GEOBAROMETRY OF GRANITOIDS AND U-PB DETRITAL ZIRCON GEOCHRONOLOGY OF OVERLYING SEDIMENTARY COVER IN THE NORTHERN SALINIAN BLOCK, CALIFORNIA

The Northern Salinian block (NSB) preserves a contact between early Mesozoic to late Cretaceous crystalline basement and Tertiary sedimentary rocks. The plutonic basement ranges from biotite- and hornblende-bearing tonalite to granodiorite, intruded into Sur Series pendant and screen metasedimentary rocks. Sedimentary rocks rest on the basement, and consist of conglomerate, chert, and turbidite sequences, which range from Paleocene to Miocene in age. The NSB originated some 150-200 km southeast of its current location, and is conjectured to be the upper plate of the Late Cretaceous Southern Sierra Nevada detachment system.

To determine the nature of the basement-sedimentary contact and the original tectonic setting of these rocks, we conducted a field, geochemical, and geochronological study of the granitoid basement and Paleocene deep-marine sedimentary cover at Montara Mountain and Bodega Head. At both locations, granitoids display early brittle-ductile shear zones crosscut by a late brittle deformation. Preliminary petrographic observations of the shear zone rocks show undulose extinction in quartz and brittle fracture in plagioclase, overprinted by a cataclastic fabric. It is not clear whether this deformation occurred during San Andreas-related deformation or during unroofing of the Southern Sierra Nevada. To approximate the paleogeographic location of the NSB, the Al-in-hornblende geobarometer of Schmidt (1992) was used on the granitoids, yielding crystallization pressures of 6.8 to 7.6 kbar at Montara and 3.9 kbar at Bodega Head. While the Bodega Head pressure is consistent with both the previously published data for the Southern Salinian block and the hypothesized upper crustal section in the Cummings Valley area east of Bakersfield (Wood and Saleeby, 1997), the Montara Mountain pressures are surprisingly high, similar to rock found in the footwall of the Southern Sierra detachment located in the Techachapi mountains. Additional data will be presented on U-Pb geochronology of detrital zircons from sandstones collected at Montara. Together, the NSB granitoid basement and overlying sedimentary rocks may preserve some of the history of extension and formation of a supradetachment basin during exhumation of the Southern Sierra Nevada.