Geoscience Reference
In-Depth Information
Fig. 11.6
Shear wave velocity in the upper mantle along the
cross-section shown. Note low velocities at shallow depth
under the western Pacific backarc basins, replaced by high
velocities, presumably slabs, at greater depth. The eastern
Pacific is slow at all depths. The Atlantic is fast below 400 km,
possibly due to accumulated slabs. The fast regions above 200
km are cratonic roots.
degree 2 are found in the lower mantle
and at degree 6 in the upper mantle; the latter
is controlled, in part, by the distribution of cra-
tons and trenches. Some have interpreted these
resultsintermsof
degree 2 convection in
the lower mantle
and degree 6 dominated
convection in the upper mantle.
The overall distribution of hotspots correlates
with very long wavelength low seismic veloci-
ties in the upper mantle and in the deep lower
mantle (1700 km CMB) but
hotspot locations
are no better correlated with lower
mantle tomography than are ridge loca-
tions
. Global correlations are very poor in the
mid-mantle, below 1000 km depth, and at shorter
wavelengths and decrease very rapidly into the
transition zone. Some hotspots correlate with
fast velocities in the 400--500 km interval.
There are strong negative correlations
between hotspot positions and the 0--180 Ma
reconstructed slab locations; hotspots apparently
do not originate in mantle that has been cooled
or blocked by slab. Normal mantle, cooled by
subduction will, of course, correlate with the
Izu-Bonin, Mariana, New Hebrides and Philippine
trenches in the transition zone.
Cratons
Quantitative correlations between cratons and
tomography confirm the impressions one gets
from visual interpretations (Polet and Anderson,
1995;
Wen
and
Anderson,
1997;
Becker
and
Boschi, 2002).
Hotspots
The relationship between hotspots and seismic
tomography has been investigated extensively,
with mixed results. Visually, some LVZ in the
mantle appear to correlate with hotspots but
these are not statistically significant.
Excellent
correlations at spherical harmonic
