Étude pétro-structurale des chevauchements ductiles himalayens sur la transversale de l'Everest–Makalu (Népal oriental)

Abstract

Tectonic and microtectonic data in eastern Nepal indicate that the major observed thrusting (100 km) on the Main Central Thrust (MCT) postdates the Barrovian metamorphism of the High Himalaya gneisses. This result, at variance with the famous "reverse metamorphism model," better explains the abnormal metamorphic superpositions in the Himalayas and accounts for the lack of high-pressure assemblages under the thick, allochtonous High Himalaya Tibetan slab.Pressure and temperature estimates by microprobe analysis on plagioclase, biotite, garnet, kyanite, sillimanite, and cordierite assemblages are presented for samples collected along the MCT shear zone and across the gneiss slab in the Everest–Makalu area. Since there is very little difference in pressure at the front of the slab (Kathmandu Klippe) and its root, these estimates support the existence of important late metamorphic thrusting. The decrease of pressure towards the top of the gneiss pile, combined with a small temperature increase, explains the kyanite–sillimanite transition. The reverse metamorphism model, which implies refolded isograds, predicts heat loss by conduction throughout the sole of the thrust; pressure–temperature variations and kyanite–sillimanite transition phases more likely reflect a late heat supply in the upper part of the gneisses. Intrusion of leucogranitic bodies, confined to the interface with the Tethyan sediments, could account for this heat supply.A new tectonic evolution model of the Himalayan intracrustal thrusts is discussed. Without completely denying the existence of a reverse metamorphism synchronous with the phases of early shearing, it can be shown that the metamorphic zonation seen at present was governed by the structure of the later shearing.