Download Tectonics of the lower mantle

The 2003 Bullerwell Lecture
Tectonics of th
Abstract
Understanding the nature of the Earth’s
core–mantle boundary region has
implications for a wide range of Earth
processes. Here I discuss seismic
properties of this region, using new
imaging techniques developed by myself
and colleagues. Our interpretations are
guided by linked geodynamical modelling,
and cumulatively suggest that many of the
seismic properties – heterogeneity and
anisotropy – of the lowermost mantle can
be explained by subduction processes that
extend to the base of the mantle.
D″
In their early seismic models Jeffreys and
Bullen (1940) divided the Earth into concentric shells based on changes in velocity gradients. Regions of the Earth were
named with letters and the lower mantle
(from 670 km to 2890 km deep) was
divided into D, D′ and D″ (read Ddouble-prime). D″ is the only commonly
used vestige of this nomenclature and
refers to the lowermost few 100 kms of
mantle. This shell shows the largest lateral variations in seismic velocities in the
lower mantle. Variations in the ratio of
P-wave to S-wave velocities suggest that
the region may be compositionally different from the overlying mantle. In certain
areas there are thin (<30 km) basal layers
of ultra-low velocities at the CMB. A
weak (<3%) discontinuity in seismic
velocities lies on average nearly 270 km
above the CMB and is visible as it
reflects seismic energy back to the surface. However, this discontinuity shows
considerable variability in both strength
and distance from the CMB. There is
also mounting evidence for more discrete
small-scale velocity anomalies that scatter seismic energy. Finally, in many areas
D″ exhibits seismic anisotropy, evidence
of which lies in the observation of two
orthogonally polarized and independent
shear-waves. A good review of these
observations can be found in an AGU
monograph entitled The Core-Mantle
Boundary Region (Gurnis et al. 1998).
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The Bullerwell Lecture is an annual award given by the British Geophysical
Association. Michael Kendall here presents the 2003 lecture.
T
here are many unanswered questions
about the nature of the Earth’s deep interior and, contrary to recent Hollywood
films, it is unlikely that we will ever be able to
answer these questions with first-hand observations. Instead we must use other imaging
methods to probe the deep Earth, the primary
one being seismology.
The base of the Earth’s mantle lies nearly
3000 km beneath us and marks a dramatic
boundary between the overlying silicate or
“rocky” mantle and the molten-iron outer core.
This interface is in many ways as dramatic as the
boundary between lithosphere and atmosphere,
where we live. Despite its apparent remoteness,
the nature of the core–mantle boundary (CMB)
affects the Earth’s behaviour in many ways: thermal coupling with the core affects the Earth’s
magnetic field, lateral heterogeneity affects the
Earth’s moment of inertia and hence its length of
day, and thermal instabilities may instigate mantle plumes that rise to form hot spots like Iceland
and Hawaii. But there is still much to be learned
about the composition of the lower mantle.
Experiments in mineral physics provide constraints on the composition and mineralogy at
depth that aid in the interpretation of seismic
observations, but there remain many unknowns.
Another long-standing question in geophysics
concerns mantle convection, notably its continuity through the depth of the mantle. Geochemical
arguments suggest only limited mass transfer
between the upper and lower mantle, yet geodynamical simulations suggest convection across
the entire mantle. As a result, seismologists have
sought to test this issue by looking for evidence
of slabs of subducted lithosphere in the lower
mantle (e.g. Grand et al. 1997).
Here I summarize recent work by myself and
colleagues to investigate the structure and properties of the lowermost mantle (see box, left).
Dense broadband seismic networks now produce large datasets, which in turn demand new
techniques for analysis. Seismic migration techniques borrowed from oil-industry data processing are used to image the fine-scale velocity
discontinuity structure of what appears to be
cold slab-like material at the base of the mantle
beneath northern Asia. In separate work we
look for evidence of seismic anisotropy in the
lower mantle beneath the northern Pacific
Ocean. Anisotropy refers to directional variations in seismic speed at a given point and may
result from the preferred alignment of minerals,
oriented inclusions, or layering; it is likely that
more than one of these mechanisms may be
operating at one time. Such effects are caused by
deformation processes and, as such, evidence of
anisotropy offers insights into the dynamical
nature of the lower mantle. Together, these
methods map the velocity structure, seismic
properties and lateral variation of this remote
but significant region of our planet.
