Rift Floor Research Articles

Groundwaters of the Central Ethiopian Rift: diagnostic trends in trace elements, δ18O and major elements

The Ethiopian Rift (a major portion of the Great East African Rift) is characterized by a narrow elongated depression bounded by highlands from both sides. This topographic configuration leads to a monsoon redistribution which resulted in an arid rift floor and humid high rainfall highlands. The rifting and associated volcanism also caused a thinning of the crust and facilitates influx of CO2 and other mantle gases as diffuse sources or along faults from deeper sources. Groundwaters in the rift floor are usually of high mineral content (high F, U, As and salinity) while those on the plateau are of low mineral content. Among many factors, groundwater availability and quality in the rift floor aquifers is the function of their connection to the aquifers in the high rainfall plateau and the residence time of groundwater prior to reaching the rift floor. This entails the need for addressing one basic hydrologic question in such a setting: at what depth and rate does recharge from the high rainfall highland reach the lowland rift aquifers? This study uses spatial variations in trace elements and relates them to 14C variations, thereby investigating the suitability of using trace elements as proxies for residence time estimation of groundwaters of relatively short (1,000–2,000 years) residence time. This work also investigates the behavior of trace element trends along the groundwater flow path in a rifted setting and compares them with such trends in sedimentary aquifers elsewhere. The comparison shows a clear difference in behavior of trace elements along the groundwater flow path when compared with such variations in big sedimentary basins with no prominent rifting and volcanism, suggesting the need of calibrating the relation between trace elements and any direct residence time indicators. An integrated use of major elements, trace elements, and environmental isotopes reveals that the main recharge of the aquifers originates from mountain blocks and that recharge takes place via fractures with no evidence of evaporation prior to recharge. Redox processes appear to play a limited role in trace element geochemistry of groundwaters in the region. Progressive trends in trace element composition along the groundwater flow path suggest continuous groundwater flow from the plateau.

Geochemical evidence of lithospheric thinning in the southern Main Ethiopian Rift

Lithospheric thinning is a fundamental process associated with the transition from continental to oceanic regimes during continental rifting. Precisely how and when this lithospheric thinning proceeds are first order controls on rift basin evolution. The Main Ethiopian Rift, part of the ∼ 2000 km long East African Rift System, is the archetypical modern example of continental rifting, and a key location in which to study the evolution of the lithosphere during extension. This study explores lithospheric modification in the interface region between the Main Ethiopian rift and northward propagating rifting from Kenya through a major and trace element study of rift initiation and maturation using the 19 myr magmatic record preserved in this region. Initial rifting in southern Ethiopia is coincident with the eruption of basalts along the rift shoulders that are characterized by deep fractionation trends (0.5 GPa) and poorly developed magmatic pathways. The earliest of these basalts are derived from melting columns where the aluminum phase is garnet-dominated (Tb N/Yb N ∼ 1.8–2) and has geochemical characteristics interpreted as melting of the lithospheric mantle. The transition from initial rift shoulder magmatism to Quaternary magmatic–tectonic fault belts on the modern rift floor at Arba Minch (6°N) is coincident with a shallowing of the melting column (Tb N/Yb N ∼ 1.3–1.7), less significant contributions from the lithospheric mantle, and the establishment of a shallow fractionation regime (0.1 GPa). At Chencha (∼ 6.3°N) newly dated (12.32 ± 0.17 Ma) magmatism on the rift shoulder has similar fractionation paths to contemporaneous magmatism to the south (0.5 GPa), but is derived from a different, shallower mantle source (Tb N/Yb N ∼ 1.3–1.5) that we interpret results from lithospheric thinning associated with the now-inactive Chow Bahir rift. Between 6.5 and 8°N, significant surface faulting and shallow magmatic fractionation paths (0.1 GPa) in the dominant Quaternary structure of the Main Ethiopian Rift (the Wonji Fault Belt and Silti-Debre Zeyit Fault Zone), highlights the strong connection between magmatism and extensional tectonics in these structures. Along the eastern rift margin, Wonji Fault Belt magmas are derived from a dominantly shallow melting column (Tb N/Yb N ∼ 1.4–1.7) that is similar in composition to the older rift shoulder lavas at Chencha. Adjacent to the western rift margin, magmas erupted in the Silti-Debre Zeyit Fault Zone are interpreted to have erupted through a thicker lithosphere as these magmas are derived from a deeper melting column (Tb N/Yb N ∼ 1.7–2.1) that contains some minor apatite. The inferred variations in lithospheric thickness in southern Ethiopia outlined in this study illustrate the interaction between northward and southward rift propagation in addition to lateral variations across the rift floor as extension migrates into zones of focused magmatic intrusion. The results of this investigation show that geochemical techniques can be applied to probe the history of lithospheric modification during rifting and provide new constraints for models of rift development.

