Development of San Leandro synform and neotectonics of the San Francisco Bay block, California

High-resolution, 24-channel seismic-reflection data show a stratified synfo rm beneath south San Francisco Bay. These seismic-reflection data reveal an eastward-dipping bedrock surface that is about 40 m deep (subbottom) benea th the western south bay, and that reaches a maximum observed depth of 500- 800 m (subbottom) below the eastern half of the south bay. An angular uncon formity cuts both the synform and underlying bedrock. The age of the unconf ormity is unknown but may be Pleistocene, when these strata forming the syn form were presumably exposed subaerially during lowered sea levels. The syn formal strata, the unconformity, and some generally flat-lying and overlyin g strata are folded near the eastern shore of the bay. This folding may res ult from movement on the Hayward fault (fault interactions and localized st rain partitioning) or from compressional deformation in the East Bay Hills related to NE-SW ('fault-normal') convergence between the Pacific and North American plates. In general, reflections from sediment overlying the unconformity are flat l ying (except near the eastern short of the bay), whereas reflections beneat h the unconformity dip eastward. The overlying, flat sediment section fills a shallow basin that is coincident with an elliptical residual gravity low . This low appears to be related to the deeper sedimentary, synformal secti on based on the spatial correlation between the east-dipping reflections an d the gravity anomaly. Projecting the east-dipping reflections to the cente r of the gravity low suggests that the total section of flat-lying and dipp ing reflections in the synform may exceed 1000 m. Modeling of the gravity l ow suggests a total tow-density section, about 1.5 km thick, at the center of the synform relative to the surrounding bedrock of presumed Franciscan C omplex. (C) 1999 Elsevier Science Ltd. All rights reserved.