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after magmatism and uplift and propagation of the rift towards south in the east
African region.
Episodic extension in Red Sea and Gulf of Aden captured ponded melt and
resulted in pulses of basaltic magmatism in east Africa (Ebinger and Sleep 1998).
Reactivation of pre-plume basins and initial extensions in the northern Kenyan Rift
about 25 Ma ago probably created a conduit for the distribution of the plume
material towards south beneath the eastern and western rift systems (Figs.
14.5
and
14.7
a, b). They thought that this additional extension perhaps caused outward
ow
as far south as Madagascar and the western rift, where magmatic centres are situated
near the Mesozoic rifts.
Ebinger and Sleep (1998) thought that the steep gradient at the lithosphere
-
asthenosphere boundary along the Tanzanian Craton and mobile belt boundary per-
haps resulted in
flow, which explains the generation of more than 30 Ma old
carbonatitic and kimberlititc magmas obtained from small melt volume at greater
depths (Boyd and Gurney 1986; Dawson and Smith 1988). Ebinger and Sleep con-
sidered that the reliefs present in the lithosphere and asthenosphere boundary before
the emplacement of the plume materials probably diverted it and helped decompres-
sion melting along the craton margins preserving their thick strong cores (Fig.
14.7
b).
The scheme of Ebinger and Sleep implied the rejuvenation and thermal subsi-
dence of continental rifts. They further thought that ponded plume material con-
tributed to regional uplift, but the sills intruding into sedimentary strata, which
underplated at the base of the previously thin crust could locally augment or inhibit
the uplift. This would however, depend on density contrast. Genik (1992) thought
that many Mesozoic
Palaeozoic rift basins lying along the paths of plume material
show late Cenozoic reactivation and igneous intrusions. Cof
-
n et al. (1986) opined
the presence of anomalous upper mantle velocity of 7.8 km per second below the
western Indian Ocean basin, which suggested elevated mantle temperature above
the south-eastern lobe (Fig.
14.7
).
The studies of Ebinger and Sleep suggested that the location of continental
ood
basalts perhaps did not coincide with the centre of plume head and the presence of
several discrete magmatic provinces could be related to a single plume. Their model
of Ethiopian plume also supported a plume origin for the east African rift systems.
Rogers et al. (1998) studied volcanism in the Eastern Birunga province. They
noted that the magmas were derived from the mantle lithosphere with the source
age of 1 Ga and 0.5 Ga and the youngest ages corresponded to the deepest magma
sources. The magma production rate in Birunga was low (0.04 km
3
per year),
re
15 Ma) of the lithosphere by the East African plume.
They suggested that conductive heating and melting of the lithosphere at the vol-
atile-enriched mantle solidus resulted in the production of lava. According to the
model of Turner et al. (1996), the amount of melt produced, is represented by the
melt thickness, which depended on the duration of heating, the potential temper-
ature of the underlying mantle plume (T
p
) and the thickness of the lithosphere. A
melt thickness of 1
ecting prolong heating (10
-
2 km can be calculated for the Birunga province from the above
total volume estimates integrated over the outcrop area of the whole province
(
-
3,500 km
3
). If it is assumed that East Africa arrived over the plume during the
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