Spatial patterns of Late Quaternary river incision along the northern Tian Shan foreland

Journal Pre-proof Spatial patterns of Late Quaternary river incision along the northern Tian Shan foreland

Honghua Lu, Dengyun Wu, Huiping Zhang, Yuanxu Ma, Xiangmin Zheng, Youli Li PII:

S0169-555X(20)30072-6

DOI:

https://doi.org/10.1016/j.geomorph.2020.107100

Reference:

GEOMOR 107100

To appear in:

Geomorphology

Received date:

5 September 2019

Revised date:

9 February 2020

Accepted date:

16 February 2020

Please cite this article as: H. Lu, D. Wu, H. Zhang, et al., Spatial patterns of Late Quaternary river incision along the northern Tian Shan foreland, Geomorphology(2020), https://doi.org/10.1016/j.geomorph.2020.107100

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2020 Published by Elsevier.

Journal Pre-proof 1 2 3 4 5 6 7 8 9 10 11

Spatial patterns of Late Quaternary river incision along the northern Tian Shan foreland Honghua Lu 1,2 *, Dengyun Wu 1, Huiping Zhang 3, Yuanxu Ma 4, Xiangmin Zheng 1, Youli Li 5 1

Key Laboratory of Geographic Information Science of Ministry of Education, School of Geographic Sciences, East China Normal University, Shanghai 200241, China

2

Institute of Eco-Chongming (IEC), East China Normal University, Shanghai 200062, China

3

Institute of Geology, China Earthquake Administration, Beijing 100029, China

4

Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, Beijing 100094, China

5

Key Laboratory of Earth Surface Processes of Ministry of Education, Peking University, Beijing 100871, China

12 *Corresponding author. E-mail address: [email protected] (H. Lu)

of

13 14

Abstract: River-incision rate is widely used to track changes in tectonics or climate over time

16

and space. However, the feasibility of utilizing the spatial variations of river incision to

17

reconstruct past tectonic and climatic processes remains unclear. Here, we focus on the spatial

18

patterns of river incision along the northern Chinese Tian Shan foreland. Three alluvial fans FP,

19

FeH, and FlH are determined as the alluvial fan context of river incision and terrace classification

20

providing the geomorphological framework across the foreland region. Four rivers (i.e. the Kuitun,

21

Jingou, Manas, and Urumqi Rivers from the western, central, and eastern part of the foreland) are

22

used to reconstruct the paleogeomorphology from the reference, which is the best-preserved

23

terrace of each river system with ages clustering in different parts of the Latest Pleistocene-Early

24

Holocene. The depth of incision constrained by the reference terrace of each river is obtained by

25

comparing the present-day topography and the reconstructed one. The resulting profile of channel

26

incision (depth and rate) of each analyzed river displays an overall decreasing-downstream trend

27

from the maximum where the river exits from the mountain range, to zero. Such a trend has been

28

attributed to the progressive lowering of the river gradient that was caused by the adjusted ratio of

29

sediment input versus water discharge induced by climate change. Superimposing onto the

30

decreasing-downstream trend, an obvious step can also easily be observed onto the profile of

31

channel incision of each river, downstream of which channel incision is significantly less. The step

32

occurs near the thrust fault controlling growth of the outermost anticline through which each river

33

cuts, thereby implying the key role of local rock uplift in forming the river-incision step. The

34

former observation implies that, at the same timescale, more river incision at the exit from the

Jo ur

na

lP

re

-p

ro

15

Journal Pre-proof 35

mountain range does not necessarily mean stronger climatic forcing of incision there. We thus

36

propose that, in a foreland setting, it should be the temporal pattern of river incision rather than its

37

spatial variation that is helpful for unraveling the change in the forcing factor of downcutting.

38

Key words: Fluvial terrace; River-incision rate; Spatial pattern; the Tian Shan

39

1. Introduction

41

River erosion is one of the main surface processes shaping the topography of an active orogenic

42

belt (e.g., Burbank and Pinter, 1999; Whipple, 2004; Burbank, 2005; Jolivet et al., 2014; Zhang et

43

al., 2014; Bufe et al., 2016). During erosion, river downcutting can form canyons and terrace

44

sequences; these fluvial features are commonly used to reconstruct the regional geomorphological

45

evolution and reveal its forcing mechanism (e.g., Yang et al., 2015; Bridgland et al., 2017; Stokes

46

et al., 2017; Lu et al., 2018a; Nie et al., 2018). River incision has causally been linked with

47

changes in tectonic and climatic factors, base level variations, rock strength as well as autogenic

48

adjustment of river systems (e.g., Ferrier et al., 2013; Bufe et al., 2017a, 2017b; Malatesta et al.,

49

2017, 2018; Stokes et al., 2017; Malatesta and Avouac, 2018). The rate of river incision is often

50

calculated using terrace data (i.e. the height of the terrace surface above the present-day riverbed

51

and the terrace formation age) and this in turn has been used to quantify tectonic uplift (e.g.,

52

Burbank et al., 1996; Leland et al., 1998; Yang and Li, 2011; Pan et al., 2013; Jochems and

53

Pederson, 2015; Bufe et al., 2017b; Demoulin et al., 2017) or to unravel the strength of climatic

54

forcing of downcutting and its variations (e.g., Hancock and Anderson, 2002; Ferrier et al., 2013;

55

Lu et al., 2018a; Nie et al., 2018). In time and space, however, river incision can be a highly

56

variable process that may be interrupted when the riverbed aggrades (Burbank and Anderson,

57

2012; Yang and Li, 2012; Finnegan et al., 2014). Correspondingly, incision rates derived from

58

terrace data possibly depend on the considered spatial and temporal scales. Thus, understanding

59

the spatiotemporal variations of river-incision rate is of primary importance for the reconstruction

60

of past tectonic and climatic processes.

Jo ur

na

lP

re

-p

ro

of

40

61 62

Some previous studies (e.g., Howard et al., 1994; Poisson and Avouac, 2004; Malatesta et al.,

63

2018) have explored the spatial geometry of incision. At the mountain front of an active orogenic

64

belt, the incision geometry has causally been linked with the evolution of the equilibrium gradient

Journal Pre-proof (S) of transport-limited rivers induced by climate variations (e.g., Bull, 1991; Poisson and Avouac,

66

2004; D'Arcy and Whittaker, 2014; Malatesta and Avouac, 2018; Malatesta et al., 2018). However,

67

the feasibility of utilizing the spatial variations of river incision in a foreland setting to track past

68

tectonic and climatic processes still requires further investigation. Over similar timescales, can

69

differences in river incision between sites along a single river system or several different but

70

adjacent river systems be used to differentiate variations in the strength of climatic or tectonic

71

forcing on river incision? How is rock uplift from local anticlinal growth recorded by the profile

72

of incision (depth and rate) along the course of a river? Can any trend of river incision be observed

73

along the range strike? The Tian Shan and its surrounding area provide an excellent natural

74

laboratory for exploration of these key questions.

ro

of

65

75

The modern Tian Shan has been built largely by basinwards thrusting along range-bounding faults

77

(Figure 1A) (Avouac et al., 1993; Deng et al., 2000; Lu et al., 2010a, 2015). A series of transverse

78

rivers emanating from the high range of the Tian Shan flow across the piedmonts and incise

79

deeply into the piedmont sedimentary bedrock strata (Figure 1B). We focus on the northern

80

Chinese Tian Shan foreland, where the catchment-fan systems exhibit similar alluvial fan systems

81

and canyons with well-developed terrace staircases preserved on both sides. For some of the main

82

piedmont rivers, the fluvial terrace sequences have been classified, and their formation ages have

83

been numerically dated (e.g., Molnar et al., 1994; Poisson, 2002; Poisson and Avouac, 2004; Yuan

84

et al., 2006; Lu et al., 2010b, 2014, 2015, 2017, 2018a; Yang et al., 2013; Gong et al., 2014, 2015;

85

Fu et al., 2017; Malatesta et al., 2018; Su et al., 2018). These existing fluvial geomorphological

86

studies provide an opportunity for integration with new results from this study to quantify regional

87

rates of river incision. This allows further investigation of incision variability along the northern

88

Chinese Tian Shan foreland, and in turn, a fuller investigation of the tectonic and climatic drivers

89

for the observed patterns.

Jo ur

na

lP

re

-p

76

90 91

In this paper, four rivers, i.e. the Kuitun, Jingou, Manas, and Urumqi Rivers from the western,

92

central, and eastern part of the foreland, are studied. Considering the interaction of alluvial fan and

93

river terrace archives (Mather et al., 2017) and the potential role of alluvial fan in constructing a

94

regional geomorphic framework (Lu et al., 2010b; Harvey et al., 2016), we first determine the

Journal Pre-proof timing of alluvial fan abandonment via incision of piedmont rivers, and use this to construct a

96

geomorphologic framework for the foreland region. Then, the inset river terrace sequences

97

preserved at the mountain front are classified and described. For each of the studied piedmont

98

rivers, a terrace is selected as a reference surface for reconstructing the paleogeomorphology (the

99

previous floodplain). The depth of incision is obtained by comparing the present-day topography

100

and the reconstructed one and then is combined with the reference terrace age in order to quantify

101

the average river incision rate. Finally, we analyze the spatial pattern of river incision (depth and

102

rate). The outcomes of this work should be helpful for understanding of the spatial variations of

103

river incision across the mountain front of an orogenic belt.

ro

104

of

95

2. General setting

106

2.1.Geologic evolution of the Tian Shan

107

The ancestral Tian Shan was formed during the Permian after experiencing two Paleozoic

108

accretion events within paleo-Asia (Windley et al., 1990; Sobel et al., 2006). Throughout the

109

Mesozoic, the Tian Shan range existed as a mountainous topography (Hendrix et al., 1992), and

110

deformation within the range mainly occurred in the Latest Triassic, Latest Jurassic, and Late

111

Cretaceous (Hendrix et al., 1992; Dumitru et al., 2001). During the Paleogene, relative tectonic

112

stability in the Tian Shan (Bullen et al., 2001, 2003; Sobel et al., 2006) resulted in the beveling of

113

topography which is expressed as regional unconformities in the Kyrgyz Tian Shan (Oskin and

114

Burbank, 2007) and the Chinese Tian Shan (Windley et al., 1990). In response to the Cenozoic

115

India-Asia collision, the Tian Shan has been tectonically reactivated with topographic growth

116

encroaching into its marginal foreland basin regions (Zhang et al., 1996; Heermance et al., 2007;

117

Lu et al., 2010a, 2015; Thompson et al., 2015, 2017). This reactivation of the range has been

118

chronologically constrained to 24±4 Ma by apatite fission track analyses undertaken on

119

sedimentary rocks exposed in an anticline of the proximal foreland area of the northern Chinese

120

Tian Shan (Hendrix et al., 1994).

