1 km and are more likely to reflect melt segregation at the base of the crust.
Whether they represent only an offset of axial topography or ridge segments that
are overlapping each other, these intermediate discontinuities, have been called
DEVALS (Deviation from Axial Linearity) or ‘‘overlapping spreading centers’’
(OSC). The presence of an OSC is the result of two ridge segments, which
propagate towards each other due to the extrusion of lava, but whose two flows fail
to meet (Macdonald et al. 1988). Hence, along the segmented ridge crest, two
evolutionary paths occur with intermittent and alternate volcanic pulses. Eventually, the dominant spreading center with its larger magma delivery will propagate
over a longer distance.
The 4th order segmentation is represented by discontinuities due to en-echelon
fissures and collapsed lava ponds running along the strike of the spreading centers.
These smaller discontinuities of less than 1 km in length have offset the ridge axis
by less than 100 m. They have a vertical relief of less than 25 m in height and a
segment length of less than 5 km. The segment’s ends are associated with talus
piles. According to submarine observations (i.e. at NEPR 13°N and SEPR 18°30
0 S,
Hekinian et al. 1985), the large fissures forming an en-echelon relay zone are
separated by older constructional highs (\10 m high). Also these 4th order segmentations are ephemeral and short-lived since they are essentially controlled by
dyke injections and hydrothermal circulation (Haymon 1996). 4th order segmentations are characterized by an amagmatic period during lithospheric cooling and
cracking which allows seawater circulation and leaching to take place. The
duration of the hydrothermal cycle was inferred to be about 100 years for the ridge
segment at 9–10°N on the EPR (Wright et al. 1995). Smaller 3rd and 4th order
discontinuities are characterized by localized magma propagation from a main
magma reservoir during dyke and sill intrusions along the ridge strike.
The segments seen on slow spreading ridge systems, such as the Mid-Atlantic
Ridge, are marked by sharper tectonic breaks or discontinuities than on fast
spreading ridges, which show more ‘‘curved’’ and settled structural appearances.
Segment discontinuities on slow spreading ridge systems compared to those from
the fast spreading East Pacific Rise differ in their 2nd order discontinuities
showing the offset of their ridge axis rift valley floor. They are deprived of
Overlapping Spreading Centers (OSCs) and instead they have developed nodal
basins at their segment ends. These structures are longer-lasting (up to millions of
years) than those from a fast spreading ridge axis. Also, the 3rd order discontinuities in slow spreading ridge areas are confined to the rift valley and are defined
by volcanic constructions, mounds and individual edifices, which are often offset
from each other. Fast spreading ridges (total spreading [4 cm/yr) show a more
uniform and linear ridge axial topography without a deep rift valley, but with a
relatively shallow graben (\300 m depth). The magma budget is more sustained
and more extended along the ridge axis than on slow spreading ridges. Lateral, offaxis magmatism giving rise to off-axial seamounts is also more prevalent for fast
spreading centers than for slower spreading ridges.
Historically, the first small scale spreading ridge discontinuity (2nd and 3rd
order) was found by the German oceanographic vessel FS Sonne in January 1982
Ridge Segmentations
169
Whether they represent only an offset of axial topography or ridge segments that
are overlapping each other, these intermediate discontinuities, have been called
DEVALS (Deviation from Axial Linearity) or ‘‘overlapping spreading centers’’
(OSC). The presence of an OSC is the result of two ridge segments, which
propagate towards each other due to the extrusion of lava, but whose two flows fail
to meet (Macdonald et al. 1988). Hence, along the segmented ridge crest, two
evolutionary paths occur with intermittent and alternate volcanic pulses. Eventually, the dominant spreading center with its larger magma delivery will propagate
over a longer distance.
The 4th order segmentation is represented by discontinuities due to en-echelon
fissures and collapsed lava ponds running along the strike of the spreading centers.
These smaller discontinuities of less than 1 km in length have offset the ridge axis
by less than 100 m. They have a vertical relief of less than 25 m in height and a
segment length of less than 5 km. The segment’s ends are associated with talus
piles. According to submarine observations (i.e. at NEPR 13°N and SEPR 18°30
0 S,
Hekinian et al. 1985), the large fissures forming an en-echelon relay zone are
separated by older constructional highs (\10 m high). Also these 4th order segmentations are ephemeral and short-lived since they are essentially controlled by
dyke injections and hydrothermal circulation (Haymon 1996). 4th order segmentations are characterized by an amagmatic period during lithospheric cooling and
cracking which allows seawater circulation and leaching to take place. The
duration of the hydrothermal cycle was inferred to be about 100 years for the ridge
segment at 9–10°N on the EPR (Wright et al. 1995). Smaller 3rd and 4th order
discontinuities are characterized by localized magma propagation from a main
magma reservoir during dyke and sill intrusions along the ridge strike.
The segments seen on slow spreading ridge systems, such as the Mid-Atlantic
Ridge, are marked by sharper tectonic breaks or discontinuities than on fast
spreading ridges, which show more ‘‘curved’’ and settled structural appearances.
Segment discontinuities on slow spreading ridge systems compared to those from
the fast spreading East Pacific Rise differ in their 2nd order discontinuities
showing the offset of their ridge axis rift valley floor. They are deprived of
Overlapping Spreading Centers (OSCs) and instead they have developed nodal
basins at their segment ends. These structures are longer-lasting (up to millions of
years) than those from a fast spreading ridge axis. Also, the 3rd order discontinuities in slow spreading ridge areas are confined to the rift valley and are defined
by volcanic constructions, mounds and individual edifices, which are often offset
from each other. Fast spreading ridges (total spreading [4 cm/yr) show a more
uniform and linear ridge axial topography without a deep rift valley, but with a
relatively shallow graben (\300 m depth). The magma budget is more sustained
and more extended along the ridge axis than on slow spreading ridges. Lateral, offaxis magmatism giving rise to off-axial seamounts is also more prevalent for fast
spreading centers than for slower spreading ridges.
Historically, the first small scale spreading ridge discontinuity (2nd and 3rd
order) was found by the German oceanographic vessel FS Sonne in January 1982
Ridge Segmentations
169