The Central Main Ethiopian Rift is younger than 8 Ma:confirmation through apatite fission-track thermochronology

Tsegaye Abebe,1 Maria Laura Balestrieri2 and Giulio Bigazzi31MASSA Spinoff, Pisa, Italy; 2CNR, Istituto di Geoscienze e Georisorse, UO Firenze, Italy; 3CNR, Istituto di Geoscienze e Georisorse, Pisa,

Italy

Introduction

The Main Ethiopian Rift (MER) is aroughly NE-oriented segment of theEast African Rift system, which ex-tends from the Afar to the Kenya rift.The MER is composed of northern,central and southern segments. It hascommonly been suggested that riftpropagation in the MER progressednorthwards (Wolfenden et al., 2004;Mackenzie et al., 2005; Keir et al.,2006), although its evolution is notwell understood. An alternativehypothesis implies southward propa-gation (Buck, 2006; Rogers, 2006).Bonini et al. (2005) suggested a heter-ogeneous time–space evolution, withinitial extension in the Southern MERat 20–21 Ma, followed by extension inthe Northern MER at � 11 Ma andfinally formation of the Central MERat about 5–6 Ma. Irregular rift pro-pagation due to the presence of pre-existing structures has also beenproposed by Keranen and Klemperer(2008).The Central MER is bounded by the

Yerer-Tullu-Wellel volcanotectoniclineament to the north and theGoba–Bonga lineament to the south(Fig. 1). It is also bounded to the eastand west by fault escarpments (someof them with an offset of more than

1500 m) such as the Munesa andGuraghe rift margins. Tertiary toQuaternary volcanics are the onlyrocks exposed in the MER, apartfrom minor fluvio-lacustrine sedi-ments, mostly of Quaternary age, thatwere deposited on the rift floor. Oneexception is an outcrop of Precam-brian crystalline rocks covered byMesozoic sediments at the base ofthe Guraghe escarpment, near thevillage of Kella (Figs 1 and 2) about80 km south-west of Addis Ababa.Apart from Kella, basement rocks

in the MER are exposed only in itsextreme south and in the northernAfar. Low-temperature thermochrono-logical data related to rift-inducedexhumation are available only fromthe Southern MER (Pik et al., 2008).By modelling apatite U–Th ⁄He data,Pik et al. (2008) excluded significantrift-related denudation before 20 ±2 Ma in the Southern MER.No thermochronological constraints

exist for the Central MER so far. Inthis work, apatite fission-track (AFT)analysis has been applied to twosamples of the Kella basement rocks(Fig. 3). The presence of these crys-talline rocks offers a unique opportu-nity to date rifting in this segment ofthe MER.

Geological setting

The exposure of pre-Tertiary rocks atKella is a block about 4 km long and1.5 km wide dipping 20� W, boundedby the NE–SW trending Guragheborder fault (Figs. 1 and 2) and to

the south by a NW trending transver-sal fault. It is interpreted as a crustalblock intimately associated with rif-ting-related extension (Di Paola et al.,1993). The Precambrian basementrocks that outcrop at the base of theKella block are approximately 90 mof strongly altered biotite–alkali feld-spar gneisses cross-cut by quartz–feld-spathic pegmatite veins (Fig. 3).In the Blue Nile river gorge located

about 200 km north of Kella, theMesozoic sedimentary succession at-tains a total thickness of more than1200 m (Jepsen and Athearn, 1997).At Kella, only about 200 m of Meso-zoic sediments are present (Fig. 3).Thus, the condensed Mesozoic succes-sion at Kella reaches only one-sixth ofthe Blue Nile �type� section (Merlaet al., 1979).Recent geological and geochrono-

logical investigations (Abebe et al.,2005; Bonini et al., 2005 and refer-ences therein) distinguished fourmajor volcanic episodes in the CentralMER:

1 32–29 Ma: The main part of theEthiopian plateau volcanics consistsof the Trap Series (Hofmann, 1997;Pik et al., 1998; Ukstins et al.,2002). They cover an area of morethan 600 000 km2 and locallyreaches thicknesses exceeding3000 m (Mohr and Zanettin, 1988).This episode has been attributed tomelting caused by the activity ofone or twomantle plumes impingingon the base of the continentallithosphere under the Afar or

ABSTRACT

An isolated block of Precambrian basement rocks and Mesozoicsediments is exposed at Kella along the western margin of theCentral Main Ethiopian Rift (MER), surrounded by Tertiary toQuaternary volcanic rocks. Apatite fission-track thermochrono-logy on two basement samples yielded ages of 7.2 ± 1.0 Maand 6.7 ± 3.0 Ma and a long mean track length (>14.5 lm).Rapid Late Miocene cooling is attributed to denudation relatedto rifting. Despite the paucity of data, due to the absence of

suitable lithologies in the area, our data confirm that theCentral MER is younger than 8 Ma as recently proposed on thebasis of field evidence and radiometric dating of volcanics. Thisimplies that the Central MER formed after the Northern MER,indicating a diachronous development of this third arm of theRed Sea–Gulf of Aden–Ethiopian Rift system.

