East African Rift Research Articles

Phylogenetic diversity of Actinobacteria from Momela soda lakes, Arusha National Park, Tanzania

The Momela soda lakes consist of seven small, hypersaline, alkaline lakes, situated in the East African rift valley at Arusha National Park, Tanzania. The lakes are fed by separate underground water sources with slightly varying mineral contents resulting in colour variation and supporting different kinds of prokaryotic and eukaryotic species. In this study, the diversity of Actinobacteria in surface water and sediments of five Lakes were investigated using culture-dependent and culture-independent molecular techniques. A total of 34 out of 112, and 13 out of 85, representatives of Actinobacteria isolates and clones, respectively, were selected for gene sequencing using the CD-HIT program. Analysis of their 16S rRNA gene sequences displayed the presence of species affiliated to 15 different genera, namely Mycobacterium, Rhodococcus, Microbacterium, Isoptericola, Dietzia, Leucobacter, Jonesia, Nesterenkonia, Micrococcus, Streptomyces, Hoyosella, Norcadiopsis, Cellulomonas, Bogoriella, and Agromyces. The results showed 5 and 12 putative new Actinobacteria isolates and clones, respectively. This is the first report of isolation of bacteria from the genus Mycobacterium from a soda lake globally, as well as the genera Hoyosella, Isoptericola, Jonesia, Micrococcus, Leucobacter and Agromyces from a soda lake in East Africa. Because Actinobacteria are known as a source of biotechnologically important compounds, the species revealed set a platform to search for novel bioactive compounds.

A Comprehensive Methodology for Monitoring Evaporitic Mineral Precipitation and Hydrochemical Evolution of Saline Lakes: The Case of Lake Magadi Soda Brine (East African Rift Valley, Kenya).

Lake Magadi, East African Rift Valley, is a hyperalkaline and saline soda lake highly enriched in Na+, K+, CO32–, Cl–, HCO3–, and SiO2 and depleted in Ca2+ and Mg2+, where thick evaporite deposits and siliceous sediments have been forming for 100 000 years. The hydrogeochemistry and the evaporite deposits of soda lakes are subjects of growing interest in paleoclimatology, astrobiology, and planetary sciences. In Lake Magadi, different hydrates of sodium carbonate/bicarbonate and other saline minerals precipitate. The precipitation sequence of these minerals is a key for understanding the hydrochemical evolution, the paleoenvironmental conditions of ancient evaporite deposits, and industrial crystallization. However, accurate determination of the precipitation sequence of these minerals was challenging due to the dependency of the different hydrates on temperature, water activity, pH and pCO2, which could induce phase transformation and secondary mineral precipitation during sample handling. Here, we report a comprehensive methodology applied for monitoring the evaporitic mineral precipitation and hydrochemical evolution of Lake Magadi. Evaporation and mineral precipitations were monitored by using in situ video microscopy and synchrotron X-ray diffraction of acoustically levitated droplets. The mineral patterns were characterized by ex situ Raman spectroscopy, X-ray diffraction, and scanning electron microscopy. Experiments were coupled with thermodynamic models to understand the evaporation and precipitation-driven hydrochemical evolution of brines. Our results closely reproduced the mineral assemblages, patterns, and textural relations observed in the natural setting. Alkaline earth carbonates and fluorite were predicted to precipitate first followed by siliceous sediments. Among the salts, dendritic and acicular trona precipitate first via fractional crystallization—reminiscent of grasslike trona layers of Lake Magadi. Halite/villiaumite, thermonatrite, and sylvite precipitate sequentially after trona from residual brines depleted in HCO3–. The precipitation of these minerals between trona crystals resembles the precipitation process observed in the interstitial brines of the trona layers. Thermonatrite precipitation began after trona equilibrated with the residual brines due to the absence of excess CO2 input. We have shown that evaporation and mineral precipitation are the major drivers for the formation of hyperalkaline, saline, and SiO2-rich brines. The discrepancy between predicted and actual sulfate and phosphate ion concentrations implies the biological cycling of these ions. The combination of different in situ and ex situ methods and modeling is key to understanding the mineral phases, precipitation sequences, and textural relations of modern and ancient evaporite deposits. The synergy of these methods could be applicable in industrial crystallization and natural brines to reconstruct the hydrogeochemical and hydroclimatic conditions of soda lakes, evaporite settings, and potentially soda oceans of early Earth and extraterrestrial planets.

