A Tale of Three Divides: Investigating the erosional signature of divide motion along the Blue Ridge Escarpment

Abstract:

River network reorganization may be an important mechanism for shaping the topography of post-orogenic landscapes. For example, river captures produce transient waves of incision that create high-relief knickzones within growing river basins and low-relief relict surfaces in shrinking river basins. The gradual motion of drainage divides is similarly thought to leave a topographic signature that is identifiable with analytical mapping techniques. Such topographic evidence has been used along the Blue Ridge Escarpment in the Appalachian Mountains to suggest that the major drainage divide is moving towards the west through both gradual divide migration and punctuated river captures. We use ¹⁰Be-derived basin-averaged erosion rates to test whether the asymmetric topography does indeed reflect divide motion. We first target two putative capture sites, the upper South Fork Roanoke River and the Dan River in Virginia, USA, both of which have knickzones associated with ~300 m of relief. To test the hypothesis that the knickzones were formed by a wave of rapid incision following river capture, we measure erosion rates in tributaries draining into points both upstream of and within the knickzones. In the Dan River we find that the erosion rate within the knickzone is approximately three times faster than upstream of the knickzone, supporting the river-capture hypothesis. In contrast, we find no significant difference in erosion rates above and below the Roanoke knickzone, and instead find that the knickzone is spatially coincident with a lithologic contact. The uniform erosion rates in the Roanoke seem to conflict with other lines of evidence that suggest past river capture, including barbed tributaries and cobbles with provenance outside the present-day watershed. We use river-profile simulations to explore whether variability in erodibility, river capture, or a combination of both are viable mechanisms to form the Roanoke knickzone without producing measurable spatial differences in erosion rates. Finally, we target the upper Pee Dee and New Rivers, in North Carolina, USA, where asymmetric topography across the divide suggests divide migration towards the New River. We find that the upper Pee Dee River is eroding on average two times faster than the New River, suggesting a horizontal divide migration rate of ~30 m/Myr.