Multiple Climatic Cycles Imprinted on Regional Uplift-Controlled Fluvial Terraces in the Lower Yalong River and Anning River, SE

Geomorphology 250 (2015) 95–112

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Geomorphology

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Multiple climatic cycles imprinted on regional uplift-controlled ﬂuvial terraces in the lower Yalong River and Anning River, SE Tibetan Plateau

Zexin He a, Xujiao Zhang a,⁎, Shuyan Bao a, Yansong Qiao b,c, Yuying Sheng a,XiaotongLiua,XiangliHea, Xingchen Yang b, Junxiang Zhao d,RuLiua,ChunyuLub a School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China b Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China c Key Laboratory of Neotectonic Movement and Geohazard, Beijing 100081, China d The Institute of Crustal Dynamics, China Earthquake Administration, Beijing 100085, China article info abstract

Article history: The development of fluvial systems on the southeastern margin of the Tibetan Plateau is linked to significant and Received 8 April 2015 rapid late Cenozoic uplift. The relatively complete fluvial terrace sequence preserved along the Yalong River val- Received in revised form 13 August 2015 ley and that of its tributary, the Anning River, provides an excellent archive for studying the development of ter- Accepted 18 August 2015 races in rapidly uplifting mountainous areas. This study reveals that terrace development is predominantly Available online 23 August 2015 controlled by multiscale climate cycles and long-term uplift, as shown by terrace dating, sedimentary characteristics, and incision rates. At least six alluvial terrace units were identified in 20 transverse sections through the Keywords: Terrace development terraces along about a 600 km length of river and were dated using Electron Spin Resonance (ESR) and Optically Tibetan Plateau Stimulated Luminescence (OSL). The climatostratigraphic positions of the terrace deposits and their respective Yalong River age constraints suggest that fluvial aggradation was concentrated during Marine Isotope Stages (MIS) 32, 22, Anning River 18, 4, 2, and the Younger Dryas (YD) and that incision occurred during the succeeding cold-to-warm transitions. Multiclimate cycle The changes in fluvial style marked by terraces 6, 5, and 4 predominantly occurred in synchrony with the 100-ka Uplift-driven valley incision Milankovitch climate cycles, while terraces 3 and 2 were controlled by the obliquity-driven 41-ka climate cycles. Finally, the aggradation of terrace T1 occurred in response to the YD stadial. During the intervening time between 0.72 and 0.063 Ma, terraces either did not form or were not preserved, which may suggest that uplift rates varied through time and influenced terrace formation/preservation. The progressive valley incision recorded by these fluvial terraces cannot be entirely explained by climate cycling alone. Temporal and spatial variations in incision rates indicate that the continuing long-term incision has been driven by uplift. The temporal distribution of the incision rates reveals two rapidly uplifting stages in the southeastern Tibetan Plateau, including an accelerated uplift that has been taking place since 0.06 Ma. The spatial distributions of differing incision rates reflect the geomorphological response to crustal shortening and differential uplift in this region. © 2015 Elsevier B.V. All rights reserved.

1. Introduction aggradation are controlled by either the intrinsic dynamics of the fluvial system or by extrinsic variables, including climatic cycling, regional up- Fluvial terraces represent fluvial bedforms/barforms of channel lift, and changes to the river's base level (Schumm, 1973; Antoine et al., and floodplains (Merritts et al., 1994) and are found in river valley 2000; Lewin and Macklin, 2003; Vandenberghe, 2003; Bridgland et al., sides around the world, spanning the entirety of the late Cenozoic and 2004). In addition, such variations in fluvial style (river capture/diver- recording hundreds to millions of years of fluvial changes. The land- sion) may be caused by changes in the size of the river itself occurring forms and sediments together provide a unique archive of climate on different temporal and spatial scales (Merritts and Vincent, 1989; change, tectonic activities, volcanic activities, geomorphological evolu- Hovius, 1999; Houben, 2003; Erkens et al., 2009). Although various in- tion, palaeohydrogeology, and ancient human activities (Bridgland, ternal factors can lead to changes in the behaviour of a fluvial system 2000, 2006; Westaway et al., 2004, 2006, 2009; Boenigk and Frechen, as a result of varying sediment supply and transport capacities, many 2006; Scharer et al., 2006; Starkel et al., 2007; Veldkamp et al., 2007, scholars have noted that terrace formation from such autogenic controls 2015; Erkens et al., 2009; Perrineau et al., 2011; Hu et al., 2012; Stokes tends to occur on relatively small temporal (10–1000 years) and spatial et al., 2012a,b). Variation in the patterns of fluvial incision and (10–100 m) scales (Brown, 1991; Blum and Törnqvist, 2000; Maddy et al., 2001; Houben, 2003). Moreover, difficulties remain in identifying fl ⁎ Corresponding author. the internal factors that caused the changes in uvial style that created E-mail address: [email protected] (X. Zhang). relatively older terraces because of the limitations of terrace

