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Comment on ‘Gigantic rockslides induced by fluvial incision in the Diexi area along the eastern margin of the Tibetan Plateau’ by Zhao et al. (2019) Geomorphology 338, 27–42
Journal Pre-proof Comment on ‘Gigantic rockslides induced by fluvial incision in the Diexi area along the eastern margin of the Tibetan Plateau’ by Zhao et al. (2019) Geomorphology 338, 27–42
Xuanmei Fan, Ali P. Yunus, John D. Jansen, Lanxin Dai, Alexander Strom, Qiang Xu PII:
S0169-555X(19)30454-4
DOI:
https://doi.org/10.1016/j.geomorph.2019.106963
Reference:
GEOMOR 106963
To appear in:
Geomorphology
Received date:
8 August 2019
Revised date:
5 November 2019
Accepted date:
16 November 2019
Please cite this article as: X. Fan, A.P. Yunus, J.D. Jansen, et al., Comment on ‘Gigantic rockslides induced by fluvial incision in the Diexi area along the eastern margin of the Tibetan Plateau’ by Zhao et al. (2019) Geomorphology 338, 27–42, Geomorphology(2019), https://doi.org/10.1016/j.geomorph.2019.106963
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Comment on ‘Gigantic rockslides induced by fluvial incision in the Diexi area along the eastern margin of the Tibetan Plateau’ by Zhao et al. (2019) Geomorphology 338, 27–42 Xuanmei Fan1*, Ali P. Yunus1*, John D. Jansen1,2, Lanxin Dai1, Alexander Strom1,3, Qiang Xu1 1
State Key Laboratory of Geohazard Prevention and Geoenvironment Protection,
Chengdu University of Technology, Chengdu, Sichuan, China, 610059. GFÚ Institute of Geophysics, Czech Academy of Sciences, Prague, Czechia.
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Geodynamics Research Centre, Volkolamskoe Shosse 2, 125080 Moscow, Russia
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* Corresponding author: Prof. Xuanmei Fan ([email protected]) and Dr. Ali P. Yunus
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([email protected]) Abstract
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Zhao et al. (2019) examine gigantic landslides in the Diexi area along the eastern
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margin of the Tibetan Plateau and propose their successive failure based on a knickpoint migration conceptual model. They postulate that a major river knickpoint
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(KpMJ) identified by them in the middle reaches of the Minjiang River (Sichuan, China)
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was initiated at the Longmenshan Fault and subsequently propagated ~ 85 km to its present position upstream of the Diexi lake. They then argue that this retreating
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knickpoint left in its wake an inner gorge that undercut and destabilized hillslopes, triggering a series of large landslides in the Diexi area. We question this interpretation based on our high-resolution landslide mapping, an analysis of knickpoints (i.e., profile convexities > 30 m high) in the Minjiang channel network, and field observations of lacustrine sediments and epigenetic gorges associated with the Diexi landslides. We confront the model proposed by Zhao et al. (2019) with three key arguments: 1) Major profile convexities in the Diexi area, including KpMJ, are associated with landslide or debris flow deposits and there is no basis for connecting explicitly any of these to long-distance knickpoint retreat; 2) the giant Diexi paleolandslide predates the debris avalanches at KpMJ, therefore the latter cannot have been the trigger for landsliding in this area; and 3) the spatial distribution of 666
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Journal Pre-proof mapped knickpoints in the Minjiang River mainstem and tributaries is not consistent with simple long-distance propagation of an ‗incisional wave‘ initiated at the Longmenshan Fault.
Key words: knickpoint, landslides, lacustrine sediments, Tibetan plateau.
1. Introduction
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The complex interplay between tectonic uplift, river incision, and mechanisms that
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trigger landslides has inspired a large body of research with much yet to be unraveled. This is especially so for the role of river incision driven by retreating knickpoints: how
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they are initiated and how they propagate the signal of base level throughout
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landscapes (Kirby and Whipple, 2012). Zhao et al. (2019) investigate large landslides
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in the Diexi area along the middle reaches of the Minjiang River, Sichuan, China (Fig. 1) (jiang means river in Mandarin Chinese). They identify a major river profile
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convexity (KpMJ) upstream of Diexi lake, which they postulate is a knickpoint that has migrated ~ 85 km upstream, initiating at the Longmenshan Fault zone. Zhao et al.
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(2019) go on to ascribe fluvial knickpoint retreat as the main prerequisite for giant
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slope failures in this region. They suggest that the long-distance retreat of KpMJ, in particular, undercut and destabilized hillslopes in its path thereby triggering the Diexi landslides.
The landslide inventory and in-depth descriptions provided by Zhao et al. (2019) enrich the landslide database in this region. We appreciate their insightful analysis of the role of geological structures on landslides, particularly wedge-type failures. Nevertheless, the interactions between river incision, tectonic uplift, and triggering mechanism of landslides are still unclear, especially the causes and effects of knickpoint migration. Based on our extensive field and topographic analyses in the Diexi valley, we question the claim of Zhao et al. (2019) that a knickpoint triggered at
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Journal Pre-proof the Longmenshan Fault has migrated ~ 85 km to a position upstream of the Diexi lake. We propose that the knickpoint referred to by Zhao et al. (2019) as KpMJ is instead a breached debris flow dam with no relationship to long-distance knickpoint retreat. We build our case upon two key analyses:
1) River damming by large landslides. By independently mapping the distribution of all river profile convexities (knickpoints) located in the Minjiang channel network
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upstream of the Longmenshan Fault, we suggest that all major convexities (> 30 m
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high) in the Diexi area are associated with landslide accumulations of various types. The profile convexity at KpMJ is categorically a debris fan deposit, not a retreating
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knickpoint.
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2) Spatial distribution of knickpoints. We conduct an automated analysis of knickpoints and according to the scaling relationship between knickpoint retreat and
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drainage area, we produce a χ-transform plot of Minjiang river profiles to examine the spatial distribution of 666 knickpoints upstream of the Longmenshan Fault. We show
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that knickpoint distribution is not consistent with simple long-distance transmission of
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base level fall initiated at the Longmenshan Fault.
2. River damming by large landslides We independently re-mapped the landslides in the Diexi region (the same areas studied by Zhao et al., 2019) using an airborne LiDAR high-resolution (50 cm) digital elevation model (Fig. 1) and we validated our mapping via detailed field observations (Figs. 2 and 3). We set out to investigate the spatial distribution of river profile convexities along the Minjiang River and its tributaries using the software package, TopoToolbox (Schwanghart and Scherler, 2014, 2017), along with the 30 m resolution digital elevation model, ALOSW3D (Fig.1). The results of our landslide mapping agree in general with that of Zhao et al. (2019) with two important exceptions: 1) We find
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Journal Pre-proof notable differences in the size of some landslides, such as the Diexi-S rock slope failure, which we interpret as being much smaller (cf. Fig. 2 in Zhao et al., 2019). In this case, we suggest that the catchment accumulation area has been misinterpreted as a landslide scarp; and 2) Three epigenetic gorge sections—vital for understanding the evolution of the Minjiang valley—were not identified by Zhao et al. (2019) (cf. their
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Fig. 3 with our Fig. 1b below).
