Combining 121 new fission track and (U‐Th)/He ages with published thermochronologic data, we investigate the Late Cretaceous‐Cenozoic exhumation/cooling history of the eastern Tibetan Plateau, Qinling, Daba Shan, and Sichuan Basin of east central China. The Qinling orogen shows terminal southwestward foreland growth in the northern Daba Shan thrust belt at 100–90 Ma and in the southern Daba Shan fold belt at 85–70 Ma. The eastern margin of Tibetan Plateau experienced major exhumation phases at 70–40 Ma (exhumation rate 0.05–0.08 mm/yr), 25–15 Ma (≤1 mm/yr in the Pengguan Massif; ~0.2 mm/yr in the imbricated western Sichuan Basin), and since ~11–10 Ma along the Longmen Shan (~0.80 mm/yr) and the interior of the eastern Tibetan Plateau (Dadu River gorge, Min Shan; ~0.50 mm/yr). The Sichuan Basin records two basin‐wide denudation phases, likely a result of the reorganization of the upper Yangtze River drainage system. The first phase commenced at ~45 Ma and probably ended before the Miocene; >1 km of rocks were eroded from the central and eastern Sichuan Basin. The second phase commenced at ~12 Ma and denudated the central Sichuan Basin, Longmen Shan, and southern Daba Shan; more than 2 km of rocks were eroded after the lower Yangtze River had cut through the Three Gorges and captured the Sichuan Basin drainage. In contrast to the East Qinling, which was weakly effected by late Cenozoic exhumation, the West Qinling and Daba Shan have experienced rapid exhumation/cooling since ~15–13 Ma, a result of growth of the Tibetan Plateau beyond the Sichuan Basin.
We measured the apatite etch rate vR in 5.5 M HNO3 at 21 °C as a function of orientation. Results for Durango apatite evidence that vR varies by a factor >5 with angle to the c-axis. Our measurements also provided track etch rates vT and surface etch rates vS. However, these cannot be combined for calculating track etching or counting efficiencies. By inserting the measured etch rates in a recent model, we calculate the geometries and dimensions of surface tracks in different apatite faces. The proposed model must be recalibrated for different etching protocols and adapted for other minerals. We submit that the new model justifies reviewing track counting efficiencies based on the existing (vB-vT) etch model. We anticipate that this will have an effect on practical aspects of fission track dating. Single-track step-etch data show that the confined track lengths increase with etch time at a decreasing average rate vL that differs from the track etch rate vT and the apatite etch rate vR. Both vT and vL exhibit large track-to-track differences that produce irreducible length variation related to the latent-track structure resulting from formation and annealing. Step etching and track width measurements are effective for reducing or eliminating procedure-related artifacts from track length data, and so for accessing more fundamental track properties.
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