Stable isotope variation in tooth enamel from Neogene hippopotamids: monitor of meso and global climate and rift dynamics on the Albertine Rift, Uganda

The Neogene was a period of long-term global cooling and increasing climatic variability. Variations in African-Asian monsoon intensity over the last 7 Ma have been deduced from patterns of eolian dust export into the Indian Ocean and Mediterranean Sea as well as from lake level records in the East African Rift System (EARS). However, lake systems not only depend on rainfall patterns, but also on the size and physiography of river catchment areas. This study is based on stable isotope proxy data (18O/16O, 13C/12C) from tooth enamel of hippopotamids (Mammalia) and aims in unravelling long-term climate and watershed dynamics that control the evolution of palaeolake systems in the western branch of the EARS (Lake Albert, Uganda) during the Late Neogene (7.5 Ma to recent). Having no dietary preferences with respect to wooded (C3) versus grassland (C4) vegetation, these territorial, water-dependant mammals are particularly useful for palaeoclimate analyses. As inhabitants of lakes and rivers, hippopotamid tooth enamel isotope data document mesoclimates of topographic depressions, such as the rift valleys and, therefore, changes in relative valley depth instead of exclusively global climate changes. Consequently, we ascribe a synchronous maximum in 18O/16O and 13C/12C composition of hippopotamid enamel centred around 1.5–2.5 Ma to maximum aridity and/or maximum hydrological isolation of the rift floor from rift-external river catchment areas in response to the combined effects of rift shoulder uplift and subsidence of the rift valley floor. Structural rearrangements by ~2.5 Ma within the northern segment of the Albertine Rift are well constrained by reversals in river flow, cannibalisation of catchments, biogeographic turnover and uplift of the Rwenzori horst. However, a growing rain shadow is not obvious in 18O/16O signatures of the hippopotamid teeth of the Albertine Rift. According to our interpretation, this is the result of the overriding effect of evaporation on 18O/16O responding to aridification of the basin floor by a valley air circulation system through relative deepening of the valley. On the other hand, a synchronous arid pulse is not so clearly recorded in palaeosol data and mammalian fauna of the eastern branch of the EARS. This discrepancy indicates that rift mesoclimates may represent an underestimated aspect in previous palaeoclimate reconstructions from rift valley data and represent a clear limitation to attempts at global climate reconstructions. The results of this study also suggest that using 18O/16O data as a proxy to rain shadow evolution must take into account relative basin subsidence to properly document mountain range uplift.

Evolution, pattern, and partitioning of deformation during oblique continental rifting: Inferences from lithospheric‐scale centrifuge models

Oblique rifting is investigated through centrifuge experiments that reproduce extension of a continental lithosphere containing a preexisting weakness zone. During extension, this weakness localizes deformation, and different rift obliquity is obtained by varying its trend with respect to the stretching direction. Model results show that deformation is mostly controlled by the obliquity angle α (defined as the angle between the orthogonal to the rift trend and the extension direction). For low obliquity (α < 45°), rifting is initially characterized by activation of large, en echelon boundary faults bordering a subsiding rift depression, with no deformation affecting the rift floor. Increasing extension results in the abandonment of the boundary faults and the development of new faults within the rift depression. These faults are orthogonal to the direction of extension and arranged in two en echelon segments linked by a complex transfer zones, characterized by strike‐slip component of motion. In these models, a strong strain partitioning is observed between the rift margins, where the boundary fault systems have an oblique‐slip motion, and the valley floor that away from the transfer zones is affected by a pure extension. Moderate obliquity (α = 45°) still results in a two‐phase rift evolution, although boundary fault activity is strongly reduced, and deformation is soon transferred to the rift depression. The fault pattern is similar to that of low‐obliquity models, although internal faults become slightly oblique to the orthogonal to the direction of extension. Deformation partitioning between the rift margins and the valley floor is still observed but is less developed than for low‐obliquity rifting. For high obliquity (α > 45°), no boundary faults form, and the extensional deformation affects the rift depression since early stages of extension. Dominance of the strike‐slip motion over extension leads to the development of oblique‐slip and nearly pure strike‐slip faults, oblique to both the rift trend and the orthogonal to the extension direction, with no strain partitioning between the margins and the rift floor. These results suggest that oblique reactivation of preexisting weaknesses plays a major role in controlling rift evolution, architecture, and strain partitioning, findings that have a significant relevance for natural oblique rifts.

