Global Tectonics iii Global Tectonic s T H E L A T E P H I L I P K E A R E Y Formerly of the Department of Geology University of Bristol UK K E I T H A . K L E P E I S Department of Geology University of Vermont Burlington, Vermont, USA F R E D E R I C K J . V I N E School of Environmental Sciences University of East Anglia Norwich, UK T H I R D E D I T I O N A John Wiley & Sons, Ltd., Publication This edition fi rst published 2009, © 2009 by Philip Kearey, Keith A. Klepeis, Frederick J. Vine Blackwell Publishing was acquired by John Wiley & Sons in February 2007. Blackwell’s publishing program has been merged with Wiley’s global Scientifi c, Technical and Medical business to form Wiley-Blackwell. 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Includes bibliographical references and index. ISBN 978-1-4051-0777-8 (pbk. : alk. paper) 1. Plate tectonics–Textbooks. I. Klepeis, Keith A. II. Vine, F. J. III. Title. QE511.4.K43 2008 551.1'36–dc22 2007020963 A catalogue record for this book is available from the British Library. Set in 9.5 on 11.5 pt Dante by SNP Best-set Typesetters Ltd., Hong Kong Printed and bound in Singapore by Markono Print Media Pte Ltd 01 2009 Contents Preface ix 2.8 The mantle 30 Acknowledgments x 2.8.1 Introduction 30 The geologic timescale and stratigraphic column xi 2.8.2 Seismic structure of the mantle 30 2.8.3 Mantle composition 31 2.8.4 The mantle low velocity 1 Historical perspective 1 zone 31 2.8.5 The mantle transition zone 32 1.1 Continental drift 2 2.8.6 The lower mantle 32 1.2 Sea fl oor spreading and the birth 2.9 The core 33 of plate tectonics 6 2.10 Rheology of the crust 1.3 Geosynclinal theory 7 and mantle 33 1.4 Impact of plate tectonics 8 2.10.1 Introduction 33 2.10.2 Brittle deformation 34 2.10.3 Ductile deformation 36 2 The interior of 2.10.4 Lithospheric strength profi les 37 2.10.5 Measuring continental the Earth deformation 39 9 2.10.6 Deformation in the mantle 41 2.1 Earthquake seismology 10 2.11 Isostasy 42 2.1.1 Introduction 10 2.11.1 Introduction 42 2.1.2 Earthquake descriptors 10 2.11.2 Airy’s hypothesis 43 2.1.3 Seismic waves 10 2.11.3 Pratt’s hypothesis 43 2.1.4 Earthquake location 11 2.11.4 Flexure of the lithosphere 44 2.1.5 Mechanism of earthquakes 12 2.11.5 Isostatic rebound 45 2.1.6 Focal mechanism solutions 2.11.6 Tests of isostasy 46 of earthquakes 12 2.12 Lithosphere and asthenosphere 48 2.1.7 Ambiguity in focal 2.13 Terrestrial heat fl ow 51 mechanism solutions 14 2.1.8 Seismic tomography 17 2.2 Velocity structure of the Earth 19 2.3 Composition of the Earth 21 3 Continental drift 54 2.4 The crust 22 2.4.1 The continental crust 22 3.1 Introduction 55 2.4.2 Upper continental crust 23 3.2 Continental reconstructions 55 2.4.3 Middle and lower continental 3.2.1 Euler’s theorem 55 crust 23 3.2.2 Geometric reconstructions 2.4.4 The oceanic crust 24 of continents 55 2.4.5 Oceanic layer 1 24 3.2.3 The reconstruction of 2.4.6 Oceanic layer 2 25 continents around the 2.4.7 Oceanic layer 3 26 Atlantic 56 2.5 Ophiolites 27 3.2.4 The reconstruction of 2.6 Metamorphism of oceanic Gondwana 57 crust 28 3.3 Geologic evidence for 2.7 Differences between continental and continental drift 58 oceanic crust 29 3.4 Paleoclimatology 60 v vi CONTENTS 3.5 Paleontologic evidence for 6 Ocean ridges 121 continental drift 61 3.6 Paleomagnetism 64 6.1 Ocean ridge topography 122 3.6.1 Introduction 64 6.2 Broad structure of the upper 3.6.2 Rock magnetism 64 mantle below ridges 125 3.6.3 Natural remanent 6.3 Origin of anomalous upper mantle magnetization 65 beneath ridges 127 3.6.4 The past and present 6.4 Depth–age relationship of geomagnetic fi eld 66 oceanic lithosphere 128 3.6.5 Apparent polar wander 6.5 Heat fl ow and hydrothermal curves 67 circulation 129 3.6.6 Paleogeographic reconstructions 6.6 Seismic