A DEFINITIVE 6 MA START DATE FOR CARVING OF THE NORTHEASTERN COLORADO PLATEAU CANYONLANDS

The timing of onset of incision of the iconic landscape of the northeastern Colorado Plateau, including the Canyonlands and Arches National Parks, remains poorly constrained to between 10 and 6 Ma. Knowing more precisely when incision first started is key to answering outstanding questions regarding the driving mechanisms and complex interrelationships behind the rise of the Colorado Plateau and development of its spectacular incised landscape. For example, what is the relative role of isostatic rebound as a result of river incision versus longer-term geodynamic processes to overall uplift of the plateau? This may be seriously overestimated if incision started at 6 Ma, rather than at the usually assumed 10 Ma. Also did the start of incision in the northeastern Colorado Plateau predate integration of the Colorado River through the Grand Canyon, perhaps as a response to tectonic surface uplift? Or did later river integration and base-level fall act to trigger upstream incision?

To date more precisely the onset of incision we present new low-temperature thermochronologic results from bedrock and borehole samples in the northeastern Colorado Plateau. These data show that rapid river incision began here at 6 Ma (5.90±0.59 Ma (2σ) constrained from thermal modeling; 6.0±2.3 Ma (2σ) independently constrained from age-elevation relationship break-in-slope) with incision rates increasing from 15-50 m/Myr to 160-200 m/Myr. This new time constraint has several important implications. First, the coincidence in time with 5.97-5.3 Ma integration of the lower Colorado River through the Grand Canyon to the Gulf of California favors downstream river integration triggering carving of the upper Colorado River canyonlands. Second, it implies integration of the entire Colorado River system in less than 2 million years. Third, rock uplift of the plateau driven by the flexural isostatic response to river incision is restricted to just the last 6 Ma. Fourth, previous estimates of upper Colorado River incision rates based on 10-12 Ma basalt datum levels are too low. This changes the dependency of incision rate on measured time interval from a non-steady-state negative power-law dependence to a near steady-state dependence meaning that long-term upper Colorado river incision rates can provide a reliable proxy for rock uplift rates.