THE MIDCONTINENT RIFT, NORTH CENTRAL USA

The Midcontinent Rift System (MRS) of central North America is a Proterozoic flood basalt province formed along a 2000 km continental rift activated by the arrival of a mantle plume approximately 1.1 Ga ago. The MRS province consists largely of mafic igneous and overlying sedimentary rocks with a thickness of at least 30 km in the vicinity of Lake Superior where it is exposed at the surface. Although most surface exposures of igneous rocks occur around Lake Superior the rift extends beneath Phanerozoic sediments SW from Lake Superior through Iowa into Nebraska and Kansas. Shorter arms extend SE into Michigan and N into Ontario. The thick volcanic section is intruded by numerous small dikes and sills as well as the large Duluth layered complex. Exposures of the intrusions are best in and near Duluth and along the north shore of Minnesota with Lake Superior. The geochemistry of mafic lavas, dikes, and sills is governed largely by the interaction of plume melt with adjacent mantle.