Late Quaternary Climatic and Tectonic Mechanisms Driving River Terrace

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Late Quaternary climatic and tectonic mechanisms driving river terrace development in an area of mountain uplift: A case study in the Langshan area, Inner Mongolia, northern China

Liyun Jia, Xujiao Zhang, Zexin He, Xiangli He, Fadong Wu, Yiqun Zhou, Lianzhen Fu, Junxiang Zhao

PII: S0169-555X(15)00028-8 DOI: doi: 10.1016/j.geomorph.2014.12.043 Reference: GEOMOR 5060

To appear in: Geomorphology

Received date: 22 July 2014 Revised date: 22 December 2014 Accepted date: 24 December 2014

Please cite this article as: Jia, Liyun, Zhang, Xujiao, He, Zexin, He, Xiangli, Wu, Fadong, Zhou, Yiqun, Fu, Lianzhen, Zhao, Junxiang, Late Quaternary climatic and tectonic mechanisms driving river terrace development in an area of mountain uplift: A case study in the Langshan area, Inner Mongolia, northern China, Geomorphology (2015), doi: 10.1016/j.geomorph.2014.12.043

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Late Quaternary climatic and tectonic mechanisms driving river terrace development in an area of mountain uplift: a case study in the Langshan area, Inner Mongolia, northern China

Liyun Jiaa, Xujiao Zhang, a,*, Zexin Hea, Xiangli Hea, Fadong Wua, Yiqun Zhoua,

Lianzhen Fua, Junxiang Zhaob a School of Earth Sciences and Resources, China University of Geosciences,

Beijing 100083, China b The Institute of Crustal Dynamics, China Earthquake Administration, Beijing

100085, China

*Corresponding author. Tel:+86 10 82322082; E-mail: zhangxj @ cugb.edu.cn.

Abstract

The Langshan Range is located in the western Yin Mountain orogenic belts and the western Hetao fault-depression zone in Inner Mongolia, northern

China. This area is on the northwestern margin of the East Asian monsoon region. The ACCEPTEDﬂuvial terraces in the transverse MANUSCRIPT drainage of the Langshan Range represent a primary geomorphic response to local tectonic uplift and climatic changes. The terrace evolution was reconstructed using a combination of optically stimulated luminescence (OSL) dating and terrace tread measurements. The terraces, designated T4 through T1, were abandoned at about 58.00, 46.25, 32.19, and 15.79 ka BP, respectively. Their aggradation occurred primarily during cold periods of the last glacial stage, and incision

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occurred primarily during shifts from cold to warm climate stages. Geomorphic analysis showed the terrace heights were controlled by the tectonic uplift in the area. Differences in river incision rates and terrace geomorphic features indicate that the uplift of the Langshan Range included a component of tilting north to south during the period of 58.00 - 41.28 ka BP, whereas the uplift of the Langshan area tended to be equal on a regional scale after 32.19 ka BP.

Key words: the Langshan Range; ﬂuvial terrace; tectonic uplift; climate cycle; dating

1. Introduction

Fluvial terraces preserve excellent records of geologic processes in the form of geomorphologic features and fluvial deposition as a result of intrinsic and external actions (Schumm, 1977). Intrinsic changes tend to occur on relatively short time scales (10-1000 a) and produce small landforms (on a scale of 10-1000 m) resulting from adjustments along individual river reaches.

(Vandenberghe, 2002; Maddy et al., 2001a, 2008). The primary factors affecting riverACCEPTED terrace evolution on MANUSCRIPT greater spatiotemporal scales consist of external processes, i.e., tectonic activity, climatic variations and base level changes.(e.g., Bridgland, 2000; Vandenberghe and Maddy, 2001;

Vandenberghe, 2002, 2003). Climatic changes result in alternating incision and deposition caused by changes in the ratio of stream flow to sediment flux.

Such processes may explain terrace development in areas of minor tectonic activity but cannot explain the development of multistage terraces (Bull, 1979,

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1990; Xu and Zhou, 2007). Sustained tectonic uplift plays an important role in terrace development by providing the dynamics for river incision (incision is also controlled by sea level changes in low-elevation coastal zones).

General understanding of the forces driving fluvial terrace development in areas of mountain uplift has evolved. During the nineteenth and twentieth centuries, the pangeosyncline theory was the basis for models of crustal uplift and its control over fluvial terrace development (e.g., Chambers, 1848; Home,

1875; Whitaker, 1875). Presently, most geologists agree that fluvial terrace development in areas of mountain uplift is driven by the combined effects of tectonic uplift and climatic change (e.g., Maddy et al., 2000; Bridgland et al.,

2004; Bridgland and Westaway, 2008; Westaway et al., 2009; Wang et al.,

2010; Herfried et al., 2012; Hu et al., 2012; Viveen et al., 2013; Ren et al., 2014) or by downcutting because of tectonic activity and aggradation for climatic cyclicity (Starkel, 2003; Westaway, 2009). Others have concluded that the primary factor controlling terrace development is tectonic uplift and that climate change can ACCEPTEDbe ignored in areas of rapidMANUSCRIPT uplift (e.g., Cheng et al., 2002; Sun,

2005, Zhang et al., 2014a). In fact, the climate and the geologic setting both count for a great deal as leading factors driving terrace formation.

