Global Tectonics, 3rd Edition

Preface ix

Acknowledgments x

The geologic timescale and stratigraphic column xi

1 Historical perspective 1

1.1 Continental drift 2

1.2 Sea floor spreading and the birth of plate tectonics 6

1.3 Geosynclinal theory 7

1.4 Impact of plate tectonics 8

2 The interior of the Earth 9

2.1 Earthquake seismology 10

2.1.1 Introduction 10

2.1.2 Earthquake descriptors 10

2.1.3 Seismic waves 10

2.1.4 Earthquake location 11

2.1.5 Mechanism of earthquakes 12

2.1.6 Focal mechanism solutions of earthquakes 12

2.1.7 Ambiguity in focal mechanism solutions 14

2.1.8 Seismic tomography 17

2.2 Velocity structure of the Earth 19

2.3 Composition of the Earth 21

2.4 The crust 22

2.4.1 The continental crust 22

2.4.2 Upper continental crust 23

2.4.3 Middle and lower continental crust 23

2.4.4 The oceanic crust 24

2.4.5 Oceanic layer 1 24

2.4.6 Oceanic layer 2 25

2.4.7 Oceanic layer 3 26

2.5 Ophiolites 27

2.6 Metamorphism of oceanic crust 28

2.7 Differences between continental and oceanic crust 29

2.8 The mantle 30

2.8.1 Introduction 30

2.8.2 Seismic structure of the mantle 30

2.8.3 Mantle composition 31

2.8.4 The mantle low velocity zone 31

2.8.5 The mantle transition zone 32

2.8.6 The lower mantle 32

2.9 The core 33

2.10 Rheology of the crust and mantle 33

2.10.1 Introduction 33

2.10.2 Brittle deformation 34

2.10.3 Ductile deformation 36

2.10.4 Lithospheric strength profiles 37

2.10.5 Measuring continental deformation 39

2.10.6 Deformation in the mantle 41

2.11 Isostasy 42

2.11.1 Introduction 42

2.11.2 Airy’s hypothesis 43

2.11.3 Pratt’s hypothesis 43

2.11.4 Flexure of the lithosphere 44

2.11.5 Isostatic rebound 45

2.11.6 Tests of isostasy 46

2.12 Lithosphere and asthenosphere 48

2.13 Terrestrial heat flow 51

3 Continental drift 54

3.1 Introduction 55

3.2 Continental reconstructions 55

3.2.1 Euler’s theorem 55

3.2.2 Geometric reconstructions of continents 55

3.2.3 The reconstruction of continents around the Atlantic 56

3.2.4 The reconstruction of Gondwana 57

3.3 Geologic evidence for continental drift 58

3.4 Paleoclimatology 60

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