Abstract In the rifts of East Africa well and outcrop data record that clastic sedimentation is dominant though some carbonates are present Fluvial and lacustrine environments are most commonly preserved. Seismic reflection character, tied to wells, permits some prediction of lithology. In particular, shale-prone lacustrine sediments are characterized by relatively slow seismic velocities and continuous high amplitude reflections. Fluvio-deltaic sediments tend to have relatively fast velocities and discontinuous, weak reflections. Within a rift, accommodation space may be developed in subbasins at higher elevations than lake level. Tectonic uplifts may occur on one basin margin, while subsidence continues on another, consequently, transgressions and regressions of a lake's margin do not necessarily correlate all the way around a basin. This is particularly the case when comparing flexural and axial margins. Strike sections of half grabens offer important perspectives on the evolution of sediments in rifts. Along-strike linkage of boundary faults leads to rapid growth of the basinal area permitting widespread lacustrine conditions to be established. In some areas (e.g., Lokichar Fault, northern Kenya and the Lupa Fault, Rukwa Rift) the linkage of relatively large fault systems (and their basins) into one continuous boundary fault (and one basin) is revealed by the presence of along-strike or longitudinal anticlines and synclines. These structures are on lapped by syn-rift deposits. Simple tectonic evolution and lake water level fluctuations tend to produce symmetric sedimentary responses on both axial margins of a simple half graben. Asymmetric responses may be the result of changes in the sediment supply to one axis of the lake (leading to a transgression or regression of the lake). Most thick, extensive lacustrine source rocks are formed under anoxic conditions in deep lakes. Lake water depth is commonly controlled by the tectonic subsidence rate along the boundary fault, climate, and sedimentation rate. The differences in apparent source rock quality between Lakes Tanganyika and Rukwa can be explained by the effects of axial sedimentation. Poor quality source rocks in the Rukwa Rift resulted from high (axial) sediment supply into a relatively short rift. By contrast the Lokichar Basin had neighboring basins, which diverted some of the sediment supply to the region. As a result, better developed, fine-grained lacustrine deposits are found in the Lokichar Basin.

Abstract Tectonic inversion has affected the Rukwa, Turkana, and Anza Graben areas to some degree. It is weakest in the Rukwa Rift where anticlines, probably of various origins, provide ambiguous evidence for inversion. In the Turkana area evidence for inversion is strong. It occurs in the youngest rift basins, but seems to have been excluded from the older rift basins. Strengthening and disruption of older faults by igneous intrusions is one possible mechanism for inhibiting inversion on older boundary faults. Inversion in the East African Rift System (EARS) has occurred at least twice, in the Pliocene and Pleistocene, the last phase probably ending in the last 100 thousand years. Inversion features of Paleogene age are very well developed in the southeastern Anza Graben, but decrease in intensity to the northwest The features also show evidence for multiple phases of inversion, that have alternated with extension. In all three areas the inversion episodes developed near the end of rifting. The timing of inversion structures suggests that any driving mechanism must be: of relatively short duration; most effective at the end of rifting; and capable of alternating with periods of extension. Also, the timing of inversion broadly coincides with periods of extensional activity in the Red Sea-Gulf of Aden. However, the periods of plate motion were much longer than were the episodes of inversion. Consequently, the forces associated with plate motion must be insufficient by themselves to cause inversion, otherwise it would be of longer duration. Perhaps stresses sufficient to cause inversion only arise when regional plate forces are added to local rift-generated forces (such as the gravitational potential of the rift flanks, reduction of buoyancy forces under the rift, and variations in basal shear stress at the base of the crust or lithosphere).