A fluvial terrace record of late Quaternary folding rate of the Anjihai anticline in the northern piedmont of Tian Shan, China
Crustal shortening of the Tian Shan Range in northwest China has largely been accommodated by fold-and-thrust belts that mark the northern and southern mountain boundaries, which were formed during the basinward propagation of the deformation front of Tian Shan. Investigating the deformation rates of active folds (anticlines) in the mountain piedmonts within different timescales is important for understanding the mountain building and front propagation processes of the Tian Shan. In this study, we investigate the late Quaternary folding rate of the Anjihai anticline, one of the major anticlines in the northern piedmont of Tian Shan. Our study is based on optically stimulated luminescence (OSL) dating and topographic measurements of folded river terraces. Three well-preserved fluvial terraces across the Anjihai anticline were identified and dated to be 3.6 ± 0.1 ka, 9.0 ± 0.6 ka and 53.3 ± 2.2 ka, respectively. These terrace ages are combined with shortening and uplift of the terraces estimated using an area-conservation method and the terrace height profiles to evaluate the average shortening and uplift rates. Our results show that the minimum shortening rate of the Anjihai anticline is ~ 0.4 mm/a between 53 ka and 9 ka and ~ 1.1 mm/a over the past 9 ka, and the corresponding minimum uplift rates are ~ 0.5 mm/a and ~ 1.4 mm/a, respectively. If a local sedimentation rate of 0.3 mm/a is taken into consideration, the shortening rate is ~ 0.7 mm/a between 53 ka and 9 ka and ~ 1.3 mm/a since 9 ka, and the corresponding uplift rates are ~ 0.8 mm/a and ~ 1.5 mm/a, respectively. These data suggest that the Anjihai anticline has accommodated ~ 20-25% of the total shortening across the whole eastern Tian Shan (~ 5 mm/a) in the Holocene. Our results suggest that the Holocene is a relatively tectonically active time interval since the late Pleistocene. The late Quaternary shortening rate of the Anjihai anticline is much higher than its long-term average shortening rate (0.2 mm/a) since its formation at 7.4 Ma. A similar situation has also been observed for other active folds in the northern and southern piedmonts of Tian Shan. The higher late Quaternary folding rate is possibly attributed to the gradual propagation of the Tian Shan towards its northern and southern foreland basins in the Quaternary.