Anatomy of a detachment fault damage zone at a nearly amagmatic mid-ocean ridge: observations from outcrop to map scale

The eastern Southwest Indian Ridge (SWIR) is a melt-poor end-member region of the global MOR system. The available magma focuses to axial volcanoes, leaving >50 km-wide, nearly amagmatic ridge sections, where seafloor spreading occurs via large offset detachment faults. We present map to outcrop scale observations of the deformation associated to one of these detachments, in the 64°35'E region of the SWIR. This active detachment fault presently has a horizontal offset of ~4 km (Cannat et al., 2019), and accommodates nearly all the plate divergence (14 km/million year; Patriat and Segoufin, 1988). We focus on the lower slopes of the footwall, where this active fault currently emerges at an angle of ~35°. The emergence is traceable across a length of ~20 km on side-scan sonar and shipboard bathymetry data. It locally shows undulations at a wavelength of ~1-4 km. High-resolution bathymetry at and near the emergence area shows two morphological domains. In one domain, the exhumed fault surface bears distinct corrugations that trend at an 18° to 33° angle to the spreading direction, extends up to 300 m, are spaced by ~15-300 m, and are ~1 m to ~40 m in amplitude. In the other domain, the exhumed fault is not corrugated. Remotely operated vehicle (ROV) dive observations at the outcrop scale show discrete planar fault planes and brecciated and fractured rock forming the top ~1-4 meters of the corrugated exposures. In contrast, the non-corrugated fault exposures show up to ~8 m of gouge-bearing micro-brecciated domains, including several up to 1 m thick horizons of semi-brittle sheared serpentinites. Dive observations further suggest that: (1) there are several sigmoidal intercalations of such gouge-bearing horizons forming the upper few tens of meters of the non-corrugated fault zone, and (2) the horizons of sheared serpentine originated as brittle cracks that served as hydrous fluid pathways into the fault damage zone. We propose: (1) that the absence of corrugations is related to the overall weaker semi-brittle rheology of the emerging fault in this domain, compared to the purely brittle corrugated domain; and (2) that the two domains represent damage developed in distinct conditions of temperature and hydrothermal fluid availability. At the broader map scale, the non-corrugated domain to the east emerges about 1.2 km farther south than the corrugated domain, and the trace of emergence thus draws an indentation between the two domains. Given the ~35° fault emergence angle in the two domains, we infer that their across-fault distance is ~650 m. The detachment damage zone may thus be at least that broad, and comprised of distinct, probably anastomosing domains of more localized deformation, which would preferentially be exposed at the seafloor. This damage zone anatomy would be consistent with seismic refraction observations (Momoh et al., 2017) in the area.