A 10 Myrs long record of mantle exhumation at the eastern Southwest Indian Ridge

Although detachment faulting is thought to be a fundamental, widespread style of accretion of oceanic lithosphere, the kinematic evolution of faulting and the link between deformation and magmatic emplacement are still poorly known. Here we use newly acquired geological and geophysical data from the eastern Southwest Indian Ridge (SWIR) to address this question. The eastern SWIR displays the widest expanses known to date of seafloor with no evidence for a volcanic upper crustal layer. Deep tow TOBI sidescan sonar images and dredged rock samples were collected over two areas with contrasting styles of non volcanic seafloor. In the first survey area (centered at 62°30’E) several broad smooth elongated and symmetric ridges are oriented SW-NE, oblique to the spreading direction (~NS). TOBI images show that the hillsides of these ridges, facing toward and away from the axial valley, correspond to smooth surfaces with mass wasting features and almost no evidence of volcanism. Some of these ridges display highly backscattering fuzzy textures with rounded shape features of unknown origin. Serpentinized mantle-derived peridotites were systematically dredged in those areas. Volcanic textures are only observed at the eastern and western ends of these ridges. The second survey area (centered at 64°35’E) covers the edge between volcanic and non volcanic seafloor. There, ridges are perpendicular to the spreading direction and display mainly an asymmetric shape. TOBI sidescan sonar images show that the inward facing gentle slope of these ridges are smooth and sedimented surfaces covered locally by small sized volcanic patches. Outward facing slopes are steeper and may be covered with volcanic textures. Serpentinized mantle-derived peridotites were also systematically dredged outside these volcanic areas. We interpret the successive non volcanic ridges as the result of large low angle normal faults which exhume mantle rocks alternatively to the northern and to the southern flank of the SWIR. The occurrence of volcanic terrain on top of the detachment faults questions the relationship between shifts in the polarity of these detachment faults and magmatic emplacements.