Normal vs. strike-slip faulting during rift development in East Africa: The Malawi rift

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 with the formation of half-grabens bounded by northwest-, north-, and northeast-striking normal faults. Owing to slightly oblique rifting, the northwest-striking faults in the northernmost rift segment show a small dextral oblique-slip component, whereas north- and northeast-oriented faults in the central part of the rift display a sinistral oblique-slip component. This first event resulted in block faulting and basin subsidence, which is largely responsible for the present-day basin morphology of Lake Malawi. A major change in fault kinematics occurred because of rotation of the extension direction and permutation of the principal compressive and intermediate axes. The structural pattern inherited from the first rifting phase was no longer suitably oriented to accommodate extensional deformation, and strike-slip faulting assumed a major role. The strike-slip regime amplified uplift of basement ridges within the rift in regions of local transpression, but it also created alluvial basins because of local transtension. This new kinematic style is compatible with the recent seismicity. Older faults that show mainly the first deformational 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.