DETRITAL ZIRCON AGES FROM UPPER CRETACEOUS–PALEOGENE FOREARC AND SUBDUCTION-ZONE STRATA OF SOUTHERN CALIFORNIA AND ENVIRONS COMPARED TO CORDILLERAN IGNEOUS BASEMENT AGES: IMPLICATIONS FOR LARAMIDE TECTONICS

Multiple detrital zircon (DZ) studies of Upper Cretaceous–Paleogene forearc and subduction-zone strata over the past two decades have helped elucidate the tectonic evolution of California. Here we integrate new DZ ages for southern California with data from previous studies and compare the results to a compilation of igneous crystallization ages for the North American (NA) Cordilleran magmatic arc. The updated DZ dataset allows samples of forearc strata, Franciscan complex, and Pelona-Orocopia-Rand schist (PORS; Franciscan relatives thrust beneath NA during the Laramide orogeny) to be assigned to various groups with contrasting age spectra. Based on the igneous age database and zircon Th/U, most groups appear to be derived from sources in southern California and adjacent areas, either from the Sierra Nevada-Peninsular Ranges magmatic arc and its wallrocks or regions farther inboard. However, some samples of Franciscan and PORS exhibit DZ age spectra apparently exotic to southern California and environs, suggesting southward transport of sediment within the Franciscan trench. The most common exotic DZ pattern resembles the Type E distribution of Dumitru et al. (2016), with an inferred source in Idaho; in addition, a few samples exhibit DZ spectra similar to those of some Franciscan samples from the Diablo Range. Type E patterns are also found in the Swakane Gneiss of Washington and have been used as evidence that the Swakane protolith is a direct correlative of the PORS that originated in southern California. However, a southerly origin of the Swakane is not required by the DZ data if Type E DZs originated in Idaho and were distributed widely along the NA margin. The igneous age compilation indicates a rapid inland sweep of magmatism at ca. 80 Ma from northern Nevada to central Mexico, coeval with onset of the Laramide orogeny. Once inboard, the arc remained stationary through the end of the Laramide event at ca. 40 Ma. The ~3000-km-length of the inboard arc suggests that shallowing of subduction resulted largely from plate-scale processes rather than local oceanic plateaus/rises. Nonetheless, plateau subduction may have locally enhanced slab-flattening; e.g., leading to tectonic erosion of the base of the NA plate and emplacement of PORS at the latitude of southern California.