Impact of structural and autocyclic basin-floor topography on the depositional evolution of the deep-water Valparaiso forearc basin, central Chile     

Jane Laursen  William R. Normark 《Basin Research》2003,15(2):201-226

The Valparaiso Basin constitutes a unique and prominent deep‐water forearc basin underlying a 40‐km by 60‐km mid‐slope terrace at 2.5‐km water depth on the central Chile margin. Seismic‐reflection data, collected as part of the CONDOR investigation, image a 3–3.5‐km thick sediment succession that fills a smoothly sagged, margin‐parallel, elongated trough at the base of the upper slope. In response to underthrusting of the Juan Fernández Ridge on the Nazca plate, the basin fill is increasingly deformed in the seaward direction above seaward‐vergent outer forearc compressional highs. Syn‐depositional growth of a large margin‐parallel monoclinal high in conjunction with sagging of the inner trough of the basin created stratal geometries similar to those observed in forearc basins bordered by large accretionary prisms. Margin‐parallel compressional ridges diverted turbidity currents along the basin axis and exerted a direct control on sediment depositional processes. As structural depressions became buried, transverse input from point sources on the adjacent upper slope formed complex fan systems with sediment waves characterising the overbank environment, common on many Pleistocene turbidite systems. Mass failure as a result of local topographic inversion formed a prominent mass‐flow deposit, and ultimately resulted in canyon formation and hence a new focused point source feeding the basin. The Valparaiso Basin is presently filled to the spill point of the outer forearc highs, causing headward erosion of incipient canyons into the basin fill and allowing bypass of sediment to the Chile Trench. Age estimates that are constrained by subduction‐related syn‐depositional deformation of the upper 700–800 m of the basin fill suggest that glacio‐eustatic sea‐level lowstands, in conjunction with accelerated denudation rates, within the past 350 ka may have contributed to the increase in simultaneously active point sources along the upper slope as well as an increased complexity of proximal depositional facies.  相似文献   

'Contracted out': some implications of the casualization of academic labour in geography     

Caitríona Ní Laoire  Nicola Jane Shelton 《Area》2003,35(1):92-100

This paper presents the initial findings of a survey of geographers working on fixed-term or other temporary contracts (FTCs), with particular reference to their characteristics, achievements and employment conditions. The results suggest that job satisfaction among contract geographers is related to career stage, and that structural features of the academic labour market combine to restrict many highly qualified academic staff to FTCs for longer than may be appropriate. This phenomenon is highly gendered, with women over-represented among the more junior ranks of contract staff and men dominating the senior ranks. Prolonged employment on FTCs can result in negative effects on the health, welfare and personal finances of many contract staff, and also on staff morale and productivity. The paper concludes that underlying structural inequalities in the academic labour market need to be addressed.  相似文献   

Discussion of Blundy and Holland's (1990) “Calcic amphibole equilibria and a new amphibole-plagioclase geothermometer”     

Jane M. Hammarstrom  E-an Zen 《Contributions to Mineralogy and Petrology》1992,111(2):264-266

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Fluid inclusions as petrogenetic indicators in granulite xenoliths,Pali-Aike volcanic field,Chile     

Jane Selverstone 《Contributions to Mineralogy and Petrology》1982,79(1):28-36

Granulite xenoliths within alkali olivine basalts of the Pali-Aike volcanic field, southern Chile, contain the mineral assemblage orthopyroxene + clinopyroxene + plagioclase + olivine + green spinel. These granulites are thought to be accidental inclusions of the lower crust incorporated in the mantle-derived basalt during its rise to the surface. Symplectic intergrowths of pyroxene and spinel developed between olivine and plagioclase imply that the reaction olivine+plagioclase = Al-orthopyroxene + Al-clinopyroxene + spinel (1) occurred during subsolidus cooling and recrystallization of a gabbroic protolith of the granulites.Examination of fluid inclusions in the granulites indicates the ubiquitous presence of an essentially pure CO₂ fluid phase. Inclusions of three different parageneses have been recognized: Type I inclusions occur along exsolution lamellae in clinopyroxene and are thought to represent precipitation of structurally-bound C or CO₂ during cooling of the gabbro. These are considered the most primary inclusions present. Type II inclusions occur as evenly distributed clusters not associated with any fractures. These inclusions probably represent entrapment of a free fluid phase during recrystallization of the host grains. IIa inclusions are found in granoblastic grains and have densities of 0.68–0.88 g/cm³. Higher density (=0.90–1.02 g/cm³) IIb inclusions occur only in symplectite phases. Secondary Type III CO₂+glass inclusions with =0.47–0.78 g/cm³ occur along healed fractures where basalt has penetrated the xenoliths. Type III inclusions appear related to exsolution of CO₂ from the host basalt during its ascent to the surface. These data suggest that CO₂ is an important constituent of the lower crust under conditions of granulite facies metamorphism, indicated by Type I and II fluid inclusions, and of the mantle, as indicated by Type III inclusions.Correlation of fluid inclusion densities with P-T conditions calculated from both two-pyroxene geothermometry and reation (1) indicate emplacement of a gabbroic pluton at 1,200–1,300° C, 4–6 kb; cooling was accompanied by a slight increase in pressure due to crustal thickening, and symplectite formation occurred at 850±35° C, 5–7 kb. Capture of the xenoliths by the basalt resulted in heating of the granulites, and CO₂ from the basalt was continuously entrapped by the xenoliths over the range 1,000–1,200° C, 4–6 kb. Examination of fluid inclusions of different generations can thus be used in conjunction with other petrologic data to place tight constraints on the specific P-T path followed by the granulite suite, in addition to indicating the nature of the fluid phase present at depth.  相似文献   

