Plio-Quaternary volcanotectonic activity in the northern ......extensive et le magmatisme se sont concentr6s dans une bande 6troite, le Faisceau de Failles de Wonji. Deux zones-cl~s

Pergamon Joumal of African Earth Sciences, Vol. 29, No. 4, pp. 679-698, 1999

© 2000 Elsevier Science Ltd P I 1 : 8 0 8 9 9 - 5 3 6 2 ( 9 9 ) 0 0 1 2 4 - 4 All rights . . . . . . ed. Printed in Great Britain

0899-5362/00 $- see front matter

Plio-Quaternary volcanotectonic activity in the northern sector of the Main Ethiopian Rift: relationships with oblique rifting

MARIO BOCCALETTI, 1 ROBERTO MAZZUOLI, 2 MARCO BONINI, 3'* TERESA TRUA 2 and BEKELE ABEBE 4

~Dipartimento di Scienze della Terra, Universit~ degli Studi di Firenze, via La Pira 4, 50121 Firenze, Italy

2Dipartimento di Scienze della Terra, Universit~ degli Studi di Pisa, via S. Maria 53, 50126 Pisa, Italy

3CNR Centro di Studio di Geologia dell'Appennino e delle Catene Perimediterranee, via G. La Pira 4, 50121 Firenze, Italy

4Department of Geology and Geophysics, Addis Ababa University, PO Box 1176, Addis Ababa, Ethiopia

ABSTRACT-- Deformation and magmatism within the - 90 km wide northern Ethiopian Rift system is concentrated along a narrow zone - the Wonji Fault Belt. Two key areas (the Nazret-Dera and Asela-Ziway areas), located along the eastern margin of the north-northeast to northeast trending Main Ethiopian Rift, have been investigated in order to reconstruct the recent tectonomagmatic evolution of the northern branch of the Main Ethiopian Rift. In these areas, Early Pleistocene volcanic products (Wonji Group) overlie Pliocene volcanic rocks (Eastern Margin Unit). Detailed stratigraphical reconstructions have revealed the presence of several tectonomagmatic units which can be correlated between the two study areas. The stratigraphical and petrological study of these units outlined (1) the bimodal composition (basalts-pantellerites) of the oldest and youngest units and the unimodal character (pantellerites) of the products erupted during the intervening period; (2) the mainly fissural origin of the ignimbrites and oldest basalts; and (3) a mafic/felsic volumetric ratio of 1:5. The geological data suggest that, around the Pliocene-Quaternary boundary, a change in the stress field occurred in this Main Ethiopian Rift sector, passing from a direction of extension roughly orthogonal to the rift shoulders, to oblique rifting related to an east-west trending extension. In this framework the change in the style of volcanism observed in the Nazret-Dera and Asela- Ziway areas can be related to the change of the stress field. A new geodynamic model is presented for the Late Pliocene to Recent evolution of this sector of the Main Ethiopian Rift. According to this model, a large volume of rhyolitic products was erupted during an oblique rifting phase, following a previous period of pure extension. The change in the tectonic regime favoured partial melting of the underplated basalts as a decrease in the pressure and an elevation of isotherms occurred. © 2000 Elsevier Science Limited. All rights reserved.

RI~SUMI~--Dans la partie septentrionale du Rift Ethiopien large d'environ 90 km, la d~formation extensive et le magmatisme se sont concentr6s dans une bande 6troite, le Faisceau de Failles de Wonji. Deux zones-cl~s (Nazret-Dera et Asela-Ziway), Iocalis6es sur la bordure orientale du Rift Majeur Ethiopien de direction NNE ~ NE, ont 6t~ examin6es pour reconstituer I'~volution tectono- magmatique r6cente de la branche septentrionale du Rift Majeur Ethiopien. Les produits volcaniques du Pleistocene inf~rieur du Groupe de Wonji y recouvrent les roches volcaniques plioc~nes de I'Unit~ de la Bordure Orientale. Les reconstitutions stratigraphiques d~taill~es r~v~lent la presence de plusieurs unit~s tectono-magmatiques que I'on peut corr~ler d'une zone ~ l'autre. L'~tude

* Corresponding author [email protected] (M. Bonini)

Journal o f African Earth Sciences 679

M. BOCCALETTI et al.

stratigraphique et petrologique des unites souligne (1) la composition bimodale (basaltes- pantellerites) des unites les plus anciennes et les plus recentes et le caractere unimodal (pantellerites) des produits emis dans la periode intermediaire, (2) I'origine surtout fissurale des ignimbrites et des basaltes les plus anciens, et (3) un rapport volumique mafique/felsique de 1:5. Les donnees geologiques dans ce secteur du Rift Majeur Ethiopien suggerent qu'~ la limite Pliocene- Quaternaire, le champ de contraintes s'est modifie, passant d'une extension ~ peu pros perpendiculaire aux bords du rift ~ une extension est-ouest se traduisant par un mouvement de rift oblique. Dans ce cadre, le changement de style volcanique observe dans les zones de Nazret- Dera et d'Asela-Ziway peut 6tre lie au changement de champ de contraintes. Nous proposons un module geodynamique nouveau de I'evolution de ce secteur du Rift Majeur Ethiopien du Pliocene Superieur au R~cent. Selon ce modele, succedant ~ une premiere periode d'extension pure, un grand volume de produits rhyolitiques a et~ 6mis durant la phase de rifting oblique. Le changement de regime tectonique a favoris6 la fusion partielle de basaltes en sous-placage & la suite d'une chute de pression et d'une ~levation des isothermes. © 2000 Elsevier Science Limited. All rights reserved.

