t propagated fro m sout h to north.
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Fig. 9 . Schemati c geologica l sectio n Sergip e Basin . Depth-converted schemati c sectio n o f the seismic profile . A volcanic plu g is interprete d nea r the southeas t extremit y o f th e profile , markin g the transitio n to oceani c crust .
Fig. 10 . PROB E Seismic Line 5. Segment of a regional seismic profile showing line interpretation of the proto-oceanic crust basinwards o f the shelf-edge , an d offse t o f the Moh o reflector associated wit h a fractur e zone . Se e Figure 4 fo r line location an d Table 1 for listing o f abbreviations .
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1
Table 1 . Definitions o f abbreviations used i n text an d figure s Abbreviation
Definition
BLU BU CC CFB DIMCS DPA FAA-BA FAA-BA TH D GT GETECH ISO ISO 1V D ISO TH D JSAT KT KW LEU LK LOEU LU MK OC OC/POC OCB PAR PBS POC PT RM S SB SDR SP TCC TOC TPOC TWTT UM UPOC
Base o f Librevill e Uni t (may b e angula r unconformity) Break-up unconformity Congo Crato n (high-grade , refractor y Archea n basement ) Congo Fol d Belt (Lat e Proterozoi c metasediments ) Deep-imaging multichanne l seismi c (large-source , long-apertur e MCS) Dip-azimuth gravit y display Free-air anomal y (sea ) - Bougue r anomaly (land ) displa y Total horizonta l derivativ e o f FAA-B A Gabon Troug h (gentl y folded , layered, high-velocit y rock s dividin g CFB/POC) Geophysical exploratio n technolog y Isostatic gravit y display First vertica l derivativ e o f IS O Total horizonta l derivativ e o f IS O Jacuipe-Sergipe-Alagoas Transfor m Faul t Upper Cretaceous-Lowe r Tertiar y Mega-Sequenc e Boundar y Kribi Wedg e (collapse d block s o f pre-break-up, Lowe r Cretaceou s sediments ) Libreville Erosiona l Unconformit y (top of Librevill e unit ; possibly coeva l wit h LOEU ) Lower Cretaceou s Mega-Sequenc e Boundary Loiret Erosiona l Unconformit y (possibly coeva l wit h LEU) Libreville Unit Middle Cretaceous Mega-Sequenc e Boundar y Oceanic Crus t (relativel y thin , monotonous O C generate d afte r abou t 11 8 Ma) Oceanic crust/proto-oceani c crus t boundary Ocean-continent crusta l boundary Pan-African rejuvenatio n of Cong o Crato n Pre-break-up sedimentar y rock s Proto-oceanic crus t (first-forme d OC ; relativel y thick , layered , an d deformed ) Platform (Lowe r t o Middl e Cretaceous , carbonat e platfor m constructed o n LU ) Reflection Moh o Salt (evaporites ) Base o f sal t Seaward-dipping reflector s (eithe r volcani c flow s o r Ogoou e offla p sequence ) Shot poin t on seismi c profile s Top o f Cong o Crato n Top o f oceani c crus t Top o f proto-oceani c crus t Two-way trave l tim e Upper mantl e Upper proto-oceani c crus t (ma y be Oceani c Laye r 2 ) Sedimentary wedg e block s (post-break-u p sedimentar y unit s infillin g PO C lows )
w
The magneti c characte r o f protoceanic crus t and the high-interva l seismi c velocitie s withi n th e dipping wedge s (e.g . Talwan i & Abre u 2000 ) clearly support a volcanic origin. The dip directions of th e African SD R argu e for a n easterly an d poss ibly centralize d sourc e (Loire t volcani c plug?) . Seismic boundar y relations sugges t a discrete event (or serie s o f events ) tha t prograde d westerl y ove r pre-existing an d pre-subside d crust . However , th e accepted mode l fo r the SD R formation indicates an oceanic spreadin g centr e origi n an d onlappin g o f previous crus t o r sediment s i n a landwar d direc tion. I t shoul d b e note d tha t th e SD R wedge s described her e ar e quit e differen t fro m th e larg e SDR wedges observed alon g volcanic margins (e.g.
Hinz 1981 ; Mutte r e t al. 1982 , Larse n e t al. 1988 ; Talwani & Abreu 2000) . Figure 1 2 (detail o f PROB E lin e 20 ) show s that between th e Bilabo Basin an d the Loiret plug there are seismi c package s characterize d b y fla t uppe r boundaries, lac k o f interna l seismi c structure , an d moderately high-interva l velocities . Thes e obser vations sugges t tha t thes e reflector s migh t corre spond t o carbonat e reefa l structure s forme d nea r wave base . Toward s th e oceani c side , th e high relief Loire t volcani c plu g wa s passively onlappe d by sedimentar y sequences , probabl y date d a s Lat e Cretaceous t o Tertiary, indicatin g that this positiv e feature wa s expose d fo r a lon g perio d o f time , either subaeriall y o r unde r reduce d bathymetri c
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Fig. 11 . Lin e 20E with Loiret plug and gravity and magnetic profiles. Shipboard gravity and magnetic profiles observed along PROB E Lin e 20 . Th e regiona l seismi c profil e i n th e Gabo n margi n indicate s tha t th e Loire t volcani c plu g i s characterized b y a stron g magneti c anomal y nea r th e transitio n fro m continenta l t o oceani c crust . Se e Figur e 4 fo r line locatio n an d Tabl e 1 for listin g o f abbreviations.
