Sedimentary record of Triassic intraplate extension in North China: evidence from the nonmarine NW Ordos Basin, Helan Shan and Zhuozi Shan   总被引：1，自引：0，他引：1  

Bradley D. Ritts  Andrew D. Hanson  Brian J. Darby  Lynde Nanson  Adrian Berry 《Tectonophysics》2004,386(3-4):177-202

The Helan Shan and Zhuozi Shan of the NW Ordos basin, China, contain thick (up to 4 km) sequences of nonmarine Triassic strata. These rocks represent a major intraplate sedimentary basin, the paleogeography, tectonic setting and provenance of which are poorly understood and controversial. Studies of the sedimentary geology of the basin, supported by new palinspastic reconstruction of younger deformation, demonstrate that the basin filled from three sides by fluvial, lacustrine-deltaic and alluvial fan depositional systems. The basin forms a westward-thickening wedge that reaches its maximum thickness along the western margin of the Helan Shan and thins to a relatively constant 600–800 m east of the Zhuozi Shan. The stratigraphy of the basin is strongly asymmetric; alluvial fan strata are restricted to the extreme western margin of the basin and interfinger with axial fluvial deposits low in the section and deep lacustrine facies high in the section. Much of the eastern part of the basin is dominated by west-flowing meandering river and deltaic systems. Large structures of Triassic age have not been identified in the Helan Shan or Zhuozi Shan, but small Triassic normal faults have been documented in the western and central Helan Shan. These characteristics most strongly support an extensional origin for the Triassic basin in NW Ordos. The basin is interpreted to have been a north-trending half graben, bound along its western margin by an east-dipping normal fault, presently concealed beneath Quaternary cover west of the Helan Shan. The eastern margin, now found in the Zhuozi Shan, has simple ramp-margin geometry. Driving mechanisms for this extension are not obvious due to limited documentation of Triassic structure throughout the region, but probably relate to far-field stresses from the Qinling or Jinsha active margins interacting with the stable Ordos block.  相似文献   

Changes in the abundance of the seagrassesZostera marina L. (eelgrass) andRuppia maritima L. (widgeongrass) in San Diego, California, following and El Niño Event     

Megan R. Johnson  Susan L. Williams  Carolyn H. Lieberman  Arne Solbak 《Estuaries and Coasts》2003,26(1):106-115

Changes in environmental conditions can be accompanied by shifts in the distribution and abundances of organisms. When physical factors become unsuitable for growth ofZostera marina (eelgrass), which is a dominant seagrass species in North America, other more ruderal seagrass species, includingRuppia maritima (widgeongrass), often increase in abundance or replace the dominant species. We report the proliferation of widgeongrass into eelgrass beds in Mission Bay and San Diego Bay in San Diego, California, during the 1997 to 1998 El Niño Southern Oscillation (ENSO). Widgeongrass persisted in these eelgrass beds at least one year after a return to non-ENSO conditions and an increase in eelgrass density. We suggest that a warming of the water in two bays in San Diego by 1.5–2.5°C could result, in a permanent shift in the local seagrass vegetation from eelgrass to widgeongrass. This shift, could, have substantial ecosystem-level ramifications.  相似文献   

Constructing the News: The Role of Local Newspapers in Environmental Risk Communication*     

Sarah E. L. Wakefield  Susan J. Elliott 《The Professional geographer》2003,55(2):216-226

Effective risk communication is central to good environmental risk management. While studies have shown that newspapers are the primary source of information to the public regarding environmental issues, little is known about how environmental news is used as a risk‐communication tool. This article explores the role of local information systems in risk communication, using newspaper content analysis as well as in‐depth interviews with journalists and community residents to develop a case study of an environmental assessment process for a nonhazardous industrial‐waste landfill. Results indicate that risk messages were chosen and shaped by journalists on the basis of their own exigencies. In addition, while newspapers were a major source of risk information, their impact was mitigated by resident distrust and access to other information sources, most notably their own personal information networks. These results have implications for environmental policy, as decision makers often use—either passively or actively—print media as a risk‐communication tool.  相似文献   

The limits to “New Regionalism”: (Re)Learning from the media industries     

Susan Christopherson 《Geoforum》2003,34(4):413

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Simple shear of deformable square objects   总被引：1，自引：0，他引：1  

Susan H. Treagus  Labao Lan 《Journal of Structural Geology》2003,25(12):1993-2003

Finite element models of square objects in a contrasting matrix in simple shear show that the objects deform to a variety of shapes. For a range of viscosity contrasts, we catalogue the changing shapes and orientations of objects in progressive simple shear. At moderate simple shear (γ=1.5), the shapes are virtually indistinguishable from those in equivalent pure shear models with the same bulk strain (R_S=4), examined in a previous study. In theory, differences would be expected, especially for very stiff objects or at very large strain. In all our simple shear models, relatively competent square objects become asymmetric barrel shapes with concave shortened edges, similar to some types of boudin. Incompetent objects develop shapes surprisingly similar to mica fish described in mylonites.  相似文献   

Genetic diversity and structure of natural and transplanted eelgrass populations in the Chesapeake and Chincoteague Bays   总被引：1，自引：0，他引：1  

