Pre-eruptive physical conditions of El Reventador volcano (Ecuador) inferred from the petrology of the 2002 and 2004–05 eruptions

A petrological study of the eruptive products of El Reventador allowed us to infer the magmatic processes related to the 2002 and 2004–05 eruptions of this andesitic stratovolcano. On November 3, 2002, El Reventador experienced a highly explosive event, which was followed by emplacement of two lava flows in November–December 2002. Silica contents range from 62 to 58 wt.% SiO₂ for the November 3 pyroclastic deposits to 58–56 and 54–53 wt.% SiO₂ for the successive lava flows. In November 2004 eruptive activity resumed supplying four new lava flows (56–54 wt.% SiO₂) between November 2004 and August 2005.     

Intrusive relationships between young and old Archaean gneisses: evidence from Ivisǎrtoq,southern West Greenland

Relict discordant relationships between the c.3000–2800 Ma old Nǔk granitic (s.l.) gneisses and the c.3700 Ma old Amǐtsoq gneisses of Ivisǎrtoq clearly indicate that the Nǔk gneisses originated as intrusive sheets and bosses. The older gneisses are preserved as continuous banded units which are injected by thin granitic dykes and sills and become progressively disrupted by increasing proportions of intrusive Nǔk granitic material until they occur merely as discrete xenoliths and enclaves dissemminated within a younger gneiss matrix. The Nǔk granites were emplaced into the cores of developing large scale folds within intercalated oceanic and ancient continental portions of the Archaean crust, previously interleaved by thrusting.     

Geomechanical simulation to predict open subsurface fractures

Geomechanical simulation of the evolution of a geological structure can play an important role in predicting open fracture development for all stages in that structure's development. In this work, three such geomechanical simulations are used to predict the evolving stress and strain fields, including dilational and compactional changes in the rock fabric in developing fault and fold systems. Their consequences for open fracture development and flow are addressed. These simulated stress and strain fields show considerable spatial and temporal heterogeneity that is consistent with deformation patterns observed in both natural examples and in laboratory-deformed analogues. But the stress and strain states that develop are neither co-axial nor do they bear a simple relationship to one another. The dilational and compactional strains, manifest as open fracturing or sealing, represent some significantly increased or significantly decreased flow rates. However, open-fracture predictions based on such geomechanical simulations are extremely difficult to validate with any degree of confidence as there is little direct evidence of sub-surface fracture distributions. In this context we also discuss possible integration of seismic anisotropy measurements, as an independent measure of open fracture alignment, to support the geomechanically derived fracture predictions. The focus of this work is on volumetric strains in fault zone evolution, though folding is also addressed.     

Contrasting responses of dimictic and polymictic lakes to environmental change: a spatial and temporal study

Although comparative analyses between dimictic and polymictic lakes have noted the key role of mixing regime in governing water quality, limnologists have historically focused on dimictic lakes, leaving polymictic lakes relatively understudied. In this study, we investigated whether the effects of agricultural development on water quality differed between dimictic and polymictic lakes in a naturally nutrient-rich region of Alberta, Canada. Through a spatial limnological analysis of 36 sites, we found that the relationship between surface water total phosphorus concentration and the percent of agriculture (% Agr) in the catchments differed between dimictic and polymictic lakes, where the proportion of variance explained was much more pronounced in the dimictic (79% explained) than in the polymictic systems (7% explained). Paleolimnological analyses of subfossil chironomids in surface sediment samples (0–1 cm) from 18 of the 36 study lakes, and in sediment core profiles from the dimictic and polymictic basins of a eutrophic lake, showed that water quality differed between mixis groups. We found that the surface sediment chironomid assemblages differed significantly between polymictic and dimictic lakes. Detailed analyses of the sediment cores demonstrated that the two basin types differed in their responses to land-use change through time, as only the dimictic basin showed a parallel increase in anoxia with increasing agricultural development. We suggest that in naturally-fertile landscapes, external nutrient loading exerts a more notable effect on dimictic lakes, whereas internal nutrient loading is more important in polymictic systems.     

Assessment of Offshore New Jersey Sources of Beach Replenishment Sand by Diversified Application of Geologic and Geophysical Methods

Beach replenishment serves the dual purpose of maintaining a source of tourism and recreation while protecting life and property. For New Jersey, sources for beach sand supply are increasingly found offshore. To meet present and future needs, geologic and geophysical techniques can be used to improve the identification, volume estimation, and determination of suitability, thereby making the mining and manag ing of this resource more effective. Current research has improved both data collection and interpretation of seismic surveys and vibracore analysis for projects investigating sand ridges offshore of New Jersey. The New Jersey Geological Survey in cooperation with Rutgers University is evaluating the capabilities of digital seismic data (in addition to analog data) to analyze sand ridges. The printing density of analog systems limits the dynamic range to about 24 dB. Digital acquisition systems with dynamic ranges above 100 dB can permit enhanced seismic profiles by trace static correction, deconvolution, automatic gain scaling, horizontal stacking and digital filtering. Problems common to analog data, such as wave-motion effects of surface sources, water-bottom reverberation, and bubble-pulse-width can be addressed by processing. More than 160 line miles of digital high-resolution continuous profiling seismic data have been collected at sand ridges off Avalon, Beach Haven, and Barnegat Inlet. Digital multichannel data collection has recently been employed to map sand resources within the Port of New York New Jersey expanded dredge-spoil site located 3 mi offshore of Sandy Hook, New Jersey. Multichannel data processing can reduce multiples, improve signal-to-noise calculations, enable source deconvolution, and generate sediment acoustic velocities and acoustic impedance analysis. Synthetic seismograms based on empirical relationships among grain size distribution, density, and velocity from vibracores are used to calculate proxy values for density and velocity. The seismograms are then correlated to the digital seismic profile to confirm reflected events. They are particularly useful where individual reflection events cannot be detected but a waveform generated by several thin lithologic units can be recognized. Progress in application of geologic and geophysical methods provides advan tages in detailed sediment analysis and volumetric estimation of offshore sand ridges. New techniques for current and ongoing beach replenishment projects not only expand our knowledge of the geologic processes involved in sand ridge origin and development, but also improve our assessment of these valuable resources. These reconnaissance studies provide extensive data to the engineer regarding the suitability and quantity of sand and can optimize placement and analysis of vibracore samples.     

