Is there a gap in the stagnant Pacific slab in the mantle transition zone under the Changbaishan volcano?

Some researchers consider that the Changbaishan volcanism is caused by hot and wet upwelling flow in the big mantle wedge (BMW) above the stagnant Pacific slab in the mantle transition zone (MTZ) beneath Northeast (NE) Asia, whereas a recent study suggests that hot upwelling flow from the lower mantle passes through a gap in the Pacific slab in the MTZ and then enters the upper mantle, causing the Changbaishan volcanism. In this article we review recent body-wave tomographic studies of the upper mantle and the MTZ under NE Asia and discuss whether a gap does exist in the Pacific slab under the Changbaishan volcano. To date, only one teleseismic tomographic model inferred from relative travel-time residuals shows a low-velocity anomaly (a gap) in the MTZ to the west of the Changbaishan volcano, whereas all the regional and global tomographic models derived from absolute travel-time residuals show a continuous high-velocity anomaly (i.e., the stagnant Pacific slab) in the MTZ from the region east of the Changbaishan volcano to the North-South Gravity Lineament (NSGL) in NE China. Although a few tomographic models show that seismic velocity is slightly lower to the west than to the east of the Changbaishan volcano, the velocity is still higher than the average velocity in the MTZ. Receiver function studies show that, the average depth of the boundary between the upper mantle and the lower mantle (BULM) is 660km globally, whereas it is 670km under NE China, and the average MTZ thickness is 250km globally, whereas it is 260km under NE China. If these global average values are taken as the criteria, then the receiver function results illustrate a depressed BULM and a thickened MTZ from the region east of the Changbaishan volcano to the NSGL. These receiver-function results are in good agreement with the continuous high-velocity anomaly in the MTZ imaged by most tomographic studies, suggesting that the front of the subducting Pacific slab has reached the boundary between the Songliao basin and the Great Xing'an range. Integrating all findings from high-temperature and high-pressure experiments, numerical modeling and petrological, geochemical and geophysical studies till now, we deem that a thinned MTZ and a slab gap do not exist to the west of the Changbaishan volcano. To date, much more pieces of evidence support the BMW origin of the Changbaishan volcanism.