Slow Fluid Leakage along Faults: A Multidisciplinary Study on the Mud Volcanoes of the Bolle della Malvizza (Southern Apennines, Italy)

Abstract

This study presents a multidisciplinary investigation of the Bolle della Malvizza mud volcanoes, located in the southern Apennines fold-and-thrust belt (Italy), aimed at characterizing these structures and constraining processes and sources of mud and gas leakage. Twelve main vents are present, continuously and slowly ejecting mud, saltwater, and gases, including CH₄ and CO₂. We carried out different investigations, including (i) stratigraphic and structural surveys, (ii) topographic and morphometric evaluations using a digital elevation model obtained by drone photogrammetry, (iii) geochemical measurements of CO₂ flux, radioactivity and soil pH and (iv) geophysical surveys including electrical resistivity tomography, induced polarization and self potential. Nine main groups of mud volcanoes are present in the area, varying in size (from a few centimeters to 13 meters) and height (from ∼3 to 15 cm). These mud eruptive vents are aligned along the ENE-WSW and N-S directed normal faults. The geogenic CO₂ flux is low when compared to other non-volcanic emissions in the southern Apennines. The electrical resistivity tomography reveals conductive volumes interpreted as clay-rich layers alternating with resistive bodies of clay-marly rocks, and conductive layers corresponding to shallow and deep aquifers. The induced polarization data highlights high-chargeability zones linked to clay-rich bodies and a narrow vertical conduit connecting deeper conductive zones to shallow levels. Self-potential data show a pronounced negative anomaly aligned with the main vents, spatially matching the high-chargeability conduit and a resistivity inflexion in the electrical section. Ground deformation modelling and Monte Carlo simulation suggest a source at ∼110 m depth, with a negative volume change of ∼5 × 10⁵ m³. We propose a conceptual model in which deep fluids slowly ascend along damage zones of two major faults, interacting with the surficial aquifer and the clayey host rock, and accumulate in a shallow reservoir that gradually releases muddy fluids to the surface, forming mud volcanoes that are continuously eroded during rainfall events.

Keywords

structural geology / geophysics / geochemistry / southern Apennines / non-volcanic degassing / mud volcanoes

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