Distributed Extension Across the Ethiopian Rift and Plateau Illuminated by Joint Inversion of Surface Waves and Scattered Body Waves

AbstractThe East African Rift System provides a rare location in which to observe a wide scope of rifting states. Well‐defined active narrow rifting in the Main Ethiopian Rift (MER) transitions to incipient extension and eventually pre‐rifted lithosphere through the northwestern flank of the Ethiopian Plateau (EP). Although the MER is well studied, the off‐axis region has received less attention. We develop Rayleigh wave phase velocity maps, Ps receiver functions, and H‐κ stack surfaces, and jointly invert these data using a trans‐dimensional, hierarchical Bayesian inversion algorithm to create shear velocity profiles across the MER and EP. All shear velocities observed are slower than the PREM global average, a reflection of the elevated temperatures that persist from plume impingement. In the EP, we find a shallow mantle slow shear velocity lineament parallel to the MER axis, amidst otherwise faster shear velocities. The crust is shallow in the MER, and also in the northwestern‐most EP flank. Thicker crust found elsewhere throughout the plateau is caused by crustal underplating and flood basalt emplacement. Shear velocities more reduced than the already low regional average, in concert with surficial volcanic features, geodetic observations, and slow P‐ and S‐wave anomalies, support off‐axis extension in the Ethiopian plateau, requiring reevaluation of the localization of continental breakup in the narrow MER.

Mid‐Pleistocene to Recent Crustal Extension in the Inner Graben of the Northern Kenya Rift

AbstractMagmatic continental rifts often constitute nascent plate boundaries, yet long‐term extension rates and transient rate changes associated with these early stages of continental breakup remain difficult to determine. Here, we derive a time‐averaged minimum extension rate for the inner graben of the Northern Kenya Rift (NKR) of the East African Rift System for the last 0.5 m.y. We use the TanDEM‐X science digital elevation model to evaluate fault‐scarp geometries and determine fault throws across the volcano‐tectonic axis of the inner graben of the NKR. Along rift‐perpendicular profiles, amounts of cumulative extension are determined, and by integrating four new 40Ar/39Ar radiometric dates for the Silali volcano into the existing geochronology of the faulted volcanic units, time‐averaged extension rates are calculated. This study reveals that in the inner graben of the NKR, the long‐term extension rate based on mid‐Pleistocene to recent brittle deformation has minimum values of 1.0–1.6 mm yr−1, locally with values up to 2.0 mm yr−1. A comparison with the decadal, geodetically determined extension rate reveals that at least 65% of the extension must be accommodated within a narrow, 20‐km‐wide zone of the inner rift. In light of virtually inactive border faults of the NKR, we show that extension is focused in the region of the active volcano‐tectonic axis in the inner graben, thus highlighting the maturing of continental rifting in the NKR.

Characterising the contemporary stress orientations near an active continental rifting zone: A case study from the Moatize Basin, central Mozambique