Figure 1. Diexi study area and landslide inventory map: (a) KpMJ (Zhao et al., 2009) shown relative to regional distribution of landslides, including the Diexi paleolandslide, and maximum extent of the dammed Diexi lake (cyan) reconstructed from lacustrine sedimentation up to 2370 m a.s.l. (b) Local Diexi area, including two epigenetic gorge sections where the Minjiang River has been laterally displaced by landslides, knickpoints identified from the 30-m digital elevation model, ALOS AW3D, and location of the 160 m-deep drillhole (filled orange rectangle) into lacustrine sediments
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Examples of rivers temporarily dammed by landslide deposits are well-documented (e.g., Hewitt, 1998, 2006; Hungr et al., 2001; Kun-Ting et al., 2018; Wei et al., 2018) and are a relatively common occurrence in settings, such as Diexi, which are tectonically-active and high-relief (Korup, 2006; Korup et al. 2010a,b). According to Zhao et al. (2019): “the major knickpoint KpMJ is 20 km from the Diexi landslide, where there are no
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landslides, faults or lithologic boundaries, which suggests that it was not generated by these factors and must have occurred because a knickpoint propagated from
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downstream. The knickpoint likely initiated at the intersection of the Minjiang River
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and the Longmenshan fault belt, which is an active thrust fault with upheaval of the
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eastern Tibetan Plateau to the west (p. 39)”.
In other words, Zhao et al. (2019) seems to draw a direct line of evidence between
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absence of landslide deposits at KpMJ and long-distance knickpoint retreat. A key
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corollary of their model is that KpMJ predates the giant landslides downstream, which
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were triggered by the destabilizing effect of knickpoint retreat.
We raise three points in response to the statement cited above. First, our field observations at KpMJ reveal no visible bedrock in the channel boundary at this site. The channel and valley sides do, however, show remnants of multiple large debris flow fans, stemming from the adjoining tributary valleys (Fig. 1a). These tributaries have large catchment areas (37 and 2.1 km2 for right and left bank tributaries, respectively) and it seems clear (Fig. 2a,b) that large debris flow fans temporarily dammed the Minjiang River at the KpMJ location and are responsible for forming the profile convexity at ~ 2321 metres above sea level (m a.s.l.) or higher.
Second, the debris flow deposits at KpMJ overlie lacustrine sediments at 2370 m a.s.l 5
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Zhao et al., 2019).
Figure 2. Views at the KpMJ location, showing remnants of debris flow fans from adjoining right and left tributary valleys: (a) Minjiang River channel incising debris flow deposits at the apex of KpMJ; (b) terraces formed by the debris flow deposits at the mouth of tributary catchment (45 m thick); and (c) debris flow deposits overlying older lacustrine sediments.
Third, we identify another set of lacustrine sediments (Fig. 3) ~ 12 km upstream of the location of KpMJ (Fig. 1) at an elevation of 2460 m a.s.l. Given that these deposits
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lacustrine sediments has not been determined for either lake.
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~ 12 km upstream of the KpMJ.
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Figure 3. Lacustrine sediments (32ᵒ 16' 13.071 N, 103ᵒ 44' 37.295 E) at 2460 m a.s.l.
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These three sets of observations contradict the model presented in Figure 14 of Zhao et al. (2019): the profile convexity at KpMJ is a debris fan deposit, not a retreating
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knickpoint; KpMJ postdates the giant Diexi paleolandslide, thereby defusing their argument regarding hillslope debuttressing in the knickpoint‘s wake. The implication of multiple lakes is that river damming is not restricted to reaches downstream of retreating knickpoints. On the contrary, river damming is likely to be very commonplace in this landscape (Hewitt, 2006)—a point we emphasise in the next section.
3. Spatial distribution of knickpoints Knickpoints are convexities in river long profiles that occur as two main types: static and mobile. Both are relevant here. Static knickpoints develop most commonly at sites of less erodible substrate, such as strong, unjointed rock (e.g., Jansen, 2006), or
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slope, substrate erodibility, and sediment load (Gardner, 1983; Howard et al., 1994;
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Hayakawa et al., 2008; Jansen et al., 2011; Yunus et al., 2016). Zhao et al. (2019) interpret KpMJ as a mobile knickpoint produced by fault-related base level fall, but as
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we note in the section above, KpMJ is associated with debris flows and no bedrock
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outcrop is visible. This does not strictly preclude the possibility that a mobile
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knickpoint has propagated from downstream to the KpMJ location, hence we test that
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scenario below.
In our analysis of the spatial distribution of knickpoints, we conduct an experiment to
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test the model of Zhao et al. (2019) that KpMJ is genetically linked to the propagation
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of a knickpoint initiated by base level fall at the Maoxian-Wenchuan component of the Longmenshan Fault ~ 85 km downstream. A knickpoint generated in this way will propagate upstream throughout the channel network at a rate that is scaled to drainage area (a proxy for discharge), bed slope, substrate erodibility, and sediment load (Whipple, 2004). Unlike Zhao et al., we analyse the entire Minjiang catchment upstream of the Longmenshan Fault. We use TopoToolbox (Schwanghart and Scherler, 2014), a MATLAB-based topographic analysis software package to automate the extraction of knickpoints from the ALOSW3D 30 m DEM. Following the approach of Schwanghart and Scherler (2017), we analyse the spatial distribution of all knickpoints in the Minjiang catchment, applying a minimum knickpoint height threshold of 30 m. Our analysis yields several important results (Figs. 4 and 5) and
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1) We identify a total of 666 knickpoints (> 30 m high) in the Minjiang channel network (>9 km2 accumulation area), some several hundred metres in height. A small fraction of these profile convexities may be due to DEM error; however, field observations confirm the dimensions of the Diexi valley knickpoints and this lends confidence to our analysis.
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2) KpMJ (~ 35 m high) is not among the most pronounced knickpoints in the Minjiang
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channel network (falling within the smallest quartile) and its plotting position in Figure 5 brings no suggestion of any special significance. The height of river profile
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convexities identified by Zhao et al. (2019) at Diexi (KpD) and Yingpinya (KpY) are
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55 m and 255 m, respectively (the Diexi dam having incised appreciably over time).
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3) The spatial distribution of the 666 knickpoints is scattered fairly uniformly across the drainage network (Figs. 4 and 5). A simple fault-triggered incision wave propagating upstream from the Longmenshan Fault would produce a dense cluster
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of knickpoints with similar χ-value; however, the scattered distribution in Figure 5b
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indicates categorically that the distribution of major knickpoints is not the result of a
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simple knickpoint retreat.
