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Found under sea: an old neighbour

New Delhi, Feb. 24: Scientists have discovered a hidden microcontinent, a chunk of ancient land that hugged India 750 million years ago but now remains submerged in the Indian Ocean beneath Mauritius.

A new study suggests that the microcontinent was sandwiched between southern India and Madagascar for several hundred million years before tectonic and volcanic activities broke it apart, and an ocean emerged to submerge it.

An international team of geologists has used sand grains from the beaches of Mauritius and gravity measurements in the Indian Ocean region to reconstruct plate tectonics and show that India’s Lakshadweep islands and Mauritius were both once part of this microcontinent. The scientists have named the microcontinent Mauritia, and have described their findings today in the journal Nature Geoscience.

“This is an unexpected discovery, never proposed before,” Lewis Ashwal, a senior geologist and a study team member from the University of Witwatersrand in South Africa, told The Telegraph.

The scientists found that minerals called zircons in the beach sand had ages between 1,970 million years and 690 million years, an indication that the zircons were part of ancient continental crust brought to the surface by plumes of lava.

Their gravity measurements also suggest that the old continental crust beneath Mauritius, now surrounded by a relatively younger oceanic crust — only five to 10 million years — was part of the microcontinent Mauritia.

The plate tectonics reconstruction suggests that Mauritia was tucked between southern India and Madagascar which about 750 million years ago lay between latitudes 30 North and 50 North.

“This is an exciting finding — it changes long-standing notions that India and Madgascar were fused together,” said Gopalakrishna Rao Parthasarathy, a senior scientist at the National Geophysical Research Institute, Hyderabad, who was not associated with the study. “A microcontinent between India and Madagascar might change the ancient geological history of western India.”

The researchers suggest the 750 million-year-old continental crust of Mauritia was once part of an extensive belt of high mountains — similar to present-day Andes — that extended from Rajasthan through the Seychelles and Madagascar into Mauritia.

“This belt would have been along the margins of the ancient supercontinent called Rodinia which broke apart to form Gondwana,” Ashwal said. Mauritia was likely exposed on the Earth’s surface between 750 million years and 100 million years.

The study suggests that about 83 million years ago, soon after magmatic activity in the region, India along with Seychelles and some Mauritian continental fragments separated from Madagascar while Mauritius remained with Madagascar.

However, Mauritius and other parts of Mauritia were “gradually transferred” to the Indian plate, and by 70 million years ago, all of Mauritia was part of the Indian plate, the researchers wrote in the supplementary material with their research paper.

“We can’t say when India precisely lost (connection with) Mauritia,” Ashwal said. Scientists say tectonic and volcanic activity would have led to the submergence of Mauritia. “Undersea volcanoes lead to the spread of the ocean floor — this expanded the ocean that submerged Mauritia,” said Mamilla Venkateshwarlu, a scientist at the NGRI, Hyderabad, who has been studying 1,100 million year old rocks in India to determine its position in the supercontinent Rodinia.