A 3‐D Seismic Tomographic Study of Spreading Structures and Smooth Seafloor Generated by Detachment Faulting—The Ultra‐Slow Spreading Southwest Indian Ridge at 64°30′E

At ultra‐slow spreading ridges, with full spreading rates less than ∼20 mm/yr, spreading is accommodated both by highly spatially and temporally segmented magmatism, and tectonic extension along large‐scale detachment faults that exhume ultramafic material to the seafloor. In the most magma‐poor regions, detachment faulting alternates in polarity over time, producing a “flip‐flopping” effect of subsequent detachment dips. The resulting seafloor in these regions displays a morphology termed “smooth seafloor” comprising elongate, broad ridges with peridotite/serpentinite lithologies. We conducted tomographic travel‐time inversion of a 3‐D wide‐angle seismic data set acquired over a region of smooth seafloor around 64°30′E along the Southwest Indian Ridge (SISMOSMOOTH; Cruise MD199), to produce a seismic velocity volume through the crustal section and into the uppermost mantle. We observe patterns of velocity anomalies that correspond with variations in the bathymetry arising from the mode of spreading and are interpreted as changes in the degree of alteration with depth resulting from spatial and temporal variations in fluid‐rock interaction, controlled by faulting and tectonic damage processes. The detachment faults do not show simple planar structures at depth but instead mirror the shapes of the bathymetric ridges that they exhume. Magmatic input is overall highly limited, but there is one region on the lower part of an exhumed detachment footwall where a thickness of volcanic material is observed that suggests a component of syn‐tectonic volcanism, which could contribute to detachment abandonment. Plain Language Summary Ultra‐slow spreading ridges are the slowest spreading type of mid‐ocean ridge. At these mid‐ocean ridges, instead of spreading through volcanism, the plate separation dominantly takes place along large shallow‐dipping “detachment” faults. This process results in the formation of topographic ridges of “smooth seafloor” where mantle rocks are exposed. We use signals from sound sources at the sea surface recorded on receivers on the seabed to map variations in sound speed in the upper ∼4 km below the seabed at an ultra‐slow spreading location on the Southwest Indian Ridge. We observe patterns in the sound speed variation that we interpret in the context of spreading via detachment faults, which allow varying amounts of fluid to access the subsurface, leading to varying degrees of chemical alteration. At depth, the detachment faults show vari