Influence of dissolution/reprecipitation reactions on metamorphic greenschist to amphibolite facies mica 40Ar/39Ar ages in the Longmen Shan (eastern Tibet)          下载免费PDF全文

Laura Airaghi  Clare J. Warren  Julia de Sigoyer  Pierre Lanari  Valérie Magnin 《Journal of Metamorphic Geology》2018,36(7):933-958

Linking ages to metamorphic stages in rocks that have experienced low‐ to medium‐grade metamorphism can be particularly tricky due to the rarity of index minerals and the preservation of mineral or compositional relicts. The timing of metamorphism and the Mesozoic exhumation of the metasedimentary units and crystalline basement that form the internal part of the Longmen Shan (eastern Tibet, Sichuan, China), are, for these reasons, still largely unconstrained, but crucial for understanding the regional tectonic evolution of eastern Tibet. In situ core‐rim ⁴⁰Ar/³⁹Ar biotite and U–Th/Pb allanite data show that amphibolite facies conditions (~10–11 kbar, 530°C to 6–7 kbar, 580°C) were reached at 210–180 Ma and that biotite records crystallization, rather than cooling, ages. These conditions are mainly recorded in the metasedimentary cover. The ⁴⁰Ar/³⁹Ar ages obtained from matrix muscovite that partially re‐equilibrated during the post peak‐P metamorphic history comprise a mixture of ages between that of early prograde muscovite relicts and the timing of late muscovite recrystallization at c. 140–120 Ma. This event marks a previously poorly documented greenschist facies metamorphic overprint. This latest stage is also recorded in the crystalline basement, and defines the timing of the greenschist overprint (7 ± 1 kbar, 370 ± 35°C). Numerical models of Ar diffusion show that the difference between ⁴⁰Ar/³⁹Ar biotite and muscovite ages cannot be explained by a slow and protracted cooling in an open system. The model and petrological results rather suggest that biotite and muscovite experienced different Ar retention and resetting histories. The Ar record in mica of the studied low‐ to medium‐grade rocks seems to be mainly controlled by dissolution–reprecipitation processes rather than by diffusive loss, and by different microstructural positions in the sample. Together, our data show that the metasedimentary cover was thickened and cooled independently from the basement prior to c. 140 Ma (with a relatively fast cooling at 4.5 ± 0.5°C/Ma between 185 and 140 Ma). Since the Lower Cretaceous, the metasedimentary cover and the crystalline basement experienced a coherent history during which both were partially exhumed. The Mesozoic history of the Eastern border of the Tibetan plateau is therefore complex and polyphase, and the basement was actively involved at least since the Early Cretaceous, changing our perspective on the contribution of the Cenozoic geology.  相似文献   

The End of Development: A Global History of Poverty and Prosperity     

Laura N. H. Verbrugge 《社会与自然资源》2018,31(9):1080-1081

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Coda: Making Geography Relevant     

Dan Trudeau  Laura Smith  Holly R. Barcus 《The Professional geographer》2018,70(2):333-337

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Climate change and the long-term viability of the World’s busiest heavy haul ice road     

Mullan  Donal  Swindles  Graeme  Patterson  Tim  Galloway  Jennifer  Macumber  Andrew  Falck  Hendrik  Crossley  Laura  Chen  Jie  Pisaric  Michael 《Theoretical and Applied Climatology》2017,129(3-4):1089-1108

Theoretical and Applied Climatology - Climate models project that the northern high latitudes will warm at a rate in excess of the global mean. This will pose severe problems for Arctic and...  相似文献   

Bringing globalization to the countryside: Special Economic Zones in India          下载免费PDF全文

Mohammad Amir Anwar  Pádraig Carmody 《Singapore journal of tropical geography》2016,37(2):121-138

Special Economic Zones (SEZs) are important vectors of neoliberal globalization in India. Despite facing widespread resistance against the proposed land acquisition for these zones, they are still being promoted across the country. We argue that the wealth redistribution to the country's elites and the fractured resistance movements enable neoliberalism and its practices to grow in the countryside. Using a private sector SEZ in Gurgaon as a case study, this article explores how special economic zoning, as a neoliberal policy, has been implicated in the spatialized production of poverty. We also show that the main actors who promote neoliberalism in India (the state and the large‐scale urban private sector) have found a seemingly unlikely ally in rural India in the form of farmers with large landholdings, rural elites who are willing to let go of their land under certain conditions. The data for the article was collected in India in 2009–10.  相似文献   

Mineralogy of suspended sediment in three karst springs     

Ellen?K.?HermanEmail author  Jennifer?H.?Tancredi  Laura?Toran  William?B.?White 《Hydrogeology Journal》2007,15(2):255-266

Springs in karstic carbonate rocks frequently carry a sediment load as well as a dissolved load. Analysis of morphology and mineralogy of suspended sediment from three contrasting karst springs reveals a suite of clastic particles that reflect both source areas and processes that take place within the aquifer. Nolte Spring in Lancaster County, Pennsylvania, USA, discharges sediment of apparently precipitated calcite, indicating that at some point in the aquifer or vadose zone, water exceeds saturation with respect to calcite. Sediment morphologies and chemical conditions in the aquifer point to two different scenarios for this precipitation. The other two springs, Arch Spring in Blair County, Pennsylvania and Bushkill Spring in Northampton County, Pennsylvania, show no evidence of calcite precipitation. Arch Spring discharges mainly layer silicates, while Bushkill Spring discharges mainly silica.