Susan L. Williams  Robert J. Orth 《Estuaries and Coasts》1998,21(1):118-128

The objective of this study was to gain baseline population data on the genetic diversity and differentiation of eelgrass (Zostera marïna L.) populations in the Chesapeake and Chincoteague bays. Natural and transplanted eelgrass beds were compared using starch gel electrophoresis of allozymes. Transplanted eelgrass beds were not reduced in genetic diversity compared with natural beds. Inbreeding coefficients (F_IS) indicated that transplanted eelgrass beds had theoretically higher levels of outcrossing than natural beds, suggesting the significance of use of seeds as donor material and of seedling recruitment following transplantation diebacks. Natural populations exhibited very great genetic structure (F_ST=0.335), but transplanted beds were genetically similar to the donor bed and each other. Genetic diversity was lowest in Chincoteague Bay, reflecting recent restoration history since the 1930s wasting disease and geographical isolation from other east coast populations. These data provide a basis for developing a management plan for conserving eelgrass genetic diversity in the Chesapeake Bay and for guiding estuary-wide restoration efforts. It will be important to recognize that the natural genetic diversity of eelgrass in the estuary is distributed among various populations and is not well represented by single populations.  相似文献   

Adaptation, Spatial Heterogeneity, and the Vulnerability of Agricultural Systems to Climate Change and CO2 Fertilization: An Integrated Assessment Approach   总被引：1，自引：1，他引：1  

John M. Antle  Susan M. Capalbo  Edward T. Elliott  Keith H. Paustian 《Climatic change》2004,64(3):289-315

In this paper we develop economic measures of vulnerability to climate change with and without adaptation in agricultural production systems. We implement these measures using coupled, site-specific ecosystem and economic simulation models. This modeling approach has two key features needed to study the response of agricultural production systems to climate change: it represents adaptation as an endogenous, non-marginal economic response to climate change; and it provides the capability to represent the spatial variability in bio-physical and economic conditions that interact with adaptive responses. We apply this approach to the dryland grain production systems of the Northern Plains region of the United States. The results support the hypothesis that the most adverse impacts on net returns distributions tend to occur in the areas with the poorest resource endowments and when mitigating effects of CO₂ fertilization and adaptation are absent. We find that relative and absolute measures of vulnerability depend on complex interactions between climate change, CO₂ level, adaptation, and economic conditions such as relativeoutput prices. The relationship between relative vulnerability and resource endowments varies with assumptions about climate change, adaptation, and economic conditions. Vulnerability measured with respect to an absolute threshold is inversely related to resource endowments in all cases investigated.  相似文献   

Comparing different methods of bed shear stress estimates in simple and complex flow fields     

Pascale M. Biron  Colleen Robson  Michel F. Lapointe  Susan J. Gaskin 《地球表面变化过程与地形》2004,29(11):1403-1415

Bed shear stress is a fundamental variable in river studies to link ?ow conditions to sediment transport. It is, however, dif?cult to estimate this variable accurately, particularly in complex ?ow ?elds. This study compares shear stress estimated from the log pro?le, drag, Reynolds and turbulent kinetic energy (TKE) approaches in a laboratory ?ume in a simple boundary layer, over plexiglas and over sand, and in a complex ?ow ?eld around de?ectors. Results show that in a simple boundary layer, the log pro?le estimate is always the highest. Over plexiglas, the TKE estimate was the second largest with a value 30 per cent less than the log estimate. However, over sand, the TKE estimate did not show the expected increase in shear stress. In a simple boundary layer, the Reynolds shear stress seems the most appropriate method, particularly the extrapolated value at the bed obtained from a turbulent pro?le. In a complex ?ow ?eld around de?ectors, the TKE method provided the best estimate of shear stress as it is not affected by local streamline variations and it takes into account the increased streamwise turbulent ?uctuations close to the de?ectors. It is suggested that when single‐point measurements are used to estimate shear stress, the instrument should be positioned close to 0·1 of the ?ow depth, which corresponds to the peak value height in pro?les of Reynolds and TKE shear stress. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

Ecofeminism in the twenty-first century   总被引：1，自引：0，他引：1  

Susan Buckingham 《The Geographical journal》2004,170(2):146-154

This paper considers the influence of ecofeminism on policy concerning gender (in)equality and the environment during the past 20 years. It reviews the broad contours of the ecofeminist debate before focusing on the social construction interpretation of women's relationship with the environment. It will argue that there have been substantial policy shifts in Europe and the UK in both the environmental and equalities fields, and that this is in part a result of lobbying at a range of scales by groups informed by ecofeminist debates. Nevertheless, the paper cautions that these shifts are largely incremental and operate within existing structures, which inevitably limit their capacity to create change. As policy addresses some of the concerns highlighted by ecofeminism, academic discourse and grass roots activity have been moving on to address other issues, and the paper concludes with a brief consideration of contemporary trajectories of ecofeminism and campaigning on issues that link women's, feminist and environment concerns.  相似文献   

Three-dimensional strains in fold-hinge zones—Discussion     

Susan H. Treagus 《Tectonophysics》1979,53(1-2)

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