Northeastern Section - 54th Annual Meeting - 2019

Paper No. 33-5
Presentation Time: 8:00 AM-12:00 PM


WILLIAMS, Garnet J. and TORAMAN, Erkan, Department of Geology, St. Lawrence University, 23 Romoda Drive, Canton, NY 13617

Understanding conditions and deformation mechanisms operating in mid-crustal shear zones is crucial for overall assessment of rheological evolution of orogenic belts. Strain history can be complex owing to reactivation of these zones during different phases of orogenic history. In the field, these shear zones often exhibit conflicting kinematic shear sense and variable deformation conditions. Forming one of the major tectonic boundaries in the Adirondacks, the CCSZ separates the upper amphibolite facies rocks of the Adirondack Lowlands from the granulite facies units in the Adirondack Highlands. The CCSZ has a prolonged history with variable kinematics and the timing and conditions of different deformation phases are highly debated. We applied kinematic and vorticity analysis on the northern end of the CCSZ within the Stone Valley area (Colton, NY), where mylonitic rocks are exposed over 3.5 km long transect along the Raquette River. Various kinematic indicators, such as asymmetric porphyroclasts and composite S/C/C’ fabrics, dominantly show top-to-NNW sense of motion (normal fault), although some samples a minor component of top-to-SSE motion (reverse fault). Preliminary results of vorticity analyses show a range of kinematic vorticity number (Wk) from 0.2 to 0.4, indicating pure shear dominated deformation. We interpret our results as mylonites recording top-to-NNW and top-to-SSE motion contemporaneously in one deformation episode as oppose to during different episodes of deformation. Combined with previous geochronological data, these results further suggest observed deformation fabrics formed during Ottawan extensional collapse of the orogen.