INFLUENCE OF MECHANICAL STRATIGRAPHY ON THE 3-D STRUCTURAL ARCHITECTURE OF THE NORTHERN APPALACHIAN FOLD-THRUST BELT; ROSENDALE, NEW YORK

Results of new geologic mapping and cross-section balancing, combined with re-analysis of a remarkable photographic archive compiled by Princeton University students during the early 1900s (i.e., prior to reforestation of the region), provide insight into the influence of mechanical stratigraphy on the 3-D structural architecture of the northern Appalachian foreland fold-thrust belt. This portion of the fold-thrust belt lies in the NW-SE-trending southern arm of the New York recess, a convex to the foreland map-view curve in the Appalachian orogen. Deformation involves a mechanically rigid strut of Late Silurian through Middle Devonian sedimentary strata that is sandwiched between thick, relatively ductile units of Ordovician shale (below) and Middle Devonian shale (above). Near Rosendale, once a major natural-cement quarry district in southeastern New York, the rigid strut thins markedly along strike. This stratigraphic change gives rise to dramatic, along-strike transitions in fold-thrust belt structure. For example, as the strut thins northward, the width of the fold-thrust belt decreases and the pin-line migrates toward the hinterland. Structural trendlines curve as a result of this change. As expected from buckling theory, thinning of the strut results in a decrease in the wavelength and amplitude of first-order folds. Similarly, there is a decrease in the spacing of thrusts as the rigid strut thins, as observed in sandbox models. Stratigraphically controlled along-strike changes in structural architecture appear to be accommodated by lateral ramping and by the dying out of faults into fault-propagation folds. Preliminary balanced cross sections also suggest that fold-thrust belt deformation near Rosendale occurred above two regional detachments, which lie roughly 20 and 170 m below the base of the strut, respectively, and that out-of-the-syncline faults play an important role in accommodating shortening in the strut. Work in progress may reveal whether there is transfer of slip between the two detachments in association with along-strike stratigraphic changes.