PRE-ALLEGHANIAN CONTRACTIONAL DEFORMATION IN CENTRAL PENNSYLVANIA

Recent mapping of the Cambro-Ordovician carbonates in the core of the Nittany anticline found a set of early contractional structures overprinted by Alleghanian deformation. The structures included thrust faults, associated fault-bend folds, and multiple sets of cleavage. These structures indicate contraction directions ranging from 030-210 to 065-245 (i.e. NNE-SSW to ENE-WSW) which is roughly perpendicular to the Alleghanian contractional directions in the area. Subsequent reconnaissance mapping has found evidence for similar structures in the overlying Silurian and Devonian strata (and apparently into the base of the Mississippian along the Allegheny front), though the intensity of cleavage and apparent fault displacements are significantly lower.

As many as three distinct cleavages can be seen in single outcrops in the Nittany valley, and two distinct cleavage directions have been observed in single outcrops in the Devonian siltstones and shales. Displacements on pre-Alleghanian thrusts within the Ordovician carbonate sequence have displacements ranging up to more than 2 km, whereas displacements on thrusts in the overlying strata appear to be no more than ½ km, and the thrusts in the Ordovician section are more closely spaced. Thus, the shortening of the Ordovician section appears to have been greater than that of the overlying strata.

The tectonic origins of this event are not obvious. It is proposed that the structures in the Ordovician carbonates formed initially as the core of a plateau fold related to the Acadian deformation in eastern New York and New England. Further expansion of the deformation including propagation of the thrust surfaces upward and formation of additional cleavage sets may be related to distal effects of early phases of the Alleghanian orogeny recognized in early folding in the Narragansett basin and in the fractures of the Appalachian-wide stress field (Engelder & Whitaker, 2006).