Hierarchical cluster analysis of hydrochemical data as a tool for assessing the evolution and dynamics of groundwater across the Ethiopian rift

The hydrochemical evolution and flow pattern of groundwater is analyzed in a complex closed rift basin on the basis of multivariate hierarchical statistical cluster analysis. The analysis was made on the basis of water samples collected from over 120 wells and limited surface waters. The result revealed dissimilar hydrochemical phases in the rift and highlands representing different groundwater flow regimes. Five groundwater zones with different hydrochemical facies were identified. Both the conventional hydrochemical and multivariate statistical analyses indicate that the central sector of the rift represents a complex groundwater system influenced by thermal waters and alkaline lakes. It also demonstrates that the close similarity of lakes and groundwater with high fluoride, sodium, Total Dissolved Solids (TDS) and pH. The waters associated with acidic volcanic rocks are characterized by a positive alkalinity residual of calcite. When they concentrate due to the effect of climate, the precipitation of calcite causes a decrease in the chemical activity of calcium and increase in alkalinity. These waters become NaHCO3-dominated with distinctly different from highland systems characterized by Ca-MgHCO3 type waters with low TDS. Alkaline waters are intimately linked to acidic volcanics while fresh waters are associated with dominantly basic volcanics. Waters with similar hydrochemical facies as highland waters are evident in local fractures of the rift floor. The zonation of hydrochemical facies on the basis of hierarchical cluster analysis indicates increase in ionic concentration and pH along the groundwater flow path. Key words: Ethiopian rift, groundwater evolution, hierarchical cluster analysis, hydrochemistry, volcanic rocks.

Comparative assessment of the water balance and hydrology of selected Ethiopian and Kenyan Rift Lakes

AbstractThe study area is part of the East African Rift system, characterized by a cluster of lakes occupying an extremely faulted rift floor with geothermal manifestations. Some of the lakes illustrated contrasting water levels and size evolution over the last few decennia, believed to have been caused by various natural and anthropogenic factors. The relative importance of these factors, however, is unknown. This study attempts to present the hydrology of the lakes in a broader context, by giving more emphasis to lake water level fluctuations and to the water balance. These factors have far‐reaching implications in regard to future management of the lake basin water. It also provides information on the relation of the groundwater with the lakes, and with the local and regional groundwater flow system from the adjacent highlands to the floor of the Rift. The methods utilized in this study include conventional hydrogeological field surveys, and hydrometeorological and data analyses, coupled with digital image processing and spatial analysis under a Geographic Information System environment. Ancillary supporting information has been obtained from environmental isotopes and hydrochemical data. The study results indicate the terminal Ethiopian lakes changed in size and water level significantly over the last half century. In contrast, the Kenyan lakes only exhibited slight changes. The lakes in both countries exhibit a striking similarity in their subsurface hydraulic connection, and are strongly governed by complex rift geological structures. Groundwater plays a vital role in the water balance of the study lakes. The study results indicate that future sustainable use of the study lakes demands that serious attention be given to the role of the groundwater component of the lake water balances.

Geophysical exploration of the Boku geothermal area, Central Ethiopian Rift

The Boku central volcano is located within the axial zone of the Central Ethiopian Rift near the town of Nazareth, Ethiopia. An integrated geophysical survey involving thermal, magnetic, electrical and gravimetric methods has been carried out over the Boku geothermal area in order to understand the circulation of fluids in the subsurface, and to localize the “hot spot” providing heat to the downward migrating groundwaters before they return to the surface. The aim of the investigations was to reconstruct the geometry of the aquifers and the fluid flow paths in the Boku geothermal system, the country's least studied. Geological studies show that it taps heat from the shallow acidic Quaternary volcanic rocks of the Rift floor. The aquifer system is hosted in Quaternary Rift floor ignimbrites that are intensively fractured and receive regional meteoric water recharge from the adjacent escarpment and locally from precipitation and the Awash River. Geophysical surveys have mapped Quaternary faults that are the major geologic structures that allow the ascent of the hotter fluids towards the surface, as well as the cold-water recharge of the geothermal system. The shallow aquifers are mapped, preferred borehole sites for the extraction of thermal fluids are delineated and the depths to deeper thermal aquifers are estimated.

