This chapter reviews the present-day composition of the continental crust, the methods employed to derive these estimates, and the implications of the continental crust composition for the formation of the continents, Earth differentiation, and its geochemical inventories. We review the composition of the upper, middle, and lower continental crust. We then examine the bulk crust composition and the implications of this composition for crust generation and modification processes. Finally, we compare the Earth's crust with those of the other terrestrial planets in our solar system and speculate about what unique processes on Earth have given rise to this unusual crustal distribution.

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Composition of the Continental Crust

This essential reference for graduate students and researchers provides a unified treatment of earthquakes and faulting as two aspects of brittle tectonics at different timescales. The intimate connection between the two is manifested in their scaling laws and populations, which evolve from fracture growth and interactions between fractures. The connection between faults and the seismicity generated is governed by the rate and state dependent friction laws - producing distinctive seismic styles of faulting and a gamut of earthquake phenomena including aftershocks, afterslip, earthquake triggering, and slow slip events. The third edition of this classic treatise presents a wealth of new topics and new observations. These include slow earthquake phenomena; friction of phyllosilicates, and at high sliding velocities; fault structures; relative roles of strong and seismogenic versus weak and creeping faults; dynamic triggering of earthquakes; oceanic earthquakes; megathrust earthquakes in subduction zones; deep earthquakes; and new observations of earthquake precursory phenomena.

The Mechanics of Earthquakes and Faulting

3.01 – Composition of the Continental Crust

The overall mechanics of fold-and-thrust belts and accretionary wedges along compressive plate boundaries is considered to be analogous to that of a wedge of soil or snow in front of a moving bulldozer. The material within the wedge deforms until a critical taper is attained, after which it slides stably, continuing to grow at constant taper as additional material is encountered at the toe. The critical taper is the shape for which the wedge is on the verge of failure under horizontal compression everywhere, including the basal decollement. A wedge of less than critical taper will not slide when pushed but will deform internally, steepening its surface slope until the critical taper is attained. Common silicate sediments and rocks in the upper 10-15 km of the crust have pressure-dependent brittle compressive strengths which can be approximately represented by the empirical Coulomb failure criterion, modified to account for the weakening effects of pore fluid pressure. A simple analytical theory that predicts the critical tapers of subaerial and submarine Coulomb wedges is developed and tested quantitatively in three ways: First, laboratory model experiments with dry sand match the theory. Second, the known surface slope, basal dip, and pore fluid pressures in the active fold-and-thrust belt of western Taiwan are used to determine the effective coefficient of internal friction within the wedge,/x = 1.03, consistent with Byerlee's empirical law of sliding friction,/at, = 0.85, on the base. This excess of internal strength over basal friction suggests that although the Taiwan wedge is highly deformed by imbricate thrusting, it is not so pervasively fractured that frictional sliding is always possible on surfaces of optimum orientation. Instead, the overall internal strength apparently is controlled by frictional sliding along suboptimally oriented planes and by the need to fracture some parts of the observed geometrically complex structure for continued deformation. Third, using the above values of/at, and/x, we predict Hubbert-Rubey fluid pressure ratios X = Xt, for a number of other active subaerial and submarine accretionary wedges based on their observed tapers, finding values everywhere in excess of hydrostatic. These predicted overpressures are reasonable in light of petroleum drilling experience in general and agree with nearby fragmentary well data in specific wedges where they are available. The pressure-dependent Coulomb wedge theory developed here is expected to break down if the decollement exhibits pressure-independent plastic behavior because of either temperature or rock type. The effects of this breakdown are observed in the abrupt decrease in taper where wedge thicknesses exceed about 15 km, which is the predicted depth of the brittle-plastic transition in quartz-rich rocks for typical geothermal gradients. We conclude that fold-and-thrust belts and accretionary wedges have the mechanics of bulldozer wedges in compression and that normal laboratory fracture and frictional strengths are appropriate to mountain-building processes in the upper crust, above the brittle-plastic transition.