(Received 10/3/98: revised version received 19/12/98: accepted 27/11/98)

INTRODUCTION

The Ethiopian Rift (ER) is part of East African Rift System (EARS) and comprises a series of rift zones extending over a distance of about 1000 km from the Afar Triple Junction at the Red Sea-Gulf of Aden intersection to the Kenya Rift (Fig. 1). The ER is associated with huge volumes of volcanic products which were erupted since its birth. Volcanism started during the Eocene-Late Oligocene and it has been matching the tectonic evolution of the rift up to recent times (Merla eta l . , 1973; Levitte etal . , 1974; Morton eta l . , 1979; Davidson and Rex, 1980; Wol- deGabriel et al., 1990; Ebinger et al., 1993; Abebe et al., 1998b). In the ER two main sectors can be dist inguished: a northeastern sector (Main Ethio- pian Rift: MER), where the recent volcanic products are markedly prevalent; and a southwestern sector (Southwestern Ethiopian Rift: SER), where large volumes of the oldest volcanic rocks are present. The latter (SER) is mainly characterised by north-south trending faults that are linked to the northward propagation of the Kenya Rift (Fig. 1 ).

The MER developed during the Late Miocene (Ebinger et al., 1993, and references therein) and it is also characterised by well-developed Quaternary faulting that is mostly related to the Wonji Fault Belt (WFB: Mohr, 1962, 1967) (Figs 1 and 2). Quaternary rocks are commonly affected by faulting along this belt, which defines an important active volcanotectonic axis located within the broader rift valley bounded by border faults trending north-northeast to northeast (Fig. 1). The estimated extension in the MER ( - 20%, [5 factor=1.2: Ebinger eta/ . , 1993) is comparatively small when compared to Afar, where extension may exceed 100% (!3 factor=2: Berckhemer et al., 1975).

The Plio-Quaternary vblcanic products of the MER show a marked bimodal composition, with the alkali basalts and rhyolites (mainly pantellerites) dominating the sequence with scarce intermediate compositional rocks (Gasparon et al., 1993; Boccaletti et al., 1995; Trua eta/ . , 1999, and references therein). In this sector, the felsic products constitute 80% of the exposed rocks (Mohr, 1992). In the northernmost sector of the MER, the central volcanic edifices are represented by composite volcanoes and by caldera str~Jctures (e.g. Mohr, 1983, 1987; Kazmin, 1980). The age and geochemical character of the volcanism changes along the MER, and its complex history and nature reflects the tectonic evolution of the rift.

This study is focused on the tectonics and volcanism of a wide sector located south of Addis Ababa, where detailed mapping at the 1:50 000 scale for two key areas (Nazret-Dera: Alula et al., 1992; and Asela-Ziway: Abebe et al., 1998a) has been carried out. Geological and petrological data acquired for this key area of the northern branch of the MER allowed the correlation in time and space of the tectonic evolution and volcanic activity since Pliocene times. On the basis of the relationships between tectonics and volcanic processes, it has been possible to reconstruct the recent tectonomagmatic history of the area and to put forward a new geodynamic model for the MER evolution.

STRUCTURAL OUTLINES

In the MER two distinct fault systems are recognisable: i) a north-northeast to northeast trending border

fault system which is well-developed, especially along the eastern margin separating the rift zone from the Somalian Plateau; and

680 Journal of African Earth Sciences

Plio-Quaternary volcanotectonic act iv i ty in the northern sector o f the Main Ethiopian Rift

Figure 1. Schematic structural sketch o f the Main Ethiopian Rift superimposed onto an elevation map. Location of the MER within the Afro-Arabian Rift system is also indicated. 1: main normal or oblique-slip faults (black squares on the downthrown side); 2: normal or oblique-slip faults with smaller vertical offset (black squares on the downthrown side); 3: main Quaternary basalts of the rift.

ii) a north-northeast-south-southwest to north- south trending, right stepping en echelon fault system (the Wonji Fault Belt: WFB) affecting the youngest volcanic rocks (Fig. 2). According to Meyer eta/ . (1975), the WFB faulting started developing at the beginning of the Early Pleistocene ( - 1.6 Ma ago), causing the important unconformity occurring between the 'Wonji Series' (Pleistocene-Holocene) and the underlying 'Nazreth Series', whose more recent products are about 1.8 Ma (Bigazzi et al., 1993)o The WFB is located between the rift borders and affects the rift floor branching off from the eastern border (Fig. 2).

