Thermal Regime and Evolution of the Congo Basin as an Intracratonic Basin - Geology and Resource Potential of the Congo Basin - page 225

variation of elevation and eustatic sea-level changes) and the
original tectonic subsidence model of McKenzie ( 1978 )
where the rifting is

0

Tectonic Subsidence

250

. Here, we use a modified
stratigraphy to take into account the recent improvements as
well as the paleoelevation and first-order eustatisc history
proposed by Linol et al. ( 2013c , d ) a more elaborated tech-
nique of backstripping that takes into account the
decompaction of sediment and its effects on density and
thermal conductivity, and a numerical model of thermal
evolution that accounts for a finite duration rifting and the
effects of the sediment blanketing on this evolution
(Lucazeau and Le Douaran 1985 ). This model is described
in Appendix 1. As the stratigraphy given by Linol et al.
( 2013c , d ) is limited to the Paleozoic, we extrapolated the
stratigraphy of the Proterozoic and Lower Phanerozoic from
the work of Kadima Kabongo et al. ( 2011a ). This new
procedure leads to a final value of the tectonic subsidence
between 1,650 and 1,750 m (Fig. 12.4 ), which can be
interpreted with the rifting of the lithosphere between 700
and 630 Ma by the same

“ instantaneous ”

500

750

1000

1.24/1.15

1250

MBD1

GLS1

1500

1750

SMB1

DKS1

2000

700 650 600 550 500 450 400 350 300 250 200 150 100 50 0

Time (Ma)

Fig. 12.5 Tectonic Subsidence of the basement for the four deep wells
drilled in the Intracratonic Congo Basin. A two-stages rifting model is
also shown with a thinning factor ʲ ¼ 1.24 during the Neo-Proterozoic

and 1.15 during reactivation in Lower Triasic

factor of 1.4 that we used in the
previous study. As the subsidence in the oldest period is only
constrained by extrapolation of the stratigraphy on seismic
lines, it is difficult to promote one of the two models with
lithosphere thickness of 200 or 250 km that both produce the
expected tectonic subsidence (Fig. 12.4 ). On the other hand,
a 300 km thick lithosphere results in a time delay of about
100 Ma before the onset of subsidence that would not sup-
port the existence of a pre-PanAfrican sequence (Fig. 12.4 ).
The initial basin forming extensional event can be related
to rifting (Fig. 12.1 ) and the formation of aulocogens in the
late Neo-Proterozoic, such as the NE-trending Sangha
aulocogen (Kadima Kabongo et al. 2011a ). The relationship
between subsidence initiation and landward dipping exten-
sional structures observed here is similar to intracratonic
basins of North America (Illinois and Michigan Basins),
South America (Parana Basin) and the West Siberian Basin
(Armitage and Allen 2010 ; Allen and Armitage 2012 ).

ʲ

uplift (Linol 2013 , unpublished; Linol et al. 2013c , d ), as we
discuss in Sect. 12.4 , is potentially is a good proxy for the
contributions of such mantle perturbations to the subsidence
evolution. The long-term subsidence is more likely caused
by the thermal relaxation of a thick lithosphere (Armitage
and Allen 2010 ; Crosby et al. 2010 ; Kadima Kabongo et al.
2011b ). In Atlantic-type margins or rifted-basin like the
North-Sea where the lithosphere is thin, the initial subsi-
dence is high and the thermal subsidence is short-lived.
The CB however formed within a region of thick litho-
sphere. The duration of the subsidence is therefore related
to the diffusion time constant ˄ of the lithosphere:

L 2
ˀ

˄ ¼ ð 12 2 Þ

:

2

ʺ

where L is the thickness of the lithosphere and the thermal

diffusivity. The time constant of the subsidence for a 200 km
thick lithosphere is therefore four times that of a 100 km
thick lithosphere, which would explain the long-lived
subsidence.
Two recent works analysed the tectonic subsidence of the
CB. Crosby et al. ( 2010 ) showed that the tectonic subsidence
inferred from the M

ʺ

12.4 Subsidence Anomalies

The subsidence of the CB shows several anomalies that
cannot be obviously related to a Neo-Proterozoic rifting
(Fig. 12.4 ). A significant increase of the subsidence during
the Permian to Triassic can be related to a second period of
extension that would be coeval with the formation of early
Permian rift basins along the northern African margin
(Guiraud et al. 2005 ). This rifting event may have led to a
general tensional regime across Gondwana (Guiraud et al.
2005 ), and hence altering subsidence within a suite of
intracratonic basins including the CB and Berkine Basin
(Algeria) to the north (Boote et al. 1998 ; Yahi et al. 2001 ).
In this case, the subsidence of the CB can be decomposed in

bandaka 1 and Gilson 1 oil wells
(Fig. 12.1 ) can be well explained by the Cambrian rifting
(560-490 Ma) of a 200 km thick lithosphere and a litho-
spheric thinning factor

'

1.2. They used a finite difference
numerical model that includes the cooling of the lithosphere
when the lithosphere is thinning. Kadima Kabongo et al.
( 2011b ) estimated a slightly higher

ʲ '

1.4, in agreement
with the crustal thinning inferred from their gravity study.
However, they used a different stratigraphic interpretation
based on Kadima Kabongo et al. ( 2011a ), a simplified
backstripping of the sediment load (constant densities, no

ʲ '