The Chako antiform: A folded segment of the Greater Himalayan sequence, Nar valley, Central Nepal Himalaya

Recent and previously published mapping in the Nar valley, north of the Annapurna massif in central Nepal, recognised an enigmatic metamorphic culmination of the Greater Himalayan sequence, surrounded by rocks correlated with the Tethyan sedimentary sequence. The map area is re-interpreted as two structural levels within the Greater Himalayan sequence. The Lower Level is correlated to the well-studied Greater Himalayan sequence of the Annapurna region, based on characteristic rock types, high-strain zones with south-verging shear-sense indicators, and high-grade metamorphism. The rocks of the Upper Level, previously mapped as the sub-greenschist or zeolite facies Tethyan sedimentary sequence, are garnet-bearing schists. Petrography and garnet-biotite thermometry suggest the Upper Level equilibrated at amphibolite facies (500-650 °C). These results support the recent contention that the Upper Level rocks are a component of the Greater Himalayan sequence, and thus indicate that components of the Himalayan metamorphic core vary considerably along strike. Structural, metamorphic and age constraints are integrated into a cohesive regional tectonometamorphic model. The Lower and Upper Levels both experienced D1 deformation and peak metamorphism before ∼20 Ma. The Lower and Upper Levels were juxtaposed along the synmetamorphic Chame detachment after 20 Ma during retrograde metamorphism. After ∼19 Ma, the Phu detachment placed the unmetamorphosed Tethyan sedimentary sequence above the Lower and Upper Levels. The entire package was then subsequently folded, after 19 Ma, by a non-cylindrical antiform-synform pair with a ∼25 km wavelength creating an apparent dome. The proposed tectonometamorphic model reconciles previously contradictory interpretations of the transition between metamorphic core and overlying sediments in the Nar valley.