Tectonics of the San Francisco Bay Region
- Trevor Dimitru
- George Hilley
The mountain ranges in the San Francisco Bay region have been generated mainly by components of shortening across the late Cenozoic San Andreas transform plate boundary between the North American and Pacific plates. Mountains are generated by the region-wide component of shortening across the boundary and by more localized bends, step-overs, and irregularities in the San Andreas fault system. The active thrust faults that accommodate the shortening represent a significant earthquake hazard in the region, although they are considerably less important as a hazard than the major strike-slip faults.
The shortening in the San Francisco region is also important in that it has uplifted and exposed rocks from the pre-San Andreas period of Franciscan subduction. We are undertaking thermochronologic analyses of San Andreas and Franciscan tectonics throughout the Bay region. This webpage highlights results from Mt Diablo anticline, an active structure about 40 km east of San Francisco. Details are available in:
Unruh, J.R., T.A. Dumitru, and T.L. Sawyer, 2007, Coupling of early Tertiary extension in the Great Valley forearc basin with blueschist exhumation in the underlying Franciscan accretionary wedge at Mt. Diablo, California, Geological Society of America Bulletin, v. 119, p. 1347–1367.
Mt Diablo anticline is an actively growing structure that has been uplifted by major post-3.5 Ma motion on the Mt Diablo blind thrust fault. The core of the anticline exposes parts of the older Coast Range fault, which forms the fundamental contact between high-pressure rocks of the Franciscan subduction complex and unmetamorphosed rocks of the Great Valley forearc basin.
Integrated apatite FT and apatite He data from the anticline have been modeled to draw two main conclusions about this area. First, the 3.5 Ma burial temperatures of samples as indicated by the data indicate that about 4 km of section have been eroded off the top of the anticline in post-3.5 Ma time. This constraint has guided the construction of retrodeformable cross-sections across the anticline. These reconstructions define a broad, flat-crested, asymmetric anticline with a maximum structural relief of about 4 km. A maximum of 8 km of slip is needed on the Mt Diablo thrust to generate the anticline and related structures. This is significantly less than some alternative reconstructions that do not honor the FT and He data. This implies slower mean shortening rates and reduced seimic risk from the blind thrust, if it is assumed that mean 3.5 to 0 Ma slip rates can be extrapolated to modern slip rates (which cannot be rigorously demonstrated) .
Second, Franciscan samples exposed beneath the Coast Range fault yield early Tertiary FT ages, much younger than the ages of Great Valley forearc basin samples above the fault. These ages record early Tertiary rise and exhumation of the high-pressure Franciscan rocks. An extensional graben system was active in the forearc basin in early Tertiary time, evidence of an extensional state of stress. This indicated that high-pressure rocks in this area where exhumed by synsubduction extensional processes, rather than erosional processes.