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Distinctive Observations of Mantle Superplumes: Insights from Global Shear Velocity Models

This work explores unique observations at the base of the Earth's mantle, particularly related to the African and Pacific superplumes. We discuss the anticorrelation between bulk sound velocity and shear velocity, the potential anticorrelation between density and shear velocity, and the significance of ULVZs. Key findings include strong lateral velocity gradients, the relationship with upper mantle structures, and anisotropic behavior. Our conclusions indicate that superplumes display sharp edges, with velocity contrasts typically not exceeding 5-6%, and undergo changes in strength and structure as they rise into the upper mantle.

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Distinctive Observations of Mantle Superplumes: Insights from Global Shear Velocity Models

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  1. 3D S velocity models Berkeley Harvard Scripps Caltech U. Texas

  2. SAW24B16 Mégnin and Romanowicz, 2000

  3. Global S velocity models

  4. Distinctive observations at the base of the mantle associated with the African and Pacific “superplumes” : • bulk sound velocity/shear velocity are anticorrelated(Su and Dziewonski, 1997; Masters et al., 2000) • density and shear velocity may be anticorrelated (Ishii and Tromp, 2000) • ULVZ’s (Garnero and Helmberger, 1996) • anisotropy • strong lateral velocity gradients • relation to upper mantle structure

  5. Depth = 2800 km S velocity P velocity Tkalcic et al., 2002

  6. Bulk sound velocity Masters et al., 2000

  7. Vs Vf r Ishii and Tromp, 2000

  8. Shear wave splitting measured on S-diffracted Vinnik, Bréger and Romanowicz,, 1998

  9. ISOTROPIC VELOCITY RADIAL ANISOTROPY = (VSH/VSV)2 Panning and Romanowicz, 2004

  10. Bréger and Romanowicz, 1998

  11. Sharp boundary of the African Superplume Ni et al., 2002

  12. dlnVs ~ -3- -12% Wang and Wen, 2005

  13. Coupled Spectral element method and normal modes 1D modes SEM Capdeville et al.,GJI 2002, 2003

  14. Forward modeling of fine scale structure Background: SAW24B16

  15. Indian Ocean Paths - Sdiffracted Corner frequencies: 2sec, 5sec, 18 sec

  16. -3% +3% -2% +2% CSEM Synthetics Toh et al. 2004

  17. Toh et al., EPSL, 2005

  18. SAW24B16 Mégnin and Romanowicz, 2000

  19. Upper mantle:Q - lower mantle: Vsh Degree 2 only Anelastic trans. zone Elastic near CMB Romanowicz and Gung, 2002

  20. AB CD Hawaii Q-1 “Pacific Superplume”

  21. MN ST Q-1 Elastic SAW24B16 African“superplume”

  22. Anisotropy versus attenuation Hawaii Central Pacific

  23. Montagner and Tanimoto, 1991

  24. Attenuation tomography Hotspot distribution QRLW8 Weighted by buoyancy flux

  25. Conclusions • “Superplumes” have sharp edges in D” • Velocity contrast may not be larger than ~5-6% on average • radial anisotropy in D” consistent with flow direction changing from horizontal to vertical in the superplumes

  26. Conclusions 2 • “Superplumes” become narrower and “weaker” in mid mantle • low velocity “structures” continue into the upper mantle, consistent with rising hot material spreading under the lithosphere • Consistent with dense cores surrounded by hot upwellings

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