With time, convective processes in the Earth's mantle will tend to align crystals, grains and inclusions. This mantle fabric is detectable seismologically, as it produces an anisotropy in material properties—in particular, a directional dependence in seismic-wave velocity. This alignment is enhanced at the boundaries of the mantle where there are rapid changes in the direction and magnitude of mantle flow1, and therefore most observations of anisotropy are confined to the uppermost mantle or lithosphere2,3 and the lowermost-mantle analogue of the lithosphere, the D″ region4. Here we present evidence from shear-wave splitting measurements for mid-mantle anisotropy in the vicinity of the 660-km discontinuity, the boundary between the upper and lower mantle. Deep-focus earthquakes in the Tonga–Kermadec and New Hebrides subduction zones recorded at Australian seismograph stations record some of the largest values of shear-wave splitting hitherto reported. The results suggest that, at least locally, there may exist a mid-mantle boundary layer, which could indicate the impediment of flow between the upper and lower mantle in this region.
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.
Montagner, J.-P. Where can seismic anisotropy be detected in the Earth's mantle? In boundary layers .... Pure Appl. Geophys. 151, 223–256 (1998).
Silver, P. G. Seismic anisotropy beneath the continents: Probing the depths of geology. Annu. Rev. Earth Planet Sci. 24, 385–432 (1996).
Savage, M. K. Seismic anisotropy and mantle deformation: What have we learned from shear wave splitting? Rev. Geophys. 37, 65–106 (1999).
Kendall, J.-M. & Silver, P. G. in The Core-Mantle Boundary Region (eds Gurnis, M., Wysession, M., Knittle, E. & Buffett, B.) 97–118 (Geodynamics series 28, American Geophysical Union, Washington DC, 1998).
Karato, S. & Wu, P. Rheology of the upper mantle: A synthesis. Science 260, 771–778 (1993).
Vinnik, L. P. & Montagner, J.-P. Shear wave splitting in the mantle Ps phase. Geophys. Res. Lett. 23, 2449–2452 (1996).
Vinnik, L. P., Chevrot, S. & Montagner, J.-P. Seismic evidence of flow at the base of the upper mantle. Geophys. Res. Lett. 25, 1995–1998 (1998).
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).
Fouch, M. J. & Fischer, K. M. Mantle anisotropy beneath northwest Pacific subduction zone. J. Geophys. Res. 101, 15987–16002 (1996).
Fischer, K. & Wiens, D. The depth distribution of mantle anisotropy beneath the Tonga subduction zone. Earth Planet. Sci. Lett. 142, 253–260 (1996).
Montagner, J.-P. & Kennett, B. L. N. How to reconcile body-wave and normal-mode reference Earth models. Geophys. J. Int. 125, 229–248 (1996).
Barruol, G. & Hoffmann, R. Upper mantle anisotropy beneath Geoscope stations. J. Geophys. Res. 104, 10757–10773 (1999).
Clitheroe, G. & Van der Hilst, R. in Structure and Evolution of the Australian Continent (ed. Braun, J. et al.) 73–78 (Geodynamics series 26, American Geophysical Union, Washington DC, 1998).
Özalaybey, S. & Chen, W. Frequency dependent analysis of SKS/SKKS waveforms observed in Australia: evidence for null birefringence. Phys. Earth Planet. Inter. 114, 197–210 (1999).
Silver, P. G. & Chan, W. W. Implications for continental structure and evolution from seismic anisotropy. Nature 335, 34–39 (1988).
Wolfe, C. J. & Silver, P. G. Seismic anisotropy of oceanic upper mantle: Shear-wave splitting methodologies and observations. J. Geophys. Res. 103, 749–771 (1998).
Kendall, J.-M. & Thomson, C. J. Seismic modelling of subduction zones with inhomogeneity and anisotropy, I: Teleseismic P-wavefront geometry. Geophys. J. Int. 112, 39–66 (1993).
Kirby, S. H., Durham, W. B. & Stern, L. A. Mantle phase changes and deep-earthquake faulting in subducting lithosphere. Science 252, 216–225 (1991).
Anderson, D. L. Thermally induced phase changes, lateral heterogeneity of the mantle, continental roots and deep slab anomalies. J. Geophys. Res. 92, 13968–13980 (1987).
Mainprice, D., Barruol, G. & Ismaı¨l, W. B. in Earth's Deep Interior: Mineral Physics and Tomography from the Atomic to the Global Scale (eds Karato, S., Fortre, A., Masters, T. G. & Stixrude, L.) 237–264 (Geophysical Monographs 117, American Geophysical Union, Washington DC, 2000).
Karato, S., Dupas-Bruzek, C. & Rubie, D. Plastic deformation of silicate spinel under the transition-zone conditions of the Earth's mantle. Nature 395, 266–269 (1998).
Meade, C. P., Silver, P. G. & Kaneshima, S. Laboratory and seismological observations of lower mantle isotropy. Geophys. Res. Lett. 22, 1293–1296 (1995).
Yeganeh-Haeri, A. Synthesis and re-investigation of the elastic properties of single-crystal magnesium silicate perovskite. Phys. Earth Planet. Inter. 87, 111–121 (1994).
Oganov, A. R., Brodholt, J. P. & Price, G. D. The elastic constants of MgSiO3 perovskite at pressures and temperatures of the Earth's mantle. Nature 411, 934–937 (2001).
Stixrude, L. in The Core-Mantle Boundary Region (eds Gurnis, M., Wysession, M., Knittle, E. & Buffett, B.) 83–96 (Geodynamics series 28, American Geophysical Union, Washington DC, 1998).
Gaherty, J. B. & Jordan, T. H. Lehmann discontinuity as the base of an anisotropic layer beneath continents. Science 268, 1468–1471 (1995).
Kusznir, N. J. Subduction body force stresses and viscosity structure at the 410 km and 660 km phase transitions. Eos 81, 1081 (2000).
Ringwood, A. E. Role of the transition zone and 660 km discontinuity in mantle dynamics. Phys. Earth Planet. Inter. 86, 5–24 (1994).
Mitrovica, J. X. & Forte, A. M. Radial profile of mantle viscosity: Results from the joint inversion of convection and postglacial rebound observations. J. Geophys. Res. 102, 2751–2769 (1997).
Hirose, K., Fei, Y., Ma, Y. & Mao, H.-K. The fate of subducted basaltic crust in the Earth's lower mantle. Nature 397, 53–56 (1999).
Widiyantoro, S., Kennett, B. L. N. & Van der Hilst, R. Seismic tomography with P and S data reveals lateral variations in the rigidity of deep slabs. Earth Planet. Sci. Lett. 173, 91–100 (1999).
Van der Hilst, R. Complex morphology of subducted lithosphere in the mantle beneath the Tonga trench. Nature 374, 154–157 (1995).
We thank K. Fischer, G. Houseman and M. Casey for comments on the manuscript, and K. Fischer for suggesting alternative models to test.
The authors declare no competing financial interests.
About this article
Cite this article
Wookey, J., Kendall, J. & Barruol, G. Mid-mantle deformation inferred from seismic anisotropy. Nature 415, 777–780 (2002) doi:10.1038/415777a
Journal of Geodynamics (2019)
Single-crystal elasticity of (Al,Fe)-bearing bridgmanite and seismic shear wave radial anisotropy at the topmost lower mantle
Earth and Planetary Science Letters (2019)
Journal of Geophysical Research: Solid Earth (2019)
Geochemistry, Geophysics, Geosystems (2019)
A Breakthrough in Pressure Generation by a Kawai-Type Multi-Anvil Apparatus with Tungsten Carbide Anvils