Terrace formation and river valley development along the lower Taohe River in central China

Journal Pre-proof Terrace formation and river valley development along the lower Taohe River in central China Hongshan Gao, Zongmeng Li, Fenliang Liu, Yajie Wu, Ping Li, Xin Zhao, Fuqiang Li, Jin Guo, Chunru Liu, Baotian Pan, Hongtai Jia

PII:

S0169-555X(19)30376-9

DOI:

https://doi.org/10.1016/j.geomorph.2019.106885

Reference:

GEOMOR 106885

To appear in: Received Date:

12 June 2019

Revised Date:

23 September 2019

Accepted Date:

23 September 2019

Please cite this article as: Gao H, Li Z, Liu F, Wu Y, Li P, Zhao X, Li F, Guo J, Liu C, Pan B, Jia H, Terrace formation and river valley development along the lower Taohe River in central China, Geomorphology (2019), doi: https://doi.org/10.1016/j.geomorph.2019.106885

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Terrace formation and river valley development along the lower Taohe River in central China

Hongshan Gao1, Zongmeng Li2*, Fenliang Liu3, Yajie Wu4, Ping Li5, Xin Zhao1, Fuqiang Li1, Jin Guo1, Chunru Liu6, Baotian Pan1, Hongtai Jia1

Key Laboratory of Western China's Environmental Systems (Ministry of

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1

Education), College of Earth and Environmental Sciences, Lanzhou University, Lanzhou, 730000, China

Henan Key Laboratory for Synergistic Prevention of Water and Soil

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2

University, Xinyang, 464000, China

School of Municipal and Surveying Engineering, Hunan City University, Yiyang,

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413000, China

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3

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Environmental Pollution, School of Geographic Sciences, Xinyang Normal

Wuwei No. 1 Middle School, Wuwei, 733000, China

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College of Resources and Environment, Tibet Agriculture and Animal

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4

Husbandry University, Linzhi, 860000, China State Key Laboratory of Earthquake Dynamics, Institute of Geology, China

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6

Earthquake Administration, Beijing, 100029, China

*

Corresponding author, E-mail: [email protected]

Highlights 

Nine river terraces have been identified and dated along the lower Taohe River.

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The development of the modern river valley in this area began at 1.4–1.14 Ma. Tectonic uplift and climate change were the main controlling factors.

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

Abstract

A flight of nine river terraces has been well developed and preserved along

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the course of the lower Taohe River. Based on optically stimulated

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luminescence (OSL), electron spin resonance (ESR), paleomagnetic and loesspaleosol stratigraphic analyses, we found that the accumulation ages of the

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gravel layers of terraces T9–T1 correspond to the loess layers L13, L9, L8, L6,

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L5, L4, L3-L2, L1-4 and L1-1, respectively. The chronologic results indicate that the formation and development of the modern river valley in this area occurred

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between 1.4 and 1.14 Ma. The topographic fluctuation caused by the Kunhuang Movement appears to have been the main reason for the reorganization of the

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river system and valley development in the area. In the context of this tectonic uplift, fluvial processes were coupled with the regional climate on a geomorphic equilibrium scale; i.e., over a glacial-interglacial cycle. That is, river aggradation appears principally to have occurred during glacial periods, while a graded state existed during interglacial periods. In addition, rapid river downcutting in

response to climate change occurred during the transition periods from glacial to interglacial climates.

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Keywords: terraces; dating; river valley development; lower Taohe River

1. Introduction The Yellow River has many tributaries in the Longzhong Basin (Fig. 1). Driven by strong tectonic activity and high topographic relief, these rivers have cut through a series of intermontane basins and formed well-developed terrace sequences, which have been covered by several to hundreds of meters of thick aeolian loess. These terrace sequences have recorded the history of landform

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evolution and environmental changes in this area. In the study of drainage evolution and river valley development, as early as the beginning of the twentieth century, Clapp (1915) proposed that the Yellow River once flowed into

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the Weihe River (Fig. 1a), and then traveled eastward during the Miocene to

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the Pliocene (Ting, 1952), or the Eocene to the Miocene (Lin et al., 2001). Another view holds that the Yellow River is an ancient river that has been

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contiguous in both its upstream and downstream area since the Early to Middle

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Miocene (Zhao et al., 2018).

However, in recent years, many studies have shown that a paleo-drainage

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system before the formation of the modern Yellow River was established at ~3.6 Ma (e.g., Guo et al., 2018). Moreover, most of the modern river valleys

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appeared in the middle and late Early Pleistocene. For example, the Liujiaxia to Lanzhou section of the Yellow River began to develop ~1.6 Ma; erosion subsequently occurred headwards through a series of basins through the processes of stream capture (Li et al., 1996). Although the oldest terrace (T11) of the Huangshui River developed at 14 Ma in the Xining Basin (Li et al., 1996;

Lu et al., 2004; Zhang et al., 2017), there was a large adjustment of the river system between 1.55 and 1.2 Ma, after which the southeastward pattern of flow was established (Lu et al., 2004). The modern valley of the upper Weihe River in the southeast of the Longzhong Basin also developed since ~1.2 Ma (Gao et al., 2016, 2017), while a terrace with an age of ~2.6 Ma, i.e., Early Quaternary, is preserved along the lower Weihe River (Sun, 2005).

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The river terraces in the Longzhong Basin have a typical dual structure, each being composed of a lower unit of coarse-grained gravels and an upper

unit of fine-grained overbank sediments overlain by thick layers of loess. This

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step-like and clear geomorphic sedimentary terrace structure is widely

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distributed in the Longzhong Basin, and has become known as the “Lanzhou style” platform, as named by Huang (1957), who suggested that the terrace

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formation here was related to large-scale tectonic uplift during the Quaternary.

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In addition, the role of climate change in the formation of terraces has also attracted much attention (Xu, 1965). Since the 1980s, the Department of

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Geography of Lanzhou University has carried out abundant large-scale, comprehensive and systematic quantitative studies on river terraces and

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Cenozoic strata in this area in an attempt to obtain a better understanding of the evolutionary history of the Yellow River valley during the Quaternary period (e.g., Li et al., 1996, 1997). In addition, many of these studies have proposed that the terraces in this region were the product of the orbital-scale glacialinterglacial cycle overprinted onto the effects of tectonic uplift (Li et al., 1997;

Pan et al., 2009). As the largest tributary of the Yellow River in the Longzhong Basin, the Taohe River, has not received sufficient attention. In one of few examples of previous research, Xu (1965) conducted a preliminary investigation into three terraces of the Taohe near Lintao. Guo et al. (2006) discussed the development and evolution of the upper Taohe valley based on four terraces near Minxian.

