TIMING AND DURATION OF INCISION ALONG THE COLORADO RIVER NEAR RIFLE, CO SUGGEST A TECTONIC DRIVER FOR POST-10 MA LANDSCAPE EVOLUTION

The pace and pattern of fluvial incision across the Colorado Plateau and Rocky Mountains remains poorly characterized despite its importance for distinguishing geodynamic processes generating and supporting high topography. Geologic relationships between Miocene basalts and river gravels along the upper Colorado River suggest that the onset of fluvial incision postdates ~11 Ma, the oldest recognized ancestral Colorado River deposits near Grand Mesa. We conducted 1D thermal modeling of the response to exhumation during river incision; models are constrained by stratigraphic estimates of material removed (~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. Comparison of our results to apatite fission-track data from the MWX borehole near Rifle, CO suggest that the onset of rapid exhumation occurred between 6 and 8 Ma and imply average incision rates of 200 to 250 m/Myr since Late Miocene time. Dating of several fluvial fan-terrace complexes developed along the northern flank of Battlement Mesa using ²⁶Al/¹⁰Be burial age techniques require up to 575 meters of incision in the past ~2 Ma yielding slightly higher incision rates of ~250-300 m/Myr. Collectively, these results suggest that incision along the western slope of the Rockies began prior to final integration of the Colorado River through Grand Canyon at ca. 6 Ma, challenging the notion that incision along the upper Colorado is a response to basin integration and drainage reorganization. Rather, our results 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.