Page 7 - Global Tectonics

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
3.6.5 Apparent polar wander oceanic lithosphere 128
6.5 Heat fl ow and hydrothermal
curves 67
3.6.6 Paleogeographic reconstructions circulation 129
based on paleomagnetism 68 6.6 Seismic evidence for an axial
magma chamber 131
6.7 Along-axis segmentation of
oceanic ridges 133

4 Sea floor spreading and
6.8 Petrology of ocean ridges 140
transform faults 72 6.9 Shallow structure of the
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
4.1.6 Magnetostratigraphy 79 7 Continental rifts and
4.1.7 Dating the ocean fl oor 84
4.2 Transform faults 84 rifted margins 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
fault offsets 89 7.3 General characteristics of
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 91 rift rocks 172
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