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Home , Continental collision, Obduction, Suture (geology)

Collision THIS VOLUME IS DEDICATED TO THE WORK OF ROBERT M. SHACKLETON F.R.S. GEOLOGICAL SOCIETY SPECIAL PUBLICATION NO. 19

Collision Tectonics

EDITED BY M. P. COWARD & ALISON C. RIES* Department of , Imperial College, London, UK and * Resources Institute, University College of Swansea, Swansea, UK

1986

Published for the Geological Society by Blackwell Scientific Publications Oxford • London • Edinburgh • Boston Palo Alto • Melbourne Published by DISTRIBUTORS

Blackwell Scientific Publications USA and Canada Osney Mead, Oxford OX2 0EL Blackwell Scientific Publications Inc. 8 John Street, London WC1N 2ES PO Box 50009, Palo Alto 23 Ainslie Place, Edinburgh EH3 6AJ California 94303 52 Beacon Street, Boston, Massachusetts 02108, USA Australia 667 Lytton Avenue, Palo Alto, California 94301, USA Blackwell Scientific Publications 107 Barry Street, Carlton, Victoria 3053, Australia (Australia) Pty Ltd 107 Barry Street, Carlton, Victoria 3053 First published 1986 British Library Cataloguing in Publication Data

© 1986 The Geological Society. Authorization to Collision tectonics. -- (Geological Society special photocopy items for internal or personal use, or publication, ISSN 0305-8719; no. 19) the internal or personal use of specific clients, 1. is granted by The Geological Society for libraries I. Coward, M.P. II. Ries, Alison C. and other users registered with the Copyright III. Geological Society of London IV. Series Clearance Center (CCC) Transactional Reporting 551.1'36 QE511.4 Service, provided that a base fee of $02.00 per copy is paid directly to CCC, 27 Congress Street, Salem, MA 01970, USA. ISBN 0-632-01211-0 0305-8719/86 $02.00.

Printed in Great Britain by the Alden Press Ltd, Oxford Contents

Preface I: WATSON,J. V ...... vii Preface II: DEWEY, J. F ...... ix Introduction: COWARD, M. P. & RIES, A. C ...... xi

PROCESSES OF COLLISION

DEWEY, J. F., HEMPTON, M. R., KIDD, W. S. F., SAROGLU, F. & ~ENGt)R, A. M. C. Shortening of continental ': the neotectonics of Eastern Anatolia -- a young collision zone ...... 3 MATTAUER, M. Intracontinental , crust-mantle d6collement and crustal-stacking wedge in the and other collision belts ...... 37 KNIPE, R. J. & NEEDHAM, D. T. Deformation processes in accretionary wedges -- examples from the SW margin of the Southern Uplands, Scotland ...... 51 HARRIS, N. B. W., PEARCE, J. A. & TINDLE, A. G. Geochemical characteristics of collision-zone magmatism ...... 67 ENGLAND, P. C. & THOMPSON, A. Some thermal and tectonic models for crustal melting in zones ...... 83 MURRELL, S. A. F. Mechanics of tectogenesis in plate collision zones ...... 95

CONTINENT- COLLISION

Himalayan-Alpine Belt TAPPONNIER, P., PELTZER, G. & ARMIJO, R. On the mechanics of the collision between and ...... 115 LE FORT, P. and magmatism during the Himalayan collision ...... 159 COLCHEN, M., MASCLE, G. & VAN HAVER, T. Some aspects of collision tectonics in the Indus Zone, Ladakh ...... 173 SEARLE, M. P. & FRYER, B. J. , tourmaline and muscovite-bearing leucogranites, gneisses and migmatites of the Higher Himalayas from Zanskar, Kulu, Lahoul and Kashmir ...... 185 COWARD, M. P., WINDLEY, B. F., BROUGHTON, R. D., LUFF, I. W., PETTERSON, M. G., PUDSEY, C. J., REX, D. C. & ASlF KHAN, M. Collision tectonics in the NW Himalayas ...... 203 HUNZII,:ER, J. C. The : a case of multiple collision ...... 221 RIcov, L. E. & SIDDANS, A. W. B. Collision tectonics in the Western Alps ...... 229 BUTLER, R. W. H., MA~HEWS, S. J. & PARISH, M. The NW external Alpine Thrust Belt and its implications for the geometry of the Western Alpine Orogen ...... 245 CHANNELL, J. E. T. Palaeomagnetism and continental collision in the Alpine Belt and the formation of late-tectonic extensional basins ...... 261

