Variations in Seismic Wave Speed and <i>V</i><sub><i>P</i></sub>/<i>V</i><sub><i>S</i></sub> Ratio in the North American Lithosphere
E. M. Golos, Hongjian Fang, Robert D. van der Hilst
Abstract
Abstract 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.