1,720,968 research outputs found
Depth of ancient seismicity along the Woodroffe Thrust (Central Australia): Constraints from pseudotachylytes in peraluminous gneisses
Full text linkThe Woodroffe Thrust (WT) is a regional‐scale mylonitic shear zone that developed during the Petermann Orogeny (630–520 Ma) in lower to mid‐crustal rocks of the Musgrave Ranges, central Australia. In the upper part the WT hosts the largest volume worldwide of tectonic pseudotachylytes (coseismic quenched frictional melts). The pseudotachylytes were only marginally reworked along the mylonitic belt marking the WT, which mainly derived from the footwall amphibolite‐facies rocks. Mid‐crustal conditions of deformation along a shallowly dipping (<6°) WT were previously inferred from estimates of the P‐T conditions of mylonitization along a regional transect in the N‐S direction of thrusting. However, the pressure estimates were subject to large uncertainties. To better constrain the ambient conditions of the ancient seismic faulting along the WT, we investigate pseudotachylytes within peraluminous gneisses, a rock type more sensitive to P‐T changes in the range of interest. Microstructural analysis allows the sequence of minerals (corundum, sillimanite, cordierite, andalusite, kyanite and garnet) developed during melt quenching and subsequent solid‐state growth to be established. Critical observations are the growth of andalusite during pseudotachylyte cooling, constraining faulting at <0.45 GPa, and of kyanite during the immediately following ductile reactivation of pseudotachylytes. Seismic faulting is inferred to have occurred at ambient conditions of ∼0.4 GPa and 450°C, that is at much shallower conditions than previously assumed. These new P‐T estimates imply an inclination of 20–25° of the WT, if the main stage of seismic faulting and mylonitization along the WT were coeval
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
The earthquake cycle in the dry lower continental crust: Insights from two deeply exhumed terranes (Musgrave Ranges, Australia and Lofoten, Norway)
Full text linkThis paper discusses the results of field-based geological investigations of exhumed rocks exposed in the Musgrave Ranges (Central Australia) and in Nusfjord (Lofoten, Norway) that preserve evidence for lower continental crustal earthquakes with focal depths of approximately 25-40 km. These studies have established that deformation of the dry lower continental crust is characterized by a cyclic interplay between viscous creep (mylonitization) and brittle, seismic slip associated with the formation of pseudotachylytes (a solidified melt produced during seismic slip along a fault in silicate rocks). Seismic slip triggers rheological weakening and a transition to viscous creep, which may be already active during the immediate post-seismic deformation along faults initially characterized by frictional melting and wall-rock damage. The cyclical interplay between seismic slip and viscous creep implies transient oscillations in stress and strain rate, which are preserved in the shear zone microstructure. In both localities, the spatial distribution of pseudotachylytes is consistent with a local (deep) source for the transient high stresses required to generate earthquakes in the lower crust. This deep source is the result of localized stress amplification in dry and strong materials generated at the contacts with ductile shear zones, producing multiple generations of pseudotachylyte over geological time. This implies that both the short- and the long-term rheological evolution of the dry lower crust typical of continental interiors is controlled by earthquake cycle deformation. This article is part of a discussion meeting issue 'Understanding earthquakes using the geological record'
Hydration and petrologic changes of inner portions of the subducting oceanic lithosphere facilitated by intermediate-depth faulting
No full textThe subduction-zone rheology and seismic behaviour of hydrated oceanic slabs have been widely studied, while the unaltered, dry portions of the subducting lithosphere remain less understood despite their role in earthquake generation and slab pull through eclogitization. We present field-based evidence from the ophiolitic Lanzo Massif (Western Alps), a slice of oceanic lithosphere composed of mantle peridotite and small volumes of gabbro which largely escaped hydration and Alpine metamorphism, representing a mechanically rigid block in the subduction complex. At intermediate depths, this dry lithosphere locally developed pseudotachylyte-bearing faults (e.g. the Moncuni locality) and widespread meso-to micro-faults (e.g., the Mt. Arpone locality). At Mt. Arpone, faults offset the gabbroic dykes by several centimetres: they contain sub-micrometric "annealed" ultramafic ultracataclasite of olivine, pyroxene and spinel, locally overgrown by chlorite and orthopyroxene. These features indicate faulting under dry conditions, which produced a tectonic porosity and enabled localized fluid infiltration. This led to discontinuous hydration of peridotite and to the widespread transformation of cataclastic gabbroic plagioclase into high-pressure