Two-dimensional thermal modelling of the early tectonometamorphic evolution in central Himalaya

The Higher Himalayan Crystallines is a major structural feature of the Himalayan belt characterized by a typical collisional and metamorphic evolution from Eocene to Miocene. We use two-dimensional thermal modelling to examine the velocity and the duration of the two main phases of metamorphism in the Higher Himalayan Crystallines. Our calculations suggest that the Eohimalayan metamorphism is not a simple consequence of the burial of the Indian plate below the upper Himalayan wedge. The M1 metamorphic conditions (700 °C, 1 GPa) recorded in the central Himalaya between 37 and 32 Ma corresponds to a period of thermal relaxation of the Higher Himalayan Crystallines. During this period, part of the Himalayan convergence had taken place in the Himalayan wedge along south-vergent thrusting structures, located north and south of the future Main Central Thrust (MCT) zone. Relatively high temperature (~600 °C) recorded in the MCT between 23 and 18 Ma (Neohimalayan metamorphism) may be explained by high heat production rates, ~3 mu W/m3, in the upper Himalayan crust and high exhumation rates, greater than 3 mm/year, of the Higher Himalayan Crystalline rocks. By analogy with modern accretionary wedges, we propose that the Eohimalayan phase corresponds to the period of the formation of the Himalayan wedge through several internal and external thrusting. During the subsequent Neohimalayan phase, the Himalayan wedge reached a thermal equilibrium, leading to the localisation of the Himalayan convergence only along the MCT zone.