Terra Nova, 22, 470–476, 2010

Correspondence: Maria Laura Balestrieri,

Sezione di Firenze, Istituto di Geoscienze e

Georisorse, Via G. La Pira, 4, Firenze,

50121, Italy. Tel.: 055 2757494; fax:

055290312; e-mail: balestrieri@igg.cnr.it,

marialaura.balestrieri@unifi.it

470 � 2010 Blackwell Publishing Ltd

doi: 10.1111/j.1365-3121.2010.00968.x

Afar–Northern Kenya (Ebinger andSleep, 1998; George et al., 1998).These volcanic successions are miss-ing at the Kella block.

2 �11–8 Ma: After a long period ofless voluminous and more localvolcanic activity related to thedevelopment of shield volcanoes(Corti, 2009), basaltic–trachytic lavaflows and associated pyroclastics

were formed. They constitute thelargest part of the Guraghe escarp-ment and extend for several tens ofkilometres west of the escarpment(see Fig. 2, unit 4). Their averagethickness at the Guraghe margin is600–700 m. Rocks of this volcanicsuccession are also absent at theKella block.

For both volcanic events (1) and(2), widespread time-correlative basal-tic units occur on the western andeastern margins of the Central MER,constraining the lateral extent of theseevents. New K–Ar ages from alongthe well-exposed volcanic sections ofthe Guraghe margin and the Ghiberiver canyon, located 75 km west ofthe Guraghe rift margin, permittedBonini et al. (2005) to reconstruct anideal transect across the Ethiopianplateau and to demonstrate theplateau-like nature of volcanic event(2). They concluded that volcanicevent (2) was a pre-rift volcanic phase.

3 5–3 Ma: Several layers of pyroclas-tic rocks associated with trachyticand rhyolitic lava domes and flowstogether with some important cen-tral volcanoes (Fig. 1, unit 3; Fig. 2unit 5) were formed in this episode,and cover the MER shoulders andfloor. Radiometric ages from boththe eastern and the western marginsof the MER range between 5.2 and2.6 Ma (see Corti, 2009 and refer-ences therein). These highly differ-entiated central and ⁄or fissuraleffusive and explosive eruptionproducts indicate that volcanicactivity during this period was veryintensive and vigorous. At Kella(Fig. 3), the Mesozoic sediments areoverlain by a thin (approximately50 cm) layer of Pliocene ash-falldeposits strongly inclined towardsW, followed by a horizontal ignim-brite dated at 3 Ma (K–Ar on alkalifeldspars, Di Paola et al., 1993).

4 <3 Ma: The products of thisepisode consist of uncompactedpumiceous fall and flow deposits,rhyolitic–trachytic lava flows form-ing central volcanic edifices, fissuralbasaltic lava flows with associatedscoria and phreatomagmatic cones,and interbedded lacustrine deposits(e.g. Di Paola, 1976; Berhe, 1978;Kazmin et al., 1980; Bigazzi et al.,1993; Le Turdu et al., 1999). Thesedeposits are generally confined tothe MER floor (Fig. 1: units 4, 5, 6;Fig. 2: units 6, 7, 8).

The MER is bounded by discontin-uous boundary faults that give rise tomajor fault escarpments separatingthe rift depression from the Somalianand Ethiopian Plateaus. These faultsare widely spaced and characterizedby large vertical offsets (>1 km; Boc-

Fig. 1 Simplified geological map of central Ethiopia, modified after Jepsen andAthearn (1997). (1) Pre-Tertiary sediments and crystalline basement, (2) Oligocene(32–29 Ma) and lower Miocene (12–8 Ma) plateau volcanics, (3) Miocene–Pliocenerift-shoulder trachytic–rhyolitic volcanics and pyroclastic layers, (4) Plio-Pleistocenerift floor, (5) Quaternary central volcanics and basaltic lava flows, associated scoriacones and phreato-magmatic deposits, (6) Quaternary lacustrine sediments andinterbedded pyroclastics, (7) faults, (8) major rift border faults, (9) major transversaltectonic lineaments in the basement, (10) Wonji Fault Belt segments. Red square: areashown in Fig. 2. In the inset: YTVL, Yerer-Tullu-Wellel volcanotectonic lineament,MER, Main Ethiopian Rift.

