Terrace sequence along the Yushanguxi River in the southern piedmont of Tian Shan and its relationship to climate and tectonics in northwestern China

Accepted Manuscript Terrace sequence along the Yushanguxi River in the southern piedmont of Tian Shan and its relationship to climate and tectonics in northwestern China

Chuanyong Wu, Wenjun Zheng, Zhuqi Zhang, Qichao Jia, Jingxing Yu, Huiping Zhang, Guihong Han, Yuan Yao PII: DOI: Reference:

S0169-555X(18)30164-8 doi:10.1016/j.geomorph.2018.04.009 GEOMOR 6380

To appear in:

Geomorphology

Received date: Revised date: Accepted date:

31 December 2017 12 April 2018 16 April 2018

Please cite this article as: Chuanyong Wu, Wenjun Zheng, Zhuqi Zhang, Qichao Jia, Jingxing Yu, Huiping Zhang, Guihong Han, Yuan Yao , Terrace sequence along the Yushanguxi River in the southern piedmont of Tian Shan and its relationship to climate and tectonics in northwestern China. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Geomor(2017), doi:10.1016/ j.geomorph.2018.04.009

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ACCEPTED MANUSCRIPT Terrace sequence along the Yushanguxi River in the southern piedmont of Tian Shan and its relationship to climate and tectonics in northwestern China Chuanyong Wua, Wenjun Zhengb, Zhuqi Zhangc, Qichao Jiad, Jingxing Yuc, Huiping

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Zhangc, Guihong Hana, Yuan Yaoa Earthquake Agency of the Xinjiang Uygur Autonomous Region, Urumqi 830011, China.

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Guangdong Provincial Key Laboratory of Geodynamics and Geohazards, School of Earth Science and

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Geological Engineering, Sun Yan-Sen University, Guangzhou 510275, China.

State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing

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100029, China.

China Earthquake Disaster Prevention Center, China Earthquake Administration, Beijing 100029, China.

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Abstract: Controversies persist regarding the formation of terraces under the control of tectonic factors or climatic

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changes. This work focuses on the Yushanguxi River in the southern piedmont of Tian Shan, which is an intense

tectonic uplift area where the terraces are very developed and the river has deeply downcut. Nine main terraces are

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distinguished (labelled T1 to Th, from youngest to oldest) based on the interpretation of a high-resolution remote

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sensing image, field investigations and detailed surveying with differential GPS. The results of the determination of 10Be exposure and

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C show abandoned ages of ~2.1 ka for T1, ~4.1 ka for T2, ~4.2 ka for T3, ~8.2 ka for T4,

~18.1 ka for T5, ~18.8 ka for T6, ~102.1 ka for T7, ~100.6 ka for T81, ~113.9 ka for T82, ~144.6 ka for Th1, ~210.7 ka for Th2, and ~284.3 ka for Th3. Since ~18 ka, the incision rate began to increase from ~0.6 mm a-1 to ~12 mm a-1, which is obviously higher than the fault slip rate of ~0.7 mm a-1. We suggest that the rapid downcutting along the

Yushanguxi River during the Holocene has mainly been caused by frequent climate fluctuations.

Keywords: Tian Shan; Terrace sequence; River incision; Climate fluctuation

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1. Introduction The essence of the formation of river terraces is that river erosion and aggradation rates should alternate or change with time (Burbank and Anderson, 2012). The two main factors of climatic fluctuation and tectonic movement can both drive changes in erosion and aggradation. Be exposure dating, optically stimulated

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C methods; therefore, terrace systems are among the best objects to

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luminescence (OSL) and

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Terrace ages can be accurately determined by using

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quantitatively study the temporal evolution of climate or regional tectonics. Over the past few decades, many researchers have undertaken a large amount of work on the sequence and

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deformation features of the river terraces in the Tian Shan area (e.g. Xu et al., 1992; Avouac and

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Peltzer, 1993; Avouac et al., 1993; Molnar et al., 1994; Zhang et al., 1995; Li et al., 1999; Deng et al., 2000; Scharer et al., 2006; Lu et al., 2010; Yang et al., 2013; Gong et al., 2014). However,

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some controversies regarding these terraces persist, especially regarding whether the formation of

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the terraces has been mainly controlled by tectonic factors (e.g. Formento-Trigilio et al., 2003; Yang et al., 2013; Gong et al., 2014) or climatic changes (e.g. Pan et al., 2003; Bookhagen et al.,

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2006; Hetzel et al., 2006; Lu et al., 2010, 2018).

