Processing and interpretation of the gravity field of the East African Rift: implication for crustal extension

Compilation of new and existing gravity data were undertaken to assess the nature of the crust beneath the East African Rift System. Using 3D gravity modeling code crustal model of gravity profiles across two sectors of the rift were computed. The results are discussed in light of the structure of the rift system.The results of the 3D modeling of gravity profiles across the two rift zones revealed northward thinning of the crust. The maximum crustal attenuation occurs beneath the Afar depression, indicating the Afar rift undergoes an intense fragmentation of the crust resulting from faulting and magmatic activity. However, our computed crustal thickness below the Afar depression falls within an upper bound compared to elsewhere below tectonically active rift zones. This can be explained in terms of crustal accretion resulting from an impact of the Afar mantle plume since 30 Ma ago.The residual gravity obtained using high-cut filtering techniques reveals significant density contrast between the northern and southern sectors of the rift. The northern part of the rift is characterized by regular patterns of positive gravity anomalies, which can be interpreted in terms of a zone of crustal thinning through which relatively dense materials have intruded the overlying crust. In contrast, south of the Main Ethiopian Rift, the anomalies are characterized by random patterns and low amplitudes. The along-rift-axis variation in gravity anomalies implies that the style of crustal deformation changed progressively, beginning with regionally distributed crustal deformation, such as the one we observe within the more juvenile and wider southern segment of the rift, to localized deformation within the active and narrow rift zones of the northern sector of the Ethiopian Rift. We suggest that the key parameters controlling along-rift-axis variation in gravity anomalies are the rate of crustal extension, faulting and magmatic activities.     

Anatomy of a river drainage reversal in the Neogene Kivu–Nile Rift

The Neogene geological history of East Africa is characterised by the doming and extension in the course of development of the East African Rift System with its eastern and western branches. In the centre of the Western Rift Rise Rwanda is situated on Proterozoic basement rocks exposed in the strongly uplifted eastern rift shoulder of the Kivu–Nile Rift segment, where clastic sedimentation is largely restricted to the rift axis itself. A small, volcanically and tectonically controlled depository in northwestern Rwanda preserved the only Neogene sediments known from the extremely uplifted rift shoulder. Those (?)Pliocene to Pleistocene/Holocene fluvio-lacustrine muds and sands of the Palaeo-Nyabarongo River record the influence of Virunga volcanism on the major drainage reversal that affected East Africa in the Plio-/Pleistocene, when the originally rift-parallel upper Nile drainage system became diverted to the East in order to enter the Nile system via Lake Victoria. Sedimentary facies development, heavy mineral distributions and palaeobiological controls, including hominid artefacts, signal a short time interval of <300–350 ka to complete this major event for the sediment supply system of the Kivu–Nile Rift segment.     

Spatial and temporal geochemical variability in lacustrine sedimentation in the East African Rift System: Evidence from the Kenya Rift and regional analyses

Many previous studies on lacustrine basins in the East African Rift System have directed their attention to climatic controls on contemporary sedimentation or climate change as part of palaeoenvironmental reconstruction. In contrast, this research focuses on the impact of tectonism and volcanism on rift deposition and develops models that help to explain their roles and relative importance. The study focuses on the spatial and temporal variability in bulk sediment geochemistry from a diverse range of modern and ancient rift sediments through an analysis of 519 samples and 50 major and trace elements. The basins examined variously include, or have contained, wetlands and/or shallow to deep, fresh to hypersaline lakes. Substantial spatial variability is documented for Holocene to modern deposits in lakes Turkana, Baringo, Bogoria, Magadi and Malawi. Mio‐Pleistocene sediments in the Central Kenya Rift and Quaternary deposits of the southern Kenya Rift illustrate temporal variability. Tectonic and volcanic controls on geochemical variability are explained in terms of: (i) primary controlling factors (faulting, subsidence, uplift, volcanism, magma evolution and antecedent lithologies and landscapes); (ii) secondary controls (bedrock types, rift shoulder and axis elevations, accommodation space, meteoric and hydrothermal fluids and mantle CO ₂); and (iii) response factors (catchment area size, orographic rains, rain shadows, vegetation densities, erosion and weathering rates, and spring/runoff ratios). The models developed have, in turn, important implications for palaeoenvironmental interpretation in other depositional basins.     

