Numerical modeling of the late Cenozoic geomorphic evolution of Grand Canyon, Arizona

The late Cenozoic geomorphic evolution of Grand Canyon has been influenced by three primary tectonic and drainage adjustment events. First, incision into the Paleozoic strata of the southwestern margin of the Colorado Plateau began at 16 Ma in response to relief production along the Grand Wash Fault. Second, the ancestral Upper Colorado River reversed drainage and became integrated with the Lower Colorado River basin through Grand Canyon between 5.5 and 6 Ma. Third, the Colorado River was influenced by Plio- Quaternary normal faulting along the Hurricane and Toroweap Faults. Despite the relatively firm constraints available on the timing of these events, the geomorphic evolution of Grand Canyon is still not well constrained and many questions remain. For example, was there a deeply-incised gorge in western Grand Canyon before Colorado River integration? How and where was the Colorado River integrated? How have incision rates varied in space and time? In this paper, I describe the results of a numerical modeling study designed to address these questions. The model integrates the stream power model for bedrock channel erosion with cliff retreat and the flexural-isostatic response to erosion. The model honors the structural geology of the Grand Canyon region, including the variable erodibility of rocks in the Colorado Plateau and the occurrence of Plio-Quaternary normal faulting along the Hurricane-Toroweap Fault system. We present the results of two models designed to bracket the possible drainage architectures of the southwestern margin of the Colorado Plateau in Miocene time. In the first model, we assume a 13,000 km2 drainage basin primarily sourced from the Hualapai and Coconino Plateaux. The results of this model indicate that relief production along the Grand Wash fault initiated the formation of a large (700 m) knickpoint that migrated headward at a rate of 15 km/Myr prior to drainage integration at 6 Ma to form a deep gorge in western Grand Canyon. This model also illustrates that integration of the Colorado River increased the rate of knickpoint migration to 60 km/Myr, resulting in rapid incision of eastern Grand, Marble, and Glen Canyons down to the level of the Redwall Limestone from 6-4 Ma. Widening of Grand Canyon by cliff retreat triggered flexural- isostatic rebound and renewed river incision of up to 400 m in Plio-Quaternary time. Plio-Quaternary normal faulting significantly dampened incision rates in western Grand Canyon relative to eastern Grand Canyon. As an alternative, we also consider the results of a model in which no incision in western Grand Canyon is assumed prior to 6 Ma. In that model, headward erosion prior to 6 Ma was not significant (by assumption), but the remaining results of the model are similar to that of the first model for the post-6 Ma period, illustrating the robustness of the post-integration behavior of the model with respect to pre-integration drainage scenarios. The results of the first model illustrate that headward erosion could be sufficient to capture the ancestral Upper Colorado River east of the Shiwitz Plateau, but the limited volume of Miocene clastic debris in the Grand Wash Trough and adjacent basins requires that slow rates of cliff widening and/or significant sediment storage in western Grand Canyon be invoked in order for this model to be consistent with the stratigraphic record.