Structure, paleolimnology and basin history of the East Kivu Graben, Lake Kivu, Rwanda from offshore seismic reflection data

Lake Kivu, located at the topographic high point of the western branch of Africa's Great Rift Valley, is regularly subjected to large-magnitude seismic events, violent volcanic discharges, and possible limnic eruptions which pose substantial geohazards to the ~two million Congolese and Rwandan people living around its basin. Although most of the western branch of Africa's Great Rift Valley is amagmatic, Lake Kivu is bordered to the north and south by two volcanic provinces. Subaqueous springs, charged by the active Virunga Magmatic Complex at the north end of the lake, currently inject gas into the deep- hypolimnion, where it remains in solution at high concentrations. Rapid expulsion of these gases into the atmosphere, possibly induced by seismic, meteorologic, or volcanic events, would be devastating to the local population. To assess the potential danger, it is critical to understand the basin's structure, and the effects of magmatism and climate changes on the lake. Information about past basin structural, and lake surface elevation changes are preserved in the sediments under the lake; these provide information about the region's tectonic and climatic past. Between 2010 and 2013, Syracuse University's Department of Earth Sciences acquired ~1100 km of marine seismic reflection data and six Kullenberg sediment cores from the eastern basin of the lake. Interpretation of these seismic data show that the eastern basin of Lake Kivu is a half-graben, bounded by east-dipping normal faults. Most subsidence is accommodated at the west side of the basin by slip along two north striking, offset growth faults which are located east of Idjwi and Iwawa Islands. Strain between these faults is accommodated by oblique strike-slip along a northeast striking, transfer fault. Oblique slip along other northeast-striking faults may accommodate extension between more recent, north striking fault segments. This could account for the irregular shape of Lake Kivu's basin, islands, and shoreline. The >1.5 km sedimentary section observed in the marine seismic reflection data records extreme climate changes through the late Pleistocene and Holocene. Truncated surfaces indicate that the lake has experienced at least three periods of desiccation. Two of these events correlate to prolonged times of aridity interpreted within the sedimentary records of other African Great Lakes. Between these desiccation periods, the maximum lake surface has varied over a range of >400 m, controlled at different times by climate or the basin's structural configuration. At ~12 ka, there was a transgression from a lowstand of ~370 m below the current lake level, to a level above the current water position. This transgression is reported as induced by a sudden, rapid expansion of the Virunga Volcanic Complex which blocked the northern outlet of the lake, forcing it to a higher, southern spill point through Bukavu Bay; however data presented herein show that it was likely caused by a change in the regional climate. Sediment core data indicate that the transgression occurred over a time span of ~100 years which was likely too short of a period for such a large magmatic expansion. Seismic data show that for most of the lake's past, prior to this -370 m lowstand, the lake level was at ~310 m below the modern lake level, indicating that the lowstand underfilled the lake basin. In this circumstance, the basinal spill point did not control the lake level. Other East African Lakes also transgressed from low stands at approximately the same time, indicating that there was a regional change to wetter climate conditions. Inflation of the Virunga Volcanic Complex, which likely occurred gradually before, or during the lowstand, raised the topographic threshold to the north forcing the lake to spill out of the southern, Bukavu Basin when the transgression occurred.