Metamorphic core complexes (MCCs) are considered to be a hallmark of large-magnitude crustal extension, with their characteristic high-strain mylonitic fabrics attributed to simple-shear strain downdip of a detachment fault. However, some MCCs exhibit pure-shear–dominated mylonitic fabrics temporally decoupled from regional extension, which may be related to magmatically enhanced buoyant doming of the lower crust. We tested the viability of buoyant doming for the formation of MCCs in the North American Cordillera by investigating the kinematics and conditions of mylonitic shear in the Ruby Mountains–East Humboldt Range (REH) MCC. Field observations and geochronology demonstrate an ~10 m.y. gap between midcrustal attenuation and regional extension in the brittle upper crust. Mylonites in the REH record general shear strain with >80% bulk attenuation at strain rates of 10–13 to 10–12 s–1 and temperatures of 400–600 °C. The REH mylonites developed at the culmination of 40–29 Ma magmatism involving mantle-derived mafic intrusions and leucogranite crustal melts prior to post–17 Ma detachment faulting. We posit diapirism driven by thermal and melt buoyancy could have generated shear zones along the diapir flanks at our documented strain rates. Characteristics of the buoyant doming model are expressed in many low- to moderate-melt-fraction MCCs globally, and the pre-extensional high-strain mylonitic fabrics may therefore be an important mechanism for localizing temporally decoupled brittle detachment faulting.