Jo ur

na

lP

re

-p

105

121 122

2.2. Tectonics and geomorphology in the foreland

123

The continued growth of the Tian Shan range has seen thrust propagation into its foreland, with

124

the development of several fold-and-thrust belts (Figure 1A). In the northern Chinese Tian Shan

Journal Pre-proof 125

foreland, these are referred to as Belts I to III, numbered sequentially outwards from the mountain

126

front (Figure 1B). Stratigraphically, the folds in Belt I are composed of Mesozoic- Cenozoic

127

sedimentary rocks, whereas those of Belts II and III only consist of Cenozoic strata. The fold

128

strata trend approximately EW or W-NW (Figure 1B), implying a N-S crustal shortening. Using

129

the top of the Dushanzi Formation (Pliocene) as the reference surface, the total magnitude of

130

crustal shortening across the northern Chinese Tian Shan foreland has been estimated to 8-15 km

131

(Deng et al., 2000; Lu et al., 2010a), approximately 6-12 % of the total shortening of 125 ± 30 km

132

across the orogenic belt at the longitude of Manas (Avouac et al., 1993).

of

133

Most of rivers in the northern Tian Shan piedmont originate from active glaciers in the axial parts

135

of the range. Thus, the hydrographs of these piedmont rivers are dominated by summer meltwater

136

peaks (Poisson and Avouac, 2004; Liu et al., 2011; Lu et al., 2018a). These north-flowing river

137

systems cut through the fold-and-thrust belts in the northern Tian Shan foreland. Here,

138

well-preserved river terrace staircases have developed documenting the history of anticline

139

deformation (e.g., Molnar et al., 1994; Lu et al., 2019). Undeformed late Quaternary river terraces

140

are preserved across the anticlines of the most proximally positioned Belt I, implying that the folds

141

are presently tectonically inactive (e.g., Deng et al., 2000; Lu et al., 2018a). In contrast, late

142

Quaternary deformation further out into the foreland is concentrated within the anticlines and

143

thrust faults of Belts II and III, resulting in faulting and folding of river terraces (e.g., Avouac et al.,

144

1993; Molnar et al., 1994; Daëron et al., 2007; Gong et al., 2015; Su et al., 2018; Lu et al., 2019;

145

Qiu et al., 2019). Using the river terraces as the reference surfaces, late Quaternary shortening

146

rates of Belts II and III have been estimated to be 1-2 mm/yr (Lu et al., 2019). Strong active

147

tectonic deformation is further associated with high seismicity along the northern Chinese Tian

148

Shan foreland. At least one major history earthquake, the Manas Ms 7.7 earthquake of 23rd

149

December 1906, occurred in the central part of the foreland (Lu et al., 2018b). A recent earthquake,

150

the Hutubi Ms 6.2 earthquake of the 8th December 2016, occurred in the eastern part of the

151

foreland (Lu et al., 2018b).

Jo ur

na

lP

re

-p

ro

134

152 153

2.3. Quaternary climate

154

The climate in the Tian Shan range and its foreland regions has experienced a complex history

Journal Pre-proof during the late Cenozoic (e.g., Zhang and Sun, 2011; Lu et al., 2013, 2016). The present climate is

156

controlled by the westerly air masses resulting in semi-arid conditions (Xu et al., 2010). Along the

157

north piedmont of the Chinese Tian Shan, the annual precipitation is commonly < 450 mm/yr (Xu

158

et al., 2010; Liu et al., 2011). The high-elevation ridges of the Tian Shan are currently dominated

159

by glaciation and periglacial processes (Cui et al., 1998). Similarly, the range has been extensively

160

and repeatedly glaciated during the late Quaternary, as recorded by multiple moraines (Cui et al.,

161

1998; Xu et al., 2010; Malatesta and Avouac, 2018). In the headwaters of the Manas River in the

162

northern Chinese Tian Shan, the periods of the Quaternary glaciation have been determined as the

163

Hongshanzui stage (MIS 12-16), the Haxionggou stage (MIS 7-9), the Hustai stage (MIS 6), and

164

the Wudate stage (MIS 2-4); the latter three stages correspond to the Gaowangfeng stage, the

165

Xiawangfeng stage, and the Shangwangfeng stage in the headwaters of the Urumqi River,

166

respectively (Xinjiang Institute of Geography, 1986; Zhao et al., 2006; Xu et al., 2010). Large

167

volumes of clastic sediments produced by glacial and periglacial processes have thus accumulated

168

in the glaciated upper parts of the catchments (Cui et al., 1998), a large amount of which has been

169

transported into the foreland basin during the interglacial stages (Molnar et al., 1994; Malatesta

170

and Avouac, 2018; Malatesta et al., 2018).

171

na

lP

re

-p

ro

of

155

3. Methods

173

This work aims to understand spatial patterns of river-incision rate constrained by a single river

174

terrace level across the northern Chinese Tian Shan foreland. The study does not analyze temporal

175

patterns of incision rate based on multiple river terrace levels. We calculate the long-term average

176

of river-incision rate using the height of the terrace surface above the present-day riverbed (i.e.

177

depth of river incision) in combination with the terrace formation age. The age data constraining

178

chronologically the formation of the river terraces preserved in the foreland are available now.

179

Thus, it is the precondition for the following analysis to determine the depth of river incision and

180

its distribution. Here, we obtain river incision depth by comparing the present-day topography and

181

the reconstructed paleogeomorphology (the previous floodplain) from the reference terrace

182

(geomorphic marker) using the method of Burbank and Anderson (2012). As such, our work

183

comprises four components: (i) constructing the fluvial geomorphological framework; (ii)

184

determining the reference terrace for each river; (iii) reconstructing the paleogeomorphic surface

Jo ur

172

Journal Pre-proof 185

(the paleo-floodplain); (iv) calculating the depth and rates of river incision. Using a combination

186

of published fluvial geomorphological research and the spatial distributions of river systems in the

187

foreland region, we selected the Kuitun, Jingou, Manas, and the Urumqi Rivers from the western,

188

central, and eastern part of the foreland for investigation.

189

3.1. Constructing a fluvial geomorphological framework

191

A geomorphological framework is a fundamental requirement for understanding the regional

192

landscape evolution and its tectonic-climatic forcing mechanisms. This has been undertaken by

193

fieldwork spanning 2006 to 2018, during which we have conducted detailed investigations along

194

the northern Chinese Tian Shan mountain front. First, we determine episodes of alluvial fan

195

development based on our previous work (Lu et al., 2010b) as well as the new data, which are

196

used as a geomorphological framework for classification of river terrace. Then, we classify the

197

sequence of river terraces preserved at the mountain front of each analyzed river based on the

198

following geomorphic characterization: (1) geomorphology of the terrace surface (height above

199

the modern riverbed, extent of dissection, and distribution etc.); (2) sedimentology (color,

200

thickness, grain size, and roundness etc.) of terrace sediments overlying the bedrock strath; (3)

201

sedimentology of loess deposits (if any) covering the terrace surfaces; and (4) geomorphic

202

mapping of the fan-terrace landforms and tectonic structures based on the field investigations and

203

remote sensing of Google Earth imagery.

ro

-p

re

lP

na

Jo ur

204

of

190

205

A chronology for the geomorphological framework of the study area was established using

206

optically stimulated luminescence (OSL) and electron spin resonance (ESR) dating methods.

207

These were applied to fluvial terrace sediments from the Toutun and Hutubi River in the eastern

208

part of the foreland adjacent to the river systems analyzed in this work (Figure 2). OSL and ESR

209

samples were taken from homogeneous aeolian loess covering the fluvial sediment or interlayers

210

of silt, very fine sand or fine sand within the fluvial sediments (Table S1 and Figure S1). When

211

sampling, a 20-cm-long, 5-cm-diameter steel pipe with one end covered with opaque material was

212

driven into the sampled layer using a plastic hammer. In order to ensure maximal shielding, only

213

the middle part of each sample was analyzed in the lab. Following the procedures of Lin et al.

214

(2005), preparation and measurements of the ESR samples were carried out in the State Key

Journal Pre-proof Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration. The

216

OSL dates were determined in the Institute of Crustal Dynamics of China Earthquake

217

Administration according to the procedures of Rees-Jones (1995) and Wang (2006). Throughout

218

the broader piedmont region, there are now numerous studies (Tables S2 to S4) that constrain the

219

timing of fluvial landscape development. This chronology includes OSL, post-IR infrared

220

stimulated luminescence (p-IR IRSL), ESR, AMS radiocarbon (AMS 14C), and cosmogenic

221

nuclide (CN 10Be) surface exposure dating data. Collectively, these published regional

222

chronological data (Tables S1 to S4; Figure 3) are used to help constrain fluvial landscape

223

development within this study. Details (including the sampling information, the lab procedure, etc.)

224

on the new and published chronology are documented in the Appendix Data.

ro

of

215

225

3.2. Determining the reference terrace for each analyzed river

227

In this work, we use river terraces as geomorphic markers from which to reconstruct

228

paleogeomorphic surfaces. Some previous studies (e.g., Dibiase, 2014; Finnegan et al., 2014) have

229

proposed that quantification of bedrock river incision rates from river terrace data exhibit a

230

negative power-law dependence on the measured interval over time scales of 104-107 years,

231

despite of the controversy over whether such a dependence exists: the exclusion of the modern

232

river as a datapoint to calculate the rate of river incision may avoid this bias (Gallen et al., 2015).

