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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Ground water geochemistry of ²²⁸Ra, ²²⁶Ra and ²²²Rn     

Philip T. King  Jacqueline Michel  Willard S. Moore 《Geochimica et cosmochimica acta》1982,46(7):1173-1182

²²⁸Ra, ²²⁶Ra, and ²²²Rn activities were determined on over 150 ground water samples collected from drilled, public water supply wells throughout South Carolina. A wide range of aquifer lithologies were sampled including the crystalline rocks of the Piedmont and sedimentary deposits of the Coastal Plain. A significant linear relationship between log ²²⁸Ra and log ²²⁶Ra (n = 182, r = 0.83) was indistinguishable between Piedmont and Coastal Plain ground water. Median ${}^{228}\text{Ra}{}^{226}\text{Ra}$ activity ratios for the Piedmont, 1.2, and Coastal Plain, 1.3, ground water are close to estimated average crustal ${}^{232}\text{Th}{}^{238}\text{U}$ activity ratios of 1.2 to 1.5 corresponding to Th/U weight ratios of 3.5 to 4.5. A linear correlation was also found between log ²²²Rn and log ²²⁶Ra for Piedmont (n = 68, r = 0.62) and Coastal Plain (n = 89, r = 0.64) ground water. However, the median ${}^{222}\text{Rn}{}^{226}\text{Ra}$ activity ratio for Piedmont ground water, 6100, was much higher than for Coastal Plain ground water, 230. Higher excess ²²²Rn activities may be due to greater retention of ²²⁶Ra by the chemically active Piedmont aquifers compared to the more inert sand aquifers sampled in the Coastal Plain. The relationship between log ²²⁸Ra and log ²²⁶Ra was used to predict total Ra (${}^{228}\text{Ra +}{}^{226}\text{Ra}$) distributions in Appalachian and Atlantic and Gulf Coastal Plain ground water. Predictions estimate that 2.4% of Appalachian and 5.3% of Atlantic and Gulf Coastal Plain ground water supplies contain total Ra activities in excess of the 5 pCi/l limit established by the U.S. Environmental Protection Agency. These predictions also indicate that 40–50% of these ground water wells may be overlooked using the presently suggested screening activity of 3.0 pCi/l of ²²⁶Ra for ²²⁸Ra analysis.  相似文献   

The RbSr isotope system as an index of origin and diagenetic evolution of southern Pacific red clays     

Norbert Clauer  Michel Hoffert  Anne-Marie Karpoff 《Geochimica et cosmochimica acta》1982,46(12):2659-2664

Morphological, mineralogical, chemical and RbSr isotopic studies have been made on Fesmectites (nontronites) from southern Pacific red clays cored near the Marquisas Islands. These minerals have at the top of the core, an ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio of 0.70917 ± 0.00007, which indicates an authigenic origin in isotopic equilibrium with seawater. Weak leaching experiments with 1N HCl show that the nontronites also contain a volcanic component with a lower ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio which, combined with the morphology of the particles, suggests a transportation by bottom currents of clay formed elsewhere.During burial, the nontronites experience diagenetic modifications resulting in an increase in Fe, K and Rb contents, a concomitant decrease of Mg, Ca, Ti, Na and Sr, and a preferential migration of radiogenic ⁸⁷Sr from the clays into the surrounding pore waters.The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio of the Sr adsorbed on the outermost surfaces of the nontronites does not change with depth in the core, and is, therefore, independent of diagenetic influence, which is rather characterized by the ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of the interstitial waters. The isotopic composition of both the adsorbed Sr and that of the pore fluids may yield useful information on the crystallization environment and subsequent history of deep sea red clays.  相似文献   

Fault patterns by space remote sensing and the rotation of western Oregon during Cenozoic times     

Michel Chaplet  Jean Chorowicz  Franois Roure 《Earth and Planetary Science Letters》1987,81(4)

We used MSS-Landsat images (bands 6 and 7) as well as previous studies to interpret fault patterns in western Oregon in terms of the rotations detected by paleomagnetism. These fault zones show a great concentration between the Cascade Range and the Idaho Batholite and have several distinct trends. The north-south striking structures are attributed to the Basin and Range province, while the large NW-SE right-lateral fault zones are interpreted as resulting from an extension between the Cascades Arc and the Olympic-Wallowa lineament. The latter was a paleoplate boundary during pre-Eocene times. This extension, beginning during Late Eocene/Oligocene times and continuing through Recent times, is accompanied by a migration of the rotation pole from southeast to northwest, and by a clockwise rotation of the Coast Range—Klamath Mountains—Cascade Range block, induced by the subduction of the Farallon plate.  相似文献   

