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1.
V. P. Gaur Hemraj Survanshi S. K. Shrivastava K. V. Nambiar 《Journal of the Geological Society of India》2016,88(5):603-608
A circular structure, termed as cauldron of volcanic origin, was located near Mohar village in Shivpuri district (M.P.) in the year 2000. Subsequently, the same structure was called as Dhala structure of impact origin. There may be debate over the origin and evolution of this circular structure, but it is characterized by a unique lithological set-up within the Bundelkhand craton. The circular structure is defined by annular disposition of igneous and sedimentary rocks. This includes a set of felsic volcanic rocks and associated breccias named as Mohar Formation, exposed in the outer rim of the circular structure. The inner part of the circular structure has sedimentary sequence, termed as Dhala Formation.The field relations indicate that the Mohar and Dhala foarmations are younger than Bundelkhand granitoid complex but older than Kaimur Group. This period in Indian stratigraphy corresponds to Semri Group which consists of Porcellanite Formation, the rocks of which have formed due to deposition of volcanic ash.The geochronological data and field relations between different litho-units indicate that the Mohar volcanism which generated large volume of volcanic ash was a possible source for the formation of Porcellanite Formation. The deposition of sedimentary sequence in main Vindhyan basin was continued, whereas the volcanic activity in Mohar area continued till H ≈ 1.0 Ga. Since, acid volcanic activity has been reported in different parts of the world at H ≈ 1.0 Ga., it is possible that the Mohar acid volcanic activity is not an isolated event; instead it may be a part of global volcanic activities around H ≈ 1.0 Ga. 相似文献
2.
A roving creel survey of the recreational shore fishery along the 16.4-km coastline in the Goukamma Marine Protected Area on the south coast of South Africa was conducted from 2009 to 2011. Some 838 patrols were stratified equally among months, areas and years, but intentionally biased towards weekends. Angler densities at Buffalo Bay and Groenvlei were 0.59 and 0.28 anglers km?1, respectively. Weekend densities were double to quadruple weekday densities and fishing during winter was more popular than during summer. Area, habitat and distance to access points explained variation in angler densities. Shannon–Wiener diversity in catches declined from 2.18 in an earlier (1993–2002) survey to 1.79. Although the order of species abundance in the catches remained largely unchanged, blacktail Diplodus capensis dominance increased to 57.3% by number, at the expense of galjoen Dichistius capensis. Habitat explained 27% of the variance in catch composition. The catch per unit effort (CPUE) for the top nine species ranged from 0.19 to 6.35 fish 100-h?1. The CPUE of all species, except spotted grunter Pomadasys commersonnii, declined. Blacktail and galjoen CPUE declined by 17% and 77%, respectively. The total catch estimate was 2 986 fish y?1. Transgressions of size limits were common. The results suggest that the fishery is overexploited and that catch rates are declining. 相似文献
3.
贝塞尔曲线在虚拟森林景观单树几何模型构建中的应用 总被引：10，自引：0，他引：10
虚拟景观模型的生成,需要对实际景观进行抽象和简化。树几何模型和数字高程模型(DEM)是构成虚拟森林景观的两大要素。而树几何模型的构建又是林业GIS三维可视化的基础。在树几何建模中,树干及树枝形状的可视化表达具有突出意义。由于实际树种的复杂性,在计算机中建树模型,通常采用基于过程建模的方式,而这将面临用户如何对模型进行交互式控制的问题：参数太少,不足以描述树形态特征,而参数太多,则模型难以生成和理解。为了解决这个问题,我们引进贝塞尔(Bezier)曲线这种基于控制点的自由曲线来描述树木中的树干以及树枝形状,利用对曲线控制点的变动来控制曲线的形状,进而生成形态各异的树形状,对此,我们基于ActiveX技术生成Bezier曲线控件。结果表明,该方法能够表示多种形状的树干与树枝,具有操作简捷、计算速度快,可视化效果良好的优点。 相似文献
4.
