Ductile deformation history of the Diana Syenite within the Carthage-Colton mylonite zone, northwest Adirondacks, New York

Graham B. Baird ‘98
Department of Earth Sciences, University of Northern Colorado, Greeley, CO 80639; graham.baird@unco.edu

The Adirondacks of northern New York State is the southern-most extension of the Grenville Province of eastern Canada. The Carthage-Colton mylonite zone is a complex structure that separates the Adirondack lowlands from the Adirondack highlands, the two terranes that comprise the Adirondacks. The central and southwestern portions of the Carthage-Colton mylonite zone and adjacent areas are comprises of the Diana Syenite. South of Harrisville, New York, the Diana Syenite contains two distinct ductile structures associated with the Carthage-Colton mylonite zone: penetrative proto-mylonitic to mylonitic fabrics, and discrete centimeter thick planar ultramylonite ductile shear zones. Kinematic indicators, which allow the type of deformation to be assessed, indicate that both the penetrative fabric in the Diana Syenite and the discrete ductile shear zones developed as a result of significant oblique left-lateral thrusting of the Adirondack lowlands to the southeast over the Adirondack highlands. The discrete ductile shear zones display mutual cross-cutting relationships with the penetrative fabric as well as possessing a complex regional orientation pattern. Numerical modeling of the rotation of passive planes in simple shear demonstrate that the discrete shear zones likely developed during the same deformation event to that of the penetrative fabric and not by two separate deformation events. This is significant as it exhibits the importance of the Carthage-Colton mylonite zone as a contractional structure in the Grenville Province. Evidence from this work and the work of others indicate that extension must have occurred along the Carthage-Colton mylonite zone subsequent to the ductile structures described here – probably near the end of Grenville tectonism.