Can lower mantle slab-like seismic anomalies be explained by thermal coupling
between the upper and lower mantles?
Department of Geophysics
Utrecht University, Utrecht, The Netherlands
Department of Geophysics
Charles University, Prague, Czech Republic
Arie P. van den Berg and Nicolaas J. Vlaar
Department of Geophysics
Utrecht University, Utrecht, The Netherlands
Below subduction zones, high resolution seismic tomographic models
resolve fast anomalies
that often extend into the deep lower mantle.
These anomalies are generally interpreted as slabs
penetrating through the 660-km seismic discontinuity,
evidence in support of whole-mantle convection.
thermal coupling between two flow systems
separated by an impermeable interface might provide
an alternative explanation of the tomographic
We have tested this hypothesis within the context of
model of mantle convection in which
an impermeable boundary is imposed at a depth of 660 km.
When an increase in viscosity alone is imposed across the
impermeable interface, our results demonstrate the dominant
role of mechanical coupling between shells, producing lower
mantle upwellings (downwellings) below upper mantle
However, we find that the effect of mechanical coupling
can be significantly
weakened if a narrow low viscosity zone exists beneath the 660-km
In such a case,
both thermally induced 'slabs' in the
and thermally activated plumes that rise from
the upper/lower mantle boundary
are observed even though mass transfer between the shells
does not exist.
Despite recent developments in seismic tomography and convection
modelling, the impact of the boundary between the upper and lower
mantles on the convective transfer of heat is not yet clear.
While it has been argued that high resolution seismic
that resolve seismically fast, slab-like structures
extending deep into the
support the concept of the whole mantle
[Grand et al., 1997;
van der Hilst et al., 1997;
Bijwaard et al., 1998],
there exists other evidence that suggests an impermeable,
or partially impermeable boundary between the upper
and lower mantles [Hofmann, 1997].
Seismic anisotropy observed near a depth of 660 km
suggests horizontal alignment of crystals and, therefore,
some degree of flow layering
[Karato, 1998; Montagner, 1998; Cadek and van den
interpretations of the geoid show that the model with
an impermeable interface at 660 km or deeper
can satisfy the data equally well
as the whole-mantle model
[Wen and Anderson, 1997; Cadek et al., 1998]
and some resistance to up- and downgoing flows
the amplitudes of surface dynamic topography
[Thoraval et al., 1995].
Finally, flow-blocking of the circulation at
a depth of 670 km
has been shown to improve the prediction of the observed surface
heat flux [Pari and Peltier, 1998].
The strongest indication that slabs extend into
the lower mantle
has been provided by seismic tomography.
However, the images provided by this method do not necessarily
yield direct dynamical information about the radial style
of the circulation.
If there is an impermeable interface
at 660 km, cold subducted lithosphere that has accumulated above
the boundary at 660 km
can cool the underlying material and initiate
a downwelling instability into the lower mantle.
In such a case, it would be difficult on the basis of
seismic tomography alone
to discriminate between 'thermal slabs'
(with no material exchange between the upper and lower mantles)
and slabs penetrating into the lower mantle.
Thermal coupling between
partially or fully separated upper and lower mantle
flow systems thus could be an alternative explanation
of the tomographic results.
Slab-like downwelling structures
have previously been simulated
in convection models that include a phase transition boundary at 660 km
and a high lower mantle viscosity but essentially imply
whole-mantle flow [e.g. Bunge et al., 1998].
In the present paper, we will test whether
continuous slab-like structures across the boundary at 660 km can develop
the exchange of mass across the discontinuity
Simulating convective flow
in a model with
an impermeable boundary between the upper and lower mantles,
we show that
thermal coupling can indeed be important
and that 'thermal slabs' can arise
for certain viscosity profile.
This study should not be regarded as advocating purely two
Its only intention is to state that the concept of an impermeable
or partially impermeable boundary between the upper and lower
mantle should not be rejected on the basis of seismic tomographic
Newtonian flow in an axisymmetric shell
An impermeable boundary
(zero radial velocity
and continuous tangential stress)
is prescribed at a depth of 660 km.
The boundary stays at the same depth and is not deformed.
Free-slip stress conditions are imposed
at the upper and lower boundaries of the shell.
The Boussinesq approximation with constant
parameters (except for viscosity) is adopted.
The surface Rayleigh number is 10**7.
Two viscosity profiles are considered: a two layered profile
with a steep increase by a factor of 30 at a depth of 660 km (model 1),
and a profile with a thin low viscosity zone below
the 660 km depth in addition to the lower mantle jump in viscosity
(model 2), see Figure 1.
The calculations have been carried out with a
semi-spectral code developed by
The resolution of the model is 10 km in radius and
about 2 degrees in latitude.
