Detailed knowledge of the lithospheric structure is essential for a better understanding of the tectonic evolution of the Xing'an‐Mongolian Orogenic Belt (XMOB). We constrained the lithospheric structure across the XMOB by receiver function imaging and shear wave splitting analysis from a dense seismic array. The lithosphere‐asthenosphere boundary (LAB) is coherently imaged in different tectonic blocks. The mid‐lithospheric discontinuity is also identified at ∼75–100 km depth over the deeper LAB (∼110–130 km) beneath the western side of the North‐South Gravity Lineament (NSGL), roughly at the same depth as the LAB beneath the eastern side of the NSGL. The distinct variations in both the lithospheric structure and fast shear‐wave polarization direction across the NSGL are likely caused by different tectonic processes in the two sides, indicating that the NSGL might represent the western boundary influenced by the subduction of the (Paleo‐)Pacific Plate. Lithosphere thinning in the XMOB is evidenced to have been limited mainly to the east of the NSGL.

We obtained a panoptic view of the pattern of mantle flow in the “Big Mantle Wedge” associated with the stagnation of the subducting Pacific slab. We applied a simultaneous inversion of multiple waveforms method to measure SKS‐wave splitting parameters for a dense seismic array in trans‐continental Northeast Asia. We identify distinct patterns separated by the North‐South Gravity Lineament (NSGL): to the west, the ENE fast axis orientation with small δt reflects limited frozen anisotropy in the stable lithosphere; while to the east, local variation of anisotropy is superimposed on the NNW‐dominated fast direction, suggesting contribution of other mechanisms rather than the only asthenospheric flow. Within a distance of 200 km crossing the NSGL, we reveal a gradual clockwise rotation in the fast direction, and the keel‐deflected flow caused by the 60–80 km reduction in the lithosphere thickness across the NSGL might contribute to the variation of anisotropy.