Ground cracks in Ethiopian Rift Valley: facts and uncertainties

No accurate relationship has been obtained in this study between ground cracks in Ethiopian Rift Valley and the most common causes of earth fissures such as aquifer-system compaction and increased horizontal seepage stresses. This is due to the fact that the level of groundwater withdrawal responsible for these processes is still negligible in Ethiopia. If aquifer-system compaction and increased horizontal seepage stresses have a certain role, then it should be through the long-term effect of groundwater flow from basins to neighboring lakes. The ground cracks appeared also not to have a direct link with active faulting or distant earthquakes. Structurally, the Ethiopian Rift Valley is dominated by NE–SW-trending tensional faults, but no evidence is obtained in this study to associate the process of surface cracking with major tectonics. However, an aseismic elastic strain, which originates at depth and propagates upward through sediments without the formation of bedrock faults, could result in conditions conducive to the development of cracks. Then, fissures might ultimately be created after heavy rainfalls by near-surface processes such as piping and hydrocompaction along water-line sources.     

A pantelleritic welded ash-flow tuff from the Ethiopian Rift Valley

Fantale (Lat. 8° 58 N., Long. 39° 54 E) is a typical Quaternary silicic strato-volcano situated on the floor of the northern part of the Main Ethiopian Rift. Following the growth of the main central cone, a voluminous ash-flow tuff was erupted in association with the development of the 4 km summit caldera. On the upper parts of the volcano the tuff is virtually restricted to three major eruptive pathways descending from the caldera. The tuff is more extensive on the lower flanks and forms a continuous sheet surrounding the volcano.The tuff is welded throughout, even when it is less than 1 m thick. It shows typical vitroclastic texture and is markedly eutaxitic in the intensely welded sections. The degree of welding appears to he largely independent of the total thickness of the tuff. Microscopic observations provide confirmatory evidence of postdepositional vesiculation of the tuff and also suggest that recrystallisation has not induced the development of cavities in the tuff.The detailed analytical data presented suggest that the chemical composition of the tuff was modified by recrystallisation which produced relative depletion in SiO₂, total Fe, Cl, and Na₂O and enrichment in Al₂O₃, K₂O and CaO. The trace elements Ba, La, Nb, Rb, and Zr were not affected by this process. Study of serial samples from three sections through the tuff has led to the identification of five of the constituent flow units of the tuff. The units are approximately homogeneous in composition but differ from each other. The earliest unit is more silicic and pantelleritic, whereas the later flow units are more trachytic in composition. This data is interpreted in terms of the progressive emptying of a small but strongly zoned high-level magma chamber.The presence of primary laminar flowage structures, postdepositional vesiculation effects, intense welding in thin units and high initial dips suggests that the Fantale tuff was deposited from a series of dense, perhaps partially fluidized, magma pulses.     

The origin of high bicarbonate and fluoride concentrations in waters of the Main Ethiopian Rift Valley, East African Rift system

Thermal waters in the Main Ethiopian Rift Valley are characterized by high Na, bicarbonate and fluoride concentrations, and near-neutral to alkaline pH. Sodium, bicarbonate and fluoride are positively correlated in the waters. The principal reason for the bicarbonate in the area is the high rate of carbon dioxide outgassing. This, combined with acid volcanics, geothermal heating, low Ca and low salinity, is also one of the causes of high fluoride in this part of the active volcanic zone of the East African Rift. Evaporative concentration is responsible for the high salinity, alkalinity and fluoride in the closed-basin lakes of the region. The waters are undersaturated with respect to fluoride and anhydrite. Calcium tends to be fixed in Ca bearing minerals such as calcite and epidote, which are abundant in the system. Hence, it appears that fluoride is a mobile component in acid volcanic geothermal systems.     

