Abstract: Neogene Nappes in the Hayward Area, Central Coast Ranges, California

R. W. Graymer, D. L. Jones, E. E. Brabb

Upper Mesozoic and Cenozoic strata in the East Bay Hills east of the Chabot fault near Hayward are arranged in a series of stacked thrust sheets of unknown structural affinity that formed during Neogene time and were folded during Quaternary compressive deformation. On the east, Upper Cretaceous rocks overlie Miocene and Pliocene strata along a west dipping thrust fault that joins the Moraga-Miller Creek thrust and continues south as the Palomares fault. Structurally above, an unnamed fault system imbricates Upper Cretaceous Redwood Canyon Formation and the younger (late Campanian) Pinehurst Shale in the upper San Leandro Reservoir area. To the southwest, the Pinehurst Shale reappears in Garin Park east of the Chabot fault, where it is structurally overlain by mid-Cretace us rocks equivalent to the Oakland Conglomerate (Cenomanian?) along an east dipping thrust fault. Regional mapping indicates that these unnamed east and west dipping structures that bound the Pinehurst Shale may connect, and thus they define an upper plate which appears to be entirely detached. The Chabot fault, a right-lateral transtensional structure that cuts Quaternary gravels, appears to post-date and bound the stacked thrust faults on the west, as it clearly truncates the east dipping thrust that bounds the Pinehurst Shale. Therefore the formation of the stacked thrust faults occurred in Pliocene or Pleistocene time. The faults were probably subsequently folded by Quaternary compression related to the Mission and Hayward fault systems, which postdate and cut the Chabot fault. Based on these differing structural styles, it appears that the Quaternary tectonic history of the East Bay Hills has alternated between times of transpressive and transtensional deformation.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California