Late Permian/early Triassic orogeny in Japan: piling up of nappes, transverse lineation and continental subduction of the Honshu block   总被引：1，自引：0，他引：1  

Michel Faure  Jacques Charvet 《Earth and Planetary Science Letters》1987,84(2-3)

Field surveys in the Oga-Atetsu and Yamaguchi areas of Southwest Japan have been conducted in order to precise the structure of the Permian orogen. A stack of nappes is recognized comprising from top to bottom: (1) the Oga nappe which is considered to be a seamount complex, (2) HP Sangun metamorphics, (3) the Permian Yakuno ophiolite, and (4) the Permian detrital Maizuru group which is interpreted as the sedimentary cover of a continental block, called here the Honshu block, outcropping as the Older Granite. This stack of nappes is overthrust by the Paleozoic Hida basement consisting of HT gneisses, granites and late Carboniferous shallow-water sediments. Microtectonic analysis of the Sangun schists shows that the subhorizontal schistosity bearing a submeridian lineation was formed during the synmetamorphic phase. Asymmetric pressure shadows, shear bands and sigmoidal minerals show that the synmetamorphic deformation corresponds to a ductile shear from north to south. The Permian/early Triassic orogeny is interpreted as the result of a collision between the Hida gneiss (or South China block) and the Honshu block, the intervening oceanic area gave rise to southward directed nappes. The Permian orogenic belt extends at least from Taiwan to central Japan.  相似文献   

Magma and hydrothermally driven sector collapses: The 3100 and 11,500 y. B.P. eruptions of la Grande Decouverte (la Soufrière) volcano, Guadeloupe, French West Indies     

Georges Boudon  Michel P. Semet  Pierre M. Vincent   《Journal of Volcanology and Geothermal Research》1987,33(4)

We recently reported (Boudon et al., 1984) on an eruption similar to that of May 18, 1980 at Mount St. Helens, that took place about 3100 years ago at la Soufrière, Guadeloupe. During the course of detailed geological mapping of the deposits of this event, older debris flow and blast deposits were recognized in the northern sector of the mapped area. Uncarbonized wood fragments in the debris flow have yielded ages ca. 11,500 y. B.P. The deposits extend from an amphitheater crater westward to the caribbean shore about 10 km downslope from the volcano. The deposits and crater structure suggest that they are the result of catastrophic flank failure like the event 3100 years ago. Unlike the latter activity, however, no magmatic component is found in the deposits.  相似文献   

Concentration of gold in in situ laterites from Mato Grosso     

Dominique Michel 《Mineralium Deposita》1987,22(3):185-189

The gold concentration studied is located in lateritic soils overlying Precambrian schists of the Cuiaba Group in Mato Grosso, Brazil. The following five horizons may be recognized from bottom to top: (1) a gray-blue altered schist horizon, (2) a red argillaceous alterite, (3) a horizon characterized by iron oxihydroxide-rich pebbles and quartz fragments in an iron oxihydroxide-rich matrix and clays, (4) an iron crust, and (5) the present soil. The most significant gold content is found in the third horizon just below the iron crust. According to geological study and morphological observations of the gold particles, the gold ore mined today is the result of two combined processes, i.e., the ferrallitic alteration of quartz lodes enclosed in schists and the effect of the red argillaceous alterite which acts as an impervious structure preventing the largest gold grains from migrating downward during their mechanical concentration.  相似文献   

International Comparison of Absolute Gravity Instruments and Workshop “Absolute Gravity Measurements”     

Michel Louis 《Journal of Geodesy》1984,58(1):V-V

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Late Hercynian remagnetization of Tournaisian series from the Laval syncline,Armorican Massif,France     

Jean-Bernard Edel  Michel Coulon 《Earth and Planetary Science Letters》1984,68(2):343-350

