The Eastern Branch of the East African Rift System diverges from a single, c. 50 km wide rift in southern Kenya to a c. 200 km wide zone in northern Tanzania, where it is comprised of three distinct rifts with different orientations. The western part of this zone contains two rift branches: the Natron-Man yara-Balangida and Eyasi-Wembere rifts. Each rift contains individual basins that are defined here on the basis of structural and geophysical interpretations. These basins are shallow (<3km) and total extension across the bounding faults is small. New K/Ar age determinations on basalts from the western rift basins show that volcanism and sedimentation began in the area at c. 5 Ma. Major fault escarpments were present by c. 3 Ma and the present-day rift escarpments developed later than c. 1.2 Ma. Pre-rift volcanism produced large shield volcanoes of a basalt-trachyte-phonolite association that now lie on the rift flanks. Volcanism after the main phase of rift faulting produced volatile- and alkali-rich explosive centres which are active today, and have no equivalent in southern Kenya. The change in morphology of the Eastern Branch of the East African Rift System, and the style of volcanism in northern Tanzania, may be the result of the transition from the rifting of Proterozoic Mozambique Belt lithosphere to the rifting of cratonic Archaean lithosphere.

Long (>50km) normal faults bound one or both sides of narrow basins within the East African Rift System, but the dimensions and internal geometry of individual basins vary along its length. We examine basins in N Tanzania that developed in Archaean and Late Proterozoic (Pan-African) crust, and the relationship of Neogene-Recent faulting and volcanism to pre-existing lithospheric structure. In northern Tanzania the c . 50 km-wide Eastern (Gregory) Rift splays into three seismically active arms, with active extension distributed across a 200 km wide zone. We use new gravity and aeromagnetic data, and existing seismic, gravity, heat flow, and geochemical data to model lithospheric structure beneath the Archaean craton and Proterozoic orogenic belts, and its influence on rift development in N Tanzania. Depths to source bodies determined from Euler deconvolution of aeromagnetic data (1 km grid) indicate that c . 40 km wide basins are less than 3.5 km deep, with basin depths decreasing to the south, consistent with depths estimated from gravity anomalies. Asymmetric basins in the Archaean and zone of reworked Archaean and Proterozoic nappes are bounded by unusually long (100 km) border faults associated with seismicity to depths >25 km. Estimates of flexural rigidity, or effective elastic thickness (TJ, suggest that the lithosphere beneath the Tanzania craton, including the western rift arm, is stronger (64 ± 5 km) than that beneath the Proterozoic belt and the transition zone (30 ± 4 km), with the lowest T e values found beneath the central rift arm (23 +2 4 km). Heat flow, seismicity, and mantle xenolith data also indicate that the lithosphere beneath the Archaean craton was and is colder and stronger than the post-Archaean lithosphere. Geophysical and geochemical data suggest that (a) pre-existing heterogeneities in the Archaean crust influenced the orientation of border faults bounding basins, and (b) that topography at the base of the lithosphere guided the location of rifting in Tanzania, producing a broader rift zone. These results indicate the persistence of a deep cratonic root despite the impingement of a mantle plume in Cenozoic time.

International audienceNew intermediate-resolution, normal-incidence seismic reflection profiles from Lake Tanganyika's central basin capture dramatic evidence of base-level change during two intervals of the late Pleistocene. Four seismically-defined stratigraphic sequences (A­D) tied to radiocarbon-dated sediment cores provide a chronology for fluctuating environmental conditions along the Kalya Platform. Stacked, oblique clinoforms in Sequence C are interpreted as prograding siliciclastic deltas deposited during a major regression that shifted the paleo-lake shore ~21 km towards the west prior to ~106 ka. The topset-to-foreset transitions in these deltas suggest lake level was reduced by ~435 m during the period of deposition. Mounded reflections in the overlying sequence are interpreted as the backstepping remnants of the delta system, deposited during the termination of the lowstand and the onset of transgressive conditions in the basin. The youngest depositional sequence reflects the onset of profundal sedimentation during the lake level highstand. High amplitude reflections and deeply incised channels suggest a short-lived desiccation event that reduced lake level by ~260 m, interpreted as a product of Last Glacial Maximum (32­14 ka) aridity. Paleobathymetric maps constructed for the two interpreted regressions reveal that despite the positive lake-floor topography created by the Kavala Island Ridge Accommodation Zone, Lake Tanganyika remained a large, mostly connected water body throughout the late Pleistocene. The results of this analysis further imply that Lake Tanganyika is the most drought resistant water body in the East African tropics, and may have acted as a refuge for local and migrating fauna during periods of prolonged aridity

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The largest natrocarbonatite lava flow eruption ever documented at Oldoinyo Lengai, NW Tanzania, occurred from March 25 to April 5, 2006, in two main phases. It was associated with hornito collapse, rapid extrusion of lava covering a third of the crater and emplacement of a 3-km long compound rubbly pahoehoe to blocky aa-like flow on the W flank. The eruption was followed by rapid enlargement of a pit crater. The erupted natrocarbonatite lava has high silica content (3% SiO 2 ). The eruption chronology is reconstructed from eyewitness and news media reports and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data, which provide the most reliable evidence to constrain the eruption's onset and variations in activity. The eruption products were mapped in the field and the total erupted lava volume estimated at 9.2±3.0×10 5 m 3 . The event chronology and field evidence are consistent with vent construct instability causing magma mixing and rapid extrusion from shallow reservoirs. It provides new insights into and highlights the evolution of the shallow magmatic system at this unique natrocarbonatite volcano.