Geological & Tectonic Zones of Nepal: Terai to Tethys

Nepal sits on the collision front of the Indian and Eurasian plates, where the ocean once known as the Tethys Sea closed and its floor was crushed into the Himalaya. Around 55 million years ago the northward-drifting Indian plate began colliding with Eurasia, and that collision is still active today. As a result, the country's geology is not arranged in random blocks but in five long, roughly east-west (longitudinal) belts that run the full length of Nepal, each formed at a different time by a different process and then piled on top of the next by faulting.

The Department of Mines and Geology (DMG), Nepal's official geoscience body, recognises these five geological regions from north to south as the Tethys Himalaya, the Higher Himalaya, the Lesser Himalaya, the Siwalik (Chure), and the Terai. This same five-fold tectonic division is the standard framework in stratigraphy and tectonics overviews of the Nepal Himalaya (for example Upreti and colleagues) and is a core topic for SEE, +2, BSc geology and Lok Sewa examinations.

The zones broadly mirror Nepal's physiography: the flat Terai in the south rises through the Chure hills and the midlands to the high snow peaks and the arid trans-Himalayan valleys in the north. Understanding this tectonic layout explains where Nepal's limestone, magnesite, coal and gemstones occur, why the Chure is so landslide-prone, and why the whole country lies in a high seismic-hazard belt.

The engine behind Nepal's geology is the continuing convergence of India into Eurasia, which in central Nepal proceeds at roughly 4 to 5 centimetres (about 40 to 50 millimetres) per year. A large part of this shortening is absorbed on the Main Himalayan Thrust (MHT), the master detachment that dips gently northward beneath the entire mountain chain and on which the Indian plate slides under the Himalaya. The four surface faults that separate the tectonic zones — from south to north the MFT, MBT, MCT and STDS — all branch upward from, or relate to, this deep MHT.

The Main Frontal Thrust (MFT), also called the Himalayan Frontal Thrust (HFT), is the youngest and southernmost fault; it carries the Siwalik hills over the Terai plain and marks the active leading edge of the mountains. Behind it, the Main Boundary Thrust (MBT) pushes the older Lesser Himalaya over the Siwalik. The Main Central Thrust (MCT) then thrusts the crystalline Higher Himalaya over the Lesser Himalaya and was active around 20 million years ago.

Uniquely, the northern boundary is not a thrust but a set of normal faults: the South Tibetan Detachment System (STDS), a north-dipping low-angle normal fault that drops the Tethys sediments down relative to the Higher Himalaya beneath them. This unusual pairing of the MCT (compression) below and the STDS (extension) above helped extrude the hot Higher Himalayan slab toward the surface — a mechanism central to modern Himalayan tectonics.

Because the shallow part of the MHT beneath the Lesser Himalaya is locked and steadily storing elastic strain, Nepal accumulates energy that is released in great earthquakes. The magnitude 7.8 Gorkha earthquake of 25 April 2015 (12 Baishakh 2072 BS) ruptured a portion of this locked MHT, and the historic 1934 Nepal-Bihar earthquake did so earlier, underlining that these thrusts are living faults, not museum pieces.

The Terai is the southernmost and youngest tectonic zone, a flat, fertile plain generally lying below about 200 metres above sea level and forming the northern edge of the Indo-Gangetic plain. Geologically it is a thick pile of Recent to sub-Recent alluvium — boulders, gravel, sand, silt and clay — shed from the rising Himalaya and spread by the Ganga and its tributaries. This alluvial cover can be over one kilometre thick (around 1,500 metres in places) and conceals the buried Siwalik-equivalent rocks and the northern edge of the Indian shield beneath it.

The Terai is bounded to the north by the Main Frontal Thrust, along which the Chure hills are actively rising over the plain. Its economic value lies less in hard-rock minerals than in its deep sediments and groundwater: the zone hosts construction aggregates (sand and gravel), rich agricultural soils, and is the part of Nepal with the greatest potential for petroleum and natural gas exploration in the buried foreland-basin sequence.

