Geothermal activity is prolific across the Himalaya-Tibet orogen, yet the principal tectonic mechanism controlling upper-crust hydrothermal circulation across this region remains unresolved. We use regional-scale geochemical sampling of spring systems and integrate classical fluid geothermometry, helium (3He/4He) isotopes, and stable isotopes of water with the mapped distributions of active fault traces to characterize regional heat source(s) and permeability pathways for hydrothermal flow. We utilize helium-isotope measurements from 196 localities and aqueous chemistry along with stable isotope data from 142 springs, yielding a dataset of 127 spring systems with both isotopic and chemical analyses. Stable isotopes of water (δ18O – δD) cluster near the local meteoric water line for nearly all springs sampled across the Tibetan Plateau, demonstrating that there is minimal rock-water interaction and meteoric recharge is the principal source of fluids for geothermal reservoirs of the region. In total, 70% of springs in our database are recognized as likely being associated with active fault systems, and elevated temperatures are positively correlated with fault control. Fifty-seven springs (40% total) across the region yield quartz-geothermometry estimates greater than 120°C, with four of those geothermal systems greater than 180°C. Southern Tibet contains the greatest density of high-temperature geothermal systems (those for which quartz-geothermometry yields modeled temperatures greater than 120°C), with model temperatures up to 215°C, that all have crustal 3He/4He ratios and are associated with major north-trending normal faults or conjugate northeast and northwest-striking strike-slip faults. Even the hottest springs in northern Tibet (quartz-geothermometry temperature of 152°C) are not as hot, but most of these high-temperature systems are located close to active northeast-striking strike-slip faults. Observed 3He/4He ratios are higher in northern Tibet, requiring some mantle volatile contribution. We attribute the principal control for geothermal systems in Tibet to be active extensional and transtensional fault zones that locally elevate permeability to allow deep (~8-14 km) circulation of aqueous solutions to mid-upper crustal depths across a region with normal-to-elevated heat flux.

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