Assembly and preservation of lower, mid, and upper orogenic crust in the Grenville Province—Implications for the evolution of large hot long-duration orogens

Abstract

It is argued that the Grenville Province is a large hot long-duration orogen with a plateau in the hinterland, remnants of which are preserved in the hangingwall of the Allochthon Boundary Thrust and characterised by metamorphism from ca. 1090 to 1020 Ma (Ottawan phase of the Grenvillian Orogeny). Hinterland rocks are grouped into three tectonic units on the basis of their Ottawan metamorphic signatures, the allochthonous High Pressure Belt, the allochthonous Medium–Low Pressure Belt, and an orogenic lid lacking evidence for penetrative metamorphism. P– T and geochronological data indicate Ottawan metamorphism developed under a relatively high geothermal gradient and was followed by slow cooling, compatible with some form of channel flow. Metamorphic rocks in the Parautochthonous Belt in the footwall of the Allochthon Boundary Thrust, also divided into medium and high-pressure units, were metamorphosed from ca. 1000 to 980 Ma (Rigolet phase) under a lower geothermal gradient and underwent rapid cooling. Their evolution is interpreted to record advance of the orogen into its former foreland after channel flow had ceased. The Allochthon Boundary Thrust is thus a material focal plane separating high-grade rocks derived from opposite sides of the orogen metamorphosed at different times under different P– T–t gradients. Preservation of the Orogenic Lid and low pressure segments of the allochthonous Medium–Low Pressure Belt is a result of gravitational collapse of the orogenic plateau, initiated in late Ottawan time, and the formation of a crustal-scale horst-and-graben architecture. This study emphasises the importance of gravitational collapse during the prolonged compressional phase, a feature not presently accommodated in numerical models of large hot long-duration orogens.