Formation of Dabashan arcuate structures: Constraints from Mesozoic basement deformation in South Qinling Orogen, China

The Dabashan arcuate structures are composed of the NW-striking Northern Dabashan Belt belonging to the southern margin of the South Qinling Orogen and the curved Southern Dabashan Belt located in the northern margin of the Yangtze Block with the arcuate Chengkou-Fangxiang Fault as their boundary. The deep understanding of the orogenesis is indispensable to unravel the mechanism of the Dabashan arcuate structures, as the basement preserves important clues on the deformational characteristics as well as the timing and orogenic processes. Here we report integrated field investigation, structural analysis, and 40Ar/39Ar thermochronological data in the Wudang and Ankang Uplifts in the South Qinling Orogen to decipher the forming processes and mechanisms of the Dabashan arcuate structures. The field and microscopic observations reveal a dominantly top-to-south (southwest) shear sense at low to medium temperatures. The basement deformation expanded southward (southwestward) in the form of duplex. During the continued shortening, the basement was uplifted due to thrust stacking within duplex systems. According to the deformation temperatures, and the new and published 40Ar/39Ar thermochronological data, the basement recorded the ca. 248-212 Ma orogenesis for the Wudang Uplift and the ca. 222-161 Ma orogenesis for the Ankang Uplift. Based on the tectonic evolution of the study area, the formation of composite Dabashan arcuate structures included three stages. Firstly, the arcuate master fault accompanied by a series of NW-striking faults came into being at ca. 450-400 Ma based on previous studies. Secondly, the scissor-like convergence between the South China Block and North China Block reactivated the extensional structures and formed the general features of the Northern Dabashan Belt at ca. 250-160 Ma. Finally, the deformation expanded toward the foreland and formed the curved Southern Dabashan Belt with obstruction by the Hannan Uplift and Shennongjia-Huangling Uplift in the front at ca. 160-100 Ma.