10Be dating of the Kuitun River terraces in the northern Chinese Tian Shan foreland: Insights into fluvial evolution and tectonic shortening pattern

Characterizing active deformation of a piedmont structure is particularly helpful for understanding the dynamics of deformation of its adjacent active orogenic belt. This work focuses on fluvial geomorphology of the Kuitun River, which flows northwards out of the high mountains and perpendicularly cuts the E-W-striking Dushanzi anticline, a structure of the outermost fold-and-thrust belt in the northern Chinese Tian Shan foreland. Four episodes of alluvial fan development (fans F1-F4, sequentially younger in abandonment age) were identified. Based on the fluvial geomorphological framework constructed by these four alluvial fans, six terraces (terraces T-1-T-6, sequentially increasing height above the riverbed) were then defined. The topographic analyses and 10Be dating of these alluvial landforms show that the Kuitun River originally flowed northwestwards before at least 80 ka. Subsequent rock uplift caused by fold growth could have resulted in an eastward deflection, after which the river began to flow northeastwards to construct fan F2. At ~67 ka, accumulation of fan F-2 ended, and the Kuitun River deflected westwards to flow northwards and began to construct fan F3. The final stabilization of the Kuitun River channel occurred at ~13 ka due to intense river incision forced by climate change, when the river began to incise a 200-m deep canyon and form a flight of strath terraces flanking the valley. By using geomorphic surfaces as the references, the Late Pleistocene and Holocene shortening rates of the Dushanzi anticline were estimated to be 0.71 + 0.17/-0.16 mm/yr (over the past ~67 kyr) and 1.6 + 0.39/-0.37 mm/yr (over the past 13 kyr), respectively. When combined with the long-term shortening rate of ~0.44 mm/yr since initiation of fold growth, it is inferred that the Dushanzi anticline likely experienced an acceleration of deformation during the late Quaternary. With respect to the entire orogenic belt, both fluvial geomorphic records and GPS surveys indicate that most of N-S crustal shortening (likely > 70%) across the Tian Shan has been absorbed in the piedmonts of the range, a typical active orogenic belt in the interior of Asia.