Strain and permeability gradients traced by stable isotope exchange in the Raft River detachment shear zone, Utah

Combined geochronological and stable isotope data of quartzite mylonite from the footwall of the Raft River detachment shear zone (NW Utah, USA) reveal that an important phase of ductile deformation and infiltration of meteoric water in the shear zone occurred in Miocene time. 40Ar/39Ar release spectra are complex, and plateau ages decrease systematically from 31.1 ± 0.8 Ma at the top to 20.2 ± 0.6 Ma at the bottom of the quartzite mylonite section, capturing a segment of the ∼40–15 Ma geochronologic record that has been documented regionally and is likely related to partial to total overprinting of Eocene white mica 40Ar/39Ar ages in the Miocene. Hydrogen stable isotope values of syn-kinematic muscovite range from −123‰ to −88‰ and suggest that meteoric water infiltrated the detachment shear zone during mica (re)crystallization and mylonite development. Bulk stable isotope analyses from fluid inclusions in quartz support a meteoric origin for the fluid (low D/H and 18O/16O ratios). Quartz and muscovite oxygen isotope analyses show varying degrees of 18O depletion, suggesting spatially variable time-integrated interaction of meteoric fluids with recrystallizing shear zone minerals. The overall pattern of D/H and 18O/16O ratios indicates that fluids were channelized along restricted layers or shear zones within the deforming detachment system. The variability in 18O/16O ratios of both quartz and muscovite and the fluid-rock isotopic exchange results can be explained by variations in the shear zone permeability (confined versus diffuse flow) along with strain variations along the transport direction (from flattening to constriction).