Diapirism during regional compression: The structural pattern in the Chibougamau region of the archean Abitibi belt,Québec     

Erich Dimroth  Wulf Mueller  Réal Daigneault  Harold Brisson  Alain Poitras  Michel Rocheleau 《International Journal of Earth Sciences》1986,75(3):715-736

The major fold pattern of the Chibougamau region in the Archean Abitibi Belt, Quebec, is the result of two fold phases. The principal F₁-synclines first formed as subsiding fault troughs, and were shortened in NS direction and molded around volcanic centers that had been consolidated by large synvolcanic plutons; they have a general easterly direction but curve locally to the NE or SE. N-trending F₁-folds are locally present in the anticlinoria and are thought to have been produced by the upwelling of basement gneiss and/or synvolcanic plutons. The F₂-folds have rather regular easterly trends; their axial plane schistosity S₂ is the predominant planar fabric element. Basin margin faults were transformed into east-trending thrust faults during the F₁- and F₂-folding.The superposed schistosities S₃ and S₄ are conjugate shear planes related to NS shortening. They and the kink bands produce only minor strain except at a few localities. Synkinematic and late kinematic diapiric plutons pierce the structural pattern. The F₁-folds, derived from the paleogeographic pattern during the uplift of volcanic islands and during the subsidence of fault basins, may also be interpreted as diapiric. Thus, diapirism took place during the whole period of NS compression.ENE trending left-lateral shear belts and their associated first and second order faults, displaced the Kenoran structures in late Archean and early Proterozoic time.

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Zusammenfassung Der Faltenbau des Chibougamaugebietes im Archaischen Abitibi Belt, Quebec, ist das Resultat zweier Faltungsphasen. Die gro\en F₁-Synklinalen bildeten sich zuerst als absinkende Verwerfungströge, die später in NS-Richtung eingeengt wurden und gleichzeitig dabei an die verfestigten Vulkanzentren gepre\t wurden. Die gro\en synvulkanischen Plutone konsolidierten die vulkanischen Zentren. Die F₁-Falten streichen generell OW, biegen aber lokal in NO- und SO-Richtung um. Nord-streichende F₁-Falten finden sich lokal in den Antiklinorien und werden auf das Aufsteigen der basalen Gneisse und/oder der synvulkanischen Plutone zurückgeführt. Die F₂-Falten haben ein regelmä\iges Ost-Streichen; ihre der Faltenachsenfläche parallele Schieferung S₂ ist das deutlichste planare Strukturelement. Die Randbrüche der Verwerfungsbecken wurden während der F₁- und F₂-Faltungsphase in Oststreichende überschiebungen umgewandelt. Die S₃ und S₄-Schieferungen sind konjugierte Scherflächen in NS-Kompression. S₃ und S₄ und die Knickbänder verursachen nur lokal eine erhebliche Verformung. Synkinematische und spätkinematische Plutone durchbrechen diese Strukturen und die F₁-Falten, erzeugt durch den Aufstieg von Vulkaninseln und das Absinken von Verwerfungsbecken können ebenfalls als diapirische Strukturen gedeutet werden. Der Diapirismus überdauerte die ganze Periode der NS-Kompression.Linkshändig rotierte ONO-streichende Scherzonen und die dazugehörigen Verwerfungen erster und zweiter Ordnung versetzen die Kenorischen Strukturen während des späten Archaikums und des frühen Proterozoikums.