Older collision belts HOSSACK, J. R. & COOPER, M. A. Collision tectonics in the ... 287 HAWKESWORTH, C. J., MENZIES, M. A. & VAN CALSTEREN, P. Geochemical and tectonic evolution of the Damara Belt, Namibia ...... : ...... 305 DALY, M. C. The intracratonic Irumide Belt of Zambia and its bearing on collision orogeny during the Proterozoic of Africa ...... 321 SHACKLETON, R. M. Precambrian collision tectonics in Africa ...... 329

CONTINENT- ARC COLLISION

MILSOM, J. & AUDLEY-Cr~ARLES, M. G. Post-collision isostatic readjustment in the Southern Banda Arc ...... 353 vi Contents

CORDILLERAN COLLISION

JONES, D. L., SILBERLING,N. J. & CONEY, P. J. Collision tectonics in the Cordillera of western N America: examples from Alaska ...... 367 DALZIEL, I. W. D. Collision and Cordilleran orogenesis: an Andean perspective ...... 389

INDEX ...... 405 Preface I

The orogenic belts that result from the collision of crustal plates form a worldwide system whose origins go back for several hundred million years. The spectacular topographical forms of these belts were familiar to man long before the emergence of geology as an independent science. The geologists of the eighteenth and early nineteenth century felt no doubts about the need to examine fundamental structures such as belts on a global scale. More than a quarter of the 180 papers published in the first three volumes of the Quarterly Journal of the Geological Society of London (1845-7) were concerned with overseas ; and the same volumes included a substantial paper by R. I. Murchison on the relationships of the Lower Palaeozoic of Scandinavia and the Baltic area and another, even more comprehensive, on the structure of the Alps, Apennines and Carpathians. The relevance of direct observation in the field was equally well understood; in volume 3 of the Quarterly Journal, Daniel Sharpe demonstrated the implications of rock with respect to the effects of compression and Henry de la Beche included in his Presidential Address an interesting analysis of Appalachian folds as indicators of crustal shortening. At the present day, the range of phenomena to be accounted for in the development of collisional orogenic belts and the diversity of scales on which they must be examined, continue to challenge the ingenuity of earth scientists. The massive build-up of factual data and the advent of many specialized techniques make it increasingly hard for an individual geologist to master any major topic as a whole and discussion meetings involving a number of contributors have, to a large extent, replaced the single authoritative review as a means of arriving at a general picture. The William Smith Lectures of the Geological Society, established in 1944 to provide annual occasions for reviews of topics of current interest, have accordingly evolved into William Smith Meetings built around the principal lecture but providing ample scope for contributions from other specialists. The William Smith Meeting of April 1983, of which the present volume is the outcome, touched on many aspects of the collision process, clarified many new fields of investigation and highlighted some of the problems that remain to be solved. Many of those who took part are former students or colleagues of Professor R. M. Shackleton, the William Smith Lecturer, whose address formed the culmination of the meeting. This special volume therefore stands not only as a record of the progress of research in an important branch of earth science but also as a personal tribute to an outstanding field geologist, whose experience and acumen have become legendary.

JANET WATSON t (President 1982-4)

