GEOLOGICAL SYNTHESIS AND TECTONIC IMPLICATIONS OF THE SOUTHEASTERN SECTION OF THE GLENS FALLS, NY 2-DEGREE SHEET

In 1983 Doug Rankin initiated preparation of the Glens Falls 2-deg Sheet covering Jim Thompson's fieldwork as well as the SE Adirondack Highlands studied by McLelland. The eastern Adirondack region contains isoclinally folded granulites (ca 1070-1050 Ma) that commonly dip east and sweep NNE from Warrensburg, NY, passing beneath Paleozoic cover near Crown Point, NY. South of Warrensburg the units strike SE then swing SW around a series of large open, upright folds that plunge gently ESE and are coaxial with earlier isoclines. The signature rocks of this package are ca. 1300 Ma migmatitic metapelites and tonalites also present in the southern Adirondacks, with which the eastern sequence connects near Glens Falls. The eastern sequence also contains large volumes of ca 1050 Ma Lyon Mt. Granite (LMG), and terminates to the west against the underlying orthogneiss-marble terrain of the central Highlands. Throughout the region rocks exhibit pronounced E-W ribbon lineation and intense mylonitization is common. Megacrystic feldspars with well-developed tails indicate a dominant top-side-down-to-the-east normal sense of shear paralleling ribbon lineation. Locally, in shear zones, LMG and late pegmatites are caught up in, and drawn out parallel to, ribbon lineation. These features are thought to reflect a ca 1050 Ma orogenic collapse along NNE low angle detachment zones that exhumed the central Adirondack terrane. The E-W ribbon lineations and upright folds in both regions are attributed to extensional faulting that also drew early isoclinal axes into approximate parallelism.

Along- strike projection of the ca 1050 Ma Tawachiche shear zone of SW Quebec places this major, east-dipping, ca. 1050 Ma detachment zone (it exhumed the Morin and Lac Taureau Terranes) within the Champlain Valley just a few miles east of the Adirondacks, and we believe that the eastern Adirondack shear zones are closely related to this feature . To complete this picture, we note the down-to-the-west normal displacement along the west dipping ca. 1050 Ma Carthage-Colton shear zone that juxtaposes the Adirondack Lowlands against the Highlands in the NW Adirondacks. Together these late detachments define a Shuswap-scale and Shuswap-style core complex that exhumed the Adirondack core and probably the entire Central Granulite Terrane.