Extensional detachment systems separate hot footwalls from cool hanging walls, but the degree to which this thermal gradient is the product of ductile or brittle deformation or a preserved original transient geotherm is unclear. Oxygen isotope thermometry using recrystallized quartz-muscovite pairs indicates a smooth thermal gradient (140 °C/100 m) across the gently dipping, quartzite-dominated detachment zone that bounds the Raft River core complex in northwest Utah (United States). Hydrogen isotope values of muscovite (δD Ms ~-100‰) and fl uid inclusions in quartz (δD Fluid ~-85‰) indicate the presence of meteoric fl uids during detachment dynamics. Recrystallized grain-shape fabrics and quartz c-axis fabric patterns reveal a large component of coaxial strain (pure shear), consistent with thinning of the detachment section. Therefore, the high thermal gradient preserved in the Raft River detachment refl ects the transient geotherm that developed owing to shearing, thinning, and the potentially prominent role of convective fl ow of surface fl uids.
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. 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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 18 O/ 16 O ratios). Quartz and muscovite oxygen isotope analyses show varying degrees of 18 O depletion, suggesting spatially variable time-integrated interaction of meteoric fluids with recrystallizing shear zone minerals. The overall pattern of D/H and 18 O/ 16 O ratios indicates that fluids were channelized along restricted layers or shear zones within the deforming detachment system. The variability in 18 O/ 16 O 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).
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 (δ2Hms) 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 δ2Hms ≥ −154‰ in quartzite mylonite of the western segment of the detachment system. These δ2Hms values are distinctively lower than in localities farther east (δ2Hms ≥ −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.
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