Variations In Seismic Wave Speed And V-P/V-S Ratio In The North American Lithosphere

Seismic wave speed is controlled by a number of factors, including temperature and chemical composition, as well as the presence of volatiles and partial melt. Tomography provides a powerful constraint on wave speed variations, but if only V-P or V-S variations are imaged, it is challenging to separate the competing effects of these factors and make a full interpretation of seismic anomalies. In this study, we generate models of variations in the V-P/V-S ratio, which introduce new constraints on geologic structures, compositions, and processes. We invert P and S wave arrival times, as well as Rayleigh wave phase velocities, utilizing the sensitivity of Rayleigh waves to both V-P and V-S to form mutually constrained but independent models of V-P and V-S structure at lithospheric depths below the continental United States and Southeastern Canada. From this we can examine variations in V-P/V-S, highlighting a distinct pattern of anomalies which are less readily observed in V-P or V-S alone. A clustering analysis is performed to relate 1-D profiles of wave speed as a function of depth to tectonic provinces. While the first-order structure of V-P and V-S appears to be dominated by the thermal contrast between the Eastern and Western United States, the strongest control over V-P/V-S ratio perturbations within the mantle lithosphere appears to be the presence of melt. Certain higher-V-P/V-S anomalies within the cratonic interior may reflect compositional anomalies and variations in Moho structure. This work provides a continental-scale framework for future quantitative analyses of thermal and compositional heterogeneity, and for targeted geologic interpretation.