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 continental 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 substantial 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 of any single-axis model of is coincident with the onset and progression of continental Ethiopian Rift magmatism. Further, we reveal heterogeneity rifting in north central Ethiopia [Hart et al., 1989; in the crustal structure, and by inference the active rifting WoldeGabriel et al., 1990; Chernet and Hart, 1999]. Pre-rift processes, beneath the WFB and Silti-Debre Zeyit Fault

Figure 1. Shaded relief digital elevation model of the central MER, constructed using NASA-JPL Shuttle Radar Topography Mission (SRTM) data with a 90 m resolution. The individual 1 by 1 degree tiles were corrected for holes and combined to produce a larger picture of the MER. The projection is based on the WGS-84 datum. (a) Cinder cone frequency in the Wonjii Fault Belt and Silti-Debre Zeyit Fault Zone in the north central MER. Cinder cones are shown as small dots [Berhe and Wondm-Agennehu, 1978; Benvenuti et al., 2002; Abebe et al., 2005]. The Boru-Toru Structural High [Bonini et al., 2005] is labeled BTSH. Northward toward Dofan, our field investigations indicate that cinder cone activity is much reduced compared with the more southerly section of the study area, but no geologic mapping exists for this region. (b) Sample locations (dots), roads (solid lines), towns and cities (starred), and lakes (grey) overlain. The large volcanic features of Dofan, Fantale, Kone, Bosetti, Gedemsa, Tullu Moye, and Ziqualla are labeled and represented by grey circles. The Wonjii Fault Belt (WFB) and Silti-Debre Zeyit Fault Zone (SDFZ) extend beyond the edges of this diagram but are roughly shaded in white.

3 of 21 B10201 ROONEY ET AL.: LITHOSPHERE IN THE ETHIOPIAN RIFT B10201 volcanic activity is dominated by a brief ( 1 my [Hofmann and simple shear [Wernicke, 1985]. Active rifting models et al., 1997]) episode of continental floodbasalt volcanism explain extension as the result of asthenospheric upwelling that extruded up to a 2 km thick succession over an area of [Morgan, 1971; Burke and Dewey, 1973]. Shortcomings of 600,000 km2 [Mohr, 1983; Mohr and Zanettin, 1988]. The these end-member models prompted recent work that com- flood basalt series are dominated by transitional to tholeiitic bines processes associated with active and passive rifting mafic lavas [Pik et al., 1998] with interlayered felsic vol- into hybrid models [Zeyen et al., 1997; Courtillot et al., canics in the upper levels [Ayalew et al., 1999]. Isolated 1999; Huismans et al., 2001; Ziegler and Cloetingh, 2004]. bimodal shield volcanoes, composed of mafic and lesser Strictly kinematic models of continental breakup [e.g., felsic eruptives, rise up to 1200 m above the flood basalt Wernicke, 1985; Lister et al., 1986] often ignore the effects plateau [Kieffer et al., 2004]. Shield volcanoes have erupted of melt in the rifting process, and in doing so they ignore the episodically with activity recorded at 30 Ma (Simien), 23 Ma observation that the average tectonic force needed to initiate (Choke and Guguftu) and 10 Ma (Guna) [Kieffer et al., and maintain breakup may be an order of magnitude greater 2004]. The onset of rifting within the central Ethiopian Rift at than that which is commonly available [Kusznir and Park, 11 Ma [Wolfenden et al., 2004] was coincident with a shift 1987; Hopper and Buck, 1993; Zeyen et al., 1997; Buck, in magmatism toward felsic and subordinate basaltic erup- 2004]. In Ethiopia, magmatic intrusion facilitating litho- tives located in discrete bimodal volcanic centers within the spheric rupture and extension was first outlined by Mohr rift [Hart et al., 1989; Chernet and Hart, 1999], which also [1987], who suggested asthenospheric material is diapira- generated the widespread ignimbrites that currently blanket cally emplaced into the lithosphere. Ebinger and Casey the rift floor. Subsequent to 3 Ma magmatic activity was [2001] presented a geodynamic model for the MER that further focused within the rift, now centering on WFB and suggested the initial stage of lithospheric extension is SDFZ. accommodated by slip along rift border faults. Over time, [8] Magmatism and lithospheric modification have been extensional stresses become focused in the central portion coupled during the 30 Ma history of the Ethiopian volcanic of the rift valley through dike intrusion and associated province. The widespread magmatic event at 30 Ma, asso- faulting as the rift border faults become inactive. These ciated with the Afar plume, is linked to infiltration of melt geodynamic models are excellent tools in the study of and the thermomechanical erosion of the lower lithosphere continental rifting and new data collected during the recent recorded in metasomatized mantle xenoliths [Bedini et al., Ethiopia Afar Geoscientific Lithospheric Experiment 1997]. Magmatism and lithospheric modification were once (EAGLE) [Maguire et al., 2003] project have brought into again coupled as the onset of rifting at 11 Ma was accompa- sharp focus, the role of magmatism and faulting during nied by a shift in regional magmatism, which became focused rifting and the transition toward seafloor spreading. In on the newly formed rift. More recently, pervasive magmatic particular, these new data shed light on how magmatism intrusions and diking have further modified the lithosphere interacts with and modifies the continental crust. beneath the WFB and SDFZ [e.g., Rooney et al., 2005; [11] EAGLE controlled-source seismological data indi- Kendall et al., 2006]. In this study we will concentrate on cate limited lower-crustal thinning northward toward Afar the effects of modern magmatism as it modifies the crustal with almost complete replacement of the lower crust by portion of the lithosphere within the WFB and SDFZ. By intrusive mafic volcanism [MacKenzie et al., 2005; Maguire focusing on more evolved volcanic products we will examine et al., 2006]. Such magmatic replacement of the crust has magmas that have spent time differentiating within the crust, also been suggested farther south on the basis of teleseismic recording lithospheric processes. receiver function analyses [Dugda et al., 2005]. Magneto- telluric investigations reveal shallow (<1 km) and mid- 2.3. Geophysical/Geodynamic Background crustal zones of high conductivity, interpreted as partially [9] Most passive margins world-wide are considered to molten material located beneath the surface manifestation of be magmatic, on the basis of the observation that they are Quaternary volcanism within the MER. The upper crust characterized by thick sequences of extruded and intruded within these zones is imaged as seismically fast [Keranen et igneous rocks that were emplaced prior to or in conjunction al., 2004; MacKenzie et al., 2005] and dense [Mahatsente et with the onset of rifting [e.g., Menzies et al., 2002]. al., 1999; Cornwell et al., 2006], suggestive of cooled mafic However, the breakup history of most magmatic passive intrusions. Partial melt is also thought to reside within margins remains enigmatic as the ocean-continent boundary vertically oriented dikes in the lithosphere on the basis of is concealed by thick seaward dipping reflectors [e.g., seismic anisotropy studies across the MER [Kendall et al., Holbrook and Kelemen, 1993] and rendered obscure 2005]. Low velocity P and S wave anomalies dominate the through erosion and thermal subsidence. The northern East Ethiopian upper mantle structure [Benoit et al., 2003; African Rift System provides the perfect setting in which to Bastow et al., 2005], and indicate the presence of partial study the early stages of continental breakup. Within the melt in the upper 300 km of the mantle [Bastow et al., region magmatic and tectonic features are well-exposed, 2005]. We therefore have the opportunity to couple geo- and in some cases are currently active (e.g., Dabbahu Rift in chemical and geophysical results to produce a synthesis Afar [Wright et al., 2006]). model of incipient continental breakup. [10] The processes of lithospheric extension and rupture have been a source of much debate that centered, in large part, on active versus passive rifting models [Sengor and 3. Methods: Sampling and Sample Preparation Burke, 1978; Turcotte and Emerman, 1983]. Passive rifting [12] Fresh samples of basalt, trachybasalt and basaltic models, where extension is generated by far-field plate trachyandesite (Figure 2) were collected in 2002 and 2003 stresses, include models of pure shear [McKenzie, 1978] from cinder cones and associated flows from 41 localities

4 of 21 B10201 ROONEY ET AL.: LITHOSPHERE IN THE ETHIOPIAN RIFT B10201 along the WFB (Figure 1). The samples were cut into slabs, trimmed to remove surface alteration and polished to remove saw marks. Slabs were then crushed using a porcelain jaw-crusher and powdered for <1 minute with a tungsten-carbide disc mill. [13] Twenty-one samples were dissolved and analyzed at Duke University using a VG PlasmaQuad-3 ICP-MS for trace elements (including REE) and an ARL-Fisons Spec- traspan 7 DCP for major elements and selected trace elements. All trace element data were obtained by ICP- MS analysis, excluding Sr and Ba which were undertaken by DCP. These data are presented in Table 1; analytical data on the remaining 20 primitive samples (>6% MgO) are presented by Furman et al. [2006]. [14] Elemental analysis of minerals was undertaken at The Pennsylvania State University using a Cameca SX- 50 electron microprobe equipped with 4 wavelength spec- trometers and one energy dispersive spectrometer. The analyses used a 12 nA sample current and a 15 kV accelerating voltage. The width of the electron beam was 20 microns for analysis of all minerals excluding spinels Figure 2. Total alkali-silica diagram [Le Bas et al., 1986] where a beam width of 1–3 microns was used. Counting showing all Quaternary MER samples including the more times were 20 seconds per element peak and 10 seconds primitive WFB (Wonjii Fault Belt) basalts presented by background for major elements while minor elements (0.1– Furman et al. [2006] and SDFZ [Rooney et al., 2005]. 1%) had a counting time of 60 seconds per peak and 30 seconds background. The data are presented in auxiliary material1 Tables S1–S4 and include unpublished microprobe analyses from both the primitive samples described transitional basalts with subordinate trachybasalts and ba- by Furman et al. [2006] and the evolved samples which are saltic trachyandesites (Figure 2). A distinct Daly Gap is the focus of this study. evident, with intermediate material (54–62 wt.% SiO2) rare. We focus here on products with 46–51 wt.% SiO2. [18] Major element patterns are broadly consistent with 4. Results fractionation of the observed mineral phases. Among mafic 4.1. Petrography lavas, major element trends appear to reflect removal of [15] All samples are hypocrystalline and many are vesic- olivine, clinopyroxene and plagioclase feldspar. Samples ular. The fabric of most samples is inequigranular but studied here exhibit positive trends of Na2O, K2O, MnO observed textures include porphyritic, glomeroporphoritic, and P2O5 with decreasing MgO, while CaO and TiO2 seriate and rarely poikilitic. Plagioclase crystals in some exhibit negative trends among samples with <5 wt.% flows exhibit trachytic textures, but in the majority of MgO. Examination of Figure 3 reveals patterns consistent samples the crystals are unaligned. Intergrowths of all three with dominant crystal fractionation such as the rise in TiO2 dominant phenocryst phases (olivine, plagioclase feldspar and Al2O3 (not shown) concentrations followed by a fall in and subordinate clinopyroxene) are observed. These phe- both. The decreasing CaO/Al2O3 values indicate clinopyr- nocrysts are set in a fine-grained matrix of plagioclase oxene is an important fractionating phase among samples feldspar, opaque oxides and to a lesser extent clinopyroxene with >6% MgO, whereafter plagioclase becomes increas- and olivine. Examples of olivine, clinopyroxene and pla- ingly important (Figure 4). The final fractionating phases gioclase feldspar crystals in disequilibrium (distinguished are Fe-Ti oxides, indicated by a drop in TiO2 at about by resorption textures) are common. 5% MgO. [16] Olivine core compositions range from Fo52 in N-03 [19] Two fractionation trends are distinguished by CaO/ to Fo77 in N-24 and contain NiO and Cr2O3 below the Al2O3 values: Trend A, which is spatially restricted to instrument detection limit (0.10 wt.%). Clinopyroxene samples surrounding Lake Besaka, and Trend B, which compositions straddle the diopside-augite boundary and comprises the majority of samples in the region. Such trends are enriched in Al O ( 7.25%) and TiO ( 3.5%). Pla- are the result of variable clinopyroxene and plagioclase 2 3 2 gioclase feldspars range primarily from An50 –85; notable fractionation, addressed in later sections. A third field, exceptions are two crystals of anorthosite in N-3. Opaque defined by low MgO and high CaO/Al2O3 encompasses oxides in the matrix are rich in both FeO (65–70 wt.%) and samples from the fissure eruption at Fantale and other flows TiO2 (19–27 wt.%). between Fantale and Dofan (N-24 from south of Tullu Moye also plots in this field). 4.2. Major Elements [17] Quaternary volcanic flows and scoria associated with 4.3. Trace Elements cinder cones along the WFB are predominantly alkaline to [20] Abundances of compatible trace elements Sc, Cr and Ni decrease with decreasing MgO, consistent with olivine 1Auxiliary materials are available at ftp://ftp.agu.org/apend/jb/ and clinopyroxene fractionation (Figure 5). Vanadium abun- 2006jb004916.