Simulations were initially carried out for
the viscosity model 1 shown in Fig. 1 characterized by a
steep increase in viscosity at a depth of
Figure 2 (left column) shows two snapshots of
the temperature distribution
after a statistically steady state
has been reached.
The flow pattern is rather stable with a small number
of convection cells.
The cells in the upper and lower mantles are mostly
coupled mechanically. This viscous coupling is
much stronger than that due to
the thermal effect of material lying above
or below the boundary, and it
forces the development of
lower mantle downwellings below the upwellings in the upper mantle
and vice versa. But even in this case, a downwelling in the lower
mantle can occasionally develop below an upper
mantle subducted cold slab,
suggesting a continuous slab across 660 km
(see Fig. 2, top left panel,
the equatorial area).
This particular feature may be rather stable and last for
years. However, the typical flow pattern is mainly characterized
by the mechanical coupling (Fig. 2, bottom left panel).
The existence of strong mechanical coupling controlling
the interaction between upper and lower mantle circulations
is in agreement with the results of previous studies of fully layered
Richter and McKenzie  have shown that
in the isoviscous case mechanical coupling at an impermeable
interface dominates thermal coupling.
Antisymmetric structures with upwellings below downwellings
(and vice versa) are also characteristic of a model with
temperature dependent viscosity [Christensen and Yuen, 1984;
Matyska, results to be published].
However, a thermally induced downwelling can occasionally
develop in the lower layer.
Cserepes et al.  have studied the effect of a viscosity
contrast between the lower and upper layer on the type
of coupling at the impermeable boundary.
They have concluded
if the viscosity contrast of the layers
A high viscosity contrast is required
to reduce it significantly and to enhance the effect of
Temperature and stress dependence of viscosity should
also decrease stress coupling across the boundary and, thus,
increase the appearance of thermal coupling.
Here we suggest
another possible mechanism that can weaken shear coupling
between upper and lower mantle shells,
namely, the presence of a low viscosity zone located below the
The existence of such a low viscosity zone located either above
or below the 660-km
discontinuity has recently been suggested by several authors
inferences of viscosity from the geoid
[Forte et al., 1993; Pari and Peltier, 1996;
Panasyuk and Hager, 1998;
Cadek et al., 1998].
Motivated by the above studies, we have tested the effect of such
a low viscosity zone on
mantle circulation within the context of a layered convection
model. We locate
the low viscosity zone in a narrow depth interval below the 660-km
(see model 2 in Fig. 1). We have found that
a viscosity contrast of three orders of magnitude with
respect to the upper mantle value is sufficient to suppress
mechanical coupling between the upper and lower flow
The thermal effect of masses stored above or below the boundary
both thermally induced 'slabs' in the lower mantle
and 'thermal plumes'
in the upper mantle are observed as characteristic features
of the temperature field (Fig. 2, right column).
In contrast to the broad
low viscosity zone
studied by Cserepes and Yuen ,
low viscosity layer decreases the shear stress at the boundary without
allowing the development of vigorous small-scale circulation.
Both the plumes in the lower mantle
and the downwellings in the upper mantle are quite stable.
The latter cool the lower mantle material, giving
rise to downwellings in the lower mantle ('thermal slabs').
These slabs are stable for about 100 million years before
they are swept by lateral flow in the lower mantle
and detached from their upper mantle extensions.
The upwellings behave in a similar way.
The heads of the lower mantle plumes,
flattened below the 660-km boundary, initiate
the development of very narrow
upwellings in the upper mantle which migrate towards
the center of the
Note that the presence of the low viscosity zone below the
impermeable interface makes heat transport through
the boundary more efficient than in model 1 which results
in a smaller
temperature jump over the mid-mantle thermal boundary layer
(Fig. 3). However, even in model 2 the temperature jump
is about 800 K which is rather high.
Including internal heating or increase of
thermal conductivity with depth
1998] would probably bring the average geotherm closer
to reality, without disturbing the effects of thermal
coupling discussed above.
Figure 4 shows details of several slab-like structures developed
in model 2. 'Slabs' apparently penetrating the whole mantle
(panels a and c),
a thick blob reaching mid-lower mantle depth (b)
as well as downwelling structures offset from the slabs in the upper
can be observed. We suggest that these temperature patterns
are remindful of features imaged
by seismic tomography [van der Hilst, 1997;
Grand et al., 1997; Bijwaard et al.,
The results of our study suggest that
the concept of a fully impermeable boundary between the upper
and lower mantle is not necessarily in contradiction with seismic
tomographic images of mantle heterogeneity.
The slab-like structures in the lower mantle
are observed under several conditions,
but they directly underlie the upper mantle
slabs only if the viscous coupling is decreased.
Then, thermally induced continuous
slab-like structures become characteristic features of the flow.