zoisite-paragonite symplectites. Thermodynamic modelling suggests that plagioclase breakdown during faulting occurred under blueschist-facies conditions. Recent petrological studies show that this reaction is associated to volume reduction and the formation of reactive porosity, making plagioclase the most intensely eclogitized mineral in the studied samples. Trace element analyses reveal that fluid infiltration drove the internal (closed-system) redistribution of fluid-mobile elements. Eclogitization of the Lanzo peridotite and gabbro only occurred where fluids were present, either from limited oceanic hydration, or from subduction-related fluid infiltration through faults and the porosity created by the metamorphic breakdown of gabbroic plagioclase. The Arpone blueschist-facies ultracataclasites may represent seismic structures that did not evolve into frictional melting; they may be the precursors to the earthquake-generating faults that developed at Moncuni
Fluid infiltration along reactive porosity networks at the subduction interface: Evidence from the eclogite-facies Erro-Tobbio Unit (Ligurian Alps)
No full textIn subduction zones, seismicity and rock rheology are strongly influenced by the presence of fluids. However, the mechanisms governing fluid extraction and transport along the subduction interface are still debated. The metaperidotite of the Erro-Tobbio Unit (Ligurian Alps) records fluid-rock interaction and associated deformation that occurred at intermediate subduction depths. Fluid pathways are represented by reaction bands of metamorphic
olivine (Ol2) and Ti-clinohumite, formed by the breakdown of brucite (Brc) and antigorite (Atg) at 1.8–2.5 GPa and 500–650 ◦C. Field and microstructural analyses allowed the role of deformation in the development of ORBs to be assessed. In-situ determination of trace elements by LA-ICP-MS allowed the scale of fluid circulation along
reaction-induced channels to be constrained.
The reaction bands occur within both meta-peridotite cores and wrapping horizons of prograde antigorite mylonites. The meta-peridotite shows two main sets of reaction bands: Set1, oriented at high angle to the bounding mylonites, and Set2, trending N-S, parallel to the mylonites (where only Set2 is present). A decrease in spacing between reaction bands of Set2 highlights a strain gradient from the undeformed meta-peridotite cores to the mylonites. This gradient, associated with oriented growth of Ol2 in the mylonites, suggests that the reaction bands developed during mylonitization. Brc relics mostly localized within the reaction bands indicate that formation
of Ol2 was originally controlled by the Brc distribution. Such channel networks have been described in previous works as the result of channel-forming reactive porosity related to initial breakdown of thermodynamically favoured domains. The formation of the reaction bands may be therefore related to formation of low-Si fluids in Brc-rich areas, which triggered Atg breakdown and formation of porosity by flowing along planes consistent with the stress field.
Later stage, Al-free serpentine (Atg + chrysotile/lizardite) locally replaces Ol2 along a pervasive network of microcracks exploiting the reactive sites of Set1-Set2 structures. An enrichment in fluid-mobile elements (As, Sb, Ba, W, Li, B) in prograde Ol2 and later-stage Al-free serpentine provides evidence of two stages of infiltration of slab-derived fluids, indicating open-system conditions during eclogite-facies deformation and during retrogression. We speculate that the Erro-Tobbio Unit was subducted at an early stage of Adria-Europe convergence and settled atop the interface within the deep slow-earthquakes depth range, above the continuously under-riding and dehydrating Alpine slab. In this environment, the Erro-Tobbio Unit was affected by intense chemical exchange due to fluid fluxing from the underlying subducting units
How to quake a subducting dry slab at intermediate depths: Inferences from numerical modelling
No full textThe origin of intermediate-depth subduction seismicity is a topic of research since long time. While plate unbending is considered as one of the main stress loading mechanisms, the processes responsible for earthquake nucleation are still unclear and depend on the question of whether failure occurs in the wet dehydrating portion of the slab or in the predominantly dry portion. Recently, the seismogenic portions of subducting oceanic slabs have been proposed to consist of dominantly dry metaperidotite that deforms by seismic brittle failure in absence of fluid-mediated embrittlement. In this work, we quantify by numerical modelling the differential stress achievable during unbending of a subducting slab. We show that the presence of discrete hydrated domains in a dry, strong slab amplifies the differential stress to high seismogenic values (ca. 3-4 GPa in the 100-200 km depth range) at intermediate depths. We also consider the effects of low temperature plasticity in olivine that can hinder the build-up of high differential stress to the first 100 km of depth
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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