Geoscience Reference
In-Depth Information
igneous rocks preferred orientation can be
caused by grain rotation, recrystallization in a
nonhydrostatic stress field or in the presence of
a thermal gradient, crystal setting in magma
chambers, flow orientation and dislocation-
controlled slip. Macroscopic fabrics caused by
banding, cracking, sill and dike injection can also
cause anisotropy. Eclogites and basalts are much
less anisotropic; anisotropy can therefore be used
as a petrological tool.
Plastic flow induces preferred orientations
in rock-forming minerals. The relative roles of
deviatoric stresses and plastic strain have been
long debated. In order to assure continuity of a
deforming crystal with its neighbors, five inde-
pendent degrees of motion are required (the Von
Mises criterion). This can be achieved in a crys-
tal with the activation of five independent slip
systems or with a combination of fewer slip sys-
tems and other modes of deformation. In silicates
only one or two slip systems are activated under
a given set of conditions involving a given tem-
perature, pressure and deviatoric stresses. The
homogenous deformation of a dominant slip sys-
tem and the orientation of slip planes and slip
directions tend to coincide with the flow plane
and the flow direction.
Mantle peridotites typically contain more
than 65% olivine and 20% orthopyroxene. The
high-P wave direction in olivine (Figure 20.2) is
along the
a
-axis [1001], which is also the domi-
nant slip direction at high temperature. The low-
est velocity crystallographic direction is [0101],
the
b
-direction, which is normal to a common
slip plane. Thus, the pattern in olivine aggregates
is related to slip orientations. There is no such
simple relationship with shear waves and, in
fact, the S-wave anisotropy of peridotites is small.
Orthopyroxenes also have large P-wave aniso-
tropies and relatively small S-wave anisotropies.
The high-Vp direction coincides with the [1001]
pole of the unique slip plane and the inter-
mediate Vp crystallographic direction coincides
with the unique [0011] slip line (Figure 20.2).
In natural peridotites the preferred orientation
of olivine is more pronounced than the other
minerals. Olivine is apparently the most ductile
and easily oriented upper-mantle mineral, and
therefore controls the seismic anisotropy of the
01.0
0.8
0.6
0.4
0.2
0
−
.2
.4
−
−
.6
−
.8
−
1.0
0
40
80
120
160
200
240
280
320
360
Azimuth
Fig. 20.1
Azimuthal anisotropy of Pn waves in the
Pacific upper mantle. The
unique anisotropy
of
the
Pacific
upper mantle has also been mapped with
surface waves (after Morris
et al.
, 1969).
Simpleshearinacrystalrotatesallthelines
attached to the crystal except those in the slip
plane. This results in a bulk rotation of crystals
so that the slip planes are aligned, as required
to maintain contact between crystals. The crys-
tal reorientations are not a direct result of the
applied stress but are a geometrical requirement.
Bulk anisotropy due to crystal orientation is
therefore induced by plastic strain and is only
indirectly related to stress. The result, of course,
is also a strong anisotropy of the viscosity of
the rock, and presumably attenuation, as well as
elastic properties. This means that seismic tech-
niques can be used to infer flow in the mantle.
It also means that mantle viscosity inferred from
postglacial rebound is not necessarily the same
as that involved in plate tectonics and mantle
convection.
Peridotites from the upper mantle display
a strong preferred orientation of the domi-
nant minerals, olivine and orthopyroxene. They
exhibit a pronounced acoustic-wave anisotropy
that is consistent with the anisotropy of the
constituent minerals and their orientation. In
