Reservoir connectivity in a continental rift: Insights from geodetic observations during the 2024-2025 dike intrusions at Fentale, Main Ethiopian Rift

Direct observations of dike intrusions during continental magmatic rifting are rare. Therefore, magma plumbing systems and associated hazards in continental rifts are not well understood. The 2024-2025 rifting event in the Fentale-Dofen magmatic segment of the Main Ethiopian Rift involved the prolonged intrusion of a ~50 km long dike into ~35 km thick continental crust lasting over 3 months, accompanied by deflation of a ~6 km deep magma reservoir beneath Fentale. Satellite-based Interferometric Synthetic Aperture Radar (InSAR) observations at regular intervals throughout the intrusion allow us to monitor the co-evolution of the magma source and the intrusion using surface deformation data, in the absence of ground-based instrumentation.

Modelled dike volumes (>1 km³) are 4-9 times larger than the volume loss of the deflating magma reservoir beneath Fentale. At other systems, this volume mismatch has been attributed to host rock rigidity, reservoir geometry, and magma compressibility. While the total dike to source volume ratio is typically reported, this ratio can vary during the diking event due to changes in gas content and compressibility, or involvement of multiple sources. Temporally-dense displacement measurements of the intrusion at Fentale present an opportunity to investigate the evolution of the dike to source volume ratio during a continental rifting event, providing a novel constraint on the conditions for magmatic storage and transport.

We propose that tracking the geodetic volume balance between the dike intrusion sink and reservoir source over time could be used as a tool to reveal changes to the magmatic system, in the absence of other observations (i.e., seismological or petrological). We present a timeseries of intrusion to source volume ratio, derived from analytic kinematic models of surface displacements. We use the relative volumes as a proxy to infer whether and how the mechanical properties of the magma, or the magma source(s) being tapped by the dike changed over time. We show that the volume balance timeseries suggests a change in the magmatic system during the intrusion, possibly related to deeper changes in the plumbing system that caused emissions of methane and carbon dioxide in January 2025 and a ~19 km deep non-double-couple earthquake in February 2025.

Pre-diking inflation and post-diking ground uplift around Fentale points towards magmatic recharge and re-pressurisation of a reservoir that is distinct from the co-diking shallower deflating source. The interpretation of a single magma source feeding a lateral dike intrusion may be insufficient to explain the geodetic observations of the intrusion, where the spatial and temporal connectivity of magmatic reservoirs is not trivial. Continuous monitoring of deformation will contribute to our understanding of threshold conditions for reservoir failure, with implications for forecasting the spatio-temporal likelihood of future intrusions.