Quaternary Faulting and Volcanism in the Main Ethiopian Rift

Journal of African Earth Sciences 48 (2007) 115–124 www.elsevier.com/locate/jafrearsci

Quaternary faulting and volcanism in the Main Ethiopian Rift

B. Abebe a,*, V. Acocella b, T. Korme c, D. Ayalew a

a Department of Earth Sciences, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia b Dip. Scienze Geologiche Roma TRE, Largo S.L. Murialdo, 1, 00146 Roma, Italy c Regional Centre for Mapping of Resources for Development (RCMRD), Nairobi, Kenya

Received 5 July 2005; received in revised form 3 March 2006; accepted 6 August 2006 Available online 21 February 2007

Abstract

The Main Ethiopian Rift (MER) is associated with bimodal Quaternary magmatism. Field, remote sensing, and geochronology data are used to examine the relationships between axial acidic volcanoes and basaltic eruptions. Two main Quaternary magmatic episodes are recognizeable in MER: (a) basaltic flows followed by ignimbrites and silicic centers in the rift floor (2–1 Ma) and (b) axial silicic volcanoes and basalts since 650 Ka. The first episode consists mainly of basaltic flows related to the Afar Stratoid and outcrops in the central and northern MER. Scattered silicic centers developed subsequently along the rift floor. In the second episode, spatial and temporal correlation between rift localization and silicic centers becomes more evident. The silicic centers are located at the intersection of the WFB with earlier structures, especially E–W faults. With ageing, these centers become faulted and allow basalts to erupt right through the volcanic edifice, suggesting a decrease in the amount of differentiation in the magma chambers. This style of evolution appears to be characteristic of continental rifts prior to the onset of drifting. Ó 2007 Elsevier Ltd. All rights reserved.

1. Introduction age of the rift. In particular, the northern part of the EARS, that is the Main Ethiopian Rift (MER), provides Understanding how regional tectonics may control vol- the opportunity to study, in a restricted area, the variation canism is crucial in defining the evolution of rift zones. This in volcanism (mainly composition and volumes) as a func- requires the definition of the overall rift structure and the tion of the amount of the extension along the rift. In fact, related volcanic features. Of particular importance are the rate and the total amount of extension of the MER the understanding of the architecture and segmentation increase northwards, being largest in the Afar triple junc- of the main fault zones, as well as their kinematics. Funda- tion. As a result, the MER itself gets wider and deeper mental volcanic parameters to consider include the compo- northwards. At the same time, the erupted products, show- sition and age of the erupted products, as well as the type ing an overall bimodal composition, change in volume and of volcanoes. An interesting feature marking the evolution composition along MER. The northermost part is mainly of continental rifts is that, at a general scale, the type of characterized by widespread basaltic lava flows, associated volcanism changes accordingly with the amount of exten- with shield volcanoes or erupted fissures (Hayward and sion, reflecting the involvment and differentiation of vari- Ebinger, 1996; Lahitte et al., 2003); these basalts constitute ous volumes of magma (Latin and Waters, 1991; Metcalf most of the floor of the northern MER (e.g., Chernet et al., and Smith, 1995). 1998). Conversely, felsic central volcanoes, usually charac- The East African Rift System (EARS) provides the terized by calderas, are responsible for the emission of rhy- opportunity to investigate the compositional variation of olite ignimbrites and predominate in the central and erupted magma with the amount of extension, that is the southern part of the rift (WoldeGabriel et al., 1990; Cher- net et al., 1998; Ebinger and Casey, 2001; Acocella et al., * Corresponding author. 2003). These deposits are usually intercalated with sedi- E-mail address: [email protected] (B. Abebe). mentary syn-rift deposits (Le Turdu et al., 1999).