Hydrogeological framework and occurrence of groundwater in the Ethiopian aquifers

Comprehensive hydrogeological investigations have been carried out in the Ethiopian volcanic terrain and associated quaternary sediments. The occurrence and distribution of groundwater are systematically analysed in relation to the geomorphological and geological setting within one of the world’s thickest volcanic provinces. The study starts from regional conceptualization to a more focussed analysis of four selected areas with distinct hydrogeological features. The chosen areas represent highland trap volcanic sequence characterized by multi-layer aquifers, intermountain sedimentary graben, fractured rift floor volcanics boarded by steep high-altitude plateau, and vast rift floor flood plain covered with alluvial sediment with adjoining gently-sloping pediments. The result revealed extreme variations in aquifer hydraulic characteristics and recharge rates. The flow and occurrence of groundwater is strongly controlled by the structure and geomorphological setup of the volcanic rocks and associated sediments. Groundwater flow in the rift and escarpment areas is largely controlled by faults. The recharge varies in a wide range between a few mm to 400 mm/yr. Despite the occurrence of permeable rocks and high recharge rates in some highlands adjacent to steep escarpments, the groundwater reserve is low due to the fast release of the recharged water to the rift plains through large open faults. Wide basic and acidic volcanics, alluvio-colluvial and lacustrine deposits form shallow unconfined aquifers, with transmissivity varying between 27 and 135 m 2/d. The thick trap series volcanics interbedded with river gravels and sands form deep confined and semi-confined aquifers locally with artesian conditions. Intermountain grabens and rift floor sediments associated with fractured volcanics form the largest aquifers under water table and semi-confined conditions. High well yields in deeper volcanic aquifers are often associated with regional fault lines and permeable sediments interbedded with the volcanics. Limited water quality analysis indicates that the groundwaters are Ca–MgHCO 3 type in the highlands, Na–HCO 3 type in the rift with local SO 4 and Cl enrichment. Hard waters are dominant in the basic volcanics. The highland waters have relatively lower ionic concentration (TDS ranging from 50 to 1200 mg/l). The rift waters have high salinity and fluoride, which is the major national water quality problem.

Control of rift obliquity on the evolution and segmentation of the main Ethiopian rift

The main Ethiopian rift has evolved in two stages, with successive activation of differently oriented fault systems. Analogue modelling of the Earth’s lithosphere demonstrates that such a rift evolution requires neither magma weakening nor a change in plate kinematics. The main Ethiopian rift is an active rift in the break-up stage, and it marks the incipient boundary between the Nubia and Somalia plates1. Rifting started with the activation of large boundary faults and diffuse volcanism, followed by focused magmatism and faulting in the rift floor, along step-like segments oblique to the rift axis that now act as a protoridge for future seafloor spreading2. This concentration of volcano-tectonic activity has been thought to be either magma assisted2,3 or controlled by a change in rift kinematics, with a late oblique rifting phase that would have caused the development of the step-like fault segments that focused magma upwelling4,5. Geodetic6,7, seismic8 and stress-field9 data confirm current oblique rifting kinematics, but plate kinematics models do not predict a change in Nubia–Somalia motion in the past 11 million years10. Here, I use lithospheric-scale analogue models of oblique rifting to analyse the development of the main Ethiopian rift. I find that neither magma weakening nor a change in plate kinematics are required to simulate a two-phase evolution with successive activation of differently oriented fault systems. I conclude that rift evolution and segmentation are controlled by rift obliquity, independent of magmatic processes.

Neogene extension and basin deepening in the West Antarctic rift inferred from comparisons with the East African rift and other analogs

The West Antarctic rift system differs from other volcanically active rift systems in two unusual respects: (1) the rift floor lies 1000–2000 m lower in elevation than others, and (2) four interior ice-filled troughs extend between 1500 m and 2555 m below sea level. Two troughs are more than twice the depth of Lake Baikal, the world9s deepest lake. The Marie Byrd Land dome, by contrast, compares closely with the intra-rift Kenyan and Ethiopian domes in the East African rift. Comparisons between these rift systems suggest (1) that the West Antarctic interior is relatively cool and volcanically inactive, (2) that there is likely to have been an episode of extension during Neogene time in the deep interior basins, and (3) that dome uplift and basin subsidence have greatly changed the West Antarctic landscape over the past 25 m.y.

