Exhumed Himalayan-type syntaxis in the Grenville orogen, northeastern Laurentia

Abstract

A deep-seated analog of the syntaxis developed in the Tibetan Plateau occurs in the Grenville Orogen of eastern 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 that overprinted compressional structures related to the Ottawan Orogeny (the last orogenic phase of what is considered Grenvillian). A northeast-striking dextral system at least 35-km wide developed in the Reading Prong of New York (locally known as the Hudson Highlands), New Jersey, and Pennsylvania. U-Pb SHRIMP zircon geochronology and Ar/Ar thermochronology on the lowest grade cataclasites constrain the age of movement between 1008 and 876 Ma. A 60-km-wide, east-west striking, sinistral shear system developed across the 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 950 Ma leucogranites. The timing, geometric relationships, and shear sense of the Adirondacks and Reading Prong shear systems suggest a conjugate system within a syntaxis with bulk compression directed ENE–WSW. This tectonic scenario invokes a component of strike-parallel deformation during the Ottawan Orogeny and provides a kinematic mechanism for an otherwise enigmatic, synchronous, late (ca. 930 Ma) extensional event including the Carthage–Colton mylonite zone in the northwest Adirondacks and Canada.