Extensional tectonics and mid-Paleozoic massive sulfide occurrences in Europe

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.

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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.

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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.

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Density and viscosity gradients in zoned magma chambers, and their influence withdrawal dynamics

The liquid being sampled from a draining reservoir of density-stratified fluid, such as an erupting zoned magma chamber, is derived from a relatively thin withdrawal layer adjacent to the level of the chamber outlet. This is a consequence of the buoyancy force associated with the density gradient inhibiting vertical motion so that the opportunity for widely separated density levels (compositions) to be tapped and mingled syneruptively is suppressed.Density gradients in zoned chambers of 0.02 – 10 kgm⁻³/m are suggested by data from caldera-forming eruptions. Viscosity gradients can be specified for a given density gradient using calculated relationships between viscosity and density. Published compositional and geothermometric data are used to show that zoned high-silica rhyolites decrease in viscosity upward because of the roofward concentration of dissolved volatiles. Other zoned calc-alkaline magmas increase in viscosity upward because of decreasing temperature and concentration of network modifying cations.A method is developed of calculating the scale of the withdrawal layer thickness, δ, for given kinematic viscosity, eruption rate, and density and viscosity gradients. The method is systematized by the identification of specific flow regimes describing the action of either viscous or inertial forces in balancing the buoyancy force. Thin withdrawal layers are favoured by small eruption rates, small viscosity, and by large density gradients. For particularly steep density gradients, however, the consequently large viscosity gradient plays a role in determining the withdrawal layer thickness. Withdrawal layer thicknesses of the order of 100 m are calculated for typical pyroclastic eruptions of zoned acid magma, and are mostly independent of the viscosity gradient.The vertical scale at which a zoned chamber is instantaneously being tapped during an eruption is equal to the scale of the withdrawal layer thickness. Thus, an eruption that causes collapse of a caldera block through a height that is less than that of the withdrawal layer scale will produce magmas from deeper levels than that to which the chamber roof sinks. In this case the eruption is said to oversample the chamber with respect to the amount of caldera collapse and will produce an essentially constant range of compositions throughout. Alternatively, if the caldera collapse distance is much greater than δ then the selective withdrawal process leads to successive levels of the chamber being “skimmed off” (on a scale δ). This allows the compositional stratigraphy of the chamber to be inverted by the eruptive process, with little opportunity for syneruptive mixing between diverse magma compositions. The geological record shows that most calderas associated with zoned magmas collapsed through vertical distances in excess of 100 m (the characteristic estimate for δ) and, in agreement with our modelling of selective withdrawal, show smooth correlations between composition, or temperature, and the order of eruption.     

The role of interactions among environmental conditions in controlling historical fisheries variability

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.     

Tertiary plate tectonics and high-pressure metamorphism in New Caledonia

The sialic basement of New Caledonia is a Permian-Jurassic greywacke sequence which was folded and metamorphosed to prehnite-pumpellyite or low-grade greenschist facies by the Late Jurassic. Succeeding Cretaceous-Eocene sediments unconformably overlie this basement and extend outwards onto oceanic crust. Tertiary tectonism occurred in three distinct phases.

1. (1) During the Late Eocene a nappe of peridotite was obducted onto southern New Caledonia from northeast to southwest, but without causing significant metamorphism in the underlying sialic rocks.

2. (2) Oligocene compressive thrust tectonics in the northern part of the island accompanied a major east-west subduction zone, at least 30 km wide, which is identified by an imbricate system of tectonically intruded melanges and by development of lawsonite-bearing assemblages in adjacent country rocks; this high-pressure mineralogy constituted a primary metamorphism for the Cretaceous-Eocene sedimentary pile, but was overprinted on the Mesozoic prehnite-pumpellyite metagreywackes.

3. (3) Post-Oligocene transcurrent faulting along a northwest-southeast line (the sillon) parallel to the west coast caused at least 150 km of dextral offset of the southwest frontal margin of the Eocene ultramafic nappe.

At the present time, the tectonics of the southwest Pacific are related to a series of opposite facing subduction (Benioff) zones connected by transform faults extending from New Britain-Solomon Islands south through the New Hebrides to New Zealand and marking the boundary between the Australian and Pacific plates. Available geologic data from this region suggest that a similar geometry existed during the Tertiary and that the microcontinents of New Guinea, New Caledonia and New Zealand all lay along the former plate boundary which has since migrated north and east by a complex process of sea-floor spreading behind the active island arcs.     

Normalization of metal concentrations in estuarine sediments from the Gulf of Mexico

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.     

Software and sociology in UK astronomy

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.     

Greenhouse warming and vernalisation of high-chill fruit in Southern Australia

Most deciduous fruit trees need sufficient accumulated chilling, or vernalisation, to break winter dormancy. Inadequate chilling due to enhanced greenhouse warming may result in prolonged dormancy, leading to reduced fruit quality and yield. The potential impact of warming on chill accumulation has been analysed using the Utah vernalisation model and temperature data from over 400 climate stations in southern Australia. Two experiments were performed: (i) a sensitivity study where temperatures were increased at all sites by either 1, 2 or 3 °C; (ii) a scenario study for the year 2030 where temperatures were increased according to spatially- and seasonally-varying warming scenarios derived from five global climate models under enhanced greenhouse conditions.The sensitivity study shows that warming causes greater reduction in chilling at sites with a higher present mean temperature and/or a wider diurnal temperature range. In the scenario study, two warming scenarios for the year 2030 were considered: a low (high) warming scenario which assumes a low (high) rate of increase of greenhouse gas emission, a low (high) global climate sensitivity to increased emissions, and a low (high) regional temperature response. The low warming scenario is less than 1 °C in southern Australia and is unlikely to affect the vernalisation of high-chill fruit, except for pome-fruit grown in south-west Western Australia. The high warming scenario exceeds 1.5 °C and would significantly increase the risk of prolonged dormancy for both stone-fruit and pome-fruit at many sites.     

The effect of dissolved water on the oxidation state of iron in natural silicate liquids

Ferric-ferrous ratios have been measured in 22 experiments on three natural compositions equilibrated at known temperature (950°–1100° C) and oxygen fugacity, and at water-saturated conditions over a pressure range from 0.05 to 0.2 GPa. There does not appear to be any reaction between the melt and the capsule material that affects the redox state of the iron in the melt. An empirical expression for the anhydrous behavior of the redox state of iron in each of these compositions has also been determined at 1 bar as a function of temperature and oxygen fugacity. A direct comparison of the hydrous ferric-ferrous values with the calculated anhydrous values shows that the dissolution of water in a per-alkaline rhyolite, andesite, and an augite minette has no effect on the redox state of the iron in these melts. This result parallels the effect of water on sulfide speciation in basaltic melts, and confirms published results on experimental hydrous basalts.