An Outline of the Structural Set-Up of the Kumaun Himalaya

Abstract The Kumaun Himalaya embraces all the four Himalayan physiographic-geologic subprovinces. (1) The northern Tethys Zone constituted of Late Precambrian to Cretaceous sediments, is demarcated against the Great Himalaya to the south by the steeply-dipping Malari Thrust. This synclinorial zone, exhibiting Jura-type folding, is split up by imbricate thrusts in northeastern Kumaun giving rise to schuppen structures. Along the indo-Tibetan border, large-scale thrusting has brought Ophiolitic melange and a sheet of ultrabasics of the Kailas-Mansarovar area in Tibet over the Cretaceous flysch of Malla Johar. (2) The Great Himalaya is a colossal tectonic slab of the granite-injected katazonal early Precambrian metamorphics of the Vaikrita Group penetrated by Tertiary granite. It is demarcated by the Malari and Vaikrita thrusts. Exhibiting homoclinal structure, the Great Himalaya is characterized by plastic deformation and fluidal folding throughout its extent. The Valkrita Thrust marks abrupt change in the grade of metamorphism from the katazonal metamorphics to epizonal metasediments of the Lesser Himalayan Munsiari Formation at the base. (3) The mylonitized and retrograded metamorphics and Precambrian augen gneisses and synkinematic granodiorites of the Munsiari Formation, sandwiched between the Vaikrita and Munsiari Thrusts at the base of the Great Himalaya, represent the root of the Almora Nappe and its four klippen that cover vast areas of the Lesser Himalaya in the eastern half of Kumaun. The Munsiari extends northwest to Join up with the Jutogh of Himachal Pradesh. Imbricately underlying the Almora nappe iS the second thrust sheet of weakly metamorphosed early Riphean flysch penetrated by voluminous granitic porphyroids of the Ramgarh Group. The northwestern prolongation of this nappe is involved, together With the underlying quartzites, in tectonic imbricate-splitting. One of the sheets of this schuppen zone extends northwest to join up With the Chail Nappe of Himachal. In the inner Lesser Himalaya the crystalline klippen are underlain by a vast thrust sheet of Berinag quartzites (=Nagthat Formation) and penecontemporaneous basic volcanics. This Berinag Nappe is the base-cut and beheaded northern extension of stupendous Krol Nappe that imbricately underlies the Ramgarh and/or Almora nappes of the outer Lesser Himalaya. The synclinorial Krol Nappe comprises, in addition to the lithological Unit of the Berinag Nappe several other formations. Under the nappes in the inner Lesser Himalaya complexly folded and faulted autochthonous Early Riphean flysch (Damtha Group) and Middle Riphean to Vendian Tejam Group (Deoban + Mandhali Formations) are exposed. (4) The molasse subprovince of the Siwalik is an autochthon, separated from the Ganga plams presumably by a deep Himalayan Frontal Fault. The single-range homoclinal Slwalik Unit of eastern sector splits up westward into two major ranges showmg Jura-type folding and thrusting with the intervening synclines filled With subrecent gravels, giving rise to intermontane flat plains or 'duns'.