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