PUZZLING INTERACTIONS BETWEEN THE NORTHERN AND CENTRAL SEGMENTS OF THE SINUOUS APPALACHIAN FORELAND FOLD-THRUST BELT IN NEW YORK AND PENNSYLVANIA

Three types of regional map-view curves occur in the Appalachian fold-thrust belt: basin-controlled (due to along-strike changes in pre-deformational stratigraphy), impingement-controlled (due to the impingement by or against rigid crustal blocks), and intersection-controlled (due to overlap of two deformation belts). One of these curves, the New York recess, links the N-S-trending arm of the Northern Appalachians and the E-W-trending arm of the Pennsylvania salient (a basin-controlled curve in the Central Appalachians). The recess contains aspects of all three types of curves; it reflects the shape of the continental margin established at the end of the Precambrian. Folds, faults, and fabrics cross-cut within the New York recess in puzzling ways. First, the regional trend of the northern Lackawanna synclinorium is nearly N-S, but outcrop-scale structures within the hinge zone, as well as on adjacent portions of the New York Plateau, trend nearly E-W. This relation developed because the N-trending northern synclinorium is a salt-collapse structure whose trend reflects dissolution of Silurian evaporates in a pre-existing basement trough, whereas the E-W-trending structures are a consequence of pre-collapse Alleghanian layer-parallel shortening. Second, the occurrence of two cleavages (N-S- and NE-SW-trending) in the southern Hudson Valley suggests overprinting of two layer-parallel shortening events within an intersection orocline formed where the orogen impinged on a cratonic corner. The N-S-trending cleavage appears to offset the NE-SW-trending cleavage, even though the N-S-trending fabric initiated first. We suggest this relation reflects positive-feedback during pressure solution on cleavage domains. Specifically, once a clay selvage forms, it remains active and localizes more pressure solution, even if the domain is no longer perpendicular to shortening. If the solution rate in older, thicker cleavage domains remains faster than that in younger, thinner cleavage domains, then the older cleavage will appear to offset the new cleavage. Third, both north-south-trending (Hudson Valley) faults and east-west-trending (PA salient) faults crop out in the Mohawk Valley. Faults of one set were not reactivated by movement on the other, suggesting that, in the distal foreland, non-coaxial deformation belts can overlap without obvious interference.