Unraveling Fluid‐Rock Coupling in Mudstone Fault Zones: Experimental Perspectives From Southwestern Taiwan

Abstract Fluids are crucial in influencing the physical and chemical properties of rock masses. Beyond lubricating and reducing friction, fluids alter mineral phases and cementation through fluid‐rock interactions, affecting mechanical properties. This study focuses on southwestern Taiwan, where mudstone blocks are highly deformed due to the presence of the Chishan and Chegualin faults. The deformations have damaged tunnels and roads. We hypothesize that fluid activity strongly impacts fault rocks by altering mineral phases and element concentrations under varying temperatures and salinities. Thus, the study aims to explore the coupling mechanisms between fluids and rocks and their impact on fault behavior through X‐ray diffraction, scanning electron microscope, and inductively coupled plasma optical emission spectroscopy. The results revealed that fluids increase quartz dissolution, pyrite oxidation, feldspar weathering, and clay transformation. Moreover, the increased temperature and reduced salinity enhance quartz dissolution and clay formation, suggesting rock weakening under these conditions. It implies that the types and concentrations of ions in the fluid and the types and contents of minerals in the rocks change before and after the fluid‐rock reaction. Consequently, the mechanical strength of the Chishan Fault is weaker and deforms faster than the Chegualin Fault, aligning with GPS literature. The risk and damage warrant ongoing monitoring and investigation.