Abstract:

 In tectonic studies, the Longmen Mountains fault zone is also called the Longmen Mountains foldthrust zone or nappe structure. Many researchers suggested that the 2008 Wenchuan Ms 8 earthquake was generated by the YingxiuBeichuan fault which is the central branch of the Longmen Mountains fault zone. Based on analysis of tectonic evolution and structural features in shallow subsurface of this fault zone, we did a new study of the relationship between the fault zone and the Ms 8 quake, and proposed a different view on this issue. Geological data indicate that the Longmen Mountains fault zone had undergone three evolution stages with varied dynamic conditions, they were the foreland foldthrust of SongpanGanzi orogenic belt (T3J), the orogene (KE) and the uplift at the eastern margin of the Tibetan Plateau (NQ). During the first and second stages, the fault zone developed in a progressive manner. And in the third stage, the fault zone was strongly reformed to the current shape. Now the Longmen Mountains fault zone lies in the upper crust of the SongpanGanzi Block and overthrusts onto the Yangtze Block in the east, where the ductile middle and lower crust as well as lithospheric mantle were thickened and plunged beneath the Yangtze Block, forming a structural style of a crocodile mouth biting the Yangtze Block in view on the vertical profile. Although the epicenters of the main shock and aftershocks of the Wenchuan Ms 8 event were distributed along the YingxiuBeichuan fault, its entire source body comprising these ruptures was located in the Yangtze Block below the Longmen Mountains. Such an inconsistency suggested that the causative structure of the Ms 8 quake was not the Longmen Mountains fault zone, and instead, it should be a highangle fault in the crust of the Yangtze Block. As the SongpanGanzi Block moved towards southeast and its frontal crocodilemouthshaped structure bit the Yangtze Block, the latter was cut off to generate a new highangle fault along which the rightlateral and thrust movement occurred, causing the Wenchuan Ms 8 event in 2008.

Key words: Wenchuan Ms 8 earthquake, Longmen Mountains fault zone, seismogenic structure, crocodile mouth structure, high-angle fault at the depth