Imaging the mantle transition zone beneath East Asia using teleseismic receiver functions: implications for thermal and compositional heterogeneities

The mantle transition zone (MTZ) plays a critical role in regulating material flux and mixing between the upper and lower mantle. Variations in MTZ structure are closely associated with mantle temperature and compositional heterogeneities, making it essential to constrain these variations to better understand mantle dynamics. To investigate the MTZ structure beneath East Asia, where the stagnant Pacific slab and intraplate volcanism provide a unique geodynamic setting, we examined over 40,000 P-to-S converted waves from 18 seismic networks in Korea, China, and Japan. Common conversion point (CCP) stacking revealed significant regional variations in MTZ discontinuities and heterogeneous lateral variations in converted wave amplitudes. The average depths of the 410-km (d410) and 660-km (d660) discontinuities were 416 ± 10 km and 668 ± 11 km, respectively, yielding a mean MTZ thickness of 252 ± 14 km with notable localized deviations. CCP-stacked profiles at 0.2 Hz and 0.5 Hz highlighted the structural complexities, revealing evidence for multi-layered MTZ boundaries. Converted wave amplitudes exhibited lateral variations on scales of 200 km and larger; however, these variations in amplitudes did not correlate with the variations in discontinuity depths. A pronounced depression of the d660 beneath the Korean Peninsula and northeastern China likely reflects the influence of the cold, stagnant, and near-horizontal Pacific slab. The accompanying d410 depression beneath the Korean Peninsula suggests compositional heterogeneity, possibly linked to water and carbon enrichment. Near-normal MTZ discontinuity structures beneath the Ulleung and Jeju intraplate volcanoes indicate limited influence from the stagnant Pacific slab, potentially due to a slab tear or a localized thermal anomaly near the base of the MTZ. Mantle upwelling through such a tear is likely hydrous rather than purely thermal, consistent with the near-normal d410. These results demonstrate that regional MTZ anomalies reflect interactions among mantle upwelling, slab-derived heterogeneities, and volatile-rich components.