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841.
The West Cycladic Detachment System (WCDS) has been mapped from the western Cycladic islands to Lavrion (mainland Greece), where several top‐to‐SSW low‐angle normal faults at different structural levels are observed. Near the detachment horizon, hydrothermal fluid flow originating from Miocene intrusions lead to high‐temperature carbonate strata‐bound Pb–Ag–Zn ore deposits. Zircon (U–Th)/He cooling ages from the mineralized footwall are 7.1 ± 0.6 to 7.9 ± 0.6 Ma. Carbon‐ and O‐isotope analyses of the carbonate host indicate high water‐to‐rock ratios during brittle deformation‐induced metasomatism, and that this interaction, unrelated to proximity of the intrusions, plays the dominant role in the mineralization. The Late Miocene geology of Lavrion is strikingly similar to Serifos island on the SE termination of the WCDS, both characterized by strong localization of detachment faulting and the interaction of brittle deformation with the injection of fluids related to granitoid intrusions. 相似文献
842.
François Nicot Philippe Gotteland David Bertrand Stéphane Lambert 《国际地质力学数值与分析法杂志》2007,31(13):1477-1515
Geo‐composite cellular structures are an efficient technological solution for various applications in civil engineering. This type of structure is particularly well adapted to resisting rockfalls and can act as a defensive structure. However, the design of such structures is for the most part empirically based; this lack of research‐based design stagnates optimization and advanced development. In this paper, the mechanical behaviour of a geo‐composite cellular structure is investigated using a multi‐scale approach, from the individual cell made up of an assembly of rocky particles contained in a wire netting cage to the entire structure composed of a regular array of cells. Based on discrete modelling of both the cell and structure scales, a computational tool has been developed for design purposes. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
843.
Yu Liu Hiroki Kurokawa Chang Liu David H. Brooks Jingping Dun Takako T. Ishii Hongqi Zhang 《Solar physics》2007,240(2):253-262
Vector magnetograms taken at Huairou Solar Observing Station (HSOS) and Mees Solar Observatory (MSO) reveal that the super
active region (AR) NOAA 10486 was a complex region containing current helicity flux of opposite signs. The main positive sunspots
were dominated by negative helicity fields, while positive helicity patches persisted both inside and around the main positive
sunspots. Based on a comparison of two days of deduced current helicity density, pronounced changes associated with the occurrence
of an X10 flare that peaked at 20:49 UT on 29 October 2003 were noticed. The average current helicity density (negative) of
the main sunspots decreased significantly by about 50%. Accordingly, the helicity densities of counter-helical patches (positive)
were also found to decay by the same proportion or more. In addition, two hard X-ray (HXR) “footpoints” were observed by the
Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) during the flare in the 50 – 100 keV energy range. The cores
of these two HXR footpoints were adjacent to the positions of two patches with positive current helicity that disappeared
after the flare. This strongly suggested that the X10 flare on 29 October 2003 resulted from reconnection between magnetic
flux tubes having opposite current helicity. Finally, the global decrease of current helicity in AR 10486 by ∼50% can be understood
as the helicity launched away by the halo coronal mass ejection (CME) associated with the X10 flare.
Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users. 相似文献
844.
David J.A. Evans 《Quaternary Science Reviews》2009,28(3-4):183-208
Controlled moraines are supraglacial debris concentrations that become hummocky moraine upon de-icing and possess clear linearity due to the inheritance of the former pattern of debris-rich folia in the parent ice. Linearity is most striking wherever glacier ice cores still exist but it increasingly deteriorates with progressive melt-out. As a result, moraine linearity has a low preservation potential in deglaciated terrains but hummocky moraine tracts previously interpreted as evidence of areal stagnation may instead record receding polythermal glacier margins in which debris-rich ice was concentrated in frozen toe zones. Recent applications of modern glaciological analogues to palaeoglaciological reconstructions have implied that: (a) controlled moraine development can be ascribed to a specific process (e.g. englacial thrusting or supercooling); and (b) controlled moraine preservation potential is good enough to imply the occurrence of the specific process in former glacier snouts (e.g. ancient polythermal or supercooled snouts). These assumptions are tested using case studies of controlled moraine construction in which a wide range of debris entrainment and debris-rich ice thickening mechanisms are seen to produce the same geomorphic features. Polythermal conditions are crucial to the concentration of supraglacial debris and controlled moraines in glacier snouts via processes that are most effective at the glacier–permafrost interface. End moraines lie on a process–form continuum constrained by basal thermal regime. The morphological expression of englacial structures in controlled moraine ridges is most striking while the moraines retain ice cores, but the final deposits/landforms tend to consist of discontinuous transverse ridges with intervening hummocks, preserving only a weak impression of the former englacial structure. These are arranged in arcuate zones of hummocky moraine up to 2 km wide containing ice-walled lake plains and lying down flow of streamlined landforms produced by warm-based ice. A variety of debris entrainment mechanisms can produce the same geomorphic signature. Spatial and temporal variability in process–form relationships will lead to the sequential development of different types of end moraines during the recession of a glacier or ice sheet margin. 相似文献
845.