http://dx.doi.org/10.1016/j.geomorph.2015.08.010 0169-555X/© 2015 Elsevier B.V. All rights reserved. 96 Z. He et al. / Geomorphology 250 (2015) 95–112 preservation and dating accuracy. Thus, climate change, regional uplift, the Yalong River valley and that of its tributary, the Anning River, offer and base level variation should be considered the primary causes of ter- the critical evidence that can help in understanding terrace formation race formation. In the lower reaches of a river, eustatic change is likely in uplifting areas. Thus, revealing the relationship between regional up- to result in an alternation of cutting and filling processes (Maddy, lift and geomorphological evolution may be possible through an exam- 1997; Karner and Marra, 1998; Tömqvist, 1998; Maddy et al., 2001; ination of the ages and formation processes of the terraces in the Bridgland and Westsway, 2008a). Additionally, river downcutting southeastern margin of the Tibetan Plateau. The fluvial style and terrace tends to occur in downstream areas and progresses upstream via development of the lower Yalong River and Anning River basins differ knickpoint recession during glacial epochs, while fluvial aggradation from those of other rivers worldwide, which may be a result of the espe- may occur during interglacial stages, accompanying a rise in sea level. cially intensive uplift of the Tibetan Plateau. In the region, terraces of the Notably, however, the above trends only apply to the downstream River Yangtze farther downstream in and around the Sichuan basin reaches of rivers near the coasts (Antoine et al., 2000, 2003; Maddy have been documented in detail (Li et al., 2001). Previous studies fo- et al., 2001). As such, the larger-scale terrace generation that occurs in cused on the shorter timescale terrace formation in the upper branch, valleys far from the coast is likely to be more closely related to climate lower of the Yalong River, and its tributary, Anning River (Li et al., cycles and uplift. 1984; Wang et al., 1998; Xu et al., 2003; Cheng, 2010); the older terraces The Quaternary is characterised by high amplitude rhythmic remain to be seen. fluctuations in global temperatures, and the glacial–interglacial climate This study enables comparison of terrace sequences between the cycle is thought to be the main force driving the filling–cutting behav- lower Yalong River and the Anning River basin; obtains the first quanti- iour in river valleys (Vandenberghe, 2003, 2008; Bridgland and tative chronology for the long timescale terrace staircase in this region Westsway, 2008a). Previous studies have shown that Milankovitch by Electron Spin Resonance (ESR) and Optically Stimulated Lumines- scale (Antoine, 1994; Vandenberghe and Maddy, 2001; Pan et al., cence (OSL) dating methods; and combines geomorphological, sedi- 2003; Maddy et al., 2005) and sub-Milankovitch scale (Starkel, 2002; mentary, and stratigraphic evidence to characterise the local fluvial Hu et al., 2013) climate cycles strongly influence this filling–cutting be- sequences. Analysis of these data can provide a conceptual model for haviour. This model further suggests that the shift is influenced by direct fluvial terrace formation in the rapidly uplifting mountain areas around climatic forcing, such as peak precipitation, as well as indirect forcing, the margin of the Tibetan Plateau. Additionally, our analyses have such as vegetation cover (Vandenberghe, 2003), which together induce attempted to use the incision by these river systems to quantify uplift. changes in river discharge and sediment delivery. During glacial periods, a cold environment leads to sparse vegetation cover, increasing 2. Study area the generation and deposition of massive coarse sediments caused by the limited transport capability of river systems. However, The Yalong River, which has a total length of 1571 km and a fall in while the climate-driven river aggradation and incision may directly altitude of 3180 m, is the largest tributary of the Jinsha River. Its source correlate with terrace generation in regions where the crust is relatively is on the southern flank of Bayan Har Mountain, and it feeds into the stable (Bridgland, 2000; Maddy et al., 2001; Bridgland et al., 2004; Jinsha near Panzhihua city in Sichuan Province. The study area for this Westaway et al., 2009), the formation of large-scale, multilevel fluvial investigation comprises the lower reaches of the Yalong River and its terraces in strongly uplifting mountain areas remains controversial. tributary, the Anning River (Fig. 1A,B). The topography in this region The interaction of uplift and climate change in the development of is dominated by alpine valleys with a relief of more than 2000 m, and fluvial terraces in intensely uplifting mountainous areas is still poorly the northwest of the area is (on average) higher than the southeast. understood (Pan et al., 2009) and has become an increasingly popular Specifically, the middle reaches of the Anning is characterized by rela- focus for studies in fluvial geomorphology. Some authors have sug- tively broader valley areas, while structurally it represents a fault gested that terrace development in these regions has been largely con- basin (Fig. 1C). The study area is located at the intersection of the area trolled by intermittent uplift, and accordingly, little attention has been affected by the Indian Ocean monsoon, southeast monsoon, and plateau paid the contribution of climatic cycles (Cheng et al., 2002; Lu et al., monsoon and experienced glacial activity during the Quaternary period 2004; Cunha et al., 2005, 2008; Sun, 2005). However, other researchers (Wang et al., 2012). The river systems were glaciated, which is signifi- claim that such terraces are the result of the interaction between cli- cantly important to constrain the timing of sediment supply linked to mate and uplift affecting fluvial sedimentation and incision (Pan et al., climate change. 2003; Starkel, 2003; Bridgland et al., 2004; Maddy et al., 2008; Wang The basement rocks in the study area primarily consist of rocks of et al., 2010). Still further studies have suggested that terrace staircases ages from the Archaean to Cenozoic, in addition to early Carboniferous record repeated incision in response to progressive epeirogenic uplift, and Cretaceous. Quaternary units here comprised the lacustrine strata which, alternating with aggradation, is thought to have been triggered of the early Pleistocene Xigeda Group, glacial deposits of the middle by climatic changes (Maddy, 1997; Pan et al., 2003). Overall, regional Pleistocene Dajingliangzi Group, and fluvial facies of the late Pleistocene uplift is widely considered a dominant driver of terrace development Tongzilin Group (Chen and Zhao, 1988). The Xigeda Lake basin, which in rapidly uplifting mountain areas, but a key question remains of how developed during the Pliocene and early Pleistocene, was replaced by strongly climate change, especially in the form of multiple climatic cy- Yalong and Anning (Jiang et al., 1999). Structurally, the study area ex- cles, affect the formation of river terraces in rapidly uplifting mountain tends across the Songpan–Ganzi orogenic belt and the Yangtze block areas. In the typical rapidly uplifting mountain of the Alaknanda valley and represents a complex tectonic deformation system combining the near the Central Himalaya, the fluvial aggradation and incision were N–S trending Anninghe–Zemuhe arcuate fault zone and the NNE- modulated by monsoon variability and the role of tectonics was subor- striking Jinping mountain thrust belt (Zhang et al., 2003)(Fig. 1A,B). dinate, limited to providing accommodation space and post-deposition As a result, active faults are widely distributed, and tectonic activity is modification of the fluvial landforms (Juyal et al., 2010). intense in the study area. Under the background of continual India–Eur- The development of fluvial systems on the southeastern margin of asia collision during the Miocene, the southeastern margin of the Tibet- the Tibetan Plateau is related to rapid and significant late Cenozoic uplift an Plateau underwent major tectonic deformation and uplift, as its (Zheng et al., 2013). The Yalong River is one of the major river systems lateral extrusion was blocked by the rigid Yangtze block. The Tibetan in this region and is characterised by alpine gorges crossing the two Plateau has been experiencing accelerated uplift since the beginning major topographic levels of China (Li et al., 2001). The area has experi- of the Quaternary period (Pan et al., 2004), with the intensity of tectonic enced strong crustal deformation, intense uplift, and frequent climate deformation and uplift amplitude varying from plate to plate. fluctuations (influenced by the plateau monsoon and Asian monsoon) Previous studies (Qiao et al., 2013) have revealed that the southeast- throughout the Quaternary. As a result, the river terraces preserved in ern margin of the Tibetan Plateau underwent strong tectonic Z. He et al. / Geomorphology 250 (2015) 95–112 97