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Within the continental region, climate changes caused by the alternation of interglacial and glacial periods can lead to river downcutting and form river terraces (Chatters and Hoover et al., 1992; Martin, 1992; Zhang et al., 1995; Starkel, 2003; Hu et al., 2013). Structural factors are mainly reflected in areas of intense tectonic movement. When the crust is relatively stable, rivers are dominated by lateral erosion. A river should quickly downcut when the crust is uplifted. In the northern and southern piedmonts of Tian Shan, for example, a series of river terraces formed because of the intermittent uplift of active faults and folds (Deng et al., 2000; Yang and Li, 2005;

ACCEPTED MANUSCRIPT Li et al., 2012; Yang et al., 2013). In many cases, terrace formation cannot be attributed to a single factor, and the two effects of climate change and tectonic movement may both play an important role (Bridgland, 2000; Maddy et al., 2001). Previous studies usually surveyed transverse sections of terraces to discuss their geneses. Transverse sections cannot adequately reflect tectonic

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information, so distinguishing the effects of climatic and tectonic factors is difficult.

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The Yushanguxi River, which is located in the southern piedmont of Tian Shan (Fig. 1), has a

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deeply incised valley. Terraces are widely developed along the river valley, and the Maidan Holocene active fault displaced these terraces. The location is a natural laboratory for exploring

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the scientific question of terrace genesis. In this study, we survey the transverse sections and 10

Be

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longitudinal profiles of these terraces to obtain terrace heights and fault displacements.

exposure ages and 14C dating methods are synthetically applied to determine the terrace sequence

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along the Yushanguxi River. Then, we compare the river’s aggradation and incision processes

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with glacial-interglacial cycles and evaluate the possible contributions of climatic changes and tectonic movement to the alluvial deposition and river incision.

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2. Geological setting

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The Tian Shan is a typical intra-plate orogenic belt within the Eurasian continent and has been reactivated by the remote effects of the India-Eurasia collision since the Cenozoic era (Fig. 1; Molnar Tapponnier, 1975; Tapponnier and Molnar, 1979; Avouac et al., 1993; Zhang et al., 1996; Yin et al., 1998; Deng et al., 2000). Frequent earthquake activity (Fig. 1) and GPS observation data (e.g. Abdrakhmatov et al., 1996; Reigber et al., 2001; Zhang et al., 2003; Yang et al., 2008; Zubovich et al., 2010) indicate that the present-day tectonic deformation of the Tian Shan is very intense. The N-S crustal shortening rate across the western Tian Shan is approximately 18-20 mm

ACCEPTED MANUSCRIPT a-1 (e.g. Abdrakhmatov et al., 1996; Avouac and Burov, 1996; Molnar and Ghose, 2000; Zhang et al., 2003; Yang et al., 2008; Zubovich et al., 2010). Because of the intense crustal shortening and tectonic uplift, the average altitude of the Tian Shan exceeds 3500 m (Avouac and Burov, 1996; Deng et al., 2000), and foreland basins formed in the northern and southern piedmonts of Tian

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Shan (Hendrix et al., 1994; Deng et al., 2000; Chen et al., 2002; Zhang, 2004). With the outward

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expansion of Tian Shan to the foreland, Cenozoic strata were folded and uplifted to form several

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rows of thrust fault-fold belts (Fig. 1; Avouac et al., 1993; Liu et al., 1994; Zhang et al., 1996; Yin et al., 1998; Burchfiel et al., 1999; Deng et al., 2000; Fu et al., 2003; Guan et al., 2007).

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Glaciers are widely developed in the Tian Shan area (Zhao et al., 2010) because of the high

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elevation. A series of inland rivers that originate from the glaciers transversely cut the foreland thrust fault-fold belts and flow into the Tarim and Junggar Basins (Fig. 1). The piedmont is the

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et al., 2013; Gong et al., 2014).

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most developed site for river terraces (Molnar et al., 1994; Deng et al., 2000; Lu et al., 2010; Yang

The Aheqi valley lies in a generally east-west direction in the southern Tian Shan piedmont

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(Fig. 1). Its length is ~150 km, its N-S width is 10-50 km, and the valley has a trumpet shape,

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being narrow in the west and wide in the east (Fig. 2a). The Aheqi valley is a Cenozoic basin in which the Cenozoic sedimentary thickness is 3000-4000 m (Xiao et al., 2008). The Maidan Holocene active fault controls the northern boundary of the valley (Wu et al., 2014). The ENE-trending Maidan fault, which is also called the South Tian Shan range-front fault, is a boundary structure between the southwestern Tian Shan area and Tarim Basin (Allen et al., 1999). Fault scarps and sinistral offsets can be widely observed in Late Quaternary geomorphic surfaces along the fault, indicating that the Maidan fault is a thrust fault with a sinistral strike-slip

ACCEPTED MANUSCRIPT component. A palaeoearthquake study showed that a strong earthquake occurred along this fault during the Late Holocene (Wu et al., 2014). Because of the continuous thrust movement of the fault, the Cenozoic strata have undergone obvious tectonic deformation, forming low hills on the landform (Figs. 2b and 3). The average altitude of the range in the northern area of the Aheqi

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valley exceeds 4000 m, with perennial snow and glaciers, and the average altitude is only ~2000

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m in the southern valley. The huge terrain fall causes eroded materials within the range to be

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transported to the piedmont by rivers and rapidly deposit, forming alluvial landforms (Fig. 3). The Tuoxkan River, which flows from west to east, is a first-class river in the Aheqi valley. A

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series of N-S tributaries flow into the Tuoxkan River along its northern bank (Figs. 2a and 3).

