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FLUID-ROCK INTERACTION AND THERMOMECHANICS OF DETACHMENTS: RAFT RIVER, NW UTAH
Stable isotope thermometry using quartz-muscovite mineral pair yields a relatively large (140°C) and smooth temperature variation over the 100 m section, from 485°C at the base to 345°C at the top. White mica δD values as low as -120 per mil, and fluid-inclusions δD values around -85 per mil indicate that mica interacted with a surface fluid at high temperature.
Microstructural analysis of the quartzite mylonite shows regime 2 microstructures and recrystallization by subgrain rotation and grain boundary migration. Quartz crystallographic preferred orientation measured using EBSD (30 samples) reveals strong crystallographic orientation indicating flow in the dislocation creep regime. Symmetrical c-axis cross-girdles suggest that a significant component of coaxial flow (pure shear) occurred during deformation and recrystallization. The recrystallized grain size ranges from 35 to 40 microns in all studied samples. Application of quartzite dislocation creep flow laws lead to rather constant flow stress and the varying temperature imply an increase in strain rate from top to bottom, which is consistent, at least qualitatively, with finite strain distribution in the quartzite.
Based on this study, we envision three factors that could have produced the observed transient thermal gradient preserved in the Raft River shear zone: (1) the dipping detachment system juxtaposes hot rocks from the footwall and cold rocks from the hanging wall, such that a steep transient geotherm is established; (2) coaxial thinning of the footwall shear zone leads to a high thermal gradient; (3) perturbation of the geotherm by the influx of cool surface fluids down to the detachment also steepens the thermal gradient.