Geoscience Reference
In-Depth Information
thestressesinvolvedinplatetectonicsmayhave
different orientations and magnitudes than the
stresses involved in surface loading experiments.
We do not know the thickness of the plate or how
well it is coupled to the underlying mantle. We
do not even know the sign of the
basal drag
force
.
In a convecting or cooling mantle there is
a
surface thermal boundary layer
(TBL)
through which heat must pass by conduction.
The thickness of the thermal boundary layer is
controlled by such parameters as conductivity
and heat flow and is not related in a simple way
tothethicknessoftheelasticlayerortheplate.
Since temperature increases rapidly with depth
in the conduction layer, and viscosity decreases
rapidly with temperature, the lower part of the
boundary layer probably lies below the elastic
lithosphere; that is, only the upper part of the
thermal boundary layer can support large and
long-lived elastic stresses. Unfortunately, the con-
duction layer too is often referred to as the litho-
sphere. In a chemically layered Earth there can
be TBLs between internal layers. These TBLs act as
thermal bottle-necks and slow down the cooling
of an otherwise convective mantle.
Continental cratons
have high seismic velocity
roots, or keels, extending to 200--300 km depth.
These are referred to as
archons
. They persist
because of low density and high viscosity, and
because they are protected from high stress. They
are more than just part of the strong outer shell.
These keels can last for billions of years.
Most models of the Earth's mantle have an
upper-mantle low-velocity zone
,LVZ,overlainbya
layer of higher velocities, referred to as the LID.
The LID is also often referred to as the litho-
sphere. Seismic stresses and periods are much
smaller than stresses and periods of geological
interest. If seismic waves measure the
relaxed
modulus
in the LVZ and the high-frequency or
unrelaxed modulus in the LID, then, in a chem-
ically homogenous mantle, the LID should be
much thicker than the elastic lithosphere. If the
LID is chemically distinct from the LVZ, then
one might also expect a change in the long-term
rheological behavior at the interface. If the LID
and the elastic lithosphere turn out to have the
same thickness, then this would be an argument
for chemical, water or crystallographic control,
rather than thermal control, of the mechanical
properties of the upper mantle.
In summary, the following 'lithospheres'
appear in the geodynamic literature (
' When I make
a word do a lot of work like that,' said Humpty Dumpty,
' I always pay it extra.'
).
(1) The elastic, flexural or
rheological lithosphere
.
This is the closest to the classical definition
of a rocky, or strong, outer shell. It can be
defined as that part of the crust and upper
mantle that supports elastic stresses of a given
size for a given period of time. The thickness
of this lithosphere depends on stress and load
duration.
(2) The
plate
. This is that part of the crust and
upper mantle that translates coherently in
the course of plate tectonics. The thickness
of the plate may be controlled by chemi-
cal or buoyancy considerations or by stress,
as well as by temperature, but there is no
known way to measure its thickness. Plates
are ephemeral.
(3) The
chemical
or
compositional lithosphere
. The
density and mechanical properties of the
lithosphere are controlled by chemical com-
position and crystal structure as well as tem-
perature. If chemistry and mineralogy dom-
inate, then the elastic lithosphere and LID
may be identical. If the lithosphere, below
the crust, is mainly depleted peridotite or
harzburgite, it may be buoyant relative to the
underlying mantle. A cratonic root, or
archon
,
is often called the
continental lithosphere
or
sub-
continental lithospheric mantle
(
SCLM
) and has
been proposed as a geochemical reservoir.
(4) The
thermal boundary layer
or
conduction layer
should not be referred to as the litho-
sphere, which is a mechanical concept, but
if the lithospheric thickness is thermally
controlled, the thickness of the lithosphere
should be proportional to the thickness of the
thermal boundary layer. If TBLs get too thick,
they can sink, or delaminate.
(5)
The seismic LID
isaregionofhighseismic
velocity that overlies the low-velocity zone.
At high temperatures the seismic moduli
measured by seismic waves may be relaxed,