Paleomagnetic investigations have been conducted on Tournaisian volcanics and sediments from the Laval syncline, in order to evaluate the consequences of the Late Variscan tectonic and thermic phases. On the southern flank of the syncline, anisotropy measurements have yielded maximum susceptibility in the schistosity planes. Thermal demagnetization exhibits only remagnetizations, characterized by a large range of blocking temperatures (from 350° to 670°C). Two phases of remagnetization seem to follow each other. The first may have occurred during the Tournaisian major orogenic phase. The second has taken place during the latest Stephanian/earliest Permian, in relation with the latest Variscan thermal event. All results are in favour of a clockwise rotation by 15–40° of the Laval syncline after the latest overprints. According to results from other regions of Brittany, this clockwise rotation involved the whole Armorican Massif. This motion may be related to the latest compression phase which has tightened the Ibero-Armorican arc, well before the Permo-Triassic opening of the Bay of Biscay.  相似文献   

Mise en evidence par le paleomagnetisme d'une importante rotation antihoraire des Vosges meridionales entre le Viseen terminal et le Westphalien superieur     

Jean-Bernard Edel  Michel Coulon  Marie-Paule Hernot 《Tectonophysics》1984,106(3-4)

The volcano-sedimentary formations from the southern Vosges are subdivided in two main series: a lower Visean series characterized by a volcanism of spilite-keratophyre type, and an upper Visean series which includes a normal volcanic association of shoshonitic tendency. Paleomagnetic study of 50 sites sampled in both series, but mostly in the upper one, yields three types of directions of characteristic magnetizations. The first type corresponds to Tertiary and Quaternary remagnetizations with low apparent blocking temperatures (350°–500°C, titano-maghemites?). The second group is formed by remagnetizations which have taken place during late Carboniferous-early Permian times, and which show high blocking temperatures of magnetite and mostly titano-haematites. The mean direction is D = 16°, I = 7°, α₉₅ = 9° for 13 sites, (λ = 43°N, φ = 165°E). The last group is represented by primary magnetizations of latest Visean age and post-Sudetic remagnetizations, with blocking temperatures of magnetite and haematite. The mean direction D = 323°, I = −17°, α₉₅ = 9° for 18 sites, (λ = 25°N, φ = 228°E), deviates from about 60° from the theoretical direction, calculated with the early Carboniferous, European pole position. This deviation is interpreted as resulting from a counterclockwise rotation of the southern Vosges between late Visean and Westphalian times. One consequence may be the formation of the variscan “V”, due to the anticlockwise rotation of the eastern branch of the chain. The northwesterly directions show a variation of the inclinations which may indicate that the rotation was preceded by a relatively significant drift of the Vosges to the north.

Résumé

Les terrains volcano-sédimentaires des Vosges méridionales se subdivisent en deux séries principales: la série du Viséen inférieur caractérisée par un volcanisme du type spilite-kératophyre et la série du Viséen supérieur qui comporte une association volcanique normale à tendance shoshonitique. L'étude paléomagnétique de 50 sites échantillonnés dans les deux séries, avec une prédominance dans la série supérieure, met en évidence trois types de directions d'aimantations caractéristiques, Le premier type correspond à des réaimantations d'áge Tertiaire à Quaternaire, à températures de blocage apparentes basses (350°–500°C, titano-maghemites?). Le second groupe est f'orme par des réaimantations mises en place au Carbonifère supérieur-Permien inférieur, à température de blocage haute de magnétite et surtout de titanohématites. La direction moyenne est D = 16°, I = 7°, α₉₅ = 9° pour 13 sites. (λ = 43°N, φ = 165°E). Le dernier groupe est représenté par des aimantations primaires, d'âge Viséen supérieur et des réaimantations post phase Sudète II, à température de blocage de magnetite et d'hématite. La direction moyenne D = 323°, I = −17°, α₉₅ = 9° pour 18 sites (λ = 25 °N, φ = 228°E), dévie de prés de 60° de la direction théorique calculée à partir du pôle européen au Carbonifère inférieur. Cette déviation est interprétée comme résultant d'une rotation antihoraire des Vosges méridionales entre le Viséen supérieur et le Westphalien. Une des conséquences en serait la formation du “V” varisque. par suite de la rotation antihoraire de la branche orientale de la chaîne. Les directions nord-ouest présentent une variation en inclinaison qui semble indiquer que la rotation antihoraire était précédée par une dérive relativement importante des Vosges vers le Nord.  相似文献