1,720,981 research outputs found
Normal-sense shear zones in the core of the Higher Himalayan Crystallines (Bhutan Himalaya): evidence for extrusion?
No full textRecent fieldwork in western Bhutan, dedicated to unravelling the tectonic structure of the mid-crustal rocks, indicates a complex deformation pattern in the Greater Himalayan Slab (GHS). A system of normal shear zones, striking
NE-SW and steeply to moderately dipping to the SE, has been recognized
within this extruding slab or wedge of crystalline rocks. The zones are
characterized by well developed shear-sense indicators pointing to a
top-down-to-SE sense of shear. The main Barrovian metamorphic minerals
are bent and stretched by extensional shear bands and associated
deformation mechanisms indicate a range of brittle-ductile deformation
conditions. Normal shear zones are concentrated in the middle-upper part
of the GHS and indicate a thrust-transport-parallel lengthening of the
core itself. Vorticity analysis highlights a non-coaxial flow with pure
and simple shear acting together during deformation (mean vorticity
number bracketed between 0.63 and 0.76). These data, when compared to
available data near the tectonic boundaries of the GHS, indicate an
increasing component of pure shear towards the interior of the GHS. The
ages of zircon overgrowths and monazites from a slightly deformed
granite, 20.5 ± 0.5 Ma, and a mylonitic granite deformed into the shear
zones, 17.0 ± 0.2 Ma, bracket the age of shear zone formation at close
to 17 Ma. According to our data, the normal shear zones could well
accommodate the pure shear component of deformation localized in the
inner part of the extruding wedge/slab and is compatible with a channel
flow model
Leucogranite intruding the South Tibetan Detachment in western Nepal: implications for exhumation models in the Himalayas
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
Pressure-Temperature-Deformation-time(P-T-d-t) exhumation history of the Voltri Massif HP-complex, Ligurian Alps, Italy
No full textAn integrated structural, petrological, and geochronological/thermochronological study was undertaken to constrain the tectonic controls on the exhumation of the Voltri Massif high-pressure (HP) complex, located in the Ligurian portion of the Western Alps (Italy). Petrotextural analyses were performed to identify the pressure-temperature interval of (1) peak metamorphism (the D1-M1 stage) to eclogitic/blueschist facies conditions and (2) the main retrogressive event (the D2-M2 stage) to greenschist facies conditions. U-Pb SHRIMP dating on a zircon rim (33.8 ± 0.8 Ma) and titanite grains (29 ± 5 Ma), coupled with 40Ar-39Ar analyses on phengite (∼64 Ma to ∼34 Ma) placed temporal constraints on the exhumation path from the D1-M1 to the D2-M2 stages at the Eocene-Oligocene boundary. Apatite fission track thermochronology, with older ages of 23.9 ± 4.9 Ma, confirms the existence of a regional, Late Oligocene to Miocene cooling/exhumation event for the Voltri Massif (the D3-M3 stage). The compilation of this pressure-temperature-deformation-time path supports a change in the exhumation history: (1) an initial stage, from the D1-M1 to the D2-M2, which was nearly isothermal with highly variable exhumation rates, and (2) a second stage, from D2-M2 to upper crustal levels, which was accomplished by cooling and moderate exhumation rates of ∼1–2 mm yr−1. This two-stage path can be reconciled with the dominant tectonic mechanisms responsible for exhumation of HP rocks in the Voltri Massif area. At the regional scale, this path is consistent with major geodynamic reorganization in the Mediterranean region at the Eocene-Oligocene time boundary, which involved a switch from synorogenic events during transpressive kinematics at the Alpine-Apennine plate boundaries, to postorogenic processes related to crustal thinning and opening of back-arc basins
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
Dynamics in the Sesia HP terrane: Combined petrochronological and structural analysis
Full text linkHP terranes dominated by continental crust represent the end result of a sequence of processes that operate at lithosphere scale, i.e. rifting, subduction/accretion, return flow/exhumation. To under¬stand the dynamics of the subduction channel in complex terranes of this kind, the effects from each stage must be investigated separately, linking the observations and data from kilometers down to micrometer scale. This task recommends an integrative approach.
Here we focus on the assembly of the Sesia Zone (SZ), a key element of the internal Western Alps. This terrane comprises two main polymetamorphic base¬ment units and thin trails of a cover sequence that includes post-Permian syn- to post-rift metasediments; the latter show no pre-Alpine metamorphic imprint. The tectonic scenario of Babist et al. (2006) recognizes five main phases in the Alpine structural evolution; their model helped us select areas for detailed structural work and sampling. Our first goal was to relate the early convergent structures (D1, D2) to the P-T evolution and to establish a robust time-frame for the HP-dynamics within and between the tectonic slices. Within the subduction/extrusion channel, problems addressed include the question of tectonic mixing, i.e. temporal and spatial scales of relative and absolute movement of the slices, and the conditions and timing of their final juxtaposition prior to the rapid exhumation of the Sesia Zone as a whole.
Mono- and polymetamorphic sediments from different slices display unequivocal evidence of several HP-stages separated in time. Successive stages under eclogite facies conditions occurred between 86 – 65 Ma, as shown by LA-ICP-MS and SHRIMP data on growth zones in accessory allanite, monazite, zircon, and titanite. By using mutual inclusions and overgrowth relationships, the age-data on allanite and monazite can be tied to the multistage evolution of an individual sample. For different rocks, these (over)growth stages can be related to D1- and D2-deformation when micro-, meso- and megastructural observations are combined. Thermobarometry indicates intermittant decompression by ~0.8 GPa between HP phases, hence pressure cycling (aka yo-yo tectonics, Rubatto et al., 2011). This tectonic mobility occurred prior to the final juxtaposition of slices and their exhumation, which involved at least two major deformation phases and lead to widespread retrogression at amphibolite to green-schist facies conditions.
Our approach combining structural, petrological, and geochronological techniques yields some field-based constraints on the duration and rates of the dynamics within a subduction channel. It may be useful to compare these to insights from numerical models, provided the latter take into account the specific conditions of the plate convergence, which turns out to have been highly oblique in the present case
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
Late Oligocene high-temperature shear zones in the core of the Higher Himalayan Crystallines (Lower Dolpo, western Nepal)
Full text linkA high‐temperature shear zone, Toijem shear zone, with a top‐to‐the‐SW sense of shear affects the core of the Higher Himalayan Crystallines (HHC) in western Nepal. The shear zone developed during the decompression, in the sillimanite stability field, of rocks
that previously underwent relatively high‐pressure metamorphism deformed under the kyanite stability field. PT conditions indicate that the footwall experienced higher pressure (∼9 kbar) than the hanging wall (∼7 kbar) and similar temperatures (675°–700°C). Monazite growth constrains the initial activity of the shear zone at 25.8 ± 0.3 Ma, before the onset of the Main Central Thrust zone, whereas the late intrusion
of a crosscutting granitic dike at 17 ± 0.2 Ma limits its final activity. Monazites in kyanite‐bearing gneisses from the footwall record prograde metamorphism in the HHC from ∼43 to 33Ma. The new data confirm that exhumation of the HHC started earlier in western Nepal than in other portions of the belt and before the activity of both the South Tibetan Detachment System (STDS)
and Main Central Thrust (MCT) zones. As a consequence,
western Nepal represents a key area where the channel‐flow‐driven mechanism of exhumation, supposed to be active from Bhutan to central‐eastern Nepal, does terminate. In this area, exhumation of crystalline units occurred by foreland propagation of ductile and, subsequently, brittle deformation
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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