Metamorphic core complexes (MCCs) in the North American Cordillera reflect the effects of lithospheric extension and contribute to crustal adjustments both during and after a protracted subduction history along the Pacific plate margin. While the Miocene-to-recent history of most MCCs in the Great Basin, including the Raft River-Albion-Grouse Creek MCC, is well documented, early Cenozoic tectonic fabrics are commonly severely overprinted. We present stable isotope, geochronological (⁴⁰Ar/³⁹Ar), and microstructural data from the Raft River detachment shear zone. Hydrogen isotope ratios of syntectonic white mica (δ²H_ms) from mylonitic quartzite within the shear zone are very low (−90‰ to −154‰, Vienna SMOW) and result from multiphase synkinematic interaction with surface-derived fluids. ⁴⁰Ar/³⁹Ar geochronology reveals Eocene (re)crystallization of white mica with δ²H_ms ≥ −154‰ in quartzite mylonite of the western segment of the detachment system. These δ²H_ms values are distinctively lower than in localities farther east (δ²H_ms ≥ −125‰), where ⁴⁰Ar/³⁹Ar geochronological data indicate Miocene (18–15 Ma) extensional shearing and mylonitic fabric formation. These data indicate that very low δ²H surface-derived fluids penetrated the brittle-ductile transition as early as the mid-Eocene during a first phase of exhumation along a detachment rooted to the east. In the eastern part of the core complex, prominent top-to-the-east ductile shearing, mid-Miocene ⁴⁰Ar/³⁹Ar ages, and higher δ²H values of recrystallized white mica, indicate Miocene structural and isotopic overprinting of Eocene fabrics.