Abstract

In the California Coast Ranges east of San Francisco, post–3.5 Ma folding and erosion of the Mount Diablo anticline have created unusual three-dimensional exposures of late Mesozoic–Cenozoic stratigraphic and structural relations among rocks of the Franciscan subduction complex, Coast Range ophiolite, and Great Valley forearc basin. These relations offer new insights into the kinematics of Late Cretaceous–early Tertiary synsubduction extension, attenuation, and blueschist exhumation within the ancestral California forearc region. Map relations and subsurface data reveal that marine strata of the Great Valley forearc basin northeast of Mount Diablo accumulated in an ∼100-km-long, north-south–trending, extensional graben system. Normal faults in the graben system cut steeply downsection through Late Cretaceous and early Tertiary growth strata and terminate against the Clayton–Marsh Creek fault, which is at a low angle to bedding and is interpreted to be the basal detachment for part of the graben system. The Clayton–Marsh Creek fault in turn is linked to a system of faults that juxtapose blueschist facies rocks of the underlying ancestral Franciscan accretionary prism against attenuated remnants of forearc crust that were never deeply buried, including the Coast Range ophiolite and basal Great Valley strata. These faults are the local expression of the Coast Range fault in this area. Franciscan rocks exposed beneath the faults were metamorphosed at ≥20 km depths after 108 Ma, based on jadeitic pyroxene and 108 Ma detrital zircons (U-Pb) in metagraywackes. Apatite fission-track data indicate that these blueschist facies rocks were subsequently uplifted and exhumed from ∼9 km to ∼3 km depths in latest Cretaceous–early Tertiary time, coeval with the subsidence and extension in the structurally overlying forearc graben system. Much of their earlier rise from ≥20 km to 9 km depth presumably also occurred coeval with graben development, although our data cannot determine this directly. These relations demonstrate that the California forearc around Mount Diablo was in an extensional regime undergoing active sedimentation during major Franciscan exhumation, and thus support models for exhumation of Franciscan blueschist facies rocks via synsubduction extensional processes, rather than via erosional processes.

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