Tectonic Controls on Hydrothermal Activity and Fluid Geochemistry along the Honghe Fault, SE Tibetan Plateau

The Red River Fault Zone (RRFZ) is a large-scale dextral strike-sip fault formed by the India-Eurasia collision, playing a crucial role in the tectonic evolution of the Southeastern Tibetan Plateau. As a deep-seated structure, the RRFZ acts not only as a pathway for deep material migration but also controls the geothermal activity and earthquake genesis. In this study, we utilize continuous hydrogeochemical data from eight hot springs along the fault over a two-year period to investigate the interplay between tectonic activity and fluid geochemical processes.

Isotopic signatures (δD, δ¹⁸O) identify meteoric recharge as the primary fluid source, with solute acquisition governed by water-rock interactions at depth. Hydrochemical facies analysis reveals a distinct zonation: the seismically active northern segment is characterized by Na-HCO3·SO4 waters, whereas the central-southern segments are dominated by Na-HCO3 and Na·Ca-HCO3 waters. Geothermometry estimates show that reservoir temperatures in the northern segment (308.6–329.7°C) are significantly higher than those in the central-southern segments (207.3–290.4°C). This thermal anomaly correlates spatially with the locus of maximum tectonic strain and elevated seismicity (M>5), suggesting a strong coupling between crustal deformation and deep fluid circulation.

Time-series analyses further elucidate the permeability dynamics of the fault system. Significant hydrogeochemical anomalies and shifts in estimated circulation depths were documented prior to the Myanmar and Eryuan earthquakes. Pre-seismic variations in the northern segment were dominated by Na⁺ and SO₄²⁻ fluxes, indicative of enhanced crustal permeability facilitating the upward migration of deep-derived components. In contrast, the southern segment exhibited more pronounced responses in HCO₃⁻ . These spatio-temporal discrepancies highlight that segment-specific lithological and structural controls modulate fluid pathways and mixing processes. Our findings demonstrate that hydrogeochemical proxies are robust tools for deciphering the active tectonics of crustal-scale fault systems, offering critical insights into mass and energy transfer within the SE Tibetan orogen.