The Langshan area is located in the western Yin Mountain orogenic belts and in the western Hetao fault-depression zone in Inner Mongolia, northern

China. Earthquakes occur frequently in this area as a result of intense tectonic activity that began in the late Cenozoic (Wang et al., 1984；Sun et al.,1990;

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Deng et al., 2002；Cheng et al., 2006; Li, 2006). In addition, this area is located on the northwestern margin of the East Asian monsoon region. Large-scale normal faulting affecting the piedmont zone(Research group of Active fault system around Ordos Massif, 1988；Chen, 2002; Ran et al., 2003) has created the fault-block depression of the Hetao basin and has led to uplift of the

Langshan Range and development of a transverse drainage network associated with further uplift of the Langshan Range. A system of river terrace sequences developed along these transverse drainages, and the terrace surface geomorphology and the underlying fluvial sediments reflect not only the tectonic uplift but also are very sensitive to climatic changes and thus provide good records for the study of this area.

In this study, four to five river terraces along one transverse drainage of the Langshan Range were identified during field investigations of 25 representative river valleys. These terraces were designated T5 through T1.

Their evolution was reconstructed using a combination of optically stimulated luminescenceACCEPTED (OSL) dating and mapping MANUSCRIPT of the terrace treads. The climatic and tectonic mechanisms driving the river terrace development and the relationships between the terraces and a palaeolake were studied. In addition, a model of the spatial relationships generated during uplift was developed based on an analysis of the rates of river incision and the geomorphic features of the terraces. Below, we summarise the uplift of the Inner Mongolian plateau and the evolution of the Hetao plain and Yellow River during the late

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Quaternary.

2. Geologic and geomorphic setting

The Langshan Range is located in the western Yin Mountains, northwest of the Hetao basin, through which the Yellow River flows. The Langshan

Range is elongated in the NE-SW direction, is 370 km wide at its base, and rises 1500-2200 m above sea level (Fig. 1A). It is bordered on the south by the

Hetao basin, which lies at an elevation of 500-1200 m lower than the mountain crest (Fig. 1C); this elevation difference is marked by several fault scarps (Fig.

1B). The mountain’s northern flank slopes gently down to the Inner Mongolian plateau, and to the west lie the Boketai and Yamalek deserts (Fig. 1A).

Fig. 1(JPG)

The basement rocks in this area primarily consist of an Archaean group and the Proterozoic Langshan (Cha'ertaishan) group. Jurassic breccia and

Cretaceous brick-red gravel strata overlie these older rocks along an angular unconformity. Palaeogene, Neogene, and Quaternary deposits are distributed inACCEPTED the valleys and piedmont MANUSCRIPT of the Langshan Range. A Quaternary upper Pleistocene series (Qp3)is exposed primarily along the Langshan piedmont and consists of diluvial, alluvial, and lacustrine sediments. Due to the

Yanshan movement during the Mesozoic, the crust in the Langshan area thickened and underwent lateral shearing, and a NE-trending orogenic belt formed owing to northsouth extension and thrust nappes. Meanwhile, a set of small NE-trending faults developed. During the Eocene (E2), for deformation of

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the Himalaya, large-scale normal faults developed, the Hetao fault-block basin began to form, and the uplift of the Langshan Range block began. The

Quaternary was a period of extensive vertical offset in the area when relative movement between the uplifted Langshan Range block and the down thrown piedmont basin block was measured at a maximum of 1.00 mm/a (Research group of Active fault system around Ordos Massif, 1988), and the transverse drainage network developed on the Langshan Range (Fig. 1C).

3. Methods

3.1. Measurement

The river terrace elevations were measured using a high-resolution global positioning system (differential GPS), and the thicknesses of the alluvial deposits were measured by tape. Not all of the alluvial deposit thicknesses could be measured because of human modification of the deposits.

3.2. Field investigation

3.2.1. The valley characteristics in the transverse drainage of the Langshan area ACCEPTED MANUSCRIPT

Most valleys in the transverse drainage of the Langshan Range trend

NW-SE, nearly perpendicular to the main boundary normal faults. The valleys developed on the Langshan Range along the topography and extend onto the

Hetao plain (Fig. 2). Typically, these valleys, in succession from the headwater areas to the lower reaches, consist of gullies, gorges, V-shaped valleys, and wide valleys. Our study of the river terraces primarily focused on the wide

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valleys, which are primarily located in the downstream areas (Figs. 1C, D; 2).

The Langshan Range may be divided into three sections (designated the southern, middle, and northern sections) based on their various geomorphic characteristics (Fig. 2). Most of the valleys south of Hariganna (southern section) are mature and wide. The terrain between Hariganna and

Dabatuyinsumu (middle section) is very steep, and the valleys in this section are primarily deep gorges. The valleys north of Dabatuyinsumu are mostly

V-shaped. The valley walls are steep and are underlain by exposed bedrock, but several valleys at lower elevations are relatively wide.

Fig. 2（JPG）

3.2.2. Characteristics of the river terraces in the Langshan area

The well-developed river terraces are primarily located in the downstream portions of the valleys in the southern and northern sections. In this study, we selected 25 representative valleys to investigate, as shown in Fig. 2.

For convenience, the 25 river valleys were assigned numbers in succession from south toACCEPTED north; these number designations MANUSCRIPT are used to refer to the valleys in the discussions below.

