Aseismic Deep Slab and Mantle Flow Beneath Alaska: Insight From Anisotropic Tomography

We present high‐resolution 3‐D images of P wave velocity (Vp), azimuthal anisotropy (AAN), and radial anisotropy (RAN) down to 900‐km depth beneath Alaska obtained by inverting a large number of high‐quality arrival time data from local earthquakes and teleseismic events simultaneously. Our results show that the high‐Vp Pacific slab has subducted down to 450‐ to 500‐km depths. A prominent slab gap is revealed at depths of 65–120 km near the Wrangell volcanic field, which is likely a slab tear acting as a channel that provides ascending mantle materials to generate magmas feeding the surface volcanoes. In the back‐arc mantle wedge near the eastern slab edge, the AAN exhibits trench‐parallel fast‐velocity directions (FVDs), which may reflect along‐strike mantle flow. The FVDs in the subducting Pacific slab are nearly east‐west, which may indicate fossil anisotropy formed at the mid‐ocean ridge. A negative RAN is revealed within the subducting slab, which may be caused by the fast plate subduction with a steep dip angle. Trench‐normal FVDs of the AAN are revealed in the mantle below the Pacific slab, which may reflect mantle flow entrained by the subducting slab. A positive RAN is revealed in the mantle beneath the Yakutat slab, indicating that its shallow subduction flattens the mantle flow below the slab to be subhorizontal. Along‐strike FVDs of the AAN around the eastern slab edge may indicate the edge‐induced toroidal mantle flow. Key Points The Pacific slab has subducted down to 450‐ to 500‐km depths beneath Alaska A slab gap exists near the Wrangell volcanic field Anisotropies exist in the mantle wedge and subslab mantle, reflecting mantle flow caused by the slab geometry and subduction