STRUCTURE OF THE EAST END OF THE NITTANY VALLEY, CENTRAL PENNSYLVANIA

Recent mapping of the east end of the Nittany Valley in central Pennsylvania has found the structure to be much more complex than previously thought. In addition to a complex sequence of non-coaxial contractional episodes associated with Alleghanian orogenesis, there is a set of pre-Alleghanian structures. The apparent anticlinal structure of the Nittany Valley is the result of juxtaposition of correlative strata from different structural levels by triangle zone backthrusting.

Pre-Alleghanian contractional structures include thrust faults and multiple sets of cleavage. These structures indicate shortening in a NE-SW direction which is roughly parallel to the local axis of the Nittany Valley and perpendicular to the Alleghanian contraction. Although these structures are cut and overprinted by the Alleghanian deformation, these cleavages are more prominent than the later Alleghanian cleavages.

Alleghanian deformation includes at least three definable stages in this area. The first stage includes thrusting and accompanying fault-bend folding of the Ordovician and younger strata with transport in a NNW direction. There is a minor backthrusting component to the deformation. The second stage involves thrusting and folding involving the entire Cambro-Ordovician section followed by triangle zone development by backthrusting across the thrust system. Primary transport is northwestward, with backthrusts verging southeastward. The third stage in this area is primarily expressed by oblique backthrusting on earlier triangle zone structures and the development of ESE-verging backthrusts that cut obliquely across earlier triangle zone structures.