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La struttura profonda del Margine orientale della Sardegna dall'interpretazione di sismica a riflessione ed a rifrazione
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Rapid uplift of a lithospheric Sliver Near the Vema FZ (Central Atlantic) due to change in the pole of rotation
No full textThe Vema F.Z. offsets the Mid Atlantic Ridge by about 320 km at 10ø45' N. The southern flank of the transform valley is bounded by a prominent transverse ridge (VTR), reaching a minimum depth of ~500 m. Several lines of evidence indicate that the north facing wall of the VTR exposes a complete, relatively undisturbed section of uplifted oceanic lithosphere. The length of the VTR is ~300 km, similar to the length of offset of the transform. The VTR starts abruptly about 140 km from the MAR axis, in crust roughly 10 m.y. old, assuming an half spreading rate of 1.5 cm/y. This age marks the end of the uplift phase of the VTR. The time of initiation of uplift is marked probably by an abrupt change of orientation of sea floor morphostructural fabric, from parallel to the present ridge axis to ~5§ to 10§ oblique. This change of orientation took place about 12 Ma, and is related to a change in the position of the pole of rotation that caused extension across the transform and triggered flexure of a lithospheric slab and uplift of the transverse ridge. The ~4 km uplift of the VTR took place in roughly 2 m.y. with an average rate of uplift of 2 mm/y. Uplift rates appear to have been similar throughout the 300 km length of the VTR. The flexure and rapid uplift did not cause major internal deformation of the lithospheric slab
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
Contrasting Ultra-Slow Ridges near the Bouvet Triple Junction in the South Atlantic
No full textTwo ultraslow mid ocean ridges, i.e., the westernmost portion of the Southwest Indian Ridge (SWIR) and the easternmost portion of the American-Antarctic Ridge (AAR), meet the southernmost portion of the Mid Atlantic Ridge (MAR) at the Bouvet Triple Junction. Although both the SWIR and the AAR have very slow spreading rates, i.e., 7.5 mm/y and 10 mm/y, respectively, they display strongly contrasting morphostructural and petrological characteristics. The AAR axial zone is deeper than normal (>4000m); some of its segments are oblique, being oriented roughly 45 degrees from the direction of spreading; basalt and peridotite chemistry suggests a relatively low degree of melting of the mantle below the AAR. In contrast, the SWIR axial zone is shallower than normal (2500m); the last segment (Spiess Ridge) is particularly anomalous, reaching ~400 m below sea level. Basalt and peridotite chemistry of this portion of SWIR suggests high extents of melting of the upper mantle. The H2O content of the SWIR basaltic glasses ranges between 0.6% and 1.0% and is significantly higher than in normal MORB. The high extent of melting at the SWIR segments is probably related not only to a mantle thermal anomaly, but also to its high volatiles content. Mantle thermal structure and composition, rather than spreading rate, are the main factors determing the structure of these ridges
Steady-state creation of crust-free lithosphere at cold spots in mid-ocean ridges
No full textMid-ocean ridges create oceanic lithosphere consisting normally of basaltic crust a few kilometers thick overlying a peridotitic mantle. However, lithosphere free of basaltic crust formed during the past ;30 m.y. at an ;50-km-long stretch of Mid-Atlantic Ridge southof the Romanche Fracture Zone, giving rise to a .500-km-long strip of ocean floor exposing mostly mantle peridotites that have undergone an unusually low (#5%) degree of melting, mixed with peridotites that reacted with a small fraction of basaltic melt. Thislithosphere contains ,10% of scattered gabbroic pockets, representing melt frozen above 25 km depth within a relatively cold subaxial lithosphere. Numerical modeling excludesdry melting below this crust-free lithosphere, because of the cooling effect of the longoffset Romanche transform combined with a regional mantle thermal minimum; however, modeling allows a limited extent of hydrous melting. This unusual lithosphere, unable to expel the melt fraction, characterizes cold spots along mid-ocean ridges
Crust-Free “Constipated” Lithosphere created at cold spots in Mid Ocean Ridges
No full textA ~50 km stretch of the MAR south of its intersection with the Romanche transform lacks a basaltic crust and exposes mantle ultramafics. We tested the hypothesis that the absence of a basaltic crust reflects a temporary phase of a-magmatic extension by sampling the ocean floor along ridge-parallel profiles taken at increasing distances from the MAR axis, in lithosphere of increasing age. The basaltic crust is nearly absent in a ~50 Km wide strip of ocean floor on the southern side of the Romanche FZ extending from ridge axis for over 400 km along a sea floor spreading flow line. Assuming a spreading half rate of 1.7 cm/yr, crust-free lithosphere formed and spread for roughly 30 m.y., excluding for this area a model of alternating cycles of igneous injection and a-magmatic extension at ridge axis. A ~30 m.y. steady-state formation and spreading of lithosphere that lacks a basaltic crust can be explained by the effect of a regional mantle thermal minimum combined with the thermal effect of a long-offset, slow-slip transform. The basaltic crust-free lithosphere created at the MAR close to the Romanche FZ consists of "normal" mid ocean ridge peridotites (N-MORP) that, however, have undergone an unusually low (<5%) degree of melting, mixed with "metasomatic" peridotites (M-MORP) that have reacted with a small fraction of basaltic melt. This lithosphere contains <10% of scattered gabbroic pockets. These melt impregnations and gabbroic pockets represent melt produced by small degree of melting, frozen within the top ~20 Km of the relatively cold subaxial lithosphere. This unusual lithosphere, unable to expel the melt fraction, ("constipated lithosphere") should be common in "cold spots" along mid ocean ridges
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