PENNSYLVANIA SALIENT OF THE APPALACHIANS: EVIDENCE AND IMPLICATIONS FOR AN ORIGIN BY MULTIPLE DIRECTIONS OF ALLEGHANIAN TECTONIC TRANSPORT

Most models for the Pennsylvania Salient of the Appalachians consider only foreland evidence. Recent models utilize only a single 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 arcuation was accomplished with almost no tangential stretching of its distal parts and with only about 15 degrees of paleomagnetic rotation despite 40-60 degrees change in regional strike. A new contour map of fold axis azimuths around the arc highlights some generally unrecognized geometric aspects: isoazimuth contours run parallel to each other rather than radial. Persistence of this foreland pattern across most of the Piedmont suggests Piedmont tectonic motion vectors must be added to origin constraints. Compilation of such vectors from 46 individual study areas shows two populations: NNW east of the Susquehanna River and WNW in the west with an overprinted area between. Each group contains both Taconian and Alleghanian age components and is normal to its part of the arc, the eastern set being slightly older. The proposed model involves a first stage of Alleghanian gross decollement with NNW transport in the eastern segment. A minor N directed push followed. This mega-sheet was then transported WNW to form the Blue Ridge or western segment. Overprinting in the axial region caused greater total shortening and uplift to form the Juniata / Nittany Culmination and an outward bulge in that part. Strike-slip strain of the north edge of the mega-sheet around the old corner of the craton produced a series of right-stepping, NNE-trending en echelon structures: Blue Ridge, Juniata / Nittany Culmination, a deeply buried ramp structure, the greater Anthracite Synclinorium, and Shawangunk Ridge. The model accounts for the lack of tangential stretching, the known transport patterns of both foreland and Piedmont, and many minor structural details. The sequence of motions agrees with changing plate motions determined for other parts of the Appalachians.