Gigantic rockslides induced by fluvial incision in the Diexi area along the eastern margin of the Tibetan Plateau

Accepted Manuscript Gigantic rockslides induced by fluvial incision in the Diexi area along the eastern margin of the Tibetan Plateau

Siyuan Zhao, Masahiro Chigira, Xiyong Wu PII: DOI: Reference:

S0169-555X(19)30149-7 https://doi.org/10.1016/j.geomorph.2019.04.008 GEOMOR 6736

To appear in:

Geomorphology

Received date: Revised date: Accepted date:

26 October 2018 26 February 2019 9 April 2019

Please cite this article as: S. Zhao, M. Chigira and X. Wu, Gigantic rockslides induced by fluvial incision in the Diexi area along the eastern margin of the Tibetan Plateau, Geomorphology, https://doi.org/10.1016/j.geomorph.2019.04.008

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ACCEPTED MANUSCRIPT Gigantic rockslides induced by fluvial incision in the Diexi area along the eastern margin of the Tibetan Plateau Siyuan Zhao a,*, Masahiro Chigira b, Xiyong Wu c a

Department of Geophysics, Graduate School of Science, Kyoto University, Gokasho, Uji, Kyoto 611-0011,

Japan Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto 611–0011, Japan

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Faculty of Geosciences and Environmental Engineering, Southwest Jiaotong University, Chengdu, 611756,

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China

Corresponding author address:

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Name: Siyuan Zhao

Address: E-305D, Division of Geohazards, Disaster Prevention Research Institute, Kyoto University, Gokasho,

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Uji, Kyoto, Japan E-mail: [email protected] Tel: +81-774-38-4099

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Fax: +81-774-38-4105

ACCEPTED MANUSCRIPT Gigantic rockslides induced by fluvial incision in the Diexi area along the eastern margin of the Tibetan Plateau

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1. Introduction

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Abstract A number of large landslides, which have strongly affected fluvial processes in the upstream catchment of the Minjiang River along the eastern margin of the Tibetan Plateau, Sichuan, were investigated with regards to both geological structure and topographic development. Topographic analysis suggested that there is a major knickpoint that formed not as a result of landslides but via tectonic activity and that the knickpoint propagated upstream, forming an inner gorge and undercutting and destabilizing nearby slopes. The effects of the knickpoint propagation and inner gorge formation on slope stability are dependent on the geometrical relationships between the river and geological structures. When the geological trend is normal or highly oblique to the river axis, landslides generally do not occur, but gigantic rockslides have occurred on one side of the Minjiang valley, which can be attributed to a wedge structure consisting of bedding planes and joints with intersections dipping valleyward and tight folds with hinges plunging valleyward. When the beds are planar, moderately dipping, and strike nearly parallel to the river, buckling deformation commonly occurs on cataclinal slopes, often transforming into catastrophic failure. Landslide dams form another type of knickpoint on the river channel that gradually disappear from the downstream to upstream as a result of river erosion. Our findings strongly suggest that the study of slope development by river incision must consider geological structures and that an understanding of geological structures and river incision history can provide a conceptual model for the prediction and mitigation of geohazards in tectonically active drainage basins. Keywords: Minjiang River, Rockslide, Geological structure, Fluvial incision, Knickpoint.

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Many large rockslides and deep-seated gravitational slope deformations (DGSDs) have occurred along major rivers in rapidly uplifting areas, where high-relief hillslopes and narrow river valleys are common (Schmidt and Montgomery, 1995; Burbank et al., 1996; Ambrosi and Crosta, 2006; Korup et al. 2010; Agliardi et al., 2013). These mass movements are the result of the interaction of fluvial incision and hillslope processes; large-scale slope instability is commonly induced by the formation of an inner gorge (Korup and Schlunegger, 2007; Chigira, 2009; Hiraishi and Chigira, 2011; Tsou et al., 2014). An inner gorge is a valley created in a previously established valley, and a mark of a new phase of river incision that divides the upper gentler slopes and lower steepened slopes with a convex slope break on the rim (Kelsey, 1988). It is formed by upstream migration of a knickpoint, a sharp changing point in a longitudinal river profile (Clark et al., 2005; Crosby and Whipple, 2006; Harkins et al., 2007; Schmidt et al., 2015). It has been studied that knickpoints are created not only by river rejuvenation (Whipple, 2004; Hiraish and Chigira, 2011), but also by landslide dams (Korup 2006; Ouimet et al., 2007; Korup et al., 2010). Landslide dams are thus the result of knickpoint propagation in some cases and also the cause of knickpoint formation in other cases. Another important factor for mass movements is geological structure, which has not been appropriately considered in the context of fluvial incision and slope destabilization. Only a few studies (Chigira, 2009; Tsou et al., 2015) investigated the mechanisms of mass movements concerning geological structure on the interaction between fluvial processes and slope processes. Arai and Chigira (2018) also reported that a slope underlain by downslope-dipping thrust fault became destabilized by river erosion at its foot and