AbstractThis paper presents the first comprehensive analysis of the present‐day stress from boreholes near an active continental rifting zone in the Moatize Basin, Mozambique. The state of present‐day stress in this area that is located ca. 100 km away from the Eastern African Rift System (EARS) is poorly understood, and most of our knowledge is from earthquake focal mechanisms that provide stress information from the deeper part of the lithosphere, and to a lesser extent from surface geological features in the Malawi region. Considering the limited reliability of earthquake‐derived stress orientations near plate boundaries, poor coverage of low to moderate magnitude earthquakes in eastern Africa, and ambiguity about the latest activity of geological structure; other well‐established methods are required to shed light on the active tectonics of EARS. In this study, we analyse stress orientation using log data from 95 vertical boreholes in a mine site to investigate the neotectonic stress pattern of the region. Analysis of 17.9 km of televiewer logs resulted in interpretation of 1188 stress‐related borehole failures. The results indicate a mean regional trend of 045° ± 31° for the maximum horizontal stress (SHmax). Our investigation reveals that the regional state of stress in the study area is controlled by superposition of stress sources that act at very different spatial scales. The consistency between our results and predictions by plate‐scale geodynamic models of stress orientations confirm that the regional pattern of stress in this area is mainly controlled by first (>500 km) and second (distances between 500 and 100 km) order stress sources (i.e., large tectonic forces and lateral density variations). However, high‐resolution data used in this study reveal that third (between 100 and 1 km) and fourth (<1 km) order stress sources from stiffness contrasts, rock fabric and geological structures have a great impact on the stress perturbations at smaller and local scales.

A first regional model of the past Earth's magnetic field from Africa for the last 4000 years

The understanding of the Earth's magnetic field variations over time on the African continent is fundamental for several reasons. For instance, the most important feature of the present geomagnetic field, the South Atlantic Anomaly (SAA) characterized by weaker geomagnetic strength values than those expected for their latitudes, may have emerged in South Africa at the beginning of the second millennium CE. Here, we first selected the available volcanic and archaeomagnetic data following a set of three criteria inspired by the FAIR principles. We then built a first regional geomagnetic model for Africa covering the last 4000 years, using a revised version of the spherical cap harmonic (SCH) analysis in 2 dimensions. The new regional model shows, at the Earth's surface, the westward migration of the SAA from the Indian Ocean over Africa since 1100 CE. In addition, the regional model is tested as a paleomagnetic dating tool by re-dating previous archaeomagnetic data from Africa and thus can be used to date other African archeological sites and the numerous active and dormant volcanoes of the East African System. Plain language summaryThe spatial and temporal evolution of the Earth's magnetic (geomagnetic) field is based on direct historical or instrumental data and the indirect information extracted from rocks and archeological artifacts (palaeo- and archaeomagnetic data). While the present field is well sampled at both spatial and temporal domains, this is not the case for the past evolution of the geomagnetic field, where the paleomagnetic data are not globally distributed. There are large areas in the Southern Hemisphere yet to be investigated. However, recent data from Africa enables us the building of the first regional geomagnetic model for the last 4000 years. The new regional model sheds light on the past evolution of the most remarkable feature of the present geomagnetic field, the South Atlantic Anomaly (SAA), showing details of the westward migration of the SAA from the Indian Ocean over Africa since 1100 CE. Moreover, the new model refines the use of paleomagnetism as a dating tool for volcanic units from East African Rift System and for the abundant African archeological sites improving the knowledge of the history of human civilizations.

Reconstructing the Environmental Context of Human Origins in Eastern Africa Through Scientific Drilling

Paleoanthropologists have long speculated about the role of environmental change in shaping human evolution in Africa. In recent years, drill cores of late Neogene lacustrine sedimentary rocks have yielded valuable high-resolution records of climatic and ecosystem change. Eastern African Rift sediments (primarily lake beds) provide an extraordinary range of data in close proximity to important fossil hominin and archaeological sites, allowing critical study of hypotheses that connect environmental history and hominin evolution. We review recent drill-core studies spanning the Plio–Pleistocene boundary (an interval of hominin diversification, including the earliest members of our genus Homo and the oldest stone tools), and the Mid–Upper Pleistocene (spanning the origin of Homo sapiens in Africa and our early technological and dispersal history). Proposed drilling of Africa's oldest lakes promises to extend such records back to the late Miocene. ▪ High-resolution paleoenvironmental records are critical for understanding external drivers of human evolution. ▪ African lake basin drill cores play a critical role in enhancing hominin paleoenvironmental records given their continuity and proximity to key paleoanthropological sites. ▪ The oldest African lakes have the potential to reveal a comprehensive paleoenvironmental context for the entire late Neogene history of hominin evolution.

