Résumé
When breakup occurs within a cratonic lithosphere, the interactions between inherited structures, active tectonics, present mantle rheology and magmatic intrusions are different factors evocated to explain the surface deformation. However, their different contributions remain poorly constrained. The starting point to enlighten our understanding of such complex geodynamic processes is to image the different interfaces in terms of geometry and nature. We present here a collaborative work in an incipient lithospheric breakup to constrain both Vp/Vs ratio and velocity profiles from receiver functions. About 35 seismological stations were installed for 2 years in the Natron lake region, and 10 short period instruments were added for 9 months in the Manyara area to record local and teleseismic events. We have analysed more than a hundred teleseismic events to compute the receiver function, and we finally obtain a Moho map of the region as well as azimuthal distribution of converted phases. The stations located on the edge of the rift and near the craton present a clear mid lithospheric interface at about 70 km depth. Below this interface, the mantle presents a velocity 10% slower than above, and is characterized by a clear conversion phase at 7s in the receiver functions. We first model those data to perfectly fit the signal, and more precisely the transverse component, which shows a strong and coherent pattern. We discuss this interface in terms of craton-rift interaction, mantle nature and partial melting.