D″ discontinuities and the thermal
properties of slabs
In the migration study we use shear-wave data
recorded by broadband three-component seismic stations in networks across Europe (figure
1). Most are permanent stations, but some were
deployed in a temporary network designed to
image the deep Earth (Kendall and Helffrich
2001). Large deep-focus earthquake events in
the northwest Pacific are 68–82° away from the
European stations and image the base of the
mantle beneath northern Asia. A seismic discontinuity in the D″ region will produce a
reflection (SdS) that will appear on a seismogram as a precursor to ScS (see figure 2). A wellknown velocity discontinuity lies roughly
280 km above the CMB in our study region
(see, for example, Lay and Helmberger 1983),
but dense seismic arrays or networks allow us
to map detailed variations in the topography of
this discontinuity. We look for seismic signals
reflected from this region using stations whose
bounce points cluster in regions roughly the size
of a Fresnel zone (i.e. the lateral resolution
expected for these waves) (figure 1).
We use a seismic migration technique developed by Thomas et al. (1999) to stack the seismic signals, thereby pinpointing reflector
locations. Travel-times are calculated for raypaths between stations and hypothetical reflection points in a 3-D grid that extends roughly
1500 km2 across the CMB and nearly 700 km
above the CMB. Seismic data for each cluster
are time-shifted and summed for each of the
hypothetical reflection points in the grid. The
data will sum coherently should a grid point lie
at an actual reflection point. An advantage of
this approach is that is allows us to accurately
April 2004 Vol 45
The 2003 Bullerwell Lecture
e lower mantle
1: Stations and events used to image the lowermost mantle beneath northern Asia. The reflection points are grouped into eight elliptically shaped regions,
roughly the size of a Fresnel zone. Smaller circles are colour coded according to individual earthquakes (see legend). The underlying colours show the
tomographic model of Grand et al. (1997); blue indicates higher than average seismic velocities and red means lower than average velocities.
2: Seismic phases that can be
used to image the lowermost
mantle. A discontinuity in the D″
region will reflect seismic energy
back to the surface (the SdS
phase). At epicentral distances
less then 80°, only the corereflected phases, ScS, will sample
the D″ region, while at greater
distances both the S and ScS
phase will sample D″. The phase
SKS transits the mantle as an
S-wave and the core as a P-wave.
Analogous phases exist for
P-waves (e.g. PcP, PKP, etc). UMA
refers to upper-mantle anisotropy.
April 2004 Vol 45
UMA
660
75°
90°
ScS
S
SdS
S
ScS
SKS
core
SKS
D″
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The 2003 Bullerwell Lecture
frequency seismic energy. Such signals will
appear as seismic reflections and tests with synthetic seismograms show that these signals will
be imaged with our seismic migration technique
(Thomas et al. 2004). There is a qualitative
agreement between the migration results for the
data and those for the synthetic waveforms calculated in regions of cold slab accumulations.
This suggests that the thermal structure of subducted material can explain our observations:
we are simply seeing reflections from the top
and bottom of the cold slabs.
top structure
(206 to 316 km)
320
280
distance from CMB (km)
3: Interpolated topography
of two D″ discontinuities
observed beneath northern
Asia. The top surface marks
an abrupt increase in
seismic velocity, while the
bottom marks an abrupt
decrease in seismic velocity
closer to the base of the
mantle.
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bottom structure
120 (5585 km)
80
40
70°N
90°E
80°E
4: A snapshot of the temperature field from a mantle convection calculation (Lowman et al. 2003), in
which slab-like downwellings accumulate at the base of the mantle. These features spread laterally in
D″ and exhibit strong undulations in their upper surfaces, but less topography on their lower surfaces.
