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Published November 28, 2003 | public
Journal Article

Further constraints on the African superplume structure


It is well established that there is a large-scale low velocity structure in the lowermost mantle beneath Africa, extending from the Southeastern Atlantic Ocean to the Southwestern Indian Ocean with a volume greater than 10 billion km^3 (>7000 km long, 1000 km across and 1200 km high) [Earth Planet. Sci. Lett. 206 (2003) 119]. This low velocity structure is often called the African superplume. Various studies also require sharp boundaries for the plume. However, as for its height and shear velocity reduction, there has been some controversy, especially concerning the velocities at the core–mantle–boundary (CMB). Here, we present an assortment of phases involving S_diff, SKS, S and S_(c)S with both vertical and horizontal paths sampling a 2D corridor through the structure. Travel time and waveform modeling of these seismic phases argues for a model with shear velocity reduction of approximately 3% within the superplume (which is basically a 200 km thick layer low velocity layer beneath the Southern Atlantic Ocean, and a 1200 km high structure beneath South Africa), and against a model of a substantially reduced low velocity layer (up to 10%, 300 km) beneath the superplume. We also analyzed P_diff and the differential times of P_(c)P–P and compared them with Sdiff and S_(c)S–S observations along the same great circle paths. The P-velocity is not very anomalous, at most −0.5%, much smaller than −1% as expected from a thermal anomaly with −3% lower S-velocity [Geophys. Res. Lett. 27 (2000) 421], thus again arguing for a chemical origin which was suggested from the modeling of African superplume sharp sides [Science 296 (2002) 1850].

Additional Information

© 2003 Published by Elsevier B. V. Accepted 11 July 2003. We thank the IRIS and GEOSCOPE data centers for providing digital seismic waveform data, particularly we are grateful for the efforts by Rick Carlson, Tim Grove, David James, Paul Silver and Tom Jordan who ran the South Africa Array from 1997 to 1999. Detailed comments by the anonymous reviewer and Michael Wysession are very helpful for us to improve the manuscript. This work was supported by NSF Grant EAR-0215644 and EAR-0229885. Contribution number 8923 of the Division of Geological and Planetary Sciences, California Institute of Technology.

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