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They suggested that the upper Taohe and the upper Minjiang were once the same river, and that the modern course of the Taohe (Fig. 1a) was formed

during the Late Pleistocene (i.e., 50–30 ka). Based on field investigations of the

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spatial distribution and sedimentary structure of the river terraces in the lower

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Taohe River Basin, we made a detailed study of the formation of the terraces of the Taohe using paleomagnetic, optically stimulated luminescence (OSL),

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electron spin resonance (ESR) and loess-paleosol climatostratigraphic

2. Study area

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analyses.

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The Taohe River, with a length of 637 km, is the tributary with the largest annual discharge into the upper Yellow River (Fig. 1). It originates in the eastern

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Xiqing Mountains and flows from west to east across the Gannan Plateau. Deeply incised meanders are well-developed, and grasslands are widely distributed, along this river section. The Taohe turns to the northwest after Minxian County and is characterized by wandering channels and gorges. This section lacks a floodplain owing to the construction of reservoirs. The Taohe

then flows out of the Haidian Gorge and enters the Longxi Loess Plateau (LLP), with a wide river valley and a floodplain, and well-developed terraces. However, the channel of the lowermost section from Xiabuzi to the confluence of the Taohe and Yellow rivers narrows to become a canyon near the Maolong Gorge. [Fig. 1. hereabouts] Our study area is mostly located in the Lintao-Tangwang Basin, which is

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connected with the Linxia Basin in the west, the Longxi Basin in the east, and the Lanzhou Basin in the north (located southwest of the Longzhong Basin). All

these basins and sub-basins are characterized by thick sedimentary Cenozoic

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strata deposited between the Middle and Late Paleogene (or Oligocene), and

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the Quaternary (Fang et al., 2003). In addition, the southern Lintao-Tangwang Basin is bounded by the northern flank of the western Qinling Orogen. There

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are only a few small basins that are dominated by Early Cenozoic deposits. A

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large number of NNW and NWW sinistral strike-slip and thrust faults were formed in this area under the long-term compressional environment (Yuan et

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al., 2013; Fig. 1a), resulting in the intense uplift of the surrounding mountains. The intermontane basins received their materials from their surrounding

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mountain ranges, leading to the deposition of thick, red, fluvio-lacustrine sediments. Intense post-Miocene thrust fault activity of the north frontal fault zone of the western Qinling Mountains caused the cessation of foreland basin development in the Longzhong Basin. After that, the Longzhong Basin and the NE Tibetan Plateau (TP) have undergone a history of punctuated Quaternary

uplift, proposing that uplift occurred: (1) from 3.6 to 1.5 Ma, named the Qingzang Movement, that resulted in massive molasse deposits around the TP’s margin and the synchronous occurrence of faulted basins within the TP (Li, 1991; Li et al., 2014, 2015); (2) from 1.2 to 0.6 Ma, named the Kunhuang Movement, during which the Yellow River entered the TP by cutting through the Jishi Gorge (Li, 1991), and the Kunlun Pass area was uplifted above 3000 m

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asl (Cui et al., 1998); and (3) at ~ 0.15 Ma, named the Gonghe Movement,

which is represented by the unconformity into the folded fluvial sediments of the Gonghe Formation, and rapid downcutting of the Yellow River into the Gonghe

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Formation to form the Longyang Gorge (Li, 1991). These strong tectonic events

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shaped the tectonic deformation framework of the region, its characteristic mountains and river landscapes, and its climatic-ecologic environmental

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patterns (Fang et al., 2003).

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Climatologically, the Lintao Basin is located at the intersection between the region principally affected by the East Asian Monsoon (EAM), the arid region of

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northwestern China, and the Qinghai-Tibet alpine region. It is located at the apex of East Asia’s ‘monsoon triangle’. Influenced by monsoonal atmospheric

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circulations, the Lintao Basin possesses a typical continental monsoonal climate, with a clear seasonality and significant variability in precipitation (300– 600 mm; Li et al., 2013). These characteristics were enhanced during the glacial and interglacial periods of the Quaternary (Li et al., 2014). In addition, the temperate semi-arid grassland-covered landscape provides the environmental

conditions necessary for (aeolian) dust deposition and loessification. 3. River terraces in the lower Taohe River We conducted field observations to identify the position of each of these terraces, the thickness of capping gravels, overbank deposits and overlying loess, and the number, character and distribution of paleosols within the loess. Strath and gravel elevations were surveyed using a differential GPS system,

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and uncertainties were estimated to be <5 cm. Based on field investigations

into the terraces from the Haidian Gorge to the Xiabuzi section of the Taohe

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River, nine terraces have been found to have developed in this area (Fig. 1b).

All the terraces are strath terraces except T1 (which is a fill terrace). The

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terraces are covered by thick layers of loess, which can provide a frame of

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reference and dating control for the chronologic identification of terrace sequences.

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There are five terraces in the canyon section below the Haidian Gorge Reservoir (Fig. 2). Terrace T1 is distributed in a narrow strip; its tread is ~3 m

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above river level (arl). Terrace T2 is well-developed near the villages that lie between Xiazhuang and Suolin in Nanping Prefecture. The strath of Terrace T2

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is ~19 m arl and is composed of Cretaceous red-purple sandstone. The fluvial deposits are composed of a ~2 m thick gravel deposit, overlain by 8–10 m of loess. Terrace T3 is distributed in the Yanjiaxiaozhuang area and has a strath ~33 m arl. The gravel of Terrace T3 is ~5–6 m thick and is overlain by an 18– 20 m thick layer of loess. Terrace T4 is distributed in the environs of Xiazhuang

village; its strath is ~60 m arl, while its gravel deposit is ~3 m thick and is mantled by 10–15 m of loess. T5 is the highest terrace in this area and is welldeveloped along the hills behind Suolin Village. Its strath is 115 m arl, while its gravel is ~3 m thick and is covered by loess. [Fig. 2. hereabouts] The Taohe flows into the LLP in the section between the Haidian Gorge

