Complex architecture of mud volcano systems: New insights on flow pathways unravel intricate fluid circulation

Mud volcanoes (MVs) are important gateways for fluid migration, particularly in subduction zones. We investigate five MVs in the Gulf of Cadiz and the Mediterranean Ridge, revealing underappreciated fluid pathways responsible for fluid expulsion. By utilizing pore water geochemistry, advection-diffusion modeling, and high-resolution seismic profiles, we trace fluid origins, quantify fluxes, and constrain migration pathways, focusing on the Ginsburg MV. The Cl-depleted summit fluids originate from clay dehydration within the Allochthonous Unit of the Gulf of Cadiz (AUGC) and are channeled by central conduits, reaching high advection velocities (2.8−15 cm/yr). The Cl-enriched moat fluids exhibit slower advection velocities (0.3 cm/yr) and show additional evaporite effects. To constrain moat fluid sources, fluid formation temperatures have been calculated using water isotopes and Mg-Li geothermometer. The resulting low temperatures suggest source depths atop the AUGC (∼0.8 kmbsf), consistent with seismic data across the Ginsburg MV (showing high-amplitude reflections at the same depths) but different from the summit sites, where the source is deeper within the AUGC (∼2.2 kmbsf). We relate moat seepage occurrence to fractures formed due to edifice subsidence, marked by stacked enhanced reflectors. Upon comparison with all analyzed MVs, we suggest that peripheral seepage of MV edifices is a widespread process that appears in a specific evolutionary stage, during which it represents an important component of fluid budget. This phenomenon, which has been observed in terrestrial MVs, is rarely described for their submarine counterparts but has wide biogeochemical implications for fluid budget in subduction zones and the sustenance of seafloor biological communities.