RATES AND POSSIBLE CAUSES OF LATE CENOZOIC INCISION OF THE WESTERN GREAT PLAINS

The western Great Plains of the continental U.S. is characterized by ongoing fluvial incision. Relief along most of the river valleys in the central Great Plains is less than a few 10’s of meters. But to the west, relief increases progressively reaching magnitudes of 700-1200 m along the Rocky Mountain front. Much of this relief has been produced by fluvial downcutting since Miocene time (5-10 Ma). Magnitudes of erosion measured from topographic markers including perched volcanic flows and ash layers in terrace deposits yield maximum long term rates of 40 - 80 m/m.y. over the last ~10 Ma and maximum shorter term rates of 100 - 150 m/m.y over the last 600,000 years (Dethier, 2001). The increase in rates of incision is consistent with compilations from the Rocky Mountains and most likely occurred around 3-4 Ma. Tectonic uplift, climate change, and base level fall have all been proposed as the cause of this fluvial incision. Support for tectonic forcing of incision comes from the pattern of incision and interfluve gradients that parallel the northward propagation of the Rio Grande Rift. In addition, gravity modeling (Angevine and Flanagan, 1987), crustal thickness estimates from P-wave velocity interpretations (Sheehan et al., 1995), flexural modeling (Roy et al., 1999), and post-depositional tilt analyses of the western Great Plains (Leonard, 2002; McMillan et al., 2002) all require long-wavelength buoyant subsurface loads beneath the region. On the other hand, the lack of a well-defined Cenozoic paleoelevation history hampers our ability to distinguish between the effects of climate and tectonic uplift. A global change at ~3 Ma (Zhang et al., 2001) from a monotonic climate to a strongly oscillating regime, perhaps with a concomitant change in vegetative cover has also been invoked to explain the production of relief (e.g. Molnar and England, 1990; Gregory and Chase, 1994). This event is supported by the observed increase in incision rates during the Pleistocene. Finally, incision due to knickpoint retreat caused by a base-level fall farther down the Missouri-Mississippi River system has also been proposed (Zaprowski et al., 2001). Regardless of the cause, the Great Plains has the most continuous continental record of deposition and incision and thus is a key location for untangling the Cenozoic tectonic and climate history of the U.S. interior.