Mise en évidence d'une tectonique compressive Éocène-Oligocène dans l'Ouest de la Cordillère orientale de Colombie, d'après la structure en duplex des gisements d'émeraude de Muzo et de Coscuez (Evidence for an Eocene-Oligocene compressive tectonics in the western part of Eastern Cordillera of Colombia, from the duplex structure of the Muzo and Coscuez emerald deposits)

The structures of the emerald deposits of the western part of Eastern Cordillera of Colombia (thrusts, ramps, duplexes, tear faults) have been formed by three deformational events forming a single thrust-tectonics phase, synchronous with the emerald-forming hydrothermalism. As this hydrothermalism is Late Eocene-Early Oligocene in age, it is also the age of the thrust tectonics

Consistent patterns of common species across tropical tree communities

Trees structure the Earth’s most biodiverse ecosystem, tropical forests. The vast number of tree species presents a formidable challenge to understanding these forests, including their response to environmental change, as very little is known about most tropical tree species. A focus on the common species may circumvent this challenge. Here we investigate abundance patterns of common tree species using inventory data on 1,003,805 trees with trunk diameters of at least 10 cm across 1,568 locations1,2,3,4,5,6 in closed-canopy, structurally intact old-growth tropical forests in Africa, Amazonia and Southeast Asia. We estimate that 2.2%, 2.2% and 2.3% of species comprise 50% of the tropical trees in these regions, respectively. Extrapolating across all closed-canopy tropical forests, we estimate that just 1,053 species comprise half of Earth’s 800 billion tropical trees with trunk diameters of at least 10 cm. Despite differing biogeographic, climatic and anthropogenic histories7, we find notably consistent patterns of common species and species abundance distributions across the continents. This suggests that fundamental mechanisms of tree community assembly may apply to all tropical forests. Resampling analyses show that the most common species are likely to belong to a manageable list of known species, enabling targeted efforts to understand their ecology. Although they do not detract from the importance of rare species, our results open new opportunities to understand the world’s most diverse forests, including modelling their response to environmental change, by focusing on the common species that constitute the majority of their trees.Publisher PDFPeer reviewe

Géologie et Art Roman : pierres romanes du Conflent (Pyrénées-Orientales)

Le Conflent (Pyrénées-Orientales) renferme cent édifices religieux romans du Xe-XIIIe siècles, intacts, plus ou moins modifiés ou sous forme de minces vestiges. La nature pétrographique des matériaux qui les composent a été relevée systématiquement, en référence à la géologie de la région. 92 édifices ont pu être classés en fonction de : (1) la nature de ces matériaux, en particulier selon qu’ils sont pour l’essentiel de provenance locale, et donc reflètent l’environnement géologique proche (82 édifices), ou en partie au moins de provenance lointaine, et alors sans rapport avec cet environnement (10 édifices) ; (2) de la qualité de la mise en œuvre de ces matériaux, du simple appareil de moellons et de blocs aux appareils les plus soignés (24 édifices sont construits, au moins partiellement, en pierres de taille). La répartition géographique des différents types d’édifices est brièvement discutéeEl Conflent (Pirineus Orientais) té cent edificis religiosos romans dels segles X-XIII; intactes, més ο menys modificats ο sota la forma de prims vestigis. La natura petrogràfica dels materials que els composen va ser revelada sistemàticament, en referència a la geologia de la regió. Noranta-dos edificis van poder ser classificats en funció de: (1) la natura d’aquests materials, en particular segons que són per a l’essencial de provinença local, i doncs reflecteixen el medi ambient geològic próxim (82 edificis), ο en part al menys de provinença llunyana, i aleshores sense relació amb aquest medi ambient (10 edificis); (2) la qualitat de l’aplicació d’aquests materials, del simple aparell de códols i de llocs als aparells més curats (24 edificis són construïts, al menys parcialment en pedra de talla). La repartició geogràfica dels diferents tipus d’edificis és breument discutidaConflent (Pyrénées-Orientales) holds hundred Χ th–XIII th centuries Romanesque religious edifices, intact, more or less modified or as small remains. The petrographical kind of the materials which make them up was systematically taken down, with a reference to the geology of the area. 92 edifices were able to be listed according to: (1) the kind of these materials, in particular from those mainly of local origin, reflections of a close geological environment (82 edifices), to those at least partly of remote origin, without any link with this environment (10 edifices); (2) The quality of the implementation of these materials, from a simple rag-stones and blocks bond to the most elaborate bonds. (24 edifices have been built, at least partly, of ashlars). The geographical distribution of the various types of edifices is briefly tackle

