DETRITAL ZIRCON GEOCHRONOLOGY INDICATES THAT UPPER CRETACEOUS TO EOCENE SEDIMENTARY AND METASEDIMENTARY ROCKS OF SOUTHERN CALIFORNIA, SALINIA, AND THE SUR-OBISPO TERRANE ARE NOT FAR TRAVELED

The Franciscan subduction complex, Great Valley forearc basin, and Sierran magmatic arc of central California east of the San Andreas fault define an exceptionally well preserved ancient convergent plate-margin sequence. Similar, but highly disrupted, lithotectonic belts also occur in southern California and in former parts of southern California translated to the NW by the San Andreas system (Salinia and Sur-Obispo terrane). One prominent characteristic of the southern California domain is the presence of the relatively high-P/low-T, eugeoclinal Pelona-Orocopia-Rand schists. Most workers consider these rocks to be a subduction complex plated beneath North American crust during low-angle subduction related to the Laramide orogeny. U-Pb detrital zircon ages and Ar/Ar metamorphic ages indicate that deposition of the schist protolith and underthrusting (i.e., subduction) occurred from ca. 90 Ma to at least 60 Ma. The zircon ages further demonstrate that detritus comprising the older parts of the schist protolith was derived primarily from the Early to Late Cretaceous magmatic arc along the continental margin, whereas younger detritus appears to have been eroded largely from latest Cretaceous, Jurassic, and Proterozoic basement farther to the east. Sedimentary rocks of southern California, Salinia, and the Sur-Obispo terrane, which comprise fragments of a Late Cretaceous-Eocene forearc basin at shallow structural levels within the upper plate of the schists, exhibit detrital zircon age distributions displaying exactly the same evolution of provenance with time as the schists. This implies that both units were deposited during progressive tectonic and/or erosional denudation of the arc, accompanied by development of integrated drainages extending into the craton. The distinctive, coupled evolution of zircon suites in the schists and sedimentary units argues against a far-traveled origin for any of the units sampled here. Furthermore, the evidence from the schist of rapid, continuous subduction beneath southern California from 90 to 60 Ma appears inconsistent with the Baja-British Columbia hypothesis, which postulates oblique plate convergence partitioned predominantly into dextral strike-slip at this time.