Deep CO2 emissions facilitated by localized shear deformation: A case study of the Karakoram fault system, western Tibet

Abstract

Strike-slip faults play a significant role in creating deeply penetrating fractures with high permeability, thus promoting rapid migration of CO₂-rich fluids to the surface. However, there are rare observations regarding how strike-slip movement could affect deep CO₂ emissions. Here, we focus on the Karakoram fault system (KKFS), western Tibet, to estimate diffuse soil CO₂ fluxes and to unravel potential controlling factors for CO₂ emissions. Average CO₂ fluxes of geothermal fields range in 22–2475 g m⁻² d⁻¹, significantly higher than the across-fault profiles (6–116 g m⁻² d⁻¹). A mass balance model based on δ¹³C-CO₂ and CO₂ concentration of soil gases reveals that deep carbon constitutes 49.1–91.5 % (average = 73.9 %) and 0.2–40.5 % (average = 25.5 %) of soil-gas carbon released from geothermal fields and across-fault profiles, respectively. Deep carbon could be produced by thermal decomposition of crustal rocks considering CO₂-rich fluids with radiogenic helium isotopes. Strikingly, higher CO₂ fluxes preferentially occur in geothermal fields along a bending segment of the KKFS, where localized shear deformation is prominent as documented by high slip rates over geological timescales, dense splay faults, clustering of earthquake events, and elevated strain rates. We suggest that high stress acting on the KKFS bend could enhance the deformation and fracturing of fault zone rocks, leading to production of metamorphic CO₂ and efficient release of CO₂-rich fluids through the highly permeable fault system. Our results could shed new light on CO₂ origins and fluxes of strike-slip faults that are characterized by spatially heterogeneous strain partitioning and thus localized enhanced shear deformation.