1,720,979 research outputs found
Calculated phase equilibria for high-pressure serpentinites and compositionally related rocks close to the MgO–Al2O3–SiO2–H2O (MASH) system
No full textThe metamorphic evolution of serpentinite has a fundamental bearing on geochemical cycles, mechanical behaviour and seismic properties of the Earth. Phase equilibria in the system MgO-Al2O3-SiO2-H2O (MASH) provide a 'backbone' for phase equilibria in larger systems that are applicable to serpentinites and also many whiteschists. Phase equilibria for MASH, and departures from MASH, are calculated using the Holland and Powell data set, tc-ds62, and new activity-composition relations for antigorite and talc. Phase relations are presented using PT projections and compatibility diagrams to highlight the role of P, T and composition on the equilibrium mineral assemblages stable in these chemical systems at high pressure. These phase relations provide the context for pseudosection and other modelling focused on individual rock compositions
Deformation history of ultra high-pressure ophiolitic serpentinites in the zermatt-saas zone, créton, upper valtournanche (Aosta valley, western alps)
No full textDetailed multiscale structural analyses and mapping (1:20 scale) integrated with petrological investigation were used to study a portion of the Zermatt-Saas serpentinites that crop out in upper Valtournanche (north-western Italy). Results are synthesized in a foliation trajectory map that displays the transposed original lithostratigraphy of a serpentinite body exposed at Créton. The serpentinite body comprises magnetite sheets and rare, decimetre-thick, diopsidite layers and lenses. Moreover, veins and aggregates of Ti-chondrodite and Ti-clinohumite, olivine-rich layers and lenses, veinlets of olivine, and layers of dark pyroxenite are embedded in the serpentinites. Serpentinites and associated rocks record three relative age groups of ductile structures: D1 consists of rare folds and S1 foliation; D2 is a group of isoclinal folds and a very pervasive foliation (S2), which is the dominant structure; D3 includes a crenulation and shear zones affecting S2. The detailed meso-structural and microstructural analyses allowed individuating the metamorphic environment of successive deformation stages and correlating the resulting tectono-metamorphic investigation with those already inferred in surrounding areas. In addition, metre-to submillimetre-sized pre-D2 structural, mineralogical, and textural relics have been clearly identified in spite of the strong transposition imposed during the development of S2 high pressure-ultra-high pressure foliation
UHP Ti-chondrodite in the Zermatt-Saas serpentinite: Constraints on a new tectonic scenario
No full textWe focus on the key role of different Ti-humite minerals in subducted serpentinites as possible indicators of extreme pressure conditions. The occurrence of Ti-chondrodite and/or Ti-clinohumite assemblages in the eclogitized serpentinites of the Zermatt-Saas Zone (ZSZ) of the Western Alps allows the recrystallization of such rocks at UHP conditions (P = 2.8-3.5 GPa, T = 600-670 °C) to be determined. Such conditions are similar to those registered by the nearby Cignana unit, a main Alpine area for UHP metamorphism, where coesite and microdiamond have been found. In ZSZ serpentinites, the new UHP assemblage predates the previously recognized HP-UHP paragenesis, which was recently dated at 65 Ma. This finding opens up a new interpretation for the petrologically and structurally well-constrained HP/ UHP records, especially because all other ages for HP-UHP metamorphism in the ZSZ are much younger, and for the size of UHP units. Our findings suggest that ophiolites in the axial zone of collisional belts are a mosaic of oceanic lithosphere slices that recorded contrasted thermal and mechanical evolutions during their physical trajectories in the subduction wedge
Birth and death of oceanic basins: Geodynamic processes from rifting to continental collision in Mediterranean and circum-Mediterranean orogens
Full text linkThe study of the evolution of ocean basins from birth to death is crucial for the understanding of the geodynamic evolution of orogenic systems. Exhumed ophiolite-bearing orogenic belts represent significant fossil analogues of different types of modern oceanic basins, allowing detailed multiscale and multidisciplinary investigations. Such investigations are highly important to our understanding of the ancient and modern geodynamic processes connected to the different stages of complete tectonic evolution, from rifting to subduction, collision and exhumation.</jats:p
Metamorphic evolution of the Saka Unit (Central Pontides, Northern Turkey): new implications for the Mesozoic convergence-related processes in the Intra-Pontide suture zone
Full text linkIn Northern Turkey, the Intra-Pontide suture (IPS) zone represents an east-west trending belt of deformed and/or metamorphic rocks bounded by the Istanbul-Zonguldak Terrane (IZ) to the north and the Sakarya Composite Terrane (SK) to the south (e.g. Göncüoğlu et al., 1997). Despite its importance for the geodynamics reconstructions of the Black Sea and Eastern Mediterranean areas during the Mesozoic, the IPS zone has been poorly investigated; only recently the role of its metamorphic units during the syn-collisional evolution was explored (Marroni et al., 2014; Frassi et al., 2016; 2017)
In order to provide new insights on the Mesozoic-Tertiary geodynamic reconstruction of the southern margin of the Laurasia, we present new data from the Saka Unit from the eastern portion of the IPS. Using a multidisciplinary approach that includes lithological, structural, metamorphic and petrographic investigations, we constrained the P-T-t path during the Mesozoic subduction and consequent exhumation and accretion to Laurasia. Following the determination of mesostructures, we analysed a series of samples of metabasite and metapelite. Mineral chemistry of phases at equilibrium in the pervasive S2 foliation was determined, and PT determinations were performed through pseudosection calculation of three samples. PT conditions are within the epidote-amphibolite facies, with pressures of 1.2 ± 0.15 GPa and Temperatures 600 ± 50 °C, thus higher than those determined previously, suggesting that S2 could be a composite foliation where a progressive evolution acquired during onset of exhumation was recorded. A comparison with the PT evolutions of the nearby Daday and Domuz-Dag units, shows that they record lower T at D2 at different ages, and that the pre-D2 conditions, when they are preserved, are different in the three units.
References
Frassi C., Göncüoğlu M.C., Marroni M., Pandolfi L., Ruffini L. Ellero A., Ottria G. & Sayit K. 2016. The Intra-Pontide Suture Zone in the Tosya-Kastamonu area, Northern Turkey. J. of Maps. 12, 211-219
Frassi, C, Marroni, M., Pandolfi, L., Göncüoğlu, M. C., Ellero, A., Ottria, G., Sayit, K., McDonald, C.S., Balestrieri, M.L. & Malasoma, A. 2018. Burial and exhumation history of the Daday Unit (Central Pontides, Turkey): implications for the closure of the Intra-Pontide oceanic basin. Geol. Mag., 155, 356-376.
Göncüoğlu, M. C., Marroni, M., Sayit, K., Tekin, U. K., Ottria, G., Pandolfi, L. & Ellero, A., 2012. The Ayli Dag ophiolite sequence (central-northern Turkey): A fragment of middle Jurassic oceanic lithosphere within the Intra-Pontide suture zone. Ofioliti, 37, 77–91.
Marroni, M., Frassi, C., Göncüoğlu, C. M., Di Vincenzo, G., Pandolfi, L., Rebay, G., Ellero, A. & Ottria, G. 2014. Late Jurassic amphibolite-facies metamorphism in the Intra-Pontide Suture Zone (Turkey): an eastward extension of the Vardar Ocean from the Balkans into Anatolia? J. Geol. Soc., 171, 605–608
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Full text linkWe 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
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