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Fig. 2.4
Lithospheric model (in
pale grey
) N-S cross-section
from southern Africa (Agulhas
Fault) to Sahara (Chad), showing
the Central and Southern African
Shields capped by a sub-
continental Cretaceous Kalahari
peneplain. Effective elastic
thicknesses of the lithosphere
(Te; in km), shown in orange
lines, are taken from Hartley and
Allan (
1994
) and Doucoure and
de Wit (
1996
), derived using the
same techniques through spectral
analysis
Shortly after Africa emerged out of Pangea/Gondwana,
over a period of some 80 million years (ca. 200-120 Ma) that
heralded the opening of the Indian and South Atlantic
Oceans, the conterminous Central and Southern African
Shields were uplifted during the Kalahari epeirogeny
(de Wit
2007
), and then peneplained to form the Kalahari
Plateau (Fig.
2.4
). During the end-Mesozoic to Cenozoic this
plateau became covered by sands and a hard-cap of calcretes
and silcretes (the Kalahari Group). Above the CB, this sur-
Topic), but across the Southern African Shield this surface
remains mostly intact, probably because,
2.3
Regional Geology and Geochronology
of Central Africa
The CB of central Africa is completely surrounded by
peneplained Precambrian basement, including Archean cra-
tonic blocks and numerous Paleo- to Meso-Proterozoic
mobile belts (Figs.
2.2
,
2.3
,
2.4
and
2.5
). This complex
assemblage comprises the Kasai, Cuango, Ntem, Bouca,
Mboumou-Uganda and Tanzanian Cratons (ca. 2.5-3.5 Ga;
Cahen et al.
1984
), the CAMB (ca. 2.0-2.3 Ga; Carvalho
et al.
2000
), and the West Central African Mobile Belt
(WCAMB; ca. 2.0-2.5 Ga; Feybesse et al.
1998
).
These oldest blocks apparently all amalgamated during
Eburnian (in Africa; Trans-Amazonian in South America)
orogenesis, ca. 1.8-2.2 Ga, along: an east-west suture in
southern Uganda, cutting across the central Rwenzori
Mountains, and collectively known as the Rwenzori Fold
Belt (Tanner
1973
); the Rusizian and Ubendian-Usagaran
Belts in Uganda and western Tanzania (Lenoir et al.
1995
;
Tack et al.
2010
; Link et al.
2010
; Fernandez-Alonso et al.
2012
; Boniface et al.
2012
; Lawley et al.
2013
,
2014
), across
central Angola (Doucoure et al.
1999
; de Carvalho et al.
2000
; Jelsma et al.
2012
), and north-south along the South
Atlantic margin where it forms the basement to the
Neoproterozoic West Congo Belt and its Braziliano counter-
part in Brazil (Feybesse et al.
1998
; Tack et al.
2001
; Toteu
et al.
2001
; Pedrosa-Soares et al.
2008
).
We have drawn roughly defined Eburnian sutures along
the CAMB and WCAMB, where major continental accretion
may have occurred to consolidate the SWCS (Figs.
2.3
and
2.6
). There are widespread areas in the CAMB (e.g.
Central Shield Zone of Carvalho et al.
2000
; Jelsma et al.
2012
) where regions of Eburnian deformation, granulite-
grade metamorphism and crustal melting have significantly
affected Archean granite-greenstone fragments. This broad
largely,
it
is
directly underlain by Precambrian basement rocks.
Whilst a large amount of deep local geophysical and
geochemical data has enabled a firm understanding of the
3-D structure beneath the Southern African Shield (e.g.
Evans et al.
2011
; Bell et al.
2003
; Stankiewicz and de Wit
2013
, and references therein), including variations in depth
to Moho and the bottom of the lithosphere, this is not the
case for the CAS (e.g. Begg et al.
2009
; Buiter et al.,
2012
;
see also Raveloson et al., Chap.
1
, this Topic). Geophysical
data is particularly scarce across the CS. Beneath the CB,
there is no hard information about variations in depth to
Moho or the lithosphere structure and its thickness, other
than by inference from (mostly) Cretaceous diamondiferous
kimberlites that have intersected the mantle lithosphere
Topic); and from low resolution seismic tomography that
suggest the thickness of the lithosphere beneath the cratons
embedded in the CS varies between 130-200 km (e.g.
enough to contain the transition into the general lithospheric
mantle stability field of diamonds (ca.
120 km). But the
geophysics has not as yet tested potential lithospheric
variations across the boundaries between, for example, the
three central Congolese Cratons of the CS.
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