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Tectonic divisions with sketch and problems

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.

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.

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.

Tentative locations

ThrustBetweenTentative locationAge of last movement
MFT / HFTTerai and SiwalikAlong the southern foot of the Churiya hillsActive (Holocene)
MBTSiwalik and Lesser HimalayaAlong the northern edge of the Siwaliks / foot of the Mahabharat rangeLate Pleistocene, reactivated
MCTLesser and Higher HimalayaAlong the foot of the Higher Himalaya, mostly north of KathmanduMiocene, partly active
STDSHigher Himalaya and TethysAlong the northern slopes of the Great HimalayaNormal 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.

✓ Answer checked by Claude Opus 5.5 (high effort)

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