KILOMETER-SCALE RECUMBENT FOLDING, TECTONIC ATTENUATION, AND MID-CRUSTAL FLOW IN THE ANACONDA METAMORPHIC CORE COMPLEX, SOUTHWESTERN MONTANA

The Eocene Anaconda metamorphic core complex (AMCC) is the most recently documented metamorphic core complex in the North American Cordillera. While much work has focused on constraining the nature and timing of core complex extension, earlier deformation preserved in its footwall is not as well understood. The AMCC footwall contains an anomalously thin, mid-crustal section of Mesoproterozoic Belt Supergroup and Paleozoic metasedimentary strata. While the tectonic nature of this attenuation is generally accepted, the mechanisms behind it remain enigmatic. Previous workers have hypothesized that footwall strata were attenuated along the upper limb of the Fishtrap recumbent anticline (FRA), a kilometer-scale northwest-vergent recumbent fold exposed throughout the southwestern AMCC footwall. New geologic mapping and igneous U-Pb zircon geochronology in the southwestern Anaconda Range better constrain the nature and timing of tectonic attenuation in this structurally complex area. Two generations of folds were recognized: F₁ recumbent folds associated with the FRA and F₂ west-vergent folds associated with macroscopic upright folds that refold the FRA. F₁ folds and axial planar S₁ bedding-parallel fabrics boudinage, transpose, and omit Belt strata in the southwestern AMCC footwall, suggesting that the FRA tectonically attenuated the Belt-Paleozoic section. This deformation is likely Late Cretaceous in age based on dates from cross-cutting intrusions and may be coeval with the ~75-74 Ma Lake of the Isle shear zone (LISZ). The LISZ may represent a steeply dipping median high strain zone between the northwest-vergent FRA and the pre-74 Ma southeast-vergent Mill Creek fold nappe in the eastern Anaconda Range. Integrated together, these three structures likely represent a double-gneiss dome formed during an episode of Late Cretaceous decompression documented in metamorphic mineral assemblages of the AMCC footwall. This episode is approximately coeval with Late Cretaceous decompression observed in the footwalls of other Cordilleran core complexes, and may have occurred in response to arc-root foundering or delamination of lithospheric mantle.