1,721,222 research outputs found
Effect of melt/mantle interactions on MORB chemistry at the easternmost Southwest Indian Ridge (61 to 67°E)
Full text linkThe easternmost part of the Southwest Indian Ridge (61°-67°E) is an end-member of the global ridge system in terms of very low magma supply. As such, it is a good laboratory to investigate the effect of melt/mantle interactions on the composition of erupted basalts: for a given volume of erupted basaltic melt, the volume of reacted mantle is potentially greater than at more magmatically robust ridges. We analyzed major, trace element and isotopic compositions in three groups of rocks: plagioclase-bearing ultramafic and gabbroic rocks dredged in nearly amagmatic spreading corridors; basalts from the sparse volcanic cover of these corridors (“ultramafic seafloor basalts”); and basalts dredged from the intervening, more volcanically active domains (“volcanic seafloor basalts”). Ultramafic seafloor basalts have significantly lower CaO and Al2O3 contents at a given MgO than most volcanic seafloor basalts. We propose that both types of basalts are derived from similar parental melts, but that the ultramafic seafloor basalts are more affected by reactions between these parent melts and the mantle rocks in the lithosphere below the ridge. We infer that these reactions occur in the walls of conduits that allow the aggregated melts extracted from the melting mantle to rise through the axial lithosphere and to the eruption sites. The principal effect of these reactions is to enrich the asthenospheric melts in MgO through olivine dissolution. This effect is not expected to be as noticeable, but could still play a role in basalt petrogenesis at more magmatic regions of the global slow-spreading MOR system
A 3-D seismic tomographic study of spreading structures and smooth seafloor generated by detachment faulting – the ultra-slow spreading Southwest Indian Ridge at 64˚30’E
No full textAt ultra-slow spreading ridges, with full spreading rates less than ∼20 mm/yr, spreading is accommodated both by highly spatially and temporally segmented magmatism, and tectonic extension along large-scale detachment faults that exhume ultramafic material to the seafloor. In the most magma-poor regions, detachment faulting alternates in polarity over time, producing a “flip-flopping” effect of subsequent detachment dips. The resulting seafloor in these regions displays a morphology termed “smooth seafloor” comprising elongate, broad ridges with peridotite/serpentinite lithologies. We conducted tomographic travel-time inversion of a 3-D wide-angle seismic data set acquired over a region of smooth seafloor around 64°30′E along the Southwest Indian Ridge (SISMOSMOOTH; Cruise MD199), to produce a seismic velocity volume through the crustal section and into the uppermost mantle. We observe patterns of velocity anomalies that correspond with variations in the bathymetry arising from the mode of spreading and are interpreted as changes in the degree of alteration with depth resulting from spatial and temporal variations in fluid-rock interaction, controlled by faulting and tectonic damage processes. The detachment faults do not show simple planar structures at depth but instead mirror the shapes of the bathymetric ridges that they exhume. Magmatic input is overall highly limited, but there is one region on the lower part of an exhumed detachment footwall where a thickness of volcanic material is observed that suggests a component of syn-tectonic volcanism, which could contribute to detachment abandonment
Seismicity and accretion processes along the Mid-Atlantic Ridge south of the Azores using data from the MARCHE autonomous hydrophone array
No full textThe seismicity of the South Atlantic Ocean has been recorded by the MARCHE network of 4 autonomous underwater hydrophones (AUH) moored within the SOFAR channel on the flanks of the Mid-Atlantic Ridge (MAR). The instruments were deployed south of the Azores Plateau between 32° and 39°N from July 2005 to August 2008. The low attenuation properties of the SOFAR channel for earthquake T-wave propagation result in a detection threshold reduction from a magnitude completeness level (Mc) of ~4.3 for MAR events recorded by the land-based seismic networks to Mc=2.1 using this hydrophone array. A spatio-temporal analysis has been performed among the 5600 events recorded inside the MARCHE array. Most events are distributed along the ridge between lat. 39°N on the Azores Platform and the Rainbow (36°N) segment. In the hydrophone catalogue, acoustic magnitude (Source Level, SL) is used as a measure of earthquake size. The source level above which the data set is complete is SLc=205 dB. We look for seismic swarms using the cluster software of the SEISAN package. The criterion used are a minimum SL of 210 to detect a possible mainshock, and a radius of 30 km and a time window of 40 days after this mainshock (Cevatoglu, 2010, Goslin et al., 2012). 