Surface geology and seismic-reflection data suggest that rather than having a massive rooted central core the southern part of the Appalachian orogen from the Appalachian Plateau to the Atlantic continental shelf is underlain by an eastward-dipping decollement zone. We infer that this decollement zone was a long-lived structural element, intermittently growing from east to west during late Proterozoic to late Paleozoic time. Onshore displacement along the detachment surface was episodic through this long period of time, so that reliable estimates of total shortening for the entire orogen are not possible. Palinspastic restoration of only the western margin of the orogen, the area west of the Brevard zone, suggests a minimum of 280 km (175 mi) of Alleghenian displacement. Total accumulated shortening through time within the entire orogen must be considerably greater, probably exceeding hundreds of additional kilometres.