THE COMPOSITION AND AGE OF PSEUDOTACHYLYTE AND WHITE NATURAL GLASS FROM THE HEALDSBURG - RODGERS CREEK FAULT ZONE, TAYLOR MOUNTAIN, SONOMA COUNTY, CALIFORNIA AND ITS IMPLICATION FOR THE AGE OF THE SAN ANDREAS FAULT SYSTEM

One of the main splays of the Rodgers Creek fault zone is composed entirely of bands and knots of two glasses, a typical dark gray pseudotachylyte and parallel bands of light colored glass. Microprobe, XRF, mass spectroscopy, and wet geochemical analyses show that the host rock, a high silica rhyolite tuff, and both fault glasses have similar bulk chemical compositions. The chemical variability of the light colored glass is much greater than that of the pseudotachylyte or host rhyolite. None of the original rhyolitic minerals, including the zircon, remain in the glasses. Scanning Electron Microscopy appears to show that both fault glasses may be slightly vesiculated. This seems to be the first report of pseudotachylyte in a volcanic tuff host rock, which is likely the cause of its unusual double glass. This also seems to be the first report of halite in fault glass. Ar / Ar age determinations are consistently 7.6 my. The Rodgers Creek fault is a significant member of the San Andreas fault system. We believe that our work shows that the Rodgers Creek fault and the San Andreas stress system are at least as old as the date of the fault glass generation.