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(b)
--1
(a)
Velocity (km s )
--1
Velocity (km s )
(c)
--1
Velocity (km s )
2
4
6
8
10
2
4
6
8
10
2
4
6
8
10
0
0
0
2
2
2
4
4
4
6
6
6
8
8
8
0 Ma
2 Ma
0 Ma
2.9 Ma
0 Ma
3 Ma
9 Ma
10
5 Ma
10
10
REYKJANES RIDGE
MID-ATLANTIC RIDGE
EAST PACIFIC RISE
Figure 9.22. Seismic P-wave velocity structures for the axial regions of mid-ocean
ridges: (a) Reykjanes Ridge at 60 N, (b) Mid-Atlantic Ridge at 37 N and (c) East
Pacific Rise at 9 N. All these velocity structures were determined using synthetic
seismograms. (After Bunch and Kennett (1980), Fowler (1976) and Orcutt et al .
(1976).)
in Fig. 9.22.However, an experiment on the Mid-Atlantic Ridge at 23 N, just
south of the Kane Fracture Zone, revealed near-normal oceanic crustal and upper-
mantle velocities beneath a portion of the median valley. These differences are
thought to reflect the time since the beginning of the last volcanic cycle. At 23 N,
the crust is older and cooler and so is closer to the velocity of normal oceanic
crust. None of these experiments gave results indicating the presence of a crustal
low-velocity zone.
A seismic-reflection survey shot across the axis of the Mid-Atlantic Ridge at
37 N detected a small, shallow low-velocity zone in the crust. This appears to be
a zone of extensive cracking at the injection centre associated with the extrusion
of lavas at the seabed. Seismic experiments on the Juan de Fuca Ridge produced
no evidence of crustal magma chambers.
Perhaps the most conclusive test of the presence of a partially molten zone can
be provided by shear waves, which should be greatly attenuated when they cross
such a zone. Arrays of three-component seismographs, with which it is possible
positively to identify shear waves, have been used to monitor the microearthquake
activity on the Mid-Atlantic Ridge (Fig. 9.23). There is no evidence for any spe-
cial crustal attenuation of the shear waves since the microearthquakes produced
large-amplitude shear waves. The crustal shear waves have travel times con-
sistent with their having crossed the axial zone at depths down to about 5 km
beneath the seabed. At these depths they would have had a wavelength of up
to 1 km.
All this seismic and thermal evidence strongly suggests that no large, contin-
uous crustal magma chamber (as in Fig. 9.20)isnormally present beneath the
axes of slow-spreading ridges, though the presence of small pockets of melt, up
to perhaps 2 km in diameter, cannot be excluded.
 
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