Joël Rolet

Neogene-Holocene rift propagation in central Tanzania: Morphostructural and aeromagnetic evidence from the Kilombero area

Based on field studies supplemented by remote sensing and aeromagnetic data from central Tanzania, a Phanerozoic structural history for the region can be developed and placed in a broader rift context. The major contribution of this work is the recognition of rift morphology over an area lying 400 km beyond the southern termination of the Eastern, or Kenya, Rift. The most prominent rift structures occur in the Kilombero region and consist of a wide range of uplifted basement blocks fringed to the west by an east-facing half-graben that may contain 6–8 km of sedimentary strata. Physiographic features and river drainage anomalies suggest that Holocene/Neogene deformation occurs along both rift-parallel and transverse faults, in agreement with the seismogenic character of a number of oblique faults. The present-day rift pattern of the Kilombero extensional province results from the complete overprinting of an earlier (Karoo) rift basin by Neogene- Holocene faults. The Kilombero rift zone is assumed to connect northward into the central rift arm (Manyara) of the Eastern Rift via an active transverse fault zone. The proposed rift model implies that incipient rifting propagates throughout the cold and strong lithosphere of central Tanzania following Proterozoic basement weakness zones (N140°E) and earlier Karoo rift structures (north-south). An eventual structural connection of the Kilombero rift zone with the Lake Malawi rift further south is also envisaged and should imply the spatial link of the eastern and western branches of the East African Rift System south of the Tanzanian craton.

Young rift kinematics in the Tadjoura rift, western Gulf of Aden, Republic of Djibouti

The Tadjoura rift forms the westernmost edge of the westerly-propagating Sheba ridge, between Arabia a nd Somalia, as it enters into the Afar depression. Fro m structural and remote sensing dataset, the Tadjou ra rift is interpreted as an asymmetrical south-facing half-gr aben, about 40 km-wide, dominated by a large boundary fault zone to the north. It is partially filled up by the 1-3 Ma-old Gulf Basalts which onlapped the older S omali Basalts along its shallower southern flexural margin. The m ajor and trace element analysis of 78 young onshore lavas allows us to distinguish and map four distinct basa ltic types, namely the Gulf, Somali, Goumarre and H ayyabley Basalts. These results, together with radiometric a ge data, lead us to propose a revised volcano-strat igraphic sketch of the two exposed Tadjoura rift margins, an d to discriminate and date several distinct fault n etworks of this oblique rift. Morphological and statistical an alyses of onshore extensional fault populations sho w marked changes in structural styles along-strike, in a dir ection parallel to the rift axis. These major fault disturbances are assigned to the arrest of axial fault tip propagati on against pre-existing discontinuities in the NS-o riented Arta transverse zone. According to our model, the sinist ral jump of rifting into the Asal-Ghoubbet rift seg ment results from structural inheritance, in contrast with the en echelon or transform mechanism of propagation that prevail ed along the entire length of the Gulf of Aden extensi onal system.