Geoscience Reference
In-Depth Information
Figure 8.7.
The change in seismic velocity that results from a change in mantle
temperature. Left-hand panel: mantle adiabats for 250
◦
C either side of a mantle
adiabat with 1300
◦
C potential temperature. Right-hand panel, percentage changes
in P- and S-wave velocities that result from the changes in temperature shown on
the left. The grey lines are for anharmonic effects only; the black lines also include
anelasticity. The S-wave velocity is more sensitive to temperature than is the P-wave
velocity. (Saskia Goes, personal communication 2004.)
high-resolution studies can now be performed. The models from these studies are
more detailed than that shown in Fig. 8.6(a); large numbers of constant-velocity
blocks and the shorter-wavelength signals allow the detail of major anomalies
such as subducting plates to be investigated. Overall the results from higher-
resolution studies of the upper mantle are in broad agreement with those from the
longer-wavelength studies: young oceanic regions and other tectonically active
regions have low velocities down to 250 km, whereas the continental shields have
high velocities. Some, but not all, hotspots seem to be underlain by slow regions
in the deep mantle. However, there are no plume-shaped anomalies extending
directly from the CMB to the upper mantle beneath the hotspots: the resolution
of data is not sufficient to detect a plume less than 100 km in diameter. It is
clear that some high-velocity zones cross the boundary between upper and lower
mantle (Fig. 9.60). Figure 8.6(d) (Plate 12) shows a comparison between two
models at 1325 km depth, namely a high-resolution P-wave model and a longer-
wavelength S-wave model. The P-wave velocity model has the mantle divided
into some 300 000 constant velocity blocks (dimension 2
◦
×
2
◦
×
100-200 km
in the lower mantle and as little as 0.6
◦
×
0.6
◦
×
35 km in the upper mantle) and
used P, pP and pwP data from 82 000 earthquakes. Both images show a fast
