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Quaternary Faulting and Volcanism in the Main Ethiopian Rift

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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 vol- canoes 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 tem- poral 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). 1464-343X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2006.10.005 116 B. Abebe et al. / Journal of African Earth Sciences 48 (2007) 115–124 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.
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