ALONG-STRIKE VARIATION OF DEFORMATION INTENSITY IN THE HUDSON VALLEY FOLD-THRUST BELT BETWEEN LEEDS AND COXSACKIE, NEW YORK

The Hudson Valley fold-thrust belt (HVB) is a north-south trending, 2-4 km- wide fold-thrust belt that lies between the cities of Kingston and Albany, New York. Its eastern edge is an approximately N-S trending 20 to 50 m-high cliff called the Helderberg Escarpment. Deformation in the HVB involves a sequence of Silurian-Devonian clastic and carbonate strata deposited unconformably over Middle Ordovician turbidites. New 1:10,000-scale geologic mapping (funded by the USGS EDMAP program) of a 4 km X 11 km strip along the HVB from Leeds to Coxsackie. The mapping permits characterization of the transition between a structural culmination (at Leeds), where an antiformal duplex brings basal Helderberg Group strata to the ground surface close to the foreland edge of the HVB, and a structural depression, where strata of the upper Tristates Group crop out at the surface. Three features mark the transition between the culmination and depression. First, the wavelength of folds increases from Leeds to the north, and fewer thrusts are emergent. Second, the trends of faults and fold axial traces rotate counterclockwise from N30°E near Leeds to N10°E in the northern part of the study area. Third, folds plunge to the north. The transition is not continuous, however, for a pronounced southeast-dipping lateral ramp brings deeper structural levels up to the surface at Greens Lake. The apparent decrease in deformation intensity northward, from the culmination to the depression, implies that cross-strike shortening west of the Helderberg Escarpment varies along-strike in the central HVB. Notably, the geometry of structures along Helderberg Escarpment also change from south to north in the map area. At the latitude of Leeds, where deformation is more intense west of the escarpment, the post-Taconic unconformity dips about 30°W, whereas further north, beds along the escarpment range from vertical to subhorizontal. These changes imply that the position of greatest shortening, relative to the Helderberg Escarpment, varies along the strike, perhaps in a relay fashion. Lateral ramps appear to serve as accommodation zones between shifts in the position of greatest shortening with respect to the Helderberg Escarpment.