Thermal, Elastic and Seismic Signature of High-Resolution Mantle Circulation Models

A long-standing question in the study of Earth’s deep interior is
the origin of seismic mantle heterogeneity. The challenge is to
efficiently mine the wealth of information available in complex
seismic waveforms and to separate the potential contributions of
thermal anomalies and compositional variations. High expectations
to gain new insight currently lie within the application of
high-performance computing to geophysical problems. Modern
supercomputers allow, for example, the simulation of global mantle
flow at Earth-like convective vigor or seismic wave propagation
through complex three-dimensional structures. The sophisticated
computational tools incorporate a variety of physical phenomena and
result in synthetic datasets that show a complexity comparable to
real observations. However, it is so far not clear how to combine
the results from the various disciplines in a consistent manner to
obtain a better understanding of deep Earth structure from the
expensive large-scale numerical simulations. In particular, it is
important to understand how to build conceptual models of Earth’s
mantle based on geodynamic considerations that can be
quantitatively assessed and used to test specific hypotheses. One
specific goal is to generate seismic heterogeneity from dynamic
flow calculations that can be used in global wave propagation
simulations so that synthetic seismograms can be directly compared
to seismic data without the need to perform inversions. In the
multi-disciplinary study presented here, a new method is developed
to theoretically predict and assess seismic mantle heterogeneity.
Forward modeling of global mantle flow is combined with information
from mineral physics and seismology. Temperatures inside the mantle
are obtained by generating a new class of mantle circulation models
at very high numerical resolution. The global average grid spacing
of ~25 km (around 80 million finite elements) allows for the
simulation of flow at Rayleigh numbers on the order of 10^9 and to
resolve a thermal boundary layer thickness of around 100 km. To
assess the predicted present day temperature fields, the geodynamic
flow calculations are post-processed with published
thermodynamically self-consistent models of mantle mineralogy for a
pyrolite composition to convert thermal structure into elastic
parameters. Quantitative predictions of the magnitudes of seismic
velocity and density variations are thereby possible due to the
appropriately high numerical resolution necessary to obtain
temperature variations that are consistent with the mineralogical
conversion. The resulting structures are compared to tomographic
models based on a variety of statistical measures taking into
account the limited resolving power of the seismic data. In a final
step, the geodynamic models are investigated with respect to the
influence of strong convective mass transport on the stability of
Earth’s rotation axis. This additional and independent analysis
provides information on whether strongly bottom heated isochemical
mantle circulation can be reconciled with paleomagnetic estimates
of true polar wander. One specific question that can be addressed
with this approach is the origin of two large regions of strongly
reduced seismic velocities in the lowermost mantle. Several
seismological observations are interpreted as being caused by
compositional variations. However, a large number of recent
geodynamical, mineralogical and also seismological studies argue
for a strong thermal gradient across the core-mantle boundary that
might provide an alternative explanation through the resulting
large temperature variations. Here, the forward modeling approach
is used to test the assumption whether the presence of a strong
thermal gradient in isochemical whole mantle flow is compatible
with a variety of geophysical observations. The results show that
the temperature variations deduced from the new high-resolution
mantle circulation models are capable of explaining gross
statistical features of mantle structure mapped by tomography. The
main finding is that models with strong core heating, which also
give a surface heat flux consistent with observations, yield
realistic depth profiles of root-mean-square (RMS) variations of
shear wave velocity. Most importantly, only models with a large
core contribution to the mantle energy budget are compatible with
the strong negative seismic anomalies in the large low velocity
provinces of the lower mantle. Taking into account the effects of
limited resolving power of seismic data on the magnitudes of
predicted seismic heterogeneity further improves this match to
tomographic models. This illustrates that seismic heterogeneity is
likely dominated by thermal variations and thus limits the possible
role of chemical heterogeneity in the lower mantle. Altogether, the
results strengthen the notion of strongly bottom heated isochemical
whole mantle flow with a pyrolite composition. Furthermore, these
findings give confidence in the consistency of the presented
approach and demonstrate the great potential of geophysical
large-scale high-performance simulations and their application to
seismic data and tomographic models.