Aerial view of the Kafue Rift southern boundary fault zone. The hot springs where researchers sampled gases lie in the green thickets
Michael Daly
The African continent may have begun tearing itself apart in a new location. Gases emerging from an arc of hot springs in Zambia appear to be coming from deep below Earth’s crust, in a sign that a new tectonic plate boundary could be opening up.
There are many rift valleys around the world where continents have been pulling apart for tens of millions of years, including the East African Rift valley, which runs from the Red Sea to Mozambique. Over millions of years, rifting can lead to a tectonic plate splitting in two, but it is unusual to find a site where this process may be just beginning, says Rūta Karolytė at the University of Oxford.
The Kafue Rift in Zambia is part of a 2500-kilometre-long rift zone stretching from Tanzania to Namibia and possibly out into the Atlantic Ocean. Geologists think the area may be showing signs of being in the early stages of forming a new boundary between continental plates, because of the geography, low-gravity anomalies, high sub-surface temperatures and low-level seismicity. But, until now, there has been no geochemical evidence.
Karolytė and her colleagues have now analysed gas samples collected from five hot springs and three geothermal wells in central Zambia. The team found that helium and carbon isotope ratios in the gases are the same as those that exist deep beneath Earth’s crust, suggesting that fluids from the mantle, up to 190 kilometres below the springs, are making their way to the surface. This implies a tear in the region’s tectonic plates.
“What our data confirms is that this system is currently ‘awake’ and geologically active,” says Karolytė. “Having an active rift developing doesn’t necessarily mean that in 100 million years you’re going to have an ocean there. But it is a possibility.”
The earliest stages of continental rifting release gases that have accumulated in the rocks for millions of years. Among these is helium, a critical resource for high-tech industry and medicine. In the Kafue Rift, this process has created concentrations of helium of up to 2.3 per cent in the fluids reaching the surface – high enough that the region is already attracting interest from industry.
“It’s hard to find these tectonic conditions that are just right to concentrate and release helium in a way that it can be captured,” says Karolytė.
Patrice Rey at the University of Sydney, Australia, says, despite the absence of active volcanoes and significant seismicity, he agrees there are many signs in the region that the landscape is tectonically active.
The new geochemical evidence from the hot springs reveals that the Kafue Rift is an early-stage continental rift where mantle fluids rich in primordial helium-3 are rising through faults, he says. “It is reasonable to think that the Kafue Rift may evolve into a plate boundary sometime in the future.”
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