The mechanical anisotropy of Four-mile gneiss has been investigated in a series of uniaxial and triaxial, compression and extension experiments performed at confining pressures Pc up to 400 MPa, constant strain rates ϵ from 1.6¿10-6 to 1.5¿10-4 s-1, and temperatures T from 25¿ to 800 ¿C on cylindrical and notched samples oriented with respect to foliation (S) and lineation (L). Differential stresses measured both at the onset of yielding and at failure vary with specimen orientation, with maximum compressive strengths exhibited by samples cored perpendicular to S and minimum strengths exhibited by samples cored at 45¿ to both S and L. While failure strengths are influenced most strongly by the orientation of S, they appear to depend upon the orientation of L as well. An orthorhombic failure criterion, generalized from a nonlinear Mohr-Coulomb relation, has been considered with quadratic and linear stress terms resembling those of invariants J2 and J1, respectively, and material parameters estimated by nonlinear regression methods. Satisfactory fits were achieved for results at T=25 ¿C as well as T=700 ¿C. Fracture strengths are relatively insensitive to changes in T and ϵ and the anisotropy exhibited at T=700 ¿C is remarkably similar to that measured at T=25 ¿C.

Relatively small reductions in strength observed at elevated temperatures are probably due to the influence of thermally induced microcracks. Mechanisms of deformation and sources of anisotropy have been identified by examining microstructures developed in deformed specimens and observing their relationships to those fabric elements initially present in the starting material. Throughgoing shear fractures developed in samples shortened in all orientations with respect to S and L by the coalescence of microcracks in feldspar and quartz grains, as reported for isotropic granites. However, inelastic strains within mica grains were accommodated by slip, frictional sliding, and kinking, and deformation of favorably oriented micas appears to have led to local stress concentrations in neighboring phases that result in nucleation of tensile microcracks. Both S and L are defined by the preferred orientations of micas, and a simple model involving crack nucleation around oriented mica grains is proposed to explain the anisotropy observed. ¿ American Geophysical Union 1990