Neoproterozoic to Early Paleozoic polyorogenic deformation in the southeastern margin of the Yangtze Block: Constraints from structural analysis and 40Ar/39Ar geochronology

The southeastern margin of the Yangtze Block is a suitable region to decipher Neoproterozoic to Early Paleozoic polyorogenic deformation of the South China Block. Based on 1:50,000-scale geological mapping, four deformational phases were identified through structural analysis and 40Ar/39Ar dating. The earliest event (D1) corresponds to nearly E-W-striking folding and top-to-the south thrusting, developed during Neoproterozoic orogeny caused by the collision between the Yangtze Block and the Cathaysia Block. The D2 event is characterized by the nearly E-W-striking top-to-the north thrusting. The D3 deformation is marked by the NNE-striking folding, thrusting and dextral shearing. The D3 structures constitute a positive flower pattern, with opposing kinematics, rooted in the Jiangwan dextral shear zone. The D4 event is the nearly E-W-striking ductile dextral shearing. 40Ar/39Ar geochronology shows that the NNE-trending dextral shearing and the nearly E-W-striking dextral shearing occurred at 449 ± 4 Ma and 429 ± 3 Ma, respectively. Combining with previous geochronology, all the D2, D3, D4 events are related to the Early Paleozoic orogeny. Both the D2 top-to-the north thrusts and D3 NNE-striking positive flower structures are syn-orogenic products. These incompatible syn-orogenic structures indicate that the orientation of the principal stress changed from the N-S-striking to E-W-striking at ∼450 Ma. The nearly E-W-striking ductile dextral shearing (D4) is the outcome of post-orogeny.