THE CARTHAGE COLTON SHEAR ZONE, A HIMALAYAN STYLE CRUSTAL CHANNEL IN THE GRENVILLE?

The Carthage Colton Shear Zone (CCSZ) is a major zone of ductile shear in the Grenville Province that separates the Adirondack Lowlands Terrane (Central Metasedimentary Belt) and the Adirondack Highlands Terrane (Central Granulite Terrane). The exact nature, timing, and tectonic significance of the CCSZ during the (1070-1055 Ma) Ottawan Orogeny remain a matter of debate. We propose that this structure represents the top of a crustal channel similar to that proposed for the Nanga Parbat and Namche Barwa massifs in the Himalaya. These channels are triggered by compression coupled with rapid surface denudation. In the Himalayan case (Nanga Parbat, and Namche Barwa), these channels bring granulite facies gneisses rapidly to the surface, resulting in elevated geothermal gradients and marked thermal anomalies across the bounding shear zones. Rapid exhumation of the granulite core results in the formation of pressure-release anatectic melts. In the Nanga Parbat region, these granite bodies are concentrated along the bounding shear zones. In the proposed Adirondack analog, the west-dipping CCSZ represents the boundary between the exhuming granulite-facies Adirondack Highlands Terrane and the “cover” rocks of the Adirondack Lowlands. In this model, the late-syn orogenic (1050 Ma) Lyon Mountain Granitoids represent the anatectic pressure release melts. Like the Nanga Parbat region, these granitoids (A-type) are concentrated primarily in the exhuming granulite massif (Adirondack Highlands) and near to the bounding shear zone (CCSZ). Geochronologic, and geothermometric data support the existence of steep thermal gradients across the CCSZ during the Ottawan Orogeny which can only be sustained via the rapid exhumation of a hot granulite core. In addition, preliminary fluid inclusion data from a Lyon Mountain Granite adjacent to the CCSZ (Adirondack Highlands) follow a nearly isothermal P-T path that is practically identical to those published for granulite facies massifs in the Himalaya.