also permit us to verify the continuity of the outcrop exposing a portion of the
Earth’s mantle.
However before all doing this, we wanted to look closely at the data from the
first dive. In the meantime, we continued to work on another important target that
consisted in verifying if the oblique sigmoidal shaped ridges were really volcanic
in origin (see the following section on the Garrett Transform Fault’s volcanism).
Transform Fault Volcanism
Although transform volcanism was predicted by Fox and Gallo (1989) for the
oblique ridges called the Alpha, Beta and Gamma ridges in the Garrett transform,
proof of this volcanic activity had not been actually seen prior to our dives. In
addition, it was found that transform volcanism does not only take place on the
oblique ridges but it is also found in the Active Tectonic Zone (ATZ) within the
Central basin (Figs. 8.1 and 8.2). All three oblique ridges were explored during
dives GN02 (Daniel Bideau), GN07 (Hekinian) and GN12 (Hekinian) to determine
the nature and the setting of the volcanism. These oblique ridges were not active at
the same time, as was observed by the degree of alteration of the outcrops. The
most eastern ‘‘Gamma ridge’’, which was also nearer to the northern spreading
segment of the EPR, is the most active. The least active of the oblique ridges is the
‘‘Alpha ridge’’, located furthest to the west. It was also observed that the middle
‘‘Beta Ridge’’ consists of freshly erupted lava flows.
The first dive on the Oblique Ridge called ‘‘Gamma Ridge’’ took place on
Monday, January 14, 1991, with Daniel Bideau as the observer and with Triger as
pilot, and Kaioun as co-pilot. Despite some problems of navigation because the
transponder network was displaced by about a mile, meaning the submersible
could not get good fixes for its navigation, nevertheless, the decision was taken to
hold a steady course without making any detours in order to avoid complications
during the reconstruction of the dives.
At 11h06 the divers arrived on the sea floor at 3470 m depth on the east slope of
the ridge and took their first sample. Then, they took a southwest direction (240°)
up slope to reach the top of the ridge. On the summit, at 3170 m, they found very
fresh lobated flows with an ‘‘oily’’ (shiny) surface appearance. Flat and ropy sheet
flows similar to those found on recent EPR segments were sampled. The divers had
accomplished their mission since they had found evidence of the most recent
volcanism in the Garrett transform (Fig. 8.8).
The other two dives, which I made (GN07 and GN12), confirmed the findings
for the other ridges (Alfa and Beta) (Fig. 8.2). These ridges differed from the
Gamma Ridge because they showed mainly pillow lava and lobated flows forming
small mounds and hills (\30 m high).
An example of the geology encountered on one of the oblique ridges was
described during Dive GN12 on ‘‘Alpha’’. My dive’s observations started 10h43 at
3755 m depth on the eastern flank of the oblique ridge where lobated flows were
Diving in the Active Tectonic Zone
267
Earth’s mantle.
However before all doing this, we wanted to look closely at the data from the
first dive. In the meantime, we continued to work on another important target that
consisted in verifying if the oblique sigmoidal shaped ridges were really volcanic
in origin (see the following section on the Garrett Transform Fault’s volcanism).
Transform Fault Volcanism
Although transform volcanism was predicted by Fox and Gallo (1989) for the
oblique ridges called the Alpha, Beta and Gamma ridges in the Garrett transform,
proof of this volcanic activity had not been actually seen prior to our dives. In
addition, it was found that transform volcanism does not only take place on the
oblique ridges but it is also found in the Active Tectonic Zone (ATZ) within the
Central basin (Figs. 8.1 and 8.2). All three oblique ridges were explored during
dives GN02 (Daniel Bideau), GN07 (Hekinian) and GN12 (Hekinian) to determine
the nature and the setting of the volcanism. These oblique ridges were not active at
the same time, as was observed by the degree of alteration of the outcrops. The
most eastern ‘‘Gamma ridge’’, which was also nearer to the northern spreading
segment of the EPR, is the most active. The least active of the oblique ridges is the
‘‘Alpha ridge’’, located furthest to the west. It was also observed that the middle
‘‘Beta Ridge’’ consists of freshly erupted lava flows.
The first dive on the Oblique Ridge called ‘‘Gamma Ridge’’ took place on
Monday, January 14, 1991, with Daniel Bideau as the observer and with Triger as
pilot, and Kaioun as co-pilot. Despite some problems of navigation because the
transponder network was displaced by about a mile, meaning the submersible
could not get good fixes for its navigation, nevertheless, the decision was taken to
hold a steady course without making any detours in order to avoid complications
during the reconstruction of the dives.
At 11h06 the divers arrived on the sea floor at 3470 m depth on the east slope of
the ridge and took their first sample. Then, they took a southwest direction (240°)
up slope to reach the top of the ridge. On the summit, at 3170 m, they found very
fresh lobated flows with an ‘‘oily’’ (shiny) surface appearance. Flat and ropy sheet
flows similar to those found on recent EPR segments were sampled. The divers had
accomplished their mission since they had found evidence of the most recent
volcanism in the Garrett transform (Fig. 8.8).
The other two dives, which I made (GN07 and GN12), confirmed the findings
for the other ridges (Alfa and Beta) (Fig. 8.2). These ridges differed from the
Gamma Ridge because they showed mainly pillow lava and lobated flows forming
small mounds and hills (\30 m high).
An example of the geology encountered on one of the oblique ridges was
described during Dive GN12 on ‘‘Alpha’’. My dive’s observations started 10h43 at
3755 m depth on the eastern flank of the oblique ridge where lobated flows were
Diving in the Active Tectonic Zone
267