CHANGING PROVENANCE OF LATE CRETACEOUS-EARLY CENOZOIC SEDIMENTS IN SOUTHERN CALIFORNIA RELATED TO FLAT SUBDUCTION

Flat subduction is commonly called upon to explain a number of anomalous features of the Late Cretaceous-early Cenozoic geology of southern California and adjacent areas. Prominent among these are the Pelona and related schists, which appear to be trench sediments and oceanic crust underplated beneath the Cretaceous batholith and Early Proterozoic to Jurassic host rocks of southwestern North America. The schists are exposed in a NW-SE-trending belt from the southern Sierra Nevada to southwesternmost Arizona. U-Pb detrital zircon ages and Ar/Ar metamorphic ages indicate that deposition of the schist protolith and underthrusting (i.e., subduction) young from ca. 90 Ma in the northwest to less than 60 Ma in the southeast. Furthermore, the older parts of the schist appear to have been derived largely by erosion of the Cretaceous arc system, whereas the younger units include mostly far-traveled (“extra-regional”) detritus sourced from east of the Cretaceous arc. Underthrusting of the schists coincided with extinction of arc magmatism, tectonic and/or erosional denudation of the arc, and transgression of the forearc basin far inland. Cretaceous to Eocene sediments between the southern Sierra Nevada and northern Peninsular Ranges (i.e., at the same latitude as the schists) show exactly the same evolution of detrital zircon suites with time as the schists. In particular, the extra-regional detritus first appears in this area in latest Cretaceous time. To the north and south, however, the extra-regional detritus is first observed in sedimentary rocks of latest Paleocene to early Eocene age and is not as dominant as in the region between the Sierra Nevada and Peninsular Ranges. This pattern confirms other evidence indicating southern California as the part of the Cordilleran margin most impacted by shallow subduction during the Laramide orogeny. However, the origin of the NW-SE age variation within the schists remains uncertain. Possible causes are north to south impingement of an oceanic plateau or ridge system with the trench or progressive eastward migration of a rolling hinge in the subducting slab.