In order to better define the relationships between the Afar triple junction (Fig. 1, inset). The MER started to amount of extension and volcanism during Quaternary in develop during Miocene time (Davidson and Rex, 1980; MER, we use remote sensing, field and geochronological WoldeGabriel et al., 1990; Chernet et al., 1998), following data. The results suggests that MER undergoes discrete a broad doming centered on the present Afar depression evolutionary stages characterizing erupted products with (e.g. Ebinger et al., 1989). During Pliocene and Quaternary, different composition. the MER progressively deepened, evolving through a sequence of interacting half-graben segments marking the 2. Tectonic and magmatic features of the MER boundary between the Nubia and Somalia plates (Hayward and Ebinger, 1996). The MER is limited by discontinuous The East African Rift System is a Miocene-Quaternary boundary faults, active from late Miocene (WoldeGabriel intracontinental extensional system composed of several et al., 1990) and striking between NNE–SSW in the south interacting rift segments, from Mozambique to Afar and NE–SW in the north (Korme et al., 2004). The youn- (Fig. 1, inset; Davidson and Rex, 1980; Rosendhal, 1987; gest part of the MER is the axial zone (Wonji Fault Belt, Ebinger, 1989; Ebinger et al., 1989; Bosworth and Strecker, WFB), mainly formed during the Quaternary (Fig. 1; Mohr, 1997, and references therein). At Afar, the EARS joins with 1967; Meyer et al., 1975; Mohr, 1987; Boccaletti et al., 1998, the Gulf of Aden and Red Sea Rifts, both characterized by 1999; Acocella et al., 2003). Despite the overall NE–SW a more advanced extensional stage (Mckenzie et al., 1970; trend of the MER, the WFB is characterized by active Ebinger and Hayward, 1996; Manighetti et al., 2001). NNE–SSW trending extension fractures and normal faults. The Main Ethiopian Rift (MER) constitutes the north- These are often set in an en-echelon arrangement and are ernmost part of the EARS, connecting the EARS with the associated with volcanic activity.

Fig. 1. Geological sketch map of the Main Ethiopian Rift in the Quaternary. Inset map shows the position of the Main Ethiopian Rift with respect to the Red Sea and Gulf of Aden. Du, Duguna; Aw, Awasa; Co, Corbetti; Sh, Shala; Abi, Abijata; La, Langano; Al, Aluto; Ga, Gademota; BB, Bora-Berecha; Ge, Gedemsa; Bo, Bosetti; Kon, Kone; Ha, Hada; Be, Beseka; Fa, Fantale; Wo, Woldoi; Gu, Gumbi; Do, Dofan. B. Abebe et al. / Journal of African Earth Sciences 48 (2007) 115–124 117

Volcanism is mainly characterized by a bimodal basalt- stratigraphic succession and the fault pattern of these areas rhyolite association (Peccerillo et al., 2003) , with a distinct were investigated. lack of lavas of intermediate composition. Rhyolites are In some of these areas, sampling and subsequent new associated with regularly-spaced central volcanoes, usually K–Ar dating (conducted at UPSIPGP Geochronology characterized by a summit caldera, with a diameter exceed- laboratory at Orsay, France) were also performed. ing 10 km (Ebinger and Casey, 2001; Acocella et al., 2002). K–Ar dating was conducted on six volcanic units at The proportion of the rhyolite was formerly estimated to UPS-IPGP Geochronology laboratory at Orsay, France. be reaching about 90% of the total volume of the erupted Following the sample preparation procedure for the Cas- products in the central MER (e.g. Trua et al., 1999), but signol–Gillot technique (Cassignol and Gillot, 1982), the recent field investigations reveal that the basalts may con- decay constants of Steiger and Ja¨ger (1977) have been stitute up to 60% of the total volume at least in the north- used throughout. K was measured by flame emission spec- ern MER (Wolfenden et al., 2004). Basalts are usually troscopy and Ar with a mass spectrometer identical to the associated with monogenetic vents and/or fissure erup- one described by Gillot and Cornette (1986). The K–Ar tions, at the side of the main central volcano (Ebinger dates are reported in Table 1. These data were then and Casey, 2001). The MER is, therefore, mostly floored matched with published geochronological data (Zanettin by several basaltic fields, silicic domes and calderas. These et al., 1980; WoldeGabriel et al., 1990; Bigazzi et al., are interlayered and covered with Plio-Quaternary fluvio- 1993; Chernet et al., 1998). lacustrine sediments (Le Turdu et al., 1999; WoldeGabriel et al., 1992). However, the genetic relationships between 4. Results the basalts and the ryholites are still debatable (Peccerillo et al., 2003). 4.1. Quaternary faulting and volcanism

