PENNSYLVANIA SALIENT OF THE APPALACHIANS: A NEW MODEL FOR ITS ORIGIN BASED ON PIEDMONT DATA

Most models for the Pennsylvania salient of the Appalachians are based only on foreland evidence. The most recent "standard" model utilizes only a single phase of NW-directed Alleghanian transport. Most models, including that below, invoke an Eocambrian rifted corner of Laurentia that focused generally NW directed Alleghanian transport into an arcuate foreland pattern. Enigmatically, this displacement produced almost no tangential stretching of the distal parts. A new contour map of azimuthal values of fold axes around the arc shows fold geometry extending across most of the Piedmont and suggests Piedmont structure must be included in origin constraints. Compilation from 46 individual Piedmont study areas shows two populations of tectonic transport: N35W east of the Susquehanna River and N70W to its west, with an overprinted area between. Each population contains both Taconian and Alleghanian age components, each with motion normal to its segment of the arc.

The Alleghanian model proposed here involves a first-stage, eastern, mega-decollement that rose to the NNW as a series of flats and ramps. A minor N-directed push followed. Subsequently, massive Blue Ridge movements displaced this mega-sheet and much of the western foreland to the WNW. Along its northern edge, this second motion reactivated first-stage decollement -flats and utilized first-stage frontal ramps as partial lateral ramps. Absence of evaporites beneath the autochthonous Pocono Plateau produced trailing edge grabens and basins at the rear of the decollement. At the level of Silurian evaporites, this extension dropped previously folded portions of the sheet to form the Lackawana synclinorioum; displacement westward was accommodated largely by layer-parallel-shortening. Farther south, deeper level decollements dropped and cross-folded the broader Southern Anthracite basins while dissipating displacement westward as the Juniata / Nittany culminations. Downwarping not only preserved the anthracite basins but provided the added burial depth to help produce the anthracite. The model integrates transport patterns of both foreland and Piedmont, avoids the lack of tangential stretching problem, and accounts for many otherwise unexplained structural details of central Pennsylvania.