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of the presence of low-density material in the upper mantle beneath the ridge.
Because gravity models are non-unique, they need to be constrained. Figure 9.11
shows four density models for the Mid-Atlantic Ridge. In the first model, the
low-density zone is narrow and extends 200 km deep into the mantle; in the other
three models, compensation is achieved by a broad and fairly shallow low-density
zone. All four models adequately satisfy the gravity data. For reasons explained
later, the first model (Fig. 9.11(c)) is thought to be the best representation of the
density structure beneath the ridge.
The most direct evidence of the deep structure of the mid-ocean ridges has
come from earthquake seismology. Results of early studies of the Mid-Atlantic
Ridge north of 50
◦
N, using data on earthquakes recorded in Iceland and Green-
land, indicated that the upper mantle there had a very low P-wave velocity and
that this low-velocity zone extended to perhaps 250 km and was a few hundred
kilometres wide. The detailed three-dimensional S-wave-velocity structure of
the upper mantle obtained by the inversion of long-period seismic recordings
shows that, beneath the mid-ocean ridges in the depth interval 25-250 km, the
S-wave velocity is reduced by 2%-8%. Similarly,
Q
β
values are considerably
reduced in the depth interval 0-150 km, being
70 at 30-
150 km depths, compared with values of 600 at 0-80 km and 80 at 80-150 km
depths in PREM. Seismic tomography (Section 8.1) confirms the depth extent
of the low-velocity regions beneath the mid-ocean ridges as being about 250 km
(Figs. 8.6-8.9). Seismologists working with teleseismic data from earthquakes
have reported a 'gap in the lithosphere' beneath the mid-ocean-ridge system,
across that Sn (a seismic shear wave that propagates in the uppermost mantle)
is very poorly propagated. Sn propagates well across stable regions such as con-
tinental shields and deep ocean basins but only very inefficiently when its path
crosses the mid-ocean-ridge system or the concave side of an island arc. In addi-
tion, it is generally noted that magnitudes of ridge-crest earthquakes are often
lower than those of earthquakes with comparable surface-wave magnitude but
which are located away from the mid-ocean-ridge system. Frequently, surface
waves are observed from mid-ocean-ridge earthquakes for which no body waves
are detected. All of these observations can be explained by the presence of an
absorptive zone in the upper mantle beneath the mid-ocean ridges. Such a zone,
which is also limited in extent, can most readily and reasonably be explained as
being due to partial melting, occurring because there the upper mantle material
is raised above its solidus. Results of seismic-refraction experiments along and
parallel to the crest of the Mid-Atlantic Ridge also suggest that, in some places,
there is a zone of strong P- and S-wave absorption extending some 25 km on
either side of the ridge axis and indicate that the top of this zone lies at least 7 km
beneath the seabed.
Results of detailed studies of the source mechanism of large, ridge-axis earth-
quakes on the Mid-Atlantic, South West Indian and American-Antarctic ridges
and the Gorda Rise and Galapagos spreading centres indicate that all the foci
∼
300 at 0-30 km and
∼
