The East African Rift System (EARS or EAR) is one of the most significant active continental rift zones on Earth. It represents a divergent tectonic plate boundary where the African Plate is slowly splitting into the Nubian Plate (west) and the Somali Plate (east), with additional microplates involved (e.g., Victoria, Rovuma, and Lwandle).

The system stretches roughly 3,000–6,400 km (depending on inclusion of northern extensions) from the Afar Triangle in Ethiopia/northern Ethiopia (connected to the Red Sea and Gulf of Aden) southward through Kenya, Tanzania, and into Mozambique. It averages 48–64 km wide.

It features two main branches:

Eastern Rift Valley (Gregory Rift): More volcanic, runs through Ethiopia (Main Ethiopian Rift), Kenya, and into northern Tanzania. Includes features like the Kenyan Dome.

Western Rift Valley: Less volcanic but more seismically active, arcs through Uganda, Rwanda, Burundi, Tanzania, etc., hosting many deep lakes.

The rift began developing around 22–25 million years ago (Miocene), with some influences from earlier events. It propagates generally from north to south.

The Nubian and Somali plates are separating at rates of about 6–9 mm per year on average (slower in some southern areas, up to ~4.7 mm/year in places like the Turkana Rift). The Victoria microplate rotates anti- clockwise.

Rifting involves normal faulting, half-graben basins (asymmetric), crustal thinning, and upwelling of mantle material. In advanced areas (e.g., Afar), it transitions toward oceanic crust formation. Recent studies (as of 2026) highlight “necking” (significant crustal thinning) in the Turkana Rift, indicating a more advanced stage than previously thought—this could lead to a new ocean basin in millions of years.

Recent insights (2025–2026): Seismic data shows the crust in Turkana has thinned critically. Mantle pulses and climate-tectonic interactions (e.g., lake level changes affecting fault activity) are also noted.

If rifting continues successfully, it could eventually form a new narrow ocean basin separating a “Horn of Africa” microcontinent from the rest of Africa over tens of millions of years (similar to how the Red Sea formed). Not all rifts succeed—some “fail” and become inactive.

The region is a hotspot for geothermal energy, mineral resources, and paleontology (cradle of humankind fossils preserved in rift sediments). It also poses hazards from earthquakes and volcanic eruptions.

This contrasts with the Southwest African Rift (e.g., Kafue in Zambia) mentioned previously, which represents another zone of potential African Plate deformation but is less developed and separate from the main EARS.

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The Southwestern Rift of Africa: isotopic evidence of early-stage continental rifting

The paper titled “The Southwestern Rift of Africa: isotopic evidence of early-stage continental rifting” (Karolytė et al., 2026, Frontiers in Earth Science, DOI: 10.3389/feart.2026.1799564) provides the first direct geochemical evidence of active mantle-crust interaction in the Kafue Rift of Zambia.

This is a concise, high-impact original research article (published 12 May 2026) led by Rūta Karolytė (University of Oxford), with co-authors including Michael C. Daly, Peter Vivian-Neal (Kalahari GeoEnergy), and experts like Chris Ballentine and Barbara Sherwood Lollar. It delivers the first direct geochemical proof of mantle-crust connectivity in the Kafue Rift, elevating the Southwestern Rift from a geophysical hypothesis to a geochemically supported active extensional zone.

Helium accumulation mechanisms in continental rifts, particularly early-stage systems like the Kafue Rift in Zambia’s Southwestern Rift, involve a combination of source, release, migration, concentration, and trapping processes. The recent Karolytė et al. (2026) paper highlights favorable conditions here.

Sources of Helium

Crustal (radiogenic ⁴He): Dominant source. Produced by alpha decay of uranium (U) and thorium (Th) in ancient Precambrian basement rocks (common in Zambia and southern Africa). Old, U/Th-rich cratonic or mobile belt crust generates helium over hundreds of millions to billions of years.

Mantle-derived (primordial ³He): Minor but diagnostic component. In the Kafue Rift, ³He/⁴He ratios of 0.14–0.17 R/Ra (8× crustal baseline) indicate mantle fluids rising through active faults, even without surface volcanism. This mixes with abundant crustal ⁴He.

Core Data and Results

Sampling: 6 samples from fault-related geothermal wells and springs inside the Kafue Rift (e.g., Bwengwa, Gwisho, Wells 15/18/20). 2 control samples from basement springs outside the rift boundary faults (~50 km SW and ~150 km NNW).

