Etude géologique de la partie occidentale du massif hercynien des Rehamna septentrionales (Meseta marocaine). Lithostratigraphie, plissements et métamorphisme, chevauchements et nappes.

The northern Rehamna massif, halfway between Marrakech to the south and Casablanca to the north, constitutes part of the Hercynian segment of the coastal Meseta, in which it is exposed as a nucleus surrounded by post-orogenic sediments of Autunian (?), PermoTriassic (?) and Cretaceous age. The massif is composed of two great structural domains separated by an important thrust zone. The western domain is a little-deformed Cambro-Ordovician foreland ; it is considered to represent a relatively autochthonous terrain. Its western margin is one of imbricated wedges, the so-called "wedges of Oued Tarfa". The eastern domain is composed of a thick sequence of superposed thrust sheets, including those of Sidi Abdallah (with Ordovician to Devonian rocks), Skikirat (with Devonian ta Visean rocks), and Gada Jenabia (with Visean rocks only). These units have undergone a polyphase tectonic-metamorphic evolution which stands in strong contrast with that of the western domain.The western domain comprises 6 sedimentary units of detrital character, dated Middle Cambrian to Arenigian. Their paroxysmal deformation resulted in the formation of Fl-folds characterized by their approximately N-S orientation, their km-scale amplitude and half-wave length, their upright cylindrical nature and law angle of plunge, and the contemporaneous development of cleavageThe wedges of Oued Tarfa are composed of rocks identical to those of the western domain. The F1 structures are of the same type, but were truncated and thrust NW-ward during the same period of tangential deformation. The width of caver is 2-6 km, depending on the wedge.The Sidi Abdallah thrust sheet is composed of 10 formations ranging in age from Ordovician (?) to Devonian. Their paroxysmal deformation gave rise to a symmetric to overturned F1-folds with NW vergence and steep NE plunge, formed contemporaneous with metamorphism and the development of schistosity. The intensity of deformation and metamorphism increases toward the E, where F2 and F3-folds make their appearance. The first thrust episode (φ1) placed the Sidi Abdallah thrust unit on top of the western domain and imbricated the unit. The displacement was ta the NW and amounted to a minimum of 8 km. A second thrust episode (φ2) with S-ward displacement in turn gave rise to minor imbrications.The Skikirat thrust sheet is composed of nonmetamorphic rocks of Lower to Middle Devonian and upper Visean age. The first tectonic event appears to have led to the development of olistoliths ; the subsequent paroxysmal deformation gave rise to F1-folds whose orientations are at present highly variable and which are characterized by a weak-S1 cleavage. The first episode of tangential deformation caused displacement towards the NW, but is not widely observed. The second thrust event carried the Skikirat unit S-ward on top of the preceding units including the western domain. The displacement amounts to several km, locally as much as tens of km.The evidence of two distinct thrust periods in the northern Rehamna massif, following their discovery in the Jebilet massif and the southeastern Rehamna massif, demonstrates that these events played a highly important role in the late Hercynian tectogenesis of the Moroccan Meseta.Le massif hercynien des Rehamna septentrionales est constitué de deux grands domaines structuraux actuellement séparés par un important contact de recouvrement. A l’Ouest s’observe le domaine occidental autochtone au moins relatif. C’est un avant-pays à matériel cambro-ordovicien puissant, simplement déformé en vastes plis droits. La marge orientale de ce domaine est écaillée (écaille de l’oued Tarfa). A l’Est s’étend un domaine oriental, constitué de plusieurs grandes unités chevauchantes dont les deux premières ont été étudiées : l’unité de Sidi Abdallah à matériel ordovico(?) – dévonien et au-dessus l’unité de la Skikirat à matériel dévono-viséen.Ces unités ont subi une évolution tectono-métamorphique polyphasée qui contraste très fortement avec le domaine occidental.Le serrage hercynien a d’abord donné lieu à des épisodes de plissement P1 synschisteux et synmétamorphes, P2 et P3 (kinks). Ensuite un premier épisode de tectonique tangentielle a amené sur le domaine occidental l’unité de Sidi Abdallah entraînant les écailles de l’oued Tarfa para-autochtone. Les déplacements, du Sud-Est vers le Nord-Ouest sont au minimum décakilométriques.Un second épisode de tectonique tangentielle mais à déplacement du Nord vers le Sud, avec une flèche minimale également décakilométrique, conduit à la superposition de l’unité de la Skikirat à la fois sur le domaine occidental, les écailles de l’oued Tarfa et de l’unité de Sidi Abdallah.Après avoir été mise en évidence dans le Jebilet et dans les Rehamna méridionales, des deux épisodes tangentiels sont ainsi également connus dans le Rehamna septentrionales. Il s’agit donc là de phénomènes importants, voire majeurs, dans la structuration hercynienne de la Meseta marocain

