East African Rift Research Articles

ICDP workshop on the Deep Drilling in the Turkana Basin project: exploring the link between environmental factors and hominin evolution over the past 4 Myr

Abstract. Scientific drill cores provide unique windows into the processes of the past and present. In the dynamic tectonic, environmental, climatic, and ecological setting that is eastern Africa, records recovered through scientific drilling enable us to look at change through time in unprecedented ways. Cores from the East African Rift System can provide valuable information about the context in which hominins have evolved in one of the key regions of hominin evolution over the past 4 Myr. The Deep Drilling in the Turkana Basin (DDTB) project seeks to explore the impact of several types of evolution (tectonic, climatic, biological) on ecosystems and environments. This includes addressing questions regarding the region's complex and interrelated rifting and magmatic history, as well as understanding processes of sedimentation and associated hydrothermal systems within the East African Rift System. We seek to determine the relative impacts of tectonic and climatic evolution on eastern African ecosystems. We ask the follow questions: what role (if any) did climate change play in the evolution of hominins? How can our understanding of past environmental change guide our planning for a future shaped by anthropogenic climate change? To organize the scientific community's goals for deep coring in the Turkana Basin, we hosted a 4 d ICDP supported workshop in Nairobi, Kenya, in July 2022. The team focused on how a 4 Myr sedimentary core from the Turkana Basin will uniquely address key scientific research objectives related to basin evolution, paleoclimate, paleoenvironment, and modern resources. Participants also discussed how DDTB could collaborate with community partners in the Turkana Basin, particularly around the themes of access to water and education. The team concluded that collecting the proposed Pliocene to modern record is best accomplished through a two-phase drilling project with a land-based transect of four cores spanning the interval from 4 Ma to the Middle–Late Pleistocene (< 0.7 Ma) and a lake-based core targeting the interval from ∼ 1 Ma to present. The second phase, while logistically more challenging due to the lack of drilling infrastructure currently on Lake Turkana, would revolutionize our understanding of a significant interval in the evolution and migration of Homo sapiens for a time period not currently accessible from the Kenyan part of the Turkana Basin. Collectively, the DDTB project will provide exceptional tectonic and climatic data directly associated with one of the world's richest hominin fossil localities.

Distribution of magmatic intrusions and key petroleum system elements in the Pliocene–Holocene Pangani Rift Basin, Northeastern Tanzania

The sedimentary fills in the Pangani rift basin (PRB) of northeastern Tanzania have not yet been studied in detail and their petroleum potential is poorly understood. This study aims at assessing distribution of magmatic intrusions and key petroleum system elements in the PRB based on 2D seismic, remote sensing, and aeromagnetic data interpretations. Results show that evolution of the basin fill was influenced by sediment supply, magmatism, and tectonics. The PRB contains Pliocene–Holocene clastic, magmatic, and volcaniclastic deposits. The clastic sediment was sourced from the north-northeast and east-southeast of the PRB. The magmatic and volcaniclastic rocks were formed during Pleistocene–Holocene and are increasing in frequency toward the north and northeast of the basin. Tectonics and faulting created depocenters and magma conduits; magmatic intrusions and fault systems form potential petroleum traps. Seismic interpretation has shown that PRB contains potential reservoir and seal rocks for possible petroleum accumulations. Fault systems dissecting the PRB provided petroleum migration pathways from deep buried source rocks to potential reservoirs. For the first time, this study has shown that the PRB contains key petroleum system elements for petroleum accumulations to occur. Our results will be of interest to researchers working in different East African Rift basins.