4) Figure 4 demonstrates that knickpoints are extremely common in the channel network. Some of these are known to be the static lithological knickpoint type (e.g., see Fig. 10 in Kirby et al., 2003) and we acknowledge that our automated method is less accurate in the steep, low-order channels. Nevertheless, we suggest that the dominant fraction of convexities in larger streams is the result of landslides partly or fully blocking river channels (i.e. Hewitt, 2006; Korup, 2006). Moreover, such a dense array of landslide dams must severely impede the propagation of knickpoints over time (Ouimet et al., 2007; Korup et al., 2010b), especially given that landslide dams > 100 m high are common.
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Figure 4. (a) The Minjiang river catchment upstream of the Longmenshan Fault zone, showing knickpoints > 30 m high (white filled circles), and major faults (heavy black lines); note two NE-striking components of the Longmenshan Fault zone are shown: the Mao-Wen and Yingxiu-Beichuan faults. (b) The Diexi valley area, showing major knickpoints identified by Zhao et al. (2019): Diexi (KpD), Yinpingya (KpY), and KpMJ. Also shown are other major landslide dams: Diexi-S, Dragon, and Manaoding. Base map derives from the ALOSW3D 30 m DEM.
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Figure 5. (a) River longitudinal profiles (blue lines) of the Minjiang mainstem and tributary network upstream of the Longmenshan Fault extracted from ALOSW3D 30 m data (minimum drainage area threshold = 9 km2). A total of 666 knickpoints > 30 m height (white filled circles). The mainstem steepening at ~ 110 km marks the Mao-Wen branch of the Longmenshan Fault. The macro-knickpoint (~ 290 km) is a lithological knickpoint corresponding to a Mesozoic granite pluton (see Fig. 10 in Kirby et al., 2003). (b) The same river profiles plotted with a χ-transformation (Schwanghart and Scherler 2014), a proxy for the rate of knickpoint retreat. The assumed profile concavity (m/n = 0.22) is determined from a Bayesian optimization test.
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Journal Pre-proof An additional major impediment to long-distance knickpoint retreat is the occurrence of intermittent epigenetic gorges. These are bedrock channel segments created when rivers incise into valley-fill and are superimposed onto an adjacent bedrock hillslope, essentially bypassing the abandoned paleovalley, which often fills with sediment (Hewitt, 1998; Ouimet et al., 2008). Epigenetic gorges are most commonly associated with phases of rapid aggradation or river damming due to landsliding and their effect on the evolution of river profiles is profound (Ouimet et al., 2008). Channel
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entrenchment at an epigenetic gorge essentially raises the local base level
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(sometimes hundreds of metres) above the valley thalweg, forcing the river to cut
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another bedrock valley adjacent to the original (Fig. 6 and 7).
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Our mapping and field studies reveal the existence of at least three epigenetic gorge
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segments along the Minjiang River immediately downstream of the giant Diexi landslide (Fig. 1b). The pre-existing valley (paleovalley) is now filled with either
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landslide deposits or fluvial deposits (Fig. 6 and 7). We note that the original (pre-giant landslide) bedrock geometry of the valley is unknown, although our recent drilling of
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Diexi paleolake sediments (Tuanjie village, 32 ᵒ 2' 42" N, 103ᵒ 40' 3" E) (Fig. 1b) reveals
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lacustrine materials > 160 m thick (i.e., down to ~ 2007 m a.s.l.). We accept that ~ 2000 m a.s.l. (as shown in Fig. 11 of Zhao et al., 2019) was possibly the pre-landslide river bed, but we see no basis for interpreting this level as bedrock. Given that we know of no bedrock exposed anywhere in the study reaches (aside from the epigenetic gorges), we anticipate that the sedimentary cover thickness could be on the order of 101–102 m—though no data exist to verify the true depth.
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Figure. 6. Epigenetic gorge in the Minjiang River (see Fig. 1): (a) Panorama viewed from the left-bank and downstream section of the Deixi paleolandslide; (b) and (c)
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views of the new channel incising bedrock (dashed line shows position of abandoned
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section of valley).
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Figure 7. (a) Overview at Dragon, showing the epigenetic gorge relative to the palaeovalley. (b) Upstream part of the palaeovalley filled by the deposits of the
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Dragon rockslide (RS). The new epigenetic gorge passes right from the left-bank bedrock cliff (BR). (c) Left bank of the epigenetic gorge.
4. Discussion 4.1 Knickpoint retreat is not essential to landsliding Central to the model proposed by Zhao et al. (2019) is that, 1) knickpoints in this landscape incise the bedrock valley trough while propagating long distances upstream throughout the channel network, and 2) KpMJ represents ~ 85 km of knickpoint retreat from the Longmenshan Fault to its present location (Fig. 1). We question both points. The Minjiang River and its tributaries exhibit the typical
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Journal Pre-proof disturbance regime proposed in Hewitt (2006) whereby the channel network is blocked frequently by large-scale landslides (Fig. 1), and large tributary-derived debris flows (Fig. 2). Our knickpoint analysis reveals the drainage network contains 666 profile convexities (> 30 m), (Figs. 4 and 5). In the face of the multitude of landslide dams and the intermittent occurrence of epigenetic gorges (Figs. 6 and 7) pinned on bedrock high above valley troughs, long-distance knickpoint propagation is difficult to envisage (cf. Korup et al. 2010b). A more intuitive interpretation of the Minjiang river
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profile (Fig. 5a,b) is that the complex of large landslides and associated epigenetic
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gorges at Diexi have retarded knickpoint retreat; however, this hypothesis awaits
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precursor to large-scale slope failures. While we agree with Zhao et al. (2009) that earthquakes may be the dominant trigger for landsliding in the Diexi valley, apart from
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the giant landslide caused by the 1933 earthquake, the triggers for the other landslides remain unknown. Located at the eastern margin of the Tibetan Plateau, this
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high-relief and tectonically-active region is subject to strong earthquakes and extreme
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rates of rock uplift accommodated largely via thrusting and transgressive shortening along major regional faults (Kirby et al., 2003). In this landscape, river incision drives hillslope erosion that ultimately governs landscape-wide denudation rates (Whipple, 2004). However, it is also the case that rocks in this area are primarily weak and highly-fractured meta-sediments. The high frequency of seismic shaking connected with strong earthquakes, such as the 1933 Ms 7.3 Diexi earthquake (Li et al., 1986; Ren et al., 2018), the 1976 Ms 7.2 Songpan-Pingwu earthquake (Deng et al., 2013), and the 2008 Ms 8.0 Wenchuan earthquake (Gorum et al., 2010 and Fan et al., 2012), is likely to dilate pre-existing rock fractures and create new ones, thereby reducing the internal strength of hillslopes (i.e. Molnar et al., 2007). This rock damage combined with steep topography are the major causes for high concentration of large landslides
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4.2 The accuracy of DEMs used in our mapping The identification of knickpoints from river profile analysis is always subject to the accuracy of the digital elevation model (Schwanghart and Scherler, 2017) and is therefore relevant here. We list the DEMs used in Zhao et al. (2019), and those used
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in our study with their vertical uncertainties in Table 1.
