La faille normale ductile du massif du Saint Barthélémy. Age et signification de l'extension crustale dans le zone Nord-pyrénéenne

The first part of this work is composed of a synthesis about Alpine and Variscan orogenies in the Pyrenees. Recent works (ECORS profile), are presented and discussed. The second part is concerning with the Saint Barthelemy massif. The results of structural and petrological studies are shawn. The structure of the massif is interpreted to be the result of a single progressive deformation event. Structural analysis and thermobarometrical data confirm the existence of extensional tectonics. The singularity of the SBM, with the occurence of a major low angle shear zone, is shawn. In the third part, we proposed a thermotectonical madel which is integrated in the regional geological context.La première partie de ce travail correspond à une synthèse bibliographique sur les chaînes alpines et hercyniennes des Pyrénées. Les résultats récents (profil ECORS) y sont présentés et discutés. La deuxième partie concerne le Massif Nord Pyrénéen du Saint Barthélémy. Elle présente les résultats d'études structurales et pétrographiques combinées qui permettent de caractériser différents stades d'évolution. Ceux-ci s'inscrivent dans un continuum de déformation progressive qui s'effectue sous des conditions rétromorphiques. L'analyse structurale et les données thermobarométriques confirment l'existence d'une tectonique extensive. La singularité du massif du Saint Barthélémy apparaît dans la présence d'une zone de cisaillement d'ampleur régionale, la Bande Mylonitique Majeure, que l'on ne retrouve pas dans les autres MNP. Dans la troisième partie, l'ensemble des données est utilisé pour élaborer un modèle d'évolution thermotectonique, qui est ensuite confronté à l'évolution géologique régionale. Les données géochronologiques sont présentées et discutées. Différents modèles d'évolution tectonique sont proposés

Spatial and temporal evolution of Liassic to Paleocene arc activity in southern Peru unraveled by zircon U-Pb and Hf in-situ data on plutonic rocks

International audienceCordilleran-type batholiths are built by prolonged arc activity along active continental margins and provide detailed magmatic records of the subduction system evolution. They complement the stratigraphic record from the associated forearcs and backarcs. We performed in-situ U-Pb geochronology and Hf isotope measurements on zircon grains from a large set of plutonic rocks from the Coastal Batholith in southern Peru. This batholith emplaced into the Precambrian basement and the Mesozoic sedimentary cover. We identify two major periods of voluminous arc activity, during the Jurassic (200-175 Ma) and the Late Cretaceous-Paleocene (90-60 Ma). Jurassic arc magmatism mainly resulted in the emplacement of a dominantly mafic suite with εHf values ranging from − 9.5 to + 0.1. Published ages south of the Arequipa area suggest that the arc migrated southwestward out of the study area during the Middle Jurassic. After a magmatic gap of 85 Ma, arc activity abruptly resumed 90 Ma ago in Arequipa. Intrusive bodies emplaced into both basement and older Jurassic intrusions and strata. This activity culminated between 70 and 60 Ma with the emplacement of very large volumes of dominantly quartz-dioritic magmas. This last episode may be considered as a flare-up event, characterized by intense magmatic transfers into the crust and rapid relief creation. The Late Cretaceous-Paleocene initial εHf are shifted toward positive values (up to + 3.3 and + 2.6) compared to the Jurassic ones, indicating either a larger input of juvenile magmas, a lesser interaction with the ancient crust, or an increase of re-melting of young mantle-derived mafic lower crust. These magmatic fluxes with juvenile component are coeval with the onset of the crustal thickening at 90 Ma and represent a significant contribution to the formation of the continental crust in this area

U-Pb geochronology on zircon and columbite-group minerals of the Cap de Creus pegmatites, NE Spain

