Boudinage structure: some new interpretations based on elastic-plastic finite element simulations

An elastic-plastic finite element method, based on the Prandtl-Reuss equations of plastic flow and involving equivalent stresses and strains, is used to study boudinage structure. Our choice of data for the simulations was guided by published stress-strain curves for marble (matrix) and quartzite (boudin), the essential parameters being yield stress and rock ‘hardness’ (defined by the slope of the stress-strain curve). All models assume an initial fracture and slight separation and therefore only simulate post-fracture behaviour. The simulations suggest that boudin shape is determined by boudin hardness; maximum stresses are concentrated in the corners which therefore shows the most shape modification. Matrix hardness determines the amount of boudin separation. Direct comparison with natural examples is restricted to boudins suffering no significant pre-fracture plastic deformation (i.e. rectangular- and barrel-shaped boudins), although other types are likely to have the characteristics of barrel and pinch-and-swell styles. The simulations do not consider the nature and timing of boudin-defining fractures but these are important in determining the style of boudinage which ultimately develops. Some mechanical problems associated with the infilling of inter-boudin gaps by ductile rock matrix are discussed and two models proposed. The first, based on yielding fracture mechanics, is used to explain boudins with wedge-shaped (or otherwise nonmatching) ends. The second, a hydraulic model, is proposed to account for gaps between rectangular boudins that are filled by ductile rock matrix.     

Airborne dust on the Arctic pack ice,its composition and fallout rate

Dust collected from snow samples on the Arctic pack ice approximately 500 km north of Alaska indicate lower fallout rates than previously reported for Arctic stations 1400 km to the east (3.3 and 14 μg/cm² yr, respectively). Either the lower frequency of southerly (off-shore) near surface winds at our sample sites off Alaska or the unknown influx of dust with upper level air masses could account for the difference in dust fallout. Irregardless, the airborne dust contribution to Arctic deep-sea sediments north of Alaska amounts to 1% or less. A consideration of the clay mineralogy and biogenous components of Arctic dust favors a global or distant source for most of the dust, especially the less than 2 μm fraction. No obvious industrial components were detected in the dust.     

The generation of vertical thermospheric winds and gravity waves at auroral latitudes—I. Observations of vertical winds

Observations of vertical and horizontal thermospheric winds, using the OI (3P-1D) 630 nm emission line, by ground-based Fabry-Perot interferometers in Northern Scandinavia and in Svalbard (Spitzbergen) have identified sources of strong vertical winds in the high latitude thermosphere. Observations from Svalbard (78.2N 15.6E) indicate a systematic diurnal pattern of strong downward winds in the period 06.00 U.T. to about 18.00 U.T., with strong upward winds between 20.00 U.T. and 05.00 U.T. Typical velocities of 30 m s^?1 downward and 50 m s^?1 upward occur, and there is day to day variability in the magnitude (30–80 m s^?1) and phase (+/- 3 h) in the basically diurnal variation. Strong and persistent downward winds may also occur for periods of several hours in the afternoon and evening parts of the auroral oval, associated with the eastward auroral electrojet (northward electric fields and westward ion drifts and winds), during periods of strong geomagnetic disturbances. Average downward values of 30–50 m s^?1 have been observed for periods of 4–6 h at times of large and long-lasting positive bay disturbances in this region. It would appear that the strong vertical winds of the polar cap and disturbed dusk auroral oval are not in the main associated with propagating wave-like features of the wind field. A further identified source is strongly time-dependent and generates very rapid upward vertical motions for periods of 15–30 min as a result of intense local heating in the magnetic midnight region of the auroral oval during the expansion phase of geomagnetic disturbances, and accompanying intense magnetic and auroral disturbances. In the last events, the height-integrated vertical wind (associated with a mean altitude of about 240 km) may exceed 100–150 m s^?1. These disturbances also invariably cause major time-dependent changes of the horizontal wind field with, for example, horizontal wind changes exceeding 500 m s^?1 within 30 min. The changes of vertical winds and the horizontal wind field are highly correlated, and respond directly to the local geomagnetic energy input. In contrast to the behaviour observed in the polar cap or in the disturbed afternoon auroral oval, the ‘expansion phase’ source, which corresponds to the classical ‘auroral substorm’, generates strong time-dependent wind features which may propagate globally. This source thus directly generates one class of thermospheric gravity waves. In this first paper we will consider the experimental evidence for vertical winds. In a second paper we will use a three-dimensional time-dependent model to identify the respective roles of geomagnetic energy and momentum in the creation of both classes of vertical wind sources, and consider their propagation and effects on global thermospheric dynamics.     

