NEW CONSTRAINTS ON THE AGE AND DISTRIBUTION OF CENOZOIC VOLCANICS NORTH OF SAN PABLO BAY, CALIFORNIA:IMPLICATIONS FOR DISPLACEMENT ALONG FAULTS INBOARD OF THE SAN ANDREAS FAULT

Most of the volcanic fields in the San Francisco Bay Region are part of a succession of volcanic fields located along the trend of the San Andreas Fault System (SAF) that become younger to the northwest within each fault block . These fields include the Quien Sabe Volcanics (QSV), volcanic rocks in the Berkeley Hills (BHV), the Burdell Mountain Volcanics (BMV), the Tolay Volcanics (TV), the Sonoma Volcanics (SV), and the Clear Lake Volcanics. All of these fields are complex, heterogeneous, bimodal volcanic suites inferred to be the result of mantle upwelling behind a slab window trailing the Mendocino triple junction. The Sonoma Volcanic field differs from the other fields within the San Francisco Bay Region by being much more voluminous, by containing the only significant amount of ignimbrites, and by being active much longer. The BMV and the TV were once considered to be part of the SV, but with the availability of radiometric ages, it became clear that most of the volcanic rocks west of the Rodgers Creek Fault in southern Sonoma and Marin counties are older than the SV. Recent Ar/Ar results show that many K/Ar ages in the North Bay region are too old and that volcanics in the Cotati/Petaluma area correlated to the BMV are part of the TV. New ages indicate that the informally named Donnell Ranch volcanics are also part of the TV. Ranges of ages obtained for the volcanic fields are: BMV-10.59 to 11.18 Ma, TV-10.64 to 8.49 Ma, SV-8.17 to 2.5 Ma. The TV correlate with the BHV indicating about 40-45 km of displacement along the Hayward and Rodgers Creek faults. The BMV have been correlated with the QSV suggesting about 170 km of displacement along the Calaveras-Hayward-faults as well as unknown faults west of Petaluma Valley (Burdell Mtn. and related faults?). The distribution of the TV indicated by these new dates requires that displacement along the Petaluma Valley Fault is much less than previously suggested and that little or no lateral displacement along the Tolay Fault.