THE EFFECTS OF SYNTECTONIC LOADING ON THE STRUCTURAL GEOMETRY AND DEVELOPMENT OF VALLEY & RIDGE OF THE PENNSYLVANIA SALIENT

The Pennsylvania salient is an arcuate fold-and-thrust belt that was deformed during the Late Paleozoic Alleghenian orogeny. Regional cross-sections show that the structural geometry varies significantly from the 030◦-striking southwestern segment to 060◦-striking northeastern segment. In the south, the eastern part of the belt is defined by a series of imbricated Cambro-Ordovician carbonate horses with leading-edge fault-propagation style folds. These transition into the Broadtop synclinorium, and then two additional carbonate horses with similar leading-edge folds that comprise the Wills Mt. anticlinorium toward the Appalachian Structural Front (ASF). In the central and eastern parts of the salient, the structural geometry is defined by a duplex with imbricate horses of Cambro-Ordovician carbonates, that transition to an antiformal stack of two to three carbonate thrust sheets near the ASF and define the Nittany anticlinorium.

The regional variation in structural style and ramp spacing may be related to the distribution of Late Carboniferous to Permian syn-tectonic loads during thrusting. Paleo-overburden thicknesses were determined from fluid inclusion microthermometry data of CH₄±CO₂ and aqueous fluid inclusions from syn-tectonic veins. In general, using retrodeformed cross-sections, restored overburdens are typically low (<1.5 to 3.0 km) above anticlinoria such as the Nittany culmination, Nittany anticline, and Wills Mt. anticlinorium. In contrast, they larger (4.3 to 7.0 km) above synclinoria such as the Broadtop Synclinorium and the Anthracite Belt.

Areas with less syntectonic overburden were prone to develop high-amplitude fold structures and stacked duplexes. A syntectonic load is not instantly emplaced across the fold-and-thrust belts, but instead is time-transgressive, with erosion of structurally elevated topography toward the hinterland resulting in forelandward deposition. This load transfer will result in the increase of pore-fluid pressure at depth and slip along a detachment. This slip is results in the growth of fold and duplex structures that, in turn, shed sediments forelandward. In other words, load drives thrusts, and thrusts drive load. Other workers have shown that syntectonic load may control the overall structural architecture of the fold-and-thrust belt.