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171.
Cristian Vallejo Richard A. Spikings Leonard Luzieux Wilfried Winkler David Chew Laurence Page 《地学学报》2006,18(4):264-269
The determination of accurate and precise ages for the timing of collision between oceanic plateaus and continental crust requires an understanding of how the indenting and buttressing plates respond to the collision. We present geochronological, thermochronological, geochemical and isotopic analyses of magmatic rocks from the Ecuadorian Andes, which relate to the collision of the Late Cretaceous Caribbean Plateau and Great Arc sequence with NW South America. The cessation of subduction magmatism during 65–64 Ma beneath the eastern edge of Caribbean Plateau was synchronous with accelerated surface uplift and exhumation within the buttressing continental margin during 75–65 Ma. We interpret this as the collision of the leading edge of the Caribbean Plateau and arc sequence with the South American Plate at 75–65 Ma. A U/Pb (zircon) SHRIMP age of 87.10 ± 1.66 (2σ) Ma, yielded by an accreted fragment of the plateau, precludes previous estimates of collision at 85–80 Ma if the plateau erupted above the Galápagos hotspot. Terra Nova, 18, 264–269, 2006 相似文献
172.
The ability to assess the impacts of changes in the physical environment prior to their occurrence has become possible through the development of SOILCART, an interactive computer program. This program evaluates the potential soil loss of an area in both a temporal and spatial framework. SOILCART has proved its usefulness both as a research and a teaching tool. 相似文献
173.
174.
Laurence M. Page 《Geodinamica Acta》2013,26(1-2):3-16
AbstractThis study provides some of the first integrated P-T-t constraints for the evolution of the Finnmarkian tectonothermal event within the northern Scandinavian Caledonides. Samples from units within the Seve Nappe of the Singis-Tjuoltajaure region contain assemblages which allow the application of well calibrateted thermoharometers. New results include : I ) Eclogite grade rocks of the Aurek Assemblage yield temperatures and pressures in excess of 12 kb and 730° С. 2) Temperatures and pressures obtained for seven samples from the Savotjåkka Assemblage range from 571766” С and 8.9-13.6 kb. These pressures correspond to burial depths of approximately 30-45 km during the Finn-markian (190 Ma) for the outer margin of Baltica. when combined with 40Ar/39Ar data, uplift rates of .2-.4 mm/yr during the early Finnmarkian arc obtained, and 3) Within the Vidja Assemblage a pressure and temperature of 7.3 ± 1.7 kb and 616 ± 60° is obtained. These conditions are consistent with the interpretation of a late Finnmarkian (450 Ma) Intra-Seve juxtaposition of the Vidja and Aurek Assemblages after approximately 20-30 km of slow uplift from peak pressures recorded during the early Finnmarkian.This study provides new constraints which need to be considered in future tectonic models. These constraints include : I ) The Savopakte Assemblage records high pressures and high temperatures during the Finnmarkian and 2) Finnmarkian uplift rates of .2-.4 mm/yr were likely for marginal Baltica after peak metamorphism. Rates of this magnitude may be accounted for solely by slow erosion and do not require (but do not preclude) more complex tectonic interpretations. A tectonic model provided by the Late Cenozoic thrust belts of the Apennine system of the mediterranean region, may lead to new insights into the Early Paleozoic evolution of the Scandinavian Caledonides. 相似文献
175.
Jason W. Barnes Jason M. Soderblom Laurence A. Soderblom Ralf Jaumann Sebastien Rodriguez Bonnie J. Buratti Roger N. Clark 《Icarus》2011,211(1):722-731
Stephan et al. (Stephan, K. et al. [2010]. Geophys. Res. Lett. 37, 7104-+.) first saw the glint of sunlight specularly reflected off of Titan’s lakes. We develop a quantitative model for analyzing the photometric lightcurve generated during a flyby in which the specularly reflected light flux depends on the fraction of the solar specular footprint that is covered by liquid. We allow for surface waves that spread out the geographic specular intensity distribution. Applying the model to the VIMS T58 observations shows that the waves on Jingpo Lacus must have slopes of no greater than 0.15°, two orders of magnitude flatter than waves on Earth’s oceans. Combining the model with theoretical estimates of the intensity of the specular reflection allows a tighter constraint on the waves: ?0.05°. Residual specular signal while the specular point lies on land implies that either the land is wetted, the wave slope distribution is non-Gaussian, or that 5% of the land off the southwest edge of Jingpo Lacus is covered in puddles. Another specular sequence off of Kraken Mare acquired during Cassini’s T59 flyby shows rapid flux changes that the static model cannot reproduce. Points just 1 min apart vary in flux by more than a factor of two. The present dataset does not uniquely determine the mechanism causing these rapid changes. We suggest that changing wind conditions, kilometer-wavelength waves, or moving clouds could account for the variability. Future specular observations should be designed with a fast cadence, at least 6 points per minute, in order to differentiate between these hypotheses. Such new data will further constrain the nature of Titan’s lakes and their interactions with Titan’s atmosphere. 相似文献
176.
