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It's quicker by plume

Deccan Traps

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.

: The Reunion Plume, India and Africa.Reproduced courtesy, GeoExpro

Not everyone is convinced. Douwe van Hinsbergen (University of Oslo) believes2 that while the plume could have helped, it was not acting alone. He and colleagues have run geodynamic models using ocean floor magnetic anomaly data very similar to Cande and Stegman’s, but found that ‘plume push’ could only account for 3-4 cm/year of speeding-up, well short of the observed rate. Hinsbergen attributes the rest to slab-pull from the subduction zone at the other end of the Indian plate pulling more efficiently as resistance dwindled under the heat of the rising plume. India slowed later as it collided with Asia. “The slowdown [was] mainly the result of collision” van Hinsbergen says.

However, now that the motions of Africa and India can be linked, Cande says geodynamic modellers may have to change the way they run their models in future. “They have to model the whole system. They may even have to change some of the [modelling] parameters” he says.

R Dietmar Müller (University of Sydney),writing in the same issue of Nature suggests the plume could not have sped up India for the whole 15 million years, because the intensive period of Deccan eruptions only lasted for a couple of million years (near the start of India's acceleration). Cande nevertheless believes this is consistent with his findings, because the Deccan volcanic activity reflects India's observed speed. India sped up in two phases. During the first three to five million years, it was moving super-fast (c. 20 cm/year). It then slowed down to about 11-13 cm/year (still fast) and continued at around this rate for 10 million years more. In this latter period the volcanic activities related to the plume were phasing out but still active. The volcanic rocks offshore south of India provide evidence for this.

“We now have a better feeling of the role of mantle plumes in driving plates, even if it's just at the plume head stage that we can really see their impact,” says Cande. “If we can figure out what's going on at the plume head stage, the you can scale that back to the more normal plate motions, and get a better feeling of how [plumes] fit in with the mantle convection scheme.”

Further reading

  1. Indian and African plate motions driven by the push force of the Réunion plume head”, Nature (2011; doi:10.1038/nature10174). Steven C. Cande and Dave R. Stegman. See also “Plate motion and mantle plumes”, Nature News & Views by R. Dietmar Müller
  2. Acceleration and deceleration of India-Asia convergence since the Cretaceous: Roles of mantle plumes and continental collision”, Journal of Geophysical Research (2011: doi:10.1029/2010JB008051). Douwe J. J. van Hinsbergen, Bernhard Steinberger, Pavel V. Doubrovine, and René Gassmöller