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)

The high temperature plastic deformation of (Ti, V)C alloys was studied by load relaxation experiments. The deformation experiments were performed at 1200 to 1600~ on fully dense, single phase, polycrystalline TiC, TIC-25% VC, TIC-50% VC and TIC-75% VC, prepared by hot-pressing. The analysis of log a-log ~ data along with the evaluation of strain rate sensitivity ,,~ 0.9 to 1.5 and the grain size dependence ,,~ 2.6 indicates that the high temperature plastic deformation of these carbides occurs by grain-boundary diffusion, which is in accordance with hot-pressing results. Also, transmission electron microscopy of deformed samples, revealing little evidence for dislocation activity, supported this conclusion. The diffusion coefficients obtained in this study are in good agreement with those obtained for boundary diffusion, with the activation energy ~ 390 kJ mo1-1 for TiC and ~ 307 + 13 kJ mo1-1 for other compositions. A sharp decrease in the strength of (Ti, V)C alloys with the addition of VC is associated with the softening and/or melting of second phase material accumulated along the grain boundaries.

http://link.springer.com/content/pdf/10.1007%2FBF01114279.pdf

Part 2 Load relaxation studies

Spray pyrolysis of the mixed metal organometallics, Al[M(CO)3C5H5]3 (M is Cr, Mo or W), prepared in tetrahydrofuran from the mercury analogues and aluminum metal, gave mixed oxide gels on heated alumina substrates. Air was used as the propellant for M = Cr, whereas oxygen was used for M = Mo or W. Mixtures of Al[Cr(CO)3C5 H5]3 and Al(O-i-C3H8)3 having a Cr/Al ratio of 0.02 to 0.93 were used to prepare gels with intermediate dopant concentrations. When calcined at 500 °C, the gels produced amorphous coatings of two-component oxides that were characterized by SEM and EDS methods. The strength of adhesion of a E-beam evaporated chromium metal overlayer to the coating, as measured by a continuous microindentation method, increased in the order Cr < Mo ≅ W; adhesion was found to be directly proportional to the effective free energy of formation of the oxide mixture.

Adhesion of Metals to Mixed Oxide Coatings (Al & Cr, Mo, OR W) Prepared by Spray Pyrolysis of Organometallics.

Partial melting often occurs alongside sites of rapid deformation in the Earth&amp;#8217;s mantle and crust. Microstructural molten and crystalline components align in response to deformation, leading to anisotropies in mechanical, transport, and seismic properties detectable by remote sensing. Here, we investigate the co-evolution of melt, shape, and crystallographic preferred orientations (MPOs, SPOs &amp; CPOs) at early stages of experimental shear deformation, constraining their contribution to observable signatures. We characterized the microstructures of partially molten (2-4 wt% melt) olivine-basalt aggregates deformed in general shear at a temperature of 1250&amp;#176;C under a confining pressure of 300 MPa, at shear stresses of &amp;#964; = 0-175 MPa and shear strains of &amp;#947; = 0-2.3. We then used the Gassman poroelastic differential effective medium method to calculate resultant seismic anisotropy.The grain-scale network of melt pockets developed a strong preferred orientation parallel to the maximum principal stress at &amp;#947; &lt; 0.4. At higher strains, the orientation of the grain-scale melt pockets remained parallel to the maximum principal stress, but incipient, sample-scale melt bands formed at ~25&amp;#176; antithetic to the direction of shear. &amp;#160;While the orientation of individual melt pockets evolved quickly, grain SPOs and CPOs required larger strains (&amp;#947; &gt; 2) to strengthen and change. A weak SPO and CPO were induced during sample preparation, with grain long axes oriented perpendicular to the direction of maximum principal stress and a- and c-axes girdled perpendicular to the long axis of the sample. At the highest explored shear strain, a strong SPO was established and the girdled a-axes of the CPO rotated to align nearly parallel to the shear plane, developing clusters parallel to the shearing direction. &amp;#160;These results yield two key conclusions about the orientation of melt networks in deforming partially molten rocks. First, the grain-scale and sample-scale alignments of melt pockets are distinct. At the grain scale, melt pockets align approximately parallel to the maximum principal stress, but the en echelon arrangement of melt pockets yields a sample-scale MPO at ~25o to the maximum principal stress (20o to the shear plane). Second, the relative timescales of melt and solid microstructural evolution are different, and are directly reflected in changes to seismic anisotropy. The grain-scale MPO reacts to a change in the orientation of the maximum principal stress after only a small amount of strain; in contrast, CPOs and SPOs require much higher strains before responding to a change in stress conditions. Seismic anisotropy is greatest when olivine a-axes and the grain-scale orientation of melt pockets are in relatively close alignment, so anisotropy will quickly decrease with any change in the orientation of the stress field that results in a rotation of the MPO away from the orientation of olivine a-axes. Perturbations to a local stress field can thus be observed almost immediately due to a rapidly reorienting melt network, making MPOs a more valuable predictor of instantaneous change than CPO-driven anisotropies. 

Co-evolution of melt and crystal phases in experimentally sheared partially molten rocks and generation of seismic anisotropy during rapid deformation

Continuous microscratch experiments have been carried out to investigate the effect of various heat treatments on adhesion in the Cr/Al2O3 system. After samples were prepared by vapor depositing 0.32 μm thick Cr films on % MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqr1ngB % PrgifHhDYfgasaacH8srps0lbbf9q8WrFfeuY-Hhbbf9v8qqaqFr0x % c9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8fr % Fve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeaaeaaakeaadaqada % qaaiaaigdaceaIXaGbaebacaaIWaGaaGOmaaGaayjkaiaawMcaaaaa % !3C38!
 $$\left( {1\bar 102} \right)$$
 surfaces of single crystal sapphire substrates, they were annealed at different temperatures at controlled oxygen partial pressures. In the subsequent continuous microscratch tests, a conical diamond indenter with a 1 μm tip radius was driven simultaneously into and across the sample until a load drop occurred. Optical and scanning electron microscopy observations demonstrate that a portion of the Cr film delaminated from the A12O3 substrate at that point. The interfacial strain energy release rate was calculated from the load at failure combined with the size and geometry of the delaminated region. The work of adhesion was found to decrease dramatically on annealing in an H2-Ar mixture at 250°C.

David L. Kohlstedt

Papers

Part 2 Load relaxation studies

Adhesion of Metals to Mixed Oxide Coatings (Al & Cr, Mo, OR W) Prepared by Spray Pyrolysis of Organometallics.

Co-evolution of melt and crystal phases in experimentally sheared partially molten rocks and generation of seismic anisotropy during rapid deformation

Effect of heat treatment on adhesion in the Cr/Al2O3 system

Kinematic partitioning in the echo lake deformation zone, seiad ultramafic complex, klamath mountains, ca