Evolution of faulting and magmatism during volcanic rifting in the Ethiopian Rift: A case study of the Boset-Bericha Volcanic Complex

Abstract

A multidisciplinary approach was applied to unravel temporal and spatial constraints of volcanism, inferred magmatic processes and sources, as well as the relative role of tectonic deformation over time on an axial segment of a magma-assisted continental rift. The Boset-Bericha Volcanic Complex (BBVC), located in the northern Main Ethiopian Rift is the perfect location to study the influence of magmatism on rifting due to evidence for subsurface melt and the faulted stratovolcano on the surface.

Here, detailed remote sensing data (including LiDAR), field observations and volcanic rocks analysis were used to investigate the 1) lava flow chronology, 2) geochemical magma evolution and 3) fault evolution at the BBVC. Lava flows, cones, craters, fissures and faults were mapped and combined with 40Ar/39Ar dating, to constrain the relative and absolute chronology of lava flow emplacement at the BBVC. In addition, detailed geochemistry, including major and trace elements, Pb-Sr-Nd-B isotopes analysis, and fault displacement analysis, were performed.

A major finding is, that the lava flow chronology at the BBVC reveals four main eruption stages: (1) early rift floor emplacement; (2) formation of southern Gudda Volcano, which occurred within two main cycles at ~120 ka and since ~16 ka; (3) the northern Bericha Volcano, formed since ~16 ka, and (4) sporadic fissure eruptions. Lava flow geochemistry indicates trachytic-pantelleritic composition for Gudda, a rhyolitic-comenditic composition for Bericha and mafic to intermediate composition of fissure lava flows. Pb isotopes suggest a magma source with isotopic signature of the Afar plume and lower crustal assimilation. Comendites experienced around twice as much crustal assimilation as pantellerites. Further, less amphibole crystallisation but additional feldspar accumulation is found for pantellerite compared to comendite. The contrasting compositions in different edifices suggest different stages in the evolution of peralkaline silicic volcanoes with a more mature plumbing system for pantelleritic composition.

For the first time, fault displacements were analysed through dated lava flows to determine fault architecture and constrain their slip-rates. Fault surface expressions vary with time from open mode fissure/fracture, to small normal faults with openings and to large normal faults. Estimated minimum slip-rates range from 0.01-0.37 mm/yr in rift floor deposits to 0.3-4.4 mm/yr in faults cutting through young lava at Gudda.

These geochemical – structural analyses combined with chronological constraints at the BBVC reveal episodic magmatic (~120 Ka; since ~16 Ka) and tectonic activity with recent volcanism offset by fractures. Differences in magma sources and evolution (including the degree of crustal assimilation), as well as magma plumbing systems forming comenditic and pantelleritic lavas at the BBVC, can be applied to other MER volcanoes. The slip-rate estimates potentially account for a significant percentage of the total extension, indicating that faulting in the uppermost crust may accommodate shallow extension, even in magma-rich rifts.

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ORCID for Thomas Gernon: orcid.org/0000-0002-7717-2092

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