Fluid evolution and brittle tectonic reactivation of the Main Central Thrust in Lower Dolpo (Himalayan chain, Western Nepal)

The Main Central Thrust Zone (MCT) in Lower Dolpo (Western Nepal) affects both the lower part of the Higher Himalayan Crystallines (HHC) and the upper part of the underlying Lesser Himalayan Sequence (LH) giving rise to a thick zone of penetrative mylonitic deformation. A sudden change in lithology, from the upper migmatitic gneiss and kyanite-bearing gneiss to a lower complex made by garnet-bearing micaschists, white quartzites, amphibolites, paragneisses and phyllites marks the boundary between the HHC and the LH. In the MCT zone an inverted metamorphic field gradient has been confirmed and biotite, garnet, staurolite and kyanite are found progressively upward in the sequence. Shear along the MCT zone and the STDS and erosion in the front of the belt allowed the ductile extrusion of the HHC from depth up to the higher structural levels (HODGES et alii, 1992; GRUIJC et alii, 1996; 2002). This path has been recorded in the MCT zone by a spectacular main ductile fabric with mylonites showing a top to the SW sense of shear, overprinted by later ductile/brittle and brittle shear zones. Overprinting brittle-ductile shear zones point to a top-to-the SW sense of shear too, testifying a later compressive reactivation of the MCT zone after the main ductile phase at upper structural levels. To constraint P-T conditions during the later tectonic evolution of the MCT zone fluid inclusion analyses have been performed on quartz lenses from the Main Central Thrust Zone. The studied fluid inclusions, found either in isolated clusters and along trails, are twophases (liquid water + liquid carbonic fluid) carbonic inclusions at room temperature. They are characterized by quite regular negative crystal shapes even if sometimes irregular morphologies have been observed. Three different groups of fluid inclusions have been distinguished on the basis of different liquid-vapor ratio. The isochores for representative fluid inclusions, compared with the geothermobarometric data and mineral assemblages in the host rock, indicate lower pressure- temperature conditions for their trapping in accordance with the retrograde P-T evolution found in the MCT zone of Garhwal Himalaya. The presence of brittle ductile shear zones overprinting the mylonites of the MCT zone could also suggest that deformation after the MCT activity proceeded both toward the foreland and by out of sequence thrusts.