Les Pyrénées alpines sud-orientales (France, Espagne) – essai de synthèse

International audienceIn the eastern Pyrenees, the Axial Zone (AZ) shows a complex Alpine structure partly obscured by the Neogene tectonics, more particularly in the vallespir area. Five south-verging, in sequence thrusts are identified in the Hercynian basement.From N to S and top to bottom, there are : three major thrusts (the well established Aspres thrust, the more recently recognized Canigou and Vallespir thrusts) and two minor thrusts (Albères and Roc de France). These contacts delimit six tectonic units, an upper (Aspres), two middle (Canigou and Vallespir) and three lower (Albères, Roc de France and St-Laurent-de-Cerdans). A similar structure is reported further southwest in the Ribes de Freser area. The rhombic pattern drawn by the Alpine E-W, NW-SE and NE-SW directed thrusts in the eastern Axial Zone largely results from pre-alpine heritage (inverted late-Hercynian and Cretaceous normal faults, late-Hercynian mylonitic zones reactivated during the Alpine events). A S-dipping monocline affecting this thrust system limits the AZ and the South-Pyrenean Zone (SPZ), the AZ being raised with respect to the Alpine cover rocks of the SPZ. This monocline corresponds to the short limb of a large ramp anticline linked to the emplacement of the lowermost unit (St-Laurent-de-Cerdans). The SPZ is made up of the Cadí units overlain by allochthonous units (from W to E : Pedraforca, Coustouges, Bac Grillera, Biure and Empordà). The Cadí units (SPZ) together with theassociated middle basement units (AZ) form the Canigou nappe, a major unit below which are visible the lower basement units (AZ) in the Albères tectonic window (and in the small Freser window).The South-Pyrenean allochthons moved from N to S and their origin is likely to be found on the basement of the Canigou nappe (AZ). Except the Coustouges, lower Bac Grillera and (?) Biure small allochtons that may be rooted in the Aspres thrust (AZ), it is not possible to root the other allochtons (especially the large Pedraforca unit) in the AZ due to lack of suitable Alpine thrusts. Therefore, we are obliged to root them farther north, i.e. to the north of the North Pyrenean Fault (NPF). It is the same for the large South-Pyrenean Central Unit in the Central Pyrenees. Athough it invalidates the model commonly accepted since 25 years for the interpretation of the "ECORS-Pyrenees" seismic profile (with a large antiformal stack in the AZ), we show that this new interpretation is reasonable on two conditions: (i) an early (Lower Eocene s.l.), major, south-verging tangential tectonic regime should have existed in the central-eastern Pyrenees, which carried the European part of the chain (now the NPZ) together with the Cretaceous rift onto the South-Iberian margin (i.e. the AZ); this is clearly visible in the westernmost Pyrenees (Béarn, Aragon); (ii) this thrust system should have been modified by the late (Upper Eocene s.l.) formation and uplift of the AZ, due to the combined effects of the bordering monocline to the South and the NPF to the North. If this were the case, the present-day AZ forms a tectonic window under a system of upper units (NPZ and southern allochtons).La Zone axiale (ZA) des