Altération et cuirassement dans le sud-est du Rwanda : étude de géomorphologie climatique
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Bordeaux 3Disciplines:
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Abstract EN:
South - east of rwanda is formed of terraced erosion surfaces, which altitudes spread out from more than 1800 to less than 1400 meters. The s i to s iii levels are connected to byumba, butare an akagera surfaces, previously defined by rossi (1979). Loal levels s iv and s v are linked to high and low fans. They were formed essentially by: - a set of contrasted lithologic types; - a tectonic of meridian oriented folding and fractures; and - series of granitic intrusions surrounded by enclosing formations following more or less large virgations. Important phenomenons of alteration and lateritization have gene rated deep superficial deposits. Post- pliocen was marked by great climatic changes, and by drainage reorganization resulting from ri opening, and the erection of the volcanic chain in the north-west of the country. Those phenomena were accompanied by destruction an redistribution of those superficial deposits elements along high and low fans. Detritic sediment accumulations in the akagera valley behind qua tzitic ridges of mutenderi-mugogo, of rusumo and of muhororo, set up an important subsident zone. This movement finds expression in a certain transgression of lake surfaces, but also in landscape
Abstract FR:
Le sud - est du rwanda est constitue de niveaux d'aplanissement etages, dont les altitudes s'echelonnent de plus de 1800 a moins de 1400 metres: les niveaux s i a s iii correspondant aux surfaces de byumba, de butare et de l'akagera, definies par rossi (1979). Leur mise en place a ete determinee essentiellement par: - un ensemble de formations lithologiques contrastees; - une tectonique de plissements et de fracturations a orientations surtout meridiennes; et - une serie d'intrusions granitiques syntectoniques autour desquelles les formations encaissantes decrivent des virgations a plus ou moins grand rayon de courbure. D'importants phenomenes d'alteration et de cuirassement ont fourni d'epaisses formations superficielles. Le post-pliocene a ete marque par de grandes fluctuations climatiques, et par le renversement du drainage consecutif a la derniere phase d'ouverture du rift et a la mise en place de la chaine des volcans dans le nordouest du pays. Ces phenomenes se sont accompagnes de la destruction. Du remaniement et de la redistribution des elements de ces formations superficielles le long des interfluves et des hauts et bas glacis. Les sediments detritiques accumules dans la vallee de l'akagera en amont des principaux seuils quartzitiques de mutenderi- mugogo de rusumo et de muhororo, font de cette derniere une importante