STRAIN HISTORY AND FLUID INCLUSIONS OF VEINS IN THE TACONIC FLYSCH AND MELANGE OF NEW YORK AND VERMONT

Ordovician shales and greywackes (flysch) deposited in the foreland basin of the Taconic thrust system comprise a deformed belt of faulted and folded rocks, or more highly disrupted melange, adjacent to the Champlain Thrust and the Taconic Allochthon. This deformed flysch/melange belt was produced during transport of the Taconic allochthon, caused by the medial Ordovician collision of the Laurentian margin with island arc terranes. Quartz-calcite veins, common in the melange, mostly have slickenside fibers recording either reverse or normal sense of shear, represented by 120/36 and 164/68 in average orientation, respectively. The reverse-motion veins are consistently cross-cut by the normal-motion veins. Microthermometric analysis of aqueous fluid inclusions in 16 samples of these veins distributed from the New York Capital Region to near the US-Canada border yields homogenization temperatures (T_H) of 145-285°C (NY Capital Region), 199-249°C (Battenkill River), 183-221°C (Mettawee River), 152-212°C (West Haven) and 133-140°C (Highgate Springs), showing a systematic decrease of peak T_H from south to north. Fluid inclusions within the flysch/melange have remarkably consistent salinities (typically 1.4-5 wt % NaCl equiv). The salinity and its range increase (4.8-12.3 wt % NaCl equiv) in samples located close to the Beekmantown carbonates of the Cambrian-early Ordovician Laurentian shelf. In the NY Capital Region, no significant difference in T_H and salinity is found between normal- and reverse-displacement veins in the most westerly locality but, about 8 km farther east, adjacent to the boundary fault of the Taconic Allochthon, normal-motion veins show somewhat lower T_H than reverse-motion veins. This, along with the cross-cutting relationship, suggests that the reverse motion was immediately followed by the normal motion in the west, but farther east there was a longer time interval between reverse and normal displacement. We suggest that there was a regional extension of the western margin of the Taconic orogen soon after cessation of the Taconic thrusting, with extension beginning in the foreland and propagating back toward the hinterland.