5 of 21 B10201 ROONEY ET AL.: LITHOSPHERE IN THE ETHIOPIAN RIFT B10201

Table 1. Major, Minor, and Trace Element Analysis of the MER Basaltsa

Sample Volcano Lat Long SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O TiO2 MnO2 P2O5 SUM Sr Ba* Cs Rb Th U Nb Ta 1026 Dofan 9.478 40.198 47.16 15.07 14.28 5.42 9.70 3.44 0.65 3.14 0.23 0.62 99.72 395 246 0.15 14.1 1.67 0.47 28.7 1.84 1028 Dofan 9.308 40.144 52.69 14.35 13.70 2.47 6.16 4.96 1.20 2.16 0.36 0.95 99.00 443 521 0.31 28.7 3.19 0.83 49.9 2.95 1029 Dofan 9.322 40.176 46.05 13.02 16.37 5.11 9.91 3.68 0.67 3.93 0.31 1.74 100.79 459 377 0.07 11.1 1.65 0.21 30.9 1.97 1024 Dofan 9.082 39.911 47.04 14.29 16.14 4.32 9.89 3.35 0.70 3.43 0.25 0.80 100.20 464 397 0.08 12.7 1.87 0.39 31.3 1.93 1024A Dofan 9.082 39.911 47.30 13.99 15.65 4.63 9.36 3.69 0.84 3.35 0.25 0.79 99.84 444 421 0.05 17.8 1.96 0.49 31.2 1.93 1025 Dofan 9.037 40.059 47.67 16.33 13.21 4.63 10.32 3.37 0.78 2.73 0.19 0.49 99.73 454 376 0.06 15.3 1.61 0.47 25.4 1.54 1034 Fantale 8.930 39.908 47.02 13.73 15.48 4.68 9.15 3.92 0.90 3.84 0.29 1.13 100.13 495 514 0.16 19.9 2.14 0.57 36.0 2.28 1017 Fantale 8.905 39.908 47.40 13.72 15.29 4.69 9.12 3.90 0.93 3.82 0.29 1.14 100.29 501 509 0.22 21.3 2.41 0.66 36.2 2.32 1021 Fantale 8.904 39.888 46.46 13.66 15.28 4.65 9.06 3.90 0.88 3.82 0.29 1.13 99.13 484 507 0.24 21.6 2.21 0.61 35.3 2.26 N-19 Fantale 8.903 39.893 46.69 13.73 15.39 4.67 9.03 4.01 1.03 3.86 0.29 1.16 99.86 448 500 0.07 11.6 1.92 0.61 35.4 2.2 1040 Kone 8.720 39.555 48.61 14.78 13.76 4.34 8.24 4.17 1.31 3.19 0.25 0.78 99.43 390 386 0.14 18.8 2.23 0.59 37.9 2.41 1039 Kone 8.716 39.553 46.95 18.20 11.07 5.62 11.49 2.85 0.60 1.99 0.16 0.30 99.22 504 191 0.09 10.1 1.64 0.36 19.8 1.28 N-13 Kone 8.667 39.483 50.92 14.86 12.17 3.82 7.23 4.58 1.97 2.91 0.25 1.17 99.88 483 504 0.37 35.0 4.34 1.10 47.4 2.9 N-07 Gedemsa 8.317 39.108 50.20 15.02 12.94 4.06 8.04 4.32 1.53 3.21 0.25 0.67 100.23 562 359 0.32 24.8 3.19 0.88 42.5 2.6 N-06 Gedemsa 8.271 39.146 50.43 15.07 12.94 4.02 7.92 4.38 1.30 3.21 0.25 0.65 100.17 572 356 0.30 23.5 3.23 0.89 43.2 2.6 N-05 Gedemsa 8.267 39.142 49.96 14.88 12.83 4.00 7.94 4.24 1.39 3.20 0.25 0.65 99.32 564 351 0.34 24.9 3.11 0.90 43.1 2.5 N-04 Gedemsa 8.258 39.175 48.86 14.94 14.64 4.76 8.85 4.03 1.00 3.64 0.23 0.61 101.57 533 324 0.11 21.3 2.61 0.64 37.8 2.3 N-03 Gedemsa 8.254 39.188 48.16 14.82 14.28 4.64 8.81 3.89 1.19 3.57 0.24 0.68 100.25 555 325 0.18 19.9 2.73 0.75 38.1 2.4 N-25 Gedemsa 8.024 39.036 48.26 14.59 14.57 5.04 8.78 3.37 1.38 3.15 0.21 0.46 99.81 462 424 0.23 26.8 3.18 0.72 35.2 2.1 N-24 Gedemsa 8.018 39.044 46.40 14.75 14.77 5.80 10.79 3.15 0.88 2.98 0.20 0.36 100.09 408 224 0.09 11.4 1.86 0.41 25.7 1.5 N-23 Gedemsa 8.005 39.091 48.69 15.83 13.20 5.00 9.19 3.40 1.22 2.79 0.19 0.42 99.94 497 379 0.20 21.7 2.96 0.67 32.6 1.9

Table 1. (continued)

Sample La Ce Pb Pr Nd Sm Zr Hf Eu Gd Tb Dy Y Ho Er Yb Lu Co Cr Ni V Sc 1026 18.9 44.3 1.48 6.08 26.8 6.41 159 3.75 2.32 6.87 1.06 6.17 31.3 1.20 3.20 2.75 0.41 41.6 58.5 34.0 305 29.7 1028 35.3 84.4 2.21 11.34 49.1 11.26 275 6.34 4.40 11.90 1.84 10.32 39.6 2.04 5.41 4.75 0.72 14.5 8.1 11.0 36 23.4 1029 27.9 68.5 1.48 9.92 46.0 10.60 142 3.56 4.01 11.30 1.65 8.79 40.3 1.68 4.17 3.33 0.49 34.1 10.4 19.1 302 35.0 1024 25.1 57.7 1.63 7.81 34.3 7.88 169 4.10 2.85 8.29 1.26 6.96 33.0 1.36 3.59 3.08 0.46 46.9 20.6 25.2 300 33.2 1024A 24.8 56.4 2.18 7.72 33.6 7.67 169 4.09 2.74 8.10 1.22 6.73 32.1 1.34 3.45 3.01 0.46 42.2 20.2 24.3 336 32.5 1025 19.7 44.7 2.19 5.93 25.3 5.80 157 3.64 2.03 6.09 0.93 5.16 28.3 1.00 2.65 2.29 0.35 39.2 65.6 34.0 340 29.0 1034 29.3 69.7 2.53 9.60 43.2 9.90 170 3.90 3.83 10.38 1.46 8.09 36.0 1.54 3.96 3.29 0.49 36.1 9.7 14.4 300 33.5 1017 30.2 70.0 1.89 9.86 43.7 9.89 191 4.59 3.77 10.52 1.51 8.29 35.1 1.56 3.95 3.39 0.48 38.4 9.5 16.1 285 33.6 1021 29.2 68.4 1.89 9.60 42.7 9.57 166 3.99 3.72 10.20 1.46 8.12 34.7 1.51 3.87 3.34 0.47 35.5 10.4 15.7 284 32.6 N-19 28.4 66.7 1.9 8.8 41.4 8.84 162 4.0 3.47 9.80 1.51 7.45 37.3 1.42 3.59 2.99 0.43 31.9 11 20 278 28.8 1040 22.6 57.7 3.30 7.04 30.0 6.51 199 4.68 2.17 6.92 1.02 5.49 31.8 1.07 2.80 2.42 0.37 31.6 7.2 13.2 237 20.2 1039 15.3 33.4 1.53 4.45 18.8 4.40 111 2.82 1.56 4.27 0.70 3.92 20.6 0.77 1.98 1.71 0.27 62.0 110 55.9 292 32.2 N-13 40.5 90.7 2.3 11.5 50.1 10.37 275 6.1 3.40 11.29 1.77 9.07 50.7 1.72 4.75 3.89 0.61 19.4 8 16 143 21.1 N-07 32.0 70.9 3.3 9.7 39.6 8.30 230 5.3 2.84 8.87 1.42 7.17 39.6 1.41 3.59 3.15 0.46 26.6 11 20 186 22.7 N-06 32.5 72.5 3.3 9.4 40.3 8.53 234 5.4 2.93 9.19 1.45 7.23 40.4 1.44 3.65 3.22 0.47 27.9 10 20 203 23.9 N-05 32.1 70.8 3.2 8.9 39.4 8.25 232 5.2 2.84 8.96 1.42 7.07 39.9 1.39 3.52 3.12 0.47 33.1 8 28 182 22.3 N-04 28.1 61.5 2.0 8.0 34.3 7.18 192 4.5 2.63 7.64 1.20 6.02 33.4 1.13 3.00 2.60 0.40 34.1 11 22 278 27.2 N-03 28.8 63.8 2.4 8.2 35.9 7.50 192 4.4 2.67 8.02 1.26 6.27 34.2 1.23 3.09 2.66 0.41 37.4 10 24 255 26.8 N-25 32.2 62.2 4.2 8.4 34.9 7.21 220 5.0 2.27 7.98 1.23 6.14 34.8 1.20 3.21 2.95 0.43 45.8 29 37 368 31.4 N-24 17.8 39.2 1.7 5.1 22.2 4.81 139 3.2 1.70 5.22 0.85 4.38 23.3 0.81 2.18 1.80 0.27 48.4 42 45 389 35.7 N-23 25.7 56.0 3.9 7.1 30.2 6.26 209 4.8 1.99 6.47 1.05 5.35 29.2 1.01 2.76 2.41 0.37 41.4 43 43 336 28.8 aUSGS standards (W-2, Diabase; DTS-1, Dunite; DNC-1, Diabase; AGV-1, Andesite; G-2, Granite; SDC-1, Mica Schist; BIR-1, Basalt), NBS/NIST standard (688 Basalt) and two standards developed at Lamont Doherty Earth Observatory (K1919a, Basalt; AII-92, Basalt) were used. The precision based on duplicate analysis is typically better than 1% for SiO2, MgO, Fe2O3, and Al2O3; better than 2% for CaO, MnO2, TiO2, Na2O, Sc, Rb, Sr, Y, Zr, Nb, Ba, La, Ce, Pr, Nd, Sm, Gd, Ho, and Er; better than 3% for V, Tb, Dy, Yb, Lu, Hf, Ta, Th, and Eu; and better than 5% for K2O, U, Pb, Cs, Ni, and Cr.

