TECTONIC EVOLUTION OF GRAND CANYON AND UPLIFT OF THE COLORADO PLATEAU

Models for the integration of the Colorado River through Grand Canyon and mantle-driven uplift and deformation of the Colorado Plateau can be tested given sufficient understanding of how Grand Canyon incision has varied both along its length and through time. New (plus previously published) results now allow for bedrock incision rate determinations along most of the 440-km-long length of Grand Canyon over the last ~0.1-4 Ma. Imprinted on regional trends are shorter 10-km-wavelength incision variations across Quaternary normal faults due to both fault offsets and hangingwall flexures. New data from areas away from Quaternary faults now show that incision rates increase from west to east from 100 m/Ma to 160 m/Ma across the length of Grand Canyon. Temporal variations in incision rates can be accessed at a number of sites where stacked terraces have been newly dated back to ~1.5 Ma. At each location, long-term incision rates are found to be semi-steady though time. Temporally steady but spatially variable incision throughout Grand Canyon is a unique result that cannot be easily explained by variations in bedrock strength, climate, or sediment load. Steady incision rates are also inconsistent with transient knickpoint migration. Instead, incision rates, along both the Colorado and Virgin Rivers, increase in the direction of a Miocene to Recent eastward magmatic sweep involving increasingly asthenopsheric melts. The leading edge of this basaltic volcanism mirrors one of the largest gradients in mantle-velocities in the western United States which is located near Lees Ferry. We suggest that heating, melt transfer, and lithospheric removal associated with a migrating mantle boundary has produced buoyancy modification and uplift. Differential incision suggests 150 m (25 m/Ma x 6 Ma) of uplift between the Lake Mead region and western Grand Canyon and 360 m (60 m/Ma x 6 Ma) between western and eastern Grand Canyon. Some but not necessarily all of the differential uplift may be occurring across the Wheeler, Grand Wash, Hurricane, and Toroweap faults. Thus we suggest ~500 m of total surface uplift (and tilting) of the Colorado Plateau relative to sea level in the last 6 Ma based on differential incision from the Gulf of California to Lees Ferry.