DEEP-SEATED HIMALAYAN-TYPE SYNTAXIS IN THE GRENVILLE OROGEN, NY-NJ-PA

A deep-seated analog of the syntaxis that has developed in the Tibetan Plateau north of the Himalayan collision appears in the Grenville Orogen of northeastern Laurentia. During the final assembly of Rodinia, Amazonia collided with Laurentia and produced a series of large conjugate transcurrent shear systems and pervasive strike-slip deformation overprinting pervasive contractional deformation produced in the earlier Ottawan Orogeny. A northeast-trending, dextral transpressional system of anastomosing shear zones at least 35 km wide developed in the Reading Prong of NY, NJ, and PA. U-Pb SHRIMP analysis of metamorphic zircons and Ar/Ar thermochronology on the rare lowest grade cataclasites constrain the age of movement to between 1008 and ca. 876 Ma. Most movement, however, ended before ca. 915 Ma based on Ar/Ar thermochronology. A 60-km-wide east-west trending sinistral shear system developed across central Adirondack Highlands. This system overprints rocks with granulite facies metamorphic assemblages containing ca. 1050 Ma metamorphic zircons and is cut by a swarm of ca. 950 Ma leucogranites. The geometric relationships and shear sense of the Adirondacks and Reading Prong shear systems suggests a conjugate system within a syntaxis with bulk compression directed ENE-WSW. This tectonic scenario invokes a component of strike-parallel deformation subsequent to the peak Ottawan Orogeny and provides a possible kinematic mechanism for an otherwise enigmatic late (ca. 930) extensional event in the northwest Adirondacks and Canada.