A new tectonic model for the Appalachian Plateau detachment sheet of southwestern Pennsylvania

Chestnut Ridge, Laurel Hill, and Negro Mountain are structures in the north central Appalachian plateau detachment sheet called the high-fold anticlines. An understanding of the structural development of these anticlines is inferred from a network of seismic profiles that reveal a pervasively fractured and tectonically thickened Upper Devonian section, called a wedge zone. Both the thrust wedges of the Upper Devonian and the imbrication of the more competent mechanical layering of the Lower Devonian interval manifest a similar structural style consisting of doubly verging fault-related folds that merge along anticlinal axes above the salt-thickened Silurian Salina Group. From the Allegheny structural front to the western terminus of the detachment sheet, each detachment sheet anticline is always situated immediately to the foreland side of structures in the Lower Paleozoic and the basement faults that they imply. The intervening synclines are characterized by an absence of deformation in the Devonian section. From southeast to northwest in the direction of tectonic transport, there is a systematic decrease in the cross-sectional width and area of the anticlines. The position of the anticlines relative to the basement faults strongly suggests that the Devonian section was transported westward along a Salina detachment surface disrupted by salt-thickened basement growth faults. Vertically propagated basement faults provide discontinuities or stress risers that disrupt the detachment sheet such that local bends in the sheet produce fracturing or wedging in the over-riding Devonian section as it is transported laterally to the northwest. Vertical propagation of basement faults was a consequence of tectonic inversion and the development of buttress anticlines in the footwall of the Appalachian plateau detachment sheet. A portion of the total horizontal displacement of the detachment sheet (7.2 km) is allocated to the vertical growth of the anticlines. In the detachment sheet, which acts as a stress guide, anticlinal growth is a mechanism to balance forces and strengthen a section that has been weakened by pervasive fracture when passing over long-lived basement growth faults.