东非裂谷系西支构造样式及其对油气成藏的控制作用

基于东非裂谷系西支地震地质综合解释对比研究,发现该生长型裂谷盆地受基底属性影响,主要发育陡断面地堑型盆地结构,沿边界主控断裂走向发育背离型和接近型两类主要调节构造,沿边界主控断裂倾向主要发育地垒式和地堑式两类调节构造。构造样式控制了主要成藏条件：陡断面地堑式裂谷能够形成沉积范围广、厚度大的湖相优质烃源岩;裂谷间走向调节构造属于一级调节带,控制长轴辫状河三角洲的展布;裂谷内错断的边界断裂带属于次级走向调节构造,控制中小型扇三角州的发育。倾向调节构造形成的断鼻、断块圈闭,为该类盆地的主要油气藏类型。     

Tertiary Mafic Lavas of Turkana, Kenya: Constraints on East African Plume Structure and the Occurrence of High-{micro} Volcanism in Africa

The East African Rift System is important to understanding plume-initiatedrifting as manifest in the geochemistry of mafic lavas eruptedalong the rift throughout its evolution. We present new datafrom high-MgO Tertiary lavas from Turkana, northern Kenya, toinvestigate regional melt source components, to identify thedepths and degrees of melting, and to characterize spatiallyand temporally the chemical structure of the underlying mantle.The Turkana area is a region of high lithospheric extensionthat sits between two topographic uplifts thought to be surfaceexpressions of one or more upwelling mantle plumes. Thinningof local crust is believed to be accompanied by widespread removalof the mantle lithosphere, causing the asthenosphere to be inclose contact with the overlying crust. New geochemical dataon basanites, picrites and basalts (MgO >7 wt %) tightlyconstrain the primary melt source regions of Tertiary volcanism.Initial isotopic signatures (¹⁴³Nd/¹⁴⁴Nd = 0·51267–0·51283,⁸⁷Sr/⁸⁶Sr = 0·7031–0·7036) and trace elementabundances (Ce/Pb 30, La/Nb = 0·6–0·8 andBa/Nb = 3–10) in these lavas are consistent with derivationfrom sub-lithospheric sources. Basalts and picrites eruptedbetween 23 and 20 Ma have Sr–Nd–Pb–He isotopiccharacteristics indicative of high-µ influence, recordhigh depths and degrees of partial melting, and are associatedwith rift propagation to the north and south. Accordingly, theselavas sample a source region that is geochemically distinctfrom that reflected both in Oligocene Ethiopian flood basaltsand in the modern Afar region. The geochemical data supportnumerical and theoretical models as well as tomographic resultsproviding for a complex thermal structure in the mantle beneathEast Africa and are interpreted to reflect isotopically distinctplume heads beneath Tanzania and Afar that are derived fromthe chemically heterogeneous South African superplume. KEY WORDS: East African Rift System; mantle plumes; HIMU; geochemistry; Afar     

Thermal variations in the gregory rift of southern Kenya (?)

A compilation of thermal and seismic data collected over the last sixty years allows one to infer that tectonic phenomena and heat emanation could be linked in an oscillatory mode up and down the Kenyan part of the East African Rift. The seismic period is approximately 20–30 years during which time the loci of maximum intensity earthquakes move in a rhythmic pattern from south to north and back to south. Temperatures measured from hot springs also fluctuate over this time span increasing or decreasing in different sections of the rift.Spatial variations were measured by infrared radiometers from low altitude aircraft or high-altitude satellites. These reveal that individual thermal springs ranging from 35°C to 80°C, warm up greater than 5 km² of the lake bottom of Magadi (only a slightly active thermal region which, however, yields greater then 300 MW). The heated area is large enough to detect by satellite imagery, making it possible to monitor the heat budget and flux over time and relate it to tectonic activity in the rift.     

