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.

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Mechanics of fold-and-thrust belts and accretionary wedges

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

Torsion experiments on partially molten aggregates of olivine þ chromite þ 4 vol. % mid-ocean ridge basalt provide new insights into the interactions between deformation and melt segregation. When samples are sheared, melt segregates into distinct melt-rich bands oriented � 208 antithetic to the macroscopic shear plane. In one series of experiments, samples were deformed at similar shear strain rates (or stresses) to a range of finite shear strains to explore the evolution of melt-rich bands. In another series of experiments, samples were deformed to similar finite shear strains at a range of strain rates to explore the effect of strain rate (or stress) on band spacing and microstructure. We relate variations in strain rate to the compaction length and show that band spacing increases with increasing compaction length. These experiments provide new information on the evolution of melt distribution, the partitioning and localization of strain, and the scaling of experimental results to the Earth’s mantle.

Stress-driven Melt Segregation in Partially Molten Olivine-rich Rocks Deformed in Torsion

Strain localization in olivine aggregates at high temperature: A laboratory comparison of constant-strain-rate and constant-stress boundary conditions

The sintering behavior of olivine and olivine-basalt aggregates has been examined at temperatures near 1,300° C. Experimental factors contributing to rapid sintering kinetics and high-density, fine-grained specimens include: (i) the uniform dispersion of basalt throughout the specimen, (ii) a very fine, uniform particle size for the olivine powder, (iii) oxygen fugacities near the high PO2 end of the olivine stability field, and (iv) rapid heating to the sintering temperature. Olivine-basalt specimens prepared from olivine particles coated with a synthetic basalt achieve chemical and microstructural equilibrium more rapidly, as well as produce higher density and finer grain-sized aggregates, than do specimens prepared by mechanical mixing of olivine and natural basalt powders. The grain boundary mobility for olivine, measured for olivine-basalt aggregates which have undergone secondary recrystallization, is on the order of 2×10−15 (m/s)/(N/m2) in the temperature range 1,300–1,400° C. Solution-precipitation (pressure-solution) processes make an important contribution to the development of the microstructure in olivine-basalt aggregates.

Sintering of olivine and olivine-basalt aggregates

The stress dependence of olivine creep rate: Implications for extrapolation of lab data and interpretation of recrystallized grain size

To quantify the effect of hydrogen on the kinetics of Fe–Mg interdiffusion in olivine, diffusion couples composed of crystals with Mg∕(Mg+Fe) ratios of 091 and 083 were annealed under water-saturated conditions at T=1373K and P=300MPa With fO2 buffered at the Ni–NiO phase boundary, fH2O≈300MPa The resulting interdiffusivity, DFe–Mg≈1×10−16m2∕s, is approximately one order of magnitude larger than that measured under anhydrous conditions The enhancement in diffusivity in the presence of water results from an increase in the concentration of cation vacancies associated with the introduction of high concentrations of protons as point defects into the olivine structure

David L. Kohlstedt

Papers

Stress-driven Melt Segregation in Partially Molten Olivine-rich Rocks Deformed in Torsion

Strain localization in olivine aggregates at high temperature: A laboratory comparison of constant-strain-rate and constant-stress boundary conditions

Sintering of olivine and olivine-basalt aggregates

The stress dependence of olivine creep rate: Implications for extrapolation of lab data and interpretation of recrystallized grain size

Effect of H+ on Fe–Mg interdiffusion in olivine, (Fe,Mg)2SiO4