Determination Of The Shear Zones In The West Of Hainan Island, Vorticity Analysis And Their Tectonic Implications

Hainan Island is an epicontinental island in the north of the South China Sea which is located at the southern margin of the South China Block. Due to its unusual tectonic setting and complicated tectonic development, Hainan Island becomes a key place for understanding not only the continental margin accretion and evolution of East Asia,but also the formation of the South China Sea and the crustal formation of Gondwana. Therefore, it has been one of the hotspots among geologists all over the world. There are two kinds of high-strain shear zones scattered on this island, i.e., WNW-trending and NE-trending. They are identified as Triassic (242-250 Ma) and late Triassic-early Jurassic (190-230 Ma), respectively.-As a basis for Vorticity analysis and discussing the tectonic evolution history, We report the features of the structure combination, mylonite, micro structure and EBSD analysis of rocks in the ductile shear zones that lie between Changjiang and Dongfang and determinate the high-strain shear zones in the West of Hainan Island. Two ductile shear zones have been identified, one is WNW-trending, the other is NE-trending. The shear direction of them are determined to be dextral and sinistral, separately.Four frequently used methods of kinematic vorticity measurements have been explored for the above shear zones:the polar Mohr diagram, quartz c-axis fabrics and strain ratio, the extensional crenulation(i.e. C) and rotated porphyroblasts. Quantitative vorticity analyses in deformed rocks from these shear zones are combined with previous geochronological data to lead to two-stage different deformation character for WNW-trending and NE-trending shear zones. For the early stage WNW-trending shear zone, the kinematic vorticity values(Wk) for S-C fabrics range from 0.72?1, for the later stage NE-trending shear zone Wk is 0.91?1; Wk of C' mainly for the early WNW-trending shear zone is 0.21?0.47. These results of kinematic vorticity values suggest that both shear zones are general shear, with simple shear as a dominate. But pure shear component increased during the development of the extensional crenulation, as showed by the decrease of the kinematic vorticity values.The occurrence of ca.1327Ma gneissic granite which was emplaced into Chg(the Middle Proterozoic Changcheng System Gezhencun Formation) and ca.1449Ma? 1452Ma for the mylonitization granitoid suggest a Grenvillian magmatic activity. Furthermore,262?272Ma zircon U-Pb ages of mylonitization biotite granites possibly be closely related to the late Permian to middle Triassic paleo-Tethys closure.The primary analysis considered tectonic implications of the Shear Zones in the West of Hainan Ialand could be as followed:the WNW-trending and NE-trending shear zone could be compared with those inside the Indochina and South China Blocks, separately.