Abstract
The East African Rift (EAR) is a unique continental open-air laboratory to study the rift evolution, from the beginning of the lithospheric extension in the South, to the oceanisation in the North. The comprehension of rifting processes and evolution are essential to better understand the Earth geodynamic (plate tectonics, mantle plume role, crust-mantle interactions…).In this study, we focus on the North Tanzanian Divergence, which is a rift initiation zone situated at the southern tip of the Eastern Branch of the EAR. There, the rift surface expression results from the interactions between deep-mantle (mantle plume), lithospheric (inherited rheology and stratification, melting...) and crustal (dyke propagation, fault activation...) processes. However, the role of each process on the observed surface activity is still debated, as their respective signals are interlinked. In order to consider the various factors that may interact in this complex zone, a multi-disciplinary study was carried out, combining seismological, petrological and petrophysical approaches.The development of a new hybrid tomographic method for both P and S-body waves permits to image the rift zone with a better resolution (particularly at the Moho depth), the structures boundaries, as well as the mantle plume geometry. My tomographic study points out that the mantle structure limits are consistent with the surface geology (rifting basin, border faults, volcanoes). At a regional scale, the strongest velocity contrasts correspond to the lithospheric inherited structure boundaries (Tanzanian craton and Proterozoic belts), which control the propagation of the rift. The Masai block, south of the NTD, is inferred to have a strong influence in the rift evolution, especially on the volcanism distribution and on the change of the rift morphology. To discriminate which parameters are acting in the rift, the P and S results are combined in a Vp/Vs ratio model. Those images enable us to determine and locate possible zones with melt, fluid or gas presence.Additionally, the geochemical analyses of mantle xenoliths from in-rift (Pello Hills) and on-craton edge (Labait) volcanoes permit to characterize the lithospheric mantle, as well as the fluids percolating and metasomatizing the mantle. The xenoliths contain abundant hydrous minerals (amphibole and phlogopite) as isolated crystals or veins that attest to an important modal metasomatism beneath the NTD. These fluids are alkaline-rich and may be plume or subduction-related. The Pello Hills samples tend to follow an adiabat instead of a geotherm, suggesting that the volcano is directly situated above the mantle plume borders. The Labait xenoliths follow an intermediate geotherm between a cratonic and a plume-modified one, indicating that the mantle below the Labait is moderately affected by the plume.The petrophysical study on the mantle xenoliths allow us to characterise the mantle seismic properties. We particularly focus on the effect of crystal aggregates, the presence or absence of veins, and the compositions and orientation of the veins. Our results point out that the mantle below the NTD is anisotropic, and particularly in highly metasomatized zones.Despite the change of spatial and temporal scale between petrological (centimetric samples from past eruptions) and the geophysical (pluri-tenth of kilometres anomalies from present day) studies, the combination of these approaches provide relevant information about the in-rift mantle metasomatism, the influence of the mantle plume on the cratonic lithosphere and its possible erosion, and offer a confirmation of the location of supercritical fluid and melt zones.