Interactions between tectonic and magmatic processes in the East African Rift

Abstract

Continental extension is caused by a combination of mechanisms such as mechanical stretching, thinning of the lithosphere, mantle upwellings and magmatic weakening of the lithosphere. Understanding these mechanisms is key to characterize plate tectonicsand mantle dynamics. The East African Rift is an ideal locale to study continental breakup as it subaerially exposes all stages of the transition from early extension at young rift segments to incipient seafloor spreading. Using several permanent and temporarynetworks deployed over the last 30 years, this thesis provides constraints on rifting processes by imaging lithospheric structures, determining the evolution of strain accommodation, analysing geometry and kinematics of tectonic faulting and identifying themagmatic and tectonic systems beneath active rift valley volcanoes. Using S-to-P receiver functions, we identify significant lithospheric thinning beneath the northern East African Rift, suggesting a magma-assisted rifting mechanism that involves a large meltzone infiltrating the lithosphere at the lithosphere-asthenosphere boundary. The presence of melt in the crust explains the multiple active volcanoes in the rift valleys, such as the currently deforming Corbetti volcano in the southern Main Ethiopian Rift.By precisely locating earthquakes using a new local seismic velocity model and by analysing focal mechanisms, we observe that the inflation of a magma chamber in the subsurface of the Corbetti volcano creates very shallow microseismicity above the reservoir.These 0-5km depth earthquakes highlight a system of discrete strike-slip faults beneath the active vents. Slightly deeper, 10 km, seismic events near the volcano highlight north-oriented pathways for hydrothermal fluids beneath the caldera. This 10 km seismogeniclayer thickens away from the Corbetti volcano and reaches 16 km beneath intra-rift faults that are active in swarms. These intra-rift faults are located beneath the populated town of Hawassa, posing a high seismic hazard in Ethiopia. The seismogenic layeris even thicker beneath amagmatic segments of the East African Rift, such as the young Tanganyika rift, where earthquakes are detected at depths down to 42 km. Using precise re-location techniques, we suggest that these unusually deep earthquakes are mostlylocalised at the steep, normal border faults of the rift basins. This contrasts with the Main Ethiopian Rift and thus provides evidence on the evolution of strain accommodation during continental rifting. Potentially damaging earthquakes and actively deformingvolcanoes show the need for continuous monitoring of the seismic and volcanic hazard in the East African Rift.

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ORCID for Derek Keir: orcid.org/0000-0001-8787-8446

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