Research On The Electrical Structure Of Crust And Mantle Of Western Margin Of The Yangtze Plate And It’s Tectonic Implications

The western margin of the Yangtze block which is the eastern margin ofQinghai-Tibet Plateau is an important part of tectonic units in china. It locatesbwtween the east of Qinghai-Tibet Plateau and the west of the Yangtze block, which isone of the most complex structural belts in the world. The Songpan-Garze fold belt,Longmenshan collision belt and the west Sichuan foreland basin of that area havealways been the hotspots studied by geologists and geophysicists. Especially theLongmenshan collision belt which locates between the Songpan-Ganzi block and theYangtze block (also locates on the north-south seismic belt of china), is a key part ofthe resources and geological hazards study. For many years, a series of key issuesabout the crust-mantle structure of Longmenshan collision belt and the border ofwestern margin of the Yangtze Plate attracted attention and researched by geologistsand geophysicists.This thesis is supported by the National Natural Science Foundation (Project Number:40839909) and SinoProbe Deep Exploration in china (Project Number:201011043-02). Using the MT+LMT exploration mode assisted with surface geologyand regional tectonic to study the deep electrical structure of crust and mantle in thewestern margin of the Yangtze block from the two different scales (upper crust andlithosphere).It’s purpose is to explore the electrical structure of the western margin ofthe Yangtze block and study the dynamics of the geological and geodynamic.Firstly, this thesis discusses LMT one-dimensional forward modeling in a layeredsphere model under the assumption of plane wave model and study the impact of theearth’s curvature on the LMT electromagnetic response with a series of theoreticalmodel. Secondly, on the basis of two-dimensional MT forward modeling with theterrain, we focuse on the impact of topography on magnetotelluric, and prepared twomodels with the real topography on the study profiles，we analyzed the topographyeffect on MT data on this two profiles；Theoretical calculations show that the two-dimensional MT inversion code with topography can effectively eliminate thedistortion on MT data which produced by rugged terrain. Finally, we get thetwo-dimensional electrical structure model of the upper crust and the electricalstructural model of Lithosphere with the MT2d inversion code. According to MT2Delectrical section on the western margin of the Yangtze block assisted with the surfacegeology we get the following main conclusions:(1) According to electrical structure ofLuqu-Zhongjiang MT profile, we prepared the geological interpretation section map ofof the upper crust; According to electrical structure of Luqu-Hechuan LMT profile weprepared the geological interpretation section map of of the Lithosphere; According toelectrical structure of Mingshan-Guangyuan LMT profile we prepared the geologicalinterpretation section map of of the Lithosphere along the Longmenshan collision belt.(2)Initially speculated that the formation of Longmenshan collision belt is probably dueto the movement of the Songpan-Ganzi block to the South East East-South Eastdirection relative to the Yangtze Block through the low resistancelayer(High-conductivity layer)in the crust but blocked by the high speed and highresistance Yangtze Block. And the plastic deformation transfer the stress to the brittleupper crust, this leads to a series of fault thrusting on top of the Yangtze block, whichforming the unique tectonic framework of high resistance on the top and bottom withthe low resistivity on the middle.(3) Initially confirm the boundary of the westernmargin of the Yangtze Plate: the surface is bounded by Mao-wen Fault, and thelithosphere is bounded by Song-pan mantle shear zone, on the east of the Song-panmantle shear zone is the Yangtze craton lithospheric and the west of the Song-panmantle shear zone is the Qinghai-Tibet thickening lithosphere.(4)As a result of thetwo-way squeeze on the Yangtze Block and Qinghai-Tibet Plate the upper crust ofSongpan-Ganzi block thrusting on to the western margin of the Yangtze block on theeast direction, the lower crust and upper mantle to the east to the deep subductiondirection, which leads to Yangtze block wedge into the internal lithosphere of Longmenmountain, the two formed a "wedge " structural characteristics.(5) According to LMTprofile, the substance of the Qinghai-Tibet Plateau eastern edge has both the north andsouth migration direction and deep southeast migration direction.(6) According toGuangyuan-Mingshan LMT profile,we preliminary study the relationship betweenearthquake activity regular pattern and the deep structure: Wenchuan earthquake offenoccurred in the transition zone between the low resistance (low speed) and highresistance (high speed) inside the high resistance (high speed) side. This is probablythe deep background of strong earthquakes.The different electrical structure ofNorthern Section, Middle Section and Southern Section in Longmen Mountain hrustbelt is the most basic reason why the distribution of aftershocks in the southernsection of Longmen Mountain less than the other section.