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Résumé Les plis majeurs de la région de Chibougamau dans la ceinture Archéenne de l'Abitibi au Québec sont le résultat de deux phases de plissement. Les synclinaux F₁ majeurs prenaient naissance comme bassins de faille subsidents et furent comprimés en direction NS. Ils furent moulés autour des centres volcaniques qui avaient été consolidés par des plutons synvolcaniques; leur direction générale est est-ouest mais ils s'incurvent localement vers le NE ou vers le SE. Des plis F₁ de direction N sont présents localement dans les anticlinoria et nous pensons qu'ils sont le résultat de l'ascension diapirique du gneiss du socle et des plutons synvolcaniques. Les plis F₂ ont une direction EO assez régulière; leur schistosité de plan axial (S₂) est l'élément structural planaire prédominant. Les failles en bordure des bassins de subsidence furent transformées en chevauchement lors des phases de plissement F₁ et F₂.Les schistosités superposées S₃ et S₄ sont des surfaces de cisaillement conjugées en compression NS. Ces schistosités et les plis en chevron ne sont pas responsables d'une déformation substantielle, sauf à quelques endroits. Des plutons diapiriques d'âge syn-cinématique à tardi-cinématique percent ce patron structural. Le patron des plis F₁, déterminé par le patron paléogéographique durant le soulèvement d'Îles volcaniques et la subsidence des bassins de faille peut aussi Être interprété comme une structure diapirique. Des ceintures de cisaillement senestre en direction ENE et leur cortège de failles de premier et deuxième ordre ont déplacé les structures Kénoréennes tard dans l'Archéen et tÔt dans le ProtérozoÏque.

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Crustal scale balanced cross-sections of the Pyrenees; discussion     

Michel Seguret  Marc Daignieres 《Tectonophysics》1986,129(1-4)

From surface and subsurface data, line-length and area balancing were used to construct four balanced and restored sections of the Pyrenees. In the Mesozoic cover, a thin-skinned tectonic model is used. In the basement an anticlinal stack geometry is applied for the foreland part of the thrust nappes. We present and discuss three possible models for the deep structures of the belt: a thin-skinned tectonic model, a thick-skinned tectonic model and an inhomogeneous strain model. The thrusts steepen downwards and the displacements die out in ductile deformation deep in the section. Therefore, we use the inhomogeneous strain model and we equal-area balance the surface of the continental crust.Hanging-wall sequence diagrams are constructed taking into account (1) the strong N-S thickness variations of the Mesozoic cover related to the Cretaceous drift of Spain and (2) the related crustal thinning of the North Pyrenean Zone superimposed upon a previous late Hercynian rise of the lower crust.The Moho step at the vertical of the North Pyrenean Fault results from the thinning of the North Pyrenean Zone. The thickening of both the Axial Zone and the North Pyrenean Zone during the Eocene compressional event preserved the step geometry.Calculated values of the minimum shortening range from 55 km in the western part of the belt to 80 km in the eastern part. Most of the shortening occurs south of the North Pyrenean Fault in the eastern part (Axial Zone) and north of the North Pyrenean Fault in the western part (Labourd thrust).  相似文献   

Computation of apparent resistivity profiles from VLF-EM data using linear filtering1     

Michel Chouteau  Ping Zhang  Dominique Chapellier 《Geophysical Prospecting》1996,44(2):215-232

A simple filter is developed which transforms VLF-EM real magnetic field transfer functions into apparent resistivities. It is based on the relationship between the horizontal derivative of the surface electric field and the vertical magnetic field at the surface of a two-dimensional earth model. The performance of this simple autoregressive filter is tested for modelled and real survey data. The technique yields profiles of apparent resistivity very similar, both in magnitude and in wavelength, to those which would have been obtained using VLF-EM resistivity measurements or d.c. resistivity profiling. This low-pass filter has the advantage of reducing high-wavenumber noise in the data; therefore only the major features of the VLF-EM profile are displayed.  相似文献   

Vegetation cutting as a clean-up method for salt and brackish marshes impacted by oil spills: a review and case history of the effects on plant recovery     

Scott A. Zengel  Jacqueline Michel 《Marine pollution bulletin》1996,32(12):876-885

Manual cutting of marsh vegetation contaminated by oil spills is often debated as a clean-up technique. Cutting oiled marsh vegetation may be proposed to prevent the oiling of sensitive wildlife associated with marshes. Less frequently, oiled marshes may be cut to aid vegetation recovery. This paper reviews several studies of oiled marsh cutting, and presents a case history of the Grand Eagle oil spill, where marsh cutting was used as a clean-up technique. The paper focuses on the effects of cutting on oiled vegetation survival and structural recovery. We conclude that cutting is often detrimental. Marshes should not be cut when impacted by light oils, where high flushing rates are present, or where re-oiling would be likely. Cutting should only be considered for areas where oil may persist, significant impacts to wildlife are likely, and less destructive clean-up techniques have proven insufficient. Seasonality should be considered prior to cutting, since vegetation impacts may be less likely during fall and winter. Concurrent stresses, such as unusual or extreme salinity or hydrological conditions, should also be considered prior to cutting, to avoid stressing oiled vegetation beyond recovery. Finally, if vegetation cutting is used in marsh environments, substrate disturbance should be avoided.  相似文献   