Aurora C. Elmore Robert C. Thunell Richard Styles David Black Richard W. Murray Nahysa Martinez Yrene Astor 《Journal of South American Earth Sciences》2009,27(2-3):197-210
The varved sediments that accumulate in the Cariaco Basin provide a detailed archive of the region’s climatic history, including a record of the quantity of fluvial and wind-transported material. In this study, we examine the sedimentological characteristics (clay mineralogy and grain size) of both surface sediments and sinking lithogenic material collected from sediment trap samples over a three-year period from 1997 to 2000. Data from biweekly sediment trap samples show a tri-modal particle size distribution, with prominent peaks at 2, 22 and 80 μm, indicating sediment contributions from both eolian and fluvial sources. The clay mineralogy of the water column samples collected from 1997 to 1999 also shows distinctive characteristics of eolian and fluvial material. An examination of surface sediment samples from the Cariaco Basin indicates that the Unare River is the main source of riverine sediments to the eastern sub-basin. By combining these sedimentological proxies, we estimate that ~10% of the terrigenous material delivered to the Cariaco Basin is eolian, while ~90% is fluvial. This represents an annual dust accumulation rate of ~0.59 mg/cm2/yr. Since aerosols are closely linked to climate variability, the ability to quantify paleo-dust fluxes using sedimentological characteristics will be a useful tool for future paleoclimate studies looking at sub-Saharan aridity and latitudinal migration of the Intertropical Convergence Zone. 相似文献
846.
David L. COOK Meenakshi WADHWA Robert N. CLAYTON Nicolas DAUPHAS Philip E. JANNEY Andrew M. DAVIS 《Meteoritics & planetary science》2007,42(12):2067-2077
Abstract— We measured nickel isotopes via multicollector inductively coupled plasma mass spectrometry (MC‐ICPMS) in the bulk metal from 36 meteorites, including chondrites, pallasites, and irons (magmatic and non‐magmatic). The Ni isotopes in these meteorites are mass fractionated; the fractionation spans an overall range of ~0.4‰ amu?1. The ranges of Ni isotopic compositions (relative to the SRM 986 Ni isotopic standard) in metal from iron meteorites (~0.0 to ~0.3‰ amu?1) and chondrites (~0.0 to ~0.2‰ amu?1) are similar, whereas the range in pallasite metal (~–0.1 to 0.0‰ amu?1) appears distinct. The fractionation of Ni isotopes within a suite of fourteen IIIAB irons (~0.0 to ~0.3‰ amu?1) spans the entire range measured in all magmatic irons. However, the degree of Ni isotopic fractionation in these samples does not correlate with their Ni content, suggesting that core crystallization did not fractionate Ni isotopes in a systematic way. We also measured the Ni and Fe isotopes in adjacent kamacite and taenite from the Toluca IAB iron meteorite. Nickel isotopes show clearly resolvable fractionation between these two phases; kamacite is heavier relative to taenite by ~0.4‰ amu?1. In contrast, the Fe isotopes do not show a resolvable fractionation between kamacite and taenite. The observed isotopic compositions of kamacite and taenite can be understood in terms of kinetic fractionation due to diffusion of Ni during cooling of the Fe‐Ni alloy and the development of the Widmanstätten pattern. 相似文献
847.
Catastrophic landslides,glacier behaviour and moraine formation – A view from an active plate margin
James Shulmeister Tim R. Davies David J.A. Evans Olivia M. Hyatt Daniel S. Tovar 《Quaternary Science Reviews》2009,28(11-12):1085-1096
The influence of large bedrock landslides (“rock avalanches”) on the behaviour of glaciers is incompletely recognised. Here we present an example from an active tectonic margin in South Island, New Zealand where large earthquakes leave a significant imprint on glacial records. We demonstrate that terminal moraines on the western side of the Southern Alps record both ‘ordinary’ (i.e. climate-driven) and landslide-initiated glacial advances. Following consideration of the processes involved in rock avalanche-initiated moraine construction we suggest ways of determining the nature of the advance that built the terminal moraine. The implications of these observations are important in breaking the conventional linkage of individual terminal moraines with climate forcing. 相似文献
848.
Nick Porch Gregory J. Jordan David M. Price Richard W. Barnes Mike K. Macphail Mike Pemberton 《Quaternary Science Reviews》2009,28(27-28):3197-3210
This paper explores the palaeoclimatic significance of a fossil plant and insect record from Yarra Creek, on King Island, between Tasmania and the Australian mainland. The record dates, based upon a thermoluminescence chronology and other evidence, to Marine Isotope Stage 5 (MIS 5); the exact timing is impossible to ascertain given the resolution of the thermoluminescence results and the presence of an unconformity in the dated section. The presence of a cool-temperate rainforest flora, outside its modern range, and other independent evidence, suggest the sequence may represent the last interglacial (MIS 5e) rather than a later MIS 5 substage. Using coexistence methods that compare modern climatic ranges of the taxa in the assemblage we reconstruct independent beetle and plant based annual and seasonal temperate and precipitation parameters. The results imply the assemblage was deposited under a wetter summer climate and suggest conditions of enhanced temperature seasonality. It is probable that enhanced temperature seasonality is a methodological artefact reflecting the rarity of extremely equable climates (like King Island) in modern climate space. This would indicate a limitation of most methods of palaeoclimatic reconstruction that rely on modern datasets – it is only possible to reconstruct past climates as being within the range of values in that currently exist in modern climate space. 相似文献
849.
850.
David Alexander 《Astrophysics and Space Science》2007,307(1-3):197-202
The solar atmosphere displays a wide variety of dynamic phenomena driven by the interaction of magnetic fields and plasma.
In particular, plasma jets in the solar chromosphere and corona, coronal heating, solar flares and coronal mass ejections
all point to the presence of magnetic phenomena such as reconnection, flux cancellation, the formation of magnetic islands,
and plasmoids. While we can observe the signatures and gross features of such phenomena we cannot probe the essential physics
driving them, given the spatial resolution of current instrumentation. Flexible and well-controlled laboratory experiments,
scaled to solar parameters, open unique opportunities to reproduce the relevant unsteady phenomena under various simulated
solar conditions. The ability to carefully control these parameters in the laboratory allows one to diagnose the dynamical
processes which occur and to apply the knowledge gained to the understanding of similar processes on the Sun, in addition
directing future solar observations and models. This talk introduces the solar phenomena and reviews the contributions made
by laboratory experimentation. 相似文献