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Geological context and vents morphology of the ultramafic-hosted Ashadze hydrothermal areas (Mid-Atlantic Ridge 13°N)

Abstract : Recent ROV dives and high-resolution bathymetric data acquired over the Ashadze fields on the Mid-Atlantic Ridge (13 N) allow us to derive constraints on the regional and local geological setting of ultramafic-hosted hydrothermal fields. The active vent fields of Ashadze hydrothermal fields are located in the western axial valley wall, downslope from the termination of a prominent corrugated surface and in a transitional domain with respect to ridge segmentation. The study of the shipboard and ROV bathymetry shows that decameter (100 m by 60 m) to kilometer-scaled rockslides shape the axial valley wall slopes in this region. The Ashadze 1 vent field occurs on a coherent granular landslide rock mass that is elongated in an E-W direction. The Ashadze 1 vent field comprises hundreds of active and inactive sulfide chimneys. The Ashadze 2 vent field is located in a NNE-trending linear depression which separates outcrops of gabbros and serpentinized peridotites. Active black smokers in the Ashadze 2 field are located on ultramafic substratum in a 40-m diameter crater, 5-m deep. This crater recalls similar structures described at some vents of the Logatchev hydrothermal field (Mid-Atlantic Ridge 15 N). We discuss the mode of formation for these craters, as well as that for a breadcrust-like array of radial fissures identified at Ashadze 1. We propose that hydrothermalism at Ashadze can be an explosive phenomena associated with geyser-like explosions. Our study also constrains the geological and geophysical context of the ultramafic-hosted Ashadze hydrothermal system that may use the oceanic detachment fault as a preferred permeability conduit.
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Hélène Ondréas, Mathilde Cannat, Yves Fouquet, Alain Normand. Geological context and vents morphology of the ultramafic-hosted Ashadze hydrothermal areas (Mid-Atlantic Ridge 13°N). Geochemistry, Geophysics, Geosystems, AGU and the Geochemical Society, 2012, 13 (11), ⟨10.1029/2012GC004433⟩. ⟨insu-01819614⟩

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