SURFICIAL AND TOPOGRAPHIC SIGNATURE OF DRAINAGE DIVIDE MIGRATION, SOUTHERN APPALACHIAN MOUNTAINS

Unique fluvial gravel deposits preserved along asymmetric drainage divides in the southern Appalachians offer direct physical evidence of post-orogenic divide migration via repeated stream capture. Gravels are preserved in zones of muted topography along divides (wind gaps) and are frequently perched above stream gorges with “hook” or “elbow” drainage patterns and knickzones indicative of transient incision. Rounding of clasts in many deposits is consistent with the capture of large streams, generating punctuated divide “jumps” of many kilometers instead of slow, steady divide retreat by headward erosion. Survival of the gravels above the deeply-incised gorges suggests slow background Appalachian erosion, which accelerates in the captured basin by sudden connection to a lower base level. This mechanism of divide migration depends upon the initial development of divide asymmetry, which persists in a feedback loop as streams on the lower side of the divide capture elevated streams, gain stream power, and incise more quickly than the elevated streams unaffected by the base level drop. Divide retreat and transient incision can therefore continue without external energy input as long as base level contrast across the divide exists. Gravels and drainage pattern/knickzone combinations indicate migrating divides of varying scales are present throughout the Appalachians. The extensive Eastern Continental Divide/Blue Ridge Escarpment system is the best known, but its origins and tectonic context remain unclear. Smaller systems appear related to erodbility contrasts between adjacent basins, suggesting the steady exhumation of complex crustal architecture may generate zones of disequilibrium as drainage networks rearrange to maximize erosional efficiency.