Analysis of structural features displayed in Silurian and Devonian strata of the Hudson Valley region indicate that these rocks form the western edge of a now largely eroded, west-verging, fold-thrust belt (the Hudson Valley fold-thrust belt, or “HVB”). West of the Hudson River, remnants of this fold-thrust belt define a miniature valley and ridge province. Faults of the HVB die out to the west as blind thrusts in the basal units of the Catskill clastic wedge. Structural geometries displayed in the HVB vary rapidly along strike and are locally complicated by internal deformation of thrust sheets and by movement on out-of-the-syncline faults. It is proposed that exposed faults of the HVB west of the river arise from a basal detachment, here called the “Rondout detachment,” that lies above the Taconic unconformity. The Rondout detachment and the overlying thrust system have been bent into large-amplitude folds; Ordovician units beneath the Rondout detachment must have shortened during development of these folds. Area balancing of cross-sectional models suggests that the shortening affecting Ordovician units could be restricted to rock above a lower blind detachment (the “Austin Glen detachment”) positioned within Ordovician strata at a depth of ∼400 m below ground surface. The age of deformation and the tectonic setting of the HVB is enigmatic; its spatial relation to the Catskill clastic wedge and to Acadian metamorphic terranes suggests that the belt represents foreland shortening developed during the Acadian Orogeny.

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