The changing face of the Ethiopian rift lakes and their environs: call of the time

AbstractThe Ethiopian rift is characterized by many perennial rivers, and a chain of lakes that vary in size, and hydrological and hydrogeological settings. The water resources of the Rift lakes are one of the focal points for large‐scale development in Ethiopia over the last few decades. Some of the lakes and their influent rivers are used for irrigation, soda abstraction, fish farming and recreation, and also support a wide variety of endemic birds and wild animals. Ethiopia's major mechanized irrigation farms and commercial fishery are confined within the Rift region. A few of the lakes have shrunk in surface area because of excessive water abstractions, while others have expanded because of increased surface run‐off and groundwater inflows from percolated irrigation water. Excessive land degradation, deforestation and over‐irrigation have changed the hydrological setting of a few Rift lakes. Human activities, in combination with changes in climate and geology, have influenced the hydrological setting and the water quality of the lakes, with the salinity and major ion composition dramatically changed in some of them. This study tries to address the challenges associated with development of these surface‐water resources, focusing on environmental problems arising over the past few decades on three lakes (Abiyata, Beseka and Ziway) situated along the tectonically active Rift floor. The methods utilized in this study include field hydrological and hydrogeological mapping, supported by aerial photographs and satellite imagery interpretations, as well as hydrometeorological and hydrochemical data analysis, and catchment hydrological modeling. A converging‐evidence approach was adapted to reconstruct the temporal and spatial variations of the lake water levels and surface areas. The study results revealed that the major changes in the Rift Valley are related primarily to recent improper use of water from the large rivers draining the Rift Valley floor and the lake catchments, and from direct lake water abstractions aggravated intermittently by climatic and land use changes. These changes represent grave environmental consequences on the fragile Rift ecosystem that demands urgent intervention in the context of an integrated, basin‐wide water management approach. This study emphasizes lake water level changes and human influences on these changes. It also assesses human interactions and water quality changes, including land use changes and environmental repercussions on the lakes, as well as providing recommendations on how these issues should be addressed.

Groundwater origin and flow along selected transects in Ethiopian rift volcanic aquifers

The disruption of lithologies by cross-cutting faults and the variability in volcanic structures make the hydrogeology of the rifted volcanic terrain in Ethiopia very complex. Along two transects, selected due to their hydrogeologic characteristics, groundwater flow, depth of circulation and geochemical evolution have been conceptualized. The groundwater flow continuity between the high rainfall plateau bounding the rift and the rift valley aquifers depends principally on the nature of the bounding faults. Up to 50% of recharge to the rift aquifers comes from the plateau as groundwater inflow where the rift is cross cut by transverse fault zones. Recharge from the mountains is found to be insignificant where the rift is bounded by marginal grabens; channel loss and local precipitation are the principal sources of recharge to the rift aquifers in such cases. At a regional scale, there is a clear zonation in the geochemical compositions of groundwaters, the result of aquifer matrix composition differences. The environmental isotope results show that the majority of the aquifers contain modern groundwaters. In a few localities, particularly in thermal groundwaters representing deeper circulation, palaeo-groundwaters have been identified. Deeper groundwaters in the rift floor have a uniform 14C age ranging between 2,300 and 3,000 years.

The distribution and hydrogeological controls of fluoride in the groundwater of central Ethiopian rift and adjacent highlands

Occurrence of fluoride (F) in groundwater has drawn worldwide attention, since it has considerable impact on human health. In Ethiopia high concentrations of F in groundwaters used for community water supply have resulted in extensive dental and skeletal fluorosis. As a part of a broader study, the distribution of F in groundwater has been investigated, and compared with bedrock geology and pertinent hydrochemical variables. The result indicates extreme spatial variations. High F concentration is often associated with active and sub-active regional thermal fields and acidic volcanics within high temperature rift floor. Variations in F can also be related to changes in calcium concentration resulting from dissolution of calcium minerals and mixing with waters of different chemical composition originated from variable hydrogeological environment across the rift valley. The concentration of F dramatically declines from the rift towards the highlands with the exception of scattered points associated with thermal springs confined in local volcanic centers. There are also interactions of F-rich alkaline lakes and the surrounding groundwater. Meteoric waters recharging volcanic aquifers become enriched with respect to F along the groundwater flow path from highland recharge areas to rift discharge areas. Locally wells drilled along large rift faults acting as conduits of fresh highland waters show relatively lower F. These areas are likely to be possible sources of better quality waters within the rift. The result of this study has important implications on site selection for water well drilling.

