关键词: 羌塘盆地, 结晶基底, 大地电磁测深, 航磁异常, 裂谷

Abstract: The Pre-Devonian metamorphic rocks exposed in the middle part of Qiangtang Basin, Qinghai-Xizang(Tibet) Plateau were formerly termed Armugang Group or Gemuri Group, which is composed of schists, gneisses, amphibolites and blue-schists. Basing on the metamorphic ranks and deformation degree of these rocks, the Qinghai-Xizang(Tibet) Petroleum Exploration Project Management Department of CNPC has recently divided the group into Guoganjianianri, Mayer Kangri, Gomori and Armugang Formations from top to bottom. The Gomori and Armugang Formations made up the crystalline basement of the Qiangtang Basin. Recently, it has been found that this series of rocks contain also radiolarian fossils of middle to late Permian and Triassic in ages, as well as early Permian and late Triassic mafic rocks. This may indicate that tensional taphrogeny had occurred during that time in the region. The results of aeromagnetic survey show that the strong magnetic anomalies spread as stripes along Rongma-Shuanghu, Geladaindong and Purog Kangri, as well as along both the southern and northern boundaries of the Qiangtang Block. The anomaly intensity varies between 100～300nT. Based on the aeromagnetic anomalies, the basement of the Qiangtang Basin is divided into two depressions and one uplift. The northern depression is 11km deep, and the southern one is 9km. Each depression can be subdivided into smaller units. After 20km upward continuation, the spreading direction of the aeromagnetic anomaly changes from east-west-trending to south-north-trending. Along Rongma, Shuanghu area and the E90° line there exist south-north-trending transverse tectonic zones, which separate Qiangtang Basin into western and eastern secondary blocks. The results of Magnetotelluric (MT) Sounding also support the existence of this situation. Moreover, the MT profile shows that the geo-electric structures of the basement in the basin vary significantly from north to south. For example, in south Qiangtang basin many crustal high-resistance bodies are developed, and the high-conductivity layer is buried at a greater depth and deformed intensely. In northern Qiangtang, however, many low-resistance bodies are developed, and crustal high- conductivity layer is buried at a shallow depth and deformed weakly. All of these situations are the result of later tectonic transformation. The crystalline basement of the basin was intensely transformed during the later uplifting of the Plateau. The basement was filled with high- or low-resistance compositions, which changed the continuity and homogeneity of the basement.

Key words: Qiangtang Basin, Crystalline basement, Magnetotelluric sounding, Aeromagnetic anomaly, Rift