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and Daban, a section in which nine terraces have developed (Fig. 3). Terrace T1 is occupied by villages and farmland. The top of its gravel is 3–5 m arl, and is overlain by 1–2 m of loess. Terrace T2 is continuously distributed in the

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Sanjiaji, Yujing and Xintian districts. The strath of terrace T2 is ~15–20 m arl

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and is composed of Neogene mudstone; its gravel is ~2–5 m thick and is covered by 8–10 m of loess. Terraces T3, T4 and T5 show clear and continuous

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loess platforms, with straths of 35–40, 60 and 95 m arl, respectively. The

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gravels of the three terraces are ~4–8 m thick, and the upper floodplain deposits are ~4–10 m thick. They are all mantled by loess of ~35–100 m thick. The higher above

Terrace

T5

are

severely

dissected

and

distributed

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terraces

discontinuously; these are preserved only in some sections and are covered by

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loess up to 100 m thick. The strath of Terrace T6 is ~115 m arl and is exposed in some villages near Lintao County. The strath of Terrace T7 is ~130 m arl; its gravel is exposed near the villages of Wotuo and Zhuoziwan near the confluence of the Guangtong and Taohe rivers. The strath of Terrace T8 is ~160 m arl; its gravel is exposed in Tianjiahe Village, near Xintian. Gravels of this

terrace are ~200 m arl. The gravels of Terrace T9 are exposed near the villages of Liujiawan and Fengjiagoumen. [Fig. 3 hereabouts] In the reach between Daban and Xiabuzi Town in the Tangwang Basin, the local purple Cretaceous sandstone is exposed. The landscape is characterized by badlands. River terraces are poorly developed, and their gravels are

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exposed only in some reaches. Seven terraces have developed, i.e., a fill terrace (T1) and strath terraces (T2–T7). The straths of terraces T2–T7 are 13– 15, 35, 70, 95, 118 and 140 m arl, respectively. Their gravels are heavily

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cemented and are overlain by thin layers of loess. However, terraces T8 and

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T9, which have been preserved in the Lintao Basin, are not found in the

4. Dating methods

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Tangwang Basin.

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The attitudes people have towards land use and ecology are changing dramatically. It is no longer advisable to excavate large-scale trenches in, and

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extract multiple samples from, densely populated, terraced farming areas. Therefore, we used paleomagnetism and paleosol stratigraphy to establish the

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chronology of terraces T8 and T5. OSL dating was used for the lower terraces T2 and T3. ESR dating was conducted on terraces T2–T7 and T9. We analyzed the grain-size characteristics of the basal units and loesspaleosol samples of the capping aeolian stratigraphy of these terraces to help characterize loess-paleosol successions in the field. Samples for particle size

analysis were collected at 5 cm intervals. In the laboratory, samples were first treated with HCl and H2O2 to remove carbonate and organic matter, respectively. Then, the particle size was measured using a U.K. Mastersizer 2000 laser particle sizer in the Key Laboratory of Western China's Environmental Systems (Ministry of Education). Paleomagnetic samples were collected in well-exposed natural outcrops.

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For each of the samples, oriented 1000 cm3 cubic blocks of loess were collected, with geographic north marked on the top surface. In the laboratory, each bar was cut into 8 cm3 cubic samples using an electric saw. The treatment and

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testing of samples were conducted in the Key Laboratory of Western China's

paleomagnetic

samples

were

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Environmental Systems (Ministry of Education), Lanzhou University. All the measured

on

2G-760R

cryogenic

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magnetometers in magnetically shielded conditions, and were progressively

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demagnetized to 580°C. Magnetizations were effectively removed at ~350°C, such that characteristic remanence directions could be clearly identifiable at

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temperatures >350°C (Fig. 4).

[Fig. 4 hereabouts]

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OSL samples were collected by hammering steel tubes into freshly cleaned

sections under light-shielded conditions. In the laboratory, samples were first treated with HCl and H2O2 to remove carbonate and organic matter, respectively. Then the samples were wet-sieved to select medium-grained fractions with a particle size of 38–63 µm. Finally, the medium-grained samples were etched

with H2SiF6 for approximately two weeks to remove feldspars. The OSL measurements were performed in the Luminescence Dating Laboratory of the Qinghai Institute of Salt Lakes, using an automated Risø TL/OSL-DA-20 reader. The single-aliquot regenerative dose (SAR) protocol (Murray and Wintle, 2000) and the standard growth curve (SGC) method (Roberts and Duller, 2004; Lai and Ou, 2013) were used for De determination. The irradiation procedure 90

Sr/90Y beta source that was fitted on the reader. The optical

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employed a

measurements were performed after heating at a temperature of 260°C for 10

s for natural and regenerative doses and after preheating at a temperature of

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220°C for 10 s for test doses. The luminescence was stimulated by blue diodes

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at 130°C for 40 s, which was detected using a 9235QA photomultiplier tube through a 7.5 mm thick U-340 filter (Lai and Ou, 2013). For each sample, 6-12

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aliquots were measured using the SAR protocol, and corresponding De and

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growth curves were obtained. For each sample, the OSL signals decrease very quickly during the first second of stimulation, indicating that the OSL signal is

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dominated by the fast component. Recuperation of signal when no dose is given is below 5% for all samples. For all aliquots, the recycling ratios are in the

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acceptance range of 0.9–1.1. The concentrations of U, Th and K for each sample were measured using

the neutron activation analysis (NAA) method. The environmental dose rate was determined according to the conversion relation between the elemental (U, Th, and K) concentrations and the dose rates of different minerals (Aitken,

1998). The efficiency of the total dose rate was corrected according to the density of the quartz and the paleowater content for each sample. The OSL results are listed in Table 1. OSL sample sites are shown in Fig. 3. [Table 1. hereabouts] ESR dating was used to study the accumulation ages of terrace gravel layers. The superficial sediment of 50–100 cm from the side of the exposure

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was cleaned under light-shielded conditions, then sand samples from the

terrace gravel were collected, sealed and archived. Pretreatment of ESR samples was completed in the Key Laboratory of Western China's

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Environmental Systems (Ministry of Education), Lanzhou University. In the

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laboratory, the 0.125–0.25 mm particle size was sieved before the organic matter, carbonate and feldspar in the samples were removed using H2O2, HCL

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and HF, respectively (Liu et al., 2013). After that, the samples were dried at

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40°C and then put into a magnetic separator to remove any magnetic minerals and purify the quartz particles. Each sample was divided into 200 mg

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subsamples. These aliquots received additional gamma doses ranging from 0 to 12,000 Gy using a 60Co source in the laboratory at Peking University, Beijing.