233

To overcome these uncertainties, the comparison of incision between the different piedmont rivers

234

should be made at the same timescale. Thus, the reference terraces used for the reconstruction

235

should be numerically dated and most importantly should have similar formation ages. Additional

236

criteria should also be considered when choosing a reference terrace for each analyzed river

237

system: (1) the terrace should distribute widely along both banks of the river and its surface should

238

be flat without obvious erosion since abandonment caused by downcutting; (2) the terrace surface

239

should have either no or limited loess cover (< 1 m thickness); (3) terrace surfaces should not be

240

covered by alluvial fan or slope deposits. The latter three criteria will ensure no obvious

241

modification of the terrace surface morphology used for quantification of incision.

Jo ur

na

lP

re

-p

226

242 243

3.3. Reconstructing the paleogeomorphic surface (paleo-floodplain)

244

Firstly, terrace elevation were extracted from the Advanced Spaceborne Thermal Emission and

Journal Pre-proof 245

Reflection Radiometer Global Digital Elevation Model (ASTER GDEM) (v1:

246

http://www.gscloud.cn) comprising 30 m pixels with ~30 m horizontal accuracy and ~20 m

247

vertical accuracy (ASTER GDEM Validation Team, 2009). Secondly, the edge of the reference

248

terrace of each river was identified on the DEM, and then the terrace surface was reconstructed by

249

interpolation across the river valley. Finally, the reconstructed surface was used as the reference

250

for incision quantification. Noting that the quality of the results of this work may be affected by

251

the accuracy of the used DEM data (Boulton and Stokes, 2018).

252

3.4. Calculating river incision depth and rate

254

Based on the elevation difference between the present-day topography and the reconstructed

255

reference terrace surface, a map of river incision depth was generated for each river. Using this

256

map, the maximum depth of river incision was extracted along the length of the modern riverbed.

257

Where each river emerges from the mountain front, we measured the height of the reference

258

terrace surface above the modern riverbed using a hand-held TruPulse 200 Laser Rangefinder

259

(~30 cm precision) to test the reliability of the DEM derived river incision quantification. The

260

long-term average of river incision rate was then calculated through combination of the incision

261

depth data with the available geochronology. All calculations were undertaken ArcGIS 10.2. River

262

incision rate uncertainties are given based on the errors of the terrace formation ages.

263

Jo ur

na

lP

re

-p

ro

of

253

264

4. Results

265

4.1. Fluvial geomorphological framework in the foreland

266

Our previous work (Lu et al., 2010b) has identified four episodes of fan growth in the area

267

between the Anjihai and Taxi Rivers (see Figure 2 of Lu et al., 2010b), i.e. the central part of the

268

northern Chinese Tian Shan foreland. These fans have been designated F1, F2, F3, and F4,

269

occurring sequentially from the hinterland to foreland (progressively younger in abandonment

270

age). In this work, we extend the analysis area westwards to the Guertu River and eastwards to the

271

Urumqi River, thus collectively covering the whole northern Chinese Tian Shan piedmont (Figure

272

1B). The margins of the fans and their catchment areas involved field and remote sensing

273

compilation of a new morphologic map (Figure 2). The remains of the innermost fan F1 are

274

distributed very locally (Lu et al., 2010b), and are not shown in Figure 2. In contrast, the other

Journal Pre-proof 275

three fans F2-F4 are widely distributed across the foreland, here referred to as FP, FeH, and FlH,

276

respectively (Figure 2).

277 The field investigations revealed that thick aeolian loess deposits mantle the alluvial sediments of

279

FP (Figure 4). Gullies cut through the overlying loess exposing the underlying alluvium, revealing

280

an undulating FP fan surface (Figure 4). Fans FeH and FlH have no loess cover and lack any

281

obvious gully erosion into the fan surface. For each piedmont river, each respective alluvial fan

282

has been abandoned due to river incision and is geomorphologically represented as a terrace along

283

the course of a given river (Figure 4).

of

278

ro

284

4.2. Chronological framework of fluvial geomorphology

286

Figure 2 illustrates that all of the main river systems in the northern Chinese Tian Shan exhibit

287

similar fan systems and incised rivers in the foreland, most likely implying coeval fan

288

abandonment and river incision patterns across the foreland (Lu et al., 2010b; Guerit et al., 2017;

289

Malatesta et al., 2018). The complied chronology (Figure 3, Tables S2 to S4), including our new

290

age data (Table S1) of the abandonment of the fans FlH, FeH, and FP further supports this regional

291

abandonment phenomenon (Molnar et al., 1994; Poisson, 2002; Poisson and Avouac, 2004; Yuan

292

et al., 2006; Lu et al., 2010b, 2014, 2015, 2017, 2018a; Yang et al., 2013; Gong et al., 2014; Fu et

293

al., 2017; Malatesta et al., 2018; Su et al., 2018; this study). According to the OSL, AMS 14C, and

294

p-IR IRSL dating data (Table S2), fan FlH is Late Holocene (2-4 ka) in age (Figure 3A). The

295

abandonment ages of fan FeH (but developed by different river systems) based on OSL and p-IR

296

IRSL dating methods range between 12-16 ka (Figure 3B and Table S3), implying Latest

297

Pleistocene-Early Holocene abandonment. Whilst the abandonment ages of fans FlH and FeH

298

(Tables S2 to S3) form distinct cluster groups (Figures 3A and B), the dates of fan FP constrained

299

by OSL, p-IR IRSL, CN 10Be, and ESR methods (Table S4) are more scattered across different

300

piedmont rivers, ranging from hundreds of thousands to tens of thousands of years (Figure 3C).

301

These dates could imply that the fan FP contains several periods of fan aggradation and incision.

302

Despite the differences, these age data (Figure 3C, Table S4) indicate that fan FP is Middle-Late

303

Pleistocene in age.

304

Jo ur

na

lP

re

-p

285

Journal Pre-proof 305

4.3. River terrace sequences and reference terraces

306

The inset river terrace sequences of the four rivers were mapped within each river independently

307

of other. We focus our descriptions below on the river terrace that was used as the reference level

308

for subsequent incision rate quantification.

309 The Kuitun River, flowing through the western part of the northern Chinese Tian Shan (Figure 2),

311

has formed at least six terrace levels at the mountain front (Poisson and Avouac, 2004; Li et al.,

312

2012). These terraces are designated T1 to T6 increasing progressively in height above the modern

313

riverbed (Figure 4A). Terrace T4 has the best preserved geomorphic expression and is

314

best-preserved on both the east and west banks of the river (Figure 4A). The T4 surface is

315

relatively planar despite some minor post-depositional erosion and is positioned some 250 m

316

above the modern riverbed where the river emerges from the range front. The terrace surface also

317

lacks loess cover. Molnar et al. (1994) have ever reported a T4 terrace formation age of ~ 14.3-22

318

ka using cosmogenic nuclide dating (Table S3). Given that their results are relatively scattered and

319

that modelling steps (e.g. production rate values) used to calculate 10Be ages has changed

320

considerably since publication, the dates of Molnar et al. (1994) are omitted from our discussion.

321

Recently, Malatesta et al. (2018) has reported a new age of 13.4 ± 1.6 ka using p-IR IRSL dating

322

(Figure 4A; sample y7 in Figure 2 and Table S3). This age provides a better age constraint for the

323

formation of the Kuitun River terrace T4 to ~ 13 ka.

ro

-p

re

lP

na

Jo ur

324

of

310

325

Flowing through the western part of the northern Chinese Tian Shan (Figure 2), the Jingou River

326

comprises six terraces (T1 to T6) at the mountain front (Figure 4B). T5 is continuously distributed

327

on both valley sides (Figure 4B), with a flat surface and absence of loess cover. The height of the

328

T5 surface above the present riverbed is ~ 133 m where the river exits from the range front,

329

decreasing to ~ 60 m to the south of the Huoerguos anticline (Belt II). Terrace T5 has been dated to

330

~ 12.6 ± 1.3 ka using the OSL dating method (Lu et al., 2010b) (Figure 4B; sample y11 in Figure

331

2 and Table S3).

332 333

The Manas River flows through the central part of the northern Chinese Tian Shan (Figure 2).

334

When compared to the other river systems in the northern Chinese Tian Shan, the Manas River has

Journal Pre-proof the largest catchment area of ~5100 km2 and the largest annual average discharge. Six terraces (T1

336

to T6) were identified at the mountain front (Figure 4C). Thick loess deposits mantle the terrace

337

gravels of T6. In contrast, the deposits of terrace T5 are not covered by loess, and only a few

338

decimeters of alluvial silt cap the terrace gravels. The surface of terrace T5 is flat without obvious

339

post-depositional erosion. This terrace is continuously distributed along the course of the river,

340

extending northwards to an area that encompasses both the Manas and Tugulu anticlines of Belt II.

341

Where the river exits from the range, the T5 surface stands ~ 150 m above the modern riverbed.

342

Gong et al. (2014) have constrained the T5 formation to 12.4 ± 0.8 ka using OSL (sample y12 in

343

Figure 2 and Table S3). This age is consistent with two other OSL ages of 13.8 ± 0.4 ka and 14.3 ±

344

0.7 ka of Yang et al. (2013) (samples y13 and y14 in Figure 2 and Table S3, respectively).

345

Collectively, these dates suggest a mean formation age of the Manas River T5 terrace to 13.5 ± 0.7

346

ka.

-p

ro

of

335

re

347

Located in the eastern part of the northern Chinese Tian Shan (Figure 2), the Urumqi River has

349

nine terrace levels (T1 to T9) at the mountain front (Lu et al., 2014, 2015) (Figure 4D). For the

350

high terraces T5-T9, thick loess sediments cover the terrace deposits (Lu et al., 2016). In contrast,

351

no loess sediment mantles the lower T1-T4 terraces. Among the nine terraces of the Urumqi River

352

at the mountain front, terrace T4 is the best–expressed fluvial feature (Figure 4D). This terrace is

353

continuously distributed on both the east and west banks of the river, and its surface is very planar

354

and easily recognized in the field (Figure 4D). The T4 surface is positioned nearly 60 m above the

355

modern riverbed where the river exits from the range front. The T4 has not been dated. As shown

356

above, the extensive incision during Latest Pleistocene-Early Holocene along the northern Chinese

357

Tian Shan foreland has resulted in abandonment of the fan FeH and thus the formation of a ~ 12-ka

358

terrace (Figures 2 and 3, Table S3). Considering the large spatial extent of the Urumqi River T4

359

terrace and its similarity to other terraces dated to 12-ka along other river systems (Figures 2 and

360

4), we tentatively estimate the formation of terrace T4 to 12 ka, noting a need for future numerical

361

dating.