* Deceased 1985. Preface II

Robert Shackleton stands almost alone in the earth sciences as a field observer of unrivalled ability. He has a most acute geometric sense and a talent for quickly unravelling complex structures and reducing them to their fundamental essentials. These abilities, coupled with more than half a century of field experience in, among other places, Africa, Arabia, the British Caledonides, S America and Fiji, make Robert a most perceptive and formidable field-based generalist of the kind that is tragically becoming a rare species in modern earth science. Field observation has been central to his life's work and is mostly encapsulated in a set of field notebooks numbering some 150. A man of enormous energy and fitness, his interests have centred principally around and tectonics at all scales. Robert's career began in Liverpool where he gained his Ph.D in 1932 on the geology of Moel Hebog, at which time began his lifetime love of North Wales geology. Between 1932 and 1934 he was Beit Research Fellow at Imperial College and also worked for Whitehall Securities in Fiji. From 1934 to 1940 and from 1945 to 1948, Robert lectured at Imperial College and began researches on Palaeozoic rocks in Ireland, most notably those of the Dingle Peninsula. From 1940 to 1945, he was a geologist with the Geological Survey of Kenya, from which sprang a series of important papers on the Migori Gold Belt and the Kavirondo . While mapping in Kenya, his indestructability was shown by an unsuccessfully attempted vault across a charging rhino. In 1948 Robert was appointed to the Chair of Geology in Liverpool, from which base the 1950s and early 1960s saw the development and fruition of seminal work on the Dalradian rocks of the Southern Grampians, Connemara and Donegal, and the Precambrian rocks of Anglesey. During this period, two of his finest classic papers were published: 'The structural evolution of North Wales' and 'Downward facing structures of the Highland Border'. I first met Robert in the early 1960s when Liverpool and North Wales were attractive weekend spots for a young lecturer in Manchester. Many happy hours were also spent during the 1960s with Robert in Connemara, where we both had undergraduate mapping parties from Liverpool, Manchester and Cambridge. The evening venue was the infamous Dooneen Lodge in Letterfrack where astonishing cocktails were consumed, wall traverses were demonstrated, and where Robert again demonstrated his indestructability by taking a nocturnal dive from the pier: bad news -- tide was out; good news -- a thick spongy layer of wrack. In 1963 Robert moved to Leeds to join Professor W. Q. Kennedy who, with funding from Anglo American Corporation, had established a Research Institute of African Geology in Leeds. Robert's lifelong interest in African geology had begun through being persuaded by Boswell to go, while still a student, to Southern Africa for 2 months, to attend the International Geological Congress. On Kennedy's retirement, Robert was appointed Director of the Institute from 1965 to 1976. The aim was no less than to study the evolution of the African continent. This provided an unrivalled opportunity to organize research in any part of Africa. Robert mainly worked on Precambrian tectonics, in East, Central and West Africa; this work forms much of the basis of his William Smith Lecture and Paper. In his latter years at Leeds, Robert developed an interest in the relationship between orogenic fabric and plate slip vectors in the Hercynian chains.

ix x Preface

In 1976, his retirement from the Leeds Chair was a minor event in the increasing and continuous flow of his work, when he moved to a Senior Research Fellowship at the Open University. From this base, he has continued his African Precambrian interests by work in the Pan African terranes of NE Africa and the . His most recent researches have been on the structural history and emplacement mechanisms of the Oman . Robert Shackleton's special characteristic is his incisive ability to cast away peripheral nonsense and cut right through to the core strengths and weaknesses of a geological argument, witnessed by the fact that his publication list consists of a modest number of papers of gem quality, and by his sparkling contributions to discussions in the old 'parliamentary days' of the Geological Society of London. If Robert had written down everything he knows we would all be reading for a very long time, but he is a man who would rather be in the field finding things out. His place in the science has been recognized by the Silver Medal of the Liverpool Geological Society (1957), the Geological Society of London's Murchison Medal (1970) and his election to Fellowship of the Royal Society (1971). Robert Shackleton has exerted a most profound influence on several generations of colleagues and students. He not only has the finest legs in the business, but is a totally unpretentious man with a deep love of, and dedication to, geology. There is no one quite like Robert and he holds a special place in the affection of his many friends. I, like many others, have enjoyed his companionship and knowledge in the field and we are fortunate to have been counted among his friends.

JOHN F. DEWEY, Department of Earth Sciences, University of Oxford, Oxford OX1 3PR, UK Introduction

The theme of this volume was chosen by Professor Robert Shackleton; it reports papers given at a 2-day William Smith Conference on Collision Tectonics, organized by the Geological Society of London in April 1983. Robert Shackleton gave the William Smith Lecture and chose the main accompanying papers. They reflect his major interests (and achievements) over recent years, particularly in the realms of observational structural and metamorphic geology and the use of detailed field and laboratory measurements in understanding regional tectonics. The aim of the meeting and this subsequent volume was to discuss the geometries, kinematics and mechanics of plate collision, in particular the distribution, sequence and timing of deformation, magmatic and metamorphic processes within a collision zone. Most speakers chose to discuss these problems round a regional framework: the Alps, Himalayas or Namibia, but to summarize the main questions and themes (asked and sometimes partially answered) we would list the following: (a) What deformational and thermal processes operate at the base of the crust during subduction, prior to collision and how can these early processes be distinguished in collisional belts? (b) What is the sequence of deformation during continent-continent or continent- arc collision, how much occurs and is the main deformation in the footwall or hangingwall of a suture? (c) What causes large slabs of ocean floor to obduct over continental margins; is it an integral part of continent-continent collision or does it develop from some other process? (d) During continent-continent obduction what controls the deformation rate, how is the deformation partitioned into overthrusting, pure or strike-slip events and how does it relate to rates of plate movement? (e) What are the main mechanisms involved in crustal thickening and what are their implications for the subsequent magmatic, metamorphic and uplift/cooling history? (f) What geophysical, geochemical, structural, stratigraphic or petrological tech- niques should be used to determine the geometry of a suture and its origin?

