Upper Miocene nonmarine and shallow-marine strata exposed east of San Francisco Bay record a change from convergent-margin to transform-margin tectonics. During the late Miocene, the East Bay area occupied the oceanward side of a shelved fore-arc basin which was progressively incorporated in the evolving San Andreas strike-slip orogene. Patterns of deposition in the broad fore-arc basin were relatively simple: andesitic arc-derived detritus was transported the full width of the fore-arc basin from the Sierras to the East Bay area. In contrast, the wrench-tectonic regime produced complex patterns of sedimentation displaying greater local variability. Based on stratigraphic data from D. Wagner and our observations, we infer that the west-facing slope of the fore-arc basin i the East Bay area was reversed about 13 m.y. ago, with uplift of the area between the eventual traces of the San Andreas and Hayward faults. A fluvial clastic wedge was shed eastward into the East Bay area from this uplifted terrane of Mesozoic subduction-complex and fore-arc basin rocks. Initial rupturing along a "proto-Hayward fault zone" followed the uplift about 10 m.y. ago. Loci of basaltic volcanism (10 to 7 m.y.) along these fractures interfinger with the clastic wedge. A similar pattern of uplift and drainage reversal apparently heralded the onset of wrenching along the Calaveras trend 6 to 8 m.y. ago. Expansion of the strike-slip orogene segmented the outer fore-arc basin into local basins, some characterized by periodic lacustrine deposition and probable internal drainage. By he end of the Miocene, 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, serve to constrain estimates of strike-slip on the Hayward fault.

Although tectonics provide the principal control on patterns and composition of Neogene sedimentation in the East Bay area, late Miocene climatic fluctuations left a marked overprint. Red beds, typical of the lower upper Miocene and suggestive of seasonal aridity, correlate with a relatively cold period along the northeast margin of the Pacific Ocean. In contrast, overlying middle upper Miocene fluvial-lacustrine beds are gray brown and contain abundant carbonaceous debris and faunas indicative of a marked decrease in aridity. These strata correspond to a period of warming in the later Miocene prior to latest Miocene refrigeration (Messinian).

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