Documenting the geometry and magnitude of shortening at the Allegheny Front: Lycoming County, Pennsylvania, United States

ABSTRACT Based on limited geophysical and outcrop data, shortening of the strata in the Appalachian Plateau above the Silurian salt detachment was interpreted to have occurred by layer-parallel shortening (LPS) plus gentle folding. Recently acquired high-quality, three-dimensional seismic reflection and well data reveal that strata from the Silurian Salina Group and the Devonian Marcellus Shale shortened through a variety of macroscale mechanisms in the Appalachian Plateau region of Lycoming County, Pennsylvania. Geometries evident in the seismic reflection profiles and repeated stratigraphy in gamma-ray log surveys indicate north- and south-dipping wedge-style thrust faults with offsets ranging from 60 to 550 m (195 to 1800 ft). Through kinematically restored and balanced cross sections, we show that the 10%–15% LPS calculated from the rocks exposed at the surface of the Appalachian Plateau is equal to the magnitude of interpreted macroscale shortening in the Silurian–Lower Devonian units immediately above the Silurian Salina Group. Previously collected seismic reflection profiles extending south of the Nittany anticlinorium into the Valley and Ridge reveal wedge-style thrust faulting in Ordovician through Silurian units and wedging of the northernmost horses (a volume of rock completely enclosed by thrust faults) of the Cambrian–Ordovician duplex of the fold–thrust belt under the southern plateau edge. These structures allow us to interpret a pathway of triangle zones (a combination of two thrusts with the same detachment and with opposing vergence such that they form a triangular zone) that transfer slip from the basal decollement in the Cambrian Waynesboro Formation to the overlying Ordovician Reedsville detachment and then to the Silurian salt detachment in the Appalachian Plateau.