Jo ur

na

lP

348

362 363

Lithologically, the straths of the terraces preserved in the northern Chinese Tian Shan foreland are

364

spatially different. Within the anticlines in the foreland, the terraces of the four rivers are beveled

Journal Pre-proof into the underlying Mesozoic and Paleogene-Neogene sedimentary strata (Figure 5). In the areas

366

between these anticlines (i.e. synclines), most of the terraces are beveled into the Early-Middle

367

Pleistocene conglomerates (Figure 6). These erosional bedrock surfaces are clearly exposed along

368

both the western and eastern valley sides of each river (Figures 5 and 6), with the height above the

369

riverbed progressively decreasing downstream. The straths are covered with a continuous layer of

370

blackish gray gravels with a variable thickness (commonly several meters to >10 m), that is in turn

371

variably capped by silt sediments (if any) (Figures 5 and 6). These observations suggest that most

372

of the terraces preserved at the mountain front of the four rivers (Figure 4) can be defined as

373

bedrock strath terraces.

of

365

ro

374

Following the reference terrace selection criteria (section 3.2), terrace T4 of the Kuitun River,

376

terrace T5 of the Jingou River, terrace T5 of the Manas River, and terrace T4 of the Urumqi River

377

were selected as the reference levels for paleogeomorphologic surface (the paleo-floodplain)

378

reconstruction. As outlined in section 4.1, alluvial fans FlH, FeH, and FP have been abandoned by

379

river incision and now are geomorphologically expressed as the main terraces along the courses of

380

the rivers; the surfaces of these terraces correspond laterally to or grade downstream into the

381

surfaces of these alluvial fans (Figures 2 and 4). South of the deformation front, for example, the

382

reference terrace of each analyzed river (i.e. terrace T4 of the Kuitun River, terrace T5 of the

383

Jingou River, terrace T5 of the Manas River, and terrace T4 of the Urumqi River) narrows on both

384

sides of the river valley (Figures 2 and 4). After leaving the outermost structures through which

385

the rivers cut, its surface grades downstream merging into the surface of the fan FeH (Figures 2 and

386

4).

Jo ur

na

lP

re

-p

375

387 388

4.4. Incision quantification using the reference terraces

389

By comparing the present-day topography (Figures 7A, 8A, 9A, and 10A) and the reconstructed

390

one (Figures 7B, 8B, 9B, and 10B), a map of river-incision depth can be obtained (Figures 7C, 8C,

391

9C, and 10C); this map is then used to extract the depth of river incision constrained by the

392

reference terrace. Where each river exits from the mountains, river incision depth varies from 250

393

m (Kuitun River), 133 m (Jingou River), 150 m (Manas River), and 53 m (Urumqi River) (Figure

394

11A), and these were considered to be consistent with the TruPulse hand levelled measurements

Journal Pre-proof 395

taken in the field. Combination of the incision amount with the age of a given reference terrace

396

level allowed an incision rate to be calculated (Figure 11B). Maximum rates were recorded where

397

each river exits from the range front, varying from 18.7 ± 2.2 mm/yr (Kuitun River), 10.5 ± 1.1

398

mm/yr (Jingou River), 11.1 ± 0.5 mm/yr (Manas River), and 4.4 ± 0.8 mm/yr (Urumqi River),

399

(Figure 11B).

400 Three spatial patterns of river incision in the northern Chinese Tian Shan foreland can be observed.

402

Overall, river incision (depth and rate) in the northern Chinese Tian Shan foreland displays a

403

decreasing-downstream trend from the maximum where the river exits from the high mountain to

404

zero (Figures 7 to 11). Superimposed onto this downstream decreasing trend, an obvious step in

405

river-incision can easily be observed on each river (Figures 7C, 8C, 9C, 10C, and 11A). South of

406

the outermost anticline through which each river cuts, the river incises a deep valley (Figures 7C,

407

8C, 9C, and 10C) and the gradient of river incision is gentle (Figure 11A). Downstream of the

408

structure, however, the river valley is markedly less deep (Figures 7C, 8C, 9C, and 10C) and the

409

gradient is noticeably steeper (Figure 11A). The incision rate thus co-varies with the depth of river

410

incision (Figure 11B). A third spatial pattern is the contrast in incision pattern along the range

411

front where greater channel incision seems to occur on the rivers in the western part of the

412

foreland (Figure 11).

Jo ur

na

lP

re

-p

ro

of

401

413 414

5. Discussion

415

Before analyzing the controlling factors of the observed spatial patterns of river incision, we first

416

revisit the climate during late Quaternary in the Tian Shan and its role in controlling regional river

417

incision across the north piedmont of the range.

418 419 420

5.1.Latest Pleistocene-Early Holocene climate in the Tian Shan and its surrounding area

421

Three main episodes of fan growth have been identified in the northern Chinese Tian Shan

422

foreland (Figures 2 and 3; section 4.1). For each piedmont river, these alluvial fans have been

423

abandoned due to river incision. The overview of the palaeoclimatic conditions during Late

424

Quaternary in the Tian Shan and its surrounding area can provide insights into the incision and fan

Journal Pre-proof 425

abandonment. Pleistocene climate-change in the Tian Shan has been studied by Malatesta et al.

426

(2018). Here we pay close attention to the climate during Latest Pleistocene-Early Holocene due

427

to the 12-16 ka timing of the reference terrace levels and their association with the abandonment

428

of the FeH alluvial fan.

429 Despite the current semi-arid climate, a relatively warm-wet climate during the Latest

431

Pleistocene-Early Holocene has been proposed based on sedimentologic, environmental magnetic,

432

and geochemical records of lake sediments and aeolian loess in the Tian Shan (Li et al., 2011; Lu

433

et al., 2016) and broader region (Tarim Basin: Luo et al., 2007; Tibetan Plateau: An et al., 2012).

434

In the southern Chaiwopu Basin along the northern Chinese Tian Shan foreland, reduced sand

435

content, supply of finer grained sediment, and lower magnetic susceptibility (χlf) values of the

436

aeolian loess sediments (Figure 12B) reveal a gradual transition from cold-dry to relatively

437

warm-wet climate during Latest Pleistocene- Early Holocene (Lu et al., 2016). A similar

438

palaeoclimatic situation has been documented from loess deposits of the Ili Basin of the central

439

Chinese Tian Shan, where sand content indicates a dominance of a cold-dry climate before 11 ka

440

(Li et al., 2011). Post 11 ka, the intensity of aridity and coldness became weakened, recorded by a

441

reducing sand fraction, despite several fluctuations (Li et al., 2011). This paleoclimatic history

442

within the Tian Shan range has also been documented by other paleoclimatic records from its

443

surrounding area (e.g., Luo et al., 2007; An et al., 2012). In the northeastern Tibetan Plateau, the

444

Asian summer monsoon index (SMI) of Lake Qinghai sediments significantly increases to a

445

relatively high value at around 12 ka and remains relatively stable despite minor fluctuations

446

(Figure 12C), implying a relatively humid climate over the past 12 kyr (An et al., 2012). A similar

447

palaeoclimatic situation is revealed by a decreasing lake carbonate content during the past 13 kyr

448

from sediments of the Lop Nor in the eastern Tarim Basin, located to the southeast of the Chinese

449

Tian Shan (Figure 12D) (Luo et al., 2007).

Jo ur

na

lP

re

-p

ro

of

430

450 451

5.2.Climatically controlled regional river incision and its downstream decrease

452

In response to climate transition (section 5.1) from a cold-dry to a relatively warm-wet climate

453

during Latest Pleistocene-Early Holocene, the consequent hydrological changes will likely play

454

some role in driving the incision patterns, i.e. the downstream decrease of river incision (depth and

Journal Pre-proof rate) from the range front (maximum) out into the foreland basin (zero) (Figure 11). This geometry

456

has been considered by several previous studies (e.g., Poisson and Avouac, 2004; Lu et al., 2018a;

457

Malatesta et al., 2018). Climate variations can change the ratio of sediment load (Qs) versus water

458

fluxes (Qw) (e.g., Yang and Li, 2012; Malatesta et al., 2018). During the stadials (cold-dry climate),

459

large volumes of glacial and periglacial sediments accumulate in the upstream glaciated areas.

460

Periodic release of these deposits into the mountain front region results in alluvial fan

461

development during the cold-dry to warm-wet climate transition. The increasing sediment flux

462

(the higher ratio of Qs/Qw) will be expected to cause continuous aggradation and thus steepening

463

of the fans (Poisson and Avouac, 2004; Malatesta et al., 2018). After depletion of the upstream

464

source of clastic deposits, river incision becomes dominant leading to alluvial fan abandonment

465

due to reduced sediment input from the catchment (the lower ratio of Qs/Qw) (Malatesta et al.,

466

2018). Such a response to periodic climate changes will finally cause adjustment of a river’s

467

gradient to a decreased slope (e.g., Lu et al., 2018a; Malatesta et al., 2018), and formation of a

468

younger terrace with a gentler topographic gradient when the active floodplain is abandoned as a

469

terrace (e.g., Poisson and Avouac, 2004; Lu et al., 2018a). As a result, river incision constrained

470

by the terrace and the present riverbed is expected to display a trend of downstream decrease, as is

471

seen along the northern Chinese Tian Shan foreland (Figures 7-11).

ro

-p

re

lP

na

Jo ur

472

of

455

473

Tectonics may have also contributed to the observed river incision patterns. However, this can be

474

ruled out, when considering the late Quaternary activity of the anticlines in Belts I to III of the

475

foreland. Previous studies have shown that the anticlines of the most proximal structure (Belt I) in

476

the northern Chinese Tian Shan foreland have been tectonically inactive, based on the presence of

477

un-deformed late Quaternary river terraces (e.g., Deng et al., 2000; Lu et al., 2010a, 2018a). In

478

contrast, significant uplift and crustal shortening during the late Quaternary has occurred on the

479

anticlines in the more distal structures (Belts II and III) (e.g., Avouac et al., 1993; Molnar et al.,

480

1994; Yang et al., 2013; Fu et al., 2017; Lu et al., 2019) (Figure 13). Late Quaternary rock uplift of

481

the anticlines in Belts II and III has been estimated to be tens of meters based on fluvial

482

geomorphological investigations (e.g., Molar et al., 1994; Yang et al., 1995; Yang and Deng, 1998;

483

Deng et al., 2000) (Figure 13). Such a tectonic configuration must have contributed to incision

484

south of these structures in the foreland (as discussed in section 5.3), but it is unlikely to be

Journal Pre-proof 485

responsible for the observed downstream decreasing incision trend across the foreland. A similar

486

scenario comes from the Qilian Shan range front which possesses similar geomorphological and

487

tectonic contexts. Here, the riverbed and the main terrace profiles converge downstream, implying

488

a progressive lowering of river gradient (Hetzel et al., 2006). This lowering has been attributed to

489

rapid Holocene river incision caused by climate change rather than rock uplift linked with regional

490

tectonic activation (Hetzel et al., 2006).

