1,720,992 research outputs found
Isotopic and geochemical constraints to mass transfers induced by fluid-migrations : from sample to basin scale
No full textClauer Norbert, Liewig Nicole, Zwingmann Horst. Isotopic and geochemical constraints to mass transfers induced by fluid-migrations : from sample to basin scale. In: Transferts dans les systèmes sédimentaires : de l'échelle du pore à celle du bassin. Réunion spécialisée SGF-TRABAS/CNRS, Paris 27-28 septembre 1999. Résumés. Strasbourg : Institut de Géologie – Université Louis-Pasteur, 1999. p. 51. (Sciences Géologiques. Mémoire, 99
Precambrian fault reactivation revealed by structural and K-Ar geochronological data from the spent nuclear fuel repository in Olkiluoto, southwestern Finland
Full text linkIntegrated structural/geochronological studies help unraveling complex brittle deformation histories. We have
analysed the structural geological database of brittle faults from the ONKALOTM underground facility for spent
nuclear fuel in Olkiluoto in southwestern Finland. Based on the structural geological data from eleven repre-
sentative fault zones, we classify the Olkiluoto brittle structural features into four fault systems, referred to as
Fault system I to IV. The classification is based on their structural properties and tectonic history, crosscutting
relationships, fault rock mineralogical characterization and 3D modelling. Some constraints on the timing of
faulting are provided by K-Ar dates on synkinematic illite from fault gouge samples. Our results show that the
bedrock in southwestern Finland experienced numerous brittle deformation phases between ca. 1.75 and 0.9 Ga.
N-S strike-slip faults (Fault systems I and II) formed at mid-crustal levels ca. 1.79–1.75 Ga ago in response to NW-
SE/NNW-SSE compression soon after the Svecofennian orogeny. Later E-W striking oblique dextral/normal faults
(Fault system III) are tentatively associated with the Gothian orogeny 1.6 Ga ago. These three fault systems were
reactivated during NE-SW compression ca. 1.3–1.2 Ga ago, coeval with intrusion of a regional swarm of olivine
diabase sills. E-W compression at the onset of the Sveconorwegian orogeny ca. 1.1–1.0 Ga ago resulted in the
formation of SE dipping low-angle thrust faults (Fault system IV) and the selective reactivation of fault system II
and III. Overall E-W extension during the collapse of the Sveconorwegian orogen ca. 0.97–0.87 Ga ago caused the
localised reactivation of fault systems III and IV. Our research approach, which is integral to the siting process of
repositories for spent nuclear fuel, demonstrates that the basement in southwestern Finland experienced repeated
reactivation since the Mesoproterozoic, suggesting that future deformation localization is likely to be also
accommodated by reactivation of existing brittle structures rather than formation of new faults
K-Ar illite age constraints on the Proterozoic formation and reactivation history of a brittle fault in Fennoscandia
No full textK-Ar ages of authigenic illite from two drill-core gouge samples of a fault in the Palaeoproterozoic basement of Finland record two distinct faulting events. The older sample yields apparent ages from 1240 ± 26 to 1006 ± 21 Ma for four grain size fractions between 6 and <0.1 μm. The second sample is structurally younger and yields statistically distinct ages ranging from 978 ± 20 to 886 ± 18 Ma. We interpret the ages of the <0.1 m fractions, which are the youngest, as representing the actual time of faulting. XRD analysis and age modelling exclude significant age contamination of the finest dated fractions with inherited host rock components. These results provide therefore an example of meaningful isotopic dating of illite-type clay material formed during Precambrian faulting, demonstrate and constrain fault reactivation and give evidence for brittle Sveconorwegian Mesoproterozoic shortening and Neoproterozoic extension in Fennoscandia. © 2013 Blackwell Publishing Ltd
Inclined K-Ar illite age spectra in brittle fault gouges: Effects of fault reactivation and wall-rock contamination
No full textK-Ar clay fraction ages of brittle faults often vary with grain size, decreasing in the finer size fractions, producing an inclined age-grain-size spectrum. K-Ar ages and mineralogical characterization of gouges from two normal faults in the Kongsberg silver mines, southern Norway, suggest that inclined spectra derived from brittle fault rocks reflect the mixing of inherited components with authigenic mineral phases. The ages of the coarsest and finest fractions constrain faulting at c. 260-270 Ma and reactivation around 200-210 Ma, respectively. This study demonstrates how wall-rock contamination influences the K-Ar age of the coarsest size fractions and that authigenic illite and K-feldspar can crystallize synkinematically under equivalent conditions and thus yield the same K-Ar ages
Direct dating of folding events by 40Ar/39Ar analysis of synkinematic muscovite from flexural-slip planes