Table 1

Elevations of the terrace treads above riverbed levels

No. Vally name Geology Altitude above river（m） No. Vally name Geology Altitude above river（m）

point No. T1 T2 T3 T4 T5 point No. T1 T2 T3 T4 T5

1 Chaasenggaole D009-7 5 11 20 34 14 Baishitougou D060 5 10 36 - -

☆D009-7 0 6 7 5 D061 5 - 40 57

2 Budunmaodegaole D027-1 -- 15 26 -- 15 Noname gou4 D062 7 18 - 43

D033-1 9 21 27 33 16 Dabagou D065 8 14 31 56

☆D033-1 9 10 6 6 ☆D065 8 14 31 7

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D036-1 8 16 24 D066 6 11 38 45 66

D041-1 -- 18 27 35 ☆D066 6 4 13 10

D044-1 8 18 32 D067 6 13 41 59

3 Wenggeleqige D016-1 -- 12 -- 32 ☆D067 6 3 2 8

4 Wusitaigaole D049-1 5 -- 23 D069 8 - 35 -

D053-1 -- 17 -- 36 D070 - - 24 44

D060-1 5 15 21 27 35? 17 Sanguikouzi D076 4 10 25 42 51

☆D060-1 5 10 3 6 ☆D076 1 2 15 7 10

5 Geeraobaogaole D075-1 4 19 D077 6 16 24 45 103?

D081-1 -- 14 ☆D077 7 8 7 5

D083-1 18 D078 5 14 25 38

D084-1 13 ☆D078 4 2 2

6 Sumutugaole D93-1 - 14 18 Nonamegou5 D079 7 17 32- -

D94-1 16 D080 5 12 25 46

D95-1 9 18 -- 31 19 Dalikegou D082 9 - 31 46 66

D97-1 4 -- 24 -- -- D083 6 - 30 50 66

7 Harigannagaole D102-1 3 15 D084 9 17 33 60 75

D103-1 4 15 ☆D084 3 2 1.5 1

D105-1 6 14 20 Xiwugaigou D085 7 14 30 44

D106-1 5 15 19 ☆D085 1.5 3 7

D107-1 4 10 23 35 D086 11 17 21 35 44

D107-2 5 0 3 12 21 Shanda temple D087 3 8 23 54

8 Nonamegou1 D040 3 13 36 51 22 Nonamegou6 D089 7 15 30 38 57

☆D040 3 4 2 0 ☆D089 1 8 15 4

9 Nonamegou2 D042 4 10 28 - 23 Shudinggaolan D090 5 27 44 70

10 Elesitaigou D045 3 12 24 43 D091 5 9 28 48

☆D045 3 3 5 3 ☆D091 2 8 0 4

D046 8 16 35 55 D092 7 17 25 41

11 Yangguikouzi D053 7 - 34 56 24 Bulegesugou D094 10 22 33

D054 11 - 45 66 ☆D094

D055 7 - 27 - D095 7 10 21 46

12 Hongshangou D057 5 16 23 52 D096 5 16 30 43

☆D057 5 16 23 3 25 Dongwugaikou D097 12 18 20 42 57

13 Noname gou3 D059 5 13 25 43 D098 3 11 26 38 51

☆D059 13 25 43 18 ☆D098 3 Note: the “☆” represent ACCEPTEDalluvium thickness on terrace. MANUSCRIPT In the southern valleys, four terraces with average elevations of 32.88, 24,

15.43, and 5.64 m above the riverbed were identified. These were designated terraces T4, T3, T2, and T1 in order of descending elevation (Fig. 3). Their underlying alluvial deposits measure 5.67, 5.33, 7.25 and 3.5 m thick on average, respectively (Table 1). Nearly all of the T1 terraces are either fill or strath terrace, and their underlying deposits consist primarily of fluvial sand

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and gravel. Most of the T1 deposits have been heavily altered by human activities (quarrying and road construction) in the downstream valleys. The T2 terraces are primarily base terraces except for several accumulation terraces, are distributed widely throughout the study area and are laterally extensive in most of the valleys (Figs. 3, 4). The T2 deposits consist primarily of fluvial sand and gravel. The gravel clast sizes vary from one valley to the next but typically range from 0.5 to 10 cm; most of the clasts are moderately weathered. The T3 and T4 terraces among the various valleys may be accumulation, base, or erosion terraces; but base and erosion terraces predominate. It is worth noting that most of the T3 terraces are concentrated in the northern valleys and are rare in the southern valleys, where they were identified only in valleys 2 and 4. These southernmost T3 terraces are small and exhibit only thin alluvial deposits, The T4 deposits are thick and laterally extensive in the southern section (Fig. 4A).

Fig. 3(JPG) The majorityACCEPTED of the northern valleys MANUSCRIPT contain sequences of four terraces, although a few contain a fifth terrace (Fig. 3), which are primarily base and erosion terraces. The average elevations of the T5, T4, T3, T2, and T1 terrace surfaces above the riverbed level are 63.3, 46.74, 29.09, 13.46, and 6.31 m, respectively; and the T4 through T1 average alluvial deposit thicknesses are

5.33, 9.03, 5.77 and 3.96, respectively (Table 1). All of these terrace elevations above riverbed level are higher than those of their counterparts to the south, except for terrace T2. The degrees of terrace preservation and the types of 9

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alluvial deposits of terraces T4 through T1 are similar to those of the southern valleys except for terrace T3. The T3 terraces in the northern valleys are well developed and are underlain by thick, laterally extensive alluvial deposits.

Lacustrine sediments were found underlying terrace T4.

Fig. 4(JPG)

3.3. OSL dating

Based on the developmental characteristics of the terraces, such as their depositional contacts, particle cementation, degree of weathering of the alluvial gravels, and their topography, we infer that the T4 through T1 terraces may have formed primarily during the late Pleistocene. We therefore selected the optically stimulated luminescence (OSL) method for dating samples of the alluvial terrace deposits.

3.3.1 Sampling

To increase the comparability of the age estimates of the terrace sediments, all of the sampling sites were located in the downstream ends of the valleys. ACCEPTED These locations provided MANUSCRIPT well-developed alluvial deposits (Figs.

1A, B) consisting of undisturbed silt or fine sand (Fig. 5). We typically collected a sample from the top and the bottom of the deposit in the event that site conditions were good. Two batches of samples for dating were collected in

2011 and 2013, as dictated by the study scheduling. The sampling equipment and procedures were as follows:

 Steel tubes 6 cm in diameter and 25 cm long were prepared. Each was

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provided with a black plastic bag to seal one end of the tube.