6 of 21 B10201 ROONEY ET AL.: LITHOSPHERE IN THE ETHIOPIAN RIFT B10201

the HFSE increase gradually with decreasing MgO among mafic lavas, but this enrichment becomes more pronounced in samples with <5% MgO. HFSE abundances are strongly coupled with one another (e.g., Nb versus Zr) and with the rare earth elements (REE; e.g., Nb versus La). [22] REE exhibit similar trends to the HFSE, gradually increasing in samples with 10–5% MgO; below 5% MgO, REE concentrations rise rapidly. Chrondrite normalized REE patterns [see Furman et al., 2006, Figure 3] show a minor HREE depletion (Tbn/Ybn = 1.68–2.23) similar to that observed in previous studies of Ethiopian rift basalts (1.37–2.34 [Trua et al., 1999]) but not of the same magnitude as the 30 Ma HT2 plume-derived basalts (2.45–2.80 [Pik et al., 1999]). Minor positive Eu anomalies are observed in some samples from the Fantale-Dofan area (Figure 7; 1028, 1029, 1021, N-19, 1017). [23] Indicators of crustal contamination (e.g., La/Nb, Ce/ Pb; Figure 8) are generally within the range of uncontam- inated mantle-derived lavas; a few anomalously low (N-21, N-23, N-25) and high (N-13, N-19, 1017, 1021, 1024, 1028, 1029) Ce/Pb values require some modification. Significant- ly, samples with high Ce/Pb values also have anomalously high P2O5 and low Hf/Sm, suggesting a role for apatite (Figure 9); a subset of these same samples also has positive Eu anomalies (Figure 7). A group of samples collected within and south of Kone Caldera (N-22, N-20, 1036) exhibit anomalously high concentrations of the most incompatible trace elements (e.g., Cs, Rb, Nb) while less incompatible elements show no such heterogeneity (e.g., Yb, Zr; Figure 5). These same samples also have high normative nepheline contents (4.9–6.5%) suggesting they were generated by a smaller degree partial melt and/or a deeper source with respect to other Kone and WFB samples [Furman et al., 2006], similar to samples from the SDFZ. [24] The heterogeneity observed in incompatible trace element variations, despite an apparent lack of widespread Figure 3. Major element X-MgO diagrams. The more primitive data plotted here (greater than 6% MgO) are presented by Furman et al. [2006]. SDFZ data are presented by Rooney et al. [2005]. Data have been normalized on a water-free basis after FeO:Fe2O3 was calculated on the basis of a 85:15 ratio. Clear trends exist above and below 6% MgO. Greater than 6% MgO, olivine is the primary fractionating phase, highlighted by the relatively constant values of other elements. These trends are covered more fully in the text. All values are presented in wt.%. dances are nearly constant in samples with >5 wt.% MgO whereafter they decrease with decreasing MgO (Figure 5), indicting the onset of Fe-Ti oxide fractionation. [21] Large-ion lithophile elements (LILE) exhibit heterogeneous behavior: Sr and Ba (not shown) do not form coherent trends against MgO, whereas Rb exhibits a clear trend of enrichment throughout the suite, increasing rapidly Figure 4. Variation of CaO/Al2O3 with MgO. Symbols are at lower MgO contents. LILE ratios such as Rb/Sr and Ba/ the same as in Figure 3. The common fractionation trend Rb are also not correlated with MgO content (Figure 6). The from 10 to 7 wt.% MgO bifurcates into a steeper trend ‘‘A’’ anomalous behavior of the LILE are further highlighted by and a more shallow trend ‘‘B’’ which dominates the the lack of correlation between Sr and Ba versus individual fractionation path. Trend ‘‘C’’ appears to be generated by high field strength elements (HFSE), in contrast to the clear the addition of a component with high CaO/Al2O3 and positive correlation of Rb with Zr (Figure 6). Interestingly, lower MgO. Silti-Debre Zeyit Fault Zone lavas are shown Sr, Ba and Rb correlate well with La values. Abundances of for comparison [Rooney et al., 2005].

7 of 21 B10201 ROONEY ET AL.: LITHOSPHERE IN THE ETHIOPIAN RIFT B10201

Figure 5. Incompatible and compatible trace element X-MgO variation diagrams for Wonjii Fault Belt and Silti-Debre Zeyit lavas. Primitive samples plotted here are presented by Furman et al. [2006]. Values for Debre Zeyit and Butajira basalts are presented by Rooney et al. [2005]. All values are ppm unless otherwise specified. crustal assimilation, coupled with the compositional varia- the region from Afar in the north [Deniel et al., 1994] to tion documented at Kone argues for some degree of source Turkana in the south [Furman et al., 2004] but is generally heterogeneity in the WFB lavas. However, the consistent minor. Hart et al. [1989] used K/P, Ti/Yb, and Sr-Nd trends in major and most trace elements are broadly isotopic enrichment patterns to indicate significant crustal indicative of fractional crystallization of observed phases. assimilation of basalts erupted from 11-6 Ma on the Ethi- opian Plateau, also noting however, that mixing between 5. Discussion silicic and basaltic melts can have a significant impact on composition elsewhere in the rift (e.g., west central Afar). 5.1. Crustal Assimilation Other authors have also indicated a role for mixing of [25] In the study of mafic rocks erupted in continental evolved magma or solidified silicic material with fraction- settings we acknowledge the perennial problem of detecting ating basalt to produce the range of observed mafic Qua- and assessing the impact of crustal assimilation. The broad ternary lavas in the Ethiopian Rift [Gasparon et al., 1993; range of lithologies for potential crustal assimilants has Peccerillo et al., 2003]. However, the effect of crustal prompted the application of multiple petrographic (e.g., contamination on MER lavas, in particular the question of embayed feldspars, disaggregated crustal xenoliths) and whether the potential assimilant was of upper or lower geochemical (e.g., Ce/Pb, Ti/Yb, K/P, La/Nb, Sr and Pb crustal lithology, remains unresolved [e.g., Boccaletti et isotopes) indicators of crustal contamination. Crustal assim- al., 1995; Chernet and Hart, 1999; Pik et al., 1999; Trua ilation has been documented to varying degrees throughout

8 of 21 B10201 ROONEY ET AL.: LITHOSPHERE IN THE ETHIOPIAN RIFT B10201

Figure 6. Key trace element ratio and correlation plots for Wonjii Fault Belt and Silti-Debre Zeyit lavas. Primitive samples plotted here are presented by Furman et al. [2006]. Values for Debre Zeyit and Butajira basalts are presented by Rooney et al. [2005]. All values are ppm unless otherwise specified. et al., 1999; Kieffer et al., 2004]. Upper crustal contamina- is petrographic evidence for magmatic interaction. FeO-rich tion has been suggested to produce low Ti/Yb (>3,500 [Hart olivines (Fo52 – 57) in sample N-7 have anomalous Fe-Mg et al., 1989]) and high K/P (<12 [Hart et al., 1989]), exchange coefficients (KD = FeOcrystal MgOhost/FeOhost 87 86 Â Â coupled with elevated Sr/ Sr (0.706–0.765 [Hart et al., MgOcrystal) of 0.55–0.67 and are apparently xenocrysts. 1989; Pik et al., 1999; Trua et al., 1999]) and 206Pb/204Pb Samples N-7 and N-17 contain xenocrysts of more evolved ( 18.6 [Trua et al., 1999]). The upper crust, represented by minerals that are similar to the phenocryst assemblage of Ethiopian meta-volcanics [Pik et al., 1999] or by Sudanese N-19, suggesting mixing between primitive and evolved granites [Trua et al., 1999], is interpreted as playing only a minor role in Quaternary MER lava petrogenesis [Boccaletti et al., 1995; Chernet and Hart, 1999; Trua et al., 1999]. Lower crustal assimilation has been suggested to play a more substantial role in the contamination of mafic Ethio- pian lavas [Boccaletti et al., 1995; Chernet and Hart, 1999; Pik et al., 1999; Trua et al., 1999; Kieffer et al., 2004]; this assimilant is typically identified by high Ti/Yb (in excess of 2000 [Hart et al., 1989]) and La/Nb (greater than 2 [Pik et al., 1999]), and low K/P ( 3–12 [Hart et al., 1989]), Ce/Pb (>1 [Pik et al., 1999]) and206Pb/204Pb (16.93–17.63 [Trua et al., 1999]). The lower crust, represented by Sudanese granulite [Hart et al., 1989; Boccaletti et al., 1999] or Sudanese granite [Trua et al., 1999] is modeled to con- taminate Quaternary basalts in the MER by 10–13% [Boccaletti et al., 1995; Trua et al., 1999]. However , these studies did not account for heterogeneity in parental magmas [Rooney et al., 2005; Furman et al., 2006] and did not use Ethiopian crustal compositions, rendering such Figure 7. Chondrite normalized REE pattern for Wonjii numerical estimates unreliable. Fault Belt lavas. The shaded field represents all samples [26] Previous studies in the southern WFB recorded from the Wonjii Fault Belt and includes primitive samples basaltic enclaves within rhyolitic eruptives and felsic xen- presented by Furman et al. [2006]. Note mild positive Eu oliths within basaltic lavas and scoria [Peccerillo et al., anomaly for sample 1028. Normalizing factors are from 2003]. Xenoliths were not observed in our study, but there Boynton [1984].

9 of 21 B10201 ROONEY ET AL.: LITHOSPHERE IN THE ETHIOPIAN RIFT B10201

Figure 8. Crustal contamination indicators, Ce/Pb and La/Nb, are plotted for Ethiopian samples. (a) Crustal rocks from Ethiopia [Kebede et al., 1999; Sifeta et al., 2005; Tadesse and Allen, 2005; Yihunie et al., 2006] and 30 Ma flood basalts that exhibit a lower crustal contamination [Pik et al., 1999; Kieffer et al., 2004]. The WFB and SDFZ lavas lie predominantly within the mantle field but extend to lower values of Ce/Pb, which may indicate crustal contamination. Importantly, a subset of samples plots at high Ce/Pb and cannot represent contamination with crustal materials. (b) The WFB and SDFZ lavas are plotted with samples from Tullu Moye [Trua et al., 1999] and Gedemsa [Peccerillo et al., 2003]. Samples from Tullu Moye that lie within the mantle array (25 ± 5 [Hofmann et al., 1986]) plot at higher La/Nb, consistent with some lower crustal contamination of these lavas noted by Trua et al. [1999]. Gedemsa samples form an array with from low Ce/Pb to moderate Ce/Pb at relatively unenriched La/Nb (excepting 3 enclaves analyzed). Wonjii Basalts from Gedemsa and evolved rocks (>65% SiO2) from the Tulu Moye region are marked by filled symbols. basaltic magmas. Sample N-3 exhibits substantially hetero- is likely to be small. Notably, no pan-African crustal geneous populations of clinopyroxene and feldspars that are xenoliths have ever been reported in studies of MER lavas. clearly xenocrystic. These compositions are observed in [27] Ce/Pb and La/Nb values (Figure 8) can be used to felsic eruptives investigated nearby [e.g., Trua et al., 1999; evaluate potential contamination by both upper crustal Peccerillo et al., 2003]. Interestingly, these samples (N-3, (basement rocks from Ethiopia [Kebede et al., 1999; Sifeta N-7 and N-17) do not exhibit a marked decrease in Ce/Pb et al., 2005; Tadesse and Allen, 2005; Yihunie et al., 2006]) that is characteristic of nearby felsic lavas (Figure 8), and lower crustal lithologies (LT basalts [Pik et al., 1999; suggesting the volume of material mixed into these lavas Kieffer et al., 2004]). The majority of the WFB and SDFZ

10 of 21 B10201 ROONEY ET AL.: LITHOSPHERE IN THE ETHIOPIAN RIFT B10201

[29] We observe no correlation between Ce/Pb and 206Pb/204Pb, weak correlation between Ce/Pb and 87Sr/86Sr, and no correlation between 207Pb/206Pb and 87Sr/86Sr (Figure 10), suggesting any potential contaminant had isotopic ratios very similar to those of the primary basalts. Unfortunately, the lack of isotopic data for the Ethiopian pan-African crust makes it difficult to rule out this component as a potential contaminant for Quaternary lavas.

