LATE-GRENVILLE DEXTRAL TRANSCURRENT TECTONICS IN THE CENTRAL APPALACHIANS

Northeast-striking Late Grenville dextral transcurrent shearing has been identified in the western Hudson Highlands, NY and proposed to be a conjugate system to East-West-striking sinistral transcurrent shearing in the Adirondacks, NY. This event post-dates the main Grenville tectonism and is proposed to reflect tectonic escape after the main collision. The dextral shear system forms a >35 km-wide zone of steeply-dipping and anastomosing mylonite zones that sharply contrast the regional shallowly-dipping Grenville gneissic foliation. Consistent dextral kinematic indicators include C/S fabric, rotated porphyroclasts and asymmetric pegmatite boudins, en-echelon folds and a large (>5 km) drag fold along the southeastern margin of a zone. Total offset on the exposed shear zone is estimated at >150 km based on integrated strain measurements. Synkinematic bimodal plutonism accompanied the dextral shearing and includes the 1020 Ma alkaline Mount Eve granite, the 1010 Ma peraluminous Canada Hill granite and the 1008 Ma Lake Tiorati diorite. The Lake Tiorati diorite was sheared while still at high temperature forming C/S mylonite along its western edge. Magnetite and related mineralization and late pegmatite intrusions along the margins of the shear zones mark the end of the event. Recent SHRIMP dating of zircons from both the magnetite mineralized zones and an undeformed pegmatite that cross-cuts the Indian Hill dextral shear zone yield ages of ca. 980 Ma. The shear system was therefore active for approximately 25 to 35 Ma which is similar in duration to the escape tectonic systems in the Himalayan collision. The dextral shear system largely disappears beneath the Newark basin to the south but Grenville gneiss in some crystalline domes of the central Appalachian Piedmont contain dextral mylonites that predate the Paleozoic cover sequence and may be distant extensions of the system.