THE STRUCTURAL GEOMETRY AND RETRODEFORMATION OF THE CENTRAL APPALACHIAN FOLD-AND-THRUST BELT IN THE PENNSYLVANIA SALIENT

The Pennsylvania salient is a classic arcuate fold-and-thrust belt that was deformed during the Late Paleozoic Alleghenian orogeny. 38 area- and line-balanced regional cross-sections with an along-strike spacing of 5-10 km were constructed from published and unpublished seismic, well, and surface data. The main competent lithotectonic unit is the 2-3 km thick Cambro-Ordovician (CO) carbonate sequence which is detached along the Cambrian Waynesboro, Fm. The 5-7 km thick Middle-Upper Paleozoic sequence is less homogeneous, and locally exhibits significant internal deformation.

The structural geometry varies significantly across the salient. In the southwest part of the salient, the hinterland part of the fold belt is defined by a series of imbricated CO carbonate thrust sheets with leading-edge fault-propagation style folds that have a structural amplitude of 5-7 km. In the central part, the Broadtop synclinorium exhibits little to no imbrication of the CO unit, while in the western part toward the foreland, two carbonate sheets with leading-edge fault-propagation style folds comprise the Wills Mt. anticlinorium. In the central and eastern parts of the salient, the structural geometry toward the foreland is defined by a duplex with 4-5 imbricate thrust sheets of CO carbonates that transitions to an antiformal stack of two to three carbonate thrust sheets comprising the Nittany anticlinorium. Toward the hinterland, the CO carbonate sequence is faulted into broadly-spaced fault-related folds, and includes the regionally continuous (>160 km) Jacks Mt. – Berwick anticline that spans both limbs of the salient. Shortening in the CO lithotectonic unit due to folding and thrusting ranges from 26% in the south, to 51% in the Juniata Culmination, and 10% in the east. 3-dimensional models of segments of the salient were created in Move software to test the spatial continuity of structures in the cross sections.

Upon retrodeformation of the cross sections, the 060°-striking northeastern segment restoration path curves 25°-30° to the east, while the 030°-striking southwestern segment curves 20°-25° to the south. The major thrust fault underlying the presently curved Jacks Mt. – Berwick anticline structure, as well as those structures toward the hinterland, restore to a nearly straight fault traces oriented 045°-050°.