Continental drift

3.1 Introduction

3.2 Continental reconstructions

3.2.1 Euler's theorem

3.2.2 Geometric reconstructions of continents

3.2.3 The reconstruction of continents around the Atlantic

3.2.4 The reconstruction of Gondwana

3.3 Geologic evidence for continental drift

3.4 Paleoclimatology

30 30

46 48 51

55 55 55

3.5 Paleontologic evidence for continental drift 61

3.6 Paleomagnetism 64

3.6.1 Introduction 64

3.6.2 Rock magnetism 64

3.6.3 Natural remanent magnetization 65

3.6.4 The past and present geomagnetic field 66

3.6.5 Apparent polar wander curves 67

3.6.6 Paleogeographic reconstructions based on paleomagnetism 68

4 Sea floor spreading and transform faults 72

4.1 Sea floor spreading 73

4.1.1 Introduction 73

4.1.2 Marine magnetic anomalies 73

4.1.3 Geomagnetic reversals 74

4.1.4 Sea floor spreading 77

4.1.5 The Vine-Matthews hypothesis 78

4.1.6 Magnetostratigraphy 79

4.1.7 Dating the ocean floor 84

4.2 Transform faults 84

4.2.1 Introduction 84

4.2.2 Ridge-ridge transform faults 88

4.2.3 Ridge jumps and transform fault offsets 89

5 The framework of plate tectonics 91

5.1 Plates and plate margins 92

5.2 Distribution of earthquakes 92

5.3 Relative plate motions 94

5.4 Absolute plate motions 97

5.5 Hotspots 99

5.6 True polar wander 103

5.7 Cretaceous superplume 106

5.8 Direct measurement of relative plate motions 107

5.9 Finite plate motions 110

5.10 Stability of triple junctions 113

5.11 Present day triple junctions 120

6 Ocean ridges 121

6.1 Ocean ridge topography 122

6.2 Broad structure of the upper mantle below ridges 125

6.3 Origin of anomalous upper mantle beneath ridges 127

6.4 Depth-age relationship of oceanic lithosphere 128

6.5 Heat flow and hydrothermal circulation 129

6.6 Seismic evidence for an axial magma chamber 131

6.7 Along-axis segmentation of oceanic ridges 133

6.8 Petrology of ocean ridges 140

6.9 Shallow structure of the axial region 141

6.10 Origin of the oceanic crust 142

6.11 Propagating rifts and microplates 145

6.12 Oceanic fracture zones 148

7 Continental rifts and rifted margins 152

7.1 Introduction 153

7.2 General characteristics of narrow rifts 155

7.3 General characteristics of wide rifts 162

7.4 Volcanic activity 169

7.4.1 Large igneous provinces 169

7.4.2 Petrogenesis of rift rocks 172

7.4.3 Mantle upwelling beneath rifts 175

7.5 Rift initiation 176

7.6 Strain localization and delocalization processes 178

7.6.1 Introduction 178

7.6.2 Lithospheric stretching 179

7.6.3 Buoyancy forces and lower crustal flow 181

7.6.4 Lithospheric flexure 183

7.6.5 Strain-induced weakening 184

7.6.6 Rheological stratification of the lithosphere 188

7.6.7 Magma-assisted rifting 192

7.7 Rifted continental margins 193

7.7.1 Volcanic margins 193

7.7.2 Nonvolcanic margins 196

7.7.3 The evolution of rifted margins 198

7.8 Case studies: the transition from rift to rifted margin 202

7.8.1 The East African Rift system 202

7.8.2 The Woodlark Rift 204

7.9 The Wilson cycle 208

8 Continental transforms and strike-slip faults 210

8.1 Introduction 211

8.2 Fault styles and physiography 211

8.3 The deep structure of continental transforms 224

8.3.1 The Dead Sea

Transform 224

8.3.2 The San Andreas Fault 224

8.3.3 The Alpine Fault 228

8.4 Transform continental margins 230

8.5 Continuous versus discontinuous deformation 232

8.5.1 Introduction 232

8.5.2 Relative plate motions and surface velocity fields 233

8.5.3 Model sensitivities 236

8.6 Strain localization and delocalization mechanisms 239

8.6.1 Introduction 239

8.6.2 Lithospheric heterogeneity 239

8.6.3 Strain-softening feedbacks 242

8.7 Measuring the strength of transforms 246

9 Subduction zones 249

9.1 Ocean trenches 250

9.2 General morphology of island arc systems 251

9.3 Gravity anomalies of subduction zones 252

9.4 Structure of subduction zones from earthquakes 252

9.5 Thermal structure of the downgoing slab 259

9.6 Variations in subduction zone characteristics 262

9.7 Accretionary prisms 264

9.8 Volcanic and plutonic activity 271

9.9 Metamorphism at convergent margins 275

9.10 Backarc basins 279

10 Orogenic belts 286

10.1 Introduction 287

10.2 Ocean-continent convergence 287

10.2.1 Introduction 287

10.2.2 Seismicity, plate motions, and subduction geometry 289

10.2.3 General geology of the central and southern

Andes 291

10.2.4 Deep structure of the central Andes 294

10.2.5 Mechanisms of noncollisional orogenesis 297