evidence for an axial based on paleomagnetism 68 magma chamber 131 6.7 Along-axis segmentation of 4 Sea fl oor spreading and oceanic ridges 133 6.8 Petrology of ocean ridges 140 transform faults 6.9 Shallow structure of the 72 axial region 141 4.1 Sea fl oor spreading 73 6.10 Origin of the oceanic crust 142 4.1.1 Introduction 73 6.11 Propagating rifts and 4.1.2 Marine magnetic microplates 145 anomalies 73 6.12 Oceanic fracture zones 148 4.1.3 Geomagnetic reversals 74 4.1.4 Sea fl oor spreading 77 4.1.5 The Vine–Matthews hypothesis 78 7 Continental rifts and 4.1.6 Magnetostratigraphy 79 4.1.7 Dating the ocean fl oor 84 rifted margins 4.2 Transform faults 84 152 4.2.1 Introduction 84 7.1 Introduction 153 4.2.2 Ridge–ridge transform 7.2 General characteristics of faults 88 narrow rifts 155 4.2.3 Ridge jumps and transform 7.3 General characteristics of fault offsets 89 wide rifts 162 7.4 Volcanic activity 169 7.4.1 Large igneous provinces 169 5 The framework of 7.4.2 Petrogenesis of plate tectonics rift rocks 172 91 7.4.3 Mantle upwelling 5.1 Plates and plate margins 92 beneath rifts 175 5.2 Distribution of earthquakes 92 7.5 Rift initiation 176 5.3 Relative plate motions 94 7.6 Strain localization and 5.4 Absolute plate motions 97 delocalization processes 178 7.6.1 Introduction 178 5.5 Hotspots 99 7.6.2 Lithospheric stretching 179 5.6 True polar wander 103 7.6.3 Buoyancy forces and lower 5.7 Cretaceous superplume 106 crustal fl ow 181 5.8 Direct measurement of relative 7.6.4 Lithospheric fl exure 183 plate motions 107 7.6.5 Strain-induced weakening 184 5.9 Finite plate motions 110 7.6.6 Rheological stratifi cation 5.10 Stability of triple junctions 113 of the lithosphere 188 5.11 Present day triple junctions 120 7.6.7 Magma-assisted rifting 192 CONTENTS vii 7.7 Rifted continental margins 193 9.5 Thermal structure of the 7.7.1 Volcanic margins 193 downgoing slab 259 7.7.2 Nonvolcanic margins 196 9.6 Variations in subduction zone 7.7.3 The evolution of rifted characteristics 262 margins 198 9.7 Accretionary prisms 264 7.8 Case studies: the transition from 9.8 Volcanic and plutonic activity 271 rift to rifted margin 202 9.9 Metamorphism at convergent 7.8.1 The East African Rift margins 275 system 202 9.10 Backarc basins 279 7.8.2 The Woodlark Rift 204 7.9 The Wilson cycle 208 10 Orogenic belts 286 8 Continental transforms 10.1 Introduction 287 and strike-slip faults 210 10.2 Ocean–continent convergence 287 10.2.1 Introduction 287 8.1 Introduction 211 10.2.2 Seismicity, plate motions, and 8.2 Fault styles and physiography 211 subduction geometry 289 8.3 The deep structure of 10.2.3 General geology of continental transforms 224 the central and southern 8.3.1 The Dead Sea Andes 291 Transform 224 10.2.4 Deep structure of the 8.3.2 The San Andreas Fault 224 central Andes 294 8.3.3 The Alpine Fault 228 10.2.5 Mechanisms of noncollisional 8.4 Transform continental margins 230 orogenesis 297 8.5 Continuous versus discontinuous 10.3 Compressional sedimentary deformation 232 basins 302 8.5.1 Introduction 232 10.3.1 Introduction 302 8.5.2 Relative plate motions and 10.3.2 Foreland basins 302 surface velocity fi elds 233 10.3.3 Basin inversion 303 8.5.3 Model sensitivities 236 10.3.4 Modes of shortening in foreland 8.6 Strain localization and fold-thrust belts 304 delocalization mechanisms 239 10.4 Continent–continent collision 306 10.4.1 Introduction 306 8.6.1 Introduction 239 10.4.2 Relative plate motions and 8.6.2 Lithospheric heterogeneity 239 collisional history 306 8.6.3 Strain-softening 10.4.3 Surface velocity fi elds feedbacks 242 and seismicity 309 8.7 Measuring the strength of 10.4.4 General geology of the transforms 246 Himalaya and Tibetan Plateau 312 10.4.5 Deep structure 316 10.4.6 Mechanisms of continental 9 Subduction zones 249 collision 318 10.5 