Tectonometamorphic Evolution of the Great Himalayan Thrust Sheets in Garhwal Region, Kumaun Himalaya

Abstract

The so-called 'Central Crystallines' building the bulk of the Great Himalaya (=Himadri) comprise two major tectonic units. The upper unit is constituted of highgrade psammitic metamorphics associated with anatectic granites of the Vaikrita Group, and the lower unit is made up of Precambrian granites, gneisses, amphibolites, meta-sediments and their mylonitic equivalents recognized as the Munsiari Formation. The Vaikrita Group makes a huge lithotectonic slab bounded by moderately inclined Main Central (Vaikrita) Thrust in the south and the steeply dipping Trans-Himadri (Malari) Thrust in the north. Sandwiched between the Munsiari and the Main Central (Vaikrita) Thrusts, the Munsiari is a severely tectonized and drastically condensed lithotectonic sheet representing the root zone of the Lesser Himalayan crystalline nappes.

In small-scale, the Vaikrita shows extremely intricate internal deformation resulting from polyphasal folding and repeated transposition of foliation planes. The bulk non-coaxial strain in the rocks is attributable to the variation in the case of slip on the shear planes during thrust propagation. The period of thrusting is coeval with the main phase of progressive metamorphism, anatexis and attendant granite emplacement during the late Eocene to Lower Miocene period. Later deformation and retrogressive metamorphism are related to the uplift during the Middle and Upper Miocene, culminating in the Middle Pleistocene, when the Vaikrita rocks were brought to a brittle field of deformation from the earlier ductile and brittle-ductile fields.

The Vaikrita exhibits an abrupt rise in the grade of metamorphism to sillimanite zone of progressive (Barrovian type) metamorphism from a very low to medium grade of (Himalayan) metamorphism in the Munsiari. In the deeper level (∼30 km), the Vaikrita rocks underwent progressive regional metamorphism and anatexis that gave rise to granites, which extensively invaded the metamorphics. The episodic uplift of the Vaikrita Group was due to the shifting of thrust movements by relay from the Main Central (Vaikrita) Thrust, to the Munsiari Thrust and the other minor thrusts of the schuppen zone in the south. A part of the uplift can be attributed to the diapiric rise of the anatectic granites.