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991.
The age and lithologic settings of mid-Paleozoic massive sulfide deposits in Central Europe and southwest Iberia are suggestive of extensional tectonic environments. This inference is supported by the facies configurations of contemporaneous clastic sediments, the presence of deepwater lithologies and pelagic faunas, and marked mafic-felsic bimodalism in associated volcanic rocks.The massive sulfide deposits of the Moravia-Silesia area, Rammelsberg, Meggen, and other deposits appear to have formed in the same tectonic environment: one dominated by crustal extension. The important massive sulfide deposits of the southwest Iberian province formed somewhat later, but appear to be related to a phase of crustal extension initiated in late Devonian time.These two sets of rifting (sensu latu) events correlate in time with Caledonide collision in the north and subsequent Acadian collision further south. The postulated extensional events thus may relate directly to these collisions.All the areas under consideration were subsequently affected by Hercynian compressional events. The high potash granites associated with Hercynian collisions are largely the products of crustal melting and consequently have associated with them ore deposits characterized by lithophile elements such as tin, tungsten and uranium. A significant fraction of the Paleozoic metallogeny of Europe can thus be understood in terms of extensional tectonic events followed by compressional events less than 100 m.y. later.
Zusammenfassung Das Alter und die lithologische Zusammensetzung von massiven Sulfiderzlagerstätten des Devons in Mittel-Europa und Südwest-Iberien deuten auf eine Ausweitungstektonik. Diese Hypothese wird bekräftigt durch die Fazieskonfiguration gleichalter klastischer Sedimente, das Auftreten von Tiefseeablagerungen und pelagischen Faunen, und die ausgesprochen mafisch-felsische Bimodalität von assoziierten Vulkaniten.Die massiven Sulfidlagerstätten von Mähren und Schlesien, Rammelsberg, Meggen und andere Vorkommen scheinen sich unter den gleichen tektonischen Bedingungen gebildet zu haben, nämlich unter Dehnungseffekten der Kruste. Die wichtigsten massiven Sulfidlagerstätten der südwestlichen Iberischen Provinz entstanden etwas später, sie scheinen einer Dehnungsphase beginnend im höheren Devon zugeordnet zu sein.Diese zwei Phasen eines Riftings (sensu lato) stimmen zeitlich mit der kaledonischen Kollision im Norden und der darauffolgenden akadischen Kollision weiter südlich überein. Die angenommenen Dehnungsvorgänge können daher direkt mit diesen Kollisionen in Verbindung gebracht werden.Alle besprochenen Gebiete sind in der Folge von herzynischen Einengungs-Erscheinungen betroffen worden. Die hochgradig kali-haltigen Granite, die den herzynischen Kollisionen zugeordnet sind, sind größtenteils die Ergebnisse von krustalen Schmelzvorgängen und haben deshalb assoziierte Erzlagerstätten, die durch lithophile Elemente wie Zinn, Wolfram und Uran gekennzeichnet sind. Ein beträchtlicher Teil der paläozoischen Metallogenese Europas kann daher aus der Sicht distraktiver tektonischer Erscheinungen gesehen werden, deren Einengungs-Vorgänge weniger als 100 Millionen Jahre später folgten.
Résumé L'âge et la composition lithologique des gisements de sulferes massifs du Paléozoïque moyen de l'Europe Centrale et du sud-ouest de la Péninsule ibérique suggèrent un environement tectonique de distension. Cette hypothèse est appuyée par la configuration des facies des sédiments clastiques contemporains, la présence de lithologies d'eau profonde et de faunes pélagiques et par l'existence d'un net bimodalisme mafique-felsique dans les roches volcaniques associées.Les importants gisements de sulfere de Moravie et de Silésie, du Rammelsberg, de Meggen et autressemblent s'être formés sous un régime tectonique semblable, principalement sous l'effet de distensions de la croûte. Les gisements du sud-ouest de la Péninsule ibérique se sont formés un jeu plus tard, mais semblent être liés à une phase de distension crustale ayant débutéau Dévonien supérieur.Ces deux séries de ruptures (sensu latu) sont corrélés dans le temps avec la collision calédonienne au nord et la collision acadienne subséquente plus au sud.Les évènements de distension invoqués peuvent done être directement liés à ces collisions.Toutes les régions considérées ici furent affectées plus tard par la compression hercynienne. Les granites à haute teneur en potasse, liés aux collisions hercyniennes sont, en grande partie, des produits de la fusion crustale. Ils sont par conséquent associés à des gisements de minerais d'étain, de tungstène et d'uranium. Une partie importante de la métallogénie paléozoïque de l'Europe peut être ainsi comprise en termes d'événements tectoniques de distensions suivis moins de 100 millions d'années plus tard, par une compression.
