LANDSCAPE DISEQUILIBRIUM...IN THE VIRGINIA BLUE RIDGE? CONNECTING "UN-APPALACHIAN" EROSION TO A STREAM CAPTURE EVENT

The rugged northwestern margin of the Blue Ridge Upland plateau has long been viewed as a landscape sculpted by erosional processes atypical of the Appalachians at large. Major incision restricted to tributaries of the South Fork of the Roanoke River (SFRR) has locally dissected the plateau margin into a discontinuous series of low relief relict surfaces separated by deep gorges. Large knickpoints at the heads of the gorges are not associated with lithologic contacts. Upstream of the knickpoints, SFRR tributaries resemble low-gradient streams of the adjacent New River basin, where major incision is not occurring. The apparent uniqueness of SFRR erosion within the region has been difficult to rectify with the existing paradigm of dynamic equilibrium throughout the Appalachians. Alluvium of clear SFRR basin provenance now preserved atop the New River-Roanoke River divide indicates that the incision is the result of capture of SFRR headwaters from the New River basin. The capture process has tapped the potential energy of New River basin streams relative to the ~ 300 m lower Roanoke River base level, producing rapid incision limited to the courses of major streams and leaving the resulting relict surfaces largely intact. The persistence of large knickpoints in many SFRR streams indicates the drainage network remains in a state of transient adjustment to the new base level. While the onset of accelerated erosion into a relict surface is known to result from rapid surface uplift, incision of the SFRR suggests that stream capture across a highly asymmetric drainage divide can tap the potential energy of “perched” streams and drive a similar erosional regime in tectonically quiet settings. Cosmogenic dating of sediments and bedrock surfaces within the SFRR basin may further enhance understanding of the role of stream capture in producing basin-scale focused erosion in mature landscapes.