Geology Reference
In-Depth Information
Fig. 1.10
Group velocity curves
for crustal structure for the
portions of the Congo Basin
where crustal thinning may exist
(Kadima et al.
2011b
). The
black
line
shows the average group
velocities from our model for the
region of thin crust according to
Kadima et al. (
2011b
). The
colored lines
show synthetic
curves for crustal models with the
Moho at the depths noted on each
curve
5.0
4.8
4.6
4.4
4.2
4.0
3.8
3.6
3.4
Syn: 20 km
Syn: 22 km
Syn: 25 km
Syn: 27 km
Syn: 30 km
Syn: 32 km
Syn: 35 km
Syn: 37 km
Syn: 40 km
data
3.2
3.0
2.8
2.6
2.4
2.2
2.0
10
20
30
40
50
60
70
Period(s)
The proposal that a mobile belt may extend beneath the
northern part of the Congo Basin has important implications
for the origin of the basin, as discussed below. Many
geodynamic models invoking density contrasts within or
below the mantle lithosphere (e.g. Hartley and Allen
1994
;
Downey and Gurnis
2009
; Crosby et al.
2010
; Forte et al.
2010
) assume a uniformly thick cratonic lithosphere beneath
the basin. If the northern portion of the Congo Basin is
underlain by Proterozoic lithosphere and is surrounded by
Archean lithosphere to the east, south and north, then there
could exist substantial heterogeneity in lithospheric thick-
ness across the region. Continental-scale tomographic
models typically have lateral resolution of no better than
300-500 km at upper mantle depths
need to explain why a Neoproterozoic rift would form in
the interior of an Archean craton (Fig.
1.11f
). Within both
the Cenozoic East African rift system and the NE Karoo rift
system there are good examples of rift basins forming along
the margins of thick Archean lithosphere. For example, in
northern and western Tanzania, the margin of the Tanzania
Craton is fractured by many Cenozoic rift faults, and a
number of half-graben basins have formed (i.e. Lake
Manyara, Lake Eyasi basins). From the Carboniferous-
Triassic Karoo system, a good example is the Zambezi rift
that formed along the northern margin of the Zimbabwe
Craton. These rifts provide plausible analogs for understand-
ing the development of a Neoproterozoic rift under the
Congo Basin within a region of cratonic lithosphere.
The tomographic models in Figs.
1.2
and
1.11
are consis-
tent with the existence of Proterozoic lithosphere beneath the
northeastern side of the Congo Basin, where lower velocities
can be seen in the 50-100 km depth interval. However, the
models do not support the proposal that Proterozoic litho-
sphere extends beneath the entire northern portion of the
basin. In Figs.
1.2
and
1.11
, there is a region of faster
velocities beneath the northwestern side of the basin,
suggesting that the lithosphere there may be cratonic.
Hence, while Proterozoic lithosphere likely extends to the
NW under the northeastern side of the basin, it probably does
not extend all
100 km, and there-
fore variations in lithospheric thickness associated with a
Proterozoic mobile belt under the northern part of the Congo
Basin may not be resolved in those models.
A second geodynamic implication of the proposal that
Proterozoic lithosphere lies beneath the northern part of the
Congo Basin concerns the Neoproterozoic rifting event that
may have initiated basin subsidence. The location of the rifts
are shown in Fig.
1.11
and are found 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
towards the northeast. The rifts may have formed along the
northern border of the SWCS, as opposed to within the
middle of a single large Archean craton, alleviating the
>
the way across the basin to southern
Cameroon.
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