Landslides in the Ethiopian highlands and the Rift margins

Landslide hazard is one of the crucial environmental constraints for the development of Ethiopia, representing a limiting factor for urbanization and infrastructures. The high relief and the rugged topography induced by a strong Plio-Quaternary uplift, the occurrence of clayey horizons within the sedimentary sequences, the dense network of tectonic fractures and faults, the thick eluvial mantles on volcanic outcrops, and the thick colluvial–alluvial deposits at the foot of steep slopes are the predisposing factors for a large variety of mass movements. Heavy summer rainfall is the main triggering factor of most landslides, some of which undergo a step-like evolution with long-lasting quiescence intervals. First generation movements are commonly restricted to shallow phenomena, such as soil slips or mud flows in eluvial–colluvial material. Fast moving slope failures, such as rock slides, topplings and falls, are also triggered by earthquakes. To mitigate the landslide risk, any first priority measure should include adequate drainage of slopes in order to reduce water infiltration. On the other hand, appropriate site selection for buildings, transferring risky settlements, accurate geological control of works, and education campaigns are all strongly recommended.     

Structure of the Ethiopian lithosphere: Xenolith evidence in the Main Ethiopian Rift

The lithospheric and sublithospheric processes associated with the transition from continental to oceanic magmatism during continental rifting are poorly understood, but may be investigated in the central Main Ethiopian Rift (MER) using Quaternary xenolith-bearing basalts. Explosive eruptions in the Debre Zeyit (Bishoftu) and Butajira regions, offset 20 km to the west of the contemporaneous main rift axis, host Al-augite, norite and lherzolite xenoliths, xenocrysts and megacrysts. Al-augite xenoliths and megacrysts derived from pressures up to 10 kb are the dominant inclusion in these recent basalts, which were generated as small degree partial melts of fertile peridotite between 15 and 25 kb. Neither the xenoliths nor the host basalts exhibit signs of carbonatitic or hydrous (amphibole + phlogopite) metasomatism, suggesting that infiltration of silicate melts resulting in pervasive Al-augite dyking/veining dominates the regional lithospheric mantle. Recent geophysical evidence has indicated that such veining/dyking is pervasive and segmented, supporting the connection of these Al-augite dykes/veins to the formation of a proto ridge axis. Al-augite xenoliths and megacrysts have been reported in other continental rift settings, suggesting that silicate melt metasomatism resulting in Al-augite dykes/veins is a fundamental processes attendant to continental rift development.     

Peralkaline magma evolution and the tephra record in the Ethiopian Rift

The 3.119 ± 0.010 Ma Chefe Donsa phreatomagmatic deposits on the shoulder of the Ethiopian Rift mark the northern termination of the Silti-Debre Zeyit Fault Zone, a linear zone of focused extension within the modern Ethiopian Rift. These peralkaline pumice fragments and glass shards span a wide range of glass compositions but have a restricted phenocryst assemblage dominated by unzoned sanidine. Glass shards found within the ash occupy a far more limited compositional range (75–76 wt% SiO₂) in comparison with the pumice (64–75 wt% SiO₂), which is rarely mingled. Thermodynamic modeling shows that liquids broadly similar to the least evolved glass composition can be achieved with 50–60 % fractionation of moderately crustally contaminated basalt. Inconsistencies between modeled solutions and the observed values of CaO and P₂O₅ highlight the important role of fluorine in stabilizing fluor-apatite and the limitations of current thermodynamic models largely resulting from the scarce experimental data available for the role of fluorine in igneous phase stability. On the basis of limited feldspar heterogeneity and crystal content of pumice at Chefe Donsa, and the difficulties of extracting small volumes of Si-rich melt in classical fractional crystallization models, we suggest a two-step polybaric process: (1) basaltic magma ponds at mid-upper-crustal depths and fractionates to form a crystal/magma mush. Once this mush has reached 50–60 % crystallinity, the interstitial liquid may be extracted from the rigid crystal framework. The trachytic magma extracted at this step is equivalent to the most primitive pumice analyzed at Chefe Donsa. (2) The extracted trachytic liquid will rise and continue to crystallize, generating a second mush zone from which rhyolite liquids may be extracted. Some of the compositional range observed in the Chefe Donsa deposits may result from the fresh intrusion of trachyte magma, which may also provide an eruption trigger. This model may have wider application in understanding the origin of the Daly Gap in Ethiopian magmas—intermediate liquids may not be extracted from crystal-liquid mushes due to insufficient crystallization to yield a rigid framework. The wide range of glass compositions characteristic of the proximal Chefe Donsa deposits is not recorded in temporally equivalent tephra deposits located in regional depocenters. Our results show that glass shards, which represent the material most likely transported to distal depocenters, occupy a limited compositional range at high SiO₂ values and overlap some distal tephra deposits. These results suggest that distal tephra deposits may not faithfully record the potentially wide range in magma compositions present in a magmatic system just prior to eruption and that robust distal–proximal tephra correlations must include a careful analysis of the full range of materials in the proximal deposit.     

