Title: Imaging Mantle Transition Zone of East Asia Using Receiver Functions: Insights into Thermal and Compositional Heterogeneities

[Abstract]
The mantle transition zone (MTZ) plays a critical role in regulating material flux and mantle mixing between the upper and lower mantle. Variations in MTZ structure are closely associated with mantle temperature and compositional heterogeneities; thus, constraining its properties is essential for understanding dynamic processes and circulation patterns within Earth’s mantle. East Asia serves as an excellent natural laboratory for investigating MTZ structure due to the presence of the stagnant Pacific Plate in the MTZ and intraplate volcanism. This study examines multi-frequency teleseismic P-to-S conversions at the 410 km (d410) and 660 km (d660) seismic discontinuities, representing the upper and lower boundaries of the MTZ, respectively. Common conversion point (CCP) stacking of over 40,000 converted waves from 13 networks in China, Korea, and Japan is used to image their topographies, as well as the amplitudes of the waves at these boundaries. Our results reveal significant regional variations in MTZ topography and thickness, resolved at a lateral resolution of 200 km. The d410 and d660 depths average 416 ± 11 km and 667 ± 12 km (±1σ), i.e., 6 km and 7 km deeper than the global averages of 410 km and 660 km, respectively. The average MTZ thickness, 251 ± 16 km (±1σ), is close to the global average of 250 km but with localized deviations. CCP stacked profiles at 0.2 and 0.5 Hz frequency band highlight structural complexity, revealing evidence of multi-layered MTZ boundaries. Converted wave amplitudes are spatially heterogeneous with lateral variations at scales of 200 km and larger; however, these variations in Pds amplitude do not correlate with the variations in discontinuity depths. Notably, the d660 depression located beneath the Korean Peninsula and northeastern China reflects the influence of the cold Pacific slab on the MTZ, while the concurrent d410 depression suggests compositional heterogeneity, possibly linked to basalt and volatile accumulation. Contrasting MTZ features beneath the Ulleung and Jeju intraplate volcanoes around the Korean Peninsula suggest differing origins: shallow d660 and normal d410 beneath Ulleung may reflect hydrated upwelling through the MTZ, while depressed d410 and normal d660 beneath Jeju may suggest thermal upwelling from the MTZ. These observations contribute to our understanding of the interplay between thermal and compositional processes within the MTZ and their influence on regional tectonics and volcanism in the East Asia.