The Study On The Electrical Structure And Dynamic Significance Of The East-west Lithosphere In Xingmeng Orogenic Belt

Since the formation of the stable Jiamong block in the early Paleozoic,the Xingmeng orogenic belt has been affected by the closure of the Paleo-Asian ocean,the closure of the Mongolia-Okhotsk ocean,especially the middle and long distance subduction tectonic events of the Pacific plate.The Subduction of the Pacific plate in Mesozoic profoundly affected a series of tectonic events in Xingmeng orogenic belt,and extensive intraplate volcanic events developed in Cenozoic,including the Changbaishan volcano,Jingbohu volcano and Wudalianchi volcano.There are active shallow earthquake areas,the main shallow earthquake occurred in Songyuan earthquake area of Songliao Basin.The lithosphere of the Great Xing'an range is thickened while that of the Songliao Basin is thinned.Previous studies generally attributed the above tectonic events to the influence of the Western Pacific subduction tectonic regime,but did not systematically study the response of the Pacific subduction at different distances.Such problems as how the remote effect of subduction plate affects the Erguna and Xing'an block,and how the mid-range effect of subduction plate stagnating in the mantle transition zone affects the songliao block and Jiamusi block,all these problems need to be further discussed.In this paper,the long period magnetotoluric sounding is used to obtain the 1300km east-west deep electrical structure of the Xingmeng orogenic belt,and the medium and long range effects of the Xingmeng orogenic belt on the Pacific subduction plate are discussed in combination with the fluid content and distribution quantitatively calculated by high temperature and high pressure rock physics.In this paper,81 magnetotelluric sounding profiles with a length of 1300km laid along the eastern section of the Xingmeng orogenic belt are used to obtain high-quality long period magnetotelluric data.The points are 20km apart and pass through the Erguna block,Xing'an block,Songliao block and Jiamusi block from west to east.The phase tensor method is used to analyze the dimension of the 10000second long period magnetotelluric data.It is found that the study area is two-dimensional in high frequency band and three-dimensional in low frequency band.The identification results of electrical principal axis are consistent with the main tectonic strike in the study area,which is approximately NE45°.Based on the deviation degree and principal axis analysis of the long period magnetotelluric data,the two-dimensional and three-dimensional resistivity models of the Xingmeng orogenic belt are obtained by using nonlinear conjugate gradient algorithm.According to the inversion results:(1)The lithosphere structure of the eastern part of the Central Asian orogenic belt is obviously different,and the Erguna block lithosphere is characterized by low resistivity.The Lithosphere of Xing'an block is intact with high resistance.The lithosphere of the Songliao block is characterized by low resistivity and high resistivity from left to right,corresponding to the Songliao Basin and the Lesser Khingan Mountains.The lithosphere of jiamusi block is characterized by alternating high depth and low resistivity.(2)There are two large-scale high conductivity anomalies C1,C2 and C3 in the crust and mantle scale in the Xingmeng orogenic belt.The high conductivity anomaly C1 extends from the surface to the asthenosphere and connects to C2,C2 exists at the depth of the asthenosphere beneath the Xing'an block,and the high conductivity anomaly C3 extends from the surface to the asthenosphere scale.(3)Large scale high conductivity anomalies C2 and C3distributed below Xing'an block and Songliao block are connected with the upper lithosphere,corresponding to the basin-mountain junction or the main suture zone and fault zone in northeast China.Based on the resistivity model,regional geological background,petrophysics and high temperature and high pressure experiments,we draw the following conclusions:(1)the lithosphere of the Xingmeng orogenic belt has been strongly transformed.Combining with the tectonic background of the region,we propose a"bottom-up"understanding of lithosphere transformation.The Pacific plate was subducted westward,and the plate remained in the 410?660km depth of mantle transition zone during the horizontal movement,which continuously disturbed the asthenosphere,resulting in the upwelling of the asthenosphere and the activation of asthenosphere materials along the weak zone of the Paleozoic block boundary,which profoundly affected the evolution of the orogenic lithosphere.(2)C1,C2 and C3 high conductivity anomalies are caused by water-bearing minerals and partial melting.According to the further calculation of high temperature and high pressure experiment,the water content of C1 below the Erguna block is 400-800ppm,and the melting fraction is about 5.6-8.0%,and the effective viscosity is about 2*10²⁰?3.5*10²⁰pa.S;C2 under the Xing'an block corresponds to water content of 150-680ppm,the melting fraction is about 1.8-3.8%,and the effective viscosity is about 3.6*10²⁰-6.2*10²⁰Pa.S.The water content of the C3 under the Songliao block is more than 1000ppm,the melting fraction is 9.6?12.2%,and the effective viscosity is about 2.4*10²⁰?1.8*10²⁰Pa.S.The above calculation shows that the moisture content and melting degree of C3 are higher than C2,the effective viscosity corresponding to C3 is lower than C2.(3)We infer that the lateral heterogeneity of the fluid-related properties is caused by the different distances of the Western Pacific subduction.The C3 is located above the stagnant plate,corresponding to the strong transformation of upper mantle resulted from the large-scale plate dehydration in the medium-distance subduction setting of the Pacific plate.The C2 is controlled by the long-distance effect of Pacific plate subduction,where the dehydration of the plate is weakened,and is mainly drove by the disturbance at the end of the plate subduction.So the upper mantle structure is relatively complete.