GRAVITY, MAGNETIC AND MAGNETOTELLURIC ANALYSIS OF THE NORTHERN EDGE OF THE WYOMING CRATON, MONTANA

The Great Falls tectonic zone (GFTZ) is located in the northwestern United States mainly in northern and central Montana. The GFTZ is the northern boundary of the Wyoming Craton and the Hearne Craton lies to the north in Canada. The GFTZ is a zone of pronounced northeast- trending high angle faults and lineaments extending from northeast Idaho to Saskatchewan. Several geophysical studies have been performed within the GFTZ and the Wyoming Craton. The most prominent was the SAREX experiment which collected seismic refraction data across the GFTZ and showed that the boundary and they Wyoming Craton has thick lower crustal layer. Our preliminary analysis of the gravity and magnetic fields indicated that the GFTZ is associated with high amplitude, short wavelength SW-trending anomalies. While gravity and magnetic anomalies in the Wyoming Craton which includes residual anomaly maps created by wavelength filtering shows two distinct trends: a southwest-northeast trend possibly associated with Archean basement rocks and structures, and several northwest-trending areas that are associated with Laramide-age basement uplifts including the Wind River, Beartooth, and Bighorn mountain ranges. Additionally, we will analyze the newly acquired magnetotelluric (MT) data from the Earthscope experiment. Three profiles that trend North-South in central Montana indicate that the boundary between the GFTZ and the Wyoming Craton is associated with an electrical resistivity minimum. This preliminary analysis will expand the knowledge of this region and be the basis for more detailed two-dimensional gravity, magnetic and MT modeling.