Structural Events And Metamorphic Consequences In Almora Nappe, During Himalayan Collision Tectonics

摘要

Almora Nappe in Uttarakhand, India, is a Lesser Himalayan representative of the Himalayan Metamorphic Belt that was tectonically transported over the Main Central Thrust (MCT) from Higher Himalaya. The Basal Shear zone of Almora Nappe shows complicated structural pattern of polyphase deformation and metamorphism. The rocks exposed along the northern and southern margins of this nappe are highly mylonitized while the degree of mylonitization decreases towards the central part where the rocks eventually grade into unmylonitized metamorphics. Mylonitized rocks near the roof of the Basal Shear zone show dynamic metamorphism (M_2) reaching upto greenschist facies (～450 ℃/4 kbar). In the central part of nappe the unmylonitized schists and gneisses are affected by regional metamorphism (M_1) reaching upper amphibolite facies (～4.0-7.9 kbar and ～500-709 ℃). Four zones of regional metamorphism progressing from chlorite-biotite to sillima-nite-K-feldspar zone demarcated by specific reaction isograds have been identified. These metamorphic zones show a repetition suggesting that the zones are involved in tight F_2 - folding which has affected the metamorphics. South of the Almora town, the regionally metamorphosed rocks have been intruded by Almora Granite (560 ± 20 Ma) resulting in contact metamorphism. The contact metamorphic signatures overprint the regional S_2 foliation. It is inferred that the dominant regional metamorphism in Almora Nappe is highly likely to be of pre-Himalayan (Precambrian!) age.