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Continental Rift Evolution: from Rift Initiation to Incipient Break-Up in the Main Ethiopian Rift, East Africa

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Continental Rift Evolution: from Rift Initiation to Incipient Break-Up in the Main Ethiopian Rift, East Africa Earth-Science Reviews 96 (2009) 1–53 Contents lists available at ScienceDirect Earth-Science Reviews journal homepage: www.elsevier.com/locate/earscirev Continental rift evolution: From rift initiation to incipient break-up in the Main Ethiopian Rift, East Africa Giacomo Corti Consiglio Nazionale delle Ricerche, Istituto di Geoscienze e Georisorse, via G. La Pira, 4, 50121, Firenze, Italy article info abstract Article history: The Main Ethiopian Rift is a key sector of the East African Rift System that connects the Afar depression, at Received 15 January 2009 Red Sea–Gulf of Aden junction, with the Turkana depression and Kenya Rift to the South. It is a magmatic rift Accepted 23 June 2009 that records all the different stages of rift evolution from rift initiation to break-up and incipient oceanic Available online 3 July 2009 spreading: it is thus an ideal place to analyse the evolution of continental extension, the rupture of lithospheric plates and the dynamics by which distributed continental deformation is progressively focused Keywords: at oceanic spreading centres. continental rifting fi fl rift evolution The rst tectono-magmatic event related to the Tertiary rifting was the eruption of voluminous ood basalts continental break-up that apparently occurred in a rather short time interval at around 30 Ma; strong plateau uplift, which magmatism resulted in the development of the Ethiopian and Somalian plateaus now surrounding the rift valley, has deformation been suggested to have initiated contemporaneously or shortly after the extensive flood-basalt volcanism, East African Rift although its exact timing remains controversial. Voluminous volcanism and uplift started prior to the main Main Ethiopian Rift rifting phases, suggesting a mantle plume influence on the Tertiary deformation in East Africa. Different – Nubia Somalia kinematics plume hypothesis have been suggested, with recent models indicating the existence of deep superplume 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 existence of this whole-mantle feature and its possible connection with Tertiary rifting are highly debated. The main rifting phases started diachronously along the MER in the Mio-Pliocene; rift propagation was not a smooth process but rather a process with punctuated episodes of extension and relative quiescence. Rift location was most probably controlled by the reactivation of a lithospheric-scale pre-Cambrian weakness; the orientation of this weakness (roughly NE–SW) and the Late Pliocene (post 3.2 Ma)-recent extensional stress field generated by relative motion between Nubia and Somalia plates (roughly ESE–WNW) suggest that oblique rifting conditions have controlled rift evolution. However, it is still unclear if these kinematical boundary conditions have remained steady since the initial stages of rifting or the kinematics has changed during the Late Pliocene or at the Pliocene–Pleistocene boundary. Analysis of geological–geophysical data suggests that continental rifting in the MER evolved in two different phases. An early (Mio-Pliocene) continental rifting stage was characterised by displacement along large boundary faults, subsidence of rift depression with local development of deep (up to 5 km) 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). Progressive extension led to the second (Pleistocene) rifting stage, 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 (Wonji segments) in the thinned rift depression. The progressive thinning of the continental lithosphere under constant, prolonged oblique rifting conditions controlled this migration of deformation, possibly in tandem with the weakening related to magmatic processes and/or a change in rift kinematics. Owing to the oblique rifting conditions, the fault swarms obliquely cut the rift floor and were characterised by a typical right- stepping arrangement. Ascending magmas were focused by the Wonji segments, with eruption of magmas at surface preferentially occurring along the oblique faults. As soon as the volcano-tectonic activity was localised within Wonji 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. E-mail address: giacomo.corti@unifi.it. 0012-8252/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.earscirev.2009.06.005 2 G. Corti / Earth-Science Reviews 96 (2009) 1–53 Overall the above-described evolution of the MER (at least in its northernmost sector) 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. © 2009 Elsevier B.V. All rights reserved. Contents 1. Introduction ............................................................... 2 2. Physiography and different rift segments .................................................. 3 3. Brief summary of the pre-Tertiary geology of Ethiopia ............................................ 4 4. Flood-basalt volcanism and plateau uplift ................................................. 6 4.1. Flood-basalt volcanism ....................................................... 6 4.2. Plateau uplift ........................................................... 8 4.3. Dynamics of plateau uplift and flood volcanism: Plume hypothesis for the East African Rift ....................... 8 5. Tertiary rifting .............................................................. 10 5.1. Plate kinematics setting ...................................................... 10 5.1.1. Present-day kinematics .................................................. 10 5.1.2. Geological estimates of Nubia–Somalia kinematics ...................................... 11 5.1.3. Estimates of Nubia–Somalia separation ........................................... 11 5.2. Fault pattern ............................................................ 11 5.2.1. Boundary fault system ................................................... 11 5.2.2. Wonji Fault Belt faulting .................................................. 15 5.2.3. Transverse structures ................................................... 17 5.2.4. Deformation outside the rift valley ............................................. 19 5.3. Summary of the evolution of volcanic activity ............................................ 19 5.3.1. Northern MER ....................................................... 19 5.3.2. Central MER .......................................................23 5.3.3. Southern MER .......................................................24 5.3.4. Spatio-temporal distribution of volcanic activity .......................................24 5.4. Geophysical data constraining the crustal and mantle structure in the MER . ............................27 5.4.1. Crustal structure .....................................................27 5.4.2. Upper mantle structure ..................................................34 5.5. Seismicity and distribution of current deformation ..........................................36 6. The evolution of the Main Ethiopian Rift: Continental rifting from initiation to incipient break-up .......................38 6.1. Rift initiation: Localisation and propagation of extensional structures .................................38 6.1.1. Rift localisation and propagation ..............................................38 6.1.2. The Red Sea, Gulf of Aden, and Ethiopian rifts triple junction history . ............................40 6.1.3. Activation of boundary fault systems ............................................42 6.2. Rift maturity: Abandonment of boundary faults and development of Wonji segments .........................43 6.3. Wonji magmatic segments and continental break-up .........................................44 7. Conclusions ...............................................................48 Acknowledgments ...............................................................49 References ..................................................................49 1. Introduction Building on previous synthetic works (e.g., Hayward and Ebinger, 1996; Ebinger, 2005), in this paper I will review these new geological The Main Ethiopian Rift is a key sector of the East African Rift and geophysical findings and integrate them with old data, to provide System that connects the Afar depression, at the Red Sea–Gulf of Aden a comprehensive analysis of the evolution continental rifting in the junction, with the Turkana depression and
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