Geoscience Reference
In-Depth Information
stack of isotropic layers. The same holds true for
an isotropic solid permeated by oriented cracks
or aligned inclusions. This serves to illustrate the
trade-off between heterogeneity and anisotropy.
Not all anisotropic structures, however, can be
modeled by laminated solids.
The crystals of the mantle are anisotropic,
and rocks from the mantle show that these crys-
tals exhibit a high degree of alignment. There is
also evidence that crystal alignment is uniform
over large areas of the upper mantle. At man-
tle temperatures, crystals tend to be easily recrys-
tallized and aligned by the prevailing stress and
flow fields. But there may also be large-scale fab-
ric in the upper mantle, caused by orientation of
subducted slabs or dikes, for example.
The effects of anisotropy are often subtle and,
if unrecognized, are usually modeled as inhomo-
geneities, for example, as layering or gradients.
The most obvious manifestations of anisotropy
are:
isotropic inversions of Love and Rayleigh waves
(pseudo-isotropic inversions). This is not a
valid procedure. There is a trade-off between
anisotropy and structure. In particular, the very
low upper-mantle shear velocities, 4.0--4.2 km/s,
found by many isotropic and pseudo-isotropic
inversions, are not a characteristic of models
resulting from full anisotropic inversion. The P-
wave anisotropy makes a significant contribution
to Rayleigh wave dispersion. This must also be
allowed for in tomographic surface wave inver-
sions, but seldom is.
Since intrinsic anisotropy requires both
anisotropic crystals and preferred orientation,
the anisotropy of the mantle contains informa-
tion about the mineralogy and stress gradients.
For example, olivine, the most abundant upper-
mantle mineral, is extremely anisotropic for both
P-wave and S-wave propagation. It is readily ori-
ented by recrystallization in the ambient stress
field. Olivine-rich outcrops show a consistent pre-
ferred orientation over large areas. In general, the
seismically fast axes of olivine are in the plane
oftheflow,withthe
a
-axis, the fastest direc-
tion, pointing in the direction of flow. The
b
-
axis, the minimum velocity direction, is gener-
ally normal to the flow plane, or vertical. Pyrox-
enes are also very anisotropic. The magnitude of
the anisotropy in the mantle is comparable to
that found in ultramafic rocks (Figure 20.1). Soft
layers or oriented fluid-filled cracks also give an
apparent anisotropy. Much seismic data that are
used in upper-mantle modeling are averages over
several tectonic provinces or over many azimuths.
Azimuthal anisotropy
may therefore be aver-
aged out, but differences between vertical and
horizontal velocities are not.
(1) [
shear-wave splitting
] or birefringence
-- the two polarizations of S-waves arrive at
different times;
(2) [
azimuthal anisotropy
] -- the arrival
times, or apparent velocities of seismic waves
at a given distance from an event, depend on
azimuth; and
(3) an apparent discrepancy between Love
waves and Rayleigh waves [
Love Rayleigh
discrepancy
].
Even these are not completely unambiguous indi-
cators of anisotropy. Effects such as P to S conver-
sion, dipping interfaces, attenuation, and density
variations must be properly taken into account.
There is now growing acceptance that much of
the upper mantle may be anisotropic to the prop-
agation of seismic waves.
It has been known for some time that the
discrepancy between mantle Ray- leigh
and Love waves
could be explained if the
vertical P and S velocities in the upper man-
tle were 7--8% less than the horizontal veloci-
ties. The Love--Rayleigh discrepancy has survived
to the present, and average Earth models have
SV in the upper mantle less than SH by about
3%. Some early models were based on separate
Origin of mantle anisotropy
Nicholas and Christensen (1987) elucidated the
reason for strong preferred crystal orientation
in deformed rocks. First, they noted that in
homogenous deformation of a specimen com-
posed of minerals with a dominant slip system,
the preferred orientations of slip planes and
slip directions coincide respectively with the ori-
entations of the flow plane and the flow line.
