LATE MESOZOIC SHEAR ZONES AND ITS CHRONOLOGY IN THE DABIE MOUNTAINS,CENTRAL CHINA
Hou Quanlin1, Liu Qing1, Li Jun2, Zhang Hongyuan3
1. Graduate University of Chinese Academy of Sciences, Beijing 100049; 2. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029; 3. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037
The main shear zones of Late Mesozoic in the Dabie Mountains were characterized by extensional detachment structures. The entire tectonic system was divided by the northern Dabie metamorphic complex into the north and south extensional tectonic systems. In the north tectonic system, the Xiaotian-Mozitan shear zone displayed extentional detachment to the NNE. In the south tectonic system the displacement direction along the Shuihou-Wuhe and the Taihu-Mamiao shear zones was to the SSE, whereas displacement along the Susong-Qingshuihe shear zone was to the SW. Finite strain measurements showed that the shear displacement of the Xiaotian-Mozitan shear zone was more than 56 km, whereas that of the Susong-Qingshuihe shear zone was more than 12 km. From north to south in the south extensional tectonic system, the Flinn index (K) demonstrated a transition from pure shear to simple shear. Therefore, it might imply that the extensional environment in the south tectonic system could be controlled by magma intrusion in the North Dabie Mountains complex belt (NDMCB). The main shear zones in the Dabie Mountains displayed two deformation ages, ca.190 Ma and ca. 124 Ma (derived from mineral 40Ar-39Ar data). The earlier age (ca. 190 Ma) might be related to the ultra-high pressure(UHP)cooling and returning during the formation of the orogeny, and the later age (ca. 124 Ma) might represent the extensional detachment age after orogenesis. Strain analysis, chronology, and the mantle enrichment of platinum group elements all suggested that magma intrusion in the NDMCB could be the main cause of extensional detachment structures during the Late Mesozoic.