implications in the construction of these ridge segments, it was important to visit
them in situ in order to observe the ground-true geology.
One of the goals of the cruise was to test previous geophysical (Detrick et al.
1995) observations that showed a low gravity spreading ridge segment, which
indicates the presence of a magma chamber. In order to text such a hypothesis, it
was important to make in situ geological observations. The details on the geology,
structural setting and composition of the rocks recovered are found in Bideau et al.
1998, Gracia et al. 1998, Hekinian et al. 2000).
A sea-surface magnetic survey was also undertaken at night, in addition to the
submersible dives. Bertrand Sichler, a geophysicist from IFREMER who is particularly handy for manipulating and fixing almost any electronic instrument, was
eager to find out how a detailed magnetic survey with small track spacing (\2 km
apart) would augment our knowledge about the structure of the sea floor. More
than 2,400 km of closely spaced (1.8 km apart) magnetic profiles (40–55 km long)
were made and two-thirds were corrected for diurnal variation using a reference
magnetometer moored at 1500 m above the seafloor. Bertrand wanted to find out if
a particular structure would give a distinct magnetic signature, which eventually
could be used as criteria for exploring different types of structures. It was found
that the strongest magnetization signals near the axis are linked to the serpentinized peridotites forming a massif at the inner corner of the spreading axis. The
deficit of magnetization in the axial area, which has slightly drifted northward with
respect to the center of the segments, corresponds to the ‘‘bull’s eye’’ feature
defined previously from the gravimetric measurements of Detrick et al. (1995).
This lack of magnetization is related to a decrease in thickness of the magnetized
layer as a result of an uplift of the Curie isotherm (about 550 °C). This suggests
that magmatic activity is present at depth, even if segment OH3 is believed to be a
magma-starved segment. The more magnetized areas are found in the axial valley,
north of OH1 and south of OH3, and are probably related to the proximity of the
major transforms.
The Oceanaut dives were concentrated in three areas: (1) the axial Rift Valley
where most volcanic activity occurs, (2) on the inside corner of the rift valley
where tectonic uplift has emplaced deep seated peridotite, and (3) on the wall of
the rift valley where individual volcanoes are formed. During about 100 h of direct
submersible observations, a total of 250 rock samples, high-quality video data and
photographs were collected from the axis, the rift-valley walls and from several
off-axis volcanoes.
Axial Rift Valley
The ridge segment is 90 km long and has an hourglass shape which narrows to
about 4 km wide at the segment’s center at 2,200 m depth, and widens up to
12 km and deepens to 4,100 m depth towards the northern and southern segment
188
7 Oceanic Spreading Ridges and Sea Floor Creation
them in situ in order to observe the ground-true geology.
One of the goals of the cruise was to test previous geophysical (Detrick et al.
1995) observations that showed a low gravity spreading ridge segment, which
indicates the presence of a magma chamber. In order to text such a hypothesis, it
was important to make in situ geological observations. The details on the geology,
structural setting and composition of the rocks recovered are found in Bideau et al.
1998, Gracia et al. 1998, Hekinian et al. 2000).
A sea-surface magnetic survey was also undertaken at night, in addition to the
submersible dives. Bertrand Sichler, a geophysicist from IFREMER who is particularly handy for manipulating and fixing almost any electronic instrument, was
eager to find out how a detailed magnetic survey with small track spacing (\2 km
apart) would augment our knowledge about the structure of the sea floor. More
than 2,400 km of closely spaced (1.8 km apart) magnetic profiles (40–55 km long)
were made and two-thirds were corrected for diurnal variation using a reference
magnetometer moored at 1500 m above the seafloor. Bertrand wanted to find out if
a particular structure would give a distinct magnetic signature, which eventually
could be used as criteria for exploring different types of structures. It was found
that the strongest magnetization signals near the axis are linked to the serpentinized peridotites forming a massif at the inner corner of the spreading axis. The
deficit of magnetization in the axial area, which has slightly drifted northward with
respect to the center of the segments, corresponds to the ‘‘bull’s eye’’ feature
defined previously from the gravimetric measurements of Detrick et al. (1995).
This lack of magnetization is related to a decrease in thickness of the magnetized
layer as a result of an uplift of the Curie isotherm (about 550 °C). This suggests
that magmatic activity is present at depth, even if segment OH3 is believed to be a
magma-starved segment. The more magnetized areas are found in the axial valley,
north of OH1 and south of OH3, and are probably related to the proximity of the
major transforms.
The Oceanaut dives were concentrated in three areas: (1) the axial Rift Valley
where most volcanic activity occurs, (2) on the inside corner of the rift valley
where tectonic uplift has emplaced deep seated peridotite, and (3) on the wall of
the rift valley where individual volcanoes are formed. During about 100 h of direct
submersible observations, a total of 250 rock samples, high-quality video data and
photographs were collected from the axis, the rift-valley walls and from several
off-axis volcanoes.
Axial Rift Valley
The ridge segment is 90 km long and has an hourglass shape which narrows to
about 4 km wide at the segment’s center at 2,200 m depth, and widens up to
12 km and deepens to 4,100 m depth towards the northern and southern segment
188
7 Oceanic Spreading Ridges and Sea Floor Creation