The ALOSW3D 30 m data used in this study is a resampled version of the ALOSW3D
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5 m data used by Zhao et al. (2019). We note that the profile convexity at KpMJ is not
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apparent in either the ASTER 30 m GDEM used by Zhao et al. (2019) or the 12.5 m
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ALOS DEM used by us; that is, the convexity is only detected in the 5 m and the
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resampled 30 m version of the ALOSW3D DEM.
Zhao et al. (2019) analyse the distribution of knickpoints only along a 50 km reach of
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the Minjiang River and, in detail, the immediate area surrounding the complex of
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landslides at Diexi. From a logarithmic plot of mean channel slope versus drainage area, they visually identify three ‗major knickpoints‘ as sharp breaks in the distribution. There is nothing intrinsically incorrect about this manual, semi-quantitative approach, but the spatially restricted extent of their analysis means they are severely limited in their capacity to understand processes and drivers in the areas beyond, such as at the Longmenshan Fault. Moreover, such a spatially restricted analysis falls well short of the comprehensive geomorphic mapping that is routinely conducted nowadays, with freely-available tools such as TopoToolbox (Schwanghart and Scherler, 2014).
Table 1. Digital elevation models (DEMs) used in Zhao et al. (2019) and in our study, listing published values of root mean square error (RMSE).
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Source
RMSE (m)
References (RMSE)
(i) ALOS AW3D 5m
The Japan Aerospace Exploration Agency (JAXA)
2.13 – 8.58
Takaku et al. (2016)
(ii) ASTER GDEM 30 m
The Ministry of Economy, Trade, and Industry (METI) of Japan and the United States National Aeronautics and Space Administration (NASA)
13.75
Tachikawa et al. (2015)
This study
(i) ALOS AW3D 30 m
The Japan Aerospace Exploration Agency (JAXA)
2.14 – 6.67
Tadono et al. (2015); Shawky et al. (2019)
(ii) ALOS PALSAR 12.5 m
Alaska Satellite Facility (ASF)
4.57
(iii) LiDAR 50 cm
Sichuan Bureau of Surveying, Mapping and Geoinformation
(small area covered, used only for mapping the landslides)
Shawky et al. (2019)
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Zhao et al. (2009)
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5. Conclusions
On the basis of high-resolution landslide mapping, analysis of knickpoints (profile
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convexities > 30 m high) in the Minjiang channel network, and field observations of
Diexi area, we find that:
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lacustrine sediments and epigenetic gorges associated with giant landslides in the
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1) all major profile convexities in the Diexi area, including the KpMJ, are associated with large accumulations of mass movement debris and there is no basis for connecting explicitly any of these to long-distance knickpoint retreat; 2) the giant Diexi paleolandslide predates the debris avalanches at KpMJ, therefore the latter cannot have been the trigger for landsliding in this area; and 3) the spatial distribution of 666 mapped knickpoints in the Minjiang River mainstem and tributaries is not consistent with long-distance propagation of a simple incisional wave initiated at the Longmenshan Fault.
Large landsides in the Diexi region, in our view, are the outcome of a complex interplay of several factors: high-relief hillslopes, high seismicity, and the densely
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Acknowledgements This research was supported by the National Science Fund for Outstanding Young Scholars of China (grant no. 41622206), the Fund for Creative Research Groups of (grant
no.
41521002),
the
Fund
for
International
Cooperation
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China
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(NSFC-RCUK_NERC), Resilience to Earthquake-induced landslide risk in China (grant no. 41661134010), the National Key R & D Program of China (grant no.
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2017YFC1501002), and the Fund of SKLGP (SKLGP2016Z002). We thank W.
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Schwanghart for his kind assistance with TopoToolbox and A. Joshua West for
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discussions in the field.
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References
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Crosby, B.T., Whipple, K.X., 2006. Knickpoint initiation and distribution within fluvial networks: 236 waterfalls in the Waipaoa River, North Island, New Zealand. Geomorphology 82, 16–38. Deng, B., Liu, Shugen, Liu, Sun, Jansa, L., Li, Z., Zhong, Y., 2013. Progressive Indosinian NS deformation of the Jiaochang structure in the Songpan-Ganzi fold-belt, Western China. PloS one 8, e76732. Fan, X., van Westen, C.J., Xu, Q., Gorum, T., Dai, F., 2012. Analysis of landslide dams induced by the 2008 Wenchuan earthquake. Journal of Asian Earth Sciences 57, 25–37. Fan, X., Xu, Q., Scaringi, G., Dai, L., Li, W., Dong, X., Zhu, X., Pei, X., Dai, K., Havenith, H.-B., 2017. Failure mechanism and kinematics of the deadly June 24th 2017 Xinmo landslide, Maoxian, Sichuan, China. Landslides 14, 2129– 2146. Gardner, T.W., 1983. Experimental study of knickpoint and longitudinal profile evolution in cohesive, homogeneous material. Geological Society of America Bulletin 94, 664–672.
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Gorum, T., Fan, X., van Westen, C.J., Huang, R.Q., Xu, Q., Tang, C., Wang, G., 2011. Distribution pattern of earthquake-induced landslides triggered by the 12 May 2008 Wenchuan earthquake. Geomorphology 133, 152–167. Hayakawa, Y.S., Yokoyama, S., Matsukura, Y., 2008. Erosion rates of waterfalls in post-volcanic fluvial systems around Aso volcano, southwestern Japan. Earth Surface Processes and Landforms 33, 801–812. Hewitt, K., 2006. Disturbance regime landscapes: mountain drainage systems interrupted by large rockslides. Progress in Physical Geography 30, 365–393. Hewitt, K., 1998. Catastrophic landslides and their effects on the Upper Indus streams, Karakoram Himalaya, northern Pakistan. Geomorphology 26, 47–80. Howard, A.D., Dietrich, W.E., Seidl, M.A., 1994. Modeling fluvial erosion on regional to continental scales. Journal of Geophysical Research: Solid Earth 99, 13971– 13986. Hungr, O., Evans, S., Hutchinson, I., 2001. A Review of the Classification of Landslides of the Flow Type. Environmental & Engineering Geoscience 7, 221–238. Jansen, J.D., 2006. Flood magnitude–frequency and lithologic control on bedrock river incision in post-orogenic terrain. Geomorphology 82, 39–57. Jansen, J.D., Fabel, D., Bishop, P., Xu, S., Schnabel, C., Codilean, A.T., 2011. Does decreasing paraglacial sediment supply slow knickpoint retreat? Geology 39, 