Geology Reference
In-Depth Information
(a)
(b)
Plume material distribution
Melt thickness, km
29
29
19
19
5
45
9
9
- 1
- 1
5-
1-5
0.2-1
0-0.2
-11
-11
-21
-21
0
10
20
30
40
50
60
0 0 0 0
40
Longitude (degrees)
0 0
Longitude (degrees)
Fig. 14.7 a Plume material distribution: contours of the maximum horizontal extent of plume
material between 45 Ma ago (contour O) and present (contour 45) predicted by the interactions of
plume with the lithospheric lid (modi
ed after Ebinger and Sleep 1998). b Melt thickness in km:
the equivalent thickness of the melt produced from 30 Ma ago to the present. The calculation
includes pressure-release melting for lateral
flow of material and lithospheric thinning, caused by
the interaction of plume material and the base of the lithosphere. Plume material was assumed to
melt by 0.1 % per km of ascent, independent of depth, melting was enhanced due to steep
gradients (modi
ed after Ebinger and Sleep 1998)
early Miocene, then the lithosphere had been heated by the plume during the period
between 10 and 15 Ma.
Applying these values to the model of conductivity heating described by Turner
et al. (1996) and assuming a lithosphere thickness of 150 km gives a maximum T
p
for the subjacent plume of 1,430
C. The calculated melt thickness, which assumes
50 % melt trapped at depth, is almost certainly an overestimate, and so 1,430
°
C
should be regarded as a maximum temperature for the underlying plume. The
estimated temperature should be considerably lower than that often assumed for
mantle plume (T
p
> 1,500
°
C), but it is consistent with the location of the Birunga
province at the periphery of the East African plateau, the present-day topographic
and geophysical expression of the East African plume the presence of small vol-
umes of magmatism in and around the Birunga province dates back to 12 Ma,
which suggested that contact with the plume occurred between 10 and 15 Ma ago, if
the duration of conduction heating had been <10 Ma, then a melt thickness of 2 km
implies higher plume temperatures. However, the duration of heating should have
to be less than
°
C.
In comparison with models of melt generation for the Kenyan Rift, which Latin
et al. (1993) related to the decompression of the East African plume as a conse-
quence of extension across Kenyan Rift, it should be emphasized that the duration
of magmatism in the Birunga province was shorter. Its volume was much smaller,
and the time of onset was generally later than that of the Kenyan Rift. However, as
*
7 Ma to necessitate plume temperatures >1,500
°
Search WWH ::
Custom Search