GEOMORPHIC NOISE AND THE CASE FOR ACTIVE TECTONIC UPLIFT OF THE APPALACHIAN MOUNTAINS (Invited Presentation)

The case has mounted recently for a pulse of late Cenozoic uplift in the Appalachian Mountains and North America’s passive margin. Geologic evidence includes base level fall and transient incision based on knickpoint distribution and migration, relief production based on basinwide erosion rates, and spatiotemporal heterogeneities in low-temperature bedrock cooling rates. There is also evidence for potential drivers of rejuvenation, including geophysical anomalies in the mantle that suggest delamination and models that predict uplift patterns based on convection and flexural-isostasy that correlate to surface observations. However, many of these inferred signals are small (e.g. slow erosion, small changes in base level) and non-unique, translating to the need for signal validation above the background of noise in the topographic-erosional system. Three primary sources of geomorphic noise are lithology, dynamic evolution of drainage basins, and late Cenozoic climate change. Erodibility has an obvious influence on Appalachian topography, but can be overlooked as a cause of knickpoints or spatiotemporal variations in erosion when contrasts in bedrock mechanical properties occur within mapped formations. Drainage basin evolution, including major divide migration (e.g. eastern continental divide) and small scale stream capture influenced by erodibility, can also produce signals that can be misinterpreted as local pulses of transient erosion. Finally, climate change may induce changes in erosion and sediment transport, via changes in weathering efficiency, vegetation, frequency of stochastic events, and stream discharge, which are rarely tested beyond a rudimentary temporal comparison. Examples illustrate how each of these factors may be widespread sources of geomorphic noise. The Appalachians, at this particular time in geologic history, are not ideally suited to preserve unambiguous records of modest tectonically driven uplift. Although evidence is mounting, future work must include diligent field testing to rule out each source of noise, statistical methods to judge result significance, and reproduction of specific results using realistic models and strategic application of independent methods.