Anza Rift Research Articles

The development of rifting and magmatism in the multiply rifted Turkana Depression, East Africa: Evidence from surface-wave analysis of crustal and uppermost mantle structure

The Turkana Depression separates the uplifted Ethiopian and East African Plateaus. It was the site, in Mesozoic times, of a failed episode of NE–SW-oriented rifting (the Anza Rift), but now hosts E–W-oriented Nubia–Somalia separation at the junction between the Main Ethiopian Rift in the north and the Eastern Rift to the south. However, the time-integrated effect of these rifting phases on crustal and lithospheric mantle architecture and thermal structure is poorly understood. Utilising data from new seismograph networks in the Turkana Depression and northern Tanzania Craton, we produce a detailed anisotropic crustal and uppermost mantle shear-wave velocity model of the region. Within the Tanzania Craton, slightly lower uppermost mantle wavespeeds (4.4–4.5 km/s) compared to neighbouring regions, and coincident rift-parallel crustal anisotropy, imply the Nyanza Rift developed in relatively weak mobile belt lithosphere between two refractory Archean blocks. At upper-crustal (≲10 km) depths in the Turkana Depression, the slowest velocities (≲3.2 km/s) are attributed to thick Mesozoic-age sedimentary basins. Nowhere within the Depression is the mid-to-lower crust or lithospheric mantle associated with wavespeeds as slow, or seismic anisotropy as strong, as that observed below the melt-rich central and northern Main Ethiopian Rift (MER) and Ethiopian Plateau further north. High upper mantle wavespeeds (≳4.5 km/s), coinciding with the broadening of MER-rifting into southern Ethiopia, confirm the presence of refractory Proterozoic lithosphere acting as a rheological boundary to rift development. Thinned crustal zones associated with failed Mesozoic Anza rifting are also underlain by fast wavespeed (>4.5 km/s) mantle lithosphere, implying this area has resisted significant thermomechanical modification from Miocene-Recent extension and magmatism. Pre-existing crustal thin zones do not, therefore, necessarily represent zones of plate-weakness where subsequent phases of rifting will develop.

Active Deformation Constraints on the Nubia‐Somalia Plate Boundary Through Heterogenous Lithosphere of the Turkana Depression

AbstractThe role of lithospheric heterogeneities, presence or absence of melt, local and regional stresses, and gravitational potential energy in strain localization in continental rifts remains debated. We use new seismic and geodetic data to identify the location and orientation of the modern Nubia‐Somalia plate boundary in the 300‐km‐wide zone between the southern Main Ethiopian Rift (MER) and Eastern Rift (ER) across the Mesozoic Anza rift in the Turkana Depression. This region exhibits lithospheric heterogeneity, 45 Ma‐Recent magmatism, and more than 1,500 m of base‐level elevation change, enabling the assessment of strain localization mechanisms. We relocate 1716 earthquakes using a new 1‐D velocity model. Using a new local magnitude scaling with station corrections, we find 1 ≤ ML ≤ 4.5, and a b‐value of 1.22 ± 0.06. We present 59 first motion and 3 full moment tensor inversions, and invert for opening directions. We use complementary geodetic displacement vectors and strain rates to describe the geodetic strain field. Our seismic and geodetic strain zones demonstrate that only a small part of the 300 km‐wide region is currently active; low elevation and high‐elevation regions are active, as are areas with and without Holocene magmatism. Variations in the active plate boundary's location, orientation and strain rate appear to correspond to lithospheric heterogeneities. In the MER‐ER linkage zone, a belt of seismically fast mantle lithosphere generally lacking Recent magmatism is coincident with diffuse crustal deformation, whereas seismically slow mantle lithosphere and Recent magmatism are characterized by localized crustal strain; lithospheric heterogeneity drives strain localization.

