SHEAR ZONE NUCLEATION IN ARCHAEAN POTASSIC a GRANITE FROM BUNDELKHAND CRATON, CENTRAL INDIA: CONSTRAINTS FROM MICROSTRUCTUAL STUDIES

Abstract

Available scientific evidences suggest that the development of ductile shear zone is related to the mechanical weakening of the rock during deformation (Drury et al., 1991). Microstructural studies from the deformed rocks of the shear zones indicate a reduction in grain size compared to the undeformed host rocks (Raiinbourg et al., 2008). The change in the grain size has severe effect on the strength of the rock and is considered as a principle factor for strain localization (Drury et al., 1991). Among the several factors, cracking (Hand), and Stunitz, 2002), dynamic recrystallization during deformation (Drury et al., 1991) and 4 formation of fine grained reaction products during metamorphism ( Newman et al.. 1991) are considered as the principle factors for the grain size reduction. The availability of fluid also enhance the strain localization by promoting metamorphic reactions and formation of hydrous minerals that are susceptible to deformation compared to the dry equivalent assemblages (Rybachi and Dresen. 2000; Dirnanov et al., 2003).lt is generally agreed that localization of deformation and formation of shear zone is a two stage process that initiates with the nucleation stage followed by a widening/development stage (Gonclaves et.al-2012. Plat and Bher et.al-201 I, Mancktelow and Pinnacchioni et.al, 2007). The localization of strain may initiate in a pre-existing structural weakness that developed due to local mechanical contrast (Mancktelow and Pinnacchioniet.al..2007) or by the dynamic grain size reduction that involves localization of strain due to progressive deformation.