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981.
AbstractFour ductile shear zones were sampled in the autochthonous Thaya basement and the Upper Bíte? nappe (Moravian unit) at the Eastern margin of the Bohemian massif. In both studied units, the tectono-metamorphic evolution and the chemical mass transfer are different. Two deformational events are recognised: the first deformation stage under amphibolite facies conditions is overprinted by a second event under greenschist facies conditions.The first deformation affected the western margin of the Thaya basement and the whole Bíte? nappe: microstructures are characterised by dynamic recrystallisation of feldspars and quartz, and occurrence of myrmekites and grain-boundary migration of quartz. None or weak chemical mass transfer is related to this medium to high temperature deformation. This deformation corresponds to the thrusting of Moldanubdian units on the Brunovistulian units (Moravian nappes and autochthonous Thaya basement).The second deformation generated shear zones in the until then preserved Thaya basement and reactivated both shear zones of the western margin of the Thaya basement and those of the Bíte? nappe. This deformation is retrograde and mainly associated with chemical mass transfer: a decrease of CaO, FeO, FeO/Fe2O3 and an increase of MgO, K2O and H2O. These chemical changes are related to greenschist metamorphic reactions leading to the destabilisation of feldspars and the crystallisation of white micas and Ca-silicates. The large chemical mass transfer is associated with the circulation of a large volume of fluids. A model of progressive fluid circulation correlated with Variscan prograde and retrograde metamorphism during the collision of Moldanubian and Brunovistulian units is proposed. 相似文献
982.
F. D’Andrea S. Tibaldi M. Blackburn G. Boer M. Déqué M. R. Dix B. Dugas L. Ferranti T. Iwasaki A. Kitoh V. Pope D. Randall E. Roeckner D. Strauss W. Stern H. Van den Dool D. Williamson 《Climate Dynamics》1998,14(6):385-407
As a part of the Atmospheric Model Intercomparison Project (AMIP), the behaviour of 15 general circulation models has been
analysed in order to diagnose and compare the ability of the different models in simulating Northern Hemisphere midlatitude
atmospheric blocking. In accordance with the established AMIP procedure, the 10-year model integrations were performed using
prescribed, time-evolving monthly mean observed SSTs spanning the period January 1979–December 1988. Atmospheric observational
data (ECMWF analyses) over the same period have been also used to verify the models results. The models involved in this comparison
represent a wide spectrum of model complexity, with different horizontal and vertical resolution, numerical techniques and
physical parametrizations, and exhibit large differences in blocking behaviour. Nevertheless, a few common features can be
found, such as the general tendency to underestimate both blocking frequency and the average duration of blocks. The problem
of the possible relationship between model blocking and model systematic errors has also been assessed, although without resorting
to ad-hoc numerical experimentation it is impossible to relate with certainty particular model deficiencies in representing
blocking to precise parts of the model formulation.
Received: 16 July 1997/Accepted: 20 October 1997 相似文献
983.
Robert Emmett Roberto Llansó Jan Newton Ron Thom Michelle Hornberger Cheryl Morgan Colin Levings Andrea Copping Paul Fishman 《Estuaries and Coasts》2000,23(6):765-792
West Coast estuaries are geologically young and composed of a variety of geomorphological types. These estuaries range from large fjords to shallow lagoons; from large to low freshwater flows. Natural hazards include E1 Niños, strong Pacific storms, and active tectonic activity. West Coast estuaries support a wide range of living resources: five salmon species, harvestable shellfish, waterfowl and marine birds, marine mammals, and a variety of algae and plants. Although populations of many of these living resources have declined (salmonids), others have increased (marine mammals). West Coast estuaries are also centers of commerce and increasingly large shipping traffic. The West Coast human population is rising faster than most other areas of the U.S. and Canada, and is distributed heavily in southern California, the San Francisco Bay area, around Puget Sound, and the Fraser River estuary. While water pollution is a problem in many of the urbanized estuaries, most estuaries do not suffer from poor water quality. Primary estuarine problems include habitat alterations, degradation, and loss; diverted freshwater flows; marine sediment contamination; and exotic species introductions. The growing West Coast economy and population are in part related to the quality of life, which is dependent on the use and enjoyment of abundant coastal natural resources. 相似文献
984.
