(intrusive basalt), which had intruded into the peridotite complex. This was an
indication that we were in the presence of volcanic events that might have disrupted the mylonityzed (metamorphosed during tectonic events) peridotites.
During the entire dive, we encountered serpentinized and mylonityzed peridotite
with only a few intrusions of gabbro. When the dive ended at 16:39 (at 2551 m
depth), we had sampled 16 rocks and we had traveled 4 km over a period of about
5 h on the sea floor.
The next morning dive SP 09, made by Susanna Sichel with Dubois as a pilot
and Olivier Cipriani as a co-pilot, took place on January 2, 1998, between 3748 m
up to 2841 m depths. This was Susanna’s second dive and she was very enthusiastic about returning to the sea floor. During this dive, 19 samples were collected
of which 14 were peridotites, 2 gabbros, 2 breccia of peridotite, and 1 indurate
carbonate with recrystallized quartz veins.
Dive # 10 (SP10) made by Gleb Udintsev started late because the engineers had
found a leak in the hydraulic system of the submersible and that had to be fixed.
Gleb and the pilots reached the bottom at 11:49 on January 3, 1998 and stayed
about 5 h covering a distance of about 5 km along the slope from 2563 to 1154 m
depths. On his way up, Gleb observed a series of normal faults associated with
talus rubble. The rubble, mostly peridotite at the foot of the scarps, was imbedded
into compacted sediment and formed conglomerates. Gleb thought that this suggested that the area must have been a dynamically upraised mantle peridotite
protruding into a sequence of sedimentary layers forming the ancient deep floor of
the Equatorial Atlantic.
Three other dives were made on the south-facing flank of the St. Peter’s and St.
Paul’s Rocks massif by Eulalia Gracia, Thierry Juteau and Bertrand Sichler, where
peridotite and solidified carbonate sediment complexes were observed (Hekinian
et al. 2000).
The common observations made during all the dives going from 1154 m down
to 3627 m deep indicated that the slope forming the North Ridge, on which the St.
Peter’s and St. Paul’s islets are built, is characterized by highly fractured and
mylonitized terrain outcropping from small linear breaks deprived of abundant
sediment cover. These mylonitized rocks represent vertically uplifted transformed
mantle peridotite. The breaks that are observed on the slope of the North Ridge are
normal faults with mild relief of less than 2 m and oriented in the same general
East–west direction as the multiple equatorial fracture zones. These topographic
steps generated fan-shaped avalanches made up of poorly sorted debris grading
from sand to cobble size to large rectangular blocks ([50 cm in diameter).
The stratigraphic sequence consists of numerous serpentinized and mylonitized
peridotite outcrops intruded by occasional gabbroic units, which were observed
and sampled at depths of 3416 and 3514 m (SP09-8, -9) (Fig. 8.16). The ultramafic
rocks are interrupted by basalts and dykes, which are shallower than 2700 m and
occur at various depths (2365, 2000, 1700–1800 and 1593). Cobble-sized breccia
of mylonite (SP10-14, SP9-16, and -17) and altered basalts (SP08-3, -12) are also
found at various depths (1700-2965 m). The basaltic breccia (2727 m) consists of
Dives on the North Ridge of St. Peter and Paul’s Massif
295
indication that we were in the presence of volcanic events that might have disrupted the mylonityzed (metamorphosed during tectonic events) peridotites.
During the entire dive, we encountered serpentinized and mylonityzed peridotite
with only a few intrusions of gabbro. When the dive ended at 16:39 (at 2551 m
depth), we had sampled 16 rocks and we had traveled 4 km over a period of about
5 h on the sea floor.
The next morning dive SP 09, made by Susanna Sichel with Dubois as a pilot
and Olivier Cipriani as a co-pilot, took place on January 2, 1998, between 3748 m
up to 2841 m depths. This was Susanna’s second dive and she was very enthusiastic about returning to the sea floor. During this dive, 19 samples were collected
of which 14 were peridotites, 2 gabbros, 2 breccia of peridotite, and 1 indurate
carbonate with recrystallized quartz veins.
Dive # 10 (SP10) made by Gleb Udintsev started late because the engineers had
found a leak in the hydraulic system of the submersible and that had to be fixed.
Gleb and the pilots reached the bottom at 11:49 on January 3, 1998 and stayed
about 5 h covering a distance of about 5 km along the slope from 2563 to 1154 m
depths. On his way up, Gleb observed a series of normal faults associated with
talus rubble. The rubble, mostly peridotite at the foot of the scarps, was imbedded
into compacted sediment and formed conglomerates. Gleb thought that this suggested that the area must have been a dynamically upraised mantle peridotite
protruding into a sequence of sedimentary layers forming the ancient deep floor of
the Equatorial Atlantic.
Three other dives were made on the south-facing flank of the St. Peter’s and St.
Paul’s Rocks massif by Eulalia Gracia, Thierry Juteau and Bertrand Sichler, where
peridotite and solidified carbonate sediment complexes were observed (Hekinian
et al. 2000).
The common observations made during all the dives going from 1154 m down
to 3627 m deep indicated that the slope forming the North Ridge, on which the St.
Peter’s and St. Paul’s islets are built, is characterized by highly fractured and
mylonitized terrain outcropping from small linear breaks deprived of abundant
sediment cover. These mylonitized rocks represent vertically uplifted transformed
mantle peridotite. The breaks that are observed on the slope of the North Ridge are
normal faults with mild relief of less than 2 m and oriented in the same general
East–west direction as the multiple equatorial fracture zones. These topographic
steps generated fan-shaped avalanches made up of poorly sorted debris grading
from sand to cobble size to large rectangular blocks ([50 cm in diameter).
The stratigraphic sequence consists of numerous serpentinized and mylonitized
peridotite outcrops intruded by occasional gabbroic units, which were observed
and sampled at depths of 3416 and 3514 m (SP09-8, -9) (Fig. 8.16). The ultramafic
rocks are interrupted by basalts and dykes, which are shallower than 2700 m and
occur at various depths (2365, 2000, 1700–1800 and 1593). Cobble-sized breccia
of mylonite (SP10-14, SP9-16, and -17) and altered basalts (SP08-3, -12) are also
found at various depths (1700-2965 m). The basaltic breccia (2727 m) consists of
Dives on the North Ridge of St. Peter and Paul’s Massif
295