Sediment geochemistry and tectonic setting: Application of discrimination diagrams to early stages of intracontinental rift evolution,with examples from the Okavango and Southern Tanganyika rift basins

In this article, we have applied discriminant diagrams and bivariate plots for tectonic setting to Quaternary sediments from the East African Rift System (EARS). Sediment samples used in this study represent two different phases in early stage intracontinental rift evolution: the alluvial fan of the nascent Okavango system and a lacustrine basin within the relatively more mature Tanganyika system. The diagrams for tectonic setting for major elements place the majority of these EARS sediments within the passive margin (PM) setting. Passive margin sandstones are generally enriched in SiO₂ and depleted in Na₂O, CaO and TiO₂ reflecting their highly recycled nature. Based on major element discriminant diagrams, we propose two new fields for early stage intracontinental rift evolution (alluvial fan and lacustrine basin), within the previously defined passive margin field. The rare earth element (REE) patterns from both Okavango and Tanganyika sediments exhibit patterns similar to PAAS, with ash layers from the Rungwe volcanics in the Tanganyika samples exhibiting an enrichment in REE relative to the bulk sediment.     

Structural position of the Pleistocene Gesher Benot Ya'aqov site in the Dead Sea Rift zone

Newly discovered outcrops of the middle Pleistocene Benot Ya'aqov Formation are strongly disturbed due to recent tectonic activity along the Dead Sea Rift. The lacustrine-fluviatile sediments of this formation comprise the littoral facies of a paleo-lake that occupied the adjacent Hula Basin. Acheulian artifacts, found embedded in the formation, have typical African characteristics. The geographical position of the site (the northern extension of the East African Red Sea Rift System) is important for understanding hominid diffusion from Africa to Eurasia.     

Continental rift evolution: From rift initiation to incipient break-up in the Main Ethiopian Rift, East Africa

The Main Ethiopian Rift is a key sector of the East African Rift System that connects the Afar depression, at Red Sea–Gulf of Aden junction, with the Turkana depression and Kenya Rift to the South. It is a magmatic rift that records all the different stages of rift evolution from rift initiation to break-up and incipient oceanic spreading: it is thus an ideal place to analyse the evolution of continental extension, the rupture of lithospheric plates and the dynamics by which distributed continental deformation is progressively focused at oceanic spreading centres.The first tectono-magmatic event related to the Tertiary rifting was the eruption of voluminous flood basalts that apparently occurred in a rather short time interval at around 30 Ma; strong plateau uplift, which resulted in the development of the Ethiopian and Somalian plateaus now surrounding the rift valley, has been suggested to have initiated contemporaneously or shortly after the extensive flood-basalt volcanism, although its exact timing remains controversial. Voluminous volcanism and uplift started prior to the main rifting phases, suggesting a mantle plume influence on the Tertiary deformation in East Africa. Different plume hypothesis have been suggested, with recent models indicating the existence of deep superplume originating at the core-mantle boundary beneath southern Africa, rising in a north–northeastward direction toward eastern Africa, and feeding multiple plume stems in the upper mantle. However, the existence of this whole-mantle feature and its possible connection with Tertiary rifting are highly debated.The main rifting phases started diachronously along the MER in the Mio-Pliocene; rift propagation was not a smooth process but rather a process with punctuated episodes of extension and relative quiescence. Rift location was most probably controlled by the reactivation of a lithospheric-scale pre-Cambrian weakness; the orientation of this weakness (roughly NE–SW) and the Late Pliocene (post 3.2 Ma)-recent extensional stress field generated by relative motion between Nubia and Somalia plates (roughly ESE–WNW) suggest that oblique rifting conditions have controlled rift evolution. However, it is still unclear if these kinematical boundary conditions have remained steady since the initial stages of rifting or the kinematics has changed during the Late Pliocene or at the Pliocene–Pleistocene boundary.Analysis of geological–geophysical data suggests that continental rifting in the MER evolved in two different phases. An early (Mio-Pliocene) continental rifting stage was characterised by displacement along large boundary faults, subsidence of rift depression with local development of deep (up to 5 km) asymmetric basins and diffuse magmatic activity. In this initial phase, magmatism encompassed the whole rift, with volcanic activity affecting the rift depression, the major boundary faults and limited portions of the rift shoulders (off-axis volcanism). Progressive extension led to the second (Pleistocene) rifting stage, characterised by a riftward narrowing of the volcano-tectonic activity. In this phase, the main boundary faults were deactivated and extensional deformation was accommodated by dense swarms of faults (Wonji segments) in the thinned rift depression. The progressive thinning of the continental lithosphere under constant, prolonged oblique rifting conditions controlled this migration of deformation, possibly in tandem with the weakening related to magmatic processes and/or a change in rift kinematics. Owing to the oblique rifting conditions, the fault swarms obliquely cut the rift floor and were characterised by a typical right-stepping arrangement. Ascending magmas were focused by the Wonji segments, with eruption of magmas at surface preferentially occurring along the oblique faults. As soon as the volcano-tectonic activity was localised within Wonji segments, a strong feedback between deformation and magmatism developed: the thinned lithosphere was strongly modified by the extensive magma intrusion and extension was facilitated and accommodated by a combination of magmatic intrusion, dyking and faulting. In these conditions, focused melt intrusion allows the rupture of the thick continental lithosphere and the magmatic segments act as incipient slow-spreading mid-ocean spreading centres sandwiched by continental lithosphere.Overall the above-described evolution of the MER (at least in its northernmost sector) documents a transition from fault-dominated rift morphology in the early stages of extension toward magma-assisted rifting during the final stages of continental break-up. A strong increase in coupling between deformation and magmatism with extension is documented, with magma intrusion and dyking playing a larger role than faulting in strain accommodation as rifting progresses to seafloor spreading.     