Figure 9. Variation of P2O5 with Hf/Sm ratio. Small quantities of apatite will significantly lower the Hf/Sm ratio while elevating P2O5. Samples identified with high Ce/Pb values show deflection toward apatite assimilation. The apatite is an accessory mineral in the fractionating assemblage required to generate evolved MER lavas from primitive basalts [Trua et al., 1999].

samples have La/Nb and Ce/Pb values that plot within the range observed in mantle-derived basalts [Hofmann et al., 1986]. Samples with high Ce/Pb cannot be the result of Ethiopian crustal assimilation (Figure 8). Evolved products of the southern WFB have low Ce/Pb and moderate to low La/Nb values [Trua et al., 1999; Peccerillo et al., 2003], suggesting they are an appropriate assimilant for our samples with anomalously low Ce/Pb. Basalts erupted within the Gedemsa caldera which show petrographic evidence of interaction with intrusive sialic rocks form an array of decreasing Ce/Pb at relatively constant La/Nb (Figure 8); samples N-25, N-23 and N-21 plot within this array, suggesting contamination with evolved magmas that are co-genetic or with the Ethiopian crust. [28] Samples with elevated Ce/Pb values (1028, 1029, 1024, N-19, 1017, 1021) are spatially restricted to the region between Fantale and Dofan. These samples are characterized by unusually high CaO/Al2O3 and low MgO, and exhibit small positive Eu anomalies consistent with feldspar assimilation. The unidentified feldspar-bearing assimilant also contains apatite, suggested by anomalously high P2O5 and Ce/Pb, low Hf/Sm (Figure 9). Apatite is found in silicic rocks in the region and the fractionation of a plagioclase-rich assemblage with accessory apatite has been invoked to produce silicic rocks from mafic magmas in the MER [Trua et al., 1999]. These data suggest that the unidentified assimilant is a cumulate related to the fractionation of more primitive lavas. This interpretation supports a model whereby at least some of the silicic products of the MER are derived through fractional crystallization of primitive magma [e.g., Ayalew et al., 1999] consistent Figure 10. Trace element and isotopic ratios sensitive to with ponding and fractionation of plume-derived melts at crustal contamination. A weak correlation is observed the base of the crust as predicted from coupled petrologic- between Ce/Pb and Sr isotopes, but no such correlation is numerical modeling [Farnetani et al., 1996], and geophys- observed with Pb isotopes. Trace element data for the SDFZ ical evidence of high velocity lower crust [MacKenzie et are from Rooney et al. [2005]. Isotopic data for WFB are al., 2005]. from Furman et al. [2006] and Rooney [2006].

11 of 21 B10201 ROONEY ET AL.: LITHOSPHERE IN THE ETHIOPIAN RIFT B10201

Table 2. Xenolith and Host Lava Components With Temperature the SDFZ lavas undergo fractionation at higher temperatures a and Pressure Estimates (1126–1362°C) and at multiple levels within the crust P, Depth, [Rooney et al., 2005]. These data indicate that at least some Sample Wo En Fs Ac kbar km Fo Fa Temp fractionation occurs at deeper levels beneath the SDFZ in N-20 (C)b 46.45 41.46 10.68 1.41 1.4 5.0 comparison to the WFB, consistent with the increased clino- N-20 45.57 41.99 11.03 1.41 0.3 1.1 pyroxene fractionation recorded in mass balance models. N-22 47.85 39.24 11.49 1.41 1.1 4.0 [32] In order to further refine our working model of low- N-22 46.64 42.76 9.46 1.14 0.2 0.6 N-22 46.30 40.57 11.65 1.48 0.9 3.1 pressure fractionation, we employed a coupled linear least N-22 48.34 40.48 9.91 1.26 2.0 6.9 squares mass balance (Table 3) and thermodynamic modeling N-22 46.82 36.26 13.77 3.15 1.0 3.7 (Table 4) approach using MELTS [Ghiorso and Sack, 1995]. N-21 43.87 42.74 11.93 1.46 0.4 1.5 We chose our most primitive sample (N-21; 10.5% MgO) as N-14 44.64 41.66 12.07 1.63 1.5 5.2 N-1 46.35 42.07 10.41 1.18 1.4 5.0 a parental composition. Although this sample has low Ce/Pb N-24 (R)b 45.62 36.89 15.94 1.55 0.1 0.4 (11), other incompatible trace element ratios indicate no N-7 52.1 46.6 977 crustal contamination (e.g., La/Nb = 0.74) and compatible N-17 47.4 51.6 1034 trace element abundances suggesting only limited removal of N-17 64.8 34.7 1171 N-16 73.9 25.7 1201 olivine from a primary mantle melt (165 ppm Ni; 556 ppm N-22 71.0 28.4 970 Cr; 33 ppm Sc). Sensitivity tests indicate that to initiate N-22 81.2 18.5 1064 plagioclase growth (as required from the observed phases and N-22 73.9 25.6 1111 mass balance results) requires a water content for N-21 of N-1 77.5 22.1 1080 0.2 wt.% and pressure of 1 kbar. At lower MgO contents N-1 81.8 17.9 1156 N-1 80.5 19.2 1089 (<7% MgO), two distinct fractionation trends are evident: N-1 78.4 21.2 1100 Trend A, which is restricted to samples surrounding Lake aPressure calculated using Nimis and Ulmer [1998] with a standard error Besaka, and trend B which defines the majority of samples of 1.7 kbar. Temperatures calculated using geothermometer (standard error within the WFB. Variable proportions of fractionating phases is ±6°C; analytical uncertainty is generally less than ±35°) of Loucks generate the alternative evolution pathways for the WFB [1996]. lavas: Trend A is dominated by clinopyroxene fractionation bValues are not systematically distributed (e.g., core and rim) except where marked core (‘‘C’’) and rim (‘‘R’’). and trend ‘‘B’’ reflects increased plagioclase and eventually Fe-Ti oxide growth. To reduce the plagioclase:clinopyroxene ratio and thus replicate Trend A lavas at 1 kbar, a higher magmatic water content ( 0.5%) is required. The inferred [30] An important conclusion from these data is that the heterogeneous magmatic water contents likely reflect varia- majority of WFB lavas do not show clear evidence of tions in the parental magmas. A significant result of this interaction with the continental crust. This interpretation modeling is the requirement for low pressure ( 1 kbar) to contrasts with observations on older lava series (low-Ti satisfy the relative proportions and total mass offractionated 30 Ma flood basalts [Pik et al., 1999]; 11-6 Ma pre-rift lavas phases predicted using the mass balance method. [Chernet and Hart, 1999]). This comparison suggests either [33] Mass balance models of fractionation for lavas from that (1) crustal modification by basaltic intrusives within the the SDFZ (Figure 1) highlight an even greater role for WFB has replaced the pre-rift continental crust with more clinopyroxene fractionation relative to that inferred for any recent rift-associated magmas and/or (2) crustal residence of the Trend A or B WFB lavas (Table 2). Plagioclase is time is substantially less for Quaternary lavas than for the absent from the modeled fractionating assemblage in Debre older flood and fissure basalts [e.g., Chernet and Hart, Zeyit but is present in Butajira. The increased role of 1999]. Geophysical evidence supports the hypothesis of clinopyroxene fractionation in these basalts is consistent with substantial intrusions and crustal modification within the Sc abundances, which form much tighter arrays with de- MER [e.g., Keranen et al., 2004; Dugda et al., 2005]. creasing MgO contents in comparison to the WFB (Figure 5). [31] To document the impact of magmatism on litho- The substantial clinopyroxene fractionation may indicate spheric structure, we first determine the conditions of deeper fractionation conditions and/or increased water con- fractionation. The maximum possible storage and fraction- tent in SDFZ lavas compared to the WFB lavas. ation depth for the SDFZ and WFB lavas is limited by the [34] Previous thermodynamic studies of Quaternary mag- depth of melt generation. Previous studies indicated that matic systems within the MER have attributed the evolution these Quaternary magmas are generated at 15–25 kbar among basalts to primarily shallow fractional crystallization ( 50–90 km) [Rooney et al., 2005; Furmanet al., 2006]. [Trua et al., 1999; Peccerillo et al., 2003]. Trua et al. This melting zone overlies low velocity anomalies inter- [1999], using a more primitive parental sample than this preted as partial melt in the depth range 75–250 km study (12% MgO), modeled limited fractionation initially at [Bastow et al., 2005]. The presence of a high velocity 5 kbar followed by extensive crystallization at 2 kbar (42%; lower crustal unit interpreted as dense mafic cumulates clinopyroxene, olivine, plagioclase and titanomagnetite). It [MacKenzie et al., 2005] raises the possibility that magmas is reasonable that fractionation of quite primitive lavas may fractionate at the base of the crust ( 30–35 km). (>10 wt.% MgO) may occur at levels deeper than those Clinopyroxene barometry [Nimis and Ulmer, 1998] indi- modeled in our data set. Peccerillo et al. [2003] extend cates that neither the WFB nor the SDFZ show appreciable fractionation to lower MgO contents (0.27%) by 40% along-axis variations in the depth of fractionation. How- fractionation of olivine, clinopyroxene, plagioclase and ever, WFB lavas fractionate at depths of less than 5 km titanomagnetite. The dominance of olivine and plagioclase and at modest temperatures (1050–1150°C; Table 2) while in the fractionating assemblage is consistent with the low

12 of 21 B10201

Table 3. Mass Balance Fractional Crystallization Modeling of the Wonjii Fault Belt Basaltsa

Wonjii Belt Debre Zeyit Butajira ROONEY From N-21 From N-18 From 1030 From N-18 From 1026 From DZ-1009 From DZ-1007 From DZ-1004 From BJ-1045 From BJ-1048 From BJ-1047 to N-18 to 1030 to N-17 to 1026 to N-6 to DZ-1007 to DZ-1004 to DZ-1006 to BJ-1048 to BJ-1047 to BJ-1042 Differenceb Differenceb Differenceb Differenceb Differenceb Differenceb Differenceb Differenceb Differenceb Differenceb Differenceb

SiO2 0.13 0.12 0.01 0.22 0 0.25 0.52 0.35 0.03 0.12 0.24 ET TiO 0.07 0.06À 0.05 À0.15 0.28 0.01 À0.1 0.28À 0.31À À0.18 0.06À

2 AL.: À À À À À À Al2O3 0.11 0.01 0.01 0.09 0.03 0.38 0.18 0.17 0.01 0.08 0.3 FeO À0.02 0.06 0.02À 0.07 0.1À 1 À0.11 0.55 0.21 0.09 0.38À 0.13 MnO 0À 0À 0 0.01 0.01 0À 0.02 0 0À 0.01 0 LITHOSPHERE MgO 0.04 0.07 0.02 0.05À À0.12 0.06 0.01 0.05 0.04 0.1 0.03 CaO À0.07 0.07 0À 0.14 0.07À À0.11 0.34À 0.2 0 0.1À 0.12

13 À À Na2O 0.05 0.36 0.02 0.27 0.06 0.29 0.07 0.21 0.07 0.13 0.2 À À À À of K2O 0.26 0.43 0.04 0.64 0.08 0.23 0.3 0.12 0.18 0.39 0.01 P O À0.04 0.17 À0.03 0.06 0.29À 0.12 0.12À 0.07 0.06 0.01À 0.09À

21 2 5 Fractionated phases À À À À IN PL An73: 3.50% An71: 2.70% An65: 8.90% An71: 14.10% An54: 23.40% An71: 5.4%

CPX En43: 8% En40: 6.70% En38: 6.80% En40: 7.70% En37: 14% En41: 7% En45: 3.90% En45 : 8.9% En44 : 5.6% En45: 8.1% En34: 8.7% THE OL Fo85 : 7.10% Fo81 : 0.80% Fo74 : 2.20% Fo81 : 5.70% Fo61: 5.30% Fo85: 2% Fo83: 1.50% Fo82: 2.2% Fo86: 2.4% Fo82 : 2.6% Fo80: 2.7%

MT Fe-Ti: 5.20% ETHIOPIAN Residual liquid 81.80% 88.90% 82.40% 71.70% 52.20% 91.10% 94.60% 88.30% 86% 90% 84.80% Sr2 0.112 0.375 0.06 0.594 0.2 0.385 0.843 0.38 0.145 0.384 0.237 aThe primitive samples used here (N-21; N-18; 1030) are presented by Furman et al. [2006]. The model was used to simulate discrete steps (e.g., N-21 to N-18, N-18 to 1026) and not a continuous fractionation trend. The fractionated phases were clinopyroxene (CPX), olivine (OL), feldspar (PL), and titanomagnetite (MT). bObserved minus calculated, times the weight factor. All oxides are 1 except SiO (0.4) and Al O (0.5).

