Development of Successive Generations of Shear Zones in the Precambrian Basement of Schirmacher Hills, East Antarctica

In the Precambrian basement of the Schirmacher Hills, ductile shear zones have developed at several stages over a wide time-span of tectonothermal history. The earliest shear zones were formed under granulite facies conditions producing well-foliated gneiss. The majority of the ductile shear zones developed under amphibolite facies conditions. The sheared rock is a mylonite characterized by drastic grain refinement of the constituent minerals. The shear zones under amphibolite facies conditions are of four broad categories: an early set subparallel to the gneissic foliation, an early and a late cross-cutting types, shear zones which developed within massive rocks and lastly, those which developed at the contacts of strong rheological contracts. The last phase of deformation produced discrete shear fractures. All the different phases of ductile shearing were broadly synchronous with emplacement of quartzofeldspathic materials. Pegmatite was emplaced even along some of the discrete shear fractures. The magnitude of shear strain could be measured from the orientations of newly formed foliations and also from the rotation of early foliations.