10BE-DERIVED BARE-BEDROCK EROSION RATES ON RIDGELINES IMPLY LANDSCAPE TRANSIENCE IN THE BLUE RIDGE, SHENANDOAH NATIONAL PARK, VIRGINIA

Although orogenic activity ceased by ~200 Mya, the southern Appalachians retain characteristics of active orogens, including steep slopes, frequent mass wasting, and valleys carved by bedrock floored streams. Several studies conclude that landscape transience is responsible for post-orogenic rejuvenation of this topography, brought on by drainage capture, uplift driven by mantle dynamics, and/or climate change. Transience implies spatial variability in rates of elevation and relief change, but coupled erosion rate measurements that allow quantification of relief change are scarce in this region. We present ridgeline bare-bedrock erosion rates measured by ¹⁰Be in the Blue Ridge of Shenandoah National Park (SNP), and compare our results to ¹⁰Be-derived watershed-averaged erosion rates measured in the SNP by Duxbury et al. (2015). We collected 24 samples on bare-bedrock ridgeline exposures of four major lithologies that underlie ~55% of the SNP area. Using the CRONUS calculator (Balco et al., 2008), the mean bare-bedrock erosion rate is 7.81±0.39 m/My, ranging from 1.41 to 37.01 m/My. The erosion rates vary with lithology: phyllite and metasiltstone (Cch) = 21.01 m/My (n=5); quartzite (Cca) = 4.20 m/My (n=6); granite (Yor) = 2.21 (n=4); and megacrystic gneiss (Yos) = 5.38 m/My (n=9). Assuming steady state erosion, these rates are averaged over ~10^{5 -}10⁶ years. Overall, ridgeline erosion rates are slow and similar to bare-bedrock rates obtained from elsewhere in the region. However, the CCh mean erosion rate differs statistically from the rest of the lithologies, implying that ridgeline erosion rates are not uniform. Our ridgeline rates are slightly slower than the mean of the basin-averaged rates (10.2 m/My, n=52) reported by Duxbury et al. (2015). Comparing ridgeline rates to basin-averaged rates by lithology, ridgeline rates are significantly slower for quartzite, granite, and gneiss-floored ridgelines than basins floored by those lithologies, while rates are significantly faster for the siliciclastic-floored ridgelines. Variation in ridgeline rates by lithology and significant differences between ridgeline and basin-averaged rates imply disequilibrium in the Blue Ridge, although the data suggest that relief is not uniformly increasing.