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ESR measurement was carried out at the State Key Laboratory of

Earthquake Dynamics, Institute of Geology, China Earthquake Administration. The Ti center was selected as the dating signal because of its quick sunlight bleaching and is totally bleachable. The ESR signal intensity was measured using a BRUKER EMX-6 X-band spectrometer at low temperatures (liquid

Nitrogen 77 K). The microwave power was 5 mW, the modulation amplitude was 0.16 mT, the conversion factor was 20.48 ms, the time constant was 40.96 ms, and the spectrum resolution was 2048 bt. The concentrations of U, Th, and K used for calculating environmental dose rates were measured by laser fluorescence,

colorimetric

spectrophotometry

and

atomic

absorption

techniques, respectively. The total annual dose was finally calibrated taking into

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account the burial depth and water content (Durcan et al., 2015). The ESR

dating results are shown in Table 2. The ESR sample sites are shown in Figs. 1, and 2.

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[Table 2. hereabouts]

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5. Timing of terrace formation

5.1. Magnetostratigraphy and loess-paleosol sequences of terraces T8 and

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T5

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The gravel of Terrace T8 is clearly exposed in Tianjiahe Village, Lintao County. There is a ~100 m thick loess layer deposited on this terrace. However,

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only the lower 65 m of the section was sampled for paleomagnetic and grain size analysis because of the prior development of agricultural terraces.

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Approximately 102 paleomagnetic samples were collected at intervals of 0.25, 0.5, 1 and 2 m at heights of 0–8, 8–21, 21–50 and 50–65 m above the alluvial deposits, respectively. The magnetostratigraphic results showed that there was only one polarity reversal, at 4.5 m, above the alluvial deposits (Fig. 5). Using previous magneto-pedostratigraphic studies of the loess (Wang et al., 2006; Liu

et al., 2010), we assigned this transition to the Matuyama-Brunhes (M/B) boundary. According to existing research results for the Chinese Loess Plateau (CLP), the M/B boundary is located at the upper part of S8 (Liu et al., 2010). We therefore interpreted the basal paleosol of this section to be S8. In addition, there are three closely-arranged paleosol layers at a height of 25–38 m in this section, clearly showing the “three red layers” characteristic of S5 (Liu, 1985).

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Grain size analysis showed three obvious finer wave peaks, clearly reflecting

three layers of sub-paleosol units. We therefore assigned the 25–38 m layer to S5. Based on these analyses, we deduced that the finer grain size peaks at

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heights of 14.9–17.4 and 18.1–22.1 m may represent two sublayers of paleosol

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S6, namely S6-1 and S6-2. The 8.1–12.3 m subsurficial layer in this section was taken to be paleosol S7, and the 2.4–5.3 m subsurficial layer was taken to

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be paleosol S8 (which was accumulated at 866–814 ka; Ding et al., 2002;

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Lisiecki and Raymo, 2005) (Fig. 5). Based on this analysis, it was concluded that the abandonment of Terrace T8 most likely occurred at ~0.87 Ma.

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[Fig. 5. hereabouts]

For the Liujiagou terrace (T5), only the lower 56 m of the section was

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sampled due to the disturbance to the terrace caused by farming. Approximately 75 paleomagnetic samples were collected at intervals of 0.5, 1 and 2 m at heights of 0–19, 19–32 and 32–56 m above the alluvial deposits, respectively. However, the measured results showed that no polarity reversal appears to have occurred in this section (Fig. 5). We therefore deduced that

Terrace T5 was probably abandoned after 0.78 Ma (the M/B boundary). 5.2. OSL dating and loess stratigraphy of the lower T2 and T3 terraces It has been suggested that the loess-paleosol strata in different areas can be compared in the CLP, and the particle size analysis can be used to identify the loess and paleosol layers in the loess sequence (Liu, 1985; Li et al., 1992). Some well-developed paleosol layers could be identified in the overlying loess

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profile on the Caojiahe Terrace (T3). Based on particle size analysis (medium diameter, Md) and comparison with the loess-paleosol stratigraphy of the

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Beiyuan Section in the Linxia Basin, and profiles from the Jiuzhoutai Section in

the Lanzhou Basin, it was deduced that these paleosol layers matched best

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with S0, Sm and S1 (Li et al., 1992). Moreover, the lower paleosol S1 could be

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divided into S1-a, S1-b and S1-c, which would correspond to Marine Isotope Stage (MIS) 5a, MIS 5c, and MIS 5e, respectively (Fig. 6). However, an OSL

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age for Sample TH-01, at a height of 1.5 m above the gravels, has been obtained as 19.1 ka, which is younger than the formation age of S1 (130–71 ka;

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Ding et al., 2002; Lisiecki and Raymo, 2005). Moreover, the age of this sample is also younger than the Sample TH-02 (~38.7 ka) which was obtained from the

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basal paleosol of the lower Yawan Terrace (T2). Nevertheless, an OSL age of 71.2 ka of Sample TH-03 was obtained for the top of the basal paleosol of the Majiawan Terrace (T3; Fig. 3b), which is located on the opposite riverbank near the Caojiahe Terrace. We therefore deduced that the basal paleosol of the loess profile on Terrace T3 in this area is most likely indicative of paleosol S1,

corresponding to MIS 5. We estimated that Terrace T3 was abandoned at ~0.13 Ma (Ding et al., 2002; Lisiecki and Raymo, 2005). [Fig. 6. hereabouts] It was difficult to identify the loess-paleosol layers in the Yawan Terrace (T2) profile due to its weakly-developed paleosols (Fig. 6). However, in the Yawan Terrace (T2), the Sample TH-02 from the bottom of the basal paleosol (i.e., ~1.8

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m above the gravel layer) had an age of 38.7 ± 3.2 ka. By comparing this with

the loess-paleosol stratigraphy of the terraces T3 and T2 in the Linxia Basin (Li

et al., 1992), it could be inferred that the lowermost paleosol of Terrace T2 is

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Terrace (T2) was abandoned at ~57 ka.

