PENNSYLVANIA APPALACHIAN PLATEAUS PROVINCE ANTICLINES: CHARACTERISTICS AND SIGNIFICANCE

Early Alleghany orogeny fold-and-and thrust tectonism created 78 doubly-plunging anticlines in the Appalachian Plateaus Province of PA west and north of the Allegheny front. Anticlines occur in 3 zones: southwest (SW), west central (WC), and north (N). Long anticlines, >15 km, are equally divided among zones. WC has 26 short, <15 km, anticlines; SW, 3; and N, 1. Mean axis orientations of long anticlines are N32E (SW), N40E (WC), and N16E (N).

Analyses of anticline length, structural relief, and distance to adjacent syncline axes indicates that all dimensions are smallest in WC, while those in SW and N are larger and comparable. The range of any set of values is large. Despite variability, overall structural relief increases to the E or SE, is largest on fold right in WC and N, and on fold left in SW. Fold left and right are determined from the axis south end. Chestnut Ridge and Laurel Hill traverse both SW and WC and their dimensions change in accordance with the zone traversed.

In SW and WC, surface topographic and/or stratigraphic reflection of anticlines varies from none to excellent and is least for short anticlines. Chestnut Ridge, Laurel Hill, and Negro Mtn. (SW) have good convex-upward shape and various topographic forms related to unroofing. Many anticlines have no recognizable form or influence on drainage pattern, but have local exposure of axis-centered, older stratigraphic units in transverse valleys. Lack of surface expression derives from low structural relief with dips <5. In the N zone, folds rise to the NE and both anticline shape and erosion-resistant cap rocks are lost northeastward by unroofing, resulting in anticlinal lowlands between synclinal uplands.

Interpretations: (1) an overall southeastward increase in deformation magnitude resulted from a northwestward-directed stress, (2) the zone of greatest fold abundance, WC, is part of the Pennsylvania salient, and (3) stress orientation shifted from N58W (SW) to N50W (WC) to N29W (N). The NE rise of folds in N zone was caused by Early Cretaceous uplift in the NY Catskill Mtns., or Late Cretaceous uplift in the NY Adirondack Mtns., or both.