Geology Reference
In-Depth Information
Fig. 1.10
Logarithm of viscosity, ǜ, across the upper mantle in the model of Walzer et al. (
2004
)
discontinuity
, a surface of sharp increase of
seismic velocities and density associated with
the chemical transition of the olivine to the
“-spinel structure. The upper part of this
layer, down to
220 km depth, forms a
low-
velocity zone
(LVZ), where the seismic velocities
are slightly smaller with respect to those of
the overlying lithosphere and the underlying
lower asthenosphere (Gutenberg
1959
). This
phenomenon is probably related to the presence
of melts, either microscopically as thin inter-
granular films or macroscopically as narrow dikes
or sills (Anderson
1989
). The LVZ is very thin
beneath the cratons, whereas it approaches the
410-km discontinuity under the East Pacific Rise.
There are two key features that characterize
the asthenospheric layer. The first one is
represented by its
local
inhomogeneity, which is
clearly determined by the continuous injection of
density, chemical, and thermal anomalies through
the subduction process. Such inhomogeneity
induces both vertical and horizontal variations
in the upper mantle geotherms, so that the range
of temperatures could be as high as
400
ı
C
(Anderson
2000
). The second key feature is
the capability of this layer to flow and deform
promptly in response to external forces. In
Chap.
13
we shall show that these flows fall
in three basic categories: (a) Laminar flows,
associated with the drag exerted by an overlying
tectonic plate that is moving towards a subduction
zone; (b) Pressure-driven flows, which are
generated by lateral variations of pressure; and
(c) Temperature-driven Rayleigh-Bénard thermal
convection, which arises spontaneously as a
consequence of the vertical variability in the
temperature distribution and the basal heating of
this layer. The local presence of melts further
increases the complexity of the asthenosphere
and consequently the difficulties to find an
adequate geodynamic description of this layer.
Figure
1.10
illustrates a possible trend of the
viscosity in the upper mantle (Walzer et al.
2004
). It is evident in such a model that the
asthenosphere is characterized by a viscosity
profile with values that decrease with depth
by more than three orders of magnitude, from
ǜ
10
24
Pa s at the LAB to ǜ
Š
3.6
10
20
Pa s.
Only close to the base of this layer, at
367 km
depth, the viscosity starts increasing, reaching
the value ǜ
10
22
Pa s at its lower boundary.
However, most recent estimations of upper
mantle viscosity point to even lower values. In
mantle viscosity is between 0.5 and 1.0
10
21
Pa
s, while the asthenosphere viscosity could be
between 0.5 and 1
10
20
Pa s.
Some lines of evidence suggest that the lower
part of the asthenosphere, approximately below
300 km depth, may be the source region of