Pleistocene movements in the Gregory Rift Valley

Summary The Tertiary and post-Tertiary chronology of faulting in the Gregory Rift Valley is discussed with particular reference to areas where Middle Pleistocene deposits have been mapped. It is concluded that while strong faulting occurred at the end of the Middle Pleistocene period, the main faulting was earlier. At least three pre-Middle Pleistocene phases of faulting are recognised; the maximum movements probably occurred in late Pliocene or earliest Pleistocene times.     

Listric growth faults in the Kenya Rift Valley

Many of the major faults in the Kenya Rift Valley are curved in section, were active over considerable periods and form sets which are related in space and time. They can, therefore, be regarded as systems of listric growth faults. The Elgeyo Fault marks the western limit of rift structures at this latitude and displaces the basement surface by up to about 6 km. The Kamasia Hills are a block rotated above this fault plane. Movement on the Elgeyo Fault has been grossly continuous since at least 16 Ma ago but deposition of volcanics and sediments has generally kept pace with the growth of the escarpment. The Kaparaina Arch is a rollover anticline on the downthrown side of the Saimo Fault on the eastern side of the Kamasia Hills. On the eastern side of the rift, the block between the Bogoria and Wasages-Marmanet Faults has shown continued rotation since about 15 Ma. The Pleistocene lavas on the rift floor here show rollover into the Bogoria Fault and have formed a facing near the top of the escarpment. Area balancing calculations suggest depths to décollement of 25 km for the Elgeyo Fault, 6 km for the Saimo Fault and 12 km for the Bogoria Fault. The most direct evidence for the listric nature of the faults is provided by microearthquakes near Lake Manyara which appear to lie on fault planes connected to surface escarpments.     

Holocene extension direction along the Main Ethiopian Rift, East Africa

Assessment of the extension direction and its spatial and temporal variations is critical for evaluating a rifting process. The extension direction in the East African Rift System is a matter of debate and the NE–SW-trending Ethiopian portion is one of the most controversial areas, for which several extension directions have been proposed. Field analysis was performed along the axis of the Main Ethiopian Rift (MER), aiming at recognizing the opening direction of the NNE–SSW-trending Holocene extension fractures. The matching of pairs of asperities along the sides of these fractures allowed evaluation of the horizontal displacement and, thus, the extension direction. The collected data reveal a consistent Holocene extension direction, with a mean value of N52°W ± 20°. This NW–SE direction is constant along the MER, despite the ∼15° variations observed in its trend and the NNE–SSW trend of the extension fractures, oblique to the NE–SW trend of the MER.     