The role of hydrothermal fluids in sedimentation in saline alkaline lakes: Evidence from Nasikie Engida,Kenya Rift Valley

Saline alkaline lakes that precipitate sodium carbonate evaporites are most common in volcanic terrains in semi‐arid environments. Processes that lead to trona precipitation are poorly understood compared to those in sulphate‐dominated and chloride‐dominated lake brines. Nasikie Engida (Little Magadi) in the southern Kenya Rift shows the initial stages of soda evaporite formation. This small shallow (<2 m deep; 7 km long) lake is recharged by alkaline hot springs and seasonal runoff but unlike neighbouring Lake Magadi is perennial. This study aims to understand modern sedimentary and geochemical processes in Nasikie Engida and to assess the importance of geothermal fluids in evaporite formation. Perennial hot‐spring inflow waters along the northern shoreline evaporate and become saturated with respect to nahcolite and trona, which precipitate in the southern part of the lake, up to 6 km from the hot springs. Nahcolite (NaHCO₃) forms bladed crystals that nucleate on the lake floor. Trona (Na₂CO₃·NaHCO₃·2H₂O) precipitates from more concentrated brines as rafts and as bottom‐nucleated shrubs of acicular crystals that coalesce laterally to form bedded trona. Many processes modify the fluid composition as it evolves. Silica is removed as gels and by early diagenetic reactions and diatoms. Sulphate is depleted by bacterial reduction. Potassium and chloride, of moderate concentration, remain conservative in the brine. Clastic sedimentation is relatively minor because of the predominant hydrothermal inflow. Nahcolite precipitates when and where pCO₂ is high, notably near sublacustrine spring discharge. Results from Nasikie Engida show that hot spring discharge has maintained the lake for at least 2 kyr, and that the evaporite formation is strongly influenced by local discharge of carbon dioxide. Brine evolution and evaporite deposition at Nasikie Engida help to explain conditions under which ancient sodium carbonate evaporites formed, including those in other East African rift basins, the Eocene Green River Formation (western USA), and elsewhere.     