conditions. Figur e 1 3 show s a schemati c geose ismic sectio n tha t crosse s th e Loire t plu g i n a n oblique direction , indicatin g tha t this intrusive feature, which is probably relate d to a leaking fracture zone, i s als o characterize d b y a major offset a t the basement an d also a t the reflector corresponding t o the bas e o f th e Aptia n salt . Igneous intrusion s an d basal t extrusion s ar e related t o a numbe r o f tectonomagmati c episode s along th e Sout h Atlanti c continenta l margin s (Chang e t al. 1992) . Thes e episode s climaxe d i n the Lat e Cretaceous/Earl y Tertiar y i n northeaster n Brazil an d extended t o th e Lat e Tertiar y alon g th e West Africa n margin, forming large volcanic com plexes alon g transvers e lineament s tha t affec t bot h oceanic and continental crust. Th e Cameroon Vol canic Lin e (Fig . 4) i s a n exampl e i n Wes t Afric a (Rosendahl et al. 1991 ; Meyer s et al. 1998 ) an d in the easter n Brazilia n margin , th e Vitoria-Trindade lineament seems to be associated wit h the Abrolhos Volcanic Comple x i n th e Espirit o Sant o Basi n (Cainelli & Mohriak 1999) . The mos t strikin g aspec t o f continenta l margin s affected b y transfor m shearin g i s th e inversio n
structures affectin g previousl y deposite d sedimen tary sequences , formin g fol d belt s whic h wer e passively overlai n b y drift-phas e deposits . Thes e folded an d non-folde d sequence s ar e ofte n separ ated b y a prominent , angula r unconformity . On e example o f a fol d bel t relate d t o transpressio n along a transfor m sheare d margi n i s illustrate d i n Figures 1 4 (seismic ) an d 1 5 (lin e interpretation) . Again, a n igneous plug may b e identifie d near th e boundary betwee n th e continenta l an d oceani c crusts. This featur e is probably related t o a leaking transform zon e (Macei o fractur e zone) . Figur e 1 6 shows a schematic tectonic interpretation alon g the Pernambuco-Paraiba shea r zon e (Szatmar i & Milam 1999) . The rif t t o transform-to-drif t progressio n i n Figures 1 7 and 1 8 belies a very complicated pictur e of heterogeneou s crusta l domains , crusta l defor mation that affected pre-rif t an d syn-rift sequences , and, subsequently , b y oceani c crusta l evolutio n along th e Jacufpe-Sergipe-Alagoas transfor m corridor. Figur e 1 7 shows the palinspasti c reconstruc tion o f th e rif t basin s wit h th e mai n crusta l types presently o n land , wit h th e Wes t Africa n fractur e
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Fig. 12. Lin e 2 0 E wit h Loiret volcani c plug . Detail o f th e Loire t plug . A Moh o offse t i s marke d a t th e locatio n o f the Loiret plug , indicating that th e fractur e zon e affect s th e base o f the crus t and upper mantle. Se e Figure 4 fo r lin e location an d Tabl e 1 for listin g o f abbreviations .
Fig. 13. Schemati c geoseismi c sectio n alon g Loiret plug . Lin e interpretatio n o f regional seismi c profil e crossin g th e Loiret plug . Observ e offse t a t the bas e o f th e sal t acros s th e fractur e zone .
zone complex correspondin g t o the deformed belt s in th e Alagoa s sub-basin . A left-latera l strike-sli p zone separate d th e Transvers e Zon e (offshor e o f the Pernambuco-Paraib a Basin ) fro m th e Cong o Ductile Sheets-Sergip e sub-basin, where divergen t motion predominates. Figur e 1 8 shows the mappe d distribution o f offshor e crusta l type s an d bound aries in this region aroun d the tim e of inception of
oceanic crus t (probably middle to late Aptian). The left-lateral deformatio n belt s i n th e Alagoa s subbasin ha s a corresponden t i n th e Nort h Gabon Equatorial Guine a region. Thi s transfor m segmen t may als o b e relate d t o a n inflection of the oceani c propagators (activ e spreadin g ridge s durin g th e inception o f oceanic crust, se e Mohriak 2001 ) tha t advanced though the rif t basin s to the sout h of the
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Fig. 14. Alagoa s Basi n seismi c data . Regiona l seismi c profil e i n th e Alagoa s sub-basin . A majo r volcani c intrusion is interpreted near the boundary between continental and oceanic crust. The syn-rift sequenc e has been strongly affected by compressio n an d erosio n prio r t o th e drif t sequence .
Fig. 15. Alagoa s Basi n schemati c section . Schemati c lin e interpretatio n o f th e Alagoa s profil e showin g main strati graphic sequence s an d tectonic interpretation .
Salvador tripl e junctio n toward s th e Sergipe-Ala goas-Gabon rifts . Thi s inflectio n resulte d i n fos silization o f th e Reconcavo-Tucan o aborte d rif t branch and the successful openin g of the other rifts . With th e fina l ruptur e o f continenta l crus t (probably Lat e Aptian-Earl y Albian) , th e tectoni c stresses wer e alleviate d an d openin g o f th e Sout h Atlantic could continue unhampered in a northward direction t o join th e Centra l Atlanti c oceani c bas ins.