543–546. Kirby, E., Whipple, K., Tang, W, Chen, Z., 2003. Distribution of active rock uplift along the eastern margin of the Tibetan Plateau: Inferences from bedrock channel longitudinal profiles. Journal of Geophysical Research 108, B4, 2217. Kirby, E., Whipple, K.X., 2012. Expression of active tectonics in erosional landscapes. Journal of Structural Geology 44, 54–75. Korup, O., 2006. Rock-slope failure and the river long profile. Geology 34, 45–48. Korup, O., Densmore, A.L., Schlunegger, F., 2010a. The role of landslides in mountain range evolution. Geomorphology 120, 77–90. Korup, O., Montgomery, D.R., Hewitt, K., 2010b. Glacier and landslide feedbacks to topographic relief in the Himalayan syntaxes. Proceedings of the National Academy of Sciences 107, 5317–5322. Kun-Ting, C., Chia-Hsing, L., Xiao-Qing, C., Gui-Sheng, H., Xiao-Jun, G., Chjeng-Lun, S., 2018. An assessment method for debris flow dam formation in Taiwan. Earth Sciences Research Journal 22, 37–43. Li, T., Schuster, R.L., Wu, J., 1986. Landslide dams in south-central China, in: Landslide Dams: Processes, Risk, and Mitigation. ASCE, pp. 146–162. Molnar, P., Anderson, R.S., Anderson, S.P., 2007. Tectonics, fracturing of rock, and erosion. Journal of Geophysical Research: Earth Surface 112. Ouimet, W.B., Whipple, K.X., Royden, L.H., Sun, Z., Chen, Z., 2007. The influence of large landslides on river incision in a transient landscape: Eastern margin of the Tibetan Plateau (Sichuan, China). Geological Society of America Bulletin 119, 1462–1476. 19
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Ren, J., Xu, X., Zhang, S., Yeats, R.S., Chen, J., Zhu, A., Liu, S., 2017. Surface rupture of the 1933 M 7.5 Diexi earthquake in eastern Tibet: implications for seismogenic tectonics. Geophysical Journal International 212, 1627–1644. Schwanghart, W., Scherler, D., 2017. Bumps in river profiles: uncertainty assessment and smoothing using quantile regression techniques. Earth Surface Dynamics 5. Schwanghart, W., Scherler, D., 2014. TopoToolbox 2–MATLAB-based software for topographic analysis and modeling in Earth surface sciences. Earth Surface Dynamics 2, 1–7. Shawky, M., Moussa, A., Hassan, Q.K., El-Sheimy, N., 2019. Pixel-Based Geometric Assessment of Channel Networks/Orders Derived from Global Spaceborne Digital Elevation Models. Remote Sensing 11, 235. Tachikawa, T., Kaku, M., Iwasaki, A., 2011. Aster GDEM version 2 validation report. Report to the ASTER GDEM Version 2. Tadono, T., Takaku, J., Tsutsui, K., Oda, F., Nagai, H., 2015. Status of ―ALOS World 3D (AW3D)‖ global DSM generation, in: 2015 IEEE International Geoscience and Remote Sensing Symposium (IGARSS). IEEE, pp. 3822–3825. Takaku, J., Tadono, T., Tsutsui, K., Ichikawa, M., 2016. Validation of" AW3D" global DSM generated from Alos Prism. ISPRS Annals of the Photogrammetry, Remote Sensing and Spatial Information Sciences 3, 25. Wei, R., Zeng, Q., Davies, T., Yuan, G., Wang, K., Xue, X., Yin, Q., 2018. Geohazard cascade and mechanism of large debris flows in Tianmo gully, SE Tibetan Plateau and implications to hazard monitoring. Engineering Geology 233, 172– 182. Whipple, K.X., 2004. Bedrock rivers and the geomorphology of active orogens. Annu. Rev. Earth Planet. Sci. 32, 151–185. Yunus, A.P., Oguchi, T., Hayakawa, Y.S., 2016. Remote identification of fluvial knickzones and their imprints on landscape morphology in the passive margins of Western Arabia. Journal of Arid Environments 130, 14–29. Zhao, S., Chigira, M., Wu, X., 2019. Gigantic rockslides induced by fluvial incision in the Diexi area along the eastern margin of the Tibetan Plateau. Geomorphology 338, 27–42.
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Journal Pre-proof Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Xuanmei Fan, Ali P. Yunus, John D. Jansen, Lanxin Dai, Alexander Strom, Qiang Xu PII:
S0169-555X(19)30454-4
DOI:
https://doi.org/10.1016/j.geomorph.2019.106963
Reference:
GEOMOR 106963
To appear in:
Geomorphology
Received date:
8 August 2019
Revised date:
5 November 2019
Accepted date:
16 November 2019
Please cite this article as: X. Fan, A.P. Yunus, J.D. Jansen, et al., Comment on ‘Gigantic rockslides induced by fluvial incision in the Diexi area along the eastern margin of the Tibetan Plateau’ by Zhao et al. (2019) Geomorphology 338, 27–42, Geomorphology(2019), https://doi.org/10.1016/j.geomorph.2019.106963
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© 2019 Published by Elsevier.
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Comment on ‘Gigantic rockslides induced by fluvial incision in the Diexi area along the eastern margin of the Tibetan Plateau’ by Zhao et al. (2019) Geomorphology 338, 27–42 Xuanmei Fan1*, Ali P. Yunus1*, John D. Jansen1,2, Lanxin Dai1, Alexander Strom1,3, Qiang Xu1 1
State Key Laboratory of Geohazard Prevention and Geoenvironment Protection,
Chengdu University of Technology, Chengdu, Sichuan, China, 610059. GFÚ Institute of Geophysics, Czech Academy of Sciences, Prague, Czechia.
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Geodynamics Research Centre, Volkolamskoe Shosse 2, 125080 Moscow, Russia
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* Corresponding author: Prof. Xuanmei Fan ([email protected]) and Dr. Ali P. Yunus
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([email protected]) Abstract
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Zhao et al. (2019) examine gigantic landslides in the Diexi area along the eastern
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margin of the Tibetan Plateau and propose their successive failure based on a knickpoint migration conceptual model. They postulate that a major river knickpoint
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(KpMJ) identified by them in the middle reaches of the Minjiang River (Sichuan, China)
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was initiated at the Longmenshan Fault and subsequently propagated ~ 85 km to its present position upstream of the Diexi lake. They then argue that this retreating
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knickpoint left in its wake an inner gorge that undercut and destabilized hillslopes, triggering a series of large landslides in the Diexi area. We question this interpretation based on our high-resolution landslide mapping, an analysis of knickpoints (i.e., profile convexities > 30 m high) in the Minjiang channel network, and field observations of lacustrine sediments and epigenetic gorges associated with the Diexi landslides. We confront the model proposed by Zhao et al. (2019) with three key arguments: 1) Major profile convexities in the Diexi area, including KpMJ, are associated with landslide or debris flow deposits and there is no basis for connecting explicitly any of these to long-distance knickpoint retreat; 2) the giant Diexi paleolandslide predates the debris avalanches at KpMJ, therefore the latter cannot have been the trigger for landsliding in this area; and 3) the spatial distribution of 666
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Key words: knickpoint, landslides, lacustrine sediments, Tibetan plateau.