Spatial and Temporal Variability in the Growth and Climate Response of Treeline Trees in Alaska

In this study, we investigated the response of trees growing at the cold margins of the boreal forest to climate variation in the 20th century. Working at eight sites at and near alpine and arctic treeline in three regions in Alaska, we compared tree growth (from measured tree ring-widths) to historical climate data to document how growth has responded to climate variation in the 20th century. We found that there was substantial regional variability in response to climate variation. Contrary to our expectations, we found that after 1950 warmer temperatures were associated with decreased tree growth in all but the wettest region, the Alaska Range. Although tree growth increased from 1900–1950 at almost all sites, significant declines in tree growth were common after 1950 in all but the Alaska Range sites. We also found that there was substantial variability in response to climate variation according to distance to treeline. Inverse growth responses to temperature were more common at sites below the forest margin than at sites at the forest margin. Together, these results suggest that inverse responses to temperature are widespread, affecting even the coldest parts of the boreal forest. Even in such close proximity to treeline, warm temperatures after 1950 have been associated with reduced tree growth. Growth declines were most common in the warmer and drier sites, and thus support the hypothesis that drought-stress may accompany increased warming in the boreal forest.     

Core-mantle interactions

Several aspects of core-mantle interactions were considered during a Royal Astronomical Society Discussion Meeting on 12th May 1989, including modelling the geomagnetic field at the core surface, the morphology of the field between 1600 and 1820 AD, dynamo theory, Taylor's constraint, fluid motions at the top of the core that reproduce the observed secular variation, pressure coupling between the core and mantle and its geophysical consequences, topographic core-mantle coupling, angular momentum transfer at the core-mantle interface, the detection and implications of core oscillations, particularly those with associated fluctuations in the Earth's rotation rate, and the seismological determination of the core-mantle boundary topography from lateral inhomogeneities in the mantle.     

Yield and overflow from a finite stochastic reservoir

The paper discusses the overflow (spillage) and yield rates and the total overflow and total yield over a specified time from a finite discrete stochastic reservoir, in which the yieldY _t during the working interval (t,t+1) is a function of the storageZ _t at timet, the inflow sequence {X _t } being IID.The distribution vector of the spillage rate at timet is a telescoped version of the distribution of a certain Markovian variable whose transition matrix is derived. Formulae are given for the distribution of the total spillageW _h given suitable initial conditions, forh=1,2,3; and a simple expression derived forE(W _h ).     

The role of fluorine in the evolution of ultrapotassic magmas

Summary The distribution of F and, to a lesser extent, Cl between phlogopite, amphibole, apatite, and glass in kamafugites from the West Eifel, Germany, and South-West Uganda, and from lamprophyres (minettes) from Hopi-Navajo, Arizona has been determined. In addition, these elements have been analyzed in the same phases from mantle-derived xenoliths from the kamafugitic rocks. All halogen determinations were made using a JEOL 8600 electron microprobe. The F contents and trends in the minerals and glass in the kamafugites and lamprophyres are very similar to those reported for lamproites (Edgar and Charbonneau, 1991). The results indicate that F in the minerals of ultrapotassic magmas is much greater then that found in the xenoliths that are believed to represent likely source regions for such magmas. Chlorine is present in much lower amounts and in the same phases. The discrepancy between F in the xenoliths and in the lavas, and the preference for F to be incorporated in solid phases suggest that F is insufficient to account for the F found in ultrapotassic magmas or to provide for the reduced fluid conditions proposed by Foley (1988) for the genesis of such magmas. Based on these results, the genesis of ultrapotassic magmas may occur if they are partial melts of a mantle source that has been further enriched in F by repeated partial melting of mantle-derived xenoliths such as those of southwest Uganda. Alternatively the slightly higher Cl in the minerals of the xenoliths relative to the magmas suggests that Cl has been lost during degassing of the ascending magma. This may enrich the magmas in F by a dilution effect that may take place prior to the formation of phenocrystal F-rich phlogopites that are often present in ultrapotassic magmas.