10.3 Compressional sedimentary basins 302

10.3.1 Introduction 302

10.3.2 Foreland basins 302

10.3.3 Basin inversion 303

10.3.4 Modes of shortening in foreland fold-thrust belts 304

10.4 Continent-continent collision 306

10.4.1 Introduction 306

10.4.2 Relative plate motions and collisional history 306

10.4.3 Surface velocity fields and seismicity 309

10.4.4 General geology of the Himalaya and Tibetan

Plateau 312

10.4.5 Deep structure 316

10.4.6 Mechanisms of continental collision 318

10.5 Arc-continent collision 330

10.6 Terrane accretion and continental growth 332

10.6.1 Terrane analysis 332

10.6.2 Structure of accretionary orogens 336

10.6.3 Mechanisms of terrane accretion 342

11 Precambrian tectonics and the supercontinent cycle 346

11.1 Introduction 347

11.2 Precambrian heat flow 347

11.3 Archean tectonics 349

11.3.1 General characteristics of cratonic mantle lithosphere 349

11.3.2 General geology of

Archean cratons 350

11.3.3 The formation of Archean lithosphere 351

11.3.4 Crustal structure 355

11.3.5 Horizontal and vertical tectonics 358

11.4 Proterozoic tectonics 361

11.4.1 General geology of

Proterozoic crust 361

11.4.2 Continental growth and craton stabilization 363

11.4.3 Proterozoic plate tectonics 364

11.5 The supercontinent cycle 370

11.5.1 Introduction 370

11.5.2 Pre-Mesozoic reconstructions 370

11.5.3 A Late Proterozoic supercontinent 370

11.5.4 Earlier supercontinents 373

11.5.5 Gondwana-Pangea assembly and dispersal 374

12 The mechanism of plate tectonics 379

12.1 Introduction 380

12.2 Contracting Earth hypothesis 380

12.3 Expanding Earth hypothesis 380

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

12.4 Implications of heat flow 382

12.5 Convection in the mantle 384

12.5.1 The convection process 384

12.5.2 Feasibility of mantle convection 386

12.5.3 The vertical extent of convection 387

12.6 The forces acting on plates 388

12.7 Driving mechanism of plate tectonics 390

12.7.1 Mantle drag mechanism 391

12.7.2 Edge-force mechanism 391

12.8 Evidence for convection in the mantle 393

12.8.1 Introduction 393

12.8.2 Seismic tomography 393

12.8.3 Superswells 394

12.8.4 The D" layer 395

12.9 The nature of convection in the mantle 396

12.10 Plumes 399

12.11 The mechanism of the supercontinent cycle 401

13 Implications of plate tectonics 404

13.1 Environmental change 405

13.1.1 Changes in sea level and sea water chemistry 405

13.1.2 Changes in oceanic circulation and the Earth's climate 406

13.1.3 Land areas and climate 411

13.2 Economic geology 412

13.2.1 Introduction 412

13.2.2 Autochthonous and allochthonous mineral deposits 413

13.2.3 Deposits of sedimentary basins 420

13.2.4 Deposits related to climate 421

13.2.5 Geothermal power 422

13.3 Natural hazards 422

Review questions 424

References 428

Index 463

Color plates appear between pages 244 and 245

A companion resources website for this book is available at www.blackwellpublishing.com/kearey

As is well known, the study of tectonics, the branch of geology dealing with large-scale Earth structures and their deformation, experienced a major breakthrough in the 1960s with the formulation of plate tectonics. The simultaneous confirmation of sea floor spreading and continental drift, together with the recognition of transform faults and subduction zones, derived from the interpretation of new and improved data from the fields of marine geology and geophysics, and earthquake seismology. By 1970 the essentials of plate tectonics - the extent of plates, the nature of the plate boundaries, and the geometry and kinematics of their relative and finite motions - were well documented.

As further details emerged, it soon became apparent that plates and plate boundaries are well-defined in oceanic areas, where the plates are young, relatively thin, but rigid, and structurally rather uniform, but that this is not true for continental areas. Where plates have continental crust embedded in them they are generally thicker, older and structurally more complex than oceanic plates. Moreover the continental crust itself is relatively weak and deforms more readily by fracture and even by flow. Thus the nature of continental tectonics is more complex than a simple application of plate tectonic theory would predict and it has taken much longer to document and interpret. An important element in this has been the advent of Global Positioning data that have revealed details of the deformation field in complex areas.