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Most of the tributaries originate from the active glaciers in the middle-high range. Because of drought and low amounts of rainfall, the river’s flow strongly depends on melt water and displays

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strong seasonal variability. The Yushanguxi River is a chief tributary of the Tuoxkan River. This

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tributary originates from the glacier area in southern Tian Shan, which has an average elevation of approximately 5000 m, a large catchment area and perennial water. The modern riverbed width of

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the Yushanguxi River ranges from 300 to 500 m, and several river terraces have asymmetrically

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developed along both banks of the river (Fig. 4). 3. Methods

3.1. Division and displacement measurements of the terraces Stratified river terraces are well developed along the river valley (Figs. 4 and 5a). The terrace sequence of the Yushanguxi River was determined based on the height above the riverbed and the extent of dissection. Elevation differences between the terraces are obvious because of strong river downcutting. Almost no plants grow on the surface in the southern piedmont of Tian Shan, and the

ACCEPTED MANUSCRIPT terrace can therefore be easily identified by using remote-sensing images. We utilized a high-resolution image (source: http://earth.google.com) and field investigations to establish the sequence of fluvial terraces along the river. The terrace geometry could be accurately determined based on remote-sensing interpretation. The slope of the terrace front along the Yushanguxi River

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is relatively steep (Figs. 5a,e and 6a), and some stratigraphic sections can be observed along the

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terraces (Fig. 5e). Based on field observations, we could define the geomorphic features, the

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thickness of gravels that cap the terrace, and the underlying bedrock.

Nine main terraces were distinguished along the river valley (labelled as T1 to Th from

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youngest to oldest in Fig. 4). The thicknesses of the terrace gravels usually ranged from 5 to 12 m.

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Beneath the terrace gravels, a hard, lower Pleistocene conglomerate was exposed, which records long-term tectonic deformation from the Maidan fault (Fig. 2b). Some major terrace divisions

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contained one to several subdivisions, which we defined by using superscripts, e.g. T82. Because

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of the arid climate and minor surface erosion, the terraces were all completely preserved (Fig. 5c). We surveyed two transverse sections in both banks of the river by using differential GPS, from

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which the terrace elevations could be accurately determined. The Yushanguxi River flows roughly

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perpendicular to the Maidan fault strike. The terraces have been obviously displaced near the fault (Figs. 4 and 6b). We surveyed the deformed terraces and modern riverbed by using differential GPS. The longitudinal profiles of the terraces could adequately reflect tectonic deformation since the geomorphic surface was abandoned. 3.2. Geomorphic surface dating The terrace alluvial sediments are proximal to their source, and the sediments are therefore often very coarse. The sediments exposed in the sampling pit and terrace surface comprise

ACCEPTED MANUSCRIPT sub-round coarse gravel clasts with quartz-rich composition (Fig. 5b,d). We mainly employed the 10

Be exposure dating method to obtain the terrace ages. In addition, we found a radiocarbon

sample in the upper portion of the T5 terrace sediment layer. 3.2.1. 10Be exposure dating

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We collected 10Be surface samples on each terrace and the modern riverbed to constrain the

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abandonment ages of the terrace surfaces, and two depth-profile samples were collected in the T3

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and T4 terraces. To avoid the influence of near-source deposits and the terrain, the samples were all taken from open areas in the terraces with negligible erosion (Fig. 5c). A statistical model

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(Anderson et al., 1996) indicated that samples of no less than 30 gravels are sufficient to derive

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theoretical average ages, so we collected 50-80 gravels for each sample to determine the terrace abandonment age. The gravel diameters were usually no more than 3 cm. Two profile samples

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were taken at T3 and T4. Each profile consisted of 6 samples with depths of 0 cm, ~20 cm, ~50 cm,

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~80 cm, ~120 cm and ~180 cm (Fig. 5b,d).

