THE STRUCTURAL EVOLUTION OF THE PENNSYLVANIA SALIENT: A NEW MODEL

The origin of the Pennsylvania salient has been a matter of debate for over 150 years. A new, comprehensive deformation model is proposed here that integrates regional cross-sections in a 3D structural reconstruction, a structural retrodeformation, regional finite strain, paleomagnetic, and fracture orientation and fault slip data, radiometric dates, and syntectonic overburden reconstructed from fluid inclusion microthermometry.

The Blue Ridge and Reading Prong massifs serve as indenters that control the deformation in the salient, and initially restore to ~130 km hinterlandward of their present positions with the Blue Ridge striking ~030° and the Reading Prong striking ~045°, making a foreland-facing ‘elbow. In the Early Pennsylvanian, the Blue Ridge advances along the Blue Ridge thrust and the Reading Prong along the Yellow Breeches thrust. Motion of the massifs is driven by the impingement of the Reguibat indenter and the regional dextral transpressional fault system in the Piedmont. The structurally rising massifs shed sediments into the foreland in two broad depocenters in the areas of the future Anthracite Belt and western Valley & Ridge. In the area of the future Juniata culmination, at the juncture of the two massifs, there is minimal sedimentation. During the Late Pennsylvanian – Early Permian, the massifs advance toward the foreland along a detachment in the Cambrian. NW-directed shortening forms a ~320° striking joint set in the foreland. The earliest Valley & Ridge folds form at the ‘elbow’ by imbrication of the Cambro-Ordovician carbonate lithotectonic unit. The depocenters continue to receive sediments, and the thickness controls the future thrust imbrication style in the Valley & Ridge. Between the Mid- to Late Permian, the Valley & Ridge folds grow and advance, extending from the impinging ‘elbow.’ Continued carbonate imbrication occurs as the Blue Ridge advances toward ~300° then to ~280°, and the advancing Reading Prong rotates clockwise toward ~350°. Corresponding joint sets form in the foreland. In the Late Permian, the Valley & Ridge structures are complete and folding continues into the Plateau province. The Blue Ridge advances toward ~280° then ~290° resulting in joint sets and tear faults in the Plateau. The advancing Reading Prong results in ~360° striking joint sets in the foreland.