No full textTiming of folding is usually dated indirectly, with limited isotopic dating studies reported in the literature. The present study investigated the timing of intracontinental, multi-stage folding in Upper Proterozoic sandstone, limestone, and marble near Beijing, North China, and adjacent regions. Detailed field investigations with microstructural, backscattered electron (BSE) images and electron microprobe analyses indicate that authigenic muscovite and sericite crystallized parallel to stretching lineations/striations or along thin flexural-slip surfaces, both developed during the complex deformation history of the study area, involving repeated compressional, extensional and strike-slip episodes. Muscovite/sericite separates from interlayer-slip surfaces along the limbs and from dilatant sites in the hinges of folded sandstones yield muscovite 40Ar/39Ar plateau ages of ~158-159 Ma, whereas those from folded marble and limestone samples yield ages of 156 ± 1 Ma. Muscovite from thin flexural-slip planes on fold limbs and hinges yields ages within analytical error of ~155-165 Ma. Further muscovite samples collected from extensionally folded limestone and strike-slip drag folds yield younger ages of 128-125 Ma with well-defined plateaus. To assess the potential influence of the detrital mica component of the host rock on the age data, two additional muscovite samples were investigated, one from a folded upper Proterozoic-Cambrian sandstone outside the Western Hills of Beijing and one from a folded sandstone sampled 20 cm from folding-related slip planes. Muscovite separates from these samples yield significantly older ages of 575 ± 2 Ma and 587 ± 2 Ma, suggesting that the timing of folding can be directly determined using the 40Ar/39Ar method. This approach enables the identification and dating of distinct deformation events that occur during multi-stage regional folding. 40Ar/39Ar dating can be used to constrain the timing of muscovite and sericite growth at moderate to low temperatures (<400 °C) during folding, yielding well-defined plateau ages and thereby the age of deformation in the upper crust
K-Ar illite and apatite fission track constraints onbrittle faulting and the evolution of the northern Norwegian passive margin
Full text linkDetermining the timing of post-Caledonian brittle faulting in northern Norway is important for the understanding of the extensional tectonic evolution of the north Norwegian continental margin. Fault gouges from the Troms and Vesterålen regions of northern Norway yield Carboniferous to Permian and Carboniferous to Cretaceous K–Ar illite ages, respectively. The results show a contrast in fault activity and exhumation between the Troms and the Vesterålen regions: while major faulting in the Troms region appears to have ceased after the Permian faulting event, faulting continued into at least the Cretaceous in the Vesterålen region. The findings highlight the importance of a widespread Permian tectonic event followed by a distinct southwestward migration of post-Permian tectonic activity on the north Norwegian passive margin. Late Triassic to Early Jurassic apatite fission track ages do not show significant age offsets across major fault zones in Troms, indicating that most or all of fault activity took place prior to the Late Triassic. The thermal history models are consistent and indicate continuous cooling to about 60 °C in the Late Permian–Triassic
Initiation and development of the Pennine Basal Thrust (Swiss Alps): a structural and geochronological study of an exhumed megathrust
No full textThe Pennine Basal Thrust (PBT) is an exhumed megathrust developed during continental collision from late Eocene to Miocene. To trace its evolution, five samples, with indications for up to three microstructurally diachronous white-mica generations, were investigated by laser in-situ and step-heating 40Ar–39Ar dating. Three deformation-related crystallization ages can be distinguished: (1) D1, characterized in the PBT hanging wall by an S1 foliation defined by white mica + chloritoid, began at or before ∼38.0 Ma; (2) D2 formed a pervasive S2 cleavage and synchronous white-mica rich veins dated at ∼27 Ma; (3) D3 produced an S3 crenulation cleavage and chlorite + white-mica veins dated at ∼23 Ma. Older ages of ∼96 Ma (footwall) and ∼115 Ma (hanging wall) are interpreted as minimum ages for the detrital component. Finally, discrete faulting produced fault gouge, with an illite K–Ar age of ∼19 Ma. A simplified back-restored reconstruction provides a tectonic context for the dated structures. In this framework, D1 occurred during middle to late Eocene tectonic accretion. After late Eocene initiation of continental collision, D2 reflects Oligocene top-to-NW shearing, with both in- and out-of sequence thrusting. D3 then developed from 23 to 19 Ma during the progressive deactivation of the PBT
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
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