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Science 7 June 2002:
Vol. 296. no. 5574, pp. 1850 - 1852
DOI: 10.1126/science.1070698
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Reports

Sharp Sides to the African Superplume

Sidao Ni,* Eh Tan, Michael Gurnis, Don Helmberger

Beneath southern Africa is a large structure about 1200 kilometers across and extending obliquely 1500 kilometers upward from the core-mantle boundary with a shear velocity reduction of about 3%. Using a fortuitous set of SKS phases that travel along its eastern side, we show that the boundary of the anomaly appears to be sharp, with a width less than 50 kilometers, and is tilted outward from its center. Dynamic models that fit the seismic constraints have a dense chemical layer within an upwardly flowing thermal structure. The tilt suggests that the layer is dynamically unstable on geological time scales.

Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125, USA.
*   To whom correspondence should be addressed. E-mail: stone{at}gps.caltech.edu

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