Geology of Nepal

The Nepal Himalaya, about 800 km long and 150-250 km wide, is divided from south to north into the following physiographic zones (based on altitude and landform):

Altitude limits differ slightly between authors; the above are the common textbook ranges.

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.

Short notes on the zones

• Terai: Quaternary alluvium; flat, flood-prone.
• Siwalik: weak sandstone, mudstone and conglomerate.
• Lesser Himalaya: phyllite, slate, quartzite, limestone, dolomite.
• Higher Himalaya: gneiss, schist, migmatite and granite.
• Tethys: fossiliferous limestone, shale, sandstone.

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

Engineering problems at the boundaries

• Crushed and sheared rock: thick fault gouge, breccia, and shear zones give very low strength, causing tunnel collapse, large overbreak, squeezing, and need for heavy support.
• Landslides: the thrust zones and surrounding weathered rock are major sources of slope failure and debris flow, e.g. along the MBT near Mugling-Narayangadh road.
• Seismic hazard: these faults are active (the 1934 Bihar-Nepal and 2015 Gorkha earthquakes), so dams, bridges and tunnels need seismic design; the fault line must not be crossed by structures, and surface rupture is possible.
• Groundwater and seepage: springs, water inrush in tunnels, and high pore pressure in fractured rock.
• Foundation problems: differential rock quality, 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 along the MCT (e.g. Tatopani) may exist.

Hence a route, dam or tunnel alignment should avoid or cross the thrust zone at a right angle, with detailed investigation, drainage and support.

Higher (Great) Himalaya. Lies between the MCT and STDS; altitude about 4000-8848 m. Rocks are Precambrian to Early Palaeozoic high-grade metamorphic rocks and granites.

Lithology (three formations)

• Formation I (lower): kyanite-sillimanite schist and gneiss, with migmatite.
• Formation II (middle): calc-silicate gneiss, marble and diopside bearing rocks.
• Formation III (upper): augen gneiss, quartzite, and Manaslu and other leucogranite intrusions.

Structure: the Higher Himalaya is a thick northward-dipping sequence of rocks with a steep foliation, folded and cut by thrusts such as the MCT zone, with a large number of glacial and periglacial features.

Engineering problems: very high relief, rockfall, avalanche, glacial lake outburst floods (GLOF), glacial and moraine hazards, and the need for blasting and difficult access.

Tibetan-Tethys (Inner, Trans) Himalaya. Lies north of the Higher Himalaya, beyond the STDS; altitude about 2500-5000 m. It has a dry, rain-shadow climate.

Lithology: Cambrian to Cretaceous (and Eocene) marine sedimentary rocks, rich in fossils: limestone, shale, sandstone, slate, quartzite and marl; the Thakkhola graben has Neogene sediments. Famous ammonite fossils (shaligram) of the Kali Gandaki valley come from Jurassic shale.

Structure: thick and gently folded, with some normal faults (Thakkhola Graben).

Engineering problems: karst in limestone, frost action, permafrost, rockfall, and difficult accessibility.

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.

Siwalik (Churiya) zone. The southernmost hill belt of Nepal, 10-50 km wide, altitude about 300-1500 m (up to 2000 m), lying between the MFT and MBT. Rocks are Miocene to Pleistocene molasse sediments, eroded from the rising Himalaya, mostly weak and poorly consolidated.

Lithology (three divisions)

• Lower Siwalik: alternation of fine-grained sandstone, mudstone and siltstone; red, purple and grey colours (Miocene).
• Middle Siwalik: thick, massive, coarse "salt and pepper" sandstone with some mudstone and conglomerate lenses (Mio-Pliocene).
• Upper Siwalik: poorly cemented boulder-pebble conglomerate with sandstone and clay (Plio-Pleistocene).

Structure: the strata are folded into anticlines and synclines with strike roughly parallel to the Himalaya (WNW-ESE), often steeply dipping or overturned in the north near the MBT, and cut by thrust faults such as the MBT and MFT.

Engineering problems: soft, erodible rocks, landslides, gullying, debris flow, and flash floods; but sandstones give good aggregate.

Terai zone. A flat, low plain 60-300 m in altitude, the northern edge of the Indo-Gangetic plain, formed of Quaternary alluvium brought by the Himalayan rivers.

By the nature of deposits it is divided into:

1. Bhabar zone: a narrow belt (8-12 km wide) at the foot of the Siwalik. It consists of coarse boulders, cobbles and gravel, very permeable, so streams disappear underground.
2. Terai proper (Terai plain): finer sand, silt and clay with high water table and springs where the Bhabar water re-emerges.

Lithology: unconsolidated gravel, sand, silt and clay; thickness is over a thousand metres.

Engineering problems: floods and shifting rivers, high water table, liquefaction in earthquakes, low bearing capacity, and shortage of rock for aggregate.