2 2 3 RIFT B10201 B10201

Table 4. Thermodynamic Modeling of Wonjii Fault Belt Basalts Using MELTSa Trend ‘‘A’’ Trend ‘‘B’’ N-21 N-18 1030 N-17 N-18 1026 N-6 Parent Calc Measured Calc Measured Calc Measured Parent Calc Measured Calc Measured

Liquidus, °C 1256 1172 1177 1163 1166 1138 1149 1186 1138 1160 1106 1118 ROONEY Pressure, kbar 1 1 1 1 1 1 1 1 1 1 1 1 Log(10) f O2 6.7 7.63 7.57 7.73 7.69 8.03 - 8.46 9.03 8.77 9.44 - Buffer QFMÀ +1 QFMÀ +1 QÀFM+1 QFMÀ +1 QFMÀ +1 QÀFM+1 QFM+1 ÀQFM ÀQFM ÀQFM ÀQFM QFM SiO2 47.16 47.35 46.9 46.77 47.27 47.23 46.73 46.95 46.71 47.71 51.29 50.65 ET TiO2 1.91 2.31 2.27 2.48 2.34 2.75 2.72 2.27 2.92 3.18 2.65 3.22

Al2O3 14.19 15.46 15.85 15.91 16.43 15.28 16.40 15.87 15.44 15.25 13.53 15.10 AL.: Fe2O3 2.54 2.84 2.74 2.9 2.78 3.14 3.02 1.89 2.26 2.13 2.21 2.05

FeO 8.33 9.02 8.54 9.14 9.27 10.46 10.16 9.3 11.05 11.08 10.66 9.82 LITHOSPHERE MnO 0.18 0.23 0.19 0.22 0.2 0.24 0.21 0.19 0.27 0.23 0.39 0.25 MgO 10.51 7.35 7.45 6.83 6.87 6.24 6.28 7.44 5.44 5.48 4.05 4.03 CaO 11.09 10.46 10.93 10.1 10.06 9.29 9.06 10.95 9.32 9.81 8.08 7.94 14 Na2O 2.84 3.43 3.23 3.51 3.12 3.38 3.35 3.24 3.96 3.48 4.41 4.39 of K2O 0.67 0.85 1.14 1.28 0.78 0.92 0.98 1.14 1.56 0.66 1.06 1.30

21 P2O5 0.37 0.47 0.51 0.58 0.38 0.46 0.50 0.51 0.71 0.63 1.06 0.65

H2O 0.2* 0.25 0.25* 0.28 0.5* 0.6 0.6* 0.25 0.35 0.35 0.6 0.6* IN Fractionated phases THE OL Fo86: 6.53% Fo83: 0.86% Fo82: 2.62% Fo80: 4.55% Fo74 : 2.21% CPX Di52,Ce15,He13 : 9.51% Di50, Ce14, He13: 5.61% Di44 ,Ce16 ,He16: 2.70% Di49,Ce13,He16 : 10.42% Di36 ,Ce19 ,He23: 11.12% PL An75: 3.64% An75 : 3.43% An75: 10.25% An73: 12.18% An63: 19.60% ETHIOPIAN MT Mt32,Sp42,Usp58: 5.69% a Ghiorso and Sack [1995]. The primitive samples used here (N-21; N-18; 1030) are presented by Furman et al. [2006]. FeO:Fe2O3 for the samples is calculated on the basis of the relevant oxygen buffer and liquidus conditions. No measured water contents are available, and all water values are modeled. Measured values for samples are normalized to 100% given the modeled water contents and FeO:Fe2O3. The model was used to simulate discrete steps (e.g., N-21 to N-18, N-18 to 1026) and not a continuous fractionation trend. The starting material for each step is the measured value for that sample and not the end point of the previous model

step. A QFM + 1 oxygen fugacity dropping to QFM was used, similar to that used by Trua et al. [1999]. Initial water contents and pressure were determined by model sensitivity analysis. The model follows two broad RIFT paths corresponding to trends ‘‘A’’ and ‘‘B’’ in Figure 4. An asterisk (*) indicates that these water contents are not measured and are taken from modeled values. Fo, fosterite; Di, diopside; Ce, clinoenstatite; He, hedenbergite; An, anorthite; Mt, magnetite; Sp, spinel; Usp, ulvospinel. B10201 B10201 ROONEY ET AL.: LITHOSPHERE IN THE ETHIOPIAN RIFT B10201 modeled pressure of 0.5 kbar for samples from Gedemsa, gravity data [Mahatsente et al., 1999; Cornwell et al., slightly shallower thanmodeled in this study as expected for 2006]. At deeper levels, Kendall et al. [2005] predict a lavas erupted within an existing caldera system. region of magma injection in the mantle lithosphere beneath a relatively unstretched, but heavily intruded crust. Similar 5.2. Spatial Variations in Crustal Structure methods have also constrained the presence of melt at [35] The data presented here allow for comparison of crustal levels. Dike emplacement into the crust is consistent fractionation depths across the rift (east-west, between the with dike-derived xenoliths found in lavas from the SDFZ WFB and SDFZ) and along the length of the rift (northeast- [Rooney et al., 2005]. These data indicate that the modifi- southwest, along the WFB). This three-dimensional frame- cation of the crust beneath these active tectonomagmatic work of fractionation depths within the rift has direct belts is primarily through dike intrusion associated with the relevance to existing geophysical work that has pointed to magmatic plumbing system of the rift. the presence of melt within the crust. Analysis of tele- [37] We may gain insight into the magmatic plumbing seismic receiver functions provides information on bulk system from surface observations of eruptive activity and crustal properties: thickness and Vp/Vs ratio. Stuart et al. the relationship of these eruptives to tectonic features. The [2006] and Dugda et al. [2005] indicate that measured Vp/ basaltic cinder cones and flows within the WFB and SDFZ Vs ratios within the rift are high ( 2 in some cases) and are associated with the tail cracks of faults/fractures point to melt within the crust, but this method is unable to [Chorowicz et al., 1994; Korme et al., 1997, 2004], locate this melt precisely. Kendall et al. [2005, 2006] also suggesting a direct link between tectonics and magmatism. suggest the presence of melt within the crust to explain Basaltic cinder cones observed in the western rift also observations of seismic anisotropy beneath the MER. Anal- share this tectonomagmatic relationship [e.g., Ebinger, yses of magnetotelluric data have suggested melt within the 1989a, 1989b]. The numerous fractures and faults exposed crust beneath the SDFZ and WFB at both shallow (<5 km) in the WFB are not deep-seated features and propagate and mid-crustal depths [Whaler and Hautot, 2006]. The from the surface to a maximum estimated depth of only geochemical constraints presented here provide direct evi- 2 km [Acocella et al., 2003a, 2003b], though seismicity dence that melt resides in the crust, strengthening existing indicates some deeper fault activity (14 km [Keir et al., geophysical arguments. Moreover, we have shown that the 2006]). It is therefore plausible that these fractures inter- depth of melt fractionation does not vary along the length of sect shallow dikes or magma storage chambers, prompting the MER, but does differ across it. The consistently shallow eruption of the variably fractionated contents. fractionation conditions of Quaternary products within the WFB are in contrast to the basalts erupted in the SDFZ that 5.3. Synthesis Model of Crustal Structure and exhibit fractionation throughout the crust (Figure 11). This Implications for Rifting variation between the two tectonomagmatic zones may [38] The geochemical and geophysical data gathered in reflect a more mature magmatic plumbing system within the north central MER support a model of focused mag- the WFB, where magma quickly rises through existing matic and tectonic activity during the Quaternary, centered conduits to fractionate at shallow depths. Conversely, in on the WFB and SDFZ. The spatial distribution of these the SDFZ these conduits are less well defined and melt may fault belts, which are the surface expression of rifting take multiple routes to the surface, on occasion stagnating related crustal modification, have significant utility in as indicated by Al-augite xenoliths and the wide range in determining how extension evolves in a rift environment. fractionation depths observed in the SDFZ (Figure 12) Figure 1a shows the location of Quaternary cinder cones [Rooney et al., 2005]. and associated lava flows within the MER, reflecting the [36] Further details of crustal structure in the MER may distribution of Quaternary magmatic and tectonic activity. be gained by probing the method by which Quaternary Shallow seismicity within the rift [Keir et al., 2006] often basalts traversed the crust. Within the WFB, spatially coincides with Quaternary lava flows; epicenters cluster on related primitive basalts exhibit substantially heterogeneous the edges of flows and are less common directly beneath the trace element and isotopic compositions [Furman et al., flows (Figure 13), suggesting linkage between the rheolog- 2006]. These diverse geochemical signatures point to a ical characteristics of the upper crust and magmatism within heterogeneous source region and to a complex magmatic the rift. There is an abrupt discontinuity in the alignment of plumbing system, which allows the transport of magma cinder cones around 8°300N, coincident with a change in through the crust without the homogenization typically overall cinder cone alignment from N24°E in the south to associated with residence in larger magma-chambers. N41°E farther north. There is also a change in strike of the Smaller conduits are also consistent with the low volume MER border faults at 8°300N from N32°E ± 9° in the of basaltic material erupted in the MER. Where multiple southwest to N47°E ± 12° in the northeast [Acocella and dikes/conduits have been ‘‘captured’’ by an existing silicic Korme, 2002]. Segmentation of seismic anomalies at 75 km plumbing system, larger volumes of extrusives may be in the upper mantle presented by Bastow et al. [2005] possible, e.g., Kone caldera and the fissure eruption at mirrors this change in cinder cone alignment. Fantale. Keranen et al. [2004] suggested extension in the [39] Existing lithospheric structures exert control on ex- central MER is accommodated by magmatic intrusion into tension throughout the EARS [Ebinger et al., 1997; Petit both a ductile middle-lower crust and upper brittle crust and and Ebinger, 2000; Nyblade and Brazier, 2002]; in partic- controlled source reflection/refraction studies [MacKenzie et ular, existing basement structures in the MER have been al., 2005; Maguire et al., 2006] indicate extensive replace- reactivated during the Cenozoic [Korme et al., 2004]. East- ment of the MER lower crust by high velocity mafic west trending structures of the Yerer-Tullu Wellel volcano- material. Mafic intrusions are also consistent with observed tectonic lineament (YTVL) are observed on the rift flanks

15 of 21 B10201 ROONEY ET AL.: LITHOSPHERE IN THE ETHIOPIAN RIFT B10201

velocity anomalies, interpreted as cooled mafic intrusions that underlie the WFB and SDFZ [Keranen et al., 2004]. [40] Figure 14a outlines a synthesis model for the north central MER at 10 Ma; the northern EARS is divided into two zones associated with the northward propagating MER and the southward propagating Red Sea Rift. While some models suggest the influence of the Gulf of Aden Rift system [Chernet et al., 1998], recent work has highlighted the dominance of the Red Sea Rift [Wolfenden et al., 2004, 2005]. Although we do not completely rule out the possibility that Gulf of Aden structures extend into this region, geophysical evidence does not support such a model. Regional SKS shear wave splitting measurements have fast polarization directions that are oriented NNE-SSW, almost perpendicular to the inferred ESE-WSW Gulf of Aden trend at 9°N [Gashawbeza et al., 2004; Kendall et al., 2005, 2006]. Furthermore, seismicity in the region shows a dominance of Red Sea trends with little activity on the eastern rift margin north of 9°N [Keir et al., 2006]. A model of current crustal structure is presented in Figure 14b; the two zones apparent at 10 Ma are now connected across the Boru-Toru Structural High. We acknowledge the crustal variations along the length of MER in both models and group our observations into two regions, separated by a transfer zone at 8°300N (Figures 14a and 14b). [41] Northeast of the transfer zone the distinct rift valley Figure 11. Plot of latitude versus calculated clinopyroxene graben morphology that is evident in the southwest gives phenocryst equilibration depths based on Nimis and Ulmer way to the less well-defined Afar depression and a single [1998]. Pressure is converted to depth assuming 1 kbar = rift-centered zone of magmatism dominates (Figures 14a 3.5 km. Source data are from Table 2 and Rooney et al. and 14b). Geophysical surveys confirm a south-to-north [2005]. Phenocryst equilibration depths in the Silti-Debre thinning of the crust from 40 km to 26 km, coincident with Zeyit Fault Zone extend throughout the crust, while those from the Wonjii Fault Belt are restricted to the upper crust.

