Seismic Evidence of Slab Segmentation and Melt Focusing Atop the 410-km Discontinuity in NE Asia

The geometry of subducting slabs is largely controlled by mantle rheology and time evolving processes of surface plate boundaries. Imaging of a detailed slab distribution and its surrounding can provide information of physical, chemical, and dynamical properties of the upper mantle. Based on new high-resolution 3-D tomography of subducting Pacific slab in northeast Asia, we revealed a prominent gap within the stagnant portions of the slab showing an abrupt change in its lateral trends that follow the trace of plate junctions associated with plate reorganization at the western Pacific margin during the Cenozoic. Focused partial melting above the slab gap was inferred based on the spatial coincidence between the high Vp/Vs anomaly and the negative reflectivities above the 410-km discontinuity from local receiver function studies. The slab gap is possibly filled with low-velocity anomalies within the MTZ as evidenced by wavefield focusing of teleseismic body waves and absolute velocity imaging from previous studies. We explain the spatial coincidence between the low-velocity anomaly within the MTZ and the focused melt layer above the MTZ by the process of mantle dynamics related with secular variation of slab geometries by tearing. Isolated low-velocity anomalies within the MTZ imaged by seismic tomography without previous thermal disturbances (e.g., hot plume) are suggested to be the products of distinct MTZ compositions disturbed by former nearby slab subductions. Our results suggest a close dynamical relationship between the subducting slab and the MTZ, which promotes the formation of multi-scale chemically distinct domains in the deeper upper mantle.