The applications of the optically stimulated luminescence (OSL) dating on the fluvial and tectonic processes in the northern Tian Shan, China

Abstract

The northern Tian Shan is a natural laboratory to study the fluvial and tectonic processes since the late Quaternary. The Simplified procedures was developed for the OSL dating of young samples with dim and poorly-bleached quartz grains in this area. A rapid average incision rate of ~7 cm/yr was revealed since ~3.7 kyr in Anjihai Gorge by using the Simplified procedures. In the lower Anjihai River, three apparent age gaps of sedimentation were distinguished, in response to climatic change and tectonic factors. The out-of-sequence active Holocene Jiangnanmiao backthrusts were identified in the Qingshuihe anticline.

In the Simplified procedures, a single-aliquot regenerative-dose (SAR) protocol was applied to establish the standardized growth curve (SGC) with heated quartz grains. This SGC curve represents the growth curves of bright quartz grains. OSL dating is achieved by only measuring the natural OSL signal (Ln) and the luminescence (Tn) response to a test dose, and then projecting Ln/Tn into the SGC curve. A Finite mixture model (FMM) was applied to the large datasets. Reliable De determinations were obtained using the proposed Simplified procedures method.

There are at least 18 river terraces distributed in the Anjihai gorge, in the northern Tian Shan. Based on OSL dating results and elevation measurements, the Anjihai River has cut into the gorge to a depth of ~260 m since 3.7±0.2 kyr, with a maximum average incision rate of 7.2±0.1 cm/yr, and a rapid average incision rate of 15±0.3 cm/yr during the interval of ~3.0-2.4 kyr. The specific geomorphological characteristics and location of the stream and autogenic adjustment to the forcing factors have resulted in the Anjihai River recording an unusually rapid incision rate within the Anjihai Gorge.

A trench was excavated in the lower Anjihai River. The OSL dating results reveal at least three hiatuses in sediment aggradation since ~60 kyr: at ~35-25 kyr, ~15-11 kyr, and ~4.1-3.1 kyr. The incision events show that the sedimentary timing gaps were caused by the subsequent fluvial incision. The age hiatuses are closely correlated with climatic transitions during MIS 3a-MIS 2, MIS 2-MIS 1, and the Mid-Late Holocene, and with periods of active crustal deformation. The climatic change was the main reason for the age gaps occurring at ~35-25 kyr and ~15-11 kyr. River deflection at ~3.0 kyr in the lower reach was the most likely cause of the sedimentation hiatus at ~4.1-3.1 kyr.

The Jiangnanmiao (JNM) backthrusts occur within the Qingshuihe anticline. A tectonic wedging model is proposed to explain the slip distribution in the thrusting and folding belts in the northern piedmont. The JNM thrust is the surficial expression of a backthrust. The JNM backthrust in the Qingshuihe anticline recorded thrusting activities from ~270 kyr to the present. The JNM backthrust and the tectonic wedge system represent the active Holocene thrusting system. The Qigu-Qingshuihe anticlines have remained an active Late Quaternary faulting-and-folding belt. The slip occurred at a uniform rate of ~0.4 mm/yr during ~270-11 kyr. During the latest Quaternary, the slip accelerated, with rates from ~0.4 mm/yr to ~0.8 mm/yr.