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Sm (Li et al., 1997), corresponding to MIS 3. We deduced that the Yawan

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5.3. ESR dating of the terraces T2–T7 and T9

Combining the above dating methods and the ESR dating, the chronologic

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framework of terraces T9–T2 in the lower Taohe River was established. The gravel of Terrace T9 had an ESR age of 1.14 ± 0.17 Ma, corresponding to MIS

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34. The accumulation age of the Terrace T8 gravel was assessed as probably equivalent to L9, i.e., 936–866 ka, owing to the presence of the lowermost

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paleosol S8 within the Tianjiahe Terrace. The gravel of Terrace T7 appears to have been deposited at 798.8 ± 118.8 ka. The gravel of Terrace T6 had an ESR age of 677.8 ± 147.2 ka. Terrace T5 had two ESR ages of 611.1 ± 100.2 and 655.9 ± 66.6 ka, similar to our estimate that the Liujiagou Terrace (T5) was abandoned later than 0.78 Ma (the M/B boundary). The ESR ages of the gravel

layers of terraces T4–T3 were 498.7 ± 54.5 and 279.2 ± 31.4 ka, respectively. The ESR age of Terrace T2’s gravel layer at the Haidian Gorge site was 72.3 ± 54.3 ka, consistent with the estimate of the lowermost paleosol Sm for the Yawan Terrace (T2) in Lintao County. Terrace T1 was not dated but, according to river terrace studies previously conducted in the Longxi Basin (Li et al., 1996; Gao et al., 2016, 2017), its gravel deposition generally corresponds to L1-1; the

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Holocene paleosol which developed on the alluvial materials corresponds to S0 (from 14 ka to present; Ding et al., 2002; Lisiecki and Raymo, 2005). 6. Discussion

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6.1. Landscape evolution and river valley development

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Step-like geomorphic surfaces (e.g., planation surfaces, erosion surfaces,

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river terraces, etc.) are important archives for studying drainage basin landforms and river valley development. Many studies have shown that the TP

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has undergone a complex history of uplift during the Cenozoic (Li et al., 1996). Several planation surfaces were formed on the TP and in the Longzhong Basin.

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The most widely distributed one of them, i.e., the main planation surface (MPS),

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was developed during the Middle and Late Neogene (Li et al., 2015). For example, the MPS in the Maxian Mountains (i.e., the Xiaoshuizi planation surface) was ultimately destroyed at 6.9 Ma (Li et al., 2017). However, this planation surface (e.g., where it appears as the Tangxian planation surface) was abandoned at 3.7–3.1 Ma in the Lüliang Mountains (Pan et al., 2012; Xiong et al., 2017). These facts would indicate that even in the same catchment, the

formation and evolution of the mountainous landforms of different tectonic units can have clear differences. A great deal of information about the provenance, paleo-systems and paleochannels of ancient rivers can be extracted from sedimentary strata (e.g., Zhao et al., 2018). However, these ancient fluvial deposits, which formed during the development of the planation surfaces,

exhibit a close correlation with modern river systems.

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represent the ancient river systems and landscapes of the basin; they do not

Erosion surfaces have often developed, and been preserved, under the

MPS of the TP, but there are great differences in these erosion surfaces

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between different areas. For example, the erosion surface located along the

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divide between the Taohe and Weihe rivers fluctuates at an altitude around 2400 m asl, with an ESR age of its overlying gravel layer of 1.4 Ma (Zhang,

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2006). In addition, the “Gansu Period Peneplain” (Chen, 1947), near the

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Lanzhou Basin, is found at an altitude of 2100 m asl. This surface was abandoned at 1.7 Ma (Zhu et al., 1996). The development of erosion surfaces

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shows that there was an intermittent period of development after the uplift of the MPS. The modern geomorphologic drainage patterns and river valley

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systems have been established since the erosion surface was formed. The landscapes found below the level of these erosion surfaces, i.e., river valleys and terrace sequences, can therefore sometimes be connected with modern landscape features and processes. The rivers abruptly started vertical erosion into the planation level of 1.7 Ma old, making a typical terrace staircase

(Vandenberghe et al., 2011; Wang et al., 2012; Vandenberghe, 2016). River terrace studies conducted in the Longzhong Basin and in its adjacent areas have shown that the formation of the modern river valleys mainly occurred during the Quaternary. For example, there are nine terraces in the Lanzhou Basin. The abandonment ages of terraces T9–T1 have been dated to 1.6 Ma, 1.5 Ma, 1.2 Ma, 1.1 Ma, 0.96 Ma, 0.86 Ma, 0.13 Ma, 0.05 Ma and 0.01

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Ma, respectively (Li et al., 1996; Pan et al., 2009). In addition, an additional

series of basins in the upper Yellow River were brought into the Yellow River

Basin through the processes of stream capture and river channel connection

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(Li et al., 1996). The present southeastward flow of the Huangshui River was

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established between 1.55 and 1.2 Ma, during which a major river system adjustment occurred in the Xining Basin (Lu et al., 2004; Vandenberghe et al.,

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2011; Wang et al., 2012). During 1.2–0.5 Ma, the river system patterns of the

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Qinghai Lake, Guide and Gonghe basins also experienced adjustment, potentially related to the crustal rebound caused by river erosion (Zhang et al.,

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2014). In this study, combing with the formation age of erosion surface (i.e., ~1.4 Ma) in this area (Zhang, 2006), our results showed that the formation of

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modern river valleys in the lower Taohe occurred at 1.4–1.14 Ma. In addition, the development of modern valleys in the upper Weihe River in the Longxi Basin began at 1.2 Ma (Gao et al., 2016, 2017). Many studies have suggested that the Kunhuang Movement (1.2–0.6 Ma) had caused strong downcutting of the rivers (e.g., the Yellow and Weihe rivers) in the Longzhong Basin (Pan et al.,

2009; Gao et al., 2016, 2017). Therefore, our results might further highlight the significant control exerted over regional landform and river system evolution by the Kunhuang Movement during the late Early Pleistocene on the northeastern TP (Cui et al., 1998). By comparing adjacent terrace ages and their altitudes in the Longzhong Basin, the rate of, and any variations in, river incision trends may be inferred

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over a given time interval (Fig. 7). During the Pleistocene, the river incised to a depth of 220–500 m, and river incision rates ranged from 0.03 to 1.5 m/kyr. Although river incision rates cannot be directly correlated with surface uplift

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rates, they may to some extent reflect any trends in that surface uplift (Maddy,

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1997). The increases in the topographic relief caused by the Gonghe Movement since the Late Pleistocene was the main reason for the rapid river incision rates

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observed in this area (Li et al., 1996). In general, however, regional river incision

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rates do not show obvious evidence for differential phases (Fig. 7), perhaps indicating a continuous and stable surface uplift during the Pleistocene in this

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area.

