LONG-TERM LANDSCAPE EVOLUTION IN THE APPALACHIANS REVISITED: CUTTING THE CORD WITH THE PALEOZOIC OROGEN

The modern topography that we refer to as “The Appalachians” has long been thought to represent the roots of a once great mountain belt; however, the emerging reality appears significantly more interesting. There are aspects of the long-term evolution of this region that fit the eroding roots paradigm. Generally speaking, long-term erosion rates are faster than short-term rates. In many places the modern topography is strongly correlated with lithology. And despite sedimentary and geomorphic evidence for considerable unsteadiness, no Cenozoic cooling ages for low-temperature thermochronometers have been found, meaning no part of the landscape has experienced exhumation and erosion significant enough to reset ages during this time. These observations do indicate that the evolution of the modern Appalachians has on the whole been rather slow, but they fail to explain the nuances that separate the different parts of the range in terms of topographic expression and litho-tectonic setting. Several recent studies that take advantage of new developments in the application and interpretation of low-temperature thermochronology have profoundly changed our view of origin of topography in this region. At both ends of the range, models reveal prolonged periods (~50 Myr) of accelerated exhumation in valley floors relative to neighboring ridges and summits long after tectonic activity related to rifting had ceased. This spatially variable exhumation generated km-scale relief in both areas and indicates that there is no genetic link between high modern relief and the Paleozoic orogen. Furthermore, it is interesting to note that the central Appalachians have experienced the least post-rift exhumation, but they are currently the area with most dynamic geomorphic processes. By combining these observations it appears that the Appalachian landscape is neither uniformly decaying, nor finished evolving and hides more secrets about the processes that drive it’s continued modification.