Andrea De Stefano Nathalie Lefebvre Maya Kopylova 《Contributions to Mineralogy and Petrology》2006,151(2):158-173
A suite of 80 macrodiamonds recovered from volcaniclastic breccia of Wawa (southern Ontario) was characterized on the basis
of morphology, nitrogen content and aggregation, cathodoluminescence (CL), and mineral inclusions. The host calc-alkaline
lamprophyric breccias were emplaced at 2.68–2.74 Ga, contemporaneously with voluminous bimodal volcanism of the Michipicoten
greenstone belt. The studied suite of diamonds differs from the vast majority of diamond suites found worldwide. First, the
suite is hosted by calc-alkaline lamprophyric volcanics rather than by kimberlite or lamproite. Second, the host volcanic
rock is amongst the oldest known diamondiferous rocks on Earth, and has experienced regional metamorphism and deformation.
Finally, most diamonds show yellow-orange-red CL and contain mineral inclusions not in equilibrium with each other or their
host diamond. The majority of the diamonds in the Wawa suite are colorless, weakly resorbed, octahedral single crystals and
aggregates. The diamonds contain 0–740 ppm N and show two modes of N aggregation at 0–30 and 60–95% B-centers suggesting mantle
storage at 1,100–1,170°C. Cathodoluminescence and FTIR spectroscopy shows that emission peaks present in orange CL stones
do not likely result from irradiation or single substitutional N, in contrast to other diamonds with red CL. The diamonds
contain primary inclusions of olivine (Fo92 and Fo89), omphacite, orthopyroxene (En93), pentlandite, albite, and An-rich plagioclase. These peridotitic and eclogitic minerals are commonly found within single
diamonds in a mixed paragenesis which also combines shallow and deep phases. This apparent disequilibrium can be explained
by effective small-scale mixing of subducted oceanic crust and mantle rocks in fast “cold” plumes ascending from the top of
the slabs in convergent margins. Alternatively, the diamonds could have formed in the pre-2.7–2.9 Ga cratonic mantle and experienced
subsequent alteration of syngenetic inclusions related to host magmatism and ensuing metamorphism. Neither orogenic nor cratonic
model of the diamond origin fully explains all of the observed characteristics of the diamonds and their host rocks.
Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users. 相似文献
985.
986.
This study analyses large wood (LW) storage and the associated effects on channel morphology and flow hydraulics in three third‐order mountain basins (drainage area 9–12 km2) covered in old‐growth Nothofagus forests, ranging from the temperate warm Chilean Andean Cordillera to the sub‐Antarctic Tierra del Fuego (Argentina). Amount, characteristics and dimensions of large wood (>10 cm diameter, >1 m long) were recorded, as well as their effects on stream morphology, hydraulics and sediment storage. Results show that major differences in LW abundance exist even between adjacent basins, as a result of different disturbance histories and basin dissection. Massive LW volumes (i.e. >1000 m3 ha?1) can be reached in basins disturbed by fires followed by mass movements and debris flows. Potential energy dissipation resulting from wood dams is about a quarter of the total elevation drop in two streams, with a gross sediment volume stored behind wood dams of around 1000 m3 km?1, which appears to be of the same order as the annual sediment yield. Finally, the presence of wood dams may increase flow resistance by up to one order of magnitude. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
987.