The principles of helium exploration

Commercial helium systems have been found to date as a serendipitous by-product of petroleum exploration. There are nevertheless significant differences in the source and migration properties of helium compared with petroleum. An understanding of these differences enables prospects for helium gas accumulations to be identified in regions where petroleum exploration would not be tenable. Here we show how the basic petroleum exploration playbook (source, primary migration from the source rock, secondary longer distance migration, trapping) can be modified to identify helium plays. Plays are the areas occupied by a prospective reservoir and overlying seal associated with a mature helium source. This is the first step in identifying the detail of helium prospects (discrete pools of trapped helium). We show how these principles, adapted for helium, can be applied using the Rukwa Basin in the Tanzanian section of the East African Rift as a case study. A thermal hiatus caused by rifting of the continental basement has resulted in a surface expression of deep crustal gas release in the form of high-nitrogen gas seeps containing up to 10% 4 He. We calculate the total likely regional source-rock helium generative capacity, identify the role of the Rungwe volcanic province in releasing the accumulated crustal helium and show the spatial control of helium concentration dilution by the associated volcanic CO 2 . Nitrogen, both dissolved and as a free-gas phase, plays a key role in the primary and secondary migration of crustal helium and its accumulation into what might become a commercially viable gas pool. This too is examined. We identify and discuss evidence that structures and seals suitable for trapping hydrocarbon and CO 2 gases will likely also be efficient for helium accumulation on the timescale of the Rukwa Basin activity. The Rukwa Basin prospective recoverable P 50 resources of helium have been independently estimated to be about 138 BSCF (billion standard cubic ft: 2.78 × 10 9 m 3 at STP). If this volume is confirmed it would represent about 25% of the current global helium reserve. Two exploration wells, Tai 1 and Tai 2, completed by August 2021 have proved the presence of seal and reservoir horizons with the reservoirs containing significant helium shows. This article is part of the Energy Geoscience Series available at https://www.lyellcollection.org/cc/energy-geoscience-series

Some of the geological challenges and opportunities associated with the dynamics of the Cenozoic East African Rift System

The Cenozoic East African Rift System (EARS) is the largest continental rift valley system on Earth. Extending over a total distance of c. 4500 km, and with an average width of about 50 km, it is home to some of East Africa's largest urban populations and some of its most important transport, energy and water supply infrastructure. Rifting commenced during the Early Miocene and crustal extension has continued to the present day, posing seismic and volcanic hazards throughout its history of human occupation. Deep-seated landslides also present significant challenges for public safety, land management and infrastructure development on the flanks of rift margins. The rift floor itself poses a range of geohazards to community livelihood and engineering infrastructure, including ground fissuring and cavity collapse, flooding and sedimentation. On the positive side, the development of the EARS has created hydrocarbon and geothermal energy resources, and geomaterials for use as aggregates and cement substitutes in road and building construction. Optimizing the use of these resources requires careful planning to ensure sustainability, while land use management and infrastructure development must take full consideration of the hazards posed by the ground and the fragility and dynamism of the human and physical environment.

A multi-isotope and modelling approach for constraining hydro-connectivity in the East African Rift System, southern Ethiopia

