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1.
Giacomo Corti   《Earth》2009,96(1-2):1-53
The Main Ethiopian Rift is a key sector of the East African Rift System that connects the Afar depression, at Red Sea–Gulf of Aden junction, with the Turkana depression and Kenya Rift to the South. It is a magmatic rift that records all the different stages of rift evolution from rift initiation to break-up and incipient oceanic spreading: it is thus an ideal place to analyse the evolution of continental extension, the rupture of lithospheric plates and the dynamics by which distributed continental deformation is progressively focused at oceanic spreading centres.The first tectono-magmatic event related to the Tertiary rifting was the eruption of voluminous flood basalts that apparently occurred in a rather short time interval at around 30 Ma; strong plateau uplift, which resulted in the development of the Ethiopian and Somalian plateaus now surrounding the rift valley, has been suggested to have initiated contemporaneously or shortly after the extensive flood-basalt volcanism, although its exact timing remains controversial. Voluminous volcanism and uplift started prior to the main rifting phases, suggesting a mantle plume influence on the Tertiary deformation in East Africa. Different plume hypothesis have been suggested, with recent models indicating the existence of deep superplume originating at the core-mantle boundary beneath southern Africa, rising in a north–northeastward direction toward eastern Africa, and feeding multiple plume stems in the upper mantle. However, the existence of this whole-mantle feature and its possible connection with Tertiary rifting are highly debated.The main rifting phases started diachronously along the MER in the Mio-Pliocene; rift propagation was not a smooth process but rather a process with punctuated episodes of extension and relative quiescence. Rift location was most probably controlled by the reactivation of a lithospheric-scale pre-Cambrian weakness; the orientation of this weakness (roughly NE–SW) and the Late Pliocene (post 3.2 Ma)-recent extensional stress field generated by relative motion between Nubia and Somalia plates (roughly ESE–WNW) suggest that oblique rifting conditions have controlled rift evolution. However, it is still unclear if these kinematical boundary conditions have remained steady since the initial stages of rifting or the kinematics has changed during the Late Pliocene or at the Pliocene–Pleistocene boundary.Analysis of geological–geophysical data suggests that continental rifting in the MER evolved in two different phases. An early (Mio-Pliocene) continental rifting stage was characterised by displacement along large boundary faults, subsidence of rift depression with local development of deep (up to 5 km) asymmetric basins and diffuse magmatic activity. In this initial phase, magmatism encompassed the whole rift, with volcanic activity affecting the rift depression, the major boundary faults and limited portions of the rift shoulders (off-axis volcanism). Progressive extension led to the second (Pleistocene) rifting stage, characterised by a riftward narrowing of the volcano-tectonic activity. In this phase, the main boundary faults were deactivated and extensional deformation was accommodated by dense swarms of faults (Wonji segments) in the thinned rift depression. The progressive thinning of the continental lithosphere under constant, prolonged oblique rifting conditions controlled this migration of deformation, possibly in tandem with the weakening related to magmatic processes and/or a change in rift kinematics. Owing to the oblique rifting conditions, the fault swarms obliquely cut the rift floor and were characterised by a typical right-stepping arrangement. Ascending magmas were focused by the Wonji segments, with eruption of magmas at surface preferentially occurring along the oblique faults. As soon as the volcano-tectonic activity was localised within Wonji segments, a strong feedback between deformation and magmatism developed: the thinned lithosphere was strongly modified by the extensive magma intrusion and extension was facilitated and accommodated by a combination of magmatic intrusion, dyking and faulting. In these conditions, focused melt intrusion allows the rupture of the thick continental lithosphere and the magmatic segments act as incipient slow-spreading mid-ocean spreading centres sandwiched by continental lithosphere.Overall the above-described evolution of the MER (at least in its northernmost sector) documents a transition from fault-dominated rift morphology in the early stages of extension toward magma-assisted rifting during the final stages of continental break-up. A strong increase in coupling between deformation and magmatism with extension is documented, with magma intrusion and dyking playing a larger role than faulting in strain accommodation as rifting progresses to seafloor spreading.  相似文献   

