FAULT-RELATED PROCESSES INFERRED FROM AUTHIGENIC MINERAL GROWTH AND PHYLLOSILICATE FABRIC DEVELOPMENT ACROSS THE PUNCHBOWL FAULT, AN EXHUMED FAULT SEGMENT OF THE SAN ANDREAS FAULT SYSTEM

The Punchbowl fault is an exhumed segment of the San Andreas Fault system that is exposed in the San Gabriel Mountains of southern California and provides the opportunity to study processes occurring near the seismogenic zone of active strike-slip faults. The fault has been exhumed from a depth of 2 to 4 km and was active from 1 to 4 Ma, with an estimated right lateral displacement of >40 km. The rocks in this region are composed of Mesozoic metasedimentary and igneous rocks, and Tertiary conglomerate. In the study area, granitic rock is juxtaposed against conglomerate with a sandy matrix and sandstone. Based on bulk rock X-ray analysis the fault core has a mineralogy that is distinct from the protolith, and is composed primarily of clinochlore, laumontite, quartz and albite and lesser amounts of illite, smectite, microcline and calcite. The reaction labradorite to laumontite and albite indicates a pressure less than 300 MPa and a temperature between 150 and 300º C. Measurements of phyllosilicate fabrics across the Punchbowl fault using X-ray goniometry provide information about the permeability structure of the fault zone. In the fault core the fabric intensity decreases 30-50% from the protolith, and so the anticipated effect of better aligned phyllosilicates under faulting-induced stress conditions is being counteracted, possibly by fluid-assisted growth during fault activity. Permeability anisotropy in gouge (with higher permeability parallel to the fault than across it) has been proposed as a mechanism by which elevated fluid pressure can be maintained, but this is not supported by our observations.