MINERALOGY AND MICROFABRIC OF THE PUNCHBOWL FAULT, AN EXHUMED SEGMENT OF THE SAN ANDREAS FAULT SYSTEM

The Punchbowl fault is an exposed segment of the San Andreas System that was active between 1-4 Ma, and exhumed from a depth of 2-4 km, thereby providing the opportunity to study processes that may be analogous to those that are operative along today’s San Andreas fault. The methods used in this study are X-ray diffraction, X-ray texture goniometry, and scanning and transmission electron microscopy. Collectively these techniques characterize the microstructural and chemical changes that occurred along the Punchbowl fault and address the importance of mineral transformations and rock fabric on the hydro-mechanical behavior of the fault zone. Based on bulk rock X-ray analysis the fault rocks have a mineralogy that is distinct from the protolith, composed of various amounts of clinochlore, laumontite, quartz and albite, and lesser amounts of smectite ± illite, microcline and calcite. Significantly, most of the changes in mineralogy occurred after the cessation of faulting, and so it appears that mineralogical transformations were not very important during strike-slip faulting. Measurements of phyllosilicate fabrics across the Punchbowl fault using X-ray goniometry provide information on the permeability structure of the fault zone, and show that the phyllosilicate fabric in fault rocks is relatively weak, and therefore that permeability anisotropy is low. This eliminates the possibility that fluids were focused by a clay fabric while the fault was active. Thus, both phyllosilicate fabric and mineral transformation appear to have played a limited mechanical role during the displacement along this segment of the San Andreas fault system.