Deepmantle highviscosity flow and thermochemical structure inferred from seismic and geodynamic data

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Deep-mantle high-viscosity flow
andthermochemical structure inferredfrom
seismic and geodynamic data
NATURE VOL 410 26 APRIL 2001

• Alessandro M. Forte Jerry X. Mitrovica

Tonie Davidsen
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In short

• Use
• Seismic data
• Geodynamic data
• Find
• High viscosity flow in deep mantle
• Predict thermochemical structure
• Flow on long wavelength scale
• Supress flow-induced deformation and convective
  mixing
• Megaplumes buoyant and actively upwelling

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Outline

• A satisfying model
• Thermal and compositional effects
• Density pertubations
• Mantle viscosity
• Lower-mantle flow
• Mixing
• Deep lower-mantle
• Conclusion

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A satisfying model

• Model must predict
• Global-scale free-air gravity anomalies
• Observed tectonic-plate motions
• Dynamic topography on CMB
• Excess or dynamic ellipticity of CMB
• Model must be consistent with
• 3-D structure in tomographic models
• Especially long wavelength heteorogenity in
  bottom 1000 km mantle

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Thermal and compositional effects

• Chemical heteorogenity may bias temperature and
  density
• Anticorrelation between pertubations of Vs and Vf

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Density pertubations

• Two density pertubation approaches

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Mantle viscosity
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Lower-mantle flow
Two-layer viscosity model
EkDz
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Mixing

• Measure mixing by stretching of particles

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Deep lower-mantle

• Thermochemical heteorogeneity
• Compositional and thermal heteorogeneity at 2740
  km depth.

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SuDz
Masters
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SuDz
Masters
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Conclusion

• High viscosity regions
• Mixing low in high viscosity regions
• Relations for compositional and thermal
  heteorgeneity
• Composition strongly correlated to bulk-sound
  velocity anomalies
• Shear velocity anomalies are strongly
  anti-correlated to temperature
• Relation for iron or perovskite variations
• Dynamic of megaplumes

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Questions?