THE MID-CONTINENT RIFT – HOW FAR DOES IT GO?

Based on strong, linear gravity anomalies, the 1.1 Ma old Mid-Continent rift (MCR) is as much as 3000 km long. At a minimum, it extends from Central Kansas to its outcrops and greatest geophysical expression in the Lake Superior region, through the remainder of the Great Lakes, to eastern Ohio. Based on seismic, gravity, and magnetic studies, the amount of magmatic modification of the crust in the Lake Superior region is so great that, at best, 25% of the pre-rift crust is still present. Based on geophysical evidence, the arms of the MCR have also been inferred to extend across the central portions of Oklahoma, Ohio, Kentucky, and Tennessee. Volcanic rocks with ages of ~1.1 Ga age are also found to the south in West Texas and New Mexico, and the Central Basin Platform in the Permian basin is cored by a mafic intrusion of this same age. To the east, the nature of the interactions between the MCR and the Grenville orogen remain unclear. Overall, the MCR is easily comparable in degree of magmatic modification of the crust, structural style, geographical extent, and syn-rift basin formation to the East African rift. Much of the western arm of the MCR experienced strong tectonic inversion not long after the cessation of rifting. Centered in Iowa, a large horst block formed and extends to north and south. Further south, it is reasonable to infer that the Nemaha uplift structures in southern Kansas and central and northern Oklahoma represent rift structures reactivated as transpressive structures during Pennsylvanian intraplate deformation.