Mantle Wavespeed and Discontinuity Structure Below East Africa: Implications for Cenozoic Hotspot Tectonism and the Development of the Turkana Depression

AbstractEthiopia's Cenozoic flood basalt magmatism, uplift, and rifting have been attributed to one or more mantle plumes. The Nubian plate, however, has drifted 500–1,000 km north since initial magmatism at ∼45 Ma, having developed above mantle that now underlies the northern Tanzania craton and the low‐lying Turkana Depression. Unfortunately, our knowledge of mantle wavespeed structure and mantle transition zone (MTZ) topography below these regions is poorest, due to a historical lack of seismograph stations. The same data gap means we lack constraints on lithospheric structure in and around the NW–SE trending Mesozoic Anza rift. We exploit data from new seismograph networks in the Turkana Depression and neighboring northern Uganda to develop AFRP22, a new African absolute P‐wavespeed tomographic model that resolves whole mantle structure along the entire East African rift system. We also map MTZ thickness using Ps receiver functions. East Africa's thinnest MTZ (∼25 km thinning) underlies the northwest Turkana Depression. AFRP22 reveals a co‐located, previously unrecognized, slow wavespeed plume tail, extending from the MTZ, deep into the lower mantle. This plume may thus have contributed, along with the African Superplume, to the development of the 45–30 Ma flood basalt province that preceded extension. Pervasive sub‐lithospheric slow wavespeeds imply that Turkana's present‐day low elevation is explained best by Mesozoic and Cenozoic‐age crustal thinning. At ∼100 km depth, AFRP22 illuminates a fast wavespeed SE Ethiopian plateau. In addition to governing the northernmost limit of Mesozoic Anza rifting, the refractory nature of this lithospheric block likely minimized Cenozoic flood basalt magmatism there.

Development of Late Jurassic‐Early Paleogene and Neogene‐Quaternary Rifts Within the Turkana Depression, East Africa From Satellite Gravity Data

AbstractThe Turkana Depression (TD) is a NW trending topographic corridor within the East African Rift System between the Ethiopia‐Yemen plateau in the northeast and the East African plateau to the southwest. The Anza rift within the TD is a NW‐SE trending failed arm of a late Jurassic rift‐rift‐rift triple junction. This rift is correlated with the Sudan and South Sudan rifts. The Anza rift is intersected by the East African Rift System represented by the N‐S trending Turkana rifted zone. We image the lithospheric structure beneath the TD using satellite gravity data. We also use these data to model crustal density distribution beneath the Kino Sogo fault belt, part of the Turkana rifted zone. Our results show thinner crust (23–28 km) and lithosphere (140–150 km) beneath the TD. We interpret this as due to extension that resulted in the formation of the Kenya‐Sudan and South Sudan rifts. Our results also show that crustal depth between 0 and 4.8 km is dominated by N‐S density contrast anomalies and between 4.8 and 14.5 km by E‐W anomalies. We interpret the N‐S anomalies as due to the presence of Precambrian structure that might have facilitated strain localization during the initiation of the Kino Sogo fault belt. Differently, we interpret the E‐W anomalies as due to the presence E‐W trending faults that were formed in association with the development of the Anza rift and/or Turkana rifted zone and were later filled with Mesozoic and Cenozoic mafic dikes.

History of the development of Permian–Cretaceous rifts in East Africa: a series of interpreted maps through time

East Africa represents the most rifted portion of crust on the planet, having been subjected to numerous phases of extension from Permian to Recent times. The first rifting phase commences in the Permian as the ‘Karoo’ set of narrow half-graben, many formed by lateral shear. Peak rifting appears to be of Middle Permian age for rifts south of Tanzania. In Kenya, Ethiopia and Somalia, this rift population merges into a set of rifts with peak rifting of earliest Triassic age. A further phase of rifting is seen in the Toarcian–Aalenian. Many of these overlie Permo-Triassic rifts but others are displaced towards what will become the continental margin. Three unrelated populations of Late Jurassic–Early Cretaceous rifts are observed, including those in Somalia, a series of pull-apart basins on the Davie Ridge and a poorly documented set in southern Mozambique. The Anza Rift, with peak rifting in the Late Cretaceous associated with the building of rift shoulders kilometres in height, is proposed here to be an isolated plume-derived rift. Evaluation of the petroleum potential associated with these rifts relies on an accurate assessment of each in terms of their age and affinity to well-documented systems. Supplementary material: A table of East Africa Permian–Mesozoic rift basins and references is available at https://doi.org/10.6084/m9.figshare.c.3902653

Cenozoic extension in the Kenya Rift from low‐temperature thermochronology: Links to diachronous spatiotemporal evolution of rifting in East Africa