2.
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.  相似文献   

3.
The Blue Nile Basin, situated in the Northwestern Ethiopian Plateau, contains ∼1400 m thick Mesozoic sedimentary section underlain by Neoproterozoic basement rocks and overlain by Early–Late Oligocene and Quaternary volcanic rocks. This study outlines the stratigraphic and structural evolution of the Blue Nile Basin based on field and remote sensing studies along the Gorge of the Nile. The Blue Nile Basin has evolved in three main phases: (1) pre‐sedimentation phase, include pre‐rift peneplanation of the Neoproterozoic basement rocks, possibly during Palaeozoic time; (2) sedimentation phase from Triassic to Early Cretaceous, including: (a) Triassic–Early Jurassic fluvial sedimentation (Lower Sandstone, ∼300 m thick); (b) Early Jurassic marine transgression (glauconitic sandy mudstone, ∼30 m thick); (c) Early–Middle Jurassic deepening of the basin (Lower Limestone, ∼450 m thick); (d) desiccation of the basin and deposition of Early–Middle Jurassic gypsum; (e) Middle–Late Jurassic marine transgression (Upper Limestone, ∼400 m thick); (f) Late Jurassic–Early Cretaceous basin‐uplift and marine regression (alluvial/fluvial Upper Sandstone, ∼280 m thick); (3) the post‐sedimentation phase, including Early–Late Oligocene eruption of 500–2000 m thick Lower volcanic rocks, related to the Afar Mantle Plume and emplacement of ∼300 m thick Quaternary Upper volcanic rocks. The Mesozoic to Cenozoic units were deposited during extension attributed to Triassic–Cretaceous NE–SW‐directed extension related to the Mesozoic rifting of Gondwana. The Blue Nile Basin was formed as a NW‐trending rift, within which much of the Mesozoic clastic and marine sediments were deposited. This was followed by Late Miocene NW–SE‐directed extension related to the Main Ethiopian Rift that formed NE‐trending faults, affecting Lower volcanic rocks and the upper part of the Mesozoic section. The region was subsequently affected by Quaternary E–W and NNE–SSW‐directed extensions related to oblique opening of the Main Ethiopian Rift and development of E‐trending transverse faults, as well as NE–SW‐directed extension in southern Afar (related to northeastward separation of the Arabian Plate from the African Plate) and E–W‐directed extensions in western Afar (related to the stepping of the Red Sea axis into Afar). These Quaternary stress regimes resulted in the development of N‐, ESE‐ and NW‐trending extensional structures within the Blue Nile Basin. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

4.
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.  相似文献   

5.
论博格达俯冲撕裂型裂谷的形成与演化   总被引：66，自引：23，他引：43  
博格达裂谷带位于准噶尔与吐－哈两个前寒武纪地块之间,呈东－西走向,东端与克拉麦里－哈尔里克泥盆－石炭纪火山弧呈大角度相交。该 裂谷于早－中石炭世启动和沉降,在盆地中堆积了巨厚的陆源碎屑岩夹双峰式火山岩。裂谷的闭合发生于中石炭世末至晚石炭世。在裂谷闭合后区域构造由挤压向拉张的转折时期,发生了以辉绿岩为主的侵入活动,并伴有少量中－酸性分异产物。博格达裂谷东、西两段的演化特征有着显著差异。东段早石炭世就已开始裂离,裂离过程的火山岩以玄武岩为主,仅有少量流纹岩,裂谷盆地强烈沉降,形成深海－半深海环境,裂谷在中石炭世末至晚石炭世初即已闭合,裂谷岩系因强烈褶皱,与上覆二叠系呈明显角度不整合,显示了“突变”式闭合特征。与此不同的是,西段至中石炭世才开始明显裂离,裂离过程的火山岩以英安岩和流纹岩为主,玄武岩量较少,火山－沉积岩系均形成于浅海环境,裂谷至晚石炭世末才发生闭合,裂谷岩系因未发生强烈褶皱,故与上覆二叠系为平行不整合接触,显示了“渐变”式闭合特征。该裂谷的形成是古亚洲洋壳向先存的准－吐－哈陆块斜向俯冲,将其东南端撕裂的产物,因而可称为俯冲撕裂型裂谷。演化过程沿走向的明显不均一性是这类裂谷的重要特点。  相似文献   

