Deformation at an Archean subprovince boundary, northern Minnesota

Abstract

Structural analysis in the northern margin of the Quetico subprovince (part of the Archean Superior Province of the Canadian Shield) in Minnesota reveals that the main deformation involved polyphase folding (F1 recumbent and nappe-like, and F2 upright, east–west trending, and tight to isoclinal) during regional ductile transpression and amphibolite-facies metamorphism. A younger deformation, developed during the latter stages of regional transpression, resulted in the generation of localized ultraphyllonites along the steeply dipping Rainy Lake – Seine River fault (RLSRF), the major fault separating the Quetico subprovince from the Rainy Lake wrench zone (a wedge-shaped block between the Quetico and Wabigoon subprovinces). The transpression involved north–south shortening and east–west dextral shear. The presence of shear zones in amphibolite-facies wall rock south of the fault and in lower grade ultraphyllonites within the RLSRF suggests that localization of shear occurred by work and (or) reaction softening, possibly enhanced by the influx of fluids during regional cooling. The youngest structures in the wall rock are conjugate brittle faults oriented similarly to the youngest ductile shear structures in the RLSRF, indicating that the zone of transpression widened following the stage of strain localization, possibly due to work hardening during continued regional cooling. Widening of the zone of deformation was accompanied by an increase in the relative intensity of the north–south shortening component of transpression, revealed by chloritized necks of boudinaged quartz ribbons, quartz and calcite microfabrics, and flattening strains. Protracted ductile flow and localized greenschist-facies conditions in the RLSRF, which occurred during widening of the zone of deformation and rotation of the kinematic frame (to produce north–south shortening structures), are best explained by an influx of fluid phases.Mesostructures and quartz microfabrics in late tectonic (but synkinematic) peraluminous leucogranitoid intrusions and host schist 10 km south of the RLSRF record north–south shortening, but not east–west dextral shear, and further support late north–south shortening across the RLSRF.Tectonic settings for the RLSRF include (i) a suture between distinct lithotectonic terranes or (ii) a zone of localization of deformation within the northern margin of the Quetico subprovince following collision between the Quetico and Wabigoon terranes.