USING SUMMIT EROSION RATES MEASURED WITH 10Be TO ASSESS LANDSCAPE DISEQUILIBRIUM IN THE BLUE RIDGE, SHENANDOAH NATIONAL PARK, VIRGINIA

We have measured erosion rates using 10Be from bare-bedrock surfaces located on summits in the Blue Ridge of Shenandoah National Park, Virginia. The rate of summit lowering allows us to assess the variability of erosion rates as a function of lithology and to evaluate the likelihood of dynamic equilibrium in this ancient range. We collected 23 samples on bare-bedrock exposures of four major lithologies on nine summits, from which we determined the 10Be abundance in quartz. The mean erosion rate from all nine summits is 8.8 m/My, with rates varying from 1.6 m/My to 41 m/My. These rates suggest that summit erosion rates in this region are generally slow, and are closely related to rock type. Rates on individual lithologies were 23 m/My on Harpers Formation metasediments (n=5), 5.5 m/My on Pedlar granitoids (n=8), 4.5 m/My on Antietam quartzites (n=6), and 2.4 m/My on Old Rag granites (n=4). We compare these rates to basin-averaged erosion rates measured using 10Be by Duxbury (2009) in watersheds that include our sampled summits, as well as regional river incision rates measured previously. Assuming that our bare-bedrock erosion rates approximate summit lowering rates, the difference between these rates and fluvial incision rates imply that relief has been increasing in much of the Blue Ridge of Shenandoah National Park over time scales averaged by these measurements (~100-250 ky). The proximity of our summit erosion rate locations to previously calculated basin and fluvial incision rates suggest the Appalachians here are neither in a state of dynamic equilibrium nor undergoing a decrease in relief through rapid summit lowering, as has been previously suggested for this region. We suggest late Cenozoic climate change may have triggered temporary disequilibrium with relief growth through the combined effects of river rejuvenation through base level lowering and landscape destabilization by rapid climate fluctuation.