FOREVER YOUNG? NEW EVIDENCE FOR CONTINUING PARALLEL RETREAT OF THE WORLD'S OLDEST PASSIVE MARGIN ESCARPMENT

The Blue Ridge Escarpment (BRE) is a striking and enigmatic topographic feature of the passive margin of eastern North America, the oldest in the world. Abruptly rising 300-500 m above the Piedmont lowlands, the crest of the escarpment generally coincides with the Eastern Continental Divide (ECD) and separates steep, eastward-flowing streams of the Atlantic basin from low-gradient streams of the Blue Ridge Upland draining west-northwest, ultimately to the Gulf of Mexico. While inland erosional retreat of the BRE has effectively been confirmed by recent studies of exhumation patterns and erosion rates, the mechanism, timing, and magnitude of retreat have remained unconstrained. Numerous fluvial terraces preserved in wind gaps at the escarpment crest present a previously neglected source of key information regarding BRE evolution. A number of terraces are located immediately atop the ECD and confirm BRE migration by parallel retreat. U-Th-Pb dating of detrital terrace zircons has failed to indicate sediment origin and thus minimum retreat distance, but roundness of vein quartz/quartzite clasts suggests many 10’s of kilometers of fluvial transport from sources now seaward of, and below, the escarpment. Preservation of terrace material in the humid-temperate climate is good and implies continued retreat of the escarpment and ECD during the Cenozoic as well as considerable upland stability. Further investigation will focus on determining clast provenance through analysis of quartz microstructure. The abundance and good preservation of fluvial debris atop such a mature feature as the BRE have implications for the use of upland surficial deposits in the study of younger and potentially better preserved passive margin escarpments.