Dual-mode geochemical evolution of geothermal fluids across strike-slip and pull-apart segments of the Ganzi–Litang fault, eastern Tibetan Plateau

Study region The Ganzi–Litang fault zone (GLFZ), the eastern margin of the Tibetan Plateau. Study focus Fault segmentation can govern the circulation and geochemical evolution of geothermal fluids, but the contrasting effects of strike-slip and pull-apart structures along plateau-margin fault zones remain insufficiently quantified. We compiled 58 water and isotope samples and 28 gas samples from the GLFZ and integrated Self-Organizing Map (SOM) classification with H–O–Sr–He–C isotopic tracing, reservoir-temperature estimation, and volatile-source apportionment to evaluate segment-specific controls on recharge, water–rock interaction, mixing, and degassing. New hydrological insights for the region Geothermal fluid evolution along the GLFZ reflects a dual-mode, segment-dependent geochemical pattern. The SOM analysis categorized the geothermal waters into three hydrochemical groups, revealing both segment-related geochemical patterns and a cross-segment shallow-circulation group. In the northern Ganzi strike-slip segment, the Group 2 geochemical signature is best explained by laterally coherent deep-circulation conduits in a granitic reservoir, yielding partial-to-full water–rock equilibrium and reservoir temperatures of 200–230 °C. In the southern Xinlong–Litang pull-apart segment, fluids (Group 1 and Group 3) are more strongly diluted by shallow cold water (57–81%) and record lower deep thermal end-member temperatures, of 140–170 °C, multi-lithological water–rock interaction, and spatially localized He–CO₂ anomalies. The geochemical patterns suggest that the northern strike-slip segment favors stable, long-residence deep circulation, whereas the southern pull-apart segment is characterized by stronger shallow mixing and localized deep-volatile anomalies. The results provide a regional framework for segment-specific geothermal development and the protection of alpine water resources in composite fault zones.