Evolution of the NE Qinghai-Tibetan Plateau, constrained by the apatite fission track ages of the mountain ranges around the Xining Basin in NW China

摘要

The Laji Shan and Daban Shan which are located along the southern and northern margins of the Xining Basin in the northeastern Qinghai-Tibetan Plateau, experienced important multi-stage exhumations during the Cenozoic. The apatite fission track (AFT) dating of 19 samples along four north-south sections in the Laji Shan and Daban Shan showed that the AFT ages of the Laji Shan from 88 Ma to 12 Ma and the ages of the Daban Shan are between 60 and 32 Ma. All these AFT ages are significantly younger than their host rock formation or sediment deposition ages. Thermal modeling of the AFT data indicated that the Laji Shan had undergone important exhumation events in the Late Cretaceous, ca. 50-30 Ma, and 17-8 Ma, whereas the only significant thermal event in the Daban Shan occurred at ca. 50-30 Ma. The exhumation of the Laji Shan during the Late Cretaceous may result from the Lhasa-Qiangtang collision. During 50-30 Ma, the Central Qilian Block rotated clockwise relative to the South Qilian Block, which led to the formation of the Xining Basin. The slow uplift of the Laji Shan and Daban Shan in this period was the isostatic adjustment of the footwalls of normal faults along the two margins of the Xining Basin. The exhumation of the Laji Shan during 17-8 Ma resulted from compression due to the clockwise rotation of the South Qilian Block relative to the Central Qilian Block, causing the inversion of the Xining Basin, a marked change in the sedimentary environments and a rapid increase in sedimentation rates.