RATES OF GEOMORPHIC PROCESSES IN THE SOUTHERN APPALACHIAN BLUE RIDGE: TESTING THE DYNAMIC EQUILIBRIUM HYPOTHESIS
Weathering of plagioclase feldspar in a watershed underlain by bedrock of the Coweeta Group occurs at a rate corresponding to the depletion of plagioclase from approximately 26 meters of rock (beneath a unit area of landscape) per million years (m/Ma). Corresponding values for garnet and biotite are 61 and 28 m/Ma, respectively. Corresponding rates for plagioclase, garnet and biotite in a watershed underlain by bedrock of the Otto Formation are 21, 24 and 28 m/Ma, respectively. Most saprolitization rates determined from present-day solute fluxes are between 20-30 m/Ma and spatially uniform among several metamorphic rock units.
The volume rate of landscape reduction by physical erosion and sediment export required to maintain a steady-state weathering profile thickness would equal the saprolitization rate (25±5 m/Ma). This is essentially identical to recently published results for (1) the time-averaged rate of trunk-stream incision into debris-flow deposits, (2) the time-averaged erosion rate for the past ~10-100 ka determined for the nearby Great Smoky Mountains using cosmogenic radionuclides, and (3) the long-term uplift rate for the southern Appalachians over the past ~200 Ma determined from fission-tracks. The excellent accord between the modern chemical saprolitization rate and the long-term average erosion and denudation rate is consistent with the hypothesis that the southern Blue Ridge landscape is in dynamic equilibrium.