VARIATION OF GEOMETRY OF STRUCTURES AND STRAIN IN A CURVED OROGENIC BELT: TACONIC ALLOCHTHON, VERMONT AND NEW YORK

Shortening in the upper to middle crust via transpression, the combination of strike-slip and contractional movement in vertical shear zones, is a well-studied phenomenon. Inclined transpression, which occurs within low-angle thrust sheets, may be just as common, but is not as well examined and so represents a gap in our understanding of orogenesis. 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. It displays an internal, along-strike change in structural orientation to form NE-trending northern and southern domains and a NW-trending central domain. All three domains contain a near homogeneous slate unit with many well-distributed markers that allow quantification of the strain. The geometry of structures and strain in the northern and central domains are well characterized from previous research. The goal of this project is to conduct a similar analysis of the southern domain. We hypothesize that the southern domain is similar to the northern domain which is characterized by 1) a mean cleavage dip of ~30°, 2) an approximately down-dip stretching lineation, 3) non-coaxial, top-to-WNW flow, 4) plane strain, and 5) a mean stretch along the X-axis of 2.3. These areas bound the central domain which is characterized by 1) a mean cleavage dip of ~45°, 2) a moderately south-raking stretching lineation, 3) non-coaxial, sinistral, top-to-WNW flow, 4) flattening strain, and 5) a mean stretch along the X-axis of 1.4 and along the Y-axis of 1.1. The structural properties of the central domain are consistent with the predictions of the inclined transpression mathematical model. Preliminary results show that, like the northern domain, the southern domain has a mean cleavage dip of ~30° and an approximately down-dip stretching lineation. However, preliminary results also indicate the state of strain and strain magnitude of the southern domain are more similar to the central domain, the region of inclined transpression. Independent evidence based on stratigraphy suggests these disparities could be a result of the southern domain being at a higher structural level within the Taconic thrust sheets than the northern domain.