 The surficial, weathered 20 to 30 cm of alluvium was removed.

 The steel tube was driven into the fresh terrace deposit.

 The sediment-filled steel tube was retrieved and its ends immediately

covered with black plastic. The tube nozzle was sealed with aluminum foil

and wrapped with adhesive tape. Steps were taken to ensure avoiding light

penetration and water loss.

Fig.5(JPG)

We collected four representative samples from the T4 terraces. Samples

LS1-D016-1 and LS1-D016-2 were obtained from the bottom and top of the T4 deposits in valley 3, respectively (Fig. 5C). These intervals were selected as representing the beginning of aggradation and beginning of incision, respectively, of the T4 terrace. Sample LS2-D059-2 was obtained from the lowermost lacustrine sediments underlying the T4 terrace in valley 13 (Fig. 5G), and sample LS-D057-1 was from the bottom of the T4 deposits in valley 12

(Fig. 5F); theACCEPTEDse intervals were also selectedMANUSCRIPT as representing the approximate start of aggradation of terrace T4.

The OSL dating of terrace T3 was primarily performed on samples from the northern valleys as the result of the weak development of this terrace in the southern valleys. Sample LS-D068-1 was obtained from the uppermost T3 deposits in valley 16 (Fig. 5H) and represents the beginning of incision of this terrace. Sample LS-D078 was obtained from the lowermost T3 deposits in

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valley 17 (Fig. 5I), which represent the approximate start of aggradation of the

T3 terrace.Four samples were obtained from the T2 terrace. Sample

LS-D093-9 was obtained from a sand lens at the bottom of the T2 sediments in valley 6 (Fig. 5D) (southern section) and represents the approximate start of aggradation of the T2 terrace. Sample LS1-D033 was obtained from the top of the T2 terrace in valley 2, and sample LS1-D009 was obtained from a sand lens in the upper sediments of the T2 terrace in valley 1 (Figs. 5A, B); these samples represent the approximate beginning of incision of the T2 terrace.Sample LS-D084-1 was obtained from the bottom of the T1 deposits in the downstream of valley 19 in the Langshan Range southern (Fig. 5J), and sample LS-D045-1 was obtained from the upper T1 deposits in valley 10 (Fig.

5E). These samples may represent the aggradation and beginning of incision of the T1 terrace. Unfortunately, we could not find appropriate sites for sampling the T5 terrace due to the topography and the erosional nature of the terrace.

3.3.2. AnalysisACCEPTED MANUSCRIPT

In 2011 and 2013, the two batches of samples were transported to the

Crustal Dynamics Key Laboratory of the Institute of Crustal Dynamics, China

Earthquake Administration, for age dating. The samples received pre-treatment in the laboratory, and their OSL signals were measured using automatic Risø DA-20–TL/OSL instruments (Denmark). The protocols used to measure the quartz equivalent doses (De) of the two batches of samples

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differed. The equivalent doses (De) of the first batch of samples, which were analysed in 2011, were determined using the SAR dating protocol (Murray and

Wintle, 2000, 2003); and the equivalent doses (De) of the second batch of samples, which were analysed in 2013, were determined using the SMAR dating protocol (Wang et al., 2005) owning to the particle sizes of the silt. The environmental dose rate was measured using a variety of techniques. The

Daybreak583 thick-source α-counting technique (Aitiken, 1985) was used to measure contributions from uranium (U) and thorium (Th) decay chains, and a flame photometer was used to measure the potassium hydroxide (KOH) content. The water content was calculated to be 5-10％ of the dry sample weight. The cosmic ray contribution to the dose rate was calculated at the same time (Prescott and Hutton, 1994). The uranium (U), thorium (Th) and potassium (K) levels of the second batch of samples were measured using an element plasma mass spectrum analyser. The environmental dose rate was then calculated based on the absorbed dose rate of the quartz and the transformationalACCEPTED relationships among MANUSCRIPT the uranium, thorium， and potassium concentrations (Aitiken, 1985, 1998). Finally, the sediment ages were calculated based on the equivalent dose and environmental dose rates of the samples.

4. Results

Table 1 presents the elevations of the terrace treads above the riverbed levels and the thicknesses of the alluvial deposits underlying the terraces.

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Table 2 presents the analytical data and results of the OSL dating. Note that three samples, i.e., LS1-D016-1, LS1-D016-2, and LS-D059-2 may be undervalued for the saturation tendency of quartz singles (Table 2). Otherwise, the age of sample LS-D091-1, which was obtained from the T2 deposits in valley 23, is 51.61 ± 5.92 ka BP, which is close to the ages of the T3 deposits in other valleys, as a result of influence from the residual luminescence signal of the sample.

Table 2

Results of the OSL analysis

The Sample code Terrace α particle K2O Dose rate Equivalent dose (Gy) Age（Ka） first sequence countingrate (%) (Gy/Ka) batch （/Ksec）

LS1-D009 T2 4.36 ± 0.14 2.80 2.95 ± 0.29 99.79 ± 12.84 33.87 ± 5.13 LS1-D016-2 T4 14.86 ± 0.26 2.50 4.20 ± 0.42 243.83 ± 20.79 58.00 ± 6.78 LS1-D016-1 T4 9.46 ± 0.20 1.91 2.99 ± 0.29 230.41 ± 34.80 76.97 ± 13.15 LS1-D033 T2 6.12 ± 0.16 2.74 3.15 ± 0.31 101.40 ± 19.47 32.19 ± 6.70 LS1-D093 T2 10.71 ± 0.21 2.94 3.95 ± 0.39 163.06 ± 15.65 41.28 ± 5.16