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finally failed catastrophically. Therefore, understanding the interrelationships between fluvial incision and geologic structures is crucial in evaluating the risk of catastrophic landslides. The steep landscape along the eastern margin of the Tibetan Plateau has been established by rapid orogenic uplift of the Longmenshan fault, and has been deeply dissected by river erosion into an unincised plateau surface (Kirby et al., 2002; Godard et al., 2009; Ouimet et al., 2009; Wang et al, 2012; Zhang et al., 2017). Accordingly, the river catchment (i.e., the adjacent Minjiang and Daduhe catchments) in this abruptly changing topographic region is a good place to study the destabilization of slopes by river erosion. However, most of the previous studies in this area mainly focused on earthquake-induced landslides resulting from the 2008 Wenchuan earthquake along the Longmenshan Fault (Chigira et al. 2010; Qi et al., 2010; Gorum et al., 2011; Tang et al., 2011; Hunag et al., 2012); the distribution features of landslides and DGSDs are not well characterized and long-term slope development has not been sufficiently studied. Ouimet et al. (2007) studied the relations between landslides and landform evolution and reported that large landslides created knickpoints in the Daduhe catchment on the west of the Minjiang catchment, but no detailed landslides and geology are described along the plateau margin. To interpret landscape development by river erosion and widely predict and evaluate potential sites of catastrophic landslides, it is important to accumulate case studies of landslides and DGSDs considering internal geological structures along rivers along the eastern margin of the Tibetan Plateau. We focused on the Diexi area, a river segment where a number of prehistoric gigantic landslides are mostly concentrated in the upstream catchment of the Minjiang River (Kirby et al., 2002; Korup et al., 2010; Wang et al., 2010). In this study, we investigated rockslides in the Diexi area in the Minjiang catchment through geological field surveys, geomorphological observations, satellite image interpretations and GIS-based topographic analyses. The main objectives of this study were to (i) identify and characterize the geomorphological and geological features of large rockslides in and around the Diexi area, (ii) investigate the mechanism and history of the large rockslides, and (iii) discuss the response of valleyside slopes to river incision.

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2. Study area

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The Diexi area is located along the eastern margin of the Tibetan Plateau and bounded by the Longmenshan fault belt adjacent to the Sichuan Basin (Fig. 1). The Longmenshan fault belt consists of three NE-striking faults, the Mao-Wen Fault, the Yingxiu-Beichuan Fault and Guanxian-Anxian Fault (Burchfiel et al., 1995; Fig. 1a), of which the Yingxiu-Beichuan Fault is the major fault that generated the 2008 Wenchuan earthquake (Li et al., 2008). The Longmenshan fault belt characterized the transitional terrain of the abrupt change from the Qinghai-Tibet Plateau to Sichuan Basin. It forms among the world’s most significant continental escarpments resulting from neotectonic activity and is an area of intense river incision (Chen et al., 1994; Kirby et al., 2002; Godard et al., 2010). Elevations rise from ~600 m asl in the southeastern Sichuan Basin to peaks exceeding 5500 m asl over a horizontal distance of 50–60 km (Clark and Royden, 2000). The steep topographic escarpment adjacent to the Sichuan Basin has been deeply dissected by rivers including the trunk of the Minjiang River (Fig. 1b). The local valley-ridge relief in many places along the escarpment exceeds 3000 m (Kirby et al., 2002). This mountainous region is underlain by rocks ranging from Precambrian to Triassic in age that mainly comprise granite, phyllite and other types metamorphic rocks, dolomite, limestone, sandstone, and shale (Fig. 1b). The trunk of the Minjiang River generally flows from north to south. The river segment from Maoxian to Wenchuan is parallel to the Longmenshan Fault, whereas the part upstream of Maoxian is normal or highly oblique to the strike of the fault belt. Tight folding with E–W trending axes has developed in the Diexi area. A series of landslides have prominently clustered in this area along the river (Fig. 1b), including the Diexi landslide which is the largest rockslide along the entire length of the Minjiang River.

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These gigantic landslides are aligned along major rivers rather than tributaries, as described in the results.

Figure 1. Geomorphological and tectonic setting of the upstream catchment of the Minjiang River. (a) Topography of the Minjiang catchment along the eastern margin of the Tibetan Plateau. The study area is in a high-relief mountainous area in the hanging wall of the Longmenshan Fault, which includes the Mao-Wen and Yingxiu-Beichuan faults in this region. (b) Simplified geological map of the Longmenshan region, modified after Burchfiel et al. (1995), based on 1:200,000 geological maps (Ministry of Geology and Mineral Resources, 1991). Landslides outlined by polygons in the Diexi area were identified in this study. The largest Diexi landslide is ~45 km upstream of Maoxian. Our study region, the Diexi area, is bounded by solid and dashed rectangles, which range from 13 km downstream to 20 km upstream of the Diexi landslide.

3. Methods Topographic investigations were conducted using DEMs (digital elevation models) and satellite images.

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We focused on the mapping of landslides and gravitational slope deformations and GIS-based analysis of landscape evolution. Landslide and DGSD inventories were compiled based on interpretation of high-resolution images, which were stereopairs of PRISM images (Scene IDs: ALPSMB 219693010 and ALPSMW 219692955) with a resolution of 2.5 m taken by the ALOS of the Japan Aerospace Exploration Agency (JAXA), as well as Google Earth images. We also stereoscopically examined PRISM image pairs to identify slope breaks, terraces, and other topographic features of the study area. We used DEMs of the ASTER GDEM (resolution = 30 m) and the ALOS AW3D DSM (resolution = 5 m) to quantitatively analyze long river profile, hillslope morphology, landslide size and extent using ArcGIS version 10.3.1. We conducted field investigations for 20 days using 1:25,000 scale topographic maps developed from the 5-m mesh DEM of the ALOS AW3D. Attitudes of bedding and cleavage were specifically investigated from outcrops in the field for the study of hillslope stability. We also made detailed observations of geomorphic features including slope breaks and linear depressions or bulges of DGSDs along the Minjiang River. Field survey allowed us to conduct more specific analysis of geological structures and geomorphologic characteristics of the landslides and DGSDs. Discontinuity analysis using the software COTOP-3D, designed by Terranum (Jaboyedoff and Couture, 2003), was completed to determine the spatial orientation of the structures. This enabled us to identify structural features (e.g., joint sets, scarps of faults or landslides) and measure their attitudes using 5-m DEMs in combination with our field survey.Longitudinal river profiles were produced using the Stream Profiler tools package (Wobus et al., 2006) in the ArcGIS 10.3 software and MATLAB, which generated a log-log power law relationship between the local channel slope and upstream drainage area as follows (Flint, 1974): 𝑆 = 𝑘𝑠 𝐴−𝜃

4. Results

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where S is the local channel slope, A is the upstream drainage area (m2), ks is the steepness index (m2θ), and θ is the concavity index. Analysis was conducted using 30-m ASTER-GDEM for the entire upstream Minjiang River and 5-m ALOS DSM for the Diexi area. Knickpoints were extracted by inspecting pronounced breaks in Flint’s law relating channel slope to upstream drainage area (Whipple, 2004; Wobus et al., 2006). Slope breaks were identified in the field using observations of the rims of inner gorges; hanging valleys were also helpful in identifying slope breaks.

