DOMAINS OF L-S AND L>S TECTONITE AND EVIDENCE FOR SINISTRAL SHEAR IN THE PISECO LAKE ZONE, SOUTHERN ADIRONDACKS, NEW YORK

The Piseco Lake shear zone traces E-W across the S Adirondacks and contains variably developed domains of L-S, L>S, and L-tectonite at the outcrop to map scale. Although mafic and pelitic mylonite occurs within the PLz, largely the zone is developed in granitic rocks previously reported to be part of the AMCG suite. During the 2007 field season, an area of the PLz (32 square km) was mapped (Clockmill Corners and Powley Place region), and this area spans the southern boundary of the zone. This work was completed to examine the structural transition from inside to outside the shear zone, determine the kinematic history, and to examine relationships between L-S and L>S fabric domains and lithology. The region is dominated by granitic mylonite with foliation and lineation defined by recrystallized aggregates of quartz and feldspar. The foliation strikes ENE and is moderate to steeply dipping. Variation in the foliation dip pattern shows a gradual steepening from N to S across the strike of the zone. The southern boundary is defined by a relatively abrupt transition (over several hundred meters) to coarse banded gneisses with very shallow dipping foliation. The steepest foliation (75-80 degree dip) in the zone occurs near this transition, and we interpret the map-scale relationships as crosscutting. This implies that the gneissocity in the wall rocks predates the PLz. The lineations consistently trend about 110 and plunge 5-20 eastward. Domains of L-S and L>S fabrics were mapped. Overall, L-dominated rocks occur most frequently in the north. Detailed mapping of an L>S domain (0.8-1.4 km wide) shows structural discordance with the overall strike of the shear zone, suggesting possible lithologic control on fabric development. As well, narrow (< 1 m wide) high-strain zones defined by ultramylonite occur within parts of the shear zone with the steepest dip. These relationships are still being investigated. Kinematic analysis was done in the field in addition to using hundreds of oriented specimens in the lab. Feldspar grains with recrytallized tails defining porphyroclasts are the most common shear sense indicator, but S-C fabrics, broken feldspar grains and shear bands are also present. The shear sense indicators reveal an overall sinsitral strain history, and this is consistent with published reports on the PLz for other areas.