[Abebe et al., 1998] and may exert influence in the rift itself [Mazzarini et al., 1999; Bonini et al., 2005]. Bastow et al. [2005] noted a seismic low velocity zone in the uppermost upper mantle beneath the YTVL, suggesting it is a feature that exists to asthenospheric depths. Significantly, the northern limit of the YTVL corresponds to the shift in alignment of the rift border faults (discussed above) and to the Boru-Toru structural high, interpreted as a transfer zone between MER rift segments [Bonini et al., 2005]. The Boru- Toru Structural High thus separates the northern EARS into two zones: one to the northeast that is part of the Red Sea Rift system and one to the southwest, dominated by the MER (Figure 14). Regional geodynamic models support this division of the northern EARS, indicating that rifting in Afar commenced along the Red Sea at 26–29 Ma and propagated southward only as far as 10°N at that time [Wolfenden et al., 2005]. To the south,rifting of the MER that began at 25 Ma in Turkana [Hendrie et al., 1994; Morley, 1994] may have propagated northward forming rift Figure 12. A cartoon of crustal structure beneath the Silti- border faults in the northern MER at 11 Ma [Wolfenden et Debre Zeyit Fault Zone (SDFZ) and Wonjii Fault Belt al., 2004]. Interestingly, two low velocity seismic anomalies (WFB). This representation suggests that the magmatic are imaged in the upper mantle beneath this region [Bastow plumbing system beneath the WFB is more developed than et al., 2005], which are distinct at 75 km depth and located that beneath the SDFZ. This results in magmas within the on either side of the structural high, but merging with WFB rising toward the surface more rapidly, fractionating increasing depth. Within the upper crust, the structural high close to the surface. Within the SDFZ the magmas may also marks the site of a change in orientation of high fractionate at various crustal depths prior to eruption.

16 of 21 B10201 ROONEY ET AL.: LITHOSPHERE IN THE ETHIOPIAN RIFT B10201

Figure 13. Distribution of seismicity overlain on Quaternary basaltic magmatism in the MER. The Wonjii Fault Belt (WFB) is shaded in blue, while the Silti-Debre Zeyit Fault Zone (SDFZ) is shaded in green [Berhe and Wondm-Agennehu, 1978; Benvenuti et al., 2002; Abebe et al., 2005]. Seismicity is from Keir et al. [2006]. The area north of 9°N has not been mapped, but fieldwork there during 2003 indicated little basaltic magmatism between Fantale and Dofan (see Figure 1). The diagram illustrates a striking anticorrelation between Quaternary basaltic magmatism and modern seismicity within the rift. Within the WFB, seismicity is concentrated between Fantale and Dofan (1), at the Ankober Border Fault (2), and at the very southern end of the fault belt (3). Less intense seismicity is also distributed throughout the WFB but is concentrated at the edge of basaltic flows (white arrows). At the northern end of the SDFZ, seismicity is also observed (dashed oval). The seismicity to the west of the SDFZ at 9°N may be related to the YTVL. the transfer zone [Maguire et al., 2006], consistent with the region, the WFB may be a precursor to a seafloor spreading influence of seafloor spreading propagating southward from center [e.g., Ebinger and Casey, 2001; Keranen et al., 2004; Afar. Crustal modification within this region is dominated Casey et al., 2006]. by basaltic and silicic activity within the WFB (the SDFZ [42] Southwest of the transfer zone, the distribution of does not extend north of the transfer zone). The fraction- recent cinder cones defines two parallel zones of magma- ation data for the WFB presented here indicate a well- tism corresponding to the SDFZ in the west and the WFB in developed magmatic system that allows magma to rise to the east, neither occupying the central portion of the Rift shallow levels before fractionating in predominantly small Valley (Figure 14b). Magmatism in the SDFZ extends from magma bodies (dikes) and some larger magma chambers the embayment of the rift near the Boru-Toru Structural (e.g., nested calderas). Within this crustally thinned northern High in the north toward Butajira in the south, eventually

17 of 21 B10201 ROONEY ET AL.: LITHOSPHERE IN THE ETHIOPIAN RIFT B10201

Figure 14. Cartoon of the north central Ethiopian Rift centered on the Boru-Toru Structural High (BTSH). The modern crustal thickness and velocity anomalies are based on geophysical data discussed in the text. YTVL is the Yerer-Tullu Wellel volcanotectonic lineament [Abebe et al., 1998]. Large volcanoes are silicic centers (e.g., Fantale, Ziqualla); smaller volcanoes are basaltic cinder cones. (a) The 10 Ma configuration of the Ethiopian Rift. This configuration represents the rift prior to the breach of theBTSH by the Wonjii Fault Belt. The propagation direction of the Main Ethiopian Rift (MER) and Red Sea Rift are based on Wolfenden et al. [2004, 2005] and Bonini et al. [2005]. (b) Present-day structure of the MER. The Boru-Toru Structural High (BTSH) is breached by the Wonjii Fault Belt. To the south of the BTSH the Silti-Debre Zeyit Fault Zone and Wonjii Fault Belt dominate Quaternary magmatism, but to the north, only the Wonjii Fault Belt is present. This model suggests crustal modification along the lengths of the Wonjii Fault Belt and Silti-Debre Zeyit Fault Zone. occupying a central zone of magmatism near Lake Shala, ternary tectonomagmatic activity point to heterogeneity merging with the WFB (Figure 13). Magmatism in the WFB within the rift-based magmatic plumbing system. Geophys- decreases substantially south of the transfer zone ceasing ical evidence also points to variation between the WFB and near Lake Ziway as it merges with the SDFZ. This southern SDFZ, specifically magnetotelluric results indicate a deeper termination of the WFB before it merges with the SDFZ is (25–30 km) and more pronounced zone of melt beneath the also the site of active extension; a fissuring event recently SDFZ in comparison to shallower melt beneath the WFB occurred near Lake Ziway, generating subsidence and [Whaler and Hautot, 2006]. Despite suggested models that rupturing the main Addis Ababa to Awassa road (G. Yirgu, the majority of extension is accommodated within the WFB personal communication, 2006]. The crustal thickness [Ebinger and Casey, 2001; Casey et al., 2006], GPS ( 40 km [Maguire et al., 2006]) in the southwest is greater measurements within or across the rift have been unable than in the northeast (26 km). Evidence of melt and to locate precisely where extension is accommodated solidified intrusions in the crust to depths of 30 km are [Bilham et al., 1999; Pan et al., 2002; Bendick et al., observed beneath both belts [e.g., Keranen et al., 2004; 2006; Pizzi et al., 2006]. Interestingly, no significant Whaler and Hautot, 2006], indicating that magmatic mod- variation in fractionation depth is observed along the ification of the crustal structure is concentrated on the WFB length of the WFB from north to south regardless of the and SDFZ. These two belts are not equivalent. Fractionation observed variation in crustal thickness. within the SDFZ occurs at various depths within the crust [43] It is clear that distinct differences in crustal structure and is followed by rapid eruption of the overall less exist between the two regions described above. The dual- fractionated products [Rooney et al., 2005]. Within the focus magmatism near the rift border faults and the minimal WFB, fractionation is predominantly a shallow process crustal thinning to the southwest passes northeastward into a and generates a greater range in erupted compositions. region with much thinner crust and a single centralized belt The differences between these two zones of focused Qua- of magmatism away from the rift border faults. The com-

18 of 21 B10201 ROONEY ET AL.: LITHOSPHERE IN THE ETHIOPIAN RIFT B10201 bined geochemical, geophysical and geodynamic data set all occasional assimilation of magmatic fractionation products. point to a model whereby the crustal structure to the These results are consistent with abundant geophysical data northeast of the transfer zone is dominated by incipient suggesting the widespread replacement of the continental seafloor spreading processes while to the southwest, conti- crust by magmatic intrusion. nental rifting still controls crustal modification processes. [46] The geochemical results of Quaternary magmas, This division into two zones separated by a transfer zone is when interpreted within the existing geophysical frame- complicated by the WFB, which passes through the transfer work, preclude a single axis of extension model for the zone. We suggest that the WFB represents the southward MER. The data indicate differing magmatic histories and propagation of the Red Sea Rift system, while the SDFZ origins for the WFB and SDFZ lavas that are best explained represents the northward propagation of the MER. This by a model where two rifts join: the Southern Red Sea Rift, interpretation defines the Quaternary zones of tectonomag- in which incipient seafloor spreading may be underway has matic activity as overlapping spreading centers, generated propagated south and joined with the northward propagat- by the merger of the Red Sea Rift and MER. This hypoth- ing continental East African Rift System in the Main esis is supported by seismic data [Keir et al., 2006] that Ethiopian Rift. show ‘‘earthquake swarms’’ along the Ankober Border Fault (linkage between the Red Sea Rift and MER) and [47] Acknowledgments. This research was supported by National within the WFB. In particular, at the southern limit of Science Foundation grant EAR 0207764 (T. Furman) as part of the cross- disciplinary Ethiopia Afar Geoscientific Lithosphere Experiment (EAGLE). magmatism in the WFB a distinct ‘‘earthquake swarm’’ A George H. Deike Jr. grant to T. Furman provided additional support for has been noted (Figure 13), coincident with the recent analytical work. T.R. thanks Kassahun Ejeta, guide and driver, and Roeland fissuring event at Lake Ziway. This seismically active Doust, field assistant during fieldwork in Ethiopia. Margaret Nitz and Leigh southern tip of the WFB also displays evidence of crustal Patterson were helpful in sample preparation. We thank Julie Bryce for contributing to an earlier version of the manuscript and Clifton Rooney for contamination in WFB basalts [Trua et al., 1999]. The help in drafting a figure. We are grateful to Mark Angelone for help with increased seismic activity, in tandem with the poorly devel- the Electron Microprobe at Penn State and Emily Klein, Gary Dwyer, and oped magmatic plumbing system and evidence of crustal Mark Rudnicki of Duke University for performing DCP and ICP-MS analysis. Thanks also to Andrew Nyblade, Richard Parizek, Derek assimilation is indicative of the southward propagating tip Elsworth, David Eggler, Barry Hanan, Peter LaFemina, and Wendy Nelson of the WFB. Similarly, the enhanced seismic activity and for thoughtful comments on earlier manuscript versions. We thank Giday poorly developed young explosive magmatism at the north- WoldeGabriel and Roger Buck for careful peer reviews of the manuscript and Peter Cawood for editorial handling. Finally, we would like to thank the ern limit of the SDFZ at Chefe Donsa, suggests the EAGLE group and in particular Cindy Ebinger for the many stimulating northward propagation of the SDFZ. These two zones of discussions that improved this manuscript. tectonic and magmatic activity thus record more than simply the transition between rifting and seafloor spreading; References they are the result of the recent linkage between the two Abebe, T., F. Mazzarini, F. Innocenti, and P. Manetti (1998), The Yerer- different structural and tectonic domains defined by the Tullu Wellel volcanotectonic lineament: A transtensional structure in central Ethiopia and the associated magmatic activity, J. Afr. Earth Sci., southward propagating Red Sea Rift and northward propa- 26(1), 135–150. gating MER/EARS. Abebe, T., P. Manetti, M. Bonini, G. Corti, F. Innocenti, F. Mazzarini, and Z. Pecksay (2005), Geological map (scale 1:200,000) of the Northern Main Ethiopian Rift and its implications for the volcano-tectonic evolu- 6. Summary and Conclusions tion of the rift, Geol. Soc. of Am., Boulder, Colo. Acocella, V., and T. Korme (2002), Holocene extension direction along the [44] The combined geochemical and geophysical results Main Ethiopian Rift, East Africa, Terra Nova, 14(3), 191–197. of the EAGLE project point to two significant advances in Acocella, V., T. Korme, and F. Salvini (2003a), Formation of normal faults along the axial zone of the Ethiopian Rift, J. Struct. Geol., 25(4), 503– our understanding of continental extension and rifting in 513. East Africa: Extension is accommodated largely through Acocella, V., T. Korme, F. Salvini, and R. Funiciello (2003b), Elliptic dike intrusion in the lithosphere and this extension is calderas in the Ethiopian Rift: control of pre-existing structures, J. Vol- focused within regions of Quaternary magmatic activity in canol. Geotherm. Res., 119(1–4), 189–203. Ayalew, D. (2000), Crustal control in the genesis of Plio-Quaternary bimo- the Main Ethiopian Rift (Wonjii Fault Belt and Silti-Debre dal magmatism of the Main Ethiopian Rift (MER): Geochemical and Zeyit Fault Zone). isotopic (Sr, Nd, Pb) evidences: Discussion [modified], Chem. Geol., [45] The geochemical results of the EAGLE project 168(1–2), 1–3. Ayalew, D., G. Yirgu, and R. Pik (1999), Geochemical and isotopic (Sr, Nd significantly enhance existing geophysical images of mag- and Pb) characteristics of volcanic rocks from southwestern Ethiopia, matic intrusion. Coupled mass balance and thermodynamic J. Afr. Earth Sci., 29(2), 381–391. modeling indicate that WFB lavas require shallow fraction- Bastow, I. D., G. W. Stuart, J. M. Kendall, and C. J. Ebinger (2005), Upper- mantle seismic structure in a region of incipient continental breakup: ation conditions, indicative of a well-developed magmatic northern Ethiopian rift, Geophys. J. Int., 162(2), 479–493. plumbing system. Conversely, within the SDFZ, melt is Bedini, R. M., J. L. Bodinier, J. M. Dautria, and L. Morten (1997), Evolu- present throughout the crust and magmatic composition is tion of LILE-enriched small melt fractions in the lithospheric mantle: dominated by fractionation at deeper levels. The preserva- A case study from the East African Rift, Earth Planet. Sci. Lett., 153(1–2), 67–83. tion of source-derived geochemical heterogeneities [Rooney Bendick, R., S. McClusky, R. Bilham, L. Asfaw, and S. L. Klemperer et al., 2005; Furman et al., 2006; this work] and xenolith (2006), Distributed Nubia-Somalia relative motion and dike intrusion in evidence [Rooney et al., 2005], points to a complex mag- the Main Ethiopian Rift, Geophys. J. Int., 165, 303–310. Benoit, M. H., A. A. Nyblade, J. C. VanDecar, and H. Gurrola (2003), matic plumbing system dominated by pervasive diking Upper mantle P wave velocity structure and transition zone thickness beneath the WFB and SDFZ. There is no clear evidence beneath the Arabian Shield, Geophys. Res. Lett., 30(10), 1531, of pan-African crustal assimilation in these lavas (excepting doi:10.1029/2002GL016436. Benvenuti, M., et al. (2002), The Ziway-Shala lake basin (main Ethiopian lavas at the southern termination of the WFB), however rift, Ethiopia): a revision of basin evolution with special reference to the anomalous major and trace element ratios indicate the Late Quaternary, J. Afr. Earth Sci., 35(2), 247–269.

