Structural evolution of the High Himalayan Gneiss sequence, Langtang Valley, Nepal

Journal article

Within the High Himalayan Crystalline Sequence (HHCS) of the Langtang Valley of central Nepal, four different lithotectonic units have been identified in the field. The structurally lowest unit, immediately in the hanging wall of the Main Central Thrust (MCT) records an inverted metamorphic sequence with kyanite-grade rocks overlain by rocks containing sillimanite. Pressure-temperature-deformation histories of the different units since Himalayan collision 50 Ma ago, in combination with geochemical data, indicates that the 'apparent' metamorphic inversion is a result of the tectonic juxtaposition along high-temperature shear zones that accommodated top-to-southwest thrusting. It is also argued that the contacts between different sillimanite-grade units are also tectonic in origin and that the HHCS in the Langtang Valley does not represent a structurally undisturbed section through the crust. Internally the different units contain minor extensional and thrust faults which at the same structural level developed at similar metamorphic conditions. Deformation along the high temperature shear zones of the HHCS are coeval with the earliest of these minor fault zones at the lowest structural levels. With time, this deformation migrated upwards and minor faulting within the upper levels of the HHCS took place at chlorite-grade. These minor fault zones are interpreted to be the result of dynamic spreading of the developing orogenic wedge associated with changes in the geometry of this wedge due to underplating and accretion of material to the base of the metamorphic pile. © 1993 The Geological Society.

Authors: Reddy, S; Searle, M; Massey, J

• Journal: Geological Society Special Publication
• Volume: 74
• Issue: 1
• Pages: 375-389
• Publication date: 1993-01-01
• DOI: 10.1144/GSL.SP.1993.074.01.25
• ISSN: 0305-8719
• Language: English