Evidence of transient topographic disequilibrium in a landward passive margin river system: knickpoints and paleo‐landscapes of the New River basin, southern Appalachians

Abstract

The upper New River basin of the southern Appalachian Mountains, a major tributary of the modern Ohio River, represents the unglaciated headwaters of the Tertiary Teays River system of eastern North America. Dating of relict fluvial gravels have suggested that New River incision may be outpacing lowering of the surrounding uplands, but physical evidence of transient topographic disequilibrium has yet to be identified. We use focused topographic analysis of the upper New River basin to delineate a perched, low‐relief paleo‐landscape that is experiencing transgressive dissection due to incision by the New River and its tributaries. Accelerated incision has decoupled hillslopes from the drainage network, generating knickpoints which represent the boundary between remnants of the paleo‐landscape and actively adjusting topography downstream. Steepening of hillslopes downstream of knickpoints suggests dynamic headward migration which, along with knickpoint occurrence throughout the drainage network, is inconsistent with the development of fixed stream profile convexities atop strike‐extensive geologic contacts. In the absence of tectonic forcing, we favor a climatically‐forced drop in external base level as driver of the incision pattern we observe. Plio‐Pleistocene glacial damming and diversion of the Teays River to form the modern Ohio River lowered regional base level for the study area, potentially forcing the paleo‐landscape developed during the Teays era to adjust to the modern drainage pattern. The upper New River may therefore represent the potential for glacially‐driven drainage rearrangement to drive transient topographic evolution hundreds of kilometers away from the ice margin, long after the disappearance of ice sheets. Copyright © 2013 John Wiley & Sons, Ltd.

Rates of erosion and landscape change along the Blue Ridge escarpment, southern Appalachian Mountains, estimated from in situ cosmogenic 10Be

“…Seismic data suggest tens of millions of years ago a hidden hotspot may have passed through northern Virginia although the effect of such a passage on escarpment topography is uncertain (Chu et al , ). Field observations as well as numerical landscape analysis and modeling suggest that the area around the escarpment is significantly and episodically modified by drainage capture events and large‐scale recent uplift (dynamic topography) likely driven by mantle processes (Prince et al , , ; Prince and Spotila, ; Gallen et al , ; Rowley et al , ; Miller et al , ; Schmandt and Lin, ; Naeser et al , ). Such capture events transiently increase the rate of basin‐scale erosion and escarpment retreat in specific areas along escarpment strike as base‐level falls rapidly and incision propagates upstream.…”

“…For example, if thermochronologic data show no time–distance relationship across the lowlands and cosmogenic data indicate millennial‐timescale rates of retreat for an inland escarpment that are too slow to accommodate the distance the escarpment has moved over the time interval since rifting began, then the concept of an escarpment continuously retreating at one rate over time is not plausible. In such a case, the erosion responsible for the inland position of the escarpment either must have occurred rapidly, soon after rifting, such that a period of relative erosional stability (post retreat) coincides with the integration time of the thermochronologic data or erosion occurs episodically, through drainage capture and rapid back‐wearing of drainage divides (Prince et al , , ; Prince and Spotila, ).…”

“…Several datasets suggest that erosion and thus sediment delivery from the range has likely not been steady over timescales of 10 6 –10 8 years. For example, Pazzaglia and Brandon () show large changes over the last 10 8 years in rates of sediment delivery to the passive margin, Prince et al (, ) and Prince and Spotila () argue that drainage capture leads to rapid downcutting and consequent landscape change over 10 6 year timescales, and Naeser et al () provide thermochronologic evidence for a major Miocene dranage capture event. Modeling (Rowley et al , ) supported by mantle imaging (Schmandt and Lin, ) suggest that some Appalachian topography (on the scale of 10 2 m of uplift) is dynamically supported and quite young, 10 6 years.…”

Controls on Zero‐Order Basin Morphology

“…Coweeta is underlain by high‐grade metamorphic rocks, primarily biotite and quartz diorite gneisses, with minor schists and metasedimentary rocks that have been strongly folded and faulted throughout the basin (Hatcher, ). Authors have suggested that this area may diverge from steady state due to rejuvenation of uplift in the Miocene (Gallen et al, ) or drainage reorganization associated with escarpment retreat (Prince & Spotila, ). However, erosion rates measured in the basin are consistent with rates measured in the nearby Great Smoky Mountains, suggestive of a mountain range approaching steady state.…”

Tectonics, sedimentation and surface processes: from the erosional engine to basin deposition

“…As to the culprit of this incision of the upper New River, their interpretation points out a well‐known glacially‐forced drop in landward base level, linked with the reorganization of the Teays River system along the margin of advancing late Pliocene and Pleistocene ice sheets. The study of Prince and Spotila () thus highlights the potential of far‐field long‐lived drainage basin reorganisation over distances of hundreds of kilometers away from perturbations and over long periods of time after perturbations. This finding is well in line with current models of dynamic river basin evolution as recently put forth in the work of Willett et al (), for instance.…”