6.
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.  相似文献   

7.
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).  相似文献   

8.
The rate of lithospheric extension has previously been suggested as the most important factor governing the compositions of magmas generated in the Cenozoic Ethiopian volcanic province (CEVP). However, the distribution and chemistry of volcanic rocks extending from the western plateau margin at Addis Ababa to the rift floor in Nazret, northern sector of the main Ethiopian rift (MER), suggest that transitional magmatism in the region may have been triggered by an increase in the amount of lithospheric extension in the Early Pliocene. The rocks occur across an area of variable crustal thickness and show a general age progression from Upper Miocene (≤9 Ma) to Recent toward the rift. Alkalic basalts are extensive in the western part of the rift and along its margin but are found only locally within the rift, whereas transitional basalts are found within the rift only. Both types of basalts appear to have been derived from a common mantle source. In contrast, alkalic and transitional basalts on the Ethiopian plateau are mutually exclusive in terms of their spatial distribution, but exhibit a compositional contiguity which suggests that transitional magmas on the plateau formed at the expense of alkalic magmas, i.e. by equilibration of alkalic magmas at relatively shallow depth. The alkalic basalts bear clear record of a decrease in the degree of partial melting with time, suggesting that magmatism on the plateau was possibly triggered by a transient thermal anomaly.  相似文献   

9.
Petrological and geochemical data for basic (alkali basalts and hawaiites) and silicic peralkaline rocks, plus rare intermediate products (mugearites and benmoreites) from the Pleistocene Boseti volcanic complex (Main Ethiopian Rift, East Africa) are reported in this work. The basalts are slightly alkaline or transitional, have peaks at Ba and Nb in the mantle-normalized diagrams and relatively low 87Sr/86Sr (0.7039–0.7044). The silicic rocks (pantellerites and comendites) are rich in sanidine and anorthoclase, with mafic phases being represented by fayalite-rich olivine, opaque oxides, aenigmatite and slightly Na-rich ferroaugite (ferrohedenbergite). These rocks were generated after prolonged fractional crystallization process (up to 90–95 %) starting from basaltic parent magmas at shallow depths and fO2 conditions near the QFM buffer. The apparent Daly Gap between mafic and evolved Boseti rocks is explained with a model involving the silicic products filling upper crustal magma chambers and erupted preferentially with respect to basic and intermediate products. Evolved liquids could have been the only magmas which filled the uppermost magma reservoirs in the crust, thus giving time to evolve towards Rb-, Zr- and Nb-rich peralkaline rhyolites in broadly closed systems.  相似文献   

10.
The Lambert Rift, which is a large intracontinental rift zone in East Antarctica, developed over a long period of geological time, beginning from the Late Paleozoic, and its evolution was accompanied by magmatic activity. The latest manifestation of magmatism is eruption of alkaline olivine–leucite basalts on the western side of the Lambert Rift; Rb–Sr dating referred its time to the Middle Eocene, although its genesis remained vague. In order to solve this problem, we found geochronometer minerals in basaltic samples and 68 apatite grains appeared to be suitable for analysis. Their ages and ages of host basalts, determined by the U–Pb local method on the SIMS SHRIMP-II, were significantly different (323 ± 31 Ma) from those assumed earlier. This age corresponds to the earliest stage of crustal extension in East Antarctica and to most of Gondwana. The new data crucially change the ideas about the evolution of Lambert Rift and demonstrate the ambiguity of К–Ar dates of the alkali effusive formed under long-term rifting.  相似文献   