Fig. 1. (A) Map of the Tibetan Plateau showing principal active faults and the location of our study area (modified after Tapponnier et al., 2001); (B) topography and principal faults of the SE margin of the Tibetan Plateau regions, 1. Xianshuihe fault, 2. Anninghe fault, 3. Zemuhe fault, 4. Litang fault, 5. Jinpingshan fault, 6. Jinhe–Jinghe fault, 7. Longmenshan fault, 8. Longquanshan fault; (C) geomorphological settings of the lower Yalong River and Anning River basin and terrace section sites. deformation and uplift during 1100–720 ka. The tectonic mode of the was shown to be more than 3.3 ± 0.8 mm a−1 (Tan et al., 2010), and Anning basin switched during the Pliocene and early Pleistocene from stratigraphic studies of aeolian deposits on the Chinese Loess Plateau stable slip-extensional deformation to a strike-slip extrusion (Zhang also suggest rapid uplift of the Tibetan Plateau between 1.1 and et al., 2003), with the Xigeda Formation and Yuanmou Formation 0.9 Ma (Sun and Liu, 2000). This area has also experienced strong tec- experiencing extensive structural extrusion at this time (Chen and tonic activities since 0.06 Ma, which are known regionally as the Latest Zhao, 1988; Zhang, 1994). The Jiawa Formation and Changtai Formation Tectonic Movement (Zhang, 1999). At this time, the regional tectonic in the Litang region are truncated by an unconformity that indicates an stress field was transformed into one of tectonic compression (Zhang important uplift event (Zhang, 1994). The presence of aeolian loess in et al., 2003); while significant sinistral strike-slip faults were formed the Ganzi basin may also indicate that the southeastern margin of the in the Anning basin, and the Dajingliangzi region underwent strong Tibetan Plateau was rapidly uplifted during this period (Qiao et al., tilting uplift (Wen and Wu, 1983). However, between 720 and 63 ka 2007). The uplift rate of Mount Gongga relative to the Sichuan basin the southeastern margin of the Tibetan Plateau was in a relatively stable 98 Z. He et al. / Geomorphology 250 (2015) 95–112 state of weak extension with relatively small incision rates. During this dating method applied to river terraces (Stokes et al., 2012a,b). This stage, normal faults were formed along the boundaries of tectonic do- technique determines the time since quartz and feldspar sand grains mains, leading to the formation of a number of valley landforms. Mean- were last exposed to daylight and is routinely used to determine terrace while, the Yuexi basin, Ganluo basin, and Hanyuan basin were filled by deposits of up to 200 ka (Duller, 2004). Thus, the samples from the older sedimentary deposits during the middle and late Pleistocene (Zhang terrace deposits (T6, T5, and T4) were dated using the ESR method. Dat- et al., 2010). ing by ESR analyses of samples ESR-D-037-1 and ESR-D059-1 were per- formed by the Institute of Nuclear Technology at Chengdu University of 3. Methods Technology, while the remainder of the samples were analysed at the laboratory of Earthquake Dynamics, China Earthquake Administration. 3.1. Field investigation Seven samples from the younger terraces (T3, T2, and T1) were dated using OSL at the Institute of Crustal Dynamics, China Earthquake Stretches of river with well-developed fluvial terraces were chosen Administration. from within the lower Yalong valley and the Anning valley using 1:50,000 scale topographic maps, remote sensing images, and Google Earth images. These areas were subjected to detailed geomorphological 3.3.1. ESR dating investigation prior to field investigation. In the field, GPS handheld re- The samples received pretreatment in the laboratory, the dose rate ceivers (equipped in barometric altimeter with temperature compensa- (D′) was calculated from the concentrations of uranium (U), thorium tion of 1 m accuracy) were used in combination with 1:50,000 (Th) and potassium (K) of each sample (Aitken, 1998). Uranium and topographic maps to measure the elevations of the terrace surfaces thorium contents were obtained using a thick source Daybreak 530 (not including the thickness of colluvium and tributary fan) and the riv- Model alpha counter. The potassium oxide content was determined by erbed, as well as the thicknesses of the gravel layers beneath the terrace atomic absorption. Cosmic dose rates were calculated on the burial surface. Additionally, small-scale observations and measurements were depth of the sample (Prescott and Hutton, 1988). The ESR measure- made to document lithology, sediment structure, size, sorting, resis- ments were carried out on a BRUKER ER041XG X-band spectrometer tance to rounding, the degree of weathering, and the orientations of in a finger dewar cooled to 77 K with liquid nitrogen. The experimental AB-planes in representative gravel layers. These data were used in com- parameters were microwave power 5 mW and modulation amplitude bination with fluvial terrace geomorphology to produce transverse sec- 0.16 mT. The Ti-center intensity was measured from the top of the tions through the terraces and to identify major terrace divisions. peak at g = 1.979 to the bottom at g =1.913(Rink et al., 2007). All of Terrace correlation was executed according to the terrace elevations the samples were measured three times in different directions to obtain above river level, the continuity of landform surfaces, and sediment the average intensity. The equivalent dose (DE) values and their individ- composition and weathering characteristics (soil development and ual errors were determined from the dose response data fitted with a gravel weathering rind). single saturating exponential (SSE) function using the protocol (and the software) described by Yokoyama et al. (1985). Table 1 shows the 3.2. Sample collection ESR dating results and accompanying analytical data. This method tends to yield errors in age estimates on the order of 10–15%. In order to obtain the timing of terrace abandonment, samples for dating were collected from exposures of representative fluvial sediments, including the silt lenses and silt and clay layers that are devel- 3.3.2. OSL dating oped in the upper parts of the terraces. Before collection, at least The samples received pretreatment in the laboratory, and their OSL 20 cm thickness of the weathered surface was first removed to mini- signals were measured using automatic Risø DA-20-TL/OSL instruments mize contamination of the sample material. Sediments were obtained (Denmark). The protocols used to measure the quartz equivalent doses through the insertion of steel pipes, with a diameter of 6 cm and a (DE). The DE of samples OSL-D005-1, which were analysed in 2012, length of 20 cm, into the fluvial strata, which were sealed immediately were determined using the SAR dating protocol (Murray and Wintle, after extraction. Fourteen representative samples from the upper ter- 2000, 2003) because it consisted of coarse sand and could not be sieved race deposits were selected for dating; their locations are shown in to separate the 4–11 μm size fraction; and the DE of the other samples, Tables 1 and 2. which were analysed in 2012, were determined using the SMAR dating protocol (Wang et al., 2005) owing to the particle sizes of the silt. The 3.3. OSL and ESR dating environmental dose rate was measured using a variety of techniques. The U, Th and K level samples were measured using an element plasma As a result of the time span involved and the empirical evidence pre- mass spectrum analyser. The cosmic ray contribution to the dose rate served in the terraces, two different dating methods were used to con- was calculated at the same time (Prescott and Hutton, 1994). The envi- struct the terrace chronosequences. Quaternary sediments, especially ronmental dose rate was then calculated based on the absorbed dose the deposits older than 200 ka, can be dated by ESR, which provides a rate of the quartz and the transformational relationships among the U, first attempt at establishing an age control for early-middle Quaternary Th, and K concentrations (1998). The OSL ages, equivalent dose values, (Stokes et al., 2012a,b). Dating by OSL has become the main absolute and the dose rates obtained are presented in Table 2.

Table 1 ESR dating parameters and results of terrace samples.

Sample no. Sample location U (μg/g) TU (μg/g) K2O (%) Water (%) Equivalent dose (Gy) Dose rate (Gy/ka) Age (ka) ESR-D037-1 T6 at Jingying 2.26 ± 0.22 7.01 ± 0.70 2.06 ± 0.20 5 4414.6 4.082 1081 ± 100 ESR-D038-1 T6 at Jingying 1.89 14.0 1.76 6 3267 ± 184 3.12 1047 ± 104 ESR-D121-1 T5 at Jinchuan 6.12 33.8 1.59 22 3946 ± 512 4.28 922 ± 120 ESR-D059-1 T5 at Chengxiang 2.64 ± 0.26 8.20 ± 0.82 1.96 ± 0.19 5 4600.0 4.923 934 ± 90 ESR-D037-2 T4 at Jingying 2.63 14.4 1.87 13 2228 ± 222 3.14 710 ± 71 ESR-D060-1 T4 at Chengxiang 3.54 24.4 1.89 14 2810 ± 337 3.90 721 ± 86 ESR-D025-2 T4 at Yakou 2.53 13.7 1.41 16 1903 ± 190 2.60 732 ± 73 Z. He et al. / Geomorphology 250 (2015) 95–112 99

Table 2 OSL dating parameters and results of terrace samples.

Sample no. Sample Measuring Measuring grain size U Th K Water Environmental dose Equivalent dose Age (ka) location technique (μm) (μg/g) (μg/g) (%) (%) (Gy/ka) (Gy)

OSL-D005-1 T2 at Huatan SAR 90–125 2.21 15.2 2.23 5 3.68 66.12 ± 1.83 18.19 ± 1.89 OSL-D002-5 T2 at Tongzilin SMAR 4–11 1.92 13.1 1.92 8.19 4.03 88.18 ± 3.30 21.89 ± 2.34 OSL-D106-1 T2 at Yantangwan SMAR 4–11 3.14 13.8 2.95 12.96 5.35 118.24 ± 10.97 22.08 ± 3.01 OSL-D115-1 T3 at Yantangwan SMAR 4–11 3.15 14.4 2.99 11.04 5.50 347.15 ± 9.30 63.06 ± 6.53 OSL-D095-1 T3 at Jingjiu SMAR 4–11 2.71 13.1 2.26 9.06 4.61 320.47 ± 23.15 69.46 ± 8.57 OSL-D072-2 T3 at Lugu SMAR 4–11 3.35 13.9 1.85 16.87 4.19 344.99 ± 25.66 82.41 ± 10.27 OSL-D079-1 T3 at Lugu SMAR 4–11 2.19 12.5 2.02 13.86 3.97 163.15 ± 12.11 41.10 ± 5.12

4. Results environment and the process of soil development (Birkeland, 1999). The highly weathered red paleosols from the surfaces of terraces T6, 4.1. Lower Yalong and Anning terraces T5, and T4 confirm the temporal succession of these terraces (Fig. 3B) and indicate a warm and humid climate at the time of their formation. In order to ascertain the distributions of fluvial terraces, establish a In contrast, the brown or grey paleosols from the surfaces of terraces complete terrace sequence, and enable comparison of terrace sequences T3, T2 (Fig. 3E), and T1 indicate their more recent formation, as well between the lower Yalong and the Anning basin, 20 cross-valley sec- as their development in a relatively colder climate. tions along both of these rivers were measured and described in detail The terrace heights above the contemporary channel bed increasing through field investigation. Seven sections were measured in the gradually from the lower to the upper reach display a diffusion trend lower Yalong basin, while 13 sections were measured in the Anning (Fig. 2), and the vertical separation between terrace trends in upper basin where well-developed flights of terraces can be found. The num- reach is greater than the lower reach. Strath terraces T8 and T7 have ber of sections was lower in the former basin as a result of objective lim- highly weathered and thick red paleosols. They are best preserved in itations, including poorer terrace preservation in the V-shaped valley, the northern reaches of the lower Yalong near Jingying (Fig. 3B) and the influence of reservoir inundation caused mainly by the Ertan hydro- Qianfeng villages, but no fluvial deposits are preserved and we cannot power station, and poor access. The relative terrace heights above the obtain dating data. Several bedded karst caves (karstic system with geo- present-day rivers and the spatial distributions of the terrace sequences morphology records) are developed in the bedrock of the T8 terrace, are shown in Table 3 and Figs. 2 and 3. most likely as a result of the long-term effects of corrosive groundwater In general, terrace staircases are less prominent along the lower (Fig. 3B). The T6 is a cut or strath terrace, with bedrock consisting almost reaches of the Yalong, probably because the river expends most if not entirely of Xigeda Formation deposits. It is a well-developed terrace all of its erosive capacity in achieving the incision required to maintain with a distinct staircase topography, located 395 m above the contem- its gradient, with none spared for lateral erosion. This has created a pre- porary channel near Jingying village (Fig. 3B). The fluvial deposits com- cipitous and narrow valley with limited space for the preservation of prise a 20-m-thick channel facies sand–gravel layer that is overlain by a river sediments and alluvial landforms. Six strath terraces are identified 3- to 5-m-thick red clay intercalated by highly weathered gravels. Nota- in a few relatively broader valley areas, such as at river bends or conflu- bly, the fluvioglacial deposits discovered at the trailing edge of the T6 ences; these areas are characterised by lateral or vertical erosion, and terrace consists of a sandy gravel layer in the upper part and a only a relatively thin veneer of fluvial sediment is preserved. In particu- yellowish-brown coarse sand–gravel bed with 60% coarse sand in the lar, in the middle reaches of the lower Yalong, which is characterised by lower part. Within this sand–gravel bed, the long axes of the gravels deep canyons, the treads of the terrace staircase that are benches are typically range from 3 to 5 cm in length. The well-sorted and poorly narrow. The fluvial deposits above them are rarely conserved, meaning rounded gravels are not bedded and constitute a single component that a thick fluvial gravel layer is lacking in these cases (Fig. 3A). The with medium-coarse sand lenses and a significantly weathered red terrace surface deposits form a chronosequence that indicates the clay layer at the top. Within the light yellowish-brown coarse sandy