AbstractThe cooling history of rift shoulders and the subsidence history of rift basins are cornerstones for reconstructing the morphotectonic evolution of extensional geodynamic provinces, assessing their role in paleoenvironmental changes and evaluating the resource potential of their basin fills. Our apatite fission track and zircon (U‐Th)/He data from the Samburu Hills and the Elgeyo Escarpment in the northern and central sectors of the Kenya Rift indicate a broadly consistent thermal evolution of both regions. Results of thermal modeling support a three‐phased thermal history since the early Paleocene. The first phase (~65–50 Ma) was characterized by rapid cooling of the rift shoulders and may be coeval with faulting and sedimentation in the Anza Rift basin, now located in the subsurface of the Turkana depression and areas to the east in northern Kenya. In the second phase, very slow cooling or slight reheating occurred between ~45 and 15 Ma as a result of either stable surface conditions, very slow exhumation, or subsidence. The third phase comprised renewed rapid cooling starting at ~15 Ma. This final cooling represents the most recent stage of rifting, which followed widespread flood‐phonolite emplacement and has shaped the present‐day landscape through rift shoulder uplift, faulting, basin filling, protracted volcanism, and erosion. When compared with thermochronologic and geologic data from other sectors of the East African Rift System, extension appears to be diachronous, spatially disparate, and partly overlapping, likely driven by interactions between mantle‐driven processes and crustal heterogeneities, rather than the previously suggested north–south migrating influence of a mantle plume.

Stratigraphy, sedimentology and diagenetic evolution of the Lapur Sandstone in northern Kenya: Implications for oil exploration of the Meso-Cenozoic Turkana depression

The northern Turkana region of northwestern Kenya forms the intersection between two major rift systems in Africa, the Cretaceous–Paleogene Central African Rift System (CARS), and the eastern arm of the Paleogene–Present East African Rift System (EARS). The southern Sudanese oil-rich rift basins form part of the CARS, and their extension into the Anza Rift in northeastern Kenya makes the area of northern Turkana an important target for oil exploration. Limited past exploration activity in the area leaves the study of surface outcrops as the main avenue for understanding the reservoir potential of the fluvial deposits of these rift systems. The outcrops of these potential reservoirs, collectively referred to as “Turkana Grits” in the past, are represented on the western side of Lake Turkana by the Lapur Sandstone in the north, and by other grit formations in the central and southern parts of the basin. Isotopic age determinations on the basal parts of the “Turkana Volcanics” that overlie the Lapur Sandstone have enabled the precise dating of the upper parts of the LS at between 35 and 37Ma, while the lower part of the formation near the contact with the underlying Precambrian basement is estimated as Upper Cretaceous (Turonian–early Campanian), based on the discovery of dinosaur and other reptilian fauna. Detailed lithological logging, coupled with subsequent petrographic and sedimentological studies, have enabled the determination of the depositional environments and the diagenetic evolution of the Lapur Sandstone. The basal and uppermost parts of the formation are interpreted as distal alluvial fan environments possibly connected to the last stages of rifting characterizing the Central African Rift System. The middle part of the Lapur Sandstone corresponds to a wide braided fluvial system that can be compared to fluvial episodes of Late Cretaceous age in the Sudan region, associated to major palaeogeographical changes in northern Africa. The nearly abrupt disappearance of the Lapur upper fan system relates to the deposition of the “Turkana Volcanics” from Late Eocene, possibly as a consequence of the emplacement of the Afar Plume at 45–35Ma. In terms of diagenesis, the main cement material at the base of the Lapur Sandstone is calcite, whereas at the middle of the formation, hematite becomes the dominant cement, and at the topmost section, kaolin cement dominates. The diagenetic evolution of the sandstones has been favourable to the retention of adequate primary intergranular porosity and the creation of secondary intragranular dissolution porosity, mainly through feldspar dissolution, and thus preserving the reservoir potential of the Lapur Sandstone. The reservoir characteristics, such as the porosity and cementation style, of the Lapur Sandstone are comparable to those of the fluvial sandstone reservoirs of the southern Sudan oil fields and this should positively contribute to the overall petroleum potential of the northern Turkana region. Though the northern Turkana area has remained largely unexplored, it is hoped that the demonstration of the presence of reasonably good reservoir quality sandstones in the Lapur Sandstone will serve to encourage further interest in hydrocarbon exploration in the Turkana area.

