A GEOPHYSICAL ANALYSIS OF THE GREAT FALLS TECTONIC ZONE, MONTANA USA

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 have facilitated the collision between the Archean Wyoming and Hearne cratonic structures. Previous geophysical studies include seismic refraction data across the boundary between the Paleoproterozoic GFTZ and Archean Wyoming craton (WC) which indicates that the lower crustal thickens as it dips beneath the boundary towards the WC. To aid in determining the nature of the GFTZ, an analysis of gravity, magnetic and EarthScope magnetotelluric (MT) data was undertaken. Bouguer gravity anomaly, magnetic, and residual (band-pass filtered) gravity and magnetic maps were constructed and illustrate the broad location of the tectonic boundaries. To further define the nature of the GFTZ and the tectonic boundaries, two north-south trending gravity, magnetic and MT models were constructed across the untectonized central region of the GFTZ using constraints from existing geologic, petrologic and active source seismic studies. Coupled with the newly developed residual gravity and magnetic anomaly maps, the models provide otherwise unavailable constraints on the extent and geometries of the GFTZ/WC boundary region including the presence of a southward subducting anomaly not previously imaged, as well as providing limits on tectonization within the GFTZ. Additional geophysical analysis of these features may substantiate the findings and help re-define the subsurface extent of the GFTZ/WC boundary.