Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021 The Afar volcanic province within the East African Rift System: introduction G. YIRGU 1, C.J. EBINGER 2 & P.K.H. MAGUIRE 3 1Department of Geology and Geophysics, University of Addis Ababa, PO Box 176, Addis Ababa, Ethiopia (e-mail: yirgu, g @ geol. aau. edu. et) 2Department of Geology, Royal Holloway, University of London, Egham, TW20 OEX, UK (e-mail: c. ebinger@ gl. rhul.ac, uk) 3Department of Geology, University of Leicester, Leicester LE1 7RH, UK (e-mail: pkm @ le.ac, uk) Continental rifting processes continually reshape Over 1000000km 3 of basalts and more the Earth's surface, producing sediment-filled rift minor rhyolites cover the c. 1000km-wide basins, or rupturing the tectonic plates to form Ethiopia-Yemen plateau, which has experienced a new ocean basins. Rift architecture and tectonics maximum basement uplift of c. 1600 m above the focus volcanic and seismic hazards, as well as mean altitude of the African plate to the west (Pik geothermal energy resources, while rift systems in et al. 2003). The Ethiopian Rift, which transects Africa have controlled faunal dispersal patterns the plateau, forms the third arm of the Red Sea- and influenced human evolution in the past. The Gulf of Aden triple junction within the Eocene- response of a plate to extension and heating pro- Oligocene so-called Afar volcanic province (also vides fundamental clues into the plate rheology, known as the Ethiopia-Yemen flood basalt pro- and the underlying mantle convection patterns. A vince) (Fig. 1). Thus, the region records both the number of models have been proposed to explain evolution of Earth' s youngest flood basalt province, the success and failure of continental rift zones, its most youthful passive continental margins, as but there remains no consensus on how strain loca- well as archetypal continental rift zones. lizes to achieve rupture of initially 125-250 km- thick plates, or the interaction between the plates and asthenospheric processes. The Afar volcanic province The seismically and volcanically active East African rift system has long been a classic area for The uplifted Ethiopia-Yemen plateau has experi- investigating rifting and break-up because its sectors enced episodes of volcanism since c. 45 Ma, with encompass basins in all stages of rift to passive eruptions continuing to the present day. Earliest margin development. Its architecture is defined on volcanism in the East African rift system occurred the basis of both structural and magmatic components. in southwestern Ethiopia, southeastern Sudan, and It extends 3000 km from the Afar depression in the northern Kenya at 40-45 Ma (Davidson & Rex north to the Okavango Delta in the south, through 1980; Ebinger et al. 1993; George et al. 1998). Djibouti, Ethiopia, Kenya, Uganda, Burundi, Between c. 31 and 22 Ma, volcanism was wide- Rwanda, Democratic Republic of Congo, Tanzania, spread throughout central Ethiopia, Eritrea and Zambia, Malawi, Zimbabwe and Botswana. The Yemen where flood basalts and associated felsic East African rift system overlies one of the most pyroclastic rocks were erupted (Zumbo et al. extensive seismic velocity anomalies in the Earth's 1995; Baker et al. 1996b; Hofmann et al. 1997; mantle, extending from the core-mantle boundary Kieffer et al. 2004). The thickest sequences beneath the South Atlantic into the upper mantle exposed along the conjugate margins of the beneath East Africa (Grand et al. 1997; van der southern Red Sea were erupted between 31 and Hilst & Karason 1999; Ritsema & van Heijst 2000). 29 Ma, prior to or concurrent with the onset of The rift system developed within Archaean-Protero- rifting (e.g. Hofmann et al. 1997; Ukstins et al. zoic continental lithosphere, providing a unique 2002; Wolfenden et al. 2005). Following the volu- opportunity to examine the mechanical response of minous eruptions of flood basalts and associated strong, cold lithosphere to extension induced by asth- felsic rocks, large shield volcanoes developed on enospheric upwelling and far-field forces. the uplifted plateau hundreds of kilometres From: YIRGU, G., EBINGER, C.J. & MAGUIRE, P.K.H. (eds) 2006. The Afar Volcanic Province within the East Afiqcan Rift System. Geological Society, London, Special Publications, 259, 1-6. 0305-8719/06/$15.00 @, The Geological Society of London 2006. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021 2 G. YIRGU ETAL. 20 ° 15 ° 10 ° _5 ° -10 ...... 