The Karagwe-Ankole Belt (KAB) in the Kivus of the Democratic Republic of the Congo (DRC), Rwanda and Burundi is generally considered as a Mesoproterozoic belt, with a geodynamic evolution governed by the 1375 Ma Kibaran magmatic event, the ± 1.0 Ga Rodinia and the ± 550 Ma Gondwana (Pan-African) amalgamation events (Tack et al., 2010). Here we focus on the ± 550 Ma Gondwana event, which has been generally underestimated in previous works. This last event was first detected by “anomalously” young ages interpreted as isotopic rehomogenisations of “Lufilian” age in the granites of Rwanda, including the ± 980 Ma Tin granites which have been frequently cataclased (Gerards and Ledent (1970). Similar ages were obtained also for Pb/Pb, Rb/Sr on microcline from pegmatites (Monteyne Poulaert, 1962), U/Pb on hydrothermal muscovite (Walemba, 2001) and U/Pb on monazite in a tectonic breccia in Burundi (Brinckmann et al., 2001). We compiled the existing chronological datasets for the Karagwe-Ankole belt from the Paleoproterozoic to the late Cenozoic rifting. It evidences a multistage evolution (magmatism, mineralization, sedimentation, metamorphism, ductile and brittle deformation) and, in particular, the importance of Pan-African reactivations. In parallel, recent work in the neighboring areas (e.g. in Tanzania, Uganda and Kenya) showed that East-Central Africa appears much more unstable than previously thought as a consequence of the east Gondwana amalgamation. Fritz et al. (2013) evidenced early Pan-African formation of the East African Orogen between 650 and 615 Ma due to the collision and closure of the Mozambique ocean. Further E-W convergence occurred between 590 and 570 Ma (Fritz et al., 2013; Saalmann et al., 2016), together with interaction between the Tanzania Craton and the Bangweulu Block between 590 and 550 Ma (Boniface & Appel, 2018). Late Pan-African reactivations under E W shortening were caused by the Tanzania–Congo / Dharwar Cratons convergence at about 530 Ma (Fritz et al. 2013). Field observations at various sectors of the KAB belt evidence brittle faulting that appear unrelated to and younger than the previous “Kibaran” magmatic, deformation and mineralization events (1.375 Ga or ~1.0 Ga), but contemporaneous with the deformation of the Neoproterozoic (Cryogenian-Ediacaran) Itombwe series. Paleostress inversion allowed to reconstruct an older and widely expressed tectonic stress field characterized by a general E W horizontal compression in a strike-slip to thrust faulting regime, and a younger brittle reactivation under N-S horizontal compression. The first brittle event occurred in brittle ductile conditions and is interpreted as related to the late Pan-African event that affected the entire belt. The second brittle event is restricted to a few area and whiteness younger deformations. This finding opens important perspectives for a revision of the regional geological map and also for the better understanding of the mineral resources.

Lire plus

Tropical environments favour chemical weathering and regolith development. Weathering induces textural, mineralogical and chemical changes in rocks, modifying their strength and thus affecting slope stability. Degree of weathering is, however, not only a function of climatic conditions, but is also influenced by e.g. bedrock composition and structure, exposure length and intensity, and slope angle. To investigate the role of weathering and rock type on landslide occurrence, we focus on the Ruzizi Gorge in the Kivu Rift segment of the western branch of the East African Rift System. Stretching along the border between the DR Congo and Rwanda, development of this 40-km long bedrock river began about 10,000 years ago, rejuvenating the landscape at a very high rate, with rather invariant slope angles outside of the landslides. The gorge stretches across a region where two main types of rocks constitute the geological substrate, i.e. late Miocene to Pleistocene volcanic rocks and Mesoproterozoic metasedimentary rocks. The gorge is a hotspot of deep-seated landsides in the region, with slope failures of up to 2 km². For the present study, we sampled weathering profiles developed on both mentioned rock types, in each case with sampling points within and outside the landslides as well as within and outside the rejuvenated landscape. The chemical composition of rock and regolith samples was determined by Inductively Coupled Plasma–Optical Emission Spectroscopy (ICP–OES) analysis, and their mineralogical composition by X-Ray Diffraction (XRD) analysis and thin section observations. Geotechnical tests were used to determine mechanical properties. Overall, we observe that lithological aspects alone control regolith characteristics, and that slope angle and exposure to landscape rejuvenation hence play no significant role. In areas with volcanic rock substrate, where the largest, mostly slide-type, landslides develop, stratified weathering profiles are observed. These profiles show a greater weathering depth than those over metasedimentary rocks, where flow- and avalanche-type landslides are more common. The regolith derived from volcanic rocks has higher clay content, greater plasticity and stronger cohesion than the sandy to silty weathering material that overlies the metasedimentary rocks. These preliminary results show that weathering and rock type are more important than landscape rejuvenation in controlling the type of deep-seated landslides.

Lire plus