DUPLEXES, CLIMBING IMBRICATES, AND TRIANGLE ZONES IN RESPONSE TO CHANGING COMPRESSION DIRECTIONS AROUND THE BEND OF THE VALLEY-AND-RIDGE IN CENTRAL PENNSYLVANIA

Compressional directions in the Nittany anticline changed progressively during Alleghanian orogenesis from nearly north-south to N70W, with cleavage and thrusting directions recording at least two intermediate directions. The current outer edge of the Valley-and-Ridge follows the approximate western edge of the thrust imbrication created by the first phase of the deformation. Each successive compressional direction caused earlier thrusts to be reactivated, but the resistance of the plateau to the northwest caused backthrusting to dominate around the bend, so the bend experienced at least three generations of triangle zone development. Once the compression direction reached approximate perpendicularity to the plateau margin to the southwest, the edge of the plateau began to deform and the thrust front progressed westward. As a result, the structural transition from the Valley-and-Ridge to the Allegheny Plateau in central Pennsylvania changes along strike. To the east of the bend, the mid to late Palezoic strata are imbricated, with the north edge of the significant imbrication marking the Allegheny front. Around the bend, the transition is a great monocline with virtually all of the minor thrusts verging southeast; major backthrusts underlying this monocline create a triangle zone by cutting across the northwest verging thrust system in the core of the Nittany anticline. In this area, some of the major northwest-verging thrusts in the Nittany anticline breach the roof detachment and place Cambro-Ordovician strata over mid Paleozoic strata. At the southwest end of the bend, there is a rapid transition from the triangle zone to duplexed Cambro-Ordovician strata overlain by passively folded and mildly imbricated mid to late Paleozoic strata. The Allegheny front in this area is marked by a rapid decrease in deformation intensity accompanying the western edge of large displacements within the duplex.