Hydrothermal Degassing Through the Karakoram Fault, Western Tibet: Insights Into Active Deformation Driven by Continental Strike‐Slip Faulting.

The Karakoram fault (KKF) is an important strike‐slip boundary for accommodating deformation following the India‐Asia collision. However, whether the deformation is confined to the crust or whether it extends into the mantle remains highly debated. Here, we show that the KKF is overwhelmingly dominated by crustal degassing related to a 4He‐ and CO2‐rich fluid reservoir (for example, He contents up to ∼1.0–1.6 vol.%; 3He/4He = 0.027 ± 0.013 RA (1σ, n = 47); CO2/N2 up to 3.7–57.8). Crustal‐scale active deformation driven by strike‐slip faulting could mobilize 4He and CO2 from the fault zone rocks, which subsequently accumulate in the hydrothermal system. The KKF may have limited fluid connections to the mantle, and if any, the accumulated crustal fluids would efficiently dilute the uprising mantle fluids. In both cases, crustal deformation is evidently the first‐order response to strike‐slip faulting. Plain Language Summary: Bubbling hot springs are common in fault zones along which Earth's lithosphere cracks. Chemical and isotopic compositions of spring gases can offer key information on the subsurface connectivity of the deep‐rooting faults that is not easily visible. To assess whether the Karakoram fault (KKF) in western Tibetan Plateau is developing in the crust or extends into deeper mantle, we studied the origin and transport of spring gases and found that the KKF is overwhelmingly dominated by degassing of a crustal fluid reservoir that contains high amounts of helium (He) and CO2. This could be attributed to He‐CO2 mobilization of deforming and fracturing fault zone rocks at crustal depths, suggesting that the KKF is primarily developing in the crust and may have limited fluid connections to the mantle. Key Points: New He isotope data show that southern Karakoram fault (KKF) is overwhelmingly dominated by degassing of crustal fluidsA crustal 4He‐ and CO2‐rich fluid reservoir is identified and linked to crustal‐scale active deformation driven by strike‐slip faultingKKF may have limited fluid connections to the mantle and requires further evaluation based on He isotope and seismic data [ABSTRACT FROM AUTHOR]