- . , Dorhaudeusens , - . , , , , , . ; -, . — (sensu lato) . , . , , , , , - , . , , , no- 100 .相似文献
992.
993.
We developed categorical time-series regression models to evaluate the roles of lagged stock abundance history, hydrographic variability, and anthropogenic factors in controlling the variation in abundance of striped bass and American shad in the Potomac, Delaware, and Hudson rivers. These models can be used to evaluate directly the role of interactions of variates to produce greater than average recruitment to commercial fish stocks. Whereas hydrographic factors dominate striped bass dynamics in all three estuaries for the period 1929–1976 compared to the pollution variables tested (i.e., sewage loading, dissolved oxygen, and biological oxygen demand), American shad shows strong dependence on the anthropogenic factors compared to hydrographic variates in all three estuaries. 相似文献
994.
995.
Hugh H Mills 《地球表面变化过程与地形》1986,11(4):451-456
Downslope movements of 109 clasts ranging in intermediate diameter from 30 to 870 mm were monitored over a five-year period on hillslopes in the Valley and Ridge province of southwest Virginia. Gradients ranged from 8° to 42°. Regression on slope tangent, depth of clast base below ground surface, clast size, and clast shape explained 70 per cent of clastmovement variance. A substantial part of the unexplained variance appears due to variation in substrate and slope aspect. Additional measurements made on two steep boulder streams showed extremely low rates. 相似文献
996.
997.
Normalization of metal concentrations in estuarine sediments from the Gulf of Mexico 总被引:3,自引:0,他引:3
J. Kevin Summers Terry L. Wade Virginia D. Engle Ziad A. Malaeb 《Estuaries and Coasts》1996,19(3):581-594
Metal concentrations were examined in sediments from 497 sites within the estuaries of the Gulf of Mexico by the United States Environmental Protection Agency's Environmental Monitoring and Assessment Program (EMAP). Data were normalized for extant concentrations of aluminum to isolate natural factors from anthropogenic ones. The normalization was based on the hypothesis that metal concentrations vary consistently with the concentration of aluminum, unless metals are of anthropogenic origin. Strong linear correlations (>75% variation explained) were observed between Al and Cr, Cu, Pb, Ni, and Zn. Moderate correlations (50–75% variation explained) were observed between Al and As or Ag. Weak but significant correlations (30–40% variation explained) were observed between Al and Hg or Cd. Based on these results, the spatial extent of contamination was examined. About 39% of sites with contamination by at least one metal occurred near population centers, industrial discharge sites, or military bases. The remainder of the observed contamination represented a dispersed pattern, including the lower Mississippi River (7%) and numerous agricultural watersheds (54%), suggesting that the contamination might be from nonpoint sources. 相似文献
998.
Kevin A Pimbblet 《Astronomy& Geophysics》2002,43(1):1.25-1.26
Why don't people use Starlink software? Kevin A Pimbblet outlines the problem and some possible solutions.
I discuss the remit of Starlink's software strategy groups and an item on the agenda of one group's meeting on 26 January 2001: "Why don't people use Starlink software?" The answer was thought to be primarily a sociological effect: most people supervising UK astronomy PhD students learnt their trade when Starlink had a less than perfect reputation. I report on recommendations made to counter this effect. 相似文献
I discuss the remit of Starlink's software strategy groups and an item on the agenda of one group's meeting on 26 January 2001: "Why don't people use Starlink software?" The answer was thought to be primarily a sociological effect: most people supervising UK astronomy PhD students learnt their trade when Starlink had a less than perfect reputation. I report on recommendations made to counter this effect. 相似文献
999.
1000.
G. Fernández Mills 《Theoretical and Applied Climatology》1995,50(3-4):169-183
Summary A T-mode Principal Component Analysis (PCA) based on a network including 68 pluviometric gauges and their 12 mean monthly amounts of rainfall is attempted in order to describe the main patterns governing precipitation in Spain. The procedure is applied to a 12 × 12 intermonth covariance matrix; the unrotated components and two additional solutions deduced after varimax and oblimin rotations are presented and discussed. In all cases component scores are computed and their spatial distribution is discussed. Two regionalizations of Spanish rainfall are then obtained and compared in terms of group homogeneity. There follows a discussion concerning the main pluviometric characteristics of each region deduced from the best division.With 7 Figures 相似文献