Pyrénées orientales présente une structure alpine complexe en partie obscurcie par la tectonique néogène, en particulier dans le Vallespir. On identifie dans le socle hercynien cinq chevauchements à vergence S en séquence, soit du N vers le S et du haut vers le bas, trois chevauchements majeurs (chevauchement des Aspres, connu de longue date, chevauchements du Canigou et du Vallespir, plus récemment mis en évidence) et deux mineurs (chevauchements des Albères et du Roc de France). Ces contacts définissent six unités structurales, une unité supérieure (Aspres), deux unités intermédiaires (Canigou, Vallespir) et trois unités inférieures (Albères, Roc de France, St-Laurent-de-Cerdans). Une structuration analogue est visible plus au Sud-Ouest, dans la région de Ribes de Freser. Le découpage losangique dessiné par les chevauchements E-W, NW-SE et NE-SW), dans la ZA orientale, est largement la conséquence d’héritages préalpins (failles normales finihercyniennes et crétacées inversées, zones mylonitiques tardihercyniennes à rejeu alpin). Ce dispositif est affecté par une flexure bordière relevant la ZA relativement à la Zone sud-pyrénéenne (ZSP), laquelle est formée de terrains de couverture alpine. Cette flexure a valeur de mégapli de rampe frontale lié à la mise en place de l’unité inférieure la plus profonde (St-Laurent-de-Cerdans). La ZSP est constituée par les unités du Cadí sur lesquelles flottent des allochtones (Pedraforca, Coustouges, Bac Grillera, Biure, Empordà). Les unités du Cadí (ZSP) et les unités de socle intermédiaires (ZA) forment la nappe du Canigou, unité majeure sous laquelle les unités de socle inférieures (ZA) sont visibles dans la fenêtre des Albères (et la petite fenêtre du Freser). Les allochtones sont déplacés depuis le N et leur patrie est à chercher sur le socle de la nappe du Canigou (ZA). Si les klippes de Coustouges, Bac Grillera inférieure et (?) Biure peuvent être enracinées dans le chevauchement des Aspres (ZA), les autres (en particulier la grande unité de la Pedraforca) ne peuvent pas être enracinées dans la ZA, faute de chevauchement alpin adéquat, verticalisé ou non. Il est donc nécessaire de les enraciner plus au Nord, au N de la Faille nord-pyrénéenne (FNP), sous la Zone nord-pyrénéenne (ZNP) donc. Il en est de même pour la vaste Unité sud-pyrénéenne centrale des Pyrénées centrales. On montre que cette conclusion, nouvelle et qui invalide largement le modèle structural « standard » (un méga-anticlinal de nappes) habituellement admis pour la ZA depuis 25 ans pour l’interprétation du profil ECORS-Pyrénées, est possible à deux conditions : (i) qu’il ait existé dans les Pyrénées centro-orientales une tectonique tangentielle majeure précoce (Éocène inférieur s.l.) à vergence S qui a transporté le rift crétacé et la partie européenne de la chaîne (la ZNP actuelle) sur sa marge Sud ibérique (ZA), comme cela se voit très bien dans les Pyrénées plus occidentales (Béarn, Aragon) ; (ii) que ce dispositif précoce ait été altéré par la formation et le soulèvement tardifs (Éocène supérieur s.l.) de la ZA par le jeu conjoint de la flexure bordière au Sud et de la FNP au Nord ; la ZA actuelle apparaît en fenêtre sous un complexe d’unités tectoniques supérieures (ZNP, allochtones méridionaux)

