CHRONOSTRATIGRAPHY OF MIOCENE STRATA IN THE BERKELEY HILLS (CALIFORNIA COAST RANGE, USA) AND CALIBRATION OF STRIKE-SLIP TECTONICS DURING MIOCENE EVOLUTION OF THE SAN ANDREAS FAULT SYSTEM

The bedrock geology of the Berkeley Hills is dominantly composed of Miocene (23-5 Ma) sedimentary and volcanic rocks deposited in pull-apart basins that developed along the San Andreas Fault system. The ages of these rocks are poorly constrained by less precise and less accurate methods such as biostratigraphic and K-Ar dating. In this study, we used laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and isotope dilution-thermal ionization mass spectrometry (ID-TIMS) to conduct a provenance study and determine high precision U-Pb dates for the zircons within these sedimentary and volcanic rocks. Three formations were included in the provenance study: the marine Claremont Formation (alternating chert and shale beds with minor sandstone), the terrestrial Orinda Formation (alluvial conglomerate with sandstone beds), and the terrestrial Moraga Formation (basalt flows, dacitic tuffs and interflow sandstones). We found that the sandstone within the Claremont Formation has a wide spread of zircon ages with a peak at 185-80 Ma and a population of Miocene-aged zircons that are not seen in the Orinda and Moraga sandstone samples. These Miocene zircons were targeted using ID-TIMS to establish a maximum depositional age for the Claremont Formation. The sandstone samples from the Orinda and Moraga formations have synonymous detrital zircon age distributions suggesting similar provenance. Clasts within the Orinda Formation indicate a source consistent with a highland of Franciscan lithologies shedding detritus into the basin during both Orinda deposition and Moraga volcanism. High precision ID-TIMS dates for the zircons within the Claremont Formation, and a dacitic tuff within the Moraga Formation, constrain an interval of rapid uplift that occurred during the late Miocene. Regional stratigraphy, coupled with new radiometric constraints, suggests that the upper portion of the Monterey Group was deposited and later eroded prior to the deposition of the Orinda Formation. Further radiometric constraints for Miocene formations in the California Coast Range will be crucial to calibrating the tectonic evolution of the San Andreas Fault system.