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121.
This paper examines the status of traditional agriculture in two areas of Azilal province in the Atlas Mountains, seeking to identify what is needed to counter any breakdown in livelihood strategies and improve the quality of life enjoyed by the farmers. Irrigated terrace agriculture is an important part of livelihood strategies in the areas studied and has the potential to be more productive and sustainable. In some areas it is reported to be endangered by socioeconomic changes and highland land degradation caused by excessive shifting cultivation, sheep and goat grazing, and charcoal production. Field visits made during summer 1998 to establish the condition of irrigated terrace and runoff agriculture and the factors affecting it showed that in at least one of the villages supportive social capital has ensured that challenges are being met and beneficial innovations are being made. Considerable variation in success was evident from community to community, with some farmers maintaining adequate lifestyles and innovating, while in other communities prospects are less promising. The effects of structural adjustment have clearly been felt and may also have triggered some of the highland degradation and difficulties, which clearly threaten some irrigated terrace agriculture and other livelihood practices. 相似文献
122.
Mesozoic and Cenozoic vertical movements in the Atlas system (Algeria, Morocco, Tunisia): An overview 总被引:5,自引:0,他引:5
Dominique Frizon de Lamotte Pascale Leturmy Sami Khomsi Omar Saddiqi André Michard 《Tectonophysics》2009,475(1):9-28
The E-W trending Atlas System of Maghreb consists of weakly shortened, intra-continental fold belts associated with plateau areas (“Mesetas”), extending between the south-westernmost branch of the Mediterranean Alpine Belt (Rif-Tell) and the Sahara Platform. Although the Atlas system has been erected contemporaneously from Morocco to Algeria and Tunisia during the Middle Eocene to Recent, it displays a conspicuous longitudinal asymmetry, with i) Paleozoic outcrops restricted to its western part; ii) highest elevation occurring in the west, both in the Atlas System and its foreland (Anti-Atlas); iii) low elevation corridors (e.g. Hodna) and depressed foreland (Tunisian Chotts and Sahel area) in the east. We analyse the origin of these striking contrasts in relation with i) the Variscan heritage; ii) crustal vertical movements during the Mesozoic; iii) crustal shortening during the Cenozoic and finally, iv) the occurrence of a Miocene-Quaternary hot mantle anomaly in the west. The Maghreb lithosphere was affected by the Variscan orogeny, and thus thickened only in its western part. During the Late Permian-Triassic, a paleo-high formed in the west between the Central Atlantic and Alpine Tethys rift systems, giving birth to the emergent/poorly subsident West Moroccan Arch. During the late Middle Jurassic-Early Cretaceous, Morocco and western Algeria were dominantly emergent whereas rifting lasted on in eastern Algeria and Tunisia. We ascribe the uplift of the western regions to thermal doming, consistent with the Late Jurassic and Barremian gabbroic magmatism observed there. After the widespread transgression of the high stand Cenomanian-Turonian seas, the inversion of the Atlas System began during the Senonian as a consequence of the Africa-Eurasia convergence. Erosion affected three ENE-trending uplifted areas of NW Africa, which we consider as lithospheric anticlines related to the incipient Africa-Europe convergence. In contrast, in eastern Algeria and Tunisia a NW-trending rift system developed contemporaneously (Sirt rifting), normal to the general trend of the Atlas System. The general inversion and orogenesis of the Atlas System occurred during two distinct episodes, Middle-Late Eocene-Oligocene and Late Miocene-Pliocene, respectively, whereas during the intervening period, the Africa-Europe convergence was mainly accommodated in the Rif-Tell system. Inversion tectonics and crustal thickening may account for the moderate uplift of the eastern Atlas System, not for the high elevation of the western mountain ranges (Middle Atlas, High Atlas, Anti-Atlas). In line with previous authors, we ascribe part of the recent uplift of the latter regions to the occurrence of a NE-trending, high-temperature mantle anomaly, here labelled the Moroccan Hot Line (MHL), which is also marked by a strip of late Miocene-Quaternary alkaline magmatism and significant seismicity. 相似文献
123.
