THREE-DIMENSIONAL MAGNETOTELLURIC MODEL OF THE MIDCONTINENTAL RIFT AND PRECAMBRIAN TERRANES IN IOWA

The Precambrian of Iowa consists of Archean-aged metamorphic lithologies, Eastern Granite/Rhyolite Terrane lithologies and the Grenville-aged Proterozoic Midcontinent Rift System (MCRS). The MCRS is a 3000 km long rift that contains two branches (eastern and western) that formed 1.1 Gya as the result of a major tectonic event (e.g., mantle plume) affecting the North American lithosphere. Phanerozoic sedimentary cover prevents direct sampling of the MCRS geology except within outcrops in the Lake Superior area and scattered drill holes. As a consequence, the MCRS is primarily characterized by extrapolations from outcrops and geophysical methods, including gravity, magnetic, broadband seismic, and seismic reflection profiles. Prominent linear gravity and magnetic maximum anomalies define the rift due to its large volume of mafic rocks but gravity and magnetic minima flanked these maxima and reflect rift related sedimentary basins. Many questions remain about the origin of the MCRS including microplate interactions, mantle plumes, and passive rifting associated with the Grenville orogeny. In Iowa, the rift cuts northeast to southwest through the state, which to date has only been investigated geophysically, with the most detailed being a gravity and magnetic study (Almaz and Mickus, 2017) and a seismic reflection study (Anderson, 1992) that investigated the upper crust, lower crust and upper mantle structure of the rift. To aid in determining the origin of the MCRS and its lithospheric structure an analysis of the long period EarthScope magnetotelluric (MT) data was undertaken. The data from 41 stations with 70 km spacing, from Iowa and northeast Nebraska were analyzed. The TE and TM mode data were rotated to electrical strike for two-dimensional resistivity inversion along the four gravity/magnetic profiles of Almaz and Mickus. A 3D resistivity inversion was undertaken using the ModEM software where the impedance tensor were inverted. The final 3D MT model shows that under the MCRS, the lithosphere thins to approximately 120-140 km while to the west of the MCRS it is approximately 200 km thick. The thinned region extends to the east of the known rift suggesting if this thinning is a relic of the Proterozoic rifting, that rifting involved more of the lithosphere in at least Iowa than just under the rift.