During the last African Humid Period (AHP; 15–5 ka), many lakes in the East African Rift System (EARS) experienced pronounced lake-level variations that dramatically transformed the hydrological landscape. Currently dry, saline or marshy-wetland terminal lakes became vast waterbodies, interconnected via overflow sills resulting in the formation of a several thousand-kilometre-long chain of lakes in the EARS. A quantitative, process-based understanding of these hydrological systems can advance our interpretation of past hydroclimate variability from proxy records. Here, we provide a critical modern hydrological dataset for the data-sparse Lake Chew Bahir basin in southern Ethiopia. Driven by modern data, an isotope-enabled hydro-balance model was developed to assess how increases in rainfall modulate δ18O and 87Sr/86Sr variability. Considering a terminal Lake Chew Bahir scenario, humid conditions resulted in higher lake δ18O (∼+14‰) due to increased evaporation and longer water residence times. At the same time 87Sr/86Sr decreased from 0.7064 to 0.7061 due to an increased riverine Sr flux characterised by lower, unradiogenic 87Sr/86Sr ratios. In a modelling scenario where Lake Chew Bahir became a flow-through system with interconnectivity between lakes Abaya, Chamo, Chew Bahir and Turkana, higher lake δ18O (∼+12‰) relative to present was found, but δ18O was lower than in the terminal lake scenario. The lake water 87Sr/86Sr ratios (<0.7061) were also slightly lower. A moderate concomitant change in rainfall input δ18O of −1‰ in step with hydrological reorganisation resulted in the lowest lake δ18O (∼+5‰). Modelled δ18O values were similar to the δ18O range of endogenic carbonates from sedimentary cores from Lake Chew Bahir at the onset of the AHP, supporting the validity of our model, and suggesting that evaporation and the lake water residence time strongly influence lake water δ18O. However, the reported 87Sr/86Sr of fossil carbonates from Lake Chew Bahir during the AHP (0.7065–0.7060) could not be reproduced by our modelled scenarios without adjusting the surface-water-to-groundwater ratio, highlighting the potential role of groundwater as a water source in semi-arid regions. These results demonstrate the insights that can be gained from applying a process-based approach using O and Sr isotope hydro-balance modelling to aid interpretation of past hydro-balance and lake interconnectivity from lacustrine sedimentary records.

The unexpected Mayotte 2018–2020 seismic sequence: a reappraisal of the regional seismicity of the Comoros

The Mayotte seismic sequence that started on 10 May 2018, with a main shock of magnitude Mw 5.9 on May 15, followed by a major offshore volcanic activity, raises several questions of seismovolcanic hazards in the Comoros region. The unexpected size and duration of the crisis is an opportunity to reassess the distribution and magnitude of the seismicity near Mayotte Island, but also regionally. We present a comprehensive seismicity catalogue of the region including the Mozambique Channel, the Mozambique coast and Madagascar, based partly on previously published data in the region and partly on unpublished data from local catalogues for the Comoros and Madagascar. Our catalogue extends from 1900 onward with a completeness of magnitude 5.5 until 1980s and decreasing only recently to 4.5 after 2010. It comprises the events of magnitude Mlv ≥3.5 for the seismic sequence of Mayotte from May 2018 to October 2020 as the crisis is still ongoing. Present knowledge of the seismicity, largely partial in distribution and magnitude before 1980, makes the seismic sequence of 2018-2020 an exceptional and unprecedented seismo-volcanic event in the region. We discuss the distribution of seismicity in time and space within the context of the south eastward propagation of the East African rift system towards Madagascar.

Sediment provenance and silicic volcano-tectonic evolution of the northern East African Rift System from U/Pb and (U-Th)/He laser ablation double dating of detrital zircons

