Structural evolution of the Kilombero rift basin in central Tanzania
Detailed geological and structural investigations at the northwestern scarp of the Cenozoic Kilombero Rift allow the drawing of its structural evolution and establishment of stress conditions that prevailed at the different deformational episodes at this rift zone. The structure, where the northwestern scarp of the Cenozoic Kilombero Rift System is located, starts by ductile deformation of granitic and gabbroic rock masses leading to the formation gneissic fabric and b-mineral lineation in the rocks. This deformation is of Precambrian age (most probably a Pan African deformation) and its kinematics are characterized by 320˚/20˚ tectonic transport direction (λ1). Later the area was subjected to at least two brittle deformations (faults and joints) of Permo-Triasic and Cenozoic ages. These brittle deformations developed E-W trending (SET-1) and NE-SW trending (SET-2) conjugate discontinuities (joints and faults). All of them have very high dip angles (about 80˚) and they dip to the south and southeast, respectively. Their configurations imply kinematics with NNW-SSE trending sub-horizontal λ1, sub-vertical λ2, and ENE-WSW trending sub-horizontal λ3 This trend suggests that the stress regime that accounts for their formation had ENE trending sub-horizontal σ1 maximum compressive stress, sub-vertical σ2 intermediate compressive stress and a NNW trending sub-horizontal σ3 minimum compressive stress.
Most of these discontinuities show reverse sense of displacement different form the main Kilombero fault (which has a normal throw) though both of them have the same trend. One could argue that the discontinuities with reverse displacement pre-dates the main Cenozoic Kilombero rift, the later being formed by reactivation of the earlier. The discontinuities with reverse displacement could be of Permo-Triasic age associated with Karoo tectonics. It is also possible that the reverse discontinuities and the Kilombero rift are coeval but the reverse sense of displacement in the earlier is induced by large-scale block rotation. In this case, both the reverse discontinuities and the Kilombero rift could be of Permo Triasic age but reactivated during the Cenozoic period particularly along the main Kilombero Rift. However, both the reverse discontinuities and the Kilombero rift could also be of Cenozoic age and that the imprints of Permo-Triasic tectonics are missing in the study area. The Permo-Triastic deformation imprints could be developed further to the southeast of the area where they are not concealed by the Cenozoic cover in the Kilombero plain. Dating of these discontinuities is therefore recommended in order to distinguish those of Permo-Triasic and Cenozoic ages.