19 of 21 B10201 ROONEY ET AL.: LITHOSPHERE IN THE ETHIOPIAN RIFT B10201

Berhe, S. M., and B. Wondm-Agennehu (1978), Nazret, Ethiopian Mapping Volcanic Province Within the East African Rift System, edited by G. Yirgu, Agency, Addis Ababa, Ethiopia. C. Ebinger, and P. Maguire, Geol. Soc. Spec. Publ., 259, 95–120. Bilham, R., R. Bendick, K. Larson, P. Mohr, J. Braun, S. Tesfaye, and Gashawbeza, E. M., S. L. Klemperer, A. A. Nyblade, K. T. Walker, and L. Asfaw (1999), Secular and tidal strain across the main Ethiopian rift, K. M. Keranen (2004), Shear-wave splitting in Ethiopia: Precambrian Geophys. Res. Lett., 26(18), 2789–2792. mantle anisotropy locally modified by Neogene rifting, Geophys. Res. Boccaletti, M., A. Getaneh, R. Mazzuoli, L. Tortorici, and T. Trua (1995), Lett., 31, L18602, doi:10.1029/2004GL020471. Chemical variations in a bimodal magma system: The Plio-Quaternary Gasparon, M., F. Innocenti, P. Manetti, A. Peccerillo, and A. Tsegaye volcanism in the Dera Nazret area (Main Ethiopian Rift, Ethiopia), Afr. (1993), Genesis of the Pliocene to Recent bimodal mafic-felsic volcanism Geosci. Rev., 2(1), 37–60. in the Debre Zeyt area, central Ethiopia: Volcanological and geochemical Boccaletti, M., R. Mazzuoli, M. Bonini, T. Trua, and B. Abebe (1999), Plio- constraints, J. Afr. Earth Sci., 17(2), 145–165. Quaternary volcanotectonic activity in the northern sector of the Main Ghiorso, M. S., and R. O. Sack (1995), Chemical mass-transfer in mag- Ethiopian Rift: Relationships with oblique rifting, J. Afr. Earth Sci., matic processes. 4. A revised and internally consistent thermodynamic 29(4), 679–698. model for the interpolation and extrapolation of liquid-solid equilibria in Bonini, M., G. Corti, F. Innocenti, P. Manetti, F. Mazzarini, T. Abebe, and magmatic systems at elevated-temperatures and pressures, Contrib. Z. Pecskay (2005), Evolution of the Main Ethiopian Rift in the frame of Mineral. Petrol., 119(2–3), 197–212. Afar and Kenya rifts propagation, Tectonics, 24, TC1007, doi:10.1029/ Gregory, J. (1896), The Great Rift Valley, 405 pp., John Murray, London. 2004TC001680. Hart, W. K., G. Woldegabriel, R. C. Walter, and S. A. Mertzman (1989), Boynton, W. V. (1984), Cosmochemistry of the rare earth elements: Meteor- Basaltic volcanism in Ethiopia: Constraints on continental rifting and ite studies, in Rare Earth Element Geochemistry, edited by P. Henderson, mantle interactions, J. Geophys. Res., 94(B6), 7731–7748. pp. 63–114, Elsevier, New York. Hendrie, D. B., N. J. Kusznir, C. K. Morley, and C. J. Ebinger (1994), Buck, W. R. (2004), Consequences of asthenospheric variability on con- Cenozoic extension in Northern Kenya—A quantitative model of rift tinental rifting, in Rheology and Deformation of the Lithosphere at basin development in the Turkana region, Tectonophysics, 236(1 –4), Continental Margins, edited by G. Karner et al., pp. 1–30, Columbia 409–438. Univ. Press, New York. Hofmann, A. W., K. P. Jochum, M. Seufert, and W. M. White (1986), Nb Burke, K., and J. F. Dewey (1973), Plume-generated triple junctions: Key and Pb in oceanic basalts — New constraints on mantle evolution, Earth indicators in applying plate tectonics to old rocks, J. Geol., 81, 406–433. Planet. Sci. Lett., 79(1–2), 33–45. Casey, M., C. Ebinger, D. Keir, R. Gloaguen, and F. Mohamed (2006), Strain Hofmann, C., V. Courtillot, G. Feraud, P. Rouchett, G. Yirgu, E. Ketefo, accommodation in transitional rifts: Extension by magma intrusion and and R. Pik (1997), Timing of the Ethiopian flood basalt event and im- faulting in Ethiopian rift magmatic segments, in The Afar Volcanic Pro- plications for plume birth and global change, Nature, 389(6653), 838– vince Within the East African Rift System, edited by G. Yirgu, C. Ebinger, 841. and P. Maguire, Geol. Soc. Spec. Publ., 259, 143–164. Holbrook, W., and P. Kelemen (1993), Large igneous province on the US Chernet, T., and W. K. Hart (1999), Petrology and geochemistry of volcan- Atlantic margin and implications for magmatism during breakup, Nature, ism in the northern Main Ethiopian Rift: Southern Afar transition region, 364, 433–436. Acta Vulcanol., 11(1), 21–41. Hopper, J., and W. Buck (1993), The initiation of rifting at constant tectonic Chernet, T., W. K. Hart, J. L. Aronson, and R. C. Walter (1998), New age force: The role of diffusion creep, J. Geophys. Res., 98, 16,213–16,221. constraints on the timing of volcanism and tectonism in the northern Huismans, R. S., Y. Y. Podladchikov, and S. Cloetingh (2001), Transition Main Ethiopian Rift-southern Afar transition zone (Ethiopia), J. Volcanol. from passive to active rifting: Relative importance of asthenospheric Geotherm. Res., 80(3–4), 267–280. doming and passive extension of the lithosphere, J. Geophys. Res., Chorowicz, J., B. Collet, F. F. Bonavia, and T. Korme (1994), Northwest to 106(B6), 11,271–11,291. north-northwest extension direction in the Ethiopian Rift deduced from Kebede, T., C. Koeberl, and F. Koller (1999), Geology, geochemistry and the orientation of extension structures and fault-slip analysis, Geol. Soc. petrogenesis of intrusive rocks of the Wallagga area, western Ethiopia, Am. Bull., 106(12), 1560–1570. J. Afr. Earth Sci., 29(4), 715–734. Cornwell, D. G., G. D. Mackenzie, R. W. England, P. K. H. Maguire, L. M. Keir, D., C. J. Ebinger, G. W. Stuart, E. Daly, and A. Ayele (2006), Strain Asfaw, and B. Oluma (2006), Northern Main Ethiopian Rift crustal struc- accommodation by magmatism and faulting as rifting proceeds to break- ture from new high-precision gravity data, in The Afar Volcanic Province up: Seismicity of the northern Ethiopian rift, J. Geophys. Res., 111, Within the East African Rift System, edited by G. Yirgu, C. Ebinger, and B05314, doi:10.1029/2005JB003748. P. Maguire, Geol. Soc. Spec. Publ., 259, 307–322. Kendall, J.-M., G. W. Stuart, C. Ebinger, I. D. Bastow, and D. Keir (2005), Courtillot, V., C. Jaupart, I. Manighetti, P. Tapponnier, and J. Besse (1999), Magma-assisted rifting in Ethiopia, Nature, 433(7022), 146–148. On causal links between flood basalts and continental breakup, Earth Kendall, J.-M., S. Pilidou, D. Keir, I. D. Bastow, G. Stuart, and A. Ayele Planet. Sci. Lett., 166(3–4), 177–195. (2006), Mantle upwellings, melt migration and the rifting of Africa: In- Deniel, C., P. Vidal, C. Coulon, and P. J. Vellutini (1994), Temporal evolu- sights from seismic anisotropy, in The Afar Volcanic Province Within the tion of mantle sources during continental rifting: The volcanism of East African Rift System, edited by G. Yirgu, C. Ebinger, and P. Maguire, Djibouti (Afar), J. Geophys. Res., 99(B2), 2853–2869. Geol. Soc. Spec. Publ., 259, 55–72. Dugda, M. T., A. A. Nyblade, J. Julia, C. A. Langston, C. J. Ammon, and Keranen, K., S. L. Klemper, R. Gloaguen, and E. W. Grp (2004), Three- S. Simiyu (2005), Crustal structure in Ethiopia and Kenya from receiver dimensional seismic imaging of a protoridge axis in the Main Ethiopian function analysis: Implications for rift development in eastern Africa, rift, Geology, 32(11), 949–952. J. Geophys. Res., 110, B01303, doi:10.1029/2004JB003065. Kieffer, B., et al. (2004), Flood and shield basalts from Ethiopia: Magmas Ebinger, C. J. (1989a), Geometric and kinematic development of border from the African superswell, J. Petrol., 45(4), 793–834. faults and accommodation zones, Kivu-Rusizi rift, Africa, Tectonics, Korme, T., J. Chorowicz, B. Collet, and F. F. Bonavia (1997), Volcanic 8(1), 117–133. vents rooted on extension fractures and their geodynamic implications in Ebinger, C. J. (1989b), Tectonic development of the western branch of the the Ethiopian Rift, J. Volcanol. Geotherm. Res., 79(3–4), 205–222. East-African rift system, Geol. Soc. Am. Bull., 101(7), 885–903. Korme, T., V. Acocella, and B. Abebe (2004), The role of pre-existing Ebinger, C. J., and M. Casey (2001), Continental breakup in magmatic structures in the origin, propagation and architecture of faults in the main provinces: An Ethiopian example, Geology, 29(6), 527–530. Ethiopian rift, Gondwana Res., 7(2), 467–479. Ebinger, C. J., T. D. Bechtel, D. W. Forsyth, and C. O. Bowin (1989), Kusznir, N. and R. Park (1987), The extensional strength of the continental Effective elastic plate thickness beneath the East-African and Afar lithosphere: Its dependence on geothermal gradient, and crustal composi- plateaus and dynamic compensation of the uplifts, J. Geophys. Res., tion and thickness, in Continental Extensional Tectonics, edited by P. L. 94(B3), 2883–2901. Hancock, Geol, Soc. Spec. Publ., 28, 35–52. Ebinger, C., Y. P. Djomani, E. Mbede, A. Foster, and J. B. Dawson (1997), Le Bas, M. J., R. W. Le Maitre, A. Streckeisen, and B. Zanettin (1986), A Rifting Archaean lithosphere: The Eyasi-Manyara-Natron rifts, East chemical classification of volcanic rocks based on total Alkali-Silica Africa, J. Geol. Soc., 154, 947–960. diagram, J. Petrol., 27(3), 745–750. Farnetani, C. G., M. A. Richards, and M. S. Ghiorso (1996), Petrological Lister, G., M. Etheridge, and P. Symonds (1986), Detachment faulting and models of magma evolution and deep crustal structure beneath hotspots the evolution of passive continental margins, Geology, 14, 246–250. and flood basalt provinces, Earth Planet. Sci. Lett., 143(1–4), 81–94. Loucks, R. R. (1996), A precise olivine-augite Mg-Fe-exchange geotherm- Furman, T., J. G. Bryce, J. Karson, and A. Iotti (2004), East African Rift ometer, Contrib. Mineral. Petrol., 125(2–3), 140–150. System (EARS) plume structure: Insights from Quaternary mafic lavas of MacKenzie, G., H. Thybo, and P. Maguire (2005), Crustal velocity structure Turkana, Kenya, J. Petrol., 45(5), 1069–1088. across the Main Ethiopian Rift: Results from 2-dimensional wide-angle Furman, T., J. G. Bryce, T. Rooney, B. B. Hanan, G. Yirgu, and D. Ayalew seismic modelling, Geophys. J. Int., 162, 994–1006. (2006), Heads and tails: 30 million years of the Afar plume, in The Afar Maguire, P., et al. (2003), Geophysics project in Ethiopia studies continental breakup, Eos Trans. AGU, 84(35), 342–343.

