SEQUENTIAL DEVELOPMENT OF THE CENTRAL APPALACHIAN FOLD-THRUST BELT, PENNSYLVANIA: INSIGHTS FROM A BALANCED GEOLOGIC CROSS SECTION

We present a kinematic model for the evolution of Appalachian fold-thrust belt through the Pennsylvania salient based on a balanced geologic cross section from the southern edge of the Valley and Ridge Province to the northern Appalachian Plateau. By combining line length and area balancing of a kinematically viable cross section with layer parallel shortening (LPS) estimates in both the Valley and Ridge Province (20%) and Appalachian Plateau (13%) we document the total magnitude of shortening in both the ductilly folded upper layer and the duplexed lower layer of the fold-thrust belt. Restoration of the cross section indicates 77 km (22%) shortening between the southern margin of the Valley and Ridge province in central Pennsylvania and a pin line immediately north of the northern limit of documented LPS in the foreland. The proposed model links early LPS on the Appalachian Plateau to two fault-bend folds that have a lower décollement in the Cambrian Waynesboro Formation and an upper, subhorizontal detachment in the Silurian Wills Creek Formation (in the Valley and Ridge) and the Salina Group on the Appalachian Plateau. This upper detachment feeds displacement from these early horses in the duplex system onto the Appalachian Plateau and helps accommodate 14 km of LPS shortening. This early shortening is followed by the development of in-sequence horses that repeat the “stiff” Cambro-Ordovician sequence using both the main décollement in the Cambrian Waynesboro and the Ordovician Reedsville Formations as an upper detachment horizon. In the south, shortening in the upper Ordovician through Devonian layers is accommodated by both LPS and forced folding of the overlying ductile cover sequence. To the north we propose the Reedsville Formation becomes weaker, facilitating shorter wavelength detachment folds. The development of gentle open folds on the Appalachian Plateau, as well as the last gasp of LPS on the plateau is linked to the northern most horse in the system. This horse, which forms the broad Nittany Anticlinorium, the northern most Valley and Ridge anticline, cuts upsection from the main décollement horizon in Cambrian strata to the Silurian Salina Group on the Appalachian Plateau and feeds 10 km of slip into the broad concentric folds of the plateau.