The effect of grain texture on the occurrence of microstructural properties in subglacial till     

Jane K. Hart  Anjana Khatwa  Peter Sammonds 《Quaternary Science Reviews》2004,23(23-24):2501-2512

In this paper, we examine whether till grain size affects the range and occurrence of micromorphological features associated with subglacial shear. Our till samples were collected from two glaciers in Iceland, and varied in texture from a coarse, sandy clast-rich till (Fjallsjökull) to a fine-grained silty-sandy till (Vestari-Hagafellsjökull). We found a wide range of deformational microstructures that included skelsepic plasmic fabric, intraclasts of pre-existing eroded bedrock (basalt) and weathered clay and ‘mini-shear zones’ between clasts. We classified our micromorphological data into three classes; rotational, intermediate and linear. In addition to these observations, we performed extensive microfabric analysis at different scales on all of our samples. We found that the coarse-grained till contained a greater number and variety of microstructures than the fine-grained till. In addition, the fine-grained till showed a distinct lack of rotational structures that we attribute to the lack of significantly sized clasts in the matrix. We argue that the varied texture of the coarse-grained till provides a greater degree of perturbation within the shearing layer and so more distinct microstructures form. In a more fine-grained till, shearing is more homogeneous since there are less perturbations in the matrix and this leads to a more singular kind of microstructure. Our observations suggest that subglacial shear occurs within a multi-layered patchwork of different grain sizes, competence and pore water pressures. It is these factors that are so crucial in determining the occurrence and type of microstructural evidence we see in subglacial tills.  相似文献   

Explaining complexities of environmental management in developing countries: lessons from the Fiji Islands     

Jane Turnbull 《The Geographical journal》2004,170(1):64-77

Development assistance agencies have long urged developing countries to adopt environmental management techniques used in industrialized countries. They have ascribed shortcomings in the way developing states use those techniques to insufficient training, environmental matters being poorly integrated into economic decision making, other institutional weaknesses and lack of capacity. These reductionist explanations ignore the influence that socio-political and economic factors may have on state environmental management. They afford only a limited understanding of environmental practice in the Fiji Islands, a South Pacific archipelago and republic. The attempts of Fijian chiefly élite to maintain their power base – achieved through political participation in the state and economic development activities as much as through maintaining the communal system – shape the state's environmental management practices. Those practices have far less rational and democratic bases than one would expect either from the underlying logic of the techniques used (such as environmental impact assessment, environmental planning and protected areas), or from the way those techniques are applied in western, industrialized countries.  相似文献   

Photogrammetric recording of the Roman earthworks 'Cawthorn camps', north yorkshire     

Jane L. Stone  Michael Clowes 《The Photogrammetric Record》2004,19(106):94-110

Cawthorn Camps is a Roman site of probable late 1st and early 2nd century date comprising two forts, one with a later annexe, and a camp. The site survives as earthworks and, within the main defences, there are many slight embanked structures. The current multidisciplinary programme of research has included geophysical prospection, excavation, ground and aerial survey; it is a joint initiative by the North York Moors National Park Authority and English Heritage. The project aims to increase academic understanding, improve information available to the public and assist the production of a revised management plan for this nationally important site.
Large-scale air photographs and digital photogrammetry have been used to produce a plan, at scale 1:500, of the earthworks to a precision of ±10 cm. This plan has been used as a base map with which to rectify other key photographs and plans allowing further detailed interpretation and mapping to be undertaken. The air photographic work has also investigated some of the products available through digital photogrammetric technology, such as digital elevation models (DEMs), orthophotographs and perspective views.  相似文献   

Delineation of the distinctive nature of tertiary coal beds     

Jane C. Shearer  Timothy A. Moore  Thomas D. Demchuk 《International Journal of Coal Geology》1995,28(2-4)

