Geologic Structures of Highly Deformed Rocks at Oliver and Snyder Hills, Southeastern Adirondacks, New York

Abstract

The study area is located at Snyder and Oliver Hills, Minerva, southeastern Adirondacks in New York State. The geologic setting of the study area is just south of the Anorthosite Suite of the Adirondack Massif. The rocks from the study area are granitic gneiss and amphibolite that were affected by high temperature metamorphism and deformation. The grade of metamorphism is interpreted from the mineral assemblage as relatively high P[H2O] granulite fades metamorphism. High temperature deformation is inferred since rock-forming minerals such as quartz, feldspar and calcite show microstructures related to plastic deformation. Strong and consistent rock fabrics are also found. Subhorizontal foliations are defined by compositional layering of mafic and felsic minerals. Subhorizontal east­west-trending lineations are defined by quartz ribbons and preferred orientation of mafic minerals. These conditions suggest subhorizontal ductile shear zone deformation in this area with the shear sense of hanging wall to the east or west. To determine the shear sense, c-axis orientation from the quartz ribbon and grain shape orientation were measured. The pattern of the structural elements from these analyses is close to being symmetric rather than asymmetric, although the hanging wall to the west sense is slightly dominant. However, at an outcrop near the lithologic contact between the granitic gneiss and the amphibolite, a dragging fold of amphibolite is found. The shear sense from this outcrop is clearly hanging wall to the west. The hanging wall to the west shear sense and the symmetric pattern from the structural elements may suggest a close-to-coaxial but non-coaxial deformation or very large strain accumulation in this area. Plastic deformation microstructures from the rock-forming minerals were carefully observed, since certain temperature conditions are required to form such textures. Microstructures found in this study include formation of quartz ribbons and quartz c-axis preferred orientation by basal <a> slip in quartz, feldspar subgrain boundaries by dislocation gliding, possibly cataclastic flow of feldspar grains, and deformation twins in calcite. The temperature conditions for these microstructures are plotted onto the regional cooling curve. These results may suggest that the deformation in this area lasted for a long period of time over a wide range of temperature conditions, although the nature of deformation, whether continuous or punctuated, is not exactly known. Regionally, this study area is very highly deformed and not much literature is yet available in the areas adjacent to the study area. Hence it is hoped that the results of this study will aid as data base to future southern Adirondack studies.