COMPARISON OF LONG-TERM RATES OF INCISION ACROSS THE ROCKY MOUNTAIN OROGENIC PLATEAU

The ongoing debate over the post-Laramide landscape evolution of the Rocky Mountain Orogenic Plateau often hinges on the interpretation of late Cenozoic incision. Widespread deep canyon cutting and basin fill exhumation have been alternately used to infer epeirogenic uplift and climate change. Comparison of long-term spatially averaged rates of incision from the western Great Plains, Colorado Plateau, and central and southern Rocky Mountains helps constrain the timing of turn around from net aggradation to degradation. Whether or not the turn around time is the same between sub-regions has significant implications for invoking a common climate and/or tectonic history for the entire region.

We compiled elevation and age control data from a variety of landscape paleomarkers including incised volcanic units, dissected erosion surfaces, and perched basin fill remnants to get average erosion rates over time scales of 10⁵ to 10⁷ yrs. We compared rates from three different zones delimited by major modern drainage divides and similar patterns of erosion: 1) northern Wyoming – southern Montana (WYMT), 2) the Colorado Plateau (CP), and 3) the western Great Plains (WGP). Rates range from 4 to 350 m/m.y. with the highest rates in the CP and the lowest in the WGP. We binned the data over various time increments over the last ~35 Ma and derived the instantaneous rates for each time bin. The bin in which the rate approaches zero is interpreted as the timing of initiation of the current cycle of erosion. Additionally, we compared the cumulative magnitude of incision from successive bins to the observed maximum magnitude of incision within in each region.

Results indicate that the CP and WGP have very similar trends of high incision rates over the last 8 – 10 m.y. These rates can easily produce the observed maximum incision depths by 5-6 Ma. The WYMT region has a different history. Rates in this region remain high over the last 9 – 15 m.y. and the maximum incision depth can be reached by 9 – 10 Ma. The results suggest that 1) climate change and glaciation in the Pliestocene could not have produced all of the observed incision, 2) the turn around from net aggradation to degradation began earlier in the north, and 3) one instantaneous process is not likely responsible for regional incision.