Inference of thermodynamic state in the asthenosphere from anelastic properties, with applications to North American upper mantle

Inference of thermodynamic state and full-spectrum mechanical behavior of the lithosphere and asthenosphere is a central problem in geophysics, implicating our understanding of the convection patterns, transient responses and chemical composition of the planet. Anelasticity is responsible for significant relaxation of stress associated with seismic wave propagation in the asthenosphere, while irreversible transient creep may be important in the lithosphere. This paper focuses on the processes that may act at the time scales of seismic wave propagation, and current questions in the effort to determine the dependence of these effects on thermodynamic state. We introduce a free code library, the “Very Broadband Rheology calculator” (VBRc), designed to calculate frequency-dependent mechanical properties and easily compare different constitutive models favored by different laboratories. The methods operate only in the forward sense, starting with arrays of models of thermodynamic state, proceeding to arrays of mechanical properties. These calculations are incorporated into a Bayesian framework to infer variation in mantle thermodynamic state from Vs and Q, applied here to four locations in Western North America. The results demonstrate how well we can constrain the state, given the input models and the measurements. Results for sites in the Basin and Range, Colorado Plateau and interior craton east of the Rio Grande separate into distinct state variable ranges consistent with their tectonic environments.