--> Abstract: The Tectonic Framework, Origin, and Evolution of the San Francisco Bay Region, by R. L. Sedlock; #90958 (1995).

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Abstract: The Tectonic Framework, Origin, and Evolution of the San Francisco Bay Region

Richard L. Sedlock

In this paper I summarize our current understanding of the tectonic framework and late Cenozoic tectonic origin and evolution of the San Francisco Bay region.

The NUVEL-1 plate motion model predicts about 36 mm/yr dextral slip on faults of the San Andreas system at this latitude. There is general consensus on the slip rates on major dextral slip faults in the Bay Area (excluding San Gregorio), which sum to 35-37 mm/yr between the coast and the Central Valley. There is a long-term redistribution of slip to successively more eastward faults. Plate motion and geologic models also predict 0-6 mm/yr margin-normal contraction. Geologic estimates of modern shortening rates across the Bay Area are not yet available.

Late Quaternary subsidence rates of the San Francisco Bay and Santa Clara Valley are estimated to be 0.2-1.0 mm/yr. The age of the underlying bedrock depression is uncertain and may vary throughout the region, but a reasonable estimate for the initiation of both the bay depression and the modern ranges that bound it is about 1 Ma. Late Quaternary uplift rates for these ranges are estimated to be 0.2-2 mm/yr. Possible uplift mechanisms include block uplift, which implies identifiable block-bounding faults; and ductile deformation, which implies plastic behavior of upper crustal rocks.

Several models, some of which are mutually exclusive, are available to explain the origin of the San Francisco Bay depression and its bounding ranges. (1) NE-SW extension results in "Basin and Range" topography. (2) San Francisco Bay, San Pablo Bay, or both are pull-apart basins at releasing bends or jumps in dextral slip fault systems. (3) Some or most of the mountainous uplifts are produced at restraining bends or jumps in these fault systems. (4) NE-SW shortening causes NW-trending crustal flexure, with upthrown regions straddling a downdropped central depression. (5) NE-SW shortening builds mountains with a NW-striking structural grain, but the central bay block remains near sea level. (6) NE-SW shortening builds mountains and causes thrust loading and downward deflection of the b y block; loading and basins may be symmetrical or asymmetrical.

Surprisingly few data are available with which to evaluate these models. We are still far from understanding the origin and evolution of the San Francisco Bay region.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California