Resolving both crustal and shallow‐mantle heterogeneity, which is needed to study processes in and fluxes between crust and mantle, is still a challenge for seismic tomography. Body wave data can constrain deep features but often produce vertical smearing in the crust and upper mantle; in contrast, surface wave data can provide good vertical resolution of lithospheric structure but may lack lateral resolution and are less sensitive to the deeper Earth. These two data types are usually treated and inverted separately, and tomographic models therefore do not, in general, benefit from the complementary nature of sampling by body and surface waves. As a pragmatic alternative to full waveform inversions, we formulate linear equations for teleseismic S wave traveltimes and surface wave phase velocities and solve them simultaneously for variations in shear wave speed anomalies in the crust and upper mantle. We apply this technique to data from USArray and permanent seismic networks and present a model of seismic shear wave speed anomalies beneath the continental United States. Our joint model fits the individual data sets almost as well as separate inversions but provides a better explanation of the combined data set. It is generally consistent with previous models but shows improvements over both body wave‐only and surface wave‐only tomography and can lead to refinements in interpretation of features on the scale of the lithosphere and mantle transition zone.

Cratons, the ancient cores of the continents, have survived thermal and mechanical erosion over multiple Wilson cycles, but the ability of their margins to withstand modification during continental convergence is debated. The Proterozoic Grenville orogeny operated for ≥300M yr along the eastern edge of the proto-North American continent Laurentia, whose age varied north-to-south from ∼1.5−0.25Gyr at the time of collision. The preserved Grenville Province, west of the Appalachian terranes, has remained largely tectonically quiescent since its formation. Thick, cool, mantle lithosphere The authors have no conflicting interests to declare.

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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. Plain Language Summary We present a model of 3-D wave speed variations down to 100 km depth below the continental United States and Southeastern Canada. Our inversion includes data from body waves and surface waves and allows us to constrain both compressional (V P) and shear (V S) wave speed variations. The quantity defined by their ratio, V P /V S , can provide additional information about the chemical, thermal, or other origins of seismic anomalies within the crust and upper mantle. Patterns of V P and V S variations are similar to one another and to previous models, showing fast wave propagation in the Eastern United States and slow propagation in the west, likely controlled mainly by temperature. Meanwhile, V P /V S seems to be sensitive to other processes in the uppermost 60 km, as the east-west dichotomy is not the dominant visual feature. We suggest that partial melting within or at the base of the lithosphere is responsible for the strongest V P /V S ratio perturbations and thus plays an important role in shaping the seismic signature and the dynamics of the lithosphere. Variations in mineral composition of mantle rocks may be responsible for other features in the stable eastern portion of the continent.