Description
Previous petrologic work indicates that Upper Cretaceous sandstones in southern California were derived from the deeply dissected plutonic/metamorphic roots of the Peninsular Ranges magmatic arc. The validity of this conclusion is dependant on the accuracy of the provenance - discrimination models of W.R. Dickinson and colleagues, and A.W. Basu and colleagues. In order to test these models, Holocene sand derived from pre-Tertiary bedrock in the Peninsular Ranges was sampled and point-counted. The Peninsular Ranges represent an uplifted basement complex composed mostly of undeformed and deformed plutonic rock and lesser amounts of metasedimentary and metavolcanic rock. The results of the petrologic study of Holocene sand support the use of the provenance - discrimination models proposed by W.R. Dickinson, A. W. Basu, and their colleagues, and strengthen the interpretation that upper Cretaceous sandstones in the Peninsular Ranges were derived from the deeply dissected plutonic/metamorphic roots of the Peninsular Ranges magmatic arc. This last interpretation is significant, because it implies that the volcanic cover of the Peninsular Ranges magmatic arc was nearly stripped clean by Late Cretaceous time. Oddly, pre-Campanian volcanic detritus is absent in the San Diego area. This enigmatic relationship can be explained by tectonic removal of volcanic detritus by strike-slip faulting. The Upper Cretaceous sandstones Group were derived from the uplifted Nevada magmatic arc which is located in the Great Valley and dissected Sierra approximately 600 km north of the Peninsular Ranges. During the Late Cretaceous the Great Valley Group may have been deposited over 1000 km north of the site of deposition of Upper Cretaceous strata in the Peninsular Ranges. These relationships suggest that the uplift and dissection of the Peninsular Ranges and Sierra Nevada magmatic arcs were responses to an interregional process. As discussed by W.R. Dickinson and W.S. Snyder in Geological Society of America Memoir 151, geological and geophysical data indicate that an oceanic plate was subducted beneath the western North American continental margin during Cretaceous time. A linear chain of plutonic/volcanic rock developed along the western North American continental margin as a result of the subduction of the oceanic plate. Throughout the Cretaceous and early Tertiary this subduction/magmatic-arc system produced complicated igneous, metamorphic, structural, and stratigraphic patterns that are attributed to the Laramide orogeny. The locus of Laramide-related magmatism moved from a western to an eastern position as a result of a shallowing of the angle of subduction beginning in the Late Cretaceous. During the low-angle subduction event, older parts of the magmatic arc were stranded and uplifted in a forearc position. Uplift and dissection of the Sierra Nevada and Peninsular Ranges magmatic arcs, is thought to have developed during this stage of the Laramide orogeny.