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caletti et al., 1998). In the CentralMER, they are mainly oriented30�–40� N. The rift floor is affectedby dense NNE–SSW-trending faultswarms, with relatively small throws

(<100 m), which comprise the WonjiFault Belt (Fig. 1; Corti, 2009 andreferences therein). The Wonji FaultBelt started to develop at the begin-ning of the Quaternary (1.8–1.6 Ma,

Meyer et al., 1975), and probablyreflects the present-day stress field(Chorowicz et al., 1994; Boccalettiet al., 1998; Ebinger and Casey, 2001).A transversal E–W system is also

present in the MER, and in the studyarea it is represented by the Yerer-Tullu-Wellel volcanotectonic linea-ment (Abebe et al., 1998) and theGoba-Bonga lineament (Abbate andSagri, 1980). This system probablyrepresents inherited structural trendsassociated with the Gulf of Adenfracture system (Abebe et al., 1998).

Results and interpretation

We applied AFT analysis to two sam-ples from the basement rocks outcrop-ping in the Kella block to evaluatethe timing of their exhumation. Thestability of the tracks is mainly depen-dent on temperature: tracks shorten oranneal as temperature increases. Somedegree of annealing occurs at all tem-peratures, but its rate is accelerated,producing a reduction in the FT age,in an interval of temperatures calledthe partial annealing zone (PAZ)(Gleadow and Fitzgerald, 1987). Forapatite, the PAZ extends approxi-mately between 120 �C and 60 �C.Fission-track length measurements,combined with age data, are com-monly used to quantify the rate atwhich rocks cool below �120 �C(Gleadow et al., 1986). In general,long mean confined track lengths(>14 lm) and narrow track-lengthdistributions correspond to rapid cool-ing, while shorter mean values(<14 lm) and broad distributionsare indicative of slower cooling ormore complex cooling paths. Throughthe estimate of a palaeo-geothermalgradient, it is possible to convert thethermal history of a given rock intotime–burial-depth information.To explain the lack of the 32–29 Ma

and �11–8 Ma volcanic successions atKella and the fact that the first volca-nic unit covering the pre-Tertiary rock(except for a thin ash layer) is the3 Ma ignimbrite, WoldeGabriel et al.(1990) and Di Paola et al. (1993)proposed that the Kella block wasalready a prominent topographic fea-ture prior to the Oligocene. In thishypothesis, AFT ages as old as orolder than 30 Ma should be expectedor, if the emplacement of the overlyingignimbrite affected the track stability

Fig. 2 Simplified geological map of the Kella area, modified after Abebe et al. (2005).(1) Kella metamorphic rocks, (2) Mesozoic cover, (3) Oligocene plateau volcanics(32–29 Ma), (4) Lower Miocene plateau volcanics (Guraghe Basalts: 10.6–8.3 Ma),(5) Miocene–Pliocene rift-shoulder trachytic–rhyolitic volcanics (<5 Ma) and pyro-clastic deposits (5.2–2.6 Ma), (6) Pyroclastic deposits (2.54–1.7 Ma), (7) Quaternarylava flow (<1.6 Ma), (8) Wonji Basalts (<1.6 Ma), (9) Lacustrine sediments, (10)faults. Circles: sample locations. A–B: trace of the schematic section in Fig. 4d.

Fig. 3 Stratigraphic section of the Kella block. (1) Precambrian biotite gneiss, (2)Adigrat sandstones, (3) claystones, shales and minor limestones, (4) palaeo-fluvialdeposits, (5) stratified ash layer, (6) compacted crystal-rich pyroclastics, (7) rhyoliticlava flows. Altitude in metres above sea level. Star: location of Kella Precambrianbasement rock samples.

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in our samples, partially reset ages(between � 30 Ma and 3 Ma) wouldbe accompanied by a broad lengthdistribution and a strongly reducedmean length.The two Kella crystalline basement

samples (K1 and K2) yielded ages of7.2 ± 1.0 Ma and 6.7 ± 3.0 Ma,respectively (Table 1). Twenty-twoconfined tracks in sample K1 have amean length>14.5 lm, indicating thatour samples cooled rapidly across theentire PAZ between 7 Ma and 6 Ma.This means that before this Late

