Submarine Hydrothermal Sites in Arc Volcanic-Back Arc Environment: Insight from Recent Marine Geophysical Investigations in the Southern Tyrrhenian Sea.

Abstract:

Hydrothermal alteration processes involve mineralogical and chemical changes, which are reflected in a major modification of potential field patterns observed over hydrothermal areas. Basalt-hosted hydrothermal sites exhibit characteristic responses with magnetic lows and minima of the gravity field. Near bottom AUV (Autonomous Underwater Vehicle) based potential field surveys have become a very effective technique in deep sea exploration. Here we present results of recent ship-borne and near seafloor magnetic and gravity investigations at deep (Marsili and Palinuro seamounts) and shallow (Panarea, Basiluzzo and Secca del Capo) hydrothermal sites in the Southern Tyrrhenian Sea including multibeam bathymetry, seafloor reflectivity and seismic profiles.

At Marsili seamount, a large Fe-Mn-oxyhydroxides-rich chimney field is located at the summit (500 m depth). This site is correlated with pronounced magnetic and gravity lows (0 A/m and 2.0 g/cm³). Deep tow magnetic survey (Cruise MAVA11) revealed strong association between the complicated magnetization pattern and the main volcano-tectonic features of the ridge.

Hydrothermal manifestations at Palinuro seamount occur mainly on the western sector within the rim of a caldera structure at depth of 600m. Recent AUV based magnetic surveys (Cruise POS442, 2012 using AUV “Abyss”) detailed a magnetization low interpreted to represent the local distribution of subseafloor hydrothermal alteration (potentially massive sulfide deposits), and also mapped previously undiscovered inactive chimney fields.

Hydrothermal sites observed at the arc-related volcanic islands (Panarea, Basiluzzo, Eolo and Secca del Capo) are confined to shallow depths (less then 300m) and associated with large ochreaceous mounds, vents and chimney fields such as those observed E of Basiluzzo Island. At this site a recent magnetic survey (Cruise PANA13_ASTREA) combined with Remote Operated Vehicle (ROV) investigations revealed that the submarine geothermal field of Panarea island is actually extending in NE-SW direction along preferential tectonic trends which intercept the submerged portion of Basiluzzo. These new findings could help the volcanic hazard mitigation of Panarea archipelago in case of a new gas crisis similar that which occurred in 2002.