The tectonic evolution of the MER has been commonly related to a pure extension mechanism (McKenzie et al., 1 970; le Pichon and Francheteau, 1978; Mohr, 1983; Ebinger eta/., 1993). However, structural data indicate that the extension direction during the Quaternary was approximately orientated east-west, forming an angle of about 50 ° to the rift margins (Boccaletti etaL, 1992, 1994, 1998; Abebe, 1993; Bonini eta/., 1997). This direction of extension

is also supported by the local palaeostress field orientations determined through the analysis of the kinematic indicators on fault surfaces (Boccaletti et al., 1992, 1998). The oblique extension resulted in a left-lateral component of motion along the rift floor, which caused the WFB fault system to develop (Mohr, 1968; Gibson, 1969; Gibson and Tazieff, 1970; Boccaletti eta/. , 1992; Abebe eta/., 1998b). Quaternary oblique faulting is also coherent with the locally complex structural patterns as graben in graben, rhomb-shaped and pull-apart structures, which are kinematically compatible with the left- lateral component of motion along the rift axis (Boccaletti eta/., 1992, 1998). In addition, the WFB is composed of right-stepping, offset fault segments (see fig. 2 in WoldeGabriel et a/., 1990), which further support the occurrence of a sinistral component of displacement along the rift structure. The east-west orientated extension does not necessarily contrast with the northwest to north-northwest trending extension determined by Chorowicz et al. (1994) on the basis of north-south to N20 ° dextral

Journal of African Earth Sciences 681


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Figure 2. Structural map of the northern sector of the Main Ethiopian Rift (MER). Polygons indicate the studied areas (Nazret-Dera and Asela-Ziway). See Fig. I for location.

oblique faults affecting the rift floor of the MER. The dextral faulting, in fact, could be related to a counter-clockwise rotation of fault-bounded blocks in response to left-lateral shearing along the rift structure (Bonini et al., 1997; Boccaletti et al.,1 998).

Composite volcanoes are mainly located on the margins of the rift, whereas the caldera structures, to which large ignimbrite sheets are associated, occur on the central sector of the rift, mainly in line with structural discontinuities (e.g. Mohr, 1983,

1987; Kazmin, 1980)(Fig. 2). WoldeGabriel et al. (1990) subdivided the volcanic

products of the whole Ethiopian Rift into six main tectonomagmatic chronozones since the Eocene- Oligocene; among these, the WFB-related volcanism represents the most recent volcanotectonic activity in the MER. This detailed stratigraphical and petrological study mainly concerns the WFB-related volcanism, which is particularly widespread in the Dera- Nazaret and Asela-Ziway areas.


P//o-Quaternary volcanotectonic activity in the northern sector of the Main Ethiopian Rift

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Figure 3. Geological sketch map of the Nazret-Dera area. 1: Melkasa Unit; 2: Gedemsa Unit; 3: Dera-Sodore Unit; 4: Bofa Unit; 5: Boku-Tede Unit; 6: Keleta Unit; 7: Nazret Unit; 8: Eastern Margin Unit; 9: spatter cones; 10: lava domes; 11: normal faults (barbs on the downthrown side); 12: volcanotectonic collapse (barbs towards the collapsed side); 13: main roads.

VOLCANOLOGY AND STRATIGRAPHY

The studied key areas are located in the northern part of the MER (Fig. 2) where the rift margin on the eastern part is bounded by a succession of fault scarps that are about 500 m high (eastern margin). The two areas can be considered in the framework of different structural contexts: the Nazret-Dera area is situated between two main en echelon right- stepping north to north-northeast trending WFB segments within the floor, while the Asela-Ziway area is located close to the eastern margin, where the WFB system and the north-northeast to northeast trending border fault system interact (Fig. 2).

In both areas, the oldest volcanic rocks outcrop along the fault-scarps of the eastern margin and are overlain by Quaternary volcanic products, which can be correlated to the WFB volcanism or 'Wonji Series' or 'Wonji Group' (Meyer et al., 1975; Mohr, 1983, 1987; WoldeGabriel etal., 1990). The 'Wonji Group'

products(< 1.6 Ma: WoldeGabriel et al., 1990) are extensively exposed throughout the rift floor.

Based on geological and petrological data of the volcanic rocks from both the Nazret-Dera and Asela- Ziway areas, several tectonomagmatic units are recognised within the Wonji Group. Using the field relations, petrology and radiometric data (published and new radiometric age determinations, Table 1 ) a systematic stratigraphical framework is developed for this important sector of the MER, correlating the tectonomagmatic units of these two areas.

Nazret-Dera area The Nazret-Dera area represents the bridge zone between two right-stepping north-northeast trending sets of normal faults belonging to the WFB (Fig. 2). One of the belts is situated along the western side of Fig. 3. It affects the Gedemsa Caldera and terminates north of Nazret (Figs 2 and 3). The other


Plio-Quaternary volcanotectonic activity in the northern sector of the Main Ethiopian Rift

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fault belt originates in the eastern sector of Fig. 3 and extends northward to the Kessem River valley area (Fig. 1). The southeastern area of Fig. 3 is affected by a northeast-southwest trending fault constituting a border fault of the eastern margin.

The volcanic succession outcropping in the Nazret- Dera area is subdivided into eight main units (Figs 3 and 4). In the southeast corner of the area, the Pliocene volcanic rocks (Eastern Margin Unit: 1 .8 Ma, Table 1 ) are bounded by rift margin faults. Early



Pleistocene products ('Wonji Group') overlie the Eastern Margin Unit rocks and occupy the rift floor (Fig. 3). The central sector is characterised by the development of the youngest volcanism, which is mainly concentrated along the rift axis. The field data revealed that the volume of the felsic products is about 500 km 3, whereas those of mafic rock is much lower, being about 50 km 3. The tectonomagmatic units are described below, starting from the older products.

