Tectonic Significance of Mylonite Kinematic Indicators within the Carthage-Colton Mylonite Zone, Northwest Adirondacks, New York

The Adirondacks are a southeastern extension of the ca. 1300-1000 Ma Grenville Province in northern New York State. In the Adirondacks, the Carthage-Colton Mylonite Zone (CCMZ) is one regional scale high strain ductile shear zone that exists within the Grenville Province and divides the Adirondacks into the Adirondack Highlands and the Adirondack Lowlands. The role of the CCMZ in Grenville tectonics is not clearly understood, but it has been proposed that, following a compressional origin, it maybe an important structure accommodating channel flow or tectonic collapse.

In order to investigate the likelihood of such models, an area across the CCMZ within the ca. 1164 Ma Diana Syenite between Harrisville and Dutton Corners, New York, was investigated. This area is dominated by a southeast striking, northwest dipping mylonitic fabric with a north-northwest plunging stretching lineation. Kinematic indicators consistently indicate south-southeast directed oblique thrusting of the Adirondack Lowlands over the Adirondack Highlands. Microstructures of the mylonitic fabric, plus kinematics and structural patterns of other ductile structures in the area are consistent with this oblique thrusting. These results appear to contradict the models that require the CCMZ to be a significant extensional structure, though it can be possible to explain these ductile structures by a rotated and warped crustal detachment. However, peak metamorphic conditions were approximately 700-800 degrees C throughout the area, which indicates that these ductile fabrics probably developed during compression of the Shawinigan Orogeny at ca. 1150-1130, when both the Lowlands and Highlands were last at these metamorphic conditions. Further, the character of Lowlands and Highlands time-temperature cooling paths indicate that extension must occur along the CCMZ late in its history. Therefore, late brecciated lineaments found throughout the area may accommodate extension in this portion of the CCMZ.