A tectonic shift in our understanding of the India-Asia collision

New research might change our understanding of how plate movements formed the continents millions of years ago.

The study out of China suggests millions of years ago India was about twice as big as it is now, but it subsided under and pushed up, the Tibetan plateau.

The Earth’s surface hasn’t always looked like it does today —hundreds of millions of years ago there were only two supercontinents: Laurasia and Gondwana.

The continents and oceanic rock that make up Earth’s crust, or lithosphere, sit atop molten rock called the mantle and move very slowly over it. The present Indian subcontinent was originally part of Gondwana, which broke up about 150 million years ago.

In some places on Earth the continents crush into each other and cause mountain ranges to rise. The Himalayas were thought to have formed when the Indian Plate collided with the Asian Plate about 55 million years ago (mya).

We know that a portion of the Indian Plate subducted beneath the Himalayas and the Tibetan Plateau during this process. However, the size of this “lost” part of the continent, called Greater India, has remained uncertain.

Now new research has found that Greater India was a single plate, extending 2,000 to 3,000 km to the north of the current day Indian subcontinent, before it subducted under Asia.

Understanding the original extent Greater India is important to resolving several key questions surrounding the age of the India–Asia collision, and answering how and when the Tibetan Plateau formed.

Professor Jun Meng of China University of Geosciences, Beijing (CUGB), first author of the study published in PNAS, explains that there are two primary models for the India–Asia collision.

“The first is a multistage collision model that subdivides the oceanic basin at the leading edge of India into smaller plates that were later incorporated into the Asian plate,” he says.

This ancient oceanic basin is known as the Tethys Ocean.

“The second model says that India and Greater India existed as a single plate in the Early Cretaceous period, with the upper crust of the northern margin of Greater India forming the Himalayan thrust belt and the lower crust being subducted under Asia.”

His colleague from CUGB, Professor Chengshan Wang adds, “Our goal was to understand which of these models was more accurate.”

Their findings support the second model.

The team studied the Sangdanlin section, in southern Tibet, that was originally part of the Greater India region that formed the Himalayas. They tracked the geographical position of the northern sector of the Indian Plate through time and estimated the minimum size for Greater India.

According to the researchers, the amount of lithosphere consumed by the subduction since the start of the collision 55 mya was larger than the Indian subcontinent as it exists today. Almost 5 million km² of lithosphere has been subducted under the Asian Plate, which must have contributed to the rise of the Tibetan Plateau.

“Our findings challenge established notions of the formation of Asia’s southern margin through the coalescence of independent tectonic blocks in the Tethys Ocean,” says Professor Stuart A. Gilder of Ludwig Maximilians University, Germany.

“They could help us fill the gap in the knowledge regarding the size of the Indian plate in a Gondwana configuration and the tectonic history of India up until its collision with Asia.”