We review recent results concerning the rheology of the lithosphere with special attention to the following topics: 1) the flexure of the oceanic lithosphere, 2) deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins, 3) the rheological stratification of the continents, 4) strain localization and shear zone development, and 5) strain‐induced crystallographic preferred orientations and anisotropies in body‐wave velocities. We conclude with a section citing the 1983–1986 rock mechanics literature by category.

Fracture energy is a form of latent heat required to create an earthquake rupture surface and is related to parameters governing rupture propagation and processes of slip weakening. Fracture energy has been estimated from seismological and experimental rock deformation data, yet its magnitude, mechanisms of rupture surface formation and processes leading to slip weakening are not well defined. Here we quantify structural observations of the Punchbowl fault, a large-displacement exhumed fault in the San Andreas fault system, and show that the energy required to create the fracture surface area in the fault is about 300 times greater than seismological estimates would predict for a single large earthquake. If fracture energy is attributed entirely to the production of fracture surfaces, then all of the fracture surface area in the Punchbowl fault could have been produced by earthquake displacements totalling <1 km. But this would only account for a small fraction of the total energy budget, and therefore additional processes probably contributed to slip weakening during earthquake rupture.

Novaculites and quartzite ranging in grain size from 1.2–211 μm have been experimentally deformed at confining pressures of 350–1620 MPa under conditions of constant displacement rate and constant deviatoric stress in order to determine the effects of both grain size and pressure on the rheology of quartzite. The amount of water available to the samples was varied so that flow strengths for the entire suite of samples could be compared at several, nominally equal water concentrations; samples were vacuum dried at 800°C for 12 hours, left as is, or sealed in Pt jackets with 0.03–0.4 wt % water added. Novaculites deformed at 800°C and 10−6 /s−1 in the presence of 0.4 wt % water show a continuous decrease in flow strength with increasing confining pressure over the range 350–1590 MPa. At high confining pressures 950–1600 MPa, constant displacement rate experiments show three distinct grain size effects, corresponding to the three levels of water concentration: (1) “grain boundary hardening” for vacuum‐dried samples, (2) grain size independent strength for as is samples, and (3) “grain boundary weakening” for samples deformed in the presence of water. Although grain boundary‐dislocation interactions may lead to grain boundary hardening and grain boundary sliding to weakening, the details of the mechanical data, in combination with microstructural observations, are inconsistent with existing models of intrinsic grain boundary effects. Instead, the strength‐grain size relations are interpreted as resulting from variations in structurally incorporated water, which, in turn, are controlled by diffusion of water to and from the grain boundaries. Finally, constant stress experiments show that the activation energy Q and stress exponent n for creep do not depend on grain size, so that the different strengths observed must be incorporated in the preexponential terms. However, both Q and n show a continuous decrease with increasing amounts of available water, from Q = 300 kJ/mol and n ≃ 4 for vacuum‐heated samples, to Q = 130 kJ/mol and n = 2.6 for water‐added samples.

The basal slip systems of biotite and their mechanical expressions have been investigated by shortening single crystals oriented to maximize and minimize shear stresses on (001). Samples loaded at 45 ø to (001) exhibit gentle external rotations associated with dislocation glide. High-angle kink bands in these samples, unlike those developed in micas loaded parallel to (001), are limited to sample comers. Samples shortened pertxmdicular to (001) show no evidence of nonbasal slip and fail by fracture over all conditions tested. The mechanical respome of biotite shortened at 45 ø to (001) is nearly perfectly elastic-plastic; stress-strain curves are characterized by a ste• elastic slope, a sharply deemed yield point, and continued deformation at low (mostly < 100 MPa), relatively constant stresses at strains >1%. Stresses measured beyond the yield point are insensitive to conf'ming pressure over the range 200 to 500 MPa and exhibit weak dependencies upon strain.. rare and ten3•a•e. Assuming an exponential relationship between differential stress c•a and strain rate oe of the form oe = C exp (otC•d) exp (-Q/RT), the data collected over strain rates and temperatures of 10-7 to 10-4 s-1 and 20 ø to 400øC, respectively, are best fit by an exponential constant ct of 0.41 q' 0.08 MPa-1 and an activation energy Q of 82 + 13 lcJ/mol. A power law fits the dam equally well with n = 18 + 4 and Q = 51 q' 9 kJ/mol. Samples oriented favorably for slip in directions [100] and [110] are measurably weaker than those shortened at 45 ø to [010] and [310], consistent with the reported Burg?rs vectors <100>, 1/2 <110>, and 1/2 <110>. The anisotropy of biotite is further revealed by contrastang these plastic strengths with results of samples deformed parallel and perpendicular to (001). Previous studies have shown that biotite loaded in the (001) plane is strong prior to the nucleation of kink bands.The strength of biotite shortened perpendicular to (001) exceeds that measured parallel to (001) and is pressure dependent. Application of the results to deformation within the continental crust suggests that biotite oriented favorably for slip is much weaker than most other silicates over a wide range of geologic conditions. Its presence within foliated rocks and shear zones may limit locally the stresses that can be supported. INTRODUCtiON Layer ,s_ilicares are weak and evidence of their deformation in •he Earths crust is widespread. The d6collements of major ttu•t sheets are commonly defined by clay-bearing shales and slams [e.g., Hayes, 1891; Rodgers, 1949; King, 1950; Chapple, 1978] and localized shear strains of mylonites and ductile shear zones are frequently associated with the presence of micas [e.g., Bell and Etheridge, 1973; Hobbs et al., 1976; White et al., 1980; Wilson, 1980; Tullis et al., 1982]. On a f'm•r scale, micas within metamorphic tectonites exhibit sharp deformation and kink bands [Hobbs et al., 1976; Wilson and Be!l, 1979; Baronnet and Olives, 1983; Vernon et al., 1983; Goodwin and Wenk, 1990], strong crystallographic ...

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