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

The microstructural variation with a progressive change in the strain patte rn are described in the rocks occurring across the footwall of the Main Cen tral Thrust (MCT) in an area of the Garhwal Himalaya. In the western Garhwa l Himalaya, the MCT has brought upper amphibolite facies metamorphic rocks southward over the greenschist facies rocks of the Lesser Himalaya. The pro gressively increasing flattening strain towards the MCT changes either to p lane strain or in some cases to constrictional strain. This change in strai n 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 porphyroclas ts 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 st rain changes either to plane strain or constrictional strain, record an inc rease in finite strain ratio up to 3.8:1.9:1. This zone represents deformat ion fabrics like S-C microstructures simultaneously developed during myloni tization in an intense ductile shear zone. The above zone is either near th e MCT or adjacent to crystalline klippen occupying the core of the synforms in the footwall of the MCT. The microstructural evolution and the finite s train suggest that the MCT has evolved as the result of superposition of so uthward directed simple shear over the flattening strain. The simple shear has played an active role in the rapid translation which followed the mylon itization at deeper levels. (C) 2001 Elsevier Science Ltd. All rights reser ved.