SPATIAL VARIABILITY IN MICROSCOPIC DEFORMATION AND COMPOSITION OF THEPUNCHBOWL FAULT, SOUTHERN CALIFORNIA - IMPLICATIONS FOR MECHANISMS, FLUID-ROCK INTERACTION, AND FAULT MORPHOLOGY

We examine the distribution and nature of microstructures, geochemistr y, and mineralogy along two traverses across the Punchbowl fault, sout hern California, to determine the morphology and deformation mechanism s of the fault zone in schistose pocks. The Punchbowl fault is an exhu med fault that has two main strands of slip localization and has a tot al of 44 km of right-slip. Protolith in the study area consists of the Pelona Schist, which is primarily a quartz-albite-muscovite-actinolit e schist with thin to medium banding, and rare metabasalts. The traver ses are 1.3 km apart, and were conducted at a site with a single fault strand and a site where both principal strands of the fault are expos ed. The fault-zone thickness is a function of the type of measurement that is used to define it. For the single strand site, analysis of the distribution of microfractures shows that the fault zone consists of a roughly 40-m-thick damaged zone adjacent to the fault core. The dama ged zone is marked by an increase in veins, thin cataclasite bands, in ter- and intragranular fracturing, and alteration relative to the coun try rock. Brittle grain-size reduction occurs in a zone 10 m thick as measured from the fault core, which consists of a continuous, 10-cm-th ick, very fine-grained cataclasite that experienced repeated alteratio n, vein injection and grain-size reduction. Whole-rock geochemical ana lyses of the fault-related rocks suggest that the geochemically define d fault tone is less than 10 m thick. Volume loss at the site with one fault strand appears to have been small. The dominant alteration reac tions associated with the fault core are the hydration of hornblende a nd actinolite accompanied by the alteration of muscovite to produce a quartz +/- chlorite +/- albite +/- epidote ultracataclasite. The compo sition of the fault core is variable and locally influenced by one of the adjacent protoliths. The examination of the two fault strand sites shows that two damaged-zone fault-core structures are present. The re gion between the two strands experienced a greater degree of deformati on than the protolith, but the total deformation is much less than imm ediately adjacent to the fault cores. The total thickness of the damag ed zone around the two strands is less than 200 m. The fault core enve loped by a damaged-zone morphology, as well as the textures of the fau lt-core rocks are similar to rocks associated with the North Branch Sa n Gabriel fault, which formed in crystalline rocks with a total displa cement of 22 km. Fault thickness is less in the Pelona Schist than in the crystalline rocks, perhaps owing to more efficient strain localiza tion in the schists. Thus, the faults of the San Andreas system may be thinner in regions where schists or Franciscan rocks are the protolit h, but the main fault core may be a constant feature of the fault zone . Transformation-induced weakening is less important in this part of t he Punchbowl fault, since the protolith has a large amount of mica. Th e structure of the fault zone with two principal slip surfaces is mark ed by both chemical changes and microstructures, and indicates that so me parts of the San Andreas fault system may consist of multiple slip surfaces, each with a damaged zone, that together may create a fault z one >100 m thick, and in which slip is localized to zones meters to de cimeters thick. (C) 1998 Elsevier Science B.V. All rights reserved.