Seismic Velocity and Radial Anisotropy in the Crust and Upper Mantle of the Wyoming Craton from Surface-Wave Tomography

Cratons are old continental cores where lithosphere is largely undeformed over a few billion years. In contrast, Wyoming craton underwent pervasive deformation ca. 80–55 Ma during the Laramide orogeny and has been subsequently encroached upon by the Yellowstone hotspot over ~2.0 Ma. This research aims to address the deformation of the cratonic lithosphere and the craton-hotspot interaction using surface-wave tomography. Rayleigh and Love wave data from 82 earthquakes recorded at 103 broadband stations of USArray Transportable Array are used to construct 3-D shear-wave velocity and radial-anisotropy models. Low velocities and positive anisotropy (VSH >VSV) are observed beneath the Yellowstone hotspot and the Cheyenne belt continuously to ~200 km depth and in the central-eastern craton at 115–250 km depth. These low velocities explained by hot temperature and partial melting are probably associated with mantle upwelling. However, strong positive anisotropy in these regions suggests that the shear due to the absolute North American plate motion is the dominant mechanism for mantle deformation. Compared to the plate-motion velocity, mantle upwelling associated with low-velocity anomalies might be too small to be observed. Finger-like projections of positive anisotropy and low velocity extend from the hotspot into the craton at depths of 100-150 km, suggesting that the cratonic lithosphere is thermally eroded by hot plume materials through weak channels at the base of the lithosphere. A high-velocity lid exists under most of the craton to at least 115 km depth, which correlates with positive anisotropy. A low-velocity layer is imaged between 115 km and 190 km, where anisotropy becomes weak. These observations help us deduce the depth of lithosphere-asthenosphere boundary at ~150 km. A high-velocity anomaly trending northeast-soutwest with negative anisotropy (VSV>VSH) extends to 200 km depth in the central Wyoming, indicating mantle downwelling and lithospheric erosion. The overall pattern of velocity anomaly and radial anisotropy suggests complex small-scale mantle convection beneath the craton, which probably developed during the subduction of the Farallon plate and has been reinforced by the Yellowstone hotspot. We propose that the combination of flat-slab subduction, small-scale convection, and hotspot activity can lead to massive destruction of a cratonic lithosphere. These observations help us deduce the depth of lithosphere asthenosphere boundary at ~150 km. A high-velocity anomaly trending northeast-soutwest with negative anisotropy (VSV>VSH) extends to 200 km depth in central Wyoming, indicating mantle downwelling and lithospheric erosion. The overall pattern of velocity anomaly and radial anisotropy suggests complex small-scale mantle convection beneath the craton, which probably developed during the subduction of the Farallon plate and has been reinforced by the Yellowstone hotspot. We propose that the combination of flat-slab subduction, small-scale convection, and hotspot activity can lead to massive destruction of a cratonic lithosphere.