THE GREAT FALLS TECTONIC ZONE;A BOUNDARY BETWEEN TWO PROVINCES

The Great Falls tectonic zone (GFTZ) is a northeast trending zone of high angle faults and lineaments extending from northeastern Idaho into Saskatchewan, Canada. The GFTZ is believed to mark the collision between the Archean Wyoming and Hearne cratonic structures. Previous geophysical studies of the area have collected seismic refraction data across the boundary between the GFTZ and Wyoming craton which indicated the lower crustal layer thickens as it dips beneath the boundary towards the Wyoming craton (WC). Our preliminary gravity and magnetic data analysis indicates the GFTZ is associated with high amplitude, short wavelength SW-trending anomalies. Gravity anomalies within the WC, including residual anomalies, show several distinctive trends; 1) SW-NE trends possibly associated with Archean basement features, and 2) several NW trending features interpreted to be Laramide-age uplifts. We have collected four new gravity profiles extending across the GFTZ-WP boundary. These four new profiles are the basis of a series of two-dimensional (2D) gravity models, constrained by seismic refraction and broadband data to model the lithospheric structure across the GFTZ. In addition, these gravity models will be integrated with the 70 km spacing Earthscope transportable magnetotelluric (MT) data. 2D inversions of the long period (1-20,000 sec) MT data will provide information on the electrical resistivity structure of the GFTZ and WC that will aid in defining the region’s lithospheric structure. The integration of the new gravity and MT data with the existing geophysical data will provide new information about the nature and origin of the GFTZ-WP boundary.