Mohammed Bouabdellah Georges Beaudoin David L. Leach Fidel Grandia Esteve Cardellach 《Mineralium Deposita》2009,44(6):689-704
The Assif El Mal Zn–Pb (Cu–Ag) vein system, located in the northern flank of the High Atlas of Marrakech (Morocco), is hosted
in a Cambro-Ordovician volcaniclastic and metasedimentary sequence composed of graywacke, siltstone, pelite, and shale interlayered
with minor tuff and mudstone. Intrusion of synorogenic to postorogenic Late Hercynian peraluminous granitoids has contact
metamorphosed the host rocks giving rise to a metamorphic assemblage of quartz, plagioclase, biotite, muscovite, chlorite,
amphibole, chloritoid, and garnet. The Assif El Mal Zn–Pb (Cu–Ag) mineralization forms subvertical veins with ribbon, fault
breccia, cockade, comb, and crack and seal textures. Two-phase liquid–vapor fluid inclusions that were trapped during several
stages occur in quartz and sphalerite. Primary inclusion fluids exhibit T
h mean values ranging from 104°C to 198°C. Final ice-melting temperatures range from −8.1°C to −12.8°C, corresponding to salinities
of ∼15 wt.% NaCl equiv. Halogen data suggest that the salinity of the ore fluids was largely due to evaporation of seawater.
Late secondary fluid inclusions have either Ca-rich, saline (26 wt.% NaCl equiv.), or very dilute (3.5 wt.% NaCl equiv.) compositions
and homogenization temperatures ranging from 75°C to 150°C. The δ18O and δD fluid values suggest an isotopically heterogeneous fluid source involving mixing between connate seawater and black-shale-derived
organic waters. Low δ13CVPDB values ranging from −7.5‰ to −7.7‰ indicate a homogeneous carbon source, possibly organic matter disseminated in black shale
hosting the Zn–Pb (Cu–Ag) veins. The calculated δ34SH2S values for reduced sulfur (22.5‰ to 24.3‰) are most likely from reduction of SO4
2− in trapped seawater sulfate or evaporite in the host rocks. Reduction of sulfate probably occurred through thermochemical
sulfate reduction in which organic matter was oxidized to produce CO2 which ultimately led to precipitation of saddle dolomite with isotopically light carbon. Lead isotope compositions are consistent
with fluid–rock interaction that leached metals from the immediate Cambro-Ordovician volcaniclastic and metasedimentary sequence
or from the underlying Paleo-Neoproterozoic crustal basement. Geological constraints suggest that the vein system of Assif
El Mal formed during the Jurassic opening of the central Atlantic Ocean. 相似文献
124.
Geomorphological heritage is a widely used term in European and North-American countries, but is still scarcely mentioned
in Africa. Nevertheless, the attractiveness of the African countries is often intimately connected to its breathtaking and
endless geological landscapes. Morocco is one of those countries that has the widest diversity in landscapes and landforms,
ranging from the Mediterranean and Atlantic coasts over the Rif, Middle Atlas, High Atlas and Anti-Atlas mountain chains to
the great rocky and sandy deserts in the South. A wide variety of geological units hosting different types of important economic
mineral deposits cover a temporal range from Late Precambrian to Quaternary. A detailed geomorphological study has been carried
out in the region of Ifrane and Azrou (Middle Atlas, Central Morocco) using a combination of high resolution satellite data
and direct field observations integrated by geological maps and scientific literature. In order to describe and evaluate the
geomorphological heritage of this area, 40 geomorphosites have been selected comprising springs, karst landforms (polje, dolines,
caves, sinkholes, stone forests, cryptokarstic dolines), carbonate depositional landforms (travertines and waterfalls), fluvial
landforms (meanders, canyons, palaeo-valleys, etc.), structural landforms (triangular facets, hogbacks, cuestas, residual
outcrops, etc.) and volcanic landforms (volcanoes, caldeira, pyroclastic cones, lava tube). The results of this research have
been summarised in a thematic map, representing the geomorphosites related to various landscape units. 相似文献
125.
