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.

Within the Western Branch of the East African Rift (EAR), volcanism is highly localized, which is distinct from the voluminous magmatism seen throughout the Eastern Branch of the EAR. Voluminous magmatism in the Eastern Branch results from plume-lithosphere interactions, but the origin of magmatism in the Western Branch remains enigmatic. Previous investigations of melt generation beneath the Rungwe Volcanic Province (RVP), the southernmost volcanic center in the Western Branch, suggest plume materials are present. Here, we develop a model of tomography-based convection (TBC) with melt generation to test the hypothesis that melt beneath the RVP is sourced from plume materials. To test our hypothesis, we use seismically constrained lithospheric thickness and sublithospheric mantle structure to develop a fully adiabatic 3D thermomechanical model of TBC with melt generation using ASPECT. We test a range of mantle potential temperatures and find values ranging from 1250-1350 °C are unable to generate melt beneath the RVP. However, when the sublithospheric mantle temperature is increased by ~250 K based on constraints from shear wave velocity anomalies, decompression melt generation occurs at a maximum depth of ~150 km beneath the RVP. Our work suggests that excess sublithospheric mantle temperatures are necessary for melt generation beneath the RVP, and that shear wave velocity anomalies can provide a first order estimate of these anomalous mantle conditions. Excess sublithospheric mantle temperature in the RVP suggests the influence of a plume-source for the seismic anomalies and supports existing geochemical interpretations of a mantle plume contribution to magmatism in the RVP.