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

Abstract

[1] 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.

High‐resolution bathymetry reveals contrasting landslide activity shaping the walls of the Mid‐Atlantic Ridge axial valley

“…Our observations in the Ashadze region suggest that slope failure in the ultramafic rocks that form the axial valley wall limits the overall slope to ~18° when the angle of emergence of the exhumation fault exceeds this value. Such a low equilibrium angle could be due to the combined effects of three factors: (1) the abundance of weak hydrous minerals such as serpentine and talc present in the exhumed ultramafic rocks [ Picazo et al ., ]; (2) sub‐surface active hydrothermal processes promoting in situ mineral alteration and high temperatures [ Ondréas et al ., ]; and (3) weakness zones created by the landslide sliding surfaces and fractures (Figure a). At this point, we do not have constraints on which of these factors are preeminent, and we lack similar high‐resolution mapping for ultramafic slopes in other regions of the Mid‐Atlantic Ridge.…”

The Trans-Atlantic Geotraverse hydrothermal field: A hydrothermal system on an active detachment fault

“…Furthermore, it should be noted that not all sulfide deposits spatially associated with detachment faults and oceanic core complexes are located on the hanging wall; some are perched high above the axial valley on the exposed fault surface (e.g., the Logatchev (14°45'N, MidAtlantic Ridge) and Ashadze (13°N, Mid-Atlantic Ridge) hydrothermal fields (Petersen et al, 2010;Ondréas et al, 2012). Given the importance of detachment faulting for crustal extension at slow spreading ridges, the fundamental question that needs to be addressed is: How do detachment fault systems, and the structure at depth associated with these systems (e.g., presence of plutons and/or high permeability zones), influence the pattern of hydrothermal circulation, mineral deposition, and fluid chemistry, both in space and time, within slowly accreted ocean crust?…”

Tectonic structure, lithology, and hydrothermal signature of the Rainbow massif (Mid-Atlantic Ridge 36°14′N)

“…The first ultramafic‐hosted hydrothermal site to be discovered was Logatchev, located at 14°45′N on the MAR [ Bogdanov et al ., ]. Since then, several inactive and active ultramafic‐hosted sites have been identified along the MAR, including Menez Hom at 37°9′N, Saldanha at 36°34′N, Lost City at 30°N, Semyonov at 13°30′N, Ashadze at 12°58′N, and Nibelungen at 8.3°S [e.g., Barriga et al ., ; Beltenev et al ., ; Fouquet et al ., ; Kelley et al ., ; Melchert et al ., ; Ondréas et al ., ]. Active and inactive sites, some also likely hosted on an ultramafic basement, have been reported along the South‐West Indian Ridge (SWIR) [e.g., German et al ., ; Bach et al ., ], the Mid‐Cayman ridge [ German et al ., ], the Central Indian Ridge [ Kumagai et al ., ], and the Arctic Ridges [ Pedersen et al ., ].…”