Foliated rocks deformed under brittle conditions have shown high degrees of strength anisotropy; they are generally weakest when compressed at 45° to the foliation and strongest when compressed perpendicular to the foliation. When rocks are deformed by crystal plastic mechanisms, foliations and lattice preferred orientations of minerals may lead to viscous anisotropy. However, there have not been studies that directly test the strength anisotropy of foliated rocks deformed by ductile mechanisms. In order to determine how foliation and lattice preferred orientation of minerals affects the viscous strength of rocks, we are deforming cores of Gneiss Minuti, a fine-grained (~150μm) gneiss composed of plagioclase feldspar (45%), quartz (35%), K feldspar (15%), and biotite (5%), at varying angles to the foliation and stretching lineation. We have shortened cylinders at a strain rate of 1.6 X 10
-7/s, a temperature of 700°C, and confining pressure 1.5 GPa in a Griggs type triaxial deformation apparatus. Cylinders were collected in six orientations perpendicular, parallel and 45° to the foliation and/or the lineation. With these six orientations, it is possible to differentiate between anisotropy caused by foliation defined by dispersed biotite grains and anisotropy caused by a lattice preferred orientation in the Gneiss Minuti, an alignment of quartz <c> axes about the stretching lineation.
One experiment was performed on a cylinder with the foliation oriented 45° to the direction of compression, and parallel to the stretching lineation. In this orientation, the foliation is subjected to maximum shear stress, and the <c> axes of quartz grains are aligned favorably for slip along the basal plane in the direction of the <a> axis. This sample reached a peak stress of ~1100 MPa, and developed a set of conjugate shear zones, with a dominant shear zone cross-cutting the foliation. The minor shear zone parallel with the foliation contains interconnected biotite grains, while the major cross-cutting shear zone contains mixed, rounded recrystallized grains that are ~1-5μm, indicating that deformation was by crystal plastic mechanisms, likely diffusion creep of feldspars and mixed phases. The development of a cross-cutting shear zone in this sample suggests that the deformation mechanisms of the framework matrix of quartz and feldspar controls the strength rather than the micaceous foliation. These preliminary results indicate that at conditions which promote strain localization, foliation may not control the rheology of rocks.