Sea-Level Changes and δO Record for the Past 34,000 yr from Mayotte Reef, Indian Ocean     

Michel Colonna  Joel Casanova  Wolf-Christian Dullo  Gilbert Camoin 《Quaternary Research》1996,46(3):335-339

New sea-level and δ¹⁸O curves for the past 34,000 yr, based on uranium–thorium chronology, are proposed for the southwestern part of the Indian Ocean. The archives include cores drilled from onshore coral reefs and submersed samples from foreslope corals of Mayotte in the Comoro Islands. The Mayotte sea-level curve shows a lowstand of 145 ± 5 m below the present level during the last glacial maximum dated at 18,400 yr. This lowstand is supported by the maximum¹⁸O enrichment in the coral colonies. The residual signal (Δδ¹⁸O), controlled by sea-surface temperature changes, indicates that surface waters 18,400 yr ago were approximately 5°C cooler than present. The deglacial sea-level rise is clearly recorded, with a mean rate of about 1.7 cm yr⁻¹between 18,400 and 10,000 yr ago. The deglaciation phase is characterized by a strong¹⁸O depletion marked by two pulses related to meltwater discharges into the North Atlantic Ocean but also characterized by responses specific to the tropical Indian Ocean.  相似文献   

Analysis of the karst aquifer structure of the Lamalou area (Hérault, France) with ground penetrating radar   总被引：1，自引：0，他引：1  

Walid Al-fares  Michel Bakalowicz  Roger Gurin  Michel Dukhan 《Journal of Applied Geophysics》2002,51(2-4)

The study site at Lamalou karst spring (Hortus karst plateau) is situated 40 km north of Montpellier in France. It consists of a limestone plateau, drained by a karst conduit discharging as a spring. This conduit extends for a few dozen meters in fractured and karstified limestone rocks, 15 to 70 m below the surface. The conduit is accessible from the surface. The main goal of this study is to analyze the surface part of the karst and to highlight the karstic features and among them the conduit, and to test the performances of ground penetrating radar (GPR) in a karstic environment. This method thus appears particularly well adapted to the analysis of the near-surface (<30 m in depth) structure of a karst, especially when clayey coating or soil that absorbs and attenuates the radar is rare and discontinuous. A GPR pulseEKKO 100 (Sensors and Software) was used on the site with a 50-MHz antenna frequency. The results highlight structures characterizing the karstic environment: the epikarst, bedding planes, fractured and karstified zones, compact and massive rock and karrens, a typical karst landform. One of the sections revealed in detail the main conduit located at a depth of 20 m, and made it possible to determine its geometry. This site offers possibilities of validation of the GPR data by giving direct access to the karstic conduits and through two cored boreholes. These direct observations confirm the interpretation of all the GPR sections.  相似文献   

Nonparametric estimation of generalized covariances by modeling on-line data     

Paul Chiasson  Michel Soulié 《Mathematical Geology》1997,29(1):153-172

Structural analysis of data displaying trends may be performed with the help of generalized increments, the variance of these increments being a function of a generalized covariance. Generalized covariances are estimated primarily by parametric methods (i. e., methods searching for the best coefficients of a predetermined function), but also may be computed by one known nonparametric alternative. In this paper, a new nonparametric method is proposed. It is founded on the following principles: (1) least-squares residues are generalized increments; and (2) the generalized covariance is not a unique function, but a family of functions (the system is indeterminate). The method is presented in a general context of a k order trend in R^d, although the full solution is given only fork = I in Rⁱ. In Rⁱ, higher order trends may be developed easily with the equations included in this paper. For higher dimensions in space, the problem is more complex, but a research approach is proposed. The method is tested on soil pH data and compared to a parametric and nonparametric method.  相似文献   

