Patterns of pervasive shear strain near the boundaries of the La Ronge Domain, inner Trans-Hudson Orogen, western Canadian Shield

Abstract

The Saskatchewan segment of juvenile Paleoproterozoic crust, inner Trans-Hudson Orogen (Reindeer Zone), is widely regarded as a NW-dipping stack of litho-structural domains. The metavolcanic-metasedimentary La Ronge Domain (LRD), whose least deformed portion has the geological map pattern of an Archean greenstone belt, is underlain by metasedimentary rocks of the Kisseynew Domain and overlain by migmatitic rocks of the Rottenstone Domain. In easily accessible parts of LRD boundary walls, the horizontal patterns of tangential (boundary-parallel) strain accumulated under peak metamorphic conditions are investigated. Utilizing the strain patterns and recently published reflection seismic profiles, we estimated the direction and derived the sense of tangential shear-strain components on various horizontal scales. In all parts of boundary walls investigated, the dip shear has the sense of ductile thrusts, but the strike-shear sense varies with position, on scales ranging from 100 m to 10 km. The regional persistence of thrust shear in both LRD walls accords with tectonic scenarios of protracted horizontal convergence of juvenile crust, after initial collision between the Archean Superior and Hearne continents. The patterns of strike shear, on the other hand, are difficult to reconcile with indentation hypotheses or counter-clockwise oroclinal rotation.