Crust and upper mantle resistivity structure at middle section of Longmenshan, eastern Tibetan plateau

摘要

Longmenshan is located in the eastern edge of the Tibetan Plateau. It is one of the hotspots of geophysical and geological studies in the world for its special tectonic characters and dynamic mechanism, especially after the 2008 Wenchuan earthquake (Ms 8.0). From 2008 to 2010, a 570 km Long-period magnetotelluric (LMT) and broadband magnetotelluric (MT) sounding profile across the Longmenshan fault zone has been carried out to investigate the crust and upper mantle resistivity structure. The coupling relationship of Longmenshan with the Songpan–Ganzi block and with the Sichuan basin of Yangtze block has been studied respectively. The analysis of the deep resistivity structure is of importance for the study of the geodynamic characteristics of the Wenchuan earthquake. The inversion reveals a high–low-high resistivity layer in the Songpan–Ganzi block, a low-high resistivity layer in the Sichuan basin and a complex resistivity structure in the Longmenshan area, respectively. There is a low resistivity layer in the middle and lower crust at the depth about 20–45 km beneath the Songpan–Ganzi block, which has shown that there may be a continuous slip layer in the crust, while there is no such a layer in the crust beneath the Sichuan basin. The thrust imbrication of resistivity structure in the upper crust beneath the Longmenshan indicates that the Songpan–Ganzi block is over thrusting onto the Yangtze block. And the low resistivity layer in the lower crust stretching downward to the bottom of the lithosphere beneath Longmenshan indicates that the Songpan–Ganzi block subducts below the lithosphere of Yangtze block. The complex structure of high resistivity massif in the lithosphere beneath Longmenshan provides some important evidence for the study of the mechanism of the 2008Wenchuan earthquake.