Microstructures and strain variation across the footwall of the Main Central Thrust Zone, Garhwal Himalaya, India

Abstract

The microstructural variation with a progressive change in the strain pattern are described in the rocks occurring across the footwall of the Main Central Thrust (MCT) in an area of the Garhwal Himalaya. In the western Garhwal Himalaya, the MCT has brought upper amphibolite facies metamorphic rocks southward over the greenschist facies rocks of the Lesser Himalaya. The progressively increasing flattening strain towards the MCT changes either to plane strain or in some cases to constrictional strain. This change in strain is well recorded in the microstructures. The zone dominated by flattening strain is expressed as bedding parallel mylonites. The grain reduction in this zone has occurred by dynamic recrystallization and quartz porphyroclasts were flattened parallel to the mylonite zone. The maximum finite strain ratio observed in this zone is 2.2:1.8:1. The zone, where the flattening strain changes either to plane strain or constrictional strain, record an increase in finite strain ratio up to 3.8:1.9:1. This zone represents deformation fabrics like S–C microstructures simultaneously developed during mylonitization in an intense ductile shear zone. The above zone is either near the MCT or adjacent to crystalline klippen occupying the core of the synforms in the footwall of the MCT. The microstructural evolution and the finite strain suggest that the MCT has evolved as the result of superposition of southward directed simple shear over the flattening strain. The simple shear has played an active role in the rapid translation which followed the mylonitization at deeper levels.