It will be obvious from the papers in this volume that there is no one simple global answer to these questions, or if there is, it is as yet unknown. Each collision belt has its own character. Sometimes the plates just simply lock together with little deformation, examples being the Caledonian Suture of the Southern Uplands of Scotland or the N Kohistan Suture in Pakistan. In other examples, such as the Alps or Himalayas, the plates override for considerable distances, smearing out earlier structures in the under- lying plate and stacking up flakes of crust. Several hundred kilometres of shortening has been achieved by this overthrust process. Elsewhere, as in Namibia and probably the Kun Lun, it is the overthrust plate which thickens. In the Caledonides and Oman- Himalayan belt, ophiolite obduction long precedes continent-continent collision and therefore must not be considered as an integral part of the collision process. Sometimes a continent-continent collision is oblique, and one crustal block will appear to skid along another, with movement along a major strike-slip . In other examples one continent will indent another, extruding thickened crust from the indented block. This

xi xii Introduction extrusion will often develop along strike-slip faults; these structures should not be considered as characteristic only of oblique collision zones. The thickened crust will tend to spread under gravity. The spreading direction need not be parallel to the regional plate movement vector and hence will give rise to folds and thrusts whose orientations and deformation rates will appear unrelated to regional movements. Which block is affected by deformation and metamorphism depends on a number of factors such as the age of the crust, its thermal regime, any anisotropy in the crust and the nature of the subcrustal lithosphere. Old lithosphere may have greater strength than its younger counterpart; certainly the presence of a subduction zone beneath could add new, hotter and hence weaker material. The presence of anisotropy is important whether at the base of the crust or spread throughout the crust by the development of earlier major shear zones. Many continental margins were thinned prior to collision and could show numerous faults or shears related to this extension. The thrusts related to collision may reactivate these earlier structures. The papers in this volume have been grouped into those which principally discuss processes and mechanisms and those which build the discussion round regional examples. Special emphasis has been put on the Mediterranean-Himalayan section. Here the northward transport of India towards and eventually into the Asian Plate is well constrained by sea-floor magnetic reversal studies and palaeomagnetic work onshore. Here also an extensive southward-directed thrust system, with major detachments, has long been recognized where there has also be'en considerable crustal thickening behind the overthrust suture zone with associated lateral spreading and/or expulsion of material. It was in the Alpine segment that major features of thrust and tectonics were first recognized, but as will be seen from this volume, there is still considerable controversy over Alpine thrust geometry and its relation to regional plate movements and Alpine collision. The Pacific margin shows excellent examples of continent-arc collision and in the western USA there has been the recent realization that the tectonics involve the accretion of numerous blocks, or 'allocthonous terranes'. These blocks originated as seamounts, arcs or small continental masses, somewhere in what is now the Pacific, then skidded by or plastered themselves into N America, many of them ending their movement in the 'graveyard' of Alaska. In many of these modern belts, plate geometry and movement can be identified from seismic and palaeomagnetic data and often the suture zone is clearly visible and available for structural and petrological study. In the ancient belts, however, there are often no such constraints. It is important therefore to know what criteria can be used to recognize the results of continental collision and how to analyse these. The final papers in this volume attempt to apply these criteria to some older belts: the Caledonides and the complex reticulate pattern of shear zones and thrusts of Africa. It is the recognition of collision processes in the Early and Late Precambrian orogenic belts of Africa, which makes the theme of Robert Shackleton's paper. It has been difficult in restraining many of the authors in their enthusiasm for the subject! Robert himself is still active in research on this topic and if readers think there has been any delay in the publication of this volume it is because subsequent to the meeting, one of the editors and many of the authors were involved in collision tectonic research instigated by Robert, culminating in a Royal Society-Chinese Academy traverse of Tibet in 1985.

M.P. COWARD • A.C. RIES