The Cap de Creus granitic pegmatites in the eastern Catalan Pyrenees were dated using in situ U-Pb geochronology by laser ablation ICP-MS on zircon and columbite-group minerals (CGM), which are present in the different types of pegmatites from type I (K-feldspar pegmatites, least evolved) to type IV (albite pegmatites, most evolved) and therefore allow dating the different pegmatitic pulses. In a type III pegmatite where zircon and CGM are co-genetically associated in the same sample, both minerals were dated using zircon and tantalite reference materials, respectively, to avoid laser-induced matrix-dependent fractionation. In one sample, xenotime genetically associated with zircon was also dated. Two ages were obtained for type I and three ages for type III pegmatites. Three of these 5 ages range from 296.2 ± 2.5 to 301.9 ± 3.8 Ma and are allocated to the primary magmatic stage of crystallization and therefore to the emplacement event. Two younger ages (290.5 ± 2.5 and 292.9 ± 2.9 Ma) obtained on secondary zircon and xenotime, respectively, are interpreted as late post-solidus hydrothermal remobilization. There is no age difference between type I and type III pegmatites. The mean 299 Ma primary magmatic age allows the main late Carboniferous deformation event to be dated and is also synchronous with other peraluminous and calc-alkaline granites in the Pyrenees. However, the youngest ages around 292 Ma imply that tectonics was still active in Early Permian times in the Cap de Creus area

High-resolution imaging of the Pyrenees and Massif Central from the data of the PYROPE and IBERARRAY portable array deployments

International audienceThe lithospheric structures beneath the Pyrenees, which holds the key to settle long-standing controversies regarding the opening of the Bay of Biscay and the formation of the Pyrenees, are still poorly known. The temporary PYROPE and IBERARRAY experiments have recently filled a strong deficit of seismological stations in this part of western Europe, offering a new and unique opportunity to image crustal and mantle structures with unprecedented resolution. Here we report the results of the first tomographic study of the Pyrenees relying on this rich data set. The important aspects of our tomographic study are the precision of both absolute and relative traveltime measurements obtained by a nonlinear simulated annealing waveform fit and the detailed crustal model that has been constructed to compute accurate crustal corrections. Beneath the Massif Central, the most prominent feature is a widespread slow anomaly that reflects a strong thermal anomaly resulting from the thinning of the lithosphere and upwelling of the asthenosphere. Our tomographic images clearly exclude scenarios involving subduction of oceanic lithosphere beneath the Pyrenees. In contrast, they reveal the segmentation of lithospheric structures, mainly by two major lithospheric faults, the Toulouse fault in the central Pyrenees and the Pamplona fault in the western Pyrenees. These inherited Hercynian faults were reactivated during the Cretaceous rifting of the Aquitaine and Iberian margins and during the Cenozoic Alpine convergence. Therefore, the Pyrenees can be seen as resulting from the tectonic inversion of a segmented continental rift that was buried by subduction beneath the European plate

Feedback between deformation and magmatism in the Lloyds River Fault Zone : an example of episodic fault reactivation in an accretionary setting, Newfoundland Appalachians

Author Posting. © American Geophysical Union, 2006. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Tectonics 25 (2006): TC4004, doi:10.1029/2005TC001789.The Lloyds River Fault Zone is a 10–15 km wide amphibolite-grade shear zone that formed during the Ordovician Taconic Orogeny. It separates ophiolites and arc–back-arc complexes formed in Iapetus from a peri-Laurentian microcontinent (Dashwoods microcontinent). The Lloyds River Fault Zone comprises three high-strain zones, dominantly composed of mylonitic amphibolites, separated by less deformed plutonic rocks. Structural, age and metamorphic data suggest the Lloyds River Fault Zone accommodated sinistral-oblique underthrusting of ophiolites underneath the Dashwoods microcontinent prior to 471 ± 5 Ma at 800°C and 6 kbar. Plutonic rocks within the Lloyds River Fault Zone comprise two suites dated at 464 ± 2 plus 462 ± 2 and 459 ± 3 Ma, respectively. The younger age of the plutons with respect to some of the amphibolites, evidence for magmatic deformation, and the elongate nature of the plutons parallel to the Lloyds River Fault Zone suggest they were emplaced within the fault zone during deformation. Both intrusive episodes triggered renewed deformation at high temperatures (770–750°C), illustrating the positive feedback between deformation and magmatism. Offshoots of the plutons intruded undeformed ophiolitic gabbros outside the Lloyds River Fault Zone. Deformation localized within the intrusive sheets, coeval with static contact metamorphism of the host gabbros, leading to the development of new, small-scale shear zones. This illustrates that channeling of plutons into shear zones and nucleation of shear zones in melt-rich zones may occur simultaneously within the same fault system.This research is funded by a scholarship from the Faculty of Graduate and Postdoctoral Studies, University of Ottawa, to C.J.L. and a NSERC grant to C.v.S in his position as Adjunct Professor at the University of Ottawa

Admission criteria and management of critical care patients in a pandemic context: position of the Ethics Commission of the French Intensive Care Society, update of April 2021.

