Mantle Water Fugacity is the Dominant Factor in Total Strength and Stability/Mobility of Continental Lithosphere

Abstract:

More than half a century after the plate tectonic revolution, the physical mechanism that distinguishes tectonically active plate boundaries from stable continental interiors remains nebulous. Rock flow strength and mass density variations both contribute to stress, so both are certain to be important, but these depend ambiguously on rock lithology, temperature, and concentrations of water. High seismic velocities observed to great depths often are interpreted as evidence that geothermal variations dominate patterns of lithospheric strength. However, mantle seismic velocities are sensitive to flow-induced anelastic attenuation as well as to temperature. A more ductile mantle will propagate waves more slowly regardless of whether low viscosity is a consequence of high temperature or of high water fugacity, complicating interpretations of seismic velocity in the absence of other constraints. Here we use EarthScope’s USArray seismic data to independently constrain crustal thickness, bulk crustal lithology and Moho temperature of the lithosphere, and magnetic bottom measurements to refine the crustal geotherm. Strength models based on these quantities are then compared to integral measurements of western U.S. isostatic strength expressed as effective elastic thickness, T_e. We show that mantle water is the primary factor that distinguishes stable lithosphere of North America’s cratonic interior from actively deforming zones in the western U.S. Cordillera. Seismic and magnetic constraints on temperature and lithology variations can be reconciled with integral strength measurements only if water fugacity within the lithospheric column is permitted to vary from near-saturation in deforming, mobile lithosphere to nearly completely dry in the stable cratonic interior.