Table 3 The distribution and correlation of the terraces in the lower Yalong River and Anning River.

Classiﬁcation Terraces sections S = strath terrace F = Fill terrace (elevation / m)

Geographic coordinates T1 T2 T3 T4 T5 T6 T7 T8

Terraces of lower Yalong River Huatan 26°38′50.89″N, 101°49′26.86″E S 17 S 32 S 89 S 137 S 245 Tongzilin 26°43′32.34″N, 101°51′18.44″E S 17 S 38 S 103 S 153 S 245 S 294 Jinhe 27°42′47.29″N, 101°56′30.52″E S 13 S 43 S 109 S 170 S 239 S 363 Tianba 28°13′40.82″N, 101°51′17.13″E S 20 S 55 S 115 S 270 S 366 S 395 Jingying 28°16′17.81″N, 101°51′28.12″E S 16 S 36 S 115 S 149 S 252 S 385 S 478 Qianfeng 28°21′5.46″N, 101°52′40.54″E S 15 S 143 S 285 S 380 S 412 S 532 Luning 28°26′41.52″N, 101°52′10.80″E S 20 S 45 S 130 S 160 S 305 S 400 Terraces of Anning River Deshi 26°43′40.27″N, 101°52′30.63″E S 18 S 55 S 100 S 151 S 245 S 294 Yakou 26°47′51.00″N, 101°59′35.46″E S 39 S 90 S 136 Panlian 26°52′3.39″N, 102° 6′12.60″E F 6 F 16 F 67 F 138 S 225 S 271 Yonglang 27° 7′12.45″N, 102°14′3.16″E F 7 F 23 F 60 S 130 S 229 S 294 Jinchuan 27°11′26.67″N, 102°16′52.62″E F 11 F 27 F 50 F 110 S 242 S 306 Manshuiwan 27°19′40.28″N, 102°18′45.33″E S 18 S 31 S 50 S 148 S 228 S 280 Yantangwan 27°23′28.21″N, 102°13′0.43″E F 8 F 24 F 66 F 120 S 161 S 323 Mali 27°31′0.95″N, 102°11′32.82″E F 9 F 20 F 51 S 104 S 178 Jingjiu 27°44′32.57″N, 102°10′23.83″E F 4 F 24 F 78 Xingsheng 28° 0′45.04″N, 102° 8′24.19″E F 13 F 39 F 116 S 177 Yuehua 28° 7′54.94″N, 102° 9′23.02″E F 13 F 42 F 100 Lugu 28°17′11.88″N, 102°11′9.69″E F 14 F 39 F 97 S 173 S 237 Chengxiang 28°33′4.25″N, 102°10′59.22″E F 10 F 40 F 84 S 156 S 187 S 245 100 Z. He et al. / Geomorphology 250 (2015) 95–112

while the lower deposits are the fluvial gravels that cover the superficial sediments on the underlying slope. Topographically, the Anning basin shows a series of parallel flat surfaces distributed regularly along the valley at increasing altitudes above the present-day floodplain (Fig. 2). The widely developed strata of the Xigeda Formation in the Anning basin serve as an excellent bedrock for the filling-and-cutting processes of terrace formation. Six stepped fluvial terraces are clearly evident along the lower and upper reaches, whereas only three terraces are developed in the middle reaches because of the faulted basin (Fig. 3E). The soil features in the surface deposits of the terraces are similar to those of the Yalong terraces. The older terraces, T4 to T6, are usually classified as cut or strath terraces with highly weathered red soil; while T1 to T3 are mostly characterised as fill terraces (Leopold et al., 1964) with weakly weathered brown or grey soil. The entire terrace sequence and soil features are consistent with the Yalong terraces. This allows reliable correlation of the terraces for relative dating. Terrace T6 is a continuous stepped surface that is located ~245 m above the riverbed, which consists of gravels with a thickness of more than 5 m near Chengxiang (Fig. 3F). However, other sections show no fluvial gravels preserved in the T6 terrace. The T5 terrace comprises 3- to 4-m-thick fluvial sand and gravel deposits resting on the bedrock. The gravel weathering rinds in this terrace are up to 5–6 mm thick at the Jinchuan site (Fig. 3G). The T4 is a widespread and continuous terrace developed in the upper and lower reaches of the river basin. There- fore, it may be regarded as a geomorphological marker for terrace correlation. At Yakou, T4 has fluvial deposits with a thickness of around 10 m, consisting of relatively well-rounded and well-sorted gravels (Fig. 3H). The T3 terrace represents a broad tread upon which are Fig. 2. Longitudinal profile of terraces along the lower Yalong River (A) and Anning River many towns and villages. In particular, in the middle reaches the T3 (B). tread is 1–2 km wide and around 98 m above the river level (Fig. 3E). Exposures at Lugu (Fig. 3I) show the fluvial deposit to be ~70 m thick. gravel bed, the long axes of gravels typically range from 1 to 3 cm in The well-rounded and well-sorted gravels have a consistent orientation, length and comprise a single component of poorly rounded grains. Im- show an almost imbricate structure, and are weakly weathered with 1- portantly, these observations highlight the relationship between the to 2-mm-thick weathering rinds. These fluvial successions can be divid- contemporaneous facies of mainstream fluvial sediments and the ed into two bipartite alluvial architectures; the thicknesses of the upper fluvioglacial deposits, which together indicate a cold sediment environ- and lower fluvial deposits are 55 and 15 m, respectively. The upper fine- ment. The T5 is a cut or strath terrace with a relatively narrow geomor- grained deposits consist of grey to greyish-yellow silt and silty clay phological surface occurring near the Jingying site. It is 252 m above the layers. The T2 and T1 terraces are well-developed and continuous riverbed and consists of an 8-m-thick fluvial sand-gravel layer with sig- stepped surfaces that contain thick fluvial successions with typical bi- nificantly weathered gravels overlain by a red clay. The T4 terrace is the partite alluvial architecture (Fig. 3J). However, the base of the gravel most developed and continuous geomorphological surface in the re- layer is not observed except in the terraces in the lower reaches, gion, and it is especially well preserved in the north and south catch- which are classified as strath terraces, where no fluvial gravels remain ment areas. It is a cut or strath terrace located 153 m above the in a few sections. contemporary channel with a wide topographic platform on Xigeda For- mation bedrock at the Huatan site in Yanyuan City (Fig. 3C). On the basis 4.2. Age determination of the fluvial terraces of geomorphological correlation, the ancient strath that occurs along the right bank of the Yalong was also formed during the T4 lateral erosion Table 4 presents a comparison of formation ages of the Yalong, stage. The composition of T3 varies markedly at different locations, Anning, Yangtze, and Yellow river terraces. The ages of ESR-D037-1 from a comparatively thick sandy gravel layer to no fluvial gravel at all and ESR-D038-1, obtained from the middle and upper fluvial deposits in some instances. The terrace is classified as a bedrock terrace at the of the T6 terrace at the Jingying site (Fig. 3B), are 1081 ± 100 and Tongzilin site (Fig. 3D), but as a strath terrace at the Huatan and Jingying 1047 ± 104 ka, respectively. Yao et al. (2007) obtained an age of site. The T2 and T1 terraces have widely been adopted as road surfaces, 1290–1010 ka from the fluvial gravel layer in terrace T6 at the Mianning and several terrace deposit exposures have been created during road site. Additionally, a palaeomagnetic age obtained from the lowermost construction; these can be used to investigate the detailed alluvial archi- loess deposited on the T6 terrace tread in Ganzi was found to be tecture of the terraces. The T2 strath terrace consists of an 8.5-m-thick ~1160 ka (Qiao et al., 2007). Based on the above data, the formation fluvial deposit and is characterised by four cut-and-fill units, each com- age of the T6 terrace is estimated to be ~1100 ka. The ESR dating results prising a lower bedload deposit overlain by an alluvial floodplain depos- of the two representative samples collected from T5 at the Jinchuan it at the Huatan site (Fig. 3C). In this case, the bipartite structures (Fig. 3G) and Chengxiang (Fig. 3F) sites are 922 ± 120 and 934 ± include lower coarse-grained fluvial sediments, upper fine-grained de- 90 ka, respectively, which is consistent with the palaeomagnetic age posits, and a reactivation channel filled with gravel, which has eroded of 0.85–0.90 Ma of the lowermost loess deposited on the T5 terrace into the older fine-grained overbank deposits. The T1 is classified as a tread in Ganzi (Qiao et al., 2010). Thus, the estimated formation age of strath terrace and is distributed widely within the southern catchment, the T5 terrace is ~900 ka. Three silt samples collected from the upper- but only narrowly in the north. Within it, bipartite alluvial architecture most fluvial deposits of the T4 terraces at the Jingying (Fig. 3B), typically is developed, up to a thickness of 2 to 3 m at the Huatan site. Chengxiang (Fig. 3F), and Yakou sites (Fig. 3H) yielded ESR ages of The upper deposits comprise coarse sand layers with small pebbles, 710 ± 71, 721 ± 86, and 732 ± 73 ka. Thus, the average age of T4 Z. He et al. / Geomorphology 250 (2015) 95–112 101

