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Upper Mantle Seismic Structure Beneath Southern Africa: Constraints on the Buoyancy Supporting the African Superswell

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Abstract

We present new one-dimensional SH-wave velocity models of the upper mantle beneath the Kalahari craton in southern Africa obtained from waveform inversion of regional seismograms from an Mw = 5.9 earthquake located near Lake Tanganyika recorded on broadband seismic stations deployed during the 1997–1999 Southern African Seismic Experiment. The velocity in the lithosphere beneath the Kalahari craton is similar to that of other shields, and there is little evidence for a significant low velocity zone beneath the lithosphere. The lower part of the lithosphere, from 110 to 220 km depth, is slightly slower than beneath other shields, possibly due to higher temperatures or a decrease in Mg number (Mg#). If the slower velocities are caused by a thermal anomaly, then slightly less than half of the unusually high elevation of the Kalahari craton can be explained by shallow buoyancy from a hot lithosphere. However, a decrease in the Mg# of the lower lithosphere would increase the density and counteract the buoyancy effect of the higher temperatures. We obtain a thickness of 250 ± 30 km for the mantle transition zone, which is similar to the global average, but the velocity gradient between the 410 and 660 km discontinuities is less steep than in global models, such as PREM and IASP91. We also obtain velocity jumps of between 0.16 ± 0.1 and 0.21 ± 0.1 km/s across the 410 km discontinuity. Our results suggest that there may be a thermal or chemical anomaly in the mantle transition zone, or alternatively that the shear wave velocity structure of the transition zone in global reference models needs to be refined. Overall, our seismic models provide little support for an upper mantle source of buoyancy for the unusually high elevation of the Kalahari craton, and hence the southern African portion of the African Superswell.

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Acknowledgments

The authors wish to thank Eric Matzel for many discussions about conjugate gradient waveform inversion methodology and for use of his code, and an anonymous reviewer for helpful comments. We would also like to thank Bill Griffin for providing information on xenoliths recovered from the Kalahari craton. We thank the University of the Witwatersrand, South Africa, for providing administrative support and the Council for Geoscience, South Africa, for granting the corresponding author study leave to spend time at the University of Texas at Austin. This research was supported by NSF grants EAR-0440222, EAR-0440032, and OISE 0530062. Language editing and styling was done by Beverlie Davies.

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Authors and Affiliations

  1. Seismology Unit, Council for Geoscience, Private Bag X112, Pretoria, 0001, South Africa

    Martin B. C. Brandt

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

    Martin B. C. Brandt & Paul H. G. M. Dirks

  3. Jackson School of Geosciences, University of Texas at Austin, Austin, TX, USA

    Stephen P. Grand

  4. Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania, USA

    Andrew A. Nyblade

  5. School of Earth and Environmental Sciences, James Cook University, Townsville, Australia

    Paul H. G. M. Dirks

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  1. Martin B. C. Brandt
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Correspondence to Martin B. C. Brandt.

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Brandt, M.B.C., Grand, S.P., Nyblade, A.A. et al. Upper Mantle Seismic Structure Beneath Southern Africa: Constraints on the Buoyancy Supporting the African Superswell. Pure Appl. Geophys. 169, 595–614 (2012). https://doi.org/10.1007/s00024-011-0361-8

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