RESTORING THE CENTRAL PENNSYLVANIA VALLEY AND RIDGE FOLD-AND-THRUST BELT

In order to understand the development of the central Appalachian fold-and thrust belt, a series of five regional balanced cross sections have been constructed. The regional cross sections cover an area from the southern border of Pennsylvania to the western Anthracite belt and extend from the Appalachian Plateau province to the Great Valley province. The sections show that the structural geometry of the fold-and-thrust belt varies markedly from the ~030°-striking southern segment to ~060°-striking Juniata culmination. 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 that have a structural amplitude of 5 to 7 km. These give way to the Broadtop synclinorium in the central part of the belt, and then two additional carbonate horses with similar leading-edge folds that comprise the Wills Mt. anticlinorium toward the Appalachian Structural Front (ASF). In this area, the duplex exhibits ~40% shortening, with the deformed length of the duplex of ~80 km, and the retrodeformed length of ~130 km. In the central and eastern parts of the salient, the structural geometry is defined by a duplex with 10-11 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. In these areas, the duplex exhibits ~48 to 53% shortening, with an over 200 km retrodeformed length.

It is possible that the variation in structural style cross the region may be related to the distribution of syn-tectonic loads during deformation. To address this, paleo-overburden thicknesses were determined using fluid inclusion microthermometry data of CH₄±CO₂ and aqueous fluid inclusions from syn-tectonic veins. Restored overburdens are quite variable when plotted in deformed sections, but on restored sections reflect a general pattern of high syn-Alleghenian tectonic load toward the southeast of 4.0 to over 5.5 km. This load may be due to structural overthrusting and/or to sediment shedding off of uplifted mountains in the hinterland, (e.g., the Blue Ridge and Reading Prong). The syn-tectonic load decreases rapidly toward the hinterland, and is markedly less (~2 km) over the Nittany anticlinorium and less than 1 km at the ASF.