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Middle Mantle Seismic Structure of the African Superplume

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Abstract

I present the results of statistical hypothesis testing of Grand’s (2002) global tomography model of three-dimensional shear velocity variations for the middle mantle underneath eastern and southern Africa. I apply an F test to evaluate the validity of a model where a tilted, slow-velocity anomaly in the deepest mantle under southern Africa, known as the African superplume, is continuous with a slow-velocity anomaly in the upper mantle under eastern Africa. This null hypothesis is tested against alternative hypotheses, in which various “obstruction volumes” in the middle mantle are constrained to zero perturbation from the one-dimensional reference velocity during the tomographic inversion. I find that there is an equal probability of accepting an alternative hypothesis with a thin “obstruction volume” at 850–1,000 km depth, whereas volumes at other depths are rejected. But the alternative hypothesis, where a connection is forced at 850–1,000 km depth, is rejected. I conclude that the African superplume rises to at least 1,150 km depth, and that the upper mantle slow-velocity anomaly continues from the surface to below the mantle transition zone. I interpret the “obstruction volume” as a weakening of the superplume in the middle mantle.

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Notes

  1. MatTimes: A MATLAB-based seismic travel-time calculation and visualisation toolbox. The Department of Geosciences at Texas Tech University.

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Acknowledgments

I wish to thank Prof. Grand of the University of Texas at Austin, USA. During my visits to Texas he made his tomography data set available for my research and suggested this project to me. Prof. Grand also kindly helped me to modify his inversion code to prepare the “BLOCK models” for my hypothesis testing. Nathan Simmons set up for my use his MatTimesFootnote 1 code to calculate predicted travel times with a one-dimensional velocity model and to measure the observed phase arrivals. I thank the University of the Witwatersrand, South Africa, for providing administrative support and the Council for Geoscience, South Africa, for granting me study leave to spend time at the University of Texas at Austin. Prof. Ray Durrheim critically checked the grammar and scientific writing of the manuscript. Two anonymous reviewers and the editor suggested thoughtful improvements to the original manuscript.

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  1. Seismology Unit, Council for Geoscience, Pretoria, South Africa

    Martin B. C. Brandt

  2. School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa

    Martin B. C. Brandt

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Correspondence to Martin B. C. Brandt.

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Brandt, M.B.C. Middle Mantle Seismic Structure of the African Superplume. Pure Appl. Geophys. 170, 845–861 (2013). https://doi.org/10.1007/s00024-012-0589-y

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