Vertical lines indicate where temperature profiles are used to construct velocity models. The top
150 km of temperature field (boundary layer) is removed in order to reveal features below the plates.
position energy that is reflected from a discontinuity with complicated topography. Most previous approaches have determined discontinuity
depth on the assumption that the reflector is a
horizontal planar feature.
In accordance with previous studies, our
results show clear evidence for a reflector due
to a rapid, but modest (3%), increase in seismic
velocity roughly 280 km above the CMB.
However, the migrations reveal dramatic variations in the topography of this discontinuity:
it varies from 206 to 345 km above the CMB
(figure 3). A more surprising result is the presence of a weaker reflector between 55 and
85 km above the CMB. In contrast to the upper
discontinuity, this reflection is due to a sharp
decrease in seismic velocity; it also shows less
topography. The reflections from the upper discontinuity are stronger because they are at nearcritical angles of incidence. The critical angle is
the point of total internal reflection, which only
occurs when there is an increase in velocity
across a boundary. The weaker deeper reflections have never been observed in a single seismogram and are only visible by coherently
stacking many seismograms.
To interpret our migration results we appeal
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to recent numerical simulations of 3-D mantle
convection (Lowman et al. 2004). These models incorporate rigid moving plates and have an
Earth-like Bénard-Rayleigh number. Heating is
supplied in equal amounts from internal sources
and an isothermal bottom boundary (i.e. the
core). The models have Cartesian geometry and
the calculations do not include temperaturedependent viscosity. Further details of these simulations can be found in Thomas et al. (2004).
These geodynamical simulations predict a complicated thermal structure at the base of the
mantle (figure 4). Here cold slab-like features
crumple and deform as they collide with the
CMB. Regions of remnant slab material show
sharp decreases in temperature a few hundreds
of kilometres above the CMB and very sharp
increases in temperature much nearer the CMB.
Considerable topography appears on the upper
surface of these cold slab anomalies. In contrast,
regions without cold slab material show a simple increase in temperature near the CMB.
We have constructed velocity models based on
the temperature profiles through the thermal
structures illustrated in figure 4. Sharp positive
or negative gradients in velocity, due to sharp
temperature gradients, will backscatter low-
Seismic anisotropy
A range of seismic indicators suggest that the
lowermost mantle is anisotropic. The decay in
amplitudes of phases that diffract around the
CMB shows dramatic regional variations.
Doornbos et al. (1986) sought to explain this in
terms of a D″ thermal-boundary layer model.
More recently, D″ anisotropy has been more
clearly documented by the observation of shearwave splitting in the S/ScS phase that sweeps
through the lowermost mantle (e.g. arrivals at
90° in figure 2). There are a range of mechanisms
for seismic anisotropy (figure 5), but all require
sustained and coherent deformation in regions
hundreds of kilometres across (i.e. on the length
scale of seismic waves in the lower mantle).
Although we are still in the early stages of
mapping on a global scale the extent and nature
of anisotropy in the lower mantle, we can
loosely group past observations into two categories based on regional characteristics (figure
6). One is associated with regions where slabs
are predicted to descend into the lower mantle
(Lithgow-Bertelloni and Richards 1998). These
regions are characterized by high D″ shearvelocities and examples are the areas beneath
the Americas, Indian Ocean, northern Asia and
Alaska. The other category involves sites of
mantle upwelling (e.g. beneath the Pacific),
which are characterized by lower than average
D″ velocities. The style of anisotropy in the
paleo-slab regions appears to be transverse
isotropy. This symmetry is characterized by a
rotational invariance in seismic velocities
around a vertical symmetry axis (i.e. there is no
azimuthal variation in velocities). Core phases,
like SKS, do not seem to be affected by D″
anisotropy in these regions (a characteristic of
transverse isotropy) and the horizontally polarized shear-waves which transit the D″ region
horizontally (S/ScSH) are ubiquitously faster
than the radially polarized shear-waves
(S/ScSV). In contrast, the anisotropy of the
Pacific region appears less consistent implying
that it is probably more complicated.
