A note on the lithospheric thickness of the Kumaun - Garhwal Himalaya from change in viscosity at the lithosphere-asthenosphere boundary

The Himalaya with a strike length exceeding 2500 km is believed to have resulted from the collision of the Indian plate with the Eurasian plate during the Eocene to Miocene times. Based on the mean surface heat flow density and radioactive heat generation values of 68 mW / m2 and 2.7 μW/ m3 respectively, a part of the Kumaun- Garhwal Himalaya (Lat: 29�-31�N; Long: 79�- 81� E ) is found to have a lithospheric thickness of about 123 km, as estimated by the depth, at which the high surface value of the viscosity dropped to about 1021 poise (1020 kg/m/sec) corresponding to the viscosity-depth curve. The intersection of the mantle melting (solidus) curve with the geotherm of the Kumaun-Garhwal Himalaya at a depth of about 123 km, also provided the lithospheric thickness in respect of the Kumaun-Garhwal Himalaya.