Rock characteristics, internal structure and physical-chemical properties of Qianning segment in Xianshuihe fault zone

The deformation behavior and slipping mechanism of fault zone are the focus of current seismic research. Rock characteristics, internal structure and physical-chemical properties of fault zone are the basis and key to determine creep- or stick-slip behavior and fault slipping mechanism. This study focuses on fault rocks from Qianning segment in Xianshuihe fault zone. Rock characteristics, structure, physical properties, mineral composition and chemical element distribution of the Xianshuihe fault zone have been analyzed in detail through field geological survey, observation of optical microscope and scanning electron microscope, particle size statistics, powder X-ray diffraction analysis (XRD) and thin section X-ray fluorescence spectrum analysis (XRF). Related deformation behaviors and slipping mechanisms have been discussed. The results show that: (1) the core of fault zone is mainly composed of black fault gouge, light yellow and yellow fault breccia, gray cataclasite, and gray fault breccia, presenting a single -core symmetrical structure; (2) the black fault gouge is 3 similar to 5cm thick and shows rapid-slip structure, indicating stick-slip behavior of the fault zone. The fault gouge can be divided into 13 slip zones, and the narrowest slip zone is about 40 mu m thick, representing at least 13 paleoearthquake events; (3) the fault gouge is mainly composed of illite, kaolinite, and quartz. The content of illite-smectite (I/S) mixed layer at the edge of fault gouge is abnormally high, which presents the primary slip zone of the latest paleoearthquake. The low permeability and high pore fluid pressure caused by main clay minerals of illite and I/S mixed layer (or smectite) in fault gouge and the discovery of fault gouge injection veins indicate that there is a thermal pressurization weakening mechanism in the fault zone sliding during the earthquake process; (4) according to the degree of fragment alteration and mineral distribution characteristics in different slip zones of fault gouge, the main seismic slip zone has a tendency to migrate towards cataclasite. It is speculated that during the sliding process, the fault tends to migrate to the side of the surrounding rock with high content of weak minerals (such as illite and I/S mixed layer) and low content of strong minerals (such as calcite and kaolinite).