1,721,005 research outputs found
Replacement reactions and deformation by dissolution and precipitation processes in amphibolites
Full text linkThe deformation of the middle to lower crust in collisional settings occurs via deformation mechanisms that vary with rock composition, fluid content, pressure and temperature. These mechanisms are responsible for the accommodation of large tectonic transport distances during nappe stacking and exhumation. Here we show that fracturing and fluid flow triggered coupled dissolution‐precipitation and dissolution‐precipitation creep processes, which were responsible for the formation of a mylonitic microstructure in amphibolites. This fabric is developed over a crustal thickness of >500 m in the Lower Seve Nappe (Scandinavian Caledonides). Amphibolites display a mylonitic foliation that wraps around albite porphyroclasts appearing dark in panchromatic cathodoluminescence. The albite porphyroclasts were dissected and fragmented by fractures preferentially developed along the (001) cleavage planes, and display lobate edges with embayments and peninsular features. Two albite/oligoclase generations, bright in cathodoluminescence, resorbed and overgrew the porphyroclasts, sealing the fractures. Electron backscattered diffraction shows that the two albite/oligoclase generations grew both pseudomorphically and topotaxially at the expense of the albite porphyroclasts, and epitaxially around them. These two albite/oligoclase generations also grew as neoblasts elongated parallel to the mylonitic foliation. The amphibole crystals experienced a similar microstructural evolution, as evidenced by corroded ferrohornblende cores surrounded by ferrotschermakite rims that preserve the same crystallographic orientation of the cores. Misorientation maps highlight how misorientations in amphibole are related to displacement along fractures perpendicular to its c‐axis. No crystal plasticity is observed in either mineral species. Plagioclase and amphibole display a crystallographic preferred orientation that is the result of topotaxial growth on parental grains and nucleation of new grains with a similar crystallographic orientation. Amphibole and plagioclase thermobarometry constrains the mylonitic foliation development to the epidote amphibolite facies (˜600°C, 0.75‐0.97 GPa). Our results demonstrate that at middle to lower crustal levels the presence of H2O‐rich fluid at grain boundaries facilitates replacement reactions by coupled dissolution‐precipitation and favours deformation by dissolution‐precipitation creep over dislocation creep in plagioclase and amphibole
Structural and metamorphic inheritance controls strain partitioning during orogenic shortening (Kalak Nappe Complex, Norwegian Caledonides)
No full textThe occurrence of pre-collisional structural and metamorphic fabrics may control the development of new structures during subsequent orogenic deformation. Structural, petrological and geochronological analyses have been performed on selected samples collected along a NW-SE cross section of the Kalak Nappe Complex (KNC) exposed on Kvaløya Island (Finnmark, Norway), in order to define pre-Caledonian or Caledonian affinity of deformation fabrics. Nappes within the KNC experienced different pre-collisional tectonometamorphic histories, resulting in contrasting pre-Caledonian fabrics, which in turn controlled orogen-scale strain partitioning and metamorphic re-equilibration during Caledonian shortening. Caledonian deformation during top-to-SE-directed thrusting occurred at 550–675 °C and 0.8–1.0 GPa in the presence of fluids. Suitably-oriented pre-collisional fabrics were firstly exploited as zones of localized shearing internal to the KNC and subjected to metamorphic re-equilibration during shortening. Fold geometry during Caledonian thrusting was also controlled by the orientation of pre-Caledonian fabrics. SE-verging asymmetric folds were developed after minor tilting of pre-Caledonian upright folds with orogen-parallel hinge in the hinterland consistently with top-to-SE shearing. Shear-parallel folds displaying orogen-perpendicular hinge lines resulted from top-to-SE general shearing of pre-collisional upright folds showing pre-collisional orogen-perpendicular hinge lines. Caledonian metamorphism appears to have been accompanied by infiltration of radiogenic 40Ar-rich fluids, which affected the Ar isotopic system in synkinematic micas
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
Misoriented faults in exhumed metamorphic complexes: Rule or exception?
