Geological and structural architecture of the Kangchenjunga region, eastern Nepal

Structural-petrographic studies based on geological mapping were performed at the western fl ank of the Kangchenjunga massif, where the Lesser Himalayan Sequence (LHS) is exposed as a regional antiform (the Taplejung Window) beneath the Greater Himalayan Sequence (GHS), the latter being divided in the lower Inverted Metamorphic Sequence (IMS) and the upper Higher Himalayan Crystallines (HHC). The juxtaposition of the high-grade mid-crustal HHC over the low- to medium-grade LHS is structurally recorded by the Main Central Thrust Zone (MCTZ), roughly equivalent to the IMS and typically showing a persistent top-to-the-south sense of regional thrusting. The boundaries of the MCTZ are not identifi able by single thrusts or by lithological contacts, but they are envisaged as zones of high strain localization. The lower boundary of the MCTZ is marked by phyllonites and mylonitic schists in the uppermost portions of the LHS near to the contact with the strongly mylonitic augen gneisses of the IMS. The upper boundary of the MCTZ is roughly located at the base of the Grt-Kfs-Ky-Sil anatectic gneiss (Barun-type) in the lower portion of the HHC, characterized by ductile shearing and boudinage. The thickness of the MCTZ ranges from 6-7 km in the north-western sector to 3-4 km in the south-eastern sector of the studied area. The overprinting of late foldings, mainly N to NE - S to SW and WNW-ESE striking, is likely responsible for the regional doming geometry of the investigated area. Southward thrusting and dome formation could have been also favoured by the activity of high-angle, NE-SW striking brittle to brittle-ductile shear zones, such as the one observed in the Yampudin area.