STRAIN IN MESO-SCALE DUCTILE SHEAR ZONES: EXAMPLES FROM THE NORTHWEST ADIRONDACKS, NEW YORK

The mechanisms that produce the strain patterns associated with meso-scale ductile shear zones are not completely understood. Flattening strains within ductile shear zones are commonly observed and require components of both pure shear and simple shear to form the shear zone. The pure shear component can be manifested either by volume loss from within the shear zone or by the extrusion of material out of the shear zone. Volume loss should create bulk chemistry differences between the shear zone and the surrounding wall rock, but this is not always observed. However, extrusion could create space compatibility problems. Currently, there is debate as to the relative importance of these two mechanisms in producing oblate strains.

The Diana Syenite within the Carthage-Colton Mylonite Zone of the Northwest Adirondacks contains many excellent examples of meso-scale ductile shear zones for which strain analysis is possible. One shear zone displays strongly flattening strains; outside the shear zone strain is nearly plane strain. Some chemical differences exist between the shear zone and the wall rock, but these differences alone are not thought to be able to produce the flattening strain. Therefore, extrusion must be in part responsible for the observed oblate strain in the shear zone. The specifics of how extrusion occurs remains under investigation.

Regionally, shear zone poles are oriented in a plane perpendicular the regional lineation, however, except in rare instances, the shear zones are not folded at the outcrop scale. The temperature of shear zone formation (ca. 730 degrees C) is consistent with the shear zones developing during peak regional metamorphism. Two possible kinematic interpretations might account for the presence of these shear zones: 1) the shear zones are related to large-scale folding and help accommodate regional shortening during folding (bulk pure shear model); or 2) the shear zones developed during, and in part accommodated, significant regional lineation-parallel shear along the Carthage-Colton Mylonite Zone (bulk simple shear model). Understanding the regional significance of the meso-scale ductile shear zones will help to elucidate the mechanisms at work in the shear zones at the individual scale.