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6.01 An Overview A. B. Watts, University of Oxford, Oxford, UK © 2007 Elsevier B.V. All rights reserved. 6.01.1 Introduction 6.01.2 Isostasy and Steady-State Equilibrium 6.01.2.1 The Earth’s Hypsometric Curve and Crustal Structure 6.01.2.2 Gravity Anomalies, Crustal Structure, and Local Models of Isostasy 6.01.2.3 Departures from Local Isostasy: Flexural Isostasy 6.01.3 The Deformation of the Crust and Lithosphere 6.01.3.1 Earthquake Loading, Postseismic Relaxation, and the Short-Term (i.e., up to a Few Hundreds of Seconds) Response 6.01.3.2 Glacial and Lake Loading and Unloading, Rebound, and the Intermediate-Term (a Few Tens of Thousand Years) Response 6.01.3.3 Volcano and Sediment Loading and the Long-Term (Greater than Several Hundreds of Thousand Years) Response 6.01.4 Relationship between Load and Plate Age and Rheological Structure 6.01.4.1 The Relationship between the Long-Term Elastic Thickness and Plate and Load Age 6.01.5 Global Elastic Thickness 6.01.5.1 The Map 6.01.5.2 Correlation of Global Elastic Thickness with Temperature Structure and Shear-Wave Velocity 6.01.6 Geological Implications 6.01.6.1 Toward an Integrated Model That Relates Elastic Thickness to Load Age on Short, Intermediate, and Long Timescales 6.01.6.2 Terranes, the Wilson Cycle, and Inheritance 6.01.6.3 Tectonic Setting of Geological Features 6.01.6.4 Surface Processes and Flexural Interactions 6.01.6.5 The Relative Contributions of Lithospheric Flexure to the Earth’s Topography and Gravity Anomaly Field and Mantle Convection 6.01.7 Conclusions References 6.01.1 Introduction It has been known since the pioneering work of Joseph Barrell during the early part of the last century that the outermost layers of the Earth comprise a strong upper layer, the lithosphere, which overlies a weak lower layer, the asthenosphere. Barrell (1914a) argued that because river deltas such as the Niger and Nile lack a flanking topographic depression, they must be supported by the strength of the lithosphere. He used Pratt isostatic gravity anomalies over North America as a proxy for the magnitude of the stress differences that could be supported by the lithosphere and showed, using the equations of Darwin (1882), that stresses increase and then decrease with depth, passing by transition into the weak underlying asthenosphere. Today, we distinguish the lithosphere from the asthenosphere not only on the basis of its strength but also its physical properties such as temperature, density, and seismic velocity structure. The lithosphere, for example, is generally associated with cooler temperatures, higher average densities, and higher average S-wave velocities than the asthenosphere. Plate tectonics is based on the assumption that the lithosphere is rigid on long timescales and is moving across the surface of the Earth with the plates. The positive density contrast between the lithosphere and the asthenosphere suggests, however, that the rigid layer may be gravitationally unstable. Indeed, oceanic lithosphere – after it is created at a mid-oceanic ridge – cools, subsides, and sinks into the underlying asthenosphere, for example, at a deep-sea trench–outer-rise system. Continental lithosphere may also be unstable. In rifts (e.g., East Africa) the lithosphere is regionally heated, thinned, and uplifted and only subsides locally below sea level. In collisional systems (e.g., Himalaya, Betics), however, continental lithosphere is thickened (Molnar et al., 1998) or is infiltrated by fluids released during metamorphic reactions (Le Pichon et al., 1997). Both processes may cause dense rocks of the lower crust to enter the eclogite stability field. As a result, the lower crust becomes denser than the underlying mantle, detaches, and, as at trenches, may sink into the underlying asthenosphere. Isostatic considerations, however, suggest that the crust – which comprises the uppermost part of the lithosphere – is buoyant and is in a state of flotation on the underlying mantle. Furthermore, flexure studies suggest that when it is subject to long-term geological loads such as volcanoes and sediment, the lithosphere, rather than behaving as a number of independent floating blocks, as local models of isostasy such as Airy and Pratt predict, responds by bending – in a similar manner as would an elastic plate that overlies an inviscid fluid substrate. We are concerned in this volume with Crust and Lithosphere Dynamics which we may define as the study of how the outermost layers of the Earth respond to loads that are emplaced on, within, and below it and its implications for plate mechanics and mantle flow. Dynamics means, of course, all the loads and stresses and their resulting deformations and strains that are applied to the crust and lithosphere and the underlying asthenosphere. Unfortunately, there is no continuum that we can study that is representative of all the spatial and