RH Bennett PD Cowley A-R Childs CG Attwood L Swart TF Næsje 《African Journal of Marine Science》2017,39(4):475-489
Understanding movement behaviour is essential for effective management of fishery species. Dart tags were used to study coastal movement patterns of white steenbras Lithognathus lithognathus, an overexploited seabream (Sparidae) in South Africa. In total, 6 962 fish (190–1 080 mm fork length) were tagged throughout the species’ distributional range, in four long-term fish-tagging programmes. The predominant behaviour recorded was residency, with relatively short-ranging movements. More than 60% of the 351 recaptured fish were recaptured within 1 km of their tagging site, some of which were at liberty in excess of three years. Most of the juveniles (93.9%), subadults (71.7%) and adults (64.0%) were recaptured within 10 km. Occasional long-distance movements of up to 800 km were recorded. Recapture distances were positively, but weakly, correlated with fish fork length (n = 257 fish measured at recapture; r2 = 0.166, p < 0.001). Low levels of connectivity among coastal areas suggest that large-scale annual spawning migrations, as previously hypothesised for this species, are unlikely, which raises the possibility of multiple spawning sites. Seventy-seven percent of L. lithognathus tagged within three marine protected areas (MPAs) were recaptured within the same MPA, suggesting that area closures provide protection for L. lithognathus through the post-estuarine juvenile, subadult and adult life stages. We confirm that the country’s current network of coastal MPAs plays a vital role in sustaining this species, and suggest that additional closures, or otherwise substantial reductions in catch rates, are necessary for the species’ recovery. 相似文献
5.
The South African midwater trawl fishery targets adult horse mackerel Trachurus capensis. The bulk of the catch is taken by a single freezer-trawler, the biggest fishing vessel operating in South African waters. As fishing takes place off the south coast in ecologically sensitive areas, there are concerns about the potential impacts of this fishing operation on non-target species. Fishing behaviour and bycatch of this fishery from 2004 to 2014 were investigated by analysing observer records with regard to catch composition, volume and temporal and spatial patterns. The midwater trawl fishery was estimated to have caught 25 415 tonnes annually, with a bycatch of 6.9% of the total catch, by weight. There are species overlaps with various fisheries, namely the demersal trawl, small-pelagic, line, shark longline and squid fisheries, yet the total bycatch estimates from this fishery are generally small relative to catches taken in the target fisheries. Bycatch species with the highest average annual catches were chub mackerel Scomber japonicus, redeye roundherring Etrumeus whiteheadi, ribbonfish Lepidopus caudatus and hake Merluccius spp. Large-fauna bycatch species included sunfish Mola mola as well as a number of CITES II- and IUCN-listed species, such as Cape fur seal Arctocephalus pusillus, dusky shark Carcharhinus obscurus, smooth hammerhead shark Sphyrna zygaena and thresher sharks Alopias spp. The 97.9% observer coverage is high and the 6.9% bycatch rate low compared to other South African fisheries; however, due to the large size of the individual hauls (average of 46.3 t), the average sampling rate of 1.56% is low. Our analyses suggest that bycatch in the South African midwater trawl fishery has been lower than in other South African fisheries and similar fisheries elsewhere, but due to the combination of high catch volumes and low sampling rates, estimation errors for rare species are high and there is a substantial risk of incidental unmonitored bycatch of rare large fauna and aggregations of small fauna. This could be mitigated by spatio-temporal management of this fishery, to avoid fishing in high-risk areas, and the introduction of an electronic monitoring programme. 相似文献
6.
The declaration of a state of emergency in the South African linefishery in 2000 has had a positive impact on a few overexploited endemic seabreams (Family Sparidae). However, the population of the reef-dwelling dageraad Chrysoblephus cristiceps has been unresponsive to progressively stricter management regulations. After decades of unsustainable fishing mortality the species is in a critical state. In this paper, a method based on standardised probability of capture is used to develop an index of relative abundance for rare species, such as dageraad, which is more robust to changes in output control regulations than conventional methods. The results show a severe decline in dageraad capture probability in the south-west region of South Africa, from 8% in 1985 to <0.1% in 2011. The east region experienced a more rapid decline, from 24% in 1999 to <0.1% in 2011. A spatial comparison of the distribution of historical and recent dageraad probability of capture along the South African coastline indicates a range contraction commonly associated with the collapse of a population. The once-widespread species is now largely limited to a few locations in the Eastern Cape province and dageraad is mostly absent across its former western distribution. Although the historical decline has commonly been attributed to commercial overexploitation, the recreational sector had an estimated 3–5 times greater impact on the dageraad population than the commercial sector in recent years. Given that previous management regulations have been unsuccessful in arresting the decline of dageraad populations, assertive management is necessary to ensure the species’ sustainability. Rehabilitation of dageraad stocks will require a further decrease in fishing mortality, in conjunction with maintaining an appropriate marine protected area (MPA) network. 相似文献
7.
