MIOCENE VOLCANIC ROCKS AT BURDELL MOUNTAIN AND IMPLICATIONS FOR SLIP ALONG THE EAST BAY FAULT SYSTEM

Burdell Mountain, located 50 km north of San Francisco, California, consists of Cretaceous and Jurassic Franciscan Complex and Great Valley Sequence rocks overlain by 65 m of Tertiary marine sandstone, and 200 m of Miocene volcanic rocks. The Burdell Mountain volcanics (BMV) are approximately 200 m thick and consist dominantly of flow-banded, porphyritic andesite, and lesser amounts of volcanic breccia, volcaniclastic mudflow deposits, and minor flow-banded rhyolite. The BMV together with the underlying Tertiary marine strata generally form a moderately, northeast-dipping (~30˚) homocline that is truncated by the Burdell Mountain fault zone (BMFZ) to the east. The age and petrology of the volcanics and stratigraphic relations of the Burdell Mountain area are important to ongoing studies of long-term right-lateral displacement along the East Bay portion of the San Andreas fault system. McLaughlin et al. (1996) estimated a maximum offset of 175 km along the East Bay fault system (EBFS) based, in part, on a correlation between the BMV and similar rocks of the Quien Sabe volcanic field (QSV) in central California. However, previous radiometric ages (K/Ar and Ar/Ar) for the Burdell Mountain volcanics (11.8-13.6 ma) do not match those for the QSV (7.4-11.6 Ma). Three new Ar/Ar ages for the BMV cluster around 11.1 Ma establishing an age correlation to the QSV. The age and lithology of the rhyolite of the BMV are also similar to the Northbrae rhyolite in the Berkeley Hills suggesting a minimum right-lateral offset of 30 km between these two regions. We will test further the BMV-QSV correlation by focusing on petrographic and stratigraphic comparisons of the two regions. Few studies have focused on the BMFZ, a complex, northwest-striking, 500 m-wide, subvertical, shear zone characterized by local hydrothermal alteration and silicification. Map relations suggest that the fault zone exhibits up to 10 km of right-lateral offset of the distinctive Cretaceous Novato Conglomerate of the Great Valley Sequence, implying that the BMFZ is an important component of the EBFS. Youthful tectonic geomorphic features along the (unzoned) fault suggest late Holocene movement. Future efforts will focus on locating a trench site suitable for providing information on the age of faulting.