Abstract This book represents a summary of key results of hydrocarbon exploration activities during the 1980s and 1990s in a number of rift segments in East Africa. Surface and subsurface analysis of the rift basins involved multifold seismic reflection data, gravity and megnetic data, deep and shallow well control, and geological analysis of outcrops. The areas covered were vast, thus, the geological information gathered represents an unprecedented effort in the region that is unlikely to be repeated soon. With 14 chapters and 5 appendices, the data presented in this volume allow faults to be mapped and correlated with more confidence than before, basin evolution examined over a long time period, and some relationships between tectonics and sedimentation to be studied.

Abstract The Tertiary age East African Rift System (EARS) is predominantly located in zones of Precambrian orogenic belts avoiding stable Archean cratonic areas. The most profound influence of preexisting fabrics is exerted by Precambrian ductile fabrics. However, later predominantly extensional events of Permo-Triassic (Karroo), Jurassic, Cretaceous, and Paleogene age have also variably influenced the location and orientation of the Tertiary rift systems. The two branches of the East African Rift System have undergone different tectonic histories. In general, the western branch can be regarded as a good model of a young continental rift while the eastern branch is representative of a “failed” mature continental rift system. Both are characterized by large half graben systems filled by fluvio-deltaic and lacustrine sediments, and/or by volcanics and volcaniclastics. The basin fills can be up to 7–8 km thick. In comparison with the eastern branch, the western branch is younger (late Miocene-Recent), less volcanic-rich but more seismically active, with deeper earthquakes (down to about 30–40 km). Extension estimates for the eastern branch depend upon location, but range up to 40 km (Turkana area), maximum extension in the western branch is about 10–12 km. The eastern branch was probably initiated in the Eocene. In the Kenya and Ethiopian Rifts, older half grabens were abandoned in the Pliocene in favor of a narrow rift zone characterized by minor fault swarms, dike intrusions, and volcanic centers. There is evidence for an important active rift component (very thin mantle lithosphere, large topographic domes, anomalously low velocities in the mantle) as well as passive rifting. Deep refraction profiles indicate extension ranges between 10–40 km in the Kenya Rift (increasing northwards). However, this may be an underestimate if the crust has been inflated by intrusion of magma during extension.

Abstract The Turkana area comprises a string of half graben basins, most of which have been imaged by seismic reflection data (a total of 2,267 km of 60-fold vibroseis data) and two tested by hydrocarbon exploration wells. The Lokichar Basin is the oldest known basin (late Paleogene-middle Miocene) and has a simple half graben geometry. Basin fill is up to 7 km thick and is comprised of Paleogene-middle Miocene fluvio-deltaic sands punctuated by episodic thick lacustrine shale and sandstone deposition. Two of these thick lacustrine shales were identified in the Loperot-1 well. In particular, the upper lacustrine sequence (Oligocene-early Miocene), called the Lokone Shale Member, is well developed. The shales are rich in TOC (4.5% average) and thicken to 1 km near the boundary fault. Overlying the Lokone Shale are fluvio-deltaic sandstones that grade upwards from arkosic sandstones to sandstones having a significant vol-caniclastic component of middle Miocene age. The uppermost basin fill is middle Miocene age lava flows. Northward the lower, arkosic sequence passes into a volcanic-dominated succession of similar age (late Oligocene-early Miocene) and is exposed in the Lothidok Hills. The volcanics seem to have been deposited in an east-thickening half graben (Lothidok Basin). At the end of the middle Miocene, both the Lokichar and Lothidok Basins were abandoned and a new string of half graben basins formed (North Lokichar Basin, Turkana Basin and the Kerio Basin). The North Lokichar Basin is a west-thickening basin superimposed on the Lothidok Basin, and the Kerio Basin is a west-thickening series of half graben basins that probably had a middle Miocene-Paleogene history of extension as well. Sediments with a considerable volcaniclastic component and lava flows fill these basins. No deep water lacustrine conditions are known. Extension continued into the Pliocene, particularly in the Turkana Basin and the northern part of the Kerio Basin. Finally, the old half grabens were abandoned and a belt of minor fault swarms and volcanic centers was established east of Lake Turkana and in the southern part the lake. Pliocene, and possibly Holocene, tectonic inversion affected the Turkana and Kerio Basins.

Abstract Reconnaissance seismic reflection data show that the Lotikipi Plain is underlain by two faulted synformal basins, called the Gatome and Lotikipi Basins. Gravity data indicate the basins are oriented north-south. Ties between the seismic data and outcrops indicate that the basins are filled by basalt and rhyolite flows (Oligocene-early Miocene) capped by sedimentary deposits. The quality of the seismic data degrades significantly within the volcanic section, hence the deeper basin geometry and depth to the Precambrian basement are poorly defined. However, in the Gatome Basin a half graben basin within or beneath the volcanics is imaged, suggesting the existence of a Paleogene (more likely) or Cretaceous (less likely) sedimentary basin within or below the volcanics. Previous interpretations have suggested that volcanism preceded rifting in the Lotikipi Plain area. Such evidence has been used in support of active rifting in the Kenya Rift. However, here it is suggested that extension preceded rifting. Whether the rifting is related to the Sudan-Anza Graben (Cretaceous-Paleogene) rift system, or the eastern branch of the East African Rift System (EARS) is uncertain. Extension is unlikely to have been large enough to cause enough partial melting of the mantle purely by passive extension, because the volumes of lava in the region are large (450,000 km 3 ) and the beta factor unlikely to exceed 1.5. Consequently, an active mantle plume under the region must be assumed if the magma has ascended vertically. The lavas are thought to have been extruded at the end of rifting and into the early stages of thermal subsidence. Tectonic inversion affected the area during the late Miocene-Pliocene.

