PIEDMONT SUPERTHRUST AND SCHUYLKILL TECTONIC ZONE: KEYS TO ENIGMATIC JUNCTION OF NORTHERN AND SOUTHERN APPALACHIANS IN PA AND MD

The PA-MD Piedmont exposes ~100 km of along-strike overlap in ends of N and S Appl megathrusts, transported NNW and WNW respectively, .. but how can radial displacement cause overlap ?? The explanation involves the Schuylkill River Tectonic Zone, a 70 km-long, ~10-15 km-wide zone of NW-SE trending anomalous structures: 1) separation of Little South Mountain's basement klippe from N Appl basement, 2) separation of Taconian "Hamburg (non) Klippe" allochthons from scattered Dauphin Fm allochthons, 3) abrupt transition into the Newark Basin's steeply tilted "narrow neck," and 4) a flat Jurassic sill's edge turning into a steeply dipping dike. The zone originated as a tear fault bounding the S Appl Megathrust. Taconian facies have ~15-20 km of dextral offset along it. The N Appl megasheet sliced off the rear ~100 km of the S Appl sheet's terminus including its tear fault to emplace the whole onto the S sheet's forward edge. These modest Alleghainian-age Phase 1displacements created the deeply buried but remarkably flat Piedmont Superthrust. The sheet included the Great Valley Cambro-Ordo rocks outboard of the basement thrusts, the Great Valley Monocline of Silurian clastics draped across its boomerang-shaped leading edge. In Phase 2 a series of underlying duplexes uplifted the whole by ~10 km while preserving the flat geometry. Associated massive outward thrusting rolled the Silurian monocline forward and upward to keep pace with the rising tapering wedge of deforming foreland stratigraphy. Phase 2a continued Phase 1b's NNW displacement; Phase 2b shifted displacement to the WNW to complete the outboard part of the PA Salient with minimal tangential stretching. The Schuylkill Zone's strength anisotropy controlled details of Alleghanian and Triassic deformation. The superthrust's uplifted surface exposes an entire Appalachian Wilson Cycle from Iaptian Martic cratonic edge, Susquehanna Transform fault, Taconian Orogeny and the above Alleghanian record.