MT Research Of Lithospheric In Eastern North China Craton

This paper relies on the second subject “Model Experimental Study of MT Standard Network in the Qinghai-Tibet Plateau and North China” of the project “Experimental Study of Standard Network of Continental Electromagnetic Parameters”, which is the first project of the state special research “Technological and Experimental Study of Deep Probing”. Data acquisition is carried out in the east area of North China Craton and lithospheric electrical structure study is also proceeding here. The study area includes Beijing, Tianjin, Shanxi, Hebei, Henan, Shandong and most part of Jiangsu province, whose geographic coordinates is 113°~119°E, 35°~39°N, with 36 long-period MT observing points and 346 MT observing points in total.Dimensionality analysis shows that the whole study area manifests with strong two dimensionality and local part with certain three dimensionality, so a 2D inversion method is applied in this research. With phase tensor analysis, two dimensionality of the study area is further tested and the corresponding electrical principle axis is also obtained: NE30°for line39, NE40°for line38, NE35°for line36 and NE30°for line35. About the inversion work, five MT profiles are designed along latitudinal direction and 2D NLCG inversion algorithm is used for the TM mode. The inversion parameters are set to be Tau=3, α=1, Error Floor(TM Rho=5%, TM Phase=5%) and an iteration count number of 200. After inversion, five reliable 2D geoelectrical models are obtained.Through analysis of electrical gradient belts in the electrical structure models and a combination with the Moho depth got by some seismic data, we determine that Taihang Mountain fault is a deep big fault cutting through the Moho. Lithospheric thickness of different part in the study area is: 100 km for the nothern Taihang uplift area and 150 km for its southern part, 100 km for the nothern Qinshui depression area and 150 km for its southern part,60~70km for the Luxi uplift area, 40~50km for both Neihuang uplift area and Dongpu depression area, 110 km for Jiaodong uplift area, 90~100km for Linqing depression area, 120 km for Jiyang depression area, 80~90km for Jizhong depression area, 80~90km for Cangxian uplift area, 90~100km for Huanghua depression area area and 70~80km for Chengning uplift area.With the electrical structure study of the target area, we obtain the lithospheric thickness range of modern North China Craton and apply seismic wave method and mthod of analysing earth heat value to test the reliability of the conclusions. Then, through a comparision with the lithospheric thickness of the craton before its destruction, we know that the lithosphere thinning happened indeed. At last, we discuss the mechanism and deep dynamic reasons of the destruction of North China Craton according to the lithospheric reduction phenomenon.