Anisotropic Tomography and Dynamics of the Big Mantle Wedge.

We determine high‐resolution 3‐D tomographic images of P‐wave isotropic velocity, radial anisotropy and azimuthal anisotropy beneath NE Asia down to 800 km depth. Our results show negative radial anisotropy (i.e., Vhorizontal < Vvertical) in the asthenosphere of the big mantle wedge (BMW), which may reflect mineral alignment caused by vertical flow in the asthenosphere. Across the Tanlu fault zone (TLF), the western and eastern parts of the BMW exhibit high and low P‐wave velocities, respectively. Combining our tomographic results with surface geological features, we speculate that convection in the BMW includes upwelling asthenosphere beneath the Japan Sea and the Korean Peninsula and downwelling asthenosphere beneath the Songliao and North China basins. The downwelling asthenosphere beneath the two basins is associated with diminishing volcanism and anomalous tectonic subsidence since ∼110 Ma. The great TLF is an important boundary for the BMW structure and dynamics. Plain Language Summary: The subducting Pacific plate becomes flat in the lower part of the mantle transition zone beneath NE Asia, and a big mantle wedge (BMW) has formed above the flat slab. The BMW controls tectonic and geological activities in NE Asia, which are characterized by large‐scale sedimentary basins, lithospheric thinning, large strike‐slip faults, deep earthquakes and intraplate volcanism. However, the convection pattern in the BMW is still unclear. We determine the first 3‐D P‐wave radial anisotropy model down to 800 km depth beneath NE Asia, as well as high‐resolution tomographic images of isotropic P‐wave velocity and azimuthal anisotropy. We find predominant negative radial anisotropy in the BMW and east‐west variations of velocity structure. Combining the tomographic results with surface geological features, we speculate that convection in the BMW may include upwelling flows beneath the SW Japan Sea and the southern Korean Peninsula and downwelling flows beneath the Songliao and North China basins. Key Points: The first 3‐D radial anisotropy model beneath NE Asia is obtainedEast‐west structural variations in the big mantle wedge (BMW) are revealedA possible convection pattern in the BMW is proposed [ABSTRACT FROM AUTHOR]