TRANSITION FROM TRANSTENSION TO TRANSPRESSION: TECTONIC INVERSION WITHIN A BAND OF DEFORMATION BETWEEN THE HAYWARD, CHABOT AND MISSION FAULTS

The Hayward, Chabot, Mission, and other unnamed fault intersect in the East Bay hills between Hayward and Fremont, Northern California. The intersection and interaction of these faults has produced a broad zone of deformation between the Bay Block to the west, and the East Bay Hills Block (EBB) to the east. The near vertical to slightly east dipping Hayward Fault (HF), the westernmost of these faults, serves as a rough delineator of these two blocks and shows transpressive motion. This fault trends N40W, and is presently the most active fault in this area. The moderate to steeply east dipping Chabot fault (CF), the next fault to the east, trends N22W to the north of latitude 37°37’N. South of this Latitude, the CF trend changes to N43W in the south, roughly parallel to the HF. North of this bend, the CF intersects the slightly east dipping Mission fault (MF). The MF is the farthest to the east, trends N54W, and is currently seismically active and shows transtensive motion. The CF is an ancestral trace to the HF, recorded an earlier transtensive tectonic setting, and was preserved as the active transpressive trace of the HF developed to the west, uplifting the East Bay hills block. The HF juxtaposes the Jurassic through Cretaceous Knoxville fm. (JKk) over and alongside the Cretaceous Marin Headlands Terrane (MHT) chert, basalt, and greywacke. The CF then places folded Miocene Monterey (Tm), Briones (Tbr), Orinda (Tor), and minor upper Great Valley Sequence (GVSu) of the McConnell syncline, over JKk. The MF then puts GVSu and minor Tm over the rocks of the McConnell syncline. Provenance data and field relationships suggest that the MHT was the source terrane for the Tor, indicating that: 1) the Tor formed in a basin east of the CF in a right-lateral transtensive regime, and 2) a tectonic inversion occurred, uplifted the EBB, and creating the HF.