Eastern Margin Unit This unit is dominated by mafic and felsic volcanic rocks which are exposed along the fault-scarps of the eastern rift margin in the Nazret-Dera sector. These rocks consist of alkaline-transitional basaltic lavas, and pantellerite, dacitic and rhyolitic ignimbrite sheets, generally strongly welded, and sporadic lava domes (Table 2). Often these products are in places covered by the products of the Nazret Unit (Fig. 3). An age of 1.8 Ma has been reported by Bigazzi et al. (1993) for an ignimbrite sheet of this unit (Table 1). The base of the Eastern Margin Unit is not exposed. However, the Middle Pliocene peralkaline obsidian lavas, belonging to the uppermost part of this unit, floor the rift under the Gedemsa WFB segment (Morton et al., 1979).

Nazret Unit This unit corresponds to the 'Nazreth Series' of Meyer et al. (1975). The Nazret Unit is made up of a sequence of ignimbrite sheets interbedded with palaeosol layers, indicating frequent breaks in the volcanic activity and aphyric flood basalts. The ignimbrite units are frequently strongly welded at the base and unwelded at the top. They are mainly characterised by glassy 'f iamme', showing a pantelleritic composition, and by abundant basaltic lithic fragments (Table 2). The basaltic lavas are mainly transitional and subordinately alkaline (Table 2). The age of this unit ranges from 1.7 to 1.5 Ma (Table 1 ).

Keleta Unit This unit constitutes the basal series of the 'Wonji Series' of Meyer et al. (1975). This succession unconformably overlies the previous Nazret Unit products; Meyer et al. (1975) relate this unconformity to an important change in tectonic activity. The Keleta Unit developed after a period of volcanic quiescence and could be considered as the actual rift floor refilling. It constitutes a sequence of ignimbrite units separated by palaeosols, reaching its great- est exposed thickness ( - 1 O0 m) in the Keleta River valley. In the lower part of the sequence, welded

thin ignimbrite sheets are followed by a thick, poorly welded, usually zeolitised ignimbrite (Fig. 4). This ignimbrite is characterised by abundant lithic fragments which reach a maximum diameter in the Keleta River valley. This suggests that the Keleta zone is the probable source area. The upper part of the sequence is formed by loose ignimbrite units which are largely widespread, covering all the central sector of the rift (Fig. 4). The welded ignimbrites are pantellerite rhyolites (Table 2). The absence of recognisable volcanic centres in the area where this ignimbrite unit outcrops suggests that this unit has a fissural origin.

Boku- Tede Unit The products of this unit are related to the collapse of a huge caldera (Boku Caldera) whose remnant rims are located in the central part of the rift (Fig. 3). According to Morton etal. (1979), the Boku Caldera/ cone diameter ratio was probably close to 1, so that caldera collapse led to the destruction of the greater part of the remnant cone. The Boku-Tede products are represented by ignimbrite sheets, pyroclastic fall and surge deposits, and highly fractured lava domes with associated obsidian layers. The surge deposits cover the central and western part of the area and their maximum thickness of 2 m decreases towards east. The volcanic rocks of this unit consist of pantellerite trachytes and rhyolites (Table 2). The radiometric age determinations carried out on rhyolites and ignimbrites gave an age ranging from 0.83 Ma up to 0.51 Ma (Morton etal., 1979) (Table 1 ). These products, therefore, are roughly coeval with the following Bofa basalts, which are essentially developed in the central part of the rift area.

Bofa Unit Mafic lava flows, mainly of fissural origin, crop out in the central area of the rift and overlie the ignimbrite sequence of the Keleta Unit (Figs 3 and 4). These rocks are mainly transitional basalts, with subordinate alkaline basalts and mugearites (Table 2). The lavas of this unit are commonly porphyritic with large laths of plagioclase. The maximum thickness (150 m) of the basaltic sequence decreases from northeast to southwest in the central part of the rift. The radiometric ages range from O.61 to 0.44 Ma (Morton et al., 1979) (Table 1 ).

Dera-Sodore Unit The most widespread ignimbrite sheet (about 400 km 2) of the area belongs to this unit. It is characterised by thin layers of unwelded or poorly welded ash containing small, scattered, rounded pumice clasts and lithics. In places these layers are interbedded



with thin pal~eosols. Its composition is pantelleritic (Table 2) and it contains a few crystal fragments of sanidine. Between the villages of Sodore and Dera, and in the northeastern sector of the area of Fig. 3, pantellerite rhyolitic lava domes with associated thick lava flows and pumice fall deposits are present. The lavas are usually vesicular, whereas in places they contain layers of obsidian lava.

There are no age determinations on the products of this unit. The field data indicate that the fine- grained ignimbrite sheet overlies the Bofa basalts.

