NEW INSIGHTS INTO THE SPATIAL AND TEMPORAL DISTRIBUTION OF MAGMATISM WITHIN MATURE CONTINENTAL RIFTS (Invited Presentation)

Dyke intrusions are thought to accommodate the majority of extensional strain in magmatic rift, but until recently, the focus has been on sea-floor spreading centres in Afar and Iceland or immature settings such as Tanzania and our quantitative understanding of the link between mantle magmatism, silicic reservoirs, and surface deformation during the extension of continental crust is very limited. Here we report on recent geodetic observations from the Main Ethiopian Rift, where extension rates are 5-6 mm/yr and the continental crust is 30-40 km thick. First we focus on Corbetti, a peralkaline caldera, which lies above a focused zone of upper mantle partial melt and has been steadily uplifting at a maximum rate of 6.6±1.2 cm yr⁻¹ for more than 10 yr with a concomitant residual gravity increase of 9±3 μGal yr⁻¹. Numerical modeling shows that the observed deformation and gravity changes are best explained by the intrusion of mafic magma at ∼7 km depth into a compressible and inelastic crystal mush. The derived magma mass flux of ∼10¹¹ kg yr⁻¹ is anomalously high and at least 1 order of magnitude greater than the mean long‐term mass eruption rate. This study demonstrates that periodic and high‐rate magmatic rejuvenation of upper‐crustal mush is a significant component of the magma budget during mature continental rifting. Secondly, we focus on a seismic swarm that occurred in 2015 to the northeast of Fentale volcano. Interferograms show a pattern of dike‐induced deformation, with a volume change of 33×10⁶±0.6×10⁶m³ at a depth range of 5.4 to 8 km - the first direct observation of a dike intrusion in the MER. However, the displacements occur over a much longer time period than typical of basaltic dyke intrusions elsewhere, with an exponential decay constant of ∼83 days. Coupled source‐sink models suggest that such slow dike intrusions require a higher viscosity magma for example a peralkaline rhyolite. Together, these observations demonstrate that magmatism associated with silicic centres and slow dyke intrusions are more significant in continental rifts than previously considered.