Sulphur and carbon signatures of metamorphic processes in the Nepal Himalayas

The Nepal Himalayas result from the India-Eurasia collision and the actual shortening is accommodated by a detachment ramp, the Main Himalayan Thrust (MHT). Separating high-grade metamorphic rocks from the Greater Himalayan Sequence to the north and low-grade metamorphic rocks from the Lesser Himalayan Sequence to the south, the Main Central Thrust (MCT) shear zone, is related to the MHT at depth where large Himalayan earthquakes nucleate. The MCT zone occurs from Far-Western to Eastern Nepal, associated at mid-crustal depth with active seismicity and high electrical conductivity; it exhibits carbon-rich rock layers and numerous active hydrothermal systems. Here, based on a multidisciplinary approach that includes geology, geochemistry and geophysics, we study the various sulphur and carbon signatures in the MCT zone in the Nepal Himalayas. First, we characterise the upper LHS rocks that include alternation of graphite-rich mica-schists (the so-called “black schists”) and carbonates (mainly siliceous dolomite). In the laboratory, we determine organic and inorganic carbon contents, as well as complex electrical conductivity. Second, we concentrate on numerous thermal springs in which we measure dissolved carbon and sulphur concentrations and their isotopic compositions (δ¹³C and δ³⁴S). Third, we study the surface gaseous emissions, directly observed in the vicinity of hot springs, with the measurements of carbon dioxide (CO₂) and hydrogen sulphide (H₂S) fluxes and isotopic compositions. By comparing the signatures of carbon and sulphur sequestration and carbon and sulphur release at a large spatial scale, our work provides insights into the carbon source-to-sink duality of large orogens, the metamorphic processes and the carbon and sulphur geochemical cycles.