Hydrogeochemical characteristics of hot springs in the Longriba fault, northeastern Tibetan Plateau: Tectonic implications for geothermal fluid circulation

The Longriba fault (LRBF) is in the seismically active northeastern Tibetan Plateau, yet the tectonic dynamics at the fault remain controversial. Furthermore, no studies have provided insights into tectonic activity through geothermal fluid circulation in the LRBF. In this study, we investigated 20 hot springs in the LRBF to get a better understanding of the processes and control factors between fluid circulation and tectonic activity. Hydrogeochemical results indicate that the hot springs in the study area are predominantly recharged by atmospheric precipitation and can be classified into two groups based on their hydrochemical types. Group I was a typical HCO₃-Ca, including some HCO₃-Ca⋅Mg, HCO₃-Mg, and HCO₃-Na⋅Ca. Group II hot springs, classified as HCO₃-Na type, have high reservoir temperatures (average 94 °C), suggesting dominant reactions with silicates during deep circulation. Meanwhile, the hydrogen, oxygen, and strontium isotopes of water in the two groups also show significant differences. In addition, the Group II hot springs are all located in the southwest segment of the study area, and their gases have higher ³He/⁴He ratios (Rc/Ra ratios up to 0.43 Ra). Hydrogeochemical results, combined with geological and geophysical data, suggest that differences in recharge elevation, granite distribution, and potential lower crustal flow or partial melting contributed to the distinct hydrogeochemical characteristics of the two groups. Moreover, we supposed that the spatial distribution of ³He/⁴He implies a lower crustal flow and partial melting beneath the LRBF. One possible explanation is the presence of a lower crustal flow tributary beneath the LRBF. In the LRBF, hydrothermal circulation and deep tectonics are likely key factors in generating high-frequency earthquakes. The above results provide insights into regional tectonic activity through the hydrogeochemical characteristics of hot springs.