Geology of Nepal; significance of HFT, MBT, MCT

From south to north, Nepal is divided into five tectonic zones, separated by major thrusts and faults (discontinuities):

  S                                              N
  Terai | Sub-Him | Lesser Him | Higher Him | Tethys
        |(Siwalik)|            |            |
       MFT/HFT   MBT          MCT          STDS

1. Terai (Gangetic plain): Quaternary alluvium.
2. Sub-Himalaya (Siwalik zone): Miocene to Pleistocene molasse sediments.
3. Lesser Himalaya: Precambrian to Palaeozoic low-grade metamorphic rocks and sediments.
4. Higher Himalaya (Great Himalaya): high-grade metamorphic rocks and granites.
5. Tibetan-Tethys Himalaya: Palaeozoic to Cretaceous fossiliferous marine sediments.

Boundaries (major discontinuities)

• Main Frontal Thrust (MFT) / Himalayan Frontal Thrust (HFT): between the Terai and Siwalik.
• Main Boundary Thrust (MBT): between the Siwalik and Lesser Himalaya.
• Main Central Thrust (MCT): between the Lesser and Higher Himalaya.
• South Tibetan Detachment System (STDS): between the Higher Himalaya and the Tethys zone.

Geological sketch (simplified north-south section)

 Terai |Siwalik| Lesser Him.  | Higher Him. | Tethys
 alluv.|sandst.| phyllite,     | gneiss,     | limest.,
 gravel|mudst. | quartzite,    | schist,     | shale
       |       | limestone     | granite     |
     MFT     MBT            MCT           STDS

Engineering significance of HFT (MFT), MBT and MCT

• HFT (MFT): the youngest thrust and still active; it can cause earthquakes and surface rupture. It controls the Terai margin, flooding and changes in groundwater, and gives poor ground for structures.
• MBT: a wide zone of crushed and sheared rock between the Siwalik and the Lesser Himalaya; a major cause of landslides, and difficult ground for tunnels and roads.
• MCT: a thick shear zone of highly deformed schist and gneiss, with hot springs; it is partly active, has frequent rockfalls and landslides, and is a risk for hydropower projects.

Hence these zones are avoided for dams, tunnels and bridges unless special design is done.