NEW MECHANISMS FOR APPALACHIAN TECTONICS FROM PENNSYLVANIA’S READING PRONG MEGATHRUST

The Alleghanian Orogeny first detached the Blue Ridge Megathrust and then thrust the Reading Prong Megathrust across its N end to form the Piedmont Superthrust at depth before lifting it by ~ 10 km while transporting it inland by ~ 150(+) km.

1). Core of the Reading Prong Megathrust is a basement slab, still horizontal after massive uplift. Its basal fault propagated inland through complex Grenville structures for 65(+) km while maintaining a constant depth 2 – 4 km below the Cambrian unconformity. Control by the brittle-ductile transition seems the only option.

2). Detachment continued across Great Valley Cambro-Ordo rocks and draped Silurian clastics across them as the Great Valley Monocline, the superthrust’s leading edge.

3). Continuing advance collected mostly C-O rocks beneath both basement slab and G.V. to form a huge underthrust duplex. This uplifted the whole while maintaining its original horizontality.

4). Continuous roll-up and incorporation of Silurian and younger units at the monocline’s base allowed its original 2 - 3 km vertical limb to rise as part of the uplift. It became the giant bulldozer blade that plowed the Valley and Ridge Province ahead of a rising Piedmont.

5). Resistance to forward movement required the Piedmont hinterland and duplex welt to thicken by pervasive folding and development of S₂ schistosity. The slab’s rear edge buckled under as the Valley Forge Lineament while structures related to the Chickies Thrust telescoped its interior.

6). Buckling and kink folding continued as foreland folds. Linear hinges formed much of the PA Salient's NE arm, buried and stabilized by the Pocono clastic wedge.

7). Transport of the blunt-nosed superthrust rotated from NNW to NW to WNW to superpose two more overlapping trends of kinks fold systems and form the PA salient.

SUMMARY: Brittle-ductile detachment, underthrusting, a bottom-up duplex, and kink folding acted in concert to maintain critical taper-angles for this part of the orogen.