Tectonic evolution since the Mesozoic of the Nanzhao area in southeast of the Ordos Basin: evidence from low-temperature thermal chronology

Although previous studies indicate that the southeast margin of the prototype Ordos Basin may be located in the Nanzhao area, the southeast extent of the basin remains unknown. Therefore, a study on the tectonic evolution of the area can provide important information for the Triassic prototype Ordos Basin research. In this paper, three Triassic samples from three field sections are investigated with zircon and apatite fission tracks to study the tectonic evolution history since Mesozoic in the Nanzhao area and its relationship with the Ordos Basin. The fission track ages of zircons range from 270±15 Ma to 181±8 Ma, which are close to, or older than the stratigraphic age, and may more represent provenance area information. The apatite fission track ages range from 57±3 Ma to 47±5 Ma. Combining these ages with thermal history simulation, this paper suggests that the Nanzhao area has undergone four large-scale structural transformations since the Triassic. In the early stage, it was affected by the strong overthrust of the Qinling Orogenic Belt at the end of the Triassic; the middle stage was from Middle-Late Jurassic to Early of Late Cretaceous; the late stage was Late Cretaceous; and the last stage was Himalayan. These four stages of structural transformation have close relationship with tectonic evolution of the Qinling Orogenic Belt. In addition, through the comparison with the surrounding areas of the Ordos Basin, it is found that the structural evolution processes of the Nanzhao area and the Ordos Basin have similar timelines, which supports the view that the Nanzhao area belongs to the Ordos prototype basin from the perspective of structural evolution. © 2020, Science Press. All right reserved.