The 3D Electrical Lithosphere Structure Of The South China And Its Tectonic Implications

The South China continent is an important component of Chinese continent and global tectonics,which located in the southeastern part of the Eurasian plate where the Eurasian plate,the Pacific plate and the India plate converge.Therefore,to establish the high-resolution 3D model of South China is of great significance.Under the support of “Deep exploration technology and experimental research” we accomplished the 3D long-period magnetotelluric data acquisition focused on the lithospheric structure and tectonic problems of South China continent.Then we established the 3D electrical lithosphere structure of South China for the first time and systematically analyzed the deep structure and state.The original time series are transformed to magnetotelluric impedance tensor through fourier transform,robust estimation,remote reference processing,power spectral selection.Then high-quality magnetotelluric data of 385 sites are selected.The phase tensor decomposition algorithm is adopted to extract the dimensional information.The study area is two-dimensional structure while there is three-dimensional feature in the local and deep regions.The GB multi-frequency multi-site impedance decomposition scheme is adopted to obtain the regional impedance and the strike distribution.The principal direction of the study area is NE.The induction vectors are adopted to analyses the plane feature of resistivity anomaly qualitatively.There might exist linear high conductor in the deep.We adopt the Rhoplus analysis scheme to analyse and correct the apparent resistivity and impedance phase.The static effect is further evaluated by referring to the geological structure and the apparent resistivity of adjacent sites.Based on the former data processing,we step into quantitative inversion stage.We invert the effecetive apparent resistivity using 1D regularization inverson and obtain the lithospheric thickness distribution of the study area.The regional imdedance data are rotated according to the principal direction of the survey line.Then 2D nonlinear conjugate inversions are carried out based on the 4 selected data sets under principal axis.L-curve scheme is used to determine the appropriate regularization factor and the fitting of different data sets is evaluated.The full inpedance data of 338 magnetotelluric sites are prepared for 3D inversion.The data processing and model construction are performed on the Tremissis plaform.Finally,we perform 3D inversion using ModEM package and obtain 3D electrical structure of lithosphere scale.Comparing the results of 2D inversion and 3D inversion,the electrical characteristics are consistent in the Southeast China magmatic Belt(SCMB)while quite different in the Cathaysia Fold Belt(CFB)and the Jiangnan Orogenic Belt(JOB).Difference in inversion results is mainly derived from the data dependance on the strike direction,which is uncertain owing to the complex geological structure.That is why the 3D inversion results are more reasonable for interpretation.The 3D electrical structure of the study area shows the characteristics of stratified vertically and the blocking laterally.Then the electrical structure of South China is comprehensively studied by means of layer and block analysis.The crust is characterized by high resistivity and complex structure.The resistivity distribution pattern of the upper crust reflects the coupling between the base of basin and the Precambrian basement,while the distribution pattern of the lower crust resistivity reflects the combination of the newborn granulite facies the lower crust and ancient continental crust.The lithospheric mantle shows the partition characteristics.Resistive body R1 exists in JOB which located in the eastern margin of Yangtze block.Resistive bodies R2 avd R3 exist in CFB,and R3 extend across Shangyu-Zhenghe-Dapu fault into the SCMB,Resistive bodies R1,R2,R3 are welded by linear conductive body C1,C2,C3;there exists conductive body C4 in the SCMB and the SCMB is separated by the Minjiang fault C5.Based on the resistivity distribution pattern of the lithospheric mantle,a tectonic framework of lithosphere scale is established.Based on the electrical structure analysis,the deep distribution characteristics of deep and large faults are studied,and the contact relation between Cathaysia and the Yangtze block are discussed.The east of the two major blocks is bounded by the Lu Shao fault,and the shallow layer of the Cathaysia Block is thrust northward and westward,and the Yangtze is thrust into the Cathaysia Block in the deep.According to the lithospheric thickness,the lithosphere of Southern China area is divided into weakly thinned fragment lithosphere(type I)and strongly thinned linear lithosphere(type II).The lithospheric thickness of the thinned thinned fragment is 130~150km,and the thickness of the thin linear lithosphere is about 80 km.Based on the evidence of mantle xenoliths,the material structure of different types of lithospheric mantle is analyzed,and it is inferred that there exist water and refractory ancient lithosphere mantle(type I)in the south China and a relatively rich water,new rocky mantle(type II).On the basis of the material structure,the thermal state of the upper mantle is analyzed by the correspondence of the earth heat flow,and the high mantle region is delineated according to the location of the high conductivity zone.