Rapid exhumation of the Tianshan Mountains since the early Miocene: Evidence from combined apatite fission track and (U-Th)/He thermochronology

Combined apatite fission track (AFT) and (U-Th)/He (AHe) thermochronometries can be of great value for investigating the history of exhumation of orogenic belts. We evaluate the results of such a combined approach through the study on rock samples collected from the Baluntai section in the Tianshan Mountains, northwestern China. Our results show that AFT ages range from ∼60 to 40 Ma and AHe ages span ∼40-10 Ma. Based on the strict thermochronological constraints imposed by AHe ages, forward modeling of data derived from AFT analyses provides a well-constrained Cenozoic thermal history. The modeled results reveal a history of relatively slow exhumation during the early Cenozoic times followed by a significantly accelerated exhumation process since the early Miocene with the rate increasing from <30 m/Myr to >100 m/Myr, which is consistent with the inference from the exhumation rates calculated based on both AFT and AHe age data by age-closure temperature and mineral pair methods. Further accelerated exhumation since the late Miocene is recorded by an AHe age (∼11 Ma) from the bottom of the Baluntai section. Together with the previous low-temperature thermochronological data from the other parts of the Tianshan Mountains, the rapid exhumation since the early Miocene is regarded as an important exhumation process likely prevailing within the whole range.