gyrocompass in order for Girardeau to study the orientation of rock deformation as
well as the paleo-magnetic orientation with respect to the Earth’s geode
(Fig. 8.14c).
Despite the fact that Dives PI 16 and PI 17 were able to complete the profile
along the north-facing wall of the transform fault, it was decided to make two
additional dives in order to satisfy our questions concerning the eventual change in
lithology observed along the strike of the wall.
Therefore, a third dive (PI 18) was made along the southern wall, but about
5 km to the east of the two previous dives. Jean Francheteau was the scientific
observer, with Martinossi and Dubois as pilot and co-pilot. The objective of this
dive was to verify whether the observed lithology on the north-facing southern
wall of the fault was representative of what would be found further along. Dive PI
18 arrived on the bottom at 11:27 at 4154 m depths, which is just few meters
above the level of sample site #07 for dive PI 16, corresponding to a zone of
brecciated peridotite with an intrusion of gabbro. Indeed, Jean’s first observation
was seeing talus pile blocks of gabbro lying on sediment at 4163 m. According to
Jean’s description, the blocks were gigantic, twice the size of the Nautile, and they
were associated with a stair case type of faulting (i.e. a succession of near vertical
and almost flat ledges). This type of panorama lasted up to 3414 m depth where
the last gabbro was sampled before coming to a different lithology. At 13:27, the
dykes with their characteristic columnar jointing and planar faces were observed
against a wall of gabbro. A sample (PI18-06) of dolerite was taken in situ at
3294 m (Figs. 8.12c, 8.13, 8.14, and 8.14d). The rest of the dive progressed
towards a shallower depth up to 2397 m, and was in a complex of massive gabbros
having a blocky appearance, sharp edges, and nearly vertical walls, when they
were in place. During any tectonic motion that was strong enough to generate
earthquakes, these massive outcrops could have been broken and slumped down
slope to form large debris, sometimes the size of a house.
The last dive (PI 19) was done with Pierre Triger (pilot), Patrick Cheilan
(co-pilot) and myself. We returned to the south-facing slope of the Terevaka
transform wall to continue the observations of the previous dive, PI 18. My dive
was aimed at exploring the other side (i.e. the northern slope of the southern wall),
in order to find the extent of the various formations by comparing the stratigraphic
sequences of both sides of this wall. The first visual contact was on the sea floor at
4797 m and the dive ended at 1767 m depth (Fig. 8.14a–h). The northern side of
the wall consisted essentially of pillow lava basalts that were occasionally interrupted by dyke intrusions at 2770 and 2000 m depths. The northern flank was
abundantly sedimented with avalanche debris that was partially buried by sediment. Most of the slope was broken up except for a few isolated, freshly fractured
outcrops showing pillow lava sections and horizontally layered flows at depths
shallower than 2940 m. The slope stayed uniform and monotonous between 2600
and 2400 m, and was characterized by sediment and partially buried lava flows. At
the end of the dive near the top of the wall, at 1700–1800 m, we reached an
abundantly altered outcrop with light brown and reddish ochre coloration. We
Diving in Terevaka Transform
279
well as the paleo-magnetic orientation with respect to the Earth’s geode
(Fig. 8.14c).
Despite the fact that Dives PI 16 and PI 17 were able to complete the profile
along the north-facing wall of the transform fault, it was decided to make two
additional dives in order to satisfy our questions concerning the eventual change in
lithology observed along the strike of the wall.
Therefore, a third dive (PI 18) was made along the southern wall, but about
5 km to the east of the two previous dives. Jean Francheteau was the scientific
observer, with Martinossi and Dubois as pilot and co-pilot. The objective of this
dive was to verify whether the observed lithology on the north-facing southern
wall of the fault was representative of what would be found further along. Dive PI
18 arrived on the bottom at 11:27 at 4154 m depths, which is just few meters
above the level of sample site #07 for dive PI 16, corresponding to a zone of
brecciated peridotite with an intrusion of gabbro. Indeed, Jean’s first observation
was seeing talus pile blocks of gabbro lying on sediment at 4163 m. According to
Jean’s description, the blocks were gigantic, twice the size of the Nautile, and they
were associated with a stair case type of faulting (i.e. a succession of near vertical
and almost flat ledges). This type of panorama lasted up to 3414 m depth where
the last gabbro was sampled before coming to a different lithology. At 13:27, the
dykes with their characteristic columnar jointing and planar faces were observed
against a wall of gabbro. A sample (PI18-06) of dolerite was taken in situ at
3294 m (Figs. 8.12c, 8.13, 8.14, and 8.14d). The rest of the dive progressed
towards a shallower depth up to 2397 m, and was in a complex of massive gabbros
having a blocky appearance, sharp edges, and nearly vertical walls, when they
were in place. During any tectonic motion that was strong enough to generate
earthquakes, these massive outcrops could have been broken and slumped down
slope to form large debris, sometimes the size of a house.
The last dive (PI 19) was done with Pierre Triger (pilot), Patrick Cheilan
(co-pilot) and myself. We returned to the south-facing slope of the Terevaka
transform wall to continue the observations of the previous dive, PI 18. My dive
was aimed at exploring the other side (i.e. the northern slope of the southern wall),
in order to find the extent of the various formations by comparing the stratigraphic
sequences of both sides of this wall. The first visual contact was on the sea floor at
4797 m and the dive ended at 1767 m depth (Fig. 8.14a–h). The northern side of
the wall consisted essentially of pillow lava basalts that were occasionally interrupted by dyke intrusions at 2770 and 2000 m depths. The northern flank was
abundantly sedimented with avalanche debris that was partially buried by sediment. Most of the slope was broken up except for a few isolated, freshly fractured
outcrops showing pillow lava sections and horizontally layered flows at depths
shallower than 2940 m. The slope stayed uniform and monotonous between 2600
and 2400 m, and was characterized by sediment and partially buried lava flows. At
the end of the dive near the top of the wall, at 1700–1800 m, we reached an
abundantly altered outcrop with light brown and reddish ochre coloration. We
Diving in Terevaka Transform
279