Detachment fault fluid composition, temperature, and pressure from fluid inclusion analysis in the central Grouse Creek Mountains, Utah

Fluid inclusions were trapped in sealed microfractures in quartz in quartz-muscovite veins in and near the Red Butte pluton. The pluton lies in the footwall of the Ingham Pass detachment fault in the Grouse Creek Mountain portion of the Raft River-Albion-Grouse Creek metamorphic complex. The veins crosscut all metamorphic fabrics in the country rock and the pluton and postdate the pluton intrusion. The aqueous fluids determined from temperature measurements contain 1.8 to 5.6 wt percent salt as NaCl and 5–21 mol percent CO2. The inclusions homogenized at 185 °C–290 °C though many of the inclusions leaked or decrepitated before homogenization was reached. Pressure estimates made from the CO2 content of the fluids ranged from a high of 210 Mpa near lithostatic pressure to 60 Mpa near hydrostatic pressure. Thermal history determined by Ar-Ar dating of K-feldspar, biotite, and muscovite show that the temperature cooled to 315 °C by 21 Ma near the upper fluid inclusion temperature. Basin and Range normal faulting is dated at 13.4 Ma. The fluid inclusion characteristics compared to lithostatic pressure gradients indicate that the upper plate of the detachment fault had thinned from the maximum of near 15 km at the inception of the fault to about 8 km by the time of entrapment of the fluid inclusions. Pressure as high as lithostatic could have facilitated detachment faulting if the elevated fluid pressure was present during detachment fault displacement.