The sample pre-treatment and 10Be target were prepared at the Key Laboratory of Crustal

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Dynamics, China Earthquake Administration (CEA). Purified quartz samples (~10-35 g) were

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dissolved in HF and HNO3 by adding a 9Be spike of ~ 0.25 g in a carrier solution. A single blank sample was also processed alongside other quartz samples to control for variations in the preparation. After removing fluorides with HNO3 and HCl and removing Fe and Ti by anion exchange, pure Be atoms were separated on cation/anion exchange columns and precipitated as hydroxides. These precipitates were dried and oxidized at 900-950°C. The resultant BeO powders were mixed with equal volumes of Nb and packed in target holders for the AMS (Accelerator Mass Spectrometry) determination of

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Be/9Be at the Laboratatoire National des Nucleides

ACCEPTED MANUSCRIPT Cosmogeniques of France. The apparent age (Table 1) of each sample was calculated according to corrections by Lal (1991) and Stone (2000). The gravels in the modern riverbed and terraces were all from the same source area and had very similar migration processes, so we suggest that the inherited nuclide concentration for each surface sample can be eliminated by the concentration of

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the modern riverbed (T0). For the two profile samples from T3 and T4, the exponential term of the

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fitting function provided an estimate of the production since the terraces were abandoned. Instead

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of only using the top-surface samples, all the amalgamated samples were used to determine the ages of the surface exposures. The factor in the exponential term (0.0125 m-1, which corresponds

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to a density of 2.0 g cm-3 for a 160 g cm-2 length scale) was chosen to maximize the best fit for the

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profiles and to be consistent with previous studies that used unconsolidated sediments. Finally, we estimated ages from the concentrations at the surface by using a low-elevation, high-latitude

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production rate of 4.3 atoms a-1 g-1 SiO2 (Granger et al., 2013). This rate was adjusted for

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elevation and latitude following Stone’s (2000) formulation. The actual production rates were ~20-23 atoms a-1 g-1 SiO2 for all sites. All the calculations and error estimations were performed

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by using the CosmoCalc MS Excel calculator (Vermeesch, 2007).

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3.2.2. Radiocarbon dating

A grey-black organic charcoal was taken from the gravel layer of the T5 terrace at ~65 cm depth. This sample was dated at the State Key Laboratory of Earthquake Dynamics, Institute of Geology, CEA. The quoted age in radiocarbon years used the Libby half-life of 5568 years, and the uncertainties were reported as 1σ. The calendar dates were calculated by using OxCal v4.2 (Ramsey, 2009) and the Northern Hemisphere terrestrial radiocarbon curve IntCal13 (Reimer et al., 2013).

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4. Results 4.1. Terrace sequence of the Yushanguxi River Nine main terraces were distinguished in the hanging wall of the Maidan fault. Some terraces were buried in the footwall of the fault, and only six grade terraces could be identified (Fig. 4).

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T1 was an accumulation terrace, while the other terraces were all base terraces with a lower

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Pleistocene conglomerate bedrock.

(Table 1) and

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10

Be exposure

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The abandonment ages of the terraces were accurately defined based on the

C (Table 2) dating results. Good exponential fitting curves were found in the

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age-depth plots of the depth-profile samples that were collected from terraces T3 and T4 (Fig. 5b,d).

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After eliminating the inheritance values by using the bottom value of the asymptote in the profile, we obtained a T3 terrace abandonment age of 4.16 ± 0.41 ka and a T4 terrace abandonment age of

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8.23 ± 2.06 ka. We only took surface samples for the other terraces. The inherited nuclide

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concentration for each sample was eliminated by the concentration of the modern riverbed sample. Thus, we determined abandonment ages of 2.10 ± 0.10 ka for terrace T1, 4.06 ± 0.13 ka for terrace

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T2, 18.06 ± 0.52 ka for terrace T5, 18.76 ± 0.58 ka for terrace T6, 102.08 ± 1.82 ka for terrace T7,

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100.55 ± 1.69 ka for terrace T81, 113.90 ± 1.75 ka for terrace T82, 144.60 ± 2.20 ka for terrace Th1, 210.65 ± 2.98 ka for terrace Th2, and 284.30 ± 3.84 ka for terrace Th3 (Fig. 7a). The terrace ages that were determined with the 10Be exposure method represent the abandonment ages, which are minimum estimates. The corrected age of the

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C sample from the T5 terrace sediments was 16.956 ± 0.104 ka

(Table 2), which was basically consistent with the 10Be exposure age of 18.06 ± 0.52 ka. The T3, T4, T5 and T8 terraces had the most developed geomorphic surfaces along the Yushanguxi River,

ACCEPTED MANUSCRIPT and their ages correlated well with the regional terrace ages (Deng et al., 2000; Li et al., 2012; Yang et al., 2013; Gong et al., 2014) in the Tian Shan area, indicating that our determination results were reliable. 4.2. Offset of the terraces

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The Maidan fault, which almost perpendicularly traverses the Yushanguxi River, obviously

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dislocated the river terraces. The fault mainly consists of the main fault and back-thrust fault. The

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main N-dipping fault is characterized by large displacement, and the S-dipping back-thrust fault on the hanging wall of the main fault is characterized by a small displacement and discontinuous

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distribution on the surface (Fig. 4). A geological section along the Yushanguxi River (Fig. 2b)

