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During the last fourteen thousand years, East Africa was the locus of significant climatic and cultural changes, which highly affected landscape evolution and prehistoric ways of life. Enhanced monsoonal precipitations over the tropics, mainly driven by Earth’s precessional and eccentricity orbital parameters, were the primary causes of the African Humid Period (AHP; ~14 - ~6 ka). This humid period led to the transgression of continental lacustrine waterbodies, the increase of fluvial discharge and the spread of vegetation cover over the present-day desertic regions. The onset and the end of this wetter phase were not homogeneous across the continent, and the AHP was punctuated by centennial-scale episodes of hyper-aridity which varied in time and in space. This highly variable environmental scenario was the theatre of one of the most important socio-economic changes in human history, that of the transition between the last hunters-gatherers and the first food production societies. The concomitance of such cultural transformations and climatic disruptions raises the question of the role of changing environmental conditions on the distribution patterns of new socio-economic practices, namely herding.The Ethiopian Highlands (EH) represent a major receiver of monsoonal precipitations in East Africa. During the AHP, substantial variations of the monsoon regime affected the functioning of the hydro-systems originating in the EH. This included the Blue Nile River and Lake Abhe, the endorheic receptacle of the Awash River waters, and located in the northern East African Rift System (EARS). Additionally, the Nile valley and the EARS, constituted the main corridors for prehistoric human trajectories across the Horn of Africa. With the aim to better understand human-climate coevolution during this crucial period a multiscale and multidisciplinary approach is proposed.First, I decipher Holocene climatic variations from the functioning of the two main EH-originated hydro-systems by: a) tracking exceptional Blue Nile paleo-flood frequencies using a high resolution study of a laminated sequence from the Nile Deep Sea Fan deposits, and b) reconstructing the hydrological fluctuations of paleo-Lake Abhe (Ethiopia, Djibouti), from the study of wave-dominated clastic paleo-shoreline sequences. Second, I characterize the impact of hydro-climatic changes on littoral paleolandscape evolution along the Lake Abhe basin. And finally, I estimate the interactions between climatic induced landform dynamics and human occupations with respect to the main socio-economic shifts that occurred over the same lacustrine basin.This study improves our comprehension of the spatial-temporal patterns of Holocene hydro-climate in East Africa: a strong and stable monsoonal regime has been evidenced between ~11 and ~9 ka BP, followed by a stepwise ardification until the end of the AHP. Moreover, belated Mid-Holocene humid conditions in the Northern Rift suggest that the Congo Air Boundary had a significant role on rainfall controls in this area. Additionally, evaluating the interplay between climatic-, geomorphic- and tectonic-induced sedimentary transfer mechanisms over the investigated basins allowed us to localise some hydro-sedimentary processes decoupled from climatic forcing and to propose a diachronic palaeolandscape reconstitution of the Lake Abhe basin for this period.Early Holocene Lake Abhe waters and environments were strongly exploited by hunter-fisher-gatherers until the last lake drop around ~4.5 ka BP, when herding practices were integrated in this region. Associations between prehistoric settlements and Holocene hydrological variations demonstrate how humans were extremely reactive to fluctuating lake environments and how new socio-economic practices were often adapted to specific local to regional environmental trajectories., Au cours du Quaternaire, l’Afrique de l’Est a été le théâtre d’importants changements climatiques et culturels. L’augmentation des précipitations, principalement liée aux paramètres orbitaux de la Terre, a été responsable de la dernière Période Humide Africaine (PHA ; ~14 - ~6 ka). Une période caractérisée par la transgression des grands bassins lacustres, par l’activation des systèmes fluviatiles et par la diffusion de la couverture végétale sur des zones aujourd’hui désertiques. Cependant, le début et la fin de cette période humide n’ont pas été synchrones sur tout le