Geoscience Reference
In-Depth Information
4.4
Figure 9.10.
Rayleigh-wave
phase-velocity-dispersion
curves (see Section 4.1.3)
for oceanic lithosphere of
various ages. Shading
shows one standard
deviation in the
measurements. Notice
that, for periods less than
100 s, the phase velocity
increases with the age of
the plate. For periods
greater than 100 s, the
velocities are within error
of each other. (After
Forsyth (1977).)
4.2
4.0
100-135 Ma
20-40 Ma
10-20 Ma
0 -10 Ma
3.8
20
60
100
140
Period (s)
is dependent on the azimuth of the ray path. Velocities measured perpendicular to
the mid-ocean-ridge axis are greater than velocities measured parallel to the ridge
axis. This velocity anisotropy is believed to be caused by the preferential aligning
of olivine crystals in the mantle parallel to the direction of flow (see Section 8.1.4).
Mantle S-wave velocities are reduced beneath ridges when compared with the
ocean basins, down to about 150 km depth.
Surface-wave-dispersion curves (Section 4.1.3) for oceanic paths show that
phase velocities of Rayleigh waves are significantly reduced for young oceanic
lithosphere and asthenosphere (Fig. 9.10). Interpretation of these dispersion
curves suggests that the lithospheric thickness increases from some 30 km at
5Mato100 km at 100 Ma, whereas lithospheric S-wave velocities increase from
4.3 to 4.6 km s
−
1
with age. Similarly, asthenosphere S-wave velocities increase
from 4.1 to 4.3 km s
−
1
with age.
9.3 The deep structure of mid-ocean ridges
9.3.1 Geophysical evidence
The free-air gravity anomaly across the mid-ocean ridges is not zero, indicat-
ing that the ridges are not in total isostatic equilibrium (see Section 5.5 for the
definition and use of these terms). Partial compensation is attained by virtue