The Study Of The Crustal And Upper Mantle Structure In Northeast China From Teleseismic Receiver Function

The northeast China, which is bounded by the Sino-Korean craton and Siberia block to the south and to the north respectively and lies to the west of the marginal seas in the western Pacific, consists of several massifs which were amalgamated before Mesozoic. It developed from the so-called Paleo-Asian Ocean tectonic domain and then from the Pacific Ocean tectonic domain in the history of geological evolution. The northeast China experienced intensive extensional deformation and magmatism in late Late Mesozoic, as manifested by development of extensional basins and widespread alkali volcanism. From the slab contour delineated by seismicity, the Pacific slab has subducted westward since the Late Mesozoic, and reaches-600 km depth beneath northeast China, thus making the northeast China into an ideal region for the study of the interaction between the subduction oceanic slab and the continental mantle material.In this thesis, we first introduced briefly the geological evolution history of the northeast China and many previous studies of the crust and the upper mantle structure. Then we introduced the principal of the receiver function method. We computed a total of 70,259 receiver functions from 1,051 teleseismic events(Mb>5.0) recorded by 250 temporary broadband seismic stations and 122 permanent stations deployed in our study region, and then focused on to derive the thickness and the Poisson ratio of the crust and the topographies of two discontinuities in the mantle transition zone by analysis of the P-wave receiver functions. Based on our results, we try to image the subduction geometry of the Pacific plate and also shed light on the the orgin of the Changbaishan volcanoes.Moho depth and average crustal Poisson ratio were measured at each station in the northeast China using the H-κ grid searching method. The results show that the crustal thickness varies from 26 km to 44 km and has obvious regional differences. There is a thinner crust and a higher Poisson ratio in the middle area than the east part and west part. The thickest crust (31.4-43.5 km) and a slightly low Poisson ratio (about 0.26) is found in the Great Xing’an range and the Eergu’Na, while the thinnest crust (27.6-34.5 km) and a high Poisson ratio (0.24-0.308) is found beneath the Songliao Basin. Moreover, there is a relative high Poisson ratio beneath the Changbaishan volcanoes (0.29).The regional differences of the crustal thickness and the Poisson ratio may relate with the mechanism of the geological evolution in the northeast China. A thick crust accompany with the high elevation in the Great Xing’an range shows that the thick crust maybe the root of mountain. Less correlation between the crustal thickness and the topography implies that the Songliao Basin is likely supported by the upper mantle. The high Poisson ration indicates there is a partial melting beneath the Changbaishan volcanoes.The topography of the 410 km and 660 km discontinuities are mapped using the common conversion stack method. The result shows that the characteristics of the 660 km discontinuity in the south of the 44.5°N are different from the north. In the south, the depression as much as 41 km in a local area which locates in the front of the deep seismicity and spans 300-400 km in the E-W direction of the 660 km discontinuity supports that the Pacific slab would pile up atop the 660 km discontinuity in the depression regions and may penetrate into the lower mantle. To the west of the depression the 660 km discontinuity is uplifted as much as 12.5 km in a small area. The result infers that the hot sub-lithospheric mantle that has been entrained beneath the sinking lithosphere of the Pacific plate is escaping through a gap in the subduction slab. The escaped material produce decompression melting in the shallow and then feeds the Changbaishan volcanoes.In the north,660 km discontinuity is depressed about 10～21 km in a broad area that extends westward as much as 750 km. This phenomenon suggest that material possesses low temperature anomaly is stagnant in this area.