Miocene cooling event, both Kellabasement samples were at tempera-tures >120 �C. The entire Mesozoicsequence outcropping at Kella is onlyapproximately 200 m thick, and thebasement above the studied samples isonly 90 m, not nearly sufficient toexplain how these rocks were buriedbelow the PAZ prior to 7 Ma. Toreach a palaeo-temperature >120 �C,we have to assume the presence of acover in the Kella area, which wasremoved after 7–6 Ma, but before theemplacement of the 3 Ma ignimbrite.A likely solution is that this cover

consisted of Oligocene lavas and the�11–8 Ma volcanics that are exposedfurther south and to the west at theGuraghe escarpment (WoldeGabrielet al., 1990; Abebe et al., 2005)(Fig. 4a). It is possible that the Kellablock was already a topographic highduring Mesozoic times, as indicatedby the reduced thickness of the Meso-zoic sedimentary succession, but theconsequence of the emplacement of athick 32–29 Ma lava pile and then a�11–8 Ma lava pile was to transferalready cooled samples back belowthe base of the PAZ and to completelyreset the AFT system. Basalts have a

low thermal conductivity, which re-sults in higher temperatures in rocksburied below a cover of basalts than inones covered by sedimentary rocks

(Gallagher et al., 1994). This meansthat a cover of basalts between 2.0 and2.2 km thick (at 40 �C km)1) wouldhave been sufficient to erase com-

Table 1 Fission track analysis of apatites from basement rocks near Kella, Central Main Ethiopian Rift.

Sample Elev. (m) Lat N Long E qS · 104 (cm)2) nS qI · 104 (cm)2) nI S ⁄ I E% Age ± 1r (Ma) U (ppm) L (lm) SD (lm) n

K1 2000 8�15¢57.90¢¢ 35�28¢40.29¢¢ 3.36 122 48.0 516 150 ⁄ 100 14 7.2 ± 1.0 10.6 14.85 1.81 22

K2 2000 8�15¢03.54¢¢ 35�28¢00.13¢¢ 6.58 68 101 170 43 ⁄ 7 44 6.7 ± 3.0 22.4

Apatite grains were separated from bulk samples using standard heavy liquids (Na-polytungstate and methylene iodide) and magnetic separation techniques. It has

been shown (Malusa et al., 2005) that for samples with low spontaneous track density, which are difficult to date using the external detector method, the population

method (Gleadow, 1981) may yield reliable results. For this reason, the apatite samples in this study, which showed a very low spontaneous track density, were dated

using the population method: two aliquots of apatite (for spontaneous and induced track counting) from each sample were mounted in epoxy resin, polished and

etched with 5% HNO3 at 20 �C. Track counting was performed using a Leica Orthoplan microscope at 1250 · magnification. Lengths were measured with a Leica

microvid stage. Elev., elevation in metres; qS (qI), spontaneous (induced) track density; nS (nI): spontaneous (induced) counted tracks; E%, propagation of relative

standard error of the spontaneous and induced track counts; S ⁄ I, number of analysed crystals for spontaneous (S) and induced (I) track density determination; L, SD,

length and standard deviation of the confined track length distribution; n, number of measured confined tracks. Irradiation was performed in the Lazy Susan facility

(Cd ratio 6.4 for Au and 48 for Co) of the Triga Mark II reactor at the University of Pavia. Ages were calculated with f = 329.7 ± 6.6 determined for dosimeter glass

SRM 612 through Fish Canyon Tuff and Durango apatite standards (Hurford, 1990) using the population method. The track density on the mica external detectors

attached to glass SRM 612 during irradiation was 3.12 · 105 cm)2; 2435 tracks were counted.

(d)

(c)

(b)

(a)

Fig. 4 Schematic model showing the volcanotectonic evolution of the western MainEthiopian Rift margin and development of the Kella horst in the Central MER. (1)Precambrian biotite gneiss, (2) Mesozoic sediments, (3) 32–8 Ma volcanics, (4) 5–3 Ma volcanics, (5) 3 Ma volcanics and fluvio-lacustrine sediments. Star: location ofKella rock samples.