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We identified 27 landslides in the Diexi area (Fig. 2). There are two large lakes along the Minjiang River (Xiaohaizi and Dahaizi lakes, Fig. 2) and three more lakes in a major tributary (the Songpinggou River), which are assumed to be the results of relatively small landslides induced by the 1933 Diexi earthquake (Wang et al., 2008). Outcrops of bedrock and landslide deposits in the study area were well exposed along the Minjiang River. Five typical landslides including Manaoding, Dragon, Diexi, Yinpingya and Xinmo were studied in detail.

4.1 General geological features in the study area The geological formations in the Diexi area are mainly Carboniferous, Permian and Triassic sandstone, alternating beds of pelitic and psammitic schists, and marble (Figs. 1b and 2). Landslide deposits including some huge rock masses are present in the riverbed. Lacustrine deposits consisting of stratified silt occur in patches, as described later. The strata in the study area mainly strike E–W or NE–SW, and the area can be divided into 4 domains, I to IV, from south to north, according to the geological structures (Fig. 2). Domain I is underlain by the alternating beds of pelitic and psammitic schists, which have consistent beddings striking NE–SW and

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dipping 70° to the SE. These rocks have cleavage that strikes WNW–ESE and dips 45–70° to the SSW. From Domain I to Domain II, the bedding strikes do not change but the beds become tightly folded. Domain III is mainly underlain by marble, the beds of which generally strike WNW–ESE and dip 30–64° to the SWS. Domain IV is underlain by alternating beds of pelitic and psammitic schists with beddings striking NW–SE and dipping to the SW. We identified six faults in the Diexi area along the river and they mostly strike ENE–WSW. One near the northern margin of Domain II has a crush zone up to 10 m in width with breccias and fault gouge; the others have crush zones with a width of 10–100 cm. These faults are not near the large landslides and likely have not controlled them. Joints distributed on some outcrops might have controlled landslides as will be described later in details.

Figure 2. Geological map and landslide inventory of the Diexi area. The area was divided into four domains based on the geological structures. Five representative gigantic rockslides (Manaoding, Dragon, Diexi,

ACCEPTED MANUSCRIPT Yinpingya, and Xinmo) were focused on in different geological settings.

4.2 Characterization of the rockslide in the Diexi area

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4.2.1 Manaoding landslide

Figure 3. Detailed geological map of the Manaoding and Dragon landslides. The two cross-sections (1–1’ and 2– 2’) were completed on the two landslides, shown in Figs. 4b and 5c. Topographic and geological features are

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described in the legend in Fig. 2.

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The Manaoding landslide is seated in alternating beds of pelitic and psammitic schist (Fig. 3). The landslide has an area of 2.38 km2, a 500-m-high head scarp and massive deposits on the valley bottom (Fig. 4a). The top of the head scarp is up to 1300 m above the riverbed. Very tight and complicated folds with an E–W-trending axis are visible on the head scarp and in outcrop opposite the landslide with tens of meters of fold wavelength. Talus deposits have accumulated just below the landslide scarp, covering the foot of the scarp and the proximal portion of the massive landslide deposit (Fig. 4b). The landslide deposit forms a mound at the foot of the landslide rising 300–400 m above the riverbed. The deposit could therefore be thicker than this exposed height. It contains massive blocks of folded bedrock as large as hundreds of meters. However, they have many random open fractures and gradually change to surrounding breccias, suggesting that they are actual parts of the landslide deposit (Fig. 4c). The contours of the intersections of the remaining bedding on the landslide deposits suggest the axes plunge 10° towards S65°W (Fig. 4d). The head scarp of this landslide is too high and too steep to access, thus the geological structures on the scarp were observed using binoculars and investigated in the opposite bedrock outcrop at the side of the river where the structures are assumed to be consistent with that of Manaoding landslide (Fig. 4e). Folds and joints in the opposite outcrop and massive landslide deposit were analyzed to show the characteristics

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of the geological structure. Stereonet analysis of the bedding planes of the opposite outcrop showed that the bedding generally strikes ENE–WSW and that the fold axis plunges 15° towards S77°W (Fig. 4f). This direction is very near the fold axis attitude (S65°W/7°W) in the deposit (Fig. 4d), which suggests that the landslide body slid without significant rotation. The joints observed in the riverside bedrock have intersections nearly parallel to the fold axes.

Figure 4. Geological structures and cross-section of the Manaoding landslide. (a) Sketch of the Manaoding landslide with tight folds on the scarp. (b) Cross-section 1–1’ of the landslide. The location is shown in Fig. 3. (c) Outcrop of massive rock blocks with remaining bedding planes (thin lines); field of view is ~200 m in width observed from the riverbed. Fractures and breccias occur in the deposit. (d) Stereonet analysis of the bedding in the blocks. The density contours of the bedding intersections indicate the fold axis (S65 °W/7°W); the counting area is 1 % of the net area. (e) Outcrop of bedrock on the opposite side of the Manaoding landslide (the location is shown in Fig. 3); field of view is ~400 m in width. The thick lines refer to the joints in outcrop and the thin lines are bedding. (f) Stereonet analysis of the discontinuities in the bedrock (the solid lines are bedding planes and the dotted lines are joints). The density contours of bedding intersections indicate the plunging direction of the fold axes (S77°W/15°W).

