We have investigated the transitional, semibrittle deformation of a mica schist (~75% biotite) by shortening cylinders cored at 0¿, 45¿, and 90¿ to foliation to varying strains, at confining pressures Pc to 500 MPa, constant strain rates &egr;˙ from 1.5¿10-7 to 1.6¿10-4 s-1, and temperatures T from 25¿ to 400 ¿C. Deformation is concentrated within one or more throughgoing, millimeter-wide shear zones at all conditions; these localize at low strains (&egr;<2%) through the nucleation and coalescence of dense sets of intragranular microkink bands (MKBs). Despite distinct differences in the relative number of mica grains originated favorably for slip and kinking in different loading directions, the differential stresses required for shear zone development vary little with fabric orientation. Biotite schist undergoes a transition from strain-softening to steady strength mechanical response at confining pressures in the range 75 to 150 MPa. The pressure sensitivity of strength (characterized by the slope μ of the Mohr envelope) decreases from μ~0.5 (at Pc<100 MPa) to μ<0.1 at pressures greater than 200 MPa, reflecting the increasing contribution of glide and kinking in biotite at higher pressures. However, dilatancy associated with microcracking and void formation along MKB boundaries persists to at least 500 MPa. Within the pressure-insensitive regime (200≤Pc≤500 MPa), temperature and strain rate dependencies of strength determined in stepping tests reveal a strong history dependence to flow that cannot strictly be described by a steady state constitutive law.

Samples deformed in steps from low to high temperatures or fast to slow strain rates consistently exhibit stronger temperature and strain rate sensitivities than those deformed along T decreasing or &egr;˙ increasing paths. Path-dependent effects may reflect differences in the degree to which inherited dislocation substructures are utilized or overprinted during later deformation increments. By assuming an exponential relationship between differential stress &sgr;d and strain rate &egr;˙ of the form &egr;˙=C exp(α&sgr;d)exp(-Q/RT), we fit the data with two end-member flow laws with a single activation energy Q=89 kJ/mol, and exponential constants αss=0.15¿0.01 MPa-1 and αws=0.55¿0.04 MPa-1 that account for different responses observed along stepping paths that are strongly sensitive or weakly sensitive to T and &egr;˙, respectively. Application of the results to crustal deformation suggests that mica-rich aggregates are weaker than other common rock types throughout a broad midcrustal depth range, supporting the inference that retrograde reaction to phyllosilicates may be important in localizing crustal deformations within large faults and shear zones. ¿ American Geophysical Union 1992