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Lithospheric weakening mechanisms in non-volcanic segments of active continental rifts remain poorly understood, raising important questions about the geodynamic processes that drive magma-poor rifting. Here, we investigate the crustal and uppermost mantle structure beneath the non-volcanic Albertine-Rhino Graben (ARG) and the adjoining volcanic Edward-George Rift (EGR), East Africa. The ARG exhibits anomalous focusing of intra-rift faulting typically associated with magma-rich, early-stage rifts. Through field observations of rift structures, combined with 3D inversions and 2D forward modeling of gravity data, we investigate the potential controls on intra-rift tectonic strain in a setting with little to no magmatism. Field ground-truthing in the southern ARG reveals prominent rift-axial basement-rooted faulting that post-dates the establishment of border faults. Gravity inversion results show low-density anomalies extending from the surface to about 50 km depth beneath both the EGR and southern ARG, with the strongest anomalies under the ARG at around 15 km. 2D gravity modeling suggests that the lower crust and uppermost mantle are both thinned and less dense beneath these rift segments. In the EGR, crustal thinning and low-density anomalies align with low P-wave velocity zones, suggesting the presence of melt. Given the similar degree of crustal thinning and de-densification in the southern ARG, we infer that trapped lower-crustal melts may also exist beneath the rift, potentially contributing to the early focusing of intra-rift strain. We propose that in non-volcanic rifts, deep, unexposed (‘blind’) melts may play a key role in mechanical weakening of the lithosphere, enabling continued tectonic extension even in the absence of significant surface volcanism. 

Studies on the crustal structure of the Turkana Rift Zone (TRZ) in northern Kenya and southern Ethiopia began in the early 1980s. Initially driven by hydrocarbon exploration, these studies revealed that the rift zone comprises multiple fault-bounded basins ranging in age from the Eocene to the present. They also showed that the area hosts the intersection zone of the N-S trending basins of the Cenozoic East African Rift System (EARS) and the NW-SE-trending Mesozoic-Paleogene Central African Rift System (CARS). However, early seismic reflection and borehole data were mostly concentrated in the southern TRZ, resulting in limited subsurface data for its northern counterpart. This data gap has led to an incomplete understanding of the rift zone's regional crustal structure and how earlier CARS-related rifting influenced the development of the present-day EARS. Here, we leverage newly collected onshore and offshore subsurface industry datasets in the TRZ, spanning a 300 x 150 km region, to characterize the TRZ's crustal structure. We map several key subsurface horizons using a dense grid of 363 2-D seismic reflection profiles, which we tie to surface geology and borehole datasets. Mapping the acoustic basement produced new structure contour maps that provide high-resolution constraints on the TRZ’s crustal structure. Additionally, our isopach maps of key horizons show that strain migrated toward the modern rift axis, located along the center of Lake Turkana, following the widespread eruption of the Gombe Group basalt around 4 million years ago. Together, these results indicate that the area of maximum subsidence is collocated with the area transected by the CARS. Thus, we propose that these earlier episodes of rifting may have influenced the development and evolution of the modern EARS in the northern TRZ. These results provide crucial information for understanding tectonics in the context of hominin evolution and offer new insights into forming a divergent plate boundary. 

This is the TDEFNODE block inversion software used in the study "Constraining the Kinematics of the Victoria Microplate and the Northern Western Branch of the East African Rift System; The software is installed and run on a Linux terminal. All the files needed to run TDEFNODE for this study, along with their outputs, are included in TDEFNODE_block_inversion_modeling_NWB_Kwagalakwe_2025_v2.tar.gz, which is a revised version of TDEFNODE_block_inversion_modeling_NWB_Kwagalakwe_2025.tar.gz (Version v1). 

Abstract The oldest structures in a rift basin define incipient rift architecture, and commonly modulate the patterns of landscape evolution, sedimentation, and associated hazards in subsequent phases of rift development. However, due to deep burial beneath younger, thick syn‐rift sequences, and limited resolution of seismic imaging, critical early‐rift processes remain poorly understood. In the Tanganyika Rift, East Africa, we augment existing 2‐dimensional (2‐D) seismic reflection data with newly acquired aeromagnetic and Full‐Tensor Gradiometry data to assess the deep basin and underlying basement structure. Aeromagnetic and gravity grids show a dominance of NW‐trending long‐wavelength (>5 km) structural fabrics corresponding to the deeper basement, and dominant NW‐trending with a secondary NNE‐trending shorter‐wavelength (<3 km) fabric representing shallower, intra‐basin structures. Seismically‐constrained 2‐D forward modeling of the aeromagnetic and gravity data reveals: (a) an anomalously high‐density (2.35–2.45 g/cc) deep‐seated, fault‐bounded wedge‐shaped sedimentary unit that directly overlies the pre‐rift basement, likely of Mesozoic (Karoo) origin; (b) ∼4 km‐wide sub‐vertical low‐density (2.71 g/cc) structures within the 3.2 g/cc basement, interpreted to be inherited basement shear zones, (c) early‐rift intra‐basin faults co‐located with the modeled shear zone margins, in some places defining a persistent structurally‐controlled intra‐basin “high,” and (d) a shallow intra‐sedimentary V‐shaped zone of comparatively dense material (∼2.2 g/cc), interpreted to be a younger axial channel complex confined between the intra‐basin “high” and border fault. These results provide new insight into the earliest basin architecture of the Tanganyika Rift, controlled by inherited basement structure, and provide evidence of their persistent influence on the subsequent basin evolution. 

Abstract The fragmentation of continents results in microplates that rotate to accommodate the lateral propagation of bounding rifts. Yet, the relationships between microplate rotation rates, fault slip, and kinematics at propagating rift tips remain unknown. Here, we analyze new Global Navigation Satellite System (GNSS) data and structural geology data from the northern Western Branch of the East African Rift System that defines part of the boundary between the Nubian plate and the Victoria microplate. We resolve 0.0583 ± 0.0293°/Myr (6.48 ± 3.26 mm/yr) counterclockwise rotation of the Victoria microplate, consistent with previous studies, but with significant northwestward shift in the Euler pole relative to earlier work. Strain is largely localized on microplate‐bounding faults with 1.8–2.2 mm/yr slip rates, 7.2 × 10⁻⁸–1.28 × 10⁻⁷ y⁻¹strain rates, NE‐directed extension, and oblique‐normal fault kinematics. Most GNSS velocities are consistent with block rigidity, but three sites in the NW region of the Victoria microplate indicate possible internal deformation. 

Abstract West Africa continues to host a growing number of low and intermediate‐magnitude earthquakes (M2‐5) along its passive margins, and its continental interior. Earthquake activity in these regions raises the need to comprehend the causes and the tectonic controls of the seismicity. Unfortunately, such studies are rare. Here, we apply single‐station inversion techniques to constrain fourteen focal mechanisms, computed after compiling a set of high‐quality waveforms. We describe the connection between seismicity, the contemporary stress field, anthropogenic activity and Holocene fault scarps in the region. Our results indicate transpressive stresses acting on the inherited brittle structures in the passive margins. We also observe a compressive regime in the intracontinental failed rifts. We attribute the seismicity to the reactivation of “weak” faults in the Neoproterozoic and Mesozoic failed rifts, the passive transform structures, and the intracratonic Precambrian brittle shear zones.