Curie point depth, thermal gradient and heat flow along the Ethiopia Rift System and adjacent plateaus using spectral evaluation approach: implications for geothermal resources

The Ethiopia Rift System (ERS) is a section of the East African Rift System within Ethiopia extending from the Afar in the northeast to the Kenya border in the southwest. It is apparent that magmatism and magmatic intrusions influence the crustal shape in the ERS resulting in its thinning and the shallowing of magmatic sources at various locations within it. As a consequence, more than 31 volcanoes hosting hydrothermal structures with a conceivable potential to generate massive quantities of geothermal energy have been identified along the ERS. In this study, we map the Curie Point Depth (CDP) over the ERS based on the analysis of aeromagnetic data extracted from the World Digital Magnetic Anomaly Map. Spectral evaluation method was used to estimate the boundaries (top and bottom) of the magnetized crust. Reduced-to-pole (RTP) aeromagnetic records have been divided into 105 (50% overlap) square blocks of 200 × 200 km size. The Curie temperature (580 °C) of magnetite was used to determine the thermal gradient and the heat drift in the area. The depths obtained for the bottom of the magnetized crust are assumed to correspond to the Curie Depths, where the magnetic layer loses all its magnetization. The determined values of Curie Point Depth, geothermal gradient and heat flow for the 50% overlapped 105 blocks, respectively, range from 8.85 to 55.85 km, 10.38 to 65.54 °C/km and 25.96 to 163.84 mW/m2. Lower CPD (< 20 km) in the ERS was obtained between Mille and Gewane (southwest Afar), between Adama (Nazret) and Yerer (NMER) and between Wendo Genet and Koti (SMER) localities. These areas, showing low CPD, exhibit excessive geothermal gradient and high heat flow all of which indicate the presence of significant geothermal potential.

Re‐routing of submarine channels by Plio‐Quaternary extensional tectonics along the Tanzania margin and implications for an offshore branch of the East African Rift System

Re‐routing of submarine channels by Plio‐Quaternary extensional tectonics along the Tanzania margin and implications for an offshore branch of the East African Rift System

The drame of kalehe, DRC. Study of the interactions beteen climate change, public prevention and human safety in mounainous Kivu

The tragedy that occurred in Kalehe, Democratic Republic of Congo (DRC), in May 2023, following torrential rains, highlights the urgent need to analyze the interactions between climate change, public prevention and human safety in the mountainous region of Kivu, also known as “Mountainous Kivu”. Mountainous Kivu, at the heart of the East African Rift Valley, is directly affected by the impacts of climate change, including extreme weather events leading to flooding and landslides. Research points to an increase in extreme weather events, contributing to environmental instability and the vulnerability of local communities, most of whom depend on family farming. Weakly defined measures for adaptation and resilience to climatic disturbances, as well as gaps in public prevention policies, have amplified this vulnerability. This study explores the complex dimensions of the Kalehe tragedy, focusing on agrarian explanations. On the one hand, the study examines the dimensions of human security, considering socio-economic, political and environmental factors, in order to define measures to strengthen community resilience. Secondly, the study assesses existing public prevention policies in the region, identifying the strengths, weaknesses and gaps that contributed to the scale of the Kalehe tragedy. The methodology is based on data collection from documentary sources and unstructured interviews with resource persons in the field.

Ranking and developing ground-motion models for Southeastern Africa

The southern East African Rift System (EARS) is an early-stage continental rift with a deep seismogenic zone. It is associated with a low-to-moderate seismic hazard, but due to its short and sparse instrumental record, there is a lack of ground-motion studies in the region. Instead, seismic hazard assessments have commonly relied on a combination of active crustal and stable continental ground-motion models (GMMs) from other regions without accounting for the unusual geological setting of this region and evaluating their suitability. Here, we use a newly compiled southern EARS ground-motion database to compare six active crustal GMMs and four stable continental GMMs. We find that the active crustal GMMs tend to underestimate the ground-motion intensities observed, while the stable continental GMMs overestimate them. This is particularly pronounced in the high-frequency intensity measures (&gt;5 Hz). We also use the referenced empirical approach and develop a new region-specific GMM for southern EARS. Both the ranked GMMs and our new GMM result in large residual variabilities, highlighting the need for local geotechnical information to better constrain site conditions.