Conclusion Intraplate strike-sli p deformatio n belt s ar e gener ally associate d wit h convergen t plat e boundaries , where sedimentar y basin s an d metamorphi c belt s are affecte d b y obliqu e convergence an d continental indentation. Strike-slip deformational belt s may also occu r alon g divergen t continenta l margins , particularly during the first stages of basin development, whe n comple x structura l plat e tectonic s
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Fig. 16. Schemati c tectoni c ma p of the Sergipe-Alagoa s microplate. Thi s mode l (modifie d fro m Szatmar i & Milani 1999 ) shows the onset of left-lateral dislocation alon g the Jacuipe-Sergipe-Alagoa s transform , an d shearin g o f the Neocomia n rif t sediment s alon g th e Alagoa s sub basin.
readjustments durin g or immediately subsequent t o the syn-rif t stag e heral d th e inceptio n o f a n activ e oceanic spreadin g centre . Thes e event s ar e ofte n related t o magmati c activit y alon g transfor m frac ture zones, and may also result in tectonic episode s characterized b y large earthquakes, an d rapid uplif t or subsidenc e o f loca l block s du e t o transpressio n or transtensio n alon g thes e zones . Thes e episode s may also result i n inversion of syn-rif t trough s and development o f majo r angula r unconformitie s separating deforme d fro m non-deforme d strati graphic sequences . Whe n affecte d b y transfor m fracture zone s th e syn-rif t block s ma y b e uplifte d and erode d alon g th e principa l deformatio n zone s of the transcurrent movements, an d even the lowe r crust ma y reflec t th e prolongatio n o f th e fractur e zones toward s th e continenta l crust . Durin g th e drift stage , oceanic transfor m faults ma y leak deep sourced magma s fro m th e mantl e whic h ma y underplate th e lowe r crust , formin g igneou s plug s and sills within the oceanic crust and the rift basin , or extrud e a s volcani c rock s o n th e basi n floor . The crusta l architectur e o f passiv e continenta l margin indicate s a rather gradua l lithospheri c an d crustal thinning fro m unextende d continenta l crus t towards oceani c crust ; wherea s transfor m sheare d margins ar e characterize d b y a relativel y abrup t boundary betwee n continenta l t o oceani c crust , with crustal thickness decreasing fro m abou t 20 km in the platform to less than 1 0 km in the deep water region beyon d th e transfor m fault s (Edward s & Whitmarsh 1997) . Sheared margin s alon g th e equatoria l Sout h Atlantic ofte n exhibi t a high-standin g margina l ridge, in most cases subparalle l t o transform zones, rising 1- 3 km abov e th e basi n floor , an d usuall y
Fig. 17. Neocomia n reconstructio n wit h mai n crusta l types (Rosendahl) . Pre-break-up reconstructio n o f crustal types along the Brazilian and West African margin, show ing the transverse zone in the Pernambuco-Paraiba basin, the fractur e zon e comple x i n th e Alagoa s sub-basin , an d the Omboue divid e that marks the transition to the divergent segment of the northeast Brazil and northwest Afric a rifts. Th e reconstructio n show s th e pre-break-u p Pan gaean basemen t settin g an d onse t o f left-latera l dislo cation alon g th e Jacuipe-Sergipe-Alagoa s Transform . The Neocomian rift basins (particularly along the Alagoas sub-basin wer e affecte d b y transfor m zones, resultin g i n uplift an d erosio n alon g th e principa l deformatio n zone s of th e transcurren t movements.
separating th e continenta l crus t fro m th e oceani c crust alon g a ridge-transfor m intersection . Thes e ridges ma y tra p syn-rif t t o drif t sedimentar y sequences i n th e landwar d troughs , wherea s th e seaward troughs are often starve d of sediments an d correspond t o bathymetri c lows . Som e o f thes e ridges ma y b e associate d wit h transpressiona l movements (e.g . Ghana-Ivor y Coast ) wherea s others ma y b e associate d wit h volcani c intrusion s (e.g. Sergipe-Alagoas) . The shearing associated wit h the transform faults in th e Sout h Atlanti c sal t basin s als o resulte d i n deformation o f earlier deposited syn-rif t sequences . This wor k suggest s an d show s evidenc e tha t th e
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Fig. 18 . Aptia n reconstructio n wit h oceani c crusta l spreading centres . Post-break-u p reconstructio n showin g propagation o f oceani c spreadin g centre s tha t eventually separated th e Sout h America n an d African plates . Trans form zone s ar e characterize d i n th e Jacuipe Sergipe/Alagoas basin s an d fro m Gabo n t o Equatoria l Guinea.