Previous studies have focused on differences
between shear-waves recorded on radial and
transverse components. Many studies even
neglect the potential effects of well-known
anisotropy in shallower parts of the upper
April 2004 Vol 45
The 2003 Bullerwell Lecture
5: Schematic illustration of mechanisms for
seismic anisotropy. The upper left illustrates
anisotropy due to the preferred alignment of
minerals (LPO), the upper right, lower left and
lower right show anisotropy due to the preferred
alignment of tabular and cylindrical shaped
inclusions (SPO), and the middle diagram shows
anisotropy due to the layering of material with
contrasting seismic velocities (PTL). LPO is short
for lattice preferred orientation, SPO means
shape preferred orientation and PTL means
periodic thin layering. The nature of shear-wave
splitting in the vertical and two horizontal
directions is indicated along the edges of each
schematic cube. Vsv refers to a shear-wave
polarized in the vertical plane, while Vsh refers to
a shear-wave polarized in the horizontal plane.
no
splitting
splitting?
LPO
SPO
Vsv > Vsh
splitting?
no
splitting
PTL
Vsh > Vsv
splitting
splitting
SPO
SPO
Vsh > Vsv
Vsh > Vsv
Vsv > Vsh
60
30
0
–30
–60
6: Regions of the world where D″ anisotropy has been previously studied. Coloured lines connect events (stars) and seismic stations along great-circle paths.
Red regions are those where the style of anisotropy appears to be transverse isotropy. The light blue lines beneath the Pacific mark a region where the style of
anisotropy appears to be quite complicated. The purple region beneath the southern Pacific is an area where there seems to be little evidence of D″
anisotropy. Blue dots mark the predicted locations of slabs at the CMB (Lithgow-Bertelloni and Richards 1998).
April 2004 Vol 45
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The 2003 Bullerwell Lecture
mantle. Collectively this suggests that although
the lowermost mantle may be anisotropic, such
analysis may restrict interpretations to simple
anisotropic symmetries (e.g. transverse isotropy).
One way to address this issue involves a judicious selection of source–receiver paths that sample regions from many azimuths. In most parts
of the world this is nearly impossible because
there are too few appropriately placed sources
and receivers. An alternative approach is to consider differential splitting between S-phases
which turn above the D″ region and ScS-phases
which sample the D″ region (figure 2). One has
to be careful with such analysis because standard
techniques for estimating shear-wave splitting
cannot be used without accounting for phase distortions in the ScS phase that arise from reflection at the CMB. Using such an approach we
have found evidence for a more complex style of
anisotropy in a region of the northwest Pacific
(figure 7). The style of anisotropy is most simply
interpreted as transverse isotropy with a dipping
symmetry axis. The orientation of the anisotropy
roughly agrees with the known dip of the D″ discontinuity in this region (Thomas et al. 2002).
Mechanisms for lower-mantle anisotropy are
still a topic of some debate, largely due to our
limited knowledge of the material properties of
lower-mantle mineral assemblages. An obvious
candidate mechanism for anisotropy is the preferred alignment of minerals such as perovskite,
magnesiowustite or stishovite. Given what little
we know of the elasticities and glide systems of
minerals at lower-mantle pressures and temperatures, it has not been easy to explain transverse
isotropy in D″ with these minerals. However,
recent work (Karato 1998) has suggested that
magnesiowustite may change its glide system in
the lowermost mantle and could explain transverse isotropy in paleo-slab regions. Crystal
alignment could also be responsible for
anisotropy in more complicated regions, such as
that beneath the central and northwest Pacific.
Another candidate mechanism for anisotropy
is the preferred alignment of inclusions or finescale layering of material with contrasting
velocities. Kendall and Silver (1996) proposed
melt alignment as a cause of transverse isotropy
in D″. Using effective-medium modelling they
showed that very small volume fractions of
melt, if highly aligned, will not affect the overall velocity of the region, but will generate significant amounts of shear-wave splitting.
