Upper-mantle seismic anisotropy structure and dynamics beneath NE China inferred from SKS splitting analysis

In this study, we collect the waveform data recorded at the 207 permanent seismic stations from 2008 to 2016 and 127 Northeast China Extended Seismic Array (NECESSArray) temporary seismic stations in Northeast China. Using the minimum transverse energy grid search method of SKS wave splitting, the effective splitting results at 243 stations are obtained. Our results show that the fast direction of anisotropy is dominated by the NW-SE direction, but there are obvious contrasts within the tectonic units and between different units. In the northern part of the Great Xing/an range, the fast direction of anisotropy changes from NNE-SSW in the north to NNW-SSE in the south, and in the middle it is dominated by the NW-SE direction, whereas in the south it changes from NE-SW in the north to nearly E-W in the south. In the Songliao basin, in the northern part, the fast direction of anisotropy changes mainly from NNW-SSE in the west to NW-SE in the east, in the middle part it changes from NE-SW in the west to nearly E- W in the east, whereas in the southern part it mixes NE-SW with NW-SE. The Jiamusi block shows the fast direction changes from NW-SE in the west to NNW-SSE in the east, and the fast-slow-wave delay time gradually becomes larger. The northern Changbaishan orogenic belt illustrates a fast-direction transition from NW-SE in the north to nearly E-W in the south, the middle belt demonstrates a fast direction of NNW-SSE, whereas the southern belt shows a NW-SE direction. In the Yanshanian orogenic belt, the fast direction of anisotropy is mainly along the E-W direction, generally parallel to the strike of the belt. These results indicate that although the complicated pattern of fast directions is somewhat related to the delamination of lithospheric materials and upwelling of hot asthenospheric materials, it is more associated with the horizontal flow of the big mantle wedge due to the deep subduction of the western Pacific slab. In the margins of the Abaga and Halaha volcanic groups, the Changbaishan, Longgang, and Jingpohu volcanic areas, and the Wudalianchi volcano, the fast direction of anisotropy changes obviously, suggesting that the upwelling of hot materials could affect the horizontal flow direction of the big mantle wedge.