Water management problems in the Ethiopian rift: Challenges for development

The Ethiopian rift is characterized by many perennial rivers and lakes occupying volcano-tectonic depressions with highly variable hydrogeological setting. The rift lakes and rivers were the focal points for relatively large-scale water resources development. They are used for irrigation, soda abstraction, commercial fish farming, recreation and support a wide variety of endemic birds and wild animals. Ethiopia’s major mechanized irrigation farms and commercial fishery are confined within the rift. A few of the lakes have shrunk as a result of excessive abstraction of water; others expanded due to increased surface runoff and groundwater flux from percolated over-irrigated fields and active tectonism. Excessive land degradation and deforestation have also played a role. Human factors, in combination with the natural conditions of climate and geology have influenced the water quality. The chemistry of some of the lakes has been changed dramatically. This paper tries to present the challenges of surface water resources development with particular reference to environmental problems caused in the last few decades. The methods employed include field hydrological mapping supported by aerial photograph and satellite imagery interpretations, hydrometeorological and hydrochemical data analysis and catchment hydrological modeling. A converging evidence approach was adapted to reconstruct the temporal and spatial variations of lake levels and the hydrochemistry. The result revealed that the major changes in the rift valley are related mainly to recent improper utilization of water and land resources in the rivers draining the rift floor and the lakes’ catchment, and to direct lake water abstraction, aggravated intermittently by natural factors (climate and tectonism). These changes appear to have grave environmental consequences, which demand urgent integrated basin-wide water management practice.

Quaternary faulting and volcanism in the Main Ethiopian Rift

The Main Ethiopian Rift (MER) is associated with bimodal Quaternary magmatism. Field, remote sensing, and geochronology data are used to examine the relationships between axial acidic volcanoes and basaltic eruptions. Two main Quaternary magmatic episodes are recognizeable in MER: (a) basaltic flows followed by ignimbrites and silicic centers in the rift floor (2–1 Ma) and (b) axial silicic volcanoes and basalts since ∼650 Ka. The first episode consists mainly of basaltic flows related to the Afar Stratoid and outcrops in the central and northern MER. Scattered silicic centers developed subsequently along the rift floor. In the second episode, spatial and temporal correlation between rift localization and silicic centers becomes more evident. The silicic centers are located at the intersection of the WFB with earlier structures, especially E–W faults. With ageing, these centers become faulted and allow basalts to erupt right through the volcanic edifice, suggesting a decrease in the amount of differentiation in the magma chambers. This style of evolution appears to be characteristic of continental rifts prior to the onset of drifting.

Crustal evaluation of the northern Red Sea rift and Gulf of Suez, Egypt from geophysical data: 3-dimensional modeling

Combined 3-D interpretations of gravity and aeromagnetic data have been used in addition to continental and marine seismic profiles, well logs and geological cross-sections. The combination of gravity and magnetic data has improved the geometry and the density distribution in the 3-D calculated profiles. Results of the current work reveal possible crustal thickness and density distribution between the sedimentary cover and the upper mantle including the Moho discontinuity. The Moho depth map of the region, as obtained from the 3-D modeling exhibits various crustal thickness distributions. The type of crust changes gradually from continental to oceanic, from north to south. The zone of crustal thinning coincides mostly with zones of low-density, heated anomalous upper mantle beneath the rift floor (7 km). The eastern plateaus (the Red Sea hills) show by far the largest crustal thickness in the region (32 km). The Moho relief, as indicated from magnetic interpretation, shows a poor flattening especially in the eastern region. This is contrary to what is given by other authors. However, the present results are in good agreement with the geothermal gradient values in the Red Sea. The interpretation of magnetic data of the Red Sea Rift shows that the spreading rate of the part south of latitude 26.5° N agrees well with the theoretical model, in the order of 0.7 cm/yr. Less agreement has been obtained in the part north of latitude 27.5° N. The magnetic anomalies along the axial portion of the rift floor, as deduced from the results of the regional and residual separation and the 3-D magnetic modeling, are mainly caused by the oceanic crustal structures beneath the graben.

