First Discovering of Nanoscale Tectonics in Western of Qiongnan Paleo-Tethyan Suture Zone in North Margin of South China Sea and Its Geotectonic Significance

The western part of Qiongnan Paleo-Tethyan Suture Zone in the northern margin of the South China Sea is commonly known as the Jiusuo-Lingshui fault zone. At first, it was deduced as a near EW-direction tectonic boundary zone based on the data of geophysical field. Due to the lack of direct evidence of tectonic deformation, the existence and geological significance of the boundary zone have been controversial for a long time. Through field observation, indoor polarizing microscope and SEM (scanning electron microscope) tests on the basement rocks exposed along the near NS-direction profile of Xiaomei riverbed, Duzong reservoir, Junchang bridge, and Lipen reservoir, and Hainan Island, the authors discovered ductile shear structure in the boundary zone, crystalline quartz schist, granitic gneiss, meta-quartz sandstone, and porphyritic quartz rock. The sha = ear structures were formed by regional dynamic-thermal metamorphism and are the most common ductile deformation metamorphic rock in general orogenic belt. Furthermore, the microstructure and nano structure, which often developed in the ductile shear zone, were found through microscope and SEM. Moreover, the authors discussed nanoscale characteristics of Xiaomei ductile shear zone in Hainan Island. The study results showed that the three rocks of granite, quartz schist and granitic gneiss have many kinds of nano textures and structures. Based on field actual situation of the development degree of nano particles in this shear zone, it is known that these nano textures and structures are related to shearing action. Their formation mechanism may be as follows: one is thermal decomposition of layered silicate under shearing action, the other is brittle fracture after particle plastic deformation and grinding through shearing action. Based on analysis of the Xiaomei ductile shear zone, and after comparison of SEM test results carried out to the rock samples of ductile shear zone of Taroko deep fault zone in Taiwan, we found that the characteristics of the nano particles and ones in Xiaomei ductile shear zone are comparable. According to their relations with the regional deep fault, it could be speculated that Xiaomei ductile shear zone may be closely correlated with Jiusuo-Lingshui deep fault zone in the regional tectonic belonging. These study results can provide important evidence for determination of the tectonic property and spatial position of Jiusuo-Lingshui fault zone.