Mesostructures and deformational history of the Central Crystallines: an example from Garhwal Himalaya, India

Abstract

Structural analysis reveals that the Central Crystallines in the Garhwal region were subjected to four phases of deformations (D₁ to D₄). The D₁ deformational phase is highly obliterated and usually found as F₁ intrafolial (rootless) tight isoclinal folds in migmatites and gneisses. The D₂ deformational phase produced strong pervasive S₂ schistosity and asymmetric and open fold (F₂) plunging 20-30° towards ENE-WSW. The L₂ lineation plunge 5-10° towards east-west is well developed in medium grade metamorphic rocks. The D₁ deformations were responsible for F₃ folds reflected in large scale anticlinal and synclinal, overturned and recumbent folds, which have 10-40° plunges towards NW. The late D₃ deformational stresses were responsible for shearing along the middle limbs of F₁ folds and they ultimately initiated thrusting. The NNE­ SSW plunging mineral or stretching lineation (L₃), S₃ crenulation cleavage and S-C fabrics were developed during the dominant ductile shearing related to the late D₃ deformation. The D₄ phase characterised by brittle-ductile deformation (minor kinks, puckers, transverse/transcurrent faults, and S-C' fabrics) and extensive cataclasis along thrust- and fault-zones reflects the last episode of deformation. The structural and geochronological data indicate that D₁ and D₂ deformation episodes may be related to the Precambrian time while D₃ and D₄ are exclusively of the Tertiary age.