ALBERT

Beschreibung: La dégradation continue des sols, particulièrementen Afrique sub-Saharienne, est devenue une préoccupationmajeure pour de millions de petits agriculteursqui dépendent étroitement de la terre pourleur subsistance. Pour faire face à cette situation,plusieurs projets/programmes tant nationaux qu'internationauxont promu et mis en oeuvre des mesuresde gestion durable des terres (GDT) afin de protégerles terres agricoles et/ou réhabiliter les sols dégradés.En dépit des efforts considérables déployés par cesprogrammes et projets, la situation continue de sedégrader du fait d'une multitude de contraintes et defacteurs complexes qu'il est impératif d'appréhenderpour renverser les tendances actuelles.

Beschreibung: Le Burkina Faso connaît une dégradation continuede ses ressources naturelles en général et de ses solsen particulier depuis de nombreuses décennies. La situationest de plus en plus préoccupante, notammentpour les petits agriculteurs dont la subsistance dépendénormément de la terre. Pour faire face à cettedégradation des sols, des pratiques et des technologiesde Gestion Durable des Terres (GDT) sont promuesà travers des politiques et des stratégies misesen oeuvre par des projets et programmes d’interventiondepuis les grandes sécheresses des années 70.Malgré les efforts considérables déployés par l'État etses partenaires, les taux d’adoption des technologiesde GDT demeurent généralement faibles et varientselon les régions de façon significative. Les niveauxd'adoption, en général appréciables pendant les projetset programmes, connaissent des chutes importantesdès la fin desdits projets et programmes. Unemultitude de facteurs limitent l’adoption durable età grande échelle des technologies. Il y a entre autres: les difficultés d’accès au crédit, l’insuffisance de lamain d’oeuvre, l’inadéquation des technologies GDTaux conditions agro-écologiques et d’autres facteurssocio-économiques, biophysiques et politico-institutionnels.Surmonter ces écueils reste un défi majeur.

Beschreibung: Slab retreat, slab tearing and interactions of slabs are first-order drivers of the deformation of the overriding lithosphere. An independent description of the tectonic evolution of the back-arc and peripheral regions is a pre-requisite to test the proposed conceptual, analogue and numerical models of these complex dynamics in 3-D. We propose here a new series of detailed kinematics and tectonic reconstructions from 35 Ma to the Present shedding light on the driving mechanisms of back-arc rifting in the Mediterranean where several back-arc basins all started to form in the Oligocene. The step-by-step backward reconstructions lead to an initial situation 35 Ma ago with two subduction zones with opposite direction, below the AlKaPeCa block (i.e. belonging to the Alboran, Kabylies, Peloritani, Calabrian internal zones). Extension directions are quite variable and extension rates in these basins are high compared to the Africa-Eurasia convergence velocity. The highest rates are found in the Western Mediterranean, the Liguro-Provençal, Alboran and Tyrrhenian basins. These reconstructions are based on shortening rates in the peripheral mountain belts, extension rates in the basins, paleomagnetic rotations, pressure-temperature-time paths of metamorphic complexes within the internal zones of orogens, and kinematics of the large bounding plates. Results allow visualizing the interactions between the Alps, Apennines, Pyrenean-Cantabrian belt, Betic Cordillera and Rif, as well as back-arc basins. These back-arc basins formed at the emplacement of mountain belts with superimposed volcanic arcs, thus with thick, hot and weak crusts explaining the formation of metamorphic core complexes and the exhumation of large portions of lower crustal domains during rifting. They emphasize the role of transfer faults zones accommodating differential rates of retreat above slab tears and their relations with magmatism. Several transfer zones are identified, separating four different kinematic domains, the largest one being the Catalan-Balearic-Sicily Transfer Zone. Their integration in the wider Mediterranean realm and a comparison of motion paths calculated in several kinematic frameworks with mantle fabric shows that fast slab retreat was the main driver of back-arc extension in this region and that large-scale convection was a subsidiary driver for the pre-8 Ma period, though it became dominant afterward. Slab retreat and back-arc extension was mostly NW-SE until ∼ 20 Ma and the docking of the AlKaPeCa continental blocks along the northern margin of Africa induced a slab detachment that propagated eastward and westward, thus inducing a change in the direction of extension from NW-SE to E-W. Fast slab retreat between 32 and 8 Ma and induced asthenospheric flow have prevented the transmission of the horizontal compression due to Africa-Eurasia convergence from Africa to Eurasia and favored instead upper-plate extension driven by slab retreat. Once slab retreat had slowed down in the Late Miocene, this N-S compression was felt and recorded again from the High Atlas to the Paris Basin.