The Higher Himalayan Crystallines, Alaknanda – Dhauli Ganga Valleys, Garhwal Himalaya, India

In central parts of Uttarakhand, more than 20 km thick homoclinal NE-dipping Higher Himalayan Crystalline (HHC) belt is almost continuously exposed between Helang and Malari along the Dhauli Ganga Valley, and is thrust over the Lesser Himalaya along the Main Central Thrust (MCT). This road dip-section traverse provides an excellent opportunity to investigate the (i) ambiguity regarding the position and definition of the MCT in terms of the Munsiari Thrust (the MCT–I), and the Vaikrita Thrust (the MCT–II), (ii) position of the South Tibetan Detachment System (STDS), (iii) deformation of the HHC, based on detailed shear sense analysis exhibiting top-to-south and top-to-north shear indicators, (iv) structural control on melt accumulation of the Himalayan migmatites and leucogranite, (v) Himalayan inverted metamorphism, and (vi) assessment of channel flow or other models for the exhumation of deep-seated crustal rocks.Based on lithologies and metamorphic grade from the lower to higher structural levels northeastwards, the HHC is divisible into two main groups above the Munsiari Thrust (MCT). In the lower parts, the Munsiari Group of low to medium grade contains a highly mylonitized package of garnet mica schist/ gneiss, quartzite, amphibolite and biotite-rich phyllonite, mylonitic gneiss and augen gneiss. Delineated by the Vaikrita Thrust (VT) the overlying Vaikrita Group is comprised of the Joshimath Formation (garnet-biotite-muscovite schist/psammitic gneiss), the Suraithota Formation (kyanite-garnet-biotite schist/psammitic gneiss and amphibolite), and the Bhapkund Formation (sillimanite/fibrolite-kyanite- garnet-biotite psammitic gneiss/schist with pervasive migmatite, concordant to discordant pegmatite veins, and small tourmaline-rich leucogranite lenses/dykes and the Malari leucogranite). The Vaikrita Group is typically characterized by inverted metamorphism, where sillimanite–K-feldspar gneiss and migmatite in uppermost parts of the Bhapkund Formation was metamorphosed under upper amphibolite facies at > 650 °C. The Bhapkund Formation constitutes the footwall of the STDS, which separates it from the very low biotite-grade to unmetamorphosed quartzite and slates/phyllite of the Martoli Formation of the basal Tethyan Sedimentary zone under peak metamorphic conditions of 450 ± 50 °C.Many shear indicators can be recognized between Helang and Malari. Asymmetry of structures like S-C and S-C’ fabric, asymmetric boudins, mantled porphyroclasts, folds etc. revealed top-to-thenorth downward or northeast shear sense near the STDS and top-to-the-south upwards or southwest in large parts of the HHC and provide invaluable evidences for the direction of tectonic transport.The Helang-Joshimath-Malari traverse along the Alaknanda – Dhauli Ganga Valleys provides excellent cross-section through the HHC and can be thoroughly investigated within 7 days, starting from Helang, located about 500 km from the Delhi Airport. A road/train connection from Delhi brings you to Holy city of Haridwar, located in the proximity of the Main Frontal Thrust (MFT). Travelling further north to Joshimath (275 Km from Haridwar) and camping for two days here, one can see the section between Helang – Joshimath – Tapovan (15 km) upstream in investigating the Munsiari Thrust, Vaikrita Thrust and lower package of the HHC, i.e. the Munsiari Group rocks. One locates the MCT about 12 km downstream from Joshimath near Helang. As the whole section is about 65 km long, it would be advisable to shift the camp to Malari to investigate the upper parts of the HHC from this picturesque village. The STDS is best exposed around this village along with a fully-developed migmatite sequence (the Bhapkund Formation), inverted metamorphism and shear indicators showing both top-to-SW upwards and top-to-NE downwards shear senses. One can traverse up to Niti village to look into the Tethyan Zone (TZ) before returning to Joshimath. After completing the whole section, various tectonic models can be critically assessed for the evolution of the Himalayan metamorphic belt.