The Chemistry and Petrogenesis of a Suite of Pantellerites from the Ethiopian Rift

Data on the major and trace element chemistry of a suite oftwenty pantelleritic pitchstone lavas from the Quaternary Ethiopianvolcano Fantale is presented. This reveals a contrast betweenthe composition of the pre-caldera flows and the more siliceous,less peralkaline post-caldera lavas. Comparison with experimental and theoretical studies suggeststhat nearly all of the major and trace element variation withinthe two suites can be explained by assuming fractional crystallizationof alkali-feldspar, the most abundant phenocryst phase. Fractionationof the mafic phases appears to have been less significant. The trace element data strongly indicate that the lavas allbelong to a single suite. However, it is suggested that themajor element chemistry of the post-caldera flows was modifiedby the loss of volatiles at the time of the formation of thecaldera, an event which coincided with the eruption of a 2 km³welded ash-flow tuff.     

A preliminary study on the Middle-Lower Yangtze River Rift Valley

The famous Middle-Lower Yangtze River Rift formed in Mesozoic (Yenshanian) is a typical continental rift, which is related to the predominant NE-striking faulting, and features a unique band-netted structural pattern as a controlling factor for the extensively developed magmatism. A variety of ore deposits were formed as a result of magmatic activities. The development of the rift was principally governed by the uplifting of the upswelled zone triggered off by strong compressional stress in the upper mantle underneath the rift. Rifting, however, was superimposed upon the folding movement of late Indosinian period in the middle-lower reaches of the Yangtze River, which had more or less influence on the development of the rift, thus leading to some distinct features differing from those of typical continental rifts found elsewhere in the world.     

Obsidian from the northern sector of the Main Ethiopian Rift: implications for archeology

Obsidian is abundant in the Main Ethiopian Rift (MER). Petrological and geochemical features of obsidian from four volcanic centers in the MER, namely Birenti, Dofen, Fentale and Kone, are presented. Compositional and petrological variability is noted among the Dofen and Fentale obsidian, but not in those from Kone and Birenti where each have separate but uniform elemental composition. The Fentale and Kone obsidian were source materials for the artifacts of a number of Middle Stone Age and Later Stone Age/Neolithic sites in the region. We have yet to determine whether Dofen and Birenti were sources for archeological artifacts. The study also shows that volcanic episodes from a single center do not necessarily result in compositional variability.     

The hydrogeology of Adama-Wonji basin and assessment of groundwater level changes in Wonji wetland,Main Ethiopian Rift: results from 2D tomography and electrical sounding methods

Rise of groundwater level becomes an emerging concern at Wonji irrigation field, Main Ethiopian Rift. An integrated study based on geophysical resistivity methods is conducted at Wonji wetland to understand the link between irrigation water and the shallow aquifer system as well as to confirm the current concern of groundwater rise. The study was also intended to improve the uncertainty of understanding the hydrogeology of Wonji wetland including the extent and direction of groundwater–surface water interaction. The vertical and horizontal contacts between the different geological series of the Wonji area are resolved with 2D high-resolution geophysical imaging. Results from both VES and 2D tomography show low resistivity layers under Wonji irrigation field with narrow ranges in resistivity variation which corresponds to a homogeneous saturated layer. The geoelectric sections reveal two fault systems running NW–SE and N–S directions which impede lateral groundwater flow. Furthermore, groundwater is converged towards the Wonji irrigation site strained by these fault systems. The geophysical results show strong link between irrigation water and the shallow unconfined aquifer as well as among the local and regional flow systems.     

Local winds in the upper Rhone Valley

Balloon soundings during July and August 1979, 1981 and 1982 showed the vertical structure of the flow in the upper Rhone Valley. Between the low level winds up to a height of about 2000 m a s l and the gradient winds above 3000 m a s l, in 73 % of the 107 ascents, a counterflow was detected. It appeared more often in connection with down-valley flow (89 %) than with up-valley flow (38 %) above the ground. This flow pattern was found to be almost unaffected by the upper winds. The horizontal structure of the wind was studied with 3 ground weather stations that were separated 2 and 5 km along the valley axis. Up-valley winds occur in the average of 32 fair weather days only around noon. During the time of strongest up-slope winds, the valley wind is down-valley. That was already found in the climatic mean by Yoshino (1964) with wind shaped trees. As the wind recordings show, the down-valley flow develops first at the end of the valley and the resulting convergence zone moves down with about 2 m/s until it stops above a characteristic step near Fiesch (Fig 6). An explanation can be given by differential heating within the Rhone Valley itself and due to neighbouring valleys. The measured differences in the diurnal pressure changes of 5 stations is consistent with that hypothesis.     

