1,721,005 research outputs found
High-resolution 40Ar/39Ar chronostratigraphy of the post-caldera (<20 ka) volcanic activity at Pantelleria, Sicily Strait
Full text linkThe island of Pantelleria (Sicily Strait), the type locality for pantellerite, has been the locus of major calderaforming
eruptions that culminated, ca. 50 ka ago, in the formation of the Cinque Denti caldera produced by the
Green Tuff eruption. The post-caldera silicic activity since that time has been mostly confined inside the caldera
and consists of smaller-energy eruptions represented by more than twenty coalescing pantelleritic centers
structurally controlled by resurgence and trapdoor faulting of the caldera floor. A high-resolution 40Ar/39Ar
study was conducted on key units spanning the recent (post-20 ka) intracaldera activity to better characterize
the present-day status (and forecast the short-term behavior of) the system based on the temporal evolution of
the latest eruptions. The new 40Ar/39Ar data capture a long-term (N15 ka) decline in eruption frequency with a
shift in eruptive pace from 3.5 ka−1 to 0.8 ka−1 associated with a prominent paleosol horizon marking the only
recognizable volcanic stasis around 12–14 ka. This shift in extraction frequency occurswithoutmajor changes in
eruptive style, and is paralleled by a subtle trend of decreasingmelt differentiation index. We speculate that this
decline probably occurred (i) without short-term variations in melt production/differentiation rate in a steadystate
compositionally-zoned silicic reservoir progressively tapped deeper through the sequence, and (ii) that it
was possibly modulated by outboard eustatic forcing due to the 140 m sea level rise over the past 21 ka. The
intracaldera system is experiencing a protracted stasis since 7 ka. Coupled with recent geodetic evidence of
deflation and subsidence of the caldera floor, the system appears today to be on a wane with no temporal
evidence for a short-term silicic eruption
Anatomy of the Cycladic Blueschist Unit on Sifnos Island (Cyclades, Greece)
No full textSince 35 Ma, the kinematics of the Aegean domain has been mainly controlled by the southward retreat of the African slab, inducing back-arc extension. The main structures and associated kinematics are well constrained, but the kinematics of deformation before 35 Ma, coeval with the exhumation of blueschists and eclogites of the Cycladic Blueschist Unit, is still poorly understood. The earlier Eocene syn-orogenic evolution is strongly debated and very different geometrical interpretations and kinematic histories have been proposed in the literature. This study focuses on the high-pressure and low-temperature (HP-LT) parageneses spectacularly exposed and well preserved on Sifnos Island. The new field work provides new structural constraints on the tectonic history of HP-LT units generated in the subduction zone during the Eocene. It further shows how lithological heterogeneities localize strain within an accretionary wedge and how the localisation of strain evolves through time during exhumation. We show, through new geological and metamorphic maps, cross-sections and analyses of kinematic indicators, that Sifnos is characterized by shallow-dipping shear zones reactivating weak zones due to competence contrasts or earlier tectonic contacts. Structures and kinematics associated with these shear zones, show a top-to-the-N to −NE ductile deformation. The lower part of the tectonic pile shows a downward gradient of shearing deformation and is actually a thick top-to-the-NE shear zone, which we name the Apollonia Shear Zone. Through time shearing deformation tends to localize downward, leaving the upper part of the subduction complex preserved from late deformation. The present-day shape and topography of the island is largely controlled by late brittle faults reworking the earlier ductile shear zones. Comparing with the nearby island of Syros, we propose a new tectono-metamorphic evolution of the Cycladic Blueschist Unit, which partly explains the different degrees of retrogression observed on the Cycladic Islands
TEPHROCRONOLOGY WITH 40AR–39AR DATING OF RECENT SILICIC VOLCANISM AT PANTELLERIA ISLAND: IMPLICATIONS ON MAGMATIC SYSTEM.
No full text
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
A revised stratigraphy of the pre-Green Tuff ignimbrites at Pantelleria (Sicily Channel)
No full textPeralkaline silicic magmas were erupted at Pantelleria in a variety of eruptive typologies and magnitudes: pyroclastic flows, Plinian to strombolian pumice fallout and lava flows. The initial cycle (330- 180 ka) was characterized mainly by effusive activity, and was followed by an intermediate cycle (181 - 85 ka), characterized by a clear drift to explosive activity. This period, onto we focus, is bracketed by six ignimbrite-forming eruptions (older and intermediate of the two caldera collapses at 140 and 50 ka that characterize the volcanological history of the island, which drained silicic and variably peralkaline magma for a cumulative volume close to 6 km3 DRE. These ignimbrites lack of continuity among the few outcrops (commonly in vertical scars) and also suffer from strong welding and rheomorphism. First reconstructions and correlations [Mahood and Hildreth, 1986], although still valid for many aspects, suffer from the poor precision of K/Ar ages, especially when Ar excess and xenocrystic contamination are common, as is the case for Pantelleria lavas and pyroclastic rocks. Recently, new correlations based on paleomagnetic methods [Speranza et al., 2012] succesfully identified and correlated two welded pyroclastic breccias outcropping in the NE and SW sectors of the island with the old caldera collapse, and also demonstrated the equivalence of two other ignimbrite units previously considered as separate eruptions.
Based on 14 new 40Ar/39Ar ages (laser heating on anorthoclase feldspar separates) ages and petrographic data, we propose an updated stratigraphy for the intermediate period and to provide accurate age brackets to the correlations. Recently, the application of 40Ar/39Ar methods on much younger (age < 20 ka) silicic lavas and tephra at Pantelleria [Scaillet et al., 2011], demonstrated highly precise and contributed to unravel the complex post-Green Tuff stratigraphy.
Our results can be summarized as follows:
- The pre-La Vecchia caldera eruptive history ended with the emplacement of two trachytic ignimbrites (at 181 ± 1.2 ka and 171 ± 1.7 ka), which were emplaced only in the south sector of the island. Both were truncated by the old (La Vecchia) caldera collapse.
- The age of the old caldera collapse is now tightly constrained between 139-146 ka , i.e. the new age of the welded pyroclastic breccia (previously undated). that represents the caldera-forming eruption [Speranza et al., 2012].
- After the La Vecchia caldera collapse, the first ignimbrite eruption occurred at 123 ka with a trachy-comenditic and crystal-rich unit emplaced in the NE to NW sectors of the island.
- At 107 ka a crystal-rich trachytic ignimbrite was erupted, together with some fines-rich lobes that (K/Ar ages) were previously considered as two different units, but with similar 40Ar/39Ar ages.
- Higher up in the stratigraphic sequence, two previously considered different ignimbrites (for an inferred volume of erupted magma close to 1.5 km3 DRE) demonstrated coeval by 40Ar/39Ar dating, and together are part of an eruptive paroxysm at 85 ka comparable in magnitude to the Plinian eruption of the Green Tuff (age 50 ka),
- The the recurrence time of the above mentioned eruptions, for a cumulative erupted magma volume close to 6 km3 DRE, suggest that at the end of the inter-eruptive period 85-50 ka the production and evolution rates of pantellerite magma peaked to reach a maximum peralkalinity
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
- …
