Radial and lateral structure - New Theory of the Earth - page 90

High-velocities appear beneath cratons -- archons .
Continental low-velocities appear in tectonically
extending regions such as the Red Sea rift and
in backarcs -- tectons. Lithospheric thickening and
asthenospheric thinning with age are evident
beneath the oceans.
Low-velocity zones occur beneath ridges, tec-
tonic regions, Yellowstone, and other places at
depths less than 200 km. Yellowstone is not
a particularly prominent anomaly when placed
in the context of the western North American
upper mantle, and does not extend below 200
km depth. At depths greater than 200 km there
are low-velocity zones beneath India, Iceland and
some ridges and back-arc basins. Significant fea-
tures include a widespread and pronounced low-
velocity zone beneath the western United States,

and high-velocity anomalies associated with sub-
ducting slabs.
The upper mantle scatters seismic energy,
indicating that it is heterogenous on the scale
of seismic waves, 10 km. At depths between

800 and 1000 km there is good correlation of
seismic velocities with inferred regions of past
subduction. Below the Repetti discontinuity, at
about 1000 km depth -- Bullen's lower mantle --
the mantle is relatively homogenous and uncor-
related with surface processes. D” is heterogenous
and may be chemically distinct from D' and C.
Although geodynamicists and geochemists
are concentrating on one- and two-layer mod-
els of the mantle, high-resolution seismic tech-
niques suggest that it is actually multilayered or
laminated.

∼