MAGNETOTELLURIC ANALYSIS OF THE LITHOSPHERIC STRUCTURE IN EASTERN BRANCH OF THE MIDCONTINENT RIFT SYSTEM IN MICHIGAN, OHIO AND NORTHERN KENTUCKY

The well known Proterozoic Midcontinent Rift system (MCRS) is mainly known from outcrops in the Lake Superior region and a high amplitude gravity and magnetic maxima that extends from Lake Superior into northern Oklahoma. Recent Earthscope-based seismic and magnetotelluric (MT) analyses have further defined the lithospheric structure of the MCRS. Less known is that the MCRS extends southeastward from the Lake Superior region through Michigan, Ohio and into northern Alabama. This extension is based on a series of discontinuous linear gravity maxima thoughout this region and seismic reflection studies in Ohio. In order to better define the lithospheric and asthenospheric structure of this eastern branch of the MCRS, we analyzed Earthscope MT stations in Michigan, eastern Indiana, western Ohio, and northern Kentucky. The Earthscope MT data were collected on an approximately 70 km grid with the data collected for approximately three weeks. The data were remote referenced and processed into impedance tensors that provided data out to approximately 20,000 seconds. The data were analyzed for dimensionality using various methods (e.g., regional strike, phase tensor) and then the impedance data were inverted for an electrical resistivity structure using the program ModEM. The inversion process used a step method, first inverting the off diagonal impedance components and eight periods, then the full impedance tensor and twenty four periods and finally adding the vertical electrical field components (tippers). Preliminary models indicate a thinned lithosphere under portions of the eastern branch, with the thinning the greatest in central Ohio. Additional modeling of the data help confirm these results.