Evidence for deep mantle circulation from global tomography

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Evidence for deep mantle circulation from global
tomography
Nature, vol 386, 10 April 1997

• by R.D. van der Hilst,
• S. Widiyantoro E.R. Engdahl

Amélie Mocquet Tonie Davidsen
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Outline

• Method
• Mantle circulation
• Mid-mantle
• Lower mantle
• Deep circulation
• Conclusion

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Method
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Tomography

• Inversion technique using seismic data to get
  information about subsurface
• Mainly first-arriving P-waves, improved
  travel-time data and earthquake locations

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Resolution in the lower mantle

• Possible underestimate amplitude overestimate
  width of anomalies
• Constant-velocity blocks 2x 2x 200 km
• Investigate the continuity of structure between
  upper and lower mantle
• The pattern of convective flow at larger depth
• Uneven source/receiver distribution

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Mantle circulation
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Resulting P-wave anomalies
150 km above CMB
Upper-mantle transition zone
Mid-mantle
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Narrow structures in the mid-mantle

• Are these narrow structures real?
• Resolution tests at 1300 km

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Cold downwellings in the lower mantle

• 3-D synthetic slab model at 1900 km.
• Plate convergence
• Whole-mantle flow
• Farallon, Tethys and Pacific

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Cold downwellings in the lower mantle

• Farollon slab. Vertical mantle section (line 5)

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Deep circulation in the Earths mantle

• Line 6
• Deep structure
• Lower-mantle flow
• Connected with upper-mantle flow

2750 km
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Conclusion
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Conclusion

• Shallow mantle flow connects (at least locally)
  to structural heterogenity just above CMB
• Narrow structures become more cylindrical with
  depth
• Excellent correlation of small-scale (lt1000 km)
  structure
• Understand thermal and chemical evolution of the
  Earth

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Problems/Improvements

• Data coverage in the lowermost mantle
• Interpretation of deep structure
• Remaining differences with long-wavelength models
• Seismic imaging
• Plate reconstruction
• Numerical flow modelling
• Chemical mass-balancing

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Questions?