The Sample code U（μg/g） Th（μg/g） K(％) Dose rate Equivalent dose (Gy) Age（Ka） second (Gy/Ka) batch LS-D045-1 T1 0.868 2.93 2.37 3.08 48.60 ± 3.99 15.79 ± 2.04 LS-D057-1 T4 3.88 15.9 2.02 5.06 321.77 ± 22.91 63.61 ± 7.81 LS-D059-2 T4 5.03 14.9 2.96 6.29 438.05 ± 37.51 69.68 ± 9.17 LS-D068-1 T3 5.31 9.62 2.32 5.23 241.87 ± 13.68 46.25 ± 5.31 LS-D078-1 T3 4.42 13.3 2.93 5.89 301.73 ± 11.62 51.25 ± 5.49 LS-D084-1 T1 1.66 6.61 2.38 3.73 86.51 ± 0.48 23.22 ± 2.33 ACCEPTEDLS-D091-1 T2 3.26 13.8 MANUSCRIPT2.52 5.13 264.75 ± 14.85 51.61 ± 5.92 As shown in Table 3, the age dates of the T4 deposits range between 76.97 ± 13.15 and 58.00 ±6.78 ka BP, and the samples from the T3 and T2 deposits yield age estimate of 51.25 ± 5.49 to 46.25 ± 5.31 ka B P and 41.28 ±

5.16 to 32.19 ± 6.70 ka B P, respectively. There are two piedmont terraces in the foothills of the Langshan Range; the T2 piedmont terrace near

Chahanbuligesu yielded an age of 58.33 ± 6.71 ka BP. (Zhang et al., 2014b), which is approximately our terrace T4 age. The T1 piedmont terrace near

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Sumutu valley (valley 6) yielded an age estimateof 47.48 ± 6.89 ka BP (He et al., 2014), which is approximately our terrace T3 age estimate. These findings indicate that the river and piedmont terraces may have developed in a uniform tectonic and climatic setting.

Table 3

Terraces sequence in the transverse drainage of the Langshan area

Terrace Height above the river level Age (ka BP) Sampling point Geological time level (m) T4 32 76.97 ± 13.15 No.3 downstream Early-late Late （40°37'20.73"N；106°22'0.78"E） Pleistocene 52 58.00 ± 6.78 No.3 downstream （40°37'20.73"N；106°22'0.78"E） -- 58.33 ± 6.71 Piedmont terrace T2（Zhang et al., 2014b）（41°5'55.64"N；107°3'44.41"E） 45 65.33 ± 6.71 debris flow gullyT4（Zhang et al., 2014b）（49°45.05"N；106°34'16.84"E） 48 69.68 ± 9.17 No.9 downstream （41°0'46.08"N；106°53'12.41"E） 52 63.61 ± 7.81 No.12 downstream （40°59'27.93"N；106°51'36.89"E） T3 21 51.25 ± 5.49 No.17 downstream（T3 bottom） late Late Pleistocene (41°07'47.84"N；107°06'00.30"E) 41 46.25 ± 5.31 No.16 downstream （41°4'2.56"N；107°0'29.90"E） -- 47.48 ± 6.89 Wulanaobao（He et al., 2014）（40°45'32"N；106°29'36.26"E） T2 12 33.87 ± 5.13 No.1 downstream late Late Pleistocene （40°35'35.2"N；107°20'12.3"E） 16 41.28 ± 5.16 No.6 downstream （40°45'56.79"N；106°28'2.05"E） 21 32.19 ± 6.70 No.2 downstream （40°37'19.62"N；106°20'2.72"E） T1 2 23.22 ± 2.33 No.19 downstream Late-the last stage of late （41°10'27.67"N；107°8'57.73"E） Pleistocene 9 15.79 ± 2.04 No.10 downstream ACCEPTED MANUSCRIPT（40°55'18.91"N；106°38'11.52"E） Theoretically, the terraces attained their greatest lateral and vertical extents as the uppermost sediments were deposited, i.e., at about the time the terraces were abandoned and began experiencing incision. Consequently, we may use the ages of the uppermost terrace sediments rather than the lowermost terrace sediments to characterise the approximate ages of the terraces. Therefore, the ages of maximum development of terraces T4 through

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T1, i.e., the ages of their surfaces at average elevations of 44, 28, 14, and 6 m above riverbed level, respectively, are approximately 58.00, 46.25, 32.19, and

15.79 ka BP, respectively.

5. Discussion

5.1. Late Quaternary fluvial evolution and incision history in the Langshan area, as determined from the river terraces

We can reconstruct the aggradation and incision history of the fluvial terraces in the Langshan area by means of the elevation data and the corresponding OSL ages of the terrace treads. Table 4 and Fig. 6 show that the terraces resulted from four intermittent aggradation and cutting stages since

76.97 ± 13.15 ka.BP The northern and southern sections of the Langshan area differ substantially in terms of the geomorphology and average terrace incision and sedimentation rates.

Table 4

Terrace formation history in the Langshan area

Terraces Time（ka.B.P.） Terrace history Marine Remark Heinrich sequence ACCEPTED MANUSCRIPToxygen events Process Average sediment isotope thickness/ stage downcutting depth（m） North South

T4 76.97 ± 13.15 ~ 58.00 ± 6.78 aggradation 5.33 5.67 MIS4 River bed aggradation H6

58.00 ± 6.78 ~ 51.25 ± 5.49 incision 17.83 8.69 Rock and sediments incision T3 51.25 ± 5.49 ~ 46.25 ± 5.31 aggradation 9.03 5.33 River bed and flood plain H5 sediment 46.25 ± 5.31 ~ 41.28 ± 5.16 incision 15.76 8.75 MIS3 Rock and sediments incision T2 41.28 ± 5.16 ~ 32.19 ± 6.70 aggradation 5.77 7.25 River bed aggradation H4

32.19 ± 6.70 ~ 23.22 ± 2.33 incision 7.09 9.79 Rock and sediments incision T1 23.22 ± 2.33 ~ 15.79 ± 2.04 aggradation 3.96 3.5 MIS2 flood plain sediment H1

15.79 ± 2.04 ~ now incision 6.35 5.64 sediments incision

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The valleys underwent a stage of riverbed aggradation during the period of approximately 76.97 - 58.00 ka BP. The average thicknesses of these deposits are 5.33 m in the northern section and 5.67 m in the southern section.

