Structural geology and tectonic evolution of the Sognefjord Transect, Caledonian orogen, southern Norway: a field trip guide (GSA Field Guide 19)

Field Guide 19

Structural Geology and Tectonic Evolution of the Sognefjord Transect, Caledonian Orogen, Southern Norway—A Field Trip Guide

by Alan Geoffrey Milnes GEA Consulting Grand-Rue 7C CH-2035 Corcelles NE Switzerland Fernando Corfu University of Oslo Department of Geosciences Postboks 1047 Blindern N-0316 Oslo Norway

Field Guide 19 3300 Penrose Place, P.O. Box 9140

Boulder, Colorado 80301-9140, USA

2011

Copyright © 2011, The Geological Society of America (GSA), Inc. All rights reserved. GSA grants permission to individual scientists to make unlimited photocopies of one or more items from this volume for noncommercial purposes advancing science or education, including classroom use. For permission to make photocopies of any item in this volume for other noncommercial, nonprofit purposes, contact The Geological Society of America. Written permission is required from GSA for all other forms of capture or reproduction of any item in the volume including, but not limited to, all types of electronic or digital scanning or other digital or manual transformation of articles or any portion thereof, such as abstracts, into computer-readable and/or transmittable form for personal or corporate use, either noncommercial or commercial, for-profit or otherwise. Send permission requests to GSA Copyright Permissions, 3300 Penrose Place, P.O. Box 9140, Boulder, Colorado 80301-9140, USA. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. Copyright is not claimed on any material prepared wholly by government employees within the scope of their employment. Published by The Geological Society of America, Inc. 3300 Penrose Place, P.O. Box 9140, Boulder, Colorado 80301-9140, USA www.geosociety.org Printed in U.S.A. GSA Books Science Editors: Marion E. Bickford and Donald I. Siegel Library of Congress Cataloging-in-Publication Data Milnes, A. G. Structural geology and tectonic evolution of the Sognefjord Transect, Caledonian orogen, southern Norway : a field trip guide / by Alan Geoffrey Milnes, Fernando Corfu. p. cm. — (Field guide ; 019) Includes bibliographical references. ISBN 978-0-8137-0019-9 (pbk.) 1. Geology, Structural—Norway. 2. Orogeny—Norway. 3. Plate tectonics—Norway. 4. Geology— Fieldwork—Norway. I. Corfu, Fernando, 1949– II. Title. QE633.N8M55 2011 551.809483ʹ8—dc22 2011002497 Cover: Sognefjorden, ~50 km from the mouth, looking NE into a small side arm. Mountains in the background are ~900 m above sea level; water depth to the right of the photo is ~1300 m (locality Austrheim, Stop 3.3). Photo by Geoffrey Milnes.

10 9 8 7 6 5 4 3 2 1

Contents

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .vii Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Chapter 1. An introductory outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Autochthon-Parautochthon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Caledonian Allochthon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Late- to Post-Collisional Extensional Shear Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Lærdal-Gjende Fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Nordfjord-Sogn Detachment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Plate Tectonic Cartoon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Chapter 2. The Sognefjord transect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1. Crustal Structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2. Structural Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Eastern Segment of the Sognefjord Transect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Central Segment of the Sognefjord Transect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Western Segment of the Sognefjord Transect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3. Retro-Deformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Balancing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Depth Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 4. Eclogite Exhumation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5. An Orogenic Timetable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Chapter 3. The field trip itinerary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Day 1. Valdres-Jotunheimen. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Stop 1.1. Søndrol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Stop 1.2. Vangsmjøsi. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Stop 1.3. Øye . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Stop 1.4. Tyin Road Profile. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Stop 1.5. Tyedalen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Stop 1.6. Lorteviki–Eidsbugarden . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Day 2. Inner Sognefjorden . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Stop 2.1. Årdal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Stop 2.2. Lærdal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Stop 2.3. Eide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Stop 2.4. Sogndal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Stop 2.5. Slinde. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Stop 2.6. Hermansverk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

iii

iv

Contents Day 3. Outer Sognefjorden . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Stop 3.1. Hella . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Stop 3.2. Sæle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Stop 3.3. Austrheim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Stop 3.4. Kyrkjebø . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Stop 3.5. Råsholm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Stop 3.6. Hellebø . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Stop 3.7. Bekkeneset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Day 4. Solund . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 Stop 4.1. Losna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 Stop 4.2. Hersvik . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Stop 4.3. Hyllestad . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Day 5. Askvoll-Atløy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 Stop 5.1. Vårdalsneset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 Stop 5.2. Gjervik. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Stop 5.3. Kviteneset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Stop 5.4. Brurestakken . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Day 6. Fensfjorden-Lindås . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Stop 6.1. Kjekallevågen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 Stop 6.2. Osterfjorden . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 Stop 6.3. Holsnøy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 Stop 6.4. Stalheim. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

Preface

The field trip described in this guide starts in the “Norwegian Alps,” the high mountain massif called Jotunheimen, and runs out along Sognefjorden, the world’s longest fjord, to the islands along the west coast of Norway. Geologically, the Sognefjord transect provides a complete cross section through the Caledonian orogenic belt, of Paleozoic age, which in Norway stretches along the west coast from Stavanger to North Cape, a distance of ~2000 km. This transect in southern Norway provides a superb and exceptionally well-documented example of late collisional tectonics in an Alpine-type orogen. It provides a continuous, 250-km-long cross section from the cratonic foreland, in the east, through the heavily deformed continental margin, to the remains of the Caledonian ocean complex, in the west. Detailed structural data are available along the whole transect, together with good stratigraphic, radiometric, petrological, and geophysical control. These data have been analyzed in terms of the kinematics and relative ages of the different deformation phases, and correlated along the whole transect. Logistically, the whole route is ideal for field trips, both individually and in groups. It is easily accessible and well exposed, with good communications and different types of accommodations and services. The itinerary described in this guide is based on Excursion 28 of the 33rd International Geological Congress held at Oslo in August 2008. Many other itineraries are possible and practical, however, and it is hoped that this guide will also prove useful for planning trips on a do-ityourself basis. Some parts of the trip are mainly “stop and look,” because of the large distances to be covered, but there are some stops with short crossings of rough (possibly wet) terrain, requiring good footgear (leather walking boots or rubber boots). Also, the weather is unpredictable, and the worst has to be expected (rain, wind, temperatures down to 10 °C in summer), requiring good wind and rain protection (wind-proof, water-tight anorak and rain trousers, umbrella). Most of the trip can be carried out anytime between March and October. May–July are the best months, both weatherwise and because of the long daylight hours. The high mountain parts of the trip (Stop 1.4 to Stop 1.6) may be hindered because of snow conditions, in which case the itinerary can be completed by driving directly along the main Oslo-Bergen road E16 (kept open year round) from Stop 1.3 to Stop 2.1. For weeklong weather forecasts, consult the web site of the Norwegian Meteorological Institute and the Norwegian Broadcasting Corporation; www.yr.com/English/. A useful piece of geological equipment, apart from field book, compass-clinometer, etc., is a pair of binoculars. For the overnight accommodation at some of the localities, participants should bring along a sheet sleeping bag, although there is also the possibility of renting one when required. Each participant should have a high-visibility vest, easily accessible throughout the trip, because some road sections are on main roads. In addition to the present Field Guide, the basic data for the trip are to be found in two papers: Milnes et al. (1997) and Wennberg et al. (1998). For an overall view of the Sognefjord transect and a comparison with the Alps, see Milnes (1998). The whole area of the trip is covered by excellent 1:250,000-scale geological maps of the Norwegian Geological Survey (NGU), map sheets Årdal, Florø, and Bergen. These should be purchased before the trip (order through the NGU web site: www.ngu.no). A recent publication of the Geological Society of Norway (Ramberg et al., eds., 2008) gives an easily read and beautifully illustrated overview of the geology of Norway and its continental shelf, including several chapters relevant to the present field trip (Chapters 3–7) and an overview map of Norway at a scale of 1:2,000,000. A good road map for the whole trip is the Cappelens 1:335,000-scale map CK2 (“Sør-Norge nord”—ISBN 978-82-02-27260-9); see Cappelens web site: www.cappelenkart.no. For planning the trip it is essential to obtain up-to-date ferry v

vi

Preface timetables. These can be obtained in English from the ferry company’s web site: www.fjord1.no/en/. The timetables needed for the trip described here are numbers 14-107, 14-173, 14-251, 14-321, 14-415, and 14-431. Google provides web access to almost all types of accommodation and services along the route if you search by town name. The web address of the Norwegian telephone database is www.telefonkatalogen. no. In Norway, the emergency telephone numbers are 110 (Fire), 112 (Police), and 113 (Medical emergency).

Acknowledgments

Many people have knowingly or unknowingly contributed to the production of this guide. We think particularly of the many graduate students, researchers, and university teachers who have taken part in field trips along part or all of this transect since we started our research program in the mid-1970s. We thank them all for the many discussions and arguments, in fair weather and foul, during which the ideas set out here crystallized and, hopefully, matured. We would like to express special thanks to Michael Heim, Urs Schärer, Andreas Koestler, Thomas Dietler, Mattias Lundmark, and Ole Petter Wennberg, for their fine research and their companionship in various phases of this work. The itinerary of the field trip as described here is based on Excursion 28 of the 33rd International Geological Congress, which was held in Oslo in August 2008. We wish to thank the participants on that trip (photo below) for their interest and inspiring evening discussions over expensive beers, and for their encouragement for writing this guide. We would also like to thank Torgeir Andersen and Håkon Austrheim for providing maps and diagrams from the guide that they prepared for the 33IGC Excursion 29, which we have used to illustrate the Atløy part of our excursion (Figs. 38–40 in this guide).

The itinerary in this Field Guide was followed on Excursion 28 of the International Geological Congress (IGC) in Norway, in August 2008. Here, “on top of the world,” are the IGC participants, with lake Tyin behind and the high mountain area of Jotunheimen in the background. In the photo, from left to right: Stefano Mazzoli, Michael Szpunar, Marion (“Pat”) Bickford, Miles Osmaston, Mattias Lundmark, George DeVries Klein, Allah Bakhsh Kausar, Fernando Corfu (co-leader), Egle Sinkune, Greg Dunning, Petras Sinkunas, Florencia Bechis, and Rômulo Machado.

vii

The Geological Society of America Field Guide 19 2011

Structural Geology and Tectonic Evolution of the Sognefjord Transect, Caledonian Orogen, Southern Norway—A Field Trip Guide Alan Geoffrey Milnes GEA Consulting, Grand-Rue 7C, CH-2035 Corcelles NE, Switzerland Fernando Corfu University of Oslo, Department of Geosciences, Postboks 1047 Blindern, N-0316, Oslo, Norway

ABSTRACT The Sognefjord transect described in this field guide starts in the “Norwegian Alps,” the high mountain massif called Jotunheimen, and runs out along Sognefjorden, the world’s longest fjord, to the islands along the Norwegian west coast. Geologically, it provides a complete cross section through the Caledonian mountain belt, and represents an exceptionally well-documented example of late collisional tectonics in an Alpine-type orogen. It is comparable with the Alps as a natural laboratory for orogenic studies, being both easily accessible and well exposed, with a long history of geological research and excellent geological map coverage. The transect exposes several major tectonic structures, including the Jotun thrust complex (Days 1–2), with a demonstrable displacement of 200–300 km, the extensional Nordfjord-Sogn Shear Zone (Days 4–6), with up to 50 km of normal displacement, and the eclogitic orogenic root, with evidence for ductile rebound under predominantly gravitational forces (Day 3). The field trip starts on the cratonic foreland of the Caledonian orogen, in the east (Day 1), continues through the heavily deformed continental margin (Days 1–3), and ends in the remains of the Caledonian ocean complex, in the west (Days 4–6). Detailed structural data are available along the whole transect, together with good stratigraphic, radiometric, petrological, and geophysical control. These data have been analyzed in terms of the kinematics and relative ages of the different deformation phases, and used to reconstruct the crustal geometry at different stages backward in time (kinematic modelling) and to imitate the process of orogenic root collapse (dynamic modelling). The itinerary is based on Excursion 28 of the International Geological Congress, which was held at Oslo in August 2008. MANUSCRIPT ACCEPTED BY THE SOCIETY 3 AUGUST 2010

Milnes, A.G., and Corfu, F., 2011, Structural Geology and Tectonic Evolution of the Sognefjord Transect, Caledonian Orogen, Southern Norway—A Field Trip Guide: Geological Society of America Field Guide 19, 80 p., doi:10.1130/2011.0019. For permission to copy, contact [email protected]. © 2011 The Geological Society of America. All rights reserved.

1

 CHAPTER 1  An introductory outline

The Sognefjord transect in southern Norway cuts through the Caledonian orogenic belt, whose collisional phase culminated in Late Silurian and Early Devonian times. It provides a superb and exceptionally well-documented example of late collisional tectonics in an Alpine-type orogen (Milnes, 1998). One of the world’s longest and deepest fjords (Sognefjorden), together with its head valleys and the rugged mountains of Jotunheimen, provides a continuous, 250-km-long cross section from the cratonic foreland, in the east, through the heavily deformed continental margin to the remains of the Caledonian oceanic complex, in the west. The location of the transect is shown in Figure 1. Detailed structural data are available along the whole transect, together with good stratigraphic, radiometric, petrological, and geophysical controls. These data have been analyzed in terms of the kinematics and relative ages of the different deformation phases, and correlated along the whole cross

section. The resulting synthesis has been used, in conjunction with the other data, to carry out a retro-deformation, reconstructing the crustal geometry at different stages backward in time (Milnes et al., 1997), and as a basis for a dynamic model of orogenic root collapse (Milnes and Koyi, 2000). These papers form the basis of the present field trip, which has been run for students and visiting international groups innumerable times in the past two decades and was included as Excursion 28 of the program of the 33rd International Geological Congress (33IGC), held in Oslo in summer 2008. The latter part of the field trip diverges northward and southward from Sognefjorden, briefly studying the footwall and hanging wall of the spectacular Nordfjord-Sogn Detachment (the main theme of Excursion 29 of the 33IGC; see Andersen and Austrheim, 2008), and particularly its southward continuation as the Bergen Arc Shear Zone (Wennberg et al., 1998).

Figure 1. Sognefjord transect through the Caledonides of southern Norway. Rectangle shows the location of the tectonic map, Figure 2. D—Denmark.

3

4

Chapter 1

The overall structure displayed by the Sognefjord transect can be summarized in terms of two main collisional complexes and two major, post-collisional, extensional shear zones (Fig. 2). The collisional complexes are designated AutochthonParautochthon and Caledonian Allochthon in the legend to Figure 2. These were actively deforming in Late Silurian and Early Devonian times. The extensional shear zones postdate the collision and were active in the Middle and Late Devonian. The more easterly zone is called the Lærdal-Gjende Fault where it crosses the Sognefjord transect. The more westerly zone, which approximately follows the Norwegian Atlantic coastline where it crosses the transect, is called the Nordfjord-Sogn Detachment in Figure 2. These extensional shear zones can be used to subdivide the transect into three segments: an eastern segment (footwall and hanging wall of the Lærdal-Gjende Fault), a central segment

(footwall of the Nordfjord-Sogn Detachment), and a western segment (hanging wall of the Nordfjord-Sogn Detachment). On the field trip the Caledonian orogen (Autochthon-Parautochthon below, Allochthon above) will be studied separately in each segment: Days 1–2, eastern segment; Day 3 (with one Stop on each of Days 4 and 5), central segment; Days 4–5, western segment. The post-collisional shear zones will be studied where the transect crosses them: Days 1–2, Lærdal-Gjende Fault; Days 4–5, Nordfjord-Sogn Detachment. On Day 6 the trip follows a branch of the Nordfjord-Sogn Detachment, the Bergen Arc Shear Zone, southward from the Sognefjord transect, to study the Caledonian structures in the hanging wall. A brief introduction to each of the major tectonic elements is given below, followed by a cartoon outline of the supposed plate tectonic development of the Caledonian orogen, in comparison with the Alps, as general background.

Figure 2. Tectonic map of the Sognefjord region, southern Norway, showing the location of the Sognefjord transect and the location of the detailed structural profile, Figure 4 (center line of the transect strip). BASZ—Bergen Arc Shear Zone; BSB—Baltic Shield Basement; HFSZ— Hardangerfjord Shear Zone; LGF—Lærdal-Gjende Fault; NSD—Nordfjord-Sogn Detachment; WGC—Western Gneiss Complex. Adapted from Milnes et al. (1997).

An introductory outline

5

1. AUTOCHTHON-PARAUTOCHTHON

2. CALEDONIAN ALLOCHTHON

The lower of the two collisional complexes, known as the Autochthon-Parautochthon in the literature (cf. Roberts and Gee, 1985), is represented along the Sognefjord transect by the Baltic Shield Basement (BSB in Fig. 2) and its northwestward continuation as the Western Gneiss Complex (WGC in Fig. 2). The Baltic Shield Basement forms the whole of southern Norway. It is now part of the Baltic Shield, and it was part of a shield area, Baltica, also in late Precambrian and early Paleozoic times, forming the rigid, cratonic foreland of the Caledonian orogen. In the eastern segment of the transect it is seen in a series of windows through the allochthonous units. The Western Gneiss Complex makes up most of the central segment of the Sognefjord transect and consists of two zones that are clearly defined on a regional scale: a southeastern zone (forming the mountains below and surrounding Jøstedalsbreen, Norway’s largest ice cap), which consists of Precambrian basement similar to the Baltic Shield Basement, and a western zone in which this basement has been “Caledonized” (deformationally and metamorphically overprinted during the Caledonian orogeny—hence the term Parautochthon). The Caledonized zone of the Western Gneiss Complex contains the well-known eclogite bodies of western Norway (“e” localities in Fig. 2).

The upper collisional complex is known as the Caledonian Allochthon and contains a number of more or less far-traveled nappe units derived from the cover of the Western Gneiss Complex, the rifted margin of the ancient craton, Baltica, and parts of the oceanic area that lay outboard of this margin, known as Iapetus (cf. Stephens, 1988). The Allochthon was overthrust from the northwest onto the Autochthon-Parautochthon in Late Silurian and Early Devonian times (ca. 425–395 Ma, sometimes referred to as the Scandian orogenic phase). Thrusting took place on a basal décollement zone, sometimes referred to as the Main Caledonian Detachment Zone, with an overall displacement of 200–300 km (Hossack et al., 1985; Fossen, 1992). The term detachment is used in this guide to designate any low-angle shear zone on which the displacement is so large that it can only be deduced from regional considerations. A detachment may be extensional or contractional and is usually accompanied by a zone of intense shearing or mylonitization that may be several kilometers thick. The shear zone at the base of the Caledonian Allochthon is a contractional detachment, related to crustal shortening (and thickening), whereas the Nordfjord-Sogn Detachment (see below) is an extensional detachment, related to crustal extension (and thinning). The Caledonian Allochthon has been subdivided into Lower, Middle, and Upper nappe units on the basis of structural position and/or rock association, which reflect different paleogeographic localities within the pre-Caledonian plate tectonic configuration (Roberts and Gee, 1985). The Lower Allochthon mainly represents the sheared-off, late Precambrian–early Paleozoic cover of the Autochthon-Parautochthon, although it locally contains slices derived from both underlying and overlying units. In the eastern segment of the Sognefjord transect it is represented by strongly sheared and disrupted early Paleozoic metasediments, but southeastward it grades into a more or less coherent foreland fold-and-thrust belt, dying out and becoming autochthonous south of Oslo (see Fig. 1: Caledonian front). The Middle Allochthon in the Sognefjord transect is represented by a large mass of Precambrian basement (called the Jotun Complex) and its late Precambrian–early Paleozoic sedimentary cover (Valdres “sparagmites” overlain by early Paleozoic platform sediments up to Wenlock in age). This unit is sometimes referred to as the Jotun Nappe or Jotun-Valdres Nappe Complex. It is generally interpreted as part of the rifted continental margin of Baltica (cf. Emmett, 1996) on the basis of stratigraphic affinity with the Lower Allochthon. Locally, it contains felsic intrusions and associated dike complexes, formed in the Silurian during the initial stages of nappe development (Lundmark and Corfu, 2007). The Upper Allochthon in southern Norway is characterized by Ordovician–Early Silurian ophiolite complexes with island arc and marginal basin affinities, and associated sedimentary and intrusive rocks (e.g., Pedersen et al., 1988; Andersen and Andresen, 1994). These units are mixed with scattered continental fragments, some in a mélange-like association reminiscent of the

Figure 2. (continued).

6

Chapter 1

Pennine Zone of the Central Alps (cf. Milnes, 1978, 1998; Froitzheim et al., 1996). The Upper Allochthon represents the remains of the Iapetus oceanic complex. On the Sognefjord transect, the Upper Allochthon is only seen in the western segment, in the hanging wall of the Nordfjord-Sogn Detachment. Higher units or units derived from the Laurentian side of Iapetus have not been identified along the Sognefjord transect. Apart from the huge Jotun Nappe, the Middle and Upper Allochthons in southern Norway are commonly fragmentary and dismembered, with different tectonic units bearing different local names or their assignment being controversial. In order to clarify their affinities in this Field Guide, local names will be followed where appropriate (in parentheses), if their affinity is established, or with question marks and/or alternatives when this is not the case.

two major, late to post-collisional, extensional shear zones. The more easterly of these is known as the Lærdal-Gjende Fault where it crosses the Sognefjord transect (LGF in Fig. 2; cf. Milnes and Koestler, 1985), and the Hardangerfjord Shear Zone farther to the southwest (HFSZ in Fig. 2; cf. Fossen, 1992; Fossen and Hurich, 2005). The low-angle, NW-dipping Lærdal-Gjende Fault marks the southeastern border of what has traditionally been known as the “Faltungsgraben” (Goldschmidt, 1912). This initially enigmatic structure is now known to be a large-scale half graben, with the Lærdal-Gjende Fault marking its southeastern margin. The downthrow on this fault is estimated at ~8 km, bringing down the Middle Allochthon, which along this transect is composed of the Precambrian Jotun Complex, making up most of the high mountain area of Jotunheimen. Nordfjord-Sogn Detachment

3. LATE- TO POST-COLLISIONAL EXTENSIONAL SHEAR ZONES Lærdal-Gjende Fault Along the Sognefjord transect the Caledonian Allochthon and the underlying Autochthon-Parautochthon are cut through by

The western end of the Sognefjord transect is intersected by a much larger low-angle, extensional shear zone known as the Nordfjord-Sogn Detachment (Fig. 2; see, e.g., Andersen, 1998; Braathen et al., 2004), represented at the present erosion level by a 1–2-km-thick zone of mylonitized rocks, topped by a brittle fault (Solund Fault or Dalsfjord Fault of probable Mesozoic age;

Figure 3. Schematic cartoon of the orogenic evolution of collisional orogens, with approximate Caledonide and Alpine time limits (adapted from Milnes, 1998). Key: cf—cratonic foreland; rm—rifted continental margins and microcontinents; oc—oceanic complex; ll—spreading centers. In southern Norway the cratonic foreland of the Caledonides is represented by the Baltic Shield Basement and the Western Gneiss Complex (Autochthon-Parautochthon; see Fig. 2). The Lower Allochthon consists of the stripped-off cover of the craton, with some basement slices; the Middle Allochthon represents basement and cover units from the rifted continental margins; and the Upper Allochthon contains remnants of the oceanic complex with fragments of oceanic crust, microcontinents, subducted flakes, island arcs, etc., often in mélange-like associations.

An introductory outline see Torsvik et al., 1992). The hanging wall of the Nordfjord-Sogn Detachment contains the erosional remnants of a large Devonian basin or group of basins, with coarse clastic sediments lying on the eroded stumps of the Caledonian Allochthon (mainly the Upper Allochthon, consisting of the Solund-Stavfjord Ophiolite Complex). The footwall is built of gneisses belonging to the Western Gneiss Complex (Fig. 2), showing collision-related high-grade metamorphism and polyphase deformation, and enclosing eclogite bodies and high-pressure schists. The displacement on the Nordfjord-Sogn Detachment is estimated to be in the region of 50 km. South of the Sognefjord section this detachment extends into the North Sea, but in the Bergen area it is at least partly represented by the Bergen Arc Shear Zone (BASZ in Fig. 2), an oblique-lateral ramp in the Devonian extensional detachment system in western Norway (Wennberg et al., 1998). 4. PLATE TECTONIC CARTOON The Scandinavian Caledonides and the Alpine System are generally described, since the advent of plate tectonics, in terms of the opening and closing of an ocean within a previously consolidated super-continent, the late Precambrian continent Rodinia in the Caledonides, and the Permo-Triassic continent Pangea in the Alps. This model—or better, “comic strip”—is little more than the traditional “Wilson cycle” and is sketched with some refinements in Figure 3. For each orogen, one envisages the opening and closing of an oceanic area, with the rifting and thinning of the continental margins and the isolation or partial isolation of microcontinents during the extensional phase (“pre-orogenic

7

extension,” Milnes, 1998) and the development of subduction zones, island arcs, backarc basins, and all their corollaries during the subsequent closing phase (“pre-collisional contraction”). During the closing phase, oceanic crust is mainly subducted or obducted, but locally continues to be formed in backarc basins. The end of this phase and the start of the collisional phase is often placed shortly after the age of the youngest backarc ophiolites (Fig. 3: in the Alps, ca. 60 Ma; in southern Norway, ca. 440 Ma; Dunning and Pedersen, 1988). The phase of collisional contraction can be subdivided into early and late collisional phases, of which the early phase comprises the reconstitution of the original crustal thickness out of the faulted continental margins and microcontinents and their obducted ophiolite fragments (Fig. 3: compare the original and final frames of the “comic strip”). The structures (faults, basins, titled blocks, etc.) formed during pre-orogenic extension largely determine the geometry and dimensions of the tectonic units in the reconstituted crust. However, none of the processes in the cartoons shown in Figure 3 necessarily leads to any change in overall plate motion; any changes in this respect are mainly caused by forces outside the system. In the collisional phase, and particularly during “late collisional contraction” (Fig. 3), the approach of the cratonic forelands is eventually prevented, and a fundamental rearrangement of plate motions on a global scale is initiated (cf. Cloos, 1993). The next step in the process (not shown in Fig. 3) is the one that is the main focus in the present Field Guide and the one that is summarized for the Sognefjord transect in Chapter 2, in preparation for the description of the field localities themselves in Chapter 3.