Insights on the African Upper Mantle From Quasi‐Love Wave Scattering

AbstractThe African upper mantle is diverse, featuring several cratonic roots, active and magmatic continental rifting, abundant intra‐plate volcanism, and several oceanic hotspots offshore. Relatively small‐scale mantle convection processes, some possibly related to lithospheric thickness variations, have been proposed to account for patterns of volcanism and topography. A key constraint on such features and processes can be found via seismic anisotropy, but limited coverage of seismograph stations across the continent has resulted in sparse observations. Quasi‐Love waves, produced by scattering of Love to Rayleigh energy at lateral gradients in upper mantle seismic anisotropy, can provide information about seismic anisotropy well away from seismograph stations. We catalog 525 observations of Quasi‐Love waves across the region and back‐project to scattering points, revealing locations of lateral gradients in upper mantle seismic anisotropy. Quasi‐Love wave scattering occurs at craton edges, likely due to the contrast between lithospheric and asthenospheric anisotropy, and close to ancient orogenic belts, indicating changes in fossilized lithospheric anisotropy from past collisional events. Scattering surrounding the proposed Al‐Kufrah cratonic remnant in north‐east Africa supports its existence as cratonic lithosphere. Scattering also occurs below thin lithosphere, suggesting deviations in asthenospheric flow patterns (such as localized upwellings), notably beneath the East African Rift, oceanic hotspot tracks, and the Cameroon Volcanic Line. Quasi‐Love scattering is abundant in the Indian Ocean and beneath Madagascar, consistent with small‐scale dynamic processes in the asthenosphere that likely relate to the complex rifting history of this ocean basin and the dispersed micro‐continents within.

Seismic properties of mantle metasomatism from mantle xenoliths beneath the North Tanzania Divergence, East African Rift

We use mantle xenoliths brought to the surface by alkaline lavas to determine the chemical and physical properties of the metasomatized lithospheric mantle that contribute to the earliest rifting stage in East Africa. Our results help to interpret the seismic tomographic images in terms of vein and inclusions proportions in the lithospheric mantle. We focus on mantle xenoliths from the in-rift Pello Hill volcano in the North Tanzanian Divergence (NTD). These xenoliths reveal the presence of refractory mantle harzburgites and dunites with coarse granular to porphyroclastic textures and 6–80 % of diopside, phlogopite and amphibole-bearing veins and phlogopite-rich hornblendite xenolith. The presence of calc-potassic and FeO, TiO2-rich veins, and mineral equilibria of olivine and pyroxenes indicate that fluid/melt-rock interactions occurred at depth from 40 km to 80–90 km, and indicate the presence of a high-temperature isotherm beneath the NTD (T = 1040–1200°C). We computed the seismic properties of the mantle xenoliths with different proportions, compositions, and geometric distributions of crystallized and fluid-filled veins. Compared to vein-free peridotites, for crystallized vein-bearing xenoliths, the velocity is lowered by 2–4 % to 28–37 % for Vp and by 2–3 % to 25–29 % for Vs for 6 % to 60 % veins, respectively. For fluid-filled inclusions, hydrous melt lens-shape inclusions are the most effective parameter to reduce P velocity, compared to dry or 2.5 %–CO2 peridotitic melt. A comparison with seismic tomography velocities allows us to discuss the current state of the lithospheric mantle. The best agreement obtained between P teleseismic tomography (Vp anomalies between −9 % and −15 %) and vein-bearing peridotites (depth 40–90 km) corresponds to 12–25 % of crystallized veins or 8–15 % for fluid filled-veins for a vertical foliation and transtensional strain regime in the mantle lithosphere beneath the NTD.

Soil-Forming Factors of High-Elevation Mountains along the East African Rift Valley: The Case of the Mount Guna Volcano, Ethiopia

