Invited talk: Mantle exhumation at the Southwest Indian Ridge; preliminary results of the “SMOOTHSEAFLOOR” cruise

The eastern Southwest Indian Ridge (SWIR) is among the deepest part of the oceanic ridge system, and it is thus inferred to represent a melt-poor end-member for this system. It displays the widest expanses known to date of seafloor with no evidence for a volcanic upper crustal layer. This nonvolcanic ocean floor has no equivalent at faster spreading ridges and has been called “smooth seafloor” because it occurs in the form of broad ridges up to 2000 m high with a smooth, rounded topography with no resolvable volcanic cones on bathymetric data (Cannat et al., 2006). On R/V Marion Dufresne cruise “SMOOTHSEAFLOOR” in October 2010, we conducted a detailed geological-geophysical survey of such smooth seafloor areas, determining the rock types exposed at these ridges, searching for hydrothermal activity and mapping the volcanic, tectonic and sedimentary structures. 35 dredges and 15 CTDs have been realized and more than 1000 km long TOBI sidescan sonar images and deep towed magnetometer profiles have been collected over two contrasting survey areas up to magnetic anomaly C5n (10 Ma). The main result of our cruise is that serpentinized mantle-derived peridotites were dredged widely throughout the smooth seafloor areas while gabbros and basalts were rare. Mantle rocks were found on moderate slopes (20-35°), facing toward and away from the axial valley, at ridges with symmetric shape, as well as on gentle slopes (<15°) facing the axis at asymmetric ridges whose outward facing steeper slopes may be covered with volcanics. TOBI sidescan sonar images show that these hillsides are ancient large low angle normal faults which were covered locally by small amount of volcanics and dismembered by landslide activity. These results show that mantle exhumation has been the main process which shaped the smooth seafloor areas of the eastern SWIR for the last 10 Myrs.