Evolution of the Kivu Rift, East Africa: interplay among tectonics, sedimentation and magmatism

The seismically and volcanically active Kivu Rift, in the western branch of the East African Rift System, is a type locale for studies of high-elevation, humid-climate rift basins, as well as magmatic basin development. Interpretations of offshore multi-channel seismic (MCS) reflection data, terrestrial radar topography, lake bathymetry and seismicity data recorded on a temporary array provide new insights into the structure, stratigraphy and evolution of the Kivu rift. The Kivu rift is an asymmetric graben controlled on its west side by a ca. 110 km-long, N-S striking border fault. The southern basins of the lake and the upper Rusizi river basin are an accommodation zone effectively linking 1470 m-high Lake Kivu to 770 m-high Lake Tanganyika. MCS data in the eastern Kivu lake basin reveal a west-dipping half graben with at least 1.5 km of sedimentary section; most of the ca. 2 km of extension in this sub-basin is accommodated by the east-dipping Iwawa normal fault, which bounds an intrabasinal horst. Lake Kivu experienced at least three periods of near desiccation. The two most recent of these approximately correlate to the African Megadrought and Last Glacial Maximum. There was a rapid lake level transgression of at least 400 m in the early Holocene. The line load of the Virunga volcanic chain enhances the fault-controlled basin subsidence; simple elastic plate models suggest that the line load of the Virunga volcanic chain depresses the basin by more than 1 km, reduces flank uplift locally and broadens the depocentre. Not only do the voluminous magmatism and degassing to the lake pose a hazard to the riparian population, but our studies demonstrate that magmatism has important implications for short-term processes such as lake levels, inflow and outlets, as well as long term modification of classic half-graben basin morphology.