The Structure And Geochronology Of The Shear Zones In The Tongbai-dabie Orogenic Belt

The Tongbai orogenic belt has an overall antiformal geometry and the hinge of the antiform is subhorizontal and trends NW-SE. The Tongbai complex (TBC) in the core of the antiform is bounded by the south-dipping Yindian-Malong shear zone in the south,the subhorizontal Taibaiding shear zone at the top and the north-dipping Hongyihe-Tongbai shear zone in the north. The three shear zones have dextral,top-to-NW and sinistral movement, respectively. They are parts of a single shear zone,termed the Tongbai shear zone, that has a uniform top-to-NW sense of shear. Three samples of deformed granitoid (mylonite or protomylonite) from the shear zone have U-Pb zircon ages of 145 ± 6 Ma,142 ± 2 Ma and 131 ± 6 Ma,respectively. An L-tectonite in the TBC yielded a metamorphic age of 137 ± 8 Ma and a migmatite an age of 137 ± 1 Ma. The Tongbai shear zone is intruded by undeformed Early Cretaceous granite and dykes and deformation in the shear zone is constrained to ca. 140 to 135 Ma,synchronous with metamorphism and migmatization in the TBC. Early Cretaceous magma emplacement and the associated uplift modified the TBC into a gentle antiform and the uplift may have continued to ca. 102-85 Ma.L tectonite is well developed and widely distributed in the Tongbai complex of the Tongbai orogenic belt. The L tectonite occurs in the core of the antiform, in a zone that is 10-30 km wide and over 100 km long. Lineations in the L tectonite are sub-horizontal,parallel to the hinge of the antiform. Sheath folds are also well developed associated with the L tectonite, with the hinges parallel to the lineations. Migmatite occurs in the core of and structurally below the L tectonite and has a gradational relationship with the L tectonite. The domain of L tectonite is bounded by three ductile shear zones, on the north, at the top and on the south, respectively. Well-developed shear sense indicators indicate that the southern, the overlying and the northern shear zones have a dextral,top-to-NW and sinistral sense of shear as interpreted above. These geometrical and kinematic data indicate that the three shear zones are likely part of a single shear zone that wraps around the L-tectonite domain. The L-tectonite zone in the core moves southeast relative to the hanging wall. The development of the tectonite is interpreted to be a result of this special geometry and kinematics and reflects an orogen-parallel extension synchronous with migmatization in Early Cretaceous.Similar geometry and kinematics have been documented in the Dabie orogen to the east, which suggests that the Central Orogenic Belt in China probably experienced a uniform orogen-parallel extension and top-to-NW shearing in the ductile lithosphere in the Early Cretaceous.