Thrust Shear Sense and Shear Zone Fabrics in the Higher Himalaya, Siyom Valley, Eastern Arunachal Himalaya, India
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Thrust Shear Sense and Shear Zone Fabrics in the Higher Himalaya, Siyom Valley, Eastern Arunachal Himalaya, India Tapos Kumar Goswamia,*, Sukumar Baruahb, Bashab Nandan Mahantac, Pranjit Kalitaa and Polash Boraa Department of Applied Geology, Dibrugarh University, Dibrugarh - 786 004, India Pare Hydroelectric Project, NEEPCO Ltd., Doimukh - 791112, Arunachal Pradesh, India c Geological Survey of India, Shillong - 793 003, Meghalaya, India E-mail: [email protected]*; [email protected]; [email protected]; [email protected]; [email protected] a
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ABSTRACT The Higher Himalayan Crystallines (HHC) in the Siyom river section of eastern Arunachal Himalaya, display top-to- ESE compressional ductile shear from Pene to Menchuka. In this part of the Himalayan Metamorphic Belt (HMB), the base of the Main Central Thrust (MCT) is the southern boundary of the ductile shear fabrics in HHC. Across the MCT, a significant break in the grade of metamorphism is observed between the HHC and the Lesser Himalayan Sequences (LHS). The ductile shear fabrics documented in the HHC are primary S-C and S-C´ fabrics, asymmetric folds, porphyroclasts and porphyrobalsts, brittleductile structures and asymmetric boudins. Three phases of folding representing three deformation episodes (D1-D3) in HHC are recorded in the area. The rocks of the HHC in Siyom valley are completely transposed by the D2 deformation into NNW dipping C- plane. Ductile shear represented by grain scale structures include sigmoidal foliation, rotation of the inclusion trails in garnet porphyroblasts, asymmetric folds with consistent top-to-ESE sense of shear. The ductile to brittle-ductile shear fabrics in the area indicate that the thrust sense of shear in the HHC is consistent without any shear sense reversal from Pene to Menchuka. These compressional shear fabrics provide invaluable field evidences for constraining the evolution of this part of the Higher Himalaya. INTRODUCTION Continental collision between Indian and Asian plates (~50-60Ma ago) was followed by intense deformation, remobilisation and dismemberment of the Proterozoic Indian crust during the Cenozoic (Jain and Anand, 1988; Hodges, 2000). The Himalayan orogen is an assembly of lithotectonic terrains, named successively from south to north, the Sub Himalaya, the Lesser Himalaya, the Higher Himalaya and the Tethys Himalaya. The southern boundary of these terrains are defined by major thrusts such as the Himalayan Frontal Thrust (HFT), the Main Boundary Thrust (MBT), the Main Central Thrust (MCT) and the South Tibetan Detachment System (STDS) (Valdiya, 1980; Thakur, 1992; Srikantia and Bhargava, 1998; Jain et al., 2005; Shah et al, 2012) (Fig.1a). A considerable amount of post collisional crustal shortening has been accommodated through these thrusts (Le Fort, 1975; Mukhopadhyay et al., 1997). The MCT, in the south of the Indus Tsangpo suture zone, is a major thrust where large amount of crustal shortening has been concentrated (Singh and Thakur, 2001) (Fig.1). The MCT separates two dis
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