491 Although climate may have caused the observed incision pattern across the northern Chinese Tian

493

Shan foreland (e.g., Lu et al., 2018a; Malatesta et al., 2018) (Figure 11), care should be taken

494

when using the rate of river incision to track the past changes in tectonics and climate across the

495

mountain front of an orogenic belt. Here, we propose a conceptual model to illustrate this issue

496

(Figure 14). As discussed above, lowering of a river gradient linked to climate change will result

497

in a downstream decrease of river-incision depth (also rate) constrained by some terraces. Thus,

498

the difference in the depth and rate of river incision observed at two sites (e.g. sites a and b of

499

Figure 14) does not have any specific significance; i.e. more river incision at site ‘a’ does not

500

necessarily mean stronger climatic forcing relative to site ‘b’. Instead, the temporal variations of

501

river incision should provide better insight for unraveling the strength of climate or tectonic

502

driving factors and its variations. The enhanced river incision since formation of a terrace (e.g.

503

terrace T2 of River A in Figure 14, corresponding in time to terrace Tb of River B) must imply a

504

significant change in a forcing factor (i.e. climate and/or tectonics). Thus, enhanced synchronous

505

river-incision across the foreland (such as an increasing river-incision rate observed at sites a-d in

506

Figure 14) means that local tectonic uplift can be excluded as a forcing mechanism for regional

507

downcutting. In a setting like the northern Chinese Tian Shan foreland, the temporal pattern of

508

river incision can provide greater insight into changes in the strength of the forcing factor

509

responsible for the downcutting.

Jo ur

na

lP

re

-p

ro

of

492

510 511

5.3. Steps of river incision caused by local rock uplift

512

Superimposed onto the downstream decrease in incision pattern are localized steps of elevated

513

river-incision (Figures 7C, 8C, 9C, 10C, and 11A). Similar to these steps, knickpoints can also be

514

readily observed along the channel long profiles created by topographic swath profiling (10 km

Journal Pre-proof 515

wide) (Figure 15). For the Kuitun River, the gradient upstream of the outermost anticline (i.e. the

516

Dushanzi anticline of Belt III) is ~1.3%, whereas the downstream gradient is ~2.1% (Figure 15A).

517

Similar changes in stream gradient are also observed on the channel profiles of the other three

518

rivers (Figures 15B to D).

519 Rock strength has been considered as an important factor controlling river incision and thus

521

terrace formation (Stokes et al., 2017). However, in this study the bedrock lithology is not

522

regarded as a key factor for knickpoint formation in the riverbed profiles (Figures 11 and 15). This

523

is because the EW-trending strata that forms the anticlines are dominated by Paleogene-Neogene

524

fluvial-lacustrine mudstone and sandstone offering relatively uniform resistance to erosion

525

(Figures 1B and 5) (Sun et al., 2004; Charreau et al., 2009; Lu et al., 2010a, 2015). The profile

526

steps / channel knickpoints coincide approximately with the locations of the thrust faults that are

527

controlling the growth of the outermost anticlines through which the rivers cut (Figures 11 and 15).

528

The faulted and folded river terraces across the anticlines have revealed that the outermost

529

anticlines have experienced significant rock uplift during Late Pleistocene (e.g., Molar et al., 1994;

530

Yang et al., 1995) (Figure 13), and a maximum uplift of up to 140 m occurs in the Dushanzi

531

anticline through which the Kuitun River cuts (Figure 13A) (Molnar et al., 1994). The Holocene

532

rock uplift of these structures has been estimated to be tens of meters (Figure 13B) (Yang et al.,

533

1995; Yang and Deng, 1998; Deng et al., 2000). Rock uplift caused by thrusting and folding must

534

have led to a base level lowering and thus greater channel incision to the south of these structures

535

(e.g., Holbrook and Schumm, 1999). This locally enhanced channel incision has thus changed the

536

overall downstream decreasing trend of channel incision by forming marked steps / knickpoints in

537

the channel incision patterns (Figure 11).

Jo ur

na

lP

re

-p

ro

of

520

538 539

5.4. Incision contrasts along the range front

540

There is a possible incision contrast pattern that can be observed on the analyzed piedmont rivers

541

in the northern Chinese Tian Shan foreland (Figure 11). Where the Kuitun River exits from the

542

range front, the depth of river incision is constrained by the reference terrace T4, and is up to 250

543

m, much deeper than the other rivers (Figure 11A). As for the rates of river incision, the along

544

strike pattern seems to reveal a 4-fold easterly decrease in the mean incision rate where the river

Journal Pre-proof 545

exits from the range front, i.e. from ~20 mm/yr in the west (Kuitun River) to ~5 mm/y in the east

546

(Urumqi River). However, the Jingou and Manas Rivers display comparable incision values in the

547

foreland over the past 13 kyr (Figure 11). This observation means that any east-west pattern

548

change of river incision shows potential but requires further investigation to be clear.

549

6. Conclusions

551

The piedmont river systems in the northern Chinese Tian Shan foreland have incised deeply into

552

their underlying Quaternary alluvial units and Mesozoic / Paleogene-Neogene bedrock, creating

553

well-developed river terrace landform levels. In this work, we have focused on the spatial

554

variations of river incision (depth and rate) at the mountain front of the northern Chinese Tian

555

Shan. For the four rivers from the western, central, and eastern parts of the foreland, we have

556

obtained the depth and rates of river incision by reconstruction of the paleogeomorphology from

557

reference terraces that have formed during the Latest Pleistocene-Early Holocene. Several spatial

558

variations can be observed on the profile of channel incision (depth and rates) of each river

559

constrained by its reference terrace. Each river displays a clear trend of channel incision that

560

decreases downstream from a maximum where the river exits from the range front to zero out in

561

the foreland. Such a downstream trend is attributed to hydrological changes (adjusting the ratio of

562

sediment input Qs versus water discharge Qw) induced by climate variations during the late

563

Quaternary. An obvious step in incision superimposes onto the overall downstream-decreasing

564

trend of channel incision of each river. This step (knickpoint) occurs near to the thrust fault

565

controlling growth of the outermost anticline through which the river cuts. This observation

566

suggests an important role of tectonics for driving localised river-incision steps. Furthermore, an

567

along range front-strike pattern of incision seems to reveal a 4-fold easterly decrease in the mean

568

incision rate where the river exits from the range, i.e. from ~20 mm/yr in the west to ~5 mm/y in

569

the east, although this pattern warrants further investigation. Collectively, our results suggest that,

570

at the same timescale, greater river incision at the outlet from a mountain range does not

571

necessarily mean stronger climatic or tectonic forcing of incision. In foreland settings, attempts

572

should be made to unravel the strength of the incision forcing factors using temporal patterns of

573

river incision rather than spatial variations alone.

Jo ur

na

lP

re

-p

ro

of

550

Journal Pre-proof 574

Acknowledgements

576

This study is financially supported by the Natural Science Foundation of China (Grants 41771013,

577

41371031 and 41001002). All the data of this paper have been provided in the document

578

“Appendix Data.docx” uploaded online that readers can find and access. We are grateful to

579

Binjing Li, Zhen Wang, Lu Cheng, Tianqi Zhang, and Supei Si for their field assistance. Dr.

580

Junxiang Zhao at Institute of Crustal Dynamics of China Earthquake Administration and Jianping

581

Li at Institute of Geology of China Earthquake Administration are greatly appreciated for their

582

helps on the OSL and ESR dating. We thank Professor Song Yougui at the Institute of Earth

583

Environment, Chinese Academy of Sciences for providing the data of the Asian summer monsoon

584

index of the Lake Qinghai sediments. Professor Burbank D.W. at University of California Santa

585

Barbara is especially appreciated for his thoughtful comments, suggestions, and help with the

586

English on the earlier version of the manuscript. Alberto Gomes, an anonymous reviewer, and the

587

editor Martin Stokes are greatly appreciated for their constructive comments and suggestions on

588

the manuscript, which resulted in its considerable improvement.

