MAGNETOTELLURIC ANALYSIS OF THE MIDCONTINENTAL RIFT IN IOWA

The Proterozoic Midcontinent Rift System (MCRS) formed 1.1 Gya as the result of a major tectonic event in the North American lithosphere. The 2,500 km MCRS spans from eastern Oklahoma to Lake Superior before curving southeast through Michigan, possibly stretching as far south as Alabama. Phanerozoic sedimentary cover prevents direct sampling of 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 gravity and magnetic anomalies define the rift due to its large volume of mafic rocks. Many questions remain about the origin of the MCRS. Some proposed theories include 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) that investigated the lower crust and upper mantle structure of the rift. To further investigate this structure an analysis of EarthScope magnetotelluric (MT) data was undertaken. The data are long period (up to 20,000 seconds) collected from 41 stations with 70 km spacing, all located within Iowa and northeast Nebraska. The data were analyzed for dimensionality and geoelectric strike, then TE and TM mode data rotated for two-dimensional resistivity inversion along four profiles. A three-dimensional resistivity inversion was also completed using the ModEM software. The final two- and three-dimensional models will be compared here with previous gravity, magnetic, and broadband seismic tomographic models from the Earthscope seismic experiment within Iowa.