One of the fundamental problems in continental rift segmentation and propagation is how strain is accommodated along large rift-bounding faults (border faults) since the segmentation of propagating border faults control the expression of rift zones, syn-rift depo-centers, and long-term basin evolution. In the southern Malawi Rift, where previous studies on the early-stage rifting only assessed border fault structure from surficial and topographic expression, we integrate surface and subsurface data to investigate border fault segmentation, linkage, and growth as proxies for strain accommodation along the Bilila-Mtakataka Fault (BMF) System. We used 30 m-resolution topographic relief maps, electrical resistivity tomography (ERT), and high-resolution aeromagnetic data to characterize the detailed fault geometry and provide a more robust estimate of along-fault displacement distribution. Our results reveal a discrepancy between sub-aerial segmentation of the BMF geometry (6 segments), scarp height (5 segments) reflecting the most recent fault offset, and cumulative throw (3 segments) reflecting the long-term fault offset. We also observe that although the BMF exhibits continuity of sub-aerial scarps along its length, the throw distribution shows a higher estimate at the Northern-to-Central segment relay zone (423 m absolute, 364 m moving median) compared to the Central-to-Southern segment relay zone (371 m absolute, 297 m moving median). The ERT profiles across the relay zones suggest a shallower basement and a possible canyon-mouth alluvial fan stratigraphy at the Central-to-Southern segment relay zone, which contrasts the deeper basement and ‘simpler’ electrical stratigraphy at the Northern-to-Central relay. The results suggest a more complex long-term evolution of the BMF than was assumed in previous studies. A comparison of maximum throw-length estimates of the BMF with those of other well-studied Malawi Rift border faults and global normal fault populations suggest that although the BMF has possibly reached its maximum length, just as the other border faults, it remains largely under-displaced. We suggest that the BMF may continue to accrue significant strain as tectonic extension progresses in southern Malawi Rift, thus posing a major seismic hazard in the region.

Low-temperature thermochronology studies record Miocene aged rift initiation of the northern Malawi Rift. However, no studies are available from the southern Malawi Rift and the Shire Rift, which are thought to have initiated at a later time. Here we present thermal history models derived from new apatite fission-track and (U-Th)/He data from the footwalls of major border faults of the southern Malawi Rift, that reveal three distinct cooling episodes in the Cretaceous, Eocene–Oligocene, and Late Oligocene–Pliocene. These results suggest that the southern Malawi Rift has been accommodating strain along its border faults since the Miocene, just as in the northern Malawi Rift. The timing and rate of extensional strain was further constrained through the application of remote sensing. These results, when combined with our thermal history modeling, yield inferred deformation strain rates that support linkage between the modern Malawi Rift and the older Shire Rift. Cooling histories show that the border faults of the southern Malawi Rift have likely been active since the Late Oligocene - Early Miocene and that this activity has caused linkage and transfer of strain to the older Shire Rift which our results suggest to have been reactivated since the Miocene too. These results provide evidence of the coeval onset of extension along the full length of Malawi Rift and possibly the Western Branch of the East African Rift System.

We investigate the spatiotemporal patterns of strain accommodation during multiphase rift evolution in the Shire Rift Zone (SRZ), East Africa. The NW-trending SRZ records a transition from magma-rich rifting phases (Permian-Early Jurassic: Rift-Phase 1 (RP1), and Late Jurassic-Cretaceous: Rift-Phase 2 (RP2)) to a magma-poor phase in the Cenozoic (ongoing: Rift-Phase 3 (RP3)). Our observations show that although the rift border faults largely mimic the pre-rift basement metamorphic fabrics, the rift termination zones occur near crustal-scale rift-orthogonal basement shear zones (Sanangoe (SSZ) and the Lurio shear zones) during RP1-RP2. In RP3, the RP1-RP2 sub-basins were largely abandoned, and the rift axes migrated northeastward (rift-orthogonally) into the RP1-RP2 basin margin, and northwestward (strike-parallel) ahead of the RP2 rift-tip. The northwestern RP3 rift-axis side-steps across the SSZ, with a rotation of border faults across the shear zone and terminates further northwest at another regional-scale shear zone. We suggest that over the multiple pulses of tectonic extension and strain migration in the SRZ, pre-rift basement fabrics acted as: 1) zones of mechanical strength contrast that localized the large rift faults, and 2) mechanical ‘barriers’ that refracted and possibly, temporarily halted the propagation of the rift zone. Further, the cooled RP1-RP2 mafic dikes facilitated later-phase deformation in the form of border fault hard-linking transverse faults that exploited mechanical anisotropies within the dike clusters and served as mechanically-strong zones that arrested some of the RP3 fault-tips. Overall, we argue that during pulsed rift propagation, inherited strength anisotropies can serve as both strain-localizing, refracting, and transient strain-inhibiting tectonic structures.