Major discontinuity systems

Engineering significance

• All are major thrusts or faults: crushed, sheared and weak ground, tunnelling and slope problems.
• They are seismically active, so earthquakes, surface rupture and landslides are linked with them.
• Springs, seepage and groundwater concentrate along them.
• Structures (dams, powerhouses) should avoid crossing them.

Problems in the tectonic subdivisions

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, still active thrust; surface rupture and earthquakes; it controls the Terai margin, flooding and groundwater changes; 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 for tunnels and roads.
• MCT: the thick shear zone with highly deformed schist and gneiss, hot springs; moves partly active; high rockfall and landslides; a risk for hydropower.

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

A thrust is a low-angle reverse fault in which an older rock is pushed over a younger rock. The importance of the HFT (MFT), MBT and MCT in civil engineering is:

1. Crushed, sheared and weak rock gives low bearing capacity, tunnel collapse, large overbreak and squeezing, so extra support is needed.
2. Slope instability: landslides and debris flows are concentrated in the thrust zones.
3. Seismic risk: the thrusts are active, so earthquake loading, surface rupture and ground shaking affect dams, bridges and buildings. Structures must not be placed across them.
4. Groundwater: thrust zones are conduits for water; heavy seepage, springs and inrush in tunnels.
5. Rock contrast: rocks of different strength and permeability meet at the thrust, giving differential settlement and leakage under dams.
6. Site investigation must locate the thrust accurately, with trenching and geophysical methods, before alignments of roads, tunnels or dams are fixed.

The Main Frontal Thrust (MFT), also called the Himalayan Frontal Thrust (HFT), is the southernmost major thrust of the Nepal Himalaya. It separates the Siwalik zone in the north from the Terai (Indo-Gangetic) plain in the south and runs roughly east-west along the foot of the Churiya range.

• Movement: the Siwalik rocks (Miocene-Pleistocene) are thrust southward over the Quaternary alluvium of the Terai.
• Activity: it is the youngest and most active of the Himalayan thrusts; it is active even today, with Holocene scarps and uplifted terraces.
• Nature: a series of splays, and in places blind thrusts.
• Engineering importance: earthquake hazard, surface rupture, ground deformation, slope instability at the Churiya foot, changes in river course and groundwater, and so it affects dams, barrages, bridges and roads of the Terai.

Physiographic division is based on the surface landform, relief and altitude. Tectonic division is based on geological structure, i.e. rock types, age and bounding thrusts or faults.

Physiographic zones

Tectonic zones

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.

Physiographic divisions of the Himalaya

Siwalik (Churiya) zone. The southernmost hill belt of Nepal, 10-50 km wide, altitude about 300-1500 m (up to 2000 m), lying between the MFT and MBT. Rocks are Miocene to Pleistocene molasse sediments, eroded from the rising Himalaya, mostly weak and poorly consolidated.

Lithology (three divisions)

• Lower Siwalik: alternation of fine-grained sandstone, mudstone and siltstone; red, purple and grey colours (Miocene).
• Middle Siwalik: thick, massive, coarse "salt and pepper" sandstone with some mudstone and conglomerate lenses (Mio-Pliocene).
• Upper Siwalik: poorly cemented boulder-pebble conglomerate with sandstone and clay (Plio-Pleistocene).

Structure: the strata are folded into anticlines and synclines with strike roughly parallel to the Himalaya (WNW-ESE), often steeply dipping or overturned in the north near the MBT, and cut by thrust faults such as the MBT and MFT.

Engineering problems: soft, erodible rocks, landslides, gullying, debris flow, and flash floods; but sandstones give good aggregate.

Higher (Great) Himalaya. Lies between the MCT and STDS; altitude about 4000-8848 m. Rocks are Precambrian to Early Palaeozoic high-grade metamorphic rocks and granites.

Lithology (three formations)

• Formation I (lower): kyanite-sillimanite schist and gneiss, with migmatite.
• Formation II (middle): calc-silicate gneiss, marble and diopside bearing rocks.
• Formation III (upper): augen gneiss, quartzite, and Manaslu and other leucogranite intrusions.

Structure: the Higher Himalaya is a thick northward-dipping sequence of rocks with a steep foliation, folded and cut by thrusts such as the MCT zone, with a large number of glacial and periglacial features.

Engineering problems: very high relief, rockfall, avalanche, glacial lake outburst floods (GLOF), glacial and moraine hazards, and the need for blasting and difficult access.

Major discontinuities of the Nepal Himalaya

Siwalik zone

Lithology (three divisions)

• Lower Siwalik: alternation of fine-grained sandstone, mudstone and siltstone; red, purple and grey colours (Miocene).
• Middle Siwalik: thick, massive, coarse "salt and pepper" sandstone with some mudstone and conglomerate lenses (Mio-Pliocene).
• Upper Siwalik: poorly cemented boulder-pebble conglomerate with sandstone and clay (Plio-Pleistocene).