 CHAPTER 2  The Sognefjord transect

Although the general outline of early Caledonian and Scandian orogenesis and late–post-orogenic extensional tectonics is well established, there have been few compilations of detailed structural data across the whole belt that could aid in advancing past the conceptual stage. An exception is the ValdresJotunheim-Sognefjord–West Coast cross section, here called the Sognefjord transect, which has been studied in detail along its entire length, with numerous Ph.D. and Masters’ theses at various institutions in the 1970s, 1980s, and 1990s. These data were compiled and synthesized for the first time in the mid-1990s and were accompanied by a composite profile as geometrically correct as possible (Fig. 1 in Milnes et al., 1997, a foldout cross section with horizontal and vertical scales of 1:600,000). The line of section is plotted in Figure 2, and a reduced version of the cross section is shown in Figure 4. The latter will be used as the connecting link in the present Field Guide, with the different Stops plotted on the section at their correct structural positions. In the original version (Milnes et al., 1997) the main sources of data were summarized with reference to a series of “panels.” These are also shown in Figure 4, but the large number of detailed references that were cited are not repeated in the present guide. Within each panel, the geometry, kinematics, and history of the Scandian deformation were described in detail, and are sufficiently well documented to be correlated with neighboring panels to underpin the reconstructions. In addition, two other types of data provide important constraints. First, although there is as yet no deep reflection seismic profile along the transect, other types of geophysical data provide invaluable information on crustal structure. These will be discussed first (Section 1). Second, the absolute dating of some of the events, by stratigraphic or radiometric means, provides an orogenic timetable that is sufficiently reliable for checking the consistency and plausibility of the movement picture. This type of information will be discussed in connection with the structural synthesis (Section 2). Based on the structural synthesis, an attempt is made to retro-deform the Sognefjord crustal segment—i.e., to reconstruct crustal conditions along the transect by successively removing each deformation phase backward in time, taking into account different lines of argument related to depth of burial at different stages (Section 3). The possibilities of retro-deformation are, however, limited, only reaching back as far as the formation of the eclogites. In the final section of this introductory overview, the kinematic modeling (retro-deformation) is carried further to show how it throws light on the processes of eclogite exhumation and how it has been used for developing a dynamic model of orogenic root collapse (Section 4). This chapter ends with an overall summary

of the tectonic evolution of the South Norwegian Caledonides in the form of an “orogenic timetable” for easy reference and as a basis for discussion (Section 5). 1. CRUSTAL STRUCTURE A recent compilation of geophysical data on Moho depth under Scandinavia (Kinck et al., 1993) shows a maximum Moho depth of almost 40 km under the eastern part of the Sognefjord transect, decreasing slowly westward to <30 km at the western end (present coastline). For farther offshore, little published data are available, and the indicated level is speculative. The combination of westward rising Moho and large offshore normal faults of Permo-Triassic age (Færseth et al., 1995), together with the downthrow on the Devonian Nordfjord-Sogn Detachment, suggests that the offshore crust is greatly thinned and mainly composed of Upper Allochthon or higher units (Fig. 4). Refraction seismic data were acquired along the Sognefjord transect during an experiment with ocean bottom seismometers (Iwasaki et al., 1994), and the results confirmed the general Moho level. The survey also provided supplementary information that may have geological significance. A mid-crustal velocity discontinuity was discovered under the eastern part of the profile. This sharp velocity increase at 20 km depth fades out in a westward direction (Fig. 4). Also, the high-velocity lower crust that underlies most of Scandinavia (P-wave velocities higher than 7 km/s, Korja et al., 1993) disappears westward from about the same position. Both these phenomena may be related to the “Caledonization” of the outer edge of the Baltic Shield, a process that the retro-deformation is thought to illuminate (see below). In the upper parts of the Western Gneiss Complex, no lateral velocity variations were recognizable along the Sognefjord profile, but a marked increase in velocity was observed where the Jotun Complex, with a high proportion of rocks with mafic composition, lies within the “Faltungsgraben.” There, the high-velocity body was detected down to a depth of 6–7 km, with little evidence for unusually high velocities in the deeper parts of the crust. This strengthens the geological interpretation that the Western Gneiss Complex is the direct westward extension of the Baltic Shield (Iwasaki et al., 1994). Similar conclusions were reached during a reappraisal of the gravity and magnetic data across the Faltungsgraben in the Jotunheimen-Valdres area by Skilbrei (1990). In spite of earlier interpretations that suggested a deep “root” of high-density material (Smithson et al., 1974), the more recent modeling indicated that the Jotun Complex is <6 km thick at its deepest below the present surface. The relevant part of the 9

Figure 4 (continued on following page). Composite E-W cross section along the Sognefjord transect: (A) Western part. (B) Eastern part. Adapted from Milnes et al. (1997). No vertical exaggeration. For line of section, see Figure 2. MA—Middle Allochthon; LA—Lower Allochthon.

10

Figure 4. (continued).

11

12

Chapter 2

present cross section (Fig. 4) is thus constructed on the basis of these combined geophysical indications (refraction seismic, gravity, aeromagnetic). Reflection seismic images for parts of the Sognefjord transect may be found on the “Mobil Search” coast-parallel profiles ILP-10 and ILP-11 (positions marked in Fig. 4, together with the location of coast-parallel deep seismic profiles NSDP 84-1 and 84-4). The first images of interest are those where the coastparallel deep seismic surveys cross the Hardangerfjord Shear Zone (HFSZ in Fig. 2) at the mouth of Hardangerfjord. These images are discussed in Fossen and Hurich (2005) and a number of earlier works. As shown in Figure 2, the Hardangerfjord Shear Zone represents the southwestward continuation of the LærdalGjende Fault. A further image, which may be similar to that which would appear on the Sognefjord transect, is the prominent antiformal reflector array, which is recorded on ILP-10 directly south of the projected position of the Nordfjord-Sogn Detachment, as discussed by Færseth et al. (1995). This lies on the offshore continuation of the culmination in the main Caledonian foliation in the Western Gneiss Complex along Sognefjorden (see Figs. 2 and 3). 2. STRUCTURAL SYNTHESIS On the scale of the whole transect, detailed structural information is mainly available from a narrow strip (marked in Fig. 2), although in some parts it must be supplemented with off-section data, along strike on both sides. For purposes of correlation, we have subdivided this strip into three segments, within which a coherent deformational history can be established on the basis of available documentation. Correlation from one segment to the next is less secure, and a working hypothesis has to be set up to make retro-deformation possible. The eastern segment consists of the Faltungsgraben and the footwall of the Lærdal-Gjende Fault, the central segment lies in the footwall of the NordfjordSogn Detachment west of the Faltungsgraben, and the western segment consists of the hanging wall of the Nordfjord-Sogn Detachment. The structurally defined deformation phases in each segment are thus prefixed with a specific name: Jotunheimen for the eastern segment (Fig. 4, panels 1–4), Sognefjord for the central segment (Fig. 4, panels 5 and 6), and West Coast for the western segment (Fig. 4, panel 7). Eastern Segment of the Sognefjord Transect A correlation and interpretation of Caledonian deformation phases in the eastern segment, and across the Faltungsgraben, was carried out by Milnes and Koestler (1985). This work was completed in 1981 and was confirmed and supplemented by numerous later studies (for references, see Milnes et al., 1997). It showed that the structural history of the Jotunheimen Detachment Zone, the major zone of Caledonian deformation in this segment, could be described in terms of a general sequence of six phases, designated Jotunheimen-D1 to Jotunheimen-D6. The

“phase” Jotunheimen-D6, however, was enigmatic, and the structural features (sharp upright folds in the base of the Jotun Nappe) are thought now to be related to perturbations during Jotunheimen-D2. The characteristics of phases D1–D5 are summarized below, and the postulated tectonic significance of each phase is added in parentheses: Jotunheimen-D1: Imbrication and ductile shearing, incorporation of exotic fragments (tectonic burial of the Jotun Complex and its cover by the Upper Allochthon, including ophiolite obduction). Jotunheimen-D2: Main phase of ductile-penetrative deformation in the basal zone of the Middle Allochthon: ductile shearing in basement rocks, isoclinal folding in the late Precambrian– early Paleozoic cover, inversion of basement-cover contacts, formation of the main foliation-stretching lineation, pebble flattening-elongation, enigmatic upright folds in the nappe base (D2 is thought to represent the early stage of SEdirected thrusting in the Jotunheimen Detachment Zone). Jotunheimen-D3: En bloc movement of the Middle Allochthon on the basal thrust: main phase of ductile-penetrative deformation in the underlying metasediments of the Lower Allochthon, formation of the Lower Allochthon duplex structures ahead of the Middle Allochthon block (“bulldozer tectonics”; the late stage of SE-directed thrusting in the Jotunheimen Detachment Zone, mainly accommodated by deformation in the Lower Allochthon). Jotunheimen-D4: Development of foreland-dipping cleavage and NW-vergent asymmetrical folds, superimposed on all earlier structures (reversed, NW-directed movement on the Jotunheimen Detachment Zone, equivalent to the “mode I extension” of Fossen, 1992). Jotunheimen-D5: Truncation and displacement of the Jotunheimen Detachment Zone by movement on the Lærdal-Gjende Fault (low-angle, NW-dipping normal fault, with associated mylonites, cataclasites, and gouges, equivalent to the “mode II extension” of Fossen, 1992). The deformation phases Jotunheimen-D1 to -D4 describe the structural history of the main Caledonian detachment zone at the base of the Jotun Nappe. Out on the foreland, the duplex structures described by Hossack et al. (1985) were formed during the Jotunheimen-D3 phase (main contractional deformation in the Lower Allochthon), and this places valuable controls on the movement picture (amount, direction, and rate of displacement). The upper duplex structure has been traced northwestward until it disappears under the Middle Allochthon, at which location the imbricates lose their coherence and are cut through by the “reversed movement” cleavage of the Jotunheimen-D4 phase (cf. Hossack, 1976, Fig. 6). This top-to-NW movement was relatively minor compared with the top-to-SE thrusting. Jotunheimen-D2 + D3 corresponds to 200–300 km of SE-directed shear displacement (see above), whereas Jotunheimen-D4 displacement must be only of the order of several kilometers (Milnes and Koestler, 1985). Off-section to the southwest, Fossen and Holst (1995) estimated the amount of reversed movement (Jotunheimen-D4) in the same

The Sognefjord transect zone to have been 20–36 km. The onset of reversed movement in the main Caledonian detachment zone (Jotunheimen-D4) marks the change from crustal contraction (collision) to crustal extension at the end of the Scandian orogenic phase (cf. Fossen, 1992). With regard to the depth of the main Caledonian detachment zone at the time of movement, color alteration of conodonts from the Lower Allochthon at the southeastern end of the segment (Fig. 4, panel 1) indicates that they lay at a maximum depth of 10–13 km (Nickelsen et al., 1985), presumably at the end of Jotunheimen-D3 overthrusting. At the northwest end of the segment (Fig. 4, panel 4), metamorphic parageneses indicate maximum depths of 15–20 km for the same zone. This suggests a flat, gently NW-dipping detachment zone at this time (end of Jotunheimen-D3), with an average dip of ~5° over a distance of 100 km. This has been compared with the seismically indicated deep detachment under the Southern Appalachians (Milnes, 1987). There is nothing to suggest that any part of the main Caledonian detachment zone in the eastern segment of the Sognefjord transect reached depths greater than those required for lower amphibolite facies metamorphism during any part of its history (see below, retro-deformation). The sequence of structural phases established in the eastern segment of the Sognefjord transect, and their tectonic significance, are summarized on the right side of Figure 5. The change from crustal contraction to crustal extension is placed at the transition D3–D4, although there may have been some overlap. Jotunheimen-D1 (obduction and overthrusting of Upper Allochthon onto the margin of Baltica, the future Middle Alloch-

13

thon) is taken to represent the early collisional phase (early Scandian). During Jotunheimen-D2 and -D3, the suture zone (Upper Allochthon) became inactive and was replaced by a deep crustal detachment surfacing some 100 km east of the Baltica margin (late collisional contraction = late Scandian). At the beginning of Jotunheimen-D4, contraction ceased, and reverse movement related to late orogenic or post-orogenic crustal extension started (mode I extension), to be followed by the development of a major crosscutting, low-angle normal fault during Jotunheimen-D5 (Lærdal-Gjende Fault, mode II extension). Central Segment of the Sognefjord Transect The structural history of the central segment is well known from the “Sognefjord north shore log” (Milnes et al., 1988) (Fig. 4, panel 5) and from detailed work in the footwall of the NordfjordSogn Detachment (summarized in Milnes et al., 1997, and literature cited therein) (Fig. 4, panel 6). The 80-km-long Sognefjord shore section exposes a continuous profile through the Western Gneiss Complex, ranging from pristine, “un-Caledonized” Precambrian granites and migmatites of the Baltic Shield Basement, emerging from beneath the Faltungsgraben, in the east, to strongly “Caledonized” (originally Precambrian) gneisses, containing eclogite pods, forming the core of a regional culmination, in the west (Figs. 2 and 3). This progressive “Caledonization” from east to west (in its original position, from top to bottom) of a 25-kmthick slice of continental crust, has been subdivided into three regimes for descriptive purposes (Milnes et al., 1988; Milnes

Figure 5. Correlation table for the deformation phases recognized in the three segments of the Sognefjord transect. Adapted from Milnes et al. (1997). The fourth column shows a general correlation with contraction and extension in the upper crust in the eastern segment. Jot.—Jotunheimen; NSD—NordfjordSogn Detachment; WGC—Western Gneiss Complex.

14

Chapter 2

and Koyi, 2000). From east to west these are: Regime 1—mainly un-Caledonized, with isolated Caledonian shear zones; Regime 2—a transitional regime characterized by heterogeneous Caledonian shear zones in an anastomosing network around protolithic lenses; and Regime 3—characterized by more or less complete Caledonian overprinting, polyphase deformation, and high-grade metamorphism. This has been simplified in Figure 2 to an easterly, un-Caledonized or weakly overprinted zone (in Fig. 4 marked BSB, as it is interpreted to be the northwestward extension of the Baltic Shield Basement), and a westerly, Caledonized zone (in Fig. 4 marked Caledonized BSB), both containing part of the transitional regime. Shear zones in Regime 1 contain a mylonitic foliation and stretching lineation, which can be followed through the transitional Regime 2 into Regime 3, where they represent the main phase of post-Precambrian deformation and show amphibolite facies mineral parageneses. In Regime 3 this foliation is seen to “flow around” lenses of amphibolitized eclogite that contain an earlier eclogite facies foliation. The early, eclogite-facies deformation phase is referred to as Sognefjord-D1, and the following amphibolite facies deformation phase as Sognefjord-D2, both demonstrably of post-Precambrian, i.e., Caledonian, age. The well-defined culmination in the Western Gneiss Complex (Fig. 2), marking the change from E- to SE-dipping foliation and E-plunging lineations, to W- to NW-dipping–plunging structures, lies in the footwall of the Nordfjord-Sogn Detachment (Fig. 4, panel 6). Its western limb is characterized by progressively more intense shearing and mylonitization upward toward the detachment, and progressively lower metamorphic grades (from amphibolite to greenschist facies). The western boundary of the Western Gneiss Complex lies in the lower part of this zone of mylonitization (the detachment zone). The formation of the culmination and the detachment-related shearing in the footwall of the Nordfjord-Sogn Detachment postdates the SognefjordD2 structures, and also the widespread top-to-NW minor folds that affect the D2 foliation. Hence the structural relations in the footwall of this detachment can be combined with those of the Sognefjord shore section to give a well-defined deformational history for the Western Gneiss Complex, as follows: Sognefjord-D1: In the west (Regime 3), foliations, lineations, and folds within the eclogitic pods, some of which are certainly coeval with eclogite facies metamorphism (Andersen et al., 1994, stage 1); juxtaposition of Upper Allochthon (Lavik Mafic Complex) against the Western Gneiss Complex; in the east (Regime 1), no post-Precambrian deformation (the earliest deformation phase is Sognefjord-D2). Sognefjord-D2: Toward the west (Regime 3), main ductilepenetrative deformation (foliation, E-W-stretching lineation, major and minor isoclinal folding) at amphibolite facies (earlier eclogite facies parageneses were completely obliterated except for the cores of some mafic bodies and some other remnants: Andersen et al., 1994, stage 2); tight folding of the Upper Allochthon–Western Gneiss Complex contact; toward the east (Regime 1), isolated shear zones in the Precambrian basement, increasing in importance and intensity westward.

The Sognefjord-D2 deformation phase, where present, postdates and overprints all pegmatites, migmatites, and granites along the continuously exposed Sognefjord profile and is nowhere associated with syntectonic migmatization or signs of anatexis. Sognefjord-D3: East of the culmination and across its hinge (Regime 3), tight to isoclinal, NW-vergent folding, postdating Sognefjord-D2 structures but still at amphibolite facies, possibly coeval with the development of amphibolite facies mylonites in the lower part of the Nordfjord-Sogn Detachment zone, where kinematic indicators show top-to-W or NW movement and shear zones truncate D2 isoclines (Swensson and Andersen, 1991; Andersen et al., 1994, early stage 3). Sognefjord-D4: In the west (Regime 3), main movement on the Nordfjord-Sogn Detachment, intense mylonitization at increasingly lower metamorphic grade (Andersen et al., 1994, late stage 3) concentrated particularly in the upper part of the detachment zone; abundant kinematic indicators show top-to-W or NW movement; culmination probably developed during this period in response to movement in the detachment zone (cf. Norton, 1986; Norton et al., 1987). Sognefjord-D5 (not present along the Sognefjord transect): Open, upright E-W–trending major and minor folds to the north of Sognefjorden, affecting the detachment zone mylonites and all earlier structures (Andersen and Jamtveit, 1990; Swensson and Andersen, 1991; Andersen et al., 1994; Chauvet and Séranne, 1994; cf. Fig. 2). Kinematic indicators are generally lacking in connection with the Sognefjord-D2 structures, and the bulk deformation field during D2 is thought to have been irrotational (“coaxial” or “pure shear” deformation, Andersen et al., 1994; see also Andersen and Jamtveit, 1990; Dewey et al., 1993; Wilks and Cuthbert, 1994; Andersen, 1998). Sognefjord-D2 is thought of as being related to subvertical crustal thinning and E-W extension after the extreme crustal thickening and root formation necessary for eclogite formation (Sognefjord-D1). The currently exposed eclogites along Sognefjorden were formed at ~15 kbar and 600 °C. During Sognefjord-D2 they decompressed isothermally to ~10 kbar, corresponding to a shortening of the distance to the contemporary Earth’s surface of ~20 km. This places important constraints on the retro-deformation carried out below; its relation to other crustal processes will be discussed further there. The earlier Caledonian phase, Sognefjord-D1, is related to the juxtaposition of the Upper Allochthon and to the tectonic burial of the Western Gneiss Complex to the depths required for eclogite facies metamorphism (root formation). Phases postdating SognefjordD2, particularly D3 and D4, which both show consistent top-toW-NW shear sense, seem to be associated with crustal extension. This is particularly clear for Sognefjord-D4 (main movement phase in the Nordfjord-Sogn Detachment). The folding during Sognefjord-D5 is taken to have marked late to post-detachment N-S shortening (cf. Chauvet and Séranne, 1994). Because of the large wedge of only slightly Caledonized Baltic Shield Basement across the eastern part of the Sognefjord

The Sognefjord transect segment (Figs. 2 and 4), correlation with the structural history of the Jotunheimen segment is indirect. The Jotun Complex (Middle Allochthon) is regarded as a part of Baltica, subjacent to, and directly oceanward of, the Baltic Shield Basement, and the Lower Allochthon is interpreted as the one-time cover of the outer part of the Baltic Shield Basement (the Western Gneiss Complex), as indicated above. Hence, most of the overthrusting of the Middle and Lower Allochthons (i.e., the top-to-SE movement on the Jotunheimen detachment, Jotunheimen-D2–D3) must have taken place before the juxtaposition of the Upper Allochthon against the Western Gneiss Complex in outer Sognefjorden (SognefjordD1). At the other end of the time span, the geometry and kinematics of the main movement on the Nordfjord-Sogn Detachment (Sognefjord-D4) and the Lærdal-Gjende Fault (Jotunheimen-D5) suggest coeval activity over much of these phases, and en bloc rotation of the crustal segment between the two shear zones (footwall uplift under the Nordfjord-Sogn Detachment, synchronous with the development of the Faltungsgraben). The conclusions drawn from this discussion are shown on the correlation chart (Fig. 5). For the retro-deformation, the main point is that Sognefjord phases D1–D4 certainly occupy the same time bracket as Jotunheimen phases D3–D5, even though the detailed correlation within this time bracket remains unclear. Western Segment of the Sognefjord Transect The western segment comprises the hanging wall of the Nordfjord-Sogn Detachment (Fig. 4, panel 7; for detailed references, see Milnes et al., 1997). Its deformational history can be divided into phases that predate and postdate the prominent unconformity at the base of the Devonian sequence, which represents a period of uplift, erosion, and subsidence before the deposition of the coarse, middle Devonian conglomerates of the Solund Basin. The pre-unconformity history is best known offsection, particularly from the Askvoll-Atløy area to the north, where the present-day erosion level below the unconformity is much deeper. The geology of the Askvoll area has been studied in great detail, and the structural history is now well documented (e.g., Andersen et al., 1994; Engvik et al., 2008). The basement of the Devonian conglomerates in this area contains a well-exposed obduction mélange (Sunnfjord mélange; see Fig. 4 for this and other units mentioned below), separating the Upper Allochthon (Solund-Stavfjord Ophiolite Complex) from the underlying Middle Allochthon, a presumed fragment of Baltica (Dalsfjord Complex). This fragment, composed of crystalline rocks similar to the Jotun Complex, has a cover of presumed late Precambrian sediments (Høyvik Group) and unconformably overlying platform sediments of ?Upper Ordovician to Middle Silurian (Wenlock) age (Herland Group). The shear zones associated with obduction and overthrusting show a complex history (Osmundsen and Andersen, 1994), which is overprinted by reversed, top-to-NW shearing, NW-vergent folding, and down-to-NW normal faulting, all predating the base of Devonian unconformity. The unconformity was later affected by syndepositional normal faulting

15

related to the formation of the Devonian basins and the deposition of the Middle to Upper Devonian clastics. The basin fills were then truncated by postdepositional movement on the NordfjordSogn Detachment in places with strong deformation of the conglomerates adjacent to the fault. The Devonian sequences were also affected by large, open, upright folds with E-W trends, some confined to the hanging wall (deformation during movement on the detachment), and some affecting all units (postdetachment in age; see Fig. 2). For correlation purposes, we have subdivided this complex sequence of events into the following phases: West Coast–D1: Obduction and overthrusting with top-to-SE movement, in several phases; folding and imbrication of the overridden Herland Group sediments under greenschist facies conditions (Andersen et al., 1990, Fig. 4). West Coast–D2: Reverse movement on D1 shear zones, development of NW-vergent asymmetrical folds (F3 folds of Brekke and Solberg, 1987, and Osmundsen and Andersen, 1994); development of normal faults that predate the base of Devonian unconformity, in places reactivating D1 shear zones, still in greenschist facies (Osmundsen and Andersen, 1994; Hartz et al., 1994). Uplift and erosion West Coast–D3: Syndepositional normal faulting associated with basin formation (Osmundsen and Andersen, 1994, 2001; see also Indrevær and Steel, 1975); ?initial movement on Nordfjord-Sogn Detachment. West Coast–D4: Main top-to-W or NW movement on the Nordfjord-Sogn Detachment; truncation of basin sequences; eastward tilting of bedding and shearing along fault zone (Séranne and Séguret, 1987); pebble deformation against the detachment; open folds resulting from hanging-wall deformation. West Coast–D5: Folding about upright E-W–trending axial planes, related to N-S shortening (e.g., Torsvik et al., 1987; Osmundsen and Andersen, 1994; Chauvet and Séranne, 1994; Hartz et al., 1994). The West Coast deformational phases can be correlated with those of the Sognefjord and Jotunheimen segments, as shown in Figure 5. The obduction phase, West Coast–D1, probably correlates with the signs of obduction-related deformation noted in the Jotunheimen Detachment Zone (Jotunheimen-D1). The deformation related to the main movement on the Nordfjord-Sogn Detachment, West Coast–D4, and the subsequent and possibly related E-W upright folding, West Coast–D5, are thought to correspond with the Sognefjord phases D4 and D5. In between lies a sequence of extensional phases and an episode of uplift and erosion (unroofing down to Caledonian greenschist facies rocks) that are difficult to correlate in detail. The top-to-NW phase in the West Coast Upper Allochthon (West Coast–D2), which predates the base of Devonian unconformity, is clearly synchronous with the top-to-SE movement in the Jotunheimen Detachment Zone if the above correlations are correct. This implies important syn-contractional extension in the Upper Allochthon, which could indicate that gravitational collapse (thinning and extension)

16

Chapter 2

coeval with nappe translation, as is known from the Caledonian Allochthon in mid-Scandinavia (cf. Gee, 1975), was operative also along the Sognefjord transect. The top-to-NW phase, which postdates the unconfirmity at the base of the Devonian sequence and predates the main movement on the Nordfjord-Sogn Detachment—the West Coast–D3 phase—represents the rifting associated with the formation of the Devonian basins. 3. RETRO-DEFORMATION Reconstructing relationships backward in time by removing the effects of successively older events has been referred to as retro-deformation (e.g., Suppe, 1980, 1985), the tectonic equivalent of backstripping in basin analysis. It depends on a detailed knowledge of the deformational histories of the different exposed parts of the orogen, and the correlation of those histories across the whole transect, just as backstripping relies on detailed knowledge of, and correlation between, well logs. Along the Sognefjord transect, the “wells” are the columns in Figure 5, and the correlations that were established in the previous section. The other basic elements of retro-deformation are balancing (e.g., Hossack, 1979; Suppe, 1980; Gibbs, 1983) and depth control—i.e., the reconstruction of the Earth’s surface at each stage in the process (cf. Milnes and Pfiffner, 1977; Milnes, 1978; Pfiffner, 1986). The basic assumptions used for balancing and depth control in the present case will be discussed first, before the results of the retrodeformation (Fig. 6) are outlined. The aim of retro-deformation is to reconstruct the orogenic structure as far back in time as possible, without recourse to conceptual models. A conceptual model is based on general knowledge of geologic processes but is not bound by observed geometrical, kinematic, or dynamic constraints (e.g., Fig. 2 in this guide). Retro-deformation is an example of kinematic modeling: it uses the data and methods of modern structural geology to reconstruct crustal evolution, but it does not rely on knowledge of forces, stresses, or material properties (dynamic modeling, as discussed later). For the Sognefjord transect, retro-deformation can be reasonably carried out back to the time of formation of the Scandian eclogites in the Western Gneiss Complex. Balancing The following assumptions have been made in constructing the profiles in Figure 6: 1. The wedge of the Baltic Shield Basement on the northwest side of the Faltungsgraben (in the hanging wall of the Lærdal-Gjende Fault; see Fig. 4 and “present cross section” in Fig. 6) is assumed to be the northwestern edge of the un-Caledonized part of the crust after its initial restoration by reversing the movement on the extensional shear zones (the Lærdal-Gjende Fault and NordfjordSogn Detachment). At the eastern limit of the transect the crust is taken to be un-Caledonized throughout its thickness, and a vertical line at that position is taken as

a fixed datum line (Fig. 6, line x–y). The shape of the crustal wedge is, of course, arbitrary: The lower surface is unlikely to be planar, as drawn, and there may have been Caledonian effects southeastward from the datum line, particularly in the lower crust. The rest of the Baltic Shield Basement is assumed to have undergone ductilepenetrative deformation of the type exposed in the central segment of the transect (“Caledonized” BSB in Fig. 4). For balancing the crustal section, its cross-sectional area has been kept approximately constant throughout the process. Within the wedge a reference line has been inscribed along the present erosion surface, which remained practically undeformed throughout the orogeny (Fig. 6, line A–B). 2. The Middle Allochthon is mainly represented by the Precambrian Jotun Complex in this cross section and has also remained only slightly Caledonized throughout orogenesis in spite of 200–300 km of translation. However, it occurs today as erosional remnants of a much larger nappe, whose thickness and horizontal extent are unknown. Here, the reconstruction of the original size and shape of the Middle Allochthon is based on the assumption that the fragment of Jotun-like basement in the hanging wall of the Nordfjord-Sogn Detachment (Dalsfjord Complex) represents the western tip of the original nappe, disconnected from the main mass by movement on the detachment. The original thickness of the Middle Allochthon is taken to be ~15 km from the present thickness preserved in the Faltungsgraben (>10 km, cf. Fig. 4) and from the estimate of the metamorphic temperatures reached in the Jotunheimen Detachment Zone, giving maximum depths of ~20 km (see below). Another reference line (Fig. 6, line C–D) is inscribed in the Middle Allochthon at the present erosion level as a passive, practically undeformed marker. Depth Control Part of depth control is based on knowledge of metamorphic conditions during the different deformational phases. This knowledge is only rudimentary in many places. However, the presence of phyllites in the main Caledonian detachment zone (Fig. 4, Fortun and Vang phyllites) suggests that this zone should not be allowed to attain depths of >20 km at position B (Fig. 6), where the Baltic Shield Basement is overlain by phyllites that reached the greenschist-amphibolite facies boundary. The depth of the main Caledonian detachment (marked LA, Lower Allochthon, in Fig. 6) is a major control on the whole retro-deformation, as it never exceeded 20 km. As noted earlier, the Lower Allochthon at the eastern end of the transect probably never even sank below 10 km; because it contains the cover of the Western Gneiss Complex, it was obviously stripped off before this complex was eclogitized (present exposures representing depths of 60–70 km). Further depth control on the Middle Allochthon is provided by

The Sognefjord transect

Figure 6. Retro-deformation of the Sognefjord transect (from Milnes et al., 1997). The “present cross section” at the top is a true-scale simplification of the detailed structural cross section shown in Figure 4. NSD—Nordfjord-Sogn Detachment; LGF—Lærdal-Gjende Fault; UA—Upper Allochthon; MA—Middle Allochthon; LA—Lower Allochthon; WGC—Western Gneiss Complex.

