describing our dives, in order to provide some continuity for our geological
discoveries.
Thus, the last dive (SP06) on the East wall of the rift valley was made in
continuation of dive SP02, and was aimed at reaching the top of the wall. At
11h10, the pilot, P. Cheilan, P. Lubin, co-pilot, and I arrived on the sea floor at a
depth of 4032 m on the top of a scarp made up of sediment that was partially
burying the rocky outcrops. This sedimented plateau with sporadic outcrops of
pillow lava represented a major break in the slope and was observed at
4020–3939 m. The first peridotite rocks associated with basalt and diabase (dyke)
were seen at 3980–4000 m. After taking our sample of serpentinized peridotite
(SP06-01), we moved on along the slope.
During our progression between 4032 m and 2870 m depths, we encountered
several breaks marked by near vertical fault scarps, which exposed striated and
massive serpentinized peridotite, suggestive of a forceful emplacement. Vertical
outcrops of massive rectangular blocks of peridotite were observed and sampled at
3183-3321 m. These blocks were about 1 m thick (sample SP06-11), and showed
sub-parallel prismatic fractures. They were oriented in a N 050° direction and
contained white veins of gabbroic material (sample SP06-12) cross cutting the
serpentinized peridotite. Other rodingitized (altered) gabbros (samples SP06-2, -7,
-8, -9, and -12) intruding into the serpentinized peridotite were recovered in situ at
various depths (3994, 3640, 3504, 3404, and 3219 m) along the slope.
To my surprise, at 15h23 and at a depth of 2873 m, we arrived at the foot of a
scarp made up of a mixture of prismatic blocks looking like dykes and radial
jointed pillow fragments. This marked the transition between the peridotite-gabbro
association, which had been overlaid by a freshly slumped talus of prismatic and
radial jointed pillow flows at about 2873 m deep (sample of basalt SP06-14). This
zone is cut by a major fault oriented roughly north south, in the same direction as
the rift valley floor. Thus, the transition between the peridotite-gabbro is short and
the normal geological sequence was not observed. Indeed, the gabbroic units,
which usually represented the solidified magma chamber (gabbro) prior to the
eruption of basaltic rocks, were thin and discontinuous. In fact, most of the gabbros observed were columnar intrusions within the peridotite.
Another unexpected observation was the thin unit of a dolerite dyke intrusion
that was exposed as large columnar blocks forming a talus pile. These were
probably exposed during tectonic movements. Thus, between 2873 and 2710 m,
which marks the top of the wall, only basaltic lava flows were seen along with
occasional dyke intrusions.
From the reconstruction of this dive, I had estimated that the basalt-dolerite
(dyke) complex did not exceed 150 m in thickness. This observation will be
important for the geological reconstruction of the ITR spreading segment formations. Thus, a complete geological section from the base (4700 m) to the top
(1850 m) of the entire East Wall of the ITR in the St. Paul’s FZ area was completed during dives SP02 and SP06.
The other dives SP03 and SP04 essentially took place on the west wall of the
rift valley, and they confirmed the large size of the serpentinized peridotite
290
8 Fracture Zones and Transform Faults
discoveries.
Thus, the last dive (SP06) on the East wall of the rift valley was made in
continuation of dive SP02, and was aimed at reaching the top of the wall. At
11h10, the pilot, P. Cheilan, P. Lubin, co-pilot, and I arrived on the sea floor at a
depth of 4032 m on the top of a scarp made up of sediment that was partially
burying the rocky outcrops. This sedimented plateau with sporadic outcrops of
pillow lava represented a major break in the slope and was observed at
4020–3939 m. The first peridotite rocks associated with basalt and diabase (dyke)
were seen at 3980–4000 m. After taking our sample of serpentinized peridotite
(SP06-01), we moved on along the slope.
During our progression between 4032 m and 2870 m depths, we encountered
several breaks marked by near vertical fault scarps, which exposed striated and
massive serpentinized peridotite, suggestive of a forceful emplacement. Vertical
outcrops of massive rectangular blocks of peridotite were observed and sampled at
3183-3321 m. These blocks were about 1 m thick (sample SP06-11), and showed
sub-parallel prismatic fractures. They were oriented in a N 050° direction and
contained white veins of gabbroic material (sample SP06-12) cross cutting the
serpentinized peridotite. Other rodingitized (altered) gabbros (samples SP06-2, -7,
-8, -9, and -12) intruding into the serpentinized peridotite were recovered in situ at
various depths (3994, 3640, 3504, 3404, and 3219 m) along the slope.
To my surprise, at 15h23 and at a depth of 2873 m, we arrived at the foot of a
scarp made up of a mixture of prismatic blocks looking like dykes and radial
jointed pillow fragments. This marked the transition between the peridotite-gabbro
association, which had been overlaid by a freshly slumped talus of prismatic and
radial jointed pillow flows at about 2873 m deep (sample of basalt SP06-14). This
zone is cut by a major fault oriented roughly north south, in the same direction as
the rift valley floor. Thus, the transition between the peridotite-gabbro is short and
the normal geological sequence was not observed. Indeed, the gabbroic units,
which usually represented the solidified magma chamber (gabbro) prior to the
eruption of basaltic rocks, were thin and discontinuous. In fact, most of the gabbros observed were columnar intrusions within the peridotite.
Another unexpected observation was the thin unit of a dolerite dyke intrusion
that was exposed as large columnar blocks forming a talus pile. These were
probably exposed during tectonic movements. Thus, between 2873 and 2710 m,
which marks the top of the wall, only basaltic lava flows were seen along with
occasional dyke intrusions.
From the reconstruction of this dive, I had estimated that the basalt-dolerite
(dyke) complex did not exceed 150 m in thickness. This observation will be
important for the geological reconstruction of the ITR spreading segment formations. Thus, a complete geological section from the base (4700 m) to the top
(1850 m) of the entire East Wall of the ITR in the St. Paul’s FZ area was completed during dives SP02 and SP06.
The other dives SP03 and SP04 essentially took place on the west wall of the
rift valley, and they confirmed the large size of the serpentinized peridotite
290
8 Fracture Zones and Transform Faults