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Lithospheric Modification During Crustal Extension in the Main Ethiopian Rift

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Lithospheric Modification During Crustal Extension in the Main Ethiopian Rift JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, B10201, doi:10.1029/2006JB004916, 2007 Click Here for Full Article Lithospheric modification during crustal extension in the Main Ethiopian Rift Tyrone Rooney,1,2 Tanya Furman,1 Ian Bastow,3,4 Dereje Ayalew,5,6 and Gezahegn Yirgu5 Received 22 December 2006; revised 11 May 2007; accepted 27 June 2007; published 5 October 2007. [1] Quaternary lavas erupted in zones of tectonomagmatic extension within the Main Ethiopian Rift (MER) preserve details of lithospheric structure in the East African Rift System. Despite observed source heterogeneity, basalts, trachybasalts, and basaltic trachyandesites erupted in the Wonjii Fault Belt (WFB) and the Silti-Debre Zeyit Fault Zone (SDFZ) form coherent fractionation paths dominated by variable removal of observed phenocryst phases. Crustal assimilation is not widespread, though it is observed at the southern end of the WFB where both fault belts merge; farther north, assimilation of cumulate phases related to fractional crystallization of previous magmas is identified. Shallow fractionation conditions ( 1 kbar) within the WFB do not change from north to south. In contrast, lavas erupted within the contemporaneous SDFZ fractionate at various crustal depths. These results indicate a better developed magmatic system beneath the WFB where magmas rose quickly before undergoing more significant fractionation at near surface levels and a less developed system beneath the SDFZ. The distribution of magmatism and extant geophysical data indicate thinned crust and a single rift-centered zone of magmatic activity northeast of 8°300N, consistent with a transitional lithosphere between continental and oceanic settings. Southwest of 8°300N, thicker crust and rift- marginal axes of extension suggest lithosphere with continental affinities. The WFB is propagating southward in response to extension within the Red Sea Rift; the northward propagating SDFZ is related to rifting within the East African Rift System. This region records the unification of two rift systems, requiring care in interpreting the MER as simply transitional between continental and oceanic environments. Citation: Rooney, T., T. Furman, I. Bastow, D. Ayalew, and G. Yirgu (2007), Lithospheric modification during crustal extension in the Main Ethiopian Rift, J. Geophys. Res., 112, B10201, doi:10.1029/2006JB004916. 1. Introduction coherent geodynamic and geophysical models of continen- tal rifting in zones of active tectonism, and to interpret areas [2] Lithospheric modification during continental rifting is of ancient rifting along passive margins. The East African an axiomatic consequence of plate tectonic processes. The Rift System (EARS) stretches for over 3000 km from the processes whereby the continental crust is modified by Red Sea and Gulf of Aden southward to Mozambique and magmatism during the progressive evolution from conti- has been recognized as a major extensional feature for well nental rifting to seafloor spreading, are however poorly over 100 years [Gregory, 1896]. It is the classic example of constrained. These ambiguities generate substantial uncer- continental rifting, generated by subsidence of fault tainties in detailing thermal structure, rift-related volcanic bounded basins coupled with the uplift of rift flanks hazards, and hydrothermal resources. Rift generated litho- [Ebinger et al., 1989]. It comprises two branches flanking spheric heterogeneity also frustrates efforts to construct the Tanzania craton in central and eastern Africa, and a single arm that traverses the Ethiopian plateau and craton in the north (Main Ethiopian Rift). Ongoing research in the 1Department of Geosciences, Pennsylvania State University, University Main Ethiopian Rift (MER) has increasingly pointed Park, Pennsylvania, USA. 2 toward magmatism as a primary mechanism for extension Now at Department of Geological Sciences, Michigan State University, and associated crustal modification processes [Keranen et East Lansing, Michigan, USA. 3Department of Geological Sciences, University of South Carolina, al., 2004; Kendall et al., 2005, 2006; Rooney et al., 2005]. Columbia, South Carolina, USA. This interpretation is consistent with the transition from 4Now at Department of Earth Sciences, University of Bristol, Bristol, fault-dominated rift morphology observed in continental UK. rifts toward magma-dominated mid-ocean ridge spreading 5Department of Earth Sciences, Addis Ababa University, Addis Ababa, Ethiopia. centers. 6Now at Dessie/Kombolcha University, Dessie, Ethiopia. [3] We present new geochemical data from several key locations along the Main Ethiopian Rift between 8° and Copyright 2007 by the American Geophysical Union. 