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(1) regions associated with rifting and extension such as the NNE rift belt running
though central Europe (including the Massif Central, the Rhenish Massif and
the Rhine Graben), the Eger Graben and the Straits of Sicily,
(2) regions associated with subduction, such as western Italy and the Aeolian
Islands,
(3) back-arc basins, such as the Pannonian basin, and
(4) hotspots, such as the Canary Islands, Madeira Islands and possibly Mt. Etna.
In contrast to three dimensional models of mantle convection, which showed
downwelling in sheets and upwelling as columns, the LVA appeared to be sheet-
like stalling to NNE and dipping to west (Hoernle et al. 1995). They thought that
planar upwelling is consistent with the large-scale tectonic features, in particular the
NNE-trending rift belt extending through central Europe and Western Mediterra-
nean and with the distribution of volcanism in the area above the LVA. They
thought that the westward dip of the LVA may re
ect a combination of eastward
asthenospheric
flow and lithospheric drag. The areal extent of this upwelling
sheet along the base of the lithosphere (2,500
4,000 km) is about twice that
estimated for plume heads in the mantle. For example, White and McKenzie (1989)
proposed that plume consisted of a narrow (150
×
-
200 km across) central column,
and a wide (1,000
-
2,000 km diameter) mushroom-shaped head of anomalously hot
mantle (100
1,200 km diameter) had
been predicted for the plumes originating from the core-mantle boundary. Such
plume heads produced rocks with high
3
He/
4
He ratios. The volcanic rocks from Mt.
Etna and the Canarian Islands had relatively low-He isotopic ratios. In case of the
LVA from this region, the seismic tomography data suggested that this was a upper
mantle feature fed from the boundary layer between the upper and lower mantle,
which explained why more extensive volcanism in Europe and Mediterranean was
absent and why continental rifting is still in its incipient stage.
200
°
C above ambient). The large heads (800
-
-
14.2 Deep-Seated Plumes Underneath the East African Rift
Valleys
Rift system of East Africa is extended from the interior highland of Kenya through
Ethiopia and Afar to the ocean
floor spreading system of Red Sea and Aden
(Fig.
14.5
). The second rift system is
finally connected to mid-oceanic ridge system
of the Indian Ocean.
The uplift of continental segments of Africa is more pronounced than the rifting.
Sometimes
there are uplifts of
thousands of metres along certain zones
(500
600 km in width and several thousand km in length). The erosional surfaces
have their vertical separation along the
-
flanks of the rifts. These erosional surfaces
converge and overlap if they are traced into the neighbouring basinal areas.
Davies (1998) discussed about plume activity in relation to volcanism in East
Africa. According to him East Africa has the following interesting characteristics:
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