Spatial variability in microscopic deformation and composition of the Punchbowl fault, southern California: implications for mechanisms, fluid–rock interaction, and fault morphology

Abstract We examine the distribution and nature of microstructures, geochemistry, and mineralogy along two traverses across the Punchbowl fault, southern California, to determine the morphology and deformation mechanisms of the fault zone in schistose rocks. The Punchbowl fault is an exhumed fault that has two main strands of slip localization and has a total of 44 km of right-slip. Protolith in the study area consists of the Pelona Schist, which is primarily a quartz–albite–muscovite–actinolite schist with thin to medium banding, and rare metabasalts. The traverses 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 exposed. 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 damaged zone is marked by an increase in veins, thin cataclasite bands, inter- and intragranular fracturing, and alteration relative to the country 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-thick, very fine-grained cataclasite that experienced repeated alteration, vein injection and grain-size reduction. Whole-rock geochemical analyses of the fault-related rocks suggest that the geochemically defined fault zone is less than 10 m thick. Volume loss at the site with one fault strand appears to have been small. The dominant alteration reactions associated with the fault core are the hydration of hornblende and actinolite accompanied by the alteration of muscovite to produce a quartz ± chlorite ± albite ± epidote ultracataclasite. The composition 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 region between the two strands experienced a greater degree of deformation than the protolith, but the total deformation is much less than immediately adjacent to the fault cores. The total thickness of the damaged zone around the two strands is less than 200 m. The fault core enveloped by a damaged-zone morphology, as well as the textures of the fault-core rocks are similar to rocks associated with the North Branch San Gabriel fault, which formed in crystalline rocks with a total displacement of 22 km. Fault thickness is less in the Pelona Schist than in the crystalline rocks, perhaps owing to more efficient strain localization in the schists. Thus, the faults of the San Andreas system may be thinner in regions where schists or Franciscan rocks are the protolith, but the main fault core may be a constant feature of the fault zone. Transformation-induced weakening is less important in this part of the Punchbowl fault, since the protolith has a large amount of mica. The structure of the fault zone with two principal slip surfaces is marked by both chemical changes and microstructures, and indicates that some parts of the San Andreas fault system may consist of multiple slip surfaces, each with a damaged zone, that together may create a fault zone >100 m thick, and in which slip is localized to zones meters to decimeters thick.