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The Afar volcanic province within the 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 have controlled faunal dispersal patterns the plateau, forms the third arm of the - and influenced human evolution in the past. The 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 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 , 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, , spread throughout central Ethiopia, Eritrea and Zambia, Malawi, Zimbabwe and . 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 (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 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. Lister deep mantle anomaly beneath . This (1986) and Davis & Kusznir (2004) suggest that a volume addresses the fundamental questions of how lateral offset in the high strain zones within the mantle melt intrudes and is distributed through the upper and lower crust can be interpreted as evidence plate, and how this magma intrusion process controls for lithosphetic or crustal-scale detachment faults along-axis segmentation and facilitates continental accommodating strain until break-up occurs. Buck break-up. Seismic data and tomographic models (2004) uses numerical models to demonstrate the show that hot asthenosphere and associated crustal fundamental role of magma injection in strain intrusions occur elsewhere throughout the length of accommodation, noting that lithospheric rupture the East African rift system (e.g. Nyblade et aL can occur at a fraction of the tectonic driving 2000; Ritter & Kaspar 1997; Achauer et al. 1994). force, where magma is available. Ebinger & Casey Pre-rift flood basalts and Quaternary lavas in a large (2001) propose that magma injection localizes region surrounding the volcanically active Afar strain as rifting proceeds to break-up, leading to province have 3He/4He ratios as high as 20 Ra the abandonment of early syn-rift detachment which are thought to characterize undegassed faults. Dunbar & Sawyer (1989) use numerical mantle material originating from below the 660 km models to demonstrate the role of zones of pre-exist- discontinuity, consistent with a mantle plume ing lithosphetic weakness in the localization of (Marty et al. 1996; Pik et al. 2005). While volcanism strain at break-up, with or without the development in the Afar province and throughout East and South of lithospheric-scale detachments. Taylor et al. Africa has been attributed to the South African Super: (1999) and Hebert et al. (2001) document the plume (Janney et al. 2002; Kieffer et al. 2004; along-axis propagation of oceanic tiffing into the Furman et al. 2004, 2006a), differences in the continental lithosphere through the emplacement timing and composition of the thick flood basalt of discrete mantle upwellings and ridge jumps, sequences erupted have led to a range of geodynamic while Kusznir & Tymms (2003) have recently intro- models involving both single (Ebinger & Sleep duced models of depth-dependent stretching in 1998), modified single (Furman et al. 2004, 2006b) which upper crustal extension is much less than and double (George et al. 1998; Rogers 2006) that of the lower crust and lithospheric mantle, to plumes, the validity of which remain unresolved. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

4 G. YIRGU ET AL.

Regardless of plume head/stem location or fundamental scientific problems, the results motiv- number, magma should be generated by adiabatic ate the next stage of exploration. melting beneath the thinning lithosphere underlying the Ethiopian, Red Sea and Aden rifts. Melt should rise to shallower levels with time, with this heat References transfer process then weakening the lithosphere, enhancing strain localization (Buck 2004). The ACHAUER & THE KRISP TELESEISMIC WORKING Afar volcanic province, including continental GROUP 1994. New ideas on the Kenya rift based on the inversion of the combined dataset of the tiffing in the Main Ethiopian Rift to nascent sea- 1985 and 1989/90 seismic tomography exper- floor spreading in Afar, is a prime locale for exam- iments. Tectonophysics, 236, 305-329. ination of these processes, fundamental to the D'ACREMONT, E., LEROY, S., BESLIER, M.