Xiaomei Ductile Shear Zone on Hainan Island in a Nanoscale Perspective

The western Qiongnan suture zone, i.e., Jiusuo-Lingshui fault belt, at the northern margin of the South China Sea, is important in tectonics. To better understand the tectonic evidences and ductile shear belts, a study is done in this area. Three types of rocks are sampled from the Xiaomei ductile shear zone on Hainan island, including granite, garnet quartz schist and granitic gneiss. Their shear surfaces were observed and analyzed through scanning electron microscope (SEM) at nanoscale. The results show multiple nano-textures and nanostructures in all three kinds of rocks and a positive relationship between the development degree of nanoparticles and the stress of the rocks. The authors propose several possible formation machanism of nanoparticles on the shear surfaces in this area, which may be thermal decomposition of sheet silicates under shear stress, or brittle deformation of minerals (especially feldspar with mechanical twins) after ductile deformation, then followed by crushing and grinding of the particles to nanoscale under shear force. Samples from the Taroko ductile shear belt, a part of the Taroko fault zone in Taiwan show similar nano characteristics to the Xiaomei ductile shear zone, implying both may have been in the same tectonic setting of a deep fracture as the Jiusuo-Lingshui fault belt. This study can therefore provide important observational facts for determining the structural properties and spatial positions of the Jiusuo-Lingshui fault belt.