Geoscience Reference
In-Depth Information
Figure 10.52.
The East Arican Rift system. The African plate is slowly splitting along
the rift system. The star indicates the pole of motion between the Nubian plate and
Somalian plate. The separation is very slow - 6 mm /yr
−1
in the north and 3 mm /yr
−1
in the south. Below: detail of the Afar region showing the connection between the
extension in the Main Ethiopian Rift and the mid-ocean ridges in the Gulf of Aden
and the Red Sea. Arrows show the direction of spreading. The Danakil horst has
moved southeastwards away from Nubia - the intervening material is oceanic. Solid
lines: main border faults; grey shading, locus of extension; black ovals, magmatic
segments. X, location of section shown in Fig. 10.55(b). (Lower part after Hayward
and Ebinger (1996).)
Twoofthe best-known rift zones are the East African Rift and the Rio Grande
Rift, though there are others such as the Rhine Graben in Europe and the Baikal
Rift in Asia. The Keweenawan Rift is a North American example of an ancient
extinct continental rift.
10.4.2 The East African Rift
This long rift system stretches over 3000 km from the Gulf of Aden in the north
towards Zimbabwe in the south (Fig. 10.52). In the Gulf of Aden it joins, at a
triple junction, the Sheba Ridge and the Red Sea. Along this rift system, uplifting,
stretching, volcanism and splitting of the African continent are in progress. The
African plate is currently moving as two plates - the main Nubian plate and an
eastern Somalian plate. The rotation pole for these two plates is just off the east
coast of southern Africa (Fig. 10.52).
The crustal and upper-mantle structure of the rift system has been determined
from seismic and gravity data. Figure 10.53 shows data and models from Kenya,
where the long-wavelength Bouguer gravity anomaly and the earthquake data can
be explained by invoking the presence of anomalous low-velocity, low-density