Tectonic evolution of the Rehamna metamorphic dome (Morocco) in the context of the Alleghanian-Variscan orogeny

Structural and 40Ar/39Ar geochronological investigations of the Rehamna Massif (Meseta, Moroccan Variscan belt) provide new constraints on the tectonic evolution of the Alleghanian-Variscan orogen during the Upper Paleozoic. Three main tectonic events have been recognized: (1) Southward thrusting of an Ordovician sequence over the Proterozoic basement, its Cambrian sedimentary cover, and the overlying Devono-Carboniferous basin. This event caused subhorizontal shearing and prograde Barrovian metamorphism of the buried rocks. (2) Continuous shortening resulting in the development of a synconvergent extrusion of metamorphosed units to form a dome elongated E-W. This was responsible for synconvergent detachment of the Ordovician upper crustal sequence. The timing of these two episodes is constrained to 310–295 Ma by cooling and metamorphic amphibole and mica ages (3) A NW-WNW convergence in a direction orthogonal to the previous one and characterized by the accretion of the Rehamna dome to the continental basement in the east. Based on 40Ar/39Ar cooling ages from a syntectonic granitoid and its host rocks and metamorphic 40Ar/39Ar ages from greenschist facies mylonite, the timing of this event falls between 295 and 280 Ma. The end of the Variscan orogeny in the Moroccan Meseta is constrained by the 40Ar/39Ar cooling age of a posttectonic pluton dated at ~275 Ma. The tectonic events highlighted in Morocco coincide with the late Variscan-Alleghanian tectonic evolution of southern Europe and North America and can be correlated with the global reorganization of plates that accompanied suturing of Pangaea at around 295 Ma.Peer reviewe

Vers une meilleure intégration des écosystèmes dans la prise de décision publique: Une évaluation économique du service écosystémique de séquestration in situ du carbone

Les écosystèmes contribuent à l’atténuation du changement climatique de multiples manières, notamment en séquestrant de manière pérenne le CO2 atmosphérique in situ sous forme de carbone organique dans différents compartiments (la biomasse vivante (arbres), les sols (prairies, pelouses alpines, sols agricoles, tourbières, etc.) et les sédiments (fonds marins)). Lorsqu’un écosystème capte davantage de CO2 qu’il n’en émet au sein de l’atmosphère durant une période donnée, on dit qu’il est un puits de carbone. Cette évaluation, réalisée dans le cadre de l’Évaluation Française des Écosystèmes et des Services Écosystémiques (EFESE), montre que les écosystèmes français terrestres constituent actuellement un puits naturel de carbone d’environ 120 millions de tonnes de CO2, soit près du quart des émissions françaises annuelles. La prise en compte de ce service écosystémique dans les décisions publiques et privées constitue donc un enjeu fort

It’s all about connections: hubs and invasions in habitat networks

International audienceUnderstanding the factors underlying invasive processes, such as establishment and spread, is critical to predict the occurrence and magnitude of biological invasions. In general, those factors do not only depend on the invader itself, but also on the characteristics of the invaded area. In the metapopulation theoretical framework, landscapes are approximated as networks of discrete habitat patches. These patches are nodes of a network connected by edges that represent dispersal pathways between them. Many properties can be quantified to accurately characterize spatial heterogeneity in the connectivity between patches in a network. Node centrality for instance, measures the contribution of an individual habitat patch to landscape connectivity; highly central nodes are called “hubs”. By facilitating individual dispersal, those hubs can promote spread, but also decrease local establishment because of mechanisms like demographic stochasticity and Allee effects. By combining modeling and experimental approaches, we investigated how invasion dynamics are affected by the presence of hubs in a landscape. We first simulated invasions using a stochastic discrete-time metapopulation model. Then we challenged model predictions by performing experimental introductions of minute parasitoid wasps in artificial laboratory landscapes with patches characterized by different levels of centrality

exp_dataset

Experimental data obtained from the artificial invasions of T. chilonis, with the following treatments: L = introduction in lowC landscapes, HH = introduction in highC landscapes in the hub, HF = introduction in highC landscapes outside of the hub