Infiltration of core material may explain the
presence of melt, but it is difficult to imagine
iron-rich core material, which is twice as dense
as lower-mantle rocks, rising to heights of a few
hundred kilometres above the CMB.
Alternatively, the anisotropy may be associated
with subduction processes. There is some suggestion that when basalt (oceanic crust) sinks
into the lower mantle it will be very near its
solidus in its new phase. Should just 1% of this
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7: A region beneath the northern Pacific
where the style of D″ anisotropy is not
simply transversely isotropic. The region
is imaged using differential shear-wave
splitting between lower-mantle turning
S-phases that do not sample D″ and
core-reflected shear-waves (see S and
ScS in figure 2). The shading and arrow
displayed at each source-receiver midpoint displays the orientation of the fast
shear-wave in the vertical plane
perpendicular to the horizontal ray
direction. For example, 0 refers to a
vertically polarized fast shear-wave,
while 90 refers to a horizontally
polarized fast shear-wave.
150
160
170
180
190
60
60
50
50
40
–45– –15
–15–15
15–45
45–75
75–105
105–135
160
former-basalt exist in aligned melt pockets it
could explain the observed anisotropy. The high
strains associated with slab material interacting
with the CMB can be invoked to explain the
alignment of either crystals or inclusions.
In summary, there are clear regional variations
in the style of D″ anisotropy that are very likely
to be associated with different physical
processes. In this sense, these variations are
analogous to those observed between continental and oceanic regions in the upper-mantle
boundary layer (i.e. the lithosphere). While
these observations are interesting from a seismological point of view, they are equally if not
more valuable as constraints on the mineralogy
and geodynamics of the lower mantle.
Effect of slabs on lower-mantle structure
I have summarized recent work that investigates
the heterogeneity and anisotropy of the lowermost mantle or D″ region. Linking these seismic
observations with state-of-the-art geodynamical
modelling has offered insights into mechanisms
responsible for these observations. Our geodynamical models suggest that the thermal
structure of D″ may be very complicated. Cold
slab material can crumple and spread out at the
CMB. For the first time we have been able to
image the top and bottom of a high-velocity
anomaly at the base of the mantle and we interpret this in the context of slab thermal structure.
While there is likely to be a degree of compositional heterogeneity in D″ (e.g. originating from
former oceanic crust or primordial material), the
thermal structure of the region can explain the
discontinuity structure. Similarly, we can look to
subduction processes to explain anisotropy in
parts of the lowermost mantle. Cumulatively, our
results provide indirect evidence that slabs can
reach the CMB in places, thereby supporting the
idea of convection on a whole-mantle scale.
At this stage it is difficult to be more definitive
in our interpretations. As seismic networks grow
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we will be better able to characterize the style of
D″ heterogeneity and anisotropy. As geodynamical modelling becomes more Earth-like, we will
be better able to model and predict such heterogeneity and anisotropy. Finally, as both laboratory and theoretical experiments in mineral
physics become more realistic for the lower mantle, we will gain a better understanding of the
elasticities and deformation processes in lowermantle phases. Linking studies from these various disciplines is needed to better illuminate the
nature of the somewhat enigmatic D″ region. ●
Michael Kendall, Professor of Seismology, School of
Earth Sciences, University of Leeds, Leeds, LS2 9JT,
UK. [email protected].
Acknowledgements: This work has been done in collaboration with many people. Specifically I would
like to thank Tine Thomas, Julian Lowman, Georg
Rümpker and James Wookey for their contributions.
Part of this research has been supported by a consortium grant from the UK Natural Environment
Research Council (NER/O/S/2001/01227) and by a
standard NERC grant (GR3/11738). I am grateful
to the British Geophysical Association for selecting
me to be the Bullerwell Lecturer in 2003.
References
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April 2004 Vol 45