At about 58.00 ± 6.78 ka BP, the rivers began to incise their beds, and terrace

T4 was abandoned. The average terrace elevations above riverbed level were

17.83 and 8.69 m in the northern and southern sections, respectively. The terrace treads of T4 in the northern section are higher than those in the southern section due to their thinner deposits and higher elevations relative to the riverbed level. Around 51.25 ka BP, the river deposition rate surpassed the incision rate, and the sediments were generally deposited on the riverbeds and floodplains. The average depositional thicknesses of this stage were 9.03 and

5.33 m (northern and southern sections, respectively) until approximately

46.25 ka BP. A subsequent stage of incision (the second) occurred during the period of 46.25 - 41.28 ka BP, during which the river incised the earlier sediment andACCEPTED bedrock and created terraceMANUSCRIPT T3. The average heights of these terraces are 8.75 and 15.76 m (northern and southern sections, respectively).

The valleys then underwent another aggradation stage during the period of

41.28 - 32.19 ka BP, and a third incision stage during the period of around

32.19 - 23.22 ka BP. The average deposit thickness and decrease in height of the resulting T2 terraces were about 5.77 and 7.25 m in the northern section, respectively, and 7.09 and 9.79 m in the southern section, respectively. The

ACCEPTED MANUSCRIPT

period of 23.22 - 15.79 ka BP was a subsequent stage of valley aggradation, primarily flood plain deposition. After 15.79 ± 2.04 ka BP, there were several cycles of aggradation and incision; but the incision rate generally exceeded the depositional rate, and the average heights of the resulting T1 terraces were

6.35 and 5.64 m in the northern and southern sections, respectively. The actual evolution of these terraces may have been more complex than is summarised in this model. However, the model presented above may still be useful for understanding the evolution of the river terraces and their tectonic and climatic settings during the late Pleistocene.

Fig.6(JPG)

5.2 Mechanisms driving river terrace development in the Langshan Range during the late Quaternary

River terrace development is a result of accumulation and incision of river systems at various times, and the factors driving these processes primarily consist of changes in the internal dynamics of the fluvial system and controls external to theACCEPTED fluvial system; the latter MANUSCRIPT primarily consist of climatic, tectonic, and base level changes (e.g., Merritts et al., 1994; Vandenberghe, 1995；

Maddy et al., 2001b). In areas of mountain uplift, base and erosion terraces represent the majority; accumulation terraces are occasionally present in the wide, downstream sections of valleys (Starkel, 2003; Gao et al., 2005; Wang et al., 2009). These patterns are the products of fluvial geomorphologic evolution over long time and large spatial scales, and thus we may neglect changes in

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internal dynamics of fluvial systems (Maddy et al., 2001a; Vandenberghe,

2002) and focus on external controls to explain the development of the terraces. The terraces on the Langshan Range are typical of terraces in such areas of uplift.

5.2.1 Terrace formation and climatic changes

The Langshan area is located in western Inner Mongolia, northern China, along a margin of the East Asian monsoon region. The area is therefore sensitive to climatic changes, which may be reflected in the terrace evolution.

Fig.7(JPG)

The relationships between terrace evolution and climatic changes may be discerned by correlating the evolution with oxygen isotope curves and Heinrich events (Table 4； Fig. 7). First, based on a comparison between terrace evolution and marine isotope curves, the developmental stages of the T4, T3,

T2 and T1 terraces may be correlated with marine isotope stages MIS4, MIS3, and MIS2. The incision of the T3 terrace occurred during the shift from the

MIS4 stage ACCEPTEDto the MIS3 stage, which MANUSCRIPT suggests that the shift to a warm, wet climate enhanced the palaeohydrodynamic conditions and caused deep incision by the river system, thus affecting the T3 terrace evolution. Second, there is a clear correlation between the warm-cold climate cycles and incision-aggradation terrace cycles when the GISP oxygen isotope curves are superimposed on a plot of the terrace evolution (Fig. 7). Third, the four stages of terrace aggradation correlate, respectively, with the H6, H5, H4, and H1

ACCEPTED MANUSCRIPT

Heinrich events, which are associated with periods of colder climate.

The above findings indicate that the aggradation and incision of the river terraces may correlate with periods of cold and warm climates, respectively.

During cold stages, stream flows are reduced, and stream erosion is weakened; consequently, deposition is the primary process acting on a floodplain. During shifts from cold to warm climates, stream flows increase, thus enhancing the incision action of the streams and accelerating the terrace formation. In addition, the degree of terrace development is also affected by the climate. Sediment deposition plays an important role in terrace development, whereby the thicker the sediments, the greater the degree of terrace development. Figure 6 shows that the periods of T4 and T2 deposition spanned longer periods than those of T3 and T1 deposition, which may explain the better development of the T4 and T2 terraces in this area.

