ESTIMATES ON THE MAGNITUDE AND TIMING OF POST-OROGENIC TOPOGRAPHIC REJUVENATION OF THE SOUTHERN APPALACHIANS USING ISOSTASY AND DEFORMED COASTAL PLAIN ROCKS

There is a growing body of evidence that the southern Appalachians have experienced a Miocene topographic rejuvenation, which includes the presence of coarse Miocene clastics in the southeastern Coastal Plain, deformed Coastal Plain sediments, and transient knick points in streams within the southern Appalachian highlands. The idea that the southern Appalachians experienced Miocene rejuvenation was proposed by Dennison and Stewart (2001) and although the existence of the event has gained support since then, the magnitude and timing of the uplift still remain unclear. Two recent studies, one based on stream erosion, the other on geodynamic modeling, estimated the uplift to be about 500 meters and occurring during the Miocene. Other studies based on deformation of Coastal Plain sediments and the influx of coarse clastics are also consistent with Miocene uplift.

Wagner et al. (2102) showed that the area underlying the North Carolina Blue Ridge is in isostatic equilibrium as is the adjacent Piedmont to the east. The Piedmont, however, appears to have a doubled Moho beneath it, and is in isostatic equilibrium only if the rocks that are sandwiched between the two Mohos are eclogite. This presumed eclogitic slab may have once continued under the Blue Ridge and Gallen et al. (2013) proposed that the foundering of this slab may have triggered the uplift of the southern Appalachians. If this scenario is correct, it would have resulted in about 1 km of uplift of the Blue Ridge with respect to the Piedmont and the boundary between the two topographic provinces would coincide with the buried edge of the remaining eclogitic slab. This edge coincides with the Blue Ridge escarpment, a ~1km high escarpment that separates the Piedmont from the Blue Ridge.

At its southwestern terminus, the southern Appalachian highlands plunge underneath the Coastal Plain sediments of Alabama. The curving outcrop pattern of these Cretaceous and younger rocks are due to the rocks being folded over the uplifted crystalline core of the southern Appalachians. By retrodeforming these folded rocks, it can be shown that the uplift occurred primarily during the late Oligocene and early Miocene.