Geoscience Reference
In-Depth Information
Fig. 11.3
Map of SV velocity at
∼
250 km depth from
sixth-order spherical harmonic representation of Nataf
et al
.
(1984). Note the slow regions associated with the midocean
ridges. The fastest regions are in the far south Atlantic, some
subduction areas and northwest Africa. Other
Surface
wave tomography
maps can be accessed on the web.
The Red Sea--Afar anomaly extends to at least
400 km, but there is no evidence for a thermal
thinning of the transition region. The very slow
velocities associated with western North Amer-
ica die out by 300 km. Other data show that the
Yellowstone anomaly extends to only 200 km, and
below that depth Yellowstone is no more anoma-
lous than the rest of western North America. Most
hotspots, in fact, have low-velocity anomalies that
are cut-off at depth (Anderson, 2005) [
scoring
hotspots
].
By 400 or 450 km depth the pattern seen at
shallow depths changes dramatically. The man-
tle beneath most ridges is fast. The Polyne-
sian Anomaly, although shifted, is still present.
Global seismic structure maps
show that
the
South Pacific Superswell
is generally
underlain by low velocities but do not correlate
well with it nor are the velocities lower than
elsewhere. Eastern North America and/or the
western North Atlantic are slow. Most of South
America, the South Atlantic and Africa are fast.
The north-central Pacific is slow. Most hotspots
are above faster-than-average parts of the mantle
at this depth. The fast regions under the Atlantic,
The central Pacific and the northeastern
Indian Ocean are fast. The Arctic Ocean between
Canada and Siberia is slow. At 250 km (see
Figure 11.3), the shields are less evident than at
shallower depths and than in the regionalized
model. On the other hand, the areas contain-
ing ridges are pronounced low-velocity regions.
The central Pacific and the Red Sea are slow.
The prominent low-velocity region in the cen-
tral Pacific roughly bounded by Hawaii, Tahiti,
Samoa and the Caroline Islands is the Polyne-
sian Anomaly. This feature may be related to
extension and a possible breaking up of the
Pacific plate. The highest velocity anomalies are
in the far south Atlantic, north-west Africa to
southern Europe and the eastern Indian Ocean to
southeast Asia and are not confined to the older
continental areas.
