Tectonic setting of the Midcontinent Rift system

Abstract

The continental lithosphere which hosts the Midcontinent Rift (MCR) system comprises a crustal assemblage ranging in age from 3.6 to 1.5 Ga. The most voluminous igneous activity along the MCR occurred within Late Archean (2.7 Ga) crust, presumably due to magmatism associated with a rising mantle plume. Two major arms of the rift extend to the southeast and the southwest, where rifting, magmatism, and sedimentation were confined to a 2000 km long rift that cuts through the various crustal provinces ranging in age from 3.6 to 1.5 Ga. Tensional reactivation of pre-existing crustal discontinuities may have influenced the location of parts of the MCR; however, direct cause and effect are difficult to distinguish for want of sufficient exposures. The most active phases of magmatism and rifting occurred between 1109 and 1087 Ma; a U-Pb age of 1097 Ma for the southern end of the MCR shows that magmatic activity was synchronous throughout its length. There is evidence of rifting and magmatism prior to 1109 Ma, but there is little evidence of magmatism after 1087 Ma; rifting may have effectively ceased by that time. Rifting may have ceased as a result of compressional forces being exerted on the foreland during the Grenville collisional event, spanning 1030-970 Ma, and compressional reactivation of normal faults may have resulted in partial inversion of the MCR. Chemical data and U/Pb and Sm/Nd isotopic data for igneous rocks along the rift are similar to those for MCR rocks in the Lake Superior area, which are best explained in terms of a mantle-plume model. However, since the distal arms of the MCR were developed in Proterozoic lithosphere and did not involve nearly as much magmatism as in the Lake Superior area, mantle-plume sources may not have been as important in formation of the distal magmas. Data from the buried arms are also consistent with the mafic magmas being derived by decompression melting of Proterozoic lithosphere during rifting.