Pattern of Folded Folds in the Appalachian Piedmont Along Susquehanna River

Abstract

Structural details are presented for 22 key stations along a 30-mile cross section of the Pennsylvania Piedmont along the Susquehanna River. Three major cleavages, S 1 , S 2 , S 3 , with associated folds F 1 , F 2 , F 3 , vary systematically and independently in orientation across the region. The pattern seems to have developed by successive superposition of three deformations, D 1 , D 2 , and D 3 , each involving a characteristic style of yield. The oldest deformation, D 1 , utilized mechanisms of flowage and isoclinal folding to create the dominant schistosity of the region. In the southeast the F 1 axial planes and the associated S 1 dip steeply to the southeast but flatten northwestward to become part of a recumbent fold system suggestive of northwestward tectonic transport from a root zone in the southeast. The D 2 deformation has uniform orientations across the region and is characterized by slip folding along S 2 . As the largest F 2 structure, the Tucquan anticline involves folding of basement in the Mine Ridge area. Evidence of D 3 appears only in the extreme northwest and southeast of the Susquehanna section. The extreme southeast is characterized by intense F 3 slip folds with northwest-dipping axial planes becoming steeper to the northwest. In the extreme northwest D 3 is marked by nearly vertical S 3 fracture cleavage planes. With changing lithology the deformation planes vary in development and style but not in local orientation. All deformations cross the classic Martic Line without change, indicating that this contact predates all major folding. The over-all style of deformation seems to change with time from predominant flow to predominant fracture, and from nappelike flowage of sedimentary rocks o t basement involvement.