Structural architecture and retrodeformation of a curved fold-and-thrust belt: New model for the Pennsylvania salient, central Appalachians

ABSTRACT

The structural architecture of the Pennsylvania salient is defined by imbricated Cambrian–Ordovician carbonate thrust sheets that are passively overlain by a Middle to Upper Paleozoic cover rock sequence. The structural style of the thrust sheets varies significantly across the salient. In the south, the Broadtop synclinorium separates the eastern and western parts of the fold-and-thrust belt, which are defined by series of imbricated thrust sheets with leading-edge fault-propagation–style folds. In the central part of the salient, the structural geometry is defined by a duplex of up to eight imbricate thrust sheets that transition to an antiformal stack of multiple thrust sheets near the Appalachian structural front. To the east, the structural geometry is defined by fewer thrust sheets, but with open fault-related folds. Shortening in the Cambrian–Ordovician lithotectonic unit, due to folding and thrusting, ranges from 26% in the south to 51% in the Juniata Culmination and <3% in the east.

Based on line, area, and volume balancing and retrodeformation of the Cambrian–Ordovician lithotectonic unit, structures in the 060°-striking segment of the salient in east-central Pennsylvania (USA) rotated up to 30° counterclockwise, while structures in the 030°-striking segment in southern Pennsylvania rotated up to 25° clockwise with increasing rotation toward the hinterland. The cover rock sequence can be restored from the foreland pin line to its predeformation configuration using finite-strain data and bed lengths. Material displacement vectors derived for the cover rock sequence indicate that the Blue Ridge and Reading Prong massifs acted as indenters, pushing the Cambrian–Ordovician through Upper Paleozoic rocks toward the foreland to create the current salient curvature.