Origin of an Eastern North America Great Escarpment, Based on (U-Th)/He Dating and Geomorphic Analysis

Many passive margin great escarpments have been interpreted as the result of slow, steady erosional processes following continental rifting. Rugged escarpments may be maintained by parallel slope retreat and drainage divide migration, driven by flexure of the lithosphere associated with rift flank uplift and the isostatic response to denudation and mass transfer from thickened mountain belts to offshore basins. In eastern North America, an escarpment bounding the eastern limit of the southern Appalachian Mountains stands high and rugged, despite a prolonged period of time since cessation of tectonic activity along this passive margin. Whether this feature is the product of slow, steady erosional processes along the continental margin since rifting in the early Mesozoic and cessation of orogenesis in the late Paleozoic, or the result of recent rejuvenation, has been the subject of debate. To investigate this, we have constrained the erosional kinematics of the Blue Ridge Escarpment in Virginia and North Carolina using radiogenic helium thermochronometry and other geomorphic data. Helium ages from rocks above and extending west of the escarpment are old (157-197 Ma), consistent with very slow cooling and denudation since the early Mesozoic. In contrast, helium ages are younger along the Piedmont surface, 500 m below and to the east of the escarpment (94-137 Ma). Although this difference may partly represent an age-elevation gradient associated with either slow exhumation and cooling or an exhumed helium partial retention zone, it cannot be fully explained without lateral variations in exhumation rate. Our working interpretation is that the western Piedmont has experienced as much as several km more exhumation than the Blue Ridge upland since samples cooled through their closure temperatures, effectively bending isochrons upwards to the east. The greater magnitude of denudation east of this east-facing escarpment is not consistent with escarpment formation by rejuvenated uplift of the Blue Ridge; it invokes the wrong sense of vertical motion than expected if the escarpment and highlands to the west had been recently uplifted by faulting or rebound associated with accelerated valley incision tied to climate change. Instead, it is more simply explained by retreat of an asymmetric divide, possibly formed by flexure and isostatic rebound in response to rift-flank uplift or offshore loading. This is consistent with other evidence for westward divide migration in the form of fluvial deposits atop the Blue Ridge highland and the character of stream networks throughout the region.