STRUCTURAL ANALYSIS OF A NORMAL DUCTILE SHEAR ZONE AT SPECULATOR MOUNTAIN, CENTRAL ADIRONDACKS, NEW YORK

Recent geologic mapping in the area of Speculator Mountain in the Adirondacks of NY, revealed a ductile normal shear zone with west directed displacement. Speculator Mountain is underlain by a sequence of granitic-, charnockitic- and gabbroic-gneisses that contain penetrative foliation and ribbon mineral lineations. The foliation texture varies from protomylonite to mylonite in a nearly vertical stack of rocks. Near the base of the mountain, protomylonite occurs in charnockitic-gneiss. Moving structurally upward, the charnockitic-gneiss is overlain by granitic-gneiss with well developed mylonitic fabric. Dynamically recrystallized quartz and K-feldspar, as well as core-mantle structure in K-feldspar are evidence for ductile strain. The mylonitic foliation dips gently to moderately (10-30 degrees) westward, but in some places the foliation is locally folded about a N-S axis. Mineral lineations associated with the mylonitic foliation are defined by ribbon-shaped aggregates of recrystallized quartz, and feldspars. Biotite forms mineral streaks that parallel the ribbon lineations. The lineations trend approximately due west. In some mafic rocks, there are relict hypersthene grains with reaction rims and fringes of hornblende and biotite. The hornblende and biotite define a microscopic foliation and lineation that is parallel to the macroscopic mylonitic foliation and lineation, suggesting these index minerals formed during ductile deformation. These mineral textures also suggest that amphibolite facies metamorphism was associated with ductile deformation, and it was superimposed on rocks that were originally higher grade (presence of relict hypersthene grains). Kinematic analysis of granitic mylonite revealed abundant shear sense indicators such as type I S-C fabrics, delta- and sigma-porphyroclasts, and shear bands. The shear sense indicators show that the direction of displacement was top toward the west, with the granitic rocks at the top of Speculator Mountain displaced over the charnockitic gneiss at the base. The presence of a large ductile normal shear zone was not previously documented in this area, however, small normal shear zones occur directly to the west in the Piseco Lake shear zone, and they have the same sense of displacement.