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51.
鄂尔多斯盆地构造应力场特征及其构造背景 总被引:20,自引:9,他引:20
通过大量断层和褶皱的野外观测以及构造形成序列的确定,同时考虑盆地形成演化过程中板块动力学背景,并结合前人的研究成果,开展了鄂尔多斯盆地古构造应力场的研究。研究表明盆地区域主压应力场方向在加里东期呈NNE-SSW向和近SN向,主要是晚奥陶世以来秦岭洋盆向北俯冲并与华北板块碰撞的结果;印支期主要呈NW-SE向和NNE-SSW向、SN向,主要受中特提斯构造动力体系中羌塘地块与欧亚大陆碰撞拼贴产生的远程构造效应影响;燕山期主要呈NW-SE向,主要受古太平洋大陆板块与欧亚大陆板块碰撞远程构造效应影响,盆地西南缘呈NE-SW向;喜马拉雅期呈NNE-SSW向,主要受新特提斯构造动力体系和今太平洋构造动力体系联合作用影响,即今太平洋板块和印度板块与欧亚板块俯冲碰撞有关。 相似文献
52.
New observations of fracture nucleation are presented from three triaxial compression experiments on intact samples of Westerly
granite, using Acoustic Emission (AE) monitoring. By conducting the tests under different loading conditions, the fracture
process is demonstrated for quasi-static fracture (under AE Feedback load), a slowly developing unstable fracture (loaded
at a `slow' constant strain rate of 2.5 × 10−6 /s) and an unstable fracture that develops near instantaneously (loaded at a `fast' constant strain rate of 5 × 10−5 /s). By recording a continuous ultrasonic waveform during the critical period of fracture, the entire AE catalogue can be
captured and the exact time of fracture defined. Under constant strain loading, three stages are observed: (1) An initial
nucleation or stable growth phase at a rate of ~ 1.3 mm/s, (2) a sudden increase to a constant or slowly accelerating propagation
speed of ~ 18 mm/s, and (3) unstable, accelerating propagation. In the ~ 100 ms before rupture, the high level of AE activity
(as seen on the continuous record) prevented the location of discrete AE events. A lower bound estimate of the average propagation
velocity (using the time-to-rupture and the existing fracture length) suggests values of a few m/s. However from a low gain
acoustic record, we infer that in the final few ms, the fracture propagation speed increased to 175 m/s. These results demonstrate
similarities between fracture nucleation in intact rock and the nucleation of dynamic instabilities in stick slip experiments.
It is suggested that the ability to constrain the size of an evolving fracture provides a crucial tool in further understanding
the controls on fracture nucleation. 相似文献
53.
Multiple exposures and dynamic vulnerability: Evidence from the grape industry in the Okanagan Valley, Canada 总被引:7,自引:1,他引:7
This paper assesses the vulnerability of grape growers and winery operators in the Okanagan Valley, British Columbia to climate variability and change, in the context of other sources of risk. Through interviews and focus groups, producers identified the climatic and non-climatic risks relevant to them and the strategies employed to manage these risks. The results show that the presence of multiple exposures affects the way in which producers are vulnerable to climate change. Producers are vulnerable to conditions that not only affect crop yield, but also affect their ability to compete in or sell to the market. Their sensitivity to these conditions is influenced in part by institutional factors such as trade liberalization and a “markup-free delivery” policy. Producers’ ability to adapt or cope with these risks varies depending on such factors as the availability of resources and technology, and access to government programmes. Producers will likely face challenges associated with the supply of water for irrigation due to a combination of climatic changes and changing demographics in the Okanagan Valley, which in turn affect their ability to adapt to climatic conditions. Finally, adaptations made by producers can change the nature of the operation and its vulnerability, demonstrating the dynamic nature of vulnerability. 相似文献
54.