Helium isotopes:

• Rift samples: ³He/⁴He = 0.14–0.17 R/Ra (corrected; consistently elevated ~8× above pure crustal production of ~0.02 R/Ra).
• Basement controls: ~0.022 R/Ra (purely radiogenic crustal signature).
• Very high ⁴He concentrations (0.4–2.3 mol% in rift samples) — among the highest in EARS hydrothermal fluids.
• Negligible atmospheric contamination (⁴He/²⁰Ne ratios 856–3,240 vs. air ~0.032).

Carbon isotopes: One reliable δ¹³C(CO₂) = −3.9‰ (within/near mantle range of ~−7 to −4‰). CO₂ present in rift samples (1.5–15%) but absent in basement samples.

Major gases: Dominated by N₂ (84–98%), with crustal mobilization. No detectable CH₄. O₂ low to moderate.

Nitrogen isotopes: Trends toward enriched δ¹⁵N, consistent with crustal sources mixed with minor mantle input.

These values indicate mantle fluids (from partial melting at >60–70 km depth) ascending via active faults, while crustal gases (N₂, radiogenic ⁴He) are thermally mobilized. Low CO₂/³He ratios suggest CO₂ dissolution into groundwater — typical of low-magmatic-flux early rifting.

Figure insights (from paper): Rift samples plot similarly to early EARS segments (e.g., Northern Tanzanian Divergence Zone — NTDZ; Rukwa Rift Basin — RRB) on ³He/⁴He vs. ⁴He plots: moderate mantle He + high crustal He. Volcanically active EARS zones show higher ³He/⁴He and lower ⁴He.

Tectonic Significance

The Southwestern Rift (~2,500 km) runs from the Western EARS (via Rukwa) through Zambian rifts (Luangwa–Luano–Kafue) to Botswana’s Okavango and Namibia’s Eiseb. Prior evidence was indirect: subtle topography, fault scarps, gravity lows, heat flow >120 °C/km, and low seismicity.

This study confirms lithospheric-scale extension with mantle upwelling/degassing restricted to the rift zone. It supports partitioning of the Nubian Plate from a proposed San Plate (southern Africa). If mantle signatures appear along the full length, it strengthens the case for a nascent plate boundary potentially linking toward the Mid-Atlantic Ridge via Walvis Ridge.

Comparison to EARS: Mirrors the least magmatic, early-stage Western Branch segments (no surface volcanism, like much of the Kafue area). More advanced EARS sections (e.g., near volcanoes) show stronger mantle signatures and higher CO₂.

Mechanisms: Tectonic strain opens deep pathways for mantle fluids even without abundant magmatism (as seen globally in early rifts). Inherited crustal weaknesses and far-field stresses from mid-ocean ridges likely drive it.

Early rifting favors helium accumulation (high ⁴He, low dilution by volcanic gases) and geothermal potential (high heat flow + permeable faults). Groundwater acts as a CO₂ sink and trap for volatiles. Hydrogen is also mentioned as a possible target. One author’s industry affiliation (Kalahari GeoEnergy) notes access to sites, but the science stands independently.

This paper is a model of targeted, multi-isotope geochemistry resolving a big tectonic question. It advances understanding of how continents break up and highlights underexplored resource potential in central/southern Africa.

The full open-access article includes maps, figures, tables, and supplements for raw data. Highly recommended reading for anyone into tectonics or natural resources.

Published:  Frontiers in Earth Science

DOI: 10.3389/feart.2026.1799564

Authors: Rūta Karolytė, Michael C. Daly, Peter Vivian-Neal, Darren Hillegonds, Long Li, Barbara Sherwood Lollar, Chris J. Ballentine

Abstract

Helium and carbon isotope data (³He/⁴He = 0.14–0.17 R/R_a; δ¹³C(CO₂) = −3.9‰) from hydrothermal springs within the Kafue Rift of Zambia provide the first geochemical characterization of thermal springs along a broad extensional zone connecting the African Rift System through central Africa to Namibia. These results reveal mantle-derived fluids at the surface, and associated mobilization of crustal N₂ (84.4%–97.6%) with elevated ⁴He concentrations (0.4%–2.3%). Active hydrothermal groundwaters from outside of the Kafue Rift boundary faults show no isotopic evidence of mantle-derived helium or carbon dioxide. These geochemical compositions and spatial trends resemble those observed in other early rifts within the more thermally developed East African Rift System. The data is consistent with early stages of active lithospheric rifting, supported by previous geophysical observations globally. In addition to the regional tectonic importance of these data, these findings highlight the resource potential along central African active fault boundaries. The combination of a mantle fluid source, advective flow along crustal scale fault zones with low level seismicity, and groundwater serving as a sink for mantle CO₂ with minimal crustal fluid dilution, indicate potentially favorable conditions for both geothermal energy development and the exploration of economically significant gases in crustal fluids, particularly helium and hydrogen.