Pliocene volcano-tectonics and paleogeography of the Turkana Basin, Kenya and Ethiopia

The distribution of hominin fossil sites in the Turkana Basin, Kenya is intimately linked to the history of the Omo River, which affected the paleogeography and ecology of the basin since the dawn of the Pliocene. We report new geological data concerning the outlet channel of the Omo River between earliest Pliocene and final closure of the Turkana Basin drainage system in the latest Pliocene to earliest Quaternary. Throughout most of the Pliocene the Omo River entered the Turkana Basin from its source in the highlands of Ethiopia and exited the eastern margin of the basin to discharge into the Lamu embayment along the coast of the Indian Ocean. During the earliest Pliocene the river’s outlet was located in the northern part of the basin, where a remnant outlet channel is preserved in basalts that pre-date eruption of the Gombe flood basalt between 4.05 and 3.95 Ma. The outlet channel was faulted down to the west prior to 4.05 Ma, forming a natural dam behind which Lake Lonyumun developed. Lake Lonyumun was drained between 3.95 and 3.9 Ma when a new outlet channel formed north of Loiyangalani in the southeastern margin of the Turkana Basin. That outlet was blocked by Lenderit Basalt lava flows between 2.2 and 2.0 Ma. Faulting that initiated either during or shortly after eruption of the Lenderit Basalt closed the depression that is occupied by modern Lake Turkana to sediment and water. Several large shield volcanoes formed east of the Turkana Basin beginning by 2.5–3.0 Ma, volcanism overlapping in time, but probably migrating eastward from Mount Kulal on the eastern edge of the basin to Mount Marsabit located at the eastern edge of the Chalbi Desert. The mass of the volcanic rocks loaded and depressed the lithosphere, enhancing subsidence in a shallow southeast trending depression that overlay the Cretaceous and Paleogene (?) Anza Rift. Subsidence in this flexural depression guided the course of the Omo River towards the Indian Ocean, and also localized accumulations of lava along the margins of the shield volcanoes. Lava flows at Mount Marsabit extended across the Omo River Valley after 1.8–2.0 Ma based on estimated ages of fossils in lacustrine and terrestrial deposits, and possibly by as early as 2.5 ± 0.3 Ma based on dating of a lava flow. During the enhanced precipitation in latest Pleistocene and earliest Holocene (11–9.5 ka) this flexural depression became the site of Lake Chalbi, which was separated from Lake Turkana by a tectonically controlled drainage divide.

Evolution of the northern Main Ethiopian rift: birth of a triple junction

Models for the formation of the archetypal rift–rift–rift triple junction in the Afar depression have assumed the synchronous development of the Red Sea–Aden–East African rift systems soon after flood basaltic magmatism at 31 Ma, but the timing of intial rifting in the northern sector of the East African rift system had been poorly constrained. The aims of our field, geochronology, and remote sensing studies were to determine the timing and kinematics of rifting in the 3rd arm, the Main Ethiopian rift (MER), near its intersection with the southern Red Sea rift. New structural data and 10 new SCLF 40Ar/39Ar dates show that extension in the northern Main Ethiopian rift commenced after 11 Ma, more than 17 My after initial rifting in the southern Red Sea and Gulf of Aden. The triple junction, therefore, could have developed only during the past 11 My, or 20 My after the flood basaltic magmatism. Thus, the flood basaltic magmatism and separation of Arabia from Africa are widely separated in time from the opening of the Main Ethiopian rift, which marks the incipient Nubia–Somalia plate boundary; triple junction formation is not a primary feature of breakup above the Afar mantle plume. The East African rift system appears to have propagated northward from the Mesozoic Anza rift system into the Afar depression to cut across Oligo-Miocene rift structures of the Red Sea and Gulf of Aden, in response to global plate reorganisations. Structural patterns reveal a change from 130°E-directed extension to 105°E-directed extension sometime in the interval 6.6 to 3 Ma, consistent with predictions from global plate kinematic studies. The along-axis propagation of rifting in each of the three arms of the triple junction has led to a NE-migration of the triple junction since 11 Ma.