3. Methodology The general Quaternary volcano-tectonic setting of the MER is controlled by the en-echelon arrangement of the In order to investigate the Quaternary tectonic and vol- Wonji Fault Belt (WFB; Mohr, 1962). These NNE–NE canic relationships within the MER, an inventory of the Quaternary rift zones of the WFB form areas of active Quaternary fault patterns and volcanic products has been deformation obliquely cutting the rift floor in the MER made from detailed interpretation of stereoscopic pairs of (Fig. 1) . Even though without exposed faults, the Butajira air photographs (1:50,000) and onscreen digitization from and Debre Zeit volcanic fields constitute further off-axis Landsat imageries (28.5 m spatial resolution). Interpreta- belts of Quaternary activity, located on the western margin tions made on air photographs were transferred to topo- of MER (Figs. 1 and 2). The Butajira volcanic zone is graphic maps of the same scale and digitized. Successive restricted to a narrow marginal graben (WoldeGabriel fieldworks were carried out at selected locations, with the et al., 1990) and is marked by NNE–NE-aligned cinder aim to further investigate areas of specific interest. The cones, maars and some lava aprons devoid of exposed

Table 1 K–Ar age data of volcanic rocks around Asela: GM denotes ground mass and AF Alkali feldspars Sample Location Phase Mass K (%) 40Ara (%) 40Arb (at/g) Age (Ma) ±1r BekB (1) 1.02010 1.092 0.56 4.323E+10 0.038 0.010 8°02.460N39°07.140E GM 1.74545 1.092 0.15 1.201E+10 0.011 0.010 31.625 Mean 0.032 0.010 BekI (2) 8°02.540N39°07.510E AF 0.59680 4.536 33.05 3.059E+12 0.646 0.010 BekJ (3) GM 0.51060 2.385 31.90 4.853E+12 1.947 0.029 8°00.110N39°06.550E 1.10765 2.385 28.05 4.845E+12 1.944 0.029 Mean 1.946 0.029 NAZ 01 (4) 8°32.1230N39°14.5550E GM 1.10531 1.139 21.89 1.969E+12 1.654 0.025 1.35767 1.139 24.37 1.975E+12 1.659 0.024 Mean 1.657 0.025 NAZ 02 (5) 8°32.1010N39°14.4960E 0.31557 4.062 5.22 3.979E+12 0.938 0.024 0.33014 4.062 5.44 3.930E+12 0.926 0.021 Mean 0.932 0.023 AS 04 (6) 8°01.000N39°02.610E GM 1.09156 1.067 0.68% 6.649E+10 0.060 0.009 1.14844 1.067 0.75% 7.182E+10 0.064 0.009 Mean 0.062 0.009 a Percentage of radiogenic 40Ar in sample. b Number of atoms of radiogenic 40Ar per gram of sample. 118 B. Abebe et al. / Journal of African Earth Sciences 48 (2007) 115–124