Fig. 3. Transverse section through the terraces of the lower Yalong and Anning Rivers. (A) Jinhe profile. (B) Jingying profile. (C) Huatan profile. (D) Tongzilin profile. (E) Jingjiu profile. (F) Chengxiang profile. (G) Jinchuan profile. (H) Yakou profile. (I) Lugu profile. (J) Yantangwan profile. 102 Z. He et al. / Geomorphology 250 (2015) 95–112

Fig. 3 (continued).

terraces is considered to be ~720 ka. Samples OSL-D115-1, OSL-D095-1, which corresponds to Marine Isotope Stage (MIS) 4. We obtained and OSL-D079-1 collected from the uppermost ﬂuvial deposits of the T3 three samples (OSL-D005-1, OSL-D002-5, and OSL-D106-1) from the terrace at Yantangwan (Fig. 3J), Jingjiu (Fig. 3E), and Lugu (Fig. 3I) sites upper ﬂuvial silts of the T2 terrace in Huatan (Fig. 3C), Tonglinzi yielded OSL ages of 63.06 ± 6.53, 69.46 ± 8.57 and 41.10 ± 5.12 ka. (Fig. 3D), and Yantangwan (Fig. 3J). These yielded formation ages of Therefore, the T3 terrace is estimated to have formed around 63 ka, 18.19 ± 1.89, 21.89 ± 2.34 and 22.08 ± 3.01 ka, thus the estimated Z. He et al. / Geomorphology 250 (2015) 95–112 103

Fig. 3 (continued). age of T2 is ~20 ka. This result is also consistent with previous studies 5. Discussion (Table 4). Two samples from the fluvial deposits of the T1 terrace at the Lugu site have been dated to 7.2 ± 0.7 and 8.5 ± 0.9 ka by Cheng 5.1. Comparing the Yalong and Anning terraces (2010), which suggests that this terrace formed ~10 ka ago, which cor- relates with the formation age of the T1 terrace in the Yangtze. The wide Anning valley is characterised by a number of faulted ba- Therefore, terraces T6, T5, T4, T3, T2, and T1 are estimated to have sins and three fill terraces. These fill terraces have a thick fluvial deposit formation ages of ~1100, 900, 720, 63, 20, and 10 ka, respectively. Evi- at the base of the river valley, which developed as a result of the stream dence from previous studies (Qiao et al., 2013) indicates that the terrace transport capacity decreasing along the river during an accumulative ages in the Yalong, Anning, the middle Yangtze catchment, and the Yel- phase. The river accumulates through lateral migration of the channel, low River encompass almost the entire period from the late early Pleis- while the river incises caused by the fluvial equilibrium being broken tocene to the Holocene. Our ESR and OSL dates derived from terraces T6 by external factors. In contrast to the development of strath terraces, to T2 are consistent with these results, and this finding suggests that the the river does not incise to the bottom of the fluvial sediments in a single terrace sequence developed in the lower Yalong and Annin basins has cycle, allowing new channel and flood deposits to be stacked upon the important implications for understanding geomorphic evolution in previous fluvial deposits. After several accumulation-incision cycles, other regions. the river eventually incises to the bottom of the stacked sequence of 104 Z. He et al. / Geomorphology 250 (2015) 95–112

Fig. 3 (continued).

channel-flood deposits. Thus the sediment structure of a fill terrace is uplift is also believed to have contributed to the local fragmentation of relatively complex, consisting of several units of channel and flood de- the Anning catchment into local-scale blocks that have subsided or posits overlain on one another. At the Lugu site, the T3 terrace deposits uplifted relative to one another (Chen and Zhao, 1988). Previous analy- reach thicknesses of up to 70 m, with two sets of bipartite fluvial archi- ses indicate that the Anning switches between incision and aggradation tecture, which indicates that the fluvial equilibrium has been broken a phases in response to climate cycles, leading to the formation of corre- number of times. sponding terraces (Cheng, 2010). Therefore, we also consider that the Usually, in a simple tectonic setting of uplift or subsidence, under- vertical separation of fill terraces, as well as strath terraces, is a response standing the tectonic impact on a fluvial system is relatively straightfor- to the general large-scale uplift, which impacts the entire river system. ward (Vandenberghe et al., 2011). In some tectonic settings, such as that Comparison of the fluvial terrace sequences reveals a reasonable cor- of the Anning, however, uplift and subsidence alternate within short relation between the Yalong and Anning terraces, which is also support- catchments. As such, strath terraces and fill terraces are formed in the ed by geomorphological and sedimentological observations, gravel Anning (Table 3). Those fill terraces that occur along the Anning devel- weathering rinds, palaeosols developed within the surface deposits, oped in catchments with subsiding faulted basins. Based on the geomor- and terrace formation ages. The T8 and T7 terraces are only preserved phological distribution of alternating basins and gorges, as well as the in the Jinpingshan region. The distribution of terrace elevations varies sedimentological character of the Anning terraces themselves, we sug- regularly because the relative elevations of the adjacent river segment gest that the fill terraces are formed in subsiding basins, while strath ter- terraces are comparable (Fig. 2). The degree of weathering in the races or no terraces at all are formed in uplifting catchments. upper fluvial deposits of T6, T5, and T4 is relatively high; and gravels Aggradation may occur simultaneously in subsiding faulted basins and with thick weathering rinds are commonly observed in the red clay in uplifting catchments, resulting in differing thicknesses of fluvial de- that rests on the fluvial deposits or the bedrock. The T4 terrace is posits, or synchronous river incision in some gorges with rapid uplift, characterised by thick fluvial successions and appears as a widespread such as at the Yakou site. River incision may occur in response to an in- and continuous terrace across the region; therefore it can be used as a crease in the stream transport capacity in the river during an incision geomorphological surface marker. The T3, T2, and T1 terraces are phase. Thus, the varying thicknesses of fluvial sediments in the terraces characterised by weakly weathered gravels with thin weathering reflect the differing tectonic backgrounds. Changing incision rates, relat- rinds or no red clay cover. The age estimations (Table 4) provided by ed to the uplift and subsidence of individual blocks, result in the simul- ESR and OSL also represent a reliable source of evidence for terrace cor- taneous development of strath and fill terraces of different sizes within relation. On the whole, the terrace sequences of T6 to T1 in the down- the Anning. The fluvial equilibrium is broken caused by climatic and tec- stream catchment of the Yalong (including its tributary, the Anning) tonic changes, and the river expends its erosive capacity in achieving are well correlated despite the diversity in the spatial distributions of the incision required to maintain its gradient through the basin. General the terrace sequences. Z. He et al. / Geomorphology 250 (2015) 95–112 105

Fig. 3 (continued).

