The Upper Mantle Seismic Velocity Structure of South-Central Africa and the Seismic Architecture of Precambrian Lithosphere Beneath the Congo Basin - Geology and Resource Potential of the Congo Basin - page 15

The difference in shear wave velocity in the uppermost
parts of the mantle between Archean and Proterozoic litho-
sphere upon which this interpretation is based is not neces-
sarily well understood. It is also noted that at the depths of
interest (i.e. 50-100 km) the Proterozoic mobile belts to the
south of the Congo Shield are characterized by slower
velocities than beneath the Archean cratons (Figs. 1.2 and
1.11 ). This pattern of slower shear wave velocities in the
uppermost part of the mantle beneath Proterozoic terrains is
a common feature of many tomographic images from Pre-
cambrian shields, although not all (Darbyshire and Eaton
2010 ).
Lastly, it is of interest to compare the Precambrian litho-
spheric architecture for the Congo Basin with the
Neoproterozoic Central super-basin of Australia (e.g. Walter
et al. 1995 ), which also formed in a region where several
Archean cratons amalgamated in the Proterozoic (e.g. Betts
and Giles 2006 ; Cawood and Korsch 2008 ) and where there
has since been a prolonged history of deformation. Similar
upper mantle structure is found beneath and surrounding this
basin; at 75 km depth slow velocities are observed beneath
the Proterozoic central region, while faster velocities are
observed beneath the Archean cratons themselves. By
125 km depth, fast velocities are observed beneath most of
the Precambrian Shield (e.g. Kaiho and Kennett 2000 ;
Fishwick and Reading 2008 ). The similarity in lithospheric
structure beneath the Congo Basin and central Australia
suggests that the slower uppermost mantle wave speeds
may reflect similar processes involved in the amalgamation
and subsequent deformation of these regions.

velocities. In both models a region of slower mantle
velocities is found beneath the northeastern portion of
the Congo Basin to the south and southwest of the
Bomu Craton. This region of slower mantle velocities
does not continue beneath the northwestern part of the
Congo Basin.
This finding suggests that the lithospheric mantle
beneath the Congo Basin may not be uniform, and that
Proterozoic lithosphere may lie beneath the northeastern
side of the basin. However, it does not support the exten-
sion of Proterozoic lithosphere beneath the entire north-
ern portion of the basin. Many geodynamic models
invoking density contrasts within or below the mantle
lithosphere assume a uniformly thick cratonic lithosphere
beneath the basin. If the northeastern portion of the
Congo Basin is underlain by Proterozoic lithosphere,
then there could exist heterogeneity in lithospheric thick-
ness across the region. A second geodynamic implication
concerns the Neoproterozoic rifting event that may have
initiated basin subsidence. The proposed location of the
rifts are in the region of our velocity model where the
velocities begin to change from faster to slower values
going from the center of the basin toward the northeast.
We suggest that the rifts may have formed along the
northeastern border of a southwestern Congo shield
(SWCS), as defined by Master ( 2004 ), as opposed to
within the middle of a greater Congo Craton, alleviating
the need to explain why a Neoproterozoic rift might form
in the interior of a large Archean Craton.

Acknowledgement We thank Maarten J. de Wit and an anonymous
reviewer for helpful comments. This study has been funded by the
National Science Foundation (Grants OISE-0530062, EAR-0440032,
EAR-0824781).

Summary and Conclusions
A new shear wave velocity model of the upper mantle for
central and southern Africa developed using an inversion
of group velocity measurements from fundamental mode
Rayleigh waves has been presented. The model shows
several areas with faster velocities that correspond to
Archean cratons (Bomu, Kaapvaal, Kasai, Ntem,
Tanzanian, Zimbabwean), and areas of slower velocities
marking Proterozoic mobile belts in between. The mobile
belts that exhibit slower upper mantle velocities include
the Mesoproterozoic Kibaran, Karagwe-Ankole, Irumide
and Southern Irumide Belts, and the Neoproterozoic
Damara Belt. The Paleoproterozoic Bangweulu Block in
northern Zambia also has slower upper mantle velocities
than the Archean cratons. Within the Congo Shield, the
fastest velocities are found beneath the southern and
central portions of the Congo Basin. The depth resolution
of the models precludes estimating the thickness of the
lithosphere.
Our model compares favorably with the tomographic
model from Fishwick ( 2010 ), which is representative of a
range of tomographic models using Rayleigh wave phase

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