line of propagation of the rift segments were dissected by faults, while others were partially covered by younger products. These products mark the continuation of the stage of silicic volcanism in the area. This stage continues also during upper Pleistocene, as shown by an ignimbrite flow 5 m below the top of the main Asela escarpment, which yielded an age of 0.65 Ma (Table 1). As the entire succession is displaced by NNE-trending faults, the WFB faults also postdates this ignimbrite. Spatial and temporal association of NNE–SSW fault zones and fissural basalts with silicic centers in the Asela area indicates close relation- ship between volcanic emplacement and fault activity (Abebe et al., 1997). These data suggest that between 2 and 1 Ma (phase 1), widespread eruption of basalts and silicic pyroclastics occurred in the central and northern part of MER (Fig. 1). The basaltic terms of this sequence can be related. In the rift axis, E–W faults must have played an important role in the location of these major Quaternary silicic centers, such as Shala, Gedemsa, Bosetti, Kone and Fantale. Several of these silicic centers (Fantale, Garibaldi, Gedmsa and Awasa) also display significant caldera structures, suggesting the presence of an established magma chamber. These acidic volcanoes occur mainly at the intersection between the WFB and the pre-existing E–W struc- Fig. 2. Faults and monogenetic cone clusters on the western margin of the tures. This implies that magma chambers beneath the silicic Main Ethiopian rift between 8 and 9°N Latitude (from database GTOP30) volcanoes along the rift axis tend to occur where there is a The Quaternary Butajira and Debre Zeit volcanic zones are shown as distinct volcanic fields. Lines with ticks represent faults, ticks pointing denser network of faults and fractures. towards downthrown side; hatched zones are monogenetic cone clusters New silicic centers, such as Bora-Berecha, Kone and and alignments. Fantale formed in the late Pleistocene (Figs. 3 and 4). Sub- sequent fissural basaltic eruptions occur between silicic centers and along their flanks (phase 2) (Fig. 1). Remote faults (Fig. 2). The Debre Zeit zone consists of a cluster of sensing and field data suggest that this new phase of fault- NE aligned cinder cones and maars. ing with associated fissural basalts, skirts around the silicic The beginning of the Quaternary within MER is charac- centers and, at later stages, it cuts across them. Late Pleis- terized by the eruption of more than 10 alternating flows of tocene volcanism, therefore, focused in narrow zones (max- aphyric basalts and basaltic agglomerates overlain by silicic imum width 25 km), forming prominent volcano-tectonic pyroclastics; this sequence is typically exposed by the Asela sectors. Estimation obtained using surface area of volcanic border fault (Fig. 1). Coeval basalts also occur along the products as a measure of magma output rate (Condit and eastern margin south of Awasa (WoldeGabriel et al., Connor, 1996) indicates that this rift localization was 1990), NW of Nazreth and along the Awash Gorge (Cher- accompanied by an increase in the relative proportion of net et al., 1998)(Fig. 1). At Asela, the basal part of the sec- basalts erupted. Pressure ridges, more than 750 vents con- tion is dominated by intercalated aphyric basalts and sisting mainly of cinder cones, a few volcanic pits and agglomerates overlain by ignimbrites. The K–Ar age of maars and numerous fissure vents comprise these basaltic 1.95 ± 0.3 Ma obtained for sample 3 along the main Asela volcanic fields of more than 2900 km2 (Table 2). These escarpment (Fig. 1 and Table 1) is consistent with those include the Duguna, Butajira, Debre Zeit, Aluto-Gedemsa reported by WoldeGabriel et al. (1990) and Boccaletti and Bosetti-Fantale volcanic fields (Fig. 1). Ongoing defor- et al. (1999). These are often covered by younger ignimb- mation in MER occurs along NNE–SSW faults and vent rites products and fluvio-lacustrine sediments, again best alignments oblique to the boundary faults. The few avail- represented by the Asela section (WoldeGabriel et al., able ages (new and published) generally range between 1990). Prior to localization of strain along the present axial 0.40 and 0.005 Ma (WoldeGabriel et al., 1990; Bigazzi rift, the physiography of the rift floor must have consisted et al., 1993; Chernet et al., 1998; Temesgen, 2001), exclud- of a broad basin bounded by segmented boundary faults ing the nineteenth century eruption of Fantale (Gibson, covered by basalts, silicic lavas and pyroclastics. At around 1970). High vent density observed in late Pleistocene bas- 1.3 ± 0.3 Ma, silicic domes and calderas like Tulu and alts (Fig. 5) thus appears to coincide with increased exten- Gademota (Fig. 1) developed along the then active part sion and establishment of the well-defined axial rift zones. of the rift, close to the eastern margin, near the Asela area. The basalts are fed by fissures and aligned cinder cones As volcanism and faulting accelerated, those lying on the probably tapping shallow magma chamber/s. At the same B. Abebe et al. / Journal of African Earth Sciences 48 (2007) 115–124 119

Table 2 Surface area of Quaternary basaltic volcanic fields in the central and northern MER Basaltic Lava field in North and Central Approximate surface area MER (km2) Tosa Sucha 163 N. Abaya 359 Butajira 290 D. Zeit 83 Aluto-Gedemsa 447 Bosetti-Fantale 1587 TOTAL 2929a a Including minor basaltic fields around the rift lakes.

