Phys. Earth Planet. Inter., 118, 241-257, 2000.
Slabs in the lower mantle -- results of dynamic modelling compared with
tomographic images and the geoid
Bernhard Steinberger
Abstract
The recent increase in resolution of tomographic images of the Earth's
interior has enabled us to ``see'' slabs in the lower mantle. On the
other hand, the distribution of slabs can be inferred from subduction
history, using a dynamical model of mantle flow. Comparison of
tomographic images with model results can help to distinguish between
alternative models of subduction history, tomographic models and mantle
flow models and thus improve our understanding of the Earth.
Here a simple dynamic model of subduction is used and some results are
presented: Amounts and locations of subduction are inferred from
published models of plate motion and plate boundaries for the past 120
Ma; the latter have been interpolated on 2 Ma time intervals. Mantle
flow driven by the density anomalies corresponding to subducted slabs
is calculated with the method of Hager and O'Connell for a viscosity
structure that features an increase in viscosity from $4\cdot
10^{20}$ Pas below the lithosphere to $4\cdot 10^{22}$ Pas in the
lower part of the lower mantle. Slabs are advected in the flow field.
In the model slabs sink on average about 1700 km through the mantle
during 100 Ma; however the calculated average horizontal motion of slabs
throughout most of the mantle is only about 600-700 km; for slabs that have
sunk to the lowermost mantle it is about 1000 km. This is in accordance
with the previously
observed good correlation between subduction locations and lowermost
mantle heterogeneities. Slabs sink fastest in regions with abundant
subduction and little trench migration (e.g. below East Asia).
In the lowermost mantle, predicted slab locations tend to lie in areas
with high seismic velocities, however there are large areas of fast
seismic velocities where no slabs are predicted, and which therefore
presumably correspond to earlier subduction. In some regions
(e.g. below Japan), our model approximately reproduces the observed
shape of the slab. In other regions, where the agreement is poor, the
results can help to further constrain models of subduction history and
associated mantle flow.
Additionally, the predicted geoid is computed and compared with the actual
geoid. Predicted and observed geoid have similar magnitude and
a number of features in common, however an optimization of the fit
is not attempted here, since, in order to do this, also the hot upwellings
from the lower boundary layer (presumably in the form of plumes) would
have to be included.