Catching the Main Ethiopian Rift Evolving Towards Plate Divergence Eugenio Nicotra ( [email protected] ) University of Calabria Marco Viccaro Università di Catania Paola Donato University of Calabria Valerio Acocella Università di Roma Tre Rosanna Rosa University of Calabria Research Article Keywords: continental rifts, magmatism, volcanism, dikes Posted Date: July 13th, 2021 DOI: https://doi.org/10.21203/rs.3.rs-691934/v1 License: This work is licensed under a Creative Commons Attribution 4.0 International License. Read Full License 1 Catching the Main Ethiopian Rift evolving towards plate divergence 2 3 Eugenio Nicotra1*, Marco Viccaro2,3, Paola Donato1, Valerio Acocella V. 4, Rosanna De Rosa1 4 5 1. Università della Calabria, Dipartimento di Biologia Ecologia e Scienze della Terra, Arcavacata di 6 Rende, Cosenza, Italy 7 2. Università di Catania, Dipartimento di Scienze Biologiche Geologiche e Ambientali, Catania, Italy 8 3. Istituto Nazionale di Geofisica e Vulcanologia – Sezione di Catania, Osservatorio Etneo, Catania, Italy 9 4. Università di Roma Tre, Dipartimento di Scienze, Roma, Italy 10 11 Abstract 12 Magmatism accompanies rifting along divergent plate boundaries, although its role before 13 continental breakup remains poorly understood. For example, the magma-assisted Northern 14 Main Ethiopian Rift (NMER) lacks current volcanism and clear tectono-magmatic 15 relationships with its contiguous rift portions. Here we define its magmatic behaviour, 16 identifying the most recent eruptive fissures (EF) whose aphyric basalts have a higher Ti 17 content than those of older monogenetic scoria cones (MSC), which are porphyritic and 18 plagioclase-dominated. Despite the similar parental melt, EF and MSC magmas underwent 19 different evolutionary processes. While MSC magmas were stored at intermediate crustal 20 levels, EF magmas rose directly from the Moho without contamination, even below older 21 polygenetic volcanoes, suggesting rapid propagation of transcrustal dikes across solidified 22 magma chambers. Whether this recent condition in the NMER is stable or transient, it 23 highlights a transition from central polygenetic to linear fissure volcanism, indicative of 24 increased tensile conditions and volcanism directly fed from the Moho, suggesting transition 25 towards mature rifting. 26 1 27 Key words: continental rifts, magmatism, volcanism, dikes 28 29 INTRODUCTION 30 31 The immature stages of rifting before continental breakup can be best investigated at the 32 Main Ethiopian Rift (MER)-Afar system, characterized by the northward increase in the 33 extension and magmatic activity, from incipient continental rifting in the Southern MER, to 34 continental breakup in Afar [1, 2]. The Northern MER (NMER) between the Southern MER 35 and Afar, extending between Dofen and Gedemsa volcanoes, apparently shows 36 controversial features (Fig. 1). Here, a melt-induced anisotropy in the upper mantle supports 37 magma-assisted rifting [3-5], culminating in polygenetic volcanoes with calderas and 38 ignimbrites, and hundreds of monogenetic volcanoes. These features are at odds with the 39 negligible recent volcanic activity of the NMER, where the last eruption occurred in the 19th 40 century, and the central volcanoes in the last decades have not shown unrest [6-9]. It is 41 unclear how these features reconcile with the magmatic activity in the Southern MER and 42 in the southern and central portions of Afar, which show evidence of central and linear 43 magmatic activity, respectively. In the Southern MER, volcanic activity shows predominant 44 central volcanoes; often experiencing unrest, as at Corbetti, Aluto and Bora (Fig. 1) [7, 10, 45 11]. In Southern Afar the predominant linear magmatic activity is underlined by dike-induced 46 recent unrest at Haledeibi, Ayelu-Amoissa and Manda Hararo (Fig. 1) [2, 12-14]. The poorly 47 understood and apparently controversial magmatic features of the NMER, also with regard 48 to the contiguous rift portions, hinder properly defining the role of magma on continental 49 rifting and, in turn, the maturity of this portion of the rift, which remain both elusive. Here we 50 contribute to solve this conundrum reconstructing the recent magmatic activity of the NMER 51 considering its widespread monogenetic volcanism (Fig. 1). 2 52 53 TECTONO-MAGMATIC BACKGROUND 54 55 Mio-Pliocene continental rifting activated the border faults of the MER and was 56 accompanied by diffuse magmatic activity. During Quaternary, tectonic and volcanic activity 57 focused in magmatic systems within the rift, partially deactivating the border faults (Fig. 1) 58 [1]. Magmatic systems are focused zones of extension and volcanic activity, parallel to the 59 rift axis, accommodating >80% of the extensional strain at depth <10 km, with each system 60 associated with a polygenetic felsic volcano and several mafic monogenetic vents [15-17]. 