An evolutionary model of the western churchill province and western margin of the superior province in Canada and the North-Central United States

Abstract

Regional-scale geophysical information, which includes aeromagnetic, gravity, seismic refraction, multi-channel seismic reflection and electromagnetic induction data, is used to extend our knowledge of the Canadian Shield beneath the Phanerozoic Williston basin of south-central Canada and the north-central United States. A new tectonic map based on this information shows the Proterozoic Flin Flon-Snow Lake and La Ronge-Lynn Lake volcanic island arcs and their associated fore-arc (Kisseynew belt) and back-arc (Reindeer-South Indian Lakes belt) basins wedged between the Archean Superior craton on the east and the Archean parts of the Churchill and Wyoming cratons on the west. Along the western margin of the Superior craton the Thompson nickel belt, including its extension southwards beneath the Williston basin, is interpreted to have been successively the site of continental rifting and rupturing, an evolving continental margin, a continent-volcanic island arc “suture” zone and eventually a continental-scale strike-slip fault. The North American Central Plains electrical conductivity anomaly and closely related seismic low-velocity zones are explained by the presence in the lower crust of buried slices of hydrated oceanic-type material, situated within the southward extension of the Reindeer-South Indian Lakes remnant back-arc basin and adjoining tectonic units. A new plate tectonic model is proposed for this region that involves the rifting and rupturing of the Archean continents and the opening and closing of one or more oceanic basins. This model is shown to be consistent with most of the geological, geophysical and geochronological data that pertains to the Proterozoic evolution of the exposed Shield and similar geophysical data and subsurface geochronological information from further south.