The Crustal And Upper Mantle Structure And Anisotropy Beneath The Lower Yangtze Craton And Its Adjacent Regions

Eastern China is located at the eastern margin of the Eurasian plate, where it interacts with the Philippine Sea Plate and the Pacific Plate. The South China Block and North China Craton that collided in the Triassic are separated by the NWW-SEE trending Qinling-Dabie Orogenic Belt and the NNE-SSW trending TanLu fault. In the Mesozoic, the eastern parts of these two blocks experienced widespread lithospheric reactivation and magmatism under the extensional environment. The investigation on the deep structure will help us understand the differences of the evolution between different blocks and also the current deep processes beneath eastern China. In this study, we applied the P-wave receiver function and shear-wave splitting methods to the data recorded by the broadband temporary seismic array deployed by Nanjing University since2009to2011. We obtained the crustal thickness, the average crustal Vp/Vs ratio, the mantle transition zone structure and the crustal and upper mantle anisotropic characteristics beneath the study area.The H-k stacking results show that the crustal thickness is in the range of29.3-36.3km. From the Cathaysia Block to the SE part of the JiangNan fault, the crustal thickness gradually tends to be larger. Around the JiangNan fault, there is a～4km offset of the Moho discontinuity. From the NW part of the JiangNan fault to the North China Craton, the Moho also gradually becomes deeper. The average crustal ratio ranges from1.69to1.83with an average small than1.78. The Vp/Vs ratio at the stations around the TanLu fault reaches the highest of the study region, indicating that the local tectonics has influences on the crustal composition. The common conversion point stacking result (in the depth range of300-750km) shows that the410-km discontinuity is relatively flat while the660-km discontinuity is deeper to the north of～29°N than the south. Combined with seismic tomography studies, the results suggest that the base of the MTZ is still influenced by the cold temperature environment of the ancient stagnant slabs to the north of～29°N. The MTZ thickness under the Cathaysia Block is normal relative to the IASP91model while that under the Yangtze Craton is larger, which correlates well to the different evolutionary histories of these two blocks.The core phase splitting reveals the laterally variant anisotropic structures among different blocks. The fast polarization direction is NE-SW (Cathaysia Block), NW-SE and NEE-SWW (Yangtze Craton) and NWW-SEE (eastern North China Craton). The Fresnel zone analysis indicates that the anisotropy in the relatively shallow depth varies significantly from the Cathaysia Block to the Yangtze Craton, indicating the significantly different lithospheric structures of them. The JiangNan fault is revealed as an important boundary within the Yangtze Craton. On the other hand, there is an anisotropic layer beneath the SE part of the North China Craton, which may be related to the deep ongoing mantle process.