Mantle transition zone beneath eastern China and its tectonic implication (Invited)

We analyzed a total of 37000 receiver-function data recorded by regional broadband seismic networks of the China Earthquake Administration to study the mantle transition zone beneath eastern China. Significant topography on both the 410-km and 660-km discontinuities was clearly imaged in the 3D volume of CCP (common-conversion-point) stacked images that covers an area of 97.5-122.5E and 22.0-42.N. 3D crustal and mantle velocity models were used in computing the PS time moveouts to better image the absolute depths of the two discontinuities. We found that the 660-km discontinuity is depressed between ~20 to 40 km along the east coast of China, indicating the presence of the subducted Pacific slab in the region. More interesting, a broad 660-km was observed in most of the depressed area, which was not seen from the 410-km discontinuity. We found a strong and localized anomaly beneath the Datong volcano, one of the three active volcanoes in China, located at the northern end of the Shanxi Graben. The 410-km is depressed by as much as 20 km beneath the volcano. If the amount of the depression is caused by unmodeled low velocity anomaly in the upper mantle, it requires either a strong (5-10%) localized low velocity anomaly near the lithosphere-asthenosphere boundary or ~2% low velocity column across the entire upper mantle above the 410-km. The later could suggest a deep origin of the Datong volcano, which was attributed to edge driven convection by most of the literatures. We also found a localized depression of the 660-km at the southwest edge of the 3D CCP volume, indicating the existence of the subducted Indian plate beneath the region.