/pdf/mechanics-of-fold-and-thrust-belts-and-accretionary-wedges-39mfymsp74.pdf

Mechanics of fold-and-thrust belts and accretionary wedges

Major and trace element composition of the depleted MORB mantle (DMM)

http://web.mit.edu/wbdurham/www1/papers/89-Mei-2007-MgO.pdf

Experimental investigation of the creep behavior of MgO at high pressures

An experimental study of the effects of surface tension in homogenizing perturbations in melt fraction

Publisher Summary This chapter presents a study where constant displacement-rate and load-relaxation experiments were performed at high pressures and temperatures to investigate the rheological behavior of partially molten aggregates of fine-grained olivine with a small amount of included basaltic melt under hydrous conditions Two-phase samples with melt fractions of either 09 or 86 vol% of basalt and grain sizes of 12 or 15 μm were fabricated by hydrostatically hot-pressing powders of San Carlos olivine plus synthetic basalt Single-phase samples with a grain size of 12 μm were prepared by hot-pressing powders of San Carlos olivine Infrared spectra revealed that the olivine in the sample without basalt and in the sample with 09 vol% of basalt was saturated with water-derived species, while the olivine in the sample with 86 vol% of basalt was undersaturated because the water partitioned largely into the melt phase The difference in creep strength between the melt-free and melt-rich samples was a result of two competing effects First, the presence of melt caused the partially molten sample to be weaker than the single-phase olivine sample Second, the presence of water resulted in the hydrolytic weakening of both samples; however, because the partially molten sample was undersaturated with water, while the single-phase sample was fully saturated, the hydrolytic weakening effect was larger in the latter than in the former If both samples had been fully saturated with water, the difference in creep strength might have been larger Hence, the rheology of partially molten upper mantle rocks in an ascending mantle plume or a mid-oceanic ridge environment will depend critically on the partitioning of water between the melt and solid phases and on the permeability of the rock (that is, melt fraction)

Chapter 3 Influence of Basaltic Melt on the Creep of Polycrystalline Olivine under Hydrous Conditions

Dislocations in minerals have a long history: The first images of dislocations ever seen were obtained by Seidentopf in 1905 on rocksalt (halite) with an optical microscope. Since then, and particularly after the development of ion thinning techniques in the early 1960s, dislocations in minerals have been studied in great detail by transmission electron microscopy. While diffraction contrast images of dislocations are similar in metals and minerals, the structures of most minerals are very complex and of low symmetry, which leads to their having a great variety of slip systems that change with temperature–pressure–strain rate conditions. Thus the identification of dislocations in rock-forming minerals and the analysis of their characteristics can be used to infer geological conditions during deformation. Recently much emphasis has been placed on the nature of dislocations in high-pressure phases that occur deep in the Earth. Here findings from transmission electron microscopy are combined with atomic scale modeling. This chapter provides an overview of early and modern work on dislocations in minerals and discusses applications to different mineral groups and their geologic significance.

Chapter 95 Dislocations in Minerals

Polycrystalline Ni2SiO4 was exposed to a gradient in oxygen potential at 1336°C to cause kinetic decomposition into its component oxides, NiO and SO2. At the higher-oxygen-potential side NiO formed, and at the lower-oxygen-potential side SiO2 formed. This spatial distribution of SiO2 and NiO is consistent with diffusion data for silicate olivines which indicate that Ni diffuses much faster than either Si or O.

David L. Kohlstedt

Papers

Experimental investigation of the creep behavior of MgO at high pressures

An experimental study of the effects of surface tension in homogenizing perturbations in melt fraction

Chapter 3 Influence of Basaltic Melt on the Creep of Polycrystalline Olivine under Hydrous Conditions

Chapter 95 Dislocations in Minerals

Decomposition of Ni2SiO4 in an Oxygen Potential Gradient