1. GEOLOGY OF NEPAL HIMALAYA
The Himalaya range is about 2400 km. long and extends from Indus rive of Pakistan in the west to Assam in 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.
5.1 Geological division of Nepal
Geologically Nepal Himalayas can sub-divide in to five tectonic zones from south to north. The tectonic are 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
5.1.1. Terai zone :
Terai zone is the northern continuation of the gangatic plains of India. It extends from the indo-Nepal boarder in the south to the base of churia or siwaliks 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.
5.1.2. 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 conglomerate, and stone, silt stone, mudstone and marls. According to litho logy, this can be further divided into three zones as follows:
• Upper Siwaliks : Consists of conglomerate.
• Middle Siwaliks : Consists of coarse sandstone.
• Lower Siwaliks : Consists of fine sandstone, mudstone.
5.1.3. Lesser Himalayan zone:
This zone lies to the north from the siwaliks. Lesser Himalaya zone of Npal forms the major geological zone and is important in majority of hill population of the view. The unmetamorphosed young sedimentary rock of churia or siwaliks zone is directly come in contact with metamorphosed rock sequences of lesser Himalaya, which is separated from the siwaliks 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, synite, pegmatite etc. in this zone.
5.1.4. 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 grate metamorphic rocks in association with high grate index materials (like kyanite, silliminite, garnet etc.) like various gneisses, schist quartzite’s, marbles. At some places granite intrusion mark their upper and northern limit. this zone is high snow covered Himalayan mountain chain. This zone is characterized by extremely high relief, steep topography, rockly cliff and outcrops with little soil cover terrain.
5.1.5. Tibetan-Tethys zone;
This zone lies north from 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 fluvio-glcial 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.
5.2.1. 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 it 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 in august 28, 1916.
5.2.2. Main boundary thrust (MBT):
This is the active tectonic contact between the lesser Himalaya and the siwaliks. The MBT has been the source of very large earthquake in the past. The maximum potential earthquake in the feature has a magnitude of 8.
5.2.3. 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.
5.3Geology of Malekhu area
In course of the fieldwork, the geology of Malekhu area along Malekhu Khola have been studied.
Nepal, the rocks belonging to the Kathmandu complex are thrust over the Nuwakot Complex along the MT (Mahabharat Thrust) (Stocklin and Bhattarai 1977). 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 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 colour and medium to fine grained There are numerous faults in this formation