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151.
白干湖金矿位于新疆秋格明塔什黄山韧性剪切带东段南侧的苦水断裂与雅满苏断裂构成的楔形区内,在变形程度上不同于秋格明塔什黄山韧性剪切带,但又具有一定程度的韧性剪切特征,是脆韧性变形的转换过渡区.其构造型式多样,构造形态繁杂,多期次的构造变形相互叠加,使它成为最有利的地球化学障,成为成矿流体运移相当活跃的场所.对于本区金的活化、迁移、沉淀和富集以及金矿体的最终定位,有着极其重要的控制意义. 相似文献
152.
Forest clearing through slash and burn to open up agricultural land is an ongoing process in large parts of the Amazon Basin. This activity severely affects the structure and balance of the natural ecosystem, and also has the potential to cause substantial changes in landscape geochemistry. The latter is the topic of this study, with special attention on translocation of potentially toxic trace elements from deforested areas to downstream aquatic and terrestrial systems. Sampling of floodplain sediments and mountain soils (Inceptisols on redbed lithologies) was carried out in two adjacent Subandean river basins, with deforestation extents of ca. 1/3 and 2/3 of the basin areas. Several toxic and potentially toxic metals (e.g., Hg, Cd, Pb, Cu and Ni) and other major and minor elements showed concentration peaks at certain depths in the alluvial deposits of both basins. These peaks were associated with organic matter, and occurred just below layers of combustion residues originating from burning of in situ biomass. Downward migration of particles originating in the combustion residues is suggested to be the direct mechanisms of the metal enrichments. Further evidence of an in situ origin of the metal peaks in the sediments was provided by the geochemical composition of soils located upstream of the floodplains. Disturbed soils (i.e. soils of pasture, coffee plantations, secondary forest and recently swidden fields) were found to be similar to soils under natural forest. Moreover, trace element concentrations in floodplain deposits were similar in the two drainage basins despite the large difference in exploitation degree. Thus, no evidence was found of large scale (basin-wide) increases in trace-metal leaching or translocation as a result of the extensive deforestation and agricultural land-use that has been practiced in the Amazonian highland jungle over more than 100 a. 相似文献
153.
Debris flows are gravity-driven mass movements that are common natural hazards in mountain regions worldwide. Previous work has shown that measurements of ground vibrations are capable of detecting the timing, speed, and location of debris flows. A remaining question is to what extent additional flow properties, such as grain-size distribution and flow depth can be inferred reliably from seismic data. Here, we experimentally explore the relation of seismic vibrations and normal-force fluctuations with debris-flow composition and dynamics. We use a 5.4 m long and 0.3 m wide channel inclined at 20°, equipped with a geophone plate and force plate. We show that seismic vibrations and normal-force fluctuations induced by debris flows are strongly correlated, and that both are affected by debris-flow composition. We find that the effects of the large-particle distribution on seismic vibrations and normal-force fluctuations are substantially more pronounced than the effects of water fraction, clay fraction, and flow volume, especially when normalized by flow depth. We further show that for flows with similar coarse-particle distributions seismic vibrations and normal-force fluctuations can be reasonably well related to flow depth, even if total flow volume, water fraction, and the size distribution of fines varies. Our experimental results shed light on how changes in large-particle, clay, and water fractions affect the seismic and force-fluctuation signatures of debris flows, and provide important guidelines for their interpretation. 相似文献
154.
The turning of wind with height and the related cross-isobaric (ageostrophic) flow in the thermally stable stratified boundary
layer is analysed from a variety of model results acquired in the first Global Energy and Water Cycle Experiment (GEWEX) Atmospheric
Boundary Layer Study (GABLS1). From the governing equations in this particular simple case it becomes clear that the cross-isobaric
flow is solely determined by the surface turbulent stress in the direction of the geostrophic wind for the quasi-steady state
conditions under consideration. Most models indeed seem to approach this relationship but for very different absolute values.
Because turbulence closures used in operational models typically tend to give too deep a boundary layer, the integrated total
cross-isobaric mass flux is up to three times that given by research numerical models and large-eddy simulation. In addition,
the angle between the surface and the geostrophic wind is typically too low, which has important implications for the representation
of the larger-scale flow. It appears that some models provide inconsistent results for the surface angle and the momentum
flux profile, and when the results from these models are removed from the analysis, the remaining ten models do show a unique
relationship between the boundary-layer depth and the surface angle, consistent with the theory given. The present results
also imply that it is beneficial to locate the first model level rather close to the surface for a proper representation of
the turning of wind with height in the stable boundary layer. 相似文献
155.
The East African region has experienced major landslides in recent years. These landslides have caused many fatalities and injuries, loss of many hectares of productive farmlands and destruction to infrastructure such as roads, railways and bridges. The warm and wet climate of the landslide-prone regions causes rapid weathering and produces a regolith weaker than the underlying rock with an interface between the two layers. This interface serves as the most common plane along which landslides are initiated once it becomes saturated. Landslides in the region are associated with steep topography, human activities such as deforestation, overgrazing, and unplanned farming on steep slopes and are induced by earthquakes and high intensity of rainfall. The landslide-prone areas are agriculturally very productive and the inhabitants depend on agriculture for their livelihood. The areas also contribute substantially to the national food reserve. The landslides are therefore a burden to the economies of the individual farmers and national governments of the region. 相似文献
156.
Geophysical surveys were carried out in an arsenic contaminated area, in the Ron Phibun District in southern Thailand. Here, tin and associated minerals, i.e. arsenopyrite and pyrite, have been extracted from granites and natural processes and the mining activities led to arsenic contamination in the environment. Electrical resistivity and self-potential (SP) were used to define the distribution of arsenic contamination in the groundwater. Resistivities of 25–100 Ωm and a positive SP anomaly of 66.0 mV were observed in an area where the arsenic content in auger water at 3.5–5.0 m depths was high, 0.5–5.0 mg/l. Integrated interpretation of resistivity, seismic refraction, GPR and gravity data gave a clear image of subsurface shallow structures (< 30 m depths). There was a good correlation between the resistivity and the gravity data. A subsurface rise was found, which possibly acts as a naturally buried dam, separating a high-contaminated area from a low contaminated area. 相似文献
157.
We examined the influence of algal farming on fish assemblages in two shallow coastal lagoons in Zanzibar, Tanzania. Fish assemblages were visually investigated using a belt transect method and the line-intercept technique was used to examine the substrate composition. 101 species of fish belonging to 31 families were recorded. Algal farming affected the associated fish fauna in terms of abundance, species richness, trophic identity and fish community composition. However, the impact differed between the lagoons. Algal farms in one lagoon hosted a more abundant and diversified fish fauna than controls, whereas farms in the other lagoon exhibited lower fish densities and similar species diversity compared to controls. The discrepancies between lagoons may be an effect of differences in farming intensity and character of the substratum. 相似文献
158.
Photometer recordings of auroral emissions from N+2 and OI have demonstrated that the ratios between the emissions show a characteristic plateau just prior to the passage of a westward travelling surge. The plateau may be interpreted as a temporary (approx. 1 min) cessation in the spectral hardening of the precipitating particles associated with the surge. The characteristic energy of the plateau is about one keV above the level prior to the beginning of the surge. The results have been interpreted in terms of an instability that is switched on during the time of the plateau and then turned off when the electron density is increased above a certain limit. 相似文献
159.
160.