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Tectonic division and Lesser Himalaya geology

Short answer

Tectonic zones are Terai, Sub-Himalaya (Siwalik), Lesser Himalaya, Higher Himalaya, and Tethys Himalaya, separated by MFT, MBT, MCT, and STDS. The Lesser Himalaya zone comprises low-grade metamorphic rocks (slate, phyllite, quartzite, limestone) of the Nawakot and Kathmandu Complexes, with granites. It is intensely folded, faulted, and thrust, featuring synclinoria and klippen.

What are the tectonic divisions of Nepal? Explain the geology of the Lesser Himalaya zone.

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.

Geology of the Lesser Himalaya

Lithology

  • Nawakot Complex: slate, phyllite, quartzite, limestone, dolomite and metasandstone. Lower Nawakot Group: Kuncha, Fagfog, Dandagaon, Nourpul and Dhading formations; Upper Nawakot Group: Benighat, Malekhu and Robang formations.
  • Kathmandu Complex (nappe): the Bhimphedi Group of schist, quartzite and marble (Raduwa, Bhainsedobhan, Kalitar, Chisapani, Kulekhani and Markhu formations), overlain by the Phulchauki Group of slate, sandstone and limestone (Tistung, Sopyang, Chandragiri, Chitlang and Godavari formations).
  • Granite and gneiss bodies: Palung granite and other granites, and the Ulleri augen gneiss.

Structure: intensely folded, faulted and thrust. There are synclinoria (e.g. the Mahabharat synclinorium of the Kathmandu nappe) with klippen, and anticlinoria, with many thrusts and tectonic windows. The beds mostly dip northward.

Engineering problems: landslides in weathered phyllite and slate, karst and seepage in limestone and dolomite, and shear zones along the thrusts.

✓ Answer checked by Claude Opus 5.5 (high effort)

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