Structure: the strata are folded into anticlines and synclines with strike roughly parallel to the Himalaya (WNW-ESE), often steeply dipping or overturned in the north near the MBT, and cut by thrust faults such as the MBT and MFT.

Higher Himalaya

Lithology (three formations)

• Formation I (lower): kyanite-sillimanite schist and gneiss, with migmatite.
• Formation II (middle): calc-silicate gneiss, marble and diopside bearing rocks.
• Formation III (upper): augen gneiss, quartzite, and Manaslu and other leucogranite intrusions.

Churiya (Siwalik) range

• Altitude: about 300-1500 m (up to 2000 m).
• Lithology: sandstone, mudstone, siltstone and conglomerate (Lower, Middle and Upper Siwalik); soft, poorly consolidated.

Fore Himalaya (Midland / Mahabharat region)

• Altitude: roughly 1000-3000 m (about 2000-3000 m in the Mahabharat range).
• Lithology: phyllite, slate, schist, quartzite, limestone, dolomite, gneiss and granite (Lesser Himalayan rocks).

Trans Himalaya (Tibetan-Tethys, Inner Himalaya)

• Altitude: about 2500-5000 m (rain-shadow plateau).
• Lithology: Palaeozoic to Cretaceous fossiliferous sedimentary rocks: limestone, shale, sandstone, slate and quartzite.

The altitude limits are approximate textbook values, and vary between authors.

Lesser Himalaya (Midland, Mahabharat) zone. Lies between the MBT in the south and MCT in the north; altitude about 1000-3000 m. Width 60-80 km. Rocks are Precambrian to Palaeozoic (some up to Eocene) low-grade metamorphic and sedimentary rocks, with a few granitic intrusions.

Lithology

• Nawakot Complex: slate, phyllite, quartzite, limestone, dolomite and metasandstone (Kuncha, Fagfog, Dunga, Nourpul, Dhading, Benighat, Malekhu and Robang formations).
• Kathmandu Complex (nappe): phyllite, marble, schist and quartzite (Chandragiri, Chitlang, Tistung, Kalitar, Sopyang formations), with the Phulchoki Group.
• Granite and gneiss bodies: Palung granite, Ulleri augen gneiss, Bhimphedi group.

Structure: intensely folded, faulted and thrusted. There are synclinoria (Kathmandu and Mahabharat nappes) with klippen, and anticlinoria, with a large number of thrusts and tectonic windows. The beds mostly dip northward.

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

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 (Kuncha, Fagfog, Dunga, Nourpul, Dhading, Benighat, Malekhu and Robang formations).
• Kathmandu Complex (nappe): phyllite, marble, schist and quartzite (Chandragiri, Chitlang, Tistung, Kalitar, Sopyang formations), with the Phulchoki Group.
• Granite and gneiss bodies: Palung granite, Ulleri augen gneiss, Bhimphedi group.

Structure: intensely folded, faulted and thrusted. There are synclinoria (Kathmandu and Mahabharat nappes) with klippen, and anticlinoria, with a large number of thrusts and tectonic windows. The beds mostly dip northward.

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

Lesser Himalaya

Lithology

• Nawakot Complex: slate, phyllite, quartzite, limestone, dolomite and metasandstone (Kuncha, Fagfog, Dunga, Nourpul, Dhading, Benighat, Malekhu and Robang formations).
• Kathmandu Complex (nappe): phyllite, marble, schist and quartzite (Chandragiri, Chitlang, Tistung, Kalitar, Sopyang formations), with the Phulchoki Group.
• Granite and gneiss bodies: Palung granite, Ulleri augen gneiss, Bhimphedi group.

Structure: intensely folded, faulted and thrusted. There are synclinoria (Kathmandu and Mahabharat nappes) with klippen, and anticlinoria, with a large number of thrusts and tectonic windows. The beds mostly dip northward.

Tibetan-Tethys Himalaya

Lithology: Cambrian to Cretaceous (and Eocene) marine sedimentary rocks, rich in fossils: limestone, shale, sandstone, slate, quartzite and marl; the Thakkhola graben has Neogene sediments. Famous ammonite fossils (shaligram) of the Kali Gandaki valley come from Jurassic shale.

Structure: thick and gently folded, with some normal faults (Thakkhola Graben).

Major lithological difference: Siwalik vs Higher Himalaya

Eluvial soil vs lacustrine soil

Terai zone

Flat Quaternary alluvial plain (60-300 m), divided into:

• Bhabar: coarse boulder and gravel belt at the foot of the Siwalik.
• Terai proper: sand, silt and clay with a high water table.

Siwalik zone

Hills of 300-1500 m (up to 2000 m) between the MFT and MBT, divided into:

Siwalik also has Dun valleys (e.g. Chitwan and Dang) filled with alluvium. The Terai is the youngest, the Siwalik is older and is thrust over the Terai at the MFT.