Une application des méthodes de l'analyse statistique à l'estimation des déplacements de voix entre les deux tours des élections présidentielles de 1965

Mendès France B., Laumonier L. Une application des méthodes de l'analyse statistique à l'estimation des déplacements de voix entre les deux tours des élections présidentielles de 1965. In: Revue française de science politique, 17ᵉ année, n°1, 1967. pp. 110-114

Denudation history and palaeogeography of the Pyrenees and their peripheral basins: an 84-million-year geomorphological perspective

International audienceThis review provides a synthesis of the evolution of the Pyrenees since ~84 Ma and is uniquely focused on analysing jointly and comparatively its peripheral pro-foreland, retro-foreland and Mediterranean basins. The reconstructions adopt a geomorphological perspective focused on the waxing and waning of palaeorelief, and is underpinned by (i) the denudation history of the mountain belt encoded in the sedimentary record of its basins, (ii) rock-cooling histories inferred from low-temperature thermochronology, and (iii) the age and spatial distribution of tectonic and erosional landforms. Existing geological reconstructions of the Pyrenees commonly terminate at the end of the syntectonic collision period (early Miocene). Here, the no-less eventful post-shortening period of the last 25-30 m.y. is also addressed. Accordingly, emphasis is given to the record provided by nonmarine clastic sequences, and to the often understated depositional biochronology documented by the continental fossils they contain. Sedimentological and provenance analysis of coarse clastic deposits further documents the fine-scale palaeogeography of sources and sinks, and is correlated with different generations of eustatic, tectonic, and volcanic features, as well as extant populations of land surfaces such as rock pediments, palaeovalleys, and other landforms indicative of palaeoelevation and palaeotopography. These interconnected and age-bracketed diagnostic features are correlated with independent evidence concerning the structural evolution of the orogenic belt at crustal and lithospheric scale. They show that the Ancestral (i.e., Paleogene) Pyrenees were in many aspects dissimilar to the successor mountain range we observe today. They also suggest that, despite its prima facie topographic continuity from the Mediterranean to the Atlantic, the modern mountain range, particularly in its eastern half, is in a transient topographic state. This would appear to have been driven by large-scale asthenospheric flows contributing to regional uplift and erosion of not just the mountain range but also its foreland basins during the last ~12 m.y. Despite these early hopes for a unified understanding of the mountain range, over 200 years later we find that the investigation of the Pyrenees has suffered from the usual disconnect between geological and geomorphological approaches, between orogen-scale desktop modelling and local-scale field observations, between a focus on Alpine tectonics and a neglect of Quaternary neotectonics, with many of its consequences listed above.Restricting research to the confines of the time or spatial bundles listed above has generated persistent blind spots with an entrenched risk of confirmation bias. This review attempts to transcend some of those conventional boundaries of inquiry by operating from two key angles: (i) we address the metabolism of the orogen as a mountain range rather than just as a crustal wedge or mosaic of geological structures, and thus focus on its erosional history, topographic evolution, and landform assemblages; (ii) we articulate the geodynamic history of the Pyrenees with the palaeogeography of its Ebro, Aquitaine and Mediterranean sedimentary basins simultaneously, spanning the earliest plate convergence episodes of the latest Mesozoic to glaciation in the Pleistocene. As a result, rather than a small, asymmetrically bi-vergent orogen captured over simplifyingly short intervals of geological time, it portrays instead a mountain range evolving continuously by lateral and longitudinal growth or destruction through a succession of three quite distinct avatars defined hereafter as the Proto-Pyrenees, the Ancestral Pyrenees, and the Modern Pyrenees - respectively and schematically during the late Cretaceous, Paleogene and Neogene-Quaternary. Each of these Pyrenean ranges has displayed transient periods of crustal and topographic symmetry and asymmetry, successions of range-parallel and range-transverse drainages, the construction and unequal preservation of range-front megafans, uneven intensities of crustal uplift and depths of rock denudation, unequal depths of fluvial incision and patterns of alluvial deposition, and unequal intensities of glacial imprint in more recent time. Many place names (massifs, peaks, towns, etc.) mentioned in the text are located in the figures, but for precision and completeness it is recommended to use an Earth navigation browser.</ce:para

Synthesis and Overview

International audienceThis chapter summarises the key points addressed in the previous four. From the K–T boundary to the present, the Pyrenees as a mountain range rising between Europe and Iberia went through three successive states: the proto-Pyrenees, the ancestral Pyrenees, and the modern Pyrenees. Throughout this period, the mountain range changed in length, width, relief and elevation. The proto-Pyrenees began to emerge in the east, forming a mountain range that coincided with the eastern third of the modern Pyrenees. Mountain topography extended at the time farther east into the area now occupied by the Gulf of Lion. Growth of the mountain belt extended westward from Ypresian time and the ancestral Pyrenees appear to have attained their peak energy and peak topography between the Priabonian and the Oligocene, at a time when the Ebro Basin was entering its ~20–25-million-year period of internally drained sedimentary confinement. Mountain-front fan systems became ubiquitous on both sides of the orogen during this time interval. The chapter delivers a comparative analysis of the depositional histories in the pro- and retro-foreland basins of the orogen. Explanations to a number of recorded features are contained among the rich assemblage of geomorphological and chronostratigraphic evidence encountered in the elevated massifs and Neogene extensional basins of the eastern Pyrenees, which are explored in greater depth throughout the nine itineraries described in chapters 6 to 14 of this GeoGuide

Itinerary 4. The Vallespir Landscape: Scars of Deluge and Hydrological Disaster

International audienceItinerary 4 follows the Tech valley, with opportunities to observe Hercynian and Alpine structures as well as geomorphological features on the southern flank of the Axial Zone (e.g., two generations of pediment surfaces, P1 and P2). An even more striking feature is the geomorphological record of high-intensity meteorological events from the past years and centuries. The Vallespir lies in the humid enclave of Catalonia, i.e., in a small, semi-enclosed area sheltered from westerly weather systems by the mountain range but open to overheated and humidity-laden air masses from the adjacent Mediterranean, which penetrate via the low-lying Empordà graben and get funnelled into the Tech valley where they promote thunderstorm clouds. Thus, although autumn storms periodically generate major hydrological extremes as elsewhere in Roussillon, unlike adjacent areas summers are also rarely dry. These paroxysms have left scars in the landscape as well as in the collective memory of its residents