-O., break-up of a . BELLAHSEN, N., FOURNIER, M., ROBIN, C., MAIA, M. & GENTE, P. 2005. Structure and evolution of the eastern Gulf of Aden conjugate margins from seismic reflection data. Geo- Motivation for future studies physical Journal International, 160, 869-890, doi:10.1111/j. 1365-246X.2005.02524.x The geophysical, geochemical and geomorphologi- ACTON, G.D., TESSEMA, A., JACKSON, M. & BILHAM, cal studies presented in this volume resolve several R. 2000. The tectonic and geomagnetic signifi- issues. Yet these results provoke additional ques- cance of paleomagnetic observations from volcanic tions, and drive future research into mantle rocks from central Afar, Africa. Earth and Plane- plumes, continental rifts and continental rupture taly Science Letters, 180, 225-241. processes. Specifically: AYELE, A. & KULHANEK, O. 1997. Spatial and tem- (a) Quaternary eruptive centres are found poral variation of seismicity in the >100km from the faulted rift valleys, and from 1960 to 1993. Geophysical Journal Inter- seismic and magneto-telluric data suggest pockets national, 130, 805-810. BAKER, J., SNEE, L. & MENZIES, M. 1996a. A brief of melt persist beneath the uplifted plateau over Oligocene period of flood volcanism in Yemen: 80 km from the rift valley. Where is this melt implications for the duration and rate of continental sourced, how does it move through the plate, and flood volcanism at the Afro-Arabian triple junction. why is it restricted to the northwestern side of the Earth and Planetary Science Letters, 138, 39-55. rift valley? Are the seismic reflectors in the upper BAKER, J., THIRLWALL, M. & MENZIES, M.A. 1996b. mantle correlated with melt extraction zones? Sr-Nd-Pb isotopic and trace element evidence for (b) Were there one or multiple plume events, crustal contamination of plume-derived flood and why has the widespread volcanism persisted basalts: Oligocene flood volcanism in western long after the onset of rifting in the Red Sea, Gulf Yemen. Geochimica et Cosmochimica Acta, 60, of Aden and Main Ethiopian Rifts? 2559-2581. BATIZA, R. 1996. Magmatic segmentation at mid- (c) When was the thick underplate observed ocean ridges: A review. In: MACLEOD, C. ET AL. beneath the plateau accreted? During the period of (eds) Tectonic, Magmatic, Hydrothermal, and rapid extrusion at c. 30 Ma, or throughout the past Biological Segmentation of Mid-Ocean Ridges. 40 Ma of extrusive volcanism? What are the Geological Society, London, Special Publications, thermal implications of this underplate? 118, 103-130. (d) Are the magmatic segments mapped at the BILHAM, R., BENDICK, R., LARSON, K., MOHR, P., surface sourced by discrete upwellings at the BRAUN, J., TESFAYE, S. & ASFAW, L. 1999. lithosphere-asthenosphere boundary, or are they Secular and tidal strain across the Main Ethiopian Rift. Geophysical Research Letters, 26, 2789-2792. restricted to zones of dyke intrusion within the BOCCALETTI, M., BONINI, M., MAZZUOLI, R., ABEBE, crust? Why are the largest thermal anomalies in the B., PICCARDI, L. 8,; TORTORICI, L. 1998. Quatern- asthenosphere offset from the magmatic segments? ary oblique extensional tectonics in the Ethiopian (e) Are magmatic segments the earliest stage in Rift (Horn of Africa). Tectonophysics, 287, 97- the formation of seaward-dipping reflector sequences 116. imaged on magmatic margins worldwide? BUCK, W.R. 2004. Consequences of asthenospheric (f) Are differences in crustal structure and variability on continental rifting. In: KARNER, thickness of underplate between the northwestern G.D., TAYLOR, B., DR1SCOLL, N.W., & and southeastern sides of the rift relicts of Pan- KOHLSTEDY, D.L. (eds) Rheology and Deformation of the Lithosphere at Continental Margins. Columbia African lithospheric structure? University Press, New York, 1-30. The studies of continental break-up and flood CHAZOT, G. & BERTRAND, H. 1993. Mantle sources basalt volcanism within the Afar volcanic province and magma-continental crust interactions during presented in this volume have implications for flood early Red Sea-Aden rifting in Southern Yemen: basalt provinces and magmatic passive continental elemental and Sr, Nd, Pb isotope evidence. margins worldwide. As well as addressing Journal of Geophysical Research, 98, ] 818-1835. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

INTRODUCTION 5

CHU, D.H. & GORDON, R.G. 1999. Evidence for HOFMANN, C., COURTILLOT, V., FI~RAUD, G., motion between Nubian and Somalia along the ROCHETTE, P., YIRGU, G., KETEFO, E. & PIK, Southwest Indian Ridge. Nature, 298, 64-66. R. 1997. Timing of the Ethiopian flood basalt DAVIDSON, A. 8¢ REX, D.C. 1980. Age of volcanism event and implications for plume birth and global and rifting in southwestern Ethiopia. Nature, 283, change. Nature, 389, 838-841. 657-658. JACKSON, J. & BLENKINSOP, T. 1993. The Malawi DAVIS, M. & KUSZNIR, N.J. 2002. Are buoancy forces earthquake of March 10, 1989: Deep faulting important during the formation of rifted margins? within the East African rift system. Tectonics, 12, Geophysical Journal International, 149, 524-533. 