STRENGTH AND ANISOTROPY OF FOLIATED ROCKS WITH VARIED MICA CONTENTS

We have shortened 15 schists and gneisses with varying compositions (15-75% mica by volume) at angles (beta) of 45-degrees and 90-degrees to foliation (S) to investigate the influence of micas on the strength and anisotropy of foliated rocks. At the conditions tested (T = 25-degrees-C, P(c) = 200 MPa and epsilon = 10(-5) s-1), compressive strengths vary by a factor >4 in the beta = 45-degrees and beta = 90-degrees orientations, and individual rock types exhibit directional responses ranging from isotropic to strongly anisotropic. Trends of decreasing strength, decreasing anisotropy and increasing ductility are observed with increasing mica content. Strains in all samples were localized within inclined shear zones, accommodated by dislocation slip and grain-scale microkinking in micas and microcracking in all other silicates. Stress-strain response and grain-scale deformation microstructures both indicate that mechanical behavior is strongly influenced by the concentration and spatial aff angement of micas. Shear zone formation is associated cither with discrete brittle fracture, transitional strain softening or steady-strength ductile shear. In both brittle and transitional samples, extensive microcracking of strong quartzo-feldspathic bridges occurs, and stress drop magnitudes decrease with decreasing spacing and increasing overlap of adjacent, critically-oriented micas. Steady-strength ductile shear zones develop in samples containing domains of interconnected micas, and isotropic mechanical response is observed in those in which micas are contiguous in nearly all directions.