STRUCTURAL GEOMETRY OF THE TACONIC ALLOCHTHON: A CURVED OROGENIC BELT AND ITS ZONE OF INCLINED TRANSPRESSION, VERMONT AND NEW YORK

The Taconic allochthon is a NNE-trending stack of thrust sheets in the northeastern United States that formed during the Middle to Late Ordovician Taconic orogeny. The slate belt in the western part of the northern Taconic allochthon displays an along-strike change in structural orientation to form NNE-trending northern and southern domains and a NNW-trending central domain. The geometry of structures and the strain in the well-exposed northern and central domains have been characterized from previous research. In part because it is poorly exposed, the southern domain has not been extensively studied. The purpose of this project is to complete the characterization of the structural geometry and the strain of the Taconic slate belt by conducting a similar analysis of the southern domain. Preliminary results from field and laboratory work show that the southern domain has: 1) a mean cleavage of 013/32, 2) a mean stretching lineation of 35/106, 3) a mean stretch along the X-axis of 1.6 and along the Y-axis of 1.1. Fibers in strain fringes around subspherical rigid objects are either 1) weakly curved or straight and inclined to cleavage consistent with top-to-WNW non-coaxial flow, 2) straight and parallel to cleavage or 3) locally deformed by a late-stage crenulation cleavage. Qualitative strain indicators including boudined siltstone layers and chocolate tablet structures within veins support flattening strain. The northern domain was determined to be a standard thrust system. The properties of the central domain are consistent with the predictions of the inclined transpression mathematical model, which is the combination of contraction, strike-slip, and dip-slip movement. The cleavage and stretching lineation data within the southern domain are consistent with a standard thrust system, but the strain is more akin to that found in the inclined transpression zone. Therefore, the southern domain is interpreted to be a standard thrust system with strain characteristics that depart from the northern domain due to the area being structurally higher in the Taconic thrust sheets, as supported by independent stratigraphic evidence.