LARAMIDE DEFORMATION OF FOREARC BASIN-ACCRETIONARY PRISM STRATA REFLECTED IN DETRITAL ZIRCON U-PB PROVENANCE OF THE LATE CRETACEOUS GREAT VALLEY GROUP

The Late Cretaceous-Paleogene Laramide orogeny produced focused deformation along the southern Sierra Nevada-Salinian portion of the Cordilleran margin producing classic Laramide cratonal foreland deformation features that extend for >1000 km inboard from the margin. The cause of Laramide tectonism has been related to subduction of buoyant oceanic plateau remnants carried by the Farallon plate starting around 90 million years ago. Here we recognize a disruption in the along strike pattern of detrital zircon (DZ) age distributions from Cordilleran margin Late Cretaceous forearc deposits that can be related to localized Laramide deformation in the Sierra Nevada-Salinia arc segment. Forearc strata of the Great Valley Group have previously published DZ U-Pb age distributions that only vaguely resemble age distributions determined from the in situ Sierra Nevada batholith or detrital zircon from modern sediments of the Sacramento-San Joaquin river system. This observation, and particularly the paucity of Late Cretaceous age zircon (~105-84 Ma) in Great Valley Group forearc strata which are characteristic of equivalent forearc strata in British Columbia and Baja California, presents a strong challenge to the prevailing view that the Late Cretaceous Great Valley forearc strata was derived entirely from first cycle erosion of the Sierra Nevada batholith. The anomalous DZ age pattern is interpreted here to reflect regional emergence and erosional reworking of the Franciscan subduction complex into the forearc basin in response to the onset of Laramide deformation along this segment of the margin.