Abrupt deglaciation events and Holocene palaeoceanography from high-resolution cores,Cartwright Saddle,Labrador Shelf,Canada

The uppermost Quaternary sediments in Cartwright Saddle, Labrador Shelf, are acoustically laminated, with reflectors that can be traced over long distances. Two piston cores from the saddle record changes in sediment and meltwater delivery from the northeast margin of the Laurentide Ice Sheet (LIS) during deglaciation. Variations in sediment properties indicate a similar history of sediment accumulation over the last 12 kyr. The temporal sampling interval reaches decadal resolution in the last deglacial period 7–9 ka. Analyses of total carbonate content, sediment magnetic variables, foraminiferal species and stable isotope measurements on planktic foraminifers show that abrupt changes occurred ca. 10.9, 9.2, 8.8, 7.9 and 7 ka (with 450 yr correction). There was no distinct change in sediment character during much of the Younger Dryas chronozone. In the δ¹⁸O record, the 8.8 ka event is a dramatic 1‰ shift toward lower values, which we associate with the Noble Inlet glacial event within Hudson Strait. We do not see the pronounced low δ¹⁸O event at 7.1 ka reported off Nova Scotia, but surprisingly, neither the Nova Scotia records nor other records in the Labrador Sea capture the impressive 8.8 ka change. Serious consideration must be given to the final collapse of the LIS as the cause of the 8.2 cal. ka cold event recorded in Greenland and northwest Europe. Copyright © 1999 John Wiley & Sons, Ltd.     

Efficient non-linear seismic analysis of arch dams

An approximate procedure for seismic analysis of concrete arch dams which is based on the smeared crack method is described. Features include construction sequence modelling, water and foundation interaction, crack formation, opening and closing of joints and cracks, frictional sliding, presence of shear keys, action of internal water pressure, and a reliable solution algorithm. Complete solutions can be obtained in an hour on a fast workstation computer, allowing parameter studies to be run. Results suggest that an arch dam can suffer significant cracking during strong ground shaking and still remain stable. © 1998 John Wiley & Sons, Ltd.     

Devensian organic interstadial deposits and ice sheet extent in Buchan,Scotland

Pre-Late Devensian organic deposits in the Buchan area of northeast Scotland were investigated for their geomorphological and palaeoecological (pollen, plant macrofossils, coleoptera) properties. Close ecological agreement exists between fossil indicators and allows the inference that the environment in the vicinity of the deposits was a dwarf shrub tundra of the type met today in high latitude areas of Scandinavia and arctic Russia. The latest in a series of radiocarbon dates from the site produced determinations beyond the limits of the method, although the geomorphological and fossil evidence appears to point to an interstadial date within Oxygen Isotope Stages 5a or 5c. The site has special significance for arguments concerning the much-debated concept of ‘Moraineless Buchan’; indeed, evidence is presented which supports the concept of extensive ice sheet glaciation during the Late Devensian for this crucial geographical area. If Buchan is to be seen as a further casualty amongst other disputed ice-free enclaves, then a return to earlier models of extensive ice sheet glaciation in the Late Devensian of Scotland would seem to be necessary. © 1998 John Wiley & Sons, Ltd.     

On the response of the neutral temperature at 78°N and 90 km altitude to solar proton events

We are able to determine neutral air temperatures by examining the fading times of meteor trail echoes in the upper mesosphere/lower thermosphere. It has been suggested that solar proton events may stimulate both dynamic and aeronomic changes in the middle atmosphere and we have endeavoured to investigate this. Despite a variety of approaches to determine the background temperature above which we might expect to see enhancements under conditions of strong proton precipitation, we are unable to detect any significant changes. We have repeated the search during selected seasons and also with various proton flux thresholds, similarly to no avail. We conclude, therefore, that at 90 km altitude, 78°N and 16°E, at least, we are unable to detect enhanced neutral temperatures due to solar proton events. At best, any enhancements, predicted to be of the order of a few K only, are likely to be completely masked by the day-to-day variability of the temperature field.     

斑岩成矿系统多中心复合成矿作用模型——以西藏甲玛超大型矿床为例

青藏高原冈底斯成矿带诸多大型-超大型斑岩成矿系统的勘查和研究成果显示,传统的陆缘弧或岛弧背景的成矿作用模型不能更好的用于指导碰撞造山背景下斑岩成矿系统的勘查实践。文章以甲玛超大型矿床为实例,基于十余年的勘查实践,通过对不同类型矿体蚀变和矿化结构的精细解剖和成岩成矿年代学的系统分析,建立造山背景下斑岩成矿系统多中心复合成矿作用模型。同时,系统总结甲玛矿床地质特征,阐述成矿作用模型中地层、构造、岩浆等基础地质要素,并详细解析斑岩型钼铜矿体、矽卡岩型铜多金属矿体、角岩型铜钼矿体、Manto型富铜铅锌矿体以及独立金矿体所构成的多元矿体结构特征。最后,揭示甲玛斑岩成矿系统多中心复合成矿作用模型与传统模型的差异,提出其对相同构造背景下斑岩成矿系统勘查评价的启示。