The Fine Structure Of The Crust And Upper Mantle In The Central And Northern Parts Of Longmenshan Fault And Its Adjacent Region

The eastern margin of the Tibetan Plateau serves as an important area linking the Tibetan Plateau with the North China and Yangtze Blocks.It records the entire process of the subduction and extension of the India-Eurasia collision which is also an important site and channel for the exchange and migration of crust-mantle materials.The special geological tectonic environment in the Tibetan Plateau attracted continuous attentions from domestic and foreign scholars.The research on the tectonic and dynamics of the eastern margin of the Tibetan Plateau has never stopped.In all studies of the Tibetan Plateau,the final key issue was to analyze the way how the material has been transported out of the Tibet Plateau and to obtain a reasonable explanation.In this paper,the waveform records of 202 stations in the Sichuan Basin and its neighboring areas are collected.We used the frequency domain to extract the teleseismic P-wave receiver function,and then applied linear inversion and H-Kappa scan to obtain the information of one-dimensional S-wave velocity,crustal thickness,and wave velocity ratio below each station.Then,by collating the information on the velocity structure at the stations below,the high-resolution structures of the crust and upper mantle in the Sichuan Basin and its adjacent areas were obtained.The conclusions obtained are as follows:(1)The overall crust thickness in the study area undergoes dramatic change from East to West.There is a deposited layer in the shallow part of the Sichuan Basin,so the S-wave velocity at the depth of 10 km in this area shows a distinct low-velocity feature.Within the depth range of 20-60 km,the basin shows specificity as a block's rigidity and stability.The thickness of the crust varies from 35 to 43 km in the area,which shows a slow thickening trend from east to west,while the distribution of the wave speed ratio shows high wave speed ratio characteristics in some areas,which is presumably due to a thicker deposited ayer in the shallow areas.As the boundary between the Sichuan Basin and the eastern edge of the Tibetan Plateau,the deep conditions of the Longmenshan fault zone are much more complicated.In the upper crust depth range,the velocity of the three main faults in Longmenshan is defined by the central fault.The velocity characteristics of the Anxian-Guanxian fault are similar to those in the Sichuan Basin,and the Wenchuan-Maoxian fault has similar velocity characteristics as the Songpan-Ganzi block.In the middle-lower crust,the S-wave velocity of the Guanxian-Anxian fault increases with the depth,while there is a low-speed anomaly in the northwestern Wenchuan-Maoxian fault.In addition,the thickness of the crust in the area shows a distinctly segmented feature,and the epicenter of the Wenchuan earthquake is at a position where the wave velocity ratio has changed significantly.(2)The average crust thickness in the Songpan-Ganzi area is significantly different from that in the Sichuan Basin,which is roughly within the range of 55km-65 km.The distribution of crust thickness in the Sichuan-Yunnan diamond block is similar to that of the Songpan-Ganzi block,except that the maximum thickness of the crust in the Songpan-Ganzi area is slightly larger than that of the diamond block,and the crust thickness of the Sichuan-Yunnan block ranges between 56-63 km.There is a low-speed anomaly beneath the block of Songpan-Ganzi and Sichuan-Yunnan diamond blocks,and the ratio of the wave speed in the Songpan-Ganzi area ranges from 1.7 to 1.9.There is a certain range of high-velocity ratio areas near the Wenchuan-Maoxian fault in this block.(3)In this study,there is a low-speed anomaly in the middle-lower crust of the eastern edge of the Tibetan Plateau,and the characteristics of some high-velocity ratios are shown in the distribution of wave speeds.The preliminary speculation may be due to the fact that the crust is rich in fluids or is in a partially molten state at high temperatures.Studies have shown that the high wave speed ratio in the mid-lower crust may indicate that the area contains relatively high iron-magnesia mineral components,or with fluid or local melt related.Combining with previous studies,there are high heat flow values and low resistivity values in this area.It is speculated that there may be high temperature induced partial melting in the region.The high wave speed ratio characteristics obtained in this area confirm the characteristics of soft and easy deformation of the crust in this area.According to the above analysis,the intersection of India and Eurasia makes the crust material in the Tibetan Plateau squeeze toward the southeast.The plastic weak material existing in the middle-lower crust of the plateau may be used as a consideration to explain the material transport patterns in the area.