continent et de courts épisodes d’aridité l’ont ponctuée. A ce jour, les mécanismes de ces variabilités millénaire et centennale restent encore fortement débattus. A partir de 14 mil ans, l’augmentation d’intensité de mousson associée aux masses d’air venant de l’océan Atlantique modulée par celle venant de l’océan Indien, semble avoir été le principal moteur de la distribution des précipitations au niveau du Hauts Plateaux Ethiopiens. Ceci a fortement affecté le régime hydrologique des deux grands hydro-systèmes d’Afrique de l’Est : les bassins du Nil et du Lac Abhé. Dans ce scenario de forte variabilité environnementale, un des plus importants changementssocio-économiques de l’histoire de l’Homme a eu lieu, dans la Corne de l’Afrique : la disparition des dernières populations de chasseurs-cueilleurs et l’émergence des premières sociétés de production. La concomitance de ces bouleversements climatiques et culturels pose donc la question du rôle des changements environnementaux dans la diffusion des nouvelles formes de modèles socio-économiques.L’objectif de ce travail de thèse est de mieux comprendre la coévolution Homme-Climat pendant la Période Humide Africaine à partir d’une étude multi-échelle et multidisciplinaire. Premièrement, une reconstitution des variations hydroclimatiques est proposée sur la base d’une comparaison entre : a) les reconstitutions centennales d’intensité et de fréquence des crues du Nil à l’Holocène à partir de l’étude d’une séquence sédimentaire prélevée au sein du Delta Profond du Nil ; b) la reconstitution des fluctuations hydrologiques du paléo-Lac Abhé (Ethiopie, Djibouti) à partir de la documentation et de la datation des anciens paléo-rivages lacustres. Deuxièmement, la caractérisation de l’impact de ces changements hydrologiques sur l’évolution des paléo-paysages littoraux autour du bassin du Lac Abhé a été étudiée. Enfin, l’étude des changements paysagers et morpho-sédimentaires en lien avec les dynamiques chrono-spatiales des occupations humaines dans ce même bassin est abordé.Ce travail de thèse a permis de préciser la compréhension de la distribution spatio-temporelle des changements hydroclimatiques en Afrique de l’Est, caractérisée par une forte activité de la mousson entre ~11 et ~9 ka BP, suivi par une aridification progressive jusqu’à la fin de la PHA. Ensuite, la persistance de conditions humides dans la partie Nord du Rift à l’Holocène Moyen suggère une influence de la Zone de Convergence du Congo dans le contrôle des précipitations dans cette région. La détection des facteurs du contrôle géomorphologiques et tectoniques sur les transferts hydro-sédimentaires qui affectent l’évolution de ces bassins à permit aussi de mieux contraindre les effets de ces changements climatiques sur large et petite échelle. A partir de l’estimation de la réactivité des paléo-paysages sous le forçage de ces changements hydrologiques, nous démontrons une forte mobilité des populations en lien avec les fluctuations hydrologiques du paléolac Abhe. D’abord avec une forte exploitation des ressources aquatiques par des sociétés de chasseurs-pécheurs les et ensuite par l’intégration de l’élevage en concomitance avec l’installation de conditions plus arides. Cette étude a permis de souligner l’évidence d’une adaptation des systèmes socio-économiques Holocènes aux changements hydro-climatiques à l’échelle régionale mais aussi locale.

The seismic effective quality factor (QC) and its frequently dependences or the frequency parameter (n) and attenuation coefficient (δ) for the Earth’s crust and upper mantle of the North Tanzanian divergence zone (East African rift system) were estimated from an analysis of the earthquake coda waves recorded in the SEISMO-TANZ’07 French-Tanzanian seismic experiment. The QC values increase and the n and δ values decrease with increasing frequency and length of the lapse time window. This behavior of the attenuation parameters may be evidence that the degree of heterogeneity of the lithosphere decreases with depth. Comparison of the depth variations in the attenuation coefficient δ and the frequency parameter n with the velocity structure of the region shows that there is a distinct change in the behavior of seismic wave attenuation at velocity discontinuities. The obtained attenuation parameters were compared with the same parameters obtained in our previous studies for other continental rift systems—the Baikal rift system (Eurasia) and the Basin and Range Province (North America).