25 ° 30 ° 35 ° 40 ° 45 ° 50 ° Fig. 1. East Africa and the main elements of the northern part of the East African rift system. The approximate extent of the Afar volcanic province is shown by the ellipse. outside the faulted rift valleys (Kieffer et al. 2004). just prior to the eruption of the Ethiopia-Yemen Red Sea and Aden rifting culminated in seafloor flood basalts and associated felsic rocks at spreading at 4 and 16 Ma, respectively, effectively c. 30 Ma (e.g. Watchorn et al. 1998; d'Acremont isolating part of the volcanic plateau in Yemen et al. 2005). Rifting in the Red Sea began at and Saudi Arabia (Chazot & Bertrand 1993; 28 Ma, soon after the peak flood basalt volcanism Baker et al. 1996a; Menzies et al. 2001). Recent (e.g. Wolfenden et al. 2005). The earliest extension volcanism is mainly confined to magmatic seg- documented in the East African rift system occurred ments within the Ethiopian rift, and incipient in inactive basins west of present-day Lake Turkana spreading zones in Afar (e.g. Vidal et al. 1991; at c. 25 Ma, within lithosphere previously stretched Schilling et al. 1992; Boccaletti et al. 1998). during a Mesozoic rifting episode (Morley et al. Thus, the Afar volcanic province has experienced 1992; Hendrie et al. 1994). Extension between the a long, complex history of basaltic and explosive Nubian and Somalian plates began in the southern felsic volcanism since 45 Ma, with the volumetri- and central sectors of the Main Ethiopian Rift by cally largest eruptions tied to the onset of rifting c. 18 Ma (WoldeGabriel et al. 1990) whereas in of Africa and Arabia at c. 30 Ma. the northern sector it started at c. liMa, more than 17 Ma after initial rifting in the southern Red Sea and Gulf of Aden rifts (Wolfenden et al. 2004). Regional context of Cenozoic rifting The junction between the Red Sea and Gulf of Aden oceanic spreading centres is currently Although far from complete, systematic dating of located within the complex Afar depression west volcanic and tectonic activity provides strong con- of the Danakil microplate (Acton et al. 2000; straints on how the mantle source of the Afar volca- Ebinger & Casey 2001). These data and obser- nic province interacted with the lithosphere, and vations have led to the proposal that the East how this interaction was linked to rifting. Continen- African rift has propagated northward across older tal rifting in the Gulf of Aden initiated at c. 35 Ma, Red Sea and Aden structure, to form the Afar triple Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021 INTRODUCTION 3 junction by c. 11 Ma (Tesfaye etal. 2003; Wolfenden explain the formation of rifted margins. There are et al. 2004). Global plate motions averaged over the profound differences predicted by these various last 3.2 Ma, and current geodetic and fault slip data models for the 3-D geometry of the crust and upper predict an extensional velocity of around 4- mantle that develops during continental break-up 6 mma -1 and an extension direction of c. 105- and the initiation of oceanic rift segmentation. 109 ° in the MER (Fig. 1, Calais et al. 2005; Bilham The along-axis segmentation of oceanic rifts is et al. 1999; Chu & Gordon 1999; Keir et al. 2005). produced by along-axis variations in magma supply Lemaux et al. (2002) used seafloor spreading (e.g. Batiza et al. 1996). In contrast the regular anomaly patterns to deduce 23 km of opening along along-axis segmentation of continental rift basins is the Nubia and Somalia plates over the past 11 Ma produced by the geometrical arrangement of large with a change from NW-SE to approximately offset border fault systems whose length is dependent E-W extension at c. 3.2 Ma. Inversions of teleseis- upon the integrated yield strength of the litho- mic events show present-day E-W to WNW-ESE sphere (e.g. Hayward & Ebinger 1996; Jackson & extension directions (Ayele & Kulhanek 1997; Blenkinsop 1997). Thus, there is also a strong Foster & Jackson 1999) consistent with the predic- signal in the along-axis segmentation pattern of tions of plate kinematic data. This realignment of continental rifts. What is the genetic link between the extensional stress direction is coincident with the continental and oceanic rifting? It is apparent that development of the right-stepping en echelon mantle dynamics and magmatic processes become magmatic centres along the axis of the Northern more important as the continental lithosphere is Main Ethiopian Rift and into southwestern Afar. stretched to the point of rupture, but when and how does melt production facilitate continental break-up? Models of continental break-up Observations within the active rift systems of the The Afar mantle plume Afar volcanic province document the key processes of continental break-up: weakening of the litho- Existing geophysical and geochemical data from the sphere by mechanical stretching, intrusive heating Afar volcanic province provide compelling evidence and interactions with a dynamic asthenosphere. for rift initiation above a mantle plume, with contro- The relative importance of these processes remains versy concerning the location, depth, extent and con- debated, in large part owing to selective studies of tinuity of the hot asthenospheric material with the non-volcanic passive margins and rift zones.