An old detachment surface: mantle rocks at the sea floor. Dive number three.

The third Nautile dive of the SMARTIES cruise was designed on a ridge similar to the one studied during the 1st dive, west of the MAR axis and south of the transform fault. The studied ridge displays some scarps which could correspond to outcrops of deep rocks. The top of the scarp might correspond to an inactive detachment fault.

3D view of the dive location on the off-axis ridge. © SMARTIES cruise

The dive started around 10 a.m. (ship time) on July 23rd. The Nautile reached the bottom a little before noon, at a depth of 4177m. We started to survey to the east (N100°E) to reach the scarps observed in the bathymetry. During the beginning of the dive, the seafloor was sedimented, only showing ripple marks, revealing the presence of a strong current which could be felt in the navigation of the submersible. Then we observed and sampled meter-size, fractured blocks. We continued uphill and found a scarp and and associated outcrop of the same kind of blocky rock. The top of the ridge is very sedimented and smooth, and we did not observe more outcrops or structures. We followed a nice Dumbo octopus for a while, and saw some other life.

The pilot navigated the Nautile back to the west, and found a new outcrop aligned with the previous one. We could sample again the deep rocks similar to the earlier samples.

We left the seafloor a little before 5 p.m., and were back on board the Pourquoi Pas near 7 p.m.

By A. Briais