7 swarms with more than 15 events are identified using this approach between 32°et 39°N of latitude. The maximum number of earthquake in a swarm is 57 events. This result differs from the study of Simao et al. (2010) as we processed a further year of data and selected sequences with fewer events. Looking at the distribution of the SL as a function of time after the mainshock, we discuss the possible mechanism of these earthquakes : tectonic events with a "mainshock-aftershock" distribution fitting a modified Omori law or volcanic events showing more constant SL values. We also present the geophysical setting of these 7 swarms, using gravity, bathymetry, and available local geological data. This study illustrates the potential of hydrophone data to monitor segment-scale ridges processes in the vicinity of the Lucky Strike seafloor observatory (lat. 37°20'N), the Azores node of the EMSO (European Multidiciplinary Subsea Observatory) system
Strain localization in oceanic detachment faults : the extreme case of a magma-starved slow spreading ridge
No full textLa partie orientale de la dorsale sud-ouest indienne est particulièrement pauvre en apports magmatiques et constitue de ce fait un laboratoire naturel pour étudier l'accrétion océanique dans une configuration extrême, pour laquelle la divergence des plaques est presque complètement accommodée par le jeu de failles normales à grand rejet. Ces failles, également appelées failles de détachement, exhument des péridotites serpentinisées d'origine mantellaire sur le plancher océanique. Les mécanismes de déformation des roches mantelliques au cours de leur exhumation, avant leur passage dans le domaine de circulation de fluides hydrothermaux, sont peu connus. Nous avons étudié les textures de déformation et les mécanismes de localisation, ainsi que les assemblages minéralogiques associés, dans un corpus de presque 400 échantillons de péridotites serpentinisées draguées pendant la campagne SMoothSeafloor (2010). Tous les échantillons présentent à des degrés variables une déformation hétérogène combinant des mécanismes de déformation cassants (fractures, kinks) et plastiques (extinction ondulante, sous-joints, recristallisation dynamique). L’une des manifestations de cette déformation est la recristallisation syntectonique d’olivines et de pyroxènes au sein de kinks et de microfractures. Ces textures résultent de conditions de hautes contraintes et hautes températures en base de lithosphère, dans la zone d’enracinement des détachements. Ces observations microstructurales ont été intégrées dans un modèle thermomécanique 2D du domaine axial des dorsales lentes, qui explore deux mécanismes d'affaiblissement observés dans les échantillons: la serpentinisation (800°C). La combinaison de ces deux mécanismes permet la reproduction de failles de détachement dans une lithosphère épaisse (20-25 km), avec un relief et une fréquence qui sont consistantes avec celles observées sur notre zone d'étude.The Southwest Indian Ridge in its eastern part has particularly low magmatic inputs and is therefore a natural laboratory to study oceanic accretion in an extreme configuration for which the divergence of the plates is almost completely accommodated by large offset normal faults, also called detachment faults. These faults exhume mantle-derived peridotites. Very little is known about the deformation mechanisms that operate at the lower levels of the lithosphere, prior to serpentinization.We studied the deformation textures, analyze strain localization mechanisms, as well as mineralogical assemblages associated with these mechanisms from a corpus of almost 400 samples of serpentinized peridotites dredged during the SMoothSeafloor cruise (2010). All samples display heterogeneous deformation combining brittle (fracturation, kinks) and plastic mechanisms (undulose extinction, subgrain boundaries, dynamic recrystallization). For example, microfracturation and kinks are locally accompanied by partial recrystallization of the primary minerals into fine-grained aggregates along orthopyroxene grains. These textures result from high stress and high temperature conditions at the base of the lithosphere, where the root of the active detachment.We use these microstructural observations to constrain a 2D thermomechanical model of lithospheric extension, in which we explore two weakening mechanisms seen in the samples: serpentinization (temperatures 800°C). Combining the two, we develop detachment faults in a thick lithospheric context (20-25 km), with fault topography and offsets that are consistent with geological observations in the eastern SWIR
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