K.R. Baiju C.G. Nambiar G.N. Jadhav H. Kagi M. Satish-Kumar 《Journal of Asian Earth Sciences》2009,36(4-5):332-340
Characterization of fluid inclusions in graphite-bearing charnockites from the southwestern part of the Madurai Granulite Block in southern India reveals a probable relation with the formation and break down of graphite during the high-grade metamorphism. The first-generation monophase pure CO2 inclusions, the composition of which is confirmed by laser Raman spectroscopy, recorded moderate density (0.77–0.87 g/cc) corresponding to low tapping pressure (around 2 kb) than that of the peak granulite-facies metamorphism. The precipitation of graphite, as inferred from graphite inclusions and δ13C values of the graphite from the outcrops, is interpreted as the cause of this lowering of fluid density. An intermediate generation of pseudosecondary inclusions resulted from the re-equilibration or modification of the first-generation fluids and the CO2 formed is interpreted to be the oxidation product from graphite. The youngest generation of fluids which caused widespread retrogression of the granulites is a low-temperature (350 °C) high-saline (32.4–52.0 wt% NaCl equivalent) brine. Carbon isotope data on the graphite from the charnockites show δ13C values ranging from −11.3 to −19.9‰, suggesting a possibility of mixing of carbon sources, relating to earlier biogenic and later CO2 fluid influx. Combining the information gathered from petrologic, fluid inclusion and carbon stable isotope data, we model the fluid evolution in the massive charnockites of the southwestern Madurai Granulite Block. 相似文献
8.
9.
Mineralogical Characterization of Graphite Deposits from Thodupuzha-Kanjirappally Belt, Madurai Granulite Block, Southern India 总被引：1，自引：0，他引：1
Graphite from deposits occurring in the high-grade metamorphic rocks and their larteritized equivalents of the Thodupuzha-Kanjirappally Belt in Madurai Granulite Block, southern India is structurally fully ordered (crystallite size, Lc(002) ranging from 469 to 749 Å), possess high degree of graphitization (DG value ranging from 105 to 267 Å) and reflect crystallization at high temperature (700±100°C). Raman spectra of graphite display profiles corresponding to high crystallinity and high structural ordering. The high temperature crystallinity characteristics of graphite were not obliterated during retrogression of granulites to amphibolite facies gneisses. Preliminary carbon stable isotope results show a spread in isotope values from —11.8 to —26.8 %, which suggest more than one sources for carbon. The lighter carbon isotope values are suggestive of biogenic origin, whereas the heavier ones are probably fluid precipitated graphite. 相似文献
10.
P. Ajayakumar P. John Kurian S. Rajendran M. Radhakrishna C.G. Nambiar T.M. Mahadevan 《Gondwana Research》2006,10(1-2):18
The study region forms the western part of the Madurai block (southern block) and shares several lithological characteristics of the Proterozoic exhumed South Indian Granulite Terrain (SGT). The crustal structure of the area has been derived from gravity data, constrained partly by aeromagnetic data. The Bouguer anomaly map of the region prepared based on detailed gravity observations shows a number of features (i) the Periyar lineament separates two distinctly different gravity fields, one, a high gravity gradient tending to be positive towards the coast in south west and significant gravity lows ranging from − 85 to as low as − 150 mGal in the NE covering a large part of the Periyar plateau (ii) within the broad gravity low, three localised circular anomalies of considerable amplitude occur in the region of Munnar granite. A magnetic low region in the central part coincides with the area of retrogressed charnockites and the major lineaments suggestive of a genetic link and considerable downward extent. The crustal models indicate that the upper layer containing exhumed lower crustal rocks (2.76 gm/cc) is almost homogeneous, most part of the gravity field resulting from variations in intracrustal layers of decharnockitised hornblendic gneisses and granite bodies. Below it, a denser layer (2.85 gm/cc) of unknown composition exists with Moho depth ranging from 36 to 41 km. The structure below the region is compared with that of two other segments of the SGT from which it differs markedly. The Wynad plateau forming the western part of the Northern Block of the SGT is characterised by a heterogeneity due to the presence of contrasting crustal blocks on either side of the Bavali shear zone, possibly a westward extension of the Moyar shear zone and presence of high density material in the mid-to-lower crustal portions. The crust below the Kuppam–Palani transect has a distinctive four-layer structure with a mid-crustal low density layer. The differences in crustal structure are consistent with the different tectonic settings of the three regions discussed in the paper. It is suggested that the crustal structure below the Kuppam–Palani transect corridor is not representative of the SGT as a whole, an aspect of great relevance to intra-continental comparisons and trans-continental reconstructions of continent configurations of the Gondwanaland. 相似文献