NEOGENE - QUATERNARY RIFTING OF THE SOUTHERN MALAWI RIFT AND LINKAGE TO THE LATE CARBONIFEROUS – EARLY JURASSIC SHIRE RIFT

The timing and rate of strain accommodation provide insight into the tectonic evolution along early-stage magma-poor continental rift systems like the Malawi Rift. Previous thermal history models have supported Miocene aged rift initiation in northern Malawi Rift, but the thermal history of southern Malawi Rift and its interactions with the relic of the older Paleozoic-Mesozoic Shire Rift system just south of it remain poorly understood.

We investigated the thermal history of the southern Malawi Rift and Shire Rift border faults using low temperature thermochronology (apatite fission tracks) and estimated the strain rates (extension rates and slip rates) from the thermal history data. The timing and rates of rock uplift were further constrained through application of remote sensing fracture analyses.

Here we present new thermal history models from new apatite fission-track data, combined with previously published thermochronology data, which outline three distinct cooling episodes in the Cretaceous, Eocene–Oligocene, and Miocene–Pliocene for the southern Malawi Rift and the Shire Rift.

Modeling results from 13 samples suggest that normal fault footwall uplift related to Cenozoic rifting in the southern Malawi Rift began in the early Miocene (20-25 Ma) just as the northern Malawi Rift. This growth and propagation has caused linkage and transfer of strain between southern Malawi Rift and the older Shire Rift, which appears to have been reactivated and accommodating strain since the Pliocene. Slip rates for the southern Malawi Rift border faults were estimated at 0.10 – 0.57 mm/yr; and extension rates were estimated at 0.03 – 0.40. mm/yr. These results, when combined with our thermal history modeling results, yield inferred deformation strain rates that support linkage between the two rift systems. The differences in the magnitude of strain rates between the northern and southern Malawi rift also explains the variation in the pattern of strain accommodation across the coeval rift systems. These results provide evidence of coeval extension across the Malawi Rift, and the Western Branch of the East African Rift System which has implications for the tectonic evolution of the East African Rift system.