The Himalaya range is about 2400 km. long and extends from Indus river of Pakistan in the west to Assam in the east. The Himalaya belt is the results of collision of two tectonic plates, Indian plate and Tibetan plate on the north. Before the collision of the plates, the place was occupied by Tethys Sea. The collision of the plate s was still going on.

Geological division of Nepal

Geologically Nepal Himalayas can sub-divide into five tectonic zones from south to north. The tectonic is extending east to west and almost parallel to sub-parallel. The zones are as follows.

1.      Indo-Gangetic plain or terai zone

2.      Sub Himalayas or siwaliks or churia  zone

3.      Lesser Himalayan zone

4.      Higher Himalayan zone

5.      Tibetan-Tethys-zone

Terai zone :

Terai zone is the northern continuation of the Gangetic plains of India. It extends from the Indo-Nepal border in the south to the base of churia or Siwalik hills in the north. Geologically this zone is composed of recent alluvium deposit consisting of sediments in the north and finer sediment like silt, in the southern part.

siwalik or churia zone:

This group represents the lower hill of the Churia range which, lies between MFT (south) and MBT (north). The Siwaliks constitutes the narrow belt of 20 Km to 30 Km in width 5 Km to 6 Km in thickness runs east-west. The age range is indicated as Middle Miocene to Early Pleistocene. This zone is covered by dense forest and the fossiliferous horizons are frequently occurring in this zone. Geologically this zone is composed of loose to consolidate north dipping sedimentary rock like a conglomerate, and stone, siltstone, mudstone, and marls. According to lithology, this can be further divided into three zones as follows:

•  Upper Siwaliks: Consists of the conglomerate.

•  Middle Siwaliks: Consists of coarse sandstone.

• Lower Siwaliks: Consists of fine sandstone, mudstone.

Lesser Himalayan zone:

This zone lies to the north from the sidewalks. Lesser Himalaya zone of Nepal forms the major geological zone and is important in the majority of hill population of the view. The unmetamorphosed young sedimentary rock of churia or sidewalks zone directly comes in contact with metamorphosed rock sequences of lesser Himalaya, which is separated from the sidewalks by MBT. Geologically this zone mainly composed of low-grade metamorphic rock like slate, Phyllite, schist, quartzite, marble and sedimentary rocks limestone and dolomite, shale etc. in the south. In some region, there is some magnetic intrusion like granite, syenite, pegmatite etc. in this zone.

Higher Himalaya zone:

This zone lies north from the lesser Himalaya zone and is separated by main central thrust (MCT). This zone is made up of high-grade metamorphic rocks in association with high grate index materials (like kyanite, sillimanite, garnet etc.) like various gneisses, schist quartzite’s, marbles. At some places, granite intrusion marks their upper and northern limit. this zone is high snow-covered Himalayan mountain chain. This zone is characterized by extremely high relief, steep topography, rocky cliff and outcrops with little soil cover terrain.

Tibetan-Tethys zone:

This zone lies north from the higher Himalayan zone. This zone Is composed of fossiliferous sedimentary rocks like shale, limestone, sandstone etc. this zone has lower relief and ruggedness than higher Himalaya. This wide valley is covered with thick glacial and fluvial-glacial deposits along with recent alluvium. These deposits are very loose and fragile. Steep slopes are dominated at places. 5.2. Major tectonic zones in the Nepal Himalaya

The tectonics of Nepal Himalaya is summarized as the three tectonics zones; viz. main central thrust, main boundary thrust zone, and Himalayan frontal thrust or main frontal thrust (MFT) zone. These are the zone in Himalayan which is highly hazardous and major triggering factor for landslides hazards and other types of mass movements.

Main central thrust (MCT):

This is the tectonic contact between the higher Himalayas and lesser Himalayas. It is a north-dipping thrust fault, which at one time was a convergent plate boundary. The MCT was actives during the early phase of Himalayan orogeny but is considered to be less active then main boundary thrust (MBT). Based on historical recall the largest earthquake recorded in the MCT zone were 7.5 magnitude events on August 28, 1916.

Main boundary thrust (MBT):

This is the active tectonic contact between the lesser Himalaya and the sidewalks. The MBT has been the source of the very large earthquake in the past. The maximum potential earthquake in the feature has a magnitude of 8.

Himalaya frontal thrust or main frontal thrust (HFT) or (MFT)

This is the tectonic feature located at the boundary of the Siwalik and the Terai. This is also considered active. The maximum earthquake potential of this fault is 6.5 in magnitude.

Geology of Malekhu area

In course of the fieldwork, the geology of Malekhu area along Malekhu Khola has been studied.

In central Nepal, the rocks belonging to the Kathmandu complex are thrust over the Nuwakot Complex along the MT (Mahabharat Thrust) (Stocklin and Bhattarai 1977). The present study area covers the upper part of Nuwakot Complex constituting the footwall and the lower part of the Kathmandu Complex comprising the Hanging wall. The outcrop consists mainly of phyllite, which is associated with quartzite. The quartzite is massive, thick bedded and yellowish white in color. The phyllites are seen as dark green patches. One can also find a greenish colored metamorphic rock, amphibolite. In this zone, a hill of quartzite (Dunga Quartzite) is observed. Foliation plane, joints, and fracture are distinct in Robang Phyllites. Limestone is also exposed in the left bank of Malekhu Khola and around the suspension bridge over the Trisuli. Limestone is in gray color and medium to fine-grained There are numerous faults in this formation.