Neogene–Quaternary initiation of the Southern Malawi Rift and linkage to the reactivated Carboniferous–Jurassic Shire Rift

Low-temperature thermochronology studies record Miocene rift initiation of the Northern Malawi Rift. However, no such studies are available that constrain the onset time of rifting of the Southern Malawi Rift, and Cenozoic reactivation of the older Carboniferous-Jurassic Shire Rift. Here we present thermal history models derived from new apatite fission-track and (U-Th-Sm)/He data from the footwalls of major border faults of the Southern Malawi Rift and Shire Rift. Results reveal three distinct cooling episodes: fast cooling in the Cretaceous, slow cooling from the Late Cretaceous until the Miocene, and fast cooling starting in the Late Miocene–Pliocene. The latter phase of cooling indicates that strain along the border faults of the Southern Malawi Rift was initiated at the same time as the Northern Malawi Rift. The timing and rate of extension were further constrained using remote sensing to map the fault networks of the Southern Malawi and Shire rifts and estimate fault displacement and geometry. These results, when combined with our thermal history modeling, yield inferred deformation strain rates that support the linkage between the modern Malawi Rift and the reactivation of border faults of the older Shire Rift since the Miocene. These results support the coeval onset of extension along the full length of the Malawi Rift, with an overall southward decrease in total extension instead driven by a southward decrease in plate-scale cumulative strain rates along the Western Branch of the East African Rift System.