MICROSTRUCTURAL ANALYSIS AND STRAIN PATTERN IN MYLONITES AND IMPLICATIONS OF SHEAR SENSE INDICATORS IN EVOLUTION OF DINARA- GARHMAU SHEAR ZONE, BUNDELKHAND MASSIF, CENTRAL INDIA

摘要

A steep north dipping and left lateral brittle-ductile shear zone extending more than 50 km in strike length is traced in the northern territory of Bundelkhand Craton of Central India. The mylonitised TTG and mafic gneisses of Archean Precambrian age were emplaced in this shear zone in syntectonic phase (D4). The evolution of proto, S-C and ultra mylonites was controlled by different stages in progressive shearing under low to moderate temperature conditions in a ductile regime. The geometry and optical characters of quartz and feldspar and orientation of mylonitic foliation, stretching lineation and S-C fabrics infer that the brittle -ductile shear zone was progressively evolved in syntectonic phase under noncoaxial strain and low to moderate strain conditions. The σ_a, and σ_b type of mantled porphyroclasts were progressively deformed by crystal plastic deformation under low to medium grade metamorphic conditions and are indicative of dynamic recrystallisation. The alteration effects, undulose extinction and dynamic recrystallisation in K-feldspar and quartz imply that the moderate temperature pressure conditions were predominant during crystal plastic deformation. The presence of microfractures, extensional cracks, and kinking in K-feldspar and plagioclase are suggestive of low temperature and brittle deformation.The strain analysis reveal that the intensively deformed and elongated quartz grains belonging to high shear strain domains of central part of shear zone are characterised by higher values of two dimensional (Rs= 4.20) and three dimensional finite strain (Σs=0.598). Whereas the low values of two and three dimensional strain values are exhibited by less deformed quartz fabrics located marginal locals of shear zone. The most of the mylonite specimens showing oblate type of strain ellipsoids were experienced flattening type of strain.