Distribution Of Crustal Low Velocity Layer In Qinghai-Tibet Plateau And Its Tectonic Significance

The Qinghai-Tibet Plateau has a thick crust.In order to explain the formation of the thick crust,many dynamics models such as crustal subduction model,crustal compression overlap and thickening model,and bottom intrusion and thickening model were proposed.Therefore,the study of the crustal structure and composition of the Qinghai-Tibet Plateau has become a hot issue.In the discussion of the crustal structure of the Qinghai-Tibet Plateau,the low-velocity layer has always been focused on,and it is a problem that can not be ignored in the study of the formation and evolution of the Qinghai-Tibet Plateau.First,the geology and Geophysics of the Qinghai-Tibet Plateau are briefly introduced,and the distribution of low-velocity layers in the crust of the Qinghai-Tibet Plateau is summarized.Secondly,the basic principles of receiving function method,complex spectral ratio inversion method and H-k stacking method are introduced,and then the data processing of this paper is explained in detail.Thirdly,according to the field distribution,the stations used in this paper can be divided into the west line near 85° E,the 89.5° E near the middle Qiangtang and the east line along Qinghai-Tibet Highway.The data of the west,middle and east lines are processed separately to obtain the one-dimensional S-wave velocity structure within 100 km below each station,and the three-spline interpolation is carried out for several sections in the north-south direction to obtain the two-dimensional velocity structure image along the latitude distribution.The results show that from above mention six profiles of S-wave velocity structure maps obtained.Those profiles have crustal low-velocity layers,which support the view that lowvelocity layers generally exist in the Qiangzang-Tibelt plateau;The low-velocity layers mainly distribute in the range of 40 km from surface to underground,especially in the depth range of 15-40 km.Most of these low-velocity layers is likely blocks in latitude,and there is a certain degree of connectivity between the blocks,but they are not continuous.The shear wave velocity of the low-velocity layers is less than 3.6 km/s.Based on geophysical,geochemical and petrological data,it is considered that the strata buried in the depth of 15-50 km in the north-central Qinghai-Tibet Plateau satisfy with the condition of partial melting.Various heat-generating processes induced by the convergence,collision and subduction of the Indian continent with Eurasian continent are the sources of heat for partial melting of rocks.The low-velocity layer in the south of Bangong Lake-Nujiang suture zone is due to the collision and friction between the rigid Indian plate and the Lhasa-Gangdise crust,resulting in partial melting of the lower and middle crust of the Lhasa-Gangdise block,forming a lowvelocity-high conductivity layer,while the Qiangtang block north of the Bangong Lake-Nujiang suture zone is due to the subduction of the rigid and cold Indian plate to Bangong.Under the lake-Nujiang suture zone,the hot mantle convection is caused,and the hot material upwells in the middle of Qiangtang,which cases the middle and lower crust of Qiangtang warm and partially melt,forming a large-scale low-velocity layer.