11.
The Xainza-Dinggye rift is one of several north-south trending rifts in central and southern Tibet created by Cenozoic east-west extension during Indo-Asian convergence. The southern part of the rift cuts through the Tethyan and High Himalayas. In the Tethyan Himalaya, this rift consists of an early domal structure and a late normal fault developed during the progressive deformation. The dome is cored by leucogranitic plutons that intruded during extension. Muscovite 40Ar/39Ar ages of the mylonitic leucogranite indicate that extension in the Tethyan Himalaya began at 8 Ma or before. In the High Himalaya, the rift is controlled by a normal fault dipping to the southeast. This fault has a structural constitution similar to a detachment fault. Its lower block is made up of mylonitic High Himalayan gneiss, intruded by early mylonitic leucogranite sills and late less-deformed biotite-bearing leucogranite dikes. Mica 40Ar/39Ar ages of these leucogranites and the retrograded metamorphosed gneiss of the lower block range from 13 to 10 Ma. In the study area, the south Tibetan detachment system (STDS) is a ductile shear zone composed of mylonitic leucogranite that is intruded by less-deformed leucogranite and overlain by low grade metamorphic rocks. Mica 40Ar/39Ar ages of leucogranites in the shear zone and schist from the detachment hanging wall indicate a protracted deformation history of the STDS from 19 to 13 Ma. The Xainza-Dinggye rift is younger than the STDS because it offsets the STDS; this north-south trending rift belongs to a different tectonic system from the east-west striking STDS, and may be caused by geological process related to India–Asia convergence. This temporal and spatial relationship of the STDS to the rift may indicate an important change in tectonic regime at 13 Ma in the building of the plateau.  相似文献   

12.
中国东南部晚中生代-新生代玄武岩与壳幔作用   总被引：46，自引：2，他引：44  
中国东南部的火山活动在早中生代时期仅有很小规模,晚中生代最早的、较大规模的岩浆活动始于中侏罗世早期,至早白垩世是火山岩浆活动的鼎盛期,在近100个Ma的时间内形成了大面积分布的晚中生代火山-侵入岩,而在新生代则以面积较小的玄武岩浆喷出活动为主,局限分布于沿海一带。以晚中生代湘南、赣南和闽西南的近EW向火山岩带和浙、闽沿海地区的近NNE向火山岩带,以及新生代近NNE向火山岩带为研究对象,对这些火山岩的地球化学特征对比研究,结合时空分布,讨论了它们的起源及其与壳幔相互作用的关系,以及它们形成的构造环境,其结果显示,EW向晚中生代火山岩带（180～170Ma）的西段玄武岩独立产出,且明显属碱性系列;而中段和东段玄武岩和流纹岩伴生,其中的玄武岩均为亚碱性系列的拉斑玄武岩。它们形成于板内拉张构造环境,是中国东南部特提斯构造域向太平洋构造域转换、晚中生代大规模岩浆作用的序幕。研究表明,该火山岩带自西向东表现出不同程度的壳幔相互作用,玄武岩在成岩过程中有少量陆壳组分加入。NNE向晚中生代火山岩带（130～90Ma）主要为流纹质岩石,安山岩和玄武岩很少。即使是双峰式火山岩也以酸性岩为主,玄武岩仅占全部火山岩体积的30％以下。其中的玄武岩主要属钙碱性系列,少数属拉斑系列。它们形成于火山弧构造环境,是中国东南部受太平洋构造域影响发生大规模火山岩浆作用的主旋律。其中玄武岩岩浆成分受到了较高程度的陆壳物质混染,同时代的中性火山岩是由底侵的玄武岩岩浆和陆壳物质来源的酸性岩浆发生岩浆混合作用而形成的,反映了强烈的壳幔相互作用。NNE向新生代火山岩带,分布在浙闽沿海,以碱性系列玄武岩为主,均含幔源包体,并受NNE向大陆边缘断裂构造的控制。它们形成于板内裂谷环境,是中国东南沿海由晚中生代火山弧构造环境转换为新生代板内裂谷环境的标志,起源于软流圈地幔,并有EMII岩石圈地幔的混合组分,但基本没有受到陆壳物质的混染。  相似文献   

