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Crustal electrical structure across the Tangra-Yumco tectonic belt revealed by magnetotelluric data: new insights on the east-west extension mechanism of the Tibetan plateau

Chen Ning, Wang Xuben, Shao Changsheng, Zou Jun, Xu Zhengwei, Li Dewei, Wang Xiangpeng

Acta Geophysica

2023

Vol. 71, no. 2

783--794

Geologic evolution of the Tibetan plateau is characterized by crustal extension and horizontal movement in the post-collision stage, during which, approximate north–south (N–S) trending tectonic belts typically represented by Tangra-Yumco rift are developed. The Tangra-Yumco tectonic belt is an ideal object to investigate the deep structure and mechanism of the crustal extension. The magnetotelluric (MT) method is effective in probing crustal structures, especially for high-conductivity bodies. A MT profile of east–west direction with dense stations has been carried out across the Tangra-Yumco tectonic belt. Resistivity models independently derived from two-dimensional and three-dimensional inversions provided more detailed geophysical constraints on the mechanism of crustal extension and deformation. A significant conductor with estimated melt fraction as 3.0–7.5% in mid-lower crust was revealed under the N–S tectonic belt, where the asthenospheric upwelling through the slab-tearing window might have induced partial melting of the lithospheric mantle and lower crust. Combined with previous studies, the upward migration of hot mantle materials and the expansion of the lower crust should be the primary mechanism driving east–west (E–W) extension of the brittle upper crust with high resistivity above the depth of 30 km. According to lateral electrical discontinuity in the upper crust, we inferred that there might exist three normal faults with the reference of topography and the trend of extension of the existing faults. The expansion and deformation of the conductor might have pulled the brittle upper crust and cause significant E–W extension, leading to the formation of the approximate N–S trending rift and normal faults.