time, volcanism in axial floor of MER also consists of numerous silicic centers within the WFB (Fig. 1 and Mohr and Wood, 1976). The most magmatically productive mid- dle Pleistocene to Holocene parts of the MER are the Aluto-Gedemsa and Bosetti-Fantale (each 100 km long and 20–25 km wide) rift zones, separated by the transfer zone along the E–W Arba Gugu structures (Acocella et al., 2002)(Fig. 1). The former is a N–NNE volcano-tectonic zone (Fig. 3) whereas the Bosetti-Fantale zone is a NE–SW volcanic field flanked and partially dissected by NNE faults (Fig. 4). Aluto and Bora-Berecha are flanked by intensely faulted basalts whereas Gedemsa lies along the fault zone and is partially faulted. Both Aluto and Fig. 3. Structural sketch map of Aluto-Gedemsa rift zone. Shaded relief lit Bora-Berecha are slightly dissected by NW–SE faults. from SSW (DEM source same as Fig. 2). Symbols: Line segments represent faults with ticks on the downthrown side; thick curved and/ or Another prominent active rift zone is situated southwest closed lines represent silicic centers with or without calderas; crosses of Duguna volcano and is marked by dense faulting and denote cinder cone, spatter cones and maars. fissural basalts associated with numerous monogenetic cones (Fig. 5). The relationships between the basalts and the ryholites are still debated (Peccerillo et al., 2003). However, in some areas the two compositions are closed related, at least spatially. The basaltic products usually cluster at the northern and southern sides of the major felsic calderas (Figs. 1 and 5). In particular, a cinder cone on the Gedemsa caldera floor and mafic enclaves within some rhyolites (Peccerillo et al., 2003), which show mingling between basalts and large volume silicic magmas, are the only signs of mafic magmatism within Gedemsa. The Bosetti-Fantale rift zone is characterized by relatively low fault density and profuse basaltic volcanism with strong vent clustering and align- ment (Figs. 1 and 2). From its morphology, the nested caldera system of Hada is older than Bosetti and Kone and its rims are dismantled by faulting. Besides their wide distribu- tion around the volcano, basaltic lavas also erupted from the Bosetti complex.

4.2. Petrography, geochemistry and geochronology

The petrologic and geochemical features of the basalts in the Asela and Nazreth areas have been studied in detail elsewhere (Boccaletti et al., 1999; Trua et al., 1999) and are not discussed here. Table 3 summarizes and compares some Fig. 4. Bosetti-Fantale rift segment. Shaded topography lit from South of the petrologic and geochemical data (Trua et al., 1999) (DEM source: database GTOPO30). of the basalts from phases 1 and 2. They show distinct 120 B. Abebe et al. / Journal of African Earth Sciences 48 (2007) 115–124

Fig. 5. Clusters and alignments of monogenetic basaltic vents developed mainly on the ﬂanks of Quaternary silicic centers in the MER. Abbreviations are as in Fig. 1.

