Geoscience Reference
In-Depth Information
in which case they can be of the order of
10% less than the high-frequency or
unrelaxed
moduli. High-temperature dislocation relax-
ation and partial melting are two mecha-
nisms that decrease seismic velocities. The
boundary between the LID and the LVZ would
be diffuse and frequency dependent if ther-
mal relaxation is the mechanism. A sharp
interface would be evidence for a chemical or
mineralogical boundary.
0
20
10
6
years
3
30
×
10
6
3
×
40
10
4
×
60
30
80
100
The best evidence for cooling of the oceanic
plate, or thickening of the thermal boundary
layer, comes from the deepening of bathymetry
as a function of time. The simple square-root-
of-time relationship for bathymetry fails after
70--80 million years, indicating that the thermal
boundary layer has reached an equilibrium thick-
ness or that a thermal event prior to
0
50
100
150
Age of oceanic crust (My)
Fig. 4.1
Relationship between effective thickness of
elastic
lithosphere
(also called the
rheologica
lor
flexural lithosphere
),
age of oceanic crust and duration of loading for 1 kbar stress.
For a given load the effective elastic thickness decreases with
time. The elastic thickness generally follows the 600--700
degree C isotherm, and is roughly half the thickness of the
thermal boundary layer (TBL). The latter should not be
confused with the lithosphere and it should not be called
the
lithosphere
or even the
thermal lithosphere;
these
are unnecessary and misleading terms. The
plate
, the
tectonosphere
and the
tectosphere
are different concepts. The
dashed line is the upper bound of seismic determinations of
LID thicknesses from surface waves and the assumption of
isotropy. The LID is actually anisotropic and when this is
allowed for the thickness and shear velocity are less than in
isotropic inversions. Because of relaxation effects the elastic
model of the plate, over geological time scales, may be less
than the seismic moduli.
80 Ma
affected the parts of the oceanic lithosphere that
have been used to calculate the bathymetry--age
relation. Dikes and sills intruded into the plate
may reset the thermal age. There is some evi-
dence that the seismic LID continues to follow
the root-
t
dependency to the oldest ages. This
would mean that density and seismic velocity are
not correlated.
∼
Effective elastic thickness
The
thickness of the lithosphere
is a
more complicated concept than is generally
appreciated. Figure 4.1 gives estimates of the
thickness of the oceanic rheological lithosphere
as a function of age of crust and duration of load,
using oceanic geotherms and a stress of 1 kilo-
bar. The depth to the rheological asthenosphere
is defined as the depth having a characteristic
time equal to the duration of the load. For exam-
ple, a 30 My load imposed on 80-My crust would
yield a thickness of 45 km if the lithosphere did
not subsequently cool, or if the cooling time is
longer than the relaxation time. A 30-My load on
currently 50-My crust would give a theological
lithosphere 34 km thick with the above qualifi-
cations. The thickness of the high seismic veloc-
ity layer overlying the low-velocity zone (the seis-
mological LID) is also shown in Figure 4.1. For
load durations of millions of years, the rheolog-
ical lithosphere is about one-half the thickness
of some of the older estimates of the thickness
of the seismological lithosphere, assuming that
it is isotropic. Note that the rheological thick-
ness decreases only gradually for old loads. On
the other hand, the lithosphere appears very
thick for young loads, and the apparent thickness
decreases rapidly. The elastic or flexural thick-
ness is roughly half the thickness of the thermal
boundary layer (TBL).
Asthenosphere
The physical properties of the mantle depend
on stress, mineralogy, crystal orientation, tem-
perature and pressure. Small amounts of water
and CO
2
can have significant effects. In the outer
100--200
km
the
increasing
temperature
with
