Seismicity of Morocco for the Period 1987–1994

— A seismic data file of 3,740 earthquakes from January 1987 to December 1994 has been elaborated for Morocco and the border regions, with 10 main events registering magnitudes from 5 to 5.6. Such seismicity is particularly important for Morocco as the released seismic energy constitutes a considerable part of the total energy radiated during the 20th century. Relative seismicity maps confirm the persistence of the major features of the seismicity of Morocco. An important seismic activity is observed in the Alboran region continental crust, which absorbs the maximum deformation resulting from the convergence of the African and Iberian plates. However, in the longitude window 3.5°– 6° W at depths of 25 to 50 km, a seismic gap zone seems to take place. An explanation of this phenomenon may be provided by the slab breakoff model. Even if the seismicity of Morocco remains moderate, heavy damage is observed when the magnitude of earthquakes exceeds 4.5, especially in the case of traditional buildings.     

Silver and silver-bearing minerals at the Um Samiuki volcanogenic massive sulphide deposit, Eastern Desert, Egypt

The Um Samiuki Zn–Cu–Pb–Ag mineralisation, south Eastern Desert, Egypt is hosted by felsic volcanic rocks which form part of the 712-Ma-old, east-west-trending Shadli Volcanic Belt. Two major occurrences of massive sulphides are present at the top of rhyolitic breccia in the Western and Eastern mine areas. In each occurrence, a bornite-bearing zone is overlain by a pyrite-chalcopyrite-bearing zone and underlain by a disseminated, Cu-depleted zone. In the massive sulphide ore, sphalerite, chalcopyrite, pyrite, galena, bornite and tetrahedrite–tennantite are major minerals, whereas arsenopyrite, pyrrhotite, molybdenite and magnetite are accessory phases. Covellite and digenite are common secondary minerals. Bornite, tetrahedrite–tennantite and covellite contain high amounts of silver (averages of 1.97, 1.39 and 1.82 wt% respectively). Based on mineralogical balance calculations, bornite and covellite accommodate 80% of silver in the Um Samiuki deposit. Ag was incorporated in the crystal structure of the early-crystallised copper sulphides and sulphosalts and silver minerals. The temperature, sequential precipitation of the fluids and the structure of the crystallising phases control the distribution of silver. Post-depositional deformation and metamorphic processes caused liberation, remobilisation and redeposition of silver within the massive sulphides.Editorial handling: D. Lentz     

Apport de la gravimétrie à l'étude de la structure du bassin des Triffa (Maroc nord-oriental) : implications hydrogéologiques

Various techniques (horizontal and vertical derivatives, upward continuation, Euler deconvolution) have been applied to the gravity data from the Triffa's plain and the north flank of the Beni-Snassen massif to delineate various major geological structures such as faults and basins. These results allow the production of a structural map showing the fault systems for the survey area. This map forms the basis for planning future hydrogeological research in this region. To cite this article: D. Khattach et al., C. R. Geoscience 336 (2004).     

Zinc,lead, chlorine and FeOOH-bearing assemblages in the Apollo 16 sample 66095: Origin by impact of a comet or a carbonaceous chondrite?

Sample 66095, 89 collected from station 6 from the lunar Highlands in the Descartes Site shows evidence of mild to severe shock. These shock features are accompanied by an unusual enrichment in the volatile elements Cl, Zn and Pb and by the presence of FeOOH.FeOOH occurs in two distinct assemblages: (1) with metallic FeNi, (2) with troilite, sphalerite and two Cl bearing Zn, Fe sulfates. Lead is present exclusively in the second assemblage at the boundaries between troilite and goethite. Lead concentrations up to 0.4% were found. However, the nature of lead-bearing phase is unknown. X-ray fluorescence analyses of a 10 × 6 mm area of the thin section also yielded enhanced chlorine, sulfur and zinc contents.The formation of this unique assemblage and the introduction of the material rich in volatile elements is very probably genetically connected with an impact of a carbonaceous chondrite or a comet. The small range of the reaction between the volatile rich gases and metallic FeNi and troilite indicate a short-live-phenomenon and thus fumarolic activity is a very unlikely process.     

The mobility spectrum and the ion concentration of large ions in air mixed with N2O gas

Summary Using the divided electrode condenser it was possible to detect the large ion groups formed when small amounts of N₂O gas were mixed with atmospheric air. Eight groups appeared with mobilities ranging from 12.50×10^–4 to 0.60×10^–4 cm/sec: volt/cm. When using the whole electrode condenser the results showed an increase in the total ion concentration of these large ions when small amounts of N₂O gas were mixed with air. The results obtained in this work confirm that N₂O gas acts as a nucleus for condensation which is changed into a large ion by appropriating an electrical charge.     