[Fig. 7. hereabouts]

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6.2. Fluvial processes and climate change The channel deposits of the lower Taohe terraces are dominated by

unconsolidated and loose gravels, but these have been transformed into conglomerate in some higher terraces. The upper overbank sediments are characterized by a variety of fine particles from coarse sand to clay. The

overbank sediments generally show some thin layers of horizontal bedding, and occasionally some crossbedding. The long-term arid and semi-arid climatic conditions, combined with the widely distributed red clay and loess found in this area, have clearly been beneficial to the development of the floodplain (Xu, 1965). Some views based on climatostratigraphy suggest that coarse terrace material was typical of glacial, braided rivers, while the fine-grained sediments

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characterized interglacial, meandering rivers (Büdel, 1982). This one-to-one correlation between fluvial geomorphologic activity and climate has been countered by several arguments, including the proposition that fluvial gravels

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can be formed in different climatic environments (Vandenberghe, 2001).

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However, it has also often been shown that, except for Holocene fluvial systems, coarse-grained terrace sediments in temperate regions formed mainly during

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cold periods (Pan et al., 2009; Vandenberghe, 2015; Wang et al., 2017).

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Moreover, our results would confirm this argument, i.e., that the formation ages of the terrace gravel layers found along the lower Taohe River correspond to

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the cold and dry periods of loess accumulation. The fluvial terraces of the Longzhong Basin are generally covered by

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hundreds of meters of aeolian loess. These loess sequences (and their interbedded soils) not only record the orbital-scale climate changes experienced over the past 2.5 My, but also could record and distinguish 102– 103 yr scale climatic events (Li et al., 1992; Liu and Ding, 1998). The cold-warm and dry-wet alternation of Quaternary climatic phases in this area is expressed

by the alternation between sedimentary and pedogenetic events. Although the loess and paleosol deposits have generally the same provenance, loess represents dust accumulation under dry and cold climatic conditions, whereas paleosols are the product of relatively warm and humid climatic environments. Studies of loess-paleosol stratigraphy along the Yellow River in the Lanzhou Basin have shown that the bottom of the overlying loess profiles, i.e., the top of

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the fluvial sediments, mainly corresponds to a paleosol (or occasionally a thin layer of loess) (Pan et al., 2009); this is also true of the Linxia Basin (along the

Daxia River; Li et al., 1997), the Longxi Basin (along the Weihe River; Gao et

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al., 2016), and along the lower Taohe River. These thin layers of loess at the

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bottom of the profile have poor lateral extension and gully erosion. These phenomena indicate that river incision and terrace formation in the study area

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occurred during the transitions between glacial and interglacial periods.

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During glacial periods, the TP was a cooling influence throughout the year. An enhanced winter monsoon above the TP greatly strengthened the East

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Asian Winter Monsoon (EAWM), while the southern branch of the Westerlies was maintained throughout the year. Meanwhile, the East Asian Summer

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Monsoon (EASM) was suppressed in the Yangtze River drainage basin. The water vapor transported by the EASM was unable to reach northwestern China, making the Longzhong Basin extremely arid. Regional vegetation was dominated by grassland and dry grassland. Slope erosion was enhanced by the poor vegetation cover, and significant quantities of sediment were

transported into the river systems, resulting in river aggradation (Fig. 8a). During interglacial periods, an enhanced EASM resulted in an increase in precipitation. The vegetation cover was principally forests or forest steppe, and, as the coverage increased, the sediment yield decreased significantly, increasing the water-sand ratio. The river systems adjusted to an equilibrium dominated by lateral erosion (Fig. 8c). In addition, abundant evidence indicates

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that the climatic transition between glacial and interglacial periods was rapid (e.g., Cheng et al., 2009; Denton et al., 2010). During these transition periods,

climatic instability would have been enhanced and characterized by frequent

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fluctuations between colder and warmer periods (Cheng et al., 2009). We

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suggest that an increase in seasonal, heavy precipitation and floods, combined with the resultant increase in vegetation cover and decrease in sediment yields,

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has enhanced river downcutting and terrace formation in the Longzhong Basin

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(Fig. 8b). However, the climate changed slowly and gradually between interglacial and glacial periods (Cheng et al., 2009; Denton et al., 2010). In this

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case, the climatic instability would not be enhanced significantly and caused significant river incision and terrace formation. Until the next glacial period, the

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river process would have once again been dominated by aggradation, causing the alluvial sediments accumulated during the former interglacial period to be retransported and redeposited (Bull, 2007). [Fig. 8. hereabouts] However, a global comparative study of river terraces has shown that

fluvial incision and terrace formation in the same region may have occurred either during glacial-interglacial transitions and interglacial-glacial transitions (Vandenberghe, 2008, 2015; Bridgland and Westway, 2014). For example, it has been suggested that incision of the Thames River occurred primarily at glacial to interglacial transitions (Maddy et al., 2001). However, studies of the Somme River showed that terrace formation mainly occurred during cooling

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transitions (Vandenberghe, 2008). In addition, the terrace sequences of the Huangshui River in the northwest of the Longzhong Basin (Figs. 1a and 7)

display multiple layers of soil within the overbank sediments, indicating warm

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climatic conditions. Although river incision during glacial-interglacial transitions

