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| Titel |
Subduction to the lower mantle: A comparison between geodynamic and tomographic models |
| VerfasserIn |
Bernhard Steinberger, Trond H. Torsvik  , Thorsten Becker |
| 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 |
250048185
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| Zusammenfassung |
| It is generally believed that subduction of lithospheric slabs is a major contribution to thermal
heterogeneity in Earth’s entire mantle and provides a main driving force for mantle flow.
Mantle structure can on one hand be inferred from subduction history and geodynamic
models of mantle flow. On the other hand, seismic tomography models provide important
information on mantle heterogeneity. Yet, the two kinds of models are only similar on the
largest (1000s of km) scales and are quite dissimilar in their detailed structure. Here we
provide a quantitative assessment how good a fit can be currently achieved with a relatively
simple (viscous flow) geodynamic model. The discrepancy between geodynamic and
tomography models can indicate where a model refinement could possibly yield an improved
fit.
Our geodynamical model is based on a model of 300 Myr of subduction inferred from a
global plate reconstruction. Density anomalies are inserted into the upper mantle beneath
subduction zones, and flow and advection of these anomalies is computed with a spherical
harmonic code and for a radial viscosity structure constrained by mineral physics and surface
observations. Model viscosities in the upper mantle beneath the lithosphere are ~1020Pas,
and viscosity increases to ~1023Pas in the lower mantle above D”. Comparison with
a number of recent tomography models is assessed in terms of correlation, both
overall and as a function of depth and spherical harmonic degree. We find that,
compared to previous geodynamic and tomography models, correlation improved
significantly, both because of presumably improved plate reconstructions and improved
treatment of mantle flow. However, the highest correlation values are still limited to
lowest spherical harmonic degrees. Further improvement – in particular at spherical
harmonic degree two – is achieved, if we include a basal chemical layer in the model.
Subduction shapes this layer into two rather stable hot but chemically dense “piles”,
corresponding to the Pacific and African Large Low Shear Velocity Provinces.
With the inclusion of these, we are also able to reproduce the large-scale features
of the geoid as an outcome of our geodynamic model. Furthermore, we compute
the degree-two geoid as a function of time and infer “true polar wander”. If we
also include a stable contribution due to the compositional “piles”, we are able to
explain essential features of observed true polar wander over the past 120 Myr. |
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