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Titel |
Influence of Large Low Shear Velocity Provinces in the lower mantle on the geoid |
VerfasserIn |
Marcus Beuchert, Harro Schmeling, Meysam Shahraki |
Konferenz |
EGU General Assembly 2011
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 13 (2011) |
Datensatznummer |
250056518
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Zusammenfassung |
The influence of the two near-equatorial, antipodal Large Low Shear Velocity Provinces
(LLSVPs) in the lower mantle on global mantle dynamics is a topic of major interest in
geodynamics. It was found in seismic studies that LLSVPs exhibit excess density with
respect to the surrounding mantle which means that they are not thermal superplumes, as
previously thought, but instead constitute large domes of dense material residing at the base
of the mantle. This has important implications for the overall convection style of the Earth’s
interior. It also changes the interpretation of the strong spatial correlation between LLSVPs
and observed positive geoid anomalies. If the anomalies were hot superplumes, they
would drive a rising flow in the mantle and thus cause positive geoid anomalies
due to dynamic topography of the surface. Yet, since the anomalies were found to
exhibit excess density, such flow is expected to be much weaker and the associated
geoid anomalies would be smaller than for superplumes. Instead, the excess density
itself contributes to the positive geoid signal above LLSVPs. Even though density
anomalies in the lower mantle are in general expected to have a relatively small
influence on the geoid due to their great distance from the surface, large volumes
with wide lateral extent, as is the case for LLSVPs, could still produce a strong
geoid signal and be responsible for the observed positive geoid anomalies. Since
both density excess of the anomalies and dynamic effects (resulting in dynamic
topography) have an influence on the geoid signal, we investigated both effects on the
geoid in fully dynamic mantle convection models with Cartesian and spherical
axisymmetric geometries. In general, we found that for increasing chemical/thermal
density contribution the geoid signal above the LLSVPs decreases, but remains
significant.
Whereas steady-state was reached in the Cartesian simulations up to Ra=106, spherical
axisymmetric simulations remained in a long-term time-dependent regime for Ra=105 and
106. For some spherical axisymmetric simulations, we found long-term existence of dense
anomalies compatible with LLSVPs as observed inside Earth, even for relatively high
Rayleigh numbers. We developed a new technique that enables us to quantitatively analyze
the geoid signal above these anomalies in time-dependent simulations. A comparison of
the thus obtained global geoid with the observed one delivered a relatively good
fit. |
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