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Le destin des anomalies magnétiques océaniques en zone de subduction
No full textL'objet de cette étude est de comprendre les causes de la diminution des anomalies magnétiques océaniques dans la croûte océanique en subduction. Nous étudions l'aimantation de la croûte océanique avant et après l'entrée en subduction et étendons notre zone d'étude initiale des fosses du Japon et des Kouriles à d'autres zones de subduction afin de généraliser nos observations. Avant subduction, une perte de 20% de l'aimantation se produit entre la ride externe et la fosse pour un plancher océanique ancien, causée par le renouvellement de la circulation hydrothermale et l'altération des minéraux magnétiques. Une telle perte n'est pas observée pour un plancher océanique jeune car la flexure y est très faible. Après subduction, dans tous les cas sont observées une décroissance rapide de l'aimantation liée à la désaimantation thermique de la titanomagnétite (Tc 150-350°C) des basaltes extrusifs, suivie d'une décroissance plus lente liée à la désaimantation thermique de la magnétite (Tc 580°C) dans la croûte profonde. La première phase est plus rapidement achevée dans le plancher océanique jeune de par des différences de structure thermique. De manière générale, la disparition des anomalies magnétiques en subduction résulte de la flexure, de la fracturation et de la circulation hydrothermale avant subduction, et de la désaimantation thermique des différents minéraux magnétiques après subduction. L'eau de mer injectée dans la croûte avant subduction est piégée par la couverture sédimentaire après être entrée en subduction et pourrait significativement réchauffer le panneau plongeant par couverture thermique, s'ajoutant au gradient thermique initial et possiblement à la chaleur dégagée par la serpentinisation du coin mantellique. La vitesse de désaimantation thermique est modulée par l'épaisseur lithosphérique, le taux d'hydratation, et donc l'âge du plancher océanique.The purpose of this study is to understand the causes of the decaying seafloor spreading magnetic anomalies on subducting oceanic crust. We investigate the magnetization of the oceanic crust both before and after subduction and extend our initial study area from the Japan-Kuril subduction zone to other subduction zones to try to generalize our observations. Before subduction, a 20% loss of magnetization between the outer-rise and the trench occurs in old seafloor, caused by rejuvenated hydrothermal circulations and alteration of magnetic minerals. Conversely, such a loss of magnetization is not observed for the young seafloor because the flexure remains very limited. After subduction, both exhibit a fast decay of magnetization due to thermal demagnetization of titanomagnetite (Tc:150-350°C) in the extrusive basalt, followed by a much slower one due to thermal demagnetization of magnetite (Tc: 580 °C) in the deeper crust. However, the fast decay is more rapidly achieved in the young seafloor due to differences in the thermal structure. Overall, the magnetic anomalies in subducting oceanic crust decay as an effect of flexure, normal faulting and hydrothermal alteration before subduction, and thermal demagnetization of the different magnetic minerals after subduction. The seawater injected in the oceanic crust before subduction is trapped by the sediment cover after entering subduction and may significantly heat up the slab through thermal blanketing, adding to the thermal gradient and possibly heat released by serpentinization of the mantle wedge. The speed of thermal demagnetization is modulated by the lithospheric thickness, hydration rate, and therefore the age of the seafloor
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Fading magnetic anomalies, thermal structure and earthquakes in the Japan Trench
No full textInternational audienc
The fate of marine magnetic anomaly in subduction zones: A global appraisal
No full textInternational audienceSeafloor spreading magnetic anomalies of oceanic plates progressively decay and disappear when approaching and entering subduction. Previous studies focused on the Japan Trench showed that before subduction, the flexure of oceanic lithosphere opens normal faults and rejuvenates hydrothermal circulation which results in the alteration of magnetic minerals. After subduction, thermal demagnetization affects the magnetic minerals of the oceanic crust when the increasing temperature of the subducting slab passes their Curie temperature. No other subduction zone was investigated so far. Here we study the evolution of magnetic anomalies in five subduction zones. The major observed differences are controlled by the age of the lithosphere and related physical properties. Before subduction, magnetic anomalies of old oceanic lithosphere show a significant decay (20-30%) related to alteration whereas those of young lithosphere at the Cascadia subduction zone displays no such decay as a consequence of negligible flexure on the young Juan de Fuca plate. After subduction, the magnetic anomalies of the subducting plate show a rapid decay reflecting the thermal demagnetization of extrusive basalt, amounting for 30-40% in old oceanic lithosphere and more than 70% in the young lithosphere of Cascadia. Beyond complete thermal demagnetization of extrusive basalt, about 40% of the magnetization remains in the old lithosphere and less than 30% in the young lithosphere of Cascadia
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