THE GEOLOGY OF LAS TRAMPAS AND ROCKY RIDGES, ALAMEDA AND CONTRA COSTA COUNTIES, CALIFORNIA

Stratigraphic measurements in the hills east of San Francisco Bay (the East Bay Hills) have revealed evidence of local tectonism coincident with the transition from a convergent boundary to a transform margin at this latitude in the middle to late Miocene. Detailed mapping has revealed structures suggesting the onset of local transpression sometime after ~ 6.5 Ma.

The stratigraphy suggests a basin evolving from an open-marine environment to a restricted estuarine environment then to a fluvial environment before emergence. This sequence is interpreted as the result of local tectonism driven by the nascent San Andreas transform. The thick stratigraphic section of over 1,200 m of shallow-water deposits precludes shoaling simply due to falling sea level. The basin must have subsided to accumulate the thick sequence. Basin shallowing apparently resulted from high sediment influx.

The structure of the study area can be explained by local transpression. The East Bay Hills lie between two active dextral strike-slip faults within the San Andreas transform system; the Hayward fault to the west and the Calaveras fault to the east. The trend of the fold axes and the strike of reverse faults is parallel to the Hayward fault, indicating an initial period of deformation by contraction normal to that fault. This structure is interpreted as the result of evolving transpression within the San Andreas transform, wherein the antecedent Hayward fault accommodated the shear component of transpression by dextral strike-slip, so that only the compressive component remained to produce the fault-normal contractile deformation. The original, purely contractile deformation was then overprinted with a suite of faults driven by subsequent rupture on the Calaveras fault to the east. The association of the several faults in the study area with two active major faults within the San Andreas system, suggests that they may have the potential for modern movement.