LITHOSPHERIC STRUCTURE UNDER THE WILLISTON BASIN FROM MAGNETOTELLURIC MEASUREMENTS

The north central portion of the United States is a geological diverse region that contains a series of Archean and Proterozoic lithologies within the the Superior Craton and Trans Hudson Orogeny. In addition, tectonic events in the Paleozoic and Mesozoic in the western United States helped create the hydrocarbon rich Williston Basin in North Dakota, Montana and southern Canada. In order to investigate the nature of the lithosphere and asthenosphere in this region, recent long-period Earthscope magnetotelluric (MT) data were analyzed and modeled. The MT data were collected on a 70 km grid in portions of the United States and the data within North Dakota, eastern Montana, South Dakota and northern Nebraska were analyzed. 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 3D electrical resistivity structure using the program ModEM. During the inversion process we 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). The resultant 3D models indicate that the Trans Hudson orogeny is associated with a low amplitude electrical resistivity zone that extends to approximately 200 km in northern North Dakota and continues into South Dakota but splits in southern South Dakota, going both SW and SE into Nebraska suggesting the Trans Hudson orogeny continued into Nebraska. Additionally, the Williston Basin is associated with low resistivity values but in the center of the basin there are resistivity boundaries at 70-80 and 30-40 km which agrees with seismic receiver function studies. However, the deeper boundary dies away in the southern edge of the basin in North Dakota. These two layers may represent interfaces the boundary between an eclogitized lower crustal layer. This layer may have contributed to the initial formation of the Williston basin during the Trans Hudson orogeny.