Mantle shear velocity (Vs) structure beneath the Transportable Array (TA) in Alaska and northwestern Canada is imaged by joint inversion of Rayleigh wave dispersion and teleseismic S wave travel times. The study connects previously unsampled parts of northern and western Alaska with portions of southern Alaska imaged with earlier seismic arrays. The new Vs tomography shows contrasting lithospheric structure in the plate interior with lower Vs shallow upper mantle indicative of thinner thermal lithosphere south of the Brooks Range and along the transform margin. Higher Vs down to~200 km beneath the Brooks Range and northern coast is consistent with the presence of a cold stable lithospheric root that may help guide intraplate deformation to the south. In the subduction-to-transform transition, a potential slab fragment is imaged beneath the Wrangell volcanic field where modern subduction has slowed due to the thick buoyant crust of the Yakutat terrane.Plain Language Summary We use a groundbreaking seismic data set from the EarthScope project to investigate the structure of the upper mantle beneath Alaska and northwestern Canada to better understand the effects of ongoing subduction and distinctive blocks within the continental lithosphere. Measurements of seismic body and surface waves are used to construct seismic images from the surface down to 800-km depth. The images reveal cold thick blocks beneath northern Alaska and the Yukon Territory adjacent to warmer thinner blocks beneath younger geologic provinces to the south, suggesting that cold strong lithosphere in the north helps guide the extent of intraplate deformation driven by the southern plate boundary. The model also identifies a potential slab fragment beneath the Wrangell volcanic field, suggesting slab contributions to volcanic activity and a growing slab tear.

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Seismic anisotropy can illuminate structural fabrics or layering with length scales too fine to be resolved as distinct features in most seismic tomography. Radial anisotropy, which detects differences between horizontally (V SH) and vertically (V SV) polarized shear wave velocities, was investigated beneath Yellowstone caldera (Wyoming, United States) and Long Valley caldera (California). Significant positive radial anisotropy indicating V SH > V SV and low isotropic velocities, were found beneath both calderas at ~5-18 km depths. The positive radial anisotropy (>8%) volumes beneath the calderas are anomalously strong compared to the surrounding areas. The absence of a similar anisotropic signal in the wake of the propagating Yellowstone hotspot indicates that the radial anisotropy diminishes after the locus of voluminous silicic magmatism moves. We propose that the anisotropic volumes represent sill complexes of compositionally evolved magma, and the magma's seismic contrast with the crust would largely fade upon crystallization. The similarity of magma reservoir anisotropy in varied tectonic settings suggests that such mid-crustal sill complexes may be ubiquitous features of silicic caldera-forming magmatic systems, and that anisotropy should be considered to seismically estimate melt content and mobility. The absence of similar radial anisotropy in the lower crust beneath the calderas suggests lower melt fractions or a transition in the geometry of magma pathways. 1 GSA Data Repository item 2018264, details on data sources, analysis methods, and synthetic tests, including Figures DR1-DR10, and Tables DR1 and DR2