Shear criteria in the Grenville Province, Ontario, Canada

Abstract

The Grenville Province of Canada is a major Proterozoic orogenic belt. Mapping and structural analysis shows that the central Ontario segment of this belt is composed of a stack of imbricated thrust nappes separated by mylonite zones. The mylonite zones were generated at high metamorphic grades and contain a distinctive assemblage of mesoscopic and microscopic asymmetric structures. These structures include rotated tectonic inclusions, rotated boudins and pinch-and-swell structures, sheath folds, rotated single-crystal porphyroclasts, single crystal ‘fish’, and shear band foliation. The shear asymmetry indicated by these structures is highly consistent between members of the assemblage, and over a large area of the Grenville province, and shows that the thrust nappes were transported to the northwest. The consistency of the structures is thought to be due to cyclical dynamic recrystallization leading to steady-state foliations in the mylonites.