The authors thank C. Matyska for fruitful discussions
and G. Pari, S. Karato and anonymous referee
for comments which helped to improve the manuscript.
The work was supported by the Charles University
Grant (GAUK) No. 4/97.
Figure 1: Viscosity models used in our study:
model 1 (solid line) with the steep increase at a depth of 660 km and model
2 (dashed line) with a low viscosity zone beneath the upper/lower mantle
boundary. The dotted line marks position of the impermeable boundary
at a depth of 660 km.
Figure 2: Left column: Two snapshots of
temperature field obtained for model 1. Time difference between the upper
and lower panel is 750 Myr. Right column: The same but for model 2. Time
difference between the upper and lower panel is 150 Myr.
Figure 3: Average temperature profiles obtained
for model 1 (solid line) and 2 (dashed line). The dotted line marks position
of the impermeable boundary at 660 km.
Figure 4: Details of several slab-like structures
developed in model 2.
Bijwaard, H., W. Spakman, and E. Engdahl, Closing the gap between
regional and global travel time tomography, J. Geophys. Res., 103,
Bunge, H., M. Richards, C. Lithgow-Bertelloni, J. Baumgardner,
S. Grand, and B. Romanowicz, Science, 280, 91-95, 1998.
Christensen, U., and D.A. Yuen, The interaction of a subducting
slab with a chemical or phase boundary, J. Geophys. Res., 89,
Cserepes L., M. Rabinowicz, and C. Rosemberg-Borot, Three-dimensional
infinite Prandtl number convection in one and two layers with
implications for the Earth's gravity field, J. Geophys.
Res., 93, 12009-12025, 1988.
Cserepes, L., and D.A. Yuen, Dynamical consequences of mid-mantle viscosity
stratification on mantle flows with an endothermic phase transition,
Geophys. Res. Lett., 24, 181-184, 1997.
Cadek, O., and A.P. van den Berg,
Layered mantle convection with a composite Newtonian and
non-Newtonian rheology: consequences for seismic anisotropy,
EOS, abstract supplement AGU 78(46), F660, 1997.
Cadek, O., D.A. Yuen, and H. Cizkova,
Mantle viscosity inferred from geoid and seismic tomography by genetic
algorithms: Results for layered mantle flow,
Phys. Chem. Earth, 23 (9-10), 865-872, 1998.
Cizkova, H., Modeling the dynamical processes
in the Earth mantle, PhD thesis, Charles University, Prague 1996.
Forte, A.M., A.M. Dziewonski, and R.L. Woodward, Aspherical structure
of the mantle, tectonic plate motions, nonhydrostatic geoid, and topography
of the core-mantle boundary, in Dynamics of the Earth's Deep Interior
and Earth Rotation, J.-L. Le Mouel, D.E. Smylie and T. Herring (eds.),
pp 135-166, Geophys. Monograph No. 72, AGU, Washington, D.C., 1993.
Grand, S.P., R.D. van der Hilst, and S. Widiyantoro, Global
seismic tomography: A snapshot of convection in the Earth,
GSA Today, 7(4),
Hofmann, A.W., Mantle geochemistry: the message from oceanic
volcanism, Nature, 385, 219-229, 1997.
Hofmeister, A., Mantle values of thermal conductivity and the
geotherm from phonon lifetime, Science, 283, 1699-1706, 1999.
Karato, S., Seismic anisotropy in the deep mantle, boundary layers
and the geometry of mantle convection, Pure Appl. Geophys.,
151, 565-587, 1998.
Where can seismic anisotropy be detected in the Earth's mantle?
In boundary layers..., Pure. appl. geophys., 151, 223-256,
Panasyuk, S.V., and B.H. Hager, A model of transformational superplasticity
in the upper mantle,
Geophys. J. Int., 133, 741-755, 1998.
Pari, G., and W.R. Peltier,
The free-air gravity constraint on
subcontinental mantle dynamics, J. Geophys. Res., 101,
Pari, G., and W.R. Peltier, Global surface heat flux anomalies from
seismic tomography-based models of mantle flow: Implications for
mantle convection, J. Geophys. Res., 103, 23743-23780, 1998.
Richter, F., and D. McKenzie, On some consequences and possible
causes of layered mantle convection, J. Geophys. Res., 86,
Thoraval, C., Ph. Machetel, and A. Cazenave, Locally layered convection
inferred from dynamic models of the Earth's mantle, Nature, 375,
van der Hilst, R.D., S. Widiyantoro, and E.R. Engdahl,
Evidence for deep mantle circulation from global tomography,
Nature, 386, 578-584, 1997.
Wen, L., and D.L. Anderson, Layered mantle convection: A model for
geoid and topography, Earth Planet. Sci. Lett., 146, 367-377,