Mapping of seafloor hydrothermally altered rocks using geophysical methods: Marsili and Palinuro seamounts, Southern Tyrrhenian Sea

Hydrothermal alteration processes involve mineralogical, chemical, and textural changes as a result of hot
aqueous #uid-rock interaction under evolving boundary conditions. These changes affect the physico-chemical
properties of the rocks, enabling high-resolution geophysical prospecting to be an important tool in the
detection of sea#oor hydrothermal alteration. Here we present the results of recent geophysical investigations
of the Marsili and Palinuro volcanic complexes, southern Tyrrhenian Sea, during the 2010 TIR10 and
2011 MAVA2011 cruises by the R/V Urania. The new dataset includes a dense grid of multibeam bathymetry;
sea#oor re#ectivity, magnetic and gravity lines; and high-resolution single (CHIRP) and multichannel seismic
proYles. The surveys were focused on areas known to host intense hydrothermal alteration in order to provide
a more detailed description of the Marsili and Palinuro hydrothermal systems. Ground-truthing was based on
earlier discoveries of hydrothermal vents and their associated deposits, and on direct observations made by
ROV dives. High-resolution morpho-bathymetry, sonar re#ectivity, rock magnetization, and density distribution
together enabled us to assess the extent of sea#oor hydrothermal alteration and its relationship to local volcanic
and tectonic structures. Hydrothermal alteration associated with the Marsili seamount is largely distributed
along primary volcano-tectonic structures at the ridge crest. By contrast, at Palinuro hydrothermal alteration
is mostly associated with secondary volcanic structures such as collapsed calderas and volcanism reactivation
along ring faults. In particular, evidence for intense hydrothermal activity occurs at Palinuro where volcanotectonic
features interact with regional tectonic structures.