Boundary Layers in the Mantle; a Tomographer’s View

1. Boundary Layers in the Mantle;a Tomographer’s View Adam M. Dziewoński In cooperation with: Gőran Ekstrőm Yu Jeffrey Gu Bogdan Kustowski VLAB Workshop, August 10, 2007

2. ScS – S elephant;the need for global 3-D thinking

3. Power spectra of three recent models Harvard Caltech Berkeley Surface T.Z. C.M.B

4. Power spectra of the three models;a closer look Harvard Caltech Berkeley

5. It cannot be that simple Grand et al., 1997

6. Diverse data sets is what made the progress possible

7. Data – the critical element Three types of data are needed: 1. Fundamental mode surface waves to resolve the near surface structure; 2.Overtone data to resolve structure in the transition zone and 3. Travel time data to resolve the lower mantle structure. Only three research groups (Berkeley, Caltech/Oxford and Harvard) use this, or equivalent, combination of data

8. Fundamental mode surface waves Rayleigh 75 seconds Love 35 seconds

9. Sensitivity kernels from Nettles (2005)

10. Long-period body wave waveforms from Gu et al. (2001)

11. Mantle wave waveforms from Gu et al. (2001)

12. Rayleigh waves 1-D sensitivity kernels from Ritsema et al., 2004

13. Body wave travel times

14. Depth resolution of different data subsets Moho CMB 650 km

15. Models from different data subsets 120 km 600 km 1600 km 2800 km After Ritsema et al., 2004

16. Surface Boundary Layer

17. Recent velocity models are very well correlated in the top 200 km, or so, of the mantle Attenuation low attenuation high attenuation Velocity

18. Radial anisotropy S-velocity at 150 km Azimuthal anisotropy 75 sec Rayleigh waves

19. From Peru Trench to Japan Trench from Nettles, 2005

20. Thickness of the continental lithosphere/tectosphere

21. Very sharp bottom of the cratonic lithosphere Shapiro & Ritzwoller (2002) From Kustowski et al. (2006)

22. Questions: • What processes cause radial anisotropy in a 100 Ma old oceanic lithosphere at a depth of 150 km? • Why are the isotropic velocity anomalies correlated with the age of the oceanic plate down to a depth of 200 km, even though plate cooling models prefer 100 km thickness? • What is the thickness of the continental lithosphere

23. Transition Zone

24. Degree-2 Upper Mantle Signal Shifts of the Spectral Peaks Plotted at the Pole Positions Masters et al., 1982

25. Stagnant slabs are common from Fukao et al. (2001)

26. Patterns of velocity anomalies above and below the 670 km discontinuity are not similar From Gu et al. (2001)

27. Power spectra of three modelswith good depth resolution Harvard Caltech Berkeley

28. Harvard Caltech Berkeley 600 km depth 800 km depth

29. Additionalevidence Change in the stress pattern near the 650 km discontinuity

30. Model S362ANIDepth 750 km l Strong, but spatially limited fast anomalies in the lower mantle may represent regions of limited penetration of subducted material accumulated in the transition zones

31. Pacific “superplume” QRLf12 (Q-1) SAW24B16 (Vs) EPR Romanowicz and Gung, 2002 The slow velocities in a superplume and high attenuation in the transition zone could be explained if transition zone is also a thermal boundary layer.

32. Two stage plume generation Courtillot et al. (2003)

33. Conclusions • Overtone/waveform data are critical for resolution of the transition zone structure. • The change in the spectrum across the discontinuity is as sharp as can be resolved at the present time. • We conclude that the transition zone is a boundary layer that could be penetrated by episodic events, but does not permit steady state circulation across the 650 km discontinuity.

34. Lowermost Mantle

35. Dziewonski, 1984

36. Equatorial Cross-section Dziewonski (1984) and Woodhouse and Dziewonski (1984)

37. Megaplumes span whole lower mantle 3-D view of +0.5% and -0.5% isosurface of S-velocity model of Masters et al. (2000). The upper surface is truncated at 800 km depth and lower – at CMB

38. Model of shear and bulk sound velocities Usually, models of the shear and compressional velocity are obtained independently. However, P-velocity depends both on shear modulus and bulk modulus. To isolate this interdependence, Su and Dziewonski (1997) formulated the inverse problem for a joint data set and derived 3-D perturbations of bulk sound velocity and shear velocity

39. K-μ Inversion550 km Depth At this depth, the shear and bulk sound velocities show high correlation; both show fast values in the Western Pacific and South America, likely associated with the ponding of subducted slabs.

40. K-μ Inversion2800 km Depth Near the CMB, there is a distinct negative correlation between K/ρ and μ/ρ under the Pacific and African Superplumes.

41. Bulk Sound and Shear Velocity Anomalies Correlation between the bulk sound and shear velocity anomalies changes from +0.7 in the transition zone to –0.8 in the lowermost mantle. From Su and Dziewonski, 1997.

42. K-μ Inversion2800 km Depth In the vicinity of the “China High”, the correlation remains positive. This is likely an expression of compositional heterogeneity, perhaps, combined with a thermal one.

43. Questions: • How have the super-plumes formed? • What part of the anomalies is caused by compositional rather than thermal variations? • Why do the super-plumes continue across the D” without an apparent change in the amplitude of the anomaly?