MYLONITE REACTIVATION AND BRITTLE AND DUCTILE DEFORMATION WITHIN THE CARTHAGE-COLTON SHEAR ZONE AT STONE VALLEY, COLTON NY

The Grenville age Carthage-Colton Shear Zone (CCSZ) is a 1–15-km-wide, NE-SW trending zone of brittle and ductilely deformed rocks in northern New York State. It has been interpreted as an extensional collapse feature whose last (ca. 1050 Ma) movement resulted in down dropping of upper amphibolite facies rocks of the Adirondack Lowlands against granulite facies rocks of the Adirondack Highlands. Both macro- and micro-scale kinematic indicators have been identified along the entire length of the CCSZ and indicate variable movement with reverse, oblique, and normal fault kinematic indicators identified by a numerous previous workers. Stone Valley, north of Colton (NY), is named for a canyon-like section of the Raquette River in which the river has down cut through the blanketing glacial deposits, exposing a nearly continuous, east-west trending, cross section across the CCSZ. In this study we examine the lithology, rheology, and structure of the CCSZ within Stone Valley and present evidence for both brittle and ductile deformation and mylonite reactivation. The main rock units identified within the field area include: pelitic and calc-silicate gneisses, quartzite, and marble of the Adirondack Lowlands; and complex granitic-tonalitic migmatitic and mylonitic orthogneisses of the Adirondack Highlands. Breccia in contact with marble contains boulder to cobble sized clasts of mylonite, marble, and leuco- and calc-silicate gneisses, within a diopside-rich matrix. In the northern section of the study area the breccia unit contains multiple, meter-scale blocks of mylonite. These blocks display a highly variable orientation with respect to the surrounding mylonite and record early ductile deformation, likely related to the Shawinigan Orogeny (ca. 1200-1150 Ma). The pyroxene-rich nature of the matrix suggests limited hydrothermal fluid present during the brittle phase of deformation. The data also reveal that the mylonite zone at Stone Valley has had a complex and variable history, with a non-linear (anastomosing?) foliation on the sub-kilometer scale.