摘要
本论文以中国地质大学(北京)完成的0线、100线、500线、700线、800线、900线、1000线等八条南北向大地电磁测深(MT)剖面数据为基础,系统应用现代的MT数据处理技术与二维反演算法,对观测数据统一进行处理与反演,获得剖面控制区域新的二维壳幔电性结构模型。大地电磁场观测数据的阻抗张量分解结果表明,研究区域的地下介质电性结构近似为二维结构,电性主轴方位近东西向,这说明青藏高原的主构造方向为东西向。通过对大地电磁测深响应的分析以及约束反演验算,证明了论文所采用的大地电磁数据的探测深度超过100公里。研究结果表明:研究区壳幔电性结构模型中,沿南北向横向存在的电性梯度带与畸变带,基本与区域地质资料圈定的大断裂带吻合,反映了区内断裂构造带的深部结构特征。青藏高原地下100Km深度范围内,地壳可大致分为三个电性层,其中第二个电性层为高导层,即中、下地壳高导层。中、下地壳高导层在各个地块内的分布有较大差别:喜马拉雅地块内的壳内高导层规模较小;拉萨-冈底斯地块内的壳内高导层分布最广泛,规模大,产状向北倾斜;而羌塘地块内的壳内高导层明显分隔为南、北两部分,分别位于南羌塘与北羌塘。青藏高原的壳内高导层可能是由于岩石的部分熔融或者部分熔融与水流体共同作用的结果。在拉萨-冈底斯地块,壳内高导层可能是印度板块向北俯冲的电性痕迹,高导层的成因与板块俯冲过程中由于摩擦生热导致岩石部分熔融和脱水作用有关;在羌塘地块,壳内高导层可能是由于幔源物质上涌的烘烤作用导致中、下地壳岩石温度升高,发生部分熔融现象的结果;羌塘地块内未见明显的板块俯冲痕迹。印度板块俯冲的前缘并没有越过班公-怒江缝合带,在班公-怒江缝合带南侧刚性的印度岩石圈下插至上地幔,造成幔源物质上涌,形成班公-怒江缝合带附近大规模的高导体。金沙江缝合带附近也存在大规模高导体,并且北侧的高导层明显向南倾斜,可能时亚洲板块向南俯冲的证据。沿缝合带东西向连续分布的大规模高导体,有可能是青藏高原下地壳物质向东“逃逸”的证据;其中班公-怒江缝合带可能是最重要的“通道”。
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".