NEOTECTONIC INFLUENCES ON THE LONGITUDINAL PROFILE OF THE COLORADO RIVER SYSTEM IN GRAND CANYON AND THE ROCKY MOUNTAINS

The Colorado River system offers a sensitive gauge for evaluating landscape evolution of the Colorado Plateau- Rocky Mountain region. Its longitudinal profile exhibits a double concave-up shape with a major knickpoint near Lee's Ferry, Arizona. Below the knickpoint, the profile is the product of river integration (across the Kaibab uplift) since 6 Ma, driven by a combination of lake spill over and headward erosion that linked Colorado Plateau drainages to Miocene paleo-drainages in western Grand Canyon. The knickpoint is viewed here as a transient feature, as suggested by different incision rates above and below it, and by systematic convex profiles of tributaries below, but not above, the knickpoint. Below the knickpoint, Grand Canyon differential incision rates (150-175 m/Ma in eastern Grand Canyon; 50-75 in western Grand Canyon; 20-30 m/Ma in the Lake Mead region) reflect fault-dampening of apparent incision rates as western blocks move downwards relative to eastern blocks across Neogene normal faults. The Rocky Mountain concave portion of the Colorado River profile seems to have evolved earlier and somewhat separately. It also exhibits differential incision across knickpoints. Quaternary incision rates of ~ 140 m/Ma are recorded from the Lee's Ferry knick point to upstream major knickpoints in the Gunnison (Black Canyon) and Colorado (Glenwood Canyon) rivers. Incision rates within the knickpoint regions is > 400 m/Ma; above the knickpoints are low gradient reaches with incision rates of 80-100 m/Ma that may be relict features of a Miocene low-relief Rocky Mountain drainage system. Differential incision data suggest the knickpoints are likely transient features migrating upstream, although locally resistant substrate may control their present position. Portions of the upper Colorado River may date back to 10 Ma based on gravels beneath dated basalts, but rapid exhumation along the river is constrained by AFT data in the MWX well to have initiated at ~ 6 Ma. Onset of rapid incision in the late Miocene is not readily explained by climate change at ~3 Ma, nor by upstream propagation of incision driven by integration of the Lower Colorado river. Instead, we hypothesize that the onset of rapid incision and exhumation at 6 Ma may be a response to epeirogenic uplift related to the Aspen mantle anomaly in central Colorado.