INTERPRETATION OF MARINE SEISMIC DATA OF THE EASTERN GRABEN OF LAKE KIVU, RWANDA

The Lake Kivu Rift is a single-segment rift basin at the highest point on the western branch of Africa’s Great Rift Valley. The lake is one of only two locations along this branch with extensive magmatic activity, in this case from the Virunga volcanic system at the lake’s north end. This unique, ~ 485 m deep lake with its vigorous hydrothermal system provides much of the water and most of the solutes to the much larger, Lake Tanganyika. Marine seismic reflection data were acquired from 2010 through 2012 in the eastern basin of the lake. These data reveal a half-graben, extensional basin bounded by a set of border faults with a dip angle of 40 to 52 degrees toward the east. Some of these faults have relief on the lake floor and are currently active. A sediment section reaching a thickness of over 1 km on the western side of this basin shows evidence of erosional truncation revealing at least three prolonged periods of desiccation which occurred at vastly different times. These may correspond to dry climatic periods recorded in sediment archives of other East African lakes. The sediment on top of all but the most recent of these erosional horizons show transgressive onlap suggesting a slow recovery. Beach deposits in the deeper sediment show that the lake was ~ 300 meters below the current level prior to the Holocene; then, after the most recent period of desiccation, newer beach deposits, carbonate ooids and wave cut features show that the lake recovered to a stable lake level significantly lower than the prior lake, at ~ 370 meters below the modern lake level. Sediments deposited above this depth do not show evidence of onlap within the survey area suggesting that the lake rose to a level above its current state relatively quickly. Current work is being done to correlate these marine seismic data to core data in order to constrain the timing of these events and to estimate the rate of extension in this basin.