night, Felix and Perron with the help of François, the boatswain, pulled back and
rolled-up the cable on the winch and then we resumed our voyage at full speed to
get to our target zone.
We arrived on the site of the dives in the evening of February 20th. The weather
was sunny and the sea calm. The normal routine of finding the ridge axis and
preparing the next morning’s diving target started with the navigation team who
deployed the three bottom transponders on the sea floor and then calibrated them
with respect to geographic coordinates obtained by a Global Positioning System
(GPS). This would take a few hours while the vessel circled around each transponder
in order to have sufficient, coherent satellite fixes for the accuracy of positioning.
The main objective of the diving program was to perform a detailed geological
survey on the four off-axial seamounts that had been discovered on the previous
‘‘Clipperton’’ cruise. A total of 11 dives were performed. Most of the dives were
made on the off-axis seamounts and only three took place on the ridge axis. The
three dives that took place in the axial graben of the ridge axis were to localize and
evaluate the degree of hydrothermal alteration on the rocks during fluid circulation. It was found that the eastern wall of the axial graben was the site of intense
hydrothermal alteration. Greenish gray faulted scarps with abundant talus were
seen. Dead hydrothermal chimneys that had been partially fractured and dislocated
were found associated with staircase-faulted scarps.
The off-axis seamounts visited are located between 4 and 18 km from the ridge
axis. The two nearest seamounts to the ridge axis are located near 12°43
0 N (called
the Southeastern seamount) and near 12°50
0 N (called the Northeastern seamount).
A third seamount was further away, about 10 km to the west of the ridge axis, and
was named the Clipperton seamount (Fig. 7.15).
On Leg 3 of this cruise, we had the privilege of having James Franklin, a
specialist of ore deposits from the Geological Survey of Canada, on board. Jim had
spent most of his life in the field exploring and finding ore deposits in Canada and
other places in the World. His knowledge, as a land-trained mining geologist, was
helpful in order to make some comparisons with land deposits. From the dives
conducted on the Southeastern seamount, where we discovered one of the most
extensive hydrothermal deposits, we learned that several conditions were necessary for such deposits to exist: (1) the path followed by the hydrothermal fluid to
carry the metals in solution to the surface must be canalized through a narrow
zone, (2) it helps if the area of hydrothermal circulation is associated with faults
and fissures, and (3) it is also better if the hydrothermal activity is moderate and
constant rather than very high and rapid.
Off-Axial Seamounts on the EPR at 12°50
0 N and 11°20
0 N
When spreading takes place at the ridge axis, it sometimes happens that the
lithosphere, during its motion away from the axis, could also carry the magma
away from the main ridge-centered upwelling zone due to lateral subsurface flows
Cyana Dives on the EPR at 12°50
0 N
223
rolled-up the cable on the winch and then we resumed our voyage at full speed to
get to our target zone.
We arrived on the site of the dives in the evening of February 20th. The weather
was sunny and the sea calm. The normal routine of finding the ridge axis and
preparing the next morning’s diving target started with the navigation team who
deployed the three bottom transponders on the sea floor and then calibrated them
with respect to geographic coordinates obtained by a Global Positioning System
(GPS). This would take a few hours while the vessel circled around each transponder
in order to have sufficient, coherent satellite fixes for the accuracy of positioning.
The main objective of the diving program was to perform a detailed geological
survey on the four off-axial seamounts that had been discovered on the previous
‘‘Clipperton’’ cruise. A total of 11 dives were performed. Most of the dives were
made on the off-axis seamounts and only three took place on the ridge axis. The
three dives that took place in the axial graben of the ridge axis were to localize and
evaluate the degree of hydrothermal alteration on the rocks during fluid circulation. It was found that the eastern wall of the axial graben was the site of intense
hydrothermal alteration. Greenish gray faulted scarps with abundant talus were
seen. Dead hydrothermal chimneys that had been partially fractured and dislocated
were found associated with staircase-faulted scarps.
The off-axis seamounts visited are located between 4 and 18 km from the ridge
axis. The two nearest seamounts to the ridge axis are located near 12°43
0 N (called
the Southeastern seamount) and near 12°50
0 N (called the Northeastern seamount).
A third seamount was further away, about 10 km to the west of the ridge axis, and
was named the Clipperton seamount (Fig. 7.15).
On Leg 3 of this cruise, we had the privilege of having James Franklin, a
specialist of ore deposits from the Geological Survey of Canada, on board. Jim had
spent most of his life in the field exploring and finding ore deposits in Canada and
other places in the World. His knowledge, as a land-trained mining geologist, was
helpful in order to make some comparisons with land deposits. From the dives
conducted on the Southeastern seamount, where we discovered one of the most
extensive hydrothermal deposits, we learned that several conditions were necessary for such deposits to exist: (1) the path followed by the hydrothermal fluid to
carry the metals in solution to the surface must be canalized through a narrow
zone, (2) it helps if the area of hydrothermal circulation is associated with faults
and fissures, and (3) it is also better if the hydrothermal activity is moderate and
constant rather than very high and rapid.
Off-Axial Seamounts on the EPR at 12°50
0 N and 11°20
0 N
When spreading takes place at the ridge axis, it sometimes happens that the
lithosphere, during its motion away from the axis, could also carry the magma
away from the main ridge-centered upwelling zone due to lateral subsurface flows
Cyana Dives on the EPR at 12°50
0 N
223