Resolving time-space histories of Late Cenozoic bedrock incision along the Upper Colorado River, USA

The upper Colorado River basin drains the western slope of the Rocky Mountains province of North America and hosts a rich record of fluvial terraces and volcanic deposits that provide an archive of bedrock river incision since the Late Miocene. Here we present new geochronology from fluvial deposits and associated volcanic units using multiple methods (luminescence, cosmogenic-burial, ⁴⁰Ar/³⁹Ar basalt and detrital sanidine, and U/Pb detrital zircon) to reconstruct a detailed record of bedrock incision in the upper Colorado River over timescales ranging from Miocene to Late Pleistocene. Late Miocene (ca. 11–8 Ma) basalt flows perched 800–1700 m above the Colorado River, and the oldest known ancestral Colorado River deposits preserved beneath basalt flows on Grand Mesa, provide the basis for average, long-term (∼10⁷ yr) incision rates. Rates range from ∼110–160 m/Ma between Grand Junction (downstream) and Glenwood Canyon (upstream), with rates decreasing upvalley. Intermediate-scale (∼10⁵–10⁶ yr) incision rates are ∼200–320 m/m.y. over the past 1–3 Ma, and these rates are faster than the average over the past 8–11 Ma. Short-term (<100 ka) bedrock incision rates are highly variable, and range from ∼250–725 m/m.y. The large variation in rates is interpreted to reflect the inherently unsteady nature of climate-driven incision when measured over time intervals (10³–10⁴ yr) that are shorter than the duration of ∼100 ky climate cycles. Finally, episodes of anomalously rapid (∼2000–3700 m/m.y.) short-term bedrock incision appear to be the consequence of stream piracy and drainage reorganization and are confined to specific reaches of the system. Collectively, these data provide a substantially refined picture of the history of incision through space and time that is consistent with headward erosion of the Colorado River into the uplifting flanks of the Rocky Mountains since at least the Late Miocene. We argue that the long-term pace of incision is not completely accounted for by isostatic rebound in response to exhumation, and that ongoing differential uplift driven by mantle buoyancy sets the background rates of fluvial incision (100–150 m/m.y.). Pulses of more rapid incision are a consequence of non-steady incision related to climatically-driven changes in erosion efficiency and/or drainage reorganization.