Petrological records of creeping and locally stick-slip deformation along Qianning segment of Xianshuihe fault

The Xianshuihe fault, as the northern boundary controlling the large-scale eastward extrusion of Tibetan Plateau, is one of the most seismically active faults within the mainland of China and even in the world. However, geodetic studies have shown that it is extensively creeping at shallow depths. Understanding the deformation behavior of both creep- and stick-slip and what controls their occurrence are critical to evaluate the seismic activity of the Xianshuihe fault. This study focuses on a similar to 800m-wide rock-structure outcrop along Qianning section of Xianshuihe fault near Longdeng Town in western Sichuan. Structural features, mineral composition and geochemical properties of fault rocks in this outcrop were analyzed in detail through field geological investigation, optical and scanning electron microscopy, powder X-ray diffraction and micro area X-ray fluorescence spectroscopy analyses. The study profile is composed of a similar to 100m-wide sandstone damage zone and a similar to 700m-wide fault rock section. The latter contains highly developed fault breccias and interspersed thin fault gouge layers, which generally show pressure solution structures. However, local fault rocks along this profile show rapid-slip characteristics. It suggests that the fault zone experiences local stick-slip during extensive creeping deformation. Fluid-rock interactions within the fault zone alter feldspar and muscovite present in fault rocks into clay minerals (content 5% similar to 11%) dominated by illite. The new formed clays show oriented distribution in dense foliations. These interactions and clay fabric promote weakening and creeping deformation of the fault at shallow depths (above 4km). On the other hand, fluid influx leads to the precipitation of abundant carbonate minerals (content 12% similar to 33%, calcite and dolomite) and the enrichment of Ca elements where fractures are locally developed in the fault zone. These results in an increase content of strong minerals (quartz, feldspar, calcite and dolomite) within fault rocks, locally strengthening the creeping fault and promoting the fault behavior switch from aseismic creeping slip to seismic stick slip. Our study results help to better understand the seismic behavior of creeping faults and provide scientific basis for regional earthquake prevention and disaster reduction.