Differences of Deformation in the Deep Part of the Longmen Shan Fault Belt, Eastern Tibetan Plateau: Evidence from Stress Condition in Fault Drilling Cores

As the eastern boundary of the Tibetan Plateau, the Longmen Shan fault (LMSF) has had a long activity history and long-term deformation, and the fault rocks have recorded multiple large earthquakes. The core data obtained by the Wenchuan earthquake Fault Scientific Drilling Project (WFSD) shows that there are many different fault zones developed in the deep part of the LMSF. Because the deformation and stress environment of the fault zone are not clear, it is difficult to properly understand the formation process of the LMSF, as well as the mode of fault activity and the mechanism of earthquakes. The syntectonic quartz/calcite veins from the drilling cores can yield deformation and stress field information, and the deformation of the fault zone and the seismogenic environment can be recognized.

The WFSD-2 borehole is located in the southern segment of the Yingxiu-Beichuan fault, the Wenchuan earthquake main surface rupture zone. A series of fault zones and their associated quartz and calcite veins are present in the cores at 600-1100 m and 1200-2100 m-depths, respectively. The real characteristics of 20 faults and 299 veins in the cores are obtained by imaging log data, well deviation and core scanning data. The veins mainly intersect the fault zones at small angles, mainly along the Riedel shear Y, R1, P and R2 planes. Based on the striations and the angle between faults and veins, the fault slip direction and kinematics properties are obtained. At depths of 600-1100 m, the maximum principal compressive stress direction is N320°-360°E, and the fault is dominated by left-lateral slip (with thrust) while at 1200-2100m, it is N270°-320°E, and the fault is dominated by right-lateral slip (with thrust). The results not only indicate that the fault kinematics properties change with depth, but also, according to previous studies, that the LMSF was mostly left-lateral in the Late Triassic and right-lateral in the Cenozoic. This suggests that the upper segment has a long history, and that the faults in deeper sections are relatively new, reflecting that the growth of the LMSF has a front-expanding feature.