MOHAMED NAJIB ZAGHLOUL SALVATORE CRITELLI FRANCESCO PERRI GIOVANNI MONGELLI VINCENZO PERRONE MAURIZIO SONNINO MAURICE TUCKER MARIANGELA AIELLO CATERINA VENTIMIGLIA 《Sedimentology》2010,57(2):312-350
The Middle to Upper Triassic redbeds at the base of the Ghomaride and Internal ‘Dorsale Calcaire’ Nappes in the Rifian sector of the Maghrebian Chain have been studied for their sedimentological, petrographic, mineralogical and chemical features. Redbeds lie unconformably on a Variscan low‐grade metamorphic basement in a 300 m thick, upward fining and thinning megasequence. Successions are composed of predominantly fluvial red sandstones, with many intercalations of quartzose conglomerates in the lower part that pass upwards into fine‐grained micaceous siltstones and massive mudstones, with some carbonate and evaporite beds. This suite of sediments suggests that palaeoenvironments evolved from mostly arenaceous alluvial systems (Middle Triassic) to muddy flood and coastal plain deposits. The successions are characterized by local carbonate and evaporite episodes in the Late Triassic. The growth of carbonate platforms is related to the increasing subsidence (Norian‐Rhaetian) during the break‐up of Pangea and the earliest stages of the Western Tethys opening. Carbonate platforms became widespread in the Sinemurian. Sandstones are quartzose to quartzolithic in composition, testifying a recycled orogenic provenance from low‐grade Palaeozoic metasedimentary rocks. Palaeoweathering indices (Chemical Index of Alteration, Chemical Index of Weathering and Plagioclase Index of Alteration) suggest both a K‐enrichment during the burial history and a source area that experienced intense weathering and recycling processes. These processes were favoured by seasonal climatic alternations, characterized by hot, episodically humid conditions with a prolonged dry season. These climatic alternations produced illitization of silicate minerals, iron oxidation and quartz‐rich red sediments in alluvial systems. The estimated burial temperature for the continental redbeds is in the range of 100 to 160 °C with lithostatic/tectonic loading of ca 4 to 6 km. These redbeds can be considered as regional petrofacies that mark the onset of the continental rift valley stage in the Western Pangea (Middle Triassic) before the opening of the western part of Tethys in the Middle Jurassic. The studied redbeds and the coeval redbeds of many Alpine successions (Betic, Tellian and Apenninic orogens) show a quite similar history; they identify a Mesomediterranean continental block originating from the break‐up of Pangea, which then played an important role in the post‐Triassic evolution of the Western Mediterranean region. 相似文献
126.
The aim of this paper is to describe the mafic rocks that crop out in the central-western Mesorif Zone (External Rif Belt), and discuss their geodynamic signification. Basalt flows, olistoliths and breccias occur in Oxfordian–Berriasian deposits of Mesorif units ascribed to the distal part of the African paleomargin. The climax of volcanic activity is observed at the northern border of a Kimmeridgian carbonate platform progressively dismembered during the Tithonian–Berriasian. In spite of the alteration of the basalts, their petrological and geochemical characters point to E-MORB affinities. The studied gabbro massifs (Bou Adel, Kef el Rhar west and north) occur as restricted slivers or klippes within the Senhadja nappe or mélange of the internal Mesorif, which overlies the basalt-bearing units and other, more external Mesorif units. The compositions range from troctolitic olivine gabbro to ferrogabbro with frequent ortho- to heteradcumulate textures; they display typical tholeiitic affinity. The gabbro massifs are crosscut by trondjhemite dykes and overlain by metabasalts, fault-scarp breccias, ophicalcites, marbles and radiolarites. Composition featuring initial near liquid composition, display multi elements patterns close to those of E-MORB, with a weak Eu negative anomaly and evidence of slight crustal contamination. These gabbro massifs were regarded as Jurassic–Cretaceous intrusions, locally dated (K–Ar) at 166 ± 3 Ma. Conversely, we assume they represent discrete samples of a Jurassic–Cretaceous oceanic basement (ophiolites), emplaced tectonically in the Senhadja nappe (mélange) of the central Mesorif. The correlation of both these types of mafic rock associations (paleomargin basalts and ophiolite klippes) with the serpentinites of the eastern Mesorif (Beni Malek) and Oran mountains (Algeria) is then briefly discussed. We conclude that the previous hypothesis of an intramargin “Mesorif suture zone” must be reconsidered, being challenged by that of a major, syn-collisional “Oran-Mesorif Strike-Slip Fault”. In the latter hypothesis, the newly described Mesorif oceanic klippes would represent allochthonous remnants of the Ligurian–Maghrebian (Tethyan) oceanic domain. 相似文献
127.
