Geology Reference
In-Depth Information
basins like the Congo. A larger proporion,
though, are of erosional origin like those des-
cribed from East Africa. These plains have been
eroded across a wide range ofdifferent rock ypes
and sructures ranging from the Precambrian
crystalline rocks of Tanzania to late Cretaceous
and Eocene sands of Nigeria and the Ivory Coast
which were planed during the African early Ter-
iary cycle of erosion (King, 1967).
beween the westen and easten arms of the Rift
Valley. Burke & Dewey (1974) and McConnell
(1977) have noted that Africa has been staionary
for the past 25-30 myr during which ime swells
and basins, rifing and magneism have all
developed.
This mechanism, as the cause of the East Afri-
can Rift Valley, differs from the commonly held
noion that the Rift Valley is part of the global
system associated with coninental breakup
(Windley, 1977).
n explanaion of the structural patten
in East Africa
Formaion of the East African rift
system
The age of the rifing and associated volcanism in
the Gregoy Rift in southen Kenya has been
determined by Crossley (1979). There acivity
commenced in the mid-Miocene (15-12 myr BP ) ,
significantly later than in the northen secion of
the rift (30-20 myr BP ) , and earlier than in the
Tanzanian deormaion to the south which is
attributed to the Pliocene (Downie & Wilkinson,
1972). Major rejuvenaions in southen Kenya
(5-2.5 and 0.6-0 myr BP ) do not correspond with
the 2-1.5 myr BP movements recorded in north-
ern Kenya (Crossley, 1979); how e ver, they appear
to be contemporaneous with the Tanzanian rift-
ing. This last Pleistocene phase created much of
the present day rift topography resuling in the
Basin and swell structures together with rift fault-
ing are both widely recognised on a large scale in
Africa today (Figures 2.8 and 2.10). These are
apparently recurrent sructural patterns and
within East Africa are repeated on a smaller scale.
It has been proposed that the sructures are
related through a common cause (Burke &
Dewey, 1974; McConnell, 1977).
The basin and swell structure develops when a
coninental plate is staionary over rising mantle
plumes or hot spots causing melting in the upper
mantle and lower crust. The consequent increase
in volume results in a swell and stretches the outer
brittle skin of the crust, which in some cases is
relieved by rifing and possibly volcanism (Figure
2.10). Basins occupy the areas between the swells.
The Lake Victoria basin, for example, lies
Swell / upwar p
Swell/ upwarp
Basin / downwarp
i
i
Precambri.n rocks
GREGORY RIFT VALLEY
WESTERN RIFT VALLEY
(m)
6000
Kilimanjuo
Ml.
Rwanda
Burundi
Lake Victoria Basin
Serengeti
Pit
ins
2000
Figure 2.10. Generalised secion across East Africa showing the effect of warping and
subsequent tensional auling.
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