Geoscience Reference
In-Depth Information
Figure 10.55.
Sections
across the East African
Rift system, showing the
changes in crustal
structure on going from
(c) continental rifting in
Kenya (at 1-2
◦
SinFig.
10.52)to(b) transitional
rifting in Ethiopia (location
in Fig. 10.52 inset), where
the extension is confined
to a narrow zone in the
centre of the rift valley
rather than on the border
faults and magmatism is
organized in oceanic-style
segments, to (a) seafloor
spreading (Asal rift in
Fig. 10.52 inset). (C. J.
Ebinger, personal
communication 2003.)
over the last 5 Ma. This is all a contrast to the East African Rift, where the narrow,
shallow and wider, deep anomalies are aligned.
Seismic-refraction data indicate that the crust in the central part of the rift is
about 30 km thick, which is some 20 km thinner than the crust beneath the Great
Plains and 10-15 km thinner than the crust beneath the Colorado Plateau. The
upper-mantle P-wave velocity beneath the rift is only 7.7 km s
−
1
(Fig. 10.56).
Results of teleseismic time-delay studies of the upper mantle show that P- and
S-wave velocities down to 145 km beneath the rift are 7%-8% lower than those
beneath the Colorado Plateau and the Great Plains. The presence of melt within
the upper mantle is consistent with much of the seismic data but is not required
in order to explain it - the temperatures are presumed to be close to the solidus.
The gross crustal structure for the rift is simple, having just two layers. The
discontinuity between the upper and lower crustal layers gives rise to a strong
reflection in the seismic-refraction data. The amplitude of this reflection has