The soils of the high-elevation mountains along the East African Rift Valley are poorly understood. Assessing the potential of soils for agriculture, climate change mitigation, and environmental functioning requires insight into how they relate to the factors influencing soil formation. Between 3000 and 4120 m a.s.l., 85 soil profiles of Mount Guna were described and sampled. Standard physicochemical analyses were done on all pedons. Additionally, X-ray diffraction, Alox and Feox content, and P fixation were performed on six selected profiles. Soils on Mount Guna included Andosols, Phaeozems, Leptosols, Regosols, Cambisols, Luvisols, and Vertisols. With increasing elevation, clay content, bulk density, and pH decreased while the C:N ratio remained constant. In contrast, sand, silt, silt-to-clay ratio, SOC, Ntotal, and SOCS increased. With a factor analysis, the soil-forming factors’ elevation/climate could be disentangled from the factor’s parent material as these affect topsoil and subsoil differently. In the ordination based on climate/elevation and parent material, Andosols and Vertisols stood out while other Reference Soil Groups (RSG) showed indistinct patterns. Soil erosion appeared as an additional soil-forming factor not accounted for by the factor analysis. The distribution of the RSG was significantly associated with elevation belts (p &lt; 0.001), lithology (p &lt; 0.001), and landcover (p &lt; 0.003). On the summital ridge, the Andosols were crucial for groundwater storage due to high precipitation. Shallow and stony soils in the mid-elevation belt contributed to runoff generation. Average soil carbon stock ranged from 8.1 to 11 kg C m−2 in the topsoil and from 29.2 to 31.9 kg C m−2 in the upper meter, emphasizing the global importance of high-elevation areas for carbon sequestration.

An Autonomous Thermal Camera System for Monitoring Fumarole Activity.

The Kenyan part of the East African Rift System hosts several geothermal fields for energy production. Changes in the extraction rate of geothermal fluids and the amount of water re-injected into the system affect reservoir pressure and production capacity over time. Understanding the balance of production, natural processes and the response of the geothermal system requires long-term monitoring. The presence of a geothermal system at depth is often accompanied by surface manifestations, such as hot water springs and fumaroles, which have the potential for monitoring subsurface activity. Two thermal camera timelapse systems were developed and installed as part of a multi-sensor observatory in Kenya to capture fumarole activity over time. These cameras are an aggregation of a camera unit, a control unit, and a battery charged by a solar panel, and they monitor fumarole activity on an hourly basis, with a deep sleep of the system in between recordings. The article describes the choice of hardware and software, presents the data that the cameras acquire, and discusses the system's performance and possible improvement points.

Mobilization rates of landslides in a changing tropical environment: 60-year record over a large region of the East African Rift

The impact of humans on Earth surface processes strongly increased since the offset of the 20th century, often surpassing the importance of natural drivers. In most mountainous regions, landslide mobilization rates may have increased due to human disturbances of the landscape such as deforestation, mining, and road construction. However, assessing the impact of these human disturbances is not straightforward, especially in the Global South where data-scarcity is commonplace. In this study, we use historical aerial imagery to assess the mobilization rates of rapidly-formed landslides and their sensitivity to human impacts over six decades and 21,000 km2 in the North Tanganyika-Kivu Rift region (NTK Rift) in Africa. This tropical region is characterized by rapid societal changes which have drastically altered the natural environment. By estimating the volume of thousands of rapidly-formed landslides, we find that the average mobilization rate in the landscapes rejuvenated by the retreat of rifting-related knickpoints is three times higher as compared to the relict landscapes (ca. 31 [29.2–34.2] m3 km2 year−1 vs 13 [11.9–13.7] m3 km2 year−1). These mobilization rates are dominated by deep-seated landslides. Our mobilization rates are relatively low as compared to observations in other mountainous regions. Yet, the tropical NTK rift also has a more moderate topography and degree of tectonic activity. In addition, our research covers an unprecedented large area and a long period, as such being not biased by local extreme events. In rejuvenated landscapes, roughly 5 % of the sediment mobilization by rapidly-formed landslides is linked to human activity, while in relict landscapes this figure rises to 18 %, notably due to mining and road construction. The role of human activity is limited as compared to the recent occurrence of some large landslides, which seem linked to natural causes and dominate the overall mobilization rates. Moreover, the limited role of human activity must be balanced with the fact that the NTK Rift, although highly populated, remains relatively untouched by major road infrastructure constructions. While previous studies have found that deforestation has a large impact on the landslide risk in the region, its impact on the mobilization rates is much less important. Overall, our results significantly contribute to a better understanding of landslide mobilization and its controlling factors in the context of human-induced environmental change, especially by providing much-needed long-term observations for a currently under-researched type of environment.