1. Introduction
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The complex interplay between tectonic uplift, river incision, and mechanisms that
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trigger landslides has inspired a large body of research with much yet to be unraveled. This is especially so for the role of river incision driven by retreating knickpoints: how
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they are initiated and how they propagate the signal of base level throughout
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landscapes (Kirby and Whipple, 2012). Zhao et al. (2019) investigate large landslides
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in the Diexi area along the middle reaches of the Minjiang River, Sichuan, China (Fig. 1) (jiang means river in Mandarin Chinese). They identify a major river profile
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convexity (KpMJ) upstream of Diexi lake, which they postulate is a knickpoint that has migrated ~ 85 km upstream, initiating at the Longmenshan Fault zone. Zhao et al.
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(2019) go on to ascribe fluvial knickpoint retreat as the main prerequisite for giant
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slope failures in this region. They suggest that the long-distance retreat of KpMJ, in particular, undercut and destabilized hillslopes in its path thereby triggering the Diexi landslides.
The landslide inventory and in-depth descriptions provided by Zhao et al. (2019) enrich the landslide database in this region. We appreciate their insightful analysis of the role of geological structures on landslides, particularly wedge-type failures. Nevertheless, the interactions between river incision, tectonic uplift, and triggering mechanism of landslides are still unclear, especially the causes and effects of knickpoint migration. Based on our extensive field and topographic analyses in the Diexi valley, we question the claim of Zhao et al. (2019) that a knickpoint triggered at
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Journal Pre-proof the Longmenshan Fault has migrated ~ 85 km to a position upstream of the Diexi lake. We propose that the knickpoint referred to by Zhao et al. (2019) as KpMJ is instead a breached debris flow dam with no relationship to long-distance knickpoint retreat. We build our case upon two key analyses:
1) River damming by large landslides. By independently mapping the distribution of all river profile convexities (knickpoints) located in the Minjiang channel network
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upstream of the Longmenshan Fault, we suggest that all major convexities (> 30 m
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high) in the Diexi area are associated with landslide accumulations of various types. The profile convexity at KpMJ is categorically a debris fan deposit, not a retreating
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knickpoint.
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2) Spatial distribution of knickpoints. We conduct an automated analysis of knickpoints and according to the scaling relationship between knickpoint retreat and
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drainage area, we produce a χ-transform plot of Minjiang river profiles to examine the spatial distribution of 666 knickpoints upstream of the Longmenshan Fault. We show
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that knickpoint distribution is not consistent with simple long-distance transmission of
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base level fall initiated at the Longmenshan Fault.
2. River damming by large landslides We independently re-mapped the landslides in the Diexi region (the same areas studied by Zhao et al., 2019) using an airborne LiDAR high-resolution (50 cm) digital elevation model (Fig. 1) and we validated our mapping via detailed field observations (Figs. 2 and 3). We set out to investigate the spatial distribution of river profile convexities along the Minjiang River and its tributaries using the software package, TopoToolbox (Schwanghart and Scherler, 2014, 2017), along with the 30 m resolution digital elevation model, ALOSW3D (Fig.1). The results of our landslide mapping agree in general with that of Zhao et al. (2019) with two important exceptions: 1) We find
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Journal Pre-proof notable differences in the size of some landslides, such as the Diexi-S rock slope failure, which we interpret as being much smaller (cf. Fig. 2 in Zhao et al., 2019). In this case, we suggest that the catchment accumulation area has been misinterpreted as a landslide scarp; and 2) Three epigenetic gorge sections—vital for understanding the evolution of the Minjiang valley—were not identified by Zhao et al. (2019) (cf. their
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Fig. 3 with our Fig. 1b below).
Figure 1. Diexi study area and landslide inventory map: (a) KpMJ (Zhao et al., 2009) shown relative to regional distribution of landslides, including the Diexi paleolandslide, and maximum extent of the dammed Diexi lake (cyan) reconstructed from lacustrine sedimentation up to 2370 m a.s.l. (b) Local Diexi area, including two epigenetic gorge sections where the Minjiang River has been laterally displaced by landslides, knickpoints identified from the 30-m digital elevation model, ALOS AW3D, and location of the 160 m-deep drillhole (filled orange rectangle) into lacustrine sediments
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Examples of rivers temporarily dammed by landslide deposits are well-documented (e.g., Hewitt, 1998, 2006; Hungr et al., 2001; Kun-Ting et al., 2018; Wei et al., 2018) and are a relatively common occurrence in settings, such as Diexi, which are tectonically-active and high-relief (Korup, 2006; Korup et al. 2010a,b). According to Zhao et al. (2019): “the major knickpoint KpMJ is 20 km from the Diexi landslide, where there are no
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landslides, faults or lithologic boundaries, which suggests that it was not generated by these factors and must have occurred because a knickpoint propagated from
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downstream. The knickpoint likely initiated at the intersection of the Minjiang River
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and the Longmenshan fault belt, which is an active thrust fault with upheaval of the
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eastern Tibetan Plateau to the west (p. 39)”.
In other words, Zhao et al. (2019) seems to draw a direct line of evidence between
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absence of landslide deposits at KpMJ and long-distance knickpoint retreat. A key
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corollary of their model is that KpMJ predates the giant landslides downstream, which
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were triggered by the destabilizing effect of knickpoint retreat.
We raise three points in response to the statement cited above. First, our field observations at KpMJ reveal no visible bedrock in the channel boundary at this site. The channel and valley sides do, however, show remnants of multiple large debris flow fans, stemming from the adjoining tributary valleys (Fig. 1a). These tributaries have large catchment areas (37 and 2.1 km2 for right and left bank tributaries, respectively) and it seems clear (Fig. 2a,b) that large debris flow fans temporarily dammed the Minjiang River at the KpMJ location and are responsible for forming the profile convexity at ~ 2321 metres above sea level (m a.s.l.) or higher.
Second, the debris flow deposits at KpMJ overlie lacustrine sediments at 2370 m a.s.l 5
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Zhao et al., 2019).
Figure 2. Views at the KpMJ location, showing remnants of debris flow fans from adjoining right and left tributary valleys: (a) Minjiang River channel incising debris flow deposits at the apex of KpMJ; (b) terraces formed by the debris flow deposits at the mouth of tributary catchment (45 m thick); and (c) debris flow deposits overlying older lacustrine sediments.
Third, we identify another set of lacustrine sediments (Fig. 3) ~ 12 km upstream of the location of KpMJ (Fig. 1) at an elevation of 2460 m a.s.l. Given that these deposits
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lacustrine sediments has not been determined for either lake.
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~ 12 km upstream of the KpMJ.
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Figure 3. Lacustrine sediments (32ᵒ 16' 13.071 N, 103ᵒ 44' 37.295 E) at 2460 m a.s.l.
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These three sets of observations contradict the model presented in Figure 14 of Zhao et al. (2019): the profile convexity at KpMJ is a debris fan deposit, not a retreating
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knickpoint; KpMJ postdates the giant Diexi paleolandslide, thereby defusing their argument regarding hillslope debuttressing in the knickpoint‘s wake. The implication of multiple lakes is that river damming is not restricted to reaches downstream of retreating knickpoints. On the contrary, river damming is likely to be very commonplace in this landscape (Hewitt, 2006)—a point we emphasise in the next section.
