Geoscience Reference
In-Depth Information
slab-related geoid. Dense delaminated lower con-
tinental crust may also contribute to density and
velocity anomalies in the upper mantle.
Fig. 11.7
Cross-section illustrating the low upper-mantle
shear velocities under midocean ridges and western North
America. Stable regions are relatively fast in the shallow
mantle.
Azimuthal anisotropy
The velocities of surface waves depend on posi-
tion and on the direction of travel. If an ade-
quately dense global coverage of surface-wave
pathsisava lable,then
azimuthal aniso-
tropy of the upper mantle
,aswellaslat-
eral heterogeneity can be studied (Tanimoto and
Anderson, 1984, 1985, Forsyth and Vyeda, 1975).
Global azimuthal anisotropy
may corre-
late with convective motions in the mantle; it
is a further constraint on mantle geodynamics.
Azimuthal anisotropy can be caused by ori-
ented crystals or a consistent fabric caused by, for
example, dikes or convective rolls in the shallow
mantle. In either case, the azimuthal variation
of seismic-wave velocity is telling us something
about convection in the mantle.
Figure 11.9 is a map of the azimuthal results
for 200-s Rayleigh waves. The lines are oriented in
the maximum velocity direction, and the length
of the lines is proportional to the anisotropy. The
therefore appears that a combination of slabs and
broad thermal anomalies in the upper mantle
can explain the major features of the degree 4--9
geoid. The longer wavelength part of the geoid,
degrees 2 and 3, correlates with seismic veloci-
ties in the deeper part of the mantle. Figure 11.8
shows the actual distribution of Love-wave phase
velocities and that computed from the geoid
assuming a linear relationship between velocity
and geoid height. Most subduction regions are
slow at short periods, presumably because of the
presence of back-arc basins and hot, upwelling
material above the slab.
Slabs are colder than normal mantle and
therefore they can be denser. Intrinsically
denser minerals also occur in the slab because
of temperature-dependent phase changes. The
phase-change effect leverages the role of temper-
ature with the result that slabs confined to the
upper mantle can explain the magnitude of the
