THE BLUE RIDGE FRONT: A FAULT SCARP REVISITED

The Blue Ridge Escarpment (BRE), constituting an ~500 m wall that delineates the topographic boundary between the Piedmont and Blue Ridge geomorphic provinces of the Southern and Central Appalachians, has puzzled geologists for much of the past two centuries. Various theories have been proposed to explain its origin and evolutionary development, yet there has been no agreement to date on the origin of this striking geomorphic feature. One of the more historically argued hypotheses suggests that the escarpment is undergoing westward parallel erosional retreat; however, rock types on either side of the BRE show no obvious difference in resistance to erosion. Reinterpretation of seismic reflection profiles suggests that the BRE may be structurally controlled by an east dipping normal fault. Significant offset (~700 m down to the SE) in a prominent layered sequence at ~7 km depth had been imaged on the SEISDATA and COCORP seismic profiles. This offset had previously been interpreted as a Late Proterozoic normal fault underlying the continuous Blue Ridge Thrust sheet. Spatial coincidence, similarity in magnitude, and consistent polarity of offset of this fault with the surface expression of the BRE suggest they may represent the down-dip continuation of a post-Paleozoic fault system that formed the BRE. Previous interpretations show little evidence of a continuous, through-going thrust sheet, and preliminary reprocessing of the SEISDATA section suggests that similar offsets may exist in the shallow subsurface above the Appalachian detachment, further supporting a fault origin for the BRE. A similar basement offset is also observed in the COCORP GA1 seismic profile ~20 km to the southwest however is not present on the ADCOH 1 profile (~20 km northeast along strike) perhaps due to transfer of offset to the nearby Marietta-Tryon Graben System. Although portions of the escarpment further north show evidence for parallel retreat, this research suggests, at least for the Southern Appalachians, a post-Paleozoic fault may be the mechanism for the current position of the BRE. If correct, this observation would imply that the Southern Appalachians have been tectonically active in Cenozoic time.