Arc–continent collision 330 9.1 Ocean trenches 250 10.6 Terrane accretion and 9.2 General morphology of island continental growth 332 arc systems 251 10.6.1 Terrane analysis 332 9.3 Gravity anomalies of subduction 10.6.2 Structure of accretionary zones 252 orogens 336 9.4 Structure of subduction zones 10.6.3 Mechanisms of terrane from earthquakes 252 accretion 342 viii CONTENTS 11 Precambrian tectonics 12.5.3 T he vertical extent of convection 387 and the supercontinent 12.6 The forces acting on plates 388 cycle 12.7 D riving mechanism of 346 plate tectonics 390 11.1 Introduction 347 12.7.1 Mantle drag mechanism 391 11.2 Precambrian heat fl ow 347 12.7.2 Edge-force mechanism 391 12.8 E vidence for convection in 11.3 Archean tectonics 349 11.3.1 General characteristics of the mantle 393 cratonic mantle lithosphere 349 12.8.1 Introduction 393 11.3.2 General geology of 12.8.2 Seismic tomography 393 Archean cratons 350 12.8.3 Superswells 394 11.3.3 The formation of Archean 12.8.4 The D” layer 395 lithosphere 351 12.9 T he nature of convection in 11.3.4 Crustal structure 355 the mantle 396 11.3.5 Horizontal and vertical tectonics 358 12.10 Plumes 399 11.4 Proterozoic tectonics 361 12.11 T he mechanism of the 11.4.1 General geology of supercontinent cycle 401 Proterozoic crust 361 11.4.2 Continental growth and craton stabilization 363 11.4.3 Proterozoic plate tectonics 364 13 Implications of 11.5 The supercontinent cycle 370 plate tectonics 11.5.1 Introduction 370 404 11.5.2 Pre-Mesozoic reconstructions 370 13.1 Environmental change 405 11.5.3 A Late Proterozoic 13.1.1 C hanges in sea level and sea supercontinent 370 water chemistry 405 11.5.4 Earlier supercontinents 373 13.1.2 C hanges in oceanic circulation 11.5.5 Gondwana–Pangea assembly and the Earth’s climate 406 and dispersal 374 13.1.3 Land areas and climate 411 13.2 Economic geology 412 13.2.1 Introduction 412 13.2.2 A utochthonous and 12 The mechanism of allochthonous mineral plate tectonics deposits 413 379 13.2.3 D eposits of sedimentary 12.1 Introduction 380 basins 420 13.2.4 Deposits related to climate 421 12.2 Contracting Earth hypothesis 380 13.2.5 Geothermal power 422 12.3 Expanding Earth hypothesis 380 13.3 Natural hazards 422 12.3.1 Calculation of the ancient moment of inertia of the Earth 381 12.3.2 Calculation of the ancient radius of the Earth 382 Review questions 424 12.4 Implications of heat fl ow 382 References 428 12.5 Convection in the mantle 384 Index 463 12.5.1 The convection process 384 12.5.2 F easibility of mantle convection 386 Color plates appear between pages 244 and 245 A companion resources website for this book is available at www.blackwellpublishing.com/kearey The mechanism of plate tectonics ix Preface As is well known, the study of tectonics, the branch Thus one moves from consideration of the fundamentals of geology dealing with large-scale Earth structures of plate tectonics, which are best illustrated with refer- and their deformation, experienced a major break- ence to the ocean basins, to continental tectonics, culmi- through in the 1960s with the formulation of plate nating in Precambrian tectonics, and a discussion of the tectonics. The simultaneous confi rmation of sea fl oor possible nature of the implied convection in the mantle. spreading and continental drift, together with the recog- The book is aimed at senior undergraduate students nition of transform faults and subduction zones, derived in the geological sciences and postgraduate students and from the interpretation of new and improved data from other geoscientists who wish to gain an insight into the the fi elds of marine geology and geophysics, and subject. We assume a basic knowledge of geology, and earthquake seismology. By 1970 the essentials of plate that for a full description of geophysical