Himalayan uplift by means of mineral dating and petrological study.

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The Himalayas heve been uplifted since Neogene. The uplift process of the Himalayas, as well as the relationship between the uplift and the movement of the Main Central Thrust zone, was investigated on the basis of metamorphic and tectonic studies in central Nepal and mineral datings by K-Ar and fission track methods. The following conclusions are reached:1. The Himalayas have undergone the polymetamorphism of barrovian-type metamorphism in Paleogene and shear-heating metamorphism in Neogene apart from some episodes of metamorphism in older ages.2. The meso- to micro- structural analyses on rocks of the Lesser and Higher Himalayas show that four stage of deformation are recognized in central Nepal and that activity of the second stage, which is due to thrusting of the Main Central Thrust zone, is most intense and extensive.3. The K-Ar ages of 61.5<plus-minus>3.1 Ma (hornblende) and 56.7<plus-minus>2.8 Ma(biotite) of the Himalayan gneisses are considered to show the cooling stage of the Paleogene metamorphd st4. Fission track datings on apatite and garnet from the Himalayan gneisses were unsuccessful owing to deficiency of their uranium content.5. Fission track ages of 14 zircons from 4 areas in the Himalayan gneiss zone have been dated to be 2.2-0.6 Ma with errors of 0.2-0.1 Ma. These ages are considered to show the cooling stage of the Neogene metamorphism.6. The relationsphi of these fission track ages to the elevation where the rock samples were collected indicates that the uplift rates of the Himalayan gneiss zone during 2.2-0.6 Ma are 3.24mm/y and 2.39mm/y for the ridge areas and 0.83mm/y and 0.47mm/y for the valley areas.7. Under the assumption that the anealing temperature of zircon is 200゜C and the geothermal gradient is 30゜C/km, the uplift rates during Quaternary are estimated to be 3.5-6.1mm/y. Therfore, the uplift rate of the Himalayas was accelerated in Quaternary.