QUARTZ RIBBONS FROM THE PISECO LAKE SHEAR ZONE: DO THEY TRACK THE LAST PULSE OF DUCTILE DEFORMATION?

The southern Adirondack mountains of New York are underlain by metaigneous and metasedimentary rocks that were deformed in a system of mostly sinistral shear zones during the late stage of the Ottawan orogeny. The shear system is generally east-west striking and has east-west trending subhorizontal mineral elongation lineations. In the southern ADK, the Piseco Lake shear zone (PLsz) strikes east-west and is up to 20 km wide. The zone is developed within highly deformed granitic rocks with chemistry suggestive of an arc affinity. New field results show that the structure has a steeply dipping (5-7 km wide) mylonite zone (southern domain) that merges across strike with a broad open foliation arch (Piseco dome, northern domain). Sinistral kinematic indicators are common throughout the zone. In the region of Piseco Lake, the trend of the dome axis is asymmetric to the adjacent mylonite zone, and consistent with sinistral shear. Steeply dipping mylonite with transcurrent strain history and adjacent asymmetric domes indicates overall sinistral transpression. As reported by earlier researchers, the sheared granitic rocks contain quartz ribbons that are 10's of cm long, mm's in width, and < mm thick. Regardless of the degree of L- and S-tectonite development, the quartz ribbons are ubiquitous within the PLz. The quartz ribbons do not exhibit a typical dynamically recrystallized texture, but they are made up of elongated optically continuous grains, that are up to 5 cm long. Some ribbons merge to form grains with a bifurcated shape. A quartz petrofabric analysis was completed at several locations within the southern and northern domains to search for c-axis orientation consistency within samples and the region. For all locations, quartz c-axes are oriented sub-perpendicular to macroscopic lineation and they plot as single asymmetric girdles. The asymmetry of the girdles is consistent with sinistral shear sense. Since the quartz fabrics in these samples are not typical of dynamically recrystallized quartz, the results suggest that they may have formed by annealing during the waning stage of sinistral, non-coaxial shear. It is likely that these quartz ribbons track the last pulse of ductile deformation in the Piseco Lake shear zone.