Hydrocarbon potential of the Meso-Cenozoic Turkana Depression, northern Kenya. I. Reservoirs: depositional environments, diagenetic characteristics, and source rock–reservoir relationships

Major oil exploration efforts started in the 70s in the Meso-Cenozoic Anza Rift and Cenozoic Turkana Depression of northern Kenya. Thick piles of fluvio-lacustrine sandstones and shales infill these different rift basins. West of Lake Turkana, the Auwerwer/Lomerimong Formation is part of the Palaeogene–middle Miocene age, 7 km-thick fluvio-lacustrine infill of the Lokichar half-graben. East of Lake Turkana, the 220 m-thick Sera Iltomia Formation is of possible late Mesozoic–basal Palaeocene age, and comprises sandstones and mudstones with conglomeratic layers. The poorly dated Sera Iltomia Formation may represent either the early phase of Cenozoic East African rifting in northern Kenya or the Meso-Cenozoic Anza Rift. The sandstones of these two formations exhibit different sediment sources and consequent reservoir quality. The Sera Iltomia sandstones are immature and basement-derived. While the sources of clastic material from the Auwerwer/Lomerimong section originated from both volcanic and basement terrains. Palaeocurrent data for the Sera Iltomia and Auwerwer/Lomerimong basement-derived sandstones suggest a source to the south and south-east of Lake Turkana. The volcanic-derived clastic rocks forming part of the Auwerwer/Lomerimong section suggest a sediment source to the south-southeast of the Lokichar Basin, linked to the lower Miocene Samburu Basalts Formation. Evidence for significant burial diagenesis is absent in both. In the Auwerwer/Lomerimong sandstones, calcite–analcite precipitation and calcite cementation significantly reduced the porosity from initial values of 40–45% to values which ranges up to 15%. In the Sera Iltomia sandstones, different early diagenetic events are recorded by calcite, quartz or kaolin cements. Quartz overgrowths and kaolin precipitation are local phenomena, and did not induce significant porosity reduction. In some cases, calcite cementation completely occluded the initial porosity, but in other cases it has helped preserve significant porosity by limiting mechanical compaction. This cementation resulted in a higher porosity (up to 33%) than in the Lomerimong sandstones and a better potential to form reservoirs of good quality. Comparison with potential reservoirs encountered in the Anza Rift wells demonstrate that volcaniclastic-rich sandstones are more frequently affected by pore cementation rather than the arkosic-type sandstones. The complex stratigraphic and structural organization of the Turkana Depression/Anza Rift system illustrates the potential variability in sediment source, diagenesis and reservoir–source rock relationships when exploring for reservoirs in continental rift systems.

Fault mechanisms and tectonic implication of the 1985-1987 earthquake sequence in south-western Ethiopia

Integrated inversions of short-period P, broadband P, and long-period P &s waves are done for fault mechanisms, focal depths, seismic moments, and source-time functions from the largest four earthquakes of the 1985 and 1987 earthquake sequence in south-western Ethiopia. These earthquakes had similar normal-faulting mechanisms. The general trends of the fault planes follow the Main Ethiopian Rift which is in agreement with foreshock-aftershock distribution, surface breaks and geology. Despite the morphological discontinuity of the Main Ethiopian Rift at its southern tip, the mode of deformation of the continental crust under study shows its extension southward. There are no significant strike-slip components trending NW–SE in all the mechanisms which would have been associated with the Aswa Fault Zone in southern Sudan or Anza Rift in northern Kenya. We also infer that the relatively broad fracture zone at the southern extreme of the Main Ethiopian Rift demonstrates the early stage of the break-up between the Nubia and Somalia plates in comparison with the Main Ethiopian Rift proper and the Afar Depression. The main shock of the sequence (Mw = 6.3) ruptured at a depth of 6.8 km, shallower than expected since the depth of earthquakes generally increase southward from the Afar Depression. The shallow depth of earthquake occurrence is supported by surface deformations with an overall trend in the direction of the Main Ethiopian Rift.

Structural framework and denudation history of the flanks of the Kenya and Anza Rifts, East Africa

The Kenya Rift in central Kenya (between ∼1° and 2° north latitude) is flanked by vast areas of Proterozoic crystalline rock where traditional stratigraphic markers useful for constraining Mesozoic to Recent structures are absent. Apatite fission track age and length data from about 100 samples collected from (1) relief profiles in north‐south trending mountain ranges, and (2) east‐west transects between mountain ranges constrain the structural framework of Cretaceous through Tertiary faulting in these flanking areas. Data from the profiles reveal regionally consistent age/elevation trends characterized by three distinct intervals, of 600 to 1200 m altitude, within which the apatite apparent ages are nearly concordant. The three isochronous age/elevation intervals yield apparent ages of ∼180, ∼115, and ∼65 Ma and record times of cooling starting at ≥220, 140–120, and 70–60 Ma. We interpret the cooling to be related to episodes of relatively rapid denudation separated in time by periods of slower exhumation. The isochronous intervals of crust are regionally consistent and relative offsets between the intervals allow fault displacements and block tilts to be estimated, when used in conjunction with the variation of fission track age and length along the transects. The fault block geometry revealed by the displaced isochronous layers indicates that normal faults related to rifting, with relative displacements >1 km, extend at least 100 km east of the Kenya Rift Valley. The data indicate a consistent direction of block tilting for the segment of the rift studied, which is similar to the asymmetric extension observed for other sections of the East Africa Rift System and other narrow rifts. The regional block faulting in this area probably occurred during early to middle Tertiary time associated with the late stages of extension in the Anza Rift and/or the early stages of extension of the Kenya Rift, in a setting similar to the present morphology of northern Tanzania. Therefore focusing of faulting within a central rift valley in Kenya appears to have followed a phase of regional extension with minor total strain.

