Large-scale flow toward low-velocity anomalies reconciles seismic and geodynamic constraints in the deepest mantle beneath Alaska

Jonathan Wolf 

Deep mantle downwellings are typically located away from the two Large Low-Velocity Provinces (LLVPs) in Earth's mantle. Geodynamic models based on global seismic tomography generally predict that convective flow at the core-mantle boundary (CMB) spreads laterally away from downwelling regions and towards LLVPs. While this offers a framework for understanding large-scale deformation in the lowermost mantle, it has yet to be confirmed by seismic constraints. This study investigates seismic anisotropy and wave reflections in the deepest mantle beneath Alaska, linking a known reflector to the inferred deformation. To capture large-scale deformation, the analysis utilizes waves with long raypaths through the deepest mantle. Mantle shear direction is then determined using global wavefield simulations that incorporate mineral physics constraints. The inferred North-South shear direction agrees with findings for an adjacent region beneath the northeastern Pacific Ocean, together forming a continuous ~3000 km x 3000 km region where the inferred southward shear aligns with geodynamic model predictions. These results support a first-order framework of lowermost mantle deformation driven by flow away from downwellings and toward LLVPs.