17

18

Chapter 2

the Ordovician-Silurian cover of the Dalsfjord Complex (Herland Group, Fig. 4), which also underwent only low-grade metamorphic conditions. The other type of depth control, which has to be applied to rock masses that do not undergo internal deformation, is that the erosion level at each stage in the retro-deformation must lie above the previous one; i.e., once-eroded parts are not allowed to reappear in the cross section at some later time. This is particularly important for the Jotun Complex, as parts must have been eroding away throughout much of the later stages of the orogeny (Fig. 6). Results The results of the exercise are shown in Figure 6 as a sequence of frames (Milnes et al., 1997). At the top, the constructed present-day cross section (from Fig. 4) is presented in a simplified form. The retro-deformation starts with a Late Devonian reconstruction (Fig. 6, frame 4, ca. 365 Ma), after the end of movement on the Nordfjord-Sogn Detachment and the LærdalGjende Fault. Because there is little information on the erosion level at this time, the erosion surface was drawn just slightly above the present level (Fig. 6, A–B and C–D). From this starting point, the first step is the removal of the effects of the Mode II extension in the upper crust (the extensional shear zones, i.e., the Lærdal-Gjende Fault and the Nordfjord-Sogn Detachment), including the removal of the coarse clastic sequences in the Middle Devonian basins. This reconstructs conditions at the end of the Mode I extension phase (Fig. 6, frame 3, ca. 385 Ma). Focusing on the Western Gneiss Complex, this removes the Sognefjord-D4 deformation and places the eclogites at a depth of 30 km by movement in the footwall of the Nordfjord-Sogn Detachment, a crustal-scale extensional detachment with a displacement of ~50 km (Fig. 6). Strong footwall uplift must have taken place and must have resulted in a high mountain chain, because it was true uplift. In the hanging wall, in contrast, the basal Devonian is at the Earth’s surface today, it was at the Earth’s surface during deposition, and it lies at the Earth’s surface at this stage of orogenic evolution, i.e., strong uplift and erosion did not take place. In comparison, the footwall loses 30 km of crust by erosion (or rather, gains it, during the retrodeformation step from frame 4 to frame 3). The next step is to remove the Mode I extension and to reconstruct conditions at the end of contraction (Fig. 6, frame 2, ca. 395 Ma). This pushes the Middle Allochthon (Jotun Complex) to its farthest outreach over the foreland. For this step we have used an estimated 20 km of reversed movement on the main Caledonian detachment (the Jotunheimen-D4 phase). At deeper levels, this step removes the asymmetrical, top-to-W or NW folds (Sognefjord-D3) in the Western Gneiss Complex. This step in the retro-deformation has relatively little effect on eclogite depth. It shows that at the end of the contractional phase, the presently exposed eclogites lay at ~40 km and were almost completely retrograded to amphibolite facies because the all-pervasive Sognefjord-D2 deformation was nearing completion.

The last step is the reconstruction of conditions at the time of eclogite formation (Fig. 6, frame 1, ca. 410 Ma). This is more speculative but is still subject to important constraints. The metasediments in the Jotunheimen Detachment, which is in its later phase of development (Jotunheimen-D3), must remain at shallow depths, as must the top of the rigid wedge of the Baltic Shield (Fig. 6, point B). But the eclogites must move downward to 60–70 km at the same time as the main deformation in the surrounding gneisses (Sognefjord-D2) is removed. This leads to a deep root, which can only be achieved by a process of downfolding of the thrust zone separating the Baltic Shield basement from the Upper Allochthon, after the Middle Allochthon has passed and after the Lower Allochthon has been stripped off. It is the destruction of this root that lifts the eclogites from a 60 to 70 km depth to a 40 km depth, which produces the Sognefjord-D2 deformation, with bulk subvertical shortening and bulk horizontal E-W extension. This process started to take place while the horizontal contraction of the rheological upper crust was continuing. 4. ECLOGITE EXHUMATION The retro-deformation of the Sognefjord transect (Fig. 6) is a typical example of kinematic modeling. It is not concerned with the forces involved, only with the probable geometry of the crustal segment at different points of time, based on the structural data and their interpretation in terms of strain and movement. No hypotheses concerning material properties or stress distributions are involved, no conceptual models related to plate tectonics are used, and no preconceived ideas about the end result of the reconstruction are entertained. The end result is determined by how far back in time the available data can be reasonably interpreted using the accepted techniques of modern structural geology. One of the most interesting results of the Sognefjord retro-deformation was that it provided new insights into the exhumation of the well-known Norwegian eclogite bodies (Milnes et al., 1997). Up to that time the exhumation and preservation of the eclogites had been explained in terms of the extensional collapse of the Caledonian orogen and by rapid uplift of the Western Gneiss Complex in the footwall of the Nordfjord-Sogn Detachment. Exhumation was conceived to be a direct result of the lateto post-orogenic extensional tectonics. With regard to eclogite formation, the process envisaged by most workers is that of in situ eclogitization. The eclogite parageneses in the mafic pods are considered to be the remnants of a pervasive eclogite facies metamorphism that affected the whole crustal root at its deepest extent, toward the end of orogenic contraction. With regard to eclogite exhumation, the Western Gneiss Complex has served as one of the main examples of the process of uplift and unroofing that results from crustal extension, because it lies now in the footwall of a spectacular extensional shear zone, the NordfjordSogn Detachment. Although the large-scale situation is clearly more complicated, the Western Gneiss Complex has even been regarded as a huge metamorphic core complex (Krabbendam and Dewey, 1998).

The Sognefjord transect However, already in the early 1990s it had been recognized that a major phase of pervasive deformation and amphibolite facies metamorphism in the Western Gneiss Complex postdated eclogite formation and predated the process of eclogite exhumation caused by crustal extension (the latter process is often referred to as “extensional collapse,” cf. Dewey, 1988). At an early stage it was recognized that the amphibolite facies overprinting showed no evidence of major non-coaxial deformation, whether contractional or extensional (e.g., Andersen and Jamtveit, 1990). Along Sognefjord, this phase of deformation was later labeled Sognefjord-D2, characterized by the absence of consistent kinematic indicators and suggesting a general “pure shear” strain regime (Milnes et al., 1997). This led directly to the idea that the root that had been reconstructed by retrodeformation (Fig. 6, frame 1) was gravitationally unstable and “collapsed upwards,” carrying the enclosed eclogites upward and decompressing them isothermally from ~20 kbar to 10 kbar pressure corresponding to a depth decrease of 20–30 km. Hence, in Milnes et al. (1997), eclogite exhumation was interpreted in terms of three successive phases, each dominated by a different process: (1) ductile rebound of the orogenic root (Fig. 6, frames 1 and 2, exhumation rate 2–3 mm a–1), (2) buoyant flexure at the transition from contractional to extensional tectonics in the upper crust (Fig. 6, frames 2 and 3, exhumation rate ~1 mm a–1), and (3) crustal extension along major detachments, or “extensional collapse” (Fig. 6, frames 3 and 4, exhumation rate 1–2 mm a–1). In later articles the first two exhumation processes were grouped together, as they could not be clearly distinguished (Milnes, 1998; Milnes and Koyi, 2000). Of the total of ~60 km of exhumation, which is estimated for the eclogites in outer Sognefjorden, about half was accomplished by buoyancy-driven processes. The field relations along the Sognefjord transect indicate that the component of eclogite exhumation from ductile rebound (1) took place before any movement on major extensional detachments, (2) proceeded more rapidly than the later extension-related exhumation, and (3) contributed an amount of exhumation of the same order of magnitude as the later extension. The structural analysis of the Sognefjord transect showed that an orogenic root formed at the end of collision and suggested the idea that the root was gravitationally unstable and collapsed by a mechanism of ductile rebound as contractional deformation ceased (Milnes et al., 1997). Subsequently, dynamic modeling was carried out using the geometry produced by the retro-deformation and inserting realistic material properties and time limits (Koyi et al., 1999; Milnes and Koyi, 2000). The models showed that the process of ductile rebound of an orogenic root, which had been earlier suggested as a possibly important orogenic process, but never demonstrated on a field example (e.g., Platt, 1993; Avouac and Burov, 1996), was indeed a plausible explanation of the relations observed along Sognefjorden. In the model (Fig. 7) the outcrops now exposed along the shores of Sognefjorden are indicated as they were toward the end of the process. In this Field Guide, the profile indicated in the model is the object of study during Day 3.

19

Figure 7. Dynamic model, based on the structural analysis and retrodeformation of the Sognefjord transect, showing the strain distribution resulting from the ductile rebound of an orogenic root (from Milnes and Koyi, 2000). The deformation in this numerical model is caused solely by gravitational forces derived from the unstable low-density, low-viscosity root. (A) Starting point of the model, dimensioned according to the retro-deformation of the Sognefjord transect (cf. Fig. 6, frame 1). (B) Strain distribution when the gravity-driven collapse (ductile rebound) of the orogenic root was almost complete (cf. Fig. 6, between frames 2 and 3). (C) Enlargment of part of B, showing the Sognefjord profile through the Western Gneiss Complex and the three structural regimes studied on Day 3 of the field trip.

20

Chapter 2

Figure 8. An orogenic timetable for the South Norwegian Caledonides (after Milnes et al., 1997). NSD— Nordfjord-Sogn Detachment; LGF—Lærdal-Gjende Fault; KGOC—Karmoy-Gullfjellet Ophiolite Complex; SSOC—Solund-Stavfjord Ophiolite Complex; WGC—Western Gneiss Complex; OSLO—continental sedimentation in the Oslo graben; defm—deformation; metm—metamorphism; sedm—sedimentation.

5. AN OROGENIC TIMETABLE In order to visualize the complicated temporal relationships discussed in this chapter, and to provide a basis for discussion during the field trip, an orogenic timetable is included in which the events discussed are plotted against a geological time scale (Fig. 8). The amount of radiometric and strati-

graphic data from southern Norway is relatively small and subject to relatively large uncertainties, and the early Paleozoic time scale itself is not well established. The present compilation, therefore, is only a rough outline and should not be taken too literally. We estimate that a margin of error of ca. 5–10 Ma must be assumed before significant age differences can be identified.

 CHAPTER 3  The field trip itinerary

The field trip is subdivided into six “Days,” each containing a number of “Stops,” based on the itinerary of Excursion 28 of the 33rd International Geological Congress (33IGC) held at Oslo in August 2008. The approximate location of the overnight stops and the route of the excursion are shown in Figure 9. Clearly, the days and stops described here can be combined in different ways, and, correspondingly, overnight stops can be chosen at different locations. There are various types of accommodations available along the route: camping sites, youth hostels, pensions, hotels; and these can now be researched on the Internet. As the latter part of the itinerary is dependent on ferry times, planning needs to

be based on current ferry timetables, which can be downloaded from web site www.fjord1.no. The geological part of the 33IGC trip started at Fagernes and ended at Gudvangen (Fig. 9), but with small adjustments the trip could conveniently be started in Oslo (a 3 h drive to Fagernes) and ended in Bergen (omitting the last 33IGC locality, Stop 6.4 in this Field Guide). The sequence of days and stops can, of course, be arranged in different ways. However, the 33IGC sequence is used as a basis, as it proved to be practical, allowing sufficient time for discussion on the outcrops, stops for buying provisions, etc., with a comfortable margin. In the following, the precise location of each Stop,

Dale Leirvik

Sogndal

Eidsbugarden

Brekkestranda Fagernes Gudvangen

Figure 9. Geological map of part of southern Norway, showing the route taken on Excursion 28 of the 33IGC in August 2008, which formed the basis for the present Field Guide. The overnight stops are indicated with open rings. The itineraries for Days 1–6 are detailed in this chapter of the guide. “Day 0” and “Day 7” were the 33IGC travel days, from Oslo to the first overnight stop (Fagernes), and from the last overnight stop (Gudvangen) back to Oslo, respectively.

21

22

Chapter 3

with instructions on how to find the best outcrops, is given before the geological descriptions. Figures 4A and 4B together form a complete geological cross section along the Sognefjord transect (from Milnes et al., 1997). The structural positions of the excursion stops have been plotted on the appropriate parts of this cross section, together with a route map, at the beginning of the itinerary for each day. DAY 1. VALDRES-JOTUNHEIMEN The stops on Day 1 (Fig. 10) show different aspects of the thrust zone at the base of the Caledonian Allochthon, in the eastern segment of the Sognefjord transect (called the Jotunheimen Detachment Zone in Fig. 4). The overall structure can be described as a “tectonic sandwich.” The “slices of bread” above and below (the Jotun Complex, and the Baltic Shield Basement, respectively) behaved in a rigid manner and show practically no signs of Caledonian deformation, whereas the “butter” between represents the main Caledonian thrust zone and consists of intensely sheared phyllites and other metasediments, mainly of Paleozoic age, belonging to the Lower Allochthon. Southeast of the Sognefjord transect the main Caledonian thrust zone grades into a foreland fold-and-thrust belt with structural relations similar to the Jura Mountains, the foothills of the Canadian Rockies, and the Valley and Ridge Province of the Southern Appalachians. In fact, it has been suggested that the tectonic sandwich of this area is an exhumed equivalent of the Appalachian deep detachment, which was postulated on the basis of reflection seismic profiling (Milnes, 1987; cf. Iverson and Smithson, 1982; Hatcher et al., 1987). The mechanics of the thin-skinned, foreland fold-and-thrust belt in southern Norway, which extends southeastward to the south of Oslo, can thus be understood in terms of “bulldozer tectonics,” i.e., the critical tectonic taper model (e.g., Chapple, 1978; Dahlen, 1990; Malavieille, 2010), with the rigid Jotun Complex of the Middle Allochthon as the upper plate, causing the push from behind (the “bulldozer”). The displacement on the thrust zone is estimated at 200–300 km, with overthrusting from NW to SE, but there is clear evidence of late-stage, reversed movement on the same zone, with a top-to-NW sense of movement, possibly with a displacement of 10–20 km. At an even later stage the LærdalGjende Fault developed, also with top-to-NW kinematics, cutting discordantly through the main Caledonian thrust zone (Heim et al., 1977; Koestler, 1983; Lutro and Tveten, 1996). The complicated structural history of the main Caledonian thrust zone and its later extensional overprinting will be the theme of Day 1, together with some aspects of the Precambrian history of the Jotun Complex above and the Baltic Shield Basement below. Stop 1.1. Søndrol Location Stop 1.1 lies on the main E16 road, ~40 km in direction Bergen from Fagernes. From the road intersection and bridge at the extreme eastern end of lake Vangsmjøsa (Hemsingbru), continue

on E16 along the steep southern shore of the lake (tunnels, cliff sections) until the landscape opens out. After 3 km, there is a sharp curve in the road with a parking area on the right (677809N, 47892E) and a wide view of the lake and the surrounding mountains. High-visibility vests must be worn! Description This is a viewpoint stop, serving as an introduction to the sandwich tectonics, which will be studied on Day 1. In the mountains south of the lake (Figs. 11A and 11B) a prominent break of slope can be seen (actually, it can be followed across country to the south for several tens of kilometers), with Precambrian crystalline rocks of the Jotun Complex in the cliffs above (Middle Allochthon), and argillaceous metasediments (Vang phyllites, Lower Allochthon) forming the gentle slopes below (Fig. 11C). This line marks the top of the main Caledonian thrust zone in this area. Most of the movement (estimated at 200–300 km), however, must have taken place within the phyllites, as there is commonly very little sign of mylonitization in the Jotun crystallines at the contact (e.g., Fig. 11D, 1-m-thick mylonite). The contact between the Vang phyllites and the underlying Precambrian basement of the Baltic Shield (outcropping here in the Vang window) is not well exposed but has a similar character: heavily deformed phyllites and quartzites lying on undeformed Precambrian granites and migmatites (see Stop 1.3). North of lake Vangsmjøsi the top of the main Caledonian thrust zone is less easy to identify because an isoclinally folded sequence of Precambrian arkose (local name “Valdres sparagmites”) and other metasedimentary units occur below the Jotun Complex crystalline rocks, which, however, still form the upper parts of the mountain slopes. These relationships will be studied in more detail at Stops 1.4 and 1.5 (see particularly Fig. 14B for a schematic representation of relations in the thrust zone north of the lake and its head valley). Stop 1.2. Vangsmjøsi Location Return along E16 to the road intersection at Hemsingbru (~3 km) and turn left across the bridge. Continue up the winding road to the next intersection (Hen) and turn left again. The road now rises to a fine viewpoint above Vangsmjøsi (view similar to Stop 1.1), and then descends again to lake level, following the northern shore of the lake. Stop 1.2 consists of road sections just before (east of) a prominent, very deep road cut (678061N, 47373E). Description From the viewpoint above Vangsmjøsi down to the lakeshore, the road follows the contact between the Precambrian basement of the Vang window and the overlying phyllites (better seen at Stop 1.3). Afterward, the contact lies beneath the lake, and the road outcrops show typical phyllite relationships: highly heterogeneous and irregularly foliated-folded at hand-specimen scale, but very monotonous and uniform at the outcrop scale and

The field trip itinerary

23

Eidsbugarden

1.5

1.6 1.4 1.3

1.2 1.1 1

Fagernes

1.5

1.4

1.6

1.2 1.3

1.1

Figure 10. Route map for Day 1, with the structural positions of the excursion stops plotted on a reduced copy of the geological cross section shown in Figure 4.

24

Chapter 3

A

B

Fig. 11A

Fig. 11D

Fig. 11C

C

D

Figure 11. (Stop 1.1.) (A) View of the mountain Grindane from Stop 1.1, showing the basal thrust of the Jotun Nappe (Middle Allochthon) and the buildings of Søndrol farm (base camp for mapping in the 1970s). (B) Field book sketch of the mountains Grindane and Bergsfjellet made at Stop 1.1, showing the basal thrust (see Fig. 11A) and the main tectonic units: Precambrian Jotun crystalline complex (brown), Precambrian basement of the Vang window (red), and Paleozoic phyllites of the main Caledonian thrust zone (light green). The main movement direction of the Jotun Nappe relative to the basement was from the NW. (C) Contact of Jotun crystallines (forming the overhang) on Paleozoic phyllite on the mountain Grindane (see Fig. 11B), marked by ~1-m-thick mylonite (geologist Michael Heim). (D) The same contact ~10 km to the SW, on the mountain Rankonøsi (geologist Ondrej Voborny).

The field trip itinerary larger. Except for some prominent masses of typical dark quartzite (lenses, boudins, isoclinal fold hinges), mappable horizons are lacking, and the only constant structure is the phyllite foliation, which, although irregular, seems to dip consistently to the SE, i.e., toward the foreland, in the direction of the main movement. This is the wrong direction for development in a top-to-SE thrust zone. Also, the angle between the shear zone margins and the phyllite foliation, a regional feature in this area, would indicate a much lower shear strain than expected in a thrust zone of this magnitude. In the road outcrops to be studied here (Figs. 12B and 12C), the general SE dip of the main foliation is well seen, together with well-developed shear bands indicating top-to-NW sense of shear. This strong evidence of reverse movement in the main Caledonian thrust zone, which at this Stop practically obliterates any structures related to the main thrusting (top-to-SE), is a typical feature of the Lower-Middle Allochthon in southern Norway, as illustrated by the diagrams taken from Fossen (1992) (Figs. 12D and 12E). The summit of the mountain directly above these outcrops, Skutshorn (see Fig. 12A), consists of Jotun crystallines, and the prominent light-colored cliffs below consist of Valdres sparagmites (coarse, current-bedded arkoses seen in blocks to the west of the road cut) involved in a series of recumbent, isoclinal folds (Heim, 1979), together forming the Middle Allochthon. These folds are truncated at the basal contact, along which a mélange-like zone of heterogeneous lithology separates the sparagmites (Middle Allochthon) from the phyllites (Lower Allochthon) below. The mélange-like zone will be studied in detail at Stop 1.4, together with the basal contact of the zone, known as the Valdres thrust, inside the main Caledonian thrust zone. Since the Valdres thrust truncates the overlying folds, it is thought that the structure above the thrust represents an early stage in the thrust movement, which was later cut through and transported passively on top of the phyllites. The early part of the top-to-SE (contractional) thrusting belongs to the Jotunheimen-D2 phase, and the later part to the Jotunheimen-D3 phase (see Fig. 4). The reversed movement in the thrust zone belongs to Jotunheimen-D4 (Fossen’s “mode I extension”), and development of the Lærdal-Gjende Fault (see Stop 1.5) represents Jotunheimen-D5 (Fossen’s “mode II extension”), i.e., both developing under the post-collisional extension. Stop 1.3. Øye Location Continue westward along the road north of lake Vangsmjøsi to the intersection with E16 at Eidsbru and park near the store (678309N, 46721E). There are outcrops on both sides of E16 and along the local roads on each side of the main road; see locality sketch (Fig. 13A). High-visibility vests must be worn!

25

phyllite is exposed in the road cut along the secondary road just east of the E16 (Fig. 13A). The contact to the basement runs along (but is hidden below) the road. Several outcrops on the western side of E16, south of the store and along the small river, display a complex contact zone between migmatitic gneisses and the “Øye granite.” Migmatitic gneiss, exposed along the river (Figs. 13C and 13D), was formed during the Gothian orogeny at ca. 1520 Ma, as indicated by a U-Pb age for zircon and monazite determined on an associated gneiss near the location of Stop 1.1 (Corfu, 1980a, 1980b). The river outcrop also shows a crosscutting pegmatite dike, which is likely related to the Øye granite. Another set of exposures south of the intersection with E16 displays the contact zone between relatively fine-grained gray, massive granite and country rock xenoliths (Fig. 13E). More homogeneous, porphyritic granite is visible farther south in fresh road cuts at the right-hand turn of the road. The granite is part of a large pluton some 20 km long and 5 km wide, which extends to the northwest and occupies a major window through phyllite on Fillefjell up to ~1300 m. The relatively fine-grained border facies seen at this locality gives way abruptly to the more common coarse-grained appearance with up to 3-cm K-feldspar megacrysts in a 0.5–1 cm matrix of blue quartz, biotite, and plagioclase (Fig. 13C). One such outcrop is exposed a few hundred meters farther east (but for practical reasons cannot be visited during the field trip). The granite has been dated by U-Pb zircon and titanite to 930 Ma (Corfu, 1980a) and belongs to a suite that is widespread in the basement of southern Norway and southwestern Sweden. The late- to post-tectonic suite is also coeval with the Rogaland anorthosite and appears to be the result of melting following lithospheric delamination and/or crustal underthrusting (Andersson et al., 1996; Duchesne et al., 1999; Lundmark and Corfu, 2008a). The Caledonian effects on the granite are minimal. Titanite U-Pb ages have not been affected, and neither have the Rb-Sr ages on muscovite found on post-magmatic joints. Deformation is not evident even in the outcrops closest to the phyllite. The rapid gain in elevation of the contact between basement and phyllite, from <466 m at Vangsmjøsi to >1300 m on Fillefjell, can be viewed as local basement uplift in the footwall of the Hardangervidda Shear Zone (Fossen and Hurich, 2005). Other authors have speculated that the basement in these windows along the southeast side of the “Faltungsgraben” may not be fully autochthonous, and may even be allochthonous (the “Windows allochthon” of Rice, 2005). There is no positive evidence that the basement in these windows could be allochthonous, although antiformal stacks due to underplating below décollements are common in analogue models (e.g., Malavieille, 2010, Fig. 2), and we prefer to assume that it represents more or less autochthonous Baltic Shield basement. Stop 1.4. Tyin Road Profile

Description The outcrops at this location exemplify the lithological diversity of the Baltic Shield Basement (Fig. 13B) and its lack of deformation near the contact with the overlying phyllite. The

Location From Stop 1.3, follow E16 up to Tyinkrysset (convenient stop for fuel and provisions), and from there take national road 53

26

Chapter 3

WNW

A

ESE

B

Stop 1.2

WNW

ESE

C E

N

D

Figs. 12B and 12C

Figure 12. (Stop 1.2.) (A) Field party on the opposite side of lake Vangsmjøsi from Stop 1.2, which lies at the base of the mountain Skutshorn. (B) Part of the road section at Stop 1.2: typical aspect of the Paleozoic phyllites of the main Caledonian thrust zone (geologist Ole Petter Wennberg). (C) Close-up of shear bands in Figure 12B, looking NE, indicating top-to-NW sense of shear (cf. Fig. 12D). These belong to the Jotunheimen-D4 deformation phase of Milnes et al. (1997), and correspond with the D2 phase of Fossen (1992) (see Figs. 12D and 12E). (D) Schematic view of the two principal phases of deformation in the main Caledonian detachment zone (Fossen, 1992). The D1 phase of Fossen corresponds with Jotunheimen-D3, and the D2 phase of Fossen corresponds with Jotunheimen-D4 (cf. Fig. 5). (E) Regional overview of top-to-NW structures (D2 = Jotunheimen-D4) in the main Caledonian detachment zone SW of the present area (from Fossen, 1992). WGR—Western Gneiss Region; UBN—Upper Bergsdalen Nappes; LBN—Lower Bergsdalen Nappes; HRNC—Hardangervidda-Ryfylke Nappe Complex.

The field trip itinerary

Fig. 13D

A

27

C

Fig. 13D

Fig. 13E

Fig. 13E from stop 1.2

Øye granite

E16

B Metasediment Amphibolite

Fig. 13C

Early crust Gabbro

Granite

Granitic gneiss

Late mafic dike

Sheared anorthosite

Early mafic dikes

E D

F

Figure 13. (Stop 1.3.) (A) Topographic map of the area around Stop 1.3. The contact between the basement and the overlying graphitic and/or quartzitic schists of the Vang Formation is covered by the road from Stop 1.2, where it intersects with E16. The black dashed line marks the contact zone between the Øye granite (to the south) and the country rocks (to the north). (B) Pictorial representation of relationships between the different lithological elements and structures in the autochthonous Precambrian basement, including the Øye granite (marked Granite; from Milnes and Koestler, 1985). (C) Schematic representation of contact relationships of Øye granite. The normally coarse-grained and K-feldspar porphyritic granite (a) gradually becomes finer grained and equigranular (b, c) as the contact is approached. The country rock (d) consists of amphibolite (A) invaded by multiple sets of granite and pegmatite veins (G) (from Corfu, 1980b). (D) Amphibolitic country rock cut by granitic veins and pegmatite. (E) Details of the contact zone: composite xenoliths with country rock and early granitic veins. (F) The stave church from Øye, just south of Stop 1.3, was built about A.D. 1200. It was dismantled in 1747, but many pieces were found later in the foundations of the new church and used to rebuild the original church at a slightly different location in the 1950s (looking ENE, toward lake Vangsmjøsi and Stop 1.2).

