A NEOGENE TO RECENT RESURGENCE OF TOPOGRAPHIC RELIEF IN THE SOUTHERN APPALACHIANS: GEOMORPHIC EVIDENCE FROM THE CULLASAJA RIVER BASIN, NORTH CAROLINA

Geoscientists have long puzzled over the evolution of topographic decay in post-orogenic mountain ranges. The Southern Appalachians, for example, are thought to have been tectonically quiescent for > 200 Ma, yet exhibit many characteristics associated with tectonically active settings, such as locally high relief, steep hillslopes, and frequent mass-wasting. Different hypotheses have been posited to explain the evolution of this landscape. Some argue that these features are typical of decaying orogens and over long periods of time the landscape slowly weathers away and relief declines. Alternatively, some hypothesize that the modern topographic form is the result of more recent external forcing and has been rejuvenated. We present results from the unglaciated Cullasaja River basin in southwestern North Carolina that suggest this landscape is adjusting to a series of base level falls, making recent external forcing an attractive explanation for the region’s rugged terrain.

Using a high-resolution lidar DEM and GIS analyses we have identified evidence of five different base level falls preserved as substantial bedrock knickpoints in the drainage network of the Cullasaja River basin. Knickpoints are widely accepted as a mechanism by which a base level fall is transmitted to and imposed upon a landscape, as they represent the mobile interface between the ‘relict landscape’ above and the actively adjusting landscape below. Analysis of the spatial position of knickpoints and topographic metrics from 25 tributary basins supports the hypothesis that knickpoints are propagating upstream, forcing a landscape response in their wake.

Assuming a range of basin average erosion rates for the Southern Appalachians and using a volume of eroded mass for time substitution, we estimate that the knickpoints entered the Cullasaja basin between the Late Miocene and Pliocene, and have sustained horizontal retreat rates of 2 – 15 mm/yr. We reconstructed the longitudinal river profile above the highest Cullasaja knickpoint to test for post-Miocene relief generation. We find that incision rates are double that of assumed basin average erosion with ~ 500 m of incision occurring since the Miocene. Our results demonstrate that the Southern Appalachians have experienced a phase of rejuvenation during the Neogene and relief is increasing.