break-up o f th e subequatoria l Sout h Atlanti c involved transfor m faul t dislocatio n prio r t o fina l disruption of the continents by oceanic propagator s along th e norther n secto r o f the Aptia n sal t basin . Left-lateral dislocatio n alon g th e Jacuipe-SergipeAlagoas Transfor m (JSAT ) move d th e Sout h American plat e a t least 8 0 km SSW relative t o the African plat e (Rosendah l e t al., in press). This was followed b y a chang e i n plat e motio n a t abou t 120 Ma, resultin g i n fina l openin g o f th e Sout h Atlantic alon g E-W flow-line s fro m abou t 11 7 Ma onwards. Thes e episode s ar e marke d b y extrusio n of volcani c rock s formin g wedge s o f seaward dipping reflectors . Subsequently , durin g th e drif t stage, th e transfor m zone s ma y als o connec t th e upper mantl e t o th e surfac e o f oceani c basins . Recent dredgin g o f some seamount s alon g fractur e zones indicate d th e presenc e o f mantl e peridotite s rather tha n basalts (Gasperin i e t al. 2001) . Using primarily gravit y an d seismic data , a corridor o f proto-oceani c crus t (POC ) ha s bee n defined alon g th e easter n Brazilia n an d Wes t African margin . Th e corrido r divide s continenta l from oceani c crus t (OC) . Seaward o f the POC/O C boundary, the palaeo Mid-Atlantic Ridge generated
relatively unifor m O C betwee n NE-SW-trendin g flow-lines. Th e thicknes s o f typica l O C range s from about 4.5 t o 6. 5 km, wit h mos t o f th e varia bility occurrin g acros s fractur e zones , wher e th e top o f th e mantl e (th e Mohovici c discontinuity ) may sho w antiforma l structure s an d larg e vertica l offsets betwee n proto-oceani c an d oceani c crusts . In contrast , PO C i s u p t o 1 5 km thic k an d charac terized b y wedge s o f seaward-dippin g reflector s which initiall y wer e forme d subaerially . This typ e of crus t show s evidenc e fo r block-faultin g an d compartmentalization by fracture zones; i t present s a seismicall y layere d signatur e o f package s tha t mimic sedimentar y layers. It is suggested that POC was generate d b y slo w spreadin g a s th e plat e motion direction s evolve d fro m dislocatio n alon g the Jacuipe-Sergipe-Alagoa s Transfor m t o full fledged spreadin g alon g th e NE-S W flow-lines . Thus, the POC along northeastern Brazi l and West West Afric a i s th e produc t o f a n earl y generatio n of oceani c magmatis m an d leak y transfor m fault s connecting propagator s tha t eventuall y separate d the tw o plates , leavin g a n aborte d thir d branc h of the Sout h Atlanti c rif t system , whic h correspond s to th e onshore Reconcavo-Tucan o basin . We than k th e government s o f Cameroon , Equatoria l Guinea, Gabon, and Sao Tome-Principe fo r permitting the PROBE fieldwor k t o b e done . W e ar e gratefu l t o J.-C . Sibuet fo r th e Infreme r data , an d GETEC H fo r allowin g us t o us e proprietar y data . W e expres s ou r gratitud e t o Agip, Amoco, Ark Geophysical, Geco, and Jebco for support an d assistance i n the analysis of the African dataset . We als o than k Petrobra s managemen t an d acknowledg e that thi s manuscrip t has benefite d fro m discussion s wit h explorationists an d colleague s a t Petrobras E & P an d at Petrobras Researc h Center . Critica l reading o f the manuscript ha s bee n kindl y provide d b y F . Salvin i an d F . Storti, an d the y ar e thanke d fo r th e man y constructiv e suggestions. Finally , w e than k F . Stort i fo r hi s stead y encouragement t o prepare this work, and his cooperation , helpfulness, an d patienc e durin g th e editin g proces s o f the manuscript .
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Index Note: page s i n italics refer t o Figure s an d Table s
