Crustal Thinning, Fluid Flow, and the Preservation of a High Transient Geotherm in the Raft River Detachment, NW Utah

The Raft River metamorphic core complex is bounded to the east by a Miocene detachment that localized in a Proterozoic quartzite which rests unconformably on Archean crystalline basement. Well-exposed sections of this quartzite allow detailed sampling of the approximately 100 m thick quartzite mylonite. The quartzite uniformly displays regime 2 microstructures and the recrystallized grain size concentrates narrowly between 35 and 40 microns in all samples. Quartz crystallographic preferred orientation measured using EBSD (15 samples) shows symmetrical patterns indicative of dominant pure shear.

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 (7 samples). Delta D of white mica yields values around -120 to -125 per mil, which indicates that mica interacted at high T with a surface fluid. The varying temperatures and the rather constant flow stress imply an increase in strain rate by two orders of magnitude from top to bottom (from flow law), which is inconsistent with the finite strain distribution in the quartzite. This suggests that deformation was diachronous and involved acceleration downward through the detachment zone, with strain rate scaling to temperature to preserve the same stress (strength of the extending crust). The question of whether a 140°C /100 m gradient ever existed during the development of the detachment is open, but numerical modeling (using the Ellipsis code) of rapid crustal extension does not preclude the existence of very large transient geotherms that can be preserved by rapid exhumation. The quartzite was substantially thinned in the detachment and the circulation of surface fluids within the detachment may have a cooling effect that promotes the high transient geotherms, as long as hot material is advected from below by continued thinning.