Heterogeneous Strain Distribution in the Malawi (Nyasa) Rift, East Africa: Implications for Rifting in Magma-Poor, Multi-Segment Rift Systems

Half-graben basins bounded by border faults typify early-stage continental rifts. Deciphering the role that intra-rift faults play in rift basin development is challenging as patterns of early-stage faulting are commonly overprinted by subsequent deformation; yet the characterization of these faults is crucial to understand the fundamental controls on their evolution, their contribution to rift opening, and to assess their seismic hazard. By integrating multiple offshore seismic reflection data sets with age-dated drill core, late-Quaternary and cumulative faulting patterns are characterized in the Central and South Basins of the Malawi (Nyasa) Rift, an active, early-stage rift system. Almost all intra-rift faults offset a late-Quaternary lake lowstand surface, suggesting they are active and should be considered in hazard assessments. Fault throw profiles reveal sawtooth patterns indicating segmented slip histories. Observed extension on intra-rift faults is approximately twice that predicted from hanging wall flexure of the border fault, suggesting that intra-rift faults accommodate a proportion of the regional extension. Cumulative and late-Quaternary throws on intra-rift faults are correlated with throw measured on the border fault in the Central Basin, whereas an anticorrelation is observed in the South Basin. Viewed in a regional context, these differences do not relate solely to the proposed southward younging of the rift. Instead, it is inferred that the distribution of extension is also influenced by variations in lithospheric structure and crustal heterogeneities that are documented along the rift axis. We carried out a detailed fault analysis of the Malawi (Nyasa) Rift integrating drill core and seismic reflection data65% and 49% of the observed extension on intra-rift faults in the Central and South Basins is explained by regional tectonic extensionSpatio-temporal patterns of throw indicate the migration of extension onto intra-rift faults may be ongoing in the Central Basin