STRUCTURE AND TECTONIC EVOLUTION OF THE BELT BASIN, MONTANA/IDAHO FROM GEOPHYSICAL CONSTRAINTS

The Middle Proterozoic Belt Basin, located in Montana, Idaho, Washington, and British Columbia, is one of the deepest basins in North America. Although the western part of the basin was removed during late Proterozoic rifting and development of the western North American passive margin, it remained largely undeformed until the Mesozoic. Magnetotelluric (MT) data were collected along a 140 km transect across the central Belt Basin from near Bonners Ferry, Idaho, to near Kalispell, Montana. A portion of the transect is coincident with earlier deep-crustal seismic reflection profiles (COCORP). A 2-D resistivity model was generated from the MT data down to 50 km depth. The model is characterized by two highly conductive subhorizontal horizons, a shallow eastern layer at 10-15 km depth and a partially overlapping, deeper western one at 25-35 km depth. The shallower eastern layer underlies a resistivity/seismic velocity stratigraphy that can be correlated with lower Belt (Prichard-Ravalli Groups) and upper Belt (Middle Belt Carbonate-Missoula Groups). This layer is inferred to correlate with sub-Prichard strata. The deeper western layer fits in a similar geophysical stratigraphy and is interpreted to represent the same stratigraphic horizon as the eastern layer. Using the geophysical and surface geologic data, a structural section has been constructed in which the Purcell Trench fault (PTF) can be traced for 100 km, dipping 25-30^o east down to 20 km depth. Restoration of the Belt/basement contact infers 27 km of Eocene top-to-the-east detachment along the PTF. Reversal of this displacement reveals the crustal structure as it was at the end of late Mesozoic Cordilleran thrusting with thrust-repetition of the conductive horizon. Reconstruction of the conductive horizon implies about 220 km of Cordilleran thrust shortening. 3-D inversion of additional MT data suggests the high-conductivity layer extends basin wide. The enhanced conductivity is attributed to a few percent of interconnected sulfides. These sulfides may have formed in a shallow-marine setting as a result of microbial-mediated sulfate reduction. Alternatively, they may originate from a large sedimentary exhalative event, similar to, but preceding that which formed the world-class Sullivan deposit in the northern Belt Basin.