Structural analysis of the northern Tongbai Metamorphic Terranes, Central China: Implications for Paleozoic accretionary process on the southern margin of the North China Craton

The Tongbai Mountain located at the junction between the Qinling and the Dabie mountains, is a part of the Qinling–Tongbai–Dabie–Sulu Composite Orogenic System containing arc–continent accretion-dominated and continent–continent collision-dominated orogenic belts. The Paleozoic accretion-dominated events are mainly preserved in the northern Tongbai Metamorphic Terranes, such as the Kuanping Group at the southern margin of the North China Craton where the Erlangping Tectonic Unit and the Qinling Complex Unit define the nature of arc, from north to south. Based on structural analysis and available geochronological data, four principal episodes of deformation (D1, D2, the boundary shear zones and D3) are distinguished in these metamorphic terranes following the Paleozoic peak metamorphism at ca. 440 Ma. The D1 deformation took place at ca. 440–430 Ma, and is characterized by a nearly WNW–NW trending penetrative foliation (S1) developed in all of these metamorphic terranes. The D2 is marked by an E–W trending pop-up structure developed at ca. 425–415 Ma. The simultaneously opposite ductile strike-slip shearing at ca. 405–395 Ma indicates that the Qinling Complex Unit was extruded laterally to the east at the end of Early Devonian. The D3 event is defined by WNW-trending open folds, and has occurred during the Late Triassic and Early Jurassic. The first three episodes of deformation are related to Paleozoic accretionary tectonics with strong structural imprints of the arc–continent collision process, while the D3 deformation belongs to the Mesozoic continent–continent collision-related deformation. We distinguish that not all of the deformation features in the northern Tongbai Metamorphic Terranes are related to the Paleozoic accretionary process on the southern margin of the North China Craton.