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TOBI surveys (06 October)

We arrived at our first study area yesterday. This zone is located in the axial valley of the southwest Indian Ridge to 28 ° S and 62 ° E. Ocean ridges to slow opening rate, and those ultra-slow rate in particular, are characterized by the presence of a rift valley on the edge of two plates move apart from each other (here African plate and the plate Antarctica). In contrast, dorsal faster rate, as the East Pacific Rise, are characterized by a large bulge in the bathymetric plate boundary.

Unlike the traditional view of oceanic ridges as a system primarily volcanic, the eastern part of the Southwest Indian Ridge shows large areas several tens of kilometers long with no evidence of volcanic. The most accurate bathymetric maps in our possession (with a resolution of about one hundred meters) show large ripples over 2 km in height with a texture, roughness, very low without any evidence of volcanic edifice. Some dredging on the seabed in these areas suggest that they are made of mantle rock. But how these mantle rocks formed a few miles deep have they been implemented? This is one of the questions we are trying to solve by collecting new images of the floor either with a resolution of about 100m but with a resolution of about 5m.

Recorder TOBI

The acquisition of these images is made by a sonar made in England called TOBI ( http://www.noc.soton .ac.uk / nmf / usl_index.php? page = tb ). This machine over 2 tons was towed near the bottom (about 500 to 600m above sea level) by the Marion Dufresne at very low speed (2-3 knots). Every 4 seconds TOBI sends a wave to the seafloor which modifies and returns to the sonar. It records the wave to build images of the seafloor topography at a resolution of about 5m. These images form successive bands of about 6 km wide. We planned to collect these bands on 4 parallel profiles of 60 km in length to cover the entire width of the axial valley on an area of nearly 1500 km 2 . It should be 20H to acquire such a bunch of sonar images.

Image TOBI

For now we are only at the end of the first profile and the results are already very interesting. We have seen many, many flaws and here or circular forms that could be a few volcanoes, but it's a large area very homogeneous appearance which intrigues us. This will be the subject of dredging attempts to determine the nature of the rocks that constitute it.

In four days we have completed the mapping of our initial study area and we can get a first idea of the nature of these surfaces non-volcanic deep in the valley of the Southwest Indian Ridge.