Controls on Early‐Rift Geometry: New Perspectives From the Bilila‐Mtakataka Fault, Malawi

We use the ∼110‐km long Bilila‐Mtakataka fault in the amagmatic southern East African Rift, Malawi, to investigate the controls on early‐rift geometry at the scale of a major border fault. Morphological variations along the 14 ± 8‐m high scarp define six 10‐ to 40‐km long segments, which are either foliation parallel or oblique to both foliation and the current regional extension direction. As the scarp is neither consistently parallel to foliation nor well oriented for the current regional extension direction, we suggest that the segmented surface expression is related to the local reactivation of well‐oriented weak shallow fabrics above a broadly continuous structure at depth. Using a geometrical model, the geometry of the best fitting subsurface structure is consistent with the local strain field from recent seismicity. In conclusion, within this early‐rift, preexisting weaknesses only locally control border fault geometry at subsurface. Plain Language Summary This study investigates the controls to fault and rift geometry for a young rift system. Our case study is a large fault in southern Malawi. The geometry of the fault at the surface, the scarp and structures within the rocks the fault cuts through, is measured. We also calculate the scarp height. The orientation and height of the fault scarp implies that local stresses influence its geometry, not regional stresses. We develop a geometrical model to link the scarp and a fault at depth. The best fit model is consistent with the hypothesis that the local stress influences the current fault geometry. We propose that a weak zone at depth may also influence the fault geometry, but weaknesses near the surface only locally influence fault orientation. Key Points The segmented 110‐km long Bilila‐Mtakataka fault scarp is oriented oblique to the current regional extension direction The surface expression is compatible with the upward propagation of a buried weak zone that fits the local stress field High‐grade metamorphic foliation locally influences scarp trend at the surface