Seismic Anisotropy Evidence for Ductile Deformation of the Forearc Lithospheric Mantle in Subduction Zones

Seismic anisotropy provides important information on the structure and geodynamics of the Earth. The forearc mantle wedge in subduction zones mainly exhibits trench-parallel azimuthal anisotropy globally, which is inconsistent with the model of olivine a axis aligning with the slab-driven corner flow. Its formation mechanism is currently unclear. Here we present high-resolution 3-D P wave anisotropic tomography of the Tohoku subduction zone. We suggest that ductile deformation of the forearc lithospheric mantle of the overriding plate induces the trench-parallel azimuthal anisotropy and positive radial anisotropy (i.e., horizontal velocity > vertical velocity) in Tohoku. Our results provide the first seismic anisotropic evidence for the slab-mantle decoupling at a common depth of ~70 km. On the basis of the high-resolution seismic images, we propose a geodynamic model suggesting that the forearc mantle wedge anisotropy is produced via ductile deformation of dry olivine or hydrous antigorite lithospheric mantle, which accords well with the trench-parallel shear wave splitting measurements dominant in subduction zones globally.