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 4

since the Early Paleozoic compressional event (Kadima
et al. 2011b ), as expected for post-rift thermal relaxation of
the lithosphere.

relationship between gravity and geoid fields for a number of
intra-cratonic basins worldwide. Crosby et al. ( 2010 )
modeled the subsidence and gravity signature of the basin
using a lithospheric thickness of 220 km interpreted from
seismic tomography images, and argued that the long wave-
length gravity signature is likely the result of recent convec-
tive drawdown beneath the basin in response to the
surrounding upwellings, consistent with the mantle convec-
tion model of Forte et al. ( 2010 ). However, based on a
variety of seismic models (see below), Buiter et al. ( 2012 )
conclude that the seismological results do not provide
supporting evidence for a first-order role of the sub-
lithospheric mantle in the recent subsidence of the Congo
Basin. In contrast to these models, Kadima et al. ( 2011b )
argue that if the effects of the low-density sediments are
removed from the gravity field, then the residual gravity
anomaly can be attributed to 8-10 km of relative crustal
thinning beneath the deepest portions of the basin.
A review of both continental and global-scale seismic
tomography models that inculcate the Congo Basin has
been recently published by Buiter et al.( 2012 ). Using simple
averaging, the authors assessed uncertainties in lithospheric
thickness estimates from a range of geophysical models, and
also computed mean velocities, along with the associated
standard deviations, to investigate the role of the
sublithospheric mantle in the subsidence of the basin. The
review showed generally thick (~200 km) lithosphere
beneath much of the basin, but suggested that the uncertain-
ties between the different tomographic models provide little
evidence of upwelling on the flanks of the basin (Buiter et al.
2012 ). We briefly summarize the key features from several
of the continental-scale models of Africa.
All continental-scale seismic tomography models show
fast, craton-like upper mantle structures beneath some parts
or else most of the Congo Basin (e.g. Fishwick 2010 ;
Priestley et al. 2008 ; Ritsema and van Heijst 2000 ; Pasyanos
and Nyblade 2007 ; Sebai et al. 2006 ). The models by
Fishwick ( 2010 ) and Priestley et al. ( 2008 ) use the same
basic technique, comprising waveform inversion
measurements for fundamental and first four higher mode
Rayleigh waves. Sebai et al. ( 2006 ) also measured and
inverted phase velocities for fundamental and higher mode
Rayleigh waves, in addition to Love waves. Ritsema and van
Heijst ( 2000 ) modeled phase velocity measurements from
fundamental mode Rayleigh waves only. In contrast, the
model by Pasyanos and Nyblade ( 2007 ) is based on group
velocity measurements from fundamental mode Rayleigh
waves.
At the shallowest depths (100-200 km depth) displayed
in the models constructed using phase velocities or wave-
form inversion techniques, the velocity structure is
reasonably similar, with fast velocities found beneath much

1.3 Previous Geophysical Studies

A number of geophysical techniques have been applied to
the Congo Basin and surrounding regions in order to
improve knowledge of the subsurface structure. Petroleum
exploration of the basin led to the acquisition of the seismic
reflection profiles mentioned previously, as well as a number
of gravity and aeromagnetic surveys. More recently, satellite
gravity missions such as the Gravity Recovery and Climate
Experiment (GRACE), have provided global knowledge of
the gravity and geoid fields. Interpretation of the seismic
reflection data identified a series of sub-basins separated by
basement highs (e.g. Lawrence and Makazu 1988 ; Daly et al.
1992 ), however a re-interpretation of the seismic profiles by
Kadima et al. ( 2011a ) suggests that salt structures may be
present in the central basin and therefore the basement highs
might not be as pronounced as originally thought. These
various interpretations remain to be tested.
Complementing the seismic reflection data, Kadima et al.
( 2011a ) presented results from an aeromagnetic survey over
the central and northern parts of the basin and a Bouguer
anomaly map for the entire basin (Fig. 1.1 ). The aeromag-
netic survey was carried out in 1984, and shows low mag-
netic gradients in the central part of the basin corresponding
to deep basement, and higher magnetic gradients to the
northeast and southwest, where the basement is interpreted
to be less deep (Kadima et al. 2011a ). The gravity database
used by Kadima et al. ( 2011a ) comes from the Bureau
Gravimetrique International (BGI) in Toulouse, France,
and incorporates data from a variety of sources. The
Bouguer anomaly map is dominated by NW-SE trending
anomalies.
At a subcontinental scale the most striking feature in the
gravity data is a long wavelength negative free air gravity
anomaly (~ -40mGal) that coincides with the basin. Hartley
and Allen ( 1994 ) proposed that this anomaly was unlikely to
be supported by variations in crustal structure, and Hartley
et al. ( 1996 ) suggested that the correlations between gravity
anomalies and topography are indicative of convective pro-
cesses in the underlying mantle. A number of recent studies
have discussed the cause of the long wavelength gravity
anomaly and the potential relationship to basin formation.
Downey and Gurnis ( 2009 ) suggested that there must be an
anomalously dense body, possibly made of eclogite, within
the mantle beneath the basin. A similar interpretation involv-
ing a dense body in the lower crust or upper mantle was
proposed by Braitenberg and Ebbing ( 2009 ) to explain the