Evapotranspiration and canopy resistance of grass in a Mediterranean region

Summary A simple method for estimating actual evapotranspiration (ET) could become a suitable tool for irrigation scheduling. Resistance models can be useful if data on canopy resistance to water vapor flow (rc) and on aerodynamic resistance (ra) are available. These parameters are complex and hard to obtain. In this studyrc is analysed for a reference crop (grass meadow). Canopy resistance is dependent on climate, weather (radiation, atmospheric vapor pressure deficit, aerodynamic resistance), agronomic practices (irrigation, grass cutting) and time scale (hour, day). Anrc model, proposed by Katerji and Perrier (KP model), using some meteorological parameters as inputs, is presented. Canopy resistance calculated according to the KP model was used to estimate a referenceET _ref on hourly and daily time scales.TheET _ref estimated using the KP model on a daily time scale was compared with a model proposed by Allen, Jensen, Wright and Burman (AJWB model) — in whichrc depends on leaf area index only — and with direct measurements from a weighing lysimeter. The results show an underestimation of 18% for the AJWB model against an underestimation of 2% for the KP model. Since the hypotheses are the same for both models and aerodynamic resistance plays a secondary role, the better results obtained by the KP model are due torc modelling.With 11 Figures     

Earth Fissures Caused By Groundwater Withdrawal In Sarir South Agricultural Project Area,Libya

arir South agricultural well field consists of 157 production wells constructed in double rows running east to west. Each well was designed to pump 76 l/sec to irrigate 80 hectares of winter and summer crops. Large volumes of water have been pumped from the aquifer since 1975. By 1990, water levels in the most heavily pumped areas had declined more than 6 meters. These large water-level declines have caused compaction of compressible fine-grained deposits within the aquifers. Earth fissures were reported in the Sarir South agricultural project as early as 1982. These fissures were mainly caused by lowering of the piezometric heads due to the withdrawal of groundwater. The fissures range in width from 2.5 cm to 100 cm on the surface and, generally, become narrower with depth. However, the total depth is not known for any of the fissures. Development of new cracks or fissures near the old ones has also been observed. Most of the recent fissures occur parallel and immediately adjacent to the older fissures. It appears that these fissures tend to develop into a linear fissure system in a north-south direction. Résumé. Le champ captant du sud de Sarir, destiné à l'irrigation, est constitué de 157 puits d'exploitation, disposés en deux rangées orientées est-ouest. Chaque puits est équipé pour pomper 76 l/sec pour irriguer 80 ha de cultures d'hiver et d'été. Des volumes considérables d'eau ont ainsi été pompés dans l'aquifère principal depuis 1975. Depuis 1990, le niveau piézométrique dans les zones les plus sollicitées est descendu de plus de 6 m. Cet abaissement important a provoqué dans les aquifères une compaction des dépôts compressibles à grains fins. Dès 1982, le sol de cette région s'est fissuré. Ces fissures proviennent pour la plupart de l'abaissement de la surface piézométrique sous l'effet du pompage de l'eau souterraine. Ces fissures sont larges de 2.5 à 100 cm, en surface et se rétrécissent en profondeur. Leur profondeur totale n'est pas connue. De nouvelles fissures apparaissent au voisinage des anciennes. La plupart d'entre elles sont parallèles et adjacentes aux anciennes. Engin, les fissures tendent à former un ensemble linéaire orienté nord-sud.     

Late Eocene marginal marine deposits and paleoenvironment characterisation from the Maadi Formation (Northern Eastern Egypt)

Two beds containing relatively thick and highly concentrated oyster shell banks occurring in the densely fossiliferous Wadi Al Abraq outcrop (Upper Eocene Maadi Formation, Cairo-Sukhna Road, Egypt). Such shell beds are almost monospecific. The upper level consists mainly of Ostrea clotbeyi Bellardi, topped by a considerable concentration of Carolia placunoides Cantraine, while in the lower level occurs Ostrea multicostata Deshayes. In both beds, the upper contact with the overlying marl bed (about 2 m thick) is sharp and sometimes erosive. Similarly, the contact with the underlying shale (1.5 m thick) is sharp and erosional. The shell beds are composed mainly of loosely packed oyster shells and their fragments (usually with a high proportion of disarticulated specimens) set in the sandy limestone matrix. The collected oysters show bioerosion and skeletobiont assemblages. Bioerosion trace fossils comprise ten ichnospecies while skeletozoans comprise five taxa. In general, shells of O. clotbeyi show a higher incidence of bioerosion than O. multicostata. The shells' chaotic orientation and their moderate fragmentation indicate that the shell bed formation was associated with high energy events. On the other hand, the low frequency of articulated specimens suggests that the shell beds are parautochthonous oyster banks' remnants. The taphonomic features of the studied assemblage indicate deposition in a shallow-water, wave-dominated environment.     

开鲁盆地砂岩型铀矿中黄铁矿与铀矿化成因关系探讨

开鲁盆地位于松辽盆地西南部,是中国北方砂岩型铀矿勘查的重点地区,自钱家店铀矿床发现以来,盆地内上白垩统姚家组目的层中相继发现了一些具有工业价值的铀矿床.为查明该层位中黄铁矿成因及其形成机制,探讨其与铀矿化之间的关系,本研究对含矿层砂岩中黄铁矿与铀矿物矿物学特征、黄铁矿S同位素开展细致研究.研究表明:(1)开鲁盆地姚家组砂岩中铀主要以独立铀矿物及吸附铀形式存在,独立铀矿物以沥青铀矿为主,含少量的钛铀矿及部分铀石,多数沿黄铁矿周边生长.吸附态的铀与黏土矿物密切相关.(2)姚家组砂岩中黄铁矿主要以草莓状、胶状及粒状产出,多与沥青铀矿共生,其中黄铁矿S同位素(d34S=–55.6‰～23.2‰),平均值–20.87‰,变化范围很大,说明硫的分馏程度较高,硫的来源范围较广.(3)分析表明黄铁矿具有细菌硫酸盐还原作用及热化学硫酸盐还原作用两种成因,并探讨了这2种成因黄铁矿的形成机制.综合前人研究,结合研究区成矿地质背景,认为黄铁矿为铀成矿作用提供了发生还原反应所需要的还原剂,且黄铁矿及铀矿物的形成与区内热流体存在紧密联系.