589 590

na

lP

re

-p

ro

of

575

References

592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609

An, Z.S., Colman, S.M., Zhou, W.J., Li, X.Q., Brown, E.T., Timothy Jull, A.J., Cai, Y.J., Huang, Y.S., Lu, X.F., Chang, H., Song, Y.G., Sun, Y.B., Xu, H., Liu, W.G., Jin, Z.D., Liu, X.D., Cheng, P., Liu, Y., Ai, L., Li, X.Z., Liu, X.J., Yan, L.B., Shi, Z.G., Wang, X.L., Wu, F., Qiang, X.K., Dong, J.B., Lu, F.Y., Xu, X.W., 2012. Interplay between the Westerlies and Asian monsoon recorded in Lake Qinghai sediments since 32 ka. Sci. Rep. 2, 619–625. ASTER GDEM Validation Team, 2009. ASTER GDEM Validation Summary Report. Avouac, J.-P., Tapponnier, P., Bai, P., You, M., Wang, G., 1993. Active thrusting and folding along the northern Tien Shan and late Cenozoic rotation of the Tarim relative to Dzungaria and Kazakhstan. J. Geophys. Res.: Solid Earth 98 (B4), 6755–6804. Boulton, S.J., Stokes, M., 2018. Which DEM is best for analysing fluvial landscape development in mountainous terrains? Geomorphology 310, 168–187. Bridgland, D.R., Demir, T., Seyrek, A., Daoud, M., Romieh, M.A., Westaway, R., 2017. River terrace development in the NE Mediterranean region (Syria and Turkey): Patterns in relation to crustal type. Quat. Sci. Rev. 166, 307–323. Bull, W.B., 1991. Geomorphic responses to climate change, New York: Oxford University Press. Bufe, A., Paola, C., Burbank, D.W., 2016. Fluvial bevelling of topography controlled by lateral channel mobility and uplift rate. Nat. Geosci. 9 (9), 706–710. Bufe, A., Burbank, D.W., Liu, L.G., Bookhagen, B., Qin, J.T., Chen, J., Li, T., Thompson, J.A.,

Jo ur

591

Journal Pre-proof Yang, H.L., 2017a. Variations of lateral bedrock erosion rates control planation of uplifting folds in the foreland of the Tian Shan, NW China. J. Geophys. Res.: Earth Surf. 122 (12), 2431–2467. Bufe, A., Bekaert, D.P.S., Hussain, E., Bookhagen, B., Burbank, D.W., Thompson, J.A., Chen, J., Li, T., Liu, L.T., Gan, W.J., 2017b. Temporal changes in rock-uplift rates of folds in the foreland of the Tian Shan and the Pamir from geodetic and geologic data. Geophys. Res. Lett. 44, 10,977–10,987. Bullen, M.E., Burbank, D.W., Garver, J.I., Abdrakhmatov, K.Ye., 2001. Late Cenozoic tectonic evolution of the northwestern Tien Shan: New age estimates for the initiation of mountain building. Geol. Soc. Am. Bull. 113 (12), 1544–1559. Bullen, M.E., Burbank, D.W., Garver, J.I., 2003. Building the northern Tien Shan: Integrated thermal, structural, and topographic constraints. J. Geol. 111, 149–165. Burbank, D.W., Leland, J., Fielding, E., Anderson, R.S., Brozovic, N., Reid, M.R., Duncan, C., 1996. Bedrock incision, rock uplift and threshold hillslopes in the northwestern Himalayas. Nature 379, 505–510. Burbank, D.W., Pinter, N., 1999. Landscape evolution: The interactions of tectonics and surface processes. Basin Res. 11, 1–6. Burbank, D.W., 2005. Cracking the Himalaya. Nature 434, 963–964. Burbank, D.W., Anderson, R.S., 2012. Tectonic Geomorphology (2nd ed.), Chichester, U.K.: John Wiley. Charreau, J., Chen, Y., Gilder, S., Barrier, L., Dominguez, S., Augier, R., Sen, S., Avouac, J.-P., Gallaud, A., Graveleau, F., Wang, Q.C., 2009. Neogene uplift of the Tian Shan Mountains observed in the magnetic record of the Jingou River section (northwest China). Tectonics 28, TC2008, doi:10.1029/2007TC002137. Cui, Z.J., Xiong, H.G., Liu, G.N., Zhu, C., Yi, C.L., 1998. Geomorphic processes and sedimentary characters of the cryosphere in the central Tian Shan (in Chinese), Shijiazhuang: Science and Technology Press. Daëron, M., Avouac, J.-P., Charreau, J., 2007. Modeling the shortening history of a fault tip fold using structural and geomorphic records of deformation. J. Geophys. Res.: Solid Earth 112, B03S13, doi:10.1029/2006JB004460. D'Arcy, M., Whittaker, A.C., 2014. Geomorphic constraints on landscape sensitivity to climate in

641 642 643 644 645 646 647 648 649 650 651 652 653

tectonically active areas. Geomorphology 204, 366–381. Demoulin, A., Mather, A., Whittaker, A., 2017. Fluvial archives, a valuable record of vertical crustal deformation. Quat. Sci. Rev. 166, 10–37. Deng, Q.D., Feng, X.Y., Zhang, P.Z., Xu, X.W., Yang, X.P., Peng, S.Z., Li, J., 2000. Active Tectonics of the Tian Shan Mountains (in Chinese), Beijing: Seismology Press. Dibiase, R.A., 2014. River incision revisited. Nature 505, 294–295. Dumitru, T.A., Zhou, D., Chang, E.Z., Graham, S.A., Hendrix, M.S., Sobel, E.R., Carroll, A.R., 2001. Uplift, exhumation, and deformation in the Chinese Tian Shan. In: M.S. Hendrix, G.A. Davis (Eds.), Paleozoic and Mesozoic Tectonic Evolution of Central and Eastern Asia: From Continental Assembly to Intracontinental Deformation. Geol. Soc. Am. Memoir 194, 71–99. Ferrier, K.L., Huppert, K.L., Perron, T., 2013. Climatic control of bedrock river incision. Nature 496, 206–211. Finnegan, N.J., Schumer, R., Finnegan, S., 2014. A signature of transience in bedrock river

Jo ur

na

lP

re

-p

ro

of

610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640

Journal Pre-proof

na

lP

re

-p

ro

of

incision rates over timescales of 104–107 years. Nature 505, 391–394. Fu, X., Li, S.H., Li, B., Fu, B.H., 2017. A fluvial terrace record of late Quaternary folding rate of the Anjihai anticline in the northern piedmont of Tian Shan, China. Geomorphology 278, 91–104. Gallen, S.F., Pazzaglia, F.J., Wegmann, K.W., Pederson, J.L., Gardenr, T.W., 2015. The dynamic reference frame of rivers and apparent transience in incision rates. Geology 43 (7), 623–626. Gong, Z.J., Li, S.H., Li, B., 2014. The evolution of a terrace sequence along the Manas River in the northern foreland basin of Tian Shan, China, as inferred from optical dating. Geomorphology 213, 201–212. Gong, Z., Li, S.H., Li, B., 2015. Late Quaternary faulting on the Manas and Hutubi reverse faults in the northern foreland basin of Tian Shan, China. Earth Planet. Sci. Lett. 424, 212–225. Guerit, L., Barrier, L., Jolivet, M., Fu, B., Métivier, F., 2016. Denudation intensity and control in the Chinese Tian Shan: new constraints from mass balance on catchment-alluvial fan systems. Earth Surf. Proc. Lands. 41 (8), 1088–1106. Hancock, G.S., Anderson, R.S., 2002. Numerical modeling of fluvial strath-terrace formation in response to oscillating climate. Geol. Soc. Am. Bull. 114, 1,131–1,142. Harvey, A.M., Stokes, M., Mather, A., Whitfield, E., 2016, Spatial characteristics of the Pliocene to modern alluvial fan successions in the uplifted sedimentary basins of Almería, SE Spain: review and regional synthesis. Geological Society, London, Special Publications, 440, 65–77. Heermance, R.V., Chen, J., Burbank, D.W., Wang, C., 2007. Chronology and tectonic controls of Late Paleogene-Neogene deposition in the southwestern Tian Shan foreland, NW China. Basin Res. 19, 599–632. Hendrix, M.S., Graham, S.A., Carroll, A.R., Sobel, E., Mcknight, C.L., Schulein, B.J., Wang, Z., 1992. Sedimentary record and climatic implications of recurrent deformation in the Tian Shan: Evidence from Mesozoic strata of the north Tarim, south Junggar, and Turpan basins, northwest China. Geol. Soc. Am. Bull. 104, 53–79. Hendrix, M.S., Dumitru, T.A., Graham, S.A., 1994. Late Oligocene-early Miocene unroofing in the Chinese Tian Shan: An early effect of the India-Asia collision. Geology 22 (6), 487–490. Hetzel, R., Niedermann, S., Tao, M., Kubik, P.W., Strecker, M.R., 2006. Climatic versus tectonic control on river incision at the margin of NE Tibet: 10Be exposure dating of river terraces at the mountain front of the Qilian Shan. J. Geophys. Res.: Earth Surf. 111, F03012, doi:10.1029/2005JF000352. Holbrook, J., Schumm, S.A., 1999. Geomorphic and sedimentary response of rivers to tectonic deformation: a brief review and critique of a tool for recognizing subtle epeirogenic deformation in modern and ancient settings. Tectonophysics 305 (1-3), 287–306. Howard, A.D., Dietrich, W.E., Seidl, M.A., 1994. Modeling fluvial erosion on regional to continental scales. J. Geophys. Res.: Solid Earth 99, 13971–13986. Jochems, A.P., Pederson, J.L., 2015. Active salt deformation and rapid, transient incision along the Colorado River near Moab, Utah. J. Geophys. Res.: Earth Surf. 120, 730–744. Jolivet, M., Barrier, L., Dominguez, S., Guerit, L., Heilbronn, G., Fu, B., 2014. Unbalanced sediment budgets in the catchment–alluvial fan system of the Kuitun River (northern Tian Shan, China): Implications for mass-balance estimates, denudation and sedimentation rates in orogenic systems. Geomorphology 214 (2), 168–182. Leland, J., Reid, M.R., Burbank, D.W., Finkel, R., Caffee, M., 1998. Incision and differential

