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

While 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 similar to 2.5 m-thick breccia, surrounded by similar to 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 (similar to 10 mu m 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. Plain Language Summary Large earthquakes are less likely to occur along active faults with slow slip that can release tectonic stress continuously (thus called "creeping" fault). However, numerous large earthquakes still occur along a typical creeping section of the Xianshuihe fault in eastern Tibet. Here, we better constrain the structural, mineralogical, and geochemical properties of fault rocks formed by earthquakes in the creeping Qianning segment of the Xianshuihe fault to understand its earthquake mechanism. The study outcrop consists of 3-5 cm-thick black fault gouge, similar to 2.5 m-thick breccia, and similar to 12 m damage zone. Intense fluid-assisted alteration of silica minerals and thermal decomposition of carbonate minerals result in the passive enrichment of quartz and feldspar (total 63%-73%) in the black fault gouge. The fault breccia contains abundant carbonate minerals (21%-53%), resulting from carbonate precipitation associated with the influx of hydrothermal fluids enriched in CO2 at depth. Therefore, these fluid-assisted chemical reactions lead to more abundant strong mineral phases (quartz, feldspar, and carbonates, 64%-87%) than weak clays (12%-36%) within the fault core, which locally strengthen the fault and promote the occurrence of earthquakes on this creeping fault.