Analysis on multi period activity and evolution of Yingxiu-Beichuan fault in Longmenshan area

As the boundary between the Yangtze Craton and the Qinghai-Tibet Plateau, the Longmenshan tectonic belt is known for its typical fault-slip systems and strong Cenozoic activities. Due to the complexity of the structure of the orogenic belt, the tectonic evolution and formation mechanism of the Longmenshan tectonic belt have been controversial. This paper essentially focuses on the geological isotopic chronology limitation of the fault activity of the Longmenshan Central Fault (Yingxiu-Beichuan Fault). The K-Ar dating of the illite in the fault gouge sample on the fault surface reveals that the direct age of the fault would be about 111 Ma. Combined with the previous age results, the following results are obtained: The Yingxiu-Beichuan fault was a multi-stage active fault in the Mesozoic, which was active in the Late Triassic (229–216 Ma), Early Jurassic (190–171 Ma) and the end of Early Cretaceous (130–110Ma). This paper analyzes the evolution of the central fault through regional geological evolution, seismic profile analysis and structural physical simulation experiment, and the following main results are obtained: Animaqing Paleo-Tethys Ocean on the northern margin of the Songpan Garze Block and the Jinshajiang Paleo Tethys Ocean in the southern margin were closed successively in the Middle and Late Triassic, Such a fact essentially occurred under the influence of differential uplift, that is, the pre-existing Yingxiu-Beichuan fault changed from a normal fault to a reverse fault in the Late Triassic, and then reactivated. Under the influence of the gravity slip mechanism, the Yingxiu-Beichuan fault was active until it ceased in the Early Jurassic. Since then, affected by the closure of the Middle Tethys Ocean, in the Early Cretaceous, the fault developed again, and branch faults developed in front of the fault. During the Himalayan period, due to the collision and ccocnnection of the Eurasian plate and the Indian plate, the fault was reactivated.