Tectono-Thermochronology Study On Strike-Slip Movement In The Middle-Southern Segment Of Tan-Lu Fault Zone

The Tan-Lu fault zone is a major fault zone trending NNE in east China and extends about 2400 km. Long-term studies suggest that the large fault zone experienced complicated evolution history and recorded geodynamic processes of eastern China. Because extensional structures from late Cretaceous to Paleogene and compressional structures since Neogene are conserved well, most geologists agreed on that the Tan-Lu fault zone experienced extension and following compression since late Cretaceous. However, though lots of international geologists paid more attention to the early strike-slip history and origin of the Tan-Lu fault zone, especially the relation between it and the Dabie-Sulu orogenic belt, but their viewpoints about that were different. Some workers considered the fault zone as a syn-orogenic tectonics, and proposed different models for it, such as transform fault model, oblique plate boundary model, rotated suture line model and tear fault model. Other workers suggest that the fault zone originated as a result of circum-Pacific tectonic movement in Late Jurassic to Early Cretaceous. The reason of the debates about strike-slip time of the fault zone mostly is lack of systematic chronology work, especially lack of systematic chronological work on ductile shear zone formed at middle-deep levels.According to lots of field works, two phases of sinistral strike-slip ductile shear belts were found in the Tan-Lu fault zone. Steep mylonite foliation in the earlier shear belts strikes NE-NNE, and dips NW or SE; its stretching lineation is gentle. Steep mylonite foliation in the later shear belts strikes NE-NNE, and mostly dips SE. Its stretching lineation is gentle and dips SSW, the maximum dip angle is less than 20°. Such microstructures as S-C fabrics, rotated feldspar porphyroblasts, and mica fish and quartz c-axis fabrics all indicate sinistral shear sense.For the sake of getting exact time of the Tan-Lu strike-slip movement and understanding relation between the fault zone and the Dabie-Sulu orogenic belt, this work selected a series of mineral separates, such as hornblende, muscovite, biotite, plagioclase and K-feldspar from mylonites in middle- southern segment of Tan-Lu fault zone, for ⁴⁰Ar/³⁹Ar dating.In this study, muscovite ⁴⁰Ar/³⁹Ar ages of 181 Ma and 209.9-214.3 Ma were obtained from earlier Tan- Lu ductile shear zone respectively on the eastern margin of the Dabie belt and the western margin of the Sulu belt. The formation temperatures of all these mylonite are more than the closure temperature of muscovite, so these ages are cooling ages of the earlier Tan-Lu faulting. A maximum cooling age is closest to deformation age, so the latest formation time of Tan-Lu fault zone is 214.3 Ma (late Triassic). Based on newly-formed mineral assembles and deformation behaviors of feldspar and quartz in mylonites, and using muscovite-chlorite geothermometry, this work shows that deformation temperatures of earlier Tan-Lu faulting are separately 400-500℃ and 600-700 ℃ respectively on the eastern margin of Dabie belt and the western margin of the Sulu belt.Confining pressures are 0.25-0.42 GPa and 1.02-1.45 GPa respectively for the earlier shear zones on the eastern margin of the Dabie belt and the western margin of the Sulu belt according to geobarometry of Si-in-phengite. Therefore, the deformation temperatures and pressure ars less than that of peak metamorphism of the Dabie-Sulu orogenic belt, which suggests that the Tan-Lu fault zone was formed during exhumation of the Dabie-Sulu orogenic belt. Considering the fault zone terminates in the southern end of the Dabie belt, it is concludes that the Tan-Lu fault zone is a transfer fault formed during exhumation of the Dabie-Sulu orogenic belt.In this study, hornblende Ar/Ar ages of 131.5-143.3 Ma, muscovite 40Ar/"Ar ages of 121.2-138.8 Ma and biotite 40Ar/39Ar ages of 108.9-137.0 Ma were obtained from later ductile shear zone in the middle-southern segment of the Tan-Lu fault zone. The deformation temperature of N14 mylonite from the Zhangbaling uplift is equa...