13.
An isolated block of Precambrian basement rocks and Mesozoic sediments is exposed at Kella along the western margin of the Central Main Ethiopian Rift (MER), surrounded by Tertiary to Quaternary volcanic rocks. Apatite fission‐track thermochronology on two basement samples yielded ages of 7.2 ± 1.0 Ma and 6.7 ± 3.0 Ma and a long mean track length (>14.5 μm). Rapid Late Miocene cooling is attributed to denudation related to rifting. Despite the paucity of data, due to the absence of suitable lithologies in the area, our data confirm that the Central MER is younger than 8 Ma as recently proposed on the basis of field evidence and radiometric dating of volcanics. This implies that the Central MER formed after the Northern MER, indicating a diachronous development of this third arm of the Red Sea–Gulf of Aden–Ethiopian Rift system. Terra Nova, 00, 000–000, 2010  相似文献   

14.
A three-dimensional interpretation of the newly compiled Bouguer anomaly map of the Main Ethiopian Rift is discussed. Then, the crustal thickness distribution beneath the Main Ethiopian Rift is confirmed using a — dimensional inverse approach to gravity data interpretation. The depths to the crust-upper mantle interface form the inversion parameters. Both approaches are constrained with the results of the seismic refraction experiments of the region. The degree of ambiguity of the final model parameters is then quantified.The Bouguer anomalies along the axial portion of the rift floor, as deduced from the results of the regional and residual separation, are mainly caused by deep-seated structures. The high resolution 3-D forward modeling reveals a possible crustal thickness and density distribution beneath the graben.The results of the inversion confirm a strong crustal attenuation zone (≤ 31 km) closely associated with the rifting of the graben and an abrupt fall of the Moho interface on either side of the rift (up to 51 km) related to the formation of the western and southeastern plateaus. However, no indication of crustal separation is observed.The ambiguity analysis reveals that greater ambiguity of the model parameters exists in the southeastern plateau. There, these model parameters represent the depths to the Moho interface where the seismic control is relatively less.  相似文献   

15.
The African Rift Valleys results from Neogene-Recent breakup up the African Plate due to continental extension. Compared with the western sector, in the eastern or Gregory Rift Valley the fracturing is accompanied by a large amount of volcanic activity and, in northern Tanzania, the volcanic rocks are relatively young compared with those in Ethiopia and Kenya where the products of the earliest stages of mantle melting are buried beneath older, more-extensive volcanic formations. The volcanic rocks vary from Pliocene and early Pleistocene intraplate basalts and trachytes to late Pleistocene/Holocene nephelinites and carbonatites.  相似文献   

16.
The northeastern extremity of the East-Asian Rift Belt is designated as the Priokhotsky Rift, comprising the broadly north–south Torom (750 × 100 km) and Nizhneamursky (450 × 100 km) open faults formed by a system of northeast striking grabens associated with the closure of the Tan-Lu shear system and north–south striking grabens formed in a setting of oblique extension. Infilling of the grabens corresponding to the rift stage proper is the Eocene?Miocene coal-bearing molasse; the fields of the Miocene basalts are also related to it. The grabens of the rift belt are overlain by the Pliocene–Neopleistocene associations of rift basins in the forming plate cover of the Alpine platform.  相似文献   