Formation of hydrocarbon reservoirs in the deep Earth's crust

Formation of reservoirs in crystalline rocks is associated with the development of rifts, with the periodic axial plunge of the rift floor during the extension of the Earth's crust, and with the elevation of consolidated basement masses during a compression phase.

Topographic analysis of Devana Chasma, Venus: implications for rift system segmentation and propagation

Devana Chasma is a rift system on Venus formed in association with the Beta Regio and Phoebe Regio volcanic highlands, which are interpreted as mantle plumes. We present a new analysis of a 2500-km-long segment of Devana. Based on the rift topography, the horizontal extension across the rift boundary faults is 3–9 km. This is a lower bound on the total rift extension because the altimetry does not resolve the topographic relief across the numerous faults that are visible in radar images of the rift floor. The total extension across Devana is approximately 20 km, similar in magnitude to continental rift systems on Earth. Rift flank elevations are up to 3.1 km in the regions nearest the mantle plumes and decay strongly with increasing distance from the plumes, indicating a strong thermal component to the rift flank topography, unlike the situation usually reported for terrestrial rifts. As on Earth, there is also a flexural uplift component to the flank topography. Rift depths are up to 2.5 km below the surrounding plains, with considerable along-strike variability. There is a 600 km lateral offset along Devana Chasma near the mid-point between the two mantle plumes. Devana most likely formed as two distinct rifts due to the horizontal stresses created by outflow from the upwelling plumes. The offset zone formed as a result of the interaction between the two rift tips, which requires that upwelling at the two mantle plumes overlapped in time.

The Wonji fault belt (Main Ethiopian Rift): structural and geomorphological constraints and GPS monitoring

Abstract The Wonji Fault Belt (WFB), Main Ethiopian Rift, forms a network of faults oriented NNE-SSW with a Quaternary direction of extension oriented c . N95° E. Faults are spaced between 0.5 and 2 km, show a fresh steep scarp, recent activity and slip rates of up to 2.0 mm a −1 . This high value of deformation along the rift floor with respect to the plate separation rates suggests that most of the active strain could be accommodated by magma-induced faulting within the rift. However, the mountain front morphology associated with a displacement of 300–400 m since the Middle Pleistocene, tilted-blocks, brittle-seismic fault rock fabric and historical earthquakes with M >6 support a tectonic origin of the Asela boundary fault. Therefore, we propose a model that considers the possible coexistence of both magmatic deformation at the rift floor and brittle faulting at the rift margin. We also report the data relative to a GPS network installed in December 2004, along two transects across the WFB, between Asela and the Ziway Lake.

Evolution of the Main Ethiopian Rift in the frame of Afar and Kenya rifts propagation

The Main Ethiopian Rift (MER) has a complex structural pattern composed of southern, central, and northern segments. Ages of onset of faulting and volcanism apparently indicate a heterogeneous time‐space evolution of the segments, generally referred to as a northward progression of the rifting process. New structural, petrological, and geochronological data have been used to attempt reconciling the evolution of the distinct MER segments into a volcanotectonic scenario accounting for the propagation of the Afar and the Kenya Rifts. In this evolutionary model, extension affected the Southern MER in the early Miocene (20–21 Ma) due to the northward propagation of the Kenya Rift‐related deformation. This event lasted until 11 Ma, then deformation decreased radically and was resumed in Quaternary times. In the late Miocene (11 Ma), deformation focused in the Northern MER forming a proto‐rift that we consider as the southernmost propagation of Afar. No major extensional deformation affected the Central MER in this period, as testified by the emplacement at 12–8 Ma of extensive plateau basalts currently outcropping on both rift margins. Significant rift opening occurred in the Central MER during the Pliocene (∼5–3 Ma) with the eruption of voluminous ignimbritic covers (Nazret sequence) exposed both on the rift shoulders and on the rift floor. The apparent discrepancy between the heterogeneous propagation of the three MER segments could be reconciled by considering the opening of Central MER and the later reactivation of the Southern MER as due to a southward propagation of rifting triggered by counterclockwise rotation of the Somalian plate starting around 10 Ma.