NEW CONSTRAINTS ON THE TIMING AND RATES OF INCISION ALONG THE COLORADO RIVER NEAR RIFLE, CO: IMPLICATIONS FOR DRIVERS OF LATE CENOZOIC LANDSCAPE EVOLUTION

The pace and pattern of fluvial incision across the Colorado Plateau and Rocky Mountains remain incompletely characterized despite their importance for distinguishing geodynamic processes generating and supporting high topography. Integration of new chronologic data from the upper Colorado River system constrain the onset of exhumation and rates of subsequent incision. Geologic relationships between Miocene basalts and river gravels in the upper Colorado River drainages suggest that onset of rapid incision postdates ~10 Ma. Age-depth relationships of apatite fission-track ages in boreholes near Rifle, Co, however, provide additional constraints on the temporal progression of exhumation. We conducted 1D thermal modeling of the response to exhumation during river incision; results suggest that the onset of cooling and exhumation occurred between ~8-10 Ma. Average incision rates since then appear to be 150 – 190 m/Ma, consistent with previous estimates. We also dated several fluvial fan-terrace complexes developed along the northern flank of Battlement Mesa with ²⁶Al/¹⁰Be burial age techniques. These results suggest that incision rates over the past ~2 Ma are comparable to the long-term average. Thus, we have no evidence to suggest a transient period of enhanced incision rate. Collectively, our results imply 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. Incision rates in the Rockies have proceeded at relatively steady rates of 150 – 190 m/Ma over the last 10 Ma. These data are at odds with the suggestion that incision along the upper Colorado is a response to basin integration and drainage reorganization. Rather, our results implicate headwater surface uplift as a primary driver of sustained incision along the western slope of the Colorado Rockies throughout the Late Cenozoic.