Jo ur

654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697

Journal Pre-proof

na

lP

re

-p

ro

of

bedrock uplift along the Indus River near Nanga Parbat, Pakistan Himalaya, from 10Be and 26 Al exposure age dating of bedrock straths. Earth Planet. Sci. Lett. 154, 93–107. Li, C.X., Song, Y.G., Qian, L.B., Wang, L.M., 2011. History of climate change recorded by grain size at the Zhaosu loess section in the Central Asia since the Last Glacial period (in Chinese with English abstract). Acta Sedimentologica Sinica 29, 1170–1179. Li, Y.L., Si, S.P., Lu, S.H., Wang, Y.R., 2012. Tectonic and climatic controls on the development of the Kuitun River terraces in the northern piedmont of Tianshan Mountains (in Chinese with English abstract). Quat. Sci. 32 (5), 880–890. Lin, M., Jia, L., Ding, Y., Cui, Y., Cheng, K., Li, H., Xiao, Z., Ying, G., 2005. Dose determination in ESR dating research (in Chinese with English abstract). Seismol. Geol. 27 (4), 698–705. Lisiecki, L.E., Raymo, M.E., 2005.A Pliocene–Pleistocene stack of 57 globally distributed benthic δ18O records. Paleoceanography 20, PA1003, doi:10.1029/2004PA001071. Liu, Y., Métivier, F., Gaillardet, J., Ye, B., Meunier, P., Narteau, C., Lajeunesse, E., Han, T., Malverti, L., 2011. Erosion rates deduced from seasonal mass balance along the upper Urumqi River in Tianshan. Solid Earth 2 (2), 283–301. Lu, H.H., Burbank, D.W., Li, Y.L., Liu, Y.M., 2010a. Late Cenozoic structural and stratigraphic evolution of the northern Chinese Tian Shan foreland. Basin Res. 22, 249–269. Lu, H.H., Burbank, D.W., Li, Y.L., 2010b. Alluvial sequence in the north piedmont of the Chinese Tian Shan over the past 550 kyr and its relationship to climate change. Palaeogeogr. Palaeoclimatol. Palaeoecol. 285 (3–4), 343–353. Lu, H.H., Zhang, W.G., Li, Y.L., Dong, C.Y., Zhang, T.Q., Zhou, Z.Y., Zheng, X.M., 2013. Rock magnetic properties and paleoenvironmental implications of an 8-Ma Late Cenozoic terrigenous succession from the northern Tian Shan foreland basin, northwestern China. Global Planet. Change 111, 43–56. Lu, H.H., Zhang, T.Q., Zhao, J.X., Si, S.P., Wang, H., Chen, S.J., Zheng, X.M., Li, Y.L., 2014. Late Quaternary alluvial sequence and uplift-driven incision of the Urumqi River in the north front of the Tian Shan, northwestern China. Geomorphology 219, 141–151. Lu, H.H., Wang, Z., Zhang, T.Q., Zhao, J.X., Zheng, X.M., Li, Y.L., 2015. Latest Miocene to Quaternary deformation in the southern Chaiwopu Basin, northern Chinese Tian Shan foreland. J. Geophys. Res.: Solid Earth 120 (12), 8656–8671. Lu, H.H., Xu, Y.D., Niu, Y., Wang, Z., Wang, H., Zhao, J.X., Zhang, W.G., Zheng, X.M., 2016. Late Quaternary loess deposition in the southern Chaiwopu Basin of the northern Chinese Tian Shan foreland and its palaeoclimatic implications. Boreas 45, 304–321. Lu, H.H., Wu, D.Y., Cheng, L., Zhang, T.Q., Xiong, J.G., Zheng, X.M., Li, Y.L., 2017. Late Quaternary drainage evolution in response to fold growth in the northern Chinese Tian Shan foreland. Geomorphology 299, 12–23. Lu, H.H., Cheng, L., Wang, Z., Zhang, T., Lü, Y., Zhao, J., Li, Y., Zheng, X.M., 2018a. Latest Quaternary rapid river incision across an inactive fold in the northern Chinese Tian Shan foreland. Quat. Sci. Rev. 179, 167–181. Lu, H.H., Li, B.J., Wu, D.Y., Zhao, J.X., Zheng, X.M., Xiong, J.G., Li, Y.L., 2019. Spatiotemporal patterns of the late Quaternary deformation across the northern Chinese Tian Shan foreland. Earth-Sci. Rev. 194, 19–37. Lu, R.Q., He, D.F., Xu, X.W., Wang, X.S., Tan, X.B., Wu, X.Y., 2018b. Seismotectonics of the 2016 M 6.2 Hutubi earthquake: implications for the 1906 M 7.7 Manas earthquake in the

Jo ur

698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741

Journal Pre-proof

na

lP

re

-p

ro

of

northern Tian Shan Belt, China. Seismol. Res. Lett. 89(1), 13–21. Luo, C., Yang, D., Peng, Z.C., Zhang, Z.F., Liu, W.G., He, J.F., Zhang, P.X., 2007. Climatic and environmental records in the sediment of the Luobei Billabong in Lop-Nur, Xinjiang in recent 32 ka (in Chinese with English abstract). Quat. Sci. 27, 114–121. Malatesta, L.C., Prancevic, J.P., Avouac, J.-P., 2017. Autogenic entrenchment patterns and terraces due to coupling with lateral erosion in incising alluvial channels. J. Geophys. Res.: Earth Surf. 122, 335–355. Malatesta, L.C., Avouac, J.-P., Brown, N.D., Breitenbach, S.F.M., Pan, J.W., Chevalier, M.-L., Rhodes, E., Saint-Carlier, D., Zhang, W.J., Charreau, J., Lavé, J., Blard, P.-H., 2018. Lag and mixing during sediment transfer across the Tian Shan piedmont caused by climate-driven aggradation-incision cycles. Basin Res. 1-23, doi:10.1111/bre.12267. Malatesta, L.C., Avouac, J.-P., 2018. Contrasting river incision in north and south Tian Shan piedmonts due to variable glacial imprint in mountain valleys. Geology 46 (7), 659–662. Mather, A.E., Stokes, M., Whitfield, E., 2017. River terraces and alluvial fans: The case for an integrated Quaternary fluvial archive. Quat. Sci. Rev. 166, 74–90. Molnar, P., Brown, E.T., Burchfiel, B.C., Deng, Q.D., Feng, X Y., Li, J., Raisbeck, G.M., Shi, J.B., Wu, Z.M., Yiou, F., You, H.C., 1994. Quaternary climate change and the formation of river terraces across growing anticlines on the north flank of the Tianshan, China. J. Geol. 102, 583–602. Nie, J.S., Ruetenik, G., Gallagher, K., Hoke, G., Garzione, C.N., Wang, W.T., Stockli, D., Hu, X.F., Wang, Y., Stevens, T., Danišík, M., Liu, S.P., 2018. Rapid incision of the Mekong River in the middle Miocene linked to monsoonal precipitation. Nat. Geosci., doi:10.1038/s41561-018-0244-z. Oskin, M.E., Burbank, D.W., 2007. Transient landscape evolution of basement-cored uplifts: Example of the Kyrgyz Range, Tian Shan. J. Geophys. Res.: Earth Surf. 112, F03S03, doi:10.1029/2006JF000563. Pan, B.T., Hu, X.F., Gao, H.S., Hu, Z.B., Cao, B., Geng, H.P., Li, Q.Y., 2013. Late quaternary river incision rates and rock uplift pattern of the eastern Qilian Shan Mountain, China. Geomorphology 184 (430), 84–97. Poisson, B., 2002. Impact du climat et de la tectonique sur l'évolution géomorphologique d'un piémont - Exemple du piémont Nord du Tian Shan depuis la fin du Pléistocène, (PhD thesis). Bibliogr. Poisson, B., Avouac, J.-P., 2004. Holocene hydrological changes inferred from alluvial stream entrenchment in north Tian Shan (northwestern China). J. Geol. 112, 231–249. Qiu, J.H., Rao, G., Wang, X., Yang, D.S., Xiao, L.X., 2019. Effects of fault slip distribution on the geometry and kinematics of the southern Junggar fold -and-thrust belt, northern Tian Shan. Tectonophysics, doi: 10.1016/j.tecto.2019.228209. Rees-Jones J., 1995. Optical dating of young sediments using fine-grain quartz. Ancient TL 13 (2): 9–14. Sobel, E.R., Chen, J., Heermance, R.V., 2006. Late Oligocene–Early Miocene initiation of shortening in the Southwestern Chinese Tian Shan: Implications for Neogene shortening rate variations. Earth Planet. Sci. Lett. 247 (1–2), 70–81. Stokes, M., Mather, A.E., Belfoul, M., Faik, F., Bouzid, S., Geach, M.R., Cunha, P.P., Boulton, S.J., Thiel, C., 2017. Quat. Sci. Rev. 166, 363–379.

Jo ur

742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785

Journal Pre-proof

na

lP

re

-p

ro

of

Su, P., He, H.L., Wei, Z.Y., Lu, R.Q., Shi, F., Sun, H.Y., Tan, X.B., Hao, H.J., 2018. A new shortening rate across the Dushanzi anticline in the northern Tian Shan Mountains, China from lidar data and a seismic reflection profile. J. Asian Earth Sci. 163, 131–141. Sun, J., Zhu, R., Bowler, J., 2004. Timing of the Tianshan Mountains uplift constrained by magnetostratigraphic analysis of molasse deposits. Earth Planet. Sci. Lett. 219, 239–253. Thompson, J.A., Burbank, D.W., Li, T., Chen, J., Bookhagen, B., 2015. Late Miocene northward propagation of the northeast Pamir thrust system, northwest China. Tectonics doi:10.1002/2014TC003690. Thompson, J.A., Li, T., Chen, J., Burbank, D.W., Bufe, A., 2017. Quaternary tectonic evolution of the Pamir-Tian Shan convergence zone, Northwest China. Tectonics 36 (12), 2748–2776. Wang, X.L., 2006. On the performances of the single-aliquot regenerative-dose SAR protocol for Chinese loess-fine quartz and polymineral grains. Radia. Meas. 41 (1), 1–8. Whipple, K.X., 2004. Bedrock rivers and the geomorphology of active orogens. Annu. Rev. Earth Planet. Sci. 32, 151–185. Windley, B.F., Allen, M.B., Zhang, C., Zhao, Z.Y., Wang, G.R., 1990. Paleozoic accretion and Cenozoic redeformation of the Chinese Tien Shan Range, central Asia. Geology 18 (2), 128–131. Xu, X.K., Kleidon, A., Miller, L., Wang, S.Q., Wang, L.Q., Dong, G.C., 2010. Late Quaternary glaciation in the Tianshan and implications for palaeoclimatic change: A review. Boreas 39, 215–232. Xinjiang Institute of Geography, Chinese Academy of Sciences. (1986), Evolutions of the Tianshan Mountains (in Chinese), Beijing: Science Press. Yang, J.C., Li, Y.L., 2011. Active Tectonic Geomorphology (in Chinese), Beijing: Peking University Press. Yang, J.C., Li, Y.L., 2012. The Principle of Geomorphology (in Chinese, 3rd ed.), Beijing: Peking University Press. Yang, R., Willett, S.D., Goren, L., 2015. In situ low-relief landscape formation as a result of river network disruption. Nature 520, 526–529. Yang, X.P., Deng, Q.D., Zhang, P.Z., Feng, X.Y., Peng, S.Z., 1995. Fold deformation along the Tugulu reverse fault-fold zone since late Pleistocene, north Tian Shan (in Chinese with English abstract), In Editing Committee of the Research of Active Fault (Ed.), Research of Active Fault (IV, pp. 46–62), Beijing: Seismology Press. Yang, X.P., Deng, Q.D., 1998. The fault-propagation folding of the Dushanzi anticline, northern Tian Shan, Xinjiang (in Chinese with English abstract), In Editing Committee of the Research of Active Fault (Ed.), Research of Active Fault (VI, pp. 66–73), Beijing: Seismology Press. Yang, X.P., Li, A., Huang, W.L., 2013. Uplift differential of active fold zones during the late Quaternary, northern piedmonts of the Tianshan Mountains, China. Sci. China: Earth Sci. 56 (5), 794–805. Yuan, Q.D., Guo, Z.J., Zhang, Z.C., Wu, C.D., Fang, S.H., 2006. The late Cenozoic deformation of terraces on the north flank of Tian Shan Mountains and the tectonic evolution (in Chinese with English abstract). Acta Geologica Sinica. 80 (2), 210–216.