Abrolhos Volcani c Comple x 27 2 Absolute Plate Motio n (APM ) 23 Aktyuz-Boordin microcontinen t 57 , 59 , 6 1 Alagoas Basi n 22 4 Alleghanian-Ouachita collisio n 16 3 Alleghanian-Ouachita orogen y 179 , 179, 180 Alpine Fault , Ne w Zealan d 15 , 19 Altai, Chines e 80- 1 Altai, Mongolia , intraplat e strike-sli p faultin g i n 65-86 Altun Huhey range 68-70, 7 1 Hayrt Nuru u an d the Chines e Alta i 80- 1 Hoh Serhiy n Nuru u 75- 8 Mengildyk Nuru u 8 0 Omno Hayrha n Uula 78-8 0 physiology 6 6 regional geolog y 67- 8 Sair Uu l 7 5 Tsambagarav Massi f 70-7 5 Altun Huhe y range 68-70, 7 1 Altun Huhe y Uul 72 , 82 , 82 , 8 3 Altyn Tag h Faul t 9 , 22 , 23 , 23 Amazonia Craton , Brazi l 7 , 8 Anatolian faul t bloc k 2 , 4 Ancestral Rock y Mountain s 159-8 1 Andaman Se a 9 2 Antarctic Digital Magneti c Anomal y Projec t 11 2 Antarctic Plat e 9, 10 Anza rif t 3 1 Aptian sal t basin 21 1 Ar Hoto l syste m 7 1 Ar Hotol/Chukhtei n Faul t 75 , 8 2 Artz Bog d fault system , Mongolia 6 , 7 Asgat Nuru u 76, 76 , 77, 7 7 Athos faul t zon e 12 7 Avalon terrane 186 , 188, 190 Aviator Faul t 114 , 149, 153 Baikal basi n 8 Balleny Fractur e Zon e 9 Balleny Transfor m 11 2 Ban Luan g basin 94 , 10 1 Barrens Hill s Formatio n 188 , 189
Barrett volcan o 12 1 Bayanhaote basin 10 5 Beacon Supergrou p 125 Benioff Zon e 25 Bilabo Basi n 219 Bogd Fault , Mongoli a 7 Borborema Province , Brazi l 2 , 4 Borborema shea r zon e syste m 16 , 16, 17 , 3 1 Bort Rive r 8 9 Bort Rive r Canyo n 78- 9 Bowling Gree n faul t (Ohio ) 172- 3 Brawley-Imperial dextral strike-slip faul t zon e 127 Brimstone pull-apar t 11 9 Burning Springs-Cambridg e faul t zon e (Ohi o an d W. Virginia ) 17 3 Cameroon Volcani c Lin e 272, 213 Campbell Faul t 114 , 148, 149, 757, 153 Campos Basin , Brazi l 21 2 Cape Robert s Rift , aeromagneti c anomalie s an d faults i n 120- 5 Carajas Faul t Zon e 8 Carajas strike-sli p faul t syste m (CaSSS) 8 Carajas-Cinzento strike-sli p faul t system , Brazi l 7, 8 ' Caribbean faul t 1 9 Cat Cree k anticlin e 175-6 , 17 6 Central Victori a Lan d Boundar y 115 , 116 , 776, 117 Chahan faul t 10 5 Chain fractur e zon e 211 , 213, 213 Changning-Menglian sutur e 97 Chao Pray a plain 9 4 Chedabucto Faul t 188 , 190, 192, 194 Chiang Da o basin 94 , 99 , 70 0 Chiang Khon g basi n 94 , 97 , 10 3 Chiang Ma i basin 94 , 99, 10 0 Chiang Mua n basin 94 , 101 , 105 Chiang Ra i basi n 94 , 10 3 Chiang Rai-Phra e fold bel t 105 Chikhtein Faul t 8 0 Chilil even t (1889 ) 5 9 Chon-Aksu faul t 61 , 6 2
230
INDEX
Chon-Kemin shea r zon e 57 , 58 , 59 , 60 , 61 , 62 Chu basi n 5 5 Chukhtein Fault/A r Hoto l 75 , 8 2 Cinzento strike-sli p faul t syste m (CzSSS ) 8 Commerce faul t zon e 176- 7 computerized tomograph y (CT ) 39 Confidence Hills , Californi a 20 7 Cordilleran convergenc e 16 3 Cottage Grov e faul t syste m (Illinois ) 171-2 , 772, 173 Country Harbou r Fault (CHF ) 192 Coyote Cree k faul t 47 , 4 9 critical resolve d shea r stres s (CRSS ) values 2 8 Cross Brook Formatio n 18 9 Dash-al-Bayaz earthquak e 4 9 David Faul t 117 , 118, 129 Dead Se a Basi n 12 7 Dead Se a faul t syste m 35 , 4 7 Deep Freez e Rang e block 114 , 11 5 Diaguita deformatio n 20 6 diorite syke s 1 7 distributed strike-sli p shea r zone s 35-5 0 Doi Inthano n mountai n 9 4 Doi Ta o basi n 10 0 Doroo Nuur Basin 77 , 78 drape fold s 16 2 Dungun pull-apar t basi n 10 4 Dungun strike-sli p faul t 10 3 dykes 17 , 17 East Africa n Rif t Syste m 31 , 14 5 East Kaiba b Monocline (Utah-Arizona ) 174 , 174 Eastern Goldfield s Province , Yilgar n Block , Australia 2 El Frail e faul t syste m 197 , 202, 204, 208 Electro-optical Distanc e Measuremen t (EDM ) 21 en echelo n fault s 164- 5 Euler Deconvolution , 3 D 113 , 121, 723 Exiles Thrus t 112 , 11 5 extensional basi n 10 5 Falkland Platea u 13 9 Fang basi n 94 , 98 , 10 2 fault-and-fold zone s 162 , 163 fault-propagation fold s 16 2 fault-tip basi n (faul t ben d basin ) 102- 3 fault-wedge basi n 104— 5 fault-zone basi n 10 5 Fergana basi n 5 5 Fergana faul t 5 6 Fernando d e Noronha fractur e 21 3 Florianopolis fractur e zon e 21 3 flower structure s 7, 9 , 16 , 16 5 Fluorspar Are a faul t 17 6 forced fold s 16 2 Fore-Kungey obliqu e thrus t 59, 6 2 Fore-Terskey faul t 59 , 60 , 6 2
