17
April 2003
Continents: How low do they go?
The depth of the lithospheric roots under old continents, in effect the depth to which the Earth's crust and upper mantle travel coherently, has been a controversial topic. Some seismological models appear to indicate roots as deep as 400 km, while most other geophysical evidence points to a continental lithosphere closer to 200–250 km. Gung et al. propose a new global model of the upper mantle which reconciles the two extremes. The key to the model is the assumption that 200–400 km depth range is an area of seismic anisotropy — that is, the properties differ according to the direction of measurement. Horizontally polarized shear waves travel faster than vertically polarized waves, a situation similar to that seen under ocean bases at shallower depths. The authors conclude that continental roots do not extend much below 250 km, and that below this lies a continental asthenosphere consisting of several hundred kilometres of highly deformed rock.
Global anisotropy and the thickness of continents
YUANCHENG GUNG, MARK PANNING & BARBARA ROMANOWICZ
Nature 422, 707–711 (2003); doi:10.1038/nature01559
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Earth science: Roots of the matter
B.
L. N. KENNETT
Nature 422, 674–675 (2003); doi:10.1038/422674a
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