--- title: "Tectonic divisions with sketch and problems" subject: "Engineering Geology I" chapter: 7 key: "semester_3/engineering_geology_i:tectonic-divisions-with-sketch-and" source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/tectonic-divisions-with-sketch-and site: BCT Engineering --- # Tectonic divisions with sketch and problems Engineering Geology I · Chapter 7: Geology of Nepal - Asked in: 2065 Shrawan (old course), 8 marks; 2064 Jestha (old course), 8 marks; 2062 Baisakh (old course), 8 marks; 2061 Baisakh (old course), 8 marks (asked 4 times) - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/tectonic-divisions-with-sketch-and - Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-7 ## Question Describe the tectonic divisions of Nepal with a neat sketch. Indicate the tentative location of the tectonic boundaries such as HFT, MBT and MCT. Explain the engineering related problems (challenges) that can exist in such boundaries. ## Short answer Five tectonic zones (Terai, Sub-Himalaya, Lesser Himalaya, Higher Himalaya, Tethys) are bounded by HFT, MBT, MCT, and STDS. These active fault zones contain crushed, weak rock and high groundwater, causing landslides, tunnel collapse/squeezing, foundation problems, and seismic hazard. Engineering requires detailed investigation, drainage, and support to avoid or safely cross. ## Answer From south to north, Nepal is divided into five **tectonic zones**, separated by major thrusts and faults (discontinuities): ```text 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. #### Tentative locations | Thrust | Between | Tentative location | Age of last movement | |---|---|---|---| | MFT / HFT | Terai and Siwalik | Along the southern foot of the Churiya hills | Active (Holocene) | | MBT | Siwalik and Lesser Himalaya | Along the northern edge of the Siwaliks / foot of the Mahabharat range | Late Pleistocene, reactivated | | MCT | Lesser and Higher Himalaya | Along the foot of the Higher Himalaya, mostly north of Kathmandu | Miocene, partly active | | STDS | Higher Himalaya and Tethys | Along the northern slopes of the Great Himalaya | Normal fault, Miocene | #### Engineering problems at the boundaries - **Crushed and sheared rock:** thick fault gouge, breccia and shear zones have very low strength, causing tunnel collapse, large overbreak and squeezing, and need heavy support. - **Landslides:** the thrust zones and the weathered rock around them are major sources of slope failure and debris flow, e.g. along the MBT near the Mugling-Narayangadh road. - **Seismic hazard:** the Himalayan thrust system is active (the 1934 Bihar-Nepal and 2015 Gorkha earthquakes occurred on the Main Himalayan Thrust, which lies beneath these faults), so dams, bridges and tunnels need seismic design. Structures should not straddle an active fault, because surface rupture is possible. - **Groundwater and seepage:** springs, water inrush in tunnels, and high pore pressure in fractured rock. - **Foundation problems:** a sharp change in rock quality, with weak, weathered rocks (Siwalik mudstones, phyllite) on one side and strong rock on the other. - **Difficult access and investigation** in sheared, steep terrain. - **Hot springs** and gases may occur along the MCT (e.g. Tatopani). Hence a road, dam or tunnel alignment should avoid the thrust zone, or cross it at a right angle, with detailed investigation, drainage and support. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The MFT/HFT is located: A. North of Kathmandu at the foot of the Higher Himalaya B. On the northern slopes of the Great Himalaya C. Along the southern foot of the Churiya hills, between Terai and Siwalik D. Within the Tethys zone Answer: C. It is active (Holocene). (Key point: HFT location) 2. The MCT lies: A. Along the southern foot of the Churiya B. At the foot of the Mahabharat range C. In the Terai plain D. At the foot of the Higher Himalaya, mostly north of Kathmandu Answer: D. MBT is at the Siwalik northern edge / Mahabharat foot. (Key point: MCT location) 3. What is a major tunnelling problem at thrust zones? A. Fault gouge, breccia and shear zones giving very low strength, collapse and squeezing B. Excess hard rock C. No groundwater at all D. Perfect rock quality Answer: A. Heavy support is needed in crushed ground. (Key point: Tunnels) 4. Why do dams and bridges near these boundaries need seismic design? A. The faults are inactive but noisy B. The faults are active, with earthquakes like 1934 Bihar-Nepal and 2015 Gorkha and possible surface rupture C. Rock is too strong D. Earthquakes only occur in the Terai Answer: B. Structures should not cross the fault line. (Key point: Seismic) 5. A route, dam or tunnel should: A. Run along the thrust zone B. Ignore the thrust C. Avoid or cross the thrust zone at right angle, with detailed investigation and drainage D. Be built only in the crushed zone Answer: C. This limits exposure to weak and wet ground. (Key point: Alignment advice) ## Related in Engineering Geology I - [Geology of Nepal; significance of HFT, MBT, MCT](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Ageology-of-nepal-significance-of) — Engineering Geology I, chapter 7 - [Engineering geological problems in tectonic divisions](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aengineering-geological-problems-in-tectonic) — Engineering Geology I, chapter 7, asked 2× - [Tectonic divisions of the Nepal Himalaya](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Atectonic-divisions-of-the-nepal) — Engineering Geology I, chapter 7, asked 4× - [Discontinuities and engineering problems in Nepal Himalaya](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Adiscontinuities-and-engineering-problems-in) — Engineering Geology I, chapter 7 - [Tectonic zones and major discontinuities](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Atectonic-zones-and-major-discontinuities) — Engineering Geology I, chapter 7, asked 2× ## Related in other subjects - [Need and challenges of irrigation in Nepal](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Anepal-irrigation-need-challenges) — Irrigation and Drainage Engineering, chapter 1, asked 9×