Contrasts in the response to dextral transpression across the Quetico–Wawa subprovince boundary in northeastern Minnesota

Abstract

Folds and shear zones in the central and western Vermilion district greenstone belt (Wawa Subprovince) are consistent with deformation in a regime of regional dextral transpression. However, the shear strain associated with this deformation regime is only locally evident in the schist and migmatite of the adjacent southeastern Vermilion Granitic Complex (VGC) (Quetico Subprovince). We suggest that differential partitioning of shear and flattening strains between the two terranes was controlled by the lower ductility, higher competency contrast, and steeper dip of orogen-parallel rock units in the Vermilion district compared with the variably oriented rocks in the adjacent VGC. Contact strains adjacent to plutonic bodies played a greater role in the development of complex fold geometries in the VGC.In the central Vermilion district, northwest-directed subhorizontal shortening produced easterly striking zones of brittle-ductile dextral shear and northeast-trending en echelon folds. The en echelon folds with curvilinear axial traces formed in the greenstone and local iron formation layers. These units were steepened during folding and early stages of deformation and acted as rigid competent units during subsequent shearing. The shear zones were concentrated in units of tuff and graywacke between the more rigid, steeply dipping greenstone units.In the adjacent southeastern VGC, the same regional stress regime resulted primarily in folding with only rare indications of ductile dextral shear. The transpression event produced westerly trending F2 folds along the southern margin of the VGC. However, the F2 folds have more varied orientations along the margins of syntectonic to late-tectonic granitic plutons and were locally refolded by easterly trending F3 folds during continued northwest-directed shortening.