Gedemsa Unit As with the Boku-Tede Unit deposits, the products of this unit are related to the collapse of a caldera approximately 8 km in diameter (Gedemsa Caldera), outcropping southwest of Boku along the central western part of the rift (Fig. 3). The pre-caldera activity gave rise to pantellerite rhyolite ignimbrites (Table 2) associated with ash flows, pumice falls and surge deposits (Fig. 4). Pantellerite rhyolitic lava domes are also present in the pre-caldera sequence (Table 2). The post-caldera products are composed of resurgent pantelleritic lava domes and basaltic spatter cones scattered within the caldera depression and just outside the rims of the caldera.

The radiometric data on the Gedemsa products are controversial. According to Morton etal. (1979), the build-up of the Gedemsa cone started 0.85 + 0.07 Ma ago. However, this age seems too old as the Gedemsa Volcano is stratigraphically younger than the Boku-Tede and Bofa Units (Fig. 4). Bigazzi eta/. (1993) obtained a fission track age of 0.21 _+ 0.032 Ma carried out on the glass of a pre-caldera lava flow exposed on the northeast rim of the Gedemsa Caldera (Table 1 ). If the K/Ar age on the whole rock of a green welded tuff reported by Morton et al. (1979) was affected by excess Ar, then the 0.21 Ma fission tracks age appears to be much more acceptable according to the stratigraphical data.

Boseti Volcano On the northeastern sector of the studied area, lava flows outpoured by the Jinjimma trachyte cone (Boseti-Gudda Volcano) have been mapped. Morton et al. (1979) quote an age of 0.21 Ma for these lava flows (Table 1 ).

Melkasa Unit This unit represents the most recent volcanic activity, mainly concentrated along the rift axis (Fig. 3), and is characterised by small spatter and cinder cones associated with basaltic lava flows (Fig. 4, Table 2). The basalts are generally aphyric

or scarcely porphyritic for plagioclase, pyroxene and olivine.

The radiometric dating carried out on these rocks show that they are younger than 0.2 Ma (Morton et al., 1979) (Table 1 ).

Asela-Ziway area In the Asela-Ziway area, the relationships between the north-northeast to northeast trending border fault system and the WFB system branching off from the rift border are clearly observable (Fig. 2). Further- more, this sector constitutes a key area in which the relations between the tectonic evolution and the associated magmatism can be investigated.

The oldest volcanic rocks outcrop along a northeast-southwest trending fault scarp located in the eastern part of Fig. 5, west of Asela town. Further west, the WFB cross-cuts the rift floor. The youngest volcanism is confined to the central rift area. Also in this area, the felsic volcanic products are largely predominant (showing a volume of - 350 km 3) over the basaltic ones ( - 5 0 km3). The whole exposed volcanic succession has been subdivided into five units that are described below, starting from the most ancient products.

Eastern Margin Unit and Chilalo Volcano This unit consists mainly of tabular aphyric or sub- aphyric hawaiite and mugearite lava flows (Table 2) and associated volcanic agglomerates. Pantellerite ignimbrite sheets (Table 2) are present in the upper part of the sequence along the eastern fault escarpment (Figs 5 and 6). The thickness of basaltic lava flows ranges from 2 m up to 5 m. Pal~osols interbedded with basalts occur sporadically.

Few age determinations are available on the basalts outcropping along the rift margin. They reveal a Plio- Pleistocene age (2-1.8 Ma, Table 1) and refer to two mafic lava flows from the rift margin (Wolde- Gabriel et al., 1990). Moreover, an apparent K-Ar age of 1.8+0.1 Ma (Table 1) was obtained on a hawaiite lava flow, sampled in the Eastern Margin escarpment.

The stratigraphical data suggest that these lavas could be partially interbedded with those of the Chila- Io Volcano on the southeastern shoulder of the rift. The Chilalo central volcano has a large crater (4 km north-south; 3 km east-west) at its summit. It dis- plays basaltic-trachyandesitic lava flows (Table 2) interbedded with strombolian scoriaceous layers. Sam- ples from the lower and upper northern and western slope of the Chilalo Volcano have been dated by WoldeGabriel eta/. (1990) and Bigazzi eta/. (1993). The K-Ar radiometric determinations indicate ages ranging from 1.74 Ma up to 2.31 Ma (Table 1).


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Journal o f African Earth Sciences 689


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Figure 5. Geological sketch map of the Asela-Ziway area. 1: alluvial deposits; 2: Galo-Salen UniC 3: Aluto-Berecha Unit; 4: Tulu Moye-Hate Unit; 5: Asela Unit; 6: Eastern Margin Unit and Chilalo volcano; 7: spatter cones; 8: normal faults (barbs on the downthrown side); 9: volcanotectonic collapse; 10: main roads.

Asela Unit The products of the Asela Unit outcrop in the eastern part of the rift and overlie the Eastern Margin Unit rocks. The maximum thickness of this unit has been found along the main border fault escarpments, southwest of the Kulumsa village (Fig. 5). The rock types of this unit mainly consist of pantellerite and commendite ignimbrites (Table 2) exhibiting different textural characters due to the grade of welding. The only radiometric age available on these products is of 1.66 Ma (WoldeGabriel et al., 1990) (Table 1 ).

