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sandstones at the base of Unit B, together with the overlying
Carboniferous-Permian diamictites and black shales, have
been deformed only by the late Paleozoic-early Mesozoic
deformation. This is well illustrated by the N-trending seis-
mic profile L50 in the centre of the basin (Fig. 6.4 )by
structures such as flexures, anticlines, synclines and thrust
faults, although it would be difficult to distinguish these
from syn-depositional features related to slumping and gla-
cial processes, or even salt tectonics. The first deformation
event occurred prior to the deposition Sequence 4 and the
second affects it, mainly by reactivating the structures
underlying the lower unconformity. Each deformation
event is characterized by an erosional phase followed by
renewed sedimentation with by top-lap structures. These
deformational events are interpreted as representing intra-
plate compressional tectonic episodes related to far-field
tectonic stresses generated by Pan African collisional tec-
tonics flanking the margin of the Congo Shield for the first
event, and at the margin of Gondwana (during the formation
of the Cape Fold Belt at ca. 250 Ma) for the second one (e.g.
Daly et al. 1992 ; Kadima et al. 2011a ; Kipata et al. 2013 .
However, if the Sequence 4 also includes the Triassic, then
the second deformation would be post-Triassic and therefore
cannot be related to the Cape Fold Belt and would require a
different interpretation of the far field stresses (Linol 2013 ;
see also Linol et al., Chap. 11 , this Topic).
By contrast, Heine et al. ( 2008 ), suggested that subsi-
dence might be due to dynamic topography, consistent
with Sahagian ( 1993 ) and Burke and Gunnell ( 2008 ), who
prefer models in which the CB acquired its modern shape
through mantle (plume) driven uplift of swells surrounding
the basin.
Others prefer a model in which recent subsidence of the
CB is controlled by a late phase of the ca. 700 Ma post-rift
thermal subsidence following the Neoproterozoic extension
(e.g. Armitage and Allen 2010 ; Crosby et al. 2010 ; Kadima
et al. 2011b ), while Buiter et al. ( 2012 ) suggests that the
observed negative gravity anomaly across the CB is mainly
due to the thick (up to 9 km) sedimentary units in the basin,
and that the sub-lithospheric mantle structures did not appar-
ently play a role in the recent subsidence of the basin.
To further test a simple long lived thermal history for the
CB, Kadima et al. ( 2011b ) backstripped the effects of the CB
sediments and noted a residual NW-SE positive and narrow
gravity anomaly across the central CB, which they
interpreted as the remaining crustal thinning associated
with the Neoproterozoic rift that initiated the CB. The
paleo-rift is aligned along the Mbandaka-1 and Dekese posi-
tive Bouguer anomalies of Jones et al. ( 1960 ) and Kadima
et al. ( 2011a ). Assuming that isostasy is governed by crustal
necking and flexural response to sediment loads, Kadima
et al. ( 2011b ) obtained a best fit to the residual gravity with a
necking depth of 10 km and an equivalent lithospheric
elastic thickness of 100 km. Consequently, the linear anom-
aly is interpreted as an old tectono-thermal heritage of the
initial Neoproterozoic rifting, when denser mantle associ-
ated with Moho uplift invaded the necking zone and has
remained in place ever since (Kadima et al. 2011b ). 2D
coupled gravity and magnetic models were constructed
across this elongated NW-trending positive residual ano-
maly, along the seismic lines constrained by well data, to
test the hypothesis for a rifting process with extensional
magmatism activity prior to the basin formation (Kadima
et al. 2011a ). The main results are as follows:
￿ Modeling of two SW-NE profiles, one along the Congo
River and passing close to the Mbandaka-1 well and one
passing through the Dekese well is consistent with a
lateral change in basement density along the trend of the
profile, suggesting that the elongate positive NE-tending
residual gravity anomaly zone could correspond to a deep
crustal discontinuity injected by mafic magma during the
Neoproterozoic rifting.
￿ The modelling is also consistent with (but does not prove)
the presence of evaporite sequences in some of the deeper
units in lateral continuity with evaporite showings
6.3.5 Gravity Anomalies and Modelling
The CB is associated with a large-scale and pronounced
negative free-air gravity anomaly (Jones et al. 1960 ;
Sandwell and Smith 1997 ; Tapley et al. 2005 ). Initiation of
the CB is frequently related to the development of a NW-SE
Neoproterozoic rift and a large part of its subsequent subsi-
dence to post-rift thermal relaxation (e.g. Kadima et al.
2011a , b ; Buiter et al. 2012 ).
The large scale and amplitude of the gravity anomaly has
led various researchers of the CB to propose very different
geodynamic models to account for subsidence of the CB.
Hartley and Allen ( 1994 ) and Hartley et al. ( 1996 ), for
example, suggested the negative Bouguer gravity could be
due to the combined effect of lower density sediments in the
basin and higher density material in the lithospheric mantle
to isostatically compensate for the sediments; and indeed,
Downey and Gurnis ( 2009 ) show numerically that the topog-
raphy and negative free-air gravity data over the basin can be
explained by a high-density body within the deeper
lithosphere.
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