Table 3 by lower MgO, CaO, Ni and Cr, and higher Sr and Ba than Petrological and geochemical characteristics of the two basalts the late Pleistocene basalts east of Lake Zway and Bora- Unit Eastern Late Pleistocene Bericio. This indicates that phase 1 basalts are more frac- margin basalts basalts tionated and, therefore, had greater crustal residence time Texture Aphyric Porphyritic of the magma than that of the late Pleistocene basalts. Olivine Fo47-55 Fo74-79 High-MG numbers (63–70) in some samples (Trua et al., Plagioclase An40-57 An60-74 1999) and an increase in compatible trace element contents, MgO 3.9–4.8 wt.% 4.8–12.0 wt.% CaO 7.1–7.7 wt.% 8.7–10.5 wt.% especially Cr and Ni with increasing MgO% and olivine Ni 5–14 ppm 15–64 ppm phenocrysts (Feeley and Winer, 1999) suggest that, at least Cr 4–56 ppm 32–376 ppm some of the youngest basalts represent relatively primitive Sr 532–875 ppm 425–675 ppm magmas. Ba 410–547 ppm 183–494 ppm It should be noted that both basalts do not represent Data source: Trua et al., 1999. primary magmas and they are more fractionated. Trua et al. (1999) proposed that the overall major and trace element spread can be explained by olivine + clinopyrox- petrographic features, the older eastern margin basalts ene + plagioclase + magnetite fractionation. Furthermore, being prevalently aphyric, and the late Pleistocene axial variable degrees of contamination by lower crust have been basalts dominantly porphyritic (up to 50% of the total vol- involved in the genesis of these rocks. ume). The composition of phases in the two units show sig- K/Ar age on feldspar separates from a green, ﬁamme- niﬁcant temporal variation; the eastern margin basalts rich ignimbrite near the top of the Asela boundary fault (phase 1) have more evolved mineralogical composition scarp sets a lower limit of 0.65 Ma (Table 1) to the begin- than the axial basalts (phase 2). ning of phase 2 volcanism. Six samples from the units 1 With the localization of volcanism in narrow zones, and 3 in the central MER have been dated with K/Ar there is geochemical evidence of a systematic spatial and method at the University of Paris, Orsay. Their location temporal change in basaltic lava composition. The petro- is shown in Fig. 1. The ages obtained cluster in two distinct logic (composition of phases) and geochemical trends of periods, from 2 to 1 Ma (phase 1, samples 3, 4 and 5) and the basalts do vary with age of eruption, amount of exten- from 650 to 32 Ka (phase 2, samples 1, 2 and 6). These sion, and geographic location. The eastern margin basalts results and the ones previously published (WoldeGabriel exposed by the main scarp west of Asela are characterized et al., 1990; Bigazzi et al., 1993; Chernet et al., 1998; B. Abebe et al. / Journal of African Earth Sciences 48 (2007) 115–124 121

Temesgen, 2001) suggest that Quaternary volcanism in the improve the resolution, two main phases of Quaternary MER mainly occurred between 2 and 1 Ma and after volcanism and rifting are identified here on the bases of 0.65 Ma, decreasing in between. stratigraphic and tectonic relationships as well as new and published ages. These are 2.0–1.0 Ma and <0.65 Ma. 5. Discussion The intervening period is characterized mainly by minor differentiated products (Trua et al., 1999). The bimodal felsic-mafic nature of Quaternary volca- In general, phase 1 volcanic activity differs from phase 2 nism in the MER has been illustrated by Boccaletti et al., by its relatively abundant acidic products. Moreover, phase 1995; Trua et al., 1999 and references therein. Although 1 basalts are more fractionated and, therefore, had greater additional geochronological information will certainly crustal residence time of the magma than phase 2 basalts.

Fig. 6. Model showing the volcano-tectonic setting of the MER during the Pleistocene. (a) phase I characterized by few silicic centers and discontinuous border faults; the area between the opposite boundary faults is ﬂoored by phase I basalts and rhyolites; (b) the MER in the late Pleistocene with development of dense faults of the WFB, more silicic centers and ﬁssural basalts with monogenetic cones. 122 B. Abebe et al. / Journal of African Earth Sciences 48 (2007) 115–124