28

Chapter 3

toward Tyin and Årdal. The road profile starts 2.3 km after leaving the intersection with E16, just before a major left curve in the road (678740N, 45895E), and heads up the road from there to the shore of the lake Tyin. High-visibility vests must be worn! Description The road to Tyin provides a good section from the phyllite (Vang Formation, Lower Allochthon) to the Jotun crystallines passing through the sedimentary formations interpreted to be the cover of the crystalline rocks of the Jotun Complex (Valdres Zone, Figs. 14A and 14B) and composed of the lithologically mixed Tya Series and the more monotonous Valdres sparagmite (all Middle Allochthon). The underlying Vang Formation consists of black-gray, graphitic quartz-biotite-sericite schists, quartzose phyllites with abundant quartz veins, and rare marble, all displaying the main E- to SE-dipping schistosity with the crenulation cleavage (Fig. 14C) described at Stop 1.2. They are followed by rocks of the Tya Series, a zone resembling a tectonic mélange composed of thin black quartzites and greenish chlorite schists and phyllites with local carbonate layers and pods (Figs. 14D and 14E). The Tya Series has been correlated with the Mellsenn Formation farther to the east, a sequence of slates and quartzites containing Early Ordovician fossils (Loeschke and Nickelsen, 1968). The Valdres sparagmite (Fig. 14F) consists of light-gray to green meta-arkoses, characteristically with purple to pink feldspar clasts, black bands of heavy minerals, and local cross-bedding. The sequence is isoclinally folded. Dating of biotite, muscovite, and feldspar from sparagmite and the Tya Series by Rb-Sr yields ages of ca. 390 Ma (Schärer, 1980a, 1980b). Long stretches of the contact between sparagmite and the overlying Jotun crystallines are characterized by a prominent quartz-pebble conglomerate, which supports a depositional relationship of sparagmite on the Jotun crystallines before inversion during the thrusting process (see also Figs. 15D and 23D). Unfortunately, the conglomerate is missing at the present locality, but it will be seen on the long alternative route at Stop 1.5. In this region the Jotun crystallines consist mainly of monzonitic hornblende gneiss with local metagabbroic

units. Just at the top of the ascent, directly west of the southern tip of the lake, the crystalline consists of fine-grained greenish rocks from a late fault, probably a subsidiary of the Devonian fault system related to late Caledonian extension. The Jotun crystallines will be examined in greater detail at Stop 1.6. Stop 1.5. Tyedalen Location The program in Tyedalen (the valley that extends from lake Tyin down to Årdal, at the head of Sognefjord, along national road 53) depends on time and weather conditions. From Stop 1.4 continue along the main road toward Årdal for ~25 km to the small tarn, Holsbruvatn, and stop at a large parking area at the NW end of the tarn (679648N, 44152E). At Holsbruvatn, highvisibility vests must be worn! Description The geological map and profile shown in Figures 15A–15B are taken from the geological map at a scale of 1:50,000, which was published for this area (Koestler, 1989). The parking area lies exactly where the Lærdal-Gjende Fault crosses the road, and large in situ outcrops of the fault rock (cataclasites, Fig. 15E), as well as innumerable fresh boulders of the same material around the parking area, can be studied. The structural relations, however, are not well seen here and will be studied in detail at Stop 2.2. At a large scale, the Lærdal-Gjende Fault cuts discordantly through the basal units of the Jotun-Valdres Nappe, the Vang Formation, and the upper part of the Baltic Shield at different localities along its trace (here, the base of the nappe). It therefore postdates all the deformational structures seen earlier in Day 1 and belongs to the regional deformation phase Jotunheimen-D5 (see Fig. 5). The fine-weather, plenty-of-time program (3 h away from the vehicles) involves a short but strenuous ascent of ~300 m from the tunnel ~2 km back along the road from Holsbruvatn (679378N, 44411E), marked by the red line in Figure 15A. The

Figure 14. (Stop 1.4.) (A) Geological map of the area around Stop 1.4, showing the location of the photos along the Tyin road profile (Figs. 14C– 14F). From bottom to top: green, Vang phyllites; light yellow, Tya Series; dark yellow, Valdres sparagmites; red, brown, Jotun crystallines. (B) Pictorial summary of relationships in the deformed zone (the main Caledonian thrust zone) between the autochthonous Baltic Shield Basement and allochthonous Jotun crystalline complex (from Milnes and Koestler, 1985). The position of the Tyin road profile is shown in red. The main top-to-SE movement on the Valdres thrust took place after the formation of the Valdres-S2 foliation (belonging to what is now called the Jotunheimen-D2 deformation phase; see Fig. 5) and before the top-to-NW reversed movement in the Vang phyllites seen at Stop 1.2 (earlier called Vang-D2; correlated now with the regional phase Jotunheimen-D4; see Fig. 5). (C) Start of the Tyin road profile: a prominent cliff on the left of the road, driving up, showing the contact between the Vang Formation (below) and the base of the Tya Series (above). The contact itself is known as the Valdres Thrust (geologist Geoff Milnes standing on the thrust). At this locality the Vang Formation consists of black graphitic schists (with typical folded quartz veins). The Tya Series begins with a succession of light- to dark-gray, generally rusty quartzites, also with typical quartz veining. Figures 14C–14E are a series of photos of the road section above this locality (see Fig. 14A). (D) Tya Series: gray quartzite and quartzitic schist, followed by black rusty schists (possibly a tectonic intercalation of the Vang Formation); above these, gray rusty quartzites and rusty dark schists. (E) Near the top of the Tya Series: rusty marble in sericite-chlorite schists. (F) Valdres sparagmite lying above the Tya Series. Sparagmites are light-gray meta-arkoses, commonly banded, with typical purple to pink feldspar clasts and feldspathic veins. There is good evidence that the Tya Series is in inverted stratigraphic position and represents the cover of the sparagmites and Jotun basement before thrusting. The words base, top, below, and above in these captions (Figs. 14C–14F) refer to present-day positions. This is not obvious at the present locality, but local examples of inverted current bedding have been observed (and heftily discussed!) in the sparagmites along this section.

The field trip itinerary

B A

29

Jotun Complex (Precambrian)

Valdres zone Valdres S2

Fig. 14F Fig. 14E Figs. 14C and D

Vang S2

Sparagmite

Tya Series Valdres Thrust Vang Fm. Phyllite/ schist Quartzite

Vang Thrust Basement Complex (Precambrian)

D

C

Vang F m

. Tya Seri e

E

s

F

A Stop 1.5

cro ss se ig.

,F

on

cti B

15

22 21 20 Jotun (NW of LGF)

7 LGF

15 14 13 Jotun (SE of LGF)

6 sparagmite

5

4 3 2 Tya Series

23 Vang Fm.

B

NW

SE

D C

Lær

dal-

G

eF jend

ault

E

The field trip itinerary upper part of this traverse is seen in Figure 15C (below the word Lærdal on the marked trace of the Lærdal-Gjende Fault). The profile starts in mylonitic Valdres sparagmite, containing greenish schist zones (mylonitized basement?) and enters heterogeneous, deformed Jotun crystalline toward the top. At this position, on a prominent rock terrace, deformed conglomerates similar to the characteristic quartz conglomerates of the Bygdin area (Hossack, 1968, see Fig. 15D) are encountered, interleaved with heterogeneously sheared Jotun crystallines. In the one area where relatively undeformed relations can be observed (Grønsennknipa, Hossack, 1972) the Bygdin conglomerate lies in stratigraphic contact with Jotun Complex rocks and is overlain by the Valdres sparagmites. Along the deformed base of the Middle Allochthon, this sequence is often reversed, suggesting widespread tectonic inversion of the stratigraphy, admittedly now dismembered and in places isoclinally folded. In bad weather but with plenty of time, a series of roadside stops can be made along the almost continuous road section as one drives back along national road 53 toward Tyin and Stop 1.6, using Figure 15A as a guide. Different stops show Valdres sparagmites with inverted(?) current bedding, Tya Series comparable with Stop 1.4, and Vang phyllites with dominant top-to-NW structures. The distance from Stop 1.1 to Stop 1.5 is 40 km as the crow flies, across strike, and the relations observed in Day 1 can be extrapolated at least 100 km along strike, in spite of all the local heterogeneities and variations. In the area of the Vang window, it is estimated that the phyllites lay at a depth of ~10 km at the end of thrusting, in Tyedalen perhaps 10–15 km, and on the NW side of the Faltungsgraben (e.g., Stop 2.6) ~15–20 km (new biotite, small garnets), suggesting a gently NW-dipping midcrustal detachment at the time of the main top-to-SE thrusting (Jotunheimen-D3 phase, Fig. 5). Stop 1.6. Lorteviki–Eidsbugarden Location Drive back on national road 53 to the southern end of lake Tyin (the top of the profile of Stop 1.4) and then take to the left (minor road 252). Stop 1.6 is on the shore by the hut Lorteviki (679040N, 45923E). Description The Lorteviki outcrops (Fig. 16A) display crystalline rocks of the Jotun Complex in various stages of shearing (Figs. 16B

31

and 16C). Gneiss samples collected in the vicinity have yielded a U-Pb zircon age, indicating magmatic crystallization at ca. 1690 Ma. These data also yield evidence for a severe disturbance at ca. 910 Ma, coinciding with the age of titanite, and interpreted as indicating the time of amphibolite facies metamorphism and deformation (Schärer, 1980a). The syenitic to monzonitic gneisses are locally intruded by younger gabbros, giving ages of 1250 Ma (Schärer, 1980a). Most of the deformation seen in these gneisses is probably Sveconorwegian. In this region the Caledonian overprint was weak and localized to discrete shear zones and faults (Fig. 16B). It is recorded by the growth of green biotite, but it did not manage to completely reset the Rb-Sr system of the original brown biotite or of feldspar (Schärer, 1980a), and it did not affect U-Pb in zircon or titanite. Both Lorteviki and Eidsbugarden (tourist hotels at the end of road 252) offer good views of the majestic Jotunheimen mountains (Figs. 16D and 16E), which consist largely of twopyroxene, monzonitic to dioritic gneisses (mangerite to jotunite) with local peridotite lenses and gabbros (see geological map of Koestler, 1989). Lundmark et al. (2007) determined protolith ages of 1660–1630 Ma for variably retrograded high-grade gneisses, also documenting a complex Sveconorwegian history with high-grade metamorphism, at least two episodes of anatectic melting at 954 and 933 Ma, and the generation of granitic pegmatites repeatedly between 950 and 925 Ma. DAY 2. INNER SOGNEFJORDEN The main emphasis of Day 2 (Fig. 17) is on the structure and content of the large half-graben that is traditionally known as the “Faltungsgraben” (after Goldschmidt, 1912): the Lærdal-Gjende Fault (the southeastern bounding fault of the half-graben), the Jotun Complex (mainly Precambrian crystalline rocks that have been little affected by the Caledonian orogeny but contain some Caledonian intrusions) occupying the core of the half-graben, and the main Caledonian thrust zone, which emerges from under the Jotun Complex on the northwestern side of the half-graben (the continuation of the thrust zone studied on Day 1). The Jotun Complex in this region can be subdivided into three main parts. The Lower Jotun Nappe, which we traversed on Day 1, consists of plutonic rocks variously deformed and metamorphosed (at amphibolite facies) during the Sveconorwegian orogeny, but without Caledonian intrusives. The Upper Jotun Nappe reached granulite facies conditions and anatexis

Figure 15. (Stop 1.5.) (A) Part of the geological map, 1:50,000-scale sheet 1517 IV, Hurrungane (Koestler, 1989), showing the area of Stop 1.5 (the exact program depends on time and weather conditions). Legend: 20–22—lithological units in the Jotun Complex to NW of the fault zone; 7—cataclasites of the Lærdal-Gjende Fault (LGF) zone (cf. Fig. 15E); 13–15—lithological units in the Jotun Complex to SE of the fault zone; 6—Valdres sparagmites underlying the Jotun Complex (Bygdin-type conglomerates indicated with ring symbols, cf. Fig. 15D); 2–5—lithological units in the Tya Series; 23—phyllites of the Vang Formation (Paleozoic) underlying the Valdres Thrust (black toothed line). (B) Geological cross section along the line marked in Figure 15A (legend same as in Fig. 15A). (C) Distant view across Tyedalen, looking NNE, showing the trace of the Lærdal-Gjende Fault. In the background are the mountains of the main Jotunheimen range (photo by Michael Heim). (D) Photo of typical Bygdin-type quartz conglomerate (from the type area at Bygdin to SE of Tyedalen); see unit 6 in Figures 15A and 15B. (E) Close-up of typical hard and coherent cataclasite from the Lærdal-Gjende Fault at the Holsbruvatn locality (unit 7 in Figs. 15A and 15B).

32

Chapter 3

A Læ

rda

l-G

de jen

Fau

lt

Eidsbugarden

Stop 1.5

Stop 1.6 Stop 1.4

B

Mafic dike

Early mylonite Syenite

Gabbro

Fig. 16C Cataclas

ite

Syenite

Late mylonites and shear zones

Gabbro Thrust or modified erosional surface

C

D

E Figure 16. (Stop 1.6.) (A) Geological map of the Tyin area, showing the location of Stop 1.6 and the tourist hotels at Eidsbugarden (convenient overnight accommodations). The location of Stop 1.5 and the end of the Tyin road profile (Stop 1.4) are also shown. Color legend as in Figure 14A. (B) Pictorial summary of Precambrian relations in the Jotun Complex in the Tyin area (from Milnes and Koestler, 1985). (C) Mylonitic gneiss of the Jotun Complex, presumably resulting from Sveconorwegian deformation. (D) Participants on an excursion following the Uppsala Caledonide Symposium in 1981 stand on mylonitized amphibolite facies, monzosyenitic gneiss at Lorteviki, while admiring the parent pyroxene gneisses of the Jotunheimen in the background. (E) Cluster of buildings at Eidsbugarden, at the end of lake Bygdin, with the high mountains of Jotunheimen in the background (mainly of pyroxene granulites, with some peridotite).

The field trip itinerary

33

Eidsbugarden Sogndal Leikanger

2.6

2.4 2.1a

2.3

2.5

2.1b 2.2

Faltungsgraben

2.5 2.4 2.6

2.3

2.2

2.1

Figure 17. Route map for Day 2, and the structural position of the excursion stops plotted on a reduced copy of the geological cross section shown in Figure 4. UA—Upper Allochthon; MA—Middle Allochthon.

34

Chapter 3

and was deformed during the Sveconorwegian (late Precambrian) orogeny, and was later intruded by Silurian granite. The Upper Jotun Nappe can be subdivided into two parts with different compositions and histories. The segment northeast of Årdal (forming the Jotunheimen mountains proper, see Day 1) comprises 1.66–1.63 Ga mangerites and jotunites, with gabbro and peridotite, metamorphosed up to granulite facies, with local anatexis and deformation during the Sveconorwegian orogeny (950–900 Ma). This segment contrasts with the segment centered in inner Sognefjorden that is dominated by 965 Ma anorthosites, which underwent high-grade metamorphism between 950 and 890 Ma, and which were much more strongly affected by Caledonian events than the peripheral parts. The distinction is seen in the density of Silurian granitic dikes, the degree of Caledonian hydration and retrogression, and the degree of resetting of U-Pb systems (Lundmark and Corfu, 2007, 2008a, 2008b; Lundmark et al., 2007). From the point of view of regional tectonics, controversial questions are: What lies at depth below the Jotun Complex? Does the main Caledonian thrust zone studied on Day 1 really correlate with the more complex zone of phyllites, sparagmites, mylonitized basement, mafic schists, and garnet schists that emerges on the other side (e.g., Stop 2.6)? Is the Precambrian of the Baltic Shield Basement really continuous with the Precambrian basement of the Western Gneiss Complex of the Jøstedalsbreen Massif, without any intervening disturbance or, as some have postulated, suture? Our mapping led us to answer both the latter questions in the affirmative, but in spite of the deep incision of Sognefjorden, the base of the half-graben is not exhumed, and there is an “exposure gap” of at least 25 km along the Sognefjord transect (between Stop 2.2 to a short distance past Stop 2.4). However, in inner Hardangerfjorden (Fig. 2), far to the southwest of the present area, there is continuity of exposure at least between the Lower and Middle Allochthon on both sides of the Faltungsgraben. In Figure 4 the reconstruction of the base of the Jotun Complex (approximately the base of the Middle Allochthon) as descending to a maximum of ~5 km below sea level, where it is truncated by the Lærdal-Gjende Fault, is based on a recent reassessment of the gravity data (Skilbrei, 1990) and the “mid-crustal discontinuity” on an interpretation of the Sognefjord refraction seismic profile (Iwasaki et al., 1994). Based on rather clear stratigraphic and lithological indications, the continuity of the Jotunheimen detachment and the Baltic Shield Basement beneath the area of inner Sognefjorden prior to the development of the Lædal-Gjende Fault is the simplest hypothesis, and there are few, if any, indications that a more complicated situation (such as that postulated by Rice, 2005) may have prevailed. Stop 2.1. Årdal Location From Eidsbugarden, drive back to the intersection with national road 53, and turn right toward Årdal (passing on the way

Stop 1.5). For Stop 2.1 there are two alternatives (see Fig. 18A). The 2.1a site (679762N, 43784E) is a parking area on the side of the road, high over the fjord, ~4 km after driving past Stop 1.5. From the parking area, walk up the old mountain road that starts directly on the southeast side of national road 53. It offers numerous exposures along the road and an incomparable view of the fjord, but it is not recommended in rainy periods because of the potential for rockfall. The alternative (or additional) site 2.1b is a large picnic area beside the main road along Årdalsfjorden (678610N, 42686E). The road between sites 2.1a and 2.1b provides excellent views of the geological relationships in the 1000-m-high rock walls on the northwest side of the fjord and lake Årdalsvatnet (Figs. 18B and 18C). Details can also be examined at the ferry quay at Fodnes, where the ferry to Mannheller will be taken later in Day 2. Description The main focus at this stop is the Årdal intrusion and the related dike complex, a swarm of predominantly medium- to fine-grained leucocratic dikes intruding the partly retrograded granulite facies rocks of the Upper Jotun Nappe. The dikes are mainly granitic in composition, and they commonly contain xenoliths and range in size from centimeters to several tens of meters across, locally >200 m. They tend to be subparallel, north-south striking, and eastward dipping. Dikes in the central Upper Jotun Nappe generally exhibit a foliation defined by biotite, and show boudinage and pinch-and-swell structures as well as shear-induced folding (Figs. 18C and 18D; cf. Lundmark and Corfu, 2007). The dikes had historically been considered to be Caledonian intrusives (e.g., Goldschmidt, 1916; Schärer, 1980b), but an Rb-Sr whole rock age of ca. 900 Ma (Koestler, 1982) changed that perception. Recent U-Pb work, however, has demonstrated that the dikes are indeed Silurian in age (427 Ma), showing that the earlier Rb-Sr isochron was a result of extensive crustal contamination and incomplete mixing, also evident in the large amount of inherited zircon xenocrysts present in the rocks (Lundmark and Corfu, 2007, 2008b). The latter paper concludes that “the geometry of the dyke complex was primarily controlled by Caledonian pre- to syn-magmatic faults reflecting a top-to-southeast, non-coaxial, strain field” (Lundmark and Corfu, 2008b, p. 987). The strain field is interpreted to reflect nappe translation, and the age of the Årdal dike complex is therefore a minimum age for the Caledonian thrusting of the Jotun Complex in western Norway. Rheological changes induced by the magmatism permitted further hydration and deformation of the country rocks in the vicinity of the dikes in a continued top-to-SE, non-coaxial strain field, possibly reflecting continued translation of the Upper Jotun Nappe and its emplacement on top of the Lower Jotun Nappe. The final major modification of the architecture of the nappe complex was the development of top-to-NW normal faults (Lundmark and Corfu, 2008b), such as the Lærdal-Gjende Fault to be studied at Stop 2.2.

The field trip itinerary

A

35

Stop 2.1a Fig. 18B

Stop 2.1b

Stop 2.2

B

C

D

Figure 18. (Stop 2.1.) (A) Geological map of parts of inner Sognefjorden (from Lutro and Tveten, 1996), showing the location of Stop 2.1 (alternative sites 2.1a and 2.1b) and Stop 2.2. The red area approximately marks the core of the Årdal intrusion, but the intense veining extends far outside the core into the surrounding granulites (marked in brown). (B, C) Views of the steep rock walls on the northwest side of lake Årdalsvatnet, from the road between sites 2.1a and 2.1b. (D) Typical appearance of the granitic dikes in the vein network surrounding the Årdal intrusion, which is of Silurian age. Various generations can be recognized, showing variable amounts of deformation. Note also the relationship between fold size and vein thickness (geologist Fernando Corfu).

36

Chapter 3

Stop 2.2. Lærdal Location The best continuous profile through the Lærdal-Gjende Fault zone, including the footwall and hanging wall, occurs along the old main road from the village of Lærdalsøyri toward the now abandoned ferry quay at Revsnes. This runs along the southern shore of Lærdalsfjorden, whereas the present main road, national road 5, enters a tunnel on the north side of the fjord. Drive to the new ferry quay at Lærdalsøyri, along the old road, and park in the large parking area. The road profile of Stop 2.2 starts at the first outcrops on the old road, near the quay, and runs for ~500 m northwestward around the bay to where the road disappears into the first tunnel (677530N, 41678E; Fig. 19A). Note the ferry crossing between Stops 2.2 and 2.3; allow 20 min to drive from Stop 2.2 through the tunnel to the Fodnes ferry quay, and consult the current ferry timetable in order to judge how much time can be spent at Stop 2.2 (ferry route 14-107 Fodnes-Mannheller; see web site www.fjord1.no). If the ferry timetables for the trip have not been downloaded previously, a current ferry timetable pamphlet for the whole region should be obtained on board this first ferry in the itinerary. Description The profile starts in veined migmatitic gneisses of the Baltic Shield basement, with no sign of deformation postdating the veining (no Caledonian overprinting). It then passes through sequences of metasediments, increasingly mylonitized, grading into mylonites and ultramylonites derived from crystalline protoliths, and ends at the tunnel entrance in post-mylonitic cataclasites and in a 20-cm-wide zone of fault gouge (Figs. 19B– 19D). Cataclasites and country rock of the Jotun Complex can be viewed by traversing along the track that extends from the tunnel entrance along the fjord shore. Across the fjord, the topographic trace of the fault can be clearly seen (Fig. 19E). As in Tyedalen (Stop 1.5), part of the cataclasite in the fault zone is more resistant than many high-grade gneisses and stands out as a cliff or ridge across the mountainside (see also Fig. 15E). It is estimated that the cataclasite, up to 200 m thick, marks the main fault core, representing most of the hanging wall–down, top-toNW movement, probably with 20–30 km of displacement (see Fig. 4). The narrow zone of brittle reactivation and gouge formation is probably much younger (Mesozoic, coeval with tectonic events responsible for the development of the North Sea Basin; cf. Andersen et al., 1999) and of much less importance. The kine-

matics and significance of the mylonites and ultramylonites are problematic and will be a main point of discussion: Are they all related to the Lærdal-Gjende Fault, i.e., top-to-NW, or are they earlier, thrust related, i.e., top-to-SE? Stop 2.3. Eide Location From Lærdal, drive back through the Fodnes tunnel to the ferry crossing between Fodnes and Mannheller. The ferry crossing and the new highway between Mannheller and Kaupanger (national road 5) provide many opportunities to view (but not to stop and scrutinize!) various expressions of the Årdal dyke complex. After passing the industrial area north of Kaupanger, take the side road to Vestrheim, and then turn immediately to the right on a short dirt road that leads to a quarry (and horse training facility). Stop 2.3 (678679N, 40366E) is a partially active quarry; the extent and range of the visit may have to be reduced if quarrying is in progress. Description The walls of the quarry at Stop 2.3 (for location, see Fig. 20A), and especially the numerous blocks lying around, give a very good overview of the dominant features of the high-grade metamorphosed, 965 Ma Jotun anorthosite-gabbro-troctolite suite (Lundmark and Corfu, 2008a). The youngest major phase of the suite, a meta-troctolite, commonly exhibits primary compositional layering, and decimeter-scale orbicular coronas made up of shells of spinel, pyroxene, garnet, and amphibole surrounding olivine nodules in a plagioclase matrix (Griffin, 1971; Figs. 20B–20D). Griffin (1971) and Griffin et al. (1985a) suggest that the coronas formed by two stages of subsolidus reaction during magmatic cooling in the mid- to lower crust (<9 kbar), possibly accompanied by progressive burial to ~10 kbar, followed by a flattening of the coronas that define a granulite facies foliation. U-Pb geochronology shows that emplacement of the anorthosite massif was followed by several episodes of high-grade metamorphism, concluded by an event of metasomatism at 890 Ma. One of these high-grade events, at 930 Ma, coincides with the emplacement of the Rogaland anorthosite and the formation of granites, such as the Øye granite seen at Stop 1.3, across the Baltic Shield (Fig. 13C). This late Sveconorwegian period reflects high lowercrustal heat flow together with orogen-perpendicular extension and is attributed to removal of overthickened lithosphere below the orogen, asthenospheric upwelling, and accompanying uplift

Figure 19. (Stop 2.2.) (A) Part of provisional geological map sheet 1417II, Lærdalsøyri, at a scale of 1:50,000 (Lutro et al., 1986), hand-colored to show the location of the Stop 2.2 road section in relation to the Baltic Shield Basement (in red), the Lærdal-Gjende Fault zone (in yellow, marking the southeastern margin of the Faltungsgraben), and the Jotun Complex (uncolored, northwest of this fault). (B) Fieldbook sketch of the southeastern end of the Lærdal road section. (C) Fieldbook sketch of the northwestern end of the Lærdal road section (continuation of Fig. 19B). (D) Close-up of the gouge zone at the tunnel entrance (northwestern end of the road section; see Fig. 19C), looking SW (geologists Hans-Ruedi Pfeifer and colleague). (E) View toward northeast from the location of Figure 19D, showing the trace of the Lærdal-Gjende Fault on the other side of Lærdalsfjorden.

A

B

C

Fig. 19D

D

E

alLærd

Gjen

ault

de F

38

Chapter 3

Sogndal

A

Stop 2.4

B

Stop 2.3

Kaupanger

from Mannheller

C

1m

20 cm

D

plag

sp

gt hbl cpx

opx

2 cm

ol

The field trip itinerary (Lundmark and Corfu, 2008a). The meta-troctolite at Stop 2.3 is strongly retrogressed, especially along faults (Fig. 20C). This is probably the Caledonian hydration that accompanied the emplacement of the Årdal dyke complex. Stop 2.4. Sogndal Location From Stop 2.3, continue along national road 5 toward Sogndal. Just before reaching the bridge over the fjord, turn into a parking area near a hardware store (678968N, 39935E). Stop 2.4 is for studying the outcrops behind the parking area and back along the cycle road. Description The mylonitic rocks shown in the outcrops belong to the Eide-Fimreite Zone (Fig. 21A), whose microstructures and deformation mechanisms were recently studied in detail (Kruse, 1998; Kruse and Stünitz, 1999). The deformation within the Eide-Fimreite Zone occurred at granulite to amphibolite facies conditions and produced mylonitic rocks with N-S–oriented linear structures. This zone is interpreted as part of a large-scale shear zone, which was subparallel to the basal thrust zone of the Middle Allochthon in which the mylonites were formed under greenschist to lower amphibolite conditions (to be studied at Stop 2.5) and contain a NW-SE–trending lineation. The EideFimreite Zone was either a very high-grade Caledonian precursor of the main Caledonian thrust zone or a Precambrian shear zone that was reactivated during the Caledonian thrusting; the age relations have not yet been clarified. The cliff section from the parking lot and for ~100 m along the cycle road display highly deformed felsic gneisses, with significant local variations in the degree of deformation (Figs. 21B and 21C). There are local gabbroic pods and black layers that are interpreted as being highly deformed dolerite dikes. The felsic gneisses resemble those seen at lake Tyin, and both units can be assigned to the Lower Jotun Nappe, cropping out on the two sides of the Faltungsgraben. Stop 2.5. Slinde Location These shore outcrops lie on a segment of the old main road Rv (national road) 55, on the northern shore of Sogndalsfjorden

39

(a side arm of Sognefjorden), ~10 km southwest of the town of Sogndal (Fig. 22A). Exit the highway ~200 m before the new national road 55 tunnel and drive ~200–300 m along the old road to the barrier. Walk past the barrier to the right bend in the road; the Stop 2.5 outcrops lie on the shore down to the left, exactly at the curve (678257N, 38760E). Description Where the main Caledonian thrust zone (Jotunheimen Detachment in Fig. 4) emerges from beneath the Jotun Complex (Middle Allochthon) on the NW side of the Faltungsgraben, it has become much wider and more complex than the same zone to the southeast (Day 1). The crystalline rocks at the base of the Jotun Complex are much more heavily deformed and mylonitized, and interleaved with mylonitized metasediments. However, unmylonitized remnants in the form of tectonic lenses are preserved, providing glimpses of the original protoliths. In such lenses and less affected parts, “Valdres sparagmite” and “Bydgin conglomerate” have been reported, but also mafic rocks that may have been derived from the Jotun basement or may be remnants of mafic volcanics within the cover sequence (one locality with pillow lavas has been reported, and remnants of ophiolite complexes have been speculated). Stop 2.5 illustrates on a small scale the general situation: The outcrops show a spectacular, low-grade (cf. Stop 2.4) mylonite zone with numerous tectonic lenses of different rock types derived from the Jotun Complex, and the progressive mylonitization of these can be studied in detail (Figs. 22B and 22C). In some zones, clear kinematic indicators can be seen in sections parallel to a strong SE-plunging mineral lineation (Fig. 22D) and can be interpreted as indicating top-toSE kinematics using classical sense-of-shear criteria (Fig. 22E). Stop 2.6. Hermansverk Location Driving westward from Stop 2.5, through the new road tunnel, national road 55 follows the shore to the peninsula on which the Sognefjord Hotel stands, just before the town of Hermansverk. Before reaching the hotel, take a clearly visible left turn to the small boat harbor. Drive around the harbor to its western end, and park (P on sketch map, Fig. 23B; 678450N, 38477E). The outcrops to be studied lie along the southern shore of the hotel peninsula, toward the small lighthouse.

Figure 20. (Stop 2.3.) (A) Geological map of the Kaupanger-Sogndal area, showing the location of Stops 2.3 and 2.4 (from Lutro and Tveten, 1996). Red—Årdal intrusion; brown—Jotun Complex granulites; orange—Caledonized basal zone of the Jotun Nappe. (B) Typical appearance of meta-troctolite, with bands of disrupted elongated mafic lenses. One thicker mafic layer (or dike) is evident. The layering is also cut by a late E-dipping shear zone. Note the change in color, from a purple (right) to a greenish hue (left), the latter reflecting advanced retrogression of the high-grade assemblage. (C) Close-up of the meta-troctolite and its typical lensoid planar fabric. The features at the left of the picture appear to indicate incipient disruption of a thicker mafic layer into the new plane of gneissosity. (D) A typical corona nodule (left) and details of the mineral distribution (right; Fig. 4 in Griffin, 1971). Olivine (ol) and plagioclase (pl) react to form (Al) orthopyroxene (opx) and clinopyroxene (cpx) + spinel (sp); a second reaction of these new minerals with plagioclase forms garnet (gt) + (low-Al)orthopyroxene and clinopyroxene; amphibole forms in a third reaction involving the pyroxenes (Griffin, 1971); hbl—hornblende.