and geochemi- tectonics – the extent of plates, the nature of the plate cal methodology it will be necessary to refer to other boundaries, and the geometry and kinematics of their texts. We have attempted to provide insights into the relative and fi nite motions – were well documented. trends of modern research and the problems still out- As further details emerged, it soon became apparent standing, and have supplied a comprehensive list of ref- that plates and plate boundaries are well-defi ned in erences so that the reader can follow up any item of oceanic areas, where the plates are young, relatively thin, particular interest. We have included a list of questions but rigid, and structurally rather uniform, but that this is for the use of tutors in assessing the achievement of not true for continental areas. Where plates have conti- their students in courses based on the book. These are nental crust embedded in them they are generally thicker, mainly designed to probe the students’ integrative older and structurally more complex than oceanic plates. powers, but we hope that in their answers students will Moreover the continental crust itself is relatively weak make use of the references given in the text and material and deforms more readily by fracture and even by fl ow. on relevant websites listed on the book’s website at: Thus the nature of continental tectonics is more complex http://www.blackwellpublishing.com/kearey than a simple application of plate tectonic theory would The initial impact of the plate tectonic concept, in the predict and it has taken much longer to document and fi elds of marine geology and geophysics and seismology, interpret. An important element in this has been the was quickly followed by the realization of its relevance advent of Global Positioning data that have revealed to igneous and metamorphic petrology, paleontology, details of the deformation fi eld in complex areas. sedimentary and economic geology, and all branches of The other major aspect of plate tectonics in which goescience. More recently its potential relevance to the progress initially was slow is the driving mechanism Earth system as a whole has been recognized. In the past, for plate motions. Signifi cant progress here had to await processes associated with plate tectonics may have pro- the development of new seismologic techniques and duced changes in seawater and atmospheric chemistry, advances in laboratory and computer modeling of con- in sea level and ocean currents, and in the Earth’s climate. vection in the Earth’s mantle. These ideas are briefl y reviewed in an extended fi nal Since 1990, when the fi rst edition of Global Tectonics chapter on the implications of plate tectonics. This appeared, there have been many developments in our extension of the relevance of plate tectonics to the understanding of Earth structure and its formation, par- atmosphere and oceans, to the evolution of life, and pos- ticularly in relation to continental tectonics and mantle sibly even the origin of life on Earth is particularly grat- convection. As a consequence, approximately two-thirds ifying in that it emphasizes the way in which the of the fi gures and two-thirds of the text in this third biosphere, atmosphere, hydrosphere, and solid Earth are edition are new. The structure of the book is largely interrelated in a single, dynamic Earth system. unchanged. The order in which data and ideas are pre- K.A. KLEPEIS sented is in part historical, which may be of some interest F.J. VINE in itself, but it has the advantage of moving from simple to more complex concepts, from the recent to the distant A companion resources website for this book is avail- past, and from the oceanic to the continental realms. able at http://www.blackwellpublishing.com/kearey ix