Syn-orogenic Deformation Pattern And Evolution Of The Southern Segment Of The Tan-Lu Fault Zone As Well As Its Formation Mechanism

The Dabie and Sulu orogens between the North China and the Yangtze plates were left-laterally offset about 400 km along the NE-striking Tan-Lu Fault Zone. The fault zone terminates abruptly at the southeastern corner of the Dabie Orogen, suggesting unique origin of the fault zone which remains controversial.Our LA-ICP-MS zircon dating results indicate that protolith ages of the Zhangbaling Group in the northern segment of Zhangbaling uplift are 754-753 Ma, in the Lujiang area are 751-749 Ma and in the east and south of the Dabie Orogen are 751-748 Ma. We also obtained 802-754 Ma for protolith ages of the Feidong Complex in the southern segment of the Zhangbaling uplift. The dating work demonstrates that protolith ages of the dated ortho-metamorphic rocks from the Zhangbaling Group and Feidong Complex are middle Neoproterozoic, and they belong to the lower cover of the Yangtzee Plate rather than the basement rocks as proposed before.Our detailed work suggests that syn-collisional deformation structures of the Tan-Lu Fault Zone are preserved as sinistral ductile shear belts in the Zhangbaling Group which located in the east of the Dabie Orogen and Lujiang area. The Tan-Lu ductile shear belts experienced two phases of deformation in the east of the Dabie Orogen. The first phase deformation (D1) exhibits as sinistral ductile shear belts. Microscope observations reveal that quartz in the mylonites shows widespread dynamic recrystallization by BLG and SR while feldspar shows undulose extension and brittle fracturing without dynamic recrystallization. Quartz C-axis plots indicate dominantly basal plane slip with a few of prism plane slip during the deformation. Hence, these microstructures suggest deformation temperatures of 350-400℃. The sinistral ductile shear zone was involved in the second phase deformation (D2), and deformed into NE-SW trending tightly folds and thrusts. Muscovite samples in the Zhangbaling Group yielded 40Ar/39Ar ages of 236-230 Ma for the first phase deformation (D1, Middle Triassic) and 225-217 Ma for the second phase deformation(D2, Late Triassic). We also obtained muscovite 40Ar/39Ar ages of 230-209 Ma from the Susong Complex and Xiangfan-Guangji Shear zone, suggesting 230 Ma (Late Triassic) for onset of crustal exhumation in the Dabie Orogen. Therefore, the sinistral shearing (D1) of the Tan-Lu Fault Zone occurred during the crustal subduction stage in the Dabie Orogen while the shortening deformation (D2) was synchronous with the exhumation stage.The Tan-Lu shear zone in Lujiang area strikes predominantly NE-SW and dips steeply to the SE with horizontal elongation lineation and sinistral shear sense. Quartz in the mylonites shows widespread dynamic recrystallization of BLG and SR while feldspar presents crystal brittle fracturing and stretching. Quartz C-axis plots indicate dominantly basal plane slip with a few of prism plane slip during the deformation. These microstructures suggest deformation temperatures of 350-400℃. Muscovite 40Ar/39Ar ages of 239 Ma dated before suggest the middle Triassic sinistral shearing for the Tan-Lu shear zone.Structures in the Zhangbaling Group and lower Sinian strata in the northern segment of the Zhangbaling Uplift formed in a flat-lying ductile detachment zone with a shear sense of top to the SSW. Quartz in the mylonites shows BLG and SR dynamic recrystallization while feldspar shows brittle fracturing and elongation. These demonstrate that the deformation occurred under low-grade greenschist facies conditions. Muscovite 40Ar/39Ar ages of 245-236 Ma dated before demonstrate that the flat-lying shearing occurred in the middle Triassic which was synchronous with the deep subduction stage. The Feidong Complex was deformed by a series of NNE-striking sinistral ductile shear belts in Late Jurassic, and the earlier ductile fabrics are only preserved between the sinistral belts in the southern part. The earlier foliation was deformed by open folds with NEE-NE trending. We infer that the Indonesian deformation in the Feidong Complex behave as a top-to-the-SWW flat-lying ductile detachment zone which is similar to the Zhangbaling Group in the northern segment.Syn-collisional folds and thrusts in the Yangtze Plate exhibit evidence for large scale dragging by the sinistral fault zone. Trends of the fold axes and thrusts in the Yangtze Plate change from NEE-SWW to NE-SW as they approach the Tan-Lu Fault Zone while those in the North China Plate are perpendicular to the Tan-Lu fault zone without obvious evidence for dragging. These phenomena also suggest that the North China Plate is more rigid than the Yangtze Plate.Based on these work, we propose an indentation-induced continent-tearing model for the origin of the Tan-Lu Fault Zone. We suggest that the present southern boundary of the North China Plate represents its original shape, with a promontory in front of the Dabie Orogen. At the oceanic subduction stage, the overriding North China Plate promontory led to vertical tearing of the subducting oceanic plate along the Tau-Lu Fault Zone, which formed along the eastern boundary of the promontory. Once the rigid North China Plate indenter collided with the passive Yangtze Plate along the Dabie Orogen, the oceanic slab tear propagated into the Yangtze Plate lithosphere, resulting in long-distance, low-angle subduction of the Yangtze Plate underneath the North China Plate indenter and NNE-ward, horizontal motion of the torn Yangtze Plate east of the Tau-Lu Fault Zone until the final collision at the Sulu Orogen. Thus, the indentation induced the vertical tearing of the subducting Yangtze Plate along the Tan-Lu Fault Zone. Following break-off of the oceanic slab, the SE-directed crust extrusion of the Dabie Orogen during the exhumation stage overprinted the shortening deformation on the Tau-Lu Fault Zone to the southeast of the Dabie Orogen.