A

Stop 2.4

Stop 2.3

A′ Airport Sogndal

Stop 2.3

´

Stop 2.4

B

C

Figure 21. (Stop 2.4.) (A) Map of the area near Sogndal, showing the distribution of mylonitic deformation at the base of the Jotun Nappe and a cross section between points A and Aʹ (adapted from Kruse, 1998). The Eide-Fimreite Zone is the local name for the mylonitized zone between Stops 2.3 and 2.4, north of the Storehaugen fault. (B) Highly sheared high-grade mylonitic gneisses of the Jotun Nappe (pen is 15 cm long). (C) View along the outcrops of heterogeneously sheared high-grade rocks at Stop 2.4 (along the cycle road).

Stop 3.1

A

Stop 2.6 Stop 2.5

Sogndal 4

Stop 2.4 Stop 2.3

N

B

C

E D

Figure 22. (Stop 2.5.) (A) Part of the Årdal geological map (scale 1:250,000) (Lutro and Tveten, 1996), showing Stops 2.5 and 2.6 at the NW margin of the Faltungsgraben. Stops 2.3 and 2.4, and the first stop of Day 3 (Stop 3.1), are shown for comparison. Key: red-brown—Jotun Complex; orange—strongly deformed basal zone of the Jotun Nappe (local name: Turtagrø zone); green-yellow—various Paleozoic metasediments and metavolcanics underlying the Jotun nappe; beige-pink—Precambrian crystalline rocks of the Western Gneiss Complex, mainly granites, gneisses, and migmatites. (B, C) Two photographs of the outcrops at Stop 2.5, showing the mylonitic gneisses with tectonic lenses (remnants of the protolith; mainly crystalline rocks[?] derived from the Jotun Complex). (D) Mylonitic rocks with well-developed shear bands. The view is toward the NNE, and the shear bands indicate top-to-ESE shearing (cf. Fig. 22E). This shearing is probably to be correlated with the JotunheimenD2 and/or -D3 phases SE of the Faltungsgraben (cf. Fig. 5 and Day 1). (E) Interpretation of shear bands in terms of late-stage movement in mylonite zones (from Hanmer and Passchier, 1991) (cf. Fig. 22D).

A

Stop 2.6

B

C

D

Jotun crystalline Bygdin-type quartz-conglomerate

Figure 23. (Stop 2.6.) (A) View from the southern shore of Sognefjorden, looking NE along the NW margin of the Faltungsgraben toward Stop 2.6. Stop 2.5 is hidden behind the largest tree, so the mountains in the background contain all of the main Caledonian Detachment Zone (the general SE dip is clearly visible). On the southern shore, mylonitic Valdres-type metasediments are seen at this point (geologist Thomas Dietler), probably directly overlying sericite schists and phyllites equivalent to those exposed at Stop 2.6. (B) Fieldbook sketch of the outcrops to be visited at Stop 2.6, with some structural data plotted; the cars can be parked at P, at the western end of the small boat harbor. For location, see Figure 22A. (C) General view looking SW along the NW margin of the Faltungsgraben as it is seen in the mountain area around Sognefjell, ~80 km along strike from Figure 23A (Skåravatn, Jotunheimen). In the foreground and middle distance are Valdres-type sparagmites. (D) Strongly deformed Bygdin-type conglomerate (underneath the hammer), underlying Jotun crystallines on the NW side of the Faltungsgraben (geologist Peter Padget). The locality is on Nobbafjelli, on the mountain ridge in the background of Figure 23C.

The field trip itinerary Description As indicated in the introduction to Day 2, the Jotunheimen Detachment is thicker and much more complicated when it emerges from beneath the Jotun Complex on the northwestern side of the Faltungsgraben. Nevertheless, it still contains remnants of units recognizable from Day 1: The Fortun phyllites look exactly like the Vang and Tyedalen phyllites, the quartz conglomerates are similar to those in Tyedalen and Bygdin (e.g., Fig. 23D), and the arkoses show features typical of the Valdres sparagmites (e.g., Figs. 23C and 23D). Unfortunately, none of these easily recognizable lithologies occur along the Sognefjord profile, and the lithologies exposed at Stop 2.6 (Fig. 23A) do not help to demonstrate the correlation. The silvery and greenish phyllites exposed on the hotel peninsula lie near the base of the main Caledonian thrust zone in this area (the Sognefjord Hotel itself is built on Precambrian migmatites) and still show only greenschist facies metamorphic assemblages. From the lighthouse there is a fine view westward along Sognefjorden. At this point, ~100 km from the entrance, the fjord is ~3.5 km wide, and the water depth is just over 1000 m. DAY 3. OUTER SOGNEFJORDEN Northwest of the “Faltungsgraben,” and covering a large part of western Norway, except for a narrow coastal strip, lies the Western Gneiss Complex, consisting mainly of granites, migmatites, and banded gneisses. Most of the well-known eclogites and garnet peridotites of western Norway lie near the western edge of this felsic crystalline complex (e.g., Griffin et al., 1985b; Cuthbert et al., 2000). Marker zones are few and discontinuous, deformation is heterogeneous, and large areas are accessible only with great difficulty: These obstacles, plus a rough climate, have always made mapping slow, laborious, and piecemeal. Except for the eclogite localities and certain road sections, the structural relations remained largely unknown up to the mid-1980s. The only more or less continuous cross section is provided by Sognefjorden itself, with long segments of continuous shoreline exposures along its northern and southern waterline, complemented by relatively new road sections, particularly in the north. In the mid-1980s this situation prompted a mapping campaign that aimed at providing complete documentation of structural relations and structural variations along the northern shore, carried out at a scale of 1:5000—a kind of “linear” mapping that we subsequently compared with the structural logging of an immense drill core with a straight length of ~60 km (Dietler, 1987; Milnes et al., 1988; Milnes and Koyi, 2000). On Day 3, we simply drive along this “drill core” through the Western Gneiss Complex, which, as we shall see, represents a depth section of ~25 km through part of the Caledonian “root” after the root “collapsed” (Fig. 24). Tectonic interpretation aside, however, the “Sognefjord north shore log” (Milnes et al., 1988) shows a rather clear structural zonation (see Chapter 2, Section 2). This can be followed from pristine Precambrian basement at the eastern end, or top, of the section (Regime 1—Stops 3.1 and 3.2), through a zone of heterogeneous Caledonian ductile shear-

43

ing in the central part (Regime 2—Stops 3.3 and 3.4), to a completely “Caledonized” and complexly folded zone at the western end, or bottom, of the section (Regime 3—Stops 3.5 and 3.6, the latter an eclogite locality). At the end of Day 3 (Stop 3.7), we will look westward (given good visibility!) toward the localities of Day 4, where the Western Gneiss Complex is truncated by the Nordfjord-Sogn Detachment (Fig. 4). The stops of Day 3 can be regarded as representative “samples” collected from this immense “drill core,” illustrating the systematic structural changes that the continuous logging revealed. A structural summary of the whole profile is given under Stop 3.1 (Fig. 25A) and will be used as a reference profile throughout the day. Stop 3.1. Hella Location Ferry station Hella on national road 55 (678830N, 37097E). Stop 3.1 is the cliff section along the queue lane area for vehicles waiting for the ferries to Dragsvik-Balestrand and Vangsnes (Fig. 25B). Unless you intend to take the next ferry, park at the eastern end of the cliff section, before reaching the pay booths, and walk toward the loading ramp (kiosk), a distance of ~200 m. A ferry timetable for the crossing Hella-Dragsvik (ferry route 14-173) should be downloaded from www.fjord1.no/en/ before the trip. Description The section consists mainly of banded migmatites with boudinaged mafic layers and crosscutting pegmatitic veins. Practically no ductile effects that postdate the migmatite foliation or the veins can be seen (Fig. 25C). Figures 25D and 25E show other outcrops in the same area, with similar relations: migmatitic rocks of various types cut by pegmatitic and other types of veins (aplitic, mafic), with practically no sign of post-migmatitic or post-pegmatitic deformation. Figure 25A shows a schematic structural “log” of the whole of Sognefjorden, with the stops of Day 3 marked, as a reference for the day’s trip. Stop 3.1 lies at the eastern end of the profile, and the whole area between here and Stop 3.3 is dominated by Precambrian relationships. As shown in Figure 25A, the migmatite neosomes and the mafic and felsic veins yield intrusion ages of ca. 950–970 Ma, as do the large granitic bodies in the surrounding areas (Høyanger granite, Jølster granite; Skår, 1998; Skår and Pedersen, 2003). The migmatization, deformation, veining, and granite intrusion all belong to the late stages of the Precambrian Sveconorwegian orogeny, which affected large parts of the Baltic Shield Basement in southern Norway (Røhr et al., 2004; Bingen et al., 2005). Along Sognefjorden, areas showing these relationships are referred to as Regime 1, so that between Stop 3.1 and Stop 3.3 you will be driving through a Precambrian world. However, as shown in Figure 25A, local post-pegmatitic, ductile shear zones (varying in thickness from a few centimeters to a few hundred meters) do occur, and a small one of these can be observed at the western end of the Stop 3.1 outcrops.

44

Chapter 3

Leirvik

3.5

3.4

Leikanger

3.2

3.7 3.6

3.7

3.1

3.3

3.6

3.5 3.4 3.3

3.2

3.1

Figure 24. Route map for Day 3, with the structural position of the excursion stops plotted on a reduced copy of the geological cross section shown in Figure 4. BSB—Baltic Shield Basement; UA—Upper Allochthon.

A

(Griffin et al., 1985b)

C

B D

E

Figure 25. (Stop 3.1.) (A) Structural summary of the Sognefjord profile through the Western Gneiss Complex, showing the location of Stops 3.1–3.7 (Day 3), after Milnes and Koyi (2000). (B) Sketch map of Stop 3.1, with fieldbook diagrams of structural relations (mainly Precambrian, with some small Caledonian shear zones, after Dietler, 1987). (C) Banded migmatites with boudinaged layers (Precambrian structural relations, no Caledonian effects; from Dietler, 1987). (D) Precambrian patch migmatites cut by a pegmatitic and, later, a mafic vein (Balestrand, ca. 1 km from Stop 3.1). (E) Heterogeneous gneisses cut by Precambrian pegmatites, ~5 km west of Stop 3.1 (high plateau above Balestrand; geologist Geoffrey Milnes).

46

Chapter 3

Stop 3.2. Sæle Location From Stop 3.1, take the ferry to Dragsvik and drive to Balestrand, and then continue westward along national road 55 for ~18 km to a prominent church (Kvamsøy church) on the left. Continuing westward, a small red and white lighthouse appears on the shore to the left, and shortly afterward, around a right bend, a large road cut followed by a straight stretch of road (677800N, 35822E). On the right, at the western end of the road cut, is a shoulder where cars and small buses can park halfway off the road. Extreme care is required at this stop, even though traffic is usually very light. High-visibility vests must be worn, and two participants must keep watch 100 m from the parked vehicles in each direction, warning approaching vehicles and signaling them to slow down. Description For structural location, see Figure 25A. In this example of relations in the “Precambrian world” (Regime 1), a complicated Precambrian history of intrusion and deformation (Figs. 26A– 26C) can be worked out, with a few, small, post-migmatitic ductile shear zones (Fig. 26E). Under dry conditions, the rather steep shore outcrops can be studied (Fig. 26D), but, remember, if you fall into the fjord at this point, you will be swimming in water 1200 m deep! Stop 3.3. Austrheim Location Continue westward on national road 55, through an 8-kmlong tunnel, to the town of Høyanger, which is bypassed by the main road. From the entrance of the first tunnel after Høyanger, drive 9 km to a disused ferry quay (Nordeide) and continue past this for ~1.2 km until you see a signpost to a bathing area down a dirt track to the left (Austrheim bathing beach; see Fig. 27A). The outcrops lie on the peninsula south of the bathing area (678510N, 33665E). Water depths in mid-fjord here are ~1300 m, and water temperatures in mid-summer are in the low teens, so swimming here is a special experience! Description For structural location, see Figure 25A. Descriptively, the transition from Regime 1 (Stops 3.1 and 3.2) to Regime 2 (Stops 3.3 and 3.4) takes place by a widening of the post-migmatitic and post-pegmatitic ductile shear zones, and a corresponding shrinking of the areas showing the Precambrian relationships seen at Stops 3.1 and 3.2. At this locality, the whole eastern side of the peninsula is sheared, although the original migmatitic structure has not been completely obliterated (Fig. 27B; compare with an unsheared equivalent from nearby, shown in Fig. 27C). The small, irregular shear zones in Regime 1 have developed into thick shear belts with a constant moderate ESE dip (i.e., dipping below Regime 1) and containing a relatively constant downdip

stretching lineation (see shear zone map, Fig. 27D, and the corresponding stereograms). The profile across Figure 27A shows schematically how pegmatitic veins, mafic zones, and synmigmatitic shear zones are deformed as they pass from inside unsheared lenses into the strongly deformed shear belts that anastamose around them (cf. Stop 3.4). This shearing is clearly post-pegmatitic (and the pegmatites are post-migmatitic), but here there is no indication as to whether it is Caledonian or not. The Caledonian age of the deformation was only revealed as the field mapping of the whole profile approached completion (cf. Stop 3.6). In the original report (Milnes et al., 1988), the deformation phase during which the main post-pegmatitic foliation-lineation was formed was labeled Sognefjord-D1 but was later re-labeled Sognefjord-D2 (Milnes et al., 1997) (as shown in Fig. 5). Stop 3.4. Kyrkjebø Location Continue westward along national road 55 from Stop 3.3 until the Kyrkjebø church appears on the right. Just before the church, turn sharply left into a steep, narrow lane that extends down to a boat quay, and park on the quay (Fig. 28A). Stop 3.4 lies on the prominent little peninsula that is seen across a small bay to the west of the quay (678490N, 33435E). Description For structural location, see Figure 25A. Stop 3.4 illustrates on a small scale what was described under Stop 3.3 at a larger scale. The outcrops show a small post-migmatitic and postpegmatitic shear zone and its accompanying zones of influence (Figs. 28B, 28C, and 28E). The progressive deformation of various Precambrian elements can be studied in detail (Fig. 28B), as well as the difference between post-migmatitic and synmigmatitic shearing (compare Figs. 28C and 28D). By zones of influence we mean zones of ductile deformation related to the movement on the shear zone in both the footwall and the hanging wall, within which the original protolithic relationships can still be distinguished. During the mapping in Regime 2, we distinguished between “destructive Caledonization” (deformation that led to complete obliteration of Precambrian relationships) and “conservative Caledonization” (deformation that overprinted Precambrian structures but conserved them, i.e., did not obliterate them). However, we did not use the term Caledonization during the mapping, as the absolute age of shearing was unknown. It was, however, clearly post-pegmatitic (now known to be post950 Ma; see Fig. 25A). An important observation from the continuous logging, however, was that there were never any signs of magmatism associated with, or postdating, the shearing. Another important observation was that the lenses of “conservative” post-pegmatitic deformation usually showed a more or less welldefined stretching lineation, some as L-tectonites, which looked “undeformed” in sections perpendicular to the lineation (and correspondingly strongly deformed, sheared, and foliated in sections parallel to the lineation).

The field trip itinerary

Fig. 26B

47

B

A

C

E D

Figure 26. (Stop 3.2.) (A) Sketch map of the shoreline at Stop 3.2, with fieldbook sketches (after Dietler, 1987). (B) Western end of the road section at Stop 3.2, showing Precambrian deformation and intrusion history (see upper fieldbook sketch in Fig. 26A). (C) Close-up of intersecting Precambrian pegmatitic veins (cf. lower fieldbook sketches in Figure 26A; photo by Thomas Dietler). (D) Shore section below Stop 3.2, looking westward along Sognefjorden (geologists Thomas Dietler and Andreas Koestler). Water depth in the fjord is ~1200 m here. (E) Small-scale, post-migmatitic, probably Caledonian, ductile shear zone (photo by Thomas Dietler).

48

Chapter 3

A

Y

N X

Y

X Fig. 27B

B

Høyanger granite

D

C

Høyanger

Fig. 27C

R2-W R2-E R1 Sæle

MISSING Shear belts (Caledonian) R2-W

R2-E

Shear zones (Caledonian) R2-E

Figure 27. (Stop 3.3.) (A) Sketch map showing the location of Stop 3.3, with a fieldbook sketch showing structural relations along line X–Y, based on detailed logging of the continuous shore profile (after Dietler, 1987). (B) View of Caledonized Precambrian migmatites at the point marked in red on the profile in Figure 27A, looking north. Høyangerfjorden in the background (the Høyanger granite makes up the mountain indicated). (C) Intrusion breccia near the margin of the Høyanger granite (for location, see Fig. 27D). The Høyanger granite yielded an intrusion age of ca. 920 Ma (cf. Fig. 25A). (D) Mapped post-pegmatitic shear zones in the area between Balestrand and Høyanger; JNC—Jotun Nappe Complex; R1—Regime 1; R2-E—Regime 2 east; R2-W—Regime 2 west (Stop 3.3). The shear zones are marked in black, and the areas dominated by Precambrian migmatites with unsheared pegmatitic veins are left white within the areas mapped in detail (dashed line). Also shown are stereoplots of foliation poles (black symbols) and lineations (open symbols) from Caledonian shear belts and shear zones around Høyangerfjorden (lower hemisphere, equal area projection; adapted from Milnes et al., 1988).

The field trip itinerary

49

A Figure 28. (Stop 3.4.) (A) Sketch map of the surroundings of Stop 3.4 (after Dietler, 1987). (B) Fieldbook sketch of the Caledonian (i.e., post-migmatitic, postpegmatitic) ductile shear zone at Stop 3.4 (“destructive Caledonization”). (C) Detail of the shear zone margin (cf. Fig. 28B). (D) Detail of a syn-migmatitic (i.e., Precambrian) shear zone in the wall rock adjacent to the post-migmatitic shear zone of Figures 28B–28C (photo by Thomas Dietler). (E) Field party studying the shear zone of Figure 28B, and the ductile deformation of the Precambrian migmatites in its immediate vicinity (“conservative Caledonization”).

B C

D

E

50

Chapter 3

Stop 3.5. Råsholm Location Continue westward from Stop 3.4 along national road 55 to Vadheim and then turn left onto main road E39. Continue westward along the northern shore of the fjord for another 20 km to the hamlet of Råsholm. After passing a few houses and fields, a prominent ridge on the right descends almost to the road and ends in a now disused quarry. Just after the quarry there is a large parking area on the left (Fig. 29A, 678300N, 31708E). From the northeastern end of the parking area a small path leads down to the shore outcrops. Description For structural location, see Figure 25A. Between Stop 3.4 and here, the post-pegmatitic deformation (Sognefjord D2; cf. Fig. 4) becomes more or less penetrative. Although now dominant, it still varies in intensity from weak to strong, interspersed with strongly lineated rocks approaching L-tectonites (in which the foliation is absent and the Precambrian relations are still discernible in sections perpendicular to the lineation). Many rocks show a banded appearance, partly from heterogeneities in the original protolith (migmatites) and partly from the concordance of what are by this time strongly sheared and boudinaged pegmatites and other vein types (Figs. 29B–29D). The deformation that led to the penetrative foliation and/or lineation, which the continuous logging of the northern shoreline of Sognefjorden showed was derived from the foliation in the shear belts and shear zones at more easterly localities, was subsequently called the Sognefjord-D2 deformation phase (in Milnes et al., 1997: previously called D1 in Milnes et al., 1988). Between Stops 3.4 and 3.5, however, folding that postdates this foliation and lineation becomes a prominent structure, formed during the Sognefjord-D3 deformational phase (cf. Fig. 5; previously called D2 in Milnes et al., 1988). A typical feature of SognefjordD3 is the constant vergence of the F3 folds: asymmetrical and overturned toward the NW, as is also seen at the present Stop (see fieldbook sketch in Fig. 29A). The pervasiveness of the D2 deformation and the widespread development of the top-to-NW D3 folding were used as criteria to distinguish Regime 3 in the Sognefjord profile (cf. Fig. 25A). A special lithology at Stop 3.5 is the quartzite, interleaved with the banded gneisses at this locality and responsible for the prominent ridge and the disused quarry. The rock contains a high concentration of pink lithium-rich mica, which gives it its unusually decorative coloring (marked yellow in Fig. 29A). Stop 3.6. Hellebø Location Continue westward from Stop 3.5 along E39 to Lavik. Instead of descending to the ferry quay, continue past the village to a road intersection and turn left along minor road 607 toward Leirvik. Drive for ~4 km to the Hellebø farm, on the left, and park half on

the shoulder of the straight stretch of road just past the farm or at the lay-by a little farther (677700N, 30831E). Access to the shore outcrops is through the uncultivated and rocky scrubland west of the farm, a scramble of ~200 m southeastward at right angles to the road (Fig. 30A). When you reach the shore, turn left to a small point (Fig. 30B); the eclogite lenses (Fig. 29C) are on the other side of the small bay, north of the point. To avoid damage to cultivated land, please return to the vehicle(s) along the shore and then back across the scrubland. Description For structural location, see Figure 25A. Between Stops 3.5 and 3.6, complex meso-scale folding is characteristic of the shore section, with folds belonging to (at least) two phases postdating the Sognefjord-D2 foliation and lineation: NW-vergent folds of the type described under Stop 3.5 (Sognefjord-D3) and even later folding of a more open and irregular type. The analysis of these folds failed to reach a proper synthesis, but none of the post-D2 folds showed signs of a new axial plane foliation, so they were all included in the Sognefjord-D3 deformation phase. At this locality, excellent examples of refolded folds and folded lineations can be observed. However, the locality is better known for demonstrating the time relations between the Sognefjord-D2 foliation and the eclogite pods. The D2 foliation is observed to “flow around” the eclogitic lenses (as seen in Fig. 30C), truncating the internal eclogitic foliation within the lenses. This internal foliation, produced in deformation phase Sognefjord-D1, was formed under eclogite facies conditions, which are estimated at 600 °C and 15 kbar in outer Sognefjord. The foliation that flows around the lenses (produced in deformation phase Sognefjord-D2) was formed under amphibolite facies conditions (~600 °C and 10 kbar; see Bailey, 1989). This isothermal decompression of ~5 kbar corresponds to ~20 km of exhumation, which in our analysis took place before the extensional tectonics, which further exhumed the eclogite bodies (on the extensional Nordfjord-Sogn Detachment, which along this section falls in deformation phase Sognefjord-D4; see Fig. 5). This first phase of eclogite exhumation was solely due to the ductile rebound of the orogenic root, a gravitationally driven process (Figs. 30D and 30E and Chapter 2, Section 4), as explained in Milnes et al. (1997) and Milnes and Koyi (2000). Stop 3.7. Bekkeneset Location From Stop 3.6, continue westward along road 607 to a prominent right curve in the road with a parking area and picnic table(s) on the left-hand side, just around the corner (677660N, 30550E), with a fine view (weather permitting!) of the islands of outermost Sognefjorden. High-visibility vests must be worn at this stop! Description For structural location, see Figure 25A. This stop provides a view of the Nordfjord-Sogn Detachment (Fig. 4B), looking

The field trip itinerary

51

A

B

C

D

Figure 29. (Stop 3.5.) (A) Sketch map showing location of Stop 3.5 (after Dietler, 1987). (B) Banded gneisses with Caledonian foliation (Sognefjord-S2 of Milnes et al., 1997) and streaked out and boudinaged remnants of Precambrian pegmatitic veins. (C) As Figure 29B, but showing remnants of the Sognefjord-D2 isoclinal folds. (D) As Figures 29B and 29C, with folded and boudinaged pegmatites and mylonitic gneisses probably derived from a Precambrian migmatite protolith (beneath the hammer; cf. the right side of Fig. 28C).

A

B

C

E

D Stop 3.6 Fig. 25A

The field trip itinerary obliquely downdip, and the Devonian conglomerates in the hanging wall (Figs. 31A and 31C: Stop 4.1 will provide details of these relationships). The road section behind you shows good examples of Sognefjord-D3 folds, with NW vergence (Fig. 31B). As shown schematically in Figure 4B, the asymmetrical (top-toNW) D3 folds continue with the same vergence over the broad culmination to the west of Stop 3.7 (seen on a clear day in the cliffs above the shores of the mainland to the north). On the western side of the culmination, the main foliation (Sognefjord-S2) and the axial planes of the asymmetrical folds (Sognefjord-F3) descend into and become part of the detachment zone, and the F3 folds cannot be distinguished from folds formed in the detachment mylonites. The formation of the culmination is considered to be related to footwall uplift and hanging-wall down movement on the Nordfjord-Sogn Detachment, a deformation phase, which, along the Sognefjord profile, has been designated Sognefjord-D4 (Fig. 5). DAY 4. SOLUND On Days 4 and 5 relationships within and on both sides of the Nordfjord-Sogn Detachment (Fig. 4B) will be studied, basically as a continuation of Day 3 (Fig. 32). Although the itinerary starts in the Western Gneiss Complex in the footwall of the detachment, emphasis will be on the structural evolution of the detachment zone itself, and on the geological relations in the hanging wall. The hanging wall is composed of a “basement” complex, parts of the Solund-Stavfjord Ophiolite Complex (Upper Allochthon), unconformably overlain by a thick sequence of Devonian conglomerates. The conglomerates lie on a well-preserved stratigraphic unconformity, which in neighboring areas (to be studied on Day 5) truncates the Caledonian nappe boundaries, marking a period of uplift and deep erosion after the emplacement of the nappes and before the development of the Devonian sedimentary basins (see Chapter 2, Section 2). In this area the Devonian sequences are discordantly cut through by the Nordfjord-Sogn Detachment, and the conglomerates in the immediate vicinity of this detachment are strongly deformed, indicating that most of the movement on the Nordfjord-Sogn Detachment took place after the start of basin sedimentation in late Early Devonian times (see Fig. 8). In a simplified sequence of deformational events based

53

on structural relations in the hanging wall, the top-to-NW, downdip movement on the Nordfjord-Sogn Detachment is referred to as the West Coast–D4 phase (Fig. 5). The superposition of the Upper Allochthon on the Middle Allochthon in the hanging wall (see Day 5) is placed in the West Coast–D1 deformation phase, which is used here as a “sack” term covering all movements that predate the formation of the unconformity at the base of the Devonian sequence. Day 4 is a relatively easy day, visiting the island of Solund, which lies off the entrance to Sognefjorden, with ~2 h of the day spent looking at the spectacular geology from the decks of ferries (binoculars useful!). Stop 4.1 is a look-see “stop” from the ferry deck, as the geology of the small island of Losna (astride the Nordfjord-Sogn Detachment) slides slowly past, often only a stone’s throw away. Stop 4.2 involves a 1–2 h walk in the Devonian conglomerates and their ophiolitic “basement” (Upper Allochthon), the length of the walk depending on weather conditions, and Stop 4.3 is a collector’s stop to obtain specimens of the well-known high-pressure garnet-kyanite schists at a locality in the footwall of the Nordfjord-Sogn Detachment. The geographical and geological locations of Stops 4.1–4.3 are shown in Figures 31A and 32. Stop 4.1. Losna Location The car ferry from Rysjedalsvik to Krakhella, on Solund, crosses first Sognefjorden to Rutledal (half-hour), and then Sognesjøen to Krakhella (half-hour). From the deck of the ferry, good views of two cross sections through the Nordfjord-Sogn Detachment are obtained when the weather is fine. The ferry runs particularly close to the southern shore of the island of Losna. For the planning of Day 4 it is important to consult the current ferry timetable for ferry route 14-321 Rysjedalsvika-RutledalKrakhella (download from web site www.fjord1.no/en). Description From the first leg of the ferry trip (Rysjedalsvika-Rutledal), the panorama with the mountain Lihesten, looking northward from the ferry deck (Fig. 33A; cf. Fig. 31C), shows at a distance large-scale relations similar to those on Losna. From the

Figure 30. (Stop 3.6.) (A) Sketch map of the surroundings of Stop 3.6 (after Dietler, 1987). (B) Intense NW-vergent folding (Sognefjord-F3) affecting the main Caledonian foliation (Sognefjord-S2) and related isoclinal folds (Sognefjord-F2), looking east (participants of the 33IGC excursion; photo by Florencia Bechis). (C) Eclogite lens, showing a pronounced internal foliation (Sognefjord-S1) in its light-greenish core and amphibolitized, dark-greenish marginal zone. The lens is enclosed in quartzo-feldspathic gneisses with their main foliation (Sognefjord-S2) “flowing around” the lens margins and truncating the eclogite-internal foliation. (D) Simplified crustal section along Sognefjord, showing the location of the structural profile (Fig. 25A) and the position of the eclogites of Stop 3.6 (red dot, after Milnes and Koyi, 2000). NSB—NordfjordSogn-Bergen extensional detachment (=Nordfjord-Sogn Detachment); GLH—Gjende-Lærdal-Hardanger extensional detachment (=LærdalGjende Fault). (E) Reconstruction of the Sognefjord crustal profile at two early stages in its tectonic evolution: (a) during formation of the eclogites and their internal foliation (Sognefjord-D1 deformation phase); (b) after the following isothermal decompression (exhumation) phase and development of the main foliation under amphibolite-facies conditions (Sognefjord-D2; see Milnes and Koyi, 2000). “Fig. 2” is the present Figure 25A; “e” marks the eclogite occurrence at Stop 3.6; UC—upper crust, LC—lower crust, ML—mantle lithosphere.