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would have resulted in no further coarse-grained gravel deposits being represented within the alluvial strata of the Huangshui River terraces, the

transitions

and

the

subsequent

interglacial

period

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glacial-interglacial

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accumulation of fine-grained overbank sediments mainly occurred during these

(Vandenberghe et al., 2011). During the following interglacial-glacial transition,

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floodplain deposition appears to have ceased completely (Vandenberghe et al., 2011; Wang et al., 2017). These studies might suggest that the incision models

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for different transition periods, and their terrace preservation capacities, are different (Vandenberghe, 2008, 2015). After glacial-interglacial transitions, interglacial rivers appear to be generally meandering, with limited lateral erosion, allowing the alluvial archives of the previous glacial period to be preserved. However, after interglacial-glacial transitions, the resultant braided

glacial rivers were generally characterized by wide and shallow channels, with frequent lateral migration. As a result, any interglacial river sediments would have been strongly eroded, or even totally removed (Vandenberghe, 2015). Glacial deposits are more likely to be preserved, potentially explaining why preserved alluvial layers often correspond to glacial periods. However, Bridgland and Westaway (2008, 2014) argued that the downcutting depth

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during interglacial-glacial transitions was relatively shallow and did not necessarily lead to the abandonment of the former riverbed and terrace

formation. Clearly, the terraces that developed along the Huangshui River and

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and preservation (Vandenberghe, 2015).

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in the Longzhong Basin may represent two different models of terrace formation

Since 1.14 Ma, 13 loess layers (L13–L1) and 13 paleosol layers (S12–S0)

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have developed on the CLP (Ding et al., 2002). If each of the pedogenic and

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sedimentary periods are taken as representative of one climatic event, these layers would indicate that there were at least 26 paleoclimatic events. Nine

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terraces have developed and been preserved in the lower Taohe River since 1.14 Ma. Using chronologic dating, we found that the gravels of terraces T9–T1

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roughly correspond to L13, L9, L8, L6, L5, L4, L3-L2, L1-4 and L1-1, respectively. The ESR sample from the gravel of T3 (279 ± 31 ka) was taken from the Yanjiaxiaozhuang Section near the Haidian Gorge, while the overlying basal paleosol of Terrace T3 in the Lintao Section corresponds to S1. A possible reason for this may be that the accumulation of the T3 gravel may have been

diachronous, i.e., the gravel was deposited during the L3–L2 period, while river incision and terrace formation occurred during the transition between L2 and S1. The exact age and the diachronism of Terrace T3 require further study. Nonetheless, the accumulation of gravels and the abandonment of terrace surfaces can be roughly coupled with the development of regional loesspaleosol pairings, indicating that orbital-scale climatic changes during the

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Quaternary period were an important control on the evolution of basin landforms. Some terraces corresponding to climatic cycles (such as L11–L10)

are missing in the study area, although they exist in other reaches of the

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Longzhong Basin, such as the Yellow River terraces in Lanzhou and the Weihe

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River terraces in the Longxi Basin. These missing terraces might exist in this area but have not yet been identified. Alternatively, very slow or rapid tectonic

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uplift might have negated their potential formation and/or preservation

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(Veldkamp and van Dijke, 2000). In addition, surface denudation, river erosion and climate can also play an important role in controlling the structure and

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topography of mountainous areas (Burbank et al., 2003). More comprehensive studies are needed to analyze in detail the processes controlling terrace

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formation and their responses to tectonic activity and climatic change. Such studies will allow a further exploration of the historical process and the factors that may control the evolution of river systems and the development of river valleys. 7. Conclusions

Based on field investigations, OSL, ESR, paleomagnetic dating and loesspaleosol stratigraphy, the formation ages of the nine river terraces in this area were approximated. The accumulation of gravels corresponds roughly to the age of loess accumulation, viz.: L13 (T9), L9 (T8), L8 (T7), L6 (T6), L5 (T5), L4 (T4), L3-L2 (T3), L1-4 (T2) and L1-1 (T1). The chronologic results show that the formation and development of the modern river valley in the lower Tahoe began

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at 1.4–1.14 Ma. The increases in the topographic relief caused by the Kunhuang Movement on the northeastern TP may be the main reason for the

reorganization of the area’s river systems. Orbital-scale Quaternary climate

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change appears to have been the main factor controlling lateral erosion and

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river system downcutting overprinted onto the effects of tectonic uplift.

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Acknowledgments

This work was supported by the Second Tibetan Plateau Scientific Expedition

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(Grant No. SQ2019QZKK2305), the National Natural Science Foundation of China (Grant Nos. 41730637 and 41471008) and the Nanhu Scholars’ Program

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for Young Scholars of XYNU. We thank Wentao Qi, Jianjun Cui and Hang Cui

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for their help with the fieldwork. We also thank Jef Vandenberghe, David Bridgland and one anonymous reviewer for very constructive comments and suggestions. We would like to express our sincere gratitude to Professor Scott A. Lecce for his hard work on this manuscript. We are grateful to Edward Derbyshire for revising the language of the manuscript.

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Zhang, H.P., Zhang, P.Z., Champagnac, J.D., Peter, M., Anderson, R.S., Kirby, E., Craddock, W.H., Liu, S.F., 2014. Pleistocene drainage reorganization driven by the isostatic response to deep incision into the northeastern Tibetan Plateau. Geology 42, 303–306. Zhang, W.L., Zhang, T., Song, C.H., Erwin, A., Mao, Z.Q., Fang, Y.H., Lu, Y.,

Meng, Q.Q., Yang, R.S., Zhang, D.W., Li, B.S., Li, J., 2017. Termination of fluvial-alluvial sedimentation in the Xining Basin, NE Tibetan Plateau, and its subsequent geomorphic evolution. Geomorphology 297, 86–99. Zhang, Y., 2006. The Neogene Deposition and Environmental Evolution in Southeast Longxi Basin. Doctoral dissertation. Lanzhou: Lanzhou University. (in Chinese with English abstract)

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Zhao, X.T., Jia, L.Y., Hu, D.G., 2018. Discoveries of fluvial terraces and Neogene gravels in the Hetao area, inner Mongolia: implications for the development of the Yellow River, antiquity of Chinese rivers, and coexistence

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theory of rivers and lakes. Acta Geol. Sin. 92, 845–886. (in Chinese with

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English abstract)

Zhu, J.J., Zhong, W., Li, J.J., Cao, J.X., Wang, J.M., Wang, J.L., 1996. The

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oldest eolian loess deposition in Longxi basin – Yandonggou profile in

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Lanzhou. Sci. Geogr. Sin. 16, 365–369. (in Chinese with English abstract)

Figure captions Fig. 1. (a) Location and tectonic units and faults of the study area. The red arrow indicates the paleo-channel of the Yellow River suggested by Clapp (1915). (b) The spatial distribution of the river terraces along the lower Taohe

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River. The triangles indicate the locations of ESR samples.

ro of -p re lP na ur Jo Fig. 2. Photograph (a) and schematic cross section (b) of the terrace sequence in the lower Taohe River near the Haidian Gorge. The location of the section AA′ is shown in Fig. 1b. The triangles indicate the locations of ESR samples.

