Early stages of evolution of an axial detachment fault at the ultraslow spreading mid-ocean ridge (64°40'E SWIR)

The 64°E region of the eastern SWIR is a melt poor end member region of the MOR system. Magma focusing to axial volcanoes leaves >50 km wide along axis corridor, where seafloor spreading occurs almost fully via successive alternate polarity detachment faults (Sauter et al., 2013). The present-day south-dipping young (~300 kyrs; Cannat et al., 2019), active detachment fault cuts through an older north dipping detachment fault. The active detachment emergence can be traced over 32 km from the shipboard bathymetry data (for comparison, the scoped-shaped finely corrugated 13°20’N exhumed detachment surface at the Mid-Atlantic Ridge extends only ~5-6 km along-axis; Escartin et al., 2017). HR micro-bathymetry maps acquired on the west and east sides of this emergence line indicate an along-strike variation of the fault structure and geometry. In the east, the fault emerges at an overall angle of ~26°-30° and the emerging fault surface is irregular, with undulations at hectometer to km scales, close to parallel to spreading direction, and rare occurrences of decameter-scale corrugations, up to 20° oblique to spreading. In the west, the detachment emerges in the form of two distinct fault splays, ~400 m apart, both at an angle ~40°-50°.  This western region receives some magma input, resulting in localized patches of basalt and hummocky ridges.

Near fault deformation structures, documented by Remotely Operated Vehicle (ROV) dives and sampling, also differ between east and west. In the west, sigmoidal blocks (~5-10 m) of moderately fractured serpentinized peridotite, some with gabbro dikes, are observed below the emerging faults, which consist of <1.5 m thick zone of serpentinite breccia, and micro-breccia, with cm-thick intervals of gouge. In the east, the highly strained intervals are thicker (up to 8 m), with a greater proportion of serpentinite gouge and microbreccia. The more coherent rocks below the fault are also more pervasively fractured, with planar decimeter to meter-spaced south-dipping joints. ROV dives near the detachment breakaway offer an opportunity to study the deformation below a more mature region of the previously active detachment. Steep landslide head scarps there expose vertical sections, up to 70 m thick, with several intervals of serpentine gouge and micro-breccia, intercalated with coarser brecciated serpentinized peridotite, and with sigmoidal, meter to decameter sized blocks of serpentinized peridotite. Together, these observations point to a heterogeneous structure and to a variable thickness of the strain localization/damage zone associated with the emerging portions of the 64°40'E SWIR detachment. Based also on the seismic reflection structure of the fault zone at depth (Momoh et al., 2017), we propose that the material that emerges samples distinct regions of a kilometer-thick heterogeneously deformed damage zone, leading to different geometries and structure of the emerged fault surface(s).