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showed that the Cenozoic layer has been folded into a syncline in the hanging wall of the fault. According to the tectonic deformation features and fault distribution (Figs. 2b and 4), we suggest

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that this feature should be a fault-bend fold. The displacement of the fault could be obtained by

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measuring the scarp on the surface (Lave et al., 2000). Field investigations indicated that the T1 and T2 terraces have not been faulted. Our measurement results revealed that the vertical

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displacements of the T3, T4 and T5 terraces were ~2.8 m, ~7.3 m and ~12.5 m, respectively. The

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T71 terrace is not continuous, and we projected the terrace profile according to its overall geometry (Fig. 6b). Except for the main thrust fault, a flexural slip fault was present on the geomorphic surfaces of terrace T7 to terrace Th (Wu et al., 2014). The vertical displacement of the main thrust fault on terrace T7 was 65 ± 9 m, and that of the flexural slip fault was 5.8 ± 0.5 m; therefore, the total vertical displacement of the two faults was ~71 m on terrace T7. The T82 terrace is one of the most continuous terraces along the Yushanguxi River. On terrace T82, the vertical displacements of the main thrust fault and flexural slip fault were 78.2 ± 6.8 m and 9.1 ± 0.7 m, respectively, and

ACCEPTED MANUSCRIPT the total vertical displacement on the T82 terrace was approximately 87.3 ± 7.5 m (Figs. 6b and 7b). 5. Discussion 5.1. River incision along the Yushanguxi since the Late Quaternary

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Terrace Th exists as the highest alluvial geomorphic surface along the Yushanguxi River.

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Beneath the Th terrace, an ~171 m downcutting depth is well exposed along the eastern bank of the

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river valley (Figs. 5a and 6a). Field investigations showed that the gravel valley fill was usually less than 12 m on each terrace (Fig. 6a). The downcutting depths and filling thicknesses of the gravels

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provide a direct record of river incision and aggradation process during the Late Quaternary.

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The 10Be exposure age of 284.30 ± 3.84 ka for terrace Th3 (Table 1) could be considered the onset time of alternating incision and aggradation since the Late Quaternary. Based on the height of

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~171 m and the Th3 abandonment age of 284.30 ± 3.84 ka, we obtained an average incision rate of

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~0.60 mm a-1. The incision rate underwent obvious changes during different periods. We calculated the incision rate by using the height difference and corresponding age between two adjacent

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terraces. Before the T6 terrace, the river’s incision rate was usually less than 0.60 mm a-1. During

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the periods between terraces Th3 and Th2, terraces Th2 and Th1, and terraces T7 and T6, the incision rates were only ~0.2-0.4 mm a-1 (Table 3). The river incision rate began to increase since ~18 ka (Fig. 7a). During 18.76-18.06 ka and the Middle-Late Holocene, the incision rate of the Yushanguxi River was significantly higher than the average incision rate (Table 3). The height difference between the T3 and T1 terraces was approximately 24.5 m, and the abandonment ages as determined by the 10Be exposure method were 4.16 ± 0.41 ka for terrace T3 and 2.10 ± 0.10 ka for terrace T1 (Table 1); therefore, we obtained an incision rate of ~11.89 mm a-1 during 4.16-2.10 ka. The river

ACCEPTED MANUSCRIPT incision rate between the T3 and T1 terraces has been approximately twenty times higher than the average incision rate of ~0.60 mm a-1 since the Late Quaternary. The T1 terrace is approximately 3.5 m above the modern riverbed, the abandonment age of which as determined by 10Be exposure was 2.10 ± 0.10 ka, thus yielding an incision rate of ~1.67 mm a-1 since ~2.1 ka.

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5.2. Mechanisms that drive of the Yushanguxi River’s incision

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River incision is influenced by many factors. In addition to tectonic uplift and climatic

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fluctuations, lithology, vegetation coverage and steep degree factors can control river incision (Schumm, 1965; Bull, 1990; Mol et al., 2000; Hancock and Anderson, 2002). The southern Tian

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Shan area has a generally arid climate, and the surface has rare vegetation and is almost in a bare

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state. The terraces (except for T1) of the Yushanguxi River are base terraces, with a lower Pleistocene conglomerate bedrock. The transverse sections of the terraces showed that the terraces’

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fronts are all steep, and the longitudinal profiles (Fig. 6b) revealed that the steepness of the terraces

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before deformation was basically consistent and had no obvious differences. Therefore, we posit that the lithology, vegetation coverage and steepness have not been responsible for the river’s rapid

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incision since the Holocene.

