UNROOFING THE APPALACHIANS; INSIGHTS FROM GEOMORPHOLOGY AND U-TH/HE THERMOCHRONOLOGY

Decay-phase orogens have long unroofing histories, but the mechanisms are commonly both unsteady and non-uniform. In the Appalachians, studies of sediments deposited off-shore in the Baltimore Canyon Trough (BCT) suggest no less than 7 km of crustal material having been removed in the past 180 Ma, and no less than 1.1 km being removed in the last 20 Ma. In apparent contrast to these data, thermochronologic and cosmogenic studies suggests slow erosion over periods of time on both long and short time spans. Furthermore, recent cosmogenic erosion rate data suggest that there are erosion “hot spots”, such as knickpoints in river channels, where rapid rates of erosion are superimposed on the much slower, long-term rates. Potentially, a very important process that drives the unroofing of an orogen is the integration of drainages, a process linked to the rate of migration of drainage divides at the orogen scale. We test the hypothesis that non-steady westward migration of the drainage divide in the Appalachian Mountains from the Blue Ridge through the Ridge and Valley, and ultimately to the Allegheny escarpment is a dominant control on landscape evolution and Appalachian unroofing and is ultimately responsible for the BCT sediment record. Our study took a combined geomorphic and geochronologic approach. Using a GIS analysis of digital topography for one part of the Appalachian continental divide in the Dolly Sods area of West Virginia, it was determined that east-flowing first-order stream channels are systematically steeper than their west-flowing counterparts. Channel slope-area modeling and field observations reveals fundamental differences in channel morphology, erosional process, and erodibility across the drainage divide, suggesting active divide migration. In a parallel analysis, U/Th-He dating of single grain detrital apatites collected from river sands in New England illustrates differential temporal and spatial scales of erosion in nested watersheds of variable relief. Relative probability diagrams of multiple grain ages argue that most of the rocks in New England came through a He partial retention zone about 100 Ma, indicting a long term erosion rate of about 40 m/m.y. We conclude that drainage integration through the Appalachian landscape is a major player, along with climate change and epeirogeny in driving unroofing in this decaying orogen setting.