Abstract The Anza Graben is a northwest-southeast trending Cretaceous-Paleogene rift system. The oldest known section from the graben comes from a well in the Chalbi Desert area (northwest area), and is of Neocomian age. It provides a rare glimpse of carbonates in a lacustrine setting. Elsewhere, deposits are dominated by Late Cretaceous-Paleogene lacustrine shales and sandstones, and fluvio-deltaic sandstones. The rift geometry appears to have changed considerably with time—with overall rift activity younger to the southeast. This is clearly seen on the northeast margin boundary fault (Lagh Bogal Fault), which in the central Anza Graben is characterized by predominantly Late Cretaceous activity and in the southeastern graben by primarily Paleogene activity. The key agent affecting basin geometries was the changing activity and location of major faults. A striking feature of the Paleogene tectonic activity, in the southeastern Anza Graben in particular, is the presence of numerous large inversion anticlines which lie sub-parallel to the rift axis. These structures appear to have grown episodically during the Paleogene, alternating with periods of extension.

Abstract The Rukwa Rift is a late Tertiary rift system superimposed on a earlier Karroo (age) rift. In both rift basins extension increases towards the southeast, and probably developed under oblique extension. The influence of Precambrian and Karroo structures on the Tertiary fault pattern is very important and imparts an overall northwest-south trend to the rift. The Lupa Fault on the northeastern margin of the basin dominates the structural style. In the southeastern area this fault is listric in cross section and accommodates about 10 km of Tertiary extension. Passing northwards, the fault decreases in extension to about 2 km, the Tertiary-Recent basin fill correspondingly decreases in thickness from 7 km to 2 km. There is evidence for marked switching of paleostress directions during Tertiary to Recent time—the present state of stress indicated by earthquakes is east northeast-west southwest to northeast-southwest. Two wells have been drilled in the basin and are important correlation points for seismic data. The late Miocene-Recent rocks/sediments in the Galula-1 and Ivuna-1 wells are dominantly sandy fluvio-deltaic deposits or shallow-water lacustrine shales. Axial drainage into a relatively short rift segment kept the sediment supply high, apparently not permitting the development of extensive deep-water lacustrine shales.

Abstract The Usangu Flats are underlain by a sedimentary basin up to 1 km thick, that, from slow interval velocities, is inferred to be young and poorly compacted (Pliocene-Pleistocene). The main fault trends are east-west to northeast-southwest, which is an unusual direction for the East African Rift System (EARS) and may reflect structural development of the basin during a short period when the regional extension direction rotated to a northwest-southeast to north northwest-south southeast direction. The basin fill represents the early stages of rift development and displays faults that controlled the sedimentary section thickness without causing much rotation of the units. The initial basin fill onlapped an undulating basement topography.

Abstract Under normal conditions of rifting, boundary faults are predicted to be planar, high-angled with dips between 45° and 70°—this the case for most of the rifts of East Africa. There are, however a number of exceptions. In particular, it seems that in regions of high heat flow and strong volcanic activity half graben boundary faults tend to be lower-angle (30–45°). In the case of the Lokichar Fault (Turkana area, Kenya) lower-angled segments, located around the zone of maximum displacement on the fault, pass into lower displacement areas characterized by very low-angle fault segments (12–20°). The initiation of faults at a low angle cannot be easily explained by rock mechanics theory. Therefore, common explanations for such faults include: rotation of higher angle faults by the “domino” faulting model; rotation of large-displacement faults by isostatic instability created by the faulting; and activation of low-angled pre-existing fabrics. The Lokichar Fault geometry is inconsistent with any of the above explanations and some other cause of the low-angled nature must be found. In the Turkana area there is a coincidence between the location of the very low-angled segments and regions of intense igneous intrusive activity. If igneous intrusions do play a role in controlling fault dip then there are two possible mechanisms. Both cause reorientation of the stress axes from the simple Andersonian condition, which permits normal faults to form at a lower angle. The mechanisms are: Magma pressure during intrusion and the stresses that remain after cooling of the intrusion may locally create compressional conditions. Approaching dikes the maximum principal stress direction may swing from vertical towards the horizontal. Emplacement of igneous intrusions may heat up and weaken the lower crust permitting it to flow. It may be possible to set up a basal shear stress between the flowing and static crust causing reorientation of the principal stress axes.