Gabon basi n 214 , 216, 276, 218, 219, 223 Gabon-Rio Muni basin 211 , 272 Gawler Crato n 11 8 Global Positionin g Syste m (GPS ) 27 Graham Hil l Formatio n 18 9 Grays Poin t faul t 17 0 Great Gle n faul t 2 3 Great Glen-Wall s Boundar y faul t (GGWBF ) 23 , 24 Great Slav e Lak e shea r zon e (GSLSZ ) 2 5 Grunehogna Provinc e 135 , 136, 138, 139 Hachat Go l Canyo n 75 , 77- 8 Hangay Basi n 8 5 Hangay Bloc k 8 5 Hanmer basin , New Zealand 20 7 Hayrt Nuru u 80- 1 Hecton Min e earthquake s (1999 ) 2 0 Hercynian oroge n 3 7 Hoh Rive r Basi n 77 Hoh Serhiy n Nuru u 75-8 , 82 Hoh Serhiy n rang e 76 , 78 , 8 3 Hoh Serk h Faul t 75 , 76 , 76 , 77, 78 , 82 , 8 4 Horton Grou p 19 0 Hosgri Fault 5-6, 6 Hovd dextra l strike-sli p faul t 70 , 7 7 Hovd Rive r 8 0 Humboldt faul t 16 9 Iberian-African plat e boundar y 50 India-Eurasia collisio n 4 , 22 , 53 , 58 , 65 , 66 Indosinian orogen y 9 1 intraplate strike-sli p deformatio n belt s 1-1 1 bends an d stepovers (jogs ) 6-8 , 8 confined 5 origins 4- 5 plate convergence an d 8- 9 plate divergenc e 9-1 1 size an d mechanical significanc e 2- 4 termination zone s 5- 6 contractional 5 extensional 5 rotational 5 strike-slip 5 transfer 5 Issyk-Kul microcontinent 57, 58, 58, 59, 60, 61, 62 cenozoic structur e 58— 9 Issyk-Kul pull-apart basin 53, 57, 58, 58, 60, 61, 62 Ivorian Basi n 214 Jacuipe-Sergipe/Alagoas basi n 211, 272, 216 Jacufpe-Sergipe-Alagoas Transfor m (JSAT ) 22 6 Jargalant Nuru u 8 2 Jargalant rang e 8 2 Jean Charco t fractur e zon e 211 , 213, 273 jostling bloc k mode l 18 0 Junggar Basi n 66 , 84 , 84, 85, 8 6
INDEX
Junggar Bloc k 8 6 Kazahstan platfor m 5 3 Kemin (Kebin ) earthquak e (1911 ) 59 Kemin-Chilik faul t 5 9 Kemin-Chu earthquak e (1938 ) 5 9 Keta Basi n 21 4 Khorat basi n 10 5 Khorat Platea u 9 1 Kirkwood Faul t 12 7 Komi faul t zon e 21 8 Kribi fractur e zon e 21 5 Kungey rang e 5 8 Kunlun faul t 22 , 23 , 2 3 Kyrgyz-Terskey shea r zon e 57 , 58 Lake Basi n faul t zone s (Montana ) 175-6 , 17 6 Lake Issyk-Ku l 57 , 58 , 60 , 61 , 6 2 Lampang basi n 9 4 Landers earthquak e (1992 ) 2 0 Lanterman Faul t 110 , 112 , 148 Lanterman Rang e 11 2 Laramide Orego n 159-8 1 Larsen Basi n 117 Larsen glacie r 116 , 11 7 lattice preferred orientatio n (LPO ) 19 Leap Yea r Faul t 112 , 148 , 149, 153 Li basi n 94 , 100 , 103 Li faul t 10 3 Limpopo Bel t 14 0 lineaments 2 lithospheric fault s 19-2 8 Little Stewiack e Rive r Formatio n 188 , 189 , 190, 191, 19 3 Lochiel Formation 18 9 Lurio Bel t 140 , 141 Maceio fractur e zon e 22 2 Mackay Se a Valle y 12 1 Madagascar shea r zon e 3 1 Mae Chae m basi n 9 4 Mae Chae m faul t zon e (MACF ) 10 5 Mae Cha n faul t (MCF ) 92, 94 , 96-8 , 99 , 102 , 103, 10 5 Mae Hon g So n faul t (MHF ) 10 3 Mae Kuan g Rive r 9 9 Mae Mo h basi n 94 , 101 , 103 Mae Mo h faul t 101 , 103 Mae Pin g faul t 99-100 , 101, 105, 106 Mae Pin g faul t zon e (MPFZ ) (Wan g Cha o faul t zone) 92 , 93 , 94 , 103 , 105 Mae Sa i basi n 10 3 Mae Sarian g faul t (MSF ) 103 Mae Sarien g basi n 9 4 Mae So t basin 10 3 Mae Sot-Ma e Ramat basi n 9 4 Mae Tee p basin 10 1 Mae Th a faul t 99 , 10 0
231
Mae Tue n basi n 9 4 Magdalen Basi n 185 , 186, 186 magnetotelluric (MT ) soundings 19-2 0 Main Recen t Fault , NW Ira n 4 Main Tsambagara v Dextra l Revers e Faul t 71 , 72 , 73, 74 Malawi Rif t 12 7 mantle plume activit y i n Ros s Se a region , Antarctica 145-5 5 Maraa Uu l 7 6 Marlborough Faul t system , Ne w Zealand 6 Mary Byr d Land 145 , 146 Matador uplift/Re d Rive r Arc h (Texas ) 16 7 Maudheim Provinc e 135 , 136, 138 Maung Pa n basi n 10 2 Mawson Bai n 11 8 Mawson Glacie r 110 , 112 , 127 McMurdo Soun d Faul t 118 , 121, 125, 127, 128, 129 McMurdo Volcani c Grou p 11 2 Meander Intrusiv e Grou p 14 8 megakink 19 2 Meguma Grou p 188 , 190, 192 Meguma terran e 186 , 186, 188 , 190, 191 Mekong Rive r 9 6 Mengildyk Nuru u rang e 80 , 83, 84 mesoscopic structura l analysi s 16 4 Mesquite Basi n 12 7 Midcontinent 159-8 1 fault-and-fold zone s 162 , 163 Minas Faul t Zon e 186 , 186, 187, 188, 193, 194 moho fault s 19-2 8 Mohr-Coulomb sli p criteri a 42 , 44 Mojave deser t 20- 2 Moray Firt h Basi n 47 , 49 Mozambique bel t 17-1 8 Mozambique Ridg e 139 , 140 Muang Pa n basin 99 , 10 0 Na No i basin 9 4 Najd faul t syste m 3 5 Nam M a faul t (NMF ) 94 , 96 , 97 , 10 3 Namurian Mabo u Grou p 19 4 Nan Rive r sutur e zone 9 4 Nan-Uttaradit sutur e 91 Nemaha uplif t (Kansas ) 169-7 0 New Madri d seismi c zon e 2 , 162 , 176, 177, 777 Newfoundland-Azores-Gibraltar faul t zon e 30 , 31 Newfoundland-Azores-Gibraltar transfor m plat e boundary 3 0 Ngao basi n 101 , 102, 104 Ngao faul t 101 , 103, 104 North Armorica n Shea r Zon e (SASZ ) 2 4 North Issyk-Ku l fault 60 , 61 , 6 2 North Pyrenea n faul t 26 , 27 , 2 7 oceanic fractur e zone s 1 Ocoyic deformatio n 19 9
232
INDEX
Omno Hayrha n Uul a 78-80, 7 8 Ottuk faul t 5 9 Owl Cree k Mountains (Wyoming ) 174- 5 Pacific Plat e 9 Pai basi n 9 4 Pamir-Bailakl-Okhotsk shea r zon e 8 , 9 pan-African shearin g i n Antarctica 135-4 1 Pangaea assembl y 185-9 4 Paradox basi n 166- 7 Pelotas Basi n 21 3 Pernambuco shea r zon e 1 7 Phayao basi n 94 , 101 , 104 Phayao-Nan are a 101-2 , 103 Phitsanulok basi n 94 , 10 4 Phrae basin 94 , 10 1 Phrae faul t 10 3 Phrao basi n 94 , 99 , 100, 150 plains-type structure s 16 2 Plum Rive r faul t zon e 170 , 171 Pn anisotrop y 2 5 Pn-waves 2 4 polycrystal plasticit y mode l 2 8 Pong basi n 10 1 Pred-Terskey faul t 6 1 Priestley faul t 5 , 6, 102 , 110, 112 , 114 , 148, 151 , 153 Prince Alber t Bloc k 114 , 115 , 115, 116 , 119, 119, 128 Prince Alber t Fault Syste m 110 , 127 , 128, 129 tectonic modellin g o f 116-20 , 12 0 Prince Alber t Mountain s Tectoni c Bloc k 11 8 principal displacemen t zon e (PDZ ) 7, 136 , 139, 204, 20 5 propeller fault s 16 7 pseudo-gravity 11 3 Pua basi n 94 , 10 1 pull-apart basi n 7 , 103- 4 Puna Altiplan o 197 , 199 push-up zone s 5 0 push-ups 7 P-waves 19 , 24, 25 , 6 0 Pyrenees 26 Qailam sedimentar y basi n 2 2 Quechua deformatio n 19 9 Reconcavo basin 21 5 Red River fault system 9, 22, 221 106 Reelfoot rif t 176 , 177 Reeves Fault 118 , 12 9 Reeves Glacie r 110 , 112, 115 , 116, 12 7 rhegmatic tectonic s 2 Ribeira shea r zon e 3 1 Ribeira-AraNuai-West Cong o oroge n 2 7 Riberia transpressiv e bel t 26 , 27 Riedel experimen t 35 , 46, 47 Riedel shea r faul t 10 5
Rio Grand e Ridg e 27 2 Rio Grand e Ris e 21 3 Rio Muni basi n 21 4 Ritscherflya Supergrou p 135 , 140 sinistral strike-sli p movement , ag e o f 138- 9 deformation o f 136-8 , 137, 13 8 Roberts Ridg e 12 5 Romanche fractur e zon e 211 , 213, 213, 214 Rong Kwan g fault 10 1 Ross Oroge n 111-1 2 Ross Se a region 9 Cenozoic magmatis m 148-5 0 Cenozoic tectonic s 147-8 , 74 7 relations betwee n faultin g an d dyk e injectio n 150-1 Ross Se a Rif t 109 , 110, 111, 112, 113, 775, 118, 127, 12 8 Rough Cree k faul t syste m (Kentuck y and Illinois ) 183 Rough Cree k grabe n 17 3 Sagaing faul t 92 , 105 , 106 Sair Uu l Bloc k 75 , 76, 77 , 78 , 8 3 Salar d e Antofall a strike-sli p basi n 201, 206 Salina de l Frail e pull-apar t basin , Argentin a 197 208 fault architectur e 203- 5 geology 798 , 199-202 , 202, 203 stratigraphy 20 0 structure 202- 3 tectonic mode l 205- 6 Salween faul t (SWF ) 103 Sal ween Rive r 9 7 San Andrea s faul t system , Californi a 15 , 27 , 22 , 35,47 San Gregorio-Sur-Sa n Simeon-Hosgr i fault , Cali fornia 5 San Jacinto faul t system , California 6 Sandwich faul t zon e 170-1 , 777 Sao Paulo Platea u 272 , 213 Sayan Mountain s 66 Sergipe Basi n 279, 220, 221 Shan-Thai terran e 90 Shubenacadie Basi n 19 2 Simae basi n 9 2 SKS wave s 19 , 20, 24 , 2 8 Song Ma/Son g D a zon e 9 0 South Armorica n Shea r Zon e (SASZ ) 24 , 24, 25,
27