Terra Nova, Vol 22, No. 6, 470–476 T. Abebe et al. • The Central Main Ethiopian Rift is younger than 8 Ma

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pletely the fission tracks in the Kellabasement samples. Such a thickness isin accordance with the thickness ofthe volcanic successions of volcanicepisodes (1) and (2) (32–29 Ma and�11–8 Ma). The process that sub-sequently allows the samples to coolto temperatures below the PAZ (up toexposure) is denudation induced byrifting.According to Bonini et al. (2005),

the lateral extent and flat-lying geo-metry of the volcanics of event (2)suggest that a thin but very extensiveflood basalt sequence was erupted overa low-relief landscape in the CentralMER between �11 Ma and 8 Ma,implying that no major extensionaldeformation affected this sector of theMER prior to or during that period. Itcan therefore be concluded that majorrift structures were not formed until atleast 8 Ma. Our AFT data indicatingrift-induced denudation as young as7–6 Ma confirm this hypothesis. Vol-canic episode (3) (5–3 Ma), which islimited to the shoulders and innerparts of the MER (see also geologicalmaps of Berhe, 1978; and Merla et al.,1979), could have been triggered bythe major break-up that led to riftformation. Therefore, break-up isbracketed between 8 Ma and 5 Ma.We propose that the basement rocks atKella were finally exhumed by pro-gressive unroofing and related flexuralrebound along the Guraghe borderfault (Fig. 4b, c). Mackenzie et al.(2005) demonstrated that the displace-ment along the rift border faults canreach several kilometres and that thefaults probably extend into the crys-talline basement. Moreover, followingthe hypothesis that Kella was alreadya topographic high in Mesozoic timesand thus that the thickness of theTertiary basalts was reduced, lesseramounts of exhumation would havebeen required to expose the basementrocks with respect to the contiguousblocks. Transversal faults observed inthe field (Fig. 2) could have beenactive after 8 Ma leading to thepreferential erosion of the 30–8 Mavolcanics above the Kella block andalso to the partial removal of the5–3 Ma volcanics during and aftertheir deposition. As the Kella pre-Tertiary rocks are covered by a 3 Maignimbrite, we know that at least atthat time they were close to the surface(Fig. 4d).

Implications for the formation ofthe Central MER

Our new AFT data suggest that themain rifting phase in the CentralMER took place after 8 Ma, confirm-ing the hypothesis of Bonini et al.(2005), who placed the main riftingphase after the �11–8 Ma volcanicevent. As deformation in the NorthernMER started at c. 11 Ma (Wolfendenet al., 2004), postponing the forma-tion of the Central MER to after8 Ma means that the different MERsegments actually formed at differenttimes. Bonini et al. (2005) proposeda model to reconcile the evolutionsof the different MER segments, whichwe summarize here. Extensionalstructures started to develop in theSouthern MER during the EarlyMiocene (20–21 Ma, Fig. 5a) due to

the northward propagation of KenyaRift-related deformation. A furtherpropagation towards the north wasprobably hindered by inherited trans-versal lines, such as the Goba-Bongalineament. In the Late Miocene(11–8 Ma, Fig. 5b), the deformation,propagating southwards from theAfar, focused on the northern MER,while the Southern MER becameessentially quiescent. At that time,no major extensional deformationwas yet affecting the Central MER.Significant extensional deformationand rift opening are evident for theCentral MER at the boundary bet-ween the Miocene and Pliocene, withhigh volcano-tectonic activity after5 Ma (Fig. 5c). Finally, from 3 Mato the Present, propagation of theMER towards the south probablyreactivated the older rift structures in

(a) (b)

(c) (d)

Fig. 5 Schematic evolution of the Main Ethiopian Rift redrawn after Bonini et al.(2005). Arrows indicate the direction of propagation of the extensional deformation.GBF, Guraghe Border Fault; MBF, Munesa Border Fault; NMER, Northern MainEthiopian Rift; CMER, Central Main Ethiopian Rift; SMER, Southern MainEthiopian Rift; YTWL, Yerer-Tullu-Wellel Lineament; GBL, Goba-Bonga Linea-ment; Star: Kella location.

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the Southern MER (Fig. 5d). Boniniet al. (2005) attributed the opening ofthe Central MER and the Quaternaryreactivation of the Southern MER tothe southward propagation of riftinginduced by the clockwise rotation ofthe Somalian plate starting at about10 Ma (Collet et al., 2000). This tec-tonic scenario in which the MERdeveloped well after the beginning ofrifting in the Gulf of Aden (>20 Ma)points to a diachronous developmentof the Red Sea–MER–Gulf of Adenarea. The third arm of the triplejunction nucleated in the Afar after11 Ma (Wolfenden et al., 2004) andthen, according to the Bonini et al.(2005) model and our data, propa-gated southwards.

Acknowledgements

We are grateful to the Associate EditorMeinert Rahn and to James Wood, Cor-nelia Spiegel and an anonymous referee forthe careful revision of the manuscript,which greatly improved it, to ErnestoAbbate for providing the samples, toMarco Bonini, Giacomo Corti and Gio-vanna Moratti for helpful discussions andto Mary Helen Dickson for the Englishlanguage revision.

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Received 19 November 2009; revised versionaccepted 20 August 2010

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