Crustal structure of a transform plate boundary: San Francisco Bay and the central California continental margin

Abstract

Wide‐angle seismic data collected during the Bay Area Seismic Imaging Experiment provide new glimpses of the deep structure of the San Francisco Bay Area Block and across the offshore continental margin. San Francisco Bay is underlain by a veneer (<300 m) of sediments, beneath which 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 assemblage. 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 crust with an average velocity of 6.75 ± 0.15 km/s. The lower crust of the Bay Area Block may be oceanic in origin, but its structure and reflectivity indicate that it has been modified by shearing and/or magmatic 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 subhorizontal reflectivity observed at near‐vertical incidence resides principally in the lowermost layer, the top of which corresponds to the “6‐s reflector” of Brocher et al. [1994]. Rheological modeling suggests that the lower crust beneath the 6‐s reflector is the weakest part of the lithosphere; the horizontal shear zone suggested by Furlong et al. [1989] to link the San Andreas and Hayward/Calaveras fault systems may actually be a broad zone of shear deformation occupying the lowermost crust. A transect across the continental margin from the paleotrench to the Hayward fault shows a deep crustal structure that is more complex than previously realized. Strong lateral variability in seismic velocity and wide‐angle reflectivity suggests that crustal composition changes across major transcurrent fault systems. Pacific oceanic crust extends 40–50 km landward of the paleotrench but, contrary to prior models, probably does not continue beneath the Salinian Block, a Cretaceous arc complex that lies west of the San Andreas fault in the Bay Area. The thickness (10 km) and high lower‐crustal velocity of Pacific oceanic crust suggest that it was underplated by magmatism associated with the nearby Pioneer seamount. The Salinian Block consists of a 15‐km‐thick layer of velocity 6.0–6.2 km/s overlying a 5‐km‐thick, high‐velocity (7.0 km/s) lower crust that may be oceanic crust, Cretaceous arc‐derived lower crust, or a magmatically underplated layer. The strong structural variability across the margin attests to the activity of strike‐slip faulting prior to and during development of the transcurrent Pacific/North American plate boundary around 29 Ma.