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The longitudinal terrace profiles indicated that the terraces have experienced obvious deformations. The different displacements on the terraces (Fig. 6b) confirmed that the Maidan fault has been continuously active since at least after Th3 was abandonment. Based on the fault displacements and abandonment ages of the terraces, we obtained a fault vertical slip rate of 0.77 ± 0.17 mm a-1 during the Late Quaternary, which was obviously larger than the river incision rate of 0.23-0.60 mm a-1 before T6 was abandoned. Since ~18 ka, the incision rate of the Yushanguxi River began to increase and reached ~12 mm a-1 during 4.16-2.10 ka. However, our results indicated that

ACCEPTED MANUSCRIPT the fault slip rate has been stable on a long-term scale (Fig. 7b). The heights of the T3 and T4 terraces above the riverbed are ~28 m and ~35 m, respectively, and are obviously higher than the scarp heights of ~2.8 m and ~7.3 m on the corresponding terrace surfaces. In the two walls of the fault, the river was observed to have experienced intense incision (Fig. 6b), and the deep

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downcutting of the valley could also be observed ~4 km south (footwall) of the Maidan fault. Our

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field investigations showed that the lower Pleistocene conglomerate beneath the river gravels also

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had a low-angle dip. Therefore, we suggest that the tectonic uplift from the Maidan fault was not a main factor that affected river downcutting, especially the rapid incision during the Holocene.

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Tian Shan is located in the interior of the Eurasian continent, which is a typical inland arid

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region. The alluvial sediments in the piedmont are closely related to climate change (Zhang et al., 1995). According to the Xinjiang Institute of Geography, Chinese Academy of Sciences (1986),

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Tian Shan has mainly experienced four glacial periods since the Late Pleistocene, namely the high

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Wangfeng, lower Wangfeng, upper Wangfeng and new glaciation. Some microclimate fluctuations and changes could be further distinguished in Xinjiang and its adjacent areas based on this

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division (Fig. 8; Shi et al., 1994; Yao et al., 1996; Tang et al., 1998). Since the Holocene, the

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climate has frequently fluctuated. The Early Holocene, specifically 12-10.5 ka, marked the Dryas glacial period in the Tian Shan area; after ~10.5 ka, the temperature began to rise. Although the temperature slightly decreased during 9-8 ka, the overall warming trend was continuous. The warmest period during the Holocene occurred during 7-6 ka, at which time the temperature was 1.5°C higher than at present (Yao et al., 1996). Based on the compositions of carbonates and isotopes in Bosten Lake, Zhang et al. (2009) presented a cold climate change during 6.4-5.1 ka in the southern Tian Shan. During 5.1-4.4 ka, the climate warmed again (Mischke and Wünnemann,

ACCEPTED MANUSCRIPT 2006; Zhang et al., 2009, 2010). Because of this very short interval between the glacial and interglacial periods, insufficient erosion debris was available to accumulate in the catchment basin, and the increase in river flow and lack of sediment sources resulted in the rapid downcutting of the riverbed. The

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Be exposure results showed that the nuclide-inherited concentration of the T3

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terrace was only 3.35E+03 atoms g-1, which was much lower than that for T4 and the riverbed

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(Table 1). This low nuclide-inherited concentration indicates that the river’s carrying ability

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during the T3 period was very strong, and sediments were quickly transported through the river to the piedmont area.

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During the Holocene, the incision rate generally significantly increased in both the Tian Shan

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area (Molnar et al., 1994; Lu et al., 2010; Gong et al., 2014) and the Qilian Mountains area (Pan et al., 2003; Hetzel et al., 2006). For example, the incision rate of the Manasi River in the northern

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Tian Shan piedmont area was ~13 mm a-1 after ~4.8 ka (Gong et al., 2014). The only factors that

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could have been responsible for the rapid incision of the river were regionally climate fluctuation events and not local tectonic changes. Therefore, we suggest that the rapid incision of the

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Yushanguxi River during the Holocene was mainly caused by rapid climate fluctuations.

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When comparing the terrace abandonment age of the Yushanguxi with the climatic curve as revealed by the oxygen isotope values of the Guliya ice core since the last glacial period (Yao et al., 1996, 1999) and the marine oxygen isotope values since the Late Pleistocene (Fig. 8; Lisiecki and Raymo, 2005), river downcutting and terrace formation mainly occurred during glacial-interglacial transition stages, e.g. the Th1, T6 and T5 terraces. During the interglacial periods of MIS 9, 7, and 5, some terraces also formed during the microclimate fluctuations of glacial-interglacial transition stages, e.g. the Th3, Th2, T81 and T7 terraces (Fig. 8). This process is

ACCEPTED MANUSCRIPT consistent with the findings of previous studies in the Qilian Mountains area by Pan et al. (2003) and the northern Tian Shan area by Lu et al. (2010). Strong physical weathering during the glacial period produced large amounts of debris accumulations in the catchment basin. Loose material storage could not be transported to the piedmont by the river because of weak water flow. Under

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the warm and humid climate of interglacial periods, the melting of glaciers rapidly increased the

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water flow, and sediments were transported to the piedmont by the main river. We suggest that the

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causes were the reservation time of the sediments during glacial ages and transport time during glacial-interglacial transition stages (Bull et al., 1990; Molnar et al., 1994).