5.2.2 Terrace development and tectonic activity

Based on the above discussion, terrace development is affected by the climate, butACCEPTED we cannot explain all MANUSCRIPT terrace phenomena in terms of climatic changes. First, the terraces are elevated above the riverbed by more than 10 m, which cannot be explained by climatic driving forces alone. Second, the terrace alluvial thicknesses in the northern section are approximately equal to those of the southern section, which holds true for all four terrace developmental stages. However, the depths of incision differ between the two sections (Table 4; Fig. 6), which indicates that the terrace incision was not only

ACCEPTED MANUSCRIPT

driven by the climate. The resistance to erosion of the bedrock in the lower valleys of the northern and southern sections does not differ markedly; and if the terrace deposition and incision were only driven by climate in this area, these depths of incision would be equal. Third, as mentioned before, the river terraces and piedmont terrace may have developed in uniform tectonic and climatic settings, judging from the consistency of their OSL dating results.

Based on previous research (He et al., 2014), the piedmont terrace in the

Langshan area was controlled more by normal faulting than by stream incision.

Thus, the driving factor that should be considered in addition to climate is tectonic uplift.

As we mentioned earlier in section 2, tectonic uplift is prevalent across the

Langshan area (Research group of Active fault system around Ordos Massif,

1988; Chen, 2002; Ran et al., 2003) . This uplift may provide the continuous driving force for stream incision, resulting in the observed terrace heights above riverbed levels of more than 10 m. Furthermore, tectonic uplift is also the primary ACCEPTEDexplanation for the different MANUSCRIPT rates of incision between the northern and southern sections of the Langshan area.

5.2.3 River terrace development in the Langshan area and Palaeolake

Jilantai-Hetao

The Yellow River is the local erosional base level of the transverse drainage on the Langshan Range. The development of the Yellow River, however, was subject to several phases of river-lake cycles during the late

ACCEPTED MANUSCRIPT

Pleistocene (Li et al., 2005, 2007; Li, 2006; Chen et al., 2008; Fan et al., 2011).

As indicated by previous studies, Palaeolake Jilantai-Hetao was present in this area during the late Pleistocene (Chun, 2006; Chen, 2008；Yang, 2008) and then dried up as the subsequent dry climate and tectonic activity in this area during the late Pleistocene. Therefore, the river incision activity during the late

Pleistocene should be related to the levels of this palaeolake.

During our field investigation, we identified lacustrine sediments in the lowermost T4 terrace deposits in the downstream portions of certain valleys.

The silt beds in these lacustrine deposits yielded an OSL age of 69.68 ±9.17 ka.B.P. (Table 3). This finding indicates that the palaeolake was present along the foothills of the Langshan Range before the development of the T4 terrace.

Episodic uplift events and the dry climate during the late Pleistocene may have led to lowering of the palaeolake level, thus increasing the downcutting by rivers and accelerating the river terrace development. Thus, the primary driving mechanisms are a combination of tectonic uplift and climatic changes.

Therefore,ACCEPTED the four river terrace MANUSCRIPTs in the transverse drainage on the

Langshan Range were formed by the combined actions of climatic shifts and tectonic uplift. Tectonic uplift controlled the depth of river downcutting, and the climate controlled the aggradation of the terraces, which was also affected by stream incision.

5.3 Implications for climatic changes and tectonic uplift events in the Langshan area based on the river incision rate

ACCEPTED MANUSCRIPT

Climatic shifts and tectonic uplift controlled the terrace incision; thus, the rates of river incision may reflect the climate and tectonic uplift conditions in the area. There may be no simple relationship between fluvial incision and surface uplift (Whipple et al., 1999). Nevertheless, the rate of fluvial incision on a long time scale may be used to quantify (or at least approximate) the rate of tectonic uplift in tectonically active regions. This correlation has gained widespread acceptance during the previous decade, based on the assumption that the river gradients remain relatively constant (e.g., Maddy et al., 2001a,

2008; Pan et al., 2009; Westaway, 2009; Bridgland, 2000; Hu et al., 2012).

The rates of river terrace incision were calculated based on the OSL dating results and the terrace elevation data. The average river incision rate in the transverse drainage of the Langshan Range is close to 0.760 mm/a. Figure

6 shows the spatial and temporal disparities between the incision rates of valleys in the Langshan area. During the periods of about 58.00 - 51.25, 46.25

- 41.28, 32.19 - 23.22, and 15.79 ka BP to the present, the depths and rates of incision in theACCEPTED northern valleys were 32.28MANUSCRIPT m and 4.78 mm/a, 21.77 m and 4.38 mm/a, 13.46 m and 1.50 mm/a, and 6.31 m and 0.4 mm/a, respectively. The corresponding depths and rates in the southern valleys during those time periods were 16.45 m and 2.43 mm/a, 17.54 m and 3.53 mm/a, 15.42 m and

1.79 mm/a, and 5.64 m and 0.36 mm/a, respectively.

Note the temporal disparities between the incision rates: the rate of

2.43-4.78 mm/a during the period of 58.00 - 41.28 ka BP is greater than the

ACCEPTED MANUSCRIPT

rate of 0.36 - 1.79 mm/a after about 32.19 ka BP. The likely explanation for this disparity is the climate shift between stages MIS 4 and MIS 3 (Fig. 7), during which rising temperatures caused greater stream flows and accelerated river incision. This high river incision rate during this stage was also observed in the

Fenwei basin, Tibet, and other areas (e.g., Hu et al., 2012; Liu et al., 2013), which also indicates that the climatic change from stage MIS 4 to stage MIS 3 caused the high river incision rate.

Another point worth noting is the spatial disparities between the incision rates in the northern and southern sections. During the periods of 58.00 -

51.25 and 46.25 - 41.28 ka BP, the northern incision rates were greater than those in the southern section, whereas these northern and southern rates were approximately equal after 32.19 ka BP. These trends indicate that the

Langshan Range experienced tilting, whereby uplift in the northern section was greater than that in the southern section during the period of 58.00 - 41.28 ka

BP; after around 32.19 ka BP, uplift tended to be equal on a regional scale.

