POSSIBLE LATE OR POST-OTTAWAN EXTENSION IN THE EASTERN ADIRONDACKS NEAR WHITEHALL, NY: PRELIMINARY RESULTS

The Ottawan orogeny (ca. 1090–1030 Ma) was the final collisional event within the Grenville province in the Adirondack region of northern New York (McLelland et al, 2001). Crustal extension in the region occurred both during the late Ottawan and as much as 50–70 m.y. after the orogeny (Streepey et al., 2001). Extension was likely driven by collapse of an over-thickened crust, although post-orogenic extension may have had an alternate origin. Shear zones thought to accommodate this extensional phase are largely found in the NW Adirondacks (Carthage-Colton shear zone) and southern Ontario (Bancroft and Robertson Lake shear zones). It remains unclear if late-orogenic extension was limited to the NW Adirondack region or if it was a more wide-spread tectonic event.

In the eastern Adirondacks near Whitehall, NY, high strain gneiss is exposed. The goal of this study was to determine the kinematics and strain magnitude of the shear zone to assess if it accommodated late-orogenic extension. The exposed rock is mainly a sillimanite-garnet gneiss, although granitic gneiss and other compositions are also present. The gneiss is an L-S tectonite with a shallowly (25°) SE dipping foliation and SE trending lineation. The granitic gneiss is typically less strained than surrounding rocks and may have been intruded syn-tectonically. Analysis of >100 K-feldspar sigma porphyroclasts in the granitic gneiss show that ~45% document top-SE or normal shear while only 25% show top-NW shear. Strain measurements suggest that low strain samples are dominated by plane strain while higher strain samples fall in the flattening field. Therefore, it is possible that the shear zone acted as a ductile normal fault for part of its strain history, although a more complex evolution is likely. Assuming that the shear zone is 3-8 km thick, that strain is due to simple shear during extension (an oversimplification), and that deformation in the granitic gneiss is typical of the entire shear zone, total normal-sense displacement across the zone is estimated at 5-15 km. These preliminary results suggest that the eastern Adirondacks also accommodated significant extension, likely during late or post-Ottawan time. Ongoing structural, petrographic and geochronologic studies are expected to yield new insights into the strain history of this shear zone.