KINEMATICS OF TRANSPRESSIONAL DEFORMATION IN THE EASTERN SAN-FRANCISCO BAY-REGION, CALIFORNIA

Inversions of earthquake focal mechanisms for brittle strain show that the dextral Hayward fault forms the western boundary of a distinct se ismotectonic domain in the eastern San Francisco Bay region, East of t he Hayward fault, the seismogenic strain held is characterized by subh orizontal extension oriented northwest-southeast and subhorizontal sho rtening oriented northeast-southwest. Major folds and thrust faults ea st of the Hayward fault are oriented normal to the direction of maximu m shortening, but have formed in a predominantly strike-slip tectonic setting and exhibit a right-stepping, en echelon geometry typical of d extral wrench folds, The direction of maximum right-lateral shear stra in in this domain is similar to N20 degrees W, which generally is para llel to the strike of the dextral Calaveras, Greenville, and Concord f aults, but oriented similar to 15 degrees-25 degrees more northerly th an the strike of the San Andreas and Hayward faults to the west, The c lockwise rotation of maximum right-lateral shear strain from west to e ast across the Hayward fault is confirmed by geodetic measurements, Th e kinematic consequences of this variation in regional strain for defo rmation east of the Hayward fault include the following, (1) Dextral f aults must form left-restraining contractional stepovers in order to m aintain continuity and conserve slip along strike, thus creating local ized fold-and-thrust belts, (2) Dextral faulting accommodates a compon ent of shortening normal to the boundary between the Pacific plate and the Sierra Nevada-Central Valley microplate, thus obviating the need for a laterally continuous, boundary-parallel zone of thrust faulting along the eastern margin of the Coast Range (Coast Ranges-Sierran bloc k boundary zone) to accommodate plate-normal motion at the latitude of the East Bay domain, The transpressional kinematics of the East Bay d omain may be present in other obliquely convergent orogenic belts, and they have implications for seismic hazard assessment in the highly ur banized San Francisco Bay area.