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 65-120km 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.