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older parts of the Pacific, suggesting a change in
the flow regime or the mantle fabric.
The variation of velocities and anisotropy
with age suggests that stress- or flow-aligned
olivine may be present. The velocities depend
on temperature, pressure and crystal orientation.
An interpretation based on flow gives the veloc-
ity depth relations illustrated in Figure 20.9. The
upper-left diagram illustrates a convection cell
with material rising at the midocean range (R)
and flowing down at the trench (T).
The lower left of Figure 20.9 shows the
schematic temperature profile for such a cell. The
seismic velocities decrease with temperature and
increase with pressure. Combining the effects
of
Fig. 20.7
S velocity from about 50 to 550 km along the
great-circle path shown. Cross-sections are shown with two
vertical exaggerations. Velocity variations are much more
extreme at depths less than 250 km than at greater depths.
The circles on the map represent hotspots.
Mapping mantle flow
The combined inversion of Rayleigh waves and
Love waves across the Pacific has led to models
that have age-dependent properties; LID thick-
nesses, seismic velocities and anisotropies. In gen-
eral, the seismic lithosphere increases in thick-
ness with age and
V
SH
>
V
SV
formostofthe
Pacific. However,
V
SH
<
V
SV
for the younger and
temperature
and
pressure,
one
obtains
a
