Geoscience Reference
In-Depth Information
GREENLAND
Permian
to
Triassic
Subduction
SIBERIA
KAZAK
JAPAN
Jurassic
to
Eocene
Subduction
CHINA
INDIA
ARABIA
INDO-
CHINA
Permian to
Triassic
Fig. 6.5
The approximate locations of subducted slabs or
subduction zones during the last two supercontinent cycles.
Note the similarity with the long wavelength geoid. This
pattern also matches that of seismic tomography at a depth of
about 800--1000 km depth, suggesting that slabs bottom out
at
∼
1000 km depth.
is important for these problems. By contrast the
upper mantle has only a weak correlation with
the
l
2 and 3 geoid. The effects of pressure on
viscosity and thermal expansion are such that
we expect the lower mantle to convect very slug-
gishly. The large features in the lower mantle are
probably ancient, and not caused by recent plate
tectonic cycles. The implication is that the geoid
and the rotation axis are relatively stable. The
plates and the upper 1000 km of the mantle are
the active layers.
=
advancing continents. These are labeled 'fast'
because these are seismically fast regions of the
transition region, where cold lithosphere may
have cooled off the mantle. The arrows repre-
sent the motions of the continents over the
past 110 Myr. Most of the hatched regions are
also geoid lows. Figure 6.5 shows the inferred
locations of subduction zones over the past two
supercontinent cycles. Slabs and delaminated
lower crust have entered the mantle in these
regions, both cooling it and fertilizing it.
Polar wander
Because the Earth is a dynamic body, it is impos-
sible to define a permanent internal reference
frame. There are three reference frames in com-
mon use: the rotation axis, the geomagnetic ref-
erence frame and the hotspot reference frame.
The rotational frame is controlled by the size of
themassanomaliesandtheirdistancefromthe
axis of rotation. Upper-mantle effects are impor-
tant because lateral heterogeneity is greater than
lower-mantle or core heterogeneity and because
they are far from the center of the Earth. The
lower mantle is important because of its large
volume, but a given mass anomaly has a greater
effect in the upper mantle. The location of the
magnetic pole is controlled by convection in the
core, which in turn is influenced by the rotation
Involvement of the lower mantle
Tomographic techniques can be applied to the
problem of lateral heterogeneity of the lower
mantle [
tomography geoid lower mantle,
Hager O'Connell
]. Long-wavelength velocity
anomalies in the lower mantle correlate well
with the
l
2, 3 geoid. Phenomena such as
tides, Chandler wobble, polar wander and the
orientation of the Earth's spin axis depend on the
l
∼
=
2 component of the geoid; the lower mantle