Tulu Moye-Hate Unit The huge volumes of felsic products belonging to this unit consist of ignimbrites and minor lava domes and f lows related to caldera collapse, or to the emplacement of lava domes aligned wi th WFB faults in the western part of the central rift area (Fig. 5). These products locally overlie the Asela Unit and consis t of peralkal ine rocks: mainly pantellerites wi th subordinate comendites (Table 2). The ignimbrite sheets are usually loose at the top and occasional ly contain large glassy dark



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'fiamme'. The pantelleritic domes show locally characteristic flow banding and interbedded vesicular layers. Usually surges associated to pyroclastic fall and flow deposits are linked to the emplacement of these domes.

No age determinations have been previously published on Tulu Moye-Hate products. One K/At date is presented here for a pantellerite lava dome belonging to this unit. This sample yielded an apparent age of 1.3+0.3 Ma (Table 1).

Aluto-Berecha Unit This unit is exposed on the western part of the Asela-Ziway area and consists of pantelleritic lava domes and flows of different ages, associated with pumice fall and flow deposits (Figs 5 and 6). The oldest group is characterised by pantelleritic lavas (Table 2) with phenocrysts of alkali feldspar in a glassy groundmass (vitrophyric lavas). The associated pumice fall and pyroclastic flows deposits occasionally contain large clasts and basaltic lithics. The youngest pantelleritic products are made up of porphyritic glassy lavas with micro- phenocrysts of anorthoclase or sanidine, quartz and aegirine. The pumice fall associated with these domes are characterised by uniformly small clasts in a very scarce fine matrix. The oldest domes have been dated at 0.83 Ma by WoldeGabriel et al. (1990).

Galo-Salen Unit The recent Galo-Salen basaltic flows either have a fissure origin along N30°-40 ° structures or are related to relatively small cinder and spatter cones (Figs 5 and 6). The oldest basalts generally contain large and abundant laths of plagioclase and iso- lated crystals of pyroxene and olivine. The more recent basaltic sequence is represented by aphyric lava flows (Table 2) erupted along faults situated along the rift axis and shifted westward in relation to the older basaltic lavas. The most recent basaltic activity gave rise to spatter and cinder cones scattered in the whole Asela-Ziway area, which are sometimes aligned in a north-south direction (Fig. 5). Available K-Ar radiometric age determinations of the oldest basalt gave an age of 0.29 Ma (WoldeGabriel et al., 1990). On the basis of radiometric ages and the stratigraphical relationships, this unit can be considered coeval to the Aluto-Berecha Unit.

The above discussed stratigraphical and geological data emphasise the occurrence of a markedly similar volcanic evolution for both the Nazret-Dera and Ase!a-Ziway areas. In both areas, in fact, the oldest volcanism forming the rift shoulders (Eastern Margin Unit), as well as the last period of volcanic activity, which mainly developed along the WFB, were characterised by the interfingering of coeval acid and mafic volcanic rocks (cf. Figs 4 and 6). Similarly, between these two main volcanic episodes only felsic magmas were erupted in both areas (Figs 4 and 6). It is suggested in this study that the reconstructed volcanic evolution was strongly associated with the tectonic evolution of the rift, as will be discussed later.



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Figure 7. Total alkalis versus silica diagram (TAS: Le Bas et al., 1986) highlighting the marked bimodafity of the Nazret-Dera and Asela-Ziway volcanic products with respect to silica. The Chilalo volcanic products are also reported. IB: subalkaline-alkaline divide (#vine and Baragar, 19 71).

PETROLOGICAL CONSIDERATIONS

The representative chemical analyses of the main rock units from the Nazret-Dera and Asela-Ziway areas are shown in Table 2. The studied rocks were classified on their major element chemistry. The total alkalis versus silica (TAS) diagram (Fig. 7) illustrates that, among the analysed samples, only the Chilalo lavas display intermediate compositions, ranging from transitional basalts to hawaiites, mugearites and benmoreites. All the other units mainly consist of basalts and/or rhyolites. The most common mafic rocks in the Nazret-Dera and Asela-Ziway areas are alkaline and transitional basalts, whereas hawaiites and mugearites are rare (Fig. 7). Felsic products are peralkaline rocks (molecular [Na20 + K20]/AI203 > 1 ), mainly pantellerites with subordinate comendites (Table 2, Fig. 7).

The acid products are largely predominant in both studied areas. The proportion of mafic/felsic products is around 1:5 in this MER sector. At least 600 km 3 of ignimbrites with subordinate lavas and domes were erupted during the Quaternary in this sector of the MER. The majority of the ignimbrites were erupted along important faults during the last stages of rift formation, whereas some ignimbrite sheets are linked to caldera collapse (i.e. Gedemsa, Boku and Tulu Moye).

The compositional bimodality of volcanic products is one of the most striking characteristics of the MER. The genesis Of acid products can mainly be related to:

i) fractional crystallisation from a basaltic parent magma in shallow magmatic chambers;

ii) fractional crystallisation plus assimilation of continental crust in crustal reservoirs and during the ascent of mantle melts; and

iii) melting of the continental crust (anatexis) or partial melting of underplated basalts (lower crust).

