Terrane accretion in the internal zone of the Ungava orogen, northern Quebec. Part 2: Structural and metamorphic history

Abstract

The tectonic history of the early Proterozoic Ungava orogen is marked by structural–metamorphic episodes that both predate and postdate a collision between a magmatic arc terrane and the northern continental margin of the Superior Province. Distinct precollisional tectonic histories are documented for the rocks forming the lower plate of the Ungava orogen (the Archean Superior Province basement and an Early Proterozoic rift-to-drift margin sequence) and the orogenic upper plate (Early Proterozoic ophiolitic and magmatic arc units). The lower-plate units preserved in the external part of the orogen (Cape Smith Thrust Belt) record the development of a foreland thrust belt characterized by south-verging faults ramping up from a basal décollement located at the basement–cover contact. The plutonic core of the magmatic arc contains structures and metamorphic assemblages indicative of an episode of dextral transcurrent deformation contemporaneous with granulite-facies metamorphism and arc plutonism. The "tectonically suspect" ophiolitic and arc units were accreted to the thrust belt along south-verging faults, which reimbricated the foreland thrust belt and which resulted in at least 100 km of displacement of upper-plate units with respect to the autochthonous basement. Collisional thickening and consequent exhumation resulted in relatively high-pressure, greenschist- to amphibolite-facies metamorphism of lower-plate cover units, and in the retrogression of high-grade assemblages in the arc rocks. Postaccretion shortening resulted in folding of both the allochthonous rocks and the footwall basement.