LATE MIOCENE ONSET OF INCISION ALONG THE UPPER COLORADO RIVER NEAR RIFLE, CO

One of the persistent debates regarding the history of erosion and the development of deep canyons throughout the greater Colorado River watershed centers on the onset of Cenozoic incision. Along the upper Colorado River, in the western Rocky Mountains, the initiation of incision and exhumation provides a critical test of alternate models that attribute the generation of modern relief to either 1) base level fall resulting from drainage basin integration through Grand Canyon between 5-6 Ma, or 2) surface uplift in the Rockies that drove a downward wave of drainage integration. Geologic relationships between Miocene basalts and river gravels along the upper Colorado River suggest that fluvial incision postdates ~11 Ma, but a lower bound on when incision began remains uncertain. We conducted 1D thermal modeling of the response to exhumation during river incision; models are constrained by stratigraphic estimates of material by incision (~1500 m), by thermal conductivity of local strata and modern heat flow, and by the presence of dated fluvial terraces up to ~575 m above the modern river (Aslan et al., 2019). Comparison of model results to apatite fission-track ages and track lengths from the MWX borehole near Rifle, CO suggest that the onset of rapid exhumation occurred between 6 and 8 Ma. Sensitivity analysis of this estimate to variations in heat flow, depth of exhumation, and the magnitude of post 2Ma incision all suggest that the onset of fluvial incision pre-dates ~6 Ma. Average exhumation rates of 200 to 250 m/Myr since Late Miocene time are consistent with Quaternary incision rates (Aslan et al., 2019) over the past ~2 Ma. Our results imply that incision along the western slope of the Rockies began prior to final integration of the Colorado River through Grand Canyon, challenging the notion that incision along the upper Colorado is a singular response to basin integration and drainage reorganization. Rather, we suggest that sustained incision along the western slope of the Rockies initiated as a separate event, likely driven by long-wavelength tilting during the Late Miocene.