Rift Propagation Interacting With Pre‐Existing Microcontinental Blocks

AbstractRift propagation is a 3D thermo‐mechanical process that often precedes continental breakup. Pre‐existing microcontinental blocks and the associated lithospheric strength heterogeneities influence the style of rift propagation. Interestingly, some rifts propagate into pre‐existing blocks and eventually cut through them (e.g., the Zhongsha Block and the Reed Bank), while others bypass these microcontinental blocks forming distinct overlapping rift branches (e.g., the East African Rift System). In this study, we use 3D numerical models to investigate the interaction between microcontinental blocks and rift propagation under different far‐field extension rates. In doing so, we assess the impact of mantle lithospheric thicknesses and lower crustal rheology on the style of rift propagation. Our models reproduce the two types of rift propagation, characterized by propagating rifts that either split or bypass the pre‐existing microcontinental blocks. We find that lithospheric thickness exerts dominant control, while lower crustal rheology of microcontinental blocks and the extension rate have less effect on rift propagation. Our model results can explain rift propagation patterns, block rotation and strong lithospheric thinning in the South China Sea, the East African Rift System, and the Woodlark Basin.

A Lake Record of Geomagnetic Secular Variations for the Last 23 ka From Lake Chala: Toward a Composite Directional Lake Record of the Earth's Magnetic Field for Equatorial East Africa

AbstractThe documentation and understanding of variations in the Earth's magnetic field through time is fundamental for several disciplines, but current geomagnetic models rely on datasets heavily biased toward the mid‐ and high northern latitudes. The African continent and surrounding islands and oceans are particularly underrepresented. Here, we present a new record of paleo‐secular variation (PSV) of the inclinations over the last 23 ka from Lake Chala, situated at 3°S near Mt Kilimanjaro in eastern equatorial Africa. This groundwater‐fed crater lake is characterized by a high sedimentation rate (ca. 1 cm/10 years) and a particularly well‐constrained age model based on 210Pb and 14C dating. The magnetic mineralogy of the sediments is tested with rock magnetic analyses. The Lake Chala inclination record shows four highs and lows over 20 ka and compares well with that of Lake Malawi (10°S) between 20 and 16.2 ka, and from 9.8 to 2.6 ka. This record is linked to PSV records at Lakes Victoria and Malawi using a sequence slotting technique to generate a composite PSV model for east Africa. Analyzed at best‐possible resolutions up to 200 years, the Lake Chala PSV record not only represents an important contribution to improve our understanding of local and global features of the Earth's magnetic field. It also expands the utility of paleomagnetism as a key tool for dating and correlation both for archeological sites throughout East Africa and the many volcanoes, active or dormant, of the East African Rift System.

Blind magmatism abets nonvolcanic continental rifting

Tectonic forces alone cannot drive rifting in old and thick continental lithosphere. Geodynamic models suggest that thermal weakening is critical for lithospheric extension, yet many active rifts lack volcanism, seeming to preclude this process. We focus on one such rift, the Tanganyika-Rukwa segment of the East African Rift System, where we analyze local seismicity for shear wave anisotropy and couple the results with numerical modeling. The strongest splitting measurements are from earthquakes with paths sampling lower crustal regions of high compressional-to-shear wave velocity ratios and have fast polarization directions parallel to the local mantle flow, implying the existence of oriented melt lenses. This lower crustal magmatism and observed high surface heat flow are consistent with substantial lithospheric weakening and explain the enigmatic relief and increasing strain accommodation along the rift axis. We conclude that progressive nonvolcanic rifting is assisted by deep crustal melts yet to breach the surface.

Influence of mantle plume on continental rift evolution: A case study of the East African rift system

Mantle plume is an essential component of the mantle convection system, and its influence on the geodynamics of continental rifts is of great significance for understanding the crust–mantle interaction. The East African Rift System, as the largest continental rift in the Cenozoic and in the initial stage, provides an excellent option for studying the interaction between the mantle plume and the continental crust. Based on the data such as GPS, seismic tomography, and global crustal model, a viscoelastic-plastic 2D thermodynamic numerical model is established to reconstruct the evolution of the Afar depression, Ethiopian Rift, and Kenyan Rift. By comparing the differences between the models of the Afar depression, Ethiopian Rift, and Kenyan Rift, the relationship between the mantle plume and pre-existing structures and their influence on the evolution of continental rifts are discussed. The results show that the mantle plume can increase the depth of the rift faults, concentrate the distribution of the faults, and strengthen the control of main faults on the rifts, allowing the possibility of narrow rifts. Pre-existing structures control the fault styles and symmetry of the rifts and also the morphology of the mantle plume.

