More magmatic versus less magmatic oceanic detachment fault zone anatomy

Oceanic detachment faults (ODFs) are critical drivers of plate separation in slow-to-ultraslow spreading mid-ocean ridges (MORs). Numerous previous studies have shown that the anatomy of ODFs varies significantly between more magmatic and nearly amagmatic ridges sections. More magmatic ODFs are typically dome-shaped, corrugated, and face volcanic seafloor on the hanging wall side, while the footwall exhumes upper crustal and mantle-derived rocks, including gabbro, serpentinized peridotite, basaltic breccia, and diabase. In contrast, nearly amagmatic ODFs (e.g., at 64°E Southwest Indian Ridge SWIR) are characterized by long (up to ~95 km) broad and smooth ridges with no visible corrugations in the shipboard bathymetry. These ODFs form in alternate polarity and exhume serpentinized peridotite to the seafloor on both plates. Here, we present the anatomy of the exposed fault zone in the footwall of D1, a young active ODF in the nearly amagmatic 64°35'E region of the SWIR, utilizing shipboard bathymetry, micro-bathymetry, and ROV dive observations to document their footwall geology, deformation patterns, and along-strike variations.

The axial valley wall in the study area corresponds to the footwall of D1, and high-resolution bathymetry shows that it exposes two distinct domains. The western domain displays corrugations similar to those documented in more magmatic, domal ODFs, while the eastern domain is smooth. The western, corrugated domain also displays small offset ESE-trending antithetic normal faults and several hecto-to-kilometers-wide NNE-trending ridges, interpreted as mega-corrugations that formed due to hecto-to-kilometer-scale phacoids between linked fault splays in the detachment damage zone. These ridges and the antithetic minor faults are absent in the smooth eastern domain. Outcrop scale ROV dive observations show one significant difference in the geology of the exposed fault zone between the two domains: in the east, the fault zone comprises up to 10 m-thick intervals of serpentine microbreccia and chrysotile gouge, while in the west, these highly deformed horizons are a few decimeter-thick at the most, surrounding phacoids of less deformed serpentinites and therefore less pervasive at the outcrop scale. These observations suggest a stronger fault and footwall in the corrugated region. Microstructural observations also suggest that hydrous fluids facilitated the formation of the gouges and that non-brittle mechanisms (serpentine dissolution and precipitation) were involved.

Compared with more magmatic, domal corrugated ODFs, the smooth eastern part of our study area exposes thicker and more pervasive intervals of cataclastic microbreccia and gouge. In contrast, previous work on domal ODFs shows that strongly deformed intervals there consist mostly of talc±tremolite±chlorite±serpentine schist. Experimental studies suggest that the frictional strength of talc and chrysotile gouge at the sample scale are comparable. However, the gouge outcrops in the eastern part of the D1 footwall are thick and promote the formation of a planar (smooth) exposed fault surface. By contrast, highly deformed talc-bearing schists at domal ODFs are found around meter to decameter-scale phacoids of less deformed rocks, which are probably the cause of the observed corrugations.