Global anisotropy and the thickness of continents

Abstract

For decades there has been a vigorous debate about the depth extent of continental roots. The analysis of heat-flow, mantle-xenolith and electrical-conductivity 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 shields, 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 basins 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.

Figures

Fig. 1. (A) maps of model SAW16AN at depths of 175 km, 300 km and 400km. From left to right: V_SH, V_SV and ξ

Fig. 2. (A) Sketch illustrating our interpretation of the observed anisotropy in relation to lithospheric thickness, and its relationship to the Lehmann (L) and Gutenberg (G) discontinuities. The Hales discontinuity (H), which is also shown, is generally observed as a positive impedance embedded within the continental lithosphere in the depth range 60¨C80 km. H and G may not be related.

Fig. 3. Depth cross-sections through three continents (see locations at top) showing the SH (left) and SV (right) components of anisotropic model SAW16AN. The SH sections consistently indicate fast velocities extending to depths in excess of 220 km, whereas the SV sections do not.

Model parametrization

The model is parametrized laterally in spherical harmonics up to degree 16 and radially in 16 unevenly spaced splines.

Download

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Press Coverage

• Frankfurter Allgemeine Zeitung: Zu den Wurzeln der Kontinente

For all enquiries please contact:

Yuancheng Gung at ycgung@ntu.edu.tw or

Barbara Romanowicz at barbara@seismo.berkeley.edu