Geomorphology of the Makgadikgadi Basin (Botswana): Insight Into the Propagation of the East African Rift System

AbstractThe Southwestern Branch of the East African Rift System propagates through the Central and Southern African plateaus and ends in the Okavango Makgadikgadi Zambezi Basin. This basin hosts the Okavango Graben and the Eiseb Graben, considered as the terminus of the Southwestern Branch of the rift. To the southeast, the Makgadikgadi Basin is affected by a series of normal faults forming the Makgadikgadi Rift Zone (MRZ) which regional geodynamic significance remains unclear. Based on fieldwork and geomorphic analysis, we revisited the geomorphological features associated with paleolakes and the fault pattern within the Makgadikgadi Basin to better constrain the dynamics of the MRZ. Fault scarps and offsets along linear dunes show normal‐dip kinematics of faults, indicating a NW‐SE extension direction in the area. Furthermore, lacustrine shorelines in the basin are undeformed, proving that they post‐date the fault activity. The previously published ages of these shorelines demonstrate that the MRZ currently has a low tectonic activity. Integrated in the geodynamic framework of the region, these results suggest that present‐day deformation shifts toward the Okavango Graben north of the MRZ. We therefore propose a “zip‐opening” model to explain the propagation of the Southwestern Branch of the East African Rift System where the tip of the system progressively progresses southwestward, driven by motions of the continental plates.

Probabilistic seismic collapse risk assessment of non-engineered masonry buildings in Malawi

This study presents the most recent development of a nationwide earthquake risk model for non-engineered masonry buildings in Malawi. Due to its location within the East African Rift, Malawi experienced several moderate earthquakes that caused seismic damage and loss. Recently, a new probabilistic seismic hazard model has been developed by considering fault-based seismic sources, in addition to conventional areal sources. The most recent 2018 national census data provide accurate exposure information for Malawian people and their assets at detailed spatial resolutions. To develop seismic fragility functions that are applicable to Malawian housing stocks, building surveys and experimental tests of local construction materials have been conducted. By integrating these new developments of seismic hazard, exposure, and vulnerability modules, a quantitative seismic building collapse risk model for Malawi is developed on a national scale. For the rapid computation of seismic risk curves at individual locations, an efficient statistical approach for approximating the upper tail distribution of a seismic hazard curve is implemented. Using this technique, a seismic risk curve for a single location can be obtained in a few seconds, thereby, this can be easily expanded to the whole country with reasonable computational times. The results from this new quantitative assessment tool for seismic impact will provide a sound basis for risk-based disaster mitigation policies in Malawi.

Probabilistic seismic hazard assessment framework for Uganda: a stochastic event-based modelling approach

Uganda lies between the eastern and western arms of the East African Rift System, the largest seismically active rift above sea level. With increasing population, urbanisation and rapid construction, seismic risk in the country is escalating fast and is compounded by the high vulnerability of the building stock and inadequate disaster prevention and mitigation strategies. Hence, there is an urgent need to assess Uganda’s resilience against seismic risks. This paper presents a Monte-Carlo based probabilistic seismic hazard model for Uganda, as the first step towards the development of a seismic risk and resilience assessment framework for the country. In addition to fault segment data, earthquake catalogues are compiled for the period between 1900 and 2022 to estimate recurrence parameters for source zones in the area of interest. Area source zones incorporating focal mechanisms are used to stochastically model a national hazard framework for Uganda. A logic tree approach is applied to implement four ground motion prediction equations for both stable continental and active shallow crust geologies. Mean hazard curves, uniform hazard spectra, earthquake disaggregation and spectral pseudo-accelerations for major Ugandan cities are derived in addition to hazard maps for the country. The findings are largely consistent with previous regional studies and confirm that western Uganda is exposed to the highest level of seismicity. The model presented herein can be used to kick-start the update and continuous improvement of Uganda Seismic Design Code and the National Policy for Disaster Preparedness and Management.