temporal scales of these loads and their deformation. This makes the determination of the thermal and mechanical properties of the lithosphere and asthenosphere a difficult, if not intractable, problem. What we do have are snapshots at particular temporal and spatial scales. They range in temporal scales from up to a few hundreds of seconds (e.g., during the coseismic deformation that results from earthquake triggering), through thousands of years (e.g., during the glacial isostatic adjustment that follows the waxing and waning of an ice sheet), to several millions of years (e.g., during the flexural deformation due to volcano loading) and in spatial scales from a few, through to a few tens, to several hundreds of kilometers. Understanding the response of the lithosphere and asthenosphere system to these loads is fundamental to our understanding of plate kinematics and mechanics and to predicting the response of the crust and lithosphere to more complex loads such as those associated with the erosion and deposition of sediment. It is also of importance in quantifying the relative contribution of the crust and lithosphere to surface observables such as gravity and topography and isolating the effects of surface and subsurface processes such as those associated with landscape evolution and mantle convection. In this volume, we bring together contributions fundamental to crust and lithosphere dynamics. The volume begins with a chapter by Wessel and Müller (Chapter 6.02) on plate kinematics. These authors show how new observations and methodologies have improved the resolution of relative and absolute plate motions. By comparing these motions to predictions based on first-order rigid plate and fixed hot-spot reference frame models, the authors document the departures and their implications for the driving forces of plate tectonics. This chapter is followed by two contributions on plate mechanics. The first by Burov (Chapter 6.03) discusses the evidence that data from experimental rock mechanics have provided on the long-term rheology of the plates. By constructing strength profiles for both oceanic and continental lithosphere and then incorporating them into numerical models, he has been able to evaluate the role played by the vertically stratified rheological structure of the plates in such phenomena as subduction and rifting, lithosphere-scale folding, and the development of convective instabilities. The second by Sabadini (Chapter 6.04) examines the evidence from geodetic data for the response of the plates to both relatively short-term loads such as those associated with coseismic and postseismic deformation and the waxing and waning of ice sheets as well as relatively long-term loads such as those associated with plate boundaries and continental collision. He shows that the response of the Eurasian Plate to boundary loads is best modeled by a thin viscous sheet with spatially varying viscosity, since such a model accounts well for the geodetically constrained velocity and stress fields. The next three chapters consider the evidence from surface heat flow, borehole breakouts, and magma models for the thermal and mechanical structure of the lithosphere. In their chapter, Jaupart and Mareschal (Chapter 6.05) review the evidence based on surface heat flow data for the thermal structure of the plates. They focus on two main issues: the first concerns the degree to which surface heat flow data is dependent on plate age while the second involves mechanisms by which heat is transported at the base of the plates. They argue that while the simple oceanic plate cooling model fails to explain the depth and surface heat flow of old seafloor, it has provided useful information on heat transport mechanisms at the base of the plate and helps explain why oceans, as well as continents, tend to a thermal steady state. In their chapter, Zoback and Zoback (Chapter 6.06) review the evidence from a variety of geological and geophysical data, including in situ measurements, for the state of stress in the upper brittle crust. They show that on a continent-wide scale, stress orientation data is strikingly uniform, a fact they attribute to major forces that drive plate motions such as ridge push and trench pull. Deviations from these stress orientations are attributed to more local geological features such as those associated with lateral density contrasts, topography and crustal thickness changes, and flexural loading. In his chapter, Marsh (Chapter 6.07) examines the evidence from the pattern and style of magmatism for processes that control magmatic activity. These include lithospheric extension that can make vast tracts of the continental crust accessible to melting and subduction that cause focused convective upwellings within the phase field of the source rock. The next two chapters examine the deformation of the lithosphere in extensional and compressional settings. Ключевые слова: e, r, o