MESOZOIC-CENOZOIC DRAINAGE DIVIDES FOR SOUTHERN CALIFORNIA RIVERS, AS DETERMINED BY DETRITAL ZIRCONS, WITH IMPLICATIONS FOR EVOLUTION OF GRAND CANYON, THE COLORADO RIVER AND THEIR PRECURSORS

Central to debate about the age, origin and evolution of Grand Canyon is the history of the Colorado River and its precursors. Reversal of dextral slip along the San Andreas fault system restores southern California to a position at the downstream end of the Early Pliocene course of the Colorado River. If the Colorado River flowed across southern California to the Pacific prior to 6 Ma, then sand deposited by it would have distinctive detrital-zircon age distributions reflecting erosion of upper Paleozoic and lower Mesozoic strata of the Colorado Plateau. The latter contain 300-1100 Ma zircon that was originally transported from orogenic belts along southeastern Laurentia. Lower Paleozoic, upper Paleozoic, Triassic, Lower to Middle Jurassic, and Upper Jurassic to Cretaceous sandstone of the Colorado Plateau contains average concentrations of 300-1100 Ma zircon of 4%, 32%, 46%, 44% and 30%, respectively. Not surprisingly, Plateau-derived sand in the modern Colorado River averages 29% 300-1100 Ma zircon. Triassic to lowest Cretaceous metasedimentary wallrocks of the mid-to-Late Cretaceous southern California magmatic arc contain 34% 300-1100 Ma zircon, indicating that sand similar to that on the Plateau reached the Pacific Ocean. In contrast, only trace amounts of 300-1100 Ma zircon occur in younger southern California sandstone. Stratigraphic groupings of age distributions of 6662 detrital zircons from 167 broadly distributed sandstone samples from coastal deposits of southern California average 44-88% Cretaceous, but only 0.4-1.3% 300-1100 Ma grains, most of which can be attributed to recycling from older deposits. No individual Upper Cretaceous to Pliocene sandstone sample contains greater than 3% 300-1100 Ma zircon. Although Paleogene headwaters of southern California rivers extended into the eastern Mojave Desert, Sonora and Mogollon Highlands, our observations indicate that these headwaters did not extend as far inland as the Colorado Plateau. This conclusion conflicts with the model of a SW-flowing Arizona River during the Paleogene, but supports rapid Late Miocene-Pliocene drainage reorganization and integration of the Colorado River coincident with development of the Salton Trough and Gulf of California.