Modern sediments and sedimentary processes in Lake Rudolf (Lake Turkana) eastern Rift Valley, Kenya

Tertiary sediments around Lake Rudolf (now Lake Turkana) in the East African Rift Valley have yielded abundant palaeontological and palaeoanthropological remains. The present study provides a basis for interpreting the ancient lake environment and furthering our knowledge of rift valley lacustrine deposits. Bottom sediments in Lake Rudolf are fine-grained (average 71% clay) well laminated and have montmorillonite, kaolinite and illite as the principal clay minerals. The sediments are relatively poor in silica (40–45%) but rich in Fe₂O₃ (10%). Both mineral proportions and chemical composition change systematically over the area of the lake and delineate four sedimentological provinces: (1) iron-rich, silty kaolinitic muds (Omo Delta); (2) iron-rich, fine-grained montmorillonite muds (North Basin); (3) silty montmorillonite muds rich in Na₂O and K₂O (Central Delta); and (4) argillaceous calcite silts (South Basin). Omo Delta and North Basin sediments are derived from the volcanics of the Ethiopian plateau; the source of Central Delta sediments is the Precambrian metamorphic terrain of the rift valley margin; the South Basin has a restricted detrital input. The water in the lake is alkaline (pH 9.2) and moderately saline (TDS = 2500 p.p.m.). Comparisons with influent water from the Omo River indicate a 200-fold concentration for the lake water. Models based on equilibrium between sediments and water column account for most of the non-conservative chemical components in the lake water. Sedimentation rates are high (about 1 m per 1000 years) and the dominance of detrital sediments makes Lake Rudolf unusual in comparison with other closed-basin lakes in the African Rift Valley although some similarities with ancient rift valley deposits are suggested.     

Phanerozoic within-plate magmatism of North Asia: Absolute paleogeographic reconstructions of the African large low-shear-velocity province

The phanerozoic within-plate magmatism of Siberia is reviewed. The large igneous provinces (LIPs) consecutively arising in the Siberian Craton are outlined: the Altai-Sayan LIP, which operated most actively 400–375 Ma ago, the Vilyui LIP, which was formed from the Middle Devonian to the Early Carboniferous, included; the Barguzin-Vitim LIP (305–275 Ma); the Late Paleozoic Rift System of Central Asia (318–250 Ma); the Siberian flood basalt (trap) province and the West Siberian rift system (250–247 Ma); and the East Mongolian-West Transbaikal LIP (230–195 Ma), as well as a number of Late-Mesozoic and Cenozoic rift zones and autonomous volcanic fields formed over the last 160 Ma. The trace-element and isotopic characteristics of the igneous rocks of the above provinces are reviewed; their mantle origin is substantiated and the prevalence of PREMA, EM2, and EM1 mantle magma sources are shown. The paleogeographic reconstructions based on paleomagnetic data assume that the Iceland hot spot was situated beneath the Siberian flood basalts 250 Ma ago and that the mantle plumes retained a relatively stable position irrespective of the movements of the lithospheric plates. At present, the Iceland hot spot occurs near the northern boundary of the African large low shear velocity province (LLSVP). It is suggested that the within-plate Phanerozoic magmatism of Siberia was related to the drift of the continent above the hot spots of the African LLSVP.     

Faulting of the lithosphere during extension and related rift-flank uplift: a numerical study

In this contribution, we present a new model of passive rifting and related rift-flank uplift. The numerical model is based on a lattice spring network coupled with a viscous particle model so that we can simulate visco-elasto-plastic behaviour with dynamic fault development. In our model, we show that rift-flank uplift can be achieved best when extension in the crust is localized and the lower crust is strong so that major rift faults transect the whole crust. Uplift of rift flanks follows a smooth function whereas down-throw in the rift basin takes place in steps. The geometry of the developing faults has also an influence on the uplift; in this case, displacement along major rift faults produces higher flanks than distributed displacement on many faults. Our model also shows that the relative elastic thickness of the crust has only a minor influence on the uplift since fault depth and elastic thickness are not independent. In addition, we show with a second set of simulations and analytically that a strain misfit between the upper and lower boundaries of a stretched crust, which is created by the horizontal extension, leads to an active uplift driven by elastic forces. We compare the numerical simulations, the analytical solution and real surface data from the Albertine rift in the East African Rift System and show that our new model can reproduce realistic features. Our two-layer beam model with strain misfit can also explain why a thick crust in the simulations can have an even higher rift flank than a thin crust even though the thin crust topography has a higher curvature. We discuss the implications of our simulations for real rift systems and for the current theory of rift-flank uplift.     

