LARGE-SCALE RIGHT-SLIP DISPLACEMENT ON THE EAST SAN-FRANCISCO BAY-REGION FAULT SYSTEM, CALIFORNIA - IMPLICATIONS FOR LOCATION OF LATE MIOCENE TO PLIOCENE PACIFIC PLATE BOUNDARY

A belt of northwardly younging Neogene and Quaternary volcanic rocks a cid hydrothermal vein systems, together with a distinctive Cretaceous terrane of the Franciscan Complex (the Permanente terrane), exhibits a bout 160 to 170 km of cumulative dextral offset across faults of the E ast San Francisco Bay Region (ESFBR) fault system. The offset hydrothe rmal veins and volcanic rocks range in age from .01 Ma at the northwes t end to about 17.6 Ma at the southeast end. In the fault block betwee n the San Andreas and ESFBR fault systems, where volcanic rocks are sc arce, hydrothermal vein system ages clearly indicate that the northwar d younging thermal overprint affected these rocks beginning about 18 M a. The age progression of these volcanic rocks and hydrothermal vein s ystems is consistent with previously proposed models that relate north ward propagation of the San Andreas transform to the opening of an ast henospheric window beneath the North American plate margin in the wake of subducting lithosphere. The similarity in the amount of offset of the Permanente terrane across the ESFBR fault system to that derived b y restoring continuity in the northward younging age progression of vo lcanic rocks and hydrothermal veins suggests a model in which 80-110 k m of offset are taken up 8 to 6 Ma on a fault aligned with the Bloomfi eld-Tolay-Franklin-Concord-Sunol-Calaveras faults. An additional 50-70 km of cumulative slip are taken up less than or equal to 6 Ma by the Rogers Creek-Hayward and Concord-Franklin-Sunol-Calaveras faults. An a lternative model in which the Permanente terrane is offset about 80 km by pre-Miocene faults does not adequately restore the distribution of 8-12 Ma volcanic rocks and hydrothermal veins to a single nortbwardly younging age trend. If 80-110 km of slip was taken up by the ESFBR fa ult system between 8 and 6 Ma, dextral slip rates were 40-55 mm/yr. Su ch high rates might occur if the ESFBR fault system rather than the Sa n Andreas fault acted as the transform margin at this time. Major tran spression across; the boundary between the Pacific and North American plates at about 3 to 5 Ma would have resulted in the transfer of signi ficant slip back to the San Francisco Peninsula segment of the San And reas fault. Since that time, the ESFBR fault system has continued to s lip at rates of 11-14 mm/yr. If this interpretation is valid, the ESFB R fault system was the Pacific-North American plate boundary between 8 and 6 Ma, and this boundary has migrated both eastward and westward w ith time, in response to changing plate margin geometry and plate moti ons.