MESO- AND MEGA-SCALE VARIATION IN L- AND L-S FABRICS IN THE PISECO LAKE SHEAR ZONE, WESTERN ADRIONDACK MOUNTAINS, NEW YORK

The Piseco Lake shear zone (PLsz) is a major structure that crosses the s. Adirondack Mts. with an E-W strike. Previous researchers documented L- and L-S tectonites in this shear zone; however, the geographic distribution of fabric intensity variation was not addressed. Detailed geologic mapping (supported by AASG) across the northern transition zone of the PLsz was completed in the area of the W. Canada Cr. basin. An area about 42 sq km was covered. In the study area, the PLsz crosses mostly granitic rocks, but they are variable in texture and deformation fabric intensity. The objective of this study was to examine variations in L-, L-S, and S-tectonite fabrics within, and across the northern boundary of the shear zone. Five fabric categories were used to classify deformation fabrics in outcrops and hand samples: L, L>S, L=S, S>L and S. Using this classification, major zones of each were delineated at the map-scale, but the boundaries between zones are gradational. Foliation is defined by planar aggregates of recrystallized quartz, plagioclase, Kspar and minor mafic phases. The mineral lineations are spectacular in some rocks, and are defined by fine-grained linear aggregates of recrystallized quartz and feldspars. Lineations trend 225-270 with the maximum at 257, are shallowly plunging, and are best developed in the southern part of the study area, where A cigar-shaped domain of L-tectonite about 3.5 X 0.5 km in size occurs. The long axis of the this domain is parallel to the trend of the mineral lineations. The intensity of lineations deminishes near the shear zone boundary. That gradation is marked by map-scale domains of L>S, L=S, S>L, and S-tectonite. The foliation strikes 225-255 and dips 20-50. Locally, the foliation defines a cylindrical fold-pattern around the L-tectonite domain, with a hinge axis parallel to the lineations. Rocks containing L-S fabrics near the L-tectonite domain were examined for kinematic information. The foliation north of the L-tectonite domain dips NNW and the shear sense is sinistral based on S-C fabrics observed. The foliation south of the L-tectonite domain dips SSW and the shear sense is also sinistral. This sense of shear is consistent with kinematic information that was reported for the PLsz by earlier researchers. In conclusion, this study has demonstrated that variations in L-S fabrics that were previously described in the PLsz at the outcrop scale, also occur at the map-scale.