STRAIN VARIATION IN THE PISECO LAKE SHEAR ZONE, SOUTHERN ADIRONDACKS, NEW YORK

The Piseco Lake shear zone is a major crustal structure that generally strikes east-west, spans the width of the southern Adirondacks, dips steeply, is up to 30 km wide, developed during a major phase of sinistral transpression during the Shawinigan orogeny. Although the shear zone is the boundary between the central Adirondack Highlands AMCG rocks (1.16-1.15 Ga) and the southern Adirondack tonalitic gneisses (1.35-1.30 Ga), the core of the zone is developed in the Piseco granitoid suite (~1.18Ga). Ductile deformation fabrics in the zone are highly variable with km-scale domains of L-tectonite and broad domains of S-L mylonite. The northern margin of the shear zone is marked by a domain of L-tectonite that occurs in the core of a mylonitic foliation antiform (Piseco antiform). During this investigation, a suite of oriented samples of Piseco granitic gneiss were collected along a transect that crosses the main steeply dipping mylonite zone (20 km wide) to examine S-C mylonites and calculate the variability in shear strain. The main steeply dipping mylonite zone is not a typical ductile shear zone with gradual strain gradient from the wall-rocks to the core of the zone. To the contrary, there are several domains of well-developed L-S tectonite and L-tectonite 100’s meters wide. Within the S-L mylonites, shear strain was determined using the angle between S and C surfaces. In the north, the S-C angles are less than 15 degrees with shear strain values of 8.0-12.0 across a domain that is 10 km wide. Farther south the strain appears to be more uniformly distributed over a 12 km wide domain with S-C angles of 20-35 degrees (shear strain of 1.2-2.3). These wide domains of S-C mylonite account for a minimum of about 175 km of displacement, assuming average shear strain values. This displacement estimate does not take into account the broad domains of L-tectonite, where S-C fabrics are not apparent.