Microstructures and strain variation across the footwall of the Main Central Thrust Zone, Garhwal Himalaya, India Keser Singh * , V.C. Thakur Wadia Institute of Himalayan Geology, 33, General Mahadeo Singh Road, Dehradun, 248 001, 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 ¯attening 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 ¯attening strain is expressed as bedding parallel mylonites. The grain reduction in this zone has occurred by dynamic recrystallization and quartz porphyroclasts were ¯attened parallel to the mylonite zone. The maximum ®nite strain ratio observed in this zone is 2.2:1.8:1. The zone, where the ¯attening strain changes either to plane strain or constrictional strain, record an increase in ®nite 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 ®nite strain suggest that the MCT has evolved as the result of superposition of southward directed simple shear over the ¯attening strain. The simple shear has played an active role in the rapid translation which followed the mylonitization at deeper levels. q 2001 Elsevier Science Ltd. All rights reserved. 1. Introduction The collision of the Indian and Asian plates during the Palaeogene period produced several thrust planes. The differential movements along these tectonic planes produced composite thrust sheets. The crustal shortening, thus produced, was concentrated on a few major thrust zones located south of the Indus Tsangpo suture zone. One of the most important of these is the Main Central Thrust (MCT). Along this thrust, medium to high grade metamorphic rocks (Higher Himalayan Crystallines: HHC) were thrusted over the low grade meta-sedimentary rocks of the Lesser Himalaya (Heim and Gansser, 1939; Gansser, 1964; Le Fort, 1975; Thakur, 1987). On the basis of the presence of klippen of the HHC far south over the Lesser Himalayan rocks, the MCT zone is thought to have a cumulative displacement of at least 140±210 km (e.g. Schelling and Arita, 1991; Schelling, 1992). Studies regard- ing the crustal shortening and deformation fabrics produced during this displacement have been limited in number. The present knowledge about the MCT and other related thrusts in general, is either restricted to deformation fabrics (Bouchez and Pecher, 1981; Brunel, 1986) or strain pattern (Roy, 1980; Saklani and Nainwal, 1989; Jain and Anand, 1988; Singh, 1991). The microstructural changes with strain pattern are required to better understand the internal defor- mation associated with the MCT. The western part of the Garhwal Himalaya between Tons and Bhagirathi valleys were selected for the above studies, since strain markers are known here across the MCT (Singh, 1991). The footwall of the MCT has klippen of the thrust sheet rooted within the crystallines of the HHC. An integrated study of strain pattern, mylonitization and microstructures across the MCT zone and also its footwall hold the key to understand the nature of deformation associated with the MCT in this part of the Garhwal Himalaya. 2. Geological setting The geology of Garhwal Himalaya is dominated by a series of northward dipping and S-verging thrusts. The Main Central Thrust (MCT), one of the prominent thrusts, has long been recognized as a major intracontinental ductile shear zone with an associated inverted metamorphic zone (Heim and Gansser, 1939; Valdiya, 1980 etc.). The MCT separates two geologically distinct zones i.e., the Lesser Himalaya (south of the MCT) and the Higher Himalayan Crystallines (north of the MCT) (Fig. 1). The rocks of the Journal of Asian Earth Sciences 19 (2001) 17±29 1367-9120/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S1367-9120(00)00006-7 www.elsevier.nl/locate/jseaes * Corresponding author. Tel.: 191-135-627387; fax: 191-135-625212. E-mail address: wihg@giasdl01.vsnl.net.in (K. Singh).