DEFORMATION AND FLUID FLOW IN THE RAFT RIVER DUCTILE DETACHMENT, NW UTAH

The Raft River detachment offers excellent sections of detachment mylonites that concentrate in a mineralogically simple (quartz + white mica) quartzite. The quartzite mylonite was deformed in the regimes 2 and 3 of dislocation creep and shows extensive dynamic recystallization by subgrain rotation and grain-boundary migration. C-axis fabris have been studied using Computer Integrated Polarization microscopy. Tens of samples across the approx 50 m thick quartzite mylonite show a large degree of preferred orientation, with a general trend of more noncaxial fabric at the base, grading into more coaxial fabrics upward, although variations exist. The main intracrystalline slip systems are the basal-a, rhomb-a, and prism-a, as indicated by girdles developed normal to lineation.

Syn-deformation fluid flow is indicated by the presence of variably deformed and transposed vein sets that are concentrated within a reddish to brick color foliation-parallel layer. In addition, somes zones within the quartzite are enriched in white mica (up to 90%), suggesting that quartz was removed by dissolution. Hydrogen isotopes in white mica indicate that the fluids that interacted with the rock was meteoric with a low delta D of approx -120 per mil. Oxygen isotope values in quartz are 10 to 12 per mil, but these values are substantially reduced locally. Oxygen isotope thermometry based on quartz-mica pairs provide temperature information for sets of microstructures and c-axis fabrics. Under those conditions, the bulk dynamic permeability of the quartzite mylonite is approached through the isotopic shifts from the known initial fluid composition, and from an assessment of the duration of deformation approached through argon geochronology.