ACCEPTED MANUSCRIPT 4.2.2 Dragon landslide The area of the Dragon landslide is 0.39 km2. Bedrock is exposed in the 200-m high head scarp, which

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has a top 400–500 m above the riverbed (Fig. 3). The head scarp is crossed by a road, where we observed geological structures (Fig. 5a and b). The outcrop consists mainly of marble with subordinate pelitic and psammitic schists. Psammitic schist is exposed on the left flank of the landslide. The bedding attitudes at the scarp suggest a syncline with a half fold wavelength of >80 m, and minor drag folds were observed on the western limb of the syncline (Fig. 5a). On the bedding plane, lineations are developed (Fig. 5b) and parallel to the fold axis suggested by the stereonet analysis. Similar to the Manaoding landslide, the Dragon landslide deposit remain only on the left bank of the river with a height of 200 m. (Fig. 5c). Folds remain in the deposit, even though the beds fractured and transported as blocks. Stereonet analysis of the bedding planes, joints, faults and lineations shows that the discontinuity data from the head scarp are consistent with the data acquired from the opposite bank outcrop. The results indicate that the fold axis plunges 27° towards N79°W (Fig. 5d). Lineations observed on the limb of folds could suggest the direction of a hinge line that gently plunges towards the west (N86°W/21°W on the head scarp and N76°W/20°W in the opposite bank outcrop). There is another landslide south of the Dragon and north of the Manaoding landslide (Fig. 3) that has similar features to those of the Manaoding landslide, but it is not described in this study.

Figure 5. Geological structures and cross-section of the Dragon landslide. (a) Syncline exposed in the outcrop of head scarp. (b) Lineations observed on the bedding plane at the head scarp. (c) Cross-section 2–2’ of the landslide; the location is shown in Fig. 3. (d) Stereographic projection of bedding planes, which suggest the fold axes (N79°W/28°W). The thicker lines are of two small faults observed in the head scarp.

4.2.3 Diexi landslide

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There are prehistoric events and a 1933 event in the history of the Diexi landslide site. The 1933 Diexi earthquake triggered a series of landslides, most of which were partial failures within older landslide deposits and of relatively small size compared to the prehistoric landslides (Ling, 2015; diagonal hatching region in Fig. 6a). According to interviews with local people, the two present lakes (Fig. 2) are the remnants of larger lakes impounded by the landslide events induced by the 1933 earthquake (Wang et al., 2008). A large deadly rock avalanche occurred and buried ancient Diexi Town (four-point star in Fig. 6a) during this earthquake (Chai et al., 1995; Wang et al., 2005). However, lacustrine deposits up to an elevation of 2350 m asl on the top of the landslide deposit distributed on both sides of the river suggest that an older and much larger landslide, the Diexi landslide (Fig. 6a), had blocked the Minjiang River once long before 1933. The bottom of the lacustrine deposit to be 30,830 a, as dated by 14C (Fig. 6a; Wang et al., 2012), which implies that this largest landslide with an area of 8.50 km2 in the study area occurred over ~30,000 years ago. The large volume of the deposits has created mounds on both sides of the riverbed, and the deposit on the right bank blocked a small stream flowing eastwards and formed a depression (Fig. 6a).

Figure 6. Panoramic photograph and detailed geological and geomorphological map of the Diexi landslide. (a) Geological map (see Fig. 2 for the legend). Two cross-sections (3–3’ and 4–4’ in Fig. 7) were completed to show the history of the landslide events at Diexi. The diagonal hatching region refers to the landslide that partially failed within the body of the major Diexi landslide. The four-point star indicates the location of ancient Diexi Town. (b) Photograph of the Diexi landslide taken from the top of the landslide dam (2350 m asl). The solid lines

ACCEPTED MANUSCRIPT show the scarps on the middle and lower slope. The upper scarp (Scarp A in (a)) is separated into three portions by a small gully and talus as shown. The dashed lines are bedding planes, and the areas bounded by the dotted

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lines are joints. The location of the photograph is shown in (a).

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Figure 7. Cross-sections and stereonet analysis of the discontinuities of the Diexi landslide. (a) Cross-section 3– 3’. (b) Cross-section 4–4’. These two cross-sections show that there were two levels of minor landslides that

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occurred after the major Diexi landslide. (c) Stereonet analysis of the discontinuities (beddings and joints) of the Diexi landslide. The density contours of the structural intersections indicate the plunging direction (S45°W/34°W). (d) Stereonet analysis of the structures remaining in the landslide deposits on the opposite slope,

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which generally strikes NW–SE.

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The landslide mainly occurred in marble. Bedding planes served as slip surfaces of the large landslide (Fig. 6b). The marble beds strike N75°W and dip 38° towards the SSW. The bedding is mostly consistent, and only a few local folds can be observed in the east cliff of the landslide scarp. Three scarps, separated by small gullies in the middle portion (at an elevation of 2500–2600m asl) of the whole hillslope, appear to be connected and might be a single scarp (scarp A in Fig. 6a) created by a secondary landslide after the major event (Fig. 7a and b). The northernmost of the three scarps is in debris, whereas the other two are in marble bedrock. In addition, the NW-facing scarp west of ancient Diexi Town is well-defined. A lower scarp (scarp B in Fig. 6a) in the landslide deposit is 50–80 m lower than the lacustrine deposit and ancient Diexi Town; thus, it can be assumed that this scarp implies a later landslide after the secondary slide on scarp A (Fig. 7a and b). The northern and middle part of scarp B were buried by the subsequent landslides and talus. Bedding planes and joints (Fig. 6b) were measured to explore the structure using 5-m DEM data with COLTOP-3D. Stereonet projections of the discontinuities showed that their intersections plotted as a cluster at approximately S45°W/34° (Fig. 7c). This direction is consistent with the movement of the Diexi landslide (from NE to SW) that is assumed from the landslide morphology, which suggests that the landslide was a wedge failure. The movement direction was also estimated from the structures in the

