INVERTED METAMORPHISM OF NEPAL HIMALAYA

Nepal Himalaya is tectonically divided into five geological regions from the foot of Himalaya (Indo-Gangetic plain) up to the top (Tibetan-Tethys Himalaya). So these tectonically different setting shows a common metamorphic process called inverted metamorphism. It is considered to be one of the distinct characters of Nepal Himalaya. Generally inverted metamorphism is when the high grade metamorphic rocks settle upwards than low grade metamorphic rocks. This is because of shear heating due to the thrust movement along MCT in case of Nepal Himalaya. Due to this thrust the inverted metamorphism is distinct from foot wall of MCT (Upper part of Lesser Himalaya) up to its hanging wall (lower part of Higher Himalaya) and it is underlain by right – way – up metamorphic sequence.

The following are the points that explain the inverted metamorphism of Nepal Himalaya:

1)            Shear heating along the MCT.

2)            Large recumbent folding of metamorphic isograds.

*isograd = it is a plane of constant metamorphic grade.

 

3)            Le Fort (1975) proposed a model called “HOT IRON MODEL”. This model explains the diffusion of heat from hotter Himalayan Crystalline to a cooler Lesser Himalaya. This movement of heat from higher to lower tectonic setting is highly accepted.