Geoscience Reference
In-Depth Information
This seismic lithosphere is not the same as the
plateorthethermalboundarylayer.Itisdefined
solely on the basis of seismic velocities. In some
places, e.g. Basin and Range, it is absent, perhaps
due to delamination. The thickness of the LID
corresponds roughly to the rheological or elastic
lithosphere, e.g. the apparently strong or coher-
ent upper layer overlying the
asthenosphere
. The
thermal boundary layer
is typically twice as thick,
at least in oceanic regions. The high-velocity roots
under cratons are chemically buoyant and are
probably olivine-rich and FeO-poor. They extend
to 200--300 km depth. They are long-lived because
they are buoyant, cold, dry, high-viscosity and
protected
olivine-rich aggregates. Eclogites are highly vari-
able but can have
V
p
and
V
s
as high as 8.8 and
4.9 km/s in certain directions and as high as
8.61 and 4.86 km/s as average values. The above
suggests that corrected velocities of at least 8.6
and 4.8 km/s, for
V
p
and
V
s
, respectively, occur
in the lower lithosphere; this requires substan-
tial amounts of garnet, about 26%. The density
of such an assemblage is about 3.4 g/cm
3
. The
lower lithosphere may therefore be gravitation-
ally unstable with respect to the underlying man-
tle, particularly when it is cold. The upper mantle
under shield regions, on the other hand, is con-
sistent with a very olivine-rich peridotite which is
buoyant and therefore stable relative to 'normal'
mantle.
Anisotropy of the upper mantle is a poten-
tially useful petrological constraint, although it
can also be caused by organized heterogeneity,
such as laminations or parallel dikes and sills
or aligned partial melt zones, and stress fields.
Recycled material may also arrange itself so as
to give a fabric to the mantle. The uppermost
mantle under oceans exhibits an anisotropy of
about 7%. The fast direction is in the direction of
spreading, and the magnitude of the anisotropy
and the high velocities of P-arrivals suggest that
oriented olivine crystals control the elastic prop-
erties. Pyroxene exhibits a similar anisotropy,
whereas garnet is more isotropic. The preferred
orientation is presumably due to the emplace-
ment or freezing mechanism, the temperature
gradient or to nonhydrostatic stresses. A peri-
dotite layer at the top of the oceanic mantle is
consistent with the observations.
The anisotropy of the upper mantle, aver-
aged over long distances, is much less than the
values given above. Shear-wave anisotropies of
the upper mantle average 2--4%. Shear velocities
in the LID vary from 4.26--4.46 km/s, increas-
ing with age; the higher values correspond to a
lithosphere 10--50 My old. This can be compared
with shear-wave velocities of 4.3--4.9 km/s and
anisotropies of 1--5% found in relatively unaltered
eclogites, at laboratory frequencies. The com-
pressional velocity range in unaltered samples
is 7.6--8.7 km/s, reflecting the large amounts of
garnet.
Garnet and clinopyroxene may be impor-
tant components of the lithosphere and upper
from
plate-boundary
interactions
by
the surrounding
mobile belts
.
Uppermost mantle compressional wave veloc-
ities, Pn, are typically 8.0--8.2 km/s, and the
spread is about 7.9--8.6 km/s. Some long refrac-
tion profiles give evidence for a deeper layer
in the lithosphere having a velocity of 8.6 km/s.
The
seismic lithosphere
,orLID,appearstocon-
tain at least two layers. Long refraction profiles
on continents have been interpreted in terms
of a laminated model of the upper 100 km
with high-velocity layers, 8.6--8.7 km/s or higher,
embedded in 'normal' material. Corrected to
normal conditions these velocities would be
about 8.9--9.0 km/s. The P-wave gradients are
often much steeper than can be explained by self-
compression. These high velocities require ori-
ented olivine or large amounts of garnet. The
detection of 7--8% azimuthal anisotropy for both
continents and oceans suggests that the shallow
mantle at least contains oriented olivine or ori-
ented cracks, dikes or lens. In some places, the
lithosphere may have formed by the stacking of
subducted slabs, another mechanism for creating
anisotropy.
Typical values of
V
p
and
V
s
at 40 km
depth, when corrected to standard conditions,
are 8.72 km/s and 4.99 km/s, respectively. Short-
period surface-wave data implies STP -- stan-
dard temperature and pressure -- velocities of
4.48--4.55 km/s and 4.51--4.64 km/s for 5-My-old
and 25-My-old oceanic lithosphere. A value for
V
p
of 8.6 km/s is sometimes observed near 40 km
depth in the oceans. This corresponds to about
8.87 km/s at standard conditions. These values
can be compared with 8.48 and 4.93 km/s for
