DEEP CONNECTION BETWEEN THE EARLY GRENVILLIAN OROGENY AND MID-CONTINENT RIFTING DURING THE LATE MESOPROTEROZOIC

Far-field forces imparted by the Grenvillian orogeny during the late Mesoproterozoic have been associated with cessation and inversion of rifting in the Archean Superior Province and neighboring Paleoproterozoic terranes, located in the present North-American Midcontinent. The Elzevirian or early Shawinigan orogeny represents an early stage of the Grenvillian orogeny and has been associated with ~N-directed subduction beneath the southern edge of Laurentia, which was rotated ~90 degrees clockwise from its present orientation, and was moving southwards to the equator, overriding the subducting lithospheric slab. We propose a model in which a mantle water cycle as deep as the transition zone, sustained by subduction during the Elzevirian orogeny, may have been driving rifting in the Midcontinent ~1000 km away from the Laurentian active margin and 100-150 m.y. later, contemporaneously with but unrelated to the late Shawinigan orogeny. This mechanism appears consistent with various geological evidence, as well as with structures imaged geophysically beneath the Midcontinent Rift, such as a magmatic root beneath the crust and a largely re-stabilized Proterozoic mantle lithosphere.