APPALACHIAN PALEOTOPOGRAPHY

Recent geomorphic, geodynamic, and thermochronologic studies are contributing to an emerging reconstruction of Cenozoic Appalachian paleotopography. Here, we focus on the longitudinal profile development of Appalachian Piedmont streams that traverse the Fall Zone and attempt to reconcile these data with a rising consensus that the eastern United States, including the Appalachians, has experienced several hundred meters of dynamically-driven rock and surface uplift in the past ~30 m.y. Central Appalachian Piedmont streams are steep, convex, and contain several distinct knickpoints that tend to cluster at common elevations. Large, obvious knickpoints mostly occur below ~150 m, an elevation coincident with the base of late Miocene upland gravels that onlap the Coastal Plain and Fall Zone. We model two end member possibilities for the base level fall that has driven fluvial incision below these deposits by adopting a stream-power based knickpoint erosion law applied to several streams where reaches upstream of the 150 m knickpoint can be confidently projected to match the elevation of upland gravels. Preliminary results favor unsteady base level fall and multiple knickpoint generation; however, there are also indications that knickpoints are being differentially uplifted as they propagate through a broad, and dynamically flexing margin. Of related interest is the predicted age of knickpoints located deeper in the Appalachian foreland, along major rivers. Assuming constant model parameters and no broad crustal deformation, prominent knickpoints at 300 m and 400 m on the Susquehanna River would be Oligocene or older. Even though this is an overly simplistic assumption given the different rock types traversed by Atlantic slope streams, the results are consistent with the idea that knickpoints are long lived dynamic features propagating through the landscape and exerting a primary control on the evolution of Appalachian topography.