At the top of an oceanic core complex

The 12th dive of the Nautile took place on August 3rd, 2019. The objective was to continue the exploration of the large massif located at the intersection of the Mid-Atlantic Ridge and the Romanche Fault. The dive 12 succeeds the dives 9 and 11 on what appears as an oceanic “core complex”. During this dive, we took rock samples to determine their nature and to measure the orientations of the tectonic structures.

We arrived on the bottom, at the depth of -3889 m, at 9:52 UT to descend to the NE on a flat surface at -4,000 m. From there, we started a profile facing the slope going up towards the north and then to the south-west, reaching the summit of a small massif at -3570 m. Up to -3680 m the relief is very rugged, with many normal faults and fractures that cut out the foliation of the rocks. The foliation is sub-horizontal, corresponding to the flat upper part of the detachment fault. The main family of normal faults is vertical and oriented N160 ° E, with an orthogonal conjugate direction. Then follows a flat surface covered by sediment from which small NNO – SSE – oriented rock bars emerge, on which small various organisms cling. More continuous outcrops then resume when the slope becomes stronger, to the top of the small elongated massif following the same NNO – SSE orientation. It is covered with sediments containing boulders. We then descended the other slope until -3880 m, turned around and went up on this massif until -3540 m to make a last sampling. We left the bottom at 15:30 UT.

All rocks sampled (19 samples) are more or less deformed peridotites, except for one basalt and one olivine pyroxenite (websterite).

By M. Seyler