Ben K. Greenfield Geoffrey S. Siemering Joy C. Andrews Michael Rajan Stephen P. Andrews David F. Spencer 《Estuaries and Coasts》2007,30(4):627-640
Management actions to control invasive aquatic species can have significant ecosystem-scale effects. We evaluated the water
chemistry and nutrient effects of mechanical shredding to control water hyacinth (Eichhornia crassipes) in an agricultural slough and a tidal wetland on the Sacramento-San Joaquin River Delta, California. Shredding was conducted
with two types of shredder boats in fall of 2003 and another boat in spring of 2004. Shredding measurably affected water quality,
but specific effects varied as a function of shredding site and season. Significant increases were observed for total Kjeldahl
nitrogen and total phosphorus for all experiments. Dissolved oxygen effects varied by site, decreasing after shredding at
the agricultural slough but increasing at the tidal wetland. The increase in dissolved oxygen likely resulted from tidal incursions
from the adjacent river. A year-long time series of dissolved oxygen data indicated a negative relationship between hyacinth
abundance and dissolved oxygen concentrations. Hyacinth contained similar tissue concentrations of mercury to underlying sediments,
suggesting that plant harvesting could aid mercury remediation efforts. Simple mass calculations indicated that Delta-wide
shredding operations could cause between 0.1% and 9.6% increases in the overall abundance of carbon, nitrogen, and phosphorus
in the Delta water column. Results suggest that local effects of management actions to control invasive aquatic plants will
vary widely as a function of site-specific hydrology, but that estuary-wide effects would be limited. 相似文献
55.
Ben J. Kefford 《International Journal of Salt Lake Research》1998,7(2):153-170
In order to relate macroinvertebrate fauna to electrical conductivity (EC), a survey of selected orders of macroinvertebrates
was conducted in the Glenelg, Hopkins, Merri and Barwon River Catchments in south-west Victoria, Australia. The survey consisted
of sampling 21 paired sites at confluences of streams. There was little difference between the in-stream habitat but there
was a range of differences in EC between each of the paired sites. The dissimilarity of macroinvertebrate fauna between the
paired sites was compared to the proportional difference in EC and other water quality variables (dissolved oxygen, water
temperature, pH, NO2−N, NO3−N, PO4 and turbidity). This allowed the elimination of EC being confounded with geographical scale parameters and took into account
other water quality parameters that may be confounded with EC. A positive relationship was found between the dissimilarity
of the fauna at each of the two paired sites and the proportional difference in EC. No such relationship was found for any
other measured water quality variable. The results of this survey show that macroinvertebrate community structure is associated
with EC of the river systems investigated after eliminating large scale geographical patterns, in-stream habitat and the measured
water quality parameters. 相似文献
56.
Claude J. Allegre Dalila Ben Othman Mireille Polve Pierre Richard 《Physics of the Earth and Planetary Interiors》1979,19(4):293-306
The neodymium-strontium isotopic correlation observed in most of the Earth mantle materials is evaluated by means of direct modelling. Several geochemical models are quantitatively developed to explain the observations. The main results of this modelling are that such a correlation is not geochemically trivial and that it corresponds to specific conditions in chemical fractionation. These specific conditions seem to be satisfied by solid-liquid partitioning in magmatic conditions. The discussion of the experimental data supports a continuous convecting-magmatic fractionation model for a large proportion of the mantle. 相似文献
57.
58.
59.
This paper reviews the socio-economic and ecological context of Fijian reef fisheries. This review is deemed necessary because improved understanding of the state and trends of Fiji's coral reef fisheries on a national level is required for designing an effective management plan for Fiji's inshore reef fisheries. The most important point that emerges from our review is that despite numerous studies of Fiji's reef fisheries, the current status of reef-associated fisheries at the national level is still uncertain due, mainly, to the lack of dependable data on the subsistence fisheries. This in turn leads to uncertainty about how the continuation of fishing, in particular, fishing focused on target species for the coral reef resources trade, will affect fishing communities and the ecosystem. 相似文献
60.
Temporal and spatial zonation of macroepiphytes on Posidonia oceanica (L.) Delile leaves in a meadow off Tunisia 下载免费PDF全文
The aim of this survey was to study the cover and the composition of macroepiphytic species on the leaves of Posidonia oceanica in the east of Tunisia. Surveys were conducted in December 2009 (winter period), March (spring period) and August 2010 (summer period) in a fringing reef located in Chebba. At each sampling date, 15 adult leaves were randomly collected and divided into basal and apical parts. The inner face of each part was examined with ocular glasses and microscope to estimate species cover by orthogonal projection. PERMANOVA and ANOSIM were used to test for differences of cover between sampling dates and leaf parts. SIMPER, cluster analysis, and PCA were used to ordinate species assemblages. Comparison of epiphytic cover along leaf blades showed significant differences for all groups, except cyanophycea, with high cover of hydrozoans and bryozoans in the basal part and high cover of algae in the apical part. The species composition and cover also vary with sampling date; minimum values were detected in December and the epiphytic community was composed of a few pioneer species, whereas maximum epiphytic cover values were registered in August, with the epiphytic community being composed of a more mature and more diverse community, termed ‘climax’. The main regulatory factors for this distribution are discussed. 相似文献