Mantle plume links India and Africa’s motions, writes Monique Tsang
Geoscientist Online 20 July 2011
Image: Deccan Traps.
It has long been an unresolved case of speeding. For 15 million years during the late Cretaceous and early Cenozoic, the Indian tectonic plate scurried towards Eurasia at between 10 and 13cm a year, eventually crashing into Eurasia, forming the Himalayas and Tibetan Plateau. For decades scientists suspected that this speeding had something to do with the Réunion mantle plume beneath the Indian Ocean. Now, according to a recent paper in the journal
Nature1, geologists have found compelling evidence of its role in directing the tectonic traffic.
Steven Cande and Dave Stegman (Scripps Institute of Oceanography, California) looked for clues in a place many had overlooked – Africa. Geologists noticed 25 years ago that the ocean-ridge fracture zones along the edges of the African plate showed strange, bending patterns. Some thought they resulted from some form of complex motion of the Africa Plate, but nobody really knew. Cande and Stegman analysed the patterns’ path relative to Africa’s pivot point, and found that these bends – like “skid marks at a car crash” – showed that Africa’s motion slowed down and then sped up.
At the time the African plate was rotating counterclockwise about a pivot point sited near the present-day Canaries. Cande and Stegman discovered that at times when the Indian plate speeded up, the Africa plate slowed down – and
vice versa. This was no coincidence.
Enter the Réunion mantle plume. This plume, thought to lie today under Réunion Island east of Madagascar, wreaked a trail of fiery havoc on the Indian plate as it moved notheast, spewing massive amounts of lava to form the Deccan Traps and spawning several Indian Ocean island chains.
When the plume head first hit the lithosphere 67 million years ago near the boundary of the India and Africa plates, it spread out radially, like the head of a mushroom. This force had the effect of speeding up India (from 4 to 10-13 cm/year) and slowed down Africa ( 2 cm/year to almost zero). And when the plume began to wane 52 million years ago, India slowed back down while Africa speeded up.
“Most people have said that the slow-down of India was due to the collision of India and Eurasia. Our study [suggests] that's not necessarily true,” Cande told Geoscientist Online. “If you were to say the slow-down of India was due to the collision with Eurasia, then why did Africa speed back up? It makes more sense to say that they're both related to a common cause, which would have to be the waning of the Réunion plume head.”
The plume head was “a major force” on India's and Africa's motions partly because it was very large, extending over some 2000 kilometres, says Cande - large enough to affect a plate as huge as Africa. “The Réunion plume head is an ideal place… because it's quite far from the pivot point,” he says.