IS THE TECTONIC DEVELOPMENT OF THE GRENVILLE PROVINCE MORE LIKE THAT OF THE NORTH AMERICAN CORDILLERA?

The Grenville Orogeny encompasses many small regional tectonthermal events of limited geographical extent occurring during Early-through Late Mesoproterozic (1500-920Ma) time. These events range from compressional tectonism (continental-continental, Andean margin type), to anorogenic rifting. Tectonothermal events show up sporadically in the myriad of terranes and exotic blocks that make up the greater Grenville Province in Eastern North America. Terrane boundaries are marked by shear zones, many with large amounts of strain accumulation and complex movement histories. Adding to this complexity, metamorphic events range widely in age and intensity across this patchwork of terranes. Many of these terranes are marked by the emplacement of large volumes of AMCG magmas intruded at relatively shallow crustal depths. In the Adirondack Mountains of New York State and portions of Quebec Province (Morin Terrane), the AMCG massifs intrude after the cessation of compressive tectonism (Elzevirian Orogeny), and are interpreted to mark an eastward sweeping delamination event. To the North (Quebec to Labrador) AMCG intrusion is later (>1.01Ga) at a time when the Adirondack segment of the Grenville is in the latter stages of Ottawan compression. A further complication is that the driving force for the Ottawan Orogeny remains enigmatic as recent reconstructions fail to show any plausible continental mass outboard of Laurentia during this time. Overall, the patchwork nature of the Grenville Orogeny does not fit well with the prevailing notion of a series of island arc, and continent-contintent collisions along the eastern margin of Laurentia. This regional tectonothermal history may better fit a tectonic cycling model involving periodic collisions between arcs (collapse of back-arc basins) and oceanic ridges/hotspots along a long-standing convergent margin. Periodic compression, high grade metamorphism, and anorogenic rifting in this model could be triggered by collisions and/or changes in subduction angle. The patchwork nature of the tectonothermal history of the region may be the result of significant (km) movements along shear zones driven by oblique collisions. In this way, the Paleozoic-recent tectonic environment in the western U.S. may provide a more instructive analog for the Grenville Orogenic