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Continental subduction and exhumation: an example from the Ulten Unit, Tonale nappe, Eastern Austroalpine.
No full textSome exhumed complexes in collisional belts consist of continental basement containing slivers of mafic and ultramafic material showing evidence of UHP metamorphism (P c. 3 GPa). Their PTt history can be interpreted in terms of subduction of continental material to depths ≥ 100 km and subsequent exhumation. This type of tectonic history is illu strated by the Late Palaeozoic evolution of the Ulten Unit, Tonale Nappe, Eastern Austroalpine. The upper crustal felsic component (c. 80% by volume) incorporated mafic material at the trench, and peridotitic material at deeper levels in the subduction zone. The peridotites show evidence of a P-increasing, T-decreasing path before incorporation in the felsic material, compatible with flow in the mantle wedge above the subducting slab. After emplacement of the peridotites, which occurred at or near peak metamorphic conditions (P ≥ 2.7 GPa, T ≥ 850 °C), the complex underwent a two-stage pre-Alpine exhumation path: a first, fast stage (c. 0.1-1 cm a-1), lasting c. 30 Ma and bringing rocks from depths ≥ 100 km to approximately 25 km; and a second, slow stage (c. 0.01-0.1 cm a-1), lasting c. 100 Ma and bringing rocks to depths 200 km if attached to a mature oceanic slab that does not break-off during the early stages of continental subduction. The first exhumation stage can be accounted for by buoyancy-driven tectonic extrusion of continental slices along the subduction channel during continuing subduction. A force balance analysis shows that such a mechanism is compatible with the rheology of felsic and intermediate rocks at high temperature. The second exhumation stage is compatible with isostatic rebound and tectonic denudation following slab break-off. The conclusion that fast exhumation occurs during continuing subduction and before slab break-off is in accordance with the observed rates, which show fast movement of the rising slices with respect to the surrounding material. Slab break-off, on the other hand, generates a long-wavelength gentle upwarping of the overlying region, which is more compatible with later and slower exhumation rates. © The Geological Society of London 2005
Subduction of continental lithosphere, changes in negative buoyancy, and slab-plume interaction: consequences for slab breakoff
No full textThe time evolution of buoyancy of oceanic and continental lithosphere during subduction is estimated on the basis of a kinematic model with both constant and time-varying convergence rates. The negative buoyancy before the arrival of continental lithosphere at the subduction zone increases with increasing depth of penetration of the slab at a rate proportional to the convergence rate. The time required for the negative buoyancy to be reduced to zero by the subduction of positively buoyant continental material depends on the depth reached by the oceanic slab and the convergence rate, but is typically of the order of a few tens Ma.
If a rising plume impinges from below on a near-stationary slab in the upper mantle, the corresponding part of the slab is heated and therefore softened. The softening effect is enhanced if the slab includes continental material.
The combination of changes in negative buoyancy caused by continental subduction, and softening of a part of the slab caused by slab-plume interaction, may act as a regulator for the time of slab breakoff and consequently for the time and type variations of magmatism in the overriding lithosphere above a subduction zone.
A plausible example of this situation may be provided by the Alpine slab subduction beneath the Adria plate at Paleocene time. Here, the Tertiary convergence between Europe and Africa plates was characterized by the consumption of both oceanic and continental European crust. Some million years later (45-30 Ma ago), two contrasting magmatic suites developed in the south-eastern sector of the Alps, partially overlapping in time: a) a calc-alkaline, subduction-related suite, and b) an alkaline, plume-related volcanic suite. On the basis of geological, geophysical and geochemical data we supposed that both the magmatic suites originated by a common and primary deep mantle plume the root of which was located beneath the Cape Verde-Madeira-Canary Islands region, while the head of which was swerved and frayed by the Eurasian plate that from this latitude moved northeastwards. In the Alpine region, the plume head material might have interacted with the Alpine subducting slab causing its heating, softening, and finally its detachment. Ensuing upwelling of plume material through the so formed plate window is supposed to be the responsible for either the partial melting of the lithospheric mantle wedge and for the partial melting of the plume material itself
Slab detachment and mantle plume upwelling in subduction zones: An example from the Italian south-eastern Alps volcanism
No full textAbstract
The geochemical properties of the South-Eastern Alps volcanics (SEAV, Eocene age) call for a within-plate origin of the most primitive basalts,
in contrast to the widespread calc-alkaline magmatism which developed some million years later northwestwards along the Periadriatic Lineament.
