A STUDY OF PROGRESSIVE DEFORMATION IN THE SOUTHERN VALLEY AND RIDGE, PERRY COUNTY, PA

This study characterizes the deformational history of a ~ 700 m long exposure in the Upper Devonian Irish Springs member of the Catskill Formation in the southern Valley and Ridge Province. The study area is located along the west shore of the Susquehanna River in the hinge of the Pennsylvania salient. The sandstones, siltstones, and shales contain 3rd and 4th order folds, conjugate contraction faults, joints, cleavage, and grain-scale finite strain indicators. This suite of structures records a complex deformational history similar to the Bear Valley sequence of progressive deformation (Nickelsen, 1979) observed 40 km to the ENE. Orientation analysis of successive stages of the progressive deformation is used to constrain the orientations of the maximum shortening direction (MSD) and distinguish between a single-stage (Gray and Stamatakos, 1997) and two stage (Wise, 2004) model for the development of the Pennsylvania salient. Analysis of the orientations of structural features from each stage of the progressive deformation sequence reveals consistently oriented MSDs ranging from ~339° to 351°, which parallels findings 100km to the northwest in the hinge of the salient at the Appalachian structural front (Spiker and Gray, 1997). This is distinctly different from the CW rotation of MSDs seen in the east (e.g., Gray and Mitra, 1993) and CCW rotation of MSDs in the west (e.g., Nickelsen, 2009), suggesting that the hinge of the Pennsylvania salient coincides with a axis of no rotation of MSDs, consistent with the single-stage model (Gray and Stamatakos, 1997).