THE ORIGIN OF LARGE, DEEP, AND ANCIENT LAKES: LITHOSPHERIC CONTROLS ON SPECIES RICHNESS AND LONGEVITY OF LARGE LAKES IN THE EAST AFRICAN RIFT

Ancient tectonic lakes such as those of the East African Rift (EAR) develop some of the highest levels of biodiversity of freshwater faunas and are ideal systems for the study of adaptive radiations, with the largest and deepest lakes exhibiting the greatest species richness. The evolution of the vast array of endemic species of the lakes of the rift valley, especially the cichlid fishes, has been forced since the Miocene by both rifting processes and a dynamic climate. New seismic reflection data are presented from Lake Albert at the northern end of the western branch of the EAR. This basin, which undergoes orthogonal extension is compared to northern Lake Malawi, at the southern end of the western branch, which experiences oblique rifting. By comparing two basins experiencing similar strain rates along a common plate boundary, and rifting lithosphere of comparable thickness and rheology, it is shown that extension direction and lithosphere structural heterogeneity pre-determine the regional rift segment geometry, drainage system development, and patterns of sediment in-filling. The contrasting rift framework produces radically differing subsidence patterns and drainage networks. Oblique extension leads to severe sediment underfilling of asymmetric basins, the development of the world's deepest lakes, and to extended longevity of lakes in those rifts. Orthogonal rifting produces isolated, symmetrical structural segments, leading to sediment overfilling of the basins, shallow lakes, and the likelihood of repeated desiccation events. These results indicate that faunas in lakes formed due to orthogonal rifting are prone to repeated and catastrophic, climatically-induced extinctions, whereas the deep basins driven by oblique extension maintain habitat, and diminished but persistent refuges for enhancing and diversifying species flocks.