The East African Rift (EAR) is a type locale for investigating the processes that drive continental rifting and breakup. The current kinematics of this ∼5000 km long divergent plate boundary between the Nubia and Somalia plates is starting to be unraveled thanks to a recent augmentation of space geodetic data in Africa. Here we use a new data set combining episodic GPS measurements with continuous measurements on the Nubian, Somalian, and Antarctic plates, together with earthquake slip vector directions and geologic indicators along the Southwest Indian Ridge to update the present-day kinematics of the EAR. We use geological and seismological data to determine the main rift faults and solve for rigid block rotations while accounting for elastic strain accumulation on locked active faults. We find that the data are best fit with a model that includes three microplates embedded within the EAR, between Nubia and Somalia (Victoria, Rovuma, and Lwandle), consistent with previous findings but with slower extension rates. We find that earthquake slip vectors provide information that is consistent with the GPS velocities and helps to significantly reduce uncertainties of plate angular velocity estimates. We also find that 3.16 Myr MORVEL average spreading rates along the Southwest Indian Ridge are systematically faster than prediction from GPS data alone. This likely indicates that outward displacement along the SWIR is larger than the default value used in the MORVEL plate motion model.

[1] The kinematics of the East African Rift (EAR) is the least well-known of all major plate boundaries. Here, we show that present-day data (a GPS+DORIS geodetic solution and earthquake slip vectors) are consistent with 3.2 Myr-average spreading rates and transform-fault azimuths along the Southwest Indian Ridge and support a kinematic model that includes three subplates (Victoria, Rovuma, and Lwandle) between Nubia and Somalia. Continental rifting in the EAR appears to involve localized strain along narrow rift structures that isolate large lithospheric blocks.

International audienceContinental rifts begin and develop through repeated episodes of faulting and magmatism, but strain partitioning between faulting and magmatism during discrete rifting episodes remains poorly documented. In highly evolved rifts, tensile stresses from far-field plate motions accumulate over decades before being released during relatively short time intervals by faulting and magmatic intrusions1, 2, 3. These rifting crises are rarely observed in thick lithosphere during the initial stages of rifting. Here we show that most of the strain during the July–August 2007 seismic crisis in the weakly extended Natron rift, Tanzania, was released aseismically. Deformation was achieved by slow slip on a normal fault that promoted subsequent dyke intrusion by stress unclamping. This event provides compelling evidence for strain accommodation by magma intrusion, in addition to slip along normal faults, during the initial stages of continental rifting and before significant crustal thinning

During the development of continental rifts, strain accommodation shifts from border faults to intra-rift faults. This transition represents a critical process in the evolution of rift basins in the East African Rift, resulting in the focusing of strain and, ultimately, continental breakup. An analysis of fault and fluid systems in the younger than 7 Ma Natron and Magadi basins (Kenya-Tanzania border) reveals the transition as a complex interaction between plate flexure, magma emplacement, and magmatic volatile release. Rift basin development was investigated by analyzing fault systems, lava chronology, and geochemistry of spring systems. Results show that extensional strain in the 3 Ma Natron basin is primarily accommodated along the border fault, whereas results from the 7 Ma Magadi basin reveal a transition to intra-rift fault-dominated strain accommodation. The focusing of strain into a system of intra-rift faults in Magadi also occurred without oblique-style rifting, as is observed in Ethiopia, and border fault hanging-wall flexure can account for only a minor portion of faulting along the central rift axis (~12% or less). Instead, areas of high upper crustal strain coincide with the presence of hydrothermal springs that exhibit carbon isotopes and N 2-HeAr abundances indicating mixing between mantle-derived (magmatic) fluids and air saturated water. By comparing the distribution of fault-related strain and zones of magmatic fluid release in the 3 Ma Natron and 7 Ma Magadi basins, we present a conceptual model for the evolution of early-stage rifting. In the first 3 m.y., border faults accommodate the majority of regional extension (1.24-1.78 mm yr-1 in Natron at a slip rate ranging 1.93-3.56 mm yr-1), with a significant portion of intra-rift faulting (38%-96%) driven by flexure of the border fault hanging wall. Fluids released from magma bodies ascend along the border fault and then outward into nearby faults forming in the flexing hanging wall. By 7 m.y., there is a reduction in the amount of extension accommodated along the border fault (0.40-0.66 mm yr-1 in Magadi at a slip rate ranging from 0.62 to 1.32 mm yr-1), and regional extension is primarily accommodated in the intra-rift fault population (1.34-1.60 mm yr-1), with an accompanying transition of magmatic volatile release into the rift center. The focusing of magma toward the rift center and concomitant release of magmatic fluids into the flexing hanging wall provides a previously unrecognized mechanism that may help to weaken crust and assist the transition to intra-rift dominated strain accommodation. We conclude that the flow of magmatic fluids within fault systems plays an important role in weakening lithosphere and focusing upper crustal strain in early-stage continental rift basins prior to the establishment of magmatic segments.