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deposits; we found that platy rock fragments in the landslide deposits showed a wavy alignment, in which bedding surfaces strike NW–SE (Fig. 7d) and suggest a movement direction to the SW. In addition, the Diexi-S landslide probably occurred under the similar adverse structural condition, which is not specified in this paper. 4.2.4 Yinpingya landslide The Yinpingya landslide occurred during the 1933 Diexi earthquake on the right bank of the Minjiang River adjacent to the Diexi landslide (Figs. 6a and 8a) and struck the steep slope on the opposite bank. The area of the Yinpingya landslide is 1.23 km2, and the rock type is mainly alternating beds of pelitic and psammitic schists. This rock avalanche blocked the river and formed Dahaizi Lake. The highest elevation of the 1933 deposits on the left bank is ~2350 m asl, but we found relatively dense chaotic deposits along a road at an elevation of 2350–2380 m asl (shown in Fig. 6a). This is inferred to be a landslide deposit older than the 1933 deposit from the Yinpingya hillslope. This implies that landslides similar to the 1933 Yinpingya landslide have repeatedly occurred according to the fluvial incision. The characteristics of the geological structures of this landslide are similar to that of the Diexi slope. The landslide scar shows stepwise surfaces consisting of bedding planes and joints (Fig. 8b). COLTOP-3D analysis showed clusters of bedding planes (N87ºW/40ºS) and joints (N27ºE/63ºS) (Fig. 8c). The intersections of the two sets of discontinuities are concentrated at S1ºW/43º, which is consistent with the movement direction estimated from the landslide morphology and suggests that the landslide was a wedge failure.

Figure 8. Photograph and geological structures of the Yinpingya landslide. (a) Photograph of the landslide taken from the road on the left bank at an elevation of 2460 m asl. (b) Schematic sketch of the Yinpingya landslide showing bedding planes (Bd) and predominant joints (J). (c) Stereographic projections of bedding planes and

ACCEPTED MANUSCRIPT joints. The density contours of the intersections between the bedding planes and joints suggest the movement direction of the landslide was S1°W/43°S.

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4.2.5 Xinmo landslide Seven landslides were identified in the catchment of the Songpinggou River, a tributary of the Minjiang River (Fig. 2). They blocked the river during the 1933 earthquake (Zhao et al., 2018), and three relatively small dammed lakes still remain (Fig. 2). On 24 June 2017, the Xinmo landslide occurred 3.5 km from the Diexi landslide (Fan et al., 2017; Zhao et al., 2018). Comparing photographs taken before and after the 2017 event, we recognized that the new Xinmo landslide started on a south-facing slope along the western margin of the old landslide source area (Figs. 2 and 9a). The source area of the new landslide was rather small compared to the entire landslide area (Fig. 9b). The source rock mass rapidly struck the old landslide deposit and remobilized it covering an area of 1.72×106 m2. This could be attributed to the energy of the rock mass that rushed down from the high source area. In particular, pre-existing open fractures and buckle fold hinges on the slip surface, interpreted from satellite image analysis and field observations before and after the 2017 event suggest that the rock in the source area had already begun gravitationally deforming (e.g., buckled beds) prior to the 2017 landslide (Zhao et al., 2018).

Figure 9. Photograph and cross-section of the Xinmo landslide. (a) Panorama of the 2017 Xinmo landslide. The photograph was taken on the deposit on the valley floor on 30th July, 2017. The source area is shown with

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shading. (b) Cross-section 5–5’ of the Xinmo landslide showing the entire pre-sliding hillslope. The source area with buckling structures of the 2017 Xinmo landslide is rather small, and the slip surface is estimated. The location of the cross-section is shown in Fig. 2.

4.3 Lacustrine deposits and landslide dams Lacustrine deposits consisting mainly of horizontally laminated silt are exposed in patches in the upstream reach of the Diexi landslide (Fig. 10a). They were distributed from the Diexi landslide site and extend ~21 km towards upstream. The flat depositional surfaces of the lacustrine deposits are at elevations of ~2350 m asl (Fig. 10b), occurring only upstream of the Diexi landslide. There are terrace surfaces and thin silt layers downstream of the Diexi landslide, but they are much lower in elevation and thus are younger than the lacustrine deposits upstream of the Diexi landslide. The base of the major lacustrine sediments is at 2097 m asl, as shown by drilling on the opposite side of the Diexi landslide (Wang et al., 2012; the location is shown in Fig. 6). This suggests the thickness of the lacustrine deposits at Diexi might

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be at least 250 m. The highest elevation of the Diexi landslide deposits is greater than 2500 m asl on the left bank, suggesting that the ancient lake was probably impounded by a landslide dam hundreds of meters high from the Diexi landslide rather than from the downstream landslides. The Diexi landslide dam gradually breached over five stages within ca. 15 ka (Wang et al., 2012e) when the river eroded into landslide deposit. During the fluvial process, the landslide dam was incised and subsequent landslides occurred with their head scarps located along the current three connected scarps (Scarp A in Fig. 6). Then, the river continued to erode and caused the third landslide below the scarp in the lower portion of the hillslope (Scarp B in Fig. 6). A long time later, the Yinpingya landslide and partial failures of the Diexi landslide dam formed smaller dams that blocked the river again during the 1933 Diexi earthquake (Chai et al., 1995; Ling, 2015). Wang et al. (2008) reported that one month later the new dams breached and caused a flood downstream, with the water levels of the dammed lakes falling tens of meters. Only two lakes (Xiaohaizi at 2150 m asl and Dahaizi at 2224 m asl) remain in the Diexi area at present. (Fig. 2).

