Seismic image of the mantle transition zone beneath northeastern China: evidence for stagnant Pacific subducting slab, lithospheric delamination and mantle upwelling

We provide a comprehensive image of the mantle transition zone (MTZ) beneath northeastern China by performing Variable Bin Radius Stacking of receiver functions. A massive seismic data set consisting of over 133 000 receiver functions recorded by 1208 broad-band stations is processed. Our results reveal fine-scale topography on the 410- and 660-km discontinuities defining the upper and lower bounds of the MTZ, lateral variations in the MTZ thickness and slab interfaces within the MTZ. In particular, unambiguous images of the slab interfaces provide direct evidence for the presence of the stagnant Pacific subducting slab below the eastern portion of the study area. A widespread deepening of the 410-km discontinuity is consistent with a hot and wet low-velocity upper mantle resulting from dehydration of the stagnant slab. Prominent depressions are evident in the depth to the 660-km discontinuity, with a thickened MTZ associated with the cold stagnating slab. Localized uplifts of the 660-km discontinuity are possibly caused by partial melt under the slab. These features attest to the influence of the Pacific plate on the MTZ. Additionally, a pronounced upwarp on the 660-km interface with a thin MTZ agrees with a previously hypothesized mantle upwelling through a slab window, possibly triggered by the sinking of the stagnant slab. Moreover, the western part of the study region is characterized by alternating ups and downs of the 410-km interface, while the topography of the 660-km discontinuity is relatively flat. We propose the western region is dominated by foundering of delaminated lithospheric blocks that induced upward mantle return flows upon entrance into the MTZ.