Data from: It’s all about connections: hubs and invasion in habitat networks

During the early stages of invasion, the interaction between the features of the invaded landscape, notably its spatial structure, and the internal dynamics of an introduced population, has a crucial impact on establishment and spread. By approximating introduction areas as networks of patches linked by dispersal, we characterized their spatial structure with specific metrics and tested their impact on two essential steps of the invasion process: establishment and spread. By combining simulations with experimental introductions of Trichogramma chilonis (Hymenoptera: Trichogrammatidae) in artificial laboratory microcosms, we demonstrated that spread was hindered by clusters and accelerated by hubs but was also affected by small-population mechanisms prevalent for invasions, such as Allee effects. Establishment was also affected by demographic mechanisms, in interaction with network metrics. These results highlight the importance of considering the demography of invaders as well as the structure of the invaded area to predict the outcome of invasions

It’s all about connections: hubs and invasions in habitat networks

International audienceUnderstanding the factors underlying invasive processes, such as establishment and spread, is critical to predict the occurrence and magnitude of biological invasions. In general, those factors do not only depend on the invader itself, but also on the characteristics of the invaded area. In the metapopulation theoretical framework, landscapes are approximated as networks of discrete habitat patches. These patches are nodes of a network connected by edges that represent dispersal pathways between them. Many properties can be quantified to accurately characterize spatial heterogeneity in the connectivity between patches in a network. Node centrality for instance, measures the contribution of an individual habitat patch to landscape connectivity; highly central nodes are called “hubs”. By facilitating individual dispersal, those hubs can promote spread, but also decrease local establishment because of mechanisms like demographic stochasticity and Allee effects. By combining modeling and experimental approaches, we investigated how invasion dynamics are affected by the presence of hubs in a landscape. We first simulated invasions using a stochastic discrete-time metapopulation model. Then we challenged model predictions by performing experimental introductions of minute parasitoid wasps in artificial laboratory landscapes with patches characterized by different levels of centrality

Simulation script

The R script used to perform the simulations presented in the main text of the articl

Quelle interministérialité pour le Conseil National de l'Alimentation ?: Vers une gestion plus efficace et ciblée de l’interministérialité pour améliorer l’impact et la reconnaissance du CNA

Cette analyse s'inscrit dans un contexte de prise de poste récente de la secrétaire interministérielle du CNA et du démarrage d'une nouvelle mandature (2016-2019). Ce projet doit permettre au commanditaire de mieux appréhender les enjeux de la dimension interministérielle du CNA et de son fonctionnement. Nous proposons des préconisations et des outils pour améliorer la coordination interministérielle dans un futur proche

Bioeconomic impacts of agroforestry policies in France

Dealing with the erosion of terrestrial biodiversity has become of key importance in order to ensure ecosystems sustainability. Agricultural and forestry activities are one major anthropogenic driver of this decline. The underlying land-use changes result in the alteration of species habitats. In this context reconciling economic and ecological objectives of agricultural policies remains an ongoing challenge. In that respect, this paper presents the bioeconomic impacts of contrasted agricultural public policy scenarios. We developped a bio-economic model coupling an ecological model of bird dynamics with a micro-economic model of land management estimated with French data. We assessed the performances of the scenarios based on 5 ecological indicators accounting for various structural and functionnal characteristics while economic performances refer to land-use incomes. First our study confirms the potential long-term synergies between several ecological and economic objectives in grassland-based policies. Second we points out the non-trivial effect of agroforestry policies on agricultural biodiversity: despite a positive overall impact on the biodiversity population size, it implies important structural changes within the community. The choice between grassland-based policies such as in Agri-Environmental Schemes and agroforestry policies will thus depend on the ecological stakeholders’ preferences