On the basis of geochemical data, Gasparon eta l . (1993) and Boccaletti et al. (1995) argue that the fractional crystallisation plus assimilation process is the most probable mechanism which accounts for the genesis of the huge volumes of rhyolitic rocks (mainly ignimbrites) of the MER. However, if the felsic rocks are the result of fractional crystallisation or fractional crystallisation plus assimilation processes from more mafic magmas, this would require huge volumes of resulting cumulates stored in relatively shallow levels of continental crust, for which there is no petrological evidence (lack of mafic cumulates). Moreover, geophysical data seem to indicate the presence of softer material in the northern part of the East African Rift system (Kebede and Kulhanek, 1991 ; Hayward and Ebinger, 1996). Moreover, fractional crystallisation calculations between the most primitive basalts sampled near Asela and the pantellerites exposed in the same area yield a high degree of crystallisation (82%) with a high sum of residues (1.84), indicating that this model cannot account for the genesis of these MER felsic magmas (Trua et al., 1999). Furthermore, the overlapping Nd and Pb isotopic compositions of mafic and felsic rocks in the Asela Ziway area (Fig. 8) suggest that the rhyolites cannot be related to basaltic magma through fractional crystallisation plus assimilation processes.


P//o-Quaternary volcanotectonic activity in the northern sector o f the Main Ethiopian Rift

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Figure 8. ~43Nd/744Nd versus 2°~PbP°4Pb isotopic diagrams for Asela-Ziway and the Chilalo volcanic products (data from Trua et al., I999). Ethiopian basalts: MER (Hart et al., 1989), southern Ethiopia (Stewart and Rogers, 1996); DM, HIMU, EMI and EMIl: end-member mantle components (Zindler and Hart, 1986; Hart, 1988).

The bimodality and huge volume of the rhyolites with respect to the basaltic lavas could suggest that the partial melting of upper continental crust (anatexis) is the dominant process. However, this process must be discarded if the overlapping Nd and Pb isotopic compositions of these rocks is taken into account (Fig. 8).

On the basis of isotopic and trace element data, Trua et al. (in press) suggest that the partial melting of basaltic lower crust could be a process plausible for the generation of these rhyolitic (pantelleritic) products. Further modal equilibrium melting calculations were performed with incompatible elements such as K, Th, Ta and Zr, and are illustrated in Fig. 9. Small degrees (10%) of partial melting of the mafic lower crust (underplated magmas) here may account for the relationship between mafic and felsic rocks.

Most probably not all the rhyolites exposed in the studied areas originated by partial melting of underplated basalts. The pantellerite ignimbrites and the

resurgent lava domes and flows related to small caldera collapses may derive from fractional crystalli- sat/on plus assimilation processes in shallow crustal reservoirs from basaltic parent magmas (Gasparon et al., 1993; Boccaletti et al., 1995). The huge volumes of acid magmas (unimodal volcanism) erupted between 1.6 and 0.6 Ma imply that partial melting of underplated basalts may have been triggered by a change of tectonic regime in the intense fault systems formed along the rift axis.

GEODYNAMIC MODEL Earlier, the fact that the evolution of the ER has been commonly considered as linked to pure extensional movements along the border faults was discussed. Recently, on the basis of geological and structural data collected on the faults affecting the Rio-Quaternary volcanic rocks of the MER, an east- west direction of extension was determined (Bocca- letti eta/., 1992, 1998; Abebe, 1993; Bonini eta/.,


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Plio-Quaternary volcanotectonic act iv i ty in the northern sector o f the Main Ethiopian Rift

1997). This extension is oblique to the rift trend, inducing a sinistral component of displacement along the axis of the rift. This deformation must have acted during very recent times as it affected volcanic rocks younger than 0.20 Ma (i.e. the Gedemsa, Melkasa and Aluto-Berecha Units: Alula et al., 1992; Bocca- letti et al., 1998).

A problem arises from these considerations: was such a direction constant in time or has the stress field changed during the evolution of the rift? In fact, oblique rifting could have controlled the entire evolution of the rift, re-activating roughly northeast- southwest trending pre-existing structures within the basement. A northeast-southwest trending fabric is well-developed in the Pan-African basement where it is exposed away from the rift valley (e.g. Hackman et al., 1990).

The age of the onset of this oblique rifting process is probably related to the development of the en echelon WFB system (Bonini eta/., 1997; Boccaletti eta/., 1998), which occurred at around 1.6 Ma (Meyer et al., 1975). This period also roughly corresponds to the lower limit of the 'Wonji Group' tectonomagmatic chronozone (WoldeGabriel et al., 1990), thus suggesting a link between tectonics and magmatism. The east-west extensional direction is compatible with the present architecture of the rift structures characterised by a north-northeast to northeast trending border system and by the north to north- northeast en echelon right-stepping WFB system, mainly dominating the rift floor.

The geological and stratigraphical data indicate that between 1.8 and 1.6 Ma an important change in the volcanic style occurred. In fact the volcanic activity which produced the Eastern Margin Unit products was characterised by both basaltic lavas and rhyolitic ignimbrites, whereas from the Early Pleistocene the volcanism gave rise to pantelleritic ignimbrites with subordinate acid lavas and domes (Wonji Group: WoldeGabriel et al., 1990). The volcanism in this period (from 1.6 up to 0.6 Ma: Figs 4 and 6) became mainly unimodal in all of this sector of the MER (Nazret-Dera and Asela- Ziway areas). Basaltic volcanism resumed during the Middle Pleistocene (0.6 Ma) with eruptions of the Bofa and Galo-Salen basalts along north- south and north-northeast structures. The eruptions of basalts were accompanied by eruptions of huge volumes of pantellerite ignimbrites mainly linked to the regional faults located either along the rift axis or along the border faults. Minor volumes of ignimbrites and resurgent lava domes and flows of limited extension were outpoured in response to caldera collapse (Gedemsa, Boku and Aluto-Berecha).