Detrital zircons from two major rift basins within the East African Rift System (EARS) provide a means to evaluate not only sediment provenance and landscape dynamics in sedimentary basins, but also the timing of the silicic volcano-tectonic evolution of the rift system. We sampled from drill cores collected by the Hominin Sites and Paleolakes Drilling Project (HSPDP) in Ethiopia and Kenya to study the detrital mineral records of the Northern Awash (NA; 3.3–2.9 Ma) and West Turkana (WTK; 1.9–1.4 Ma) drill cores. We performed (U-Th)/He and U/Pb analyses on detrital zircons using single crystal laser ablation double dating (LADD) techniques. Analyses of four NA samples yielded zircon 206Pb/238U dates younger than ∼45 Ma, consistent with derivation from silicic volcanic rocks associated with EARS activity. Most of these samples lack zircon 206Pb/238U dates from ∼22–13 Ma, due to a decrease in silicic volcanism and a watershed configuration limiting delivery of silicic source materials to the sample site. NA zircon 206Pb/238U dates imply a sedimentary source from the western Afar margin, with a transition to more localized sediment reworking within the Afar Depression after a major regional tectonic reorganization and formation of a disconformity at ∼2.9 Ma. The WTK sample yielded many zircons with Cenozoic 206Pb/238U dates similar to those from the NA core, but the WTK sample also sources a small population of Neoproterozoic zircons associated with rocks from the Mozambique Belt and reworked sedimentary deposits. Despite being recorders of predominantly silicic activity, the detrital zircon U/Pb dates from both drill sites track the established timing of major volcanic phases in the EARS. A subset of zircons from both sites has concordant 206Pb/238U and (U-Th)/He dates, indicating a short duration between zircon crystallization and eruption of the host volcanic rock, but the majority of zircon (U-Th)/He dates are significantly younger than the 206Pb/238U dates for the same zircon. Some (U-Th)/He dates are even younger than the depositional age of the sedimentary sample from which it was collected. The observed spread in zircon (U-Th)/He dates likely reflects partial resetting associated with late mafic volcanism and/or hydrothermal activity within this dynamic rift environment.

Late-Holocene paleomagnetic secular variation records from Lake Turkana, East Equatorial Africa

We have carried out a paleomagnetic study of three piston cores collected from Lake Turkana. The goal is to recover a Holocene paleomagnetic secular variation (PSV) record for this lake and to correlate it with other Holocene PSV records from the East Africa Rift Valley (EARV). All three cores were sampled with u-channels and magnetic measurements of magnetic susceptibility, the natural remanence (NRM), and two artificial remanences, anhysteretic remanence (ARM) and saturation isothermal remanence (SIRM), were made on them. The remanences were routinely step-wise demagnetized and measured at 10 mT steps up to 60 mT. The NRMS had a simple pattern of demagnetization with a characteristic direction (ChRM) removed between 10 and 60 mT. ARM and SIRM demagnetization indicated that the magnetic grains were relatively soft with median destructive fields (MDF) less than 30 mT. We interpret the magnetic grains to be multi-domain (silt-sized) magnetite/titanomagnetite. The resulting magnetic records of all three cores could be correlated. A chronology for these cores was determined from four radiocarbon dates on core 4P. We also estimated the sediment ages by correlating the PSV to two other well-dated PSV records from the same region, Lakes Malawi and Victoria. PSV age estimation indicates that the radiocarbon dates are about 500 year too old; a correction for that offset causes three of the four radiocarbon ages to become consistent with the PSV age estimates. The final composite Lake Turkana PSV record for the last ~4000 years is the highest resolution directional record of PSV (~200 cm/ky) ever to be recovered from Africa.

Thermal structure of the African continent based on magnetic data: Future geothermal renewable energy explorations in Africa

The defractal method was applied to the observed magnetic data and was used to estimate Curie point depth (CPD) i.e., depth to the base of magnetic sources (DBMS), near-surface geothermal gradient (GG), surface heat production and bulk crust thermal conductivity of Africa. A range of window sizes was tested using different values of the fractal parameter (α). The calculated DBMS ranges from 19 km to 112 km in Africa, and the GG varies from 15.9 °C/km to 30 °C/km (averaging 25 °C/km). Surface heat production values vary between 0.2 μW/m2 to 5.6 μW/m2, averaging 2.5 μW/m2. The calculated bulk crust thermal conductivity ranges from 1.4 W/mK to 3.4 W/mK with an average of 2.6 W/mK. In general, shallow DBMS, high GG, and high surface heat production coincide with known geothermal, volcanic, and seismic regions in Africa. Shallow DBMS, high GG, and high surface heat production were encountered in southeastern Africa, northeastern Africa (Egypt and Red Sea), and northern Africa (Hoggar region). High bulk crust thermal conductivity values are encountered in northern Africa (Algeria, Morocco, Libya), southeastern Africa and the East African Rift (EAR) regions.

