LITHOSPHERIC RUPTURING AND MAGMATIC PROCESSES IN THE RWENZORI REGION, EAST AFRICAN RIFT

The western branch of the East African Rift System is characterized by high seismic activity. Several seismological studies have shown that the depth distribution of earthquakes reaches down to the Moho. But there was no evidence for earthquakes beneath the crust so far. In the framework of the RiftLink project a local seismic network was deployed in the Rwenzori Mountains region to investigate the structure of the crust and mantle lithosphere. Seismogram recordings reveal more than 800 local events per month with maximum hypocentral depths reaching from 20 km beneath the rift valley up to 30 km below the eastern rift shoulder. Moho depths were determined by teleseismic receiver functions and prove that the majority of the located events indeed occur within the crustal part of the lithosphere. Additionally, we detected a small group of earthquakes at depths between 53 and 60 km in the mantle. The epicentres are located NW of the Rwenzoris, in an area where the mountains are still connected to the eastern rift shoulder and where the northward propagating Lake George Rift is still in its early stage. We therefore conclude that the deep fracturing is caused by magmatic impregnation and dike intrusions in the mantle lithosphere. A careful relocation of the crustal events reveals several earthquake clusters in the same area. They are located in the middle crust and form pipe-like patterns with vertical extensions of 3 to 6 km. In several cases they exhibit a systematic migration of the earthquake hypocentres. These structures possibly are connected to magmatic feeding channels through the crust that originate from the heated and impregnated lithospheric mantle. Together with the detected deep earthquakes these observations may be indicative of rifting in its initial stage.