Geochemical cycling, tectonic drivers and environmental impacts of CH4-rich mud extrusions in subduction zones

Abstract

Subduction zones are critical interfaces for lithospheric volatile fluxes, where complex tectonic and geochemical interactions facilitate the release of gases and fluids from deep-seated reservoirs within the Earth’s crust. Mud volcanism, as a dynamic manifestation of these processes, contributes CH₄ emissions that influence the global methane budget and impact marine ecosystems. Although ∼2000 CH₄-rich mud extrusions have been documented in subduction zones globally, the geological origins and subduction-related geochemical and tectonic mechanisms driving these emissions remain poorly understood. This research examines the Makran subduction zone which hosts one of the world’s largest accretionary wedge and extensive CH₄-rich mud extrusions, as a model system. Integrated geochemical, geophysical, and geological observations reveal that thermogenic CH₄ and clay-rich fluidized muds originate from deeply buried Himalayan turbidites (underthrusted sediments), driven by organic-rich sediment maturation and high fluid overpressure. Key tectonic features, including thrust faults, overburden pressure of wedge-top sediments, normal faults, brittle fractures, and seismicity, facilitate CH₄-rich mud extrusions into the hydrosphere and atmosphere. The extruded gases are predominantly CH₄, with minor C₂H₆, C₃H₈, i-C₄H₁₀, and n-C₄H₁₀ while the mud breccia exhibits a chemical composition dominated by SiO₂, Al₂O₃, and Fe₂O₃, enriched with trace elements (Rb, Zr, and V) and clay minerals, quartz, and carbonates. Geochemical indicators suggest intense chemical weathering and mature sediments classifying the mud breccia as litharenite and sub-litharenite, indicative of deep burial and compaction. These findings model the evolution of CH₄-rich mud extrusions through three geological stages: (i) Eocene to Early Miocene pre-thermogenic formation of the CH₄-rich source, (ii) Middle Miocene to Pliocene syn-thermogenic CH₄ and fluidized mud generation, and (iii) Pleistocene to Recent post-thermogenic CH₄-rich fluidized mud migration. These findings underscore the critical yet often overlooked role of subduction-related geochemical and tectonic processes in CH₄ generation and emission, with significant implications for the global CH₄ budget and marine ecosystems.