10°N (Figure 1) in order to assess the interaction of magmas 0148-0227/07/2006JB004916$09.00 B10201 1 of 21 B10201 ROONEY ET AL.: LITHOSPHERE IN THE ETHIOPIAN RIFT B10201 Figure 1 2 of 21 B10201 ROONEY ET AL.: LITHOSPHERE IN THE ETHIOPIAN RIFT B10201 with the continental crust in this zone of active extension, Zone (SDFZ) (Figure 1). We attribute these variations to the and to examine how magmatic processes modify crustal coeval propagation of extension into the MER from both the structure. This approach focuses on geochemical indicators south (Kenya) and the north (Red Sea). of assimilation and fractional crystallization during the residence of magma within the crust. We integrate studies 2. Geologic and Geodynamic Background of more primitive rocks [Rooney et al., 2005; Furman et al., 2006] from the same eruptive locations and time periods to 2.1. Tectonic Setting chart the evolution of magmas at crustal levels. Specifi- [6] The central MER marks the transition between rifting cally, we focus on evolved (<6% MgO) lavas erupted of thick continental crust in the southern and central EARS within the Wonjii Fault Belt (WFB), considered one of the and incipient seafloor spreading in northern Afar. Recent primary extensional axes in the MER [Ebinger and Casey, volcanic and tectonic activity in the central MER is gener- 2001; Casey et al., 2006]. Our sampling approach allows ally confined to distinct faults belts [Mohr, 1962, 1967; for an evaluation of magmatic processes throughout the WoldeGabriel et al., 1990], postulated to be magmatic central MER, expanding on previous studies of single segments and precursors to mid-ocean ridge spreading volcanic centers [e.g., Peccerillo et al., 2003]. Our focus centers [Ebinger and Casey, 2001]. These belts of tectono- on suites of contemporaneous and, often, comagmatic magmatic activity and associated lithospheric modification lavas reduces the effects of source heterogeneity that may are the WFB [Mohr, 1967] and the SDFZ [WoldeGabriel et have clouded previous studies [e.g., Trua et al., 1999; al., 1990] (Figure 1). These belts are coincident with crustal Ayalew, 2000]. extension within the rift, occupying grabens on the eastern [4] This study of Quaternary eruptives probes ongoing and western margins, broadly paralleling the rift alignment crustal modification processes. Consequently, our geochem- [WoldeGabriel et al., 1990]. The WFB and SDFZ are dominated by large (up to 90 km3) felsic volcanoes ical results can be interpreted within the context of substan- tial geophysical investigations of current lithospheric (e.g., Ziqualla, Fantale, Dofan, Bosetti), some with well- structure within the MER [Keranen et al., 2004; Bastow developed calderas (Kone, 5 km diameter; Gedemsa, 8 km diameter), spaced 20–45 km apart along the fault et al., 2005; Cornwell et al., 2006; Furman et al., 2006; Maguire et al., 2006; Whaler and Hautot, 2006]. The belts. Basaltic activity is restricted to minor flows emanating alteration of continental crust through assimilation and/or from cinder cones along faults and fractures, and rare fissure the presence of fractionating melt in the form of pervasive eruptions. The evolution from more widespread magmatism dikes or large magma chambers will have a significant during the early syn-rift stage toward more restricted axial impact on crustal structure by replacing existing strata with activity was accompanied by a shift in locus of strain from magmatic products. We assess small-scale heterogeneities in the rift border faults toward the WFB [Ebinger and Casey, geochemical signatures that point to the size and distribu- 2001] and SDFZ. The current concentration of crustal tion of fractionating magma bodies within the MER. These extension within the SDFZ and WFB makes them excellent results will aid in the interpretation of existing geophysical locales for the study of rift-related lithospheric modification data sets that have indicated the presence of melt within processes. the crust [e.g., Dugda et al., 2005; Stuart et al., 2006; 2.2. Geologic History Whaler and Hautot, 2006] but have been unable to [7] The volcanics of the SDFZ and WFB erupted through determine if this melt occupies large shallow magma a substantially modified continental crust that has experi- chambers or more complex diked zones of melt intrusion. enced considerable volcanism over the past 30 Ma. Exten- Such distinctions are an important indicator of the relative sive volcanism in north central Ethiopia, linked to the Afar importance of magmatism- and fault-based extension in plume [Pik et al., 1999; Kieffer et al., 2004], is restricted to continental rifting. episodic eruptions between 30 Ma and 11 Ma (pre-rift [5] Our investigation is suggestive of a dual axis of stage). Rejuvenated magmatism from 11 Ma to present extension requiring revision
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