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6. Conclusions

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Nine main terraces were distinguished along the Yushanguxi River in the southern piedmont of Tian Shan (labelled as T1 to Th from youngest to oldest). T1 is an accumulation terrace, while 10

Be

D

the other terraces are all base terraces with lower Pleistocene conglomerate bedrock. The

PT E

exposure dating and 14C dating results showed that the river terraces were abandoned at ~2.1 ka for T1, ~4.1 ka for T2, ~4.2 ka for T3, ~8.2 ka for T4, ~18.1 ka for T5, ~18.8 ka for T6, ~102.1 ka

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for T7, ~100.6 ka for T81, ~113.9 ka for T82, ~144.6 ka for Th1, ~210.7 ka for Th2, and ~284.3 ka

AC

for Th3. The Maidan fault has been continuously active since the late Quaternary and obviously faulted these terraces. The average incision rate has been ~0.60 mm a-1 since Th3 was abandoned, which is slightly lower than the vertical fault-slip rate. The river incision rate began to increase after ~18 ka, especially during the Middle-Late Holocene, reaching ~12 mm a-1. This sudden increase in the river incision rate since the Holocene may have been caused by frequent climate fluctuations. Despite being in an intense tectonic uplift area, the formation of the Yushanguxi River’s terraces has been mainly driven by climatic factors.

ACCEPTED MANUSCRIPT Acknowledgements: This research was supported by the National Science Foundation of China (41672208, 41590861, and 41661134011) and the Fund from the State Key Laboratory of Earthquake Dynamics (LE1413). Digital Globe high-precision remote sensing imagery was obtained from Google Earth. We thank the State Key Laboratory of Earthquake Dynamics, CEA,

PT

Key Laboratory of Crustal Dynamics, CEA, and Laboratatoire National des Nucleides

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Cosmogeniques of France for their support with the sample dating. We also thank the reviewers

SC

and the editor for their valuable comments and suggestions on the manuscript. Figure and Table captions

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Fig. 1. (a) Topographic map that shows the location of Tian Shan. (b) Tectonic map that shows the

MA

geomorphic features and active faults in Tian Shan. A series of inland rivers (blue lines) that originated from glaciers flow into the Tarim and Junggar Basins and the Kazakh Platform. The

D

major active faults were adopted from Thompson et al. (2002) and Deng (2007).

PT E

Fig. 2. (a) Geological structure map of the Aheqi valley. Along the northern bank of the Tuoxkan River, a series of north-south tributaries flow into the main river. (b) Geological section map along

AC

the fault.

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the Yushanguxi River. The Cenozoic layer has been folded into a syncline in the hanging wall of

Fig. 3. Photo facing ~N that shows the geomorphic features in the northern bank of the Tuoxkan River. A series of alluvial geomorphic surfaces developed in the piedmont. Fig. 4. Terrace divisions and fault distribution near the Yushanguxi River. Nine main divisions and several subdivisions of river terraces were distinguished based on image interpretation and field investigations. Fig. 5. Terrace features and sample positions of the Yushanguxi River. (a, c) The terraces are

ACCEPTED MANUSCRIPT mostly flat and wide, without obvious erosion. (b, d) Concentrations of 10Be occur at depth for the T3 and T4 terrace surfaces along the Yushanguxi River. Depth profiles and inheritance (equivalent age) values are presented in the figure, and the data in the plots are presented in Table 1. (e) Approximately five-meter-thick gravel caps the T3 terrace. This terrace was faulted, with a vertical

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displacement of ~2.8 m. The Maidan fault, which dips ~40 degrees, can be observed within the T3

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section.

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Fig. 6. (a) Terrace-to-river cross sections that were surveyed by differential GPS; see Fig. 4 for the section locations of the two E-W purple lines. (b) Deformed terrace profiles that were surveyed by

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differential GPS. The different coloured lines represent different terraces, and the dotted line

MA

represents the inferred terrace surface. See Fig. 4 for the profile locations of the N-S purple lines. Fig. 7. (a) Relative heights of the different terraces plotted against their corresponding ages. (b)

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Vertical fault-slip rate (dashed area), which has been stable since the late Pleistocene.

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Fig. 8. Correlations between climate fluctuations and the terrace-abandonment ages of the Yushanguxi River. The climatic change curve was revealed from the oxygen isotope results of

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Yao et al. (1996, 1999) and Lisiecki and Raymo (2005).

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Table 1 Determination results of the 10Be exposure ages Table 2 Determination results of the 14C sample Table 3 Incision rates of the Yushanguxi River during different periods

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Terrace

Sample

Altitude

number

site

T0

40.9881°

Mass of pure

(m)

quartz (g)

10

10

Be

10

Be Conc.

