LATE QUATERNARY DEFORMATION IN THE NORTHEASTERN EAST BAY HILLS, SAN FRANCISCO BAY REGION

The youthful East Bay Hills structural domain contains a series of left-stepping, en echelon dextral faults and lineaments, collectively termed the "Contra Costa Shear Zone" (CCSZ), which extends about 50 km from the northern Calaveras fault to the West Napa fault. The CCSZ includes NNW-striking sections of the Reliez Valley, Lafayette, Southampton and other faults, and locally exhibits prominent geomorphic features suggestive of late Quaternary activity. Dextral slip on the northern Calaveras fault is transferred into the interior of the northern East Bay Hills via the CCSZ through complex deformation along discontinuous, NNW-striking dextral slip faults and intervening sections of NW-striking reverse faults and folds. The 1977 “Briones swarm”, a NNW alignment of small earthquakes with dextral focal mechanisms, is spatially associated with one of the lineaments of the CCSZ. Post-Miocene slip (ca. 11 to 13 km) accommodated by shortening and distributed strike-slip faulting in the northern East Bay Hills is similar to the total post-Miocene offset on the northern Calaveras fault, which suggests that most if not all of the motion on this fault steps northwestward into the East Bay Hills in a restraining geometry, rather than eastward onto the Concord fault through a releasing stepover. Detailed field mapping of three emergent marine platforms with shoreline-angle elevations of about 4 m, 12 m, and 18 m along Carquinez Strait provides additional data to test this structural model. The 12-m-high and 18-m-high platforms correlate with substage 5e (ca. 120 ka) and stage 9 (ca. 330 ka) sea-level high stands, respectively, and suggest that the East Bay Hills west of the Southampton fault have undergone block uplift at a rate of 0.05 +/- 0.01 m/ka without substantial tilting. With resolution of about 3m, the 120 ka and 330 ka shoreline angles are not displaced across the inactive Franklin fault, but exhibit substantial vertical displacement and gentle westward tilting across the NNW-striking Southampton fault. Near Carquinez Strait, the Southampton fault forms the western margin of the CCSZ and extends northward to the West Napa fault. We interpret that the CCSZ forms a complex, poorly integrated connection between the dextral northern Calaveras and West Napa faults, which is supported by recent GPS-derived surface velocities.