Abstract: Structural Geology of the East Bay Hills

Ronald C. Crane

The East Bay Hills are dominated by two active structural domains, the Hayward Fault Zone, and the Franklin High. A third domain occurs between the two and is due to the interaction of these two zones, as well as interference from the westward moving Mount Diablo Backthrust Region.

The Hayward Fault Zone encompasses the vertical, apparent strike-slip segments, as well as a 2 to 4 mile zone to the east which consists of belts of rotated vertical faults and thrusts, each with its own distinctive stratigraphy. The Hayward Fault Zone has approximately 40,000' of vertical displacement, west-side-up.

The Franklin High lies above a west-vergent backthrust system and is characterized by the presence of "island" structures. The western leading edge structure is marked by the Pinole thrust system, which swings to the east and disappears by plunge to the south. All structures south of this plunge are east directed thrust sheets.

The San Pablo Reservoir Fault, lying between the two domains to the north, is rotated in dip; either a thrust, a strike-slip, or normal fault, depending on latest movement. The Lafayette-Cull Canyon fault diagonally divides the third domain, with east vergent thrusts as far as the Calaveras Fault in the southeast portion.

The latest Holocene-Pleistocene structure is superposed on earlier Pliocene, Upper Miocene, Lower to Middle Miocene, Paleocene and Upper Cretaceous structures. Each earlier structure is modified by later structural forces. The complexity of structural detail is due to this interference, rotation and modification. Both old and recent highs are characterized by limited horizontal dimensions and abrupt plunges of up to 60 degrees. Several older highs can be distinguished based on both well and surface mapping.

Horizontal shortening due to compression is 50% to 55%. The Hayward fault zone originated approximately 35 miles west of its present location.

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