Solute transport in rivers is controlled by surface hydrodynamics and by mass exchanges with distinct retention zones. Surface and hyporheic retention processes can be accounted for separately in solute transport models with multiple storage compartments. In the simplest two component model, short term storage can be associated to in-channel transient retention, e.g. produced by riparian vegetation or surface dead zones, and the long-term storage can be associated to hyporheic exchange. The STIR (Solute Transport In Rivers) multiple domain transport model is applied here to tracer test data from three very different Mediterranean streams with distinctive characteristics in terms of flow discharge, vegetation and substrate material. The model is used with an exponential residence time distribution (RTD) to represent surface storage processes and two distinct modeling closures are tested to simulate hyporheic retention: a second exponential RTD and a power-law distribution approximating a known solution for bedform-induced hyporheic exchange. Each stream shows distinct retention patterns characterized by different timescales of the storage time distribution. Both modeling closures lead to very good approximations of the observed breakthrough curves in the two rivers with permeable bed exposed to the flow, where hyporheic flows are expected to occur. In the one case where the occurrence of hyporheic flows is inhibited by bottom vegetation, only the two exponential RTD model is acceptable and the time scales of the two components are of the same magnitude. The significant finding of this work is the recognition of a strong signature of the river properties on tracer data and the evidence of the ability of multiple-component models to describe individual stream responses. This evidence may open a new perspective in river contamination studies, where rivers could possibly be classified based on their ability to trap and release pollutants. 相似文献
988.
During the last decade there was a change in paradigm, which led to consider that terrestrial-type planets within liquid-water habitable zones (LW-HZ) around M stars can also be suitable places for the emergence and evolution of life. Since many dMe stars emit large amount of UV radiation during flares, in this work we analyze the UV constrains for living systems on Earth-like planets around dM stars. We apply our model of UV habitable zone (UV-HZ; Buccino, A.P., Lemarchand, G.A., Mauas, P.J.D., 2006. Icarus 183, 491–503) to the three planetary systems around dM stars (HIP 74995, HIP 109388 and HIP 113020) observed by IUE and to two M-flare stars (AD Leo and EV Lac). In particular, HIP 74995 hosts a terrestrial planet in the LW-HZ, which is the exoplanet that most resembles our own Earth. We show, in general, that during the quiescent state there would not be enough UV radiation within the LW-HZ to trigger the biogenic processes and that this energy could be provided by flares of moderate intensity, while strong flares do not necessarily rule-out the possibility of life-bearing planets. 相似文献
989.
Fragmented water governance arrangements complicate sustainable water management in the western U.S. where local water management organizations (LWMOs), including municipalities and irrigation groups, directly manage water conveyance systems. LWMOs make critical decisions that determine water delivery and management, but have traditionally not collaborated with one another. Our research examines the ways in which LWMOs are increasingly interconnected within a context of widespread urbanization. We draw on three years of qualitative research on LWMOs in Utah, including interviews and in-depth qualitative case studies, to analyze the conditions under which collaborations between LWMOs emerge, or have not yet occurred. Our findings indicate that historically fragmented LWMOs are engaging in cooperative efforts based on their extensive water infrastructure connections. LWMOs did not transform cooperative efforts into collaborations unless such a collaboration protected internal interests. LWMOs engaged in collaborations primarily to reduce their liabilities in the instance of infrastructure failure. 相似文献
990.
The equations of motion of the 2+2 body problem (two interacting particles in the gravitational field of two much more massive primaries m1 and m2 in circular keplerian orbit) have an integral analogous to the Jacobi integral of the circular 2+1 body problem. We show here that with 2+2 bodies this integral does not give rise to Hill stability, i.e. to confinement for all time in a portion of the configuration space not allowing for some close approaches to occur. This is because all the level manifolds are connected and all exchanges of bodies between the regions surroundingm
1,m
2 and infinity do not contradict the conservation of the integral. However, it is worth stressing that some of these exchanges are physically meaningless, because they involve either unlimited extraction of potential energy from the binary formed by the small bodies (without taking into account their physical size) or significant mutual perturbations between the small masses without close approach, a process requiring, for the Sun-Jupiter-two asteroids system, timescales longer than the age of the Solar System. 相似文献