3. Spatial distribution of knickpoints Knickpoints are convexities in river long profiles that occur as two main types: static and mobile. Both are relevant here. Static knickpoints develop most commonly at sites of less erodible substrate, such as strong, unjointed rock (e.g., Jansen, 2006), or
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Journal Pre-proof inputs of landslide debris that cannot be readily entrained (e.g., Korup, 2006). Static knickpoints do not propagate appreciably: knickpoints on strong rocks are retained within their lithological contacts and a landslide convexity will diffuse in situ as immobile debris is gradually eroded and transported (Korup et al., 2010a). Mobile knickpoints typically reflect climate-related enhanced discharge, or base level fall due to crustal deformation (Crosby and Whipple, 2006), and can be likened to transient ‗incisional waves‘ that propagate upstream at rates determined by discharge, bed
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slope, substrate erodibility, and sediment load (Gardner, 1983; Howard et al., 1994;
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Hayakawa et al., 2008; Jansen et al., 2011; Yunus et al., 2016). Zhao et al. (2019) interpret KpMJ as a mobile knickpoint produced by fault-related base level fall, but as
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we note in the section above, KpMJ is associated with debris flows and no bedrock
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outcrop is visible. This does not strictly preclude the possibility that a mobile
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knickpoint has propagated from downstream to the KpMJ location, hence we test that
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scenario below.
In our analysis of the spatial distribution of knickpoints, we conduct an experiment to
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test the model of Zhao et al. (2019) that KpMJ is genetically linked to the propagation
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of a knickpoint initiated by base level fall at the Maoxian-Wenchuan component of the Longmenshan Fault ~ 85 km downstream. A knickpoint generated in this way will propagate upstream throughout the channel network at a rate that is scaled to drainage area (a proxy for discharge), bed slope, substrate erodibility, and sediment load (Whipple, 2004). Unlike Zhao et al., we analyse the entire Minjiang catchment upstream of the Longmenshan Fault. We use TopoToolbox (Schwanghart and Scherler, 2014), a MATLAB-based topographic analysis software package to automate the extraction of knickpoints from the ALOSW3D 30 m DEM. Following the approach of Schwanghart and Scherler (2017), we analyse the spatial distribution of all knickpoints in the Minjiang catchment, applying a minimum knickpoint height threshold of 30 m. Our analysis yields several important results (Figs. 4 and 5) and
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1) We identify a total of 666 knickpoints (> 30 m high) in the Minjiang channel network (>9 km2 accumulation area), some several hundred metres in height. A small fraction of these profile convexities may be due to DEM error; however, field observations confirm the dimensions of the Diexi valley knickpoints and this lends confidence to our analysis.
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2) KpMJ (~ 35 m high) is not among the most pronounced knickpoints in the Minjiang
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channel network (falling within the smallest quartile) and its plotting position in Figure 5 brings no suggestion of any special significance. The height of river profile
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convexities identified by Zhao et al. (2019) at Diexi (KpD) and Yingpinya (KpY) are
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55 m and 255 m, respectively (the Diexi dam having incised appreciably over time).
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3) The spatial distribution of the 666 knickpoints is scattered fairly uniformly across the drainage network (Figs. 4 and 5). A simple fault-triggered incision wave propagating upstream from the Longmenshan Fault would produce a dense cluster
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of knickpoints with similar χ-value; however, the scattered distribution in Figure 5b
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indicates categorically that the distribution of major knickpoints is not the result of a
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simple knickpoint retreat.
4) Figure 4 demonstrates that knickpoints are extremely common in the channel network. Some of these are known to be the static lithological knickpoint type (e.g., see Fig. 10 in Kirby et al., 2003) and we acknowledge that our automated method is less accurate in the steep, low-order channels. Nevertheless, we suggest that the dominant fraction of convexities in larger streams is the result of landslides partly or fully blocking river channels (i.e. Hewitt, 2006; Korup, 2006). Moreover, such a dense array of landslide dams must severely impede the propagation of knickpoints over time (Ouimet et al., 2007; Korup et al., 2010b), especially given that landslide dams > 100 m high are common.
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Figure 4. (a) The Minjiang river catchment upstream of the Longmenshan Fault zone, showing knickpoints > 30 m high (white filled circles), and major faults (heavy black lines); note two NE-striking components of the Longmenshan Fault zone are shown: the Mao-Wen and Yingxiu-Beichuan faults. (b) The Diexi valley area, showing major knickpoints identified by Zhao et al. (2019): Diexi (KpD), Yinpingya (KpY), and KpMJ. Also shown are other major landslide dams: Diexi-S, Dragon, and Manaoding. Base map derives from the ALOSW3D 30 m DEM.
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Figure 5. (a) River longitudinal profiles (blue lines) of the Minjiang mainstem and tributary network upstream of the Longmenshan Fault extracted from ALOSW3D 30 m data (minimum drainage area threshold = 9 km2). A total of 666 knickpoints > 30 m height (white filled circles). The mainstem steepening at ~ 110 km marks the Mao-Wen branch of the Longmenshan Fault. The macro-knickpoint (~ 290 km) is a lithological knickpoint corresponding to a Mesozoic granite pluton (see Fig. 10 in Kirby et al., 2003). (b) The same river profiles plotted with a χ-transformation (Schwanghart and Scherler 2014), a proxy for the rate of knickpoint retreat. The assumed profile concavity (m/n = 0.22) is determined from a Bayesian optimization test.
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Journal Pre-proof An additional major impediment to long-distance knickpoint retreat is the occurrence of intermittent epigenetic gorges. These are bedrock channel segments created when rivers incise into valley-fill and are superimposed onto an adjacent bedrock hillslope, essentially bypassing the abandoned paleovalley, which often fills with sediment (Hewitt, 1998; Ouimet et al., 2008). Epigenetic gorges are most commonly associated with phases of rapid aggradation or river damming due to landsliding and their effect on the evolution of river profiles is profound (Ouimet et al., 2008). Channel
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entrenchment at an epigenetic gorge essentially raises the local base level
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(sometimes hundreds of metres) above the valley thalweg, forcing the river to cut
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another bedrock valley adjacent to the original (Fig. 6 and 7).
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Our mapping and field studies reveal the existence of at least three epigenetic gorge
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segments along the Minjiang River immediately downstream of the giant Diexi landslide (Fig. 1b). The pre-existing valley (paleovalley) is now filled with either
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landslide deposits or fluvial deposits (Fig. 6 and 7). We note that the original (pre-giant landslide) bedrock geometry of the valley is unknown, although our recent drilling of
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Diexi paleolake sediments (Tuanjie village, 32 ᵒ 2' 42" N, 103ᵒ 40' 3" E) (Fig. 1b) reveals
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lacustrine materials > 160 m thick (i.e., down to ~ 2007 m a.s.l.). We accept that ~ 2000 m a.s.l. (as shown in Fig. 11 of Zhao et al., 2019) was possibly the pre-landslide river bed, but we see no basis for interpreting this level as bedrock. Given that we know of no bedrock exposed anywhere in the study reaches (aside from the epigenetic gorges), we anticipate that the sedimentary cover thickness could be on the order of 101–102 m—though no data exist to verify the true depth.
