ORIGIN OF THE MID-CONTINENT RIFT AND MAGMA VOLUMES

The 1.1 Ga Mid-Continent Rift (MCR) has two major arms meeting in the Lake Superior region. One extends southwestward at least as far as Kansas, and the other extends southeastward through Michigan. Gravity modeling shows that the west arm has significantly more magma and that magma volume along this arm increases toward Lake Superior. Thus spreading was greater along the western arm and decreased southward, consistent with the arms being boundaries of a microplate rotating with respect to the Superior province with its rotation pole to the southwest. This tectonic setting can be viewed by analogy with younger and morphologically similar rift systems, whose tectonic settings are more easily understood because their surroundings - including seafloor with magnetic anomalies - have not been deformed or destroyed by subsequent collisions and rifting. The West and Central African rift system (WCAR) formed as part of the Cretaceous regional extension during separation of Africa and South America. Once seafloor spreading was established along the entire margin, extension soon stopped within Africa, leaving a failed rift. Shortly afterwards, ~85 Ma, motion reversed on some of the normal faults of the WCAR, probably associated with compression due to the collision of Africa with Europe. While it was active, the MCR likely resembled the present East African rift system, which has been extending for ~30 my, with one end in the Gulf of Aden and Red Sea having largely evolved to seafloor spreading, while the other has diffuse continental extension. Thus rather than the MCR being an isolated, failed, extensional zone within a stable continent, it may have been part of a larger rift system formed during a short extensional phase within the ~1.3-~0.98 Ga Grenville orogeny, perhaps extending eastward of the Grenville Front. This extensional phase would have ended on the MCR before the continental collisions that produced the Grenville front, the most inland Grenville deformation. Later reverse faulting on the MCR normal faults due to compression, perhaps from collisions around Rodina's margins, would not be unexpected because the MCR would be a weak intraplate zone due to higher crustal temperatures and faults. This reactivation might be similar to the Paleozoic deformation in the central US due to collisions at the margins of North America.