KINEMATIC EVOLUTION OF THE GEORGETOWN THRUST FOOTWALL, WEST-CENTRAL ANACONDA RANGE, SW MONTANA, USA

The Late Cretaceous Georgetown Thrust Zone, located in the west-central Anaconda Range of SW Montana, has been the subject of considerable geologic study over the past century. Deformation within this zone is interesting in that the rocks were exhumed from mid-crustal depths in the footwall of the Paleogene Anaconda metamorphic core complex. As such, it may be representative of deformation that took place at mid-crustal depths in the hinterland region of the North American Cordillera orogenic system during Late Cretaceous time. The footwall of the Georgetown Thrust is host to a structurally complex, tectonically attenuated section of Mesoproterozoic Belt Supergroup strata deformed by layer-parallel, pure-shear dominated fabrics. While the tectonic nature of this attenuation is generally accepted, the mechanisms behind it remain enigmatic. Geologists from the Montana Bureau of Mines and Geology hypothesize that footwall strata were tectonically attenuated along the thinned lower limb of the Fishtrap Recumbent Anticline (FRA), a hypothesized tens of kilometer-scale west-vergent fold nappe exposed in the Anaconda and Flint Creek ranges. To test this interpretation, 1:24,000 scale mapping was conducted in the Carpp Ridge 7.5’ quadrangle to document structures, determine kinematic evolution, and determine structural style. Microstructural analysis, including examination of shear sense indicators and documentation of micro-scale deformation, was also conducted on several samples to further resolve the progression of deformation. Preliminary mapping results indicate the existence of at least three generations of structures in the field area. Future research will assess whether footwall strata were tectonically attenuated by the FRA and whether the FRA is a major structure of regional significance in the North American Cordillera retroarc region.