Structural and stratigraphic evolution of the Anza rift, Kenya

The Anza rift is a large, multi-phase continental rift basin that links the Lamu embayment of southern Kenya with the South Sudan rifts. Extension and deposition of syn-rift sediments are known to have commenced by the Neocomian. Aptian-Albian strata have, thus far, not been encountered during limited drilling campaigns and, in at least one well, are replaced by a significant unconformity. Widespread rifting occurred during the Cenomanian to Maastrichtian, and continued into the Early Tertiary. Marine waters appear to have reached the central Anza rift in the Cenomanian, and a second marine incursion may have occurred during the Campanian. As no wells have yet reached basement in the basinal deeps, the possibility exists that the Anza rift may have initiated in the Late Jurassic, in conjunction with extension to the south in the Lamu embayment and to the north in the Blue Nile rift of Sudan. Structural and stratigraphic evolution in the Anza rift followed a pattern that has now been inferred in several rift settings. Early phases of extension were accommodated by moderately dipping faults that produced large stratal rotations. Sedimentary environments were dominantly fluvial, with associated small lakes and dune fields. Volcanic activity is documented for the early Neocomian, but its extent is unknown. This initial style of deformation and sedimentation may have continued through several of the earliest pulses of rifting. By the Late Cretaceous, a new system of steeply dipping faults was established, that produced a deep basin without significant rotation of strata in the north, and only minor rotation in the south. This basin geometry favored the establishment of large, deep lakes, which occasionally were connected to the sea. The older basins were partly cannibalized during the sedimentary in-filling of these successor basins. Early Senonian volcanism was encountered in one well, and reflection seismic evidence suggests that one or more thick, regionally extensive igneous sills were intruded, probably during the Early Tertiary. The change in rift style from early, strongly rotational, shallow basins to late, non-rotational, deep basins has been observed in the southern Gulf of Suez/northern Red Sea, the Southwestern Turkana/northern Kenyan rift, and at Anza. It therefore takes place in rifts in variable tectonic settings, with a wide range of volcanic activity and, presumably, with different driving mechanisms. The shift in deposition in each case is away from early rift-bounding faults toward the half-graben flexural margins, further in-board to the upper structural plate. This suggests at least some component of regional simple shear in the deformation history of the rifted lithosphere, either via broad shear zones or at discrete detachment surfaces.

Mesozoic-Cenozoic Stratigraphy and Rifting History of the Anza Rift, Kenya: ABSTRACT

Mesozoic-Cenozoic Stratigraphy and Rifting History of the Anza Rift, Kenya: ABSTRACT

Stratigraphy and Rifting History of the Mesozoic-Cenozoic Anza Rift, Kenya

Lithological and compositional relationships, thicknesses, and palynological data from drilling cuttings from five wells in the Anza rift, Kenya, indicate active rifting during the Late Cretaceous and Eocene-Oligocene. The earlier rifting possibly started in the Santonian-Coniacian, primarily occurred in the Campanian, and probably extended into the Maastrichtian. Anza rift sedimentation was in lacustrine, lacustrine-deltaic, fluvial, and flood-basin environments. Inferred synrift intervals in wells are shalier, thicker, more compositionally immature, and more poorly sorted than Lower Cretaceous(?)-lower Upper Cretaceous and upper Oligocene(?)-Miocene interrift deposits. Synrift sandstone is mostly feldspathic or arkosic wacke. Sandstone deposited in the Anza basin during nonrift periods is mostly quartz arenite, and is coarser and has a high proportion of probable fluvial deposits relative to other facies. Volcanic debris is absent in sedimentary strata older than Pliocene-Holocene, although small Cretaceous intrusions are present in the basin. Crtaceous sandstone is cemented in places by laumontite, possibly recording Campanian extension. Early Cretaceous history of the Anza basin is poorly known because of the limited strata sampled; Jurassic units were not reached. Cretaceous rifting in the Anza basin was synchronous with rifting in Sudan and with the breakup and separation of South America and Africa; these events likely were related. Eocene-Oligocene extension in the Anza basin reflects different stresses. The transition from active rifting to passive subsidence in the Anza basin at the end of the Neogene, in turn, records a reconfigured response of east African plates to stresses and is correlated with formation of the East Africa rift.

