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.

/pdf/composition-of-the-continental-crust-2hxz49vsod.pdf

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)

Planetary Tectonics: Strength and deformation of planetary lithospheres

Creep experiments have been carried out on single crystals of Fe2SiO4 and Co2SiO4. The crystals were internally buffered by oxide precipitates against FeO and SiO2, respectively. The following flow laws were obtained: for internally buffered fayalite, e = 1·7 × 1022σ6±1 pgO Oexp(-923±220kJmoL−1 RT), and for internally buffered cobalt olivine. where e is in units of s−1 and σ and po2 are in units of MPa. In Fe2SiO4, he observed po2 dependence of the strain rate can be explained by a model in which creep is controlled by the motion of positively charged jogs along edge dislocation lines, with the diffusion of oxygen by a vacancy mechanism as the rate-limiting step. In Co2SiO4, on the contrary, the po2 dependence suggests that creep may be glide controlled, that is, rate limited by migration of ionized kinks along the dislocations. However, defect associations, which cannot be totally ruled out, were not considered in this work.

Creep of Fe2SiO4 and Co2SiO4 single crystals in controlled thermodynamic environments

The physical properties of partially molten rocks are directly coupled to the grain-scale as well as the broader scale distribution of the melt phase. At the grain scale, if melt forms isolated pockets in a silicate matrix, its influence on plastic flow is generally relatively minor. In contrast, if the melt wets all of the grain-grain interfaces, it may dramatically lower the viscosity of the rock. At the broader scale, deformation will localize to form shear zones in melt-rich regions that commonly develop in partially molten rocks. In turn, the melt distribution evolves during plastic deformation of a partially molten rock. Melt located in triple junctions takes on a pronounced preferred orientation. In addition, melt often segregates during plastic deformation to form melt-enriched bands separated by melt-depleted regions.

This chapter develops two interrelated themes with emphasis on the relationship between melt distribution (structure) and the plastic flow (property) of partially molten rocks. The first section concentrates on thermodynamic constraints and experimental observations on the distribution of melt in a non-deforming rock, that is, in a partially molten rock exposed simply to a hydrostatic state of stress. The second section then builds on these boundary conditions to introduce theoretically predicted and experimentally determined flow laws describing plastic deformation of partially molten rocks. Finally, the third section examines the influence of deformation (i.e., a non-hydrostatic state of stress) on melt distribution and the associated implications for the rheological properties of partially molten crustal and mantle rocks.

Under a hydrostatic state of stress, the melt distribution in a partially molten rock is governed by the relative values of the solid-melt and solid-solid interfacial energies, γsm and γss. (NB: Although the term melt is used throughout this paper, the designation fluid or liquid could equally well be substituted.) The melt distribution …

Partial Melting and Deformation

To understand the processes involved in rheological weakening due to phase mixing in olivine + orthopyroxene aggregates, we conducted high-strain torsion experiments on two-phase samples at a temperature of 1200°C and a confining pressure of 300 MPa. Samples composed of iron-rich olivine plus 26% orthopyroxene were deformed to outer radius shear strains of up to γ ≈ 26. Values for the stress exponent, n, and grain size exponent, p, were determined based on least-squares fits of the strain rate, stress, and grain size data to a power-law creep equation both at smaller strains (γ ≤ 3) and at larger strains (γ ≥ 24). Microstructural observations demonstrate that, with increasing shear strain, grain size decreased and mixtures of small, equant grains of olivine and pyroxene developed. The values of n and p combined with associated changes in microstructure demonstrated that our samples deformed (i) by dislocation-accommodated grain-boundary sliding with subgrains present at lower strains and (ii) by dislocation-accommodated grain-boundary sliding with subgrains absent at higher strains. The evolution of both the mechanical and the microstructural properties observed in this study provide insights into the dynamic processes associated with rheological weakening and strain localization in the plastically deforming portion of the lithosphere.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2017JB014333

Rheological Weakening of Olivine + Orthopyroxene Aggregates Due to Phase Mixing: 1. Mechanical Behavior

The effect of electron irradiation on defects in natural quartz grains has been studied using strong-beam and weak-beam transmission electron microscopy (TEM) techniques. Damage is visible soon after the sample is exposed to the electron beam. The defect centers grow with increased irradiation, but the density of centers is independent of electron dose. Dislocation images also widen during irradiation; the rate of widening is independent of Burgers vector and dislocation nodes widen without constriction. Both of these observations indicate that the widening of dislocation images is related to vitrification of the material in the vicinity of the dislocations rather than irradiation-assisted separation of partial dislocations. This interpretation is supported by observations on grain boundaries.

David L. Kohlstedt

Papers

Planetary Tectonics: Strength and deformation of planetary lithospheres

Creep of Fe2SiO4 and Co2SiO4 single crystals in controlled thermodynamic environments

Partial Melting and Deformation

Rheological Weakening of Olivine + Orthopyroxene Aggregates Due to Phase Mixing: 1. Mechanical Behavior

Electron irradiation damage in natural quartz grains