Phyllosilicate mineral assemblages of the SAFOD Pilot Hole and comparison with an exhumed segment of the San Andreas Fault System

Abstract

This paper establishes a reference phyllosilicate data set from the SAFOD Pilot Hole for future SAFOD drilling and presents an application of these data for studies of exhumed fault segments. The chlorite assemblages in cuttings from two intervals of the SAFOD Pilot Hole are separated into two populations based on X-ray diffraction characteristics. The first population is found in granite in both the deeper and shallower interval, whereas the second population occurs only in clastic sedimentary rocks in the shallower interval. The characteristics of the first population match those for protolith and cataclasite of the exhumed Punchbowl Fault, whereas samples from intensely deformed ultracataclasite are most similar to the second. This supports previous findings that the mineral assemblages in the ultracataclasite formed after the cessation of motion along the fault, above a depth of ∼2 km, and that mineral assemblages in these exhumed fault rocks have been overprinted by post-faulting alteration. Copyright 2004 by the American Geophysical Union.