Study On Electrical Structure In The Southern And Surrounding Areas Of Songpan-Ganzi Block

The southern part of the Songpan-Ganzi block is located on the eastern margin of the Qinghai-Tibet Plateau and on the west side of the Sichuan Basin.The regional structure is complex and the earthquakes occur frequently.In order to explore the lithospheric structure of each plate and each fault zone in this area,this paper uses magnetotelluric(MT)to analyze and research the area.The data used in this paper is mainly derived from the "Three-dimensional lithospheric electrical structure study in the eastern Tibet-Western Sichuan region" under the topic "Crystal Structure and Deformation Zone Imaging of the Qinghai-Tibet Plateau" undertaken by China University of Geosciences(Beijing).Through the processing,analysis and inversion of the collected magnetotelluric data in the field,the two-dimensional electrical structure model of the three survey lines and the overall three-dimensional electrical structure model of the study area were obtained.Based on the electrical structure model and the existing geological and geophysical data in the study area,the following understandings are obtained.The southern part of the Songpan-Ganzi block and the Sichuan Basin are bounded by the Longmenshan fault zone.The differences of electrical structure between two areas are very obvious.The electrical structure in the southern part of the Songpan-Ganzi block has the characteristics of longitudinal stratification.The upper crust is characterized by highresistance brittleness,and the middle and lower crust is characterized by large-scale high-ductivity plasticity.It is speculated that the high conductor is a local molten substance or salt water fluid.The lithosphere of the Sichuan Basin and the Longmenshan fault zone showed high electrical resistance.Most of the faults in the study area are thrusting nappe faults.It is speculated that crustal shortening caused by brittle deformation such as thrusting nappe played a major role in the uplift process of Eastern Qinghai-Tibet Plateau,and crustal thickening caused by high conductivity materials in the middle and lower crust of Songpan-Ganzi block also played a major role in promoting the uplift of the plateau.Under the combined action of these two mechanisms,the whole uplift of the eastern QinghaiTibet Plateau was caused.In addition,electrical model shows that Huya fault and Longmenshan fault zone are the western boundary of Yangtze plate.Most of the epicenters of the earthquakes in the study area are concentrated on the contact boundary between the high-resistance body and the high conductor near the fault zone.The block motion on both sides of the fault causes strong deformation near the fault zone,and the stress accumulates continuously,causing frequent earthquakes.However,there is no source in the high-conductivity layer of the middle and lower crust,indicating the stress accumulation conditions required for the earthquake-free source in the plastic body.