Les chaînes de la marge occidentale du Craton Ouest-Africain,modèles géodynamiques

Until now, no satisfactory geodynamic model has been delivered concerning the three main West African orogens: Panafrican 1 (Bassaride belt), Panafrican 2 (Rokelide belt) and Hercynian (Mauritanide belt). However, since the last synthetic paper (Villeneuve, 2008), new geological, geophysical and geochronological data, from the Moroccan Sahara to Sierra Leone, allow us to propose a new geodynamical model. It includes the two Panafrican events in a single model very similar to the present western Pacific margin. An old “West African Neoproterozoic ocean” (WANO) was limited by a set of island arcs separated from the West African craton by a series of “back arc basins”. The closure of this first round of back arc basins around 650 Ma led to the Bassaride belt (Panafrican 1). Then the WANO was subducting underneath the island arcs (between 650 and 550 Ma) meanwhile a new generation of “back arc basins” opened to the east between the arcs and the craton margin. The closure of the WANO and associated island arcs and back arc basins (550 to 500 Ma) led to the Rokelide belt (Panafrican 2). The Hercynian structures involving a Palaeozoic cover (made with continental material) associated to a “greeenschist facies” metamorphism is ascribed to an intracontinental belt.     

Effects on runoff caused by changes in land cover in a Brazilian southeast basin: evaluation by HEC-HMS and HEC-GEOHMS

The Southeast Region of Brazil has undergone major changes in land cover, especially after the eighteenth century. It is currently the most populous region of the country, highly urbanized, with a high degree of industrial and agricultural development. Extensive areas of native vegetation have been replaced by pastures, crops and urban areas, which have increased runoff, causing environmental, economic and social problems related to flooding. The objective of this study was to analyze effects of land cover changes in a basin with rural and urban characteristics on the flow of its main river. Hydrological data, orbital images, soils and topographical maps were used for this purpose. Based on the land cover maps for the years of 1989, 2001 and 2015, and on the hydrological modeling performed using the Hec-HMS 4.1 software, scenarios were simulated and showed that the land cover changes in this basin significantly affect the flow behavior of the main river. The simulated runoff was calibrated using the data observed in the field during 2001, and validation was performed using data from 1989. After the calibration and validation processes, a scenario was simulated where the rainiest month of the whole series measured by the rainfall station (during December 1989) acted on the land cover of 2015. There was an increase in pasture areas and impermeable spaces in the basin, which caused a decrease in infiltration and an increase in surface runoff, and also an increase in the flow peaks and a reduction in the time of concentration. The hydrological modeling was satisfactory, since the uncertainties related to the simulation were low.