Volatile and Melt State Fingerprints in In-Situ Rheology

The material state properties that distinguish mobile lithosphere from stable continental interiors remain nebulous more than half a century after the plate tectonic revolution. Rock flow strength depends ambiguously on lithology, temperature, volatile-state and partial melting. Seismic velocities (used to image physical state) and mass density (driving lithospheric strain) are sensitive to the same state parameters so are similarly fraught with ambiguity. Low uppermost mantle seismic velocities observed in mobile belts often are interpreted as evidence that thermal variations dominate lithospheric strength, but mantle velocities are sensitive to viscoelastic attenuation, hydration, and melts as well as temperature. Here we show, with the aid of lithospheric flexural rigidity measurements, that mantle volatile state and its impact on partial melting are primary factors distinguishing stable lithosphere of North America's cratonic interior from actively deforming zones in the western U.S. Cordillera. We use EarthScope's USArray seismic data to independently constrain crustal thickness, bulk crustal lithology and temperature of the lithosphere. Strength models based on these quantities are then compared to integral measures of western U.S. isostatic strength expressed as effective elastic thickness, Te. Seismic estimates of temperature and lithology variations can be reconciled with integral strength measurements if water fugacity within the lithospheric column is permitted to vary from near-saturation in deforming, mobile lithosphere to nearly dry in the stable cratonic interior. However, the inference of water-saturation in Cordilleran mantle conflicts with other observables. Hydration state also influences the solidus temperature, and including rheological effects of mantle partial melting reduces the need for extreme hydration state. The results also indicate melts in the crust and related tectonic stressing effects are important contributors to extremely low Te in rift zones. Relationships of surface heat flow and Pn-derived temperature further suggest widespread influence of melt/volatile flux and hydration reaction thermodynamics in the Cordillera, implying volatile state plays a first-order role in lithospheric strength, stability, and epeirogeny.