Tectonostratigraphic development of the Interior Sudan rifts, Central Africa

In the Muglad, Melut and Blue Nile rift basins of Interior Sudan three major episodes of rifting, concomitant subsidence and nonmarine/nonvolcanic sedimentation are recognized. These three rifting cycles, which spanned 140 to 95 Ma (Fl), 95 to 65 Ma (F2), and 65 to 30 Ma (F3), resulted in the accumulation of up to 5400, 4200 and 5400 m of sediments, respectively. In the Muglad Basin, the best known and largest of the Sudan rift basins, each rifting cycle consists of (1) a basal sandstone unit (at least near rift margins), that is followed by (2) an upward coarsening section of lacustrine shale grading through marginal lacustrine mudstone and sandstone into fluvial mudstone and sandstone, and (3) a capping blanket of fluvial and alluvial sandstone. The shale-dominated portions of these cycles were deposited in a closed-drainage basin during active faulting. The fluvial and alluvial blanket sands were deposited in an open-drainage basin during the thermal sag phase following each tectonic cycle. The Early Cretaceous F1 intracontinental rifts of Interior Sudan were linked to major rifts/spreading centres in the Proto-South Atlantic by the dextral WSW-trending Central African Shear Zone and to the Indian Ocean via the NW-trending Anza rift in Kenya. In the Muglad Basin, F1 deformation involved high strain rates, rapid syn-rift crustal stretching and subsidence, and the formation of deep, fault-bounded tensional and transtensional pull-apart basins. During the F2 and F3 deformations, the rates of subsidence and stretching were much lower and were focused within smaller geographic areas. Structural elements include asymmetric half-grabens and less common full-grabens with central highs. The three superimposed tectonic episodes resulted in the subsidence of NNW- to NW-trending rift sub-basins; this gave rise to a wide variety of normal fault geometries, displacements, and growth histories. Planar domino-style and listric normal F1 fault arrays are modeled. The rotated F1 basement blocks typically are asymmetric and low-standing, and favour NE-directed growth and material transport. The F2 and F3 normal faults, which have both NE- and SW-directed polarities, often are listric and decouple younger syn-rift strata from older F1 rotated and locked planar basement faults. Palinspastic restorations and forward modeling of three regional cross-sections suggest that F1 faults account for 65–80% of total crustal extension. Stretching factors across the Interior Sudan rifts vary between 1.25 and 1.40. This range is considered to be a minimum since restored sections do not cross F3 depocenters. Total crustal extension across the Muglad plus the Melut rift basins exceeds 75 km.

The morphotectonic evolution of rift-margin mountains in central Kenya: Constraints from apatite fission-track thermochronology

Passive margin mountains and rift flank uplifts are common features of extended continental areas. Apatite fission-track thermochronology of Proterozoic rocks from mountain ranges east of the Kenya Rift in central Kenya reveals episodes of rapid cooling and denudation, in Early Cretaceous (130-110 Ma), Late Cretaceous-Paleocene (70-60 Ma), and possibly Miocene ( ∼ 10 Ma) times. During the Early Cretaceous ∼ 1.2–2.0 km of denudation occurred in this area, and evidence for this is preserved in the Karisia Hills. The Late Cretaceous-Paleocene episode is well documented from escarpments along the western margin of the Anza Rift including the Mathews Range. Detailed fission-track age profiles up these escarpments indicate that 2.6–3.0 km of crust was denuded from this area in < 5 Ma. These periods of rapid denudation probably contributed to the deposition of thick sedimentary sequences in the adjacent Anza Rift. A third period of cooling during the Miocene is proposed on the basis of forward modeling of the apatite age and track length data from the Mathews Range samples. Denudation associated with Miocene cooling probably only amounted to ∼ 500 m in most areas. The timing of the three denudational episodes can be correlated with intracratonic deformational events elsewhere in Africa, as well as with periods of extension in the Lamu Embayment, Anza Rift and Kenya Rift. The variation of apatite fission-track age and track length with elevation in the mountain ranges also provides a basis for reconstructing preserved parts of pre-Early Cretaceous and pre-Late Cretaceous apatite partial annealing zones. Offsets of these partial annealing zones indicate that the basement mountain ranges in central Kenya developed as rift margin mountains adjacent to the Anza Rift. Early and Late Cretaceous extension in the Anza Rift caused normal faulting with displacements of ∼ 1 km and ∼ 3° of westward tilting of crustal blocks along its flanks.

