Geochemical Characteristics of Thermal Springs and Insights Into the Intersection Between the Xiaojiang Fault and the Red River Fault, Southeastern Tibet Plateau.

During the ongoing uplift and expansion of the southeastern margin of the Tibetan Plateau, the front edge of the Sichuan‐Yunnan rhombic block (SYB) has experienced intense tectonic activity and frequent seismicity. In this study, the fluid geochemistry in the primary active faults at the front edge of the SYB was investigated, with the aim of understanding the tectonic activity and intersection relationship between the Xiaojiang fault (XJF) and the Red River fault (RRF). Thermal spring water and gases exhibit a coupled spatial distribution relationship; relatively high ion concentrations and 3He/4He ratios (Rc/Ra ratios of 0.21 to 0.62Ra) are observed along the RRF, Qujiang fault (QJF), and Shiping‐Jianshui fault (SJF). Multidisciplinary research results have indicated that mantle‐derived intrusion has been detected in the crust beneath the QJF and SJF. The current tectonic activity in the front edge of the SYB remains intense, with compressive stresses shifting toward the western side of the XJF and accumulating on the QJF and SJF. This has led to the development of fractures, enhancing the water–rock interaction and deep‐derived gas degassing along the faults. The unmixing characteristics of fluids at the intersection area of these two faults suggest the absence of conduits for fluid migration between the faults. Owing to the lower gas 3He/4He ratios, lower shear strain rates, stable reservoir temperature field, and extremely low historical seismicity in the Indo‐Chinese block, it is speculated that the current movement of the XJF may not cut through the RRF and continue southward. Plain Language Summary: Fluids serve as carriers of information regarding deep activities, and are known to migrate along active faults. Additionally, fluid geochemistry is highly sensitive to tectonic activity. Given the intense tectonic activity and frequent seismicity experienced at the front edge of the Sichuan‐Yunnan rhombic block (SYB), our study focuses on investigating the fluid geochemical characteristics of the primary active faults in this region. Significant spatial differences in fluid chemistry are observed, particularly with respect to relatively high ion concentrations and mantle He values along the Red River fault (RRF), Qujiang fault (QJF), and Shiping‐Jianshui fault (SJF). Furthermore, the lack of conduits facilitating fluid migration between the Xiaojiang (XJF) and RRF is evident from the distinct unmixed characteristics of the fluids. Multidisciplinary results indicate the presence of mantle‐derived intrusion into the crust beneath the QJF and SJF. The compressive stresses have shifted toward the western side of the XJF and are accumulated on the QJF and SJF, resulting in the observed spatial variations in fluid geochemistry. Ultimately, these spatial differences can be attributed to the unique intersection relationship between the XJF and RRF. Key Points: Mantle‐derived intrusion has been detected in the crust beneath the Qujiang fault (QJF) and Shiping‐Jianshui fault (SJF)Tectonics remains active at the front edge of the Sichuan‐Yunnan block, and stresses accumulate on the QJF and SJFThe current movement of the Xiaojiang fault may not cut through the Red River fault and continue southward [ABSTRACT FROM AUTHOR]