青藏高原东缘地壳上地幔电性结构研究进展

经过数十年的努力,中国学者针对青藏高原东缘地壳上地幔探测,累积完成超过20000 km的大地电磁测深剖面,取得了一系列重要科学数据和认识,为青藏高原东缘构造格局、地壳上地幔电性结构、地震机制和动力学研究奠定了基础.根据青藏高原东缘的主要构造和断裂分布特征,本文重点对龙门山构造带、川滇构造带和三江构造带三个构造带分区进行研究,主要依据大地电磁探测工作成果和壳幔电性结构特征,系统地对青藏高原东缘地壳上地幔电性结构、与扬子西缘接触关系、汶川地震和芦山地震的电性孕震环境及弱物质流通道等几个方面进行了梳理和分析.一是青藏高原东缘地壳表层岩块和物质沿壳内高导层向龙门山造山带仰冲推覆,表现为逆冲推覆特征的薄皮构造;二是高原东部地壳中下部及上地幔顶部向龙门山造山带和上扬子地块西缘岩石圈深部俯冲,呈现刚性的上扬子地块西缘高阻楔形体向西插入柔性青藏块体的楔形构造;三是将汶川地震和芦山地震的震源投影到大地电磁剖面上,发现震源位于剖面下方的高阻块体与低阻体之间靠近高阻体的一侧,龙门山构造带岩石圈表现出高阻、高密度和高速的"三高"特征,这种非均匀电性结构可能构成地震孕育发生条件;四是川滇和三江地区的多条大地电磁剖面探测结果表明,在青藏高原东缘中下地壳存在下地壳流和局部管道流,大地电磁结果对其空间分布形态、位置及大小进行了较好的刻画.根据研究区壳幔电性结构特征的构造解析和综合实例分析,总结了青藏高原东缘六类壳幔电性结构模型,提出了下一步重点研究领域和目标.总之,青藏高原东缘壳幔电性结构的研究对揭示研究区岩石圈结构和构造格局提供了重要依据,对油气及矿产资源远景评价提供了背景资料,对"Y"型多地震区的构造关系和发震机理研究具有重要指导意义.

Research progress in research on electrical structure of crust and upper mantle beneath the eastern margin of the Tibetan plateau

After decades of efforts, Chinese scholars have acquired a large number of important scientific data and understanding of electrical structure of the crust and upper mantle beneath the eastern margin of the Tibetan plateau, covering a total length of profiles more than 20000 km, which laid a foundation for study of the tectonic framework, electrical structure of crust and upper mantle, seismic mechanism and dynamics. This paper focuses on the three tectonic zones of Longmenshan, Sichuan-Yunnan and Sanjiang based on the main structures and fault distribution characteristics. We make a systematic review and analysis of the electrical structure of the crust and upper mantle in the eastern margin of the Tibetan plateau, its contact relationship with the western Yangtze block, electrical backgrounds of the Wenchuan and Lushan earthquakes and weak material flow channels, which are based on the results of MT surveys and characteristics of electrical structure of crust and mantle. Firstly, the surface rock and material of the eastern Tibetan plateau are overthrust along the high-conductivity layer in the crust toward the Longmenshan orogenic belt, showing thrust-nappe characteristics of thin-skin structure in the shallow crust. Secondly, the lower crust and upper mantle of the eastern Tibetan plateau are subducted to the Longmenshan orogenic belt and Yangtze block lithosphere, while the rigid western Yangtze block is inserted into the weak Tibetan plateau at depth. Thirdly, it can be found that the sources of the Wenchuan and Lushan earthquakes are located at the side of high resistivity from MT data. The Longmenshan tectonic zone lithosphere shows a "three high" characteristic of high resistance, high density and high velocity, where the non-uniform collocation of this kind of the electric medium is favorable to generation of earthquakes. Fourthly, the results of MT profiles in the Sichuan-Yunnan and Sanjiang regions also show lower crustal flow and local channel flow of middle and lower crust material developed in the eastern Tibetan Plateau, of which the spatial distribution, location and size are well depicted by MT data. Six kinds of electrical structure models of crust and mantle in the eastern margin of the Tibetan plateau are established based on the structural analysis and comprehensive study of the electrical structure, and the next key research fields and objectives are suggested. In short, the study provides an important basis for revealing the lithospheric structure and tectonic framework in the study area. It can also provide background of the evaluation of solid minerals, oil and gas resources, and has important guiding significance to the study of the tectonic relationship and mechanism of the "Y" type seismically active area.