OVERLAPPING MEGATHRUST TECTONICS JOINING NORTHERN AND SOUTHERN APPALACHIANS IN PENNSYLVANIA

The N Appalachian megathrust is a 50-60 km-wide sheet of the upper few km of the NY Promontory's cratonic surface. Megathrust mechanisms detached the upper 2-3 km of its basement surface, probably along the brittle-ductile transition; minor distortion created dilatancy and fluid filling; this concentrated more distortion, etc., to form a Rubey and Hubbert zone of excess fluid pressure for massive thrust displacement. The sheet moved NNW ̴ 150 km to develop ̴ 10 - 15 km of structural relief while maintaining most of its original horizontality. Even though that uplift W of Reading (PA) rolled the Great Valley's bounding Silurian ridge to the vertical, no known structures are capable of accommodating so large a differential motion. One explanation is a co-evolving mega-monocline (megacline ?) that draped Silurian cover across early stages of the megathrust, both rising on a series of small, flat-topped, formation-controlled duplexes at their collective base, the vertical Silurian limb becoming a bulldozer blade to continue foreland deformation.

In the vicinity of Reading the sheet thins to plunge SW into the sub-surface beneath the Lancaster Platform. Its trace continues as a zone of shallow quakes to the Susquehanna River and an abrupt cutoff at a buried transform fault. The thrust surface continued to climb SW through much thickened Lower Cambrian Lancaster Platform formations of the rift-bounded York Basin. Early Alleghanian NNW displacement of the N Appalachian sheet placed this terminus on the back of the future S Appalachian sheet. A second phase combined the two in WNW detachment and thrusting. By Mid-Triassic, the overlying N Appalachian sheet had its Potomac and Westminster terminae exposed somewhere NE of the Potomac whereas the largely concealed S Appalachian sheet continued minimal NE exposure to the Susquehanna River. Triassic megaduplex collapse created the "broad terrane" to end the Appalachian Wilson cycle.