Geology of the Malawi Rift: kinematic and tectonosedimentary background to the Chiwondo Beds, northern Malawi

Abstract

Abstract Kinematic analysis of Neogene and Quaternary faults demonstrates that the direction of extension in the Malawi Rift rotated from east-northeast to southeast. Rift development commenced at about 8·6 Ma with normal faulting causing erosion and re-sedimentation of Mesozoic deposits and formation of half-grabens bounded by northwest-, north-, and northeast-striking normal faults. Continued normal faulting led to block faulting and basin subsidence which created a riverine system and eventually formed a rift lake (Palaeo-Lake Malawi). Based on the onshore sedimentary record an age of 4·5 to 4·0 Ma appears likely for initial lacustrine sedimentation. This phase of normal faulting is largely responsible for the present-day basin morphology of Lake Malawi. A major change in fault kinematics and also in the sedimentation pattern occurred due to rotation of the principal extension and shortening axes, probably between 0·2 Ma. As a result, the structural pattern inherited from the first rifting phase was no longer suitably oriented to accommodate extensional deformation and strike-slip faulting became more important. The strike-slip regime may have caused uplift and erosion of the lake sediments and of basement ridges within the rift and along the rift flanks in regions of local transpression, but also created local sedimentary basins due to transtension. Older faults that show mainly the first deformation increment are restricted to the outermost parts of the rift. Toward the center, the faults depict an increase in strike slip components of movement, suggesting deformation propagation toward the rift center, which results in a narrowing of the active rift environments with time.