Along-rift variations in magma system geometry observed using Sentinel-1 InSAR data from the East African Rift System

Volcano deformation signals detected using InSAR can be used to infer and contrast magmatic system geometry between volcanoes, although their observations present only a brief ‘snapshot’ of the system in geological time. Thus, whilst varying deformation signals may reflect ‘permanent’ differences in magmatic system architecture between systems, they may also simply reflect temporal variability in activity within otherwise similar systems. On geological timescales, magmatic system processes are controlled by tectonics, chiefly crustal properties and mantle melt supply, whereas their shorter-term activity is controlled by the relative strengths of the interacting mafic, tectonic, and silicic parts of the system, varying on timescales of 10s, 100s and 1000s of years, respectively.

Here, using this framework, we combine systematic InSAR-based analytical modelling with additional geochemical and geophysical observations to 16 deforming volcanoes in the Eastern Branch of the East African Rift System (EARS), to assess the prevalence and ‘permanence’ of along-rift variations in magma system geometry. The EARS is characterised by a wide variety of volcanism, rift tectonics, and deformation signals; mature continental rifts with large, central silicic caldera systems, thick crust (25-40 km), and low spreading rates (2-5 mm/yr) further south give way to nascent seafloor spreading ridges further north predominantly mafic volcanism, thinner crust (15-25 km), and higher spreading rates (10-17 mm/yr). The impacts of these variations on volcano deformation signals are important for understanding how architecture and activity of magmatic systems varies along-rift, and for the first time, routinely acquired and processed Sentinel-1 InSAR data presents the opportunity assess them.

To model each signal, we compare up to 9 possible source geometries, assessing model preference using Bayesian Information Criterion (BIC). The modelled deformation sources show a systematic change in melt geometry from dominantly horizontal in the mature continental rifts to vertical in the Erta Ale Volcanic Range (EAVR), indicating structural differences in magmatic system architecture. The extent and magnitude of deformation signals in the Kenyan Rift and Central Main Ethiopian Rift (MER) are also generally larger than in the EAVR during this period. Elsewhere, differences between the Northern and Central MER are attributed to temporal variability. Overall, along-rift differences in deformation patterns are attributed to both temporal variability and permanent differences in magmatic system architecture, with the latter influenced by crustal thickness and melt supply, impacting melt residence times and fractionation.