Recently, the breaking of the Indian Plate beneath the Tibetan Plateau has been in the news. What has been discovered is evidence that the Indian Plate is splitting. This is a significant finding that enhances our understanding of natural processes and helps us interpret and predict geological events with greater accuracy. It also deepens our knowledge of orogeny—the process of mountain formation—and provides insights into how tectonic plates, especially continental plates, behave when they collide.
Earth is the only planet we know of that has active tectonic plates moving and interacting with each other. These plates are a key part of Earth’s geology and are responsible for earthquakes, volcanic eruptions, and the formation of mountains and ocean basins.
There are two main types of tectonic plates: oceanic plates and continental plates. Oceanic plates are found under the oceans and are made of dense basalt, which makes them heavier. Continental plates form the continents and are made of lighter granite.
When an oceanic plate and a continental plate collide, the heavier oceanic plate usually sinks beneath the lighter continental plate in a process called subduction. In some rare cases, the opposite happens, where the oceanic plate is forced over the continental plate; this is called obduction.
When two continental plates collide, neither is as dense as oceanic plates, but one plate may still be forced under the other. This creates large mountain ranges. Around 50–60 million years ago, the Indian plate collided with the Eurasian plate, both are continental plates. Before this collision, there was an ancient ocean called the Tethys Sea between these two plates. As a result of the collision, the sediments deposited in the Tethys Sea were compressed and uplifted, leading to the formation of the Himalayan mountain range.
When a sea completely disappears between two converging tectonic plates, the line where the two plates meet and are welded together is called a suture zone. The Indus-Tsangpo (also called Yarlung-Zangbo) Suture Zone is one such suture zone where the Indian plate collided with and is still moving under the Tibetan Plate, a part of the Eurasian Plate.
The mountain formation process (orogeny) is still active in the Himalayas. This process is driven by the continued subduction – movement of the Indian Plate under the Tibetan Plate.
A group of scientists led by Lin Liu from the Ocean University of China was studying how the lithosphere (the rigid outer layer of the Earth) transitions into the asthenosphere (the softer, more ductile layer below it) in south-eastern Tibet. Their study focussed on mapping the depths of the Indian lithosphere and the Tibetan lithosphere in the Indus-Tsangpo suture zone using seismic data and helium isotopes derived from the earth’s mantle. The mantle is the layer that lies between the Earth's crust and its outer core. Though it is mostly made of solid rocks, over a long geological period mantle acts as a viscous fluid playing a key role in the movement of tectonic plates.
The researchers found that in regions west of 90°E longitude, approximately 100 km north of the Indus-Tsangpo suture zone, the Indian lithosphere is largely intact and appears to be subducting beneath Tibet. This process forms a continuous layer beneath the Tibetan lithosphere, referred to as 'underplating.'
However, east of 90°E longitude, they observed a more complex scenario. In this region, the Indian lithospheric mantle has started breaking off and peeling away due to gravity, a process known as "delamination." And, between this delaminated Indian mantle and the Indian crust above it, a wedge of asthenosphere has formed. See this figure:
3-D representation of the subduction process and delamination of the Indian plate (Taken from the main article by Lin et.al.)
The subducting Indian plate is experiencing intense tectonic forces. In the west, it remains largely intact and is being pushed northward beneath the Tibetan lithosphere. In the east, however, gravitational forces are causing parts of the Indian plate's lithospheric mantle to break, peel away, and roll back, resulting in a torn or warped plate structure. This delamination process is further complicated by the Tibetan lithosphere thickening and growing over the detached Indian mantle.
The study team noted that the Indian plate faces a 'geometric problem,' as it is being pushed in two different directions:
- Northward, beneath the Tibetan plate.
- Eastward, toward the subduction zone beneath the Burma Volcanic Arc.
This double movement generates significant stresses, causing the Indian plate to deform, break, and roll back, leading to complex structural and dynamic changes in the lithosphere and mantle.
The delamination of the Indian lithosphere beneath southeastern Tibet is believed to have started around 8–10 million years ago. These findings provide a more detailed understanding of the tectonic processes shaping the region, offering valuable insights into geologic events, including earthquakes in the Himalayan region.
---
*With Professional Assistance for Development Action (PRADAN)
Comments