No full textLow angle normal faults and other weak faults are common in the metamorphic core of collisional orogens.
They frequently show a phyllosilicate-rich mylonitic foliation that was reactivated under brittle conditions.
Experimental and theoretical works indicate that mechanical anisotropies exert a substantial influence on
shear failure and frictional sliding, eventually inhibiting the nucleation and propagation of new Andersonian
shear fractures and favoring the localization of brittle failure along the pre-existing foliations. Metamorphic
phyllosilicate-rich rocks may show a friction coefficient varying from 0.6, at high angles to the foliation, to
0.2–0.4, for shear along the inherited foliation. To test the influence of mechanical anisotropies on the
development of non-Andersonian faults, we have applied a modified slip tendency analysis to three
misoriented phyllosilicate-rich faults of the European Alps. The analysis accounts for anisotropy in friction
coefficients, and has been named “Anisotropic Slip Tendency analysis”. Here we show that brittle deformation
along misoriented phyllosilicate-rich foliations is more probable than the development of new Andersonian
faults. The presence of a well developed network of weak, phyllosilicate-rich faults may influence the overall
structural style and mechanical properties of the brittle lithosphere in collisional orogens
Earthquake nucleation in the lower crust by local stress amplification
Full text linkDeep intracontinental earthquakes are poorly understood, despite their potential to cause significant destruction. Although lower crustal strength is currently a topic of debate, dry lower continental crust may be strong under high-grade conditions. Such strength could enable earthquake slip at high differential stress within a predominantly viscous regime, but requires further documentation in nature. Here, we analyse geological observations of seismic structures in exhumed lower crustal rocks. A granulite facies shear zone network dissects an anorthosite intrusion in Lofoten, northern Norway, and separates relatively undeformed, microcracked blocks of anorthosite. In these blocks, pristine pseudotachylytes decorate fault sets that link adjacent or intersecting shear zones. These fossil seismogenic faults are rarely >15 m in length, yet record single-event displacements of tens of centimetres, a slip/length ratio that implies >1 GPa stress drops. These pseudotachylytes represent direct identification of earthquake nucleation as a transient consequence of ongoing, localised aseismic creep
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
High-stress creep preceding coseismic rupturing in amphibolite-facies ultramylonites
Full text linkCoeval pseudotachylytes (solidified melts produced during seismic slip) and mylonites are generally regarded as the geological record of transient seismic events during dominant ductile flow. Thermal runaway has been proposed as a model to explain the pseudotachylyte-mylonite association. In the Mont Mary unit (Western Alps), pseudotachylyte fault veins occur along the amphibolite-facies (ca. 550 °C; 0.35 GPa) ultramylonitic foliation of paragneisses. These veins formed at the same metamorphic conditions of the ultramylonites, thus potentially recording thermal runaway. We analysed the microstructure of quartz in ultramylonite and of ultramylonite clasts in pseudotachylyte to investigate the possible occurrence of thermal runaway. Quartz aggregates show an evolution under constant temperature to ultrafine-grained recrystallised grain size (2.5 μm), reflecting creep under high differential stresses (> 200 MPa) and high strain rates (10−9 s−1), along very narrow foliation-parallel layers. In the ultrafine aggregates, viscous grain boundary sliding became dominant and promoted cavitation leading to disintegration of quartz aggregates and precipitation, in the pore space, of biotite, oriented parallel to the main ultramylonitic foliation. The strain rate-limiting process was aseismic fluid-assisted precipitation of biotite. The potential occurrence, at the deformation conditions of the Mont Mary ultramylonites, of thermal runaway in pure quartz layers was investigated by numerical modelling. The models predict a switch from stable flow to thermal runaway at background strain rates faster than 10−9 s−1 for critical differential stresses that are comparable to the brittle strength of rocks. Deformation of ultramylonites occurred close to the conditions for thermal runaway to occur, but based on the microstructural record we conclude that the Mont Mary pseudotachylyte-mylonite association is best explained by brittle failure, triggered by transients of high differential stress and strain rate causing a downward deflection of the brittle-ductile transition
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