REGIONAL FRACTURE SYSTEMS IN THE PENNSYLVANIA VALLEY AND RIDGE PROVINCE, A RECORD OF ALLEGHENIAN STRESS DIRECTION

Multiple regional fracture (joint and vein) sets in the Pennsylvania Valley and Ridge province are used to delineate the stress history during the Alleghenian orogeny; and indicate a complex sequence of stress reorientation events. Fracture orientations were compiled for over 1000 field sites in all exposed stratigraphic units. Facture trends were also delineated using stream segments on Lidar hillshade images. Fracture sets either cut across fold axes at an angle (cross-fold), or parallel fold axes (strike). Poles to most fractures become subvertical upon rotation of bedding to horizontal, indicating that the fractures are typically pre- to very early syn-folding.

In the 030° segment of the Valley and Ridge, three cross-fold fracture sets transect fold axes. A pre-folding set strikes 345°±5°, and merges with the dominant cross-fold set in the salient. A second pre-folding set strikes 317°±8° toward the hinterland, but the strike swings to 295°±7° at the Appalachian Structural Front (ASF). A third set in the 030° segment strikes 275°±7° and is interpreted as post-folding as poles to the fractures uncluster on unfolding. Its strike is similar to the dominant set in the Plateau province to the west.

In the outer arc of the salient (Nittany Anticline), pre-folding fracture orientations are subnormal to the fold axes, slightly fanning around the salient. However, in the inner arc, fractures transect curved fold axes with strike of 340°± 10°. In the eastern part of the salient, a pre-folding fracture set transects fold axes and strikes 310°±15°. In the Devonian rocks along the ASF, cross-fold fractures are subnormal to bedding strike, and do not change progressively around the salient, but instead occur in distinct domains that incrementally become more westerly-striking from east (005°±5°) to west (301°±10°). In contrast, fracture orientations in in the Plateau northwest of the ASF, are more consistently oriented (striking 323°±5°), do not follow the change in strike around the salient.

Each part of the salient has its own fracture history. In the 030° segment, the three fracture sets suggests a rotation of the maximum shortening direction from 345° to 275°. Within the salient, a single fracture set dominates, while in the eastern part of the salient, there was a rotation of the maximum shortening direction from 310° to 340°.