The other major aspect of plate tectonics in which progress initially was slow is the driving mechanism for plate motions. Significant progress here had to await the development of new seismologic techniques and advances in laboratory and computer modeling of convection in the Earth's mantle.

Since 1990, when the first edition of Global Tectonics appeared, there have been many developments in our understanding of Earth structure and its formation, particularly in relation to continental tectonics and mantle convection. As a consequence, approximately two-thirds of the figures and two-thirds of the text in this third edition are new. The structure of the book is largely unchanged. The order in which data and ideas are presented is in part historical, which may be of some interest in itself, but it has the advantage of moving from simple to more complex concepts, from the recent to the distant past, and from the oceanic to the continental realms.

Thus one moves from consideration of the fundamentals of plate tectonics, which are best illustrated with reference to the ocean basins, to continental tectonics, culminating in Precambrian tectonics, and a discussion of the possible nature of the implied convection in the mantle.

The book is aimed at senior undergraduate students in the geological sciences and postgraduate students and other geoscientists who wish to gain an insight into the subject. We assume a basic knowledge of geology, and that for a full description of geophysical and geochemi-cal methodology it will be necessary to refer to other texts. We have attempted to provide insights into the trends of modern research and the problems still outstanding, and have supplied a comprehensive list of references so that the reader can follow up any item of particular interest. We have included a list of questions for the use of tutors in assessing the achievement of their students in courses based on the book. These are mainly designed to probe the students' integrative powers, but we hope that in their answers students will make use of the references given in the text and material on relevant websites listed on the book's website at: http://www.blackwellpublishing.com/kearey

The initial impact of the plate tectonic concept, in the fields of marine geology and geophysics and seismology, was quickly followed by the realization of its relevance to igneous and metamorphic petrology, paleontology, sedimentary and economic geology, and all branches of goescience. More recently its potential relevance to the Earth system as a whole has been recognized. In the past, processes associated with plate tectonics may have produced changes in seawater and atmospheric chemistry, in sea level and ocean currents, and in the Earth's climate. These ideas are briefly reviewed in an extended final chapter on the implications of plate tectonics. This extension of the relevance of plate tectonics to the atmosphere and oceans, to the evolution of life, and possibly even the origin of life on Earth is particularly gratifying in that it emphasizes the way in which the biosphere, atmosphere, hydrosphere, and solid Earth are interrelated in a single, dynamic Earth system.

A companion resources website for this book is available at http://www.blackwellpublishing.com/kearey

The first two editions of Global Tectonics were largely written by Phil Kearey. Tragically Phil died, suddenly, in 2003 at the age of 55, just after starting work on a third edition. We are indebted to his wife, Jane, for encouraging us to complete a third edition. Phil had a particular gift for writing succinct and accessible accounts of often difficult concepts, which generations of students have been thankful for. We are very conscious of the fact that our best efforts to emulate his style have often fallen short.

We thank Cynthia Ebinger, John Hopper, John Oldow, and Peter Cawood for providing thoughtful reviews of the original manuscript. Ian Bastow, José Cembrano, Ron Clowes, Barry Doolan, Mian Liu, Phil Hammer, and Brendan Meade provided helpful comments on specific aspects of some chapters. KAK wishes to thank Gabriela Mora-Klepeis for her excellent research assistance and Pam and Dave Miller for their support.

Era

Period

Epoch

*Ma

Pleistocene

23.03

65.5

Neogene

Pliocene

Late Early

Cenozoic

Middle

Early

Oligocene

Late Early

Paleogene

Middle

Early

Middle

Early

Cretaceous

Late

145.5 161.2

175.6 199.6 228.0 245.0

Early

Late

Mesozoic

Jurassic

Middle

Early

Late

Triassic

Middle

Early

Continued xii THE GEOLOGIC TIMESCALE AND STRATIGRAPHIC COLUMN

Period

Epoch

Period

Epoch

Late

Permian

Middle

Early

Carboniferous

Pennsylvanian

Mississippian

Late

Devonian

Middle

Paleozoic

Early

Silurian

Late

Early

Late

Ordovician

Middle

Early

Late

Cambrian

Middle

Early

(Eon)

(Era)

Late

Proterozoic

Middle

Precambrian

Early

Late

Archean

Middle

Early

Eoarchean

*Age, in millions of years (Ma), based on the timescale of Gradstein et al. (2004)

260.4

270.6

299.0

318.1

359.2

385.3

397.5 416.0 422.9

443.7 460.9

1000 1600 2500 2800 3200 3600

-4600

*Age, in millions of years (Ma), based on the timescale of Gradstein et al. (2004)

Historical perspective

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