Characterising the variations in volcanism across the Afar region

Abstract

Continental rifting and mantle upwellings are key components in our understanding of plate tectonics and the asthenospheric mantle. While volcanism is common as rifts mature from continental rifting to ocean-spreading, our current understanding of how volcanism changes during rift evolution is limited. The Afar triangle, in East Africa, is a rift-rift-rift system that meets in a triple junction and has been proposed to be underlain by a mantle plume. Given that Afar has already undergone extensive rifting, yet remains subaerial, the region presents a unique opportunity to investigate the physical and chemical characteristics of volcanism associated with a proto-oceanic rift. Here, the volcanic evolution of three volcanoes, Alu, Dalafilla and Borale, are characterised through a combination of detailed remote-sensing mapping, petrology, major element, trace element and radiogenic isotope geochemistry. The approximate eruptive frequency and style is constrained as well as highlighting the changes in melt production and storage over the recorded eruptive history of the volcanoes. The results show that each of the volcanoes has a compositional cyclicity (basalt-trachy-andesite/rhyolite), which is likely controlled by the time required for fractional crystallisation to occur in an interconnected stacked-sill system (∼1-4 km depth). The variation in the melting extent and depth among the basalts can be attributed to potential variations in the rifting rate throughout the evolution of the volcanoes. Following this, an integrated approach combining geochemistry (>100 new observations), geophysics (shear wave velocities and Moho depth) and statistical modelling is applied to investigate the melt production beneath the region, assessing the spatial and chemical characteristics of mantle upwelling across the region. The statistical modelling shows the favoured predictive model to explain melt characteristics observed is a single heterogeneous plume, which is asymmetric around the triple junction. This, combined with results from K-means cluster analysis, indicate that different spreading rates across each rift arm may be the cause of the plume’s asymmetry. 

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