POST- TO LATE-OTTAWAN RETROGRESSION ASSOCIATED WITH EAST-WEST EXTENSION IN THE SOUTHERN ADIRONDACKS, NEW YORK

A broad (>20 km) zone of L- and L-S tectonite defines the Piseco Lake shear zone (PLz), located in the southern Adirondacks, New York. The PLz traces across the entire southern Adirondacks in a roughly east-west direction. Published zircon geochronology from rocks within the PLz constrain the metamorphism to be associated with the Ottawan orogeny. The PLz is crosscut by a series of ductile shear zones that exhibit normal and oblique normal offset. In the region of Speculator Mt., the largest (~100 m wide) of these shear zones strikes north-south and dips moderately westward. Kinematic analysis on this zone reveals top westward displacement, or normal. This shear zone crosscuts Charnockitic gneiss within the PLz, and contains minor syntectonic pegmatite. Disequilibrium textures consisting of chlorite and biotite that formed at the expense of primary hornblende and hypersthene are abundant. Additonally, garnets in minor amphibolite layers contain rims of chlorite. These observation suggest that the normal faulting was accompanied by low-grade retrogression. In other parts of the PLz, there are small (5-30 cm wide) crosscutting shear zones that strike about north-south and dip westward. These small zones exhibit the same fabrics and retrogressive metamorphic mineral textures as the larger zone near Speculator. These small shear zones transpose the L-S fabrics of the Piseco Lake shear zone, always contain granitic pegmatite parallel to the zone boundaries, and exhibit normal east-west displacement. The foliation in these small zones is defined by dynamically recrystallized quartz and feldspar, and parallel alignment of biotite and chlorite. Some of these small ductile zones form conjugate pairs with bulk extension direction trending about east-west. It is clear from field relations that these normal ductile shear zones post-date the main L-S fabrics of the PLz, therefore, they must be late- to post-Ottawan in age. It is possible that these normal shear zones and retrogression, could be associated with the apparent extensional deformation on the Carthage-Colton zone. However, the geographic distribution of these small normal zones confines them within the boundaries of the PLz. An alternate hypothesis is that the PLz experienced internal extension during exhumation and cooling, which would explain the parallelism of the displacement on the PLz and the extension direction.