Late Cenozoic out-of-Sequence Growth of the Southern Qilian Shan: Insights from Apatite Fission Track Thermochronology

Abstract:

The Qilian Shan is located in the NE active margin of the Tibetan Plateau. Its deformation pattern can be viewed as the key to constrain the dynamic evolution of the Tibetan Plateau. Studies from northern Qilian Shan revealed an image of the forward propagation of the range growth (Zheng Wj Helishan paper, Qilian 9Ma, Yumushan ~3-4Ma, Heli Shan, ~2Ma). However, researches are still sparse to reveal late Cenozoic deformation along the southern Qilian Shan. To quantify the evolution of the Qilian Shan, we therefore carry out thermochronology study at Huaitoutala region within the adjacent Qaidam basin and southern Qilian Shan transition belt.

Our thermal modeling of fission track results show that the rapid exhumation of the southern Qilian Shan was initiated at 13-15Ma ago, providing the dominated sediments to the Huaitoutala region, which was also verified by the paleocurrent data within the Huaitoutala section. Sediment recycling was further revealed by our detrital fission track data. Both the paleocurrent and lithoface data indicate that the onset of rapid rising of the Oulongbuluke Mountain was at about 6~8Ma ago. Growth strata were observed at the bottom of Qigequan formation led us to conclude that the Huaitoutala anticline began to form at about 2.5-3.5Ma ago. The folded and faulted Qigequan formation and the overlying Quaternary sediments all suggest that the faulting movement were active during Quaternary.

In summary, our study revealed an out-of-sequence pattern of late Cenozoic deformation along the southern margin of the Qilian Shan, different from the previous finding from northern Qilian Shan. 1)at 13-15Ma ago, exhumation and rapid uplift began along the southern Qilian Shan. 2)starting from 6-8Ma ago, the tectonic activity propagated southward to the Oulongbuluke Mountain region 3) at about 2.5-3.5 Ma ago, however, the deformation jumped backward to the north near Huaitoutala reservoir region, between the southern Qilian Shan and uplifted Oulongbuluke Mountain. We hypothesize here that this out-of-sequence deformation was resulted from the stress rebalance between southward expanding of the Qilian Shan and the resistance of the more rigid Qaidam basement. Our finding indicates a complex spatial and temporal evolution for this young outward growing margin of the Tibetan Plateau.