Local winds in the upper Rhone Valley

Balloon soundings during July and August 1979, 1981 and 1982 showed the vertical structure of the flow in the upper Rhone Valley. Between the low level winds up to a height of about 2000 m a s l and the gradient winds above 3000 m a s l, in 73 % of the 107 ascents, a counterflow was detected. It appeared more often in connection with down-valley flow (89 %) than with up-valley flow (38 %) above the ground. This flow pattern was found to be almost unaffected by the upper winds.The horizontal structure of the wind was studied with 3 ground weather stations that were separated 2 and 5 km along the valley axis. Up-valley winds occur in the average of 32 fair weather days only around noon. During the time of strongest up-slope winds, the valley wind is down-valley. That was already found in the climatic mean by Yoshino (1964) with wind shaped trees.As the wind recordings show, the down-valley flow develops first at the end of the valley and the resulting convergence zone moves down with about 2 m/s until it stops above a characteristic step near Fiesch (Fig 6).An explanation can be given by differential heating within the Rhone Valley itself and due to neighbouring valleys. The measured differences in the diurnal pressure changes of 5 stations is consistent with that hypothesis.     

Hydrogeochemical study in the Main Ethiopian Rift: new insights to the source and enrichment mechanism of fluoride

The central Main Ethiopian Rift suffers a severe water quality problem, characterized by an anomalously high fluoride (F) content that causes an endemic fluorosis disease. The current study, conducted in the Ziway–Shala lakes basin, indicates that the F content exceeds the permissible limit for drinking prescribed by the World Health Organization (WHO; 1.5 mg/l) in many important wells (up to 20 mg/l), with even more extreme F concentration in hot springs and alkaline lakes (up to 97 and 384 mg/l respectively). The groundwater and surface water from the highlands, typically characterized by low total dissolved solids (TDS) and Ca (Mg)–HCO₃ hydrochemical facies, do not show high F content. The subsequent interaction of these waters with the various rocks of the rift valley induces a general increase of the TDS, and a variation of the chemical signature towards Na–HCO₃ compositions, with a parallel enrichment of F. The interacting matrixes are mainly rhyolites consisting of volcanic glass and only rare F-bearing accessory minerals (such as alkali amphibole). Comparing the abundance and the composition of the glassy groundmass with other mineral phases, it appears that the former stores most of the total F budget. This glassy material is extremely reactive, and its weathering products (i.e. fluvio/volcano-lacustrine sediments) further concentrate the fluoride. The interaction of these “weathered/reworked” volcanic products with water and carbon dioxide at high pH causes the release of fluoride into the interacting water. This mainly occurs by a process of base-exchange softening with the neo-formed clay minerals (i.e. Ca–Mg uptake by the aquifer matrix, with release of Na into the groundwater). This is plausibly the main enrichment mechanism that explains the high F content of the local groundwater, as evidenced by positive correlation between F, pH, and Na, and inverse correlation between F and Ca (Mg). Saturation indices (SI) have been calculated (using PHREEQC-2) for the different water groups, highlighting that the studied waters are undersaturated in fluorite. In these conditions, fluoride cannot precipitate as CaF₂, and so mobilizes freely without forming other complexes. These results have important implications for the development of new exploitation strategies and accurate planning of new drilling sites. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Early evolution and transformation of organic matter in the active continental Jordan Rift Valley

