AFFECTS OF ALONG-STRIKE CHANGES IN STRATIGRAPHY ON FOLD-THRUST BELT STRUCTURAL STYLE AND GEOMETRY: AN EXAMPLE FROM THE NORTHERN APPALACHIAN MOUNTAINS, CENTRAL HUDSON VALLEY, NEW YORK

Detailed field mapping of Ordovician through Devonian strata in the historic Rosendale natural cement region, a 125 km2 area of south of Kingston, Ulster County, New York, provides a basis for examining the role of along-strike changes in mechanical stratigraphy on the scale and geometry of structures in a thin-skinned fold-thrust belt. In this region, deformation involves a mechanically rigid layer, or strut, composed of Late Silurian and Early Devonian carbonate and clastic sedimentary rocks, sandwiched between ductile units (Ordovician shale below and Middle Devonian shale above). The Late Silurian Shawangunk Conglomerate, the basal and most rigid unit of the strut, thins along strike over a distance of about 9 km from nearly 500 m thick at the south end of the map area to 0 m thick at the center. Our mapping reveals that: 1) the Rondout detachment, a regional detachment horizon at the base of the strut in the Hudson Valley fold-thrust belt previously interpreted as extending southwards as far as Rosendale, actually dies out to the north of Kingston; 2) as the strut thickens southward in the vicinity of Rosendale, displacement is transferred to a principal detachment lying at depth within the underlying Ordovician Martinsburg Formation; 3) thrust faults with displacements that decrease to the north ramp up from the detachment in the Martinsburg Formation and cut across the strut; 4) fault-propagation folds and fault-bend folds formed above these ramps; and 5) as the strut and the overlying stratigraphic section thickens southward, the pin line of deformation propagates westward and the amplitude and wavelength of folds increases. Thus, the loss of the Rondout detachment, the progressive overall southward thickening of the strut, and the insertion of the Shawangunk Conglomerate led to a redistribution of displacement to a stratigraphically lower detachment horizon and a change in the amplitude and wavelength of folds. Structural relationships in the map area provide insight into the transition between the Pennsylvania Valley and Ridge Province to the south, where struts are up to 1000 m thick and folds have amplitudes and wavelengths measured in kilometers, and the Hudson Valley fold-thrust belt to the north, where struts are 10s of meters thick folds have amplitudes and wavelengths measured in 10s to 100s of meters.