South Atlanti c rifte d continenta l margins , trans form zone s i n 211-26 geological an d geophysica l interpretatio n 215— 24 South Issyk-Ku l faul t 59 , 60 , 61 , 6 2 Southern Cros s Mountain s Block 11 4 Southern Ocea n 9 Southern Ocea n Fractur e Zone s 152 , 755, 154 Southern Oklahom a aulacoge n 167- 9
INDEX
Southern Troodo s transfor m fault 5 0 Southwestern Wisconsi n 17 1 Soviet endogenou s regim e mode l 2 St Marys Basin , Nov a Scoti a 185-9 4 geology 188- 9 structural evolutio n 189-9 3 Subbetic zone , Beti c Cordiller a 49, 50 Sukhothai fol d bel t 9 2 Sverdrupfjella Grou p 13 5 S-waves 19 , 20, 23 , 2 6 Tal Nuur Basin 8 0 Tal Nuu r thrust faul t 79 , 84 Talas-Fergana faul t 53 , 5 7 Taldy-Su thrus t 5 9 Tanga faul t 5 9 Tarim plat e 53 Tasman transfor m 11 2 Terror Rif t 5 , 102 , 112 Thailand, strike-sli p faultin g i n 89-106 Cenozoic tectonic s 91- 2 geological structur e 9 3 interpretation o f openin g o f sedimentar y basin s and 102- 5 Landsat T M image s 94 , 9 5 pre-Tertiary tectonic s 90- 1 regional 92- 4 sedimentary basin s an d 94-102 tertiary basin s 9 0 Thoen basi n 103 , 199 Thoen faul t 10 1 Thoen-Phrae faul t 100-1 , 702 Three Pagoda s faul t 91-2 , 103 , 106 Tibetan Platea u 9 Tien Dha n strike-sli p deformatio n 53-6 2 active crusta l movement s 59-6 1 geodetic dat a 60- 1 seismicity 59-6 0 seismotectonics 60 basement structur e o f 57- 8 cenozoic structur e o f Issyk-Kul regio n 58- 9 geological an d tectonic backgroun d 54- 5 tectonic layerin g o f lithospher e 55- 7 Tien Tari m microcontinen t 58 Tolbo Nuu r Faul t 75 , 76 , 7 8 Torbook Formatio n 18 8 Toromhon regio n 6 Toru-Aigyr faul t 5 8 transcurrent fabri c 1 6 transcurrent shea r zone s an d strai n 16-1 9 transform fault s 1 trishear fault-propagatio n foldin g 16 2 Tristan hot-spo t 21 3 Tsagaan Sala a Faul t 77 , 77 , 8 4 Tsambagarav Massi f 70-5 , 73 , 74 , 77, 82 , 82 Tucano Basi n 21 4 Uncompahgre uplif t 166- 7
233
United State s crato n 159-8 1 case studie s of Mesozoic-Cenozoi c strike-sli p 173- 6 of Palaeozoi c strike-sli p 165-7 3 geological background 161- 3 Holocene example s 176- 7 identification o f displacement s 163- 5 Urfjell Grou p 135 , 138, 139 Uttaradit faul t zon e (UDFZ ) 94 , 10 5 Valley o f Lakes , Mongoli a 66 , 6 8 Vaza-Barris faul t 21 4 Vernen earrthquak e (1887 ) 5 9 Very Lon g Baselin e Interferometr y (VLBI ) 2 1 Victoria Lan d Basi n 118 , 128 Volcan Antofall a 197 , 199, 201 Von Mise s equivalen t strai n rat e 30 Walls Boundar y fault 2 3 Walvis Ridg e 272, 213 Wang Cha o (Ma e Ping) faul t zon e 92 , 93 , 94 , 103, 10 5 Wang Nu a basi n 10 2 West Antarcti c Rif t Syste m (WARS ) 109 , 145-55 West Rive r S t Marys Faul t 190 , 19 2 Western Dronnin g Mau d Land , Antarctica , panAfrican shearin g i n 135-4 1 implications fo r Gondwana refit s 139-4 1 tectonic consequence s fo r southeas t Afric a 14 1 White Roc k Formatio n 18 8 Whittier faul t system , Lo s Angele s Basi n 6 Wiang Haen g basi n 9 4 Wiang Papa o basi n 99 , 10 0 Wilkes subglacia l basin , strike-sli p faultin g i n 109-29 aeromagnetic anomalies/faults , Cap e Robert s Rift regio n 120- 5 aeromagnetic dat a 112-1 3 aeromagnetic interpretation technique s 11 3 aeromagnetic signature s 11 3 Cenozoic transtension , Cap e Roberts Rif t regio n 125-7 crustal bloc k 113-1 6 eastern margi n 128- 9 regional obliqu e riftin g an d 127- 8 structural framewor k 111-1 2 tectonic modellin g o f Prince Albert Fault Syste m 116-20 tectonics 77 7 Transantarctic Mountain s uplif t an d 12 8 Wilson thrus t 11 2 Windsor Grou p 192 , 194 Wing Papa o basi n 101- 2 wrench fault s 15-3 2 lithospheric 28-31 , 29, 30 strain-induced mechanica l anisotrop y o f continental lithospher e 28-3 0 thermal conductivit y anisotrop y 30- 1
234 INDE
moho fault s v s lithospheri c fault s 19-2 8 transcurrent shea r zone s 20- 5 transpressional erogeni c domain s 25- 8 transcurrent shea r zone s an d strai n 16-1 9 Wuwei basi n 10 5 Yom Rive r 10 1 Zimbabwe crato n 14 0
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