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Die Rolle von fluor in der evolution ultrapotassischer magmen

Zusammenfassung Die Verteilung von Fluor and untergeordnet von Chlor zwischen Phlogopit, Amphibol, Apatit and Glas wurde an Kamafugiten aus der westlichen Eifel, Deutschland and aus Südwest-Uganda sowie an Lamprophyren (Minetten) aus Hopi-Navajo, Arizona bestimmt. Außerdem wurde der Gehalt an diesen Elementen in den gleichen Mineralphasen von Mantel-Xenolithen aus kamafugitischen Gesteinen analysiert. Die Halogenbestimmungen wurden an einer JEOL 8600 Mikrosonde durchgeführt. Die Gehalte und Verteilungstrends von Fluor in den Mineralphasen und im Glas der Kamafugite und Lamprophyre sind vergleichbar mit jenen aus Lamproiten (Edgar und Charbonneau, 1991). Die Resultate deuten an, daß der Fluorgehalt in Mineralen ultrapotassischer Magmen viel höher ist als in Xenolithen, die als Repräsentanten der Herkunftsregion derartiger Magmen gelten. Chlor tritt in den selben Mineralphasen in viel geringeren Mengen auf. Das unterschiedliche Auftreten von Fluor in den Xenolithen und in den Laven sowie der bevorzugte Einbau von Fluor in feste Mineralphasen lassen vermuten, daß dieser Gehalt an Fluor nicht ausreicht, um den hohen Fluorgehalt in ultrapotassischen Magmen und die reduzierenden Fluida-Bedingungen, wie sie von Foley (1988) für die Genese von solchen Magmen angenommen worden sind, zu erklären. Auf der Basis dieser Ergebnisse können ultrapotassische Magmen nur dann entstehen, wenn das Mantelmaterial als Herkunft für die partielle Schmelze zum Beispiel durch wiederholtes Teilaufschmelzen von Mantelxenolithen wie jenen aus Südwest-Uganda an Fluor angereichert worden ist. Als Alternative wird angenommen, daß die leicht höheren Chlorgehalte in den Mineralen der Xenolithe verglichen mit dem der Magmen durch Chlorverlust während Entgasung beim Aufstieg des Magmas zustande gekommen sind. Dadurch kam es zu einem Verdünnungseffekt, der zu einer Anreicherung von Fluor führt, vor der Bildung von F-reichem Phlogopit als Phenokristall, Kristalle, die häufig in ultrapotassischen Magmen auftreten.

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With 9 Figures     

Near-surface lithogeochemical halo as an aid to discovery of deeply buried unconformity-type uranium deposits, Athabasca Basin, Canada

Unconformity-type U deposits are being found to depths of more than 400 m where graphitic rocks underlie the Proterozoic sandstone unconformity. Exploration to date involves drilling weak electromagnetic conductors that reflect the graphitic basement. The deposits are believed to have formed through circulation of heated formation waters which discharged essentially vertically above the deposits. Geochemically detectable zones of alteration are reported for cross-sections through the Deilmann, Midwest, and McClean deposits. Information on the areal dimensions of near-surface haloes was required to justify the drilling of grids of shallow holes along conductors or favourable structures as a cost-effective exploration alternative to deep holes, and to define grid spacings.Orientation surveys were undertaken by sampling the uppermost 10 m of sandstone in a selection of holes from the Cigar Lake, Dawn Lake, and Wolf Lake areas where the deposits are, respectively, 400–430 m, 100–160 m, and 180–210 m below surface. The first two deposits have published reserves of 385 and 30–40 million pounds U₃O₈, respectively, while Wolf Lake is much smaller.Well-developed anomalies lie vertically above the richest portion of the Cigar Lake deposit in an area 150 × 600 m and a weak anomaly in many elements continues over the 1800-m length that was sampled. Total and leachable U are 13 to 8 times background with maximum values of 53 and 26 ppm, respectively. Anomalies of 14 to 8 times background are also present for V, Na₂O and Zn, and 5 to 3 times background for Sr, Y, Pb, Th, As, and P.Of the four Dawn Lake mineralized zones, only the 11B Zone was studied, with partial coverage of the 11A Zone. Anomalies 50–100 m by 250–600 m in size are often displaced up to 100 m northeast of the 11B Zone. B, MgO, Pb and both total and leachable U anomalies are 16 to 8 times background with Y, V and As 7 to 5 times background.By contrast, at the Wolf Lake deposit, only irregular variations occur for most of these elements. The unusual patterns may reflect post-alteration tectonism of the sandstone overlying the deposit as regional thrust faulting occurs nearby.Detection of near-surface lithogeochemical haloes using grids of shallow holes would facilitate discovery of deep deposits and reduce expensive, deep drilling.