Since the concept of "tectosphere" was first proposed, there have been vigorous debates about the depth extent of continental roots. The analysis of heat flow, mantle xenoliths, gravity and glacial rebound data indicate that the coherent, conductive part of continental roots is not much thicker than 200-250 km. Some global seismic tomographic models agree with this estimate but others indicate much thicker lithosphere under old continents, reaching at least 400km in depth. Here we show that the disagreement can be reconciled when taking into account anisotropy. Significant radial anisotropy with Vsh>Vsv is present under most cratons in the depth range 250-400 km, similar to that reported earlier12 at shallower depths (80-250km) under ocean basins. We propose that in both cases, this anisotropy is related to shear in the asthenospheric channel, located at different depths under continents and oceans. The seismically defined lithosphere is then at most 200-250 km thick under continents. The Lehmann discontinuity, observed mostly under continents around 200-240 km, and the Gutenberg discontinuity, observed under oceans at shallower depths (~ 60-80km), may both be associated with the bottom of the lithosphere, marking a transition to flow-induced asthenospheric anisotropy.
The model is parametrized laterally in spherical harmonics up to degree 16 and radially in 16 unevenly spaced splines.
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|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.|
|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.