Strain Transition and Microstructural-Fabric Analysis of Qaurtz-Mylonites Exposed within Pratapnagar Nappe Garhwal Himalaya, India

The Pratapnagar Thrust sheet in the Lesser Garhwal Himalaya consists of massive, white mylonitized Pratapnagar Quartzite and is delineated at its base by the Pratapnagar Thrust which brings this rock group to rest over the parautochthonous Argillocalc-arenite Group.

The estimated three-dimensional finite strain (̅Σs) in the specimens located close to the Pratapnagar Thrust was 0.937 while away from the thrust it was 0.412. The quartzite exposed within the mylonitized zone of Pratapnagar Thrust underwent non-coaxial (= simple shear) deformation revealed by asymmetrical (unpopulated) C-axes girdles and rotation of mylonitic foliation (S₃), lineation (Lm) and C-axes girdles, The variable orientation of C-axes girdles, mylonitic foliation, lineation and microstructural shear indicators (asymmetrical porphyroclasts and pressure shadows) noticed within the rock exposed along the thrust zone indicate sinistral (-ve) sense of shear movement (NE-SW) with respect to N. The quartzite away from the thrust were initialy subjected to coaxial (= pure shear) deformation as is indicated by scattered globular and tabular host quartz grains and symmetrical cross and small circle girdle patterns of C-axes. The microstructural transition and asymmetrical orientation of C-axes fabrics indicate that the mylonitized Pratapnagar Quartzite was formed by deformation under ductile flow and low grade metamorphic conditions.