Being low, flat and densely settled, the Terai's principal hazards are hydrological rather than tectonic in origin: monsoon flooding, waterlogging, riverbank cutting and, near the MFT, the potential for strong shaking and liquefaction during Himalayan earthquakes.

North of the Terai lies the Siwalik zone, known in Nepal as the Chure and internationally as the Sub-Himalaya. It is a belt roughly 10 to 25 kilometres wide of Himalayan foreland-basin deposits — an upward-coarsening sequence of mudstone, siltstone, sandstone and, in the Upper Siwalik, thick conglomerate. These sediments are Neogene to Quaternary in age, deposited from about 20 million to 2.5 million years ago as debris eroded from the growing mountains to the north.

The Siwalik is bounded by the MFT on the south and the MBT on the north, and it encloses several flat-floored inner basins called Dun valleys (such as Chitwan, Dang, Deukhuri and Surkhet). The zone is famous for its vertebrate fossils — remains of ancient elephants (Stegodon), crocodilians and early apes — recording the animals of the ancient river floodplains.

Geologically the Chure is very young, soft and only weakly cemented, which makes it Nepal's most fragile landscape. Its mudstones and loose conglomerates are highly prone to gully erosion, shallow landslides and debris flows during monsoon downpours, a fragility worsened by deforestation and unregulated extraction of sand, gravel and boulders. Protecting the Chure is therefore a national conservation priority, addressed through the President Chure-Terai Madhesh Conservation programme.

The Lesser Himalaya forms the broad midland belt of Nepal, some 60 to 80 kilometres wide, sandwiched between the MBT to the south and the MCT to the north. It is built of low- to medium-grade metasedimentary and sedimentary rocks — slate, phyllite, schist, quartzite, limestone, dolomite, shale and sandstone — that were originally deposited on the northern margin of the Indian plate. Their ages span an enormous range, from roughly 1.9 billion years old to comparatively young Tertiary rocks.

These rocks preserve some of the oldest fossils in Nepal, including stromatolites (layered structures built by microbial mats over a billion years ago) and, in younger units, nummulites of about 45 million years. Structurally the zone is complex, folded into large features such as the Kunchha-Gorkha anticlinorium, with metamorphism generally increasing toward the MCT in the north.

The Lesser Himalaya is economically the richest zone for Nepal's known mineral resources. It hosts important deposits of metallic minerals (such as iron, copper, lead-zinc and cobalt), industrial and carbonate minerals (limestone for cement, dolomite, magnesite, talc), and Nepal's principal coal and slate occurrences. Because its slopes are steep, deforested and heavily settled, the midlands are also a major zone for rainfall-triggered landslides.

The Higher (Greater) Himalaya is the crystalline backbone of Nepal, a slab of high-grade metamorphic and igneous rock roughly 5 to 15 kilometres thick that contains the country's greatest peaks. Bounded by the MCT below and the STDS above, it consists of kyanite- and sillimanite-bearing gneisses, schists, marbles and migmatites, intruded by younger High Himalayan leucogranites. Although its protolith sediments are Late Proterozoic, the rocks we see were buried to roughly 30 kilometres depth, cooked at high temperature and pressure during the collision, and then rapidly exhumed to the surface starting around 20 million years ago.

This deep metamorphism grew index minerals such as garnet, kyanite, staurolite, sillimanite, biotite and tourmaline, and the zone is the most promising source of precious and semi-precious gemstones and certain metallic minerals in Nepal. Its steep, glaciated high terrain also makes it the source area for glacial-lake outburst floods (GLOFs), rockfalls, avalanches and the large co-seismic landslides seen in 2015.

Above the STDS lies the northernmost zone, the Tethys (Tibetan) Himalaya, best developed in the Thak Khola (Mustang), Manang and Dolpa regions behind the high peaks. It is a thick succession — several kilometres of shale, sandstone and fossil-rich limestone — that was laid down in the shallow Tethys Sea on India's passive continental margin from Cambrian to Eocene times. These strata carry marine fossils such as ammonites and crinoids (the black spiral shaligram stones of the Kali Gandaki come from here) and host limestone, gypsum, rock salt and potential natural gas, but being high and arid the zone's main hazards are rockfall, debris flows and earthquake shaking.