20 of 21 B10201 ROONEY ET AL.: LITHOSPHERE IN THE ETHIOPIAN RIFT B10201

Maguire, P., et al. (2006), Crustal structure of the Northern Main Ethiopian Rooney, T., T. Furman, G. Yirgu, and D. Ayalew (2005), Structure of the Rift from the EAGLE controlled source survey: A snapshot of incipient Ethiopian lithosphere: Xenolith evidence in the Main Ethiopian Rift, lithospheric break-up, in The Afar Volcanic Province Within the East Geochim. Cosmochim. Acta, 69(15), 3889–3910. African Rift System, edited by G. Yirgu, C. Ebinger, and P. Maguire, Sengor, A. M. C., and K. Burke (1978), Relative timing of rifting and Geol. Soc. Spec. Publ., 259, 269–292. volcanism on Earth and its tectonic implications, Geophys. Res. Lett., Mahatsente, R., G. Jentzsch, and T. Jahr (1999), Crustal structure of 5(6), 419–421. the Main Ethiopian Rift from gravity data: 3-dimensional modeling, Sifeta, K., B. P. Roser, and J. I. Kimura (2005), Geochemistry, provenance, Tectonophysics, 313(4), 363–382. and tectonic setting of Neoproterozoic metavolcanic and metasedimen- Mazzarini, F., T. Abebe, F. Innocenti, P. Manetti, and M. T. Pareschi (1999), tary units, Werri area, Northern Ethiopia, J. Afr. Earth Sci., 41(3), 212– Geology of the Debre Zeyt area (Ethiopia) (with a geologic map at scale 234. 1:100,000), Acta Vulcanol., 11(1), 131–141. Stuart, G., I. D. Bastow, and C. Ebinger (2006), Crustal structure of McKenzie, D. P. (1978), Some remarks on the development of sedimentary the northern Ethiopian Rift from receiver function studies, in The Afar basins, Earth Planet. Sci. Lett., 40, 25–32. Volcanic Province Within the East African Rift System, edited by G. Yirgu, Menzies, M. A., S. L. Klemperer, C. J. Ebinger, and J. Baker (2002), C. J. Ebinger, and P. K. H. Maguire, Geol. Soc. Spec. Publ., 259, 253– Characteristics of volcanic rifted margins, in Volcanic Rifted Margins, 267. edited by M. A. Menzies et al., Spec. Pap. Geol. Soc. Am., 362, 1–14. Tadesse, G., and A. Allen (2005), Geology and geochemistry of the Neo- Mohr, P. A. (1962), The Ethiopian rift system, Bull. Geophys. Observ., 3(1), proterozoic Tuludimtu Ophiolite suite, western Ethiopia, J. Afr. Earth 33–62. Sci., 41(3), 192–211. Mohr, P. A. (1967), Major volcano-tectonic lineament in the Ethiopian rift Trua, T., C. Deniel, and R. Mazzuoli (1999), Crustal control in the genesis system, Nature, 213(5077), 664–665. of Plio-Quaternary bimodal magmatism of the Main Ethiopian Rift Mohr, P. (1983), Ethiopian flood basalt province, Nature, 303(5918), 577– (MER): Geochemical and isotopic (Sr, Nd, Pb) evidence, Chem. Geol., 584. 155(3–4), 201–231. Mohr, P. (1987), Structural style of continental rifting in Ethiopia: Reverse Turcotte, D. L., and S. H. Emerman (1983), Mechanisms of active and decollements, Eos Trans. AGU, 68, 721, 729–730. passive rifting, Tectonophysics, 94(1–4), 39–50. Mohr, P. and B. Zanettin (1988), The Ethiopian flood basalt province, in Wernicke, B. P. (1985), Uniform-sense normal simple shear of the conti- Continental Flood Basalts, edited by J. D. MacDougall, pp. 63–110, nental lithosphere, Can. J. Earth Sci., 22(1), 108–125. Kluwer Acad., Dordrecht, Netherlands. Whaler, K. and S. Hautot (2006), Magnetotelluric studies of the northern Morgan, J. P. (1971), Convection plumes in the lower mantle, Nature, 230, Ethiopian rift, in The Afar Volcanic Province Within the East African Rift 42–43. System, edited by G. Yirgu, C. Ebinger, and P. Maguire, Geol. Soc. Spec. Morley, C. K. (1994), Interaction of deep and shallow processes in the Publ., 259, 293–306. evolution of the Kenya rift, Tectonophysics, 236(1–4), 81–91. WoldeGabriel, G., J. L. Aronson, and R. C. Walter (1990), Geology, geo- Nimis, P., and P. Ulmer (1998), Clinopyroxene geobarometry of magmatic chronology, and rift basin development in the central sector of the Main rocks Part 1: An expanded structural geobarometer for anhydrous and Ethiopia Rift, Geol. Soc. Am. Bull., 102(4), 439–458. hydrous, basic and ultrabasic systems, Contrib. Mineral. Petrol., 133(1– Wolfenden, E., C. Ebinger, G. Yirgu, A. Deino, and D. Ayalew (2004), 2), 122–135. Evolution of the northern Main Ethiopian rift: Birth of a triple junction, Nyblade, A. A., and R. A. Brazier (2002), Precambrian lithospheric con- Earth Planet. Sci. Lett., 224(1–2), 213–228. trols on the development of the East African rift system, Geology, 30(8), Wolfenden, E., C. Ebinger, G. Yirgu, P. R. Renne, and S. P. Kelley (2005), 755–758. Evolution of a volcanic rifted margin: Southern Red Sea, Ethiopia, Geol. Pan, M., L. E. Sjoeberg, L. M. Asfaw, E. Asenjo, A. Alemu, and Soc. Am. Bull., 117(7–8), 846–864. A. Hunegnaw (2002), An analysis of the Ethiopian Rift Valley GPS Wright, T. J., C. Ebinger, J. Biggs, A. Ayele, G. Yirgu, D. Keir, and A. Stork campaigns in 1994 and 1999, J. Geodyn., 33, 333–343. (2006), Magma-maintained rift segmentation at continental rupture in the Peccerillo, A., M. R. Barberio, G. Yirgu, D. Ayalew, M. Barbieri, and T. W. 2005 Afar dyking episode, Nature, 442, 291–294. Wu (2003), Relationships between mafic and peralkaline silicic magma- Yihunie, T., M. Adachi, and K. Yamamoto (2006), Geochemistry of the tism in continental rift settings: A petrological, geochemical and isotopic Neoproterozoic metabasic rocks from the Negele area, southern Ethiopia: study of the Gedemsa volcano, central Ethiopian rift, J. Petrol., 44(11), Tectonomagmatic implications, J. Afr. Earth Sci., 44(3), 255–269. 2003–2032. Zeyen, H., F. Volker, V. Wehrle, K. Fuchs, S. V. Sobolev, and R. Altherr Petit, C., and C. Ebinger (2000), Flexure and mechanical behavior of cra- (1997), Styles of continental rifting: Crust-mantle detachment and mantle tonic lithosphere: Gravity models of the East African and Baikal rifts, plumes, Tectonophysics, 278(1–4), 329–352. J. Geophys. Res., 105(B8), 19,151–19,162. Ziegler, P. A., and S. Cloetingh (2004), Dynamic processes controlling Pik, R., C. Deniel, C. Coulon, G. Yirgu, C. Hofmann, and D. Ayalew evolution of rifted basins, Earth Sci. Rev., 64(1–2), 1–50. (1998), The northwestern Ethiopian Plateau flood basalts: Classification and spatial distribution of magma types, J. Volcanol. Geotherm. Res., 81(1–2), 91–111. Pik, R., C. Deniel, C. Coulon, G. Yirgu, and B. Marty (1999), Isotopic and ÀÀÀD.ÀÀÀÀAÀÀyalewÀÀÀÀÀÀ,ÀÀÀDessie/KombolchaÀÀÀÀ University, P.O. Box 1145, Dessie, trace element signatures of Ethiopian flood basalts: Evidence for plume- Ethiopia. lithosphere interactions, Geochim. Cosmochim. Acta, 63(15), 2263–2279. I. Bastow, Department of Earth Sciences, University of Bristol, Bristol Pizzi, A., M. Coltorti, B. Abebe, L. Disperati, G. Sacchi, and R. Salvini BS8 1RJ, UK. (2006), The Wonjii Fault Belt (Main Ethiopian Rift): structural and geo- morphological constraints and GPS monitoring, in The Afar Volcanic T. Furman, Department of Geosciences, Pennsylvania State University, Province Within the East African Rift System, edited by G. Yirgu, C. J. University Park, PA 16802, USA. Ebinger, and P. K. H. Maguire, Geol. Soc. Spec. Publ., 259, 191–207. T. Rooney, Department of Geological Sciences, Michigan State Rooney, T. (2006), Continental rifting in central Ethiopia: Geochemical and University, East Lansing, MI 48824, USA. ([email protected]) isotopic constraints from lavas and xenoliths, Ph.D. dissertation thesis, G. Yirgu, Department of Earth Sciences, Addis Ababa University, P.O. Penn. State Univ., Univ. Park. Box 1176, Addis Ababa, Ethiopia.

21 of 21