6. ConclusionACCEPTEDs MANUSCRIPT

 Four to five river terraces developed in the transverse drainage of the

Langshan Range. The surfaces of terraces T4 through T1 are at average

elevations of 44, 28, 14, and 6 m respectively, above the riverbed level,

and their deposits have been assigned age dates of 58.00，46.25, 32.19,

after 15.79 ka BP respectively, according to the optically stimulated

luminescence (OSL) dating.

ACCEPTED MANUSCRIPT

 Terraces T4 through T1 experienced four intermittent aggradation stages

during the periods of approximately 76.97 - 58.00, 51.61- 46.25, 41.28 -

32.19, and 23.22 - 15.79 ka BP respectively, and incision stages during the

periods of 58.00 - 51.61, 46.25 - 41.28, 32.19 - 23.22, and 15.79 ka BP to

the present, respectively.

 The four river terraces developed as a result of the combined driving

forces of climatic change and tectonic uplift. Tectonic uplift controlled the

depth of river downcutting, and climate controlled the aggradation of the

terraces, which was also affected by the incision by the river.

 The temporal and spatial disparities in the incision rates indicate that the

climate shift between stages MIS4 and MIS3 caused the high river incision

rate. The uplift of the Langshan Range included a component of tilting

whereby the northern section rose more rapidly than the southern section

during the period of 58.00 - 41.28 ka BP, whereas after about 32.19 ka BP,

the uplift tended to be equal on a regional scale.

AcknowledgACCEPTEDements MANUSCRIPT

We are grateful for the guidance of research fellow Zhonghai Wu regarding the writing and for the help in the field from Bin Guo of the Inner

Mongolia Bayannur Bureau of Geopark Administration. This paper has beneﬁted from valuable comments and suggestions by Professor David

Bridgland and anonymous reviewer, whose efforts are gratefully acknowledged.

We also thank Richard A. Marston, our editor, for his time and effort dedicated

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to improving our manuscript in format and English. This study was financially supported by the Public Welfare Profession Special Research Fund of the

China Ministry of Land and Resources (No. 201211077), the 1:50,000 pilot mapping project funded by CGS (China Geological Survey) (No.

12120114042101), and the geological heritage conservation project of

―Tectonic movement and environmental effects in Inner Mongolia Bayannur of

China during the Late Pleistocene‖.

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Figure captions:

Fig. 1.(A) General tectonic setting of the Hetao rift system and adjacent areas.

(B) Faults at base of Langshan Mountain (C) DEM map of the drainage in the

Langshan area (D) Location of sampling area in the drainage area. The location of the study area in (A) and (C) is denoted by the box.

Fig. 2. Valleys in the Langshan area. Valley designations are as follows: 1.

Chaasenggaole 2. Budunmaodegaole 3. Wenggeleqige 4. Wusitaigaole 5.

Geeraobaogaole 6. Sumutugaole 7. Harigannagaole 8. Nonamegou1 9.

Nonamegou2 10. Elesitaigou 11. Yangguikouzi 12. Hongshangou 13. Noname gou3 14. Baishitougou 15. Noname gou4 16. Dabagou 17. Sanguikouzi 18.

Nonamegou5 19. Dalikegou 20. Xiwugaigou 21. Near Shanda temple 22.

Nonamegou6 23. Shudinggaolan 24. Bulegesugou 25. Dongwugaikou

Fig. 3. RiverACCEPTED terraces in the Langshan MANUSCRIPT area

Fig. 4. Photographs of typical river terraces in the Langshan area. (A) River terraces in (valley 2) Budunmaodegaole, downstream. (B) River terraces in

(valley 12) Hongshangou, downstream. (C) River terraces in (valley 23)

Shudinggaolan, midstream. (D) Gravels on the T3 and T4 terraces of valley 13

(downstream segment) and the overlying gravels.

Fig. 5. Cross sections through the valleys in the Langshan area. (A) Valley 1

ACCEPTED MANUSCRIPT

Chaasenggaole (LS1-D009). (B) Valley 2 Budunmaodegaole (LS1-D033). (C)

Valley 3 Wenggeleqige (LS1-D016). (D) Valley 6 Sumutugou (LS1-D093).

(E) Valley 10 Elesitaigou (LS-2D045). (F) Valley 12 Hongshangou (LS-D057).

(G) Valley 13 Noname gou3 (LS-D059). (H) Valley 16 Dabagou (LS-D068). (I)

Valley 19 Dalikegou (LS-D084). (J) Valley 17 Sanguikouzi (LS-D078)

Fig. 6. Formation processes of the fluvial terraces in the Langshan area.

Fig. 7. The relationship between river terrace formation and global climatic change during the late Quaternary (oxygen isotope curves from Shackleton et al., 1983; Yao, 1999; and the present study).

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Figure 1A,B

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Figure 1C,D

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Figure 2 ACCEPTED MANUSCRIPT

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Figure 3

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Figure 4

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Figure 5A

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Figure 5B

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Figure 5C

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Figure 5D

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Figure 5E

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Figure 5F

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Figure 5G

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Figure 5H

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Figure 5I

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Figure 5J

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Figure 6

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Figure 7

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Highlights:

 We designated 4 - 5 ﬂuvial terraces in the transverse drainage of

Langshan Range.

 The fluvial evolution and incision history was reconstructed in the

Langshan area.

 We discussed the genetic mechanism of river terrace in Langshan area,

North China.

 We analysed the crustal uplift difference in Langshan area by river incision

rates.

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