Global anisotropy and the thickness of continents


For decades there has been a vigorous debate about the depth extent of continental roots1,2. The analysis of heat-flow3, mantle-xenolith4 and electrical-conductivity5 data all indicate that the coherent, conductive part of continental roots (the ‘tectosphere’) is at most 200–250 km thick. Some global seismic tomographic models agree with this estimate, but others suggest that a much thicker zone of high velocities lies beneath continental shields6,7,8,9, reaching a depth of at least 400 km. Here we show that this disagreement can be reconciled by taking into account seismic anisotropy. We show that significant radial anisotropy, with horizontally polarized shear waves travelling faster than those that are vertically polarized, is present under most cratons in the depth range 250–400 km—similar to that found under ocean basins9,10 at shallower depths of 80–250 km. We propose that, in both cases, the anisotropy is related to shear in a low-viscosity asthenospheric channel, located at different depths under continents and oceans. The seismically defined ‘tectosphere’ is then at most 200–250 km thick under old continents. The ‘Lehmann discontinuity’, observed mostly under continents at about 200–250 km, and the ‘Gutenberg discontinuity’, observed under oceans at depths of about 60–80 km, may both be associated with the bottom of the lithosphere, marking a transition to flow-induced asthenospheric anisotropy.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Figure 1: Correlation coefficient as a function of depth between model SAW24B168, an SH model, and other global tomographic S velocity models.
Figure 2: Maximum depth for which the velocity anomaly with respect to the reference model PREM28 is greater than 2%, for different S velocity models.
Figure 3: Maps of relative lateral variations in the anisotropic parameter δ ln ξ of model SAW16AN at three depths in the upper mantle.
Figure 4: Depth cross-sections through three continents (see locations at top) showing the SH (left) and SV (right) components of anisotropic model SAW16AN.
Figure 5: Sketch illustrating our interpretation of the observed anisotropy in relation to lithospheric thickness, and its relationship to the Lehmann (L) and Gutenberg (G) discontinuities.


  1. 1

    Jordan, T. H. The continental lithosphere. Rev. Geophys. Space Phys. 13, 1–12 (1975)

    ADS  Article  Google Scholar 

  2. 2

    Anderson, D. L. The deep structure of continents. J. Geophys. Res. 84, 7555–7560 (1990)

    ADS  Article  Google Scholar 

  3. 3

    Jaupart, C., Mareschal, J. C. & Guillou-Frottier, L. Heat flow and thickness of the lithosphere in the Canadian Shield. J. Geophys. Res. 103, 15269–15286 (1998)

    ADS  Article  Google Scholar 

  4. 4

    Rudnick, R., McDonough, W. & O'Connell, R. Thermal structure, thickness and composition of continental lithosphere. Chem. Geol. 145, 395–411 (1998)

    ADS  CAS  Article  Google Scholar 

  5. 5

    Hirth, G., Evans, R. L. & Chave, A. D. Comparison of continental and oceanic mantle electrical conductivity: Is the Archean lithosphere dry? Geochem. Geophys. Geosyst. 1, 2000GC000048 (2000)

  6. 6

    Masters, G., Johnson, S., Laske, G. & Bolton, B. A shear-velocity model of the mantle. Phil. Trans. R. Soc. Lond. A 354, 1385–1411 (1996)

    ADS  Article  Google Scholar 

  7. 7

    Ritsema, J., van Heijst, H. & Woodhouse, J. H. Complex shear wave velocity structure imaged beneath Africa and Iceland. Science 286, 1925–1928 (1999)

    CAS  Article  Google Scholar 

  8. 8

    Mégnin, C. & Romanowicz, B. The 3D shear velocity structure of the mantle from the inversion of body, surface and higher mode waveforms. Geophys. J. Int. 143, 709–728 (2000)

    ADS  Article  Google Scholar 

  9. 9

    Ekström, G. & Dziewonski, A. M. The unique anisotropy of the Pacific upper mantle. Nature 394, 168–172 (1998)

    ADS  Article  Google Scholar 

  10. 10

    Montagner, J. P. What can seismology tell us about mantle convection? Rev. Geophys. 32 (2), 135–137 (1994)

    ADS  Article  Google Scholar 

  11. 11

    Li, X. D. & Romanowicz, B. Comparison of global waveform inversions with and without considering cross branch coupling. Geophys. J. Int. 121, 695–709 (1995)

    ADS  Article  Google Scholar 

  12. 12

    Tong, C., Gudmundsson, O. & Kennett, B. L. N. Shear wave splitting in refracted waves returned from the upper mantle transition zone beneath northern Australia. J. Geophys. Res. 99, 15783–15797 (1994)

