This long-range survey was possible because the ship was equipped with a newly
acquired multichannel SeaBeam system, which was able to follow and map a band
of ridge axis 3 km wide at a depth of 2500 m during a single pass. This long-range
survey covered about 2,500 km of the EPR axis. The cruise was divided into two
legs: the first leg left Mazatlan, Mexico on May 10, and arrived a month later in
Hao, French Polynesia, on June 8, 1980.
The first Leg (SEARISE 1) surveyed the northern EPR between 21°N to 20°S
along the ridge axis. In addition to having a general view of the ridge, the cruise
was aimed at surveying the CYAMEX Project diving site (at 21°N) in more detail.
Further south, a few stations of water sampling and dredging were performed near
the ridge axis at 13°N and along various other segments in order to find signs of
hydrothermalism. A particular emphasis was made on sampling and obtaining
bathymetric coverage of the Galapagos Triple Junction.
The second Leg (SEARISE 2) started on July 3, 1980. It was to leave from the
Island of Hao, which at that time was an advanced military base for the French
nuclear testing that was being conducted on the Island of Mururoa, about 1000 km
further to the east. Hao is an atoll and since it was controlled by the military,
special permission was required to be on the island. Members of the French
Foreign Legion and other navy and military personnel were often in transit on the
island between Mururoa and Papeete in Tahiti. David Needham, Felix Avedik,
Jean-Louis Cheminée, Jean-Paul Foucher (a specialist in heat flow measurement),
Jean Francheteau, Bob Ballard and I joined joined the the R.V. Jean Charcot in
Hao. After having a swim off a sandy beach, we left port early in the morning (at 2
AM) and passed through the narrow channel between the coral reefs which gave
access to the open ocean. One hour later, we started setting up our watches. As
soon as we reached deep water, we switched on the SeaBeam and placed our
magnetometer into the sea.
Some interesting observations were made along the various segments of the
EPR during this long-range axial exploration. For example, this study enabled us
to better correlate the axial topography with the rock composition and some
structural settings such as accreting ridge discontinuities (segments). It was
detected that in the vicinities of these discontinuities there is a topographic low
while the highs, where the Ridge crest is bulging about 50–200 m above its ends,
are most often located in the centers of the segments. This situation is found
elsewhere along other spreading ridge segments (Francheteau and Ballard 1983).
Another interesting discovery consisted in finding that transform faults displacing
segments of the EPR were the sites of a particular type of volcanic activity giving
rise to porphyritic basaltic lava enriched in early-formed large ([0.1 mm in
diameter) plagioclase crystals (Eissen et al. 1981). This observation suggested the
presence of crystal growth in magma reservoirs.
The scientific results of these two Legs were produced after long-term research
in various national and international laboratories. One of the results concerned the
compositional variability observed in the volcanic samples collected. The existence of geochemical and petrological provinces was confirmed. These provinces
had direct implications on the partial melting of different types of mantle located
204
7 Oceanic Spreading Ridges and Sea Floor Creation
acquired multichannel SeaBeam system, which was able to follow and map a band
of ridge axis 3 km wide at a depth of 2500 m during a single pass. This long-range
survey covered about 2,500 km of the EPR axis. The cruise was divided into two
legs: the first leg left Mazatlan, Mexico on May 10, and arrived a month later in
Hao, French Polynesia, on June 8, 1980.
The first Leg (SEARISE 1) surveyed the northern EPR between 21°N to 20°S
along the ridge axis. In addition to having a general view of the ridge, the cruise
was aimed at surveying the CYAMEX Project diving site (at 21°N) in more detail.
Further south, a few stations of water sampling and dredging were performed near
the ridge axis at 13°N and along various other segments in order to find signs of
hydrothermalism. A particular emphasis was made on sampling and obtaining
bathymetric coverage of the Galapagos Triple Junction.
The second Leg (SEARISE 2) started on July 3, 1980. It was to leave from the
Island of Hao, which at that time was an advanced military base for the French
nuclear testing that was being conducted on the Island of Mururoa, about 1000 km
further to the east. Hao is an atoll and since it was controlled by the military,
special permission was required to be on the island. Members of the French
Foreign Legion and other navy and military personnel were often in transit on the
island between Mururoa and Papeete in Tahiti. David Needham, Felix Avedik,
Jean-Louis Cheminée, Jean-Paul Foucher (a specialist in heat flow measurement),
Jean Francheteau, Bob Ballard and I joined joined the the R.V. Jean Charcot in
Hao. After having a swim off a sandy beach, we left port early in the morning (at 2
AM) and passed through the narrow channel between the coral reefs which gave
access to the open ocean. One hour later, we started setting up our watches. As
soon as we reached deep water, we switched on the SeaBeam and placed our
magnetometer into the sea.
Some interesting observations were made along the various segments of the
EPR during this long-range axial exploration. For example, this study enabled us
to better correlate the axial topography with the rock composition and some
structural settings such as accreting ridge discontinuities (segments). It was
detected that in the vicinities of these discontinuities there is a topographic low
while the highs, where the Ridge crest is bulging about 50–200 m above its ends,
are most often located in the centers of the segments. This situation is found
elsewhere along other spreading ridge segments (Francheteau and Ballard 1983).
Another interesting discovery consisted in finding that transform faults displacing
segments of the EPR were the sites of a particular type of volcanic activity giving
rise to porphyritic basaltic lava enriched in early-formed large ([0.1 mm in
diameter) plagioclase crystals (Eissen et al. 1981). This observation suggested the
presence of crystal growth in magma reservoirs.
The scientific results of these two Legs were produced after long-term research
in various national and international laboratories. One of the results concerned the
compositional variability observed in the volcanic samples collected. The existence of geochemical and petrological provinces was confirmed. These provinces
had direct implications on the partial melting of different types of mantle located
204
7 Oceanic Spreading Ridges and Sea Floor Creation