4

3

cor Be Conc. -1

10 atoms

10 atoms g

3

10

-1

10 atoms g

Be exposure age (ka)

2012

25.918

101.585 ± 4.243

39.194 ± 1.637

1.94 ± 0.08

2013

30.536

130.175 ± 5.901

42.630 ± 1.933

3.436 ± 3.570

2.10 ± 0.10

2019

34.083

280.739 ± 8.949

82.369 ± 2.626

43.175 ± 4.263

4.06 ± 0.13

31.547

312.859 ± 9.909

99.172 ± 3.141

21.451

129.312 ± 4.310

60.282 ± 2.009

30.521

156.311 ± 5.116

51.214 ± 1.676

30.139

110.064 ± 4.525

36.518 ± 1.501

10.446

34.336 ± 3.273

32.871 ± 3.134

30.326

34.241 ± 2.755

11.291 ± 0.908

30.825

637.690 ± 20.979

206.875 ± 6.806

10.867

182.861 ± 6.128

168.271 ± 5.639

18.082

210.074 ± 7.121

116.181 ± 3.938

11.028

90.850 ± 3.702

82.379 ± 3.357

25.984

122.126 ± 4.184

78.5038° T1

40.9935° 78.4998°

T2

40.9844°

T4

40.9925°

2048

78.4982°

14.024 T5

40.9969°

2061

30.541

2081

31.694

T7

41.0046°

T8 T8

2

41.0057°

71.436 ± 2.696 337.766 ± 12.450

18.06 ± 0.52

397.070 ± 12.170

357.876 ± 13.807

18.76 ± 0.58

6697.130 ± 119.151

2188.670 ± 38.940

2149.480 ± 40.577

102.08 ± 1.82

2105

30.477

6592.870 ± 110.598

2163.250 ± 36.289

2124.050 ± 37.926

100.55 ± 1.69

Th

2

Th

3

41.0114°

2117

30.672

7578.29 ± 116.559

2470.77 ± 38.0020

2431.57 ± 39.6391

113.90 ± 1.75

2147

30.414

9738.340 ± 148.287

3201.910 ± 48.756

3162.710 ± 50.393

144.60 ± 2.20

2188

30.217

14492.600 ± 205.314

4796.240 ± 67.948

4757.050 ± 69.585

210.65 ± 2.98

2224

30.526

20247.400 ± 273.620

6632.840 ± 89.635

6593.640 ± 91.272

284.30 ± 3.84

AC

1

CE

78.5151° Th

1258.480 ± 38.573

30.599

78.5133° 41.0078°

47.001 ± 1.610

2100

78.5126° 1

8.23 ± 2.06

376.961 ± 10.813

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78.5086°

170 ± 42.7

100.181 ± 3.781

D

41.0099°

4.16 ± 0.41

1151.260 ± 33.023

78.4943° T6

85.4 ± 8.43

RI

78.5130°

SC

2036

NU

40.9905°

MA

T3

PT

78.5106°

78.5195°

41.0149° 78.5193°

41.0198° 78.5201°

Table 1

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Sample

Sample site

Dating age

number 14

C-01

Calibrated ages (ka)

(ka BP) T5 terrace

±

Age interval

13.975 ± 0.035

cal BP 17070 (68.2%) cal BP 16870

16.956 ± 0.104

AC

CE

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MA

NU

SC

RI

PT

Table 2

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Height

Corresponding age

Incision rate

number

(m)

age (ka)

Difference (ka)

(mm a )

T1

3.5

2.10

3.5

2.10

1.67

T3

28

4.16

24.5

2.06

11.89

T4

35

8.23

7

4.07

1.72

T5

42

18.06

7

9.83

0.71

T6

48

18.76

6

0.70

8.57

adjacent terraces (m)

-1

102.08

19

83.32

0.23

T8

75

100.55

8

—

—

T8 T8

2

83

113.90

8

13.35

0.60

Th

1

121

144.60

38

30.70

1.24

Th Th

2

148

210.65

27

66.05

0.41

Th

3

171

284.30

23

73.65

0.31

AC

CE

PT E

D

MA

NU

Table 3

RI

67 1

SC

T7

Abandoned Height difference of two

PT

Terrace

ACCEPTED MANUSCRIPT Highlights

AC

CE

PT E

D

MA

NU

SC

RI

PT

(1) The terrace sequences of the Yushanguxi River were dated by 10Be exposure and 14C dating methods. (2) Although in an intense tectonic uplift area, the terraces of the Yushanguxi River were developed mainly by the climatic factor. (3) The rapid downcutting in the northwestern China area since the Holocene is mainly driven by the frequent climate fluctuations.

Figure 1

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Figure 6