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plateau, which took place 45 million years ago may be related to an older plume,
which lies under Lake Victoria. According to them the later eruptions in southern
Ethiopia (19
12 millions year ago) might be related to the spreading in
uence of
-
the more northerly Afar plume.
The volcanism of alkalic rocks without rifting and associated with topographic
swells and ocean island chains of Africa was considered to be related to separate
plumes (Wilson and Giraud 1993; Emerick and Duncan 1982). Burke (quoted in
Ebinger and Sleep 1998) considered that the Cenozoic volcanism and high eleva-
tion in this region is related to stationary position of the African plate relative to
mantle circulation since 35 Ma ago. This resulted in the penetration of 40 mantle
plumes without creation of hot-spot tracks. The swells at Hoggar, Tibesti, Darfur
and Adamawa and eruptive centres of the Cenozoic period lie on a Mesozoic and a
Mesozoic-Palaeocene rift systems between East African and Ethiopian plateaux
(Fig. 14.5 ). Wilson and Giraud (1993) suggested preferential melting of mantle,
which was metasomatized during the breakup of Gondwana in the presence of
many plumes during the Mesozoic. Ebinger and Sleep (1998) suggested that
seismic velocity and geochemical anomalies below Red Sea, Afar and Eastern Rift
(Prodehl and Mechi 1991; Green et al. 1991). According to Ebinger and Sleep,
tomographic model suggested that the lithosphere beneath the cratonic core was
more than 200 km thick.
Very little is known of the upper mantle structure beneath the Ethiopian plateau
away from the rifts. The high 3 He/ 4 He ratios throughout a 2,000-km-wide region
centred on the Ethiopian plateau indicated the presence of a broad mantle thermal
anomaly. According to Ebinger and Sleep (1998) the largest geoid anomalies
coincided with the Ethiopian and east African plateau, where more than
8
10 5 km 3 of primary basaltic material erupted since 45 Ma ago (Latin et al.
1993). Cenozoic
×
flood basaltic magmatism took place in the Ethiopian plateau
Lava flows
Ocean
Continental
lithosphere
Zone of
melting
Plume head
Mantle
Plume
Hot thermal
boundary layer
Core
Fig. 14.6 Ebinger and Sleep
s proposed model showing how a mantle plume interacting with
topography on the base of the lithosphere, might explain scattered volcanism across a very broad
region of east Africa (see also Davies 1998)
'
 
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