The evolution of a terrace sequence along the Manas River in the northern foreland basin of Tian Shan, China, as inferred from optical dating

摘要

The Tian Shan range lies in the actively deforming part of the India-Asia collision zone. The uplift rate and deformation pattern of the Tian Shan are important for understanding the dynamics of crustal deformation in the region. The river terraces in northern Tian Shan provide key records of past changes in climate and/or regional tectonics. In this study, a terrace sequence along the Manas River in a tectonically active zone in the northern foreland basin of Tian Shan is investigated. Six river terraces were identified and dated using optically stimulated luminescence (OSL). The results show that the six terraces were abandoned at ～0.5 ka, ～1.4 ka, ～3.1 ka, ～4.0 ka, ～12.4 ka and ～19.9 ka, respectively. Together with high resolution Global Positioning System (GPS) measurements on the terrace treads, the fluvial history of Manas River is reconstructed. From ～20 ka to ～4.8 ka, the height of the fluvial bed of Manas River decreased at an average rate of 2.2 ± 0.6 mm/yr. From ～4.8 ka to the present the height of the fluvial bed decreased at an average rate of 13.5 ± 0.6 mm/yr, corresponding to intensified incision of Manas River during the late Holocene. This accelerated incision is very likely caused by the tectonic forces rather than climatic influences alone, suggesting that the tectonic uplift activity was significantly intensified since ～4.8 ka in the northern piedmont of Tian Shan. Other controlling factors on the incision of Manas River are also discussed.