Geoscience Reference
In-Depth Information
of continental extension, earthquakes nucleate in the upper 15 km of the crust
(the seismogenic layer). The implication of this thick seismogenic layer in eastern
Africa is that the crust is strong and thick. This is consistent with the low heat
flow and low temperature gradients measured over cratons (Chapter 7).
The uplift and the volcanism which started in northeast Africa
40 Ma ago
were caused by a mantle plume. Global seismic tomographic images reveal
extensive low-velocities in the mantle beneath east Africa (Plate 10). The hotter-
than-normal mantle is providing dynamic support for the elevation of the whole
region - the African superswell. About 31 Ma ago there was an outpouring of
flood basalts across a
∼
1000-km-wide area and after this, as Arabia moved north-
eastwards away from Africa, stretching began in the Red Sea and Gulf of Aden.
Seafloor spreading had started in the Gulf of Aden by 10 Ma ago and in the Red
Sea by 4 Ma ago. The East African Rift exhibits all stages of the break- up of a
continent along its length. As continental stretching starts, normal faults develop
and the lithosphere thins. To the south, where the continental rift is young and
extension is not great, the rift is characterized by border faults. At some stage in
the stretching process the continental lithosphere reaches the point of 'break-up' -
and a new ocean basin forms. Magmatic processes control the resultant oceanic
spreading whereas faulting controls the earlier continental rifting. This transition
from continental to oceanic rifting is currently taking place along the northern
Ethiopian Rift (Fig. 10.55). There the extension (geodetic data show
∼
80% of
the strain) is confined to a narrow zone within the rift valley rather than being
accommodated on the normal faults that define the
∼
100-km-long rift valley. The
planform of the volcanic activity is oceanic - the segmentation is that of a slow-
spreading ridge (Fig. 9.34 and Table 9.6). In the extreme north towards Afar (i.e.,
furthest from the rotation pole) continental break-up has already taken place and
seafloor spreading is effectively taking place, but along the rest of the rift system
the continental lithosphere is still undergoing extension. In Afar the maximum
extension may be as much as 70-100 km, but south of Afar the geological estimate
of the maximum extension which has taken place is 30 km. Thus the extension
rates decrease from north to south as the rotation pole is approached. Volcanism
along the rift is rather alkaline, which is normal for continental volcanism in
relatively undisturbed lithosphere.
∼
10.4.3 The Rio Grande Rift
The Rio Grande Rift is a much smaller feature than the East African Rift system.
Visually, the two rift systems are very similar, with platform-like rift blocks rising
in steps on each side of the central graben. Volcanism in the Rio Grande Rift began
27-32 Ma ago in the Precambrian Shield as a northeast-southwest rift opened.
Subsequently extension 5-10 Ma ago resulted in a north-south rift characterized
byathermal anomaly and crustal thinning. The present-day lithospheric and
mantle anomaly is primarily the result of westnorthwest-eastnortheast extension