The continental Upper Triassic Tadrart Ouadou Sandstone Member was deposited in an extensional setting on the Pangaean continent, strongly influenced by a low‐latitude climatic regime (10° to 20° north). Complex interaction of basin subsidence and climatically driven processes led to high facies variability and a lack of correlatable units across the Argana Valley exposures. A process‐orientated approach integrating detailed facies with architectural element analysis was undertaken, which resulted in a multistage depositional model for the Tadrart Ouadou Sandstone Member. The basin‐scale model shows that basal alluvial fan and braided river systems are confined to the centre of the Argana Valley exposures. Aeolian deposits occur throughout the sequence, but dominate in the north. After a phase of playa deposition, prominent basin‐wide fluvial incision of up to 8 m marks the onset of perennial fluvial flow. These well‐sorted, internally complex and locally highly amalgamated fluvial sandstones are widespread throughout the basin and are focused in a north to south (south‐west) flowing channel system. After a final stage of aeolian sedimentation, sandstone deposition of the Tadrart Ouadou Sandstone Member in the Argana Valley is terminated rapidly by the onlap of lacustrine mudstones of the Sidi Mansour Member. The study revealed that, except for one pronounced period of perennial conditions, sedimentation is controlled largely by ephemeral fluvial flow, alternating ground water tables, deflation processes and periods with limited periodic local run‐off. The study highlights that facies architecture in the basin is the result of complex interaction of local syn‐sedimentary tectonics and the climatic regime within the basin, but also the climate of the catchment area to the east. The data suggest a proximal to mid‐distal basin setting in the rain‐shadow to the west of a mountain range (Massif Ancien), which exerted a strong control on the depositional environments of Triassic deposits exposed in this part of South‐west Morocco. 相似文献
128.
David M. Martill Nizar IbrahimPaulo M. Brito Lahssen BaiderSamir Zhouri Robert LoveridgeDarren Naish Richard Hing 《Cretaceous Research》2011,32(4):433-446
Laminated, dolomitised marine limestones within a transgressive fluvio-lagoonal-carbonate platform sequence at Gara Sbaa in the Kem Kem region of south eastern Morocco yield well preserved marine fishes and crustaceans. A rarer terrestrial component includes delicate fern fronds, insects and a lizard. Sedimentological structures and stratigraphic context indicate initial shallow waters with in a carbonate lagoon with benthic microbial mats followed by deepening. High precision dating has yet to be accomplished, but a Late Cenomanian or Early Turonian age is indicated on the basis of faunal and sequence-stratigraphic considerations. The fish assemblage has affinities with mid-Cretaceous ichthyofaunas in South America and Lebanon, including taxa in common at generic level. 相似文献
129.
Peggy Vincent Nathalie Bardet Xabier Pereda Suberbiola Badi Bouya Mbarek Amaghzaz Saïd Meslouh 《Gondwana Research》2011,19(4):1062-1073
Though the Maastrichtian Phosphates of Morocco have yielded very rich marine vertebrate assemblages, plesiosaurs remain very scarce in these strata. The only hitherto recognized taxon was Plesiosaurus mauritanicus Arambourg, 1952, regarded here as a nomem dubium. Here we describe a new genus and species of elasmosaurid plesiosaur, Zarafasaura oceanis, which represents the first valid elasmosaurid plesiosaur described from the latest Cretaceous of Africa, and the second one from this continent. A phylogenetic analysis of plesiosauroids indicates that Zarafasaura oceanis has close affinities with elasmosaurids from the Late Cretaceous of North America and Japan. Among its distinctive suite of characters, the general shape and organisation of its squamosal and palate are unique among elasmosaurids. This new taxon completes our understanding of Late Cretaceous plesiosaur palaeobiodiversity and palaeobiogeography, and shows that Maastrichtian plesiosaurs were characterized by a quite high degree of endemism. They were also highly diversified and distributed worldwide, which supports the hypothesis of a catastrophic extinction of plesiosaurs at the K/T boundary. 相似文献
130.