assigned for communication. The support ship R.V. Le NADIR stayed nearby for
safety reasons.
Since the shore party had left for the day, and in order to make good use of our
time at sea, I decided to program a Nautile dive (dive # SP10) on the flank of the
island while the other people were sampling its top. This would allow us to have
additional observations along the northern flank of the structure on which the islets
are formed near 0°50
0 S-24°10
0 W.
The party landed at 8:00 AM on the southwestern islet, called Belmonte
according to a Brazilian sketched map. The landing was apparently risky and was
facilitated thanks to a metallic ladder that the Brazilian Navy had put on board the
support ship, LE NADIR, 2 weeks before. In order to carry out their objectives, the
party had to split up. Thierry Juteau, Eulalia Gracia and Ronan Apprioual went to
sample the rocks from the Belmonte landing area near the lighthouse where the
site was most suitable to sample, while Bertrand went to collect drill cores with his
hand-held coring machine. The best outcrops exposed are located at the northwestern corner near the lighthouse where 8 oriented samples and 15 oriented drill
cores were taken. The area consisted of yellowish-green mylonitized peridotite.
The rest of the area was extremely fractured by tectonic activity. Conglomerate
breccias and hydrothermally altered veined material were sampled. Because it was
risky to pass from one island to the other, the scientific party could only go from
the Belmonte islet to the Challenger islet and they only had enough time to explore
its southern portion. The southern part of this islet was difficult to reach therefore
only three samples were taken. The scientific party was especially interested in this
area because they found dark-colored, fine-grained amphibolite-enriched rocks,
which were tentatively interpreted by Thierry Juteau as originally being dolerite
(dykes) that had been extremely transformed during tectonic activity. The transformation (metamorphism) of these rocks into amphibolite suggests high temperatures ([400 °C) and high pressure. The main orientation of the vertical
foliation for the mylonitized peridotite and amphibolite collected from the islets is
roughly East–West, which is the same direction as the fracture zone itself.
Historical Background of the Cruise
The St. Paul cruise was made in conjunction with UBO (Université de Bretagne
Occidentale) in Brest. We had been obliged to wait for 3 years before the IFREMER administration and the French National Scientific Community accepted our
cruise. In addition, we encountered a great deal of harassment from the Brazilian
authorities. Since the expedition was taking place in Brazilian territorial waters,
we had to ask permission from the Brazilian Navy to carry out our study. This was
not an easy task: the proposal had to be translated into Portuguese and some
agreements had to be reached between the IFREMER and the Brazilian authorities.
The Brazilian authorities placed several hurdles before giving us permission to
dive in their territorial waters. One of these was to have a Brazilian scientist
284
8 Fracture Zones and Transform Faults
safety reasons.
Since the shore party had left for the day, and in order to make good use of our
time at sea, I decided to program a Nautile dive (dive # SP10) on the flank of the
island while the other people were sampling its top. This would allow us to have
additional observations along the northern flank of the structure on which the islets
are formed near 0°50
0 S-24°10
0 W.
The party landed at 8:00 AM on the southwestern islet, called Belmonte
according to a Brazilian sketched map. The landing was apparently risky and was
facilitated thanks to a metallic ladder that the Brazilian Navy had put on board the
support ship, LE NADIR, 2 weeks before. In order to carry out their objectives, the
party had to split up. Thierry Juteau, Eulalia Gracia and Ronan Apprioual went to
sample the rocks from the Belmonte landing area near the lighthouse where the
site was most suitable to sample, while Bertrand went to collect drill cores with his
hand-held coring machine. The best outcrops exposed are located at the northwestern corner near the lighthouse where 8 oriented samples and 15 oriented drill
cores were taken. The area consisted of yellowish-green mylonitized peridotite.
The rest of the area was extremely fractured by tectonic activity. Conglomerate
breccias and hydrothermally altered veined material were sampled. Because it was
risky to pass from one island to the other, the scientific party could only go from
the Belmonte islet to the Challenger islet and they only had enough time to explore
its southern portion. The southern part of this islet was difficult to reach therefore
only three samples were taken. The scientific party was especially interested in this
area because they found dark-colored, fine-grained amphibolite-enriched rocks,
which were tentatively interpreted by Thierry Juteau as originally being dolerite
(dykes) that had been extremely transformed during tectonic activity. The transformation (metamorphism) of these rocks into amphibolite suggests high temperatures ([400 °C) and high pressure. The main orientation of the vertical
foliation for the mylonitized peridotite and amphibolite collected from the islets is
roughly East–West, which is the same direction as the fracture zone itself.
Historical Background of the Cruise
The St. Paul cruise was made in conjunction with UBO (Université de Bretagne
Occidentale) in Brest. We had been obliged to wait for 3 years before the IFREMER administration and the French National Scientific Community accepted our
cruise. In addition, we encountered a great deal of harassment from the Brazilian
authorities. Since the expedition was taking place in Brazilian territorial waters,
we had to ask permission from the Brazilian Navy to carry out our study. This was
not an easy task: the proposal had to be translated into Portuguese and some
agreements had to be reached between the IFREMER and the Brazilian authorities.
The Brazilian authorities placed several hurdles before giving us permission to
dive in their territorial waters. One of these was to have a Brazilian scientist
284
8 Fracture Zones and Transform Faults