Intensive care unit professionals have experience in critical care and its proportionality, collegial decision-making, withholding or withdrawal of treatment deemed futile, and communication with patients' relatives. These elements rely on ethical values from which we must not deviate in a pandemic situation. The recommendations made by the Ethics Commission of the French Intensive Care Society reflect an approach of responsibility and solidarity towards our citizens regarding the potential impact of a pandemic on critical care resources in France, with the fundamental requirement of respect for human dignity and equal access to health care for all

Amphibole and apatite insights into the evolution and mass balance of Cl and S in magmas associated with porphyry copper deposits

Chlorine and sulfur are of paramount importance for supporting the transport and deposition of ore metals at magmatic–hydrothermal systems such as the Coroccohuayco Fe–Cu–Au porphyry–skarn deposit, Peru. Here, we used recent partitioning models to determine the Cl and S concentration of the melts from the Coroccohuayco magmatic suite using apatite and amphibole chemical analyses. The pre-mineralization gabbrodiorite complex hosts S-poor apatite, while the syn- and post-ore dacitic porphyries host S-rich apatite. Our apatite data on the Coroccohuayco magmatic suite are consistent with an increasing oxygen fugacity (from the gabbrodiorite complex to the porphyries) causing the dominant sulfur species to shift from S2− to S6+ at upper crustal pressure where the magmas were emplaced. We suggest that this change in sulfur speciation could have favored S degassing, rather than its sequestration in magmatic sulfides. Using available partitioning models for apatite from the porphyries, pre-degassing S melt concentration was 20–200 ppm. Estimates of absolute magmatic Cl concentrations using amphibole and apatite gave highly contrasting results. Cl melt concentrations obtained from apatite (0.60 wt% for the gabbrodiorite complex; 0.2–0.3 wt% for the porphyries) seems much more reasonable than those obtained from amphibole which are very low (0.37 wt% for the gabbrodiorite complex; 0.10 wt% for the porphyries). In turn, relative variations of the Cl melt concentrations obtained from amphibole during magma cooling are compatible with previous petrological constraints on the Coroccohuayco magmatic suite. This confirms that the gabbrodioritic magma was initially fluid undersaturated upon emplacement, and that magmatic fluid exsolution of the gabbrodiorite and the pluton rooting the porphyry stocks and dikes were emplaced and degassed at 100–200 MPa. Finally, mass balance constraints on S, Cu and Cl were used to estimate the minimum volume of magma required to form the Coroccohuayco deposit. These three estimates are remarkably consistent among each other (ca. 100 km3) and suggest that the Cl melt concentration is at least as critical as that of Cu and S to form an economic mineralization

Whisker Movements Reveal Spatial Attention: A Unified Computational Model of Active Sensing Control in the Rat

Spatial attention is most often investigated in the visual modality through measurement of eye movements, with primates, including humans, a widely-studied model. Its study in laboratory rodents, such as mice and rats, requires different techniques, owing to the lack of a visual fovea and the particular ethological relevance of orienting movements of the snout and the whiskers in these animals. In recent years, several reliable relationships have been observed between environmental and behavioural variables and movements of the whiskers, but the function of these responses, as well as how they integrate, remains unclear. Here, we propose a unifying abstract model of whisker movement control that has as its key variable the region of space that is the animal's current focus of attention, and demonstrate, using computer-simulated behavioral experiments, that the model is consistent with a broad range of experimental observations. A core hypothesis is that the rat explicitly decodes the location in space of whisker contacts and that this representation is used to regulate whisker drive signals. This proposition stands in contrast to earlier proposals that the modulation of whisker movement during exploration is mediated primarily by reflex loops. We go on to argue that the superior colliculus is a candidate neural substrate for the siting of a head-centred map guiding whisker movement, in analogy to current models of visual attention. The proposed model has the potential to offer a more complete understanding of whisker control as well as to highlight the potential of the rodent and its whiskers as a tool for the study of mammalian attention