Structural Control on Hydrothermal Fluid Circulations in Two Volcanoes of the Aeolian Archipelago, Stromboli and Vulcano (Italy)

Electric resistivity tomography (ERT), self-potential (SP), soil CO2 flux, and soil temperature have been used during various field surveys to investigate the inner structure, on key areas, of Stromboli and La Fossa cone, on Vulcano (Aeolian Islands, Italy). The measurements were performed on the volcanic cones with a spacing of 20 m or 5 m, along various profiles crossing some major structures of the edifices. The crater rims are underlined by sharp resistivity contrasts. SP, soil CO2 flux, and soil temperature anomalies underline these boundaries which we interpret as structural limits associated to preferential circulation of hydrothermal fluids. The former craters and some particular lithological units strikingly enclose or guide hydrothermal circulations, which we identified on the base of low electrical resistivity values (from less than 200 Ω m to less than 20 Ω m in areas probably affected by strong hydrothermal alteration of the rock). Structures on Stromboli -like an impact crater or volcanic deposit levels- and on Vulcano -like former or recent crater boundaries, regional faults, a former dome unit- have been identified and strongly influence hydrothermal fluid circulation at the scale of the volcanic edifices. An original aspect of the study we carried out is the evidence of the extension of hydrothermal circulations in the whole volcanic edifice on Stromboli as well as on Vulcano, in particular in peripheral zones of the edifices with respect to the central hydrothermal system. These new multidisciplinary studies allow proposing a global model of the fluid flow inside an active volcanic edifice.