POST-10 MA RIVER INCISION RATES IN WESTERN COLORADO: IMPLICATIONS FOR NEOGENE UPLIFT OF THE COLORADO ROCKIES

Basalt flows and volcanic ashes serve as a datum for calculating post-10 Ma river incision rates in western Colorado. The main picture that emerges from the data is one of regional variability of incision rates, which we hypothesize to reflect differential uplift of the Colorado Rockies during the Neogene. Maximum rates (100-150 m/Ma) and magnitudes (1000-1500 m) of river incision are recorded between Grand Mesa and Glenwood Canyon, and in the Flat Tops. Minimum rates (<50 m/Ma) and magnitudes (<250 m) of river incision are associated with the Browns Park-Sand Wash basin in northwestern Colorado and in Middle Park of north-central Colorado.

Differential uplift of subregional blocks of the Colorado Rockies during the late Cenozoic can be inferred by comparing incision rates and magnitudes at locations upstream and downstream of knickzones. Along the Colorado River, post-10 Ma incision rates and magnitudes incision remain fairly constant (rates >100 m/Ma; magnitudes >1000 m) from Grand Mesa upstream to Gore Canyon, and then decrease markedly in Middle Park (rates <10 m/Ma; magnitudes <100 m) across the Gore Canyon knickzone. Normal-faulting of ca. 10 Ma deposits in Middle Park shows that incision rate variations partly reflect late Cenozoic faulting. In addition, river incision in this region began ca. 9.5-8 Ma, prior to integration of the Colorado River system through Grand Canyon.

Along the Yampa River, post-10 Ma incision rates and magnitudes are low (rates 15-27 m/Ma; magnitudes < 230 m) immediately upstream of Yampa Canyon, and then increase significantly (rates 96-132 m/Ma; magnitudes ~1250 m) upstream near the headwaters. We interpret this upstream increase in river incision rate and magnitude to reflect Neogene uplift of the Yampa River headwaters relative to its lower reaches.

Assuming river incision and rock uplift magnitudes are subequal, flexural isostatic modeling suggests that isostatic adjustments account for only 10-40% of the post-10 Ma rock uplift recorded in western Colorado, and that there has been 0.85 to 1.5 km of post-10 Ma epeirogenic rock uplift. Areas with the largest magnitudes of post-10 Ma rock uplift generally overlie the Aspen Anomaly, an area of anomalously low P-wave velocities. We support the hypothesis that mantle buoyancy has produced >1 km of Neogene uplift of the Colorado Rockies.