    ADS  Article  Google Scholar 

  13. 13

    Debayle, E. & Kennett, B. L. N. Anisotropy in the Australasian upper mantle from Love and Rayleigh waveform inversion. Earth Planet. Sci. Lett. 184, 339–351 (2000)

    ADS  CAS  Article  Google Scholar 

  14. 14

    Silver, P. G. Seismic anisotropy beneath the continents: probing the depths of geology. Annu. Rev. Earth Planet. Sci. 24, 385–432 (1996)

    ADS  CAS  Article  Google Scholar 

  15. 15

    Vinnik, L. P., Makeyeva, L. I., Milev, A. & Usenko, Y. Global patterns of azimuthal anisotropy and deformations in the continental mantle. Geophys. J. Int. 111, 433–447 (1992)

    ADS  Article  Google Scholar 

  16. 16

    Fouch, M. J., Fischer, K. M., Parmentier, E. M., Wysession, M. E. & Clarke, T. J. Shear wave splitting, continental keels, and patterns of mantle flow. J. Geophys. Res. 105, 6255–6275 (2000)

    ADS  Article  Google Scholar 

  17. 17

    Bokelmann, G. H. R. Which forces drive North America? Geology 30 (11), 1027–1030 (2002)

    ADS  Article  Google Scholar 

  18. 18

    Levin, V., Menke, W. & Park, J. Shear wave splitting in the Appalachians and the Urals: a case for multilayered anisotropy. J. Geophys. Res. 104, 17975–17993 (1999)

    ADS  Article  Google Scholar 

  19. 19

    Romanowicz, B. & Gung, Y. Superplumes from the core-mantle boundary to the lithosphere: implications for heat flux. Science 296, 513–516 (2002)

    ADS  CAS  Article  Google Scholar 

  20. 20

    Shearer, P. Seismic imaging of upper mantle structure with new evidence for a 520 km discontinuity. Nature 344, 121–126 (1990)

    ADS  Article  Google Scholar 

  21. 21

    Gu, Y. M., Dziewonski, A. M. & Ekström, G. Preferential detection of the Lehmann discontinuity beneath continents. Geophys. Res. Lett. 28, 4655–4658 (2001)

    ADS  Article  Google Scholar 

  22. 22

    Revenaugh, J. & Jordan, T. H. Mantle layering from ScS reverberations. 3. The upper mantle. J. Geophys. Res. 96, 19781–19810 (1991)

    ADS  Article  Google Scholar 

  23. 23

    Leven, J. H., Jackson, I. & Ringwood, A. E. Upper mantle seismic anisotropy and lithospheric decoupling. Nature 289, 234–239 (1981)

    ADS  Article  Google Scholar 

  24. 24

    Karato, S. I. On the Lehmann discontinuity. Geophys. Res. Lett. 19, 2255–2258 (1992)

    ADS  Article  Google Scholar 

  25. 25

    Gaherty, J. B. & Jordan, T. H. Lehmann discontinuity as the base of an anisotropic layer beneath continents. Science 268, 1468–1471 (1995)

    ADS  CAS  Article  Google Scholar 

  26. 26

    Montagner, J.-P. Upper mantle low anisotropy channels below the Pacific Plate. Earth Planet. Sci. Lett. 202, 263–274 (2002)

    ADS  CAS  Article  Google Scholar 

  27. 27

    Montagner, J. P. & Anderson, D. L. Petrological constraints on seismic anosotropy. Phys. Earth Planet. Int. 54, 82–105 (1989)

    ADS  Article  Google Scholar 

  28. 28

    Dziewonski, A. M. & Anderson, D. L. Preliminary reference earth model. Phys. Earth Planet. Int. 25, 297–356 (1981)

    ADS  Article  Google Scholar 

  29. 29

    Babuska, V., Montagner, J. P., Plomerova, J. & Girardin, N. Age-dependent large-scale fabric of the mantle lithosphere as derived from surface wave velocity anisotropy. Pure Appl. Geophys. 121, 257–280 (1998)

    ADS  Article  Google Scholar 

  30. 30

    Levin, V. & Park, J. Shear zones in the Proterozoic lithosphere of the Arabian Shield and the nature of the Hales discontinuity. Tectonophysics 323 (3–4), 131–148 (2000)

    ADS  Article  Google Scholar 

Download references


We thank J. Park, B. Kennett and J. P. Montagner for constructive comments on this manuscript. This work was supported through a grant from the National Science Foundation.

Author information



Corresponding author

Correspondence to Barbara Romanowicz.

Ethics declarations

Competing interests

The authors declare that they have no competing financial interests.

Supplementary information

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Gung, Y., Panning, M. & Romanowicz, B. Global anisotropy and the thickness of continents. Nature 422, 707–711 (2003).

Download citation

Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.