International audience; The Early Pliocene Kanapoi Formation of the Omo-Turkana Basin consists of two fluvial/deltaic sedimentary sequences with an intermediate lacustrine sequence that was deposited in Paleolake Lonyumun, the earliest large lake in the basin. Overall, the geology and vertebrate paleontology of the Kanapoi Formation are well studied, but its freshwater mollusks, despite being a major component of the benthic ecosystem, have not been subjected to in-depth study. Here I present the first treatment of these mollusks, which have been retrieved mainly from the lacustrine but also from the upper fluvial sediments, with a focus on paleoecological implications. Overall, the freshwater mollusk fauna is reasonably diverse and contains the gastropods Bellamya (Viviparidae), Melanoides (Thiaridae), Cleopatra (Paludomidae) and Gabbiella (Bithyniidae), as well as the unionoid bivalves Coelatura, Pseudobovaria (Unionidae), Aspatharia, Iridina (Iridinidae) and Etheria (Etheriidae). Material is typically recrystallized and lithified and its taphonomy suggests deposition in a system with intermediate energy, such as a beach, with post-depositional deformation and abrasion. The mollusk assemblage is indicative of perennial, fresh and well-oxygenated waters in the Kanapoi region. It suggests that Paleolake Lonyumun had largely open shores with limited vegetation and that swampy or ephemeral backwaters were rare. Overall, these findings support earlier paleoecological interpretations based on the fish assemblage of Paleolake Lonyumun at Kanapoi. Moreover, mollusk assemblages from this lake are very similar across the Omo-Turkana Basin (Nachukui, Usno, Mursi and Koobi Fora Formations) suggesting that the lacustrine paleoecological conditions found in the Kanapoi Formation existed throughout the basin.

International audience; The Okavango Graben (OG) has been considered as the terminus of the southwestern branch of the East African Rift System (EARS) since the 1970's based on fault morphology and early seismic and geophysical data. Thus it is assumed to be an incipient rifting zone, analogous to the early stage of mature rifts in the EARS. Recent geodetic data and geophysical studies in the area bring new insights on the local crust and lithosphere, mantle activity and fault activity. In this study, we computed the velocities for three permanent GPS stations surrounding the graben and undertook a review of the new geophysical data available for the area. The northern and southern blocks of the graben show exclusively a low strike-slip displacement rate of about 1 mm/yr, revealing the transtensional nature of this basin. The seismic record of central and southern Africa is revealed to be instrumentally biased for the events recorded before 2004 and the OG may not represent the most seismically active area in Botswana anymore. Moreover, no significant lithosphere and crustal thinning is found in the tectonic structure as well as no strong negative Bouguer anomaly and surface heat flux. Thus the OG does not match the classical model for a rifting zone. We propose a new geodynamic model for the deformation observed west of the EARS based on accommodation of far-field deformation due to the differential extension rates of the EARS and the displacement of the Kalahari craton relative to the Nubian plate.

National audience; This study is based on a coupled basin infilling study and alandforms analysis of the Lake Albert Rift located at the northernpart of the western branch of the East African Rift.The basin infilling study is based on both subsurface data andoutcrops analysis. The objective was to (1) actualize age model,(2) to reconstruct the 3D architecture of the rift using sequencestratigraphy correlations and seismic data interpretation, (3) tocharacterize the deformation and its changes through times and(4) to quantify the accommodation for several time intervals. Theinfilling essentially consists of isopach fault-bounded units composedof lacustrine deposits wherein were characterized two majorunconformities dated at 6.2 Ma (Uppermost Miocene) and2.7 Ma (Pliocene-Pleistocene boundary), coeval with major subsidenceand climatic changes.The landforms analysis is based on the characterization and relativedating of Ugandan landforms which consist of stepped planationsurfaces and incised valleys. We here proposed a seven-stepsreconstruction of the