Tectonic Implications for a Cordilleran Orogenic Base in the Frenchman Cap Dome, Southeastern Canadian Cordillera

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The Frenchman Cap gneiss dome of the Monashee Complex in the Canadian Cordillera sits in basement rocks of the orogen. It records a stepwise downward disappearance of penetrative deformation indicative of a frozen downward-migrating base of easterly verging Cordilleran shearing. This tectonic setting is incompatible with the commonly held views that gneiss domes of the Canadian Cordillera are extensional core complexes and that the presence of gneiss domes in orogens implies vertical flow. An important structural-time marker in our study is a widely distributed suite of 1850 Ma granite dykes that allow Cordilleran-aged structures to be distinguished from the older structures. The dykes show that only the uppermost 1.5 km structurally thick carapace of basement gneiss records penetrative Cordilleran strain, whereas the lowermost 5 km thick basement section does not and instead preserves a Paleoproterozoic migmatitic gneissosity. Cordilleran high strain in the upper basement carapace is characterized by penetrative easterly verging shear strain on both the westerly dipping and easterly dipping flanks of the dome, whereas Cordilleran deformation in the lower basement is limited to a meter-scale, top-to-the-east shear zone and NNE-trending, upright folds. New and previously published U–Pb data from accessory minerals indicates that the Cordilleran structures formed between 53 and 49 Ma, immediately prior to regional cooling and extension. The dome is interpreted as an incipient upright drag fold developed during top-to-the-east shearing.