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 point defect chemistry and the kinetic properties of manganese olivine Mn2SiO4, electrical conductivity (’) of single crystals was measured along either the [100] or the [010] direction. The experiments were carried out at temperatures T=850–1200 °C and oxygen fugacities 
$$f_{{\text{O}}_{\text{2}} } = 10^{ - 11} - 10^2 $$

 atm under both Mn oxide (MO) buffered and MnSiO3 (MS) buffered conditions. Under the same thermodynamic conditions, charge transport along [100] is 2.5–3.0 times faster than along [010]. At high oxygen fugacities, the electrical conductivity of samples buffered against MS is ∼1.6 times larger than that of samples buffered against MO; while at low oxygen fugacities, the electrical conductivity is nearly identical for the two buffer cases. The dependencies of electrical conductivity on oxygen fugacity and temperature are essentially the same for conduction along the [100] and [010] directions, as well as for samples coexisting with a solid-state buffer of either MO or MS. Hence, it is proposed that the same conduction mechanisms operate for samples of either orientation in contact with either solid-state buffer. The electrical conductivity data lie on concave upward curves on a log-log plot of σ vs 
$$f_{{\text{O}}_{\text{2}} } $$

 , giving rise to two 
$$f_{{\text{O}}_{\text{2}} } = 10^{ - 11} - 10^2 $$

 regimes with different oxygen fugacity exponents. In the low-
 
$$f_{{\text{O}}_{\text{2}} } = 10^{ - 11} - 10^2 $$

 regime 
$$\left( {f_{{\text{O}}_{\text{2}} } < 10^{ - 7} {\text{atm}}} \right)$$

 , the 
$$f_{{\text{O}}_{\text{2}} } = 10^{ - 11} - 10^2 $$

 exponent, m, is 0, the MnSiO3-activity exponent, q, is ∼0, and the activation energy, Q, is 45 kJ/mol. In the high 
$$f_{{\text{O}}_{\text{2}} } = 10^{ - 11} - 10^2 $$

 regime 
$$\left( {f_{{\text{O}}_{\text{2}} } > 10^{ - 7} {\text{atm}}} \right)$$

 , m=1/6, q=1/4–1/3, and Q=45 and 200 kJ/mol for T 1100 °C, respectively. Based on a comparison of experimental data with results from point defect chemistry calculations, it is proposed that the change in m with 
$$f_{{\text{O}}_{\text{2}} } = 10^{ - 11} - 10^2 $$

 is induced by a switch in charge neutrality condition. At low 
$$f_{{\text{O}}_{\text{2}} } = 10^{ - 11} - 10^2 $$

 s, the charge neutrality condition is [e′]=[Mn
 Mn
 {
 ]; the hopping motion of electron holes h
. is the dominant conduction mechanism. At high 
$$f_{{\text{O}}_{\text{2}} } = 10^{ - 11} - 10^2 $$

 s, the charge neutrality condition is 2[V
 Mn
 ·
 ]=[Mn
 ·
 ]; the hopping motion of electron holes h
. and the migration of Mn ions associated with a counter flow of divalent Mn vacancies V
 Mn
 ·
 control electrical conduction at T 1100 °C, respectively.

Manganese olivine I: Electrical conductivity

7. Thermo-Mechanical Models of Convergent Orogenesis: Thermal and Rheologic Dependence of Crustal Deformation

The effect of interfacial reactions on the fracture toughness of the bond between a reactive metal, Ti, and a relatively stable oxide αAl 2 O 3 , was studied for heat treatments at different temperatures. The microstructure, chemistry and fracture toughness of the interfaces were characterized using transmission electron microscopy and a continuous microscratch test. The formation of intermetallic compounds resulted in a deterioration in the interfacial fracture toughness, relative to an unreacted interface. The decrease in fracture toughness was related to the presence along the interface of a brittle intermetallic phase and cavities. The cavities were produced as a result of the volume change and the oxygen released during the formation of intermetallic compounds at temperatures of 600°C and above. The strongest interface exhibited little if any reaction between the Ti and the Al 2 O 3 .

David L. Kohlstedt

Papers

Manganese olivine I: Electrical conductivity

7. Thermo-Mechanical Models of Convergent Orogenesis: Thermal and Rheologic Dependence of Crustal Deformation

The influence of interfacial reactions on the fracture toughness of TiAl2O3 interfaces

An experimental and numerical study of surface tension-driven melt flow

Dislocation density: stress relationships in natural and synthetic sodium chloride