5.2. Terrace development and multiple climatic cycles development of fluvial terraces in Yalong and Anning is considered to be closely related to climate cycles and uplift in the area. The terrace sequences of the Yalong and Anning are over the As noted by many other researchers, climate cycles play an impor- relatively small temporal and spatial scales to assume that terrace pro- tant role in driving river terrace formation in uplifting mountain areas duction is controlled by intrinsic changes to the river system. The (Starkel, 2003; Wang et al., 2010; Hu et al., 2013). Fluvial morphological confluence of the Yalong and Jinsha rivers is more than 2000 km up- and sedimentological differentiation occurs simultaneously at different stream from the mouth of the Yangtze, and so the influence of eustasy scales, which may be related to multitimescale climatic cycles on fluvial behaviour is unlikely to have controlled terrace formation (Vandenberghe, 1995, 2002, 2003). For example, longtimescale climatic (Schumm, 1993). Rather, the rapid uplift of the southeastern Tibetan cycles could be recorded by changes in fluvial accumulation and ero- Plateau – resulting from the continuous Indo-Asian collision (Chen sional processes (Bridgland, 2000; Veldkamp and Tebben, 2001). How- and Zhao, 1988; Jiang et al., 1999) – and frequent climate fluctuation – ever, the behavioural response of rivers to multitimescale climate influenced by the combination of the varying Indian Ocean monsoon, cycles, such as Milankovitch and especially sub-Milankovitch scale cy- East Asian monsoon, and plateau monsoon (Pan and Wang, 1999; cles in combination with strong uplift to form river terraces, is still poor- Qiao et al., 2010) – more likely provided the necessary crustal and ly understood. The terrace chronosequences produced here from the climatic background for changes in fluvial style. Therefore, the transverse sections, combined with ESR and OSL dating of terrace 106 Z. He et al. / Geomorphology 250 (2015) 95–112

Table 4 at MIS4 and MIS2 cold stage, while incision may have occurred during Comparison of forming ages of the Yalong River, Yangtze River, and Yellow River terraces. the cold-warm transition periods. During this period the climate cycle river Location Level Above river Forming age appears to be driven by the 41 ka obliquity reflected in the Guliya ice level (m) core curve (Yao et al., 1997). These findings suggest that the formation Yalong River Jingying T6 385 1047 ± 104 ka (ESR) of terraces T3 and T2 is related to the 41-ka Milankovitch cycle. Addi- Jingying T6 385 1081 ± 100 ka (ESR) tionally, the aggradation of the T1 terrace probably prevailed during Jingying T4 149 710 ± 71 ka (ESR) the coolest phases of the Younger Dryas (YD) interval, while the abun- Huantan T2 32 21.8 ± 2.3 ka (OSL) dance of palaeochannels may have been formed during the rapid Huantan T2 32 18.2 ± 1.8 ka (OSL) Tongzilin T2 38 15,253 ± 259a (14C) warming period at the beginning of the Holocene. This sequence of (Li et al., 1984) changes reflects shorter stadial–interstadial climatic fluctuations on Tongzilin T2 38 26,296 ± 1372a (14C) the scale of thousands of years. In support of this conclusion, the YD (Li et al., 1984) stadial and the following interstadial are well documented by an abun- Ganzi T6 103 1.16 Ma dance of large palaeochannels in the Polish River (Starkel, 2002). (Qiao et al., 2007) fl fl – Ganzi T5 60 0.85–0.90 Ma Interrelationships between glacio uvial, slope, and uvial systems (Qiao et al., 2010) in addition to the complex internal sedimentary architecture of many Anning River Manning T6 300 1290 ~ 1010 ka (ESR) terrace sediments – represent important evidence for understanding (Yao et al., 2007) the climatostratigraphic position of terraces (Starkel, 2003). The close Jinchuan T5 242 922 ± 120 ka (ESR) Chengxiang T5 187 934 ± 90 ka (ESR) connection between the terrace sequences developed in the Yalong Yakou T4 136 732 ± 73 ka (ESR) and Anning and multitimescale climatic cycles is not only expressed Chengxiang T4 156 721 ± 86 ka (ESR) by a correlation in timing, but is also documented by various deposit fa- Yantangwan T3 66 63.0 ± 6.5 ka (OSL) cies and sedimentary successions beneath the terraces. For example, Jingjiu T3 78 69.4 ± 8.5 ka (OSL) in the section at the Jingying site, terrace T6, considering the relation- Lugu T3 97 41.1 ± 5.1 ka (OSL) fl fl Yantangwan T2 24 22.0 ± 3.0 ka (OSL) ship between uvial and uvioglacial systems, indicates that river ag- Huanglianguan T2 25 2407 5 ± 450 a (14C) gradation occurred during the cold period corresponding to MIS 32 (Li et al., 1984) (Fig. 6A). The complex internal alluvial architecture beneath many ter- 14 Mianning T2 35 21,100 ± 260 a ( C) races may represent the geomorphological system's response to high- (Wang et al., 1998) Mianning T2 35 25,930 ± 700 a (14C) frequency climate cycles (Maddy et al., 2001). Generally, the coarse- (Li et al., 1984) grained fluvial sediment layer at the base of the terrace, which is Linli T2 32 25.35 ± 0.41 ka (OSL) interpreted as a bedload deposit, is related to a cold environment; (Xu et al., 2003) while the upper fine-grained layer, interpreted as an alluvial floodplain Lugu T1 9 7.2 ± 0.7 ka (OSL) deposit or overbank deposit, may date from the end of the cold period or (Cheng, 2010) Lugu T1 9 8.5 ± 0.9 ka (OSL) thebeginningofthenextwarmperiod(Vandenberghe, 2014). This de- (Cheng, 2010) posit unit suggests that the incision of terrace surface is overtopped and Yangtze River The eastern T7 240~ 1.16 Ma inundated with fines, thus the fines represent the onset of incision and Sichuan basin T6 200~ 0.86 Ma the soils the continuing incision. The thick fluvial deposits of terrace (Li et al., 2001) T5 150~ 0.73 Ma T4 110~ 0.49 Ma T3 at Lugu include two bodies of sediment as part of a bipartite se- T3 80~ 0.11–0.15 Ma quence, with aggradation of fine-grained materials above coarser T2 45~ 0.05–0.06 Ma units, and are shown in Fig. 6B. The gravel layer within the lower dual T1 10~ 0.01–0.03 Ma alluvial architecture may therefore correspond to MIS 4-2, and the Yellow River The eastern T7 216 1.24 Ma very thick gravel within the upper sediment structure probably oc- Lanzhou basin T6 128 1.05 Ma (Pan et al., 2007) T5 108 0.96 Ma curred in response to MIS 4-1. In addition to this, the higher frequency T4 100 0.86 Ma climatic fluctuations of MIS 4-1 were probably recorded by the changes T3 77 0.13 Ma in gravel size in the upper bipartite sediment structure. The sediment T2 30 0.05 Ma profiles in exposures of terrace T2 created during road construction dis- T1 15 0.01 Ma play four distinct cut-and-fill units, corresponding from upper to lower to MIS 2-1, MIS 2-2, MIS 2-3, and MIS 2-4 (Fig. 6C). The brown soil de- deposits, reveal six cyclical adjustments of fluvial aggradation and inci- veloped in the uppermost floodplain deposits is distributed on the T2 sion in the Yalong and Anning. Fig. 4 shows the close correspondence terrace surface and indicates that the onset of river incision may have between the timing of terrace formation and multitimescale climate cy- occurred during the beginning of the next warm period. The typical cling. Generally, the estimated ages for terraces T6 to T1 are broadly dual sediment structure of terrace T1 consists of a lower gravel layer contemporaneous with cold periods. Fig. 4A,B shows the correlation of and a coarse sand layer; the lowermost deposits belong to a slope facies the lower Yalong and Anning terraces T6, T5, and T4 with the marine ox- (Fig. 6D). The sedimentary successions and palaeochannel abandon- ygen isotope stages (Lisiecki and Raymo, 2005) and the average rate of ment occur in response to the YD stadial and the following rapid river incision near the Jingying site. From this, we can see that the aggra- warming at the beginning of the Holocene. Similar fluvial style changes dation stage of terraces T6, T5, and T4 may be correlated with MIS32, have also been reported in the Netherlands, Poland, and England (Kasse, MIS22, and MIS18, respectively, while the onset of river incisions prob- 1995; Rose, 1995; Huisink, 1997; Starkel, 2003). Additionally, the ter- ably occurred during the subsequent transitions from cold to warm cli- race soil can indicate the environment and the fluvial process. The mates. The middle Pleistocene transition (MPT) began 1250 ka and was fines developed on the terrace surfaces of Yalong and Anning represent complete by 700 ka, while the low-frequency (~100 ka) climate cycle the onset of incision and the soils the continuing incision, which suggest emerged 1250 ka and reached its full amplitude by 700 ka (Clark river incision may occur during the cold-to-warm transition. et al., 2006). Thus, it appears from this evidence that the fluvial system's From the previous discussion, we can conclude that climate cycles response to the 100 ka Milankovitch cycles drove terrace formation for are a key control on fluvial aggradation and incision, leading to terrace terraces T6 to T4. Fig. 4C,D shows the relationships of terraces T3, T2, formation in the lower Yalong and Anning. Terrace aggradation oc- and T1 with climate change as indicated by the Guliya ice core curve curred during the cold periods, while terrace incision occurred during (Yao et al., 1997). The elevations and terrace ages suggest that the ag- the transition to the subsequent warming periods. The terrace se- gradation episodes for terraces T3 and T2 corresponded to cold periods quences and their sedimentary characteristics record Milankovitch- Z. He et al. / Geomorphology 250 (2015) 95–112 107

