Off-rift volcanism during continental rifting: Observations and models with a focus on the Main Ethiopian Rift, East Africa

The occurrence of volcanic activity outside a deforming continental rift valley (off-rift volcanism) is a typical feature of rift systems and different hypothesis have been invoked to explain this setting. In this paper, we first summarize observations from off-rift volcanoes in the Main Ethiopian Rift (East Africa) and then we review models -from early conceptual models to modern numerical and analogue experiments-proposed to explain this process. Overall, all these models suggest that off-rift volcanism may be attributed to two main end-member processes: (1) magma migration and (2) off-rift magma production. Magma migration involves extension-parallel lateral transfer of magma from below the rift depression to the rift margins; this process can explain volcanoes formed in the footwall of major boundary faults, in the plateaus close to the rift depression (the so-called flank volcanoes). Off-rift magma generation suggests instead that magma is produced away from the rift depression and is transferred to the surface via steep tectonic structures above the area of production. In both processes, pre-existing structures, inherited from previous tectonic events, are fundamental for controlling (shallow) magma transfer and emplacement, and the final characteristics of off-rift volcanoes.