Upper mantle and mantle transition zone thermal and water content anomalies beneath NE Asia: Constraints from receiver function imaging of the 410 and 660 km discontinuities

The 410 and 660 km discontinuities (d410 and d660) bordering the mantle transization zone (MTZ) beneath NE Asia are imaged in successive circular bins with a radius of 1 degree by stacking a total of 274,593 P-to-S radial receiver functions recorded by 799 broadband seismic stations. After moveout correction based on the 1-D IASP91 Earth model, the resulting apparent depths of the discontinuities exhibit significant and spatially systematic variations. Three approximately N-S elongated narrow zones with significantly thickened MTZ are observed, which may be associated with the thermal effect and dehydration of subducted slabs. The major volcanoes in NE China are underlain by a d660 that is apparently depressed by 20 km, which can be interpreted by the presence of an anomalously high water concentration in the lower MTZ released from the stagnated slabs. Upper mantle low wavespeed anomalies west of the Datong volcanic fields are underlain by an MTZ with normal thickness, and are attributable by dehydration of the leading portion of the stagnant Pacific slab in the MTZ that is revealed in an N-S oriented narrow zone east of this area. The lateral shift of the upper mantle low wavespeed zone and the area with thickened MTZ may suggest a westward drift of the upper mantle relative to the subducted slab. An abnormally thin MTZ is observed beneath the Hangay Dome in central Mongolia, suggesting the possible existence of thermal upwelling from the lower mantle through the MTZ.