The Montana Alps: kilometer-scale recumbent folding and tectonic attenuation in the Anaconda Range, southwestern Montana

Abstract

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 strata. While the tectonic nature of this thinning is generally accepted, the mechanisms behind it remain enigmatic. Geologists from the Montana Bureau of Mines and Geology hypothesize that footwall strata were attenuated along the upper limb of the Fishtrap recumbent anticline (FRA), a kilometer-scale NW-verging recumbent fold exposed throughout the southwestern AMCC footwall. New geologic mapping in the Carpp Ridge 7.5' quadrangle and U-Pb geochronology better constrain the nature and timing of tectonic attenuation in this complex area. Two generations of folds deformed rocks in the quadrangle: F 1 recumbent folds with S 1 axial planar fabrics associated with the FRA, and F 2 upright folds with S 2 axial planar fabrics that refold the FRA. These deformations are likely Late Cretaceous in age based on dates from cross-cutting intrusions, although a foliation sub-parallel to S 1 in a 51.87 Ma granodiorite stock in the FRA hinge suggests localized Eocene deformation. Elsewhere in the field area, the same granodiorite crosscuts S1 fabrics. F 1 folds and S 1 fabrics transpose, attenuate, and omit Belt strata in the southeastern portion of the quadrangle, suggesting that recumbent folding is intimately associated with tectonic attenuation. Further, west-vergent F 1 and F 2 folds may be decoupled from regional east-vergent tectonics and instead related to gneiss-doming in the ~75-74 Ma Lake of the Isle shear zone. Gneiss-doming and associated development of the FRA may have been driven by widespread decompression of the Cordilleran middle crust during Late Cretaceous time, perhaps in response to delamination of lithospheric mantle or arc-root foundering beneath the Cordilleran magmatic arc of SW Montana.