Fluid Influx Promotes Local Strengthening of the Creeping Xianshuihe Fault, Eastern Tibet

Abstract

AbstractWhile the Xianshuihe fault displays continuous creeping behavior, it is also the most seismically active fault in the eastern Tibetan Plateau, and its earthquake mechanisms remain unclear. Here, we aim at the stick‐slip portion of the creeping Qianning segment of the Xianshuihe fault to determine the characteristics of fault rocks and how fluids at depth influence fault behavior. Field survey, optical and scanning electron microscope observations, X‐ray diffraction and fluorescence analyses, as well as carbon and oxygen isotope analyses were performed on the collected samples. The fault core consists of 3–5 cm‐thick black fault gouge and ∼2.5 m‐thick breccia, surrounded by ∼12 m damage zone. In contrast to fault breccia (1–8 cm in diameter), the black fault gouge, which represents the principal slip zone of repeated seismic events, contains angular quartz particles (∼10 μm on average) and clays dominated by illite. The fluid‐rock interactions altering silica minerals into illite, and the thermal decomposition of carbonate minerals, passively increase the relative content of quartz and feldspar (total 63%–73%) in the fault gouge. The deeply sourced CO2 (from mantle and metamorphic degassing) within the hydrothermal fluids causes carbonate precipitation in breccias (21%–53%), composed of calcite, dolomite, and aragonite. These fluid‐assisted reactions lead to more abundant strong mineral phases (quartz, feldspar, and carbonates, 64%–87%) than weak clays (12%–36%) within the fault core, and locally strengthen the fault, which inhibits slow release of stress at shallow depth and promotes seismic rupture of the fault.