DEFORMATION STYLE OF THE LEEDS-COXSACKIE SEGMENT OF THE HUDSON VALLEY FOLD-THRUST BELT

Geologic mapping at a 1:10,000-scale of a segment of the Hudson Valley fold-thrust belt (HVB) in the Leeds-Coxsackie region of New York is in progress in order to determine the variation in structural trends and deformation styles of the HVB along strike. In the study area, the HVB involves lower Devonian limestone and shale, and Paleozoic thin-skinned deformation (due to either the Avalonian or Alleghanian orogenies) produced overall west-verging folds and thrusts. Preliminary results indicate that only some of the major folds documented in the Catskill-Leeds segment of the HVB, to the south, extend across the Leeds-Coxsackie segment. Others folds die out or are truncated at faults. Specifically, at the latitude of Leeds, there are 11 first-order fold hinges within the belt, whereas at the latitude of Greens Lake, there are only 5. Furthermore, structural trends change along strike, from an average of 020° in the Catskill-Leeds segment to an average of 010° in the Leeds-Coxsackie segment. Finally, the folds plunge gently to the north between Leeds and Coxsackie, so that higher levels in the stratigraphy crop out closer to the Helderberg Escarpment in the Leeds-Coxsackie segment than they do in the Catskill-Leeds segment. These observations are compatible with a model in which the Catskill-Leeds segment is a local structural culmination in the fold-thrust belt which accommodated a higher degree of strain than the Leeds-Coxsackie segment to the south. Thus, in the Catskill-Leeds area, more uplift occurred and deeper structural levels are currently exposed than in the north. The observed change in structural trend simply reflects accommodation of differential displacement along strike—fold traces bend slightly between the Catskill-Leeds segment where most shortening occurred and the Leeds-Coxsackie segment where least shortening occurred. Overall, it appears that cross-strike strain in the HVB dies out progressively to the north, between Catskill and Coxsackie.