Crustal structure of the East African Rift zone

A review of seismological data on the crustal structure of the East African Rift zone is presented. The only refraction line is that along the Gregory Rift, which indicates a 7.5 km/sec refractor which is presumed to be the Moho. The bulk of data is provided by surface-wave dispersion studies. Some preliminary measurements of crustal and sub-Moho velocities using the University of Durham array at Kaptagat in Kenya are included.

There is now a growing body of evidence that the crust is generally of shield type over the whole rift zone. The exception is along the axis of the Gregory Rift, where a low-velocity Moho and some crustal modification is apparent. This is presumably the result of magma intrusions and suggests some crustal separation along this section of the rift. Sub-Moho velocities are probably normal outside the rifts themselves, though anomalously low upper-mantle velocities are to be associated with rifting. There is firm evidence for thinning of the lithosphere along the eastern branch of the rift. A cross-section of the Gregory Rift which is consistent with the current data is presented.     

Geodynamics of the Riphean stage in the evolution of the northern passive margin of the East European Craton

The nearly parallel northwest-trending Onega-Kandalaksha, Kerets-Leshukonsky, and Barents paleorift zones located in the northeastern part of the East European Platform are interpreted as a common structural assemblage that was formed in the Middle-Late Riphean as a result of horizontal extension of the continental margin. Therefore, it is reasonable to combine these paleorift structural features into the common White Sea Rift System instead of subdividing them into two or more systems as done previously. The White Sea Rift System originated owing to the breakup of the ancient Paleopangea supercontinent 1300–1240 Ma ago. The latter event occurred as a result of the divergence of the Baltia and Laurentia continental plates that most probably was caused by mantle spreading within the hot equatorial belt of the Earth. The diffuse rifting of that time occurred in the form of near-parallel rifts developing progressively from the inner part of the continental plate toward its margin. A pericratonic sedimentary basin eventually formed at the passive margin of Baltia as a system of roughly parallel rift zones. The geologic and geophysical data show that the passive margin of the East European Platform formed in the Riphean, a phenomenon that corresponds with a model of large-scale extension of the lithosphere after the stage of early ocean-floor spreading. In the course of this process, the brittle upper crust was detached from the ductile lower crust. The geodynamic regime of the Riphean passive margin of the East European Platform probably was similar to the regime of the present-day Atlantic-type passive margins. The White Sea Rift System differs from the transverse Mid-Russian Paleorift System both in origin and age. The Mid-Russian Paleorift System is considered to have formed in the Late Riphean as a result of transtension along a mobile zone in the ancient basement. The lithosphere of northeastern Fennoscandia has experienced horizontal extension since the Middle Riphean, a phenomenon that is closely related to the evolution of the White Sea Rift System, i.e., to the formation of the passive margin of the Baltia continent.     

The East African Rift System and the impact of orographic changes on regional climate and the resulting aridification

Several proxy data indicate an aridification of the East African climate during the Neogene, which might be influenced by the orographic changes of the East African Rift System (EARS) induced by tectonic forcing during the last 20 million years. To investigate the impact of the orography and especially of the rifts, the regional climate model CCLM is used, covering the EARS with Lake Victoria in the centre of the model domain. CCLM is driven by the ERA-Interim reanalysis and applied with a double-nesting method resulting in a very high spatial resolution of 7 km. The resolution clearly shows the shoulders and rifts of the western and eastern branch of the EARS and the Rwenzoris within the western branch. To analyse the orographic influence on climate, a new technique of modifying the orography is used in this sensitivity study. The shoulders of the branches are lowered and the rifts are elevated, resulting in a smoothed orography structure with less altitude difference between the shoulders and rifts. The changes in 2 m-temperature are very local and associated with the changes in the orography. The vertically integrated moisture transport is characterised by less vortices, and its zonal component is increased over the branches. The resulting amount of precipitation is mainly decreased west of the western branch and increased in the rift of the western branch. In the eastern branch, however, the changes in the amount of precipitation are not significant. The changes in the precipitation and temperature patterns lead to a shift of biomes towards a vegetation coverage characterised by more humid conditions in the northern part of the model domain and more arid conditions in the South. Thus, the aridification found in the proxy data can be attributed to the orographic changes of the rifts only in the northern model domain.     

