LATE CENOZOIC LANDSCAPE EVOLUTION ALONG THE WESTERN SLOPE OF THE COLORADO ROCKIES: A ROLE FOR DIFFERENTIAL ROCK UPLIFT DRIVEN BY BUOYANT MANTLE

The evolution of topography in the central Rocky Mountains lies at the center of a long standing debate over the processes by which high elevations were attained. Although most agree that buoyant mantle supports a significant fraction of present-day elevation, the timing of when such conditions were established remains uncertain. Widespread fluvial incision across the Colorado Plateau, Colorado Rockies and Great Plains during the late Neogene may reflect epeiorogenic rock uplift centered on the high Rockies, but determining whether such a signal exists is made challenging by the convolved effects of climatically enhanced incision and/or drainage basin integration. Here, we present a synthesis of new and existing geologic and geomorphic data from the western slope of the Colorado Rockies that refine the spatial and temporal patterns of incision during the Late Cenozoic. Refined estimates of the timing of fluvial incision along the Colorado River from borehole thermochronologic data suggest that incision began at ~8-10 Ma, consistent with previous geologic studies. New ⁴⁰Ar/³⁹Ar dating of basaltic volcanism in the headwaters of the White, Yampa, and Little Snake Rivers suggest that incision into Tertiary sediments of the Browns Park Formation initiated at or around the same time (~8-10 Ma). Thus, fluvial incision along the western slope appears to pre-date final integration of the Colorado River though Grand Canyon at ca. 6 Ma. Interestingly, the amount of Late Cenozoic incision appears to vary along the range, from ~1500 m along the Colorado River trunk stream to ~500 m along the Little Snake in the north. Analysis of river longitudinal profiles suggest that these variations in the depth of incision correspond to differences in channel steepness (channel gradient normalized for drainage area); channels in the Colorado River basin are ~3X as steep as those farther north. Collectively, the correlation of steep channels and deep incision are inconsistent with a simple response to climatically enhanced incision. Although a quantitative estimate of the amplitude and wavelength awaits further study, we argue that the spatio-temporal patterns of incision and channel steepness require tilting and differential rock uplift between the Colorado Rockies and the Colorado Plateau.