WestMed Landslides and hydrothermal systems in the Western Mediterranean Sea - Cruise No. M73 - July 21 - August 30, 2007 - Cadiz (Spain) - Heraklion (Greece)

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    103 pages
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    Elektronische Ressource
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    DDC:    550 Earth sciences and geology


Cruise M73 studied a variety of geodynamic processes in the Western Mediterranean Sea. Although unrelated and submitted as stand-alone research proposals, the two expeditions are linked since they contrast the "passive" margin setting of the northern Ligurian Basin with the active subduction zone setting further southeast in the Tyrrhenian Sea. During leg M73/1, the focus along western Ligurian Margin off Southern France was put on sedimentary and tectonic processes in five regions: (i) The Nice airport slide and adjacent stable slope in 15-300 m water depth; (ii) the mid-slope with both ridges and gullies where the landslide mass was traveling, including some unaffected stable slope regions, all in 500-1000 m water depth; (iii) the deeper slope (1500-2000 m water depth) adjacent to the Var Canyon with landslide scars and talus ("western slide"); (iv) an even larger slide complex further away from the canyon system ("eastern slide"); and (v) the Var Canyon region with its flanks, terraces, and channels in down to approximately 2500 m water depth. The majority of the work was dedicated to area (i) where evidence for groundwater-charging of coarse-grained delta deposits, weak sensitive clays, and faulting was found in the cores. Mapping, ROV surveys and in situ CPTu (cone penetration tests with pore pressure) deployments revealed active present-day deformation and a large societal threat. The results were condensed in an IODP drilling proposal and various manuscripts. During leg M73/2, shipboard bathymetry, shallow drilling (<5m) using the British Geological Survey (BGS) Rockdrill 1 instrument and gravity coring were used in order to investigate the local geological setting and the third dimension of three known hydrothermal sites on volcanoes of the Tyrrhenian Sea: (i) Palinuro volcanic complex; (ii) Marsili Seamount; and (iii) Panarea. Drilling at Palinuro proved the occurrence of extensive (>60 x 50 m; open to depth) massive sulfides underlying unconsolidated sediments of variable thickness. Sampling of warm (60°C) sediments in gravity cores indicates, that Palinuro is, despite its sedimentary cover and the lack of plume signals in the overlying water column, still hydrothermally active. The mineralization shows evidence for a contribution of magmatic volatiles derived from a degassing magma chamber at depth. Investigations near Panarea revealed that hydrothermal activity is more widespread than previously reported and that hydrothermal explosions are a common phenomenon in this very shallow (<100 m) environment. The size of explosion craters in the area (> 100m in diameter) suggests larger eruptions in the recent past when compared to the last crisis in 2002, an important observation for the town of Panarea with its prolific tourism. Hydrothermal activity is associated with degassing of magmatic volatiles leading to widespread deposition of massive anhydrite/gypsum (calcium sulfate) as well as native sulfur within discrete explosion craters. Sulfides are only of minor importance. The sulfates are interpreted to represent a cap forming at the interface between seawater and geothermal fluids ascending towards the seafloor.

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