deformation-erosion-sedimentation relationshipsof the Lake Albert Basin and its catchments:- 55-45 Ma: formation of laterites corresponding to the AfricanSurface during the very humid period of the Lower-Middle Eocene;- 45-22: stripping of the African Surface in response of the beginningof the East-African Dome uplift and formation of a pediplainwhich associated base level is the Atlantic Ocean;- 17-2.5 Ma: Initiation of the Lake Albert Basin around 17 Ma andcreation of local base levels (Lake Albert, Edward and George) onwhich three pediplains tend to adapt;- 18-16 Ma to 6.2 Ma: ”Flexural” stage (subsidence rate: 150-200 m/Ma; sedimentation rate 1.3 km3/Ma between 17 and 12Ma and 0.6 km3/Ma from 12 to 6 Ma) – depocenters location(southern part of Lake Albert Basin) poorly controlled by fault;- 6.2 Ma to 2.5 Ma: Rift stage 1 (subsidence rate: > 500m/Maup to 600-800 m/Ma; sedimentation rate: 2.4 km3/Ma) – Riftingclimax;- 2.5-0.4 Ma: uplift of the Ruwenzori Mountains and shifting froman alluvial system to a network of bedrock river incision – RiftStage 2 (subsidence rate: 450 to 250 m/Ma; sedimentation rate:1.5 km3/Ma);- 0.4-0 Ma: long wavelength downwarping of the Tanzanian Craton,initiation of the Lake Victoria trough, drainage network inversionand uplift of the present-day Ugandan escarpment (normalfaulting motion of the border faults) with formation of perchedvalleys associated to the Lower Pleistocene (2.5-0.4 Ma) riversnetwork.The sediment budget is successful with, between 17 and 2.5 Ma,an excess of 16 % of upstream eroded material compared to thesediment volume deposited that can be explained by the chemicalerosion prevailing at this period in Central Africa. The significant(60%) opposite difference between 2.7 and 0 Ma may be the consequenceof a high sediment supply resulting from the erosion ofthe uplifted Ruwenzori Mountains.

Segments of the Main Ethiopian Rift (MER), a key sector of the East African Rift (EAR) linking the Afar and Turkana depressions, record different stages of lithospheric evolution from initiation to continental break- up and incipient oceanic spreading. Although EAR tectonic and magmatic activity is generally ascribed to amanifestation of sublithospheric mantle processes, the number, depth of provenance and triggering mechanisms of the related mantle upwellings remain topics of debate. We present new Hf and Pb isotope data for EAR mantle xenolith clinopyroxenes from Southern Ethiopia (Mega, the Sidamo region) that compellingly testify to multiple episodes of mantle depletion and metasomatic enrichment. Radiogenic values (epsilon(Hf) up to + 1076) suggest that present-day MER lithospheric mantle domains underwent melting, possibly in the presence of residual garnet, billions of years ago, thereby fractionating (increasing) the Lu/ Hf ratio and ultimately leading to extremely high Hf-176/ Hf-177. Although the precise dating of these depletion episodes is hampered by possible metasomatic overprinting, positively correlated Lu/Hf and Hf-176/Hf-177 indicates apparent ingrowth at ca. 1.9 Ga, providing a minimum age for the delineated Proterozoic melting events. Our findings of Early Proterozoic melting episodes are complementary to previously determined Os model ages on xenoliths from the same site indicating melt extraction between 2.4 and 2.8 Ga (Reisberg et al., 2004; Chem. Geol. 208, 119-140). Taken together, the geochemical and isotopic characteristics of the Mega xenoliths therefore record melt extraction events and preserve memory of ancient mantle dynamics beneath this site, where important lithospheric discontinuities exist between the Archean/Early Proterozoic Tanzanian craton and the Late Proterozoic Panafrican mobile belt. Subsequent metasomatic reactions variably affected the Nd and Hf isotopic compositions of some samples and completely overprinted the Pb isotopic composition of the whole xenolith suite. The persistence of these geochemical heterogeneities within the investigated suites of xenoliths precludes pervasive melt refertilization, which would have homogenized existing compositions and obliterated the record of previous petrologic processes. This suggests that the MER segment considered here developed on a lithospheric section isolated by preexisting tectonic structures, far from the influence of plumes originating deep in the convecting mantle. (C) 2014 Elsevier B.V. All rights reserved.

