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Kinematics of transpressional deformation in the eastern San Francisco Bay region, California

¹ William Lettis & Associates, Inc., 1777 Botelho Drive, Suite 262, Walnut Creek, California 94596

Inversions of earthquake focal mechanisms for brittle strain show that the dextral Hayward fault forms the western boundary of a distinct seismotectonic domain in the eastern San Francisco Bay region. East of the Hayward fault, the seismogenic strain field is characterized by subhorizontal extension oriented northwest-southeast and subhorizontal shortening oriented northeast-southwest. Major folds and thrust faults east of the Hayward fault are oriented normal to the direction of maximum shortening, but have formed in a predominantly strike-slip tectonic setting and exhibit a right-stepping, en echelon geometry typical of dextral wrench folds. The direction of maximum right-lateral shear strain in this domain is N20°W, which generally is parallel to the strike of the dextral Calaveras, Greenville, and Concord faults, but oriented 15°–25° more northerly than the strike of the San Andreas and Hayward faults to the west. The clockwise rotation of maximum right-lateral shear strain from west to east across the Hayward fault is confirmed by geodetic measurements. The kinematic consequences of this variation in regional strain for deformation east of the Hayward fault include the following. (1) Dextral faults must form left-restraining contractional stepovers in order to maintain continuity and conserve slip along strike, thus creating localized fold-and-thrust belts. (2) Dextral faulting accommodates a component 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 block boundary zone) to accommodate plate-normal motion at the latitude of the East Bay domain. The transpressional kinematics of the East Bay domain may be present in other obliquely convergent orogenic belts, and they have implications for seismic hazard assessment in the highly urbanized San Francisco Bay area.

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