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

A.K. Jain, Mrinal Shreshtha, Puneet Seth, Lawrence Kanyal, R. Carosi, Chiara Montomoli, and S. Iaccarino

As a consequence of the Cenozoic India–Himalayan convergence, the Himalaya exhibits some of the most spectacular features like deformation, metamorphism and generation of migmatite/leucogranite of the Higher Himalayan Crystalline (HHC) belt, and its subsequent thrusting along the Main Central Thrust (MCT) over the Lesser Himalaya. In central parts of Uttarakhand, more than 20 km thick and homoclinal NE-dipping HHC is almost continuously exposed between Helang and Malari along the Dhauli Ganga Valley. 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, (v) Himalayan inverted metamorphism, and (vi) assessment of channel flow or other models.

Based on lithologies and grade of metamorphism from the lower to higher structural levels northwards, the HHC is divisible into two main groups above the Munsiari Thrust. In the lower parts, the Munsiari Group of low to medium grade contains garnet mica schist/gneiss, quartzite, amphibolite and biotite-rich phyllonite, mylonitic gneiss and augen gneiss. Overlying the Vaikrita Thrust, the 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.

Various shear sense indicators proliferate between Helang and Malari. Asymmetry of structures like S-C and S-C’ fabric, boudins, mantled porphyroclasts, folds etc. revealed top-to-the-north or northeast near the STDS and top-to-the-south or southwest in the large parts of the HHC and provide invaluable evidences for the direction of tectonic transport. In the upper parts of the HHC migmatite is ubiquitously distributed up to Malari in the upper parts of the Bhapkund Formation, having five different phases of melt accumulation. The oldest migmatite phase (Me1) parallels the main foliation Sm as the stromatite layers and concordant leucogranite bands. Younger melt phases Me2, Me3 and Me5 are recorded along small-scale ductile thrusts, extensional fabric and structureless patches, respectively. It is only the Me4 melting phase that is evidenced by large-scale melt migration along cross-cutting irregular veins. These were possible conduits for migration and accumulation of melt into larger leucogranite bodies like the Malari granite (19.0± 0.5 Ma).

Various tectonic models for the evolution of the Himalayan metamorphic belt can be critically assessed in this section.

The Helang-Joshimath-Malari traverse along the Alaknanda – Dhauli Ganga Valleys provides excellent cross-section through the HHC and can be thoroughly investigated within 10-days, starting from Helang, located about 500 km from Delhi Airport. A road/train connection from Delhi brings you to Haridwar. One locates the MCT about 12 km downstream at Helang. Two days can be spent 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.

Next 2 days can still be spent at Joshimath looking the HHC upstream till Suraithota (32 km milestone), its lithology, structures and mineral assemblages. As the whole section is about 65 km long, it would be advisable to shift camp to Malari and investigate the upper parts from this picturesque village. The STDS is best exposed around this village along with migmatite sequence (the Bhapkund Formation), inverted metamorphism and shear indicators showing both top-to-SW and top-to-NE shear senses. One can traverse to Niti and Sumna villages to look into the Tethyan Sedimentary Zone (TSZ) before returning to Joshimath.

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