54

Chapter 3

A Stop 4.2

Lihesten Solund

Losna Stop 4.3 Stop 4.1

Stop 3.7 Figure 31. (Stop 3.7.) (A) Geological map of the surroundings of outer Sognefjorden, showing Stop 3.7 (see Figs. 24 and 25A for position on the structural profile) and the geology of the islands of Losna and Solund, to the west (Day 4). (B) Road outcrops at Stop 3.7 (see Fig. 25A for structural position) showing Sogneford-D3 folding (view toward NE). (C) Panoramic view with parts of Lihesten, Losna, and Solund (tops ca. 700 m above sea level), looking approximately WNW from a position above Stop 3.7. Note the cliffs of Devonian conglomerate overlying mylonites and phyllonites of the Nordfjord-Sogn Detachment zone, whose upper surface is marked by the Solund Fault (see Day 4 itinerary). The prominent N-S–trending fjords and sounds crossing Solund mark the location of a series of post-Caledonian (Mesozoic?) faults, one of them clearly displacing the Solund Fault.

B

C Hersvik

Solund Fault

Lihesten

The field trip itinerary

55

Dale

4.2

4.3

4.1 Leirvik

4.2 4.1 4.3

Figure 32. Route map for Day 4, with the structural position of the excursion stops plotted on a reduced copy of the geological cross section shown in Figure 4. MA—Middle Allochthon.

56

Chapter 3

A

Stop 4.3: Hyllestad

Lihesten

lt d Fau Solun

SD

Top

of N

Leirvik

Risnes tectonic lens b

N of e as

SD

WGC

Sogneskollen granodiorite

B Fig. 33D Fig. 33C

C

E

D

Fig. 33E

The field trip itinerary second leg of the ferry crossing (Rutledal-Krakhella), one obtains a much closer view of the upper part of the Nordfjord-Sogn Detachment, with Devonian conglomerates forming the prominent cliff (Figs. 33D and 33E). Along the base of the cliff runs a sharp planar contact, mostly coinciding with the base of the conglomerates, but not everywhere. This is a late, brittle fault plane, probably of Mesozoic age, called the Solund Fault. At one locality the fault is slickensided, and the striations are almost horizontal and trend ENE-NE (Bøe, 1997). This is in sharp contrast to the kinematics of the Nordfjord-Sogn Detachment itself, represented by the intensely sheared metasedimentary rocks and greenschists that make up most of the hillsides below the fault. In these mylonitic rocks, stretching lineations and sense-of-shear indicators give a mean shear direction plunging 12° toward 289°, with a consistent top-to-NW sense of shear. These relations will be viewed “hands-on” at Stop 5.2 (Solund Fault = Dalsfjord Fault; Losna Phyllites = Askvoll Group). In some of the Devonian cliffs on Losna, the coarse bedding in the conglomerates can be distinguished (Fig. 33D) dipping gently to the SE (on Losna, mean dip of 20° to 143°). The bedding is truncated sharply by the Nordfjord-Sogn Detachment, which dips gently to the NW (on Losna, mean dip of 18° to 332°). The structural relations in the Devonian and the underlying zone of mylonitic phyllites and schists are shown in cross section in Figures 33B and 33C. Two low-strain “tectonic lenses,” which survived the mylonitization, can be distinguished. The smaller one (red), easily seen from the ferry, is of granodiorite and is a small edition of the large lens visible on the Lihesten panorama (Fig. 33A; for a detailed description of this profile, see Meidell, 1998). The larger lens (orange and green, with a complex internal structure), forming a prominent peninsula where the ferry first approaches close to the shore, consists of intensely folded metasedimentary and metavolcanic rocks, with the layering and folds sharply truncated at the lens margins (premylonitic, probably contractional, deformation phases). Based on a study of the sizes of dynamically recrystallized quartz grains in sheared quartzitic rocks (layers in the upper and lower phyllites in Fig. 33D), the stress levels could be estimated, and then, by combining these data with the estimated shear strain rate in the detachment, recalculated to give an estimate of temperature (T) conditions during movement (Bøe, 1997). The result of the study was T = 320–400 °C, which is in agreement with the lower greenschist facies metamorphism deduced from the mineral parageneses in the mylonitic rocks.

57

Stop 4.2. Hersvik Location From the ferry quay at Krakhella, drive along the main Solund road, road 606, for ~11 km, to a road intersection with a branch to the right marked Hersvik, Leknessund. Continue along this minor road until a road fork is reached (678930N, 27978E), the left fork descending a short distance to Hersvik, the right fork continuing to Leknessund. Take the right fork and park the vehicles in a large parking area around a public building, a few hundred meters from the fork (close to the letter A in Fig. 34A). Description From the parking place, an easy walking tour in rather rough but treeless country can be made, length and aim depending on the weather and on the time of the next ferry back to the mainland. The tour may include some or all of the items below: - Detailed study of the coarse, polymict, thick-bedded Devonian conglomerates of the Solund Basin, to determine the mode of formation and flow direction, and to distinguish possible tectonic effects (Fig. 34C); - Short climb to the top of Skognipa (Figs. 34A and 34B) and discussion of the significance of the huge blocks of gabbroic rocks enclosed in the conglomerates (with no signs of contact metamorphism along the well-exposed contacts); - On a clear day, binocular geology from the top of Skognipa, including an overview of the geology to be seen on Day 5 (Fig. 34E); the same view can be studied by driving along the road to Leknessund as far as a small pass just south of the hamlet; - Study of the well-exposed base of Devonian unconformity and the underlying greenschists belonging to the SolundStavfjord Ophiolite Complex, around the hamlet of Hersvik (Fig. 34D). Stop 4.3. Hyllestad Location From Solund, return with the ferry back to Rysjedalsvik. From there take national road 57 back to Leirvik. Continue north for ~3 km to a road intersection and take minor road 607 toward Hyllestad. Crossing a low pass, the road descends, with a prominent lake (Aksvatnet) on the left, toward the fjord. The stop is

Figure 33. (Stop 4.1.) (A) Panoramic view of Lihesten and the Nordfjord-Sogn Detachment zone, looking toward Hyllestad (Stop 4.3), from a position above the trace of the Rysjedalsvika-Rutledal ferry. This profile was mapped in detail by Meidell (1998). (B, C) Schematic cross sections through the upper margin of the Nordfjord-Sogn Detachment, corresponding to the view from the Rutledal-Krakhella ferry, showing the two low-strain “tectonic lenses” (adapted from Bøe, 1997). (D) View from the ferry of the island of Losna (Stop 4.1), showing steep outcrops of Devonian conglomerate overlying mylonites and phyllonites of the Nordfjord-Sogn extensional detachment zone, with the base of the Devonian cliffs marking the Solund Fault. (E) Contact between the Devonian conglomerates and the underlying mylonites on Losna. The contact is marked by a thin zone of brittle deformation, interpreted as a late, probably Mesozoic, fault, here called the Solund Fault (geologist Andreas Koestler). Farther north, e.g., at Stop 5.2, this is called the Dalsfjord Fault.

C

A

B

D

A

A´

B´

B

A´

A

B´

B

E

Alden Tviberg

Atløy

The field trip itinerary near the end of the lake, at the start of the branch road toward Kolgrov (678640N, 30250E). Description The area around Hyllestad comprises a nearly complete sequence of the Caledonian nappes and overlying sediments (Figs. 35A and 35B). The Western Gneiss Complex is the structurally lowest unit in the area, dominantly in amphibolite facies with local eclogite lenses (similar to Stops 3.6 and 3.7). The structurally overlying Hyllestad Complex comprises psammites, mafic schists, metapelites, amphibolites, and calcsilicates. The metapelites locally contain zones with kyanite + staurolite + garnet + chloritoid, which can be seen at the stop (Fig. 35C), and have provided the material for manufacturing milling-grinding stones, a tradition that extends far back in the history of the region (Fig. 35D). The Hyllestad Complex is ~1 km thick and has been correlated with the Lower Allochthon (Chauvet and Dallmeyer, 1992; Swensson and Andersen, 1991) or the Høyvik Group of the Middle Allochthon on Atløy (M. Tillung, 1999, personal commun.). The Lifjorden Complex, above the Hyllestad Complex, consists of metagraywacke, greenschist, and greenstone, with minor serpentinite, metagabbro, chert, quartzose sandstones, marble, and volcanogenic conglomerates. The rock types and deformation of the Lifjorden Complex are similar to, and probably correlative with, the Staveneset Group (Furnes et al., 1990), the metagraywacke and metavolcanic rocks that overlie the 443 Ma Solund–Stavfjord Ophiolite Complex (Dunning and Pedersen, 1988) just west of the study area. The Lifjorden Complex contains a tectonic lens within which the synorogenic Sogneskollen granodiorite (Skjerlie et al., 2000) is well preserved. A U-Pb intrusion age of 434 ± 4 Ma for this pluton was reported (Hacker et al., 2003). The Sogneskollen granodiorite resembles geochemically and isotopically the Årdal Dyke Complex in the Upper Jotun Nappe (seen on Day 2) and the dikes in the Lindås Nappe (that will be seen on Day 6), and may well be genetically related to those units (Lundmark and Corfu, 2007). DAY 5. ASKVOLL-ATLØY Day 5 (Fig. 36) provides an opportunity to take a closer look at relationships in the footwall and hanging wall of the Nordfjord-Sogn Detachment, which were not visible along Sognefjorden and on the island of Solund (Day 4). The day starts

59

with an examination of the Vårdalsnes eclogite, which is structurally positioned just below the base of the Nordfjord-Sogn Detachment, in this area it is represented by a 2–3-km-thick zone of mylonites and other fault rocks. The eclogite displays a composite fabric developed during metamorphism accompanying syn-deformational decompression of the high-pressure rocks (Andersen et al., 1994; Engvik and Andersen, 2000). The hanging wall is best exposed on the island of Atløy. There we look first at the uppermost parts of the NordfjordSogn Detachment, where greenschist facies mylonitic rocks are capped by brittle fault rocks of the Dalsfjord Fault (= Solund Fault). The hanging wall itself consists of a stack of Caledonian nappes representing the Middle and Upper Allochthons. The lower allochthonous units are made up of the Precambrian Dalsfjord magmatic suite with its unconformably overlying metasedimentary cover (Høyvik Group), itself unconformably overlain by the fossiliferous Silurian deposits of the Herland Group (all Middle Allochthon). These are in turn overlain by the discordant Sunnfjord Mélange and the Solund-Stavfjord Ophiolite Complex of the Upper Allochthon (Brekke and Solberg, 1987; Andersen et al., 1990). The uppermost units in this area are the Devonian sedimentary rocks of the Kvamshesten Group, but they are not preserved on Atløy. These rocks and structures preserve the record of a complex sequence of events, including a Late Ordovician orogenic phase, followed by multistage sedimentation, deformation, and development of unconformities during obduction of the ophiolites and Scandian thrusting. The basement of the Dalsfjord Nappe represents an analogue of the Jotun Nappe, seen on Days 1 and 2, showing a comparable record of Proterozoic magmatism, deformation, and metamorphism. Stop 5.1. Vårdalsneset Location There are several choices for accommodations at the end of Day 4: The one chosen for the 33IGC excursion was at Dale (from Stop 4.3, continue along minor road 607 to Flekke and then turn left along national road 57 to Dale). From Dale, take the car ferry to Eikeneset, and then drive westward ~7 km to Vårdal. Pass Indre Vårdal and a small creek, and turn left toward an old brick factory (some 800 m down a small lane; park on the empty area by the factory ruin; 68061N, 29506E) near boat houses and a stone quay (extreme right of Fig. 37A). The eclogite body is on the shore ~200 m farther west (Figs. 37A and 37E). If this Day is

Figure 34. (Stop 4.2.) (A) Geological map of the surroundings of Stop 4.2, showing the unconformable contact between the Devonian conglomerates (uncolored) and the underlying metasediments of the Solund-Stavfjord Ophiolite Complex (green). Within the Devonian, the main slide blocks of plutonic and metamorphic rocks are shown in brown, and occurrences of volcanic rocks in black. (B) Sections through the slide blocks shown in Figure 34A. (C) Close-up of typical polymict conglomerates of the Solund Devonian Basin. (D) Unconformity at the base of the Devonian sequence in outcrop (dashed line). (E) View from the small pass south of Leknessund, looking north, showing the area to be visited on Day 5 (Atløy). The islands of Alden and Tviberg may be remnants of ancient mountains in the erosion surface at the base of the Devonian sequence, which is extremely irregular. They are composed of different elements of the Solund-Stavfjord Ophiolite Complex.

60

Chapter 3

A

Fig. 35B

Brittle fault Solund Fault = top of NSD

Risnes tectonic lens base of NSD

Fig. 35C

B

C

Figure 35. (Stop 4.3.) (A) Schematic vertical section, showing the structural relationships of the different crustal units in the Hyllestad-Sunnfjord area (modified from Foreman et al., 2005). HP–LT— high pressure–low temperature; NSD—NordfjordSogn Detachment. (B) View of part of the Lihesten range (the mountain Gygrekjeften, summit 700 m above sea level) from Hyllestad, showing the cliffs of Devonian conglomerates overlying mylonites and phyllonites of the Nordfjord-Sogn Detachment zone. (C) Hyllestad schist, displaying the prominent white mica-garnet-kyanite mineral assemblage (diameter of coin [see arrow] is ~1.5 cm). (D) Traditional milling wheels carved out from the hard Hyllestad schist.

D

The field trip itinerary

61

Dale

5.1 5.3

5.2 5.4 Atløy Brekkestranda

5.3 5.4

5.2

5.1

Figure 36. Route map for Day 5, with the structural position of the excursion stops plotted on a reduced copy of the geological cross section shown in Figure 4. MA—Middle Allochthon.

62

Chapter 3

A Granodioritic mylonite gneiss

Eclogite facies mylonitic foliation

B

Eclogite to amphibolite facies foliation

C

Eclogite facies L>S tectonite

B

C

D

E

The field trip itinerary

63

really run in one day (as was the case of the 33IGC excursion), a close watch has to be kept on timing, as four different ferries have to be caught. These are Dale-Eikenes (ferry route 14-415, at the start of the day), Askvoll-Gjervik (ferry route 14-431, between Stops 5.1 and 5.2), Gjervik-Fure (ferry route 14-431, after Stop 5.4), and Lavik-Oppedal (ferry route 14-251, at the end of the day). Download current timetables from web site www.fjord1 .no/en before the planned field trip.

5. Amphibolite facies mylonites mainly formed under noncoaxial top-to-W movement are related to large-scale movement on the extensional detachments active during the late orogenic extension of the Caledonides. These structural relationships are taken to indicate overall coaxial deformation in the lower crust, partly coeval with extensional detachment in the upper crust during exhumation (Engvik and Andersen, 2000).

Description The Vårdalsnes eclogite is in the upper part of the Western Gneiss Complex, structurally ~3 km below the Dalsfjord Fault at the top of the Nordfjord-Sogn Detachment. The body was mapped in great detail by A.K. Engvik, and the description given here is taken from her paper (Engvik and Andersen, 2000). The eclogite occurs as layers and lenses, variably retrograded to amphibolite. It is composed of garnet and omphacite with varying amounts of barroisite, actinolite, clinozoisite, kyanite, quartz, paragonite, phengite, and rutile, cut through by quartz veins and high-grade shear zones (Figs. 37B and 37C). The rocks record a five-stage evolution connected to Caledonian burial and subsequent exhumation (Fig. 37D): 1. A prograde evolution through amphibolite facies; 2. Formation of L>S-tectonite eclogite (T = 680 ± 20 °C, P = 16 ± 2 kbar) related to the subduction of continental crust; a lack of asymmetrical fabrics and the orientation of eclogite facies extensional veins indicate that the deformation regime during formation of the L>S fabric was coaxial (with vertical stretching, see Fig 37D); 3. Formation of subhorizontal eclogite facies foliation, with a horizontal stretching direction (see Fig. 33D); disruption of eclogite lenses and layers between symmetrical shear zones characterizes the dominantly coaxial deformation regime of stage 3. Locally occurring mylonitic eclogites (T = 690 ± 20 °C, P = 15 ± 1.5 kbar) with topto-W kinematics may, however, indicate that non-coaxial deformation was also active at eclogite facies conditions; 4. Development of a widespread regional amphibolite facies foliation (T = 564 ± 44 °C, P <10.3–8.1 kbar); quartz veins and development of conjugate shear zones indicate that coaxial vertical shortening and subhorizontal stretching were active during exhumation from eclogite to amphibolite facies conditions;

Stop 5.2. Gjervik Location After leaving Vårdalsneset, drive west to Askvoll and then take the ferry to Gjervik on Atløy. From the dock at Gjervik follow the road to the intersection with road 608 and turn right (north), following 608 for ~1.5 km to the entrance of the road tunnel (680880N, 28752E). Park just before the tunnel, on the road. High-visibility vests must be worn! Description This stop illustrates the lithologies and structures in the uppermost parts of the Nordfjord-Sogn Detachment (Fig. 38A). Mylonitic gneisses of the Western Gneiss Complex on southern Altøy are structurally overlain by a succession of felsic schists and phyllonitic garnet-amphibole mica schists with lenses of coarsegrained garnet amphibolite, mafic to intermediate mylonites with local felsic schists and lenses of gabbro (Skår et al., 1994). These grade upward into the gray and green phyllonites, locally with massive sulfide mineralization, quartz schists, and thin marble zones, seen at this stop. The phyllonites show good kinematic indicators for the top-to-W shear sense on the detachment (Fig. 38B) and can be observed on shore sections below the road. Late N-S–trending joints and breccias cutting the mylonites are probably late Mesozoic fractures. Near the top of the detachment zone the mylonites are brecciated. The contact to the hanging wall lies along the brittle Dalsfjord Fault, which is exposed in the road section near the tunnel and is characterized by green and, younger, red-stained breccias and a weakly consolidated fault gouge (Fig. 38C). Geological, paleomagnetic, and isotopic work on the breccias indicate multistage reactivation of the fault in the Late Permian (248–260 Ma, the green breccias), in the Late Jurassic (less than 162 Ma, the red breccias), and in the Cretaceous (less than 96 Ma, the fault gouges) (Torsvik et al., 1992; Eide et al., 1997).

Figure 37. (Stop 5.1.) (A) Aerial view of the Vårdalsnes eclogite (from http://kart.sesam.no/3d/), showing distribution of main structural domains and locations of Figures 37B and 37C. (B) Detailed structural map of a part of the eclogite to amphibolite facies foliated area, showing the symmetrical nature of the eclogite lenses. Notice also the N-S orientation of the L>S stretching lineation in the boudins (adapted from Fig. 2 of Engvik and Andersen, 2000). (C) Detailed structural map of a part of the area with eclogite L>S tectonite. Notice the eclogite facies extensional veins oriented normal to the stretching lineation, and the N-S–oriented quartz veins associated with amphibolitization (adapted from Fig. 2 of Engvik and Andersen, 2000). (D) Pressure (P), temperature (T), and structural evolution for the Vårdalsnes eclogite. Boxes numbered 1–5 indicate P-T conditions for the five tectono-metamorphic stages described in the text. Boxes A, B, and C indicate the bulk strain regime and characteristic structures developed (adapted from Fig. 7 of Engvik and Andersen, 2000). Mg—margarite; Qtz—quartz; Zo—zoisite; Ky—kyanite; V—vapor. (E) Participants on the 33IGC excursion examine the Vårdalsnes eclogite at the site shown in Figure 37C and enjoy the splendid view across the fjord.

A

N 2 km

Fig. 38C Fig. 38B

Solund-Stavfjord Ophiolite Complex

Sunnfjord Melange

Fig. 40A Herland Group

Høyvik Group

Dalsfjord Suite top of NSD WGC

B

C

Figure 38. (Stop 5.2.) (A) Simplified geological map of Atløy with excursion localities in the hanging wall of the Nordfjord-Sogn Detachment (NSD). The accompanying cross section illustrates the structural relationships. Axial traces of large scale folds (overturned to recumbent, in blue) predate the Herland Group unconformity. In northern Atløy the Sunnfjord Mélange lies directly on the Høyvik Group. This contact is a low-angle angular unconformity. Thus, three major unconformities are preserved on Atløy: the Dalsfjord Suite–Høyvik Group contact, the Høyvik Group–Herland Group contact, and the Herland Group–Sunnfjord Mélange contact. (Figure courtesy of T.B. Andersen and H. Austrheim.) WGC—Western Gneiss Complex. (B) Low-grade mylonitic schists with top-to-W kinematic indicators exposed in the shore sections below the road at Stop 5.2 (coin is ~1.5 cm in diameter). (C) Dalsfjord Fault being inspected by participants on the 33IGC excursion.

The field trip itinerary Stop 5.3. Kviteneset Location Drive through the tunnel and continue north along road 608 for ~2.5 km, to a cattle grid. Stop near the small creek with prominent road sections, at a lefthand curve (Fig. 39B: 681140N, 28679E), a few hundred meters before reaching the northern end of the peninsula. Description This stop shows the contact between the Precambrian Dalsfjord Suite and the sedimentary Høyvik Group (Fig. 39A). The nature of the contact exposed by the road is not clear owing to the strong foliation developed during the Middle Ordovician (ca. 450 Ma) pre-Scandian deformation, but near the top of the hill to the west the depositional contact, including a basal conglomerate, is remarkably well preserved. The Dalsfjord Suite comprises banded felsic gneisses, less deformed gabbro, monzonites, and alkaline mangeritic rocks. Monzonitic rocks of the Dalsfjord Suite contain abundant mesoperthite, which is commonly found as typical clastic grains in the psammites of the Høyvik Group. Regionally the Dalsfjord Suite rocks are correlated with similar rocks in the Jotun and Lindås Nappes, an interpretation supported by U-Pb data (Corfu and Andersen, 2002). Locally, an up to 10-m-thick zone, highly enriched in Fe oxides and muscovite, possibly representing a paleo-lateritic weathering zone in the Dalsfjord gneisses, is present along the unconformity. The basal deposits consist of deformed pebbly conglomerates and a massive bluish subarkosic metasandstone (Granesund Formation). This is succeeded by quartz-rich mica schists, locally with layers of dolomitic marble (Kvitanes Formation) and metapsammites and schists (Atløy Formation). The Høyvik Group was deformed and metamorphosed in a pre-Scandian, Ordovician orogenic event, as indicated by the Ar-Ar plateau age of ca. 449 Ma for muscovite (ferri-phengite) in the main greenschist facies foliation at this locality (Andersen et al., 1998). Stop 5.4. Brurestakken Location Drive back along road 608 and continue past Gjervik, rounding the southern coast of Atløy. Stop and park at a small abandoned quarry on the right of the road, with the hill Brurestakken to the left (near the S in Sjøralden in Fig. 40A), ~1 km before Herland. The profile begins on the shores of lake Sjøralden at 680761N, 28100E (Fig. 40C) and ends on the summit of the hill Brurestakken at 6807615N, 280327E (Fig. 40D). Description This stop features the unconformity between the Høyvik Group and the overlying Herland Group, the stratigraphy of the Silurian Herland Group and the Sunnfjord Mélange, and the structures related to Scandian thrusting (Fig. 40B).

65

The Herland Group (Brekke and Solberg, 1987) consists of two formations. The base of the Sjøralden Formation is defined by fluvial conglomerates that overlie the Høyvik Group unconformably. They are followed upward by the Brurestakken Formation, a succession of shallow-marine sandstones, fine-grained wackes, mud-rich sandstones, a black mud-shale sequence, and a calcareous unit, locally with a mid-Silurian (Wenlock) shelly fauna, the diagnostic fossils being Pentamerus sp. The Brurestakken Formation comprises three coarse clastic units sandwiched between sandstone-shale and fossiliferous calcareous zones. The deformation of the Herland Group is related to layerparallel shortening associated with the obduction and emplacement of the 443 Ma (Dunning and Pedersen, 1988) SolundStavfjord Ophiolite, with a fold-and-thrust belt geometry related to the SE-directed tectonic transport during the Scandian orogeny, and late W-vergent, asymmetrical back-folds formed during the extension of the orogen (Fig. 40B). The Sunnfjord Mélange, formed during obduction and emplacement of the ophiolite, links the oceanic and continental rocks in this region. The contact between the mélange and the Herland Group is highly sheared, although locally an unconformable depositional contact is preserved (Fig. 38A). The mélange contains a variety of clasts, those in the conglomerates reflecting a bimodal source of both continental and oceanic affinity and formation in a rapidly deepening foreland basin. The rapid subsidence was probably a result of the loading on the continental margin by the advancing ophiolite nappe and its cover. DAY 6. FENSFJORDEN-LINDÅS During Days 4 and 5 we looked at the Nordfjord-Sogn Detachment and the geology of the footwall and hanging wall of this major extensional detachment. Losna (Stop 4.1) is practically the southernmost exposure of the detachment, which then descends below sea level into Sognesjøen and never reappears (except speculatively on some offshore seismic profiles, e.g., Færseth et al., 1995). However, it was recently discovered that a major extensional shear zone does come onshore to the south, in the Bergen area. This is known as the Bergen Arc Shear Zone, and at its northern end Devonian conglomerates lie unconformably on deeply eroded Caledonian Allochthons in the hanging wall (Wennberg, 1998; Wennberg et al., 1998). Although the geometry of the Bergen Arc Shear Zone is quite different from that of the Nordfjord-Sogn Detachment, the shear vector is remarkably similar: The mean orientation of the shear direction in the north (area nearest to the detachment) is found to be 20° to 276° top-to-W, as compared to 12° to 289° on Losna (Stop 4.1). However, the shear zone orientation is much steeper (mean dip 55° to 208°). This means that it is an oblique-slip shear zone, and it is interpreted as a lateral ramp, branching off from the main Nordfjord-Sogn Detachment. The minimum displacement in the shear direction is estimated to be ~16 km. A prominent set of NE-SW–striking normal faults of Devonian age (Larsen et al., 2003), which are developed in the hanging wall of the

66

Chapter 3

Høyvik

A Høyvik Group

Kvitanes

Atløy Fm. (meta-psammite + schist) Kvitanes Fm. (mica-schist + marble) Granesund Fm. (cgl., arkose) Nonconformity

Dalsfjord Suite Monzonitic gneiss + gabbro

Stop 5.3

500 m

Granesund

B

N

Stop 5.3 Figure 39. (Stop 5.3.) (A) Geological map of the Kviteneset area, Atløy, showing the nonconformity between the Dalsfjord Suite and the Høyvik Group (unpublished map, T.B. Andersen, 1985). The best preserved localities are along the top and just over on the SW side of the hill between Granesund and Høyvik (symbolized with black dots). (B) Panoramic view of Stop 5.3, looking west (from http://kart.sesam.no/3d/).

Figure 40. (Stop 5.4.) (A) Detailed geological map of the area around Brurestakken. (Unpublished map by T.B. Andersen, 1985; figure courtesy of T.B. Andersen and H. Austrheim.) (B) Restored, pre-D3 vertical section of the Herland area (from Andersen et al., 1990), showing the structural and stratigraphic relationships between the continental units (Dalsfjord Suite, Høyvik Group, and Herland Group cover) and the Sunnfjord Mélange and Solund-Stavfjord Ophiolite Complex. (C) Participants on the 33IGC excursion on the western shore of lake Sjøralden, inspecting fluvial conglomerates of the Sjøralden Formation (Herland Group) and the well-exposed unconformity above the Høyvik Group. (D) View from Brurestakken toward the islands Alden and Tviberg in the west (see also Fig. 34E). The islands are composed of units of the Solund-Stavfjord Ophiolite Complex. A white granitic pluton, related to the syntectonic Sogneskollen pluton discussed on Day 4, is visible near the shore of the first island, Tviberg (geologist Petras Sinkunas).

