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this model are of similar magnitude to the velocity jumps in the upper mantle
(at 220, 410 and 670 km). It is clear that the old, cold continental shields have
high-velocity roots or 'keels' that extend down for several hundred kilometres
(Fig. 8.6(a) at 290 km), whereas at these depths the oceanic areas have low veloc-
ities indicative of the asthenosphere. The young, hot mid-ocean-ridge systems
are associated with very low velocities as plate tectonics would predict (Fig.
8.6(a) at 60 km): the Canadian Shield has the largest positive anomaly and the
East Pacific Rise the largest negative anomaly. At 60 km depth the oldest oceanic
lithosphere in the northwestern Pacific shows up clearly as having high veloc-
ities. However, the continents are not all underlain by high-velocity mantle -
although North and South America have high velocities, Asia is characterized by
low velocities. By 700 km depth the mantle beneath the subduction zones has, for
the most part, high velocities and the oceanic regions have low velocities. Contin-
uing down into the lower mantle, there is a change in the wavelength of the hetero-
geneities: wavelengths are shorter in the outer part of the lower mantle than in the
upper mantle. The old, cold subducting Tethys and Farallon slabs show up as high-
velocity zones in the outer parts of the lower mantle (Fig. 8.6(a) at 925-1525 km;
see also Fig. 9.60(a)). The basal 800 km or so of the lower mantle is character-
ized by a merging of these shorter-wavelength anomalies into extensive lateral
anomalies. There appear to be two slow regions at the base of the lower mantle,
one beneath Africa and the other beneath the Pacific, and two fast linear regions,
one beneath India and the other beneath the Americas, which almost appear to be
encircling the Pacific. It is tempting to interpret these slow and fast anomalies as
being hot upwelling zones (plumes) and cold descending regions (a 'graveyard'
for subducting plates). Figure 8.6(c) (Plate 11) shows a comparison between two
lower-mantle body-wave models - one using P-wave travel times and the other
using S-wave travel times. There is good agreement between them. Figure 8.7
shows the effects of a change in temperature on seismic velocities. Some veloc-
ity anomalies suggest that variations in temperature of up to
250
◦
Cmaybe
present in the mantle. The standard colour scheme for tomographic images has
low velocities red and high velocities blue, which is consistent with differences in
velocity being caused by variations in temperature. Figure 8.8 (Plate 13) shows an
attenuation (
Q
) model for the upper mantle in terms of variation in the logarithm
of 1
±
Q
. Areas with higher than normal
Q
(low attenuation) show up as blue on
these maps while areas with lower than normal
Q
(high attenuation) show up as
/
←−
mantle and then increase to ±2% just above the CMB. Colour version Plate 9.
(From Masters
et al
. The relative behaviour of shear velocity, bulk sound speed
and compressional velocity in the mantle: implications for chemical and thermal
structure. Geophysical Monograph 117, 2000. Copyright 2000 American Geophysical
Union. Reprinted by permission of American Geophysical Union.)
