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 Identification of syn-tectonic sequences includes recognition of onlap surfaces, expansion of strati-graphic section across a fault (rotational and non-rotational), and unconformities. It is often helpful to consider structural units in syn-rift sequences in terms of the boundaries marking the beginning and end of a deformation phase. In this chapter, a number is used to denote the structural phase, the letter “A” denotes the lower boundary, while “B” denotes the upper boundary of the particular phase. This naming convention is particularly helpful in identifying complex boundaries where erosion, non-deposition, or an abrupt change in structural style mean that a single boundary may be representative of multiple structural phases found elsewhere in the basin. In the long-lived rifts of the Anza Graben (Cretaceous-Paleogene) and Turkana area of the Kenya Rift (Paleogene-Recent) structural evolution follows a variety of patterns at different scales and locations. These two areas have the following trends in common: The timing of rift activity (basin initiation and deactivation) tends to be earlier in one direction along the rift axis. The major faults evolve through time from alternating “polarity” to more unidirectional dips. As rifting activity decreases in an area, the minor faults tend to be abandoned first, followed by the boundary faults. The rifting episode is terminated by a phase of inversion. The mechanism for generating dominant boundary fault dip directions is not well understood, but may include: deactivation of one fault by a cross-cutting fault; and regional rotation of principal stress axes to favor activity on one fault dip direction (for example, by a basal shear stress or by igneous intrusions creating local sub-horizontal compression). Lateral migration of rift basins can be caused by lithospheric-scale strain hardening associated with relatively low strain rates. Strain hardening may also occur by rotation of strength anisotropy to less favorable orientations with respect to the principal stresses (e.g., by fault block rotation). Differences between the evolution of the Anza and Turkana Rifts include: The Turkana Rift displays a pronounced lateral shift (eastwards) in location of rift activity with time. Paleogene activity became concentrated within the Anza Trough, but the Anza Graben did not significantly change location with time. The final phase of extension in much of the Kenya Rift is characterized by minor fault swarms and abandonment of half graben boundary faults—this is not seen in the Anza Graben. In the Turkana area, some boundary faults probably changed angle with time. Very low-angle boundary fault segments may have been initiated during times of intense igneous intrusion. In the Anza Graben most low-angle faults appear to be rotated high-angle faults, commonly abandoned and cut through by younger, higher-angle faults. The influence of volcanic activity on the Kenya Rift and its absence in the Anza Graben is one factor which could help explain the differences in structural evolution between the two areas.

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).

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 Major boundary faults in East Africa tend to transfer displacement along overlapping fault segments along which displacement gradually dies out on one fault and increases on one or more others. Oblique and cross faults associated with displacement transfer are rare. Minor faults are also affected by larger transfer zones. One particularly common pattern occurs where numerous minor faults dip in the same direction as the boundary fault. If the basin flips “polarity” then the boundary faults change to the opposite side of the basin and dip in opposite directions, minor fault dips also tend to flip. Complex interfingering of minor faults with opposite dips occurs in the vicinity of such transfer zones. The Eastern Branch of the East African Rift System (EARS) and the Anza Graben have undergone considerable structural evolution with time, hence the types of transfer zone present in a rift also change with time. For example, if a rift evolves towards a dominant boundary fault dip direction with time, then the transfer zones will change from dominantly conjugate types to dominantly synthetic types. Since transfer zones are commonly either barriers to sedimentation or drainage entry points, any change in transfer zone configuration is likely to significantly affect sedimentation patterns.

Abstract Examination of the different characteristics between rifts and associated basins (simple rifts, rifts with sag basins, and rifts on passive margins), and between the different types of basin fill (predominantly continental and predominantly marine) shows there is considerable variation in the elements of the associated hydrocarbon systems. Of the estimated 200 billion bbl (31.8 × 10 9 m 3) of reserves in rifts, approximately 65% are found in rifts with sag basins filled by marine sedimentary rocks, and 24% are found in rifts with sag basins fill by continental sedimentary rocks. Next largest reserves are found in rifts and passive margins and the least in simple rifts. Sag basins have the advantage of optimal depths of burial of source rocks, the common presence of widespread seals at the base of the sag basin sequence, and a wide variety of trap types within both the rift and sag basin sequences. Marine systems tend to favor more widespread and frequent development of sealing facies and better reservoir quality. Source rock potential is almost as good in continental as in marine settings. Lacustrine source rocks in rifts are somewhat different from their marine counterparts, tending to be thicker, but more locally developed due to the strong tectonic control on their distribution. The main problem in finding hydrocarbons in both the Anza Graben and the East African Rift is finding a widespread sealing facies at the right level with respect to both traps and reservoirs. This is typical of rifts with no, or only poorly developed, sag basins.

Abstract The Turkana magmatic rift (Northern Kenya) initiated at 45 Ma as one of the nucleation zones of rifting in the East African Rift. It forms an anomalously broad-rifted zone ( c. 200 km) striking with a north—south trend and lying within a NW—SE topographic depression, floored on both sides of the Turkana area by Cretaceous rifts in the Sudan and Anza plains. From a compilation of available data, combined with newly acquired remote sensing and DEM dataset, we propose a five-stage tectono-magmatic model for the Turkana rift evolution (45–23 Ma; 23–15 Ma; 15–6 Ma; 6–2.6 Ma and 2.6 Ma—Present). The corresponding ‘restored’ maps clearly show the changing spatial distribution of magmatism and fault/basin network with time, hence supplying some clues about dynamics of continental extension. First-order basement-rooted transverse faults zones are identified and their influence on nucleation and propagation of strain is demonstrated, whereas the role of magmatic ‘soft-spots’ as concentrating strain is minimized. Blocking of deformation as well as rift jump and lateral transfer of strain are discussed in relation to various types of fault interaction (dip direction, strikes and acute/ obtuse angle of the intersecting faults). The causal links between rift nucleation ‘cells’ and inherited transverse weakness zones in the Turkana rift might also exist elsewhere along the eastern branch of the East African Rift, hence suggesting a complex and discontinuous mode of rift propagation.