Fig. 4. Correlation of river terraces in the lower Yalong River with the climate change curves (A, D) and the average rate of Yalong River's incision near the Jingying site (B, B). (A) The marine oxygen isotope proxy-climate record (Lisiecki and Raymo, 2005). (B) Reconstruction average incision rate of the Yalong River near the Jingying sites. Y1, the accelerated incision stage of the Yalong River with an average incision rate of 1.83 m/ka over the past 63 ka; Y2, the stable stage of the Yalong River with a long-term incision rate of 0.05 m/ka from 720 to 63 ka; Y3, the rapid incision stage of the Yalong River with an average incision rate of 0.62 m/ka from 1100 to 720 ka. (C) Incision by Yalong River over the past 63 ka near Jingying. (D) Comparison between the δ18O record in the Guliya ice core and the solar radiation at high latitude of the North Hemisphere. scale and sub-Milankovitch-scale climate cycles as well as lower magni- MIS 12, are not identiﬁed in our sections, despite the argument that tude climatic ﬂuctuations within a glacial period. such important climate cycles are likely to have a major impact on ter- However, the question remains as to why this terrace archive con- race formation (Bridgland and Westsway, 2008a). Our terrace record tains only six terraces, when 32 glacial–interglacial cycles (MIS 1–MIS from 1.10 to 0.72 Ma does seem to support this argument, as terraces 32) have occurred since 1.10 Ma (Fig. 4). Additionally, the most impor- appear to correlate with the larger climate cycles; however, the absence tant climate cycles, such as the severe glaciations that occurred during of terraces between 0.72 and 0.063 Ma is unexpected. One possibility is 108 Z. He et al. / Geomorphology 250 (2015) 95–112

than 100 m (Fig. 2). Such large elevations could not be caused by climate change if no rapid uplift occurred. In addition, the longitudinal profile of terraces indicates differences between the Hatan and Jingying sites (Fig. 2), which is not in accordance with terrace development being controlled by climate change alone (Pan et al., 2000). We can suppose that the elevation differences are related to tectonic activity. During the intervals of 1100 to 720 ka and 63 ka to present, rapid uplift provided the driving force for continuous downcutting by the lower Yalong and Anning. Fig. 4 shows the relationship between terrace formation and incision rates and indicates that the main period of terrace formation corresponds to the two rapid downcutting stages. The time- averaged incision rates of 0.62 m/ka for the Yalong at the Jingying site between 1100 and 720 ka led to the formation of terraces T6, T5, and T4. Likewise, the time-averaged incision rate of 1.83 m/ka since 63 ka controlled the development of terraces T3, T2, and T1. In contrast, the river incision rate is only 0.05 m/ka between 720 and 63 ka, which is in- sufficient to provide a sustained and effective driving force for continuous river downcutting; therefore, no terraces were developed during this period. Thus, the small incision rate provides a plausible explana- tion for the absence of terraces between 1100 and 63 ka. This conclusion suggests that uplift rates must exceed some threshold value in order to drive fluvial terrace formation. Our conclusions agree with previous numerical simulations (Veldkamp and van Dijke, 2000). Our analysis suggests that the changes in incision rates calculated from this area are completely different from those seen in many parts of the world that have been taken to be coupled responses to variation in the longer term climatic signal (Bridgland and Westaway, 2014). In addition, within the Jinpinshan area, the Yalong has incised much faster than the typical regional rates (Fig. 5). This finding implies regional up- Fig. 5. Temporal variation of average incision rates of the lower Yalong and Anning rivers lift difference resulting from different types of crust may be essential to based on the heights of terrace treads versus their age. (A) incision rate of the lower create accommodation space for the formation of those fluvial terraces. Yalong River; (B) incision rate of Anning River. In summary, the findings of this study appear to indicate that regional uplift has provided the driving force for continuous downcutting and that terraces formed during this time period are simply not preserved. created accommodation space for the formation of terraces along the The apparent absence of an MIS 12 terrace along the rivers Platte and Yalong and Anning rivers, while climate change represents a trigger Susquehanna in North America is believed to be most likely a result of for fluvial style changes that led to terrace formation in these basins. climate instabilities originating from the North Atlantic (Westaway, 2007; Bridgland and Westaway, 2008b), which may be similar to 5.3.2. Implication of river downcutting for the uplift of the SE Tibetan the climate-driving mechanism for the Yalong and Anning terrace Plateau formation. Although the incision rate of a river is not equal to the uplift rate (Merritts and Vincent, 1989; Hovius, 1999; Whipple et al., 1999), 5.3. Terrace development and regional uplift long-term incision rates have been used to estimate epeirogenic uplift rates and provide reliable quantitative constraints (Bridgland, 2000; 5.3.1. Regional uplift provides the driving force for continuous downcutting Clark et al., 2006; Westaway et al., 2006; Maddy et al., 2008). The results in uplift catchment described in the previous sections support the positive correlation be- Although the timing of terrace formation is controlled by climate tween Yalong downcutting and uplift. change on different timescales, we cannot assume that river valley inci- The alternation of filling and cutting in the Yalong and Anning basins sion is driven by climate cycling alone. Regional uplift is considered an led to the formation of six distinct river terraces. Time-averaged incision essential requirement for long-term incision (Bridgland, 2000; Maddy rates can be estimated from the differences in terrace elevation and et al., 2001). Regional uplift, leading to steeping in a river's longitudinal their formation ages (Fig. 4A). Fig. 5 shows the temporal and spatial var- slope, can enhance river water energy and can strongly induce river iations in cutting rates since 1100 ka. We can see from this figure that incision. the variation in fluvial incision rates is very similar in different catch- Based on the above discussions, the cyclic formation of terraces in ments, but varies significantly between sites in different tectonic set- the lower Yalong and Anning seems to be a response to climatic fluctu- tings. In both the lower Yalong and the Anning, the time-averaged ations, but climate cycle alone cannot entirely control the terrace devel- incision rates also vary noticeably between 1100 ka and today and can opment. Fig. 4 shows that not every climate cycle results in terrace be divided into three temporal stages, including two significant formation. For example, during MIS 12, the climate shows significant downcutting phases and one weak incision period. Between 1100 and fluctuation, and yet no fluvial terraces were identified from this time. 720 ka, significant downcutting of the riverbed occurred, and the However, some terraces were formed during smaller climate fluctua- time-averaged incision rates in the lower Yalong and Anning basins tions. The temporal distribution of incision rates indicates that, although are estimated to have been 0.54 and 0.35 m ka−1, respectively. The not the same along the entire course of the rivers, three stages are al- time-averaged incision rates during 720–63 ka appear to have slowed most always seen (Figs. 4Band5), and no direct relationship was to fairly constant levels of around 0.06 and 0.09 m ka−1 in the lower Ya- shown between climate fluctuation and river incision. From the per- long and Anning, respectively. Finally, the time-averaged incision rates spective of terraces as a landform feature (Fig. 3), the Yalong commonly of these rivers between 63 ka and the present day are dramatically developed six strath terraces, with remarkable vertical separation be- higher, reaching 1.71 and 1.16 m ka−1, respectively. The rapid incision tween terrace surfaces and a maximum elevation difference of more events are closely linked to tectonic activity (Zhang et al., 2003). In Z. He et al. / Geomorphology 250 (2015) 95–112 109

