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Miocene nonmarine and shallow marine strata exposed east of San Francisco Bay record a change from convergent-margin tectonics to transform margin tectonics. During the middle Miocene, the East Bay area occupied the oceanward side of a shelved forearc basin that was progressively incorporated in the evolving San Andreas strike-slip orogene. Patterns of deposition in the broad forearc basin were relatively simple: andesitic arc-derived detritus was transported the full width of the forearc basin from the Sierras to the East Bay area. In contrast, the wrench-tectonic regime produced complex patterns of sedimentation displaying greater local variability. On the basis of stratigraphic data, we infer that the west-facing slope of the forearc basin in the East Bay area was reve sed about 13 Ma with uplift of the area between the eventual traces of the San Andreas and Hayward faults on the site of the present bay. A fluvial clastic wedge was shed eastward into the East Bay area from this uplifted terrane of Mesozoic subduction complex and forearc basin rocks. Initial rupturing along the Hayward fault trend followed the uplift at about 10 Ma. Loci of basaltic volcanism (10-7 Ma) along these fractures interfinger with the clastic wedge. A similar pattern of uplift and drainage reversal apparently presaged the onset of wrenching along the nearby Calaveras trend from 8-6 Ma. Expansion of the strike-slip orogene segmented the outer forearc basin into local basins, some characterized by episodic lacustrine deposition and probable internal drainage. By the end of the M ocene, Sierran arc volcanism waned at the latitude of San Francisco Bay, and arc-derived volcaniclastics were fully supplanted by recycled Coast Range-derived detritus in the East Bay area. Certain of these Coast Range sediment sources, particularly blueschist-bearing Franciscan terranes, permit an estimate of 7-27 km (4-17 mi) of total right slip on the Hayward fault.
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