Étude sismologique du volcan Lucky Strike et du champ hydrothermal, dorsale médio-atlantique
No full textLes circulations hydrothermales au niveau des dorsales médio-océaniques sont responsables des échanges thermiques et chimiques entre la terre solide et l'océan. L'étude de la circulation hydrothermale sur les dorsales lentes est particulièrement difficile car l'emplacement et la taille de la source de chaleur magmatique et la géométrie des voies de circulation des fluides hydrothermaux sont dans la plupart des cas mal définis. Dans cette thèse, j'étudie les caractéristiques spatio-temporelles de la microsismicité sous le volcan Lucky Strike et le champ hydrothermal, Mid-Atlantic Ridge pour comprendre les circulations hydrothermales et les interactions magmato-tectoniques dans un center de dorsale lente magmatiquement riche. Dans la première partie, j'ai analysé 12 années de données de sismomètres fond de mer (OBS) à courte période enregistrées entre 2007 et 2019. Cette étude révèle une sismicité régulière et continue de faible magnitude (ML ~ -1 à 0) juste au nord du champ hydrothermal et ~0.5-2 km au-dessus du réflecteur axial de la chambre magmatique (AMC). Cette sismicité indique que les processus se produisant à la base de la circulation hydrothermale, tels que la contraction thermique des roches par un refroidissement rapide, éventuellement combinée à une expansion volumique due à la formation de minéraux hydratés et à une fissuration d'extension liée à l'écartement des plaques, sont les principales sources de sismicité dans cette région. Nous proposons donc que les changements spatiaux documentés des groupes de sismicité suivent la progression des zones d'extraction de plus forte chaleur dans la partie inférieure de la zone d'écoulement descendant hydrothermique. Le principal changement s'est malheureusement produit pendant un intervalle de données entre juin 2013 et avril 2015: le principal groupe de sismicité s'est déplacé de ~800 m vers l'est et la distribution de la sismicité est passée d'un patch proche de l'AMC à un modèle vertical en forme de tuyau. Nous proposons que ce changement est dû à des injections magmatiques récentes au-dessus de l'AMC. Nous observons également trois périodes de sismicité plus élevée : Avril-Juin 2009, Août-Septembre 2015, et Avril-Mai 2016. La période avril-juin 2009 a été la plus intense, commençant par un événement de magnitude relativement élevée (ML = 1.7), et culminant en juin après un autre événement de plus forte magnitude (ML = 1.8). Nous interprétons cette période de sismicité plus élevée comme résultant d'événements tectoniques ouvrant des canaux locaux de plus grande perméabilité pour les fluides hydrothermaux descendants. Dans la partie 2, je me concentre sur la détection de la microsismicité proche du plancher océanique près d'un site hydrothermal (Tour Eiffel) en utilisant environ un an de données d'hydrophones enregistrées entre septembre 2016 et septembre 2017 à partir d'un petit réseau d'ouverture ~150 m. Nous détectons 740 événements peu profonds dans la gamme de magnitude locale -4 à -0,5. La plupart de ces événements sont dispersés sans aucun regroupement spécifique et ont une limite de profondeur de ~150-250 m sous le fond marin, parmi lesquels seulement ~10% des événements sont situés à l'intérieur du réseau. Les taux de sismicité à Tour Eiffel sont ~80 fois moins élevés qu'à TAG (Pontbriand & Sohn, 2014), ce qui serait cohérent avec les évidences chimiques d'une précipitation d'anhydrite limitée, voire absente, enregistrée dans les fluides hydrothermaux diffus de TE. Dans la troisième partie, je présente et évalue l'utilisation de la polarisation des ondes P des événements locaux pour orienter les composantes horizontales des OBS de courte période. Nous montrons que la technique peut fournir des orientations fiables dans certains cas, mais ne sont pas fiables dans de nombreux cas, probablement liées à la structure locale sous la station.Hydrothermal circulations at Mid-Ocean Ridges are responsible for thermal and chemical exchange between the solid earth and the ocean. Studying hydrothermal circulation at slow spreading ridges is particularly challenging because the location and size of the magmatic heat source and the geometry of hydrothermal fluid pathways are in most cases poorly constrained. In this thesis, I study the spatiotemporal characteristics of microseismicity beneath Lucky Strike volcano and hydrothermal field, Mid-Atlantic Ridge to understand the hydrothermal circulations and magmato-tectonic interactions in a magmatically robust slow spreading segment center. In part-1, I analyzed 12 years of short period ocean bottom seismometer (OBS) data recorded between 2007 and 2019. This study reveals steady and continuous low magnitude seismicity (ML ~ -1 to 0) just north of the hydrothermal field and ~0.5-2 km above the axial magma chamber reflector (AMC), indicating that processes occurring at the base of hydrothermal circulation, such as thermal contraction of rocks by rapid cooling, possibly combined with volumetric expansion due to the formation of hydrous minerals and extensional cracking related to plate spreading, are the major sources of seismicity in this region. These processes are not mutually exclusive, and we thus propose that the documented spatial shifts of the seismicity clusters track the progression of enhanced heat extraction zones in the lower part of hydrothermal downflow zone. The main shift unfortunately occurred during a data gap between June 2013 and April 2015: the main seismicity cluster moved ~800 m eastward and the seismicity distribution changed from a patch close to the AMC to a vertical pipe-like pattern. We propose that this shift was driven by recent magmatic injections above the AMC. We also observe three higher seismicity periods: April-June 2009, August-September 2015, and April-May 2016. The April-June 2009 period was the most intense, starting with a relatively high magnitude event (ML = 1.7), and culminating in June after another higher magnitude (ML = 1.8) event. We interpret this higher seismicity period to have resulted from tectonic events opening enhanced local permeability channels for downgoing hydrothermal fluids. In part-2, I focus on detection of near seafloor microseismicity near one hydrothermal vent site (Tour Eiffel) using ~ 1 year of hydrophone data recorded between September 2016 and September 2017 from a small ~150 m aperture network. We detect 740 shallow events in local magnitude range -4 to -0.5. Most of these events are scattered without any specific clustering and have a depth limit of ~150-250 m beneath the sea floor, out of which only ~10% of the events are located inside the network. As a preliminary interpretation, we consider these events to have originated as a result of reaction driven cracking in response to anhydrite precipitation. Seismicity rates at Tour Eiffel are ~80 times less than at TAG (Pontbriand & Sohn, 2014), which would be consistent with the chemical evidence for limited to absent anhydrite precipitation recorded in the TE diffuse vent fluids (Wheeler et al., submitted). In part-3, I present and evaluate using P-wave polarization of local events to orient the horizontal components of short period OBSs. The technique is commonly used for broadband sites using teleseismic events, but short-period seismometers can only use local events. We show that the technique can provide reliable orientations in some cases, but in many cases the orientations are not reliable, probably related to local structure beneath the station
Anatomie des failles de détachement dans un contexte de dorsale médio-océanique presque amagmatique : observations à l'échelle de la carte et de l'échantillon sur la dorsale sud-ouest indienne, 64°40'E
No full textL'Est de la dorsale sud-ouest indienne (SWIR) est un end-member peu magmatique du système des dorsales médio-océaniques. Dans certaines zones presque amagmatiques de plus de 50 km de large, la divergence des plaques se fait le long de failles de détachement (Ocean Detachment Faults ; ODF), qui exhument des roches dérivées du manteau. Ces ODF ont une durée de vie de 1 à 2 millions d'années, puis une nouvelle ODF prend le relais avec une polarité opposée. On parle de « flip-flop detachment faulting ». Cette thèse utilise la bathymétrie, les observations de plongée ROV et des échantillons de roches pour étudier la géologie, la tectonique et les processus de déformation de ces ODF presque amagmatiques. Le mur de la vallée axiale dans la zone d'étude (64°35'E SWIR) correspond au plancher d'un ODF actif jeune, D1. Le sommet de ce mur correspond au domaine dit de « breakaway » de D1. Il comprend des blocs glissés qui exposent une séquence de déformation d' ~100m d'épaisseur, avec des niveaux de gouges et de microbrèches de serpentinite. D'après sa géométrie, on attribue cette séquence à la zone de faille de l'ODF D2. Ceci renforce l'interprétation des ODFs D1 et D2 comme flip-flop. Notre étude montre une variation significative des déformations liées à D1 le long de l'axe. A l'est de la zone d'étude, la zone de faille exhumée est lisse, à l'ouest elle présente des corrugations similaires à celles documentées dans des ODF plus magmatiques. La région corruguée présente également plusieurs crêtes de quelques centaines de mètres de large, orientées NNE, que nous interprétons comme des méga-corrugations formées en raison de l'existence de phacoïdes d'échelle comparable dans la zone d'endommagement du détachement. Le domaine corrugué a aussi un relief plus marqué et des petites failles normales