Electrical Structure Study Of The Crust Mantle In The Middle Section Of Longmen Mountain Structural Belt

This paper analysed the Magnetotelluric sounding(MT) profile data of Longmen Mountain with advanced data-processing technique, we provide rather reliable electrical structure of the crust mantle in the middle section of Longmen Mountain structural belt. Combining with the basic geology and using the advancement of MT, we discussed and did research on the hot issues of the Longmenshan crustal structure and on Basin-mountain Coupling of Longmenshan structural belt.The article first detailed the work of Longmen Mountain profile,the profile from Zhongjiang in the South, through Longmen Mountain and Songpan, accessed Ruoergai block, arrived in Luqu,Gansu, crossover foreland basin of Sichuan Basin, Longmenshan structural belt, fold belt of Songpan-Ganzi and West Qinling Orogen, and summary introduced the data processing of MT.Secondly, we focused on qualitative analysis of profile. By the rotation of tensor impedance, we certained regional structural trend of profile, Results from the analysis the best azimuth is NW-SE, roughly the same direction of the layout of the profile; All 49 points were analyzed with Mohr circle analysis method, obtained angle of deviation and angle of nonisotropy of two-dimensional, concluded that the overall performance of the two-dimensional, and Longmen Mountain has strong performance of 3D and nonisotropy; Form the two dimension degree of deviation of impedance, we can have the same conclusion of Mohr circle analysis method;made a forward simulation of tilted fault model by using two-dimension simulation programme, summed up the characteristics of tipper, by analysis of the actual tipper, it is obvious that, in comparison of inversion result of resistivity, data of tipper reflect more detailedly to fault structure with more information; by analysis of curve, apparent resistivity, phase, understand the major electrical structure features, provides a basis of the interpretation of the electrical structure. Thirdly, we discussed the inversion model and two-dimensional inversion of the basic theory, compared several commonly inversion methods, study the issues such as the dependency of initial model, the stability of convergence ,the superior and inferior of inversion results, used the Non-liner conjugate gradient method and TE&TM combined mode for two-dimensional inversion, and obtain the two-dimensional electrical structure of the crust of the profile.Fourthly, we focus on Longmenshan structural belt, introduced the basic geological conditions of main structure's layer and fault along the profile,with qualitative analysis of the results, analysis two-dimensional electrical structure of the crust, once again proves the existence of thrust tectonic of Longmen Mountain, clearly reveals the dual structure of the high resistance in lower part and the low resistance in upper part of Sichuan basin, reveals that there have resistive bed in superficial part and low resistance zone for the deep of Songpan-Ganzi fold belt, electrical characteristics of Low-High-Low; and made a note of the location and attitude of major fault.Finally, combined with MT profile, we do a simple analysis of Wenchuan earthquake ,with other information can have the conclusion of thrust tectonic and the focal depth of Wenchuan earthquake is very low, and discuss the aftershock zone of the focal depth, found that focal depth of aftershock were about 10km,it is the same as MT profile reveals.