KINEMATIC EVOLUTION OF A WEST-VERGING FOLD NAPPE IN THE GEORGETOWN THRUST FOOTWALL, SW ANACONDA RANGE, MONTANA, USA

New geologic mapping in the footwall of the Late Cretaceous Georgetown thrust has revealed that a west-verging fold nappe likely deforms Eocene-aged intrusive rocks. 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 (AMCC). 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 and Paleozoic passive margin 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 upper limb of the F₁ Fishtrap Recumbent Anticline (FRA), a hypothesized tens-of-kilometer-scale NW-verging fold nappe exposed in the Anaconda and Flint Creek ranges. To test this interpretation, 1:24,000 scale geologic mapping was conducted in the Carpp Ridge 7.5’ quadrangle to document structures, determine structural style, and determine kinematic evolution. A pervasive S₁ cleavage that is axial planar to mesoscopic SW-verging folds was documented throughout the field area and may be related to development of the FRA. A locally developed S₂ crenulation cleavage further deforms S₁, likely due to F₂ macroscopic N-trending folds that refold the FRA. New geochronologic data from cross-cutting dikes indicate that both S₁ and S₂ are likely Late Cretaceous in age, however, a 51.88 Ma mylonitic granodiorite with a foliation axial planar to the FRA hinge may indicate reactivation of the nappe at the onset of Eocene extension in the AMCC. This could be explained by contraction perpendicular to SE-directed extension in the core complex, although more data is needed to corroborate this.