These data suggest that around the Pliocene-Qua- ternary boundary (1.8-1.6 Ma) a change in the stress field occurred in this MER sector, passing from a direction of extension roughly normal to the rift shoulders to east-west extension (Bonini eta/., 1997; Boccaletti et al., 1998). This hypothesis is also supported by the fact that, along the eastern rift margin, well-developed pre-Quaternary northwest-southeast orientated dyke swarms (indicating a roughly northeast-southwest extension) are capped by Wonji Qua- ternary volcanic products (Abebe, 1993). An oblique rifting phase following a previous pure extension is also consistent with results of analogue models exploring two-stage rifting (Bonini et al., 1997). In this scenario, the proposed change of the extension direction could be related to the complex kinematic interactions between both plate boundaries and/or between EARS segments (in this case between the MER and the Kenya Rift) (e.g. Bosworth eta/., 1992).

On the basis of the above discussed data, a geodynamic evolution model illustrating the history of the northern sector of the MER has been hypothe- sised and briefly summarised below. Figure 10a sche- matically represents this for this MER sector during Miocene-Pliocene times, when the extension direction was normal to the rift axis. In this stage the border faults are active as pure normal faults. These faults acted as magma conduits, since Miocene times, in agreement with Ebinger eta/. (1993). This stage of rift formation is characterised by basaltic lavas interbedded with pantelleritic ignimbrites (bimodal vertical volcanism; Eastern Margin Unit). This could be related to the fact that some of these faults reached the upper mantle, favouring the uprising of mafic melts, whereas others reached the magmatic reservoirs where differentiated magma was stored. The magmatic chambers were probably mainly placed in the continental crust, perhaps at the brittle- ductile interface, where a regime of pure extension could have favoured the set up of magmatic reservoirs (Fig. 10a).

At the end of Pliocene times, the oblique extension due to the re-orientation of the stress field started to act (Fig. lOb). In this stage, the north-northeast to north-south faults (WFB) began to develop, mainly affecting the rift floor. These faults formed within the rift shoulders and could have favoured partial melting of the underplated basalts because of the sharp pressure decrease and the likely uprising of isotherms. This process gave rise to important volumes of felsic magmas (pantellerites and comendite ignimbrite of Asela, Nazret, Tulu Moye-Hate and Keleta Units: Figs 4 and 6).

Partial melting of underplated basalts is favoured by the presence of fluids rich in halogens in the



Figure 10. Schematic block diagram illustrating the control on magmatic evolution in the view of the two-phase tectomc model proposed for the MER evolution. Miocene-Pliocene pure extension caused the contemporaneous eruption of basalts and acidic rocks (bimodal magmatisml depending upon the depth reached by the normal faults (a). At the Pliocene- Quaternary boundary the MER was characterised by oblique rifting and, as a consequence, the WFB developed within the rift shoulders (b). A t the beginning of this phase, magmatism was characterised by unimodal acidic magmatism that become bimodal again when the faults could reach the upper mantle and basalts could rise up to the surface. The dark area indicates the underplated basaltic magma, while the stippled area indicates the partial melting of the underplated magmas.

lower crust. Such a mechanism has also been in- voked to explain the genesis of the Olkaria peralkaline rhyolites (Kenya). On the basis of the high halogen contents of these rhyolites, a volatile fluxing

mechanism promoting partial melting was proposed for their genesis (Bailey and Macdonald, 1970; Davies and Macdonald, 1987). Although no halogen data are available on the MER peralkaline rhyolites,



the harmful biological effect of fluoride on the Ethio- pian population living along the MER has been known for years. Indeed, high F contents are very common in natural waters of the MER, especially in the Lakes region (Chernet et al., 1997). The combination of the fluids and the decompression process triggered the partial melting of underplated basalts, giving rise to large volumes of acid alkaline melts. During this phase the eruptions linked to shallow magmatic chambers also persist. In this case the eruptions of rhyolitic products could be related to periods of tectonic quiescence, which favoured the ponding and consequent evolution of the magmas in magmatic reservoirs. Continuing the oblique extension up to recent times, the WFB faults reached the upper mantle allowing the uprising of basaltic magmas (Bofa, Galo-Salen and Melkasa Units), while the felsic products' eruptions persisted along this MER sector (Aluto-Berecha and Gedemsa Units), giving rise once more to bimodal volcanism.

ACKNOWLEDGEMENTS

The authors thank C. Ebinger and two anonymous reviewers for their thoughtful suggestions that improved the manuscript. The work was supported by CNR (contract 97.00307.CTO5 responsible R. Mazzuoli) and MURST 40% and 60% (M. Boccaletti) grants. Editorial handling - C. Ebinger

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Plio-Quaternary volcanotectonic activity in the northern ......extensive et le magmatisme se sont concentr6s dans une bande 6troite, le Faisceau de Failles de Wonji. Deux zones-cl~s