The field trip itinerary

67

A

Fig. 40C Fig. 40D

Fig. 40D

Fig. 40C

B C

D

68

Chapter 3

Bergen Arc Shear Zone, are thought to be related to this extensional shearing. Unfortunately, many of the most interesting outcrops of the Bergen Arc Shear Zone are difficult to access. Day 6 is conceived as a stop-and-look day following the main road toward Bergen (Fig. 41). It starts in the Western Gneiss Complex in the footwall of the Bergen Arc Shear Zone, continues through remnants of the Lower, Middle, and Upper Allochthons within the shear zone itself, partially overprinted by brittle faulting (represented by the Fensfjord Fault), and ends, in the hanging wall, in a large nappe complex, the Lindås Nappe, which is dominated by granulite facies rocks (anorthosite, gabbro, charnockite, mangerite) and whose tectonostratigraphic position is controversial (Middle or Upper Allochthon?). One subunit in the Lindås nappe, the Holsnøy subunit, contains the famous “eclogitized” Caledonian shear zones (Austrheim and Griffin, 1985; Austrheim, 1987, 1990; Bingen et al., 2004; Glodny et al., 2008), which will be studied as the final stop in this cross section. The day ends with a long drive along the main Bergen-Oslo road (E16) to Gudvangen (Sognefjorden again), with some buswindow geology and a short stop in the anorthosites of the Jotun Complex along the way. Stop 6.1. Kjekallevågen Location South of Sognefjorden, the route from the Day 5 localities follows the main E39 road toward Bergen, and takes off along road 570 following the northern shore of inner Fensfjorden for ~7 km. At this point, road 570 crosses a bridge over a small arm of the fjord (Kjekallevågen). Just after the bridge, stop at a parking place and picnic area on the left (673551N, 30449E). Description From this parking place a good view of the Bergen Arc Shear Zone is obtained. It runs from the low islands visible on the horizon to the northwest along the whole length of Fensfjorden to the deep inlet visible toward the southeast (Fig. 42A), a distance of ~50 km—all under water (except for its marginal parts). It comes to land to the southeast, and a cross section through the whole zone will be studied at Stop 6.2. The islands at its northern end, however, have been mapped in detail, to produce the structural profile a–aʹ shown in Figure 42B. One of the islands exposes a fine eclogite in the footwall of the Bergen Arc Shear Zone (Winswold, 1996) embedded in the gneisses of the Western Gneiss Complex (cf. Stop 3.6), as well as part of the transition zone into the shear zone proper (mylonitic gneisses, Figs. 42C and 42D), although most of the actual shear zone is submerged here. Although there are few data from the shear zone itself in this northern area, the structural data from the footwall and hanging wall show some interesting features (Fig. 42E). First, the prominent stretching lineation in the Western Gneiss Complex, parallel to the axes of a set of tight upright folds and developed under amphibolite facies conditions (with superficial similarities to parts of the Sognefjord profile, cf. Stops 3.4 and 3.5, although

structural mapping of this complex between Fensfjorden and Sognefjorden has not yet been carried out), has a similar orientation to the shear vector in the Bergen Arc Shear Zone (Fig. 42Ea). Nevertheless, detailed mapping along the northern shore of Fensfjorden has allowed the transition from the Western Gneiss Complex to the overprinted, lower grade mylonitic gneisses of the Bergen Arc Shear Zone proper to be described in detail (Wennberg et al., 1998). Second, the bedding data from the Devonian islands (Byrknesøyene, see Figs. 42A and 42B) show irregular orientations, with no obvious folding but rather with a constant dip of 10°–30° to the northeast (Fig. 42E, c). Third, the internal structure of the Lindås nappe in this area is dominated by a major, almost isoclinal, upright synform (Børillen synform), affecting the main amphibolite-facies foliation in the gneisses of the unit (Fig. 42E, b). The axial trace of this synform runs parallel to the trace of the Bergen Arc Shear Zone but disappears under water and the Devonian islands in the north (Fig. 42A). Based on a detailed analysis of the metamorphic and structural history of the whole area (Borthen, 1995), the deformation leading to the dominant foliation, and the later isoclinal folding, are thought to be of Caledonian age, and the lack of similar folding in the Devonian conglomerates indicates that the folding predates their deposition. Hence, on Figure 41 the cross section through Stops 6.1–6.3 shows a stratigraphic unconformity at the base of the Devonian conglomerates, truncating the Børillen synform in the Lindås Nappe. From the Stop 6.1 parking place, a spectacular geomorphological phenomenon can, however, not be seen! The very low topography of the islands on the opposite side of Fensfjorden is typical of the Norwegian strandflat, the rock plain just above present sea level, which is found all along the Norwegian west coast and which is thought to be an erosional surface of late Pliocene– Pleistocene age (the product of a combination of glacial erosion, marine erosion, and subaerial weathering; see Holtedahl, 1998; Aarseth and Fossen, 2004). In this region the strandflat is ~30 km wide and undulates between 0 and 50 m above sea level, with a mean around 20 m. What is not seen is the depth of the Fensfjorden. At the level of the Mongstad oil refinery (seen from this stop), the detailed subsurface topographic maps (made for laying the oil pipelines) show extremely steep fjord walls descending to flat fjord bottoms at depths exceeding 500 m. The mode of formation of this spectacular “hidden topography,” with steep-sided U-shaped valleys separated by large flat-topped mountains and plateaus, is still problematic. Stop 6.2. Osterfjorden Location From Stop 6.1, return along road 570 to the intersection with E39, and turn right toward Bergen. The localities follow in sequence along the main road, which follows the shore of Osterfjorden toward Knarvik and the floating bridge across Osterfjorden to Bergen: 6.2a, Bjørsvik (UTM 30795E, 67276N); 6.2b, Ostereidet, petrol station (UTM 30704E, 67263N); 6.2c,

The field trip itinerary

69

Brekkestranda

Gudvangen

6.4 6.1 6.3 6.2

6.4

6.3 Main thrust

Øygarden Gneiss Complex Lindås Nappe BASZ = Bergen Arc Shear Zone BS = Bjørillen Synform FF = Fensfjord Fault SFC = Sotra-Fedje Culmination

6.2

6.1

Major Bergen Arc

Figure 41. Route map for Day 6, with the structural position of the excursion stops plotted on a generalized cross section through the Bergen area (approximately along the line of Stops 6.1–6.3, from Milnes and Wennberg, 1997), and a reduced copy of the geological cross section shown in Figure 4 for Stop 6.4. The colors used on the Bergen cross section are as follows, from right to left: pink—Western Gneiss Complex; dark olive—Kvalsida Gneiss (possible Jotun equivalent); dark green—Major Bergen Arc Zone (ophiolites and related rocks); middle olive—Lindås Nappe (possible Jotun equivalent); violet—Ulriken subunit of Lindås Nappe: light green—Minor Bergen Arc Zone (sediments with Late Ordovician–Early Silurian fossils); pink—Øygarden Gneiss Complex (continuation of Western Gneiss Complex). On the cross section for Stop 6.4: UA—Upper Allochthon; MA—Middle Allochthon.

A

a´

Fig. 42D

a

n tio ec 41 s s . os Fig Cr in

Stop 6.1

b´ Stop 6.2

b

Figure 42. (Stop 6.1.) (A) Overview structural map of the Bergen Arc Shear Zone (BASZ) in the area south of Sognefjorden (see Fig. 41). (B) Structural profile through the Bergen Arc Shear Zone at the northern end of Fensfjorden, marked a–aʹ in Figure 42A (after Wennberg et al., 1998). (C) Field party studying mylonitic gneisses within the Bergen Arc Shear Zone at the south end of the structural profile, Figure 42B, looking SE along Fensfjordenen. The island in the background consists of Devonian conglomerates in the hanging wall of the shear zone (geologists Ole Petter Wennberg, Inger Winsvold, Sigrid Borthen). (D) Sense of shear indicators in mylonitic gneisses toward the footwall margin of the Bergen Arc Shear Zone (photo by Ole Petter Wennberg). (E) Structural data from the footwall and hanging wall of the Bergen Arc Shear Zone (from Wennberg et al., 1998).

B Fig. 42C

a

a´

D

C

E

The field trip itinerary Totland picnic area (UTM 30612E, 67245N); 6.2d, Sauvåg picnic area (UTM 30479E, 67247N). High-visibility vests must be worn at all these stops, and care must be taken to keep off the road, which is at times quite busy. Description This sequence of stops provides a brief introduction to the different tectonic units in the Bergen area, both inside and outside the Bergen Arc Shear Zone (Wennberg, 1998). These units are summarized in the general cross section shown in Figure 41, which is drawn along the line indicated in Figure 42A. The sequence of stops along the main road is marked on the structural profile, Figure 43A (line of section b–bʹ is marked in Fig. 42A), and on the geological map of the area, Figure 43B (based on the detailed structural map, scale 1:13,000, in Wennberg, 1998). Stop 6.2a is situated within the Bergen Arc Shear Zone, near its footwall margin. The late brittle part of the movement (Fensfjord Fault) is concentrated in the deep gully just before the main road to Bergen disappears into the tunnel, opposite the road intersection to Bjørsvik, but brittle deformation effects overprint the whole road section east of the gully. On each side of the gully, packets of black garnet-mica schist occur, probably remnants of the onetime cover of the Western Gneiss Complex or possibly the Lower Allochthon. The tunnel entrance marks the contact with the Kvalsida Gneiss unit, containing mylonitized rocks of Jotun affinity lying within the Bergen Arc Shear Zone. Stop 6.2b lies in the central part of the Bergen Arc Shear Zone, and the main road sections show heterogeneously banded and mylonitized Kvalsida Gneiss units (Fig. 43C). Stop 6.2c lies in the hanging wall, just outside the margin of the Bergen Arc Shear Zone. The contact between the Lindås Nappe (Fosnøy subunit) and the underlying Major Bergen Arc zone is exposed in the road section that starts at the picnic area and runs northwestward on the inside curve of the road (DANGER!). The hanging-wall margin of the Bergen Arc Shear Zone is not seen here, but it has been studied in detail along the shore of Osterfjorden and subjected to detailed structural analysis (Fig. 43D; see Wennberg, 1996). Stop 6.2d lies within the Lindås Nappe (Fosnøy subunit) approximately on the hinge of the Børillen synform (see Stop 6.1), hence the subhorizontal foliation (as opposed to the steeply dipping foliation and contact at Stop 6.2c). Crosscutting but strongly folded felsic veins of the Ostereidet dike swarm (Wennberg et al., 2001) are well exposed (Figs. 43E and 43F). One controversial point in the regional geology has been the relationship between the Lindås Nappe and the Upper Jotun Nappe that we crossed on Day 2. The two are similar in terms of lithology, metamorphism, and age, and they have both been intruded by Silurian syntectonic leucocratic granitic (to trondhjemitic?) dikes (Fig. 43F). The only apparent difference is the presence of eclogitized shear zones in parts of the Lindås Nappe (Holsnøy subunit, Stop 6.3) and their absence in the Upper Jotun Nappe. Tectonically, however, they differ in that the Lindås

71

Nappe is structurally juxtaposed on Paleozoic ophiolitic units of the Upper Allochthon (Major Bergen Arc), whereas no such (proven) units appear to be present underneath the Jotun Nappe Complex. The geochemical and isotopic composition of the Årdal dike complex, however, is best explained by the melting of sedimentary rocks similar to those invoked for the genesis of granitic rocks at this stop and for the Sogneskollen intrusion (cf. Stop 4.3) (Skjerlie et al., 2000; Lundmark and Corfu, 2007). If the Lindås and Upper Jotun have a common origin, and both were derived from the margin of Baltica, then more complex tectonic solutions are needed to bring the Lindås on top of the ophiolites (Middle Allochthon on top of Upper Allochthon!). The alternative is that they have distinct origins, as implied in Figure 41 (from Milnes and Wennberg, 1997). Stop 6.3. Holsnøy Location At Knarvik, take road 564 for Holsnøy, reached over two bridges and one intervening island. Follow the road west, and then northwestward to Rossland; continue north and then westward to Sætrevik, and from there walk to the east side of the bay (672400N, 27884E). Description The island of Holsnøy is geologically a part of the Lindås Nappe (the Holsnøy subunit, see Milnes and Wennberg, 1997) and is composed of a granulite facies AMCG (anorthosite, mangerite, charnockite, and granite) complex, locally overprinted by eclogite facies metamorphism (Figs. 44A and 44B). Ages reported so far for the magmatic emplacement of the complex range from ca. 1240 to 951 Ma with high-grade metamorphism at 929 Ma (Bingen et al., 2001; Glodny et al., 2008). The complex has generally been considered to be related to the high-grade rocks of the Upper Jotun Nappe seen on Days 1 and 2, an inference supported by the geochronological work (but see discussion for Stop 6.2). Subduction of the Baltic margin during the Caledonian orogeny led to the partial eclogitization of the granulites at 430 Ma, followed by an amphibolite facies overprint at 414 Ma. The mode of development of the various paragenetic assemblages and related structures in these rocks has become a prime example for the importance of fluids in enabling the progress of mineral reactions and, conversely, their complete inhibition under dry conditions (e.g., Austrheim and Griffin, 1985; Austrheim, 1987, 1990; Jamtveit et al., 1990, 2000; Austrheim and Boundy, 1994; Jolivet et al., 2005; Bjørnerud and Austrheim, 2006). The region has also become an important area for studying the behavior of isotopic systems and the interpretation of radiometric ages (e.g., Bingen et al., 2001, 2004; Kühn et al., 2000, 2002; Camacho et al., 2005; Glodny et al., 2008; Andersen and Austrheim, 2008). The general interpretation is that the granulites were subducted in the Silurian, reaching eclogite facies conditions (650– 750 °C and 15–17 kbar) at which they underwent brittle fracturing. The fractures were infiltrated by fluids, which promoted further

b

6.2d

B

6.2d

A

C

6.2c

Fig. 43D Askvik

6.2b

6.2a

6.2c

E

D

F

Figure 43. (Stop 6.2.) (A) Structural profile through the Bergen Arc Shear Zone (BASZ) along the Osterfjord cross section, marked b–bʹ in Figure 42A (after Wennberg et al., 1998). LC—Lindås Complex; MaBA—Major Bergen Arc; KG—Kvalside Gneiss; WGC—Western Gneiss Complex. (B) Geological map of the Osterfjorden cross section, showing the location of the short stops included in Stop 6.2 (simplified after Wennberg, 1998). (C) Heterogeneous mylonite gneisses of the Kvalside unit, in the central part of the Bergen Arc Shear Zone, at short stop 6.2b (geologist Fernando Corfu). (D) Structural summary of the superimposed fabrics owing to reversal of shear sense, across the hanging-wall margin of the Bergen Arc Shear Zone (from Wennberg, 1996). The upper zone of sinistral kinematic indicators represents the nappe movements that predate the development of the Bergen Arc Shear Zone; the lower zone of dextral kinematic indicators shows the structures developed during movement in this shear zone. The transition zone marks the hanging-wall margin of this shear zone where the two sets of shear indicators are superimposed. (E, F) Two examples of crosscutting but strongly folded felsic veins in the Lindås nappe at short stop 6.2d.

6.2b 6.2a

b´

The field trip itinerary

A

73

C

Stop 6.3

D

B

Stop 6.3

E

Figure 44. (Stop 6.3.) (A) Geological map of Holsnøy, showing the distribution of the main lithologies: red and orange for mangeritecharnockite, and dotted-brown for anorthosite to gabbro. (B) Map showing the distribution and intensity of eclogitized shear zones and the degree of eclogitization (from Jamtveit et al., 1990). (C) Classical appearance of eclogitization in the Lindås Nappe. Eclogite develops symmetrically around a central vein filling a fracture in the original granulite (from Jamtveit et al., 1990). (D) Eclogite fingers penetrate into anorthosite, starting from an eclogite shear zone at the bottom of the picture (from Jamtveit et al., 1990). (E) Participants of the 33IGC excursion study the eclogitized outcrops at Stop 6.3 (photo by George DeVries Klein).

74

Chapter 3

A

B

Stops 2.3 + 2.4

Stop 2.1

Stop 6.4

Stop 2.2

Stop 6.4

Fig. 45B

C D

Figure 45. (Stop 6.4.) (A) Regional distribution of anorthositic-gabbroic phases within the Jotun Nappe Complex (from Wanvik, 2000). (B) Geological map of the Gudvangen-Mjolfjell massif, focused especially on the industrial quality of the anorthosite in terms of purity and solubility in consideration of its potential for extracting aluminum (from Wanvik, 2000). (C) View from Stahlheim toward Gudvangen and Nærøyfjorden in the NNE. The rounded mountain top to the left, Jordalsnuten, consists of pure anorthosite and therefore has very little vegetation. (D) Anorthosite is extracted for various industrial purposes, not the least as an ornamental stone, as seen in this picture.

The field trip itinerary shearing, enabling crystallization of the eclogite facies paragenesis (Figs. 44C–44E). Subsequent deformation and/or fluid intrusion was controlled largely by eclogite, which is rheologically weaker than granulite. The eclogites occur in anastomosing shear zones, mostly trending NW, with the mineralogy of eclogitized anorthosite consisting of omphacite, garnet, kyanite, zoisite, phengite, rutile, quartz, and amphibole. Stop 6.4. Stalheim Location On the drive back to Oslo from Bergen (route E16, through Voss), leave the main highway at Stalheim and stop near the hotel (67470N, 37390E) for a brief overview of the zone. From there, descend to the valley and drive ~2 km northward, cross a small bridge, and stop on the side road to the anorthosite quarry to the left (67476N, 376398E). Description Stalheim offers an excellent view of Nærøydalen, with Gudvangen and Nærøyfjord in the background. The spectacular Nærøyfjord is ranked as a UNESCO World Heritage Site. The rounded mountaintop Jordalsnuten to the left is essentially bare of vegetation owing to its nutrient-poor anorthositic composition (Fig. 45C). The anorthosite can be examined in blocks near two quarries at the foot of Jordalsnuten. The variety present at this

75

location is highly leucocratic (<10% mafic minerals) and is generally retrogressed so that the dark minerals consist of epidote and amphibole with minor amounts of garnet, biotite, and sericite (Wanvik, 2000). It can be compared with the troctolitic variety of the Jotun Complex crystallines that we examined at Stop 2.3 (Fig. 45A). The use of anorthosite for industrial purposes has been explored for a century, starting with Goldschmidt (1917). Wanvik (2000) investigated the geology and mineral potential of the Inner Sogn occurrence (Figs. 45B and 45D) and gives the following summary of the features that make this occurrence interesting: “…anorthosite with a high anorthite content (An >70) is easily soluble in mineral acids, and the bytownite plagioclase of the Inner Sogn anorthosite makes it well suited for industrial processes based on acid leaching. The high aluminum content, ca. 31% Al2O3, has made these occurrences interesting for various industrial applications, especially as an alternative raw material for the Norwegian aluminum industry” (Wanvik, 2000, p. 103). A process currently under development and testing would co-produce aluminum, silica, and calcium and would consume CO2, which represents an interesting modern environmental application. Current mining activity produces anorthosite as a reflective material for mixing with asphalt or concrete in pavement construction, as a refractory material in industrial applications, for the manufacture of mineral wool, and for other applications.

References Cited Aarseth, I., and Fossen, H., 2004, Late Quaternary cryoplanation of rock surfaces in lacustrine environments in the Bergen area, Norway: Norwegian Journal of Geology, v. 84, p. 125–137. Andersen, T.B., 1998, Extensional tectonics in southern Norway: An overview: Tectonophysics, v. 285, p. 333–351, doi:10.1016/S0040-1951(97)00277-1. Andersen, T.B., and Andresen, A., 1994, Stratigraphy, tectonostratigraphy and the accretion of outboard terranes in the Caledonides of Sunnhordland, W. Norway: Tectonophysics, v. 231, p. 71–84, doi:10.1016/0040-1951 (94)90122-8. Andersen, T.B., and Austrheim, H., 2008, The Caledonian infrastructure in the Fjord-region of Western Norway; with special emphasis on formation and exhumation of high- and ultra-high-pressure rocks, late to postorogenic tectonic processes and basin formation: Oslo, International Geological Congress, 33rd, Guidebook to Excursion 29; available at http:// www.33igc.org/coco/Layoutpage.aspx?containerid=11435&pageid=5059. Andersen, T.B., and Jamtveit, B., 1990, Uplift of deep crust during orogenic extensional collapse: A model based on field studies in the SognSunnfjord region of West Norway: Tectonics, v. 9, p. 1097–1112, doi: 10.1029/TC009i005p01097. Andersen, T.B., Skjerlie, K.P., and Furnes, H., 1990, The Sunnfjord Melange, evidence of Silurian ophiolite accretion in the West Norwegian Caledonides: Journal of the Geological Society [London], v. 147, p. 59–68, doi:10.1144/gsjgs.147.1.0059. Andersen, T.B., Jolivet, L., and Osmundsen, P.T., 1994, Deep crustal fabrics and a model for the extensional collapse of the Southwest Norwegian Caledonides: Journal of Structural Geology, v. 16, p. 1191–1203, doi:10.1016/0191-8141(94)90063-9. Andersen, T.B., Berry, H.N., Lux, D.R., and Andresen, A., 1998, The tectonic significance of pre-Scandian 40Ar/39Ar phengite cooling ages in the Caledonides of western Norway: Journal of the Geological Society [London], v. 155, p. 297–309, doi:10.1144/gsjgs.155.2.0297. Andersen, T.B., Torsvik, T.H., Eide, E.A., Osmundsen, P.T., and Faleide, J.I., 1999, Permian and Mesozoic extensional faulting within the Caledonides of central south Norway: Journal of the Geological Society [London], v. 156, p. 1073–1080, doi:10.1144/gsjgs.156.6.1073. Andersson, M., Lie, J.E., and Husebye, E.S., 1996, Tectonic setting of postorogenic granites within SW Fennoscandia based on deep seismic and gravity data: Terra Nova, v. 8, p. 558–566, doi:10.1111/j.1365-3121.1996 .tb00785.x. Austrheim, H., 1987, Eclogitization of lower crustal granulites by fluid migration through shear zones: Earth and Planetary Science Letters, v. 81, p. 221–232, doi:10.1016/0012-821X(87)90158-0. Austrheim, H., 1990, The granulite–eclogite facies transition: A comparison of experimental work and a natural occurrence in the Bergen Arcs, western Norway: Lithos, v. 25, p. 163–169, doi:10.1016/0024-4937(90)90012-P. Austrheim, H., and Boundy, T.M., 1994, Pseudotachylytes generated during seismic faulting and eclogitization of the deep crust: Science, v. 265, p. 82–83, doi:10.1126/science.265.5168.82. Austrheim, H., and Griffin, W.L., 1985, Shear deformation and eclogite formation within granulite-facies anorthosites of the Bergen Arcs, Western Norway: Chemical Geology, v. 50, p. 267–281, doi:10.1016/0009-2541 (85)90124-X. Avouac, J.-P., and Burov, E.B., 1996, Erosion as a driving mechanism of intracontinental mountain growth: Journal of Geophysical Research, v. 101, p. 17,747–17,769, doi:10.1029/96JB01344. Bailey, D.E., 1989, Metamorphic evolution of the crust of southwestern Norway: An example from Sognefjord [Ph.D. thesis]: University of Oxford, UK, 192 p. Bingen, B., Davis, W.J., and Austrheim, H., 2001, Zircon U–Pb geochronology in the Bergen arc eclogites and their Proterozoic protoliths, and implications for the pre-Scandian evolution of the Caledonides in western Norway: Geological Society of America Bulletin, v. 113, p. 640–649, doi:10.1130/0016-7606(2001)113<0640:ZUPGIT>2.0.CO;2. Bingen, B., Austrheim, H., Whitehouse, M.J., and Davis, W.J., 2004, Trace element signature and U–Pb geochronology of eclogite-facies zircon, Bergen arcs, Caledonides of W. Norway: Contributions to Mineralogy and Petrology, v. 147, p. 671–683, doi:10.1007/s00410-004-0585-z.

Bingen, B., Skår, Ø., Marker, M., Sigmond, E.M.O., Nordgulen, Ø., Ragnhildstveit, J., Mansfeld, J., Tucker, R.D., and Liégeois, J.-P., 2005, Timing of continental building in the Sveconorwegian orogen, SW Scandinavia: Norwegian Journal of Geology, v. 85, p. 87–116. Bjørnerud, M.G., and Austrheim, H., 2006, Geophysics: Hot fluids or rock in eclogite metamorphism?: Nature, v. 440, E4, doi:10.1038/nature04714. Bøe, O., 1997, Structural evolution of the Nordfjord-Sogn Shear Zone in Solund, Losna, outer Sognefjord, Western Norway [unpublished thesis]: University of Bergen, Norway, 116 p. Borthen, S., 1995, Børillen synform in the Lindås Complex, Western Norway— Petrography, structural geology and regional significance [unpublished thesis]: University of Bergen, Norway, 143 p. Braathen, A., Osmundsen, P.T., and Gabrielsen, R.H., 2004, Dynamic development of fault rocks in a crustal scale detachment: An example from western Norway: Tectonics, v. 23, TC4010, doi:10.1029/2003TC001558. Brekke, H., and Solberg, P.O., 1987, The geology of Atløy, Sunnfjord, western Norway: Geological Survey of Norway (NGU) Bulletin, v. 410, p. 73–94. Camacho, A., Lee, J.K.W., Hensen, B.J., and Braun, J., 2005, Short-lived orogenic cycles and the eclogitization of cold crust by spasmodic hot fluids: Nature, v. 435, p. 1191–1196, doi:10.1038/nature03643. Chapple, W.M., 1978, Mechanics of thin-skinned fold-and-thrust belts: Geological Society of America Bulletin, v. 89, p. 1189–1198, doi:10.1130/0016 -7606(1978)89<1189:MOTFB>2.0.CO;2. Chauvet, A., and Dallmeyer, R.D., 1992, 40Ar/39Ar mineral dates related to Devonian extension in the southwestern Scandinavian Caledonides: Tectonophysics, v. 210, p. 155–177, doi:10.1016/0040-1951(92)90133-Q. Chauvet, A., and Séranne, M., 1994, Extension-parallel folding in the Scandinavian Caledonides: Implications for late-orogenic processes: Tectonophysics, v. 238, p. 31–54, doi:10.1016/0040-1951(94)90048-5. Cloos, M., 1993, Lithospheric buoyancy and collisional orogenesis: Subduction of oceanic plateaus, continental margins, island arcs, spreading ridges, and seamounts: Geological Society of America Bulletin, v. 105, p. 715– 737, doi:10.1130/0016-7606(1993)105<0715:LBACOS>2.3.CO;2. Corfu, F., 1980a, Geologie der Jotun Decke sowie U-Pb und Rb-Sr Geochronologie des darunterliegenden präkambrischen Schildes, Fillefjell, Südnorwegen [Ph.D. thesis]: ETH Zürich, Switzerland, 138 p. Corfu, F., 1980b, U-Pb and Rb-Sr systematics in a poly-orogenic segment of the Precambrian shield, central southern Norway: Lithos, v. 13, p. 305–323, doi:10.1016/0024-4937(80)90051-1. Corfu, F., and Andersen, T.B., 2002, U-Pb ages of the Dalsfjord Complex, SWNorway, and their bearing on the correlation of allochthonous crystalline segments of the Scandinavian Caledonides: International Journal of Earth Sciences, v. 91, p. 955–963, doi:10.1007/s00531-002-0298-3. Cuthbert, S.J., Carswell, D.A., Krogh-Ravna, E.J., and Wain, A., 2000, Eclogites and eclogites in the Western Gneiss Region, Norwegian Caledonides: Lithos, v. 52, p. 165–195, doi:10.1016/S0024-4937(99)00090-0. Dahlen, F.A., 1990, Critical taper model of fold-and-thrust belts and accretionary wedges: Annual Review of Earth and Planetary Sciences, v. 18, p. 55–99, doi:10.1146/annurev.ea.18.050190.000415. Dewey, J.F., 1988, Extensional collapse of orogens: Tectonics, v. 7, p. 1123– 1139, doi:10.1029/TC007i006p01123. Dewey, J.F., Ryan, P.D., and Andersen, T.B., 1993, Orogenic uplift and collapse, crustal thickness, fabrics and facies changes: The role of eclogites, in Prichard, H.M., Alabaster, T., Harris, N.B.W., and Neary, C.R.,eds., Magmatic Processes and Plate Tectonics: Geological Society [London] Special Publication 76, p. 325–343. Dietler, T.N., 1987, Struckturgeologische und tektonische Entwicklung der Western Gneiss Complexes im Sognefjord-Querschnitt, westliches Norwegen [Ph.D. thesis]: ETH Zürich, Switzerland, 233 p. Duchesne, J.C., Liégeois, J.P., Vander Auwera, J., and Longhi, J., 1999, The crustal tongue melting model and the origin of massive anorthosites: Terra Nova, v. 11, p. 100–105, doi:10.1046/j.1365-3121.1999.00232.x. Dunning, G.R., and Pedersen, R.B., 1988, U/Pb ages of ophiolites and arcrelated plutons of the Norwegian Caledonides: Implications for the development of Iapetus: Contributions to Mineralogy and Petrology, v. 98, p. 13–23, doi:10.1007/BF00371904.

