FLUID PATHWAYS IN DETACHMENT SHEAR ZONE: INSIGHT FROM FRACTURE AND FLUID INCLUSION ANALYSIS FROM THE RAFT RIVER DETACHMENT SHEAR ZONE (UTAH)

Geochemical evidence demonstrate that the footwall of detachment shear zones is permeated by meteoric fluids, which influences the thermomechanical behavior of the crust. Faulting in the brittle upper crust reduces the local grain size and can increase permeability, enhancing fluid flow into and along fault zones. How surface-derived water gets pumped beyond the brittle-ductile transition is, however, a conceptually challenging physiomechanical process. We present the results of a multi-scale analysis of the Miocene Raft River detachment shear zone (RRDSZ). The RRDSZ is localized in the Proterozoic Elba quartzite, which unconformably overlies an Archean basement complex, and consists of an alternating sequence of white quartzite (~90% quartz, ~10% muscovite) and muscovite-quartzite schist. Previous work demonstrates abundant evidence of chemical, physical, and isotopic water-rock interactions in the RRDSZ. Outcrop structural analysis reveal the presence of two dominant, vertical, conjugate sets of fractures (NE and NW), and three generations of quartz veins. Thin section analyses of the quartzite mylonite reveal the presence of micocracks, fluid inclusion planes, and isolated fluid inclusion clusters. Microcracks tend to be perpendicular to the mylonitc foliation, in an orientation similar to the fractures observed in outcrop. They are also filled with quartz, which has a distinct fabric, not as strong as the mylonitic fabric, suggesting that microcracks healed while the shear zone was still at conditions favorable for quartz crystal plasticity. Fluid inclusion planes are transgranular and occur in two conjugate orientations, at ~60º from the mylonitic foliation. Fluid inclusions are also present as intergranular clusters. The orientation of the fluid inclusion planes, and their host quartz crystal c-axes suggest that they are subparallel to the basal crystallographic planes of quartz. Our results show ductile overprint of brittle microstructures, which suggest that, during exhumation, the detachment shear zone may have crossed brittle-ductile transition repeatedly, providing opportunities for fluid to permeate the detachment shear zone.