The Characteristics Of Crust-mantle Electrical Structure And Dynamics Within Tibetan Plateau

Based on the magnetotelluric sounding (MT) data from eight north-south trending profiles (the line 0, line 100, line 500, line 700, line 800, line 900, etc.) finished by China University of Geosciences Beijing and with systematical application of modern data processing technology and two-dimensional algorithm to data processing and inversion, the paper presents a new two-dimensional model of crust-mantle electrical structure in profile-controlled region.As the decomposition of the magnetotelluric impedance tensor show, the underground electrical structure of research area is approximately a two-dimensional structure, in which electrical axis is nearly east-west direction, thus indicating the main structure of Tibetan Plateau is east-west direction. With MT sounding response analyzed and constrained inversion checked, the exploration depth of MT sounding used in this paper is proved to be over 100km.The results show that in crust-mantle electrical structure model of the study region, the location of electric gradient and distortion zone along the north-south direction, roughly coincides with the major fault line delineated by regional geological data, reflecting deep structural characteristics of regional faults. Within the depth of 100Km, the Tibetan Plateau crust can be broadly divided into three electrical layer, of which the second layer is the high conductivity layer, that is, the middle and lower crustal high conductivity layer. The distribution of middle and lower crustal high conductivity layer differs a lot in different plots. In the Himalayan block exists a relatively small-scaled high conductivity layer; in Lhasa - Gangdise block, there is a vast distribution of high conductivity layer, with an attitude of north-dipping; and in Qiangtang block,the high conductivity layer can be clearly divided into two parts, which are located in the South and North Qiangtang respectively.The crustal high conductivity layer within Tibetan Plateau may be due to the effect of partial melting , or a combination of partial melting and fluid effect. In Lhasa - Gangdise block, crustal high-conductivity layers might be the electrical traces of the northward underthrusting of Indian plate. And the cause of crustal high conductivity layer there is probably friction heat, which can lead to rock partial melting and dehydration in the process of underthrusting. In Qiangtang block, causes of crustal high conductivity layer might be the heating effect of upwelling mantle-derived material, leading to the warming up and thus partial melting of middle and lower crustal rock. There are no apparent underthrusting traces in Qiangtang block.The leading edge of India plate did not cross the Bangong Lake-Nu River suture zone in the underthrusting; In the south of Bangong Lake-Nu River suture zone, India's rigid lithosphere underthrusts to the upper mantle, resulting in the upwelling of mantle material, thus the large-scale high-conductors near Bangong Lake-Nu River suture zone is formed. Near Jinsha River suture zone also exists a large-scale high conductivity layer, and the north of the layer obviously tilted south, which might be the evidence of Asian plate's underthrusting southward. And the large-scale east-west direction distribution of high-conductors along the suture zone might prove that the Tibetan Plateau lower crustal materials escape eastward and the Bangong Lake-Nu River suture zone is probably the most important "passages".