Polyphase Deformation in Phanerozoic Rocks of the Central Himalayan Gneiss, Northwest India

Abstract

Multiple deformation in gneisses and schists of the Chandra Valley, Himachal Pradesh, can be divided into three phases of folding. The earliest recognizable deformation, D₁, is characterized by isoclinal folds, F₁, with a strong axial-surface foliation, S₁, containing a mineral lineation, L₁, which may lie in the kinematic a direction. The F₂ folds deform S₁ and L₁, and vary from open to very tight structures with rounded hinges, crenulation lineations, and a weak axial-surface crenulation cleavage. The consistent sense of asymmetry of F₂ folds southwest of the central crystalline axis indicates strong overthrusting from northeast. The third phase of deformation, D₃, produced the regional broad antiforms and synforms which plunge 5° NW. Calc-schistsnear Tandi contain marine fossils of Jurassic age. These calc-schists are deformed by the three deformations recognized in the gneisses, and are thus part of the central crystalline zone. The fossiliferous Spiti section is a para-autochthon tectonically overlying the central gneiss. Field relations suggest that the metamorphites formed during the Tertiary, and that the central gneiss occupies the root zone of the extensive crystalline thrusts and nappes of the Lesser Himalaya.