POST-OROGENIC UPLIFT OF THE SOUTHERN APPALACHIANS

The Appalachian Mountains are a 2,400-km-long mountain belt that records the assembly of two super continents, accretion of exotic terranes, and two major rifting events. Conventional wisdom tells us that mountain belts like the southern Appalachians stop growing after they are no longer part of an active plate boundary but that turns out to be wrong. These mountain belts can have a long history of orogeny and erosion but can also have periods of uplift that cannot be directly linked to plate-boundary processes.

Several early workers had recognized that the modern-day topographic relief of the southern Appalachians, locally up to 1500 meters, was greater than would be expected if the mountains had simply been eroding since the last major orogenic event, the Alleghanian orogeny, which ended at about 300 Ma with the assembly of Pangea. Triassic rifting might have induced thermally driven uplift along the continental margin but this event is not likely to have created topographic relief that would be maintained for 200 million years.

Recent work by McKeon et al. (2014) has shown that the southern Appalachians underwent topographic rejuvenation during the Cretaceous and our work and the work of others (e.g. Gallen et al., 2012) has shown another pulse during the Miocene. We have documented structures and geomorphic indicators that show the Miocene event in the southern Appalachians uplifted the mountains between 500 and 1,000 meters. The driving mechanisms for the uplift are not clear but recent seismic tomography results presented by Biryol et al., (2016) suggest that the upper mantle beneath the eastern US is heterogeneous and includes large areas where cold, dense mantle has detached from the lithosphere and has been replaced by warmer, more-buoyant material. This change in lithospheric density could be the mechanism that is driving topographic rejuvenation.