SUPERIMPOSED MESOSCOPIC, MAGNETIC, MICRO AND PETROFABRICS IN THE HIGH HIMALAYAN CRYSTALLINES (HHC) BHAGIRATHI VALLEY, GARHWAL HIMALAYA, INDIA: IMPLICATIONS FOR STRAIN VARIATION AND SUPERPOSED DEFORMATION ALONG THE VAIKRITA THRUST

The study aims to understand the deformation pattern across the Vaikrita Thrust (VT), Bhagirathi valley, using both macro-and-microscopic grain fabric analyses of granitic gneisses and associated leucogranites of the High Himalayan Crystallines (HHC).The mesoscopic structures across the VT shows a strong pervasive foliation that is parallel to the axial plane foliation of early asymmetric F₁ folds. This implies that at the vicinity of VT is dominated by intense simple shear and away from VT, the density of shear planes gradually decrease to become upright folds suggesting dominance of pure shear. The microstructures show high temperature deformation fabric followed by late low temperature superposed deformation fabric. The composite mesoscopic and magnetic fabric shows intersection fabric and random distribution of magnetic lineation is due to superposition of late folds over early non-cylindrical folds. The same relation was found for the parameter of degree of anisotropy (P’) suggesting that the magnetic anisotropy is controlled by intensity of deformation and it is in good agreement with the mesoscopic fabric. Shape Preferred Orientation (SPO) analysis shows that the strength of magnetic lineation (κ) for the granitic gneisses is 0.660 close to the VT, and gradually decreases as moving away from VT i.e. 0.543, 0.191 and 0.149. The quartz grain analysis shows that increase in the finite strain towards VT produces a change in foliation with development of S-C fabrics. The kinematic analysis of the granitic gneisses of the Bhagirathi valley suggests high shear strain ratio and ductile deformation near the VT. The simultaneous change in foliation with the development of S-C fabrics coincides with a change in the strain symmetry from the flattening strain to either the plane or the constrictional strain. Comparison of mesoscopic and magnetic fabric suggests interference of mild superposed deformation with upright superposed folds over pre-existing early folds. Finally the deformation pattern inferred from various methods suggesting that there is systematic increase in the strain gradient across the VT, and also strongly supports that the VT is tectonics break between low grade and high grade metamorphics associated with late stage superposed folding along the VT in Bhagirathi valley.