| The North China Craton had suffered massive damage since Mesozoic. Its failure mechanism and extent of damage are always the research focus which attract a large number of geologists and geophysicists to study. The middle section of Tancheng-Lujiang fault is always the important study area for geology in the east of North China Craton. The thesis is based on a project called “The Magnetotelluric(MT) observation network in north China”which from the national project “Deep Exploration Technology and Experimentation ”(SinoProbe). This study area is located in Shandong province, the scope of longitude is 116° to 122° and latitude is 35 ° to 38 °. There are 258 MT sites in this study area which including some tectonic units called Luxi uplift, Lingqing depression, Jiyang depression, Chengning uplift, Huanghua depression, the middle section of Tancheng-Lujiang fault and Jiaodong uplift. By using the advanced methods for data processing, data analtsis and inversion, we obtained the 2-D and 3-D electrical structure models which will be discussed with Geology and other geophysical data.Data analysis based on tensor decomposition at different frequencies shows that the regional strike direction in study area is complex. There are significant differences about strike directions from shallow to deep in the same area, which proves that the "decoupling" phenomenon exists between the crust and lithosphere in this study area. Data analysis based on phase tensor analysis shows that the shallow sediments of the study area are generally of 1-D characteristics, the mid-lower crust of the study area are strongly of 2-D characteristics with complex strike direction, and the 3-D characteristics are clearly reflected in the lower crust. Data analysis according to tensor decomposition and induction vector shows the high resistivity characteristics in the Luxi uplift, and the strike direction of Tancheng-Lujiang fault is about N30°E. MT data of two profiles were rotated to corresponding regional strike direction and 2D inversions were conducted and the reliable 2-D inversion models were obtained. Since the study area is close to the coastline, comparative analysis between the 3-D inversion with submarine topography data and the 3-D inversion without submarine topography data shows that different submarine topography has a great influence on the 3-D inversion of MT data. Therefore the submarine topography data should be used in 3-D inversion and the reliable 3-D inversion models were obtained.According to the 2-D and 3-D inversion results, the distribution of the two main faults of Tancheng-Lujiang fault zone is like the shape “八”. The west branch of Tancheng-Lujiang fault zone is west-dipping and the east branch is east-dipping. The two main faults’ cutting depth is less than 80 km, they cut through the crust but did not cut through lithosphere. In the shallow surface, the two main faults were divided into four smaller faults. There is a big difference on electrical structure between the two sides of the weat branch, the east part called Jiaodong uplift with high resistivity and the west part called Luxi uplift with high conductivity below 35 km in depth. This big difference on electrical structure not only indicates that the west branch of Tancheng-Lujiang fault zone is the boundary between North China Block and Jiao-Liao-Korea Block, but also illustrates its left-slip occurred in the west branch which shows high conductivity. The Tancheng-Lujaing Fault paovides a channel for the upper mantle material. Analysis based on electrical models shows that the thickness of lithosphere in Luxi uplift is thinner with 80 km and other secondary tectonic units are more than 100 km, which concluded that the degree of the lithosphere thinning is inconsistent in the area of North China Block. The 3-D electrical models obtained from this paper provide references for the regional hot spot issues like seismic activity, lithosphere thinning phenomenon, the power and channels about asthenosphere upwelling and so on. |
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