Mesozoic and early Tertiary rift tectonics in East Africa

A complex history of crustal extension occurred in east and central Africa during the Mesozoic and early Tertiary. Beginning in the Late Jurassic, this resulted in a large system of rifts, the Central African rift system, that spanned from central Sudan to southern Kenya. Late Jurassic rifting is best documented in the White and Blue Nile rifts of the Sudan, and records east-west extension in half-graben that were connected by large-scale shear zones and pull-apart basins. Early Cretaceous rifting re-activated Jurassic basins and spread to the large South Sudan rifts and Anza rift in Kenya. By the Late Cretaceous, the extension direction shifted to the NE-SW, and the presently observed large-scale rift geometry was established. In the early Tertiary, some Mesozoic basins were again reactivated, while other regions underwent wrench faulting and basin inversion. The large number of basins preserved in the Central African rift system can be used to construct an evolutionary model of continental rift tectonics. Early phases of extension at low strains produced alternating half-graben/accommodation zone geometries similar to those observed in most young and active continental rifts. At higher strains, some border faults were abandoned so that through-going, simpler active fault systems could evolve. This is interpreted as representing a switch from complex, oppositely dipping detachment structures, with strike dimensions of 50–150 km, to regional detachment structures that continue for hundreds of kilometers parallel to the rift. This change in the type of detachment was accompanied by a shift in the position of the subsidence away from the breakaway to a position focused further within the regional upper plate. Non-rotational, high angle, normal faulting dominates in the development of these late basin geometries. Deciphering similar rift basin histories from passive continental margins may, in many cases, exceed the limits of available reflection seismic data. East-west striking shear zones, including the Umm Hani and South Sudan shears, served as basin-terminating structures in the Late Jurassic rift system. These shear zones were later reactivated during the Cretaceous and early Tertiary, controlling the positions of some accommodation zones and “jogs” (apparent offsets) in rift trends. The South Sudan shear may have continued to play a role in Neogene extensional tectonics, as it connects “jogs” in both the Abu Gabra/Anza rift (Jurassic? to Early Tertiary) and Turkana/Chew Bahir rift (Neogene to Recent).

Crustal structure and tectonic evolution of the anza rift, northern Kenya

The Anza trough is a Mesozoic rift located in northern Kenya that appears to be the failed third arm of a paleo-triple junction which allowed the separation of Madagascar from Africa during the Jurassic. The rift is oriented NW-SE and its tectonic evolution is related to that of the Mesozoic southern Sudan rift system. We analyzed seismic and gravity data from the southwestern side of the Anza rift including the Chalbi Desert to gain a better understanding of rift structure. Gravity data delineate the main rift basins as well as a small sub-basin on the southwest side of the main rift. Normal faulting evident on the NW end of a 42-km-long, NW-SE oriented Vibroseis® profile, marks the western boundary of the sub-basin. This sub-basin is offset from the trend of the main Anza trough; the western boundary may be a complex fault zone accommodating a change in direction of the main rift trend. Gravity values increase to the NW in the faulted area, suggesting shallowing of basement. A strong NW-dipping reflection from 0.5 s to almost 3 s is interpreted as a pre- to mid-Cretaceous unconformity. The configuration of the unconformity and the normal faulting strongly resembles the half-graben geometry imaged in the East African Rift. Numerous discontinuous reflections can be seen deeper in the section between 6 and 9 s, but a distinct reflection Moho cannot be interpreted with certainty. In addition to seismic and gravity data, regional geologic and well data lead us to conclude that there are probably Jurassic marine sediments in the bottom of the Anza rift.