The tectonic interaction, linkage, and coalescence of propagating continental rift segments eventually create a through-going axial rift floor without which a break-up axis cannot develop. However, prior to linkage, interacting rifts are separated by a topographic basement-high (rift interaction zone, RIZ) which is progressively dismembered and down-thrown by the lateral propagation of rift-tip faulting and their hanging wall subsidence. Here, we explore the evolution of the Middle Shire and Nsanje RIZs located along three contiguous non-volcanic propagating rift segments: the southern Malawi Rift (SMR), Lower Shire Graben (LSG), and the Nsanje Graben (NG), East Africa. The Middle Shire RIZ is an overlapping-oblique divergent RIZ in which the NNE/N-trending SMR is propagating southwards into the shoulder of the NW-trending LSG, whereas the Nsanje RIZ is a tip-to-tip oblique RIZ in which the LSG has propagated southeast into the northern tip of the N-trending NG. We utilize field observations and a landscape evolution model with implemented fault displacement fields of two contiguous RIZs with contrasting geometries, to simulate their geomorphic evolution, and apply a static stress model to evaluate the stress transfer patterns during RIZ evolution. The model results provide insights into the natural observations in the study area, in which, with progressive extension and tip growth, the Middle Shire RIZ maintains minor basement down-throw and an unequilibrated axial stream profile, which contrasts the widespread basement burial and equilibrated axial stream profile across the Nsanje RIZ. Modeled static stress distribution predicts compounding stress concentrations at tip-to-tip RIZs (synthetic border fault interactions), favoring brittle strain localization and rift coalescence, and stress relaxation at overlapping divergent RIZs (antithetic border fault interactions), favoring stalled rift coalescence. We argue that RIZ and rift border fault geometries, and their kinematics strongly influence the pace of rift coalescence by modulating the spatial distribution of tectonic stresses necessary to promote rift-linking deformation.

We investigate the upper-crustal structure of the Rukwa-Tanganyika Rift Zone, East Africa, where earthquakes anomalously cluster at the northwestern tip of the Rukwa Rift, the eastern tip of the Mweru-Wantipa Rift, and along the Tanganyika Rift axis. The current rift tips host distributed faulting in exposed basement with little sedimentation. Here, we invert earthquake P and S travel times to produce three-dimensional upper-crustal velocity models for the region. The resulting models reveal the occurrence of anomalously high Vp/Vs ratios that extend from the surface down to ca. 10 km at the tip zones of the Rukwa and Mweru-Wantipa rifts. The spatial association of distributed faulting, upper-crustal seismicity, and thermal anomalies with the high Vp/Vs ratios suggest a weakened crust below the rift tips’ flexural zone. We propose an ongoing strain localization and crustal softening at the rift tips that is accommodated by brittle damage from bending strain, and potentially compounded by hydrothermal weakening. This setting represents a precursory phase that may initiate unilateral rift tip propagation.

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.

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.

The onshore continental margins of western Central Africa have been hosting potentially damaging earthquake events for decades; yet, the links between the seismicity, the contemporary stress field, and pre-existing faults are not well understood. Here, we analyze the regional stress fields along the coastal margin and interior cratonic areas using earthquake focal mechanisms, map and characterize the detailed structure of preexisting fault systems in outcrops, and assess the reactivation potential of the mapped structures. Our results show that the earthquakes originate under a transpressive stress regime with a horizontal maximum principal compressive stress (σ1) that is oriented NNE-SSW. We show that regional stresses acting on offshore oceanic fracture zones are compatible with those acting along with the onshore areas of the continental margin. Field observations reveal the presence of large fault systems that deform both the Precambrian basement and Phanerozoic sedimentary sequences, with widespread hydrothermal alterations of calcite veining, quartz veining, and palygorskite mineralization along the fault zones. Along the margin, the preexisting NNE-, NNW-, and N-S -trending strike-slip faults and normal faults show a high slip tendency (60 – 100 %), ), whereas in the cratonic interior, the NW- and N-S -trending thrust faults are the most likely to reactivate. We argue that favorable orientation of the preexisting faults and potentially, their hydrothermal alteration products, define the susceptibility of the faults to seismic reactivation. We propose that possible stress propagation into the near-shore and onshore tip zones of oceanic fracture zones may be driving stress loading on pre-stressed fault systems onshore.

Continental rifting is a critical component of the plate tectonic paradigm, and occurs in more than one mode, phase, or stage. While rifting is typically facilitated by abundant magmatism, some rifting is not. We aim to develop a better understanding of the fundamental processes associated with magma-poor (dry) rifting. Here, we provide an overview of the NSF-funded Dry Rifting In the Albertine-Rhino graben (DRIAR) project, Uganda. The project goal is to apply geophysical, geological, geochemical, and geodynamic techniques to investigate the Northern Western Branch of the East African Rift System in Uganda. We test three hypotheses: (1) in magma-rich rifts, strain is accommodated through lithospheric weakening from melt, (2) in magma-poor rifts, melt is present below the surface and weakens the lithosphere such that strain is accommodated during upper crustal extension, and (3) in magma-poor rifts, there is no melt at depth and strain is accommodated along pre-existing structures such as inherited compositional, structural, and rheological lithospheric heterogeneities. Observational methods in this project include: passive seismic to constrain lithospheric structure and asthenospheric flow patterns; gravity to constrain variations in crustal and lithospheric thickness; magnetics to constrain the thermal structure of the upper crust; magnetotellurics to constrain lithospheric thickness and the presence of melt; GNSS to constrain surface motions, extension rates, and help characterize mantle flow; geologic mapping to document the geometry and kinematics of active faults; seismic reflection analyses of intra-rift faults to document temporal strain migration; geochemistry to identify and quantify mantle-derived fluids in hot springs and soil gases; and geodynamic modeling to develop new models of magma-poor rifting processes. Fieldwork will begin in January 2022 and the first DRIAR field school is planned for summer 2022. Geodynamic modeling work and morphometric analyses are already underway.

We present the Malawi Active Fault Database (MAFD), a geospatial database of 114 active fault traces in Malawi, and in neighboring Tanzania and Mozambique. The MAFD has been developed from a multidisciplinary dataset: high resolution digital elevation models, field observations, aeromagnetic and gravity data, and seismic reflection surveys from offshore Lake Malawi. Active faults longer than 50 km are found throughout Malawi, where seismic risk is increasing due to its rapidly growing population and its seismically vulnerable building stock. The MAFD also provides an opportunity to investigate the population of normal faults in an incipient continental rift. We find that the null hypothesis that the distribution of fault lengths in the MAFD is described by a power law cannot be rejected. Furthermore, a power-law distribution of faults in Malawi is consistent with its thick seismogenic crust (~35 km), and low (<8%) regional extensional strain that is predominantly (50-75%) accommodated across relatively long hard-linked border faults. Cumulatively, the data and inferences drawn from the MAFD highlight the importance of integrating onshore and offshore geological and geophysical data to develop active fault databases along the East African Rift and similar continental settings, both to understand the regional seismic hazard and tectonic evolution.