1131-1139. DUNBAR, J. & SAWYER, D. 1989. Three-dimensional JANNEY, P.E., LE ROEX, A.P., CARLSON, R.W. dynamical model of continental rift propagation VILJOEN, K.S. 2002. A chemical and multi- and margin formation. Journal of Geophysical isotope study of the Western Cape melilite pro- Research, 101, 27845-27863. vince, South Africa: implications for the sources EBINGER, C.J., YEMANE, T., WOLDEGABRIEL, G., of kimberlites and the origin of the HIMU signature ARONSON, J.L. & WALTER, R.C. 1993. Late in Africa. Journal of Petrology, 43, 2339-2370. Eocene-Recent volcanism and rifting in the KEIR, D., KENDALL, J.-M., EBINGER, C. d~; STUART, southern main Ethiopian rift. Journal of the Geo- G. 2005. Variations in late syn-rift melt alignment logical Society, London, 150, 99-108. inferred from shear-wave splitting in crustal earth- EBINGER, C.J. & SLEEP, N.H. 1998. Cenozoic magma- quakes beneath the Ethiopian rift. Geophysical tism throughtout east Africa resulting from the Research Letters, 32, L23308, doi: 10.1029/2005 impact of a single plume. Nature, 395, 788-791. GL024150. EBINGER, C.J. & CASEY, M. 2001. Continental KIEFFER, B., ARNDT, N., ET AL. 2004. Flood and breakup in magmatic provinces: An Ethiopian Shield basalts from Ethiopia: magmas from the example. Geology, 29, 527-530. African superswell. Journal of Petrology, 45, FOSTER, A.N. & JACKSON, J.A. 1999. Source par- 793-834. meters of large African earthquakes; implications KUSZNIR, N.J. & TYMMS, V. 2003. Modelling sea-floor for crustal rheology and regional kinmeatics. Geo- spreading initiation and depth dependent stretching physical Journal International, 134, 422-428. at rifted continental margins. EOS, Transactions of FURMAN, T., BRYCE, J.G., KARSON, J. & IOTTI, the American Geophysical Union, 84(46) Fall A. 2004. East African Rift System plume structure: Meeting Supplement, Abstract T12A-0440. insights from Quaternary mafic lavas of Turkana, LEMAUX, J., GORDON, R.G. & ROYER, J.-Y. 2002. The Kenya. Journal of Petrology, 45, 1069-1088. location of the Nubia-Somalia boundary along the FURMAN, T., KALETA, K.M. & BRYCE, J.G. 2006a. Southwest Indian Ridge. Geology, 30, 339-342. Tertiary mafic lavas of Turkana, Kenya: constraints LISTER, C.R.B. 1986 Differential thermal stresses in on temporal evolution of the EARS and the occur- the Earth. Geophysical Journal of the Royal Astro- rence of HIMU volcanism in Africa. Journal of nomical Society, 86, 319-330. Petrology (in press). MAGUIRE, P., EBINGER, C., ET AL. 2003. Geophysics FURMAN, T., BRYCE, J., ROONEY, T., HANAN, B., project in Ethiopia studies continental breakup. YIRGU, G. & AYALEW, D. 2006b. Heads and tails: EOS, Transactions of the American Geophysical 30 Ma of the Afar plume. In: YIRGU, G., EBINGER, Union, 84, 342-343. C.J. & MAGUIRE, P.K.H. (eds) The Afar Volcanic MARTY, B., PIK, R. & YIRGU, G. 1996. Helium isoto- Province within the East African Rift System. pic variations in Ethiopian plume lavas: nature of Geological Society, London Special Publications, magmatic sources and limit on lower mantle contri- 259, 95-119. bution. Earth and Planetary Science Letters, 144, GEORGE, R., ROGERS, N. & KELLY, S. 1998. Earliest 223 -237. magmatism in Ethiopia: evidence for two mantle MENZIES, M., BAKER, J. & CHAZOT, G. 2001. Cenozoic plumes in one flood basalt province. Geology, 26, plume evolution and flood basalts in Yemen: a key 923-926. to understanding older examples. In: ERNST, R.E. GRAND, S.P., VAN DER HILST, R.D. & WIDIYANTORO, & BUCHAN, K.L. (eds) Mantle plumes: their identi- S. 1997. Global seismic tomography; a snapshot of fication through time, Special Paper Geological convection in the Earth. GSA Today, 7, 1-7. Society of America, 352, pp. 23-36. HAYWARD, N. & EB1NGER, C. 1996. Variations in the MORLEY, C.K., WESCOTT, W.A., STONE, D.M., along-axis segmentation of the Afar rift system. HARPER, R.M., WIGGER, S.T. & KARANJA, F.M. Tectonics, 15, 244-257. 1992. Tectonic evolution of the northern Kenyan HEBERT, H., DEPLUS, C., HUCHON, P., KHANBARI, Rift, Journal Geological Society of London, 149, K. & AUDIN, L. 2001. Lithospheric structure of a 333-348. nascent spreading ridge inferred from gravity NYBLADE, A., OWENS, T., GURROLA, H., RITSEMA, data: The western Gulf of Aden. Journal of Geo- J. & LANGSTON, C.A. 2000. Seismic evidence for physical Research, 106(B 11), 26345 - 26363. a deep upper mantle thermal anomaly beneath HENDRIE, D.B., KUSZNIR, N.J., MORLEY, C.K. & East Africa. Geology, 28, 599-602. EBINGER, C.J. 1994. Cenozoic extension in north- PIK, R., MARTY, B., CARIGNAN, J. & LAVI~, J. 2003. eru Kenya: a quantitative model of rift basin devel- Stability of the Upper drainage network opment in the Turkana region. Tectonophysics, (Ethiopia) deduced from (U-Th)/He thermochro- 236, 409-438. nometry: implications for uplift and erosion of Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

