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.
Subscribe to Journal
Get full journal access for 1 year
only $3.90 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Rent or Buy article
Get time limited or full article access on ReadCube.
All prices are NET prices.
Jordan, T. H. The continental lithosphere. Rev. Geophys. Space Phys. 13, 1–12 (1975)
Anderson, D. L. The deep structure of continents. J. Geophys. Res. 84, 7555–7560 (1990)
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)
Rudnick, R., McDonough, W. & O'Connell, R. Thermal structure, thickness and composition of continental lithosphere. Chem. Geol. 145, 395–411 (1998)
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)
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)
Ritsema, J., van Heijst, H. & Woodhouse, J. H. Complex shear wave velocity structure imaged beneath Africa and Iceland. Science 286, 1925–1928 (1999)
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)
Ekström, G. & Dziewonski, A. M. The unique anisotropy of the Pacific upper mantle. Nature 394, 168–172 (1998)
Montagner, J. P. What can seismology tell us about mantle convection? Rev. Geophys. 32 (2), 135–137 (1994)
Li, X. D. & Romanowicz, B. Comparison of global waveform inversions with and without considering cross branch coupling. Geophys. J. Int. 121, 695–709 (1995)
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)
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)
Silver, P. G. Seismic anisotropy beneath the continents: probing the depths of geology. Annu. Rev. Earth Planet. Sci. 24, 385–432 (1996)
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)
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)
Bokelmann, G. H. R. Which forces drive North America? Geology 30 (11), 1027–1030 (2002)
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)
Romanowicz, B. & Gung, Y. Superplumes from the core-mantle boundary to the lithosphere: implications for heat flux. Science 296, 513–516 (2002)
Shearer, P. Seismic imaging of upper mantle structure with new evidence for a 520 km discontinuity. Nature 344, 121–126 (1990)
Gu, Y. M., Dziewonski, A. M. & Ekström, G. Preferential detection of the Lehmann discontinuity beneath continents. Geophys. Res. Lett. 28, 4655–4658 (2001)
Revenaugh, J. & Jordan, T. H. Mantle layering from ScS reverberations. 3. The upper mantle. J. Geophys. Res. 96, 19781–19810 (1991)
Leven, J. H., Jackson, I. & Ringwood, A. E. Upper mantle seismic anisotropy and lithospheric decoupling. Nature 289, 234–239 (1981)
Karato, S. I. On the Lehmann discontinuity. Geophys. Res. Lett. 19, 2255–2258 (1992)
Gaherty, J. B. & Jordan, T. H. Lehmann discontinuity as the base of an anisotropic layer beneath continents. Science 268, 1468–1471 (1995)
Montagner, J.-P. Upper mantle low anisotropy channels below the Pacific Plate. Earth Planet. Sci. Lett. 202, 263–274 (2002)
Montagner, J. P. & Anderson, D. L. Petrological constraints on seismic anosotropy. Phys. Earth Planet. Int. 54, 82–105 (1989)
Dziewonski, A. M. & Anderson, D. L. Preliminary reference earth model. Phys. Earth Planet. Int. 25, 297–356 (1981)
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)
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)
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.
The authors declare that they have no competing financial interests.
About this article
Cite this article
Gung, Y., Panning, M. & Romanowicz, B. Global anisotropy and the thickness of continents. Nature 422, 707–711 (2003). https://doi.org/10.1038/nature01559
Journal of Geophysical Research: Solid Earth (2020)
Earth and Planetary Science Letters (2020)
3-D shear wave velocity model of the lithosphere below the Sardinia–Corsica continental block based on Rayleigh-wave phase velocities
Geophysical Journal International (2020)
Shear wave velocity and radial anisotropy structures beneath the central Pacific from surface wave analysis of OBS records
Earth and Planetary Science Letters (2020)
American Mineralogist (2020)