Abstract
Magmatism accompanies rifting along divergent plate boundaries, although its role before continental breakup remains poorly understood. For example, the magma-assisted Northern Main Ethiopian Rift (NMER) lacks current volcanism and clear tectono-magmatic relationships with its contiguous rift portions. Here we define its magmatic behaviour, identifying the most recent eruptive fissures (EF) whose aphyric basalts have a higher Ti content than those of older monogenetic scoria cones (MSC), which are porphyritic and plagioclase-dominated. Despite these differences, calculations highlight a similar parental melt for EF and MSC products, suggesting only a different evolutionary history after melt generation. While MSC magmas underwent a further step of storage at intermediate crustal levels, EF magmas rose directly from the base of the crust without contamination, even below older polygenetic volcanoes, suggesting rapid propagation of transcrustal dikes across solidified magma chambers. Whether this recent condition in the NMER is stable or transient, it indicates a transition from central polygenetic to linear fissure volcanism, indicative of increased tensile conditions and volcanism directly fed from the base of the crust, suggesting transition towards mature rifting.
Highlights
Magmatism accompanies rifting along divergent plate boundaries, its role before continental breakup remains poorly understood
The immature stages of rifting before continental breakup can be best investigated at the Main Ethiopian Rift (MER)-Afar system (Fig. 1A), characterized by the northward increase in the extension and magmatic activity, from incipient continental rifting in the Southern MER, to continental breakup in Afar[1,2]
The age of the Monogenetic Scoria Cones (MSC) is poorly constrained, most of them are generally coeval to the Late Quaternary activity of the central volcanoes, with the northernmost cones being covered by the ~ 168 ka Fantale ignimbrite[27] and other monogenetic cones to the south being sutured by Holocene epiclastic deposits
Summary
Magmatism accompanies rifting along divergent plate boundaries, its role before continental breakup remains poorly understood. While MSC magmas underwent a further step of storage at intermediate crustal levels, EF magmas rose directly from the base of the crust without contamination, even below older polygenetic volcanoes, suggesting rapid propagation of transcrustal dikes across solidified magma chambers Whether this recent condition in the NMER is stable or transient, it indicates a transition from central polygenetic to linear fissure volcanism, indicative of increased tensile conditions and volcanism directly fed from the base of the crust, suggesting transition towards mature rifting. A melt-induced anisotropy in the upper mantle supports magma-assisted rifting[3,4,5], culminating in polygenetic volcanoes with calderas and ignimbrites, and hundreds of monogenetic volcanoes These features are at odds with the negligible recent volcanic activity of the NMER, where the last eruption occurred in the nineteenth century, and the central volcanoes in the last decades have not shown evident unrest[6,7,8,9]. Not being the main focus of this work, we refer to literature for their characterization (e.g.,18,23–31)
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