THE FLUVIAL RECORD OF ROCKY MOUNTAIN EPEIROGENY - A REVIEW

The mean elevation of an orogen is a direct reflection of the dynamic and isostatic forces that support topography above sea level. Recognition globally that some orogens may maintain their lofty elevations principally by a dynamically buoyant mantle has lead to intense scrutiny of how the evolution of mean elevation is actually determined. For the Rocky Mountains, paleobotanical and isotopic methods provide only very broad estimates and are restricted to specific localities leading most studies to argue for river incision as a measure of both rock and surface uplift. An implicit assumption in this argument is that the incision principally reflects a steepening of river gradients through tectonic/epeirogenic processes. A review of numerous river incision studies throughout the Rocky Mountains confirms this notion at least locally, and seems to support the view that the current mean elevation of the Rockies may have been attained relatively recently. But two unrelated studies, one along the Jemez lineament in New Mexico and the other in the Black Hills of South Dakota, also draw attention to the importance of base level fall and exhumation of the Rockies over large temporal and spatial scales. In particular, the Jemez lineament study illustrates how much river incision should be apportioned to an epeirogenically-driven change in mean elevation. The conclusion is that there are significant lag times in the fluvial excavation of an orogen as broad as the Rocky Mountains. It should not be assumed that the current elevation of a river valley bottom represents the river's base level of erosion that was maintained during uplift. Nor should any data regarding the onset of incision correspond necessarily to the onset of epeirogeny. Any single measure of river incision must be carefully considered in the context of all processes that drive river incision to extract that portion which truly reflects changes in mean elevation, and by extension, epeirogenic processes driven by dynamic mantle buoyancy.