The Grenville Province as a large hot long-duration collisional orogen – insights from the spatial and thermal evolution of its orogenic fronts

Abstract

Abstract The proposition that the Grenville Province is a remnant of a large hot long-duration collisional orogen is examined through a comparative study of its present orogenic front, the Grenville Front, and a former front, the Allochthon Boundary Thrust. Structural, metamorphic and geochronologic data for both boundaries and their hanging walls from the length of the Grenville Province are compared. Cumulative displacement across the Grenville Front was minor (10 s of km) whereas that across the Allochthon Boundary Thrust was major (100 s of km), consistent with the observation that the latter boundary separates rocks with a different age, and P – T character, of metamorphism. On an orogen scale, Grenvillian metamorphism can be subdivided into two spatially and temporally distinct orogenic phases, a relatively high T Ottawan ( c . 1090–1020 Ma) phase in the hanging wall of the Allochthon Boundary Thrust, and a relatively lower T Rigolet ( c . 1000–980 Ma) phase in the hanging wall of the Grenville Front. It is argued that the structural setting and ≥50 My duration of Ottawan metamorphism are compatible with some form of channel flow beneath an orogenic plateau, with the Allochthon Boundary Thrust forming the base of the channel. Channel flow ceased at c . 1020 Ma when the Allochthon Boundary Thrust was reworked as part of a system of normal-sense shear zones, and following a hiatus of c . 20 My the short-lived Rigolet metamorphism took place in the former foreland and involved the development of a new orogenic front, the Grenville Front. Taken together, this suggests the Grenville Orogen developed as a large hot long-duration orogen during the Ottawan orogenic phase, but following gravitational collapse of the plateau the locus of thickening migrated into the foreland and active tectonism was restricted to a subjacent small cold short-duration orogen. The foreland-ward migration of the orogenic front from the Allochthon Boundary Thrust to the Grenville Front, the contrasting P–T–t character of the metamorphic rocks in their hanging walls, and the evidence for orogenic collapse followed by renewed growth, provide insights into the complex evolution of a long-duration collisional orogen.