On the basis of a 2008 survey conducted in the Msunduzi municipality in the KwaZulu-Natal province, the paper begins an exploration of the character of popular politics and citizenship in South Africa. Embracing a ‘citizen-centred’ methodology informed by participation literatures, and sensibilities to the ‘work in progress’ character of African cities from urban studies debates, the paper interrogates the mainstream liberal-participatory model of citizenship in South Africa, and the critiques of current South African politics informed by these notions, specifically the ‘racial census’ and ‘dominant party syndrome’ analyses. Taken together these views can be read as characterising South African politics as a game for individual citizens governed by liberal rules, but played by racial and/or partisan groups in exclusionary ways, thus distorting liberal democratic mechanisms of representation and accountability. The paper also examines evidence for an alternative class-based analysis of one aspect of citizenship, namely, protest against poor local governance.The paper looks to unpack this ‘liberal model versus racialised communitarian practice’ imaginary by, on the one hand, demonstrating the ways in which citizenship is not racialised, or is asymmetrically racialised. Indeed, other than party allegiances and trust in key offices, very little by way of what citizens do, believe or think of themselves follows discrete racial lines. Similar points hold for partisanship too. On the other hand, the paper does not redeem the liberal-democratic model as there is also evidence of trust in government when it is not deserved based on performance, but more importantly, evidence that citizens embrace ‘informal’ means to secure their rights. A good example of this is protestors who are also more likely to vote than non-protestors. Taken together, these findings affirm both the way in which the racial and partisan legacy of the past is being undone by new institutions and practices, and suggest the complex intersection of these with networks of personal relations which characterise the local politics of most African cities. 相似文献
177.
Shaun?M.?Baesman Laurence?G.?MillerEmail author 《Journal of Atmospheric Chemistry》2005,52(2):203-219
Large carbon kinetic isotope effects (KIEs) were measured for reactions of methyl bromide (MeBr), methyl chloride (MeCl),
and methyl iodide (MeI) with various nucleophiles at 287 and 306 K in aqueous solutions. Rates of reaction of MeBr and MeI
with H2O (neutral hydrolysis) or Cl− (halide substitution) were consistent with previous measurements. Hydrolysis rates increased with increasing temperature
or pH (base hydrolysis). KIEs for hydrolysis were 51 ± 6%0 for MeBr and 38 ± 8%0 for MeI. Rates of halide substitution increased with increasing temperature and greater reactivity of the attacking nucleophile,
with the fastest reaction being that of MeI with Br−. KIEs for halide substitution were independent of temperature but varied with the reactant methyl halide and the attacking
nucleophile. KIEs were similar for MeBr substitution with Cl− and MeCl substitution with Br− (57 ± 5 and 60 ± 9%0, respectively). The KIE for halide exchange of MeI was lower overall (33 ± 8%0) and was greater for substitution with Br− (46 ± 6%0) than with Cl− (29 ± 6%0). 相似文献
178.
Access to fresh water is one of the major issues of northern and sub-Saharan Africa. The majority of the fresh water used for drinking and irrigation is obtained from large ground water basins where there is minor contemporary recharge and the aquifers cross national borders. These aquifers include the Nubian Aquifer System shared by Chad, Egypt, Libya, and Sudan; the Iullemeden Aquifer System, extending over Niger, Nigeria, Mali, Benin, and Algeria; and the Northwest Sahara Aquifer System shared by Algeria, Libya, and Tunisia. These resources are subject to increased exploitation and may be severely stressed if not managed properly as witnessed already by declining water levels. In order to make appropriate decisions for the sustainable management of these shared water resources, planners and managers in different countries need an improved knowledge base of hydrological information. Three technical cooperation projects related to aquifer systems will be implemented by the International Atomic Energy Agency, in collaboration with the United Nations Educational, Scientific and Cultural Organization and United Nations Development Programme/Global Environmental Facility. These projects focus on isotope hydrology studies to better quantify ground water recharge and dynamics. The multiple isotope approach combining commonly used isotopes 18O and 2H together with more recently developed techniques (chlorofluorocarbons, 36Cl, noble gases) will be applied to improve the conceptual model to study stratification and ground water flows. Moreover, the isotopes will be an important indicator of changes in the aquifer due to water abstraction, and therefore they will assist in the effort to establish a sustainable ground water management. 相似文献
179.
Ruth Soto Joseph Martinod Rodrigo Riquelme Grard Hrail Laurence Audin 《Tectonophysics》2005,411(1-4):41-55
We study the Neogene tectonic activity in a sector of the Precordillera in the Andean forearc analysing aerial photographs, satellite images and fieldwork data. The interpretation of alluvial landforms, drainage organisation and evolution of intermittent river networks affecting post-Lower Miocene deposits allow us to recognize low intensity tectonic processes controlling the landscape evolution. All these geomorphological markers indicate no strike-slip offsets, but repeated and small tectonic pulses that reactivate previous structures originated under a transpressive context. The observed deformation pattern is the consequence of E–W orthogonal compression resulting in limited shortening, related to the accommodation of deformation in the Chilean forearc of the Neogene uplift of the Altiplano-Puna. 相似文献
180.
Laurence A. Coogan Simone A. Kasemann Sumit Chakraborty 《Earth and Planetary Science Letters》2005,240(2):415-424
Episodic emplacement and cooling of lavas and dikes at mid-ocean ridges leads to large fluctuations in hydrothermal fluxes and biological activity. However, the processes operating beneath the seafloor during these transient events such as permeability creation and dike cooling are poorly understood. We have developed a new approach to determine the cooling rate of the sheeted dike complex based on the extent of diffusion of lithium from plagioclase into clinopyroxene during cooling. We have calibrated this Li-geospeedometer using new high-temperature experiments to determine both the temperature dependence of the partitioning of Li between plagioclase and clinopyroxene and the diffusion coefficient for Li in clinopyroxene. Application of this method to lavas and dikes from ODP Hole 504B shows that cooling rates vary dramatically with depth in the upper oceanic crust. Extremely rapid cooling rates (> 450 °C hr− 1) in the upper part of the sheeted dike complex are sufficient to power hydrothermal megaplume formation within the overlying water column. 相似文献