Fluorine concentrations in geothermal springs as an indicator of rheological weakening promoting active deformation in Western North America

Geothermal fluids can have elevated fluorine concentrations, particularly over regions of upwelling mantle convection. Thus, fluorine (and fluorite) may be an important indicator of present and past fluid flux within the crust and upper mantle. Typically, seismic velocity data are utilized to detect temperature and hence rheological variations of the upper mantle and lower crust. However, detecting the presence of fluids is more difficult using seismic methods alone. We use GPS observations to obtain a model estimate of a continuous strain rate and velocity field within the Western U.S. We solve the force-balance equations for vertically integrated horizontal deviatoric stress, which are then used to obtain a variable distribution of effective lithospheric viscosity. The depth integral of viscosity is separately calculated using constraints from seismic velocities. The two viscosity field estimates show marked differences within Arizona and the Rio Grande rift regions, where the seismically defined model shows similar effective viscosity values to areas experiencing higher strain rates within the Great Basin. We find that differences between the two viscosity models can be reconciled if the fluid content within the upper mantle lithosphere below the actively straining regions of the Great Basin is elevated relative to areas along the edges of the Colorado Plateau and within the Rio Grande Rift, which are both relatively dry in comparison to the Great Basin. Present-day fluorine concentrations measured within geothermal springs of the Great Basin have a strong correlation with transtensional strain rates, along with a strong correlation with predicted distributions of expected elevated fluid content in the mantle. This suggests that fluorite deposits may provide a proxy for paleo-fluid input into the crust, which enhanced strain rates by decreasing the effective viscosity of the lithosphere. Fluorite deposits are found in the Basin and Range along zones that have experienced large amounts of crustal extension. U-Pb dating of these fluorite deposits holds the opportunity to test the hypothesis that paleo-fluid input can explain the timing of lithospheric weakening and the extensional collapse of the paleotopography (Nevadaplano and Mogollon highlands).