Figure 10. Photograph and distribution of lacustrine deposits upstream of the Diexi landslide. (a) Photograph of horizontally laminated lacustrine deposit, taken at the location marked in (b). The height of this outcrop is 4–5 m, and the lacustrine deposit is overlain by gravel and sand. (b) Map showing the elevations of the top flat surface of the lacustrine deposits at an elevation of ~2350 m asl, shown as gray-filled circle. Deposits with no top flat surface are shown as white-filled circles.

ACCEPTED MANUSCRIPT 4.4 River profiles and their relationships to the landslides

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4.4.1 Long River profile and knickpoints A knickpoint along the Minjiang River was identified by checking sharp break in the linear portions of the log-log scaling of the channel gradient to drainage basin area (Wobus et al, 2006; Fig. 11a). It is near the Diexi area and exhibits a pronounced abruption of the convex point on the channel profile. In addition, a number of large rockslides, as described above, cluster near the Diexi area, just downstream of this knickpoint. Moreover, the quantitative analyses of detailed river profiles for the Diexi area calculated from the DEM with higher resolution show that two knickpoints, KpY and KpD (white stars in Fig. 11b), occur exactly at the sites of the Yinpingya and Diexi landslides, respectively. The further upstream knickpoint KpMJ (large gray star in Fig. 11b) is approximately 19 km upstream from the Diexi landslide, where there are no landslide deposits and no lithological boundaries or faults. It is likely that KpMJ is the major knickpoint that was initiated by the prevailing tectonic uplift (Wallace, 1980), and continuously migrated headward caused by fluvial incision (Whipple et al, 2004). The hypothetical former riverbed prior to all the landslides can be assumed (Fig. 11c), and the two knickpoints (KpY and KpD) should be the result of the Yinpingya and Diexi landslides, respectively (Fig. 11d). Downstream of the Diexi landslide, there are five large landslides (Fig. 2), in which at least the Manaoding and Dragon landslides should have once blocked the Minjiang River with deposits hundreds of meters high and then breached. However, there are no evident knickpoints at these landslide sites (Fig. 11c).

Figure 11. Long river profile analysis of the upstream Minjiang River. (a) The reconstructed river profile A –B of

ACCEPTED MANUSCRIPT the entire length of the Minjiang River using 30-m ASTER-GDEM (upper) and the log-log plots of the channel slope versus drainage area (lower). An knickpoint upstream of Maoxian were extracted using 30-m ASTER-GDEM. It is near the Diexi area where the large landslides are concentrated. (b) Map showing the distribution of the knickpoints in the whole Minjiang catchment. (c) River profile C –D of the Diexi area calculated from 5-m ALOS DSM. One major knickpoint (KpMJ) and two minor knickpoints (KpY and KpD) were explored by assuming a hypothetical previous riverbed. The columns represent the prehistoric landslide dams. (d) Detailed map showing the knickpoints in the Diexi area in profile C–D (blue rectangle in (b)).

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4.4.2 Slope breaks

Figure 12. Typical hillslopes with slope breaks and hanging valleys on the opposite side of the Manaoding and

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Dragon landslides. The dark gray area shows the slope below the slope break and light gray area shows the slope above the slope break. (a) Photograph of the hillslope. (b) Sketch and interpretation of the hillslope. The slope break at an elevation of ~2250–2480 m asl was identified by the mouths of hanging valleys. The dark gray area

shows the area below the slope break, and the light gray area is above the slope break. (c) Incised inner

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gorge with slope breaks in the Diexi area. Gigantic landslides nearby have occurred on the left bank of the river. Black intermittent lines represent the slope breaks obtained from field observations. The inner gorge was

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assumed by connecting the slope breaks.

Slope breaks were mapped by identifying contrasts in the characteristics of the local slope angles, local relief, and erosion processes (Tsou et al, 2017). The gentler slopes above a slope break are the remnants of a previously incised valley that has been incised by rivers. Knickpoints of tributary streams on hillslopes, which leave the valley hanging above the major river, are helpful in identifying the slope break (Fig. 12a). However, slope breaks are commonly interrupted by landslides and gravitational slope deformation. Thus, we assumed the connection of slope breaks on both sides of landslides (Tsou et al, 2014). Our field survey along the Minjiang River in the Diexi area showed a slope break at an elevation of ~2300–2500 m asl by connecting the mouths of the hanging valleys and the convex rims of the remnant slopes of the previously incised valley (Fig. 12b). The major knickpoint KpMJ can be correlated to the slope breaks that bound the inner gorge (Fig. 12c). This slope break evidently passes through the middle portion of the landslides described above, which indicates that it could have destabilized the slopes where these landslides occurred.

ACCEPTED MANUSCRIPT 5. Discussion 5.1 Geological causes of gigantic rockslides in the Diexi area

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The stability of slopes is generally controlled by the relationship between the slope surface and geological structures (Cruden and Varnes, 1996). Slopes with foliations that strike normal or highly oblique to the slopes are relatively stable and commonly not subject to gravitational slope deformation and landslides (Cruden, 1989). However, the Diexi, Yinpigya, and Manaoding landslides occurred on such slopes. The Diexi landslide is a wedge failure (Hoek and Bray, 1981) in marble with two sets of discontinuities (bedding planes and joints), for the intersection of which plunges valleyward (Fig. 13a). The slip surfaces of the Diexi landslide, however, are stepped, and the sliding mass has multiple wedges (Fig. 13b). It is difficult to recognize the sliding mechanism of the Yinpingya landslide when viewed from the front, but the COLTOP analysis also suggests that the intersections of the bedding and joints plunge valleyward and that the landslide comprised of multiple wedges. The intersections of the discontinuities dip valleyward 34ºat the Diexi landslide and 43ºat the Yinpingya landslide, gentler than the original hillslope angles; thus, the intersections should have daylighted.

