TRANSITIONS IN STRUCTURAL STYLES AND TRENDS WITHIN THE NORTHERN APPALACHIAN HUDSON VALLEY FOLD-THRUST BELT NEAR CATSKILL, NEW YORK

The Hudson Valley foreland fold-thrust belt (HVB) is a narrow, 2 to 8-km wide belt of deformed upper Ordovician to Middle Devonian clastic and carbonate strata exposed along the western margin of the Hudson Valley in eastern New York State. Here, deformed strata are involved in an array of west-verging thrust faults and fault-related folds above regional-scale, sub-horizontal detachment faults in the underlying Ordovician and Silurian sequences. Recent geologic mapping at a scale of 1:10,000 along an 11-km long segment of the HVB near the town of Catskill reveals that this segment includes a doubly-plunging, first-order anticline-syncline pair that occupies nearly the entire ~ 2 km cross-strike width of the belt; the syncline lies to the east of the anticline. Comparison of this segment to adjacent segments of the HVB along strike reveals that two along-strike transitions in structural styles and trends take place near Catskill First, the 1.5 km wavelength of the first-order folds is significantly greater than that of the along-strike equivalents in adjacent regions. For example, the first-order folds in the Leeds-Coxsackie segment of the HVB located 2 km north of the Catskill study area have approximately 0.5 km wavelengths. The larger wavelength of the folds in the Catskill area may reflect a local increase in cross-strike shortening near Catskill accommodated by the propagation of thrust fault ramps up-section from the underlying Austin Glen detachment. Second, while the first-order folds near Catskill have similar structural styles and trends in the southern and northern portions of the study area, they are separated by a central, complexly deformed interval. Folds in the southern portion of the Catskill study area trend 015° and plunge northward. Folds in the northern part of the study area plunge to the south and trend 030°. Strata in the central portion of the study area are involved in a complex synclinorium along the eastern margin of the HVB that contains relatively tight folds that are cut by lateral thrust ramps. This along-strike transition in structural styles and trends is possibly the result of a relay structure between thrust faults associated with the southern ~ 015°-trending folds and the northern ~ 030°-trending folds.