Biogeographical Importance of the Livingstone Mountains in Southern Tanzania: Comparative Genetic Structure of Small Non-volant Mammals

The Livingstone Mountains (LM; also known as the Kipengere Range) found in south-western Tanzania at the northern end of Lake Nyasa are an important region for understanding the biogeography of Eastern Africa. The two branches of the East African Rift Valley meet here and the mountains might represent stepping stones for colonization and migration between different parts of the Eastern Afromontane Biodiversity Hotspot (especially the link between the Eastern Arc Mountains, EAM, and the Southern Rift Mountains, SRM), as well as an efficient barrier to gene flow for taxa living in drier savannahs in lower elevations. Here we combine new mitochondrial sequence data from 610 recently sampled rodents and shrews with available georeferenced genetic data (3538 specimens) from southern Tanzania, northern Malawi/Zambia and northern Mozambique and compare the spatial genetic structure among different taxa. There is no universal phylogeographic pattern in taxa preferring humid montane habitats. For some of them, the Makambako Gap acts as a barrier between the SRM and the EAM, but other taxa can bridge this gap. Barriers within the EAM (frequently) and within the SRM (sometimes) appear more important. The Rukwa rift between the SRM and the ARM is an important barrier that perhaps can only be crossed by taxa that are not that strictly tied to humid montane environments. For mammals living in lower-elevation savannah-like habitats, the LM can act as a strict barrier to gene flow, and together with the Ufipa Plateau, Lake Nyasa and the EAM create a very similar phylogeographic pattern with three recognizable genetic groups in most savannah-dwellers. The Livingstone Mountains thus appear to be one of the most important biogeographic crossroads in Eastern Africa.

Preliminary Seismic Hazard Analyses for the Ugandan Region

Uganda is situated between the two seismically active branches of the East African Rift Valley System, which are characterized by high levels of seismicity. A probabilistic approach has been used to assess the seismic hazard for Uganda and the surrounding areas. A probabilistic seismic hazard analysis requires the availability of an earthquake catalog, relevant ground motion prediction equations, and an outline of how the hazard calculations will be conducted. Using online sources, an earthquake catalog for Uganda and the immediate areas around Uganda was compiled spanning 108 years, from 1912 to 2020. This catalog was homogenized to moment magnitude to match with the selected ground motion prediction equations from Toro and Idriss. A logic tree accounting for the two ground motion prediction equations and dividing the study region into four seismic zones was used for calculating the seismic hazard. As an example, the seismic hazard results at two sites close to each other showed how different seismic hazards can be. Results from the probabilistic seismic hazard analyses was expressed through seismic hazard maps for peak ground acceleration at 10% probability of exceedance in 5, 10, 20, 50, 100 and 500 years, corresponding to return periods of 50, 100, 200, 500, 1000 and 5000 years, respectively. The seismic hazard map for 10% probability of exceedance in 5 years calculated PGAs from 0.02 to 0.10 g and 0.10 to 0.27 g outside of and within the western branch of the East African Rift Valley System, respectively. The estimated PGAs from previous studies at a similar probability of exceedance level are within the range of these findings, although the ranges calculated herein are wider.

Contribution to the Geothermal Exploration of Eburru Field

With the increasing global demand for clean energy, the use of geothermal energy is expected to increase in the future. The East African countries, especially those crossed by the East African Rift System, have a significant potential in geothermal energy. This paper comes therefore to bring a contribution in the exploration of this clean energy in Africa especially in the region of Eburru in Kenya. Eburru is one of the geothermal fields in Kenya, with the estimated capacity of around 250Mwe. This paper presents the results and interpretations of the processing and the analysis of some geophysical survey data especially gravity, geochemical data and geological data acquired during our field survey. For the geology studies, a geological map showing structures trending in North-South direction and also several geothermal manifestations like fumaroles, hot ground and craters were mapped. From the geophysical studies, a heat source located at 1500 meters below masl and the reservoir is estimated between 1000 above masl to 1000 below masl. The cap rock of this prospect is between 1000 to 2000 meters above masl and the density increases from the West to the East of the prospect. The interpretation of the fluids geothermometers gave the calculated temperature between 255-270 Celsius degree. By combining and interpreting the different results obtained, we found that the Eburru, with the reservoir temperatures ranged between 255-270°C, has the potential to provide geothermal energy for various utilization.

