Origin of Himalayas
Before discussing the geological division of Nepal, it's good to know about the origin of the Himalayas. Nepal is a country that does not sit on a single tectonic plate. It sits on the boundaries of two tectonic plates i.e. Indian plate and the Eurasian plate ( also called the Tibetan plate ). The Indian plate in the south and the Eurasian plate in the north started to collide millions of years ago after the vanishing of Tethys sea in between them. The collision process is still active and this collision gave birth to the Himalayas of which Nepal is the most important part. The Himalayan range is about 2400Km long and extends from the Indus River in the west to the Assam in the east. The collision process continued over years and some major and minor faults were developed within the area of our country. The study has shown the following major faults:
- Main Frontal Thrust (MFT) / Himalayan Frontal Thrust (HFT)
- Main Boundary Thrust (MBT)
- Main Central Thrust (MCT)
- South Tibetan Detachment System (STDs)
These faults have covered the whole length of Nepal extending from east to west as shown in the figure above.
Geological Division of Nepal
The collision of tectonic plates till now results in several thrusts & faults based on which Nepal can be geologically divided into the following five regions:
- The Terai Zone
- Northern (Bhabar) Zone
- Middle (Marshy) Zone
- Southern Zone
- The Siwalik Zone
- The Lower Siwalik
- The Middle Siwalik
- The Upper Siwalik
- The Lesser Himalayan Zone
- Higher Himalayan Zone
- Tibetan Tethys Zone
MFT separates Siwalik from Terai. MBT is the boundary between Siwalik and Lesser Himalaya whereas MCT separates Lesser Himalaya and Higher Himalaya. Finally, the STDs divides the Higher Himalayas & Tibetan Tethys Zone.
The development of such weak zones will continue and are prone to earthquakes. The Indian plate is moving north at around 45 mm per year and pushing under the Eurasian plate due to which the rocks at the plate boundary are continuously undergoing deformation. During the deformation of the plates, a great amount of stress is developed on the rock. Initially, the stress developed is resisted by frictional resistance. But when the stress exceeds the frictional resistance, the plates move along the fault plane releasing the accumulated energy due to which earthquake occurs.