FOLIATION INTENSITY AND ORIENTATION AND THEIR AFFECT ON ROCK STRENGTH

After an earthquake, stresses in the upper crust are redistributed into the lower crust which results in crystal-plastic deformation. This deformation is currently modeled using strength of homogeneous quartzites, rather than the heterogeneous-polyphase granitic rocks found in the mid to lower crust. Foliations are common heterogeneities in crustal rocks and cause anisotropy in rock strength under brittle conditions, but little is understood about the effect of foliations on rock strength during ductile deformation. Foliation orientation is thought to cause a viscous anisotropy due to the shear stress increasing as foliation rotates from parallel to 45 degrees to the compression direction and decreases as foliation rotates to being perpendicular to the compression direction. Micas deform easily by dislocation glide when oriented at 45 degrees to the maximum compression direction, but are much stronger at less optimal orientations because slip across (001) is extremely difficult. In order determine how foliation orientation and foliation intensity affect the strength of rocks or cause viscous anisotropy, we deformed cores of Westerly Granite and Gneiss Minuti with foliation parallel, at 45 degrees to, or perpendicular to the compression direction. Experiments were performed using a Griggs apparatus at a temperature of 800°C, confining pressure of 1.5 GPa, and strain rate of 10^-6/s. Both of these rocks have low-mica content, similar phase percentages, and foliations which are defined by the alignment of biotite grains, but the alignment of biotite grains (foliation intensity) is much less uniform in Westerly Granite than in Gneiss Minuti. Westerly Granite had an average peak stress of 970 MPa and an anisotropy coefficient of 1.1x. Gneiss Minuti had an average peak stress of 700 MPa and an anisotropy coefficient of 1.2x. Experimental data from this study and comparable previous studies indicate that: 1) there is a relationship between increasing feldspar content and increasing peak stress and 2) foliation intensity does not appear to significantly affect the strength of rocks with weak micas isolated in strong framework phases.