Stratigraphic and structural evolution of the Blue Nile Basin,Northwestern Ethiopian Plateau

The Blue Nile Basin, situated in the Northwestern Ethiopian Plateau, contains ∼1400 m thick Mesozoic sedimentary section underlain by Neoproterozoic basement rocks and overlain by Early–Late Oligocene and Quaternary volcanic rocks. This study outlines the stratigraphic and structural evolution of the Blue Nile Basin based on field and remote sensing studies along the Gorge of the Nile. The Blue Nile Basin has evolved in three main phases: (1) pre‐sedimentation phase, include pre‐rift peneplanation of the Neoproterozoic basement rocks, possibly during Palaeozoic time; (2) sedimentation phase from Triassic to Early Cretaceous, including: (a) Triassic–Early Jurassic fluvial sedimentation (Lower Sandstone, ∼300 m thick); (b) Early Jurassic marine transgression (glauconitic sandy mudstone, ∼30 m thick); (c) Early–Middle Jurassic deepening of the basin (Lower Limestone, ∼450 m thick); (d) desiccation of the basin and deposition of Early–Middle Jurassic gypsum; (e) Middle–Late Jurassic marine transgression (Upper Limestone, ∼400 m thick); (f) Late Jurassic–Early Cretaceous basin‐uplift and marine regression (alluvial/fluvial Upper Sandstone, ∼280 m thick); (3) the post‐sedimentation phase, including Early–Late Oligocene eruption of 500–2000 m thick Lower volcanic rocks, related to the Afar Mantle Plume and emplacement of ∼300 m thick Quaternary Upper volcanic rocks. The Mesozoic to Cenozoic units were deposited during extension attributed to Triassic–Cretaceous NE–SW‐directed extension related to the Mesozoic rifting of Gondwana. The Blue Nile Basin was formed as a NW‐trending rift, within which much of the Mesozoic clastic and marine sediments were deposited. This was followed by Late Miocene NW–SE‐directed extension related to the Main Ethiopian Rift that formed NE‐trending faults, affecting Lower volcanic rocks and the upper part of the Mesozoic section. The region was subsequently affected by Quaternary E–W and NNE–SSW‐directed extensions related to oblique opening of the Main Ethiopian Rift and development of E‐trending transverse faults, as well as NE–SW‐directed extension in southern Afar (related to northeastward separation of the Arabian Plate from the African Plate) and E–W‐directed extensions in western Afar (related to the stepping of the Red Sea axis into Afar). These Quaternary stress regimes resulted in the development of N‐, ESE‐ and NW‐trending extensional structures within the Blue Nile Basin. Copyright © 2008 John Wiley & Sons, Ltd.     

Formation of silica oncoids around geysers and hot springs at El Tatio, northern Chile

Siliceous oncoids, up to 4 cm in diameter, are common on the laterally extensive sinter aprons that surround the spectacular geysers and hot springs at El Tatio in northern Chile. Many of these complex oncoids developed close to geyser and spring vents that discharge boiling water. Internally the oncoids, which are composed of precipitated amorphous silica, are formed of complex arrays of spicules and concentric laminae as well as detrital volcanic grains. Spicular growth is dominant in most examples. The formation and growth of the spicules and concentric laminae were mediated by a microbial community which included filamentous microbes, mucus, and possibly bacteria. The microbes and mucus were silicified by replacement and encrustation. In some laminae the filamentous microbes lay parallel to the growth surface; in other laminae most filaments forming the thin mats were suberect. Amorphous silica precipitated between the filaments occluded porosity and commonly disguised the microbial fabric. The oncoids grew on the proximal sinter aprons around the geyser vents and hot spring pools. Most growth took place subaerially with the silica delivered to the precipitation sites by splashing water from the geysers and/or periodic shallow flooding of the discharge aprons. Unlike silica oncoids at other geothermal sites, vertical growth of oncoids that formed in some rimstone pools was not limited by water depth.     

