Research of Conductive Structure of Crust and Upper Mantle beneath the South-Central Tibetan Plateau

With super-wide band magnetotelluric sounding data of Jilong-Cuoqin profile (named line 800) finished at 2001 and funded by the Ministry of Land and Resources, and Dingri-Cuomai profile (named line 900) finished at 2004 and funded by Ministry of Education, we obtained the strike direction of each MT station through strike analysis, then traced profiles perpendicular to the main strike direction, and finally got the resistivity model of each profile by Nonlinear Conjugate Gradients (NLCG) Inversion. With these two models, we have described resistivity structure features of the crust and upper mantle of the center-southern Tibetan plateau and their relationship with Yaluzangbo Suture: the upper crust of the research area is a resistive layer whose resistivity value ranges from 200 to 3 000 Ω·m; the depth of its bottom surface is about 15-20 km in general, but the bottom surface of the resistive layer is deeper in the middle of these two profiles; at line 900 it is about 30 km deep and even at line 800 it is about 38 km deep. There is a gradient belt of resistivity at the depth of 15-45 km, with a conductive layer beneath it whose resistivity is even less than 5 Ω·m. This conductive layer is composed of individual conductive bodies. At the south of Yaluzangbo suture the conductive bodies are smaller, with the thickness of about 10 km, leaning slightly to the north, but at the north of Yaluzangbo suture the conductive bodies are bigger, with the thickness of about 30 km, leaning slightly to the north too. Relatively speaking, the conductive bodies of line 900 are thinner than those of line 800, and the bottom surface of line 900 is also at a shallower depth. At last, after analyzing the factors affecting the resistivity of rocks, we concluded that the formation of the high-conductive layer was attributed to the partial melt of rocks or hydrous fluid in them. It suggests that middle and lower crust of the center-southern Tibetan plateau is very thick and hot, flabby and waxy.