1,721,065 research outputs found
Magmatic history and evolution of the Central American Land Bridge in Panama since Cretaceous times
No full textChemical compositions for 310 igneous rocks from the Cordillera de Panama and the Sona and Azuero peninsulas were supplemented by (40)Ar/(39)Ar dating and Sr-, Nd-, Pb-, and O-isotope analysis to determine the magmatic evolution and oceanic plate inter actions over the past 100 Ma in western Panama. An initial phase of intraplate magmatism, having geochemical characteristics of the Galapagos hotspot, formed the oceanic basement of the Caribbean large igneous province from 139 to 69 Ma. Younger accreted terranes with enriched trace element patterns (accreted ocean island basalt [OIB]) were amalgamated between 70 and 20 Ma. A second magmatic phase in the Azuero and Sona peninsulas has trace element patterns (Sona-Azuero arc) suggesting the initiation of subduction at 71-69 Ma. Arc magmatism continued in the Chagres basin region (Chagres-Bayano arc) from 68 to 40 Ma. A third phase formed discrete volcanic centers across the Cordillera de Panama (Cordilleran arc) from 19 to 5 Ma. The youngest phase consists of isolated volcanic centers of adakitic composition (Adakite suite) in the Cordillera de Panama that developed over the past 2 million years. Initiation of arc magmatism at 71 Ma coincides with the cessation of Galapagos plateau formation, suggesting a causal link. The transition from intraplate to arc magmatism occurred relatively quickly and introduced a new enriched mantle source. The arc magmatism involved progressive transition to more homogeneous intermediate mantle wedge compositions through mixing and homogenization of subarc magma sources through time and/or the replacement of the mantle wedge by a homogeneous, relatively undeleted asthenospheric mantle. Adakite volcanism started after a magmatic gap, enabled by the formation of a slab window.Army Research Laboratory; Deutsche Forschungsgemeinschaft [WO 362/27-2
Multidisciplinary tephrochronological correlation of marker events in the eastern Tyrrhenian Sea between 48 and 105ka
No full textThe basal portion (2.92–5.08 m from core top) of the CET1 core located in a bathyal area of the eastern Tyrrhenian Sea is the subject of a multidisciplinary investigation, encompassing tephrostratigraphy, quantitative analyses of planktonic foraminifera and calcareous nannofossil assemblages, and δ18O measurements, supported by a 40Ar/39Ar age determination. Calcareous nannofossil assemblages allowed the attribution of the analyzed sediments to the biozone MMN21a, and the succession spans from more than 48 to ca. 105 ka, although there are at least two stratigraphic gaps and minor resedimentation episodes. A paleoclimatic reconstruction obtained via δ18O and planktonic foraminifera data identifies several of the major climatic events that occurred in the investigated time span.
A total of 13 visible tephra layers and cryptotephras are recognized and correlated with their volcanic sources (Campanian Volcanic Zone and Pantelleria volcano) and, when possible, with well-known and dated events or with widespread marker tephras.
Using a combination of biostratigraphic, oxygen isotope, and tephrostratigraphic correlations, we develop a chronostratigraphy for the main climatic events and tephra layers in the core and pinpoint tephra markers for the climatic events comprised between GS 22 and GI 24 in the eastern Tyrrhenian Sea
Early Holocene collapse of Volcan Parinacota, central Andes, Chile: Volcanological and paleohydrological consequences
No full textThe catastrophic gravitational collapse of the Old Cone of Volcan Parinacota produced a 6 km(3) debris avalanche that traveled similar to 22 km and covered more than 150 km(2). The Upper Lauca drainage, a broad high-altitude basin in the Chilean Altiplano, was permanently altered by the collapse. Although the eruptive history of Parinacota before and after-the debris avalanche is well known from petrologic and geochronologic studies, previous age limits on the debris avalanche (based on multiple chronometers) span ca. 8-20 ka. New cosmogenic surface-exposure ages from boulders atop the deposit are based on a regionally calibrated production rate of in situ Be-10 and indicate that the avalanche occurred at 8.8 +/- 0.5 ka. These data demonstrate that cosmogenic 10Be surface-exposure dating can be successfully applied to quartz-bearing, volcanic debris avalanche deposits, and that this method offers a distinct advantage over C-14 chronologies that provide only minimum or maximum age limits. The 8.8 ka exposure age for the debris avalanche (1) agrees with 14C age limits of paleosol material incorporated in the debris avalanche, (2) requires a voluminous initial phase of postcollapse volcanism with an eruptive rate exceeding that of recent cone-building episodes at most continental arc volcanoes, and (3) suggests that volcano collapse did not result in the formation of Lago Chungara, but instead led to a major expansion of a preexisting closed basin
Predict: Assessing the Seismic Response in the city of Rome, Part 1. New Data for a Geologic Overview
Full text linkThis work presents the preliminary results of the stratigraphic, paleomagnetic, micropaleontologic and geochronologic investigations carried out on the cores of three boreholes performed in the historic center of Rome within the Predict Project, aimed at evaluating the seismic response within the City through the 3D modeling of the subsoil.