The two contrasting magmatic suites that coexist in the Alpine area define binary mixing relationships in the Sr–Nd and Sr–Pb isotopic space, the
end members of which being a crustal component (e.g. lower continental crust) and a HIMU-DMM component (e.g. the SEAV). The occurrence
of a HIMU (high μ= high
238
U/
204
Pb) component, which normally traces mantle plumes of deep mantle origin, in a tectonic regime dominated
by collision tectonics (the tertiary convergence of European and Adriatic plates) can be explained by slab detachment and ensuing upwelling of
mantle material through the lithospheric gap. We combine geochemical data and geophysical modelling to unravel the evolution of the Alpine
slab after interaction with plume material and the genesis of the Alpine magmatism. The combination of changes in negative buoyancy caused by
continental subduction and softening of a part of the slab caused by slab–plume interaction may act as a regulator for the time of slab breakoff and,
consequently, for the variations of magmatism in the overriding lithosphere above a subduction zone. The thermal evolution of a subducting slab is
modified by contact with the plume material which decreases significantly the total strength of the slab and favours slab detachment. Interactions
between the HIMU component and the shallower depleted mantle can account for the geochemical characteristics of the SEAV. Counterflows of
plume material towards the top of the subducting slab may also increase heating and partial melting of the overriding mantle wedge, giving rise to
the calc-alkaline suite outcropping in the proximity of the Periadriatic Lineament.
© 2007 Elsevier Ltd. All rights reserved
HIMU-OIB magmatism in subduction zones: An example from the Italian south-eastern Alps
No full textThe geochemical features of the SE Alps volcanics (SEAV, Tertiary age) are comparable to the numerous volcanic eruptions of Tertiary-Quaternary age from the western Mediterranean area for which a plume-related origin has been assessed, and contrast to the widespread calcalkaline magmatism which developed northwestwards along the Periadriatic Lineament. The occurrence of a HIMU component, which is the hallmark of hotspot basalts, in a collision environment (the Tertiary convergence of Europe and Africa plates) is here explained in terms of slab breakoff. Evidence for the European slab breakoff comes from seismic tomography which shows that the present-day fast velocity material, interpreted as the European slab subducted below the Alpine chain, is shorter by about 300 km than the total length of the subducted slab estimated by paleotectonic reconstructions. Other piece of evidence comes from a kinematical model consisting in evaluating the time evolution of buoyancy of oceanic and continental lithosphere during subduction with both constant and time-varying convergence rates. If the subducted slab intercepts a rising plume from below the corresponding part of the slab is heated and therefore softened. The softening effect is enhanced if the slab includes continental material. The combination of changes in negative buoyancy caused by continental subduction, and softening of a part of the slab caused by slab-plume interaction, may act as a regulator for the time of slab breakoff and consequently for the time and type variations of magmatism in the overriding lithosphere above a subduction zone. In the Alpine region, we assume that the plume material interacted with the subducting slab causing its heating, softening, and finally its detachment. Ensuing upwelling of plume material through the resulting plate window is supposed to be the responsible for partial melting in the lithospheric mantle wedge and/or decompression melting of the ascending plume material. On the basis of geological, geophysical and geochemical data we conclude that both magmatic suites originated from a common and primary deep mantle plume the root of which was located beneath the Cape Verde-Madeira-Canary Islands region, while the head was dragged and frayed by the northeastward motion of the Eurasian plate
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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