Jo ur

786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827

Journal Pre-proof

na

lP

re

-p

ro

of

Zhang, H.P., Zhang, P., Champagnac, J.-D., Molnar, P., Anderson, R.S., Kirby, E., Craddock, W.H., Liu, S., 2014. Pleistocene drainage reorganization driven by the isostatic response to deep incision into the northeastern Tibetan Plateau. Geology 42 (4), 303–306. Zhang, P.Z., Deng, Q.D., Yang, X.P., Peng, S.Z., Xu, X.W., Feng, X.Y., 1996. Late Cenozoic tectonic deformation and mechanism along the Tianshan mountain, northwestern China (in Chinese with English abstract). Earthquake Res. China 12 (2), 127–140. Zhang, Z.Q., Sun, J.M., 2011. Palynological evidence for Neogene environmental change in the foreland basin of the southern Tianshan range, northwestern China. Global Planet. Change. 75, 56–66. Zhao, J.D., Zhou, S.Z., He, Y.Q., Ye, Y.G., Liu, S.Y., 2006. ESR dating of glacial tills and glaciations in the Urumqi River headwaters, Tianshan Mountains, China. Quat. Int. 144, 61–67.

Jo ur

828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871

Journal Pre-proof 872 873 874 875

-p

ro

of

Figure 1. (A) Map showing the topographic and tectonic pattern of the Tibetan Plateau and surrounding area. The “CB” and “LN” circles mark locations of two paleoclimatic studies (Figures 12B and D, respectively). CB = Chaiwopu Basin, LN= Lop Nor. (B) Stratigraphy, tectonics, and river systems in the northern Chinese Tian Shan range and its foreland, based on the 1:1, 500, 000 Chinese geological map. (C) Geological section showing the general tectonic setting (modified from Lu et al., 2010a). JFTF = Junggar Frontal Thrust Fault. Belt I: TST = Tuostai anticline, NAJH = NanAnjihai anticline, NMNS = NanManas anticline; Belt II: HEGS = Huoerguos anticline, MNS = Manas anticline, TGL = Tugulu anticline; Belt III: DSZ = Dushanzi anticline, HLAD = Halaande anticline; AJH = Anjihai anticline; SA = Saerqiaoke anticline; XH = Xihu anticline.

lP

re

Figure 2. The three main episodes of alluvial fan growth in the northern Chinese Tian Shan foreland. Fans are designated FlH (Late Holocene fans), FeH (Latest Pleistocene-Early Holocene fans), and FP (Middle-Late Pleistocene fans). Locations of literature derived dating of abandonment ages are shown by magenta, white, and blue labels. “a”, “y”, and “o” are used to distinguish the map codes of these samples (Tables S2, S3, and S4). Figure 3. Dates used to constrain the abandonment ages of fans FlH (A), FeH (B), and FP (C) by optically stimulated luminescence (OSL), AMS radio carbon (AMS 14C), post-IR infrared stimulated luminescence (p-IR IRSL), electron spin resonance (ESR), and cosmogenic nuclide (10Be) surface exposure dating methods. See Tables S2, S3, and S4 for detail.

na

877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914

Figure captions:

Jo ur

876

Figure 4. Fluvial terraces and their distributions at the mountain front of the Kuitun River (A), the Jingou River (B), the Manas River (C), and the Urumqi River (D). Age data on the formation age of the reference terraces can be found in Tables S2 and S3. Figure 5. Field photos showing planated bedrock surfaces that are clearly exposed on the sides of the rivers within the anticlines. The sequence comprises a Paleogene-Neogene mudstone and sandstone sequence overlain by blackish gray terrace gravels of variable thickness. Figure 6. Field photos showing planated bedrock surfaces that are clearly exposed on the sides of the rivers between the anticlines of the various structure belts (I-III). Here, early Quaternary conglomerates are capped by blackish gray terrace gravels of variable thickness. Figure 7. The present-day topography at the mountain front of the Kuitun River (A), the paleogeomorphology (the previous floodplain) (B) reconstructed from the reference terrace (T4), and the elevation difference between them (C). The formation age of the reference terrace T4 is

Journal Pre-proof dated to 13.4 ± 1.6 ka by post-IR infrared stimulated luminescence dating (Malatesta et al., 2018). The value (250 m) alongside the yellow star is the maximum river incision constrained by the reference terrace T4. Figure 8. The present-day topography at the mountain front of the Jingou River (A), the paleogeomorphology (the previous floodplain) (B) reconstructed from the reference terrace (T5), and the elevation difference between them (C). The formation age of the reference terrace T5 has been dated to 12.6 ± 1.3 ka by OSL dating method (Lu et al., 2010b). The value (133 m) alongside the yellow star is the maximum river incision constrained by the reference terrace T5.

-p

ro

of

Figure 9. The present-day topography at the mountain front of the Manas River (A), the paleogeomorphology (the previous floodplain) (B) reconstructed from the reference terrace (T5), and the elevation difference between them (C). The formation age of the reference terrace T5 is estimated to 13.5 ± 0.7 ka from the OSL dates of Yang et al. (2013) and Gong et al. (2014). The value (150 m) alongside the yellow star is the maximum river incision constrained by the reference terrace T5.

lP

re

Figure 10. The present-day topography at the mountain front of the Urumqi River (A), the paleogeomorphology (the previous floodplain) (B) reconstructed from the reference terrace (T4), and the elevation difference between them (C). The formation age of the reference terrace T4 is estimated to about 12 ka based on its similarity with the 12 ka terraces of the other rivers. The value (53 m) alongside the yellow star is the maximum river incision constrained by the reference terrace T4.

na

Figure 11. (A) The depth of river incision since formation of the reference terrace extracted from the map of river-incision depth (Figures 7C, 8C, 9C, and 10C). (B) The rates of river incision of the four rivers that are calculated using the depth of river incision (A) and the formation age of the reference terrace (errors of the rates are estimated from the age error). The data used in this figure are shown in Tables S5-S8. The arrows denote the steps / knickpoints of river incision (the depth and rate).

Jo ur

915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958

Figure 12. Paleoclimatic records from the Tian Shan and its surrounding area. (A) The deep-sea oxygen-isotope record (Lisiecki and Raymo, 2005). (B) Grain size and magnetic susceptibility (χlf) of the Urumqi River loess sediments in the southern Chaiwopu Basin (CB), northern Chinese Tian Shan foreland (Lu et al., 2016). (C) Asian summer monsoon index (SMI) of Lake Qinghai (QHL) sediments (northeastern Tibetan Plateau) (An et al., 2012). (D) Carbonate content from Lop Nor (LN) in the easternmost part of the Tarim Basin (northwestern China) (Luo et al., 2007). The arrows show the trends of the palaeoclimatic variation. Figure 13. Estimates of anticline uplift in the northern Chinese Tian Shan foreland during the Late Pleistocene (A) and the Holocene (B). Anticlines (gray color) have been proposed as inactive structures during the late Quaternary, and thus no estimate of rock uplift is given on these structures. Anticlines (white color), have no estimate of rock uplift, although they are tectonically active during the late Quaternary. Anticline names and published studies:: the Dushanzi anticline

Journal Pre-proof (DSZ)-Molnar et al. (1994); Yang and Deng (1998); the Halaande anticline (HLAD)-Fu et al. (2017); the Anjihai anticline (AJH)-Deng et al. (2000); the Huoerguos anticline (HEGS)-Deng et al. (2000); the Manas anticline (MNS)-Yang et al. (1995); the Tugulu anticline (TGL)-Yang et al. (1995). See Figure 1B for the full name of the abbreviation of the other anticlines.

of

Figure 14. Conceptual model showing possible spatial and temporal patterns of river incision at the mountain front of an active orogenic belt. Since abandonment of an alluvial fan owing to river incision, River A has formed three terraces, i.e. terraces T1-T3 increasing progressively in height above the riverbed, correlating in time with terraces Ta-Tc of River B, respectively. For these piedmont rivers, the depth and rate of river incision that is constrained by the terraces will be expected to decrease downstream (see inset diagrams). Since formation of terrace T2/Tb, river incision has been significantly enhanced, implying a change in the forcing mechanism (climatic or tectonic). See text for detail.

na

lP

re

-p

ro

Figure 15. Topographic swath profiles (10 km wide: locations in Figure 1B) of the four rivers showing the slope of the riverbed and the elevation differences between the outlet of the river and the locations where river incision equals zero. JFTF = Junggar Frontal Thrust Fault. Yellow stars mark the knickpoint positions on the channel profiles.

Jo ur

959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977

Journal Pre-proof 978

Declaration of interests

979 980

☒ The authors declare that they have no known competing financial interests or personal

981

relationships that could have appeared to influence the work reported in this paper.

982 983

All the authors have no interests to declare.

of

984

☐The authors declare the following financial interests/personal relationships which

986

may be considered as potential competing interests:

Jo ur

na

lP

re

-p

ro

985

987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001

Graphical abstract

Highlights: The paleogeomorphology in the foreland is reconstructed from the reference terrace. The profile of channel incision displays an overall decreasing-downstream trend. An obvious step superimposes onto the downstream-decreasing trend of incision. A possible 4-fold easterly decrease in the mean incision rate is observed. The temporal pattern of river incision is more helpful for unraveling its mechanism.