SENSE OF SHEAR IN HIGH-TEMPERATURE MOVEMENT ZONES FROM THE FABRIC ASYMMETRY OF PLAGIOCLASE FELDSPARS

A new method for determining the sense of shear in plagioclase-bearing tectonics from the (010) orientation of plagioclase feldspar is presented. The method is based on the asymmetry of the (010) plane with respect to the structural frame (foliation and lineation) and the dominant activity of the (010) slip plane in the high-temperature plasticity of plagioclase feldspar. Using examples from the Zabargad gneisses (Red Sea) the method is applied to plagioclases of An25-An45 and compared with other methods of shear-sense determination (quartz c-axis fabrics and microstructural criteria).