Geoscience Reference
In-Depth Information
0
4
5
6
0
80
h
PV
PH
SV
SH
TNA
ATL
160
SNA
240
200
320
78 9
3.5
4
4.5
5
0.8
1
1.2
h
Velocity (km
/
s)
Fig. 8.5
Velocity-depth profiles for the average Earth, as
determined from surface waves (Regan and Anderson, 1984).
From left to right, the graphs show P-wave velocities (vertical
and horizontal), Swave velocities (vertical and horizontal) and
an anisotropy parameter (1 represents isotropy).
400
600
The low-velocity zone or LVZ
A region of diminished velocity or negative veloc-
ity gradient in the upper mantle was proposed
by Beno Gutenberg in 1959. Earlier, just after
isostasy had been established, it had been con-
cluded that a weak region underlay the rela-
tively strong lithosphere. This has been called the
asthenosphere
. The discovery of a low-velocity zone
strengthened the concept of an asthenosphere.
Most models of the velocity distribution in
the upper mantle include a region of high gra-
dient between 250 and 350 km depth. Lehmann
(1961) interpreted her results for several regions
in terms of a discontinuity at 220 km (some-
times called the
Lehmann discontinuity
),
and many subsequent studies give high-velocity
gradients near this depth. Although the global
presence of a discontinuity, or high-gradient,
region near 220 km has been disputed, there is
now appreciable evidence, from reflected phases,
for its existence. The situation is complicated by
the extreme lateral heterogeneity of the upper
200 km of the mantle. This region is also low Q
(high attenuation) and anisotropic. Some upper
mantle models are shown in Figures 8.5 and 8.6.
Various interpretations have been offered for
the upper mantle low-velocity zone. This is
undoubtedly a region of high thermal gradient,
the boundary layer between the near surface
where heat is transported by conduction and
the deep interior where heat is transported by
convection. If the temperature gradient is high
enough, the effects of pressure can be overcome
800
Velocity (km
/
s)
Fig. 8.6
Shear-wave velocity profiles for various tectonic
provinces; TNA is tectonic North America, SNA is shield
North America, ATL is north Atlantic. It is difficult to resolve
the small variations below 400 km depth (after Grand and
Helmberger, 1984a).
and velocity can decrease with depth. It can
be shown, however, that a high temperature
gradient alone is not an adequate explanation.
Partial melting and dislocation relaxation both
cause a large decrease in velocity. Water and CO
2
decrease the solidus and the seismic velocities.
For partial melting to be effective the melt must
occur, microscopically, as thin grain boundary
films or, macroscopically, as dikes or sills, which
also are very small compared with seismic wave-
lengths. Melting experiments suggest that melt-
ing occurs at grain corners and is more likely to
occur in interconnected tubes. This also seems
to be required by electrical conductivity data.
However, slabs, dikes and sills act macroscopi-
cally as thin films for long-wavelength seismic
waves.
High attenuation is associated with relaxation
processes
such
as
grain
boundary
relaxation,
