Global anisotropy and the thickness of continents

Abstract

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.

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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.

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Acknowledgements

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.

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Correspondence to Barbara Romanowicz.

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Gung, Y., Panning, M. & Romanowicz, B. Global anisotropy and the thickness of continents. Nature 422, 707–711 (2003). https://doi.org/10.1038/nature01559

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