Geologic evolution

Pre-rift history: flood basalt volcanism and plateau uplift
Tertiary rifting in Ethiopia was preceded by emplacement of voluminous flood basalts that apparently occurred in a rather short time interval at around 30 Ma; strong plateau uplift, which resulted in the development of the Ethiopian and Somalian plateaus now surrounding the rift valley, initiated contemporaneously or shortly after the extensive flood basalt volcanism, although its exact timing remains controversial. Occurrence of voluminous volcanism and uplift prior to the main rifting phases has been used as an argument to suggest a mantle plume influence on the Tertiary deformation in East Africa. Recent plume models indicate the existence of a deep superplume structure originating at the core-mantle boundary beneath southern Africa, rising in a north-northeastward direction toward eastern Africa, and feeding multiple plume stems in the upper mantle. However, the characteristics (and even the existence) of mantle plume(s) beneath the African continent are debated.

Uplift of the Ethiopian/Somalian and East African plateaus has been suggested to have had a first-order impact on the climate evolution of East Africa in the last 8 Myr. Indeed, by inducing a drastic reorganization of atmospheric circulation and by forming orographic barriers to moist air, this uplift led to a progressive aridification and caused a gradual replacement of closed forest woodlands by open savanna grassland. This in turn, may have controlled the evolutionary paths of East African hominins, particularly forcing an arid-adaptive evolutionary path for Homo sapiens.

 

Traps close to Debre Lybanos (click for a larger version)

 

Traps close to lalibela (click for a larger version)

 

Rift evolution: rift localisation and activation of large boundary faults
The main rifting phases started diachronously along the Ethiopian Rift Valley in the Oligo-Miocene; rift propagation was not a smooth process but rather a process with punctuated episodes of extension and relative quiescence. The extensional deformation is controlled by the relative motion of the three major plates Arabia, Africa and Somalia; in the Afar depression, rifting is controlled by the Africa-Arabia divergence, whereas to the south extension is controlled by the Africa-Somalia motion. Rift location was most probably controlled by the reactivation of a lithospheric-scale pre-Cambrian weakness; south of the Afar depression, the orientation of this weakness (roughly NE-SW) and the Miocene (post-11 Ma)-recent extensional stress field generated by relative motion between Nubia and Somalia plates (roughly ESE-WNW) suggest that oblique rifting conditions (i.e., extension direction not orthogonal to the trend of the rift axis) have controlled rift evolution. Analysis of geological-geophysical data from the Main Ethiopian Rift suggests that continental rifting typically evolved in two different phases. An early rifting stage was characterised by displacement along large boundary faults, subsidence of rift depression with local development of deep (up to 5km) asymmetric basins and diffuse magmatic activity. In this initial phase, magmatism encompassed the whole rift, with volcanic activity affecting the rift depression, the major boundary faults and limited portions of the rift shoulders (off-axis volcanism).

 

Large normal fault close to Golja (click for a larger version)

 

Rift evolution: Abandonment of boundary faults and development of internal fault segments
Progressive extension led to the second rifting stage, well expressed in the Northern part of the rift, characterised by a riftward narrowing of the volcano-tectonic activity. In this phase, the main boundary faults were deactivated and extensional deformation was accommodated by dense swarms of faults with associated intense volcanism (tectono-magmatic segments) in the axis of the thinned rift depression. The progressive thinning of the continental lithosphere controlled this migration of deformation, possibly in tandem with the weakening related to magmatic processes. In the Main Ethiopian Rift, owing to the oblique rifting conditions, the axial fault swarms obliquely cut the rift floor and were characterised by a typical right-stepping arrangement. Ascending magmas were focused by the tectono-magmatic segments, with eruption of magmas at surface preferentially occurring along the oblique axial faults.

The two phases of evolution of rifting (click for a larger version)

 

Morphological difference between the large boundary faults (top) and the Wonji faults (bottom) (click for a larger version)

 

Continental break-up
As soon as the volcano-tectonic activity was localised within the internal tectono-magmatic segments, a strong feedback between deformation and magmatism developed: the thinned lithosphere was strongly modified by the extensive magma intrusion and extension was facilitated and accommodated by a combination of magmatic intrusion, dyking and faulting. In these conditions, focused melt intrusion allows the rupture of the thick continental lithosphere and the magmatic segments act as incipient slow-spreading mid-ocean spreading centres sandwiched by continental lithosphere. Overall the above-described evolution documents a transition from fault-dominated rift morphology in the early stages of extension toward magma assisted-rifting during the final stages of continental break-up. A strong increase in coupling between deformation and magmatism with extension is documented, with magma intrusion and dyking playing a larger role than faulting in strain accommodation as rifting progresses to seafloor spreading.

Schematic model of rift evolution in the Main Ethiopian Rift (modified after Ebinger, 2005, Astronomy and Geophysics) (click for a larger version)

 

The final stages of break-up are exposed free of cover by seawater in the Afar depression, where Quaternary axial volcanic ranges like Erta Ale are believed to behave as subaerial mid-ocean ridges. Portions of the Afar depression have been shown to be characterised by subaerial magnetic stripes similar in pattern and amplitude to those that characterize seafloor spreading centers.

 

THe erta ale range in the danakil depression (northern afar) (click for a larger version)