during geophysical investigations along the strike of the oceanic ridges. The
evidence of these ridge discontinuities was found along the East Pacific Rise with
the multichannel bathymetry system that enabled mapping of a large portion of the
sea floor in a single swath, in the early 80s. However, the fact that the strike of the
ridge was segmented was first suggested by the pioneering work of Bruce Heezen
and Mary Tharp who drew the first physiographic map of the western North
Atlantic in 1954, and later for the World’s ocean floor in 1967 (Heezen and Tharp
1968). Looking at this physiographic map of the oceans, it becomes obvious that
the entire spreading ridge system around the World was marked by ridge segment
discontinuities as well as being cross cut and displaced by transform faults, which
have offset the accreting plate boundary region for distances of hundreds of
kilometers (from about 50 up to 500 km).
The causes of ridge segmentation are multiple:
(1) The early history of continental opening formed major breaks and fracture
zones in the lithosphere, which later caused the large-scale discontinuities
during the separation of Africa and the Americas in the Equatorial Atlantic
where the largest ridge segment offsets are observed.
(2) Instability in the asthenosphere due to differences in rheology (i.e. the force of
deformation of a structure, plus the degree of plasticity and the quantity and
speed of magma flowing through the crust and upper mantle).
(3) Depending on the spreading rate and different plate tectonic motions, the ridge
segmentations will affect the mode and rate of magma delivery. For example,
ridges with slow spreading rates and starved magmatism will have a thicker,
more brittle lithosphere, which will slow magma delivery even more. Fast
spreading ridges, with their faster rate of magma delivery will show smoother
breaks between the segments.
Since 1981, scientific ships equipped with multichannel echo sounders have
enabled us to obtain detailed bathymetry of the seafloor so we have been able to
identify further types of segmentation of the spreading ridge system. The observed
topographic variability along a spreading ridge axis, the morphology and composition of the rocks, as well the geophysical observations (gravimetry, seismicity
and magnetism) and the distribution of hydrothermal activity have enhanced our
understanding of ridge segmentation. The volcanic and tectonic activities along
each portion of an individual spreading ridge segment will vary along their strike.
Generally speaking, the ends of the segments are volcanically less active than the
middle portions.
The EPR fast spreading ridge systems are made up of several segments that
have undergone different periods of volcanic, tectonic and hydrothermal activities
(Fig. 7.2). The overall observations between transform faults and large ‘‘nontransform offset’’ boundaries have permitted us to define several orders of segmentation on the fast spreading ridge systems of the Pacific Ocean.
The 1st order segmentation reflects large-scale magmatic upwelling in the
asthenosphere (Macdonald et al. 1988), which usually occurs between two
Ridge Segmentations
167
evidence of these ridge discontinuities was found along the East Pacific Rise with
the multichannel bathymetry system that enabled mapping of a large portion of the
sea floor in a single swath, in the early 80s. However, the fact that the strike of the
ridge was segmented was first suggested by the pioneering work of Bruce Heezen
and Mary Tharp who drew the first physiographic map of the western North
Atlantic in 1954, and later for the World’s ocean floor in 1967 (Heezen and Tharp
1968). Looking at this physiographic map of the oceans, it becomes obvious that
the entire spreading ridge system around the World was marked by ridge segment
discontinuities as well as being cross cut and displaced by transform faults, which
have offset the accreting plate boundary region for distances of hundreds of
kilometers (from about 50 up to 500 km).
The causes of ridge segmentation are multiple:
(1) The early history of continental opening formed major breaks and fracture
zones in the lithosphere, which later caused the large-scale discontinuities
during the separation of Africa and the Americas in the Equatorial Atlantic
where the largest ridge segment offsets are observed.
(2) Instability in the asthenosphere due to differences in rheology (i.e. the force of
deformation of a structure, plus the degree of plasticity and the quantity and
speed of magma flowing through the crust and upper mantle).
(3) Depending on the spreading rate and different plate tectonic motions, the ridge
segmentations will affect the mode and rate of magma delivery. For example,
ridges with slow spreading rates and starved magmatism will have a thicker,
more brittle lithosphere, which will slow magma delivery even more. Fast
spreading ridges, with their faster rate of magma delivery will show smoother
breaks between the segments.
Since 1981, scientific ships equipped with multichannel echo sounders have
enabled us to obtain detailed bathymetry of the seafloor so we have been able to
identify further types of segmentation of the spreading ridge system. The observed
topographic variability along a spreading ridge axis, the morphology and composition of the rocks, as well the geophysical observations (gravimetry, seismicity
and magnetism) and the distribution of hydrothermal activity have enhanced our
understanding of ridge segmentation. The volcanic and tectonic activities along
each portion of an individual spreading ridge segment will vary along their strike.
Generally speaking, the ends of the segments are volcanically less active than the
middle portions.
The EPR fast spreading ridge systems are made up of several segments that
have undergone different periods of volcanic, tectonic and hydrothermal activities
(Fig. 7.2). The overall observations between transform faults and large ‘‘nontransform offset’’ boundaries have permitted us to define several orders of segmentation on the fast spreading ridge systems of the Pacific Ocean.
The 1st order segmentation reflects large-scale magmatic upwelling in the
asthenosphere (Macdonald et al. 1988), which usually occurs between two
Ridge Segmentations
167