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Resumen Los manantiales en rocas carbonatadas karstficadas tienen frecuentemente una carga de sedimentos y de sólidos disueltos. Los análisis de la morfología y mineralogía de los sedimentos suspendidos de tres manantiales kársticos contrastados revela un conjunto de partículas clásticas que reflejan ambas áreas fuente y los procesos que tienen lugar dentro del acuífero. Nolte Spring en el Condado de Lancaster, Pennsylvania, USA, descarga sedimento de calcita aparentemente precipitada indicando que en cierto punto del acuífero o en la zona vadosa, el agua supera la saturación con respecto a la calcita. La morfología de los sedimentos y las condiciones químicas en el acuífero apuntan a dos escenarios diferentes para esta precipitación. Los otras dos manantiales, Arch Spring en el Condado de Blair, Pennsylvania y Bushkill Spring en el Condado de Norhtampton, Pennsylvania, no muestran evidencia de precitación de calcita. Arch Spring descarga principalmente filosilicatos mientras que Bushkill Spring descarga fundamentalmente Sílice.

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Résumé Les sources situées en contexte carbonaté karstique charrient fréquemment, outre les matières dissoutes, une charge solide. La morphologie et la minéralogie des sédiments en suspension issus de trois sources karstiques de caractéristiques distinctes ont été analysées : une série de particules clastiques, reflétant à la fois les secteurs des sources et les processus internes de l’aquifère, en ressort. Les sédiments issus de Nolte Spring (Comté de Lancaster, Pennsylvanie, Etats-Unis) sont constitués de calcite apparemment précipitée: ceci indique que l’eau atteint la sursaturation vis-à-vis de la calcite à un certain point de l’aquifère ou de la zone non-saturée. La morphologie des sédiments et les conditions chimiques au sein de l’aquifère mènent à deux scénarii différents pour expliquer cette précipitation. Les deux autres sources, Arch Spring (Comté de Blair, Pennsylvanie) et Bushkill Spring (Comté de Northampton, Pennsylvanie), ne présentent aucun indice de précipitation de calcite. Arch Spring exporte essentiellement des phyllosilicates, alors que la silice est majoritaire à Bushkill Spring.

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The Geography of Fish: The Use of Remote Sensing and Spatial Analysis Tools in Fisheries Research     

Laura M. Kracker 《The Professional geographer》1999,51(3):440-450

Rather than seeing the oceans as an expansive void, researchers using acoustic technologies can detect the heterogeneity of biological resources and view the marine environment as a three-dimensional landscape. Underwater remote sensing, using acoustics, provides high resolution maps of the spatial distribution of organisms in aquatic ecosystems. Analyzing the spatial pattern of species distribution within the water column and the impact of that organization on ecological processes bridges the fields of fisheries and spatial analysis. Tools and concepts familiar to geographers, such as remote sensing, GIS, and landscape ecology, contribute to the investigation of large lake and marine ecosystems.  相似文献   

Tree-Ring δD as an Indicator of Asian Monsoon Intensity     

Xiahong Feng  Haiting Cui  Kuilian Tang  Laura E. Conkey 《Quaternary Research》1999,51(3):262

Oxygen and hydrogen isotopic compositions of meteoric water are known to correlate with surface air temperature, except in tropical areas. This relationship has been described using a number of terms corresponding to specific observations, such as latitude, altitude and seasonal effects. However, these temperature effects do not seem to apply to precipitation in monsoonal areas of Asia. Questions have been raised as to whether the isotopic composition of meteoric water can be used to reconstruct paleomonsoon intensity. Tree rings of two modern spruce trees (Picea meyeri) and a 10,000-yr-old timber (Picea jezoensis) were analyzed for hydrogen isotopic composition. On average, the older tree is depleted in deuterium by 45‰ compared to the modern trees. We attribute this isotopic depletion to the strength of summer monsoons, which were more intense in the early Holocene than at present. Although this study is not definitive, it suggests that paleomonsoon intensity can be reconstructed by direct or proxy methods that yield the oxygen or hydrogen isotopic composition of meteoric water.  相似文献   

Using Diurnal Temperature Signals to Infer Vertical Groundwater‐Surface Water Exchange          下载免费PDF全文

Dylan J. Irvine  Martin A. Briggs  Laura K. Lautz  Ryan P. Gordon  Jeffrey M. McKenzie  Ian Cartwright 《Ground water》2017,55(1):10-26

Heat is a powerful tracer to quantify fluid exchange between surface water and groundwater. Temperature time series can be used to estimate pore water fluid flux, and techniques can be employed to extend these estimates to produce detailed plan‐view flux maps. Key advantages of heat tracing include cost‐effective sensors and ease of data collection and interpretation, without the need for expensive and time‐consuming laboratory analyses or induced tracers. While the collection of temperature data in saturated sediments is relatively straightforward, several factors influence the reliability of flux estimates that are based on time series analysis (diurnal signals) of recorded temperatures. Sensor resolution and deployment are particularly important in obtaining robust flux estimates in upwelling conditions. Also, processing temperature time series data involves a sequence of complex steps, including filtering temperature signals, selection of appropriate thermal parameters, and selection of the optimal analytical solution for modeling. This review provides a synthesis of heat tracing using diurnal temperature oscillations, including details on optimal sensor selection and deployment, data processing, model parameterization, and an overview of computing tools available. Recent advances in diurnal temperature methods also provide the opportunity to determine local saturated thermal diffusivity, which can improve the accuracy of fluid flux modeling and sensor spacing, which is related to streambed scour and deposition. These parameters can also be used to determine the reliability of flux estimates from the use of heat as a tracer.  相似文献   

Stress softening behaviour of HDNR bearings: modelling and influence on the seismic response of isolated structures          下载免费PDF全文

Enrico Tubaldi  Laura Ragni  Andrea Dall'Asta  Hamid Ahmadi  Alan Muhr 《地震工程与结构动力学》2017,46(12):2033-2054

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