LATE CRETACEOUS–PALEOGENE TECTONIC AND PALEOGEOGRAPHIC EVOLUTION OF SOUTHERN CALIFORNIA BASED ON DETRITAL ZIRCON FROM THE FOREARC BASIN, PELONA-OROCOPIA-RAND SCHISTS, AND FRANCISCAN COMPLEX

Previous studies of detrital zircon from the forearc basin, Pelona-Orocopia-Rand Schists, and Franciscan Complex of (pre-San Andreas) southern California have established the framework of Late Cretaceous– Paleogene drainage systems. New analyses reported here provide further resolution of this history. In general, Cenomanian–early Maastrichtian units reflect progressive west to east erosion of the mid-Cretaceous Sierra Nevada–Peninsular Ranges arc, with local modest contributions from the Jurassic arc. Some Pelona-type schist and Franciscan samples exhibit the Lemhi doublet (1.80–1.65 Ga and 1.38 Ga peaks) of Dumitru et al. (2016), but this association has not been observed in the forearc basin of southern California. Beginning in the mid-Maastrichtian, sediment dominated by mid-Cretaceous zircon was progressively replaced by that dominated by Proterozoic, Jurassic, and latest Cretaceous zircon. The mid-Maastrichtian and younger component has generally been referred to as “extraregional,” with inferred source areas in the Mojave Desert, southwest Arizona, and northwest Sonora. However, Late Jurassic >> Middle Jurassic zircon and 1.7 Ga >> 1.4 Ga zircon in the earliest “extraregional” sediment may indicate relatively local sources in the central Transverse Ranges. This relationship implies that the initial transition from arc to “extraregional” sources was due mainly to collapse of the arc and forearc basin along the Nacimiento fault rather than to far-inland headward erosion of river systems. Widespread development of rivers analogous to the Poway drainage network of the San Diego area is not evident until the late Paleocene–early Eocene. At this time, at least two major drainages existed north of the Poway system. One, likely tapping the area of the present lower Colorado River trough and Transition Zone, is characterized by subequal 1.7 Ga and 1.4 Ga zircon in addition to Middle Jurassic and/or latest Cretaceous zircon. A second system, tapping southwesternmost Arizona–northwesternmost Sonora is distinguished by Paleocene in addition to latest Cretaceous zircon and 1.7 Ga > 1.4 Ga zircon. The details of the detrital zircon age distributions have potential for constraining translations and rotations in southern California related to both the Nacimiento and San Andreas systems.