INTERPRETATION OF THERMAL HETEROGENEITY FROM SEISMIC WAVESPEED IN THE CONTINENTAL LITHOSPHERE: TOOLS AND LIMITATIONS

Recently-developed toolkits for quantitatively translating seismic wavespeed into variations in thermal, chemical, and mechanical properties present an exciting new horizon in geophysics. In the continental lithosphere, integrating such tools with seismic imaging offers the powerful possibility of channeling an extensive collection of seismic models to investigate the tectonic processes and lithosphere-asthenosphere interactions that shape the surface we live on. However, these tools are new, the results of such analyses depend on technical choices made within the workflow, and there is not yet consensus on best practices for using them. Here we demonstrate the use of interpretive tools within the southwestern United States, where strong contrasts in deformation, seismic wavespeed, and topography invite investigation of the influence of thermochemical variations on seismic wavespeed, and the contributions of asthenospheric flow and partial melt to intraplate tectonic and magmatic activity. We first present a new model of 1-D Vs depth profiles at 0.5˚ spacing across the southwestern US. This model is based on joint inversion of Rayleigh wave phase velocities and Sp receiver functions, which constrain absolute Vs as well as the location and nature of major seismic discontinuities such as the Moho and Lithosphere-Asthenosphere Boundary (LAB). A distinction is observed between the Basin and Range, where Vs is low, lithosphere is thin (60-80 km), and Vs decreases sharply from the lithosphere to the asthenosphere; and the Colorado Plateau, where lithosphere is thicker and seismically faster, and the LAB is complicated and gradual.

Subsequently, we employ two toolkits to estimate temperature from Vs: the Very Broadband Rheology calculator (VBRc), and the Whole-rock Interpretive Seismic Toolbox For Ultramafic Lithologies (WISTFUL). Depending on which tool is used, and how attenuation is parameterized within each, the inferred temperature within the upper mantle varies significantly: on the order of 200˚C. Reducing this uncertainty is essential, especially in the context of evaluating the presence and fraction of partial melting. Further constraints from mineral physics and magma thermobarometry may be useful in moving towards a quantitative thermal interpretation throughout the lithosphere.