COMPOSITE PLANAR FABRIC OF GOUGE FROM THE PUNCHBOWL FAULT, CALIFORNIA
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The structure of clay-bearing gouge from the Punchbowl Fault zone, a brittle fault of the San Andreas system in southern California, is examined at the microscopic scale. The right-lateral, oblique-slip fault consists of a single, continuous gouge zone bounded by a zone of damaged host-rock. The gouge has a statistically homogeneous, composite planar fabric that consists of a foliation defined by the preferred orientation of clay, porphyroclasts and compositional lamination, and a planar anisotropy defined by zones of localized high shear strain (shear-bands). The foliation appears to be related to the accumulation of finite strain and to rotate towards the shear-band orientation with an increase in shear strain. In the Punchbowl gouge the sense of shear on individual shear-bands, and the asymmetric disposition of the planar fabrics with respect to each other and to the boundaries of the zone, appear to be valid indicators of the overall sense and direction of shear. Local variation in preferred orientation of fabric elements exists in the gouge, and therefore statistically based sampling and analysis of the fabric is necessary to infer shear sense and direction. 1987.