International audience; In this study, we explore the origin of lower crustal seismicity and the factors controlling rift propagation using seismological data recorded within the youngest part of the East African Rift System, the North Tanzanian Divergence (NTD). Most earthquakes below Lake Manyara occur at depth ranging between 20 and 40 km and have a swarm-like distribution. Focal mechanisms of 26 events indicate a combination of strike-slip and normal faulting involving Archaean basement structures and forming a relay zone. The derived local stress regime is transtensive and the minimum principal stress is oriented N110°E. Crustal seismic tomography reveals low-velocity anomalies below the rifted basins in the NTD, interpreted as localized thermomechanical perturbations promoting fluid release and subsequent seismicity in the lower crust. SKS splitting analysis in the NTD indicates seismic anisotropy beneath 17 stations most likely due to aligned magma lenses and/or dikes beneath the rift and to the lithospheric fabrics. Our results favor a strain pattern intermediate between purely mechanical and purely magmatic. We suggest that melt products arising from a large asthenospheric thermal anomaly enhance lithospheric weakening and facilitate faulting and creeping on critically oriented inherited structures of the Precambrian lower crust. Although the crust is unlikely weakened at a point comparable to other parts of the East African Rift System, this deep-seated thermomechanical process is efficient enough to allow slow rift propagation within the eastern Tanzanian cratonic edge.

International audience; The Rungwe Volcanic Province in SW Tanzania is a densely populated area that is considered volcanically active. As part of the East African Rift System, a significant control of tectonic activity seems to exist on the location and also potential destabilization of volcanic edifices. Three large volcanoes, Ngozi, Rungwe, and Kyejo, dominate the landscape and all show contrasting eruptive behaviour in the recent geological past. Kyejo volcano is a flow-dominated volcano that had a historic lava flow eruption. Lake sediment cores, drilled in Lakes Malawi, Masoko, Rukwa, and Tanganyika, provide a record of frequent explosive eruptions in the last few tens of thousands of years. In combination with on-land stratigraphic observations, they constrain the minimum eruptive frequency of especially Rungwe and Ngozi volcanoes. Both volcanoes had Plinian-style eruptions in the Holocene. The most striking documented Rungwe eruption, the ca. 4 ka Rungwe Pumice, is a rare case of a Plinian eruption in near-wind-free conditions. Furthermore, the Rungwe Pumice, just like any other Rungwe tephra deposit, does not show any evidence of pyroclastic density current deposits. Apart from explosive eruptions at a range of scales happening every few hundred years at Rungwe, the volcano also experienced at least two sector collapse events generating debris avalanches. All existing evidence shows that the Rungwe Volcanic Province is prone to future significant explosive eruptions. To further assess, quantify and mitigate volcanic hazard risks, extensive and systematic multidisciplinary geological research, and both volcanic and tectonic monitoring are needed.

International audience; The 'wet' early to mid-Holocene of tropical Africa, with its enhanced monsoon, ended with an abrupt shift toward drier conditions and was ultimately replaced by a drier climate that has persisted until the present day. The forcing mechanisms, the timing, and the spatial extent of this major climatic transition are not well understood and remain the subject of ongoing research. We have used a detailed palaeo-shoreline record from Lake Turkana (Kenya) to decipher and characterise this marked climatic transition in East Africa. We present a high-precision survey of well-preserved palaeo-shorelines, new radiocarbon ages from shoreline deposits, and oxygen-isotope measurements on freshwater mollusk shells to elucidate the Holocene moisture history from former lake water-levels in this climatically sensitive region. In combination with previously published data our study shows that during the early Holocene the water-level in Lake Turkana was high and the lake overflowed temporarily into the White Nile drainage system. During the mid-Holocene (~ 5270 ± 300 cal. yr BP), however, the lake water-level fell by ~ 50 m, coeval with major episodes of aridity on the African continent. A comparison between palaeo-hydrological and archaeological data from the Turkana Basin suggests that the mid-Holocene climatic transition was associated with fundamental changes in prehistoric cultures, highlighting the significance of natural climate variability and associated periods of protracted drought as major environmental stress factors affecting human occupation in the East African Rift System.