77

78

References Cited

Eide, E.A., Torsvik, T.H., and Andersen, T.B., 1997, Absolute dating of brittle fault movements: Late Permian and Late Jurassic extensional fault breccias in western Norway: Terra Nova, v. 9, p. 135–139. Emmett, T.F., 1996, The provenance of pre-Scandian continental flakes within the Caledonide Orogen of south-central Norway, in Brewer, T.S., ed., Precambrian Crustal Evolution in the North Atlantic Region: Geological Society [London] Special Publication 112, p. 359–366. Engvik, A.K., and Andersen, T.B., 2000, Evolution of Caledonian deformation fabrics under eclogite and amphibolite facies at Vårdalsneset, Western Gneiss Region, Norway: Journal of Metamorphic Geology, v. 18, p. 241– 257, doi:10.1046/j.1525-1314.2000.00252.x. Engvik, A.K., Andersen, T.B., and Wachmann, M., 2008, Inhomogeneous deformation in deeply buried continental crust, an example from the eclogite-facies province of the Western Gneiss Region, Norway: Norwegian Journal of Geology, v. 87, p. 373–389. Færseth, R.B., Gabrielsen, R.H., and Hurich, C.A., 1995, Influence of basement in structuring of the North Sea basin, offshore southwest Norway: Norsk Geologisk Tidsskrift, v. 75, p. 105–119. Foreman, R., Andersen, T.B., and Wheeler, J., 2005, Eclogite-facies polyphase deformation of the Drøsdal eclogite, Western Gneiss Complex, Norway, and implications for exhumation: Tectonophysics, v. 398, p. 1–32, doi:10.1016/j.tecto.2004.10.003. Fossen, H., 1992, The role of extensional tectonics in the Caledonides of south Norway: Journal of Structural Geology, v. 14, p. 1033–1046, doi:10.1016/ 0191-8141(92)90034-T. Fossen, H., and Holst, T.B., 1995, Northwest-verging folds and the northwestward movement of the Caledonian Jotun Nappe, Norway: Journal of Structural Geology, v. 17, p. 3–15, doi:10.1016/0191-8141(94)E0033-U. Fossen, H., and Hurich, C.A., 2005, The Hardangerfjord shear zone in SW Norway and the North Sea; a large-scale low-angle shear zone in the Caledonian crust: Journal of the Geological Society [London], v. 162, p. 675–687, doi:10.1144/0016-764904-136. Froitzheim, N., Schmid, S.M., and Frey, M., 1996, Mesozoic paleogeography and the timing of eclogite facies metamorphism in the Alps: A working hypothesis: Eclogae Geologicae Helvetiae, v. 89, p. 81–110. Furnes, H., Skjerlie, K.P., Pedersen, R.B., Andersen, T.B., Stillman, C.J., Suthren, R.J., Tysseland, M., and Garmann, L.B., 1990, The Solund-Stavfjord ophiolite complex and associated rocks, west Norwegian Caledonides: Geology, geochemistry and tectonic environment: Geological Magazine, v. 127, p. 209–224, doi:10.1017/S0016756800014497. Gee, D.G., 1975, A tectonic model for the central part of the Scandinavian Caledonides: American Journal of Science, v. 275, p. 468–515. Gibbs, A.D., 1983, Balanced cross-section construction from seismic sections in areas of extensional tectonics: Journal of Structural Geology, v. 5, p. 153–160, doi:10.1016/0191-8141(83)90040-8. Glodny, J., Kühn, A., and Austrheim, H., 2008, Geochronology of fluid-induced eclogite and amphibolite facies metamorphic reactions in a subduction– collision system, Bergen Arcs, Norway: Contributions to Mineralogy and Petrology, v. 156, p. 27–48, doi:10.1007/s00410-007-0272-y. Goldschmidt, V.M., 1912, Die kaledonische Deformation der südnorwegischen Urgebirgstafel: Oslo, Skrifter Norsk Videnskaps-Akademi, mat.-naturvitenskap: Klasse, v. 19, p. 1–11. Goldschmidt, V.M., 1916, Uebersicht der Eruptivgesteine im Kaledonischen Gebirge zwischen Stavanger und Trondheim: Oslo, Jacob Dybwad, 140 p. Goldschmidt, V.M., 1917, Beretning om labradorstensfelter i Sogn. Det Norske Aktieselskab for Elektrokemisk Industri, Kristiania, Internal Report, 31 p. Griffin, W.L., 1971, Genesis of coronas in anorthosites of the upper Jotun nappe, Indre Sogn, Norway: Journal of Petrology, v. 12, p. 219–243. Griffin, W.L., Mellini, M., Oberti, R., and Rossi, G., 1985a, Evolution of coronas in Norwegian anorthosites; re-evaluation based on crystal-chemistry and microstructures: Contributions to Mineralogy and Petrology, v. 91, p. 330–339, doi:10.1007/BF00374689. Griffin, W.L., Austrheim, H., Brastad, K., Bryhni, I., Krill, A.G., Krogh, E.J., Mørk, M.B.M., Qvale, H., and Tørudbakken, B., 1985b, High-pressure metamorphism in the Scandinavian Caledonides, in Gee, D.G., and Sturt, B.A., eds., The Caledonide Orogen—Scandinavia and Related Areas: Chichester, UK, Wiley and Sons, p. 783–801. Hacker, B.R., Andersen, T.B., Root, D.B., Mehl, L., Mattinson, J.M., and Wooden, J.L., 2003, Exhumation of high-pressure rocks beneath the Solund Basin, Western Gneiss Region of Norway: Journal of Metamorphic Geology, v. 21, p. 613–629, doi:10.1046/j.1525-1314.2003.00468.x.

Hanmer, S., and Passchier, C., 1991, Shear-Sense Indicators: A Review: Geological Survey of Canada Paper 90-17, 72 p. Hartz, E., Andresen, A., and Andersen, T.B., 1994, Structural observations adjacent to a large-scale extensional detachment zone in the Hinterland of the Norwegian Caledonides: Tectonophysics, v. 231, p. 123–137, doi:10 .1016/0040-1951(94)90125-2. Hatcher, R.D., Costain, J.K., Çoruh, C., Phinney, R.A., and Williams, R.T., 1987, Tectonic implications of new Appalachian ultra-deep core hole (ADCOH) seismic reflection data from the crystalline southern Appalachians, in Matthews, D., and Smith, C., eds., Deep Seismic Reflection Profiling of the Continental Lithosphere: Special Issue of the Geophysical Journal of the Royal Astronomical Society, v. 89, p. 157–162. Heim, M., 1979, Struktur und Petrographie des Jotun-Valdres-Deckenkomplexes und der ihn unterlagenden kaledonischen Deformationszone im Gebiet des östlichen Vangsmjøsi (zentrales Südnorwegen) [Ph.D. thesis]: ETH Zürich, Switzerland, 210 p. Heim, M., Schärer, U., and Milnes, A.G., 1977, The nappe complex in the Tyin-Bygdin-Vang region, central southern Norway: Norsk Geologisk Tidsskrift, v. 57, p. 171–178. Holtedahl, H., 1998, The Norwegian strandflat. A geomorphological puzzle: Norsk Geologisk Tidsskrift, v. 78, p. 47–66. Hossack, J.R., 1968, Pebble deformation and thrusting in the Bygdin area (southern Norway): Tectonophysics, v. 5, p. 315–339, doi:10.1016/0040 -1951(68)90035-8. Hossack, J.R., 1972, The geological history of the Grønsennknipa Nappe, Valdres: Geological Survey of Norway (NGU) Bulletin 281, 26 p. Hossack, J.R., 1976, Geology and structure of the Beito Window, Valdres: Geological Survey of Norway (NGU) Bulletin, v. 327, p. 1–33. Hossack, J.R., 1979, The use of balanced cross sections in the calculation of orogenic contraction: A review: Journal of the Geological Society [London], v. 136, p. 705–711, doi:10.1144/gsjgs.136.6.0705. Hossack, J.R., Garton, M.R., and Nickelsen, R.P., 1985, The geological section from the foreland up to the Jotun thrust sheet in the Valdres area, south Norway, in Gee, D.G., and Sturt, B.A., eds., The Caledonide Orogen—Scandinavia and Related Areas: Chichester, UK, Wiley and Sons, p. 443–456. Indrevær, G., and Steel, R.J., 1975, Some aspects of the sedimentary and structural history of the Ordovician and Devonian rocks of the westernmost Solund Islands, West Norway: Geological Survey of Norway (NGU) Bulletin, v. 317, p. 23–32. Iverson, W.P., and Smithson, S.B., 1982, Master décollement root zone beneath the southern Appalachians: Geology, v. 10, p. 241–245, doi:10.1130/0091 -7613(1982)10<241:MDRZBT>2.0.CO;2. Iwasaki, T., Sellevoll, M.A., Kanazawa, T., Veggeland, T., and Shimamura, H., 1994, Seismic refraction crustal study along Sognefjord, south-west Norway, employing ocean-bottom seismometers: Geophysical Journal International, v. 119, p. 791–808, doi:10.1111/j.1365-246X.1994.tb04018.x. Jamtveit, B., Bucher-Nurminen, K., and Austrheim, H., 1990, Fluid-controlled eclogitization of granulites in deep crustal shear zones, Bergen Arcs, western Norway: Contributions to Mineralogy and Petrology, v. 104, p. 184–193, doi:10.1007/BF00306442. Jamtveit, B., Austrheim, H., and Malthe-Sørensen, A., 2000, Accelerated hydration of the Earth’s deep crust induced by stress perturbations: Nature, v. 408, p. 75–78, doi:10.1038/35040537. Jolivet, L., Raimbourg, H., Labrousse, L., Avigad, D., Leroy, Y., Austrheim, H., and Andersen, T., 2005, Softening triggered by eclogitization, the first step toward exhumation during continental subduction: Earth and Planetary Science Letters, v. 237, p. 532–547, doi:10.1016/j.epsl.2005.06.047. Kinck, E.S., Husebye, F.R., and Larsson, F.R., 1993, The Moho depth distribution in Fennoscandia and the regional tectonic evolution from Archean to Permian times: Precambrian Research, v. 64, p. 23–51, doi:10.1016/0301 -9268(93)90067-C. Koestler, A.G., 1982, A Precambrian age for the Ofredal granodiorite intrusion, central Jotun Nappe, Sogn, Norway: Norsk Geologisk Tidsskrift, v. 62, p. 225–228. Koestler, A.G., 1983, Zentralkomplex und NW-Randzone der Jotundecke, West-Jotunheimen [Ph.D. thesis]: ETH Zürich, Switzerland, 225 p. Koestler, A.G., 1989, Hurrungane, berggrunnskart 1517 IV. Geological Survey of Norway (NGU) Bedrock Maps, scale 1:50,000. Korja, A., Korja, T., Luosto, U., and Heikkinen, P., 1993, Seismic and geoelectric evidence for collisional and extensional events in the Fennoscandian shield—Implications for Precambrian crustal evolution, in Green, A.G.,

References Cited Kröner, A., Götze J.-J., and Pavlenkova, N., eds., Plate tectonic signatures in the continental lithosphere: Tectonophysics, v. 219, p. 129–152. Koyi, H.A., Milnes, A.G., Schmeling, H., Talbot, C.J., Juhlin, C., and Zeyen, H., 1999, Numerical models of ductile rebound of crustal roots beneath mountain belts: Geophysical Journal International, v. 139, p. 556–562, doi:10.1046/j.1365-246x.1999.00978.x. Krabbendam, M., and Dewey, J.F., 1998, Exhumation of UHP rocks by transtension in the Western Gneiss Region, Scandinavian Caledonides: Geological Society [London] Special Publication 135, p. 159–181. Kruse, R.H., 1998, Recrystallization and deformation mechanisms in mafic high temperature mylonites from the Jotun nappe (S. Norway) and the Ivrea zone (N. Italy) [Ph.D. thesis]: University of Basel, Switzerland, 161 p. Kruse, R., and Stünitz, H., 1999, Deformation mechanisms and phase distribution in mafic high-temperature mylonites from the Jotun Nappe, southern Norway: Tectonophysics, v. 303, p. 223–249, doi:10.1016/S0040 -1951(98)00255-8. Kühn, A., Glodny, J., Iden, K., and Austrheim, H., 2000, Retention of Precambrian Rb/Sr phlogopite ages through Caledonian eclogite facies metamorphism, Bergen Arc Complex, W-Norway: Lithos, v. 51, p. 305–330, doi:10.1016/S0024-4937(99)00067-5. Kühn, A., Glodny, J., Austrheim, H., and Råheim, A., 2002, The Caledonian tectono-metamorphic evolution of the Lindås nappe: Constraints from U/Pb, Sm/Nd and Rb/Sr ages of granitoid dykes: Norsk Geologisk Tidsskrift, v. 82, p. 45–57. Larsen, Ø., Fossen, H., Langeland, K., and Pedersen, R.-B., 2003, Kinematics and timing of polyphase post-Caledonian deformation in the Bergen area, SW Norway: Norwegian Journal of Geology, v. 83, p. 149–165. Loeschke, J., and Nickelsen, R.P., 1968, On the age and tectonic position of the Valdres sparagmite in Slidre (southern Norway): Neues Jahrbuch für Geologie und Palaontologie, Abhandlungen, v. 131, p. 337–367. Lundmark, A.M., and Corfu, F., 2007, Age and origin of the Årdal dike complex, SW Norway: False isochrons, incomplete mixing, and the origin of Caledonian granites in basement nappes: Tectonics, v. 26, TC2007, doi:10.1029/2005TC001844. Lundmark, A.M., and Corfu, F., 2008a, Late-orogenic Sveconorwegian massive anorthosite in the Jotun Nappe Complex, SW Norway, and causes of repeated AMCG magmatism along the Baltoscandian margin: Contributions to Mineralogy and Petrology, v. 155, p. 147–163, doi:10.1007/ s00410-007-0233-5. Lundmark, A.M., and Corfu, F., 2008b, Emplacement of a Silurian granitic dyke swarm during nappe translation in the Scandinavian Caledonides: Journal of Structural Geology, v. 30, p. 918–928, doi:10.1016/j.jsg.2008.03.008. Lundmark, A.M., Corfu, F., Spürgin, S., and Selbekk, R.S., 2007, Proterozoic evolution and provenance of the high-grade Jotun Nappe Complex, SW Norway: U-Pb geochronology: Precambrian Research, v. 159, p. 133– 154, doi:10.1016/j.precamres.2006.12.015. Lutro, O., and Tveten, E., 1996, Geologisk kart over Norge, berggrunnskart Årdal: Geological Survey of Norway (NGU) Bedrock Maps, scale 1:250,000. Lutro, O., Tveten, E., and Malm, O.A., 1986, Lærdalsøyri, berggrunnskart 1417 II: Geological Survey of Norway (NGU), Preliminary Bedrock Maps, scale 1:50,000. Malavieille, J., 2010, Impact of erosion, sedimentation, and structural heritage on the structure and kinematics of orogenic wedges: Analog models and case studies: GSA Today, v. 20, p. 4–10, doi:10.1130/GSATG48A.1. Meidell, C., 1998, Den strukurelle utviklingen av Nordfjord-Sognskjærsonen og tilgrensende deler av liggblokken, sørvest for Lifjorden, ytre Sogn, Vest-Norge [unpublished thesis]: University of Bergen, Norway, 161 p. Milnes, A.G., 1978, Structural zones and continental collision, Central Alps: Tectonophysics, v. 47, p. 369–392, doi:10.1016/0040-1951(78)90039-2. Milnes, A.G., 1987, The Lower Allochthon in southern Norway: An exhumed analogue of the southern Appalachian deep detachment?, in Schaer, J.-P., and Rodgers, J., eds., The Anatomy of Mountain Ranges: Princeton, New Jersey, Princeton University Press, p. 59–64. Milnes, A.G., 1998, Alpine and Caledonide tectonics—A brief comparative study: GFF (Swedish Journal of Geology), v. 120, p. 237–247. Milnes, A.G., and Koestler, A.G., 1985, Geological structure of Jotunheimen, southern Norway (Sognefjell-Valdres cross-section), in Gee, D.G., and Sturt, B.A., eds., The Caledonide Orogen—Scandinavia and Related Areas: Chichester, UK, Wiley and Sons, p. 457–474. Milnes, A.G., and Koyi, H.A., 2000, Ductile rebound of an orogenic root: Case study and numerical model of gravity tectonics in the Western Gneiss

79

Complex, Caledonides, southern Norway: Terra Nova, v. 12, p. 1–7, doi: 10.1046/j.1365-3121.2000.00266.x. Milnes, A.G., and Pfiffner, O.A., 1977, Structural development of the Infrahelvetic complex, eastern Switzerland: Eclogae Geologicae Helvetiae, v. 70, p. 83–95. Milnes, A.G., and Wennberg, O.P., 1997, Tektonisk utvikling av Bergensområdet: Geonytt 1997, no. 1, p. 6–9. Milnes, A.G., Dietler, T.N., and Koestler, A.G., 1988, The Sognefjord north shore log—A 25 km depth section through Caledonized basement in western Norway: Geological Survey of Norway (NGU) Special Publication, v. 3, p. 114–121. Milnes, A.G., Wennberg, O.P., Skår, Ø., and Koestler, A.G., 1997, Contraction, extension and timing in the South Norwegian Caledonides: The Sognefjord transect, in Burg, J.-P., and Ford, M., eds., Orogeny through Time: Geological Society [London] Special Publication 121, p. 123–148. Nickelsen, P.R., Garton, M., and Hossack, J.R., 1985: Late Precambrian to Ordovician stratigraphy and correlation in the Valdres and Synnfjell thrust sheets of the Valdres area, southern Norway; with some comments on sedimentation, in Gee, D.G., and Sturt, B.A., eds., The Caledonide Orogen—Scandinavia and Related Areas: Chichester, UK, Wiley and Sons, p. 369–378. Norton, M.G., 1986, Late Caledonian extension in Western Norway: A response to extreme crustal thickening: Tectonics, v. 5, p. 195–204, doi:10.1029/ TC005i002p00195. Norton, M.G., McClay, K.R., and Way, N.A., 1987, Tectonic evolution of Devonian basins in northern Scotland and southern Norway: Norsk Geologisk Tidsskrift, v. 67, p. 323–338. Osmundsen, P.T., and Andersen, T.B., 1994, Caledonian compressional and late-orogenic extensional deformation in the Staveneset area, Sunnfjord, Western Norway: Journal of Structural Geology, v. 16, p. 1385–1401, doi:10.1016/0191-8141(94)90004-3. Osmundsen, P.T., and Andersen, T.B., 2001, The middle Devonian basins of western Norway: Sedimentary response to large-scale transtensional tectonics?: Tectonophysics, v. 332, p. 51–68, doi:10.1016/S0040-1951 (00)00249-3. Pedersen, R.B., Furnes, H., and Dunning, G., 1988, Some Norwegian ophiolite complexes reconsidered: Geological Survey of Norway (NGU) Special Publication, v. 3, p. 80–85. Pfiffner, O.A., 1986, Evolution of the north Alpine foreland basin in the Central Alps, in Allen, P.A., and Homewood, P., eds., Foreland Basins: International Association of Sedimentologists Special Publication 8, p. 219–228. Platt, J.P., 1993, Exhumation of high-pressure rocks—A review of concepts and processes: Terra Nova, v. 5, p. 119–133, doi:10.1111/j.1365-3121.1993 .tb00237.x. Ramberg, I.B., Bryhni, I., Nöttvedt, A., and Rangnes, K., eds., 2008, The Making of a Land. Geology of Norway: Trondheim, Norsk Geologisk Forening (Norwegian Geological Association), 622 p. Rice, A.H.N., 2005, Quantifying the exhumation of UHP-rocks in the Western Gneiss Region, S.W. Norway—A branch-line—Balanced cross-section model: Austrian Journal of Earth Sciences, v. 98, p. 2–21. Roberts, D., and Gee, D.G., 1985, An introduction to the structure of the Scandinavian Caledonides, in Gee, D.G., and Sturt, B.A., eds., The Caledonide Orogen—Scandinavia and Related Areas: Chichester, UK, Wiley and Sons, p. 55–68. Røhr, T.S., Corfu, F., Austrheim, H., and Andersen, T.B., 2004, Sveconorwegian U-Pb zircon and monazite ages of granulite-facies rocks, HisarøyaGulen, Western Gneiss Region, Norway: Norwegian Journal of Geology, v. 84, p. 251–256. Schärer, U., 1980a, U-Pb and Rb-Sr dating of a polymetamorphic nappe terrain: The Caledonian Jotun nappe, southern Norway: Earth and Planetary Science Letters, v. 49, p. 205–218, doi:10.1016/0012-821X(80)90065-5. Schärer, U., 1980b, Geologie und U–Pb/Rb–Sr Geochronologie der kaledonischen Jotundecke im Tyingebiet, Südnorwegen [Ph.D. thesis]: ETH Zürich, Switzerland,125 p. Séranne, M., and Séguret, M., 1987, The Devonian basins of western Norway: Tectonics and kinematics of an extending crust, in Coward, M.P., Dewey, J.F., and Hancock, P.L., eds., Continental Extensional Tectonics: Geological Society [London] Special Publication 28, p. 537–548. Skår, Ø., 1998, The Proterozoic and Early Paleozoic evolution of the southern parts of the Western Gneiss Complex, Norway [Ph.D. thesis]: University of Bergen, Norway, 158 p.

80

References Cited

Skår, Ø., and Pedersen, R.B., 2003, Relations between granitoid magmatism and migmatization; U-Pb geochronological evidence from the Western Gneiss Complex, Norway: Journal of the Geological Society [London], v. 160, p. 935–946, doi:10.1144/0016-764901-121. Skår, Ø., Furnes, H., and Claesson, S., 1994, Proterozoic orogenic magmatism within the Western Gneiss Region, Sunnfjord, Norway: Norsk Geologisk Tidsskrift, v. 74, p. 114–126. Skilbrei, J.R., 1990, Structure of the Jotun Nappe Complex, Southern Norwegian Caledonides, Ambiguity of Gravity Modelling, and Reinterpretation: Geological Survey of Norway (NGU) Report 89, 169 p. Skjerlie, K.P., Pedersen, R.B., Wennberg, O.P., and de la Rosa, J., 2000, Volatile-phase fluxed anatexis of metasediments during late Caledonian ophiolite obduction: Evidence from the Sogneskollen Granitic Complex, west Norway: Journal of the Geological Society [London], v. 157, p. 1199– 1213, doi:10.1144/jgs.157.6.1199. Smithson, S.B., Ramberg, I.B., and Gronlie, G., 1974, Gravity interpretation of the Jotun nappe of the Norwegian Caledonides: Tectonophysics, v. 22, p. 205–222, doi:10.1016/0040-1951(74)90082-1. Stephens, M.B., 1988, The Scandinavian Caledonides: A complexity of collisions: Geology Today, v. 4, p. 20–26, doi:10.1111/j.1365-2451.1988 .tb00537.x. Suppe, J., 1980, A retro-deformable cross-section of northern Taiwan: Geological Society of China, Proceedings, v. 23, p. 46–55. Suppe, J., 1985, Principles of Structural Geology: Englewood Cliffs, New Jersey, Prentice Hall, 537 p. Swensson, E., and Andersen, T.B., 1991, Contact relationships between the Askvoll group and the basement gneisses of the Western Gneiss Region (WGR), Sunnfjord, Western Norway: Norsk Geologisk Tidsskrift, v. 71, p. 15–27. Torsvik, T.H., Sturt, B.A., Ramsay, D.M., and Vetti, V., 1987, The tectonomagnetic signature of the Old Red Sandstone and pre-Devonian strata in the

Håsteinen area, Western Norway, and implications for the later stages of the Caledonian Orogeny: Tectonics, v. 6, p. 305–322. Torsvik, T.H., Sturt, B.A., Swensson, E., Andersen, T.B., and Dewey, J., 1992, Paleomagnetic dating of fault rocks: Evidence for Permian and Mesozoic movements and brittle deformation along the extensional Dalsfjord Fault, western Norway: Geophysical Journal International, v. 109, p. 565–580, doi:10.1111/j.1365-246X.1992.tb00118.x. Wanvik, J.E., 2000, Norwegian anorthosites and their industrial uses, with emphasis on the massifs of the Inner Sogn-Voss area in western Norway: Geological Survey of Norway (NGU) Bulletin, v. 436, p. 103–112. Wennberg, O.P., 1996, Superimposed fabrics due to reversed shear sense: An example from the Bergen Arc Shear Zone, western Norway: Journal of Structural Geology, v. 18, p. 871–889, doi:10.1016/0191-8141(96 )00014-4. Wennberg, O.P., 1998, The Bergen Arc Shear Zone—Kinematic analysis and implications for the tectonic evolution of the Bergen area [Ph.D. thesis]: University of Bergen, Norway, 186 p. Wennberg, O.P., Milnes, A.G., and Winswold, I., 1998, The northern Bergen Arc Shear Zone—An oblique-lateral ramp in the Devonian extensional detachment system of western Norway: Norsk Geologisk Tidsskrift, v. 78, p. 169–184. Wennberg, O.P., Skjerlie, K.P., and Dilek, Y., 2001, Field relationships and geochemistry of the Ostereide Dykes, Western Norway; implications for Caledonian tectonometamorphic evolution: Norsk Geologisk Tidsskrift, v. 81, p. 305–320. Wilks, W.J., and Cuthbert, S.J., 1994, The evolution of the Hornelen Basin detachment system, western Norway: Implications for the style of late orogenic extension in the southern Scandinavian Caledonides: Tectonophysics, v. 238, p. 1–30, doi:10.1016/0040-1951(94)90047-7. Winswold, I., 1996, Tektonisk utvikling av Byrknesøy (Vest-Norge)— Opphevningshistorie av eklogitter i sørvestlige del av Vestre Gneiskompleks [unpublished thesis]: University of Bergen, Norway, 131 p.

Contents Preface

Day 2. Inner Sognefjorden

Acknowledgments Abstract Chapter 1. An introductory outline I Autochthon-Parautochthon

2 Caledonian Allochthon 3 I ate- to Post-Collisional Extensional Shear Zones

Lrerdal-Gjende Fault Nord{jord-Sogn Detachment 4 Plate Tectoni c Cartoon

Chapter 2. The Sognefiord transect I Crustal Stru cture

2. Structural Synthesis

Eastern Segment of the Sogne{jord Transect Central Segment o(the Sogne{jord Transect Western Segment of the Sognefjord Transect 3 Retro-Deformation

Balancing Depth Control R.esJJiJs. 4. Eclogite Exhumation 5. An Orogenic Timeta ble

Chapter 3. The field trip itinerary Day I. Valdres-Jotunheimen

Stop Stop Stop Stop Stop Stop

1.1. S¢ndrol 1.2. Vangsmj¢si 1.3. @ye 1.4. Tyin Road Profile 1.5. Tyedalen 1.6. Lorteviki-Eidsbugarden

. . THE GEOLOGICAL SOCIETY • OF AMERICA®

3300 Penrose Place • P.O. Box 9140 Boulder, CO 80301 -9140, USA

Stop Stop Stop Stop Stop Stop

2.1. 2.2. 2.3. 2.4. 2.5. 2.6.

A.rdal Lrerdal Eide Sogndal Slinde Hermansverk

Day 3. Outer Sognefjorden

Stop 3.1. Stop 3.2. Stop 3.3. Stop 3.4. Stop 3.5. Stop 3.6. Stop 3.7.

Hella Srele Austrheim Kyrkjeb¢ Rasholm Helleb¢ Bekkeneset

Day 4 . So lund

Stop 4.1. Losna Stop 4.2. Hersvik Stop 4.3. Hyllestad Day 5. Askvoll-Atl¢y

Stop 5.1. Stop 5.2. Stop 5.3. Stop 5.4.

Vardalsneset Gjervik Kviteneset Brurestakken

Day 6 . Fensfjorden-Lindas

Stop Stop Stop Stop

6.1. 6.2. 6.3. 6.4.

Kjekallevagen Osterfiorden Holsn¢y Stalheim

References Cited

ISBN 978-0-81 37-0019-9