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Details of these ESR samples are listed in Table 2.

Fig. 3. Schematic cross section (a) and spatial distribution (b) of the terrace

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sequences along the Taohe River Valley in the Lintao Basin. The location of the

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Fig. 3b is shown in Fig. 1b. YW: Yawan; CJH: Caojiahe; LJG: Liujiagou; GYHJ: Gaoyahejia; FJGM: Fengjiagoumen; TJH: Tianjiahe; MJW: Majiawan; GWJP:

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Guowangjiaping; XWJW: Xiawangjiawan.

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Fig. 4. Stepwise alternating field and thermal demagnetization diagrams of some representative samples from the Tianjiahe Terrace (T8). N, E and UP

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represent north, east and up. NRM and NRM0 are the natural and measured

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remanent magnetization, respectively. The samples T8-44, T8-4 and T8-2.5 are taken from the heights of 44, 4 and 2.5 m of the loess profile above the alluvial

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deposits on Terrace T8.

Fig. 5. Stratigraphy, grain size, declination, inclination and observed polarity

patterns for the loess sequences of terraces T8 (the Tianjiahe Terrace) and T5 (the Liujiagou Terrace). Md, Inc and Dec represent the medium grain size,

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inclination and declination, respectively.

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Fig. 6. Stratigraphy, grain size and OSL dating of the loess sequences of the

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Caojiahe Terrace (T3) and the Yawan Terrace (T2). The sample TH-03 was collected from the top of the basal paleosol of the Majiawan Terrace. The dark

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circle showed an approximately position of the sample TH-03 in the Majiawan

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Terrace. The marine oxygen isotope record (LR04 benthic δ 18O stack) is cited from Lisiecki and Raymo (2005). Locations of the OSL samples are shown in

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Fig. 3b.

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Fig. 7. Correlation of the incision rates for river terraces along the lower Taohe

River, the Yellow River in Lanzhou (Li et al., 1996; Pan et al., 2009), the Huangshui River (Zhang et al., 2017), the Daxia River (Li et al., 1997) and the

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Weihe River in the Longxi Basin (Gao et al., 2016), compared to the marine

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oxygen isotope climate record (LR04 benthic δ 18O stack; Lisiecki and Raymo, 2005) and loess-paleosol development (Ding et al., 2002). The polarity

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timescale is cited from Cande and Kent (1995). The terrace height is taken to

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be the height of the strath above the modern river level. Incision rates were calculated from one terrace to another. The tectonic events on the northeastern

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TP are derived from Li et al. (2014).

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Fig. 8. Schematic diagram of terrace development along the lower Taohe River. (a), (b) and (c) indicate the fluvial processes during the periods of glacial,

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glacial-interglacial transition and interglacial, respectively.

Table 1. OSL dating results for samples taken from the Taohe River Valley in the Lintao Basin. Water Sample

Depth

Grain

K

Th

U

ID

(m)

(µm)

(%)

(ppm)

(ppm)

Dose rate

De

OSL age

(Gy/ka)

(Gy)

(ka)

content (%)

9.5

38–63

1.96±0.051

11.5±0.319

2.86±0.103

15

3.21±0.23

61.5±2.55

19.1±1.6

TH-02

10

38–63

1.86±0.050

11.5±0.319

2.86±0.103

15

3.13±0.22

120.86±5.08

38.7±3.2

TH-03

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38–63

1.55±0.047

8.54±0.248

2.35±0.094

15

2.55±0.18

181.34±9.66

71.2±6.3

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TH-01

Table 2. ESR dating results for samples taken from terraces along the lower Taohe River. Water Sample Terrace

Location*

K (%)

Th (ppm)

U (ppm)

Dose rate

Equivalent

ESR age

(Gy/ka)

dose (Gy)

(ka)

content

ID (%) XZ

XZ-2

1.15±0.06

6.27±0.31

1.36±0.07

0.19

2.08±0.1

150.36 ±112.62

72.3±54.3

T3

YJXZ

YJXZ

1.03±0.05

5.29±0.26

1.27±0.06

0.19

1.87±0.09

522.99±52.62

279.2±31.4

T4

SHJ

XZ-3

1.37±0.07

6.31±0.32

1.41±0.07

0.19

2.3±0.12

1148.25±111.55

498.7±54.5

T5

SL

SL

1.28±0.06

5.17±0.26

1.14±0.06

0.36

2.07±0.1

1354.81±119.7

655.9±66.6

T7

ZZW

ZZW

0.71±0.04

4.44±0.22

1.48±0.07

0.19

1.56±0.08

2 1244.19±174.2

798.8±118.8

T9

LJW

LJW

0.98±0.05

4.59±0.23

1.11±0.06

1.56

1.71±0.09

7 1958.27±275.6

1142.8±170.7

T5

CBLG

CXLG-3

1.32±0.07

5.48±0.27

1.21±0.06

0.19

T6

CBLG

CXLG-2

1.66±0.08

5.62±0.28

1.22±0.06

0.17

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T2

2.15±0.11

1311.71±204.9

611.1±100.2

2.48±0.12

3 1681.58±355.4

677.8±147.2

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* Locations of the ESR samples are shown in Figs. 1b and 2. XZ: Xiazhuang; YJXZ:

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Yanjiaxiazhuang; SHJ: Shanghejia; SL: Suolin; ZZW: Zhuoziwan; LJW: Liujiawan; CBLG:

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Chubulagou.