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region although the Cameroon chain onshore witnessed discontinuous activity since
Late Cretaceous (Lee et al. 1994). Incomplete thermal equilibration of thinned
lithosphere beneath Late Cretaceous-Palaeocene rifts and Mesozoic margins might
have caused variation in depth to the 1,300
C isotherm at the base of the litho-
sphere beneath Africa 45 Ma ago and the basin became active during Palaeocene.
Sleep (1996) thought that the topographic relief at the lithosphere
°
asthenosphere
boundary before the arrival of the plume head probably resulted in the de
-
ection of
plume material away from its centre, as is expected from cratonic roots. In the
numerical models of Sleep, topographic relief at the lithosphere
asthenosphere
boundary disrupts the radial spread of buoyant plume material, which resulted in
ponding below the thinner lithosphere. According to Sleep et al. melting is
enhanced, where plume material cascades over steep relief at the lithosphere
-
-
asthenosphere boundary.
The distribution of plume material and melt beneath the central and northern
Africa was determined by Ebinger and Sleep (1998, Fig.
14.7
a, b). Assuming a
linear depth versus melting parameterization they predicted the
final thickness of
the liquid layer above the plume. In their model they did not distinguish between
plume head and tail material during slow movement of the African plate since
45 Ma ago. According to them the plume was initially centred below a site, where
earliest basalts were found and it moved to a spot, the projection of which on the
surface occurs at 36
°
E, 4
°
N. They further thought that 45 Ma ago, a circular plume
head (750 km radius) arrived at the base of lithosphere producing a thick low
density layer beneath the Ethiopian plate. They speculated a net upward movement
of linearly
flowing plume material with steep to vertical gradient having increasing
vertical velocity. This was associated with fusion related to pressure release
(Fig.
14.7
b). They further envisaged that pre-existing structural features might have
controlled location of eruptive centres. About 45 Ma ago, plume materials moved
an additional 500
800 km southward and eastward to a region of thin lithosphere
along the Indian Ocean margin and beneath the Arabian peninsula after Mesozoic
and Palaeogene rifts. They further speculated that between 35 and 10 Ma ago, a
narrow tongue of plume material moved upward beneath the central African rift
zone reaching the western Adamawa plateau 5 Ma ago.
The model of Ebinger and Sleep took into account thermal thinning of the
lithosphere above the plume material. They thought that the newly thinned litho-
sphere might have supplied melt from the eastern limb into the axis of the Red Sea
and Aden Rifts and also the adjoining margins. They further thought that these
extension along the Arabian margins more than 30 Ma ago perhaps triggered
decompression melting of ponded plume material, which led to widespread mag-
matism along the Arabian margin. The separation of Red Sea, Aden and East
African extension in Afar possibly resulted in melting due to pressure release rather
than at the centre of the plume con
-
guration. Lateral density variation within the
upper mantle should generate extensional stresses, which might have been large
enough to initiate rifting. They further established that adiabatic decompression
melting beneath the rift should enhance
flow of plume material and rift propagation.
Ebinger and Sleep (1998) thought that these mechanism might explain extension
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