RATES OF GEOMORPHIC PROCESSES IN THE SOUTHERN APPALACHIAN BLUE RIDGE: TESTING THE DYNAMIC EQUILIBRIUM HYPOTHESIS

Primary-mineral destruction rates during weathering of high-grade metasedimentary rocks in two low-order forested watersheds of the southern Appalachian Blue Ridge Mountains (U.S.D.A Forest Service Coweeta Hydrologic Laboratory, western North Carolina, U.S.A.) are determined by combining balanced stoichiometric weathering reactions (based on mineralogical observations) with stream solute flux data for major elements and REE (in mol/ha/yr) over a period-of-record of several decades. The rate at which the weathering front descends into the deeper fresh rock (volume of fresh rock converted to saprolite/ha/yr, leaving saprolitic regolith above) is determined by combining the weathering rate with the modal abundance (determined from petrographic analysis) and molar volume of each mineral, and assuming isovolumetric weathering of rock to saprolite.

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.