APPALACHIAN NORTH-SOUTH JUNCTION IN PENNSYLVANIA: INTERPLAY OF PIEDMONT SUPERTHRUST, GREAT VALLEY MONOCLINE, SCHUYLKILL TECTONIC ZONE, VALLEY FORGE MONOCLINE, AND SUSQUEHANNA TERRANE

Early Alleghanian thrusting of the N Appl Megathrust over the terminus of the S Appl Megathrust overlapped their ends at depth to form the PIEDMONT SUPERTHRUST. Its leading edge, GREAT VALLEY MONOCLINE, upended Silurian units across Great Valley Cambro-Ordovician units and fore-structures of the N Appl basement massif. This basement slab's trailing edge is the Laurentian cratonic margin, the Martic Zone of Early Cambrian platform, slope , and deep-water deposits. The Taconian Orogeny modified the zone by metamorphism, thrusting, and folding of those formations across it. Mid-Paleozoic strike-slip displacement docked the hinterland SUSQUEHANNA TERRANE against the deep-water deposits. Gondwana collision detached the sheets along the brittle/ductile transition, conjoined them, built duplexes to uplift the superthrust by ~ 10 km and drive it into the foreland. The remarkably linear ~110 km-long VALLEY FORGE MONOCLINE is one of several late-stage buckles in that sheet and its foreland belt. It terminates the Martic Zone by roll-down beneath its edge and Susquehanna and deep-water terranes by roll-up and erosion.

The ~15 km-wide SCHUYLKILL TECTONIC ZONE is a doubly allochthonous part of the N Appl megathrust, It trends WNW-ESE across Great Valley and basement massifs to disappear in roll-over monoclines at both ends. It began in Early Alleghanian time as a dextral shear zone deep in the S Appl hinterland, offsetting the end of the Reading Prong Massif, isolating Little South Mountain Klippe, adding additional separation to two types of basement, and offsetting the Taconian Hamburg Allochthon. The N Appl Megathrust then included this hinterland and its geology in NNW transport, moving that rear section onto the outer -most edges of the same sheet. Linear edges of Triassic and Jurassic basins indicate even later reactivation. This scenario solves the anomaly of radially displaced megathrust sheets joined by overlap instead of predictable extension.