G. YIRGU ETAL.

the Afar plume dome. Earth and Planetary Science flood volcanism in Ethiopia and Yemen. Earth Letters', 215, 73-88. and Planetary Science Letters, 198, 289-306. PIK, R., MARTY, B. & HILTON, D.R. 2006. How many VAN DER HILST, R.D. & KARASON, H. 1999. Compo- mantle plumes in Africa? The geochemical point of sition heterogeneity in the bottom 1000 kilometres view. Chemical Geology, 226, 100-114. of Earth's mantle; toward a hybrid convection RITSEMA, J., VAN HE1JST, H. & WOODHOUSE, J. 1999. model. Science, 283(5409), 1885-1888. Complex shear wave velocity structure imaged VIDAL, P., DEN1EL, C., VELLUT1NI, P.J., PIGUET, P., beneath Africa and Iceland. Science, 286, 1925- COULON, C., VINCENT, J. & AUDIN, J. 1991. 1928. Changes of mantle source in the course of a rift RITTER, J.R.R. & KASPAR, T. 1997. A tomography evolution. Geophysical Research Letters, 18, study of the Chyulu Hills, Kenya. Tectonophysics, 1913-1916. 278(1-4), 149-169. WATCHORN, F., NICHOLS, G. & Bosence, D. 1998. ROGERS, N. 2006. Basaltic magmatism and the geody- Rift-related sedimentation and stratigraphy, namics of the East African rift system. In: YIRGtL southern Yemen (Gulf of Aden). In: PURSER, B. G., EB1NGER, C.J. & MAGUtRE, P.K.H. (eds) The & BOSENCE, D. (eds) Sedimentary and Tectonic Afar Volcanic Province within the East African Evolution of Rift Basins. Chapman & Hall, Rift System. Geological Society, London, Special London, pp. 165-189. Publications, 259, 77-93. WOLDEGABRIEL, G., ARONSON, J.L. & WALTER, R.C. SCHILLING, J.-G., KINGSLEY, R., HANAN, B. & 1990. Geology, geochronology and rift basin devel- MCCULLY, B. 1992. Nd-Sr-Pb isotopic variations opment in the central sector of the main Ethiopian along the Gulf of Aden: Evidence for the Afar rift. Geological Society of America Bulletin, 102, mantle plume-lithosphere interaction. Journal of 439-485. Geophysical Research, 97, 10927-10966. WOLFENDEN, E., EBINGER, C., YIRGU, G., DEINO, TAYLOR, B., GOODLIFFE, A. & MARTINEZ, F. 1999. A. & AYALEW, D. 2004. Evolution of the northern How break up: Insights from Papua main Ethiopian rift: birth of a triple junction. Earth New . Journal of Geophysical Research, and Planetary Science Letters, 224, 213-228. 104, 7497. WOLFENDEN, E., EBINGER, C., YIRGU, G., RENNE, TESFAYE, S., HARDING, D.J. & KUSKY, T. 2003. Early P. & KELLEY, S.P. 2005. Evolution of the southern continental breakup boundary and the migration of Red Sea rift: birth of a magmatic margin. Geologi- the Afar Triple Junction, Ethiopia. Geological cal Society of America Bulletin, 117, 846-864. Society of America Bulletin, 115, 1053-1067. ZUMBO, V., FERAUD, G., BETRAND, H. & CHAZOT, UKSTINS, I., RENNE, P., WOLFENDEN, E., BAKER, J., G. 1995. 4°Ar/39Ar chronology of Tertiary mag- AYALEW, D. & MENZIES, M. 2002. Matching matic activity in southern Yemen during the early conjugate volcanic rifted margins: 4°Ar/39Ar Red Sea-Aden rifting. Journal of Volcanology chronostratigraphy of pre- and syn-rift bimodal and Geothermal Research, 65(3-4), 265-279.