North America's mid-continent rift: a chanced meeting of a plume and a rift

The Mid-Continent Rift System (MCRS) is a 3000-km long failed rift system that formed within the Precambrian continent of Laurentia and nearly split North America apart about 1.1 billion years ago. The MCRS can also be classified as a Large Igneous Province (LIP), made up of two distinct magmatic phases [Stein et al., 2015]. The first, large-scale syn-rift magmatism, is characterized by a large volume of flood basalt that filled a fault-controlled basin. The second, post-rift, phase consists of volcanics and sediment that were deposited in a thermally subsiding basin after faulting ended. This flood basalt filled rift geometry is a special characteristic of the MCRS that is not observed in other presently active or ancient rifts. Hence the MCRS's unusual nature likely reflects the combined effects of rifting and a mantle plume. We investigate this hypothesis with a geodynamic model by fully exploring the parameter space for a range of mantle potential T_p and plume excess T_exc temperatures under different extension scenarios and lithospheric/plume structure.

Without a plume, our rifting models failed to produce any syn-rift melt, but with an elevated T_p, significant amount of post-rift melt was generated via small scale convection. In order to produce both syn- and post-rift volumes of flood basalt over an extended period of time, an initially 150 km thick lithosphere with an elevated T_p had to be sufficiently thinned before the introduction of a thermal plume. The impinging plume head drove partial delamination of the mantle lithosphere beneath the young rift basin and surrounding regions and assisted with melt production during the rifting phase. Increasing the T_p lead to enhanced small-scale convection and prolonged melt production into the post-rift phase. Hence the MCR's extensive volcanism seems to reflect both a mantle plume and increased ambient mantle temperatures during the Neoproterozoic.

Merino, M., Keller, G.R., Stein, S., Stein, C., 2013. Variations in Mid-Continent Rift magma volumes consistent with microplate evolution: Geophysical Research Letters, 40, 1513- 1516.

Stein, C.A., Kley, J., Stein, S., Hindle, D., Keller, G.R., 2015. North America's Midcontinent Rift: When rift met LIP. Geosphere, v. 11, 1607-1616, doi:10.1130/GES01183.1.