and lava are exposed on both sides of the Cocos-Nazca spreading center. It was
also observed that the transition between dyke intrusions and erupted basalt is
gradual and interlocking. This mixed transitional zone of dykes and erupted lava
was observed throughout all the crossings and reaches up to 500 m in thickness. In
the mixed zones, the lava complex is massive and sub-horizontal, probably fed
from the underlying dykes (Francheteau et al. 1992). A significant observation was
made during dive NZ20 where the observers saw the contact between the dykes
and the gabbroic unit, which was partially buried by talus at 3046 m depth.
Combining the dive results, we were able to extrapolate the thickness of the
dyke complex to be about 1,200 m. On the other hand, the thickness of the volcanic units, including basalt and mixed dyke-basalt overlying the dykes, is only
150–200 m. The overall sequence of the volcanic rocks lies on top of a gabbroic
unit representing a fossil magma chamber exposed on the 1 million year old
lithosphere of the ancient EPR. The lower and upper mantle material on which this
gabbroic and volcanic complex lies consists of residual peridotite that had accumulated after a partial melting of the mantle giving rise to the volcanic rocks.
The composition of the rocks in relation to their lithological sequences revealed
the presence of several types of gabbroic rocks going from cumulates (heavier,
olivine enriched) up to lighter ferrogabbros and isotropic (clinopyroxene plagioclase
enriched) gabbros (Hekinian et al. 1993; Coogan et al. 2002). This rock sequence
suggested that crystal liquid fractionation took place in a confined environment.
Thus, for the first time, direct field observation on the sea floor confirmed the
existence of a fossil magma chamber on a fast spreading ridge system.
South East Pacific Rise
The ultrafast spreading segments in the South Pacific (SEPR, South East Pacific
Rise) were first investigated by the N.O. JEAN CHARCOT (Searise cruise) and by
the F.S. SONNE, both of which used multibeam echosounder bathymetric systems
(also called SeaBeam and Hydrosweep) in 1979–1980 to map the sea floor. The
scientific interest in studying the South East Pacific Rise was to find out how the
ridge’s tectonic activity and associated magmatism contributes to shape ultra-fast
spreading ridge systems. The axial region of the SEPR is interrupted (\40 km, in
length) by small ridge-discontinuities, which are small transforms or overlapping
spreading centers and/or kinks (Macdonald 1989). These spreading ridge segments
are located south of the Garrett transform (13°26
0 S) and extend for about 1000 km
up to the northern boundary of the Easter Microplate (23°S) (Fig. 7.29). The full
spreading rates of these various segments vary between 141 and 162 mm/yr
(DeMets et al. 1990). The general axial depth of the ridge decreases gradually
towards the south and varies between 2590 and 2850 m depths.
The ultrafast South Pacific Spreading Ridge (SEPR) segments are characterized
by volcanic domes. They are the closest structural settings on the sea floor to what
is observed on subaerial shield volcanoes. The bulging of the domed shaped ridge
238
7 Oceanic Spreading Ridges and Sea Floor Creation
also observed that the transition between dyke intrusions and erupted basalt is
gradual and interlocking. This mixed transitional zone of dykes and erupted lava
was observed throughout all the crossings and reaches up to 500 m in thickness. In
the mixed zones, the lava complex is massive and sub-horizontal, probably fed
from the underlying dykes (Francheteau et al. 1992). A significant observation was
made during dive NZ20 where the observers saw the contact between the dykes
and the gabbroic unit, which was partially buried by talus at 3046 m depth.
Combining the dive results, we were able to extrapolate the thickness of the
dyke complex to be about 1,200 m. On the other hand, the thickness of the volcanic units, including basalt and mixed dyke-basalt overlying the dykes, is only
150–200 m. The overall sequence of the volcanic rocks lies on top of a gabbroic
unit representing a fossil magma chamber exposed on the 1 million year old
lithosphere of the ancient EPR. The lower and upper mantle material on which this
gabbroic and volcanic complex lies consists of residual peridotite that had accumulated after a partial melting of the mantle giving rise to the volcanic rocks.
The composition of the rocks in relation to their lithological sequences revealed
the presence of several types of gabbroic rocks going from cumulates (heavier,
olivine enriched) up to lighter ferrogabbros and isotropic (clinopyroxene plagioclase
enriched) gabbros (Hekinian et al. 1993; Coogan et al. 2002). This rock sequence
suggested that crystal liquid fractionation took place in a confined environment.
Thus, for the first time, direct field observation on the sea floor confirmed the
existence of a fossil magma chamber on a fast spreading ridge system.
South East Pacific Rise
The ultrafast spreading segments in the South Pacific (SEPR, South East Pacific
Rise) were first investigated by the N.O. JEAN CHARCOT (Searise cruise) and by
the F.S. SONNE, both of which used multibeam echosounder bathymetric systems
(also called SeaBeam and Hydrosweep) in 1979–1980 to map the sea floor. The
scientific interest in studying the South East Pacific Rise was to find out how the
ridge’s tectonic activity and associated magmatism contributes to shape ultra-fast
spreading ridge systems. The axial region of the SEPR is interrupted (\40 km, in
length) by small ridge-discontinuities, which are small transforms or overlapping
spreading centers and/or kinks (Macdonald 1989). These spreading ridge segments
are located south of the Garrett transform (13°26
0 S) and extend for about 1000 km
up to the northern boundary of the Easter Microplate (23°S) (Fig. 7.29). The full
spreading rates of these various segments vary between 141 and 162 mm/yr
(DeMets et al. 1990). The general axial depth of the ridge decreases gradually
towards the south and varies between 2590 and 2850 m depths.
The ultrafast South Pacific Spreading Ridge (SEPR) segments are characterized
by volcanic domes. They are the closest structural settings on the sea floor to what
is observed on subaerial shield volcanoes. The bulging of the domed shaped ridge
238
7 Oceanic Spreading Ridges and Sea Floor Creation