Floral character in mires has changed progressively through time. In the Carboniferous, pteridophytes, sphenophytes and lycophytes were dominant but by the Permian gymnosperms were an important component of mire flora. During the early Mesozoic gymnosperms remained the characteristic mire vegetation, together with pteridophytes, and conifers became dominant during the Jurassic. Cretaceous and Paleocene vegetation are similar, with taxodiaceous flora being important in mire vegetation. From the Eocene onwards, however, angiosperms were increasingly dominant in mire communities and in the Miocene herbaceous vegetation began to play a significant role. Together with these changes in floral character at least three aspects of coal character also appear to vary sequentially with time and are distinctive in the Tertiary: (1) proportions and thickness of vitrain banding, (2) coal bed thickness and (3) proportions of carbonised material. A compilation has been made of data from the coal literature comparing older coals with those of the Tertiary, in order to give a perspective in which to examine Tertiary coals. It was found that only Tertiary coals contain significant proportions of coal devoid of vitrain bands. In addition, Tertiary coals are the thickest recorded coal beds and generally contain low percentages of carbonised material (many less than 5%) as compared to older coals. It is interesting to note that Paleocene coal beds are similar to Cretaceous coals in that they tend to be thinner and contain higher proportions of carbonised material than do younger Tertiary coals.The absence of vitrain bands in some Tertiary coal beds is thought to result from the floras dominated by angiosperms, which are relatively easily degraded as compared to gymnosperms. The thickness of Tertiary coals may be related to an increase in biomass production from the Carboniferous through to the Tertiary, as plants made less investment in producing lignin, an energy-intensive process. In addition, with less lignin in plants, easier degradation of biomass may have facilitated nutrient recycling which, in turn, led to greater biomass production. Increased biomass production may have also ‘diluted’ the carbonised material present in some Tertiary peats, leading to lower proportions in the coal. Another possible cause of decreased carbonised components in Tertiary coal is that decreasing lignin content resulted in decreased charring during fires, as lignin is particularly prone to charring. A third possibility is that the carbonised component of peat may be concentrated during coalification so that Tertiary coals, generally of lower rank than Mesozoic or Paleozoic coals, contain a smaller fraction of carbonised plant material. It is not at present clear which of these mechanisms may have affected carbonised material in peat and coal but it is clear that lignin type and content has had an important role in determining peat and coal character since the Paleozoic.  相似文献   

Changes in seasonal water transport in a Louisiana estuary,Fourleague Bay,Louisiana     

Thomas A. Denes  Jane M. Caffrey 《Estuaries and Coasts》1988,11(3):184-191

Water transport in Fourleague Bay is strongly influenced by the discharge of the Atchafalaya River and prevailing wind conditions. In three 50-h intensive surveys, we measured transport through the major inlets of the bay during the three major river discharge/weather regimes of the year: strong frontal passage, high river flow, and calm, low river flow. Wind stress caused significant changes in both transport and water levels. North winds, dominant during winter frontal passages, caused a net export of water. During the high river flow survey, southeasterly winds created an opposing hydraulic pressure gradient in the Gulf of Mexico, diverting river flow into Fourleague Bay. This resulted in an increase in water elevation and the inundation of adjacent marshes. We believe that this high-discharge, southerly wind condition is the major mechanism leading to sheet flow and sedimentation on the marsh. Calm, low-discharge conditions, typical of late summer, produced classic tidally-dominated circulation.  相似文献   

Till fabric associated with deformable beds   总被引：1，自引：0，他引：1  

Jane K. Hart 《地球表面变化过程与地形》1994,19(1):15-32

This paper investigates the development of till fabric associated with glaciers overlying deformable beds. First, the till fabrics from both modern and ancient, soft bed and hard bed subglacial environments are investigated, and then soft bedded sites with specific strain histories are examined. The results indicate that lodgement tills had a strong fabric, whilst there is a wide range of much weaker fabric strengths associated with soft bed tills. It is suggested from a theoretical study of soft bed till deposition that longitudinal and vertical processes in the subglacial deforming layer are important in fabric development. Field sites were chosen to isolate these two factors, and fabrics investigated in areas of known relative shear strain, and known relative thickness of the deforming layer. The results indicate that: (1) for a fixed deforming layer, fabric strength does increase with increasing deformation (except at very high strains where the development of a transverse fabric can lead to a reduction in the overall fabric strain); and (2) lodgement tills and soft bed tills associated with a thin deforming layer have a high fabric strength, whilst soft bed tills associated with a thick deforming layer have a low fabric strength. Where the deforming layer is thin, clasts are oriented parallel with the glacier direction either by lodging against a hard bed or ploughing against a soft bed. However, where the deforming layer is thicker and shear strains are high, melt-out processes encourage a parallel fabric, but the increased shear strain and changes in the deforming layer thickness cause the clasts to develop both a parallel and transverse fabric. These processes combine to reduce the overall fabric strength. Till fabric studies are a further sedimentological technique to separate the depositional processes associated with a homogeneous till.  相似文献