antithétiques, absentes dans la région non corruguée. Les horizons de microbrèches/gouges sont plus épais et plus continus dans la région non corrugué et nos observations de terrain suggèrent que des fluides hydratés favorisent la formation de ces gouges dans la zone de faille. Tout cela suggère une faille et un plancher D1 plus résistants à la déformation dans la région corruguée. Les corrugations font un angle de 15-25° sur la direction d'expansion NS. La résistance variable de la faille et de son plancher le long de l'axe pourrait causer une rotation locale du champ de déformation. Les microbrèches et gouges de D1 et D2 résultent de la fracturation cassante de péridotites déjà serpentinisées. Ces roches contiennent des microdomaines riches en clastes et des microdomaines foliés pauvres en clastes avec une matrice de chrysotile à grain fin dans laquelle nous avons observé des indices de dissolution et de précipitation syntectoniques. Les données expérimentales indiquent que la résistance à la friction des gouges de chrysotile à des températures 50 km wide, nearly amagmatic corridors, where seafloor spreading is accommodated mainly by large offset detachment faults (ODFs), leading to the exposure of mantle-derived rocks on the seafloor. These ODFs have a lifetime of 1-2 myr, then a new ODF takes over with an opposite polarity. This process is known as flip-flop detachment faulting. This thesis uses bathymetry, ROV dive observations, and rock samples to study amagmatic ODF's geology, tectonics, and deformation processes. The axial valley wall in the study area (64°35'E SWIR) corresponds to the footwall of a young active ODF, D1. It exposes variably deformed serpentinized peridotites with rare gabbro dikes. This south-facing wall's top comprises km-long scarps representing the degraded D1 breakaway. The outcrops along these scarps feature highly deformed serpentinite microbreccia and gouge-bearing horizons interlayered with less deformed serpentinized peridotites. These deformed lithologic packages are up to ~100m thick, related to D2 ODF zone, which strengthens the interpretation of nearly amagmatic flip-flop ODFs, consistent with prior research. Our study found significant along-strike variations in D1 footwall features. Limited data was available due to incomplete coverage of ROV high-resolution bathymetry and observation dives. The eastern area of the D1-footwall is smooth, while the western zone has higher footwall (D1) elevation and corrugations similar to those in magmatic, domal, and corrugated ODFs. The corrugated region in our study area has several hecto-to-kilometers-wide NNE-trending ridges, interpreted as mega-corrugations due to the inferred presence of hecto-to-kilometer-scale phacoids between linked fault splays in the detachment damage zone. The corrugated domain has minor-offset antithetic faults, absent in the non-corrugated region where the D1 footwall elevation is not as high. The microbreccia/gouge layers are also thicker and more extensive in the non-corrugated region, and field observations suggest that hydrous fluids in the fault zone could enhance the formation of such gouges. These characteristics suggest a more robust D1 fault and footwall in the corrugated region. Corrugations show a 15-25° clockwise orientation to the NS spreading direction. We propose this may be due to variable fault and footwall strength along the axis, possibly linked to hydrothermal fluid availability. The deformation microtextures of serpentinite microbreccia and gouges from the D1 and D2 fault zones show clast-rich microdomains formed due to the brittle fracturing of already serpentinized peridotites, while clast-poor microdomains have a foliated structure with a fine-grained chrysotile matrix. Chrysotile gouge strength is similar to commonly found mineral talc in magmatic detachment fault zones at low temperatures, but chrysotile's strength increases at higher temperatures and pressures. However, fluid-enhanced dissolution-precipitation processes weaken chrysotile gouge in deeper areas. Thus, alongside the rheology of these gouge horizons, the thickness, extent, and connectivity of gouge and microbreccia horizons also play a crucial role in determining fault strength. This study shows differences between amagmatic ODFs in the eastern SWIR and more magmatic corrugated dome-shaped ODFs. The fault zone of amagmatic ODFs has chrysotile-bearing gouges and microbreccia, while the magmatic ODFs have talc, amphibole, and chlorite-bearing serpentinites. The amagmatic ODFs have a thicker damage domain with phacoids of less deformed serpentinized peridotite. We propose that this thick damage zone is a legacy of less localized deformation in the transition between brittle and ductile domains
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
- …
