Geologic History of Mt. Diablo

Mt. Diablo is a large, actively growing anticline that has formed in a restraining stepover between two major dextral strike-slip faults of the San Andreas system. Uplift of Mt. Diablo anticline during the past 3-5 million years has produced unique 3-D exposures of normal faults that were active in the ancestral Great Valley forearc basin during late Cretaceous-early Tertiary time, coeval with plate convergence and subduction beneath western California. Stepwise restoration of Mt. Diablo anticline reveals that the Mesozoic-early Tertiary normal faults are related to low-angle structures that attenuate the ophiolitic basement and juxtapose metamorphosed blueschist-facies rocks of the Franciscan complex with relatively unmetamorphosed marine forearc sediments. Apatite fission-track analyses indicate that the Franciscan rocks were uplifted and cooled from depths of 20+ km in the subduction zone while normal faulting and extension were occurring in the overlying forearc crust. The unique stratigraphic and structural relations at Mt. Diablo support models for exhumation of Franciscan blueschists through synsubduction extension and attenuation of the overlying forearc crust, rather than uplift and erosion of the accretionary prism.

Speaker: Dr. Jeff Unruh, Lettis Consultants International