SPATIAL VARIATION IN FOLD-AND-THRUST BELT STRUCTURAL GEOMETRY AND RESTORED SYN-TECTONIC LOADS: PENNSYLVANIA VALLEY AND RIDGE

In fold-and thrust belts, syn-tectonic loads from overthrusting and/or sedimentation may drive forward propagation of thrust sheets. In order to evaluate the contribution of syn-tectonic loads in the development of the Pennsylvania Valley & Ridge province, vein samples were collected along four transects from the southern Valley and Ridge province to the anthracite belt. Fluid inclusion microthermometry and barometry of the vein minerals provides estimates of syntectonic overburdens.

Four line-balanced structural cross sections constructed through the region 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. 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 toward the Appalachian Structural Front (ASF). The deformed length of the duplex is 80 km, with a retrodeformed length of the carbonate strut 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 carbonate thrust sheets near the ASF. The deformed length of the duplex is these areas is ~98 km, with a retrodeformed length of 205 km.

Based on retrodeformed sections and fluid inclusion microthermometry data of CH₄±CO₂ and aqueous fluid inclusions, syn-Alleghenian sediment and/or thrust loads also vary across the region. In the southern segment, estimated post-Carboniferous loads range from 4 to 5 km in the east and central parts of the belt to less than 1.5 km at the ASF. Similarly, in the central and easternmost sections, maximum loads are 4 to 5 km toward the hinterland. However, sample sites at and near the ASF in the central part of the salient indicate little post-Carboniferous load. This area corresponds to a low maturity region in the Middle Devonian section in the Plateau province. It is possible that the leading duplex structure in the salient was a persistent topographic high and Permian sediments were deflected to the northeast and west.