Present-day Fault Kinematics and their Reactivation Likelihood within and South of the North Tanzania Divergence (NTD), East African Rift System: Implication for Geo-hazards Assessment

Abstract This study investigates the relationship between the North Tanzania Divergence (NTD), the southern part of the NTD and the rift structures in the eastern part of the East African Rift System (EARS). It also assesses the susceptibility of these structures to reactivation under the current stress field. Fault slip data and focal mechanisms from various sources have been used to determine the minimum horizontal stress axes (Shmin) of different faults, including the Eyasi rift border fault south of the NTD. The analysis reveals that faults trending NW-SE to NNW-SSE have a sinistral sense of movement, while those trending NE-SW generally have a dextral sense of movement. The N60°E trending faults that dip easterly are not optimally oriented with the current stress field and have a low to moderate likelihood of reactivation. This includes the NE-SW trending Eyasi rift border fault (part of the NTD) and other structures oriented N60°E south of the NTD, with a computed slip tendency (Ts) of less than 0.5. Faults trending N-S, NNW-SSE, and NNE-SSW, on the other hand, have a high to the highest likelihood of reactivation (Ts &amp;gt; 0.85). The other faults in the study area, which trend NE-SW or NW-SE, have Ts values expressed as 0.5 ≤ Ts &amp;lt; 0.85. These findings have important implications for earthquake risk assessment in the NTD and south of the NTD, as geological constraints need to be taken into account for better management and mitigation. The results could be applied elsewhere in the EARS provided that they are under E-W extensional stress regime.

Magma‐Assisted Continental Rifting: The Broadly Rifted Zone in SW Ethiopia, East Africa

AbstractThe Gofa Province and Chew Bahir Basin in the Broadly Rifted Zone (BRZ) between the southern Main Ethiopian Rift (sMER) and the northern Kenya Rift (nKR) record early volcanism and associated faulting in East Africa; however, the spatiotemporal relationships between volcanism and faulting remain poorly constrained. We applied apatite (U‐Th)/He (AHe) and zircon (U‐Th)/He (ZHe) thermochronometry to Neoproterozoic basement rocks from exhumed footwall blocks of the extensional Gofa Province and Chew Bahir Basin, and analyzed our result in the context of well‐dated regional volcanic units in the BRZ to unravel the interplay between tectonic exhumation, faulting and volcanism. Single‐grain AHe ages ranging from 1.0 to 136.8 Ma were recorded in 32 samples, and single‐grain ZHe ages from three samples range between 142.2 and 335.6 Ma. The youngest AHe ages were obtained from the Chew Bahir Basin and the narrow deformation zone in the Gofa Province. Our thermal modeling results reflect little or no significant regional crustal cooling prior to extensive volcanism, which started at about 45 Ma. Conversely, new and previously published thermal history models suggest that widespread crustal cooling related to regional extension occurred between ∼27 and 20 Ma. Thermal modeling results from subsets of samples indicate that following this initial diffuse extensional deformation, renewed exhumation occurred along a narrow zone within the Gofa Province and the Chew Bahir Basin during the middle to late Miocene (15‐6 Ma) and Pliocene (&lt;5 Ma), respectively. The crustal cooling phases follow a regional trend in volcanic episodes. For example, initial cooling between 27 and 20 Ma corresponds with the end of widespread flood‐basalt volcanism (45–28 Ma), suggesting that spatially diffuse normal faulting may have initiated shortly after the emplacement of voluminous and areally extensive flood basalts. The Miocene and Pliocene shifts in deformation along the Mali‐Dancha and Bala‐Kela basins in the Gofa Province and the Chew Bahir Basin, respectively, may indicate strain localization during the late stage of rifting and ongoing tectonic interaction between the sMER and the nKR. Our results support the notion of crustal weakening by massive volcanism as a precursor to widespread extensional faulting, and thus offer further insights into magma‐assisted deformation processes in the East African Rift System.