Upper Mantle Structure Beneath Mariana: Insights From Rayleigh-Wave Anisotropic Tomography

We determine Rayleigh-wave phase-velocity azimuthal anisotropy tomography beneath Mariana using a large number of high-quality amplitude and phase data of teleseismic fundamental mode Rayleigh waves at periods of 25-130 s. The obtained phase-velocity model is then inverted for a 3D azimuthal anisotropic shear-wave velocity (Vs) model down to similar to 300 km depth. The old (similar to 150 Ma) Pacific oceanic lithosphere near the Mariana trench is imaged as a clear high-Vs zone with a thickness of similar to 100 km, whereas obvious low-Vs anomalies exist in the mantle wedge and in the asthenosphere beneath the incoming plate. The Pacific oceanic lithosphere near the Mariana trench mainly exhibits a fast-velocity direction (FVD) of NW-SE normal to seafloor isochrons, which may reflect frozen-in lattice-preferred orientation formed at the mid-ocean ridge, whereas the dominant FVD in the subducting Pacific slab at depths >similar to 100 km is parallel to the Mariana trench, probably reflecting subduction-related structural deformation in the slab. Significant trench-normal FVDs exist in the mantle wedge at depths <similar to 100 km, whereas the deeper portion of the mantle wedge mainly exhibits trench-parallel FVDs, which may indicate a flow pattern in the mantle wedge due to the slab deep subduction and dehydration.