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Figure. 6. Epigenetic gorge in the Minjiang River (see Fig. 1): (a) Panorama viewed from the left-bank and downstream section of the Deixi paleolandslide; (b) and (c)
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views of the new channel incising bedrock (dashed line shows position of abandoned
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section of valley).
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Figure 7. (a) Overview at Dragon, showing the epigenetic gorge relative to the palaeovalley. (b) Upstream part of the palaeovalley filled by the deposits of the
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Dragon rockslide (RS). The new epigenetic gorge passes right from the left-bank bedrock cliff (BR). (c) Left bank of the epigenetic gorge.
4. Discussion 4.1 Knickpoint retreat is not essential to landsliding Central to the model proposed by Zhao et al. (2019) is that, 1) knickpoints in this landscape incise the bedrock valley trough while propagating long distances upstream throughout the channel network, and 2) KpMJ represents ~ 85 km of knickpoint retreat from the Longmenshan Fault to its present location (Fig. 1). We question both points. The Minjiang River and its tributaries exhibit the typical
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profile (Fig. 5a,b) is that the complex of large landslides and associated epigenetic
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gorges at Diexi have retarded knickpoint retreat; however, this hypothesis awaits
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We emphasise that there is no evidence that knickpoint retreat is a necessary
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precursor to large-scale slope failures. While we agree with Zhao et al. (2009) that earthquakes may be the dominant trigger for landsliding in the Diexi valley, apart from
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the giant landslide caused by the 1933 earthquake, the triggers for the other landslides remain unknown. Located at the eastern margin of the Tibetan Plateau, this
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high-relief and tectonically-active region is subject to strong earthquakes and extreme
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rates of rock uplift accommodated largely via thrusting and transgressive shortening along major regional faults (Kirby et al., 2003). In this landscape, river incision drives hillslope erosion that ultimately governs landscape-wide denudation rates (Whipple, 2004). However, it is also the case that rocks in this area are primarily weak and highly-fractured meta-sediments. The high frequency of seismic shaking connected with strong earthquakes, such as the 1933 Ms 7.3 Diexi earthquake (Li et al., 1986; Ren et al., 2018), the 1976 Ms 7.2 Songpan-Pingwu earthquake (Deng et al., 2013), and the 2008 Ms 8.0 Wenchuan earthquake (Gorum et al., 2010 and Fan et al., 2012), is likely to dilate pre-existing rock fractures and create new ones, thereby reducing the internal strength of hillslopes (i.e. Molnar et al., 2007). This rock damage combined with steep topography are the major causes for high concentration of large landslides
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4.2 The accuracy of DEMs used in our mapping The identification of knickpoints from river profile analysis is always subject to the accuracy of the digital elevation model (Schwanghart and Scherler, 2017) and is therefore relevant here. We list the DEMs used in Zhao et al. (2019), and those used
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in our study with their vertical uncertainties in Table 1.
The ALOSW3D 30 m data used in this study is a resampled version of the ALOSW3D
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5 m data used by Zhao et al. (2019). We note that the profile convexity at KpMJ is not
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ALOS DEM used by us; that is, the convexity is only detected in the 5 m and the
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resampled 30 m version of the ALOSW3D DEM.
Zhao et al. (2019) analyse the distribution of knickpoints only along a 50 km reach of
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the Minjiang River and, in detail, the immediate area surrounding the complex of
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landslides at Diexi. From a logarithmic plot of mean channel slope versus drainage area, they visually identify three ‗major knickpoints‘ as sharp breaks in the distribution. There is nothing intrinsically incorrect about this manual, semi-quantitative approach, but the spatially restricted extent of their analysis means they are severely limited in their capacity to understand processes and drivers in the areas beyond, such as at the Longmenshan Fault. Moreover, such a spatially restricted analysis falls well short of the comprehensive geomorphic mapping that is routinely conducted nowadays, with freely-available tools such as TopoToolbox (Schwanghart and Scherler, 2014).
Table 1. Digital elevation models (DEMs) used in Zhao et al. (2019) and in our study, listing published values of root mean square error (RMSE).
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Journal Pre-proof DEMs used & their resolution
Source
RMSE (m)
References (RMSE)
(i) ALOS AW3D 5m
The Japan Aerospace Exploration Agency (JAXA)
2.13 – 8.58
Takaku et al. (2016)
(ii) ASTER GDEM 30 m
The Ministry of Economy, Trade, and Industry (METI) of Japan and the United States National Aeronautics and Space Administration (NASA)
13.75
Tachikawa et al. (2015)
This study
(i) ALOS AW3D 30 m
The Japan Aerospace Exploration Agency (JAXA)
2.14 – 6.67
Tadono et al. (2015); Shawky et al. (2019)
(ii) ALOS PALSAR 12.5 m
Alaska Satellite Facility (ASF)
4.57
(iii) LiDAR 50 cm
Sichuan Bureau of Surveying, Mapping and Geoinformation
(small area covered, used only for mapping the landslides)
Shawky et al. (2019)
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Zhao et al. (2009)
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5. Conclusions
On the basis of high-resolution landslide mapping, analysis of knickpoints (profile
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convexities > 30 m high) in the Minjiang channel network, and field observations of
Diexi area, we find that:
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lacustrine sediments and epigenetic gorges associated with giant landslides in the
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1) all major profile convexities in the Diexi area, including the KpMJ, are associated with large accumulations of mass movement debris and there is no basis for connecting explicitly any of these to long-distance knickpoint retreat; 2) the giant Diexi paleolandslide predates the debris avalanches at KpMJ, therefore the latter cannot have been the trigger for landsliding in this area; and 3) the spatial distribution of 666 mapped knickpoints in the Minjiang River mainstem and tributaries is not consistent with long-distance propagation of a simple incisional wave initiated at the Longmenshan Fault.
Large landsides in the Diexi region, in our view, are the outcome of a complex interplay of several factors: high-relief hillslopes, high seismicity, and the densely
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Journal Pre-proof fractured and fragile rocks. A geophysics survey to establish the basal morphology of the bedrock valley trough, and absolute dating of the landslides and lacustrine sediments to establish the chronology on their formation are future priorities at Diexi.
Acknowledgements This research was supported by the National Science Fund for Outstanding Young Scholars of China (grant no. 41622206), the Fund for Creative Research Groups of (grant
no.
41521002),
the
Fund
for
International
Cooperation
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China
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(NSFC-RCUK_NERC), Resilience to Earthquake-induced landslide risk in China (grant no. 41661134010), the National Key R & D Program of China (grant no.
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2017YFC1501002), and the Fund of SKLGP (SKLGP2016Z002). We thank W.
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Schwanghart for his kind assistance with TopoToolbox and A. Joshua West for
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discussions in the field.
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Journal Pre-proof Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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