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)

To investigate the kinetics of diffusion of hydrogen in olivine, single crystals from San Carlos in Arizona have been annealed at temperatures between 800° and 1000°C under hydrothermal conditions at a confining pressure of 300 MPa. The hydrogen diffusivities were determined for the [100], [010], and [001] directions from concentration profiles for hydroxyl in the samples. These profiles were obtained from infrared spectra taken at 100-μm intervals across a thin slice which was cut from the central portion of each annealed crystal. The rate of diffusion is anisotropic, with fastest transport along the [100] axis and slowest along the [010] axis. The fit of the data to an Arrhenius law for diffusion parallel to [100] yields an activation enthalpy of 130±30 kJ/mol with a preexponential term of (6±3)×10−5 m2 s−1. For diffusion parallel to [001], as there are insufficient data to calculate the activation enthalpy for diffusion, we used the same value as that for diffusion parallel to [100] and determined a preexponential term of (5±4)×10−6 m2 s−1. The diffusion rate parallel to [010] is about 1 order of magnitude slower than along [001]. The measured diffusivities are large enough that the hydrogen content of olivine grains which are millimeters in diameter will adjust to changing environmental conditions in time scales of hours at temperatures as low as 800°C. As xenoliths ascending from the mantle remain at high temperatures (i.e., >1000°C) but experience a rapid decrease in pressure, and hence hydrogen fugacity, olivine grains may dehydrate during ascent. By comparison, slow rates of carbon diffusion (Tingle et al., 1988) suggest that carbon will not be lost from olivine during ascent. Thus, low hydrogen contents within olivine and within fluid inclusions in olivine cannot be taken as support for low water contents in the mantle.

Diffusion of hydrogen in olivine: Implications for water in the mantle

Hydrothermal treatment experiments have been performed on single crystals of San Carlos olivine at 1100° to 1300°C and 300- and 1500-MPa confining pressure, with the oxygen fugacity around the specimen constrained near the Fe/FeO buffer. The diffusion coefficients for the hydroxyl species giving rise to the sharp band and broadband features in the infrared spectrum of hydrothermally treated olivine have been determined to be greater than 10−10 m2 s−1 under all pressure and temperature conditions tested; the solubility of the hydroxyl species seems to vary between crystals but to show little variability for specimens from the same crystal over the temperature range investigated at 300-MPa confining pressure. Crystals hydrothermally treated at 1300°C, 300-MPa confining pressure, and 10−5 s−1 strain rate are a factor of 1.5 to 2.5 weaker than those treated in an anhydrous environment. Microstructural investigations suggest that this reduction in strength is due to enhancement of the rate of climb of dislocations in the “wet” experiments. First-order calculations suggest a stress exponent of n ∼ 2.5 and a water fugacity exponent of m ∼ 1/5 for deformation under hydrous conditions, assuming that the deformation obeys a power law relation.

The role of water in the deformation of olivine single crystals

Gem quality single crystals of dry olivine were plastically deformed in compression at differential stresses between 50 and 1500 bars, with zero confining pressure, and at temperatures between 1428° and 1650°C. When these new data are combined with existing creep data for dry olivine-bearing rock, a flow law of the form e˙ = ƒ(σ) exp (−Q/RT) is obtained, whereƒ(σ) is an empirical function that is not simply proportional to σn for constant n and Q is equal to 125 ± 5 kcal/mol. The flow law covers the stress range 50–10,000 bars and the temperature range 1100°–1700°C and is consistent with all published data to 1 decade in strain rate.

Low-stress high-temperature creep in olivine single crystals

We have analyzed the kinetics of hydrogen diffusion in ferromagnesian olivine by measuring the —H distribution in single crystals annealed under hydrothermal conditions. The hydrogen concentration evolves toward an equilibrium value in two stages. First, hydrogen diffuses rapidly (DH = 10\"10 m2/s at 1000 °C) into olivine probably charge compensated by a flux of polarons or of electrons. The resultant metastable equilibrium concentration of hydrogen is directly related to the imposed hydrogen fugacity and to the preexisting concentration of intrinsic point defects; except for electronic species, intrinsic defects diffuse significantly more slowly than hydrogen. Second, hydrogen and intrinsic point defects such as metal vacancies diffuse more slowly (DVMl = 10 12 m2/s at 1000 °C) into or out of the olivine as the concentrations of all defect species evolve toward equilibrium with the imposed thermochemical environment.

Diffusion of hydrogen and intrinsic point defects in olivine

To investigate the rheological behavior of the Earth's upper mantle, over 100 high-temperature deformation experiments have been performed on single crystals of San Carlos olivine in controlled chemical environments at a total pressure of 0.1 MPa. Constitutive equations have been determined which describe the dependence of creep rate on applied stress, temperature and oxygen fugacity in terms of power law relations. In addition, the effects of orthopyroxene activity and loading orientation on the creep behavior have been investigated. For samples of each of three compression directions, [101]c, [011]c and [110]c, buffered by orthopyroxene or magnesiowustite, either two or three power law equations are required to describe the dependence of strain rate at fixed stress on temperature and oxygen fugacity over the full range of experimental conditions. It is proposed that in each power law regime a different creep mechanism controls the creep rate. For all of the experimental conditions, the activation energy for creep is independent of the stress level and the stress exponent is constant at 3.5±0.1. Activation energies for the various creep mechanisms varied from 230 to 1000 kJ/mol; and oxygen fugacity exponents lie in the range −0.03 to 0.4. From the constitutive equations determined based on the power law equations for all of the creep mechanisms, e˙-T-fo2 deformation maps were constructed at a stress of 1 MPa for olivine.

David L. Kohlstedt

Papers

Diffusion of hydrogen in olivine: Implications for water in the mantle

The role of water in the deformation of olivine single crystals

Low-stress high-temperature creep in olivine single crystals

Diffusion of hydrogen and intrinsic point defects in olivine

High‐temperature creep of olivine single crystals 1. Mechanical results for buffered samples