Geoscience

Mechanism of Earthquakes in Southwest China

Since 45Ma or about, the crust of the India-Eurasia collision zone has shrinked at least 1,500 km due to continuous convergence and collision between the Indian and Eurasian plates. How to regulate or absorb such a large scale of convergence has been a hot subject of debates in the circle of geoscience. The Longmen Mountain fault belt lies on the eastern margin of the Tibetan Plateau and adjacent to the western margin of the Yangtze craton is an ideal research “window” for understanding the intracontinental deformation mechanism and testing the different compensation modes, and a key region for exploring the lateral accretion of the plateau, studying the interaction between basins and mountains and probing into the deep structural background of the occurrence of the M8.0 Wenchuan earthquake. Prof. Zhang Zhongjie and his colleagues from the Division of Tibet Plateau, Institute of Geology and Geophysics, CAS, on the basis of the passive-source seismic profile across the Longmen Mountain fault belt in 2006-2007, analyzed P and S receiver function images and conducted comprehensive dynamic research. They found series phenomena that can not be fully explained by two mainstream modes, escape structure and lower crustal flow. Their comprehensive research demonstrated that the resistance of Sichuan Basin to the escape flow of deep substance in the eastern Tibetan Plateau is not limited to the level of crust, but very likely extends to the whole lithosphere, and even reaches the upper mantle. In accordance with above analysis, they put forward a two-phase regulation mode for the interaction between the escape flow of deep substance at the east of Tibetan Plateau and Sichuan Basin: 1) the westward escape flow of deep substance in the Tibetan Plateau, after being rigidly resisted by Sichuan Basin, breaks into two flows; one flow bypasses Sichuan Basin and escapes towards southeast and northeast and the other flow submerges into the mantle (the near-vertical flow of asthenospheric substance below Longmen Mountain orogenic belt can be regarded as another mode of compensation for continuous convergence between the India and Eurasian plates; 2）removal of the bottom of the lithosphere caused by delamination of lithosphere or hot asthenospheric escape flow leads to gravitational buoyancy, resulting in the uplift of Longmen Mountain and formation of Longmen Mountain orogenic belt with the largest topographic gradient at the margin of Tibetan Plateau. This research result was recently published on Earth and Planetary Science Letters (Zhang et al. Seismic signature of the collision between the east Tibetan escape flow and the Sichuan Basin. Earth and Planetary Science Letters, 2010, 292: 254-264).