On the other hand, Cr and Ni content and high mg-number basaltic extrusive phase, possibly enhanced by the cooling in some axial basalts suggest that the second phase basalts of silicic magmas within the main reservoirs. As a conse- may represent relatively primitive magmas (Trua et al., quence of this cooling, new basaltic magma coming from 1999). depth does not interact with the cold felsic residual magma Therefore, there are several indications of a close rela- in the reservoirs. The injection of new basaltic magma in tionship between axial silicic centers, basaltic volcanic the upper crust will consequently increase the local crustal fields and faulting in the central and northern MER. It density, allowing more primitive basalts to become buoy- appears that Quaternary rifting in the MER is influenced ant and erupt (Glazner and Ussler, 1989). This is supported by crustal structures on diverse scales and stages, from by the fact that, even though silicic rocks still constitute the the position of silicic centers to localization of axial rift seg- greater proportion of the rift cover, basalts are becoming ments and their vent alignments. MER developed through relatively abundant and less evolved with time. episodes of volcanism and tectonism, which lead to the for- Associated with this latest basaltic phase is intense fault- mation of discrete axial rift segments during Pleistocene ing and fracturing along the rift floor, as shown by the dis- (Fig. 6b). Ongoing deformation and magmatism are con- section of the basaltic lava flows through the rift axis, centrated in narrow segments, each consisting of one or suggesting a localization of the strain due to regional tec- more silicic centers. Mahatsente et al. (1999) correlate tonics during and after basalt emplacement. These data residual gravity anomalies over the Main Ethiopian Rift suggests that the silicic centers constitute the center of with these Quaternary rhyolitic centers. The silicic centers propagation of the magmatic systems throughout the rift appear to have been mainly controlled by the interaction axis. With increasing extension and volcanism, as observed between N–NNE faults and E–W trending Tertiary faults in the last 0.65 Ma, these propagate along axis and will (Acocella et al., 2002). eventually link up, forming a more continuous deforma- The close spatial and temporal links between silicic vol- tion belt, characterized by an increase of erupted magmas, canoes and propagating faults accompanied or followed by as observed along the northern MER and in southern Afar. fissural basalts hints that acidic volcanoes play an active This mechanism is a required step in the mature stage of role in the rifting process in the Main Ethiopian Rift. Sim- development of continental rifts, anticipating the structure ilar features have been observed at the Afar (Lahitte et al., and magmatic production typical of oceanic spreading cen- 2003). Here the late Miocene-early Pliocene silicic centers ters (Ebinger and Hayward, 1996; Hayward and Ebinger, (Woldoi, Gumbi, etc) cluster along the foothills of the 1996). southeastern margin of the Afar Rift. Volcanic activity begins with highly differentiated products predating the 6. Conclusions axial fault zones. Subsequent fissure-fed basalts and monogenetic cones are spatially clustered very close to the rhyo- The relationships between late Quaternary volcanism litic volcanoes. Similarly to Afar (van Wyk de Vries and and rifting have been assessed using sensing, field and Merle, 1996; Lahitte et al., 2003), a close spatial and tem- geochronology data. The results suggest a close relation- poral association between silicic centers and rift segments ship between the late Quaternary tectonic setting and is also evident in the MER. From field relationships, late the location of silicic centers, pattern of basaltic volca- Pleistocene rhyolitic volcanoes in MER appear to guide nism to monogenetic vent alignments. In particular, two and concentrate deformation, even though most faulting main Pleistocene magmatic episodes are recognized: (a) as well as basaltic eruption is outside the edifices, especially basaltic flows followed by ignimbrites and few silicic cen- in the last 0.65 Ma. ters in the rift floor (erupted between 2 and 1 Ma) and (b) The silicic centers also seem to determine the direction axial silicic volcanoes and basalts, erupted in the last of future rift propagation (Lahitte et al., 2003) and devel- 650 Ka. The first episode consists mainly of basaltic opment of a well-defined axial rift zone (Fig. 6). We spec- flows related to the Afar Stratoid and outcrops in the ulate that the silicic centers are controlled by rift northern MER. Scattered silicic centers develop subse- tectonics and the basaltic eruptions are generally peripheral quently along the rift floor. The silicic centers appear to to these centers because of density variations between have been mainly controlled by the interaction between basaltic and silicic magmas in stratified reservoirs. Exten- N–NNE faults and E–W trending Tertiary faults. In the sion by normal faults may have created conduits for basal- second episode, spatial and temporal correlation between tic magma. We believe that the cooling of these silicic rift localization and silicic centers becomes more evident. reservoirs may have enhanced the extrusion of basaltic With ageing, these centers become faulted and allow bas- magma injected within the rift axis. Further processes that alts to erupt right through the volcanic edifices and along enhance basaltic volcanism may be mafic intrusions, exten- their flanks, suggesting a decrease in the amount of differ- sion, differentiation and hybridization of magma which can entiation in the magma chambers, possibly due to their increase mean crustal density. This, in turn allows more cooling. Basaltic eruptions increased and became less basalts to rise by buoyancy (Glazner and Ussler, 1989). evolved with time accompanied by some petrological Therefore, the small volume basalts at silicic centers like and geochemical changes such as increase in MgO, Ni Gedemsa and Bosetti suggests the beginning of a new and Cr. B. Abebe et al. / Journal of African Earth Sciences 48 (2007) 115–124 123

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