Moreover, we have integrated the investigations on the cores of four previously performed boreholes and we use this larger dataset to provide an objective element for the interpretation and validation of a large databank of paper stratigraphies of boreholes carried out for civil engineering purposes in the Roman area.
The new data are presented within an exhaustive review of state of the art on the scientific knowledge on the geology of Rome, aimed at providing an updated background for the Quaternary scientists, seismologists, engineers and professional technicians operating in this region.
Within this framework, we provide a detailed reconstruction of the chronostratigraphic setting in central Rome, highlighting a coherent picture within the glacio‐eustatic control on the sedimentary processes and providing the background geological input data for the creation of a geo‐database in a dynamic GIS environment, which is the subject of a forthcoming sister‐paper
Paleozoic metasomatism at the origin of Mediterranean ultrapotassic magmas. Constraints from time-dependent geochemistry of Colli Albani volcanic products (Central Italy)
No full textThe major processes that control the genesis of potassic volcanic rocks, like the timing ofmulti-stagemantlemetasomatism,
remain largely unclear. In an attempt to clarify the timing of the metasomatic process, a detailed geochronologic
and geochemical study has been conducted on the ultrapotassic rocks of the Colli Albani Volcanic
District (Central Italy). New 40Ar/39Ar data coupled with literature and newly performed 87Sr/86Sr, 143Nd/144Nd
and chemical data allow us to precisely delineate the time-dependent geochemical variations of the magmas
erupted at the Colli Albani Volcanic District and to better define mantle source processes responsible for their genesis.
The temporal geochemical variations observed in the Colli Albani magmas indicate that: i) the ultrapotassic
magmas originated from a metasomatized mantle source in which phlogopite is the potassium-bearing phase;
ii) the partial melting of the mantle source involved mainly phlogopite and clinopyroxene (±olivine), whereas
the role of accessory phases was less significant; and iii) the metasomatic process that led to the formation of
the phlogopite in the mantle can be reasonably related to events that have occurred during the Paleozoic Era
Shallow and deep crustal control on differentiation of calc-alkaline and tholeiitic magma
No full textThe role of changing crustal interaction and plumbing geometry in modulating calc-alkaline vs. tholeiitic magma affinity is well illustrated by the influence of 70 km thick crust beneath Volcan Parinacota. Changes in petrologic affinity correlate with periods of cone-building, sector collapse, and rebuilding of the volcano over the last 52 ka, and are well explained by changes in magma recharge regime. With increasing recharge and magma output, lavas transition from low-Fe, strongly calc-alkaline, phenocryst-rich silicic compositions to medium-Fe, near-tholeiitic, mafic, and aphanitic characteristics. Strontium isotope data show that the change in magma regime did not affect all parts of the system simultaneously; these are characterized by distinctive (87)Sr/(86)Sr ratios, which suggest an initially compartmentalized system. Relatively high ((230)Th/(232)Th) activity ratios of similar to 0.72 in early-erupted calc-alkaline lavas are consistent with interaction with high-U upper crust. Low ((230)Th/(232)Th) activity ratios of similar to 0.55 and up to 33% Th-excess in younger near-tholeiitic lavas correlate with steep REE patterns, indicating lower-crustal interaction. Thorium-excesses at the time of eruption approach the maximum that can be generated via small-degree garnet-residual melting in the lower crust or mantle and imply that transit time through the crustal column for the most tholeiitic magmas had to be short, on the order of 3x10(5) yr in the upper crust. In addition to more traditional explanations tied to magma source, expression of low-Fe 'calc-alkaline' (CA) vs. medium-Fe 'near-tholeiitic' (TH) magma series at the scale of individual volcanoes is likely to be modulated by transitions from compartmentalized, stagnant, assimilation-prone 'dirty' systems (CA) to 'clean' systems (TH) that are characterized by rapid magma throughput and minimal opportunity for upper-crustal contamination. (C) 2009 Elsevier B.V. All rights reserved
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
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