Late Shortening and Extensional Structures and Veins in the Western Margin of the Taconic Orogen (New York to Vermont)

Along the western margin of the Taconic orogen in New York and Vermont, undeformed quartz-calcite veins commonly occur in the belt of melange that formed beneath the westward-advancing Taconic Allochthon during the Middle-Late Ordovician Taconic orogeny. The veins have mineral slickenfibers recording either reverse or normal slip. In New York, the reverse-motion veins recording the latest phase of shortening are crosscut by the normalmotion veins and faults. The shortening indicated by the reverse-motion veins is correlated with the convergence along the Champlain thrust in Vermont, which is also crosscut by a significant, strike-parallel normal fault. Fluid inclusion data from the veins, complemented by stable isotope data, lead to a reconstruction of the sequence of events in the context of a cooling of the fluids, which is consistent with crosscutting relationships among the veins. Following cessation of the convergence, there was regional extension of the western margin of the Taconic orogen, analogous to modern arc-continent collisional orogens. Extension progressed to normal faulting without vein precipitation, and the normal faulting significantly modified the Allochthon-melange contact. The timing of extension is constrained to follow the late Taconic thrusting and predate the latest Silurian, based on the similar fluid temperature/salinity of the reverseand the normal-motion veins, and contrast with veins nearby in Devonian rocks. Extensional, partial collapse of the orogen was accompanied or followed by rebound of the foreland basin, perhaps due to reduction of the thrust load and/or subducted slab breakoff. The systematic gradient of homogenization temperatures exhibited by the reverse-motion veins along the orogen margin is interpreted to be caused by real differences in temperature of the vein-forming fluids. Online enhancements: appendix, figure, table.