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, S1, S2, S3, with associated folds F1, F2, F3, vary systematically and independently in orientation across the region. The pattern seems to have developed by successive superposition of three deformations, D1, D2, and D3, each involving a characteristic style of yield.

The oldest deformation, D1, utilized mechanisms of flowage and isoclinal folding to create the dominant schistosity of the region. In the southeast the F1 axial planes and the associated S1 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 D2 deformation has uniform orientations across the region and is characterized by slip folding along S2. As the largest F2 structure, the Tucquan anticline involves folding of basement in the Mine Ridge area.

Evidence of D3 appears only in the extreme northwest and southeast of the Susquehanna section. The extreme southeast is characterized by intense F3 slip folds with northwest-dipping axial planes becoming steeper to the northwest. In the extreme northwest D3 is marked by nearly vertical S3 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.

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