Towards a Volumetric Variation of Lake Kivu Gases by the Nyamulagira and Nyiragongo Volcanoes

After the volcanic eruption of January 17th, 2002, Goma Volcano Observatory (OVG) initiated carbon dioxide (CO2) monitoring in the fractures south of the Nyiragongo volcano and at the northern edge of Lake Kivu. During the period from March 17th, 2017 to June 10th, 2020, twice a week, a GA 5000 gasometer was used for CO2 in the fractures as well as in the mazukus, respectively, as part of a project funded by the Kingdom of the Netherlands through the Lake Kivu Monitoring Program (LKMP) in Rwanda. The level of volcanic activity was determined by the volume of gas emitted by the volcano and measured by a DOAS and the seismicity measured by a network of 15 seismometers managed by the OVG. The results show that the concentration of CO2 in the active fractures from recent Nyiragongo eruptions varies with volcanic activity and the occurrence of a large earthquake in the East African Rift; but in the mazukus there is almost no relationship between CO2 content and volcanic activity. The study also shows that the influence of carbon dioxide in mazukus on the carbon dioxide into the Lake Kivu waters remains a major research question. The purpose of this study is to establish the relationship between the existing Carbon Dioxide (mazukus) in the Northern watershed and that of the deep waters of Lake Kivu.

Fault Controlled Geometries by Inherited Tectonic Texture at the Southern End of the East African Rift System in the Makgadikgadi Pans, Northeastern Botswana

Fault Controlled Geometries by Inherited Tectonic Texture at the Southern End of the East African Rift System in the Makgadikgadi Pans, Northeastern Botswana

Evidence from gas-rich ultramafic xenoliths for Superplume-derived recycled volatiles in the East African sub-continental mantle

Volatile-rich fluids are believed to play a key role in the metasomatic enrichment of the East African sub-continental lithospheric mantle (SCLM), but limited data is available on key volatile tracers such as CO2 and N2. We report new CO2 and N2 isotope and relative abundance data in exceptionally gas-rich mantle peridotite and pyroxenite xenoliths from nine localities throughout the eastern branch of the East African Rift System (EARS). Importantly for our study, the xenolith localities straddle the transition between continental and oceanic lithosphere. In addition, we report major and trace element contents together with oxygen isotopes on host crystals and bulk multiple sulfur isotope data on select pyroxene crystals from the same xenolith suite. These new data are interpreted alongside previously published He, Ne and Ar isotope and relative abundance data.Pyroxenite xenoliths formed from the infiltration and crystallization of mafic melts in the lithospheric mantle. This was followed by fluids fracturing through the pyroxenites leaving gas-rich fluid inclusions. The elemental and isotope systematics of the fluid inclusion-hosted volatile species (He, Ar, N, CO2) within the mantle xenoliths are consistent with the enrichment of the EARS SCLM by CO2-rich mantle fluids from subducted carbonate-rich material. Such CO2 enrichments (CO2/3He > 7 × 109, δ13C ~0‰) are also associated with positive δ15N values (as high as +3.4‰), reinforcing the link between the metasomatic fluids and subduction of hydrothermally altered oceanic crust. Recycled signatures are also consistent with oxygen and sulfur isotope compositions of host crystals, which are distinct from compositions typical for the depleted upper MORB-source mantle.The occurrence of recycled volatile signatures in the EARS SCLM is widespread and must therefore be associated with a large-scale mantle process currently supplying material to EARS magmas. Notably, mantle xenoliths with high 3He/4He from the Ethiopian Rift are also associated with δ15N and δ13C values similar to the composition of sediments. This suggest that recycled materials have been entrained within the deep-seated African Superplume, which supplies the EARS with both primordial and recycled volatiles.