Preservation of an extreme transient geotherm in the Raft River detachment shear zone

Abstract

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.