de Physique du Globe (IPG). The diving team included scientists from several
disciplines who were familiar with the geology and the structure of the seafloor.
The goal of the project was to obtain continuous observations and sample the
exposed lower and upper mantle complex in the context of a fast (6 cm/year total
rate) spreading ridge system. Twenty-one dives were conducted on the CocosNazca spreading center and the Intra-rift ridge bounding the young spreading
ridge. The Intra-Rift ridge represents the section of the EPR over hanging the Hess
Deep. More than 400 samples were collected, with rock compositions that ranged
from basalts, dolerites, cumulates (olivine gabbros), non-cumulates (isotropic
gabbros, containing clinopyroxene, olivine and plagioclase), to ultramafics
(dunites and peridotite).
For the first time in the Pacific Ocean, a nearly complete section of a relatively
young crust and upper mantle representing a portion of a spreading ridge were
observed and served as a model of comparison with other land-based ophiolite
complexes (Fig. 7.26a, b).
After the Nautile dives, two other international cruises with the Alvin submersible investigated the upper crust (lavas and dykes) and uppermost gabbro
formations exposed along the northern rift valley scarp crust (Karson et al. 1992)
in the same general area. Furthermore, the Ocean Drilling Program (ODP) Leg 147
drilled gabbroic rocks at the crest of the intra-Rift ridge (Site 894 and 895) and has
recovered peridotites intruded by numerous gabbroic veins, at site 395, located
about 10 km southeast of Site 894 (Gillis et al. 1993).
I would like to mention in the following section a particular dive that took place
in the Hess Deep in order to show how unpredictable underwater exploration could
be even after thorough and detailed preparations.
The Intra-Rift Ridge Dive NZ08 in the Hess Deep
Dive NZ08 (standing for NaZcopac dive # 08 of the Nazcopac (NZ) cruise) took
place on the Nautile with Triger, LeBigot and myself. We started at 9:00 o’clock in
the morning of October 21, 1988. We reached the bottom at 11h08 and landed at
the foot of south facing steep slope at 5391 m in the Hess Deep near 02°15
0 N101°33
0 W in the equatorial Pacific. The depression forming the Hess Deep is
covered by sand sized debris and pelagic sediment. As usual we looked for the first
sample before starting to explore. As the submersible moved upward along the
slope of the Intra-Rift north wall we encountered several faulted steps showing
green and white colored peridotite outcrops (Fig. 7.26a, b, 7.28) crisscrossed by
variable sized massive gabbroic dykes. Slumped rocks and debris of variable size
going from cobble size to large blocks up to the size of the submersible were lying
on the slope. This was not very reassuring because these large blocks of massive
gabbros are unstable and could move at anytime during our ascension.
As we progressed upward, the large unstable blocks decreased and were
replaced by smaller debris (coarse sand and boulder size) of basaltic and dolerite
236
7 Oceanic Spreading Ridges and Sea Floor Creation
disciplines who were familiar with the geology and the structure of the seafloor.
The goal of the project was to obtain continuous observations and sample the
exposed lower and upper mantle complex in the context of a fast (6 cm/year total
rate) spreading ridge system. Twenty-one dives were conducted on the CocosNazca spreading center and the Intra-rift ridge bounding the young spreading
ridge. The Intra-Rift ridge represents the section of the EPR over hanging the Hess
Deep. More than 400 samples were collected, with rock compositions that ranged
from basalts, dolerites, cumulates (olivine gabbros), non-cumulates (isotropic
gabbros, containing clinopyroxene, olivine and plagioclase), to ultramafics
(dunites and peridotite).
For the first time in the Pacific Ocean, a nearly complete section of a relatively
young crust and upper mantle representing a portion of a spreading ridge were
observed and served as a model of comparison with other land-based ophiolite
complexes (Fig. 7.26a, b).
After the Nautile dives, two other international cruises with the Alvin submersible investigated the upper crust (lavas and dykes) and uppermost gabbro
formations exposed along the northern rift valley scarp crust (Karson et al. 1992)
in the same general area. Furthermore, the Ocean Drilling Program (ODP) Leg 147
drilled gabbroic rocks at the crest of the intra-Rift ridge (Site 894 and 895) and has
recovered peridotites intruded by numerous gabbroic veins, at site 395, located
about 10 km southeast of Site 894 (Gillis et al. 1993).
I would like to mention in the following section a particular dive that took place
in the Hess Deep in order to show how unpredictable underwater exploration could
be even after thorough and detailed preparations.
The Intra-Rift Ridge Dive NZ08 in the Hess Deep
Dive NZ08 (standing for NaZcopac dive # 08 of the Nazcopac (NZ) cruise) took
place on the Nautile with Triger, LeBigot and myself. We started at 9:00 o’clock in
the morning of October 21, 1988. We reached the bottom at 11h08 and landed at
the foot of south facing steep slope at 5391 m in the Hess Deep near 02°15
0 N101°33
0 W in the equatorial Pacific. The depression forming the Hess Deep is
covered by sand sized debris and pelagic sediment. As usual we looked for the first
sample before starting to explore. As the submersible moved upward along the
slope of the Intra-Rift north wall we encountered several faulted steps showing
green and white colored peridotite outcrops (Fig. 7.26a, b, 7.28) crisscrossed by
variable sized massive gabbroic dykes. Slumped rocks and debris of variable size
going from cobble size to large blocks up to the size of the submersible were lying
on the slope. This was not very reassuring because these large blocks of massive
gabbros are unstable and could move at anytime during our ascension.
As we progressed upward, the large unstable blocks decreased and were
replaced by smaller debris (coarse sand and boulder size) of basaltic and dolerite
236
7 Oceanic Spreading Ridges and Sea Floor Creation