figure change

Abstract

The Ionian Sea in southern Italy is at the center of active interaction and convergence between the Eurasian and African-Adriatic plates in the Mediterranean. This area is seismically active with instrumentally and/or historically recorded M w > 7.0 earthquakes, and it is affected by recently discovered long strike-slip faults across the active Calabrian accretionary wedge. Many mud volcanoes occur on top of the wedge. A recently discovered one (called the Bortoluzzi Mud Volcano or BMV) was surveyed during the Seismofaults 2017 cruise (May 2017). High-resolution bathymetric backscatter surveys, seismic reflection profiles, geochemical and earthquake data, and a gravity core are used here to geologically, geochemically, and geophysically characterize this structure. The BMV is a circular feature ' 22 m high and ' 1100 m in diameter with steep slopes (up to a dip of 22). It sits atop the Calabrian accretionary wedge and a system of flower-like oblique-slip faults that are probably seismically active as demonstrated by earthquake hypocentral and focal data. Geochemistry of water samples from the seawater column on top of the BMV shows a significant contamination of the bottom waters from saline (evaporite-type) CH 4-dominated crustal-derived fluids similar to the fluids collected from a mud volcano located on the Calabria mainland over the same accretionary wedge. These results attest to the occurrence of open crustal pathways for fluids through the BMV down to at least the Messinian evaporites at about 3000 m. This evidence is also substantiated by helium isotope ratios and by comparison and contrast with different geochemical data from three seawater columns located over other active faults in the Ionian Sea area. One conclusion is that the BMV may be useful for tracking the seismic cycle of active faults through geochemical monitoring. Due to the widespread diffusion of mud volcanoes in seismically active settings, this study contributes to indicating a future path for the use of mud volcanoes in the monitoring and mitigation of natural hazards. 1 Introduction Mud volcanoes are ubiquitous structures on the Earth's surface in both marine and continental settings and form due to a variety of causes, most of which are amenable to clayand fluid-rich subsurface levels in which overpressure contributes to driving the ascent of mixed and liquefied clay, water, and gases (e.g., CO 2 , CH 4 , and N), with the consequent Published by Copernicus Publications on behalf of the European Geosciences Union.