By G.S. MUDUR
New Delhi, August 11 – The northern edges of the Indian landmass stretch far beneath Tibet, extending to Lhasa in the east and a split in the crust called the Altyn Tagh fault in western Tibet, new studies have revealed.
Since India collided with Asia about 55 million years ago, tectonic forces have driven India underneath Asia, creating the Himalayan mountain chain. The Indian landmass continues to slip beneath the northern continental plate at the rate of 5 cm per year.
But the northern limits of the Indian landmass have never been clearly delineated.
Now, Indian scientists working with British colleagues have discovered evidence that suggests the Indian plate may have penetrated as far north as the Altyn Tagh fault in western Tibet, about 300 km north of the Karakoram range.
“We know tips of the Indian landmass lie beneath Lhasa and Altyn Tagh. We’re now trying to sketch the arc that connects these two points,” said Supriyo Mitra, a geophysicist and a team member at the Indian Institute of Technology, Kharagpur.
“These studies are part of efforts to find out how far India has penetrated underneath the Asian landmass,” said Shyam Sunder Rai, the head of geophysics at the National Geophysical Research Institute in Hyderabad. “We’re looking for the northern boundary of the Indian plate.”
The scientists studied seismic waves that pass through underground rocks to determine the depth of the Moho — the boundary between the Earth’s crust and the mantle where the properties of rocks significantly change. Their findings appear this week in the international journal Geophysical Research Letters.
“The depth of the Moho increases steadily as we move northwards,” said Rai.
The Moho is about 35 km deep near Thiruvananthapuram, 50 km deep near Manali and 70 km deep near Ladakh. It reaches a depth of 90 km near Altyn Tagh fault and then abruptly shallows to 50 km north of Altyn Tagh.
“South of the Altyn Tagh, the Indian crust sliding beneath Asia contributes to the increasing depth of the Moho. And north of the Altyn Tagh, it’s only pure Asian crust,” Mitra said.
Besides revealing how far the Indian landmass stretches beneath Tibet’s underbelly, such studies are also expected to provide new insights into the accumulation of strain in underground rocks that precede earthquakes.
Studies on continental plate positions coupled with knowledge of plate movements will help scientists simulate on computer the accumulation of strain in rocks. When the strain exceeds a critical limit the rocks rupture, causing an earthquake.
“We cannot predict earthquakes, but strain simulation studies might allow us to say how far a section of rock is close to rupture,” said Gaur.
