Coupling of Early Tertiary Graben Development in the Great Valley Fore Arc Basin with Blueschist Exhumation in the Underlying Franciscan Accretionary Wedge at Mt. Diablo, Eastern San Francisco Bay Area, California

J. R. Unruh¹ and T. A. Dumitru^2
1Lettis & Assoc., Inc, Walnut Creek, CA
²Dept. Geological and Envir. Sci, Stanford University, Stanford, CA

Stratigraphic and structural relations exposed in three dimensions by late Cenozoic uplift and folding of Mt. Diablo anticline provide new insights into the kinematics of syn-subduction attenuation of the ancestral western California fore arc. Map relations indicate that late Cretaceous and early Tertiary marine fore arc strata on the northeast limb of Mt. Diablo anticline accumulated in a north-south-trending graben system. Normal faults that bound the graben system cut steeply downsection through growth strata and terminate against the Clayton-Marsh Creek fault system, which is at a low angle to bedding and is interpreted to be the basal detachment of the graben system. The Clayton-Marsh Creek fault in turn is linked to a system of faults that juxtapose blueschist-grade rocks of the ancestral accretionary wedge (the Franciscan complex) with highly attenuated remnants of the ancestral fore arc basement (the Coast Range ophiolite). Apatite fission-track (AFT) data indicate that Franciscan rocks at Mt. Diablo were exhumed primarily in latest Cretaceous-early Tertiary time, coeval with extensional activity in the structurally overlying forearc graben system. The AFT data also provide a basis for estimating total late Cenozoic structural relief associated with growth of Mt. Diablo anticline; these relations are used to develop a series of cross-sections that illustrate the progressive structural development of Mt. Diablo since mid Tertiary time, and to reconstruct the geometry of the older extensional structures. The reconstructions suggest that the ophiolitic fore arc basement was strongly attenuated, and rocks from deep within the accretionary wedge were exhumed, beneath a long-lived, actively subsiding graben system within the fore arc basin. This interpretation is consistent with observations of attenuation of fore arc crust elsewhere in western California, and posits a kinematic link between mid-crustal and upper crustal extensional processes during subduction.

AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California