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melting. Trench and marginal sea regions (T), on
the other hand, are relatively slow above 200 km,
probably indicating the presence of a partial
melt, and fast below 400 km, probably indicating
the presence of cold subducted lithosphere. The
large size of the tectonic regions and the long
wavelengths of surface waves require that the
anomalous regions at depth are much broader
than the sizes of slabs or the active volcanic
regions at the surface. This is consistent with
broad passive upwellings under young oceans
andabundantpilingupofslabsundertrench
and old ocean regions. The latter is evidence
for layered-mantle convection and the cycling of
oceanic plates into the transition region.
Shields and young oceans are still evident
at 250 km. At 350 km the velocity variations
are much suppressed. Below 400 km, most of
the correlation with surface tectonics has disap-
peared, in spite of the regionalization, because
shields and young oceans are both slow, and
trench and old-ocean regions are both fast. Most
oceanic regions have similar velocities at depth.
Shields do not have higher velocities than some
other tectonic regions below 250 km and defi-
nitely do not have 'roots' extending throughout
the upper mantle or even below 400 km, as in
the original
tectosphere
hypothesis. In high-
resolution body-wave studies, subshield velocities
drop rapidly at 150 km depth, although velocities
remain relatively high to about 390 km.
0
A
B
100
C
M
S
T
200
D
300
400
500
600
−
0.1
0
0.1
−
0.1
0
0.1
Velocity variation (km
/
s)
Fig. 11.2
Variation of the SV velocity with depth for various
tectonic provinces. A--D, oceanic age provinces ranging from
old, A, to young, D; S, continental shields; M, mountainous
areas; T, trench and island-arc regions. These are regionalized
results.
Ryukyu, Philippine, Fiji, Tonga, Kermadec and
New Zealand arcs.
Maps of surface-wave velocity (Nakanishi and
Anderson, 1983, 1984a,b) provide the most direct
display possible of the lateral heterogeneity of
the mantle. The phase and group velocities can
be obtained with high precision and with rela-
tively few assumptions. In general, the shorter
period waves, which sample only the crust and
shallow mantle, correlate well and as expected
with surface tectonics. The longer-period waves,
which penetrate into the transition region (400--
1000 km), correlate with past subduction zones
but less well with current surface tectonics.
Spherical harmonic inversion
An alternative way to analyze tomographic data
is through a spherical harmonic expansion that
ignores the surface tectonics. This provides a less
biased way to assess the depth extent of tectonic
features but the results are similar. In both cases,
the unsampled regions are essentially filled in by
interpolation. The major tectonic features corre-
late well with the shear velocity above 50 km.
Shields and old oceans are fast. Young oceanic
regions and tectonic regions are slow. The slowest
regions are centered near the midocean ridges,
back-arc basins and the Red Sea. The hotspot
province in the south Pacific is slow at shallow
depths, but the shallow mantle in the north-
central Pacific, including Hawaii, is fast.
Regionalized inversion results
Shields (S) are faster than all other tectonic
provinces except old ocean from 100 to 250 km
depth. Below 220 km the velocities under shields
decrease, relative to average Earth, and below
400 km shields, on average, are among the slow-
est regions. At all depths beneath shields the
velocities averaged over hundreds of km can be
accounted for by reasonable mineralogies and
temperatures without any need to invoke partial
