Multiple Climatic Cycles Imprinted on Regional Uplift-Controlled Fluvial Terraces in the Lower Yalong River and Anning River, SE
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Geomorphology 250 (2015) 95–112 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph 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 character- istics, 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 geo- morphological 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 pe- riods, 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