Middle Miocene to Pleistocene sedimentary record of rift evolution in the southern Albert Rift (Uganda)

This study presents an almost complete Middle Miocene to Pleistocene sequence of synrift sediments in the western branch of the East African Rift. The studied succession is exposed in several patches on an eastward tilted block between the northern tip of the Rwenzori Block and the eastern shoulder of the Albert Rift. In this position, it reaches a maximum thickness of 600 m of which 350 m have been logged systematically by analysing lithofacies and sediment architecture. Stratigraphic subdivision of the succession relies on published biostratigraphic data of endemic mollusc associations and their correlation across East Africa. The synrift sediments encountered are siliciclastics ranging from clay to coarse gravel with gypsum and ferrugineous interlayers or impregnations. Lithofacies and architectural analysis indicate alluvial plain, delta plain, nearshore, delta front, or lacustrine depositional environments. Based on the vertical stacking pattern, prograding and retrograding trends of the depositional environments, and climatic indicators (e.g. conservation of feldspar, gypsum, and/or iron hydroxide precipitation), four evolutionary phases can be distinguished: (i) a first phase between ca. 14.5 and 10.0 Ma is characterised by bedload-dominated fluvial environment with massive sandy to gravelly bedforms, feldspar-rich sands, rare iron impregnations and relatively low accommodation space. This phase is interpreted as pre- and early synrift sedimentation under a semiarid climate. (ii) From ca. 10.0 to 4.5 Ma predominantly fine-grained siliciclastics were deposited in a distal fluvial plain to lacustrine setting characterised by limited accommodation space. Fluctuation of thin beds, dominance of clay and frequent iron impregnations point to a more humid climate with seasonality and weak tectonic activity. (iii) During the third phase between 4.5 and 2.0 Ma delta plain and nearshore deposits with frequent ferrugineous impregnations and rich mollusc associations occurred, indicating a humid period with lake-level highstands and accelerated subsidence. (iv) During the final sedimentary interval between 2.0 and 1.5 Ma gravel units reoccurred with less iron- but more carbonate and gypsum impregnations, and arkosic sandstones. This phase recorded a general aridisation trend most probably caused by the upcoming rain barrier of the Rwenzori Mountains together with accelerated rift-flank uplift and strong subsidence of the rift floor. The results of this study are of particular importance for delineating key controls on sedimentation in the Albert Rift.     

Hydrothermal hydrocarbon gases: 2, Application in the East African Rift System

Hydrocarbon gases are ubiquitous in the hydrothermal systems of the East African Rift System (EARS), though often found at very low concentrations in the ‘volcanic’ eastern branch as compared to the ‘sedimentary’ western branch. Study of the chemical and isotopic compositions of these hydrocarbons from sites in Kenya, Ethiopia, Djibouti and Uganda reveals considerable homogeneity over hundreds of km of the various rift units. Consideration of C and He isotopic evidence points to a predominantly crustal thermogenic origin for the hydrocarbons, there being no evidence of mantle inputs in either the MORB or ‘hotspot’ sectors of the EARS. Temperature information from geothermal wells has been utilised to investigate the relationship between reservoir temperatures and ratios of CH₄ to C₂H₆. The general C₁/C₂ geothermometric relationship proposed in Part 1 of this study holds reasonably well, and is shown to give results equal to or better than the ‘inorganic’ gas geothermometers presently in use, both in the wellfields and undeveloped high-enthalpy geothermal areas. Results from low-enthalpy hot spring systems are less well correlated with apparent deep temperatures, but consistent with data from similar systems elsewhere in the world.