CRUSTAL STRUCTURE OF A TRANSFORM PLATE BOUNDARY - SAN-FRANCISCO BAY AND THE CENTRAL CALIFORNIA CONTINENTAL-MARGIN

Wide-angle seismic data collected during the Bay Area Seismic Imaging Experiment provide new glimpses of the deep structure of the San Franc isco Bay Area Block and across the offshore continental margin. San Fr ancisco Bay is underlain by a veneer (<300 m) of sediments, beneath wh ich P wave velocities increase rapidly from 5.2 km/s to 6.0 km/s at 7 km depth, consistent with rocks of the Franciscan subduction assemblag e. The base of the Franciscan at 15-18 km depth is marked by a strong wide-angle reflector, beneath which lies an 8- to 10-km-thick lower cr ust with an average velocity of 6.75+/-0.15 km/s. The lower crust of t he Bay Area Block may be oceanic in origin, but its structure and refl ectivity indicate that it has been modified by shearing and/or magmati c intrusion. Wide-angle reflections define two layers within the lower crust, with velocities of 6.4-6.6 km/s and 6.9-7.3 km/s. Prominent su bhorizontal reflectivity observed at near-vertical incidence resides p rincipally in the lowermost layer, the top of which corresponds to the ''6-s reflector'' of Brocher et al. [1994]. Rheological modeling sugg ests that the lower crust beneath the 6-s reflector is the weakest par t of the lithosphere; the horizontal shear zone suggested by Furlong e t al. [1989] to link the San Andreas and Hayward/Calaveras fault syste ms may actually be a broad zone of shear deformation occupying the low ermost crust. A transect across the continental margin from the paleot rench to the Hayward fault shows a deep crustal structure that is more complex than previously realized. Strong lateral variability in seism ic velocity and wide-angle reflectivity suggests that crustal composit ion changes across major transcurrent fault systems. Pacific oceanic c rust extends 40-50 km landward of the paleotrench but, contrary to pri or models, probably does not continue beneath the Salinian Block, a Cr etaceous are complex that lies west of the San Andreas fault in the Ba y Area. The thickness (10 km) and high lower-crustal velocity of Pacif ic oceanic crust suggest that it was underplated by magmatism associat ed with the nearby Pioneer seamount. The Salinian Black consists of a 15-km-thick layer of velocity 6.0-6.2 km/s overlying a 5-km-thick, hig h-velocity (7.0 km/s) lower crust that may be oceanic crust, Cretaceou s are-derived lower crust, or a magmatically underplated layer. The st rong structural variability across the margin attests to the activity of strike-slip faulting prior to and during development of the transcu rrent Pacific/North American plate boundary around 29 Ma.