61 Magmatic systems induce rift opening through dike intrusions feeding the monogenetic 62 vents [16, 18-20]. Volcanic activity along the MER is mirrored by a similar segmentation of 63 high-velocity bodies at 10-15 km of depth, interpreted as cooled mafic intrusions [5, 21-22]. 64 Fantale, Kone, Boseti and Gedemsa, each spaced of a few tens of km, are the main 65 polygenetic central volcanoes of the NMER (Fig. 1), formed by successions of trachytic to 66 rhyolitic lava flows and ignimbrite deposits and associated with one or more caldera collapse 67 episodes. Not being the main focus of this work, we refer to literature for central volcanoes 68 characterization [e.g., 18, 23-31]. 69 Mafic monogenetic volcanoes focus along the strike of the magmatic systems, reaching 70 the highest density in the NMER. Here, at least 283 (cf. Methods; Figs. 1-2) monogenetic 71 volcanoes can be recognized. These mainly consist of basaltic scoria cones, at times 72 associated with minor lava flows and commonly generating short (<4 km long) alignments 73 parallel to the strike of the magmatic system. 74 75 RESULTS 76 Geology 3 77 We surveyed and sampled 41 of the 283 recognized monogenetic scoria and spatter 78 cones (MSC) outcropping in the NMER, between Fantale and Gedemsa central volcanoes 79 [Figs. 1 and 2; Electronic Supplementary Material (ESM) 1]. The term “monogenetic” refers 80 here to all the volcanic edifices (e.g., small-volume spatter and scoria cones, maars and 81 small lava domes) produced by a single eruption. The MSC cluster into 5 major monogenetic 82 fields (dotted ellipses in Fig. 1; cf. Chapter 3.1) along NNE-SSW elongated areas between 83 the major central volcanoes (Fig. 1). Approximately 216 out of the 283 cones can be grouped 84 into 39 alignments, whereas 67 cones remain apparently isolated (Fig. 1). In particular: a) 85 222 MSC) have been recognized between Fantale and Boseti (ca. 5 cones per 10 km2; 86 Abebe et al., 2015), of which 178 grouped into 30 eruptive episodes; b) 61 MSC have been 87 recognized between Boseti and Giano (Fig. 1), of which 38 grouped into 9 eruptive episodes. 88 These alignments are parallel to the rift direction and their length ranges between 0.3 and 89 3.6 km (average of ca. 1.67 km). Although the age of the MSC is poorly constrained, most 90 of them are generally coeval to the Late Quaternary activity of the central volcanoes, with 91 the northernmost cones being sutured by the ~168 ka old Fantale ignimbrite [27] and other 92 monogenetic cones to the south being sutured by Holocene epiclastic deposits. 93 In the NMER, we have also distinguished three eruptive fissures (EF; Fig. 2), which differ 94 from the MSC for several features: a) the EF form the longest alignments of cones, 6 to 14 95 km long, belonging to the same eruptive event, recognized through remote sensing data 96 and in the field; b) the EF are neither eroded nor partially buried and, in the field, their 97 products are generally massive and mostly aphyric, without evidence of alteration or 98 weathering; c) the EF are accompanied by a relatively larger (1-2 orders of magnitude more 99 voluminous) amount of lava with respect to the MSC. 100 The northernmost EF (herein Metehara EF) is formed by two parallel and én-échelon 101 segments (11 and 7 km long), extending from the middle (ca. up to 500 m above the rift 4 102 floor) southern slope of Fantale volcano to the scoria cones inside Lake Besaka (Fig. 2a). 103 At its northernmost tip, the EF does not affect the rim of Fantale caldera. Along the Metehara 104 EF a chain of scoria cones and a small-volume (~53×106 m3; Wadge et al., 2016) lava field 105 formed (Fig. 2a). These products have been previously associated with the two post-caldera 106 lava flows within the caldera [e.g., 32]. Nonetheless, our survey reveals that intra-caldera 107 lava flows are obsidian coulee, trachytic-rhyolitic in composition and without macroscopic 108 evidence of magma mingling/mixing (Fa1 and Fa2 samples in ESM 1-2; cf. also [27]), as 109 also recently confirmed by [31]. Historical reports and local oral traditions date this eruptive 110 event at 1810 CE [8, 33-34]. 111 The eruptive fissure cutting the Kone volcanic complex (Kone EF) consists of 5 main én- 112 échelon segments, for a total length of 25 km (Figs. 1-2). The main fissure (EFKone-1 in 113 Fig. 2b) is ~7 km long and passes through the rims of the two main Kone calderas (Fig. 2b). 114 Our survey has highlighted that the most recent lava flows filling the Kone-Korke calderas 115 were fed by three scoria cones developed along the EFKone 1 fissure system (Fig. 2b), 116 which continues to the south forming two small scoria cones on the southern lower flank of 117 Kone. The southernmost termination of the Kone EF lies south of the calderas, in the Kokoro 118 area (Fig.