Figure 13. Schematics of the inferred slope failure modes. (a) Tight folds plunging valleyward (Manaoding and Dragon landslides). (b) Wedge failure (Diexi and Yinpingya landslides). Major slip surfaces are bedding planes. (c) Buckling failure (2017 Xinmo landslide).

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The sliding materials of the Manaoding landslide are alternating beds of pelitic and psammitic schists, which are tightly folded with their hinges trending normal to the river axis. Tightly folded beds cannot slide normal to the fold axes unless a continuous decollement is formed, which was not the case for the Manaoding landslide. However, beds, even tightly folded, can slide parallel to fold hinges, and if the hinges plunge valleyward, the gravitational force can induce sliding. The Manaoding and Dragon landslides are of this case; the plunge angles are 15°and 28°, respectively. The angle 15° is rather small, but the rock mass could slide along such gentle bedding with water pressures or during strong earthquake shaking (Selby, 1993, p. 362; Cruden et al., 1999). Moreover, the joints we found may have contributed to the sliding because they also form a wedge with probably steeper intersection line (Fig. 4e). The Songpinggou River trends parallel to the strike of the beds; thus, the valley slopes are cataclinal or anaclinal slopes, which are commonly susceptible to gravitational slope deformation and landslides where the dip angles are favorable (Cruden, 1989; Chigira, 1992). The beds along the Songpiggou River catchment dip approximately 50–60° southward, thus the slopes on the left bank are mostly cataclinal underdip or dip slopes. The geological structures observed before and after the 2017 Xinmo landslide suggest that buckle folding preceded that landslide (Zhao et al, 2018; Fig. 13c). It provides a good example how gravitational slope deformation can transform into rockavalanches, such as the catastrophic landslides preceded by DGSDs as in the case of some examples Japan and Taiwan (Chigira et al., 2003; Wang et al., 2003; Chigira et al., 2013b; Tsou et al., 2011, 2015). In addition to the Xinmo example, the open fractures and warped beds observed on the slip surfaces at two landslide sites in the Songpinggou River catchment are inferred to have been formed by gravitational buckling deformation (Fig. 13c). The dip angles of 50–60° were approximately within the same range as those of alternating beds of sandstone and mudstone in Taiwan (Tsou et al., 2015), which suggests very unstable structural angles against buckling. Such gravitational deformations could convert to subsequent catastrophic slope failure and result in large rockslides (Sartori et al., 2003) during a future intense rainfall or earthquake event.

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5.2 Knickpoint migration and effects of fluvial incision on landslide occurrence

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The abrupt step (KpMJ) in the long river profile at the Diexi site was supposed to have been formed by the tectonic uplift along an E–W striking thrust fault near the Diexi landslide (Zhang et al. 2005), but we did not find the evident fault in the field. Instead, long river profiles (Fig. 11) and landslide morphology strongly suggest that the two knickpoints, KpY and KpD near Diexi formed because of the landslide dams at the Yinpingya and Diexi sites. However, the major knickpoint KpMJ is 20 km from the Diexi landslide, where there are no landslides, faults or lithologic boundaries, which suggests that it was not generated by these factors and must have occurred because a knickpoint propagated from downstream. The knickpoint likely iniated at the intersection of the Minjiang River and the Longmenshan fault belt, which is an active thrust fault with upheaval of the eastern Tibetan Plateau to the west (Kirby et al., 2002, 2003; Ouimet et al., 2009; Wang and Kirby, 2012; Fig. 1). Knickpoint migration is an important transient hillslope forcing mechanism that shapes topography (Gallen et al., 2011) and forms an inner gorge with slope breaks along its rims. In such a case, the hillslope gradually adjusts to the fluvial incision by gravitational slope deformation and landslides. Local and short-lived knickpoints are created by landslide dams as these processes operate (Korup, 2006; Ouimet et al., 2007). Hiraishi and Chigira (2005) and Tsou et al. (2017) reported that DGSDs and landslides are induced by knickpoint migration in the upstream catchment of the Kii Mountains, western Japan. In the central range of Taiwan, Tsou et al. (2014) found three series of knickpoints and slope breaks, implying three pulses of river incision, which could progressively control the distribution patterns of DGSDs and landslides associated with knickpoint migration and inner gorges.

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The large landslides in our study area are aligned immediately downstream of the major knickpoint KpMJ. Interestingly, the lacustrine deposits created by the Diexi landslide are distributed from the Diexi site several hundred meters upstream of KpMJ, which may suggest that the knickpoint propagation of KpMJ predated the Diexi landslide. Moreover, the slope breaks, which correspond to KpMJ, pass though the landslide areas of Yinpingya, Diexi, Dragon, Manaoding and other nearby landslides (Fig. 12c). This topographic condition probably causes stress redistribution and hillslope debuttressing, which has been suggested in glaciated valleys (Agliardi et al., 2001; Arsenault and Meigs, 2005; Crosta et al., 2013) and non-glaciated valleys (Chigira, 2009; Hou et al., 2014; Tsou et al., 2014). The actual landslide triggers in the study area are not known except for the landslides induced by the 1933 Diexi earthquake, but this area is seismically active (Chai et al., 1995), thus earthquakes may be the dominant trigger.

