Hydrogeochemical characteristics of geothermal springs along the central section of Altyn Tagh fault: Implications for deep fluid circulation and tectonic activity

Geothermal springs associated with fault systems may offer valuable insights into fluid-rock interactions and crustal processes. However, hydrogeochemical data on the Altyn Tagh fault, a major strike-slip fault bounding the northern Tibetan Plateau, are scarce. In this study, the chemical and isotopic compositions of 29 geothermal springs along the central section of Altyn Tagh fault have been analyzed and the results show the following: (1) The springs are primarily meteoric in recharge source, exhibiting Na-SO₄-Cl and Ca-Mg-SO₄-Cl type. Calculated circulation depths ranging from approximately c. 135 m to c. 2790 m, indicating a shallow to intermediate hydrothermal system with limited water-rock interaction. Dissolved inorganic carbon (DIC) derives primarily from deep decarbonation and carbonate dissolution. (2) Specifically, the highest reservoir temperatures, deepest circulation, and greatest deeply sourced CO₂ flux have been found from the Xorkoli to the Wuzunxiaoer section of the fault. Spatial trends reveal increasing reservoir temperatures, circulation depths, and endogenic DIC fractions with increasing distance from the fault, suggesting the influence of distal, deeper geothermal systems on spring geochemistry. (3) Comparative analysis with other strike-slip faults reveals notably shallower circulation depths along the Altyn Tagh fault, attributable to the combined effects of cold crustal thermal regime, seismic cycle, and moderate topographic relief. Locally enhanced circulation depths occur in distinct sedimentary-hydrogeological conditions and extensional step-overs areas. These findings provide new constraints on the relationship between hydrothermal circulation and fault zone processes, with implications for understanding crustal fluid dynamics along major strike-slip faults.