Geology Reference
In-Depth Information
Fig. 1.9
Depthtothe
LAB (After Plomerová
et al. (
2002
). Depths are in
km)
fact that the rate of shear deformation increases
abruptly passing from the mantle lithosphere to
the underlying asthenosphere implies a vertical
variation of seismic anisotropy that can detected
and used to constrain the LAB topography.
An interesting feature of the lithosphere
beneath the continental cratons is the presence
of thick “keels” of highly refractory peridotite
(Fig.
1.9
), which are characterized by high
seismic velocities. Furthermore, while the base
of the lithosphere is marked by a sharp reduction
of seismic velocities beneath the oceanic basins,
possibly associated with retained melts, such a
reduction seems to be absent beneath the old
Precambrian shields. In general, the seismic
velocities are higher in the sub-continental
mantle lithosphere and correlate with the age
of tectono-magmatic activity at regional scale.
Another important source of data about the
sub-continental mantle lithosphere, which has
undoubtedly helped to clarify the differences
with respect to the oceanic counterpart, is
represented by
xenoliths
. These are fragments
of mantle lithosphere carried to the surface by
explosive volcanic rocks such as kimberlites,
whose systematic study has produced a large
knowledge base about the physical properties, the
chemistry, and the geochronologic history of the
sub-continental mantle lithosphere. For example,
xenoliths have revealed that this lithosphere
is anomalously enriched in some highly
incompatible trace elements, such as potassium,
uranium, and thorium. Such enrichment explains
the relatively high production of radiogenic heat
of the continental regions with respect to the
oceans. Perhaps the most important conclusion
of these studies has been that the cratonic
mantle lithosphere is less dense than the oceanic
TBL, as a consequence of high degree partial
melting and melt removal during the Archean.
Such primary melt depletion gave to the sub-
continental mantle peridotites a compositional
buoyancy that must be considered as a key
factor preserving both the continental crust
and the underlying mantle from sinking into
the asthenosphere (e.g., Carlson et al.
2004
).
Conversely, the dry, chemically depleted, oceanic
lithosphere becomes gravitationally unstable
in so far as its thickness and average density
increase with the cooling. Therefore, the fate of
the old oceanic lithosphere is to bend downwards
and sink into the asthenosphere, possibly after
an episode of horizontal compression, forming
a
slab
. In this event, phase transitions, such as
dehydration or MORB metamorphism to eclogite
facies, will determine an increase of density,
which facilitates passive sinking into the mantle.
1.5
Asthenosphere
The
asthenosphere
is a mechanically weak solid
state fluid layer just beneath the lithosphere
(Fig.
1.1
). Its base is defined by the
410-km
