VARIATIONS IN L- AND S- TECTONITES WITHIN THE PISECO LAKE DUCTILE SHEAR ZONE, ADIRONDACK MOUNTAINS, NEW YORK

The Piseco Lake shear zone is a major ductile structure that strikes E-W in the Southern Adirondack Mountains. Previous work documented five fabric variations L>>S, L>S, L-S, S>L, and S on the northern boundary of the shear zone. S-tectonites outside of the shear zone show larger gain size and pyroxene as the major metamorphic index mineral. L- tectonites within the shear zone show a decrease in grain size and chlorite as the major metamorphic index mineral. A granitic mineral assemblage with chlorite defining the microscopic deformation fabric suggests retrograde metamorphism within the shear zone.

To augment work done at the map scale, outcrop mapping of the Ohio gorge was completed (supported by the R.E. McNair program). The Ohio gorge in located within Piseco Lake shear zone, trending east west about a kilometer in the Southwestern Adirondacks. The gorge is composed of granitic rocks that vary only in the amounts and type of mica. Within the gorge there are three apparent fabric variations L>>S, L>S, and L-S. The rocks with L>>S fabrics contain chlorite with abundant quartz lineations. The rocks with L>S fabrics contain both chlorite and biotite with some lineations either quartz or mica streaks. The rocks with L-S fabric contain mostly biotite and have stronger foliation with few lineations and many porphyroclasts. Both ó- and ä-porphyroclasts demonstrate sinistral shear sense throughout the gorge agreeing with kinematics seen throughout the mapped region. The boundaries between the different fabric zones are generally gradational, but some abrupt boundaries can be observed. The boundaries observed within the Ohio gorge show similarities to a cigar-shaped boundary at the map scale but are meters in size rather then kilometers. Ductile normal faults are also found within the gorge crosscutting the older fabric and are thus younger. Two of these ductile normal faults show an oblique motion. The first strikes 192 degrees and dips 57 degrees and has left lateral oblique motion. The second strikes 316 degrees and dips 71 degrees with right lateral oblique motion. These ductile shear zones form a conjugate pair with the bulk extension direction roughly E-W.