17.
Lahar deposits occur within a shallow marine sedimentary succession of the Pliocene La Cueva Formation in the Coastal Cordillera of central Chile (33°40′–34°15′S). Provenance studies of the abundant volcanic material in the lahar deposits suggest that they derive from denudation by mass wasting of Oligocene–Miocene volcanic rocks on the western slopes of the Main Andean Cordillera at the same latitude. Pliocene rock debris deposits preserved in the region of El Teniente (34°S) and scattered along the westernmost part of the Andes of central Chile indicate catastrophic erosive events related to the rapid uplift of the cordilleran block. This rock debris was deposited by avalanches and transformed further downslope into lahars by dilution with stream water. Lahars were channeled along the ancient drainage system that reached a shallow Pliocene sea at the site of the present Coastal Cordillera. The exceedingly rapid exhumation of active porphyry systems during the Early Pliocene in this part of the Andes may have played a role in affecting hydrothermal processes, brecciation, and diatreme formation at the porphyry systems of El Teniente and Río Blanco–Los Bronces.  相似文献   

18.
This article outlines geomorphological and tectonic elements of the Afar Depression, and discusses its evolution. A combination of far-field stress, due to the convergence of the Eurasian and Arabian plates along the Zagros Orogenic Front, and uplift of the Afar Dome due to a rising mantle plume reinforced each other to break the lithosphere of the Arabian–Nubian Shield. Thermal anomalies beneath the Arabian–Nubian Shield in the range of 150 °C–200 °C, induced by a rising plume that mechanically and thermally eroded the base of the mantle lithosphere and generated pulses of prodigious flood basalt since ∼30 Ma. Subsequent to the stretching and thinning the Afar Dome subsided to form the Afar Depression. The fragmentation of the Arabian–Nubian Shield led to the separation of the Nubian, Arabian and Somalian Plates along the Gulf of Aden, the Red Sea and the Main Ethiopian Rift. The rotation of the intervening Danakil, East-Central, and Ali-Sabieh Blocks defined major structural trends in the Afar Depression. The Danakil Block severed from the Nubian plate at ∼20 Ma, rotated anti-clockwise, translated from lower latitude and successively moved north, left-laterally with respect to Nubia. The westward propagating Gulf of Aden rift breached the Danakil Block from the Ali-Sabieh Block at ∼2 Ma and proceeded along the Gulf of Tajura into the Afar Depression. The propagation and overlap of the Red Sea and the Gulf of Aden along the Manda Hararo–Gobaad and Asal–Manda Inakir rifts caused clockwise rotation of the East-Central Block. Faulting and rifting in the southern Red Sea, western Gulf of Aden and northern Main Ethiopian Rift superimposed on Afar. The Afar Depression initiated as diffused extension due to far-field stress and area increase over a dome elevated by a rising plume. With time, the lithospheric extension intensified, nucleated in weak zones, and developed into incipient spreading centers.  相似文献   

19.
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.  相似文献   

20.
《Comptes Rendus Geoscience》2019,351(5):355-365
Located in northern Niger, the NW–SE Téfidet trough is the western branch of the Ténéré rift megasystem.Here we present a tectono-sedimentary analysis of the Téfidet trough, based on the combined use of satellite imagery, field observations and measures, and available literature. We use these data to analyse the sedimentary facies and the tectonic deformations (faults, folds, basins) in the Téfidet trough, and derive their relative chronology. Doing so, we characterize synrift and postrift deformations and their interactions with sedimentation.Altogether our analyses suggest that the Téfidet trough was affected from the Cretaceous to the Paleogene by three major tectonic periods.
  • •The first period was a rifting stage with extension and transtension during the Albian–Aptian times. The mean extension was ∼N60° and dominantly produced NW–SE-trending normal faults, a few strike-slip faults locally associated with small folds with sigmoidal axis and small reverse faults, and progressive unconformities.
  • •the second period was also a rifting time, which prevailed during the Upper Cretaceous. The regime was marked by transtensional to extensional tectonics, under a ∼N130° shortening and a ∼N60° trending stretching. The end of this period saw the closure of the Téfidet trough.
  • •the third period was a postrift stage. It was characterized by a ∼N70° extensional to transtensional regime during the Oligocene–Pliocene. It mainly produced post-sedimentary extensional faults and fractures and alkaline volcanism. We eventually discuss these deformation phases in relation with the Cretaceous Gondwana breakup and its related rifting events in West and North Africa, and with the subsequent Africa–Europe collision.
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