Fisson-track constrains on superposed folding in the Beishan orogenic belt, southernmost Altaids

The Hongyanjing inter-arc basin, is located at the central part of Beishan Orogenic College (BOC), Gansu Province, northwest China. Thick sequences of Permian sediments were strongly folded, forming extremely spectacular superposed folds. To better understand the thermal history of Hongyanjing inter-arc basin and to potentially constrain the timing of deformation, apatite fission track thermochronology method was applied on two superposed folds in the Hongyanjing Basin. Samples from the basin, yield central AFT ages ranging from ∼206 to 118 Ma. AFT peak ages were largely consistent between samples and can divided into three groups: 245, 204–170 and 112–131 Ma. Subsequent thermal history modeling of the samples from the Hongyanjing Basin can be summarized as follows: (1) thermal reheating by sedimentary burial at ∼ 260 to ∼220 Ma; (2) major cooling from ∼220 to ∼180 Ma; (3) an episode of very slow subsequent cooling from ∼180 to 65 Ma (∼80 °C) to present-day outcrop temperatures. Sediments in the Hongyanjing Basin were folded forming F1 fold during the early to late Triassic (∼240–∼220 Ma), by regional stress, and at the time that the adjacent Xingxingxia shear zone started to become active. It is further suggested that the F2 folding occurred at ∼225–219 Ma. The deformation age of F2 should be extended to 180 Ma based on our thermal history modeling for the Hongyanjing Basin, which show a rapid exhumation and cooling at the late Triassic to early Jurassic (∼220–∼180 Ma). In our interpretations, the F1 folding is therefore thought to be related to the final closure of the Paleo-Asian Ocean, while the F2 folding occurred at ∼225–180 Ma associated with a major pulse of orogenesis in the BOC.