MT sounding and lithosphere thickness in North China

Abstract: In recent years, with the development of geosciences, there has been a growing interest in the study of electrical conductivity structure of the continental lithosphere, which is because the results of solid geophysical research show that the study of electrical conductivity structure of the continental lithosphere can provide important physical grounds for research in other areas of geosciences. Magnetotelluric(MT) sounding is an indispensable geophysical method for the study of the structures of the crust and upper mantle in the context of electrical conductivity. For a long time, great efforts have been devoted to the study of MT technologies in China. At present, with the development of science and technology, major advances have been made in instrumentation, data acquisition or processing technology and inversion technology. In China, superwide-band and high-precision MT survey techniques have been applied and data processing and inversion technologies have been improved greatly and in the main are compatible with the world's advanced technologies. Therefore, an MT profile was run along the Yingxian, Shanxi, to Shanghe, Shandong, in 2001. A 2D conductivity structure model of the profile, obtained by using the advanced MT data processing method and rapid release inversion (RRI), shows the features of the conductivity structure of the lithosphere in North China. According to the electrical features, the North China lithosphere is divided into the eastern and western parts by the frontal fault of the Taihang Mountains. The eastern part is characterized by low resistivity and the western part by high resistivity. In the eastern part, the electrical structure of the upper curst corresponds roughly to a tectonic framework of alternating uplifts and depressions in the North China rift system and the lithosphere conductance reaches a maximum of 30 000 S, being far greater than that of the Andes magma arc area with strong volcanic activities and that of the Tibetan Plateau. In the western part, the lithosphere of the Taihang and Hengshan mountains, marked by high-resistance blocks, is characterized by the conductivity structure of the stable continental lithosphere. However, a group of gently west-inclined high-conductive layers, with a conductivity of 0.04—0.25 S/m and a top depth of 20 km and a bottom depth of 40 km, were discovered under the high-resistance block of Hengshan Mountain.Study indicates that the very low-resistivity feature of the crust below the North China rift basin is probably determined by the thermal structure and regime in the curst and mantle of the rift basin. According to a discussion on continental resistivity models, it can be inferred that the “asthenosphere” character defined by predecessors might not exist in the upper mantle below the Ordos block. Although the North China rift basin is a Meso-Cenozoic tectonically active area, its activity is weaker than that of younger volcanic arcs or zones with violent tectonic movement. Therefore, the asthenosphere character of the North China rift basin is not distinct. That is one of the possible reasons why there is no "high-conductive layer of the upper mantle" in the MT profile from Yingxian to Shanghe, which images the asthenosphere electrical characters. However, according to the survey results, it can be inferred that the crust and lithosphere of western North China are thicker than those of eastern North China. In order to obtain more accurate results, more intensive and precise geophysical surveys and studies remain to be conducted.