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 approximately 4,500 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. Optimising 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.

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. 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% 4He. 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 CO2. 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 CO2 gases will likely also be efficient for helium accumulation on the timescale of the Rukwa basin activity.The Rukwa Basin prospective recoverable P50 resources of helium have been independently estimated to be about 138 billion standard cubic feet (2.78 x 109 m3 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

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.

The first Miocene fossils from coastal woodlands in the southern East African Rift

The Miocene is a key time in the evolution of African mammals and their ecosystems witnessing the origin of the African apes and the isolation of eastern coastal forests through an expanding biogeographic arid corridor. Until recently, however, Miocene sites from the southeastern regions of the continent were unknown. Here we report discovery of the first Miocene fossil teeth from the shoulders of the Urema Rift in Gorongosa National Park, Mozambique, at the southern East African Rift System. We provide the first 1) radiometric age determinations of the fossiliferous Mazamba Formation, 2) reconstructions of past vegetation in the region based on pedogenic carbonates and fossil wood, and 3) description of fossil teeth from the southern rift. Gorongosa is unique in the East African Rift System in combining marine invertebrates, marine vertebrates, terrestrial mammals, and fossil woods in coastal paleoenvironments. The Gorongosa fossil sites offer the first evidence of persistent woodlands and forests on the coastal margins of southeastern Africa during the Miocene, and an exceptional assemblage of fossil vertebrates including new species. Further work will allow the testing of hypotheses positing the formation of a northeast-southwest arid corridor isolating species on the eastern coastal forests from those elsewhere in Africa.

Triassic trachytic volcanism in the Bangong–Nujiang Ocean: geochemical and geochronological constraints on a continental rifting event

The Bangong–Nujiang suture zone (BNSZ), which separates the Gondwana-derived Qiangtang and Lhasa terranes, preserves limited geological records of the Bangong–Nujiang Ocean (BNO). The timing of opening of this ocean has been hotly debated due to the rare and complicated rock records in the suture zones, which span over 100 Ma from Carboniferous–Permian to Early Jurassic time, based on geological, palaeontological and palaeomagnetic data. A combination of geochemical, geochronological and isotopic data are reported for the Riasairi trachytes, central BNSZ, northern Tibet, to constrain its petrogenesis and tectonic settings. Zircon U–Pb dating by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yields mean ages of 236 Ma. Geochemically, these rocks are high-K calc-alkaline with moderate SiO2 (59.1–67.5 wt%) and high K2O + Na2O (8.1–11.6 wt%) contents. They are enriched in light rare earth elements with negative Eu anomalies, and show enrichments in high-field-strength elements with positive ‘Nb, Ta’ anomalies, similar to the intra-continental rift setting-related felsic lavas from the African Rift System. The high positive zircon ϵHf(t) and bulk ϵNd(t) values, as well as high initial Pb isotopes, imply a heterogeneous source involving both asthenospheric and subcontinental lithospheric mantle. The field and geochemical data jointly suggest that the Riasairi trachytes within the Mugagangri Group were formed in a continental rift setting. We interpret that the continental-rift-related Riaisairi trachytic lavas as derived from the southern margin of the Qiangtang terrane, implying that the BNO would have opened by Middle Triassic time, well after the commonly interpreted break-up of the Qiangtang terrane from Gondwana.

Volcanic activity and hazard in the East African Rift Zone

Over the past two decades, multidisciplinary studies have unearthed a rich history of volcanic activity and unrest in the densely-populated East African Rift System, providing new insights into the influence of rift dynamics on magmatism, the characteristics of the volcanic plumbing systems and the foundation for hazard assessments. The raised awareness of volcanic hazards is driving a shift from crisis response to reducing disaster risks, but a lack of institutional and human capacity in sub-Saharan Africa means baseline data are sparse and mitigating geohazards remains challenging.