Brandt, Martin B.C. 2011. Imaging the African Superplume – Upper mantle,
Tomography and Moment tensor. Ph.D. thesis, Faculty of Science, University of the
Witwatersrand, Johannesburg, South Africa.
The African Superplume, African Superswell and East African Rift System are
amongst the most prominent geophysical features on Earth, but the structure,
evolution and interaction between these features is controversial. In my thesis I
conducted a range of investigations in an effort to better understand these issues. The
thesis presents the investigations into the structure and expressions of these features.
These include:
(I) A study of the upper mantle shear velocity structure beneath southern Africa to
investigate the source of the buoyancy that has powered the Superswell;
(II) Statistical hypothesis testing of middle-mantle shear velocity tomographic models
to evaluate evidence for links between the Superplume and low velocity features
in/near the transition zone; and
(III) Computation of three new regional moment tensors for South Africa to assess
crustal stress in the Kalahari craton, and its link with mantle structure and dynamics.
Waveform data were obtained for the study on the upper mantle shear velocity
structure and the moment tensor inversions from the Southern African Seismic
Experiment Kaapvaal craton array. For the statistical hypothesis testing on global
tomography images, new travel-time data from both global and AfricaArray stations
were added to Grand’s global shear velocity data set.
The principal findings of this study are summarized below.
I. The upper mantle shear velocity structure beneath the Kalahari craton is similar to
that of other shields, except for slightly slower velocities from 110–220 km depth.
The difference may be due to higher temperatures or a decrease in magnesium
number (Mg#). If the slower velocities in the deep lithosphere are due solely to a
temperature anomaly, then slightly less than half of the unusually high elevation of
the Kalahari craton can be explained by shallow buoyancy from a depleted hot
lithosphere. Decreasing the Mg# of the lower lithosphere would increase density and
counteract higher temperatures. If an excess temperature of 90 K over a 110 km depth
range and a corresponding decrease in Mg# of -2 between the Kalahari and the other
cratons are assumed, this would match the seismic velocity difference but would
result in essentially no buoyancy difference. We conclude that the high elevation of
the Kalahari craton can only be partially supported by shallow mantle buoyancy and
must have a deeper source. We determined a thickness of 250±30 km for the mantle transition zone below eastern
southern Africa, which is similar to the global average, but the corresponding velocity
gradient is less steep than in standard global models (PREM and IASP91). Velocity
jumps of 0.16±0.1 km/s (eastern) and 0.21±0.1 km/s (central) across the 410 km
discontinuity were found. Our results indicate a thermal or chemical anomaly in the
mantle transition zone, but this cannot be quantified due to uncertainty.
II. Statistical hypothesis testing on our global tomography images indicated that the
African Superplume rises from the core-mantle boundary to at least 1150 km depth,
and the upper mantle slow-velocity anomaly extends from the base of the lithosphere
to below the mantle transition zone. The model that links the African Superplume
with the slow-velocity anomaly in the upper mantle under eastern Africa has an equal
probability to an alternative hypothesis with a thin slow-velocity “obstruction zone” at
850 to 1000 km depth.
III. Finally, we calculated three regional moment tensors for South Africa and made
progress towards resolving the discrepancy between the local and moment magnitudes
we observe for the region. Moment tensors/focal mechanisms in southern Africa
change from normal faulting (extension) in the northeast near the East African Rift to
strike-slip faulting in the southwest. This confirms previous studies stating that not
only eastern Africa, but also southern Africa is being actively uplifted by lithospheric
modification at its base and/or the African Superplume.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/11988 |
| Date | 01 October 2012 |
| Creators | Brandt, Martin Barend Christopher |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf, application/pdf, application/pdf, application/pdf, application/pdf |
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