Abstract

The Malawi Rift is the southern segment of the western branch of the East African Rift System. There has been little agreement as to whether this rift is opening perpendicularly or obliquely to its north-south axis, and little agreement on a mechanism for the formation of the asymmetric basins that make up the rift. Our analysis combines Landsat image interpretation and field measurement of the latest slip direction on faults to produce a kinematic model of rift evolution.

The fault-slip data are consistently compatible with a late Cenozoic northwest-southeast-directed divergence between the Nubian and Somalian blocks (plates) across the rift. The northwest-striking Livingstone fault at the northern limit of the Malawi Rift and the northwest-striking Zambezi fault zone to the south serve as dextral intracontinental transform zones between left-stepping segments of the western branch of the East African Rift System. Within the rift, second-order northwest-striking transfer faults separate adjacent asymmetric basins. The transform and transfer faults form a pattern that is similar to overlapping, right-stepping strike-slip faults with a releasing geometry. Fault surfaces with two sets of striations indicate a progression from predominantly strike-slip toward oblique normal faulting. We hypothesize that during initial rift formation most northwest-striking faults were dextral, and that fault interaction between these structures resulted in curved stress trajectories and a pull-apart component. Arcuate north- to northeast-striking border faults and subsiding basins then formed between these northwest-striking faults in response to the right-stepping dextral shear. The maximum distance over which fault interaction is significant may control basin dimensions. As opening of the rift proceeds, slip partitioning and gravity failure result in increasing normal-slip perpendicular to major escarpments and decreasing strike-slip parallel to them.

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