Fig. 6. (A) Terrace T6 near Jingying: fluvioglacial deposits consisting of gravel and coarse sand. (B) The thick fluvial deposits beneath terrace T3 near Lugusite include bipartite sediment structures with aggradation of fine-grained materials above coarser units. (C) Sediment profiles of terrace T2 near the Huatan site display four distinct cutting-and-filling units. (D) Typical dual sediment structure of terrace T1 near the Huatan site consists of a lower gravel layer and a coarse sand layer, and the lowermost deposits belong to a slope facies.

particular, the drastic acceleration of incision rates since 63 ka may be illustrate the influence of tectonic activity on the elevation differences related to the large-magnitude earthquakes occurring in the region dur- of the terraces (Fig. 2). ing the same period (Ran et al., 2008). Based on the above discussions The temporal distribution of Yalong incision rates reveals two and previous studies (Zhang et al., 2003), the incision history of the phases of uplift and a relatively stable phase in the southeastern Tibetan lower Yalong and the Anning can be considered consistent with tempo- Plateau since 1.1 Ma ago. The two rapid uplift phases span 1.10–0.73 ral changes in tectonic activity, which demonstrates that the formation and 0.063 Ma to the present, while the relatively stable phase ranges of the terraces is sensitive to changes in uplift. Additionally, the longitu- from 0.72 to 0.063 Ma. The Yalong incision rates since 0.063 Ma are dinal profiles of terraces along the lower Yalong and Anning also greater than earlier incision rates, which indicate that the Yalong 110 Z. He et al. / Geomorphology 250 (2015) 95–112 catchment may have been in a phase of accelerated uplift since the late Anninghe–Zemuhe arcuate fault zone, when the southeast margin of Pleistocene. The formation of terrace sequences along the Yalong is a re- the Tibetan Plateau extruded clockwise, the arc space is relatively nar- sponse of uplift of the southeastern Tibetan Plateau region. row. The Jinpingshan and Gonggashan thrust belts, which are located The spatial distribution of incision rates along the Yalong reflects the within this arc space, absorb the large displacement through severe de- differences in uplift in the different river reaches, which is a significant formation and uplift. This tectonic deformation style resulted in the topic of discussion for studies into the differences in uplift and tectonic rapid thrust uplift of the Jinping Mountains, which produced significant deformation in the southeastern margin of the Tibetan Plateau. Fig. 7 downcutting in the Yalong. Uplift of the Jinchuan and Manshuiwan re- shows the spatial distribution of incision rates for the lower Yalong and gions was relatively strong during 1.10–0.72 Ma, but has been relatively the Anning. Combined with the structural information from the section weak since 0.06 3Ma, as indicated by the river downcutting rates. Based in Fig. 1, we can see that between 1.10 and 0.72 Ma the incision rates of on the fault zone distribution shown in Fig. 1 and the proximity of the the lower Yalong decline from north to south (Fig. 7, L2), and that incision Anning segment to the Zemu River fault zone, we consider it likely rates for the Anning decrease downstream (Fig. 7,L1).Thedistributionof that the Zemu River fault zone absorbed the displacement from the lat- incision rates shows a distinct relationship with the distance from the eral extrusion of the southeastern Tibetan Plateau in this region, pro- Jinpingshan thrust belt, such that the closer to the thrust belt, the greater ducing the relatively slow uplift in this river segment. the incision rates and vice versa. This distribution of Yalong incision rates During 0.72–0.06 3Ma, the downcutting rates of the Yalong catch- is significantly similar during 1.10–0.72 and between 0.063 Ma and the ment are minor but exhibit certain spatial differences. The incision present (Fig. 7, L1 and L2). The increased rates in the southern section rates along the Anning are greater than those for the downstream and of the lower Anning differ from previous rates but maintain the overall upstream regions of the Yalong, which may be linked to lithology. Strata relationship with the Jinpingshan thrust belt. However, the cause of this of the Xigeda Formation, which are extensively distributed within the increase in incision rates of the lower Anning requires further explana- Anning valley, are particularly susceptible to erosion; whereas erosion tion and linking to tectonic studies, as well as verification of its relation- rarely occurs in the bedrock channels, which are extensively distributed ship with the Jinhe-Jinghe thrust belt. Overall, the distribution of incision across the Yalong basin. rates in these two rivers indicates rapid uplift of the Jinpingshan region The overall spatial distribution of uplift inferred from the incision during 1.10–0.72 Ma and since 0.063 Ma; the uplift amplitude is greatest rates of the Yalong from 1.10 to 0.72 Ma and since 0.063 Ma, in addition within this area, and decreases toward the south and east. to findings from previous studies (Qiao et al., 2013), reveal that the The intense uplift of the Jinping Mountain region during these two Mount Gongga and Jingping Mountain areas (which are located in the periods is significantly influenced by the presence of the thrust belt. southeastern part of the Tibetan Plateau boundary zone) have the larg- Mount Gongga, which is located to the north of the Jinpingshan Moun- est uplift amplitudes and that the uplift amplitudes gradually diminish tains, has experienced rapid uplift since 1 Ma; the maximum uplift rate toward the plateau interior and edge. The spatial distribution of uplifts of the southern region is 3.3 ± 0.8 mm a−1 (Tan et al., 2010). In re- is the tectonic response to lateral extrusion and growth to the southeast sponse to the tectonic style of the southeastern margin of the Tibetan of the Tibetan Plateau. The terrace sequences of the lower Yalong across Plateau, the NNW Xianshuihe fault transforms into the nearly NS- the Tibetan Plateau landscape boundary zone, in addition to two first- striking Anning fault in the Shimian region. During the Late Cenozoic order tectonic units including the Tibetan Plateau block and the Yangtze lateral extrusion of the Tibetan Plateau, the displacement of the block, record this tectonic movement. The distribution of incision rates Xianshuihe fault was greater than that of the Anning fault (Shen et al., can be considered a response to the diversity of crustal shortening and 2000; Wang and Burchfiel, 2000); and the Xianshuihe–Anninghe fault uplift in the extrusion of the southeastern margin of the Tibetan Plateau. slip rate has decreased from the northwest to the southeast since the terminal late Pleistocene (Xu et al., 2003), while the tectonic deforma- 6. Conclusions tion style has been maintained (Wang et al., 2010). The geometric shape of the Xianshuihe–Anninghe fault affects the distribution of tec- (i) The lower Yalong and its tributary the Anning, within the rapidly tonic strain. Owing to the limited distribution of the Xianshuihe– uplifting mountainous area in the southeastern margin of the

Fig. 7. The spatial distributions of differing incision rates of the lower Yalong and Anning rivers system. Z. He et al. / Geomorphology 250 (2015) 95–112 111

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