A sequence of alternating lacustrine marls, peat and basalts was penetrated in the Notera-3 well in the northern part of the Jordan Rift, Israel. The 2781 m thick sequence, ranges from Upper Miocene to Recent, reflects high sedimentation rate in the active continental rift associated with the Dead Sea Transform. The deep burial and the relatively high geothermal gradient (40°C km⁻¹) compensate for the short time span so that coalification expressed by vitrinite reflectance consistently increases with depth, from about 0.32% Ro at 1040 m to 0.48% Ro at 2495 m.Analysis of the peat reveals that the O/C, S/C and δ¹³C of the humic acids (HA) and the heavy to light normal alkane ratios are the only parameters sensitive enough to express this slight maturation increase with depth. A sharp δ¹³C change from about − 18‰ prevailing in the uppermost meters to an average of − 27.5‰ at 15 m and deeper reflects a change in the higher plant source of the peat (from C4 to C3 plants) rather than an early diagenetic modification.The δ¹³C, O/C, S/C and N/C ratios are usually lower in the kerogens than in the corresponding HA. The decrease in the δ¹³C and the O/C ratios are explained by elimination of oxygen-containing functional groups during transformation and by polymerization effects. The gradual decrease in the ¹²C and the O/C of the HA with depth are attributed to decarboxylation coupled with kinetic effects. The N/C depletion during the transformation from HA to kerogens probably results from the breakdown of amino acids. The S/C ratio which decreases both during this transformation and also with maturation is most readily explained by the breakdown of ester sulfate-containing groups such as sulfated polysaccarids, which formed diagenetically during the humification process.     

Lake-groundwater relationships and fluid-rock interaction in the East African Rift Valley: isotopic evidence

The assessment of water resources in the Rift Valley environment is important for population, agriculture and energy-related issues and depends on a good understanding of the relationship between freshwater lakes and regional groundwater. This can be hampered by the amount of fluid-rock interaction which occurs throughout the rift, obscuring original hydrochemical signatures. However, O and H stable isotope ratios can be used as tracers of infiltration over sometimes considerable distances, while showing that the volcanic edifices of the rift floor have varying effects on groundwater flow patterns. Specific cases from Kenya and Ethiopia are considered, including Lakes Naivasha, Baringo, Awasa and Zwai.In addition to their physical tracing role, stable isotopes can reveal information about processes of fluid-rock interaction. The general lack of O isotope shifting in rift hydrothermal systems suggests a high water:rock ratio, with the implication that these systems are mature. Carbon isotope studies on the predominantly bicarbonate waters of the rift show how they evolve from dilute meteoric recharge to highly alkaline waters, via the widespread silicate hydrolysis promoted by the flux of mantle carbon dioxide which occurs in most parts of the rift. There appears to be only minor differences in the C cycle between Kenya and Ethiopia.     

The Role of Pre-existing Structures in the Origin, Propagation and Architecture of Faults in the Main Ethiopian Rift

Four major fault systems oriented N–S to NNE–SSW, NE–SW, E–W and NW–SE are identified from Landsat Thematic Mapper (TM) images and a high resolution digital elevation model (DEM) over the Ethiopian Rift Valley and the surrounding plateaus. Most of these faults are the result of Cenozoic - extensional reactivation of pre-existing basement structures. These faults interacted with each other at different geological times under different geodynamic conditions. The Cenozoic interaction under an extensional tectonic regime is the major cause of the actual volcano-tectonic landscape in Ethiopia. The Wonji Fault Belt (WFB), which comprises the N–S to NNE–SSW striking rift floor faults, displays peculiar propagation patterns mainly due to interaction with the other fault systems and the influence of underlying basement structures. The commonly observed patterns are: curvilinear oblique-slip faults forming lip-horsts, sinusoidal faults, intersecting faults and locally splaying faults at their ends. Fault-related open structures such as tail-cracks, releasing bends and extensional relay zones and fault intersections have served as principal eruption sites for monogenetic Plio-Quaternary volcanoes in the Main Ethiopian Rift (MER).