Here we describe the new Sub-Saharan Africa Geodetic Strain Rate Model v.1.0 (SSA-GSRM v.1.0), which provides fundamental constraints on long-term tectonic deformation in the region and an improved seismic hazards assessment in Sub-Saharan Africa. Sub-Saharan Africa encompasses the East African Rift System, the active divergent plate boundary between the Nubian and Somalian plates, where strain is largely accommodated along the boundaries of three subplates. We develop an improved geodetic strain rate field for sub-Saharan Africa that incorporates 1) an expanded geodetic velocity field, 2) redefined regions of deforming zones guided by seismicity distribution, and 3) updated constraints on block rotations. SSA-GSRM v.1.0 spans longitudes 22° to 55.5° and latitudes −52° to 20° with 0.25° (longitude) by 0.2° (latitude) spacing. For plates/sub-plates, we assign rigid block rotations as constraints on the strain rate calculation that is determined by fitting bicubic Bessel splines to a new geodetic velocity solution for an interpolated velocity gradient tensor field. We derive strain rates, velocities, and vorticity rates from the velocity gradient tensor field. A comparison with the Global Geodetic Strain Rate model v2.1 reveals regions of previously unresolved spatial heterogeneities in geodetic strain rate distribution, which indicates zones of elevated seismic risk.

Although the East African Rift System (EARS) is an archetype continental rift, the forces driving its evolution remain debated. Some contend buoyancy forces arising from gravitational potential energy (GPE) gradients within the lithosphere drive rifting. Others argue for a major role of the diverging mantle flow associated with the African Superplume. Here we quantify the forces driving present‐day continental rifting in East Africa by (1) solving the depth averaged 3‐D force balance equations for 3‐D deviatoric stress associated with GPE, (2) inverting for a stress field boundary condition that we interpret as originating from large‐scale mantle tractions, (3) calculating dynamic velocities due to lithospheric buoyancy forces, lateral viscosity variations, and velocity boundary conditions, and (4) calculating dynamic velocities that result from the stress response of horizontal mantle tractions acting on a viscous lithosphere in Africa and surroundings. We find deviatoric stress associated with lithospheric GPE gradients are ∼8–20 MPa in EARS, and the minimum deviatoric stress resulting from basal shear is ∼1.6 MPa along the EARS. Our dynamic velocity calculations confirm that a force contribution from GPE gradients alone is sufficient to drive Nubia‐Somalia divergence and that additional forcing from horizontal mantle tractions overestimates surface kinematics. Stresses from GPE gradients appear sufficient to sustain present‐day rifting in East Africa; however, they are lower than the vertically integrated strength of the lithosphere along most of the EARS. This indicates additional processes are required to initiate rupture of continental lithosphere, but once it is initiated, lithospheric buoyancy forces are enough to maintain rifting.

East African Rift System plate geometries and surface motions are some of the least constrained in the context of global plate motion models. In this study, we used GPS data to constrain Somalian plate rotation and to suggest a new tectonic plate geometry for the region. In addition, we tested geologic data from the Southwest Indian Ridge and new GPS data on Madagascar to determine refined kinematics of the Lwandle microplate. A zone of broad deformation was discovered, extending from the eastern boundary of the Rovuma microplate, across the Comoros Islands, and including parts of central and northern Madagascar. Madagascar is fragmenting, with southern Madagascar rotating with the Lwandle microplate and a piece of eastern and south-central Madagascar moving with the Somalian plate. Divergence of the Nubian-Somalian plate system across the East African Rift System involves both diffuse deformation and strain accommodation along narrow rift segments that bound rigid blocks.

On 3 April 2017, a Mw 6.5 earthquake struck Moiyabana, Botswana, nucleating at >20 km focal depth within the Paleoproterozoic Limpopo‐Shashe orogenic belt separating the Archean Zimbabwe and Kaapvaal Cratons. We investigate the lithospheric structures associated with this earthquake using high‐resolution aeromagnetic and gravity data integrated with Differential Interferometric Synthetic Aperture Radar (DInSAR) analysis. Here we present the first results that provide insights into the tectonic framework of the earthquake. The ruptured fault trace delineated by DInSAR aligns with a distinct NW striking and NE dipping magnetic lineament within the Precambrian basement. The fault plane solution and numerical modeling indicate that the cause of the earthquake was 1.8 m displacement along a NW striking and NE dipping normal fault, rupturing at 21–24 km depth. We suggest that this seismic event was due to extensional reactivation of a crustal‐scale Precambrian thrust splay within the Limpopo‐Shashe orogenic belt.