THE CALIFORNIA RIVER AND ITS ROLE IN CARVING GRAND CANYON

Recently published thermochronological and paleoelevation studies in the Grand Canyon region, combined with sedimentary provenance data in both the coastal and interior basins of the Cordillera, place important new constraints on the coupled paleohydrological-tectonic evolution of the southwestern United States. Review and synthesis of these data suggest that incision of Grand Canyon from a plain of low relief to a canyon of roughly its current length and depth occurred primarily in Campanian time, by a high-discharge NE-flowing river with headwaters on the NE slope of the Cordillera in California, named after its source region the California River. Laramide uplift of the Cordilleran interior and collapse of the source region into a continental borderland reversed the river's course by Paleocene time, such that its headwaters lay in the eastern Grand Canyon region and its terminus in what are now the Western Transverse Ranges and Salinian terrane; this river is referred to after its source region as the Arizona River. From Paleocene through late Miocene time, the interior of the Colorado Plateau was a closed basin separated from the Arizona River drainage by an asymmetrical divide in the Lees Ferry-Glen Canyon area, with a steep SW flank and gently sloping NE flank that drained into large interior lakes. By Oligocene time the lakes had dried up and were replaced by ergs, and by mid-Miocene time the Arizona River drainage below Grand Canyon was deranged by extensional tectonism. Increasing moisture in the Rocky Mountains in late Miocene time reinvigorated aggadation NE of the divide, which was finally overtopped at 6 Ma, catastrophically lowering base level in the plateau interior by 1500 m. This event reintegrated the former Arizona drainage through a cascade of spillover events through Basin and Range valleys, for the first time connecting sediment sources as far east as Colorado with the coast. This event increased the hydrologic discharge through Grand Canyon by perhaps two orders of magnitude from its mid-Tertiary nadir, giving birth to the modern subcontinental-scale Colorado River drainage system, and variably excavating the plateau interior.