CRETACEOUS SUBDUCTION ARCHITECTURE AND MID-MIOCENE CALIFORNIA BORDERLAND EXTENSION THROUGH THE LENS OF THE SAN ONOFRE BRECCIA

The Cretaceous Catalina schist (CS) on Catalina Island and Jurassic Willows plutonic complex (WPC) have been used to model the subduction complex and tectonically overlying forearc basement, respectively. However, the forearc basement structurally above the Catalina schist prior to extension may have a Peninsular Ranges batholith (PRB) affinity or vary across space between the PRB and WPC. Furthermore, distribution of CS clasts may also reflect progressive unroofing from beneath what was the forearc basement. Capture of a ~400 km long coastal sliver from the western margin of North America to the Pacific plate (~20-10 Ma) resulted in extension of the inner southern California borderland. This led to uplift and unroofing of the CS. The emerging highland region shed nonmarine and marine sediments derived from the CS and the overlying forearc to form a syn-tectonic deposit, the San Onofre Breccia (SOB). We have sampled the SOB from three tectonic domains to determine the relationship between the forearc basement and SOB detritus: 1) coastal deposits underlain by the Cretaceous Peninsular Ranges batholith (Dana Point); 2) deposits underlain by the WPC (displaced northern Channel Islands); and 3) inner borderland underlain by CS (Malibu coast, Palos Verdes, Catalina Island). Petrologic study of a suite of mafic clasts obtained from the Dana Point coastal region reveals similarities with the WPC and saussurite gabbro on Catalina Island and within drillholes in the SW Los Angeles Basin. White mica 40Ar/39Ar data from CS clasts at the coastal and Santa Cruz Island breccia deposits reveals that these clasts are similar to but slightly older than CS basement on Catalina Island, consistent with the expectation of an unroofing sequence. Work is ongoing to collect supplemental data from SOB at the remaining two tectonic domains. New U-Pb ages from the mafic clasts and detrital zircon provenance studies combined with data produced by previous investigations will validate the following: the WPC is an accurate analog for the forearc basement that previously overlay the CS, and low temperature CS clasts (that are more abundant throughout the inner borderland than indicated by exposures on Catalina Island) will allow us to better characterize the temperature gradient of the Catalina subduction complex prior to unroofing.