Geochemistry, genesis, and health implications of fluoriferous groundwaters in the upper regions of Ghana   总被引：9，自引：2，他引：9  

W. B. Apambire  D. R. Boyle  F. A. Michel 《Environmental Geology》1997,33(1):13-24

 The most appropriate and widely used source of drinking water for the populations of the upper regions of Ghana is groundwater. In general, groundwater quality is acceptable except for some parts of the Bolgatanga and Bongo Districts, where there are occurrences of elevated levels of natural groundwater fluoride. Concentrations of groundwater fluoride in excess of the World Health Organization (WHO) maximum guideline value (1.5 mg/l) in the Bongo area have been known since 1978. However, the effect of fluoride on people ingesting the water did not receive public and medical attention until October 1993, when health personnel were asked to investigate the cause of stained teeth in school children. The investigation established that 62% of the total population of school children in the Bongo area had dental fluorosis. Against this background, a study was initiated to understand the geochemistry, genesis, and distribution of fluoride in relation to the geology of the area. Groundwater fluoride in the upper regions ranges from 0.11 to 4.60 ppm, with the highest concentrations associated with the fluorine-enriched Bongo coarse-grained hornblende granite and syenite suite. The source of groundwater fluoride within the Bongo granitoids is dissolution of the mineral fluorite and dissolution of and anion exchange with micaceous minerals and their clay products. Applying the WHO recommended guideline values for fluoride in drinking water reveals that 49% of wells in the area deliver water below the optimum level of 0.5 mg/l F^–; these populations are thus prone to dental caries. Twenty-eight percent of the wells fall within the optimum interval for good dental health (0.5–1.5 mg/l F^–). Twenty-three percent of the wells have concentrations above the recommended maximum guideline limit of 1.5 mg/l F^–; this population is susceptible to dental and possibly skeletal fluorosis. Climatic conditions of the area suggest that the individual water consumption is in the order of 3 to 4 l which is higher than the WHO estimate of 2 l/adult/day. In addition, dietary intake for the upper region population is probably higher than WHO baseline values (0.2–0.5 mg/day). This implies that a much higher population is susceptible to developing dental and skeletal fluorosis than originally suspected. Geochemical symbol plot maps help geochemists understand factors controlling the distribution and uptake of fluoride in the upper regions, but they are of minimal value to health officials responsible for planning epidemiological studies and dental health education programs in the region. By casting fluoride data into contoured 'geochemical health-risk maps' using intake interval guidelines more closely aligned to regional climatic and dietary conditions, health officals can better judge the impacts (regional and population based) of fluoride on segments of the population, such as various sex and age groups. Received: 11 March 1997 · Accepted: 17 June 1997  相似文献   

Late holocene level fluctuations of the Lake Ilay in Jura,France: sediment and mollusc evidence and climatic implications     

Michel Magny  Jacques Mouthon  Pascale Ruffaldi 《Journal of Paleolimnology》1995,13(3):219-229

The level fluctuations of Lake Ilay, Jura (France) during the last three millennia are reconstructed from sedimentological and malacological analyses of a core that is well-dated by tree-ring, radiocarbon and pollen datings. Changes in sediment facies, in carbonate concretion assemblages and in mollusc assemblages highlight a major lowering phase atc. 1550 BP and minor lowering phases atc. 2800 BP and shortly before AD 1394. Rises in lake level developed during the early Subatlantic and betweenc. 1500 and 1000 BP. These data are in good agreement with other proxy data from higher European and American latitudes. These correlations support the climatic origin of the level fluctuations of the Lake Ilay during the late Holocene. They suggest that the mediaeval climatic optimum is centred rather in the early than the late Middle Age.  相似文献   

High-frequency magnetotelluric investigation of crustal structure in north-central Abitibi, Quebec, Canada     

Ping Zhang  Michel Chouteau  Marianne Mareschal  Ron Kurtz  Claude Hubert 《Geophysical Journal International》1995,120(2):406-418

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