APPALACHIAN TECTONICS AT THE PENNSYLVANIA NORTH-SOUTH JUNCTION

Ends of the northern and southern Appalachian megasheets overlap along strike in the PA and MD Piedmont by about 100 km. In the upper or northern sheet a transform-faulted cratonic alcove preserves atypical examples of the entire Appalachian Wilson cycle.

This mildly-deformed, little-metamorphosed, non-intruded cratonic record begins with a Neoproterozoic transform offset in the rifted edge of Laurentia. A sharp cut-off in modern earthquake patterns marks its buried location beneath a jog in the Susquehanna River. Differential subsidence of the platform edge across the transform created marked facies differences in Eocambrian and Lower Cambrian edge deposits. In Mid-Ordovician times, offshore island arcs and ribbon basins of Iapetus finally inverted their contents onto the craton to begin the onshore Taconian Orogeny as thin-skinned, mostly gravity-driven nappe flowage across the little-deformed craton. Mid-Paleozoic strike-slip docking of the Potomac Terrane largely bypassed the recessed corner.

First phase of the Alleghanian Orogeny began as thin-sheet, megathrust detachment of the upper surface of the craton along the brittle-ductile transition. Differing directions of displacement overlapped megathrust ends while a low-relief, future Great Valley monocline draped across their combined leading edge. The second phase uplifted this collective group of features by about 10 km on a series duplexes while driving the monocline forward. Its structural relief grew into a giant blade that bulldozed thin-skinned foreland folding and thrusting. Two new directions of hinterland displacement of the blade created linear segments in northern and southern foreland termini, their overlap zone joined by a sharp elbow-like bend. This slightly forward displaced, slightly uplifted zone extends across the orogen just west of the Susquehanna. The multi-direction driving mechanism produced the PA salient's radial displacement without significant stretching of its distal parts.