Figure 14. Schematic sequential model of knickpoint migration and landslides along the Minjiang River in the Diexi area. (a) A previously incised valley eroded into the low-relief paleosurface. (b) Formation of an inner

ACCEPTED MANUSCRIPT gorge and slope break due to knickpoint migration. The major knickpoint created by the tectonic uplift and its propagation is indicated by a large gray star. (c) Failure mode of the Manaoding and Dragon landslides with valleyward-plunging folds. Landslide-produced knickpoints are shown as white stars. (d) Wedge failure of the Diexi landslide. (e) A huge dammed lake formed by the Diexi landslide at least 30,000 years ago (Wang et al, 2012). (f) Current state of the Diexi area. The ancient dammed lake has disappeared, and the landslide-induced knickpoints downstream of the Diexi site have been eroded. One major knickpoint (KpMJ) and two knickpoints (KpY and KpD) remain.

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The interactions between valley slopes and fluvial erosion in the Diexi area can be summarized as shown in the time series of Fig. 14. First, tectonic uplift, probably along the Longmenshan fault belt, lowered river base level, forming a knickpoint, which propagated headward into the previously incised valley of low relief (Fig. 14a). Subsequently, an inner gorge was created and the hillslopes were undercut (Fig. 14b) and destabilized; landslides were induced on slopes with adverse geological conditions such as valleyward-plunging tight folds (Manaoding and Dragon landslides, Fig. 14c) and the wedge-shaped discontinuities in the Diexi and Yinpingya landslides (Fig. 14d). The Diexi landslides formed a large dammed lake (Fig. 14e) at least 30,000 years ago (Wang et al., 2012). Then, the lake was gradually breached during five stages (Wang et al., 2008). Later, the Diexi landslide deposit partially failed during the 1933 Diexi earthquake to form a new and relatively small dam (Chai et al., 1995; Ling, 2015). This new dam was breached soon thereafter and is no longer recognized, but the previously dammed lake remains as Xiaohaizi Lake (Fig. 2) behind the knickpoint KpD (Figs. 11b and 14f). The Yinpingya landslide was also induced by the 1933 earthquake and created a dam that still remains, forming Dahaizi Lake behind knickpoint KpY (Figs. 11b and 14f). These two knickpoints in the Diexi area (KpY and KpD are shown as small white stars in Fig. 14) are the result of large landslides, which have been delaying the continuous propagation of the major knickpoint (KpMJ) and have reduced the erosion rate. Such a landslide effect on river incision has been reported by Korup (2006). However, it is notable that large landslides such as Manaoding and Dragon should have created knickpoints similar to knickpoints KpY and KpD (Fig. 14c), but they all been eroded away by the Minjiang River (Fig. 14d and f). In contrast, the blockage caused by the Diexi landslide has not been incised up to its base yet, which is probably due to the deposits accumulated in the ancient dammed lake that extending ~21 km of the river. Moreover, the blockage made by the Manaoding and Dragon landslides likely produced deposits accumulated downstream of the Diexi dam. These deposits should protect the lowermost part of the blockage from further incision (Strom and Abdrakhmatov, 2018).

6. Conclusions

A series of at least four large landslides in a reach of 10 km along the Minjiang River occurred as a bedrock response to fluvial incision. From downstream to upstream, these are the Manaoding, Dragon, Diexi, and Yinpingya landslides. The four landslides have areas ranging from 0.39 km2 to 8.50 km2 and deposits that are hundreds of meters thick and once blocked the Minjiang River. The response of valley slopes to river incision is strongly controlled by geological structures. The Manaoding and Dragon landslides were controlled by tight folds, which plunge valleyward at 10–30°. The Diexi and Yinpingya landslides were multiple wedge failures controlled by intersections of bedding planes and joints; their intersections dip valleyward 30–50°. The Diexi landslide created the largest dam lake, forming lacustrine deposits 250 m thick. There is a major knickpoint (KpMJ) at an elevation of 2330 m asl, and 15 km upstream of the Yinpingya landslide dam. This knickpoint separates the upstream gentle riverbed and the downstream steeper riverbed, and the four gigantic landslides are located immediately downstream of

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KpMJ. The landslides induced by the 1933 Diexi earthquake at the Yinpingya and Diexi sites created two distinct knickpoints (KpY and KpD) that remain and have been inhibiting upstream propagation of KpMJ. Similarly, the two downstream landslides, Manaoding and Dragon, should have created such knickpoints but none remains because of river erosion. In summary, the major knickpoint KpMJ was probably born downstream via tectonic movement of the Longmenshan Fault and propagated upstream. Upstream migration of KpMJ formed slope breaks and an inner gorge, undercutting nearby slopes, in which the steepened slopes with structural defects became unstable and finally slid. The results suggest that continual undercutting in combination with adverse geological conditions strongly control the gigantic landslides in mountainous regions such as the Diexi area.

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Acknowledgments

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We thank Prof. Yuki Matsushi and Prof. Gonghui Wang of the Disaster Prevention Research Institute, Kyoto University and Prof. Ching-Ying Tsou of Hirosaki University for useful discussions. We also thank Prof. Xiaoning Li of Southwest University of Science and Technology and Prof. Xin Liao, Dr. Sixiang Ling and Mr. Chunwei Sun of Southwest Jiaotong University for kind support in the field. We acknowledge Prof. Fanyu Zhang of Lanzhou University and Mr. Yongdong Liu of Beijing Geoscience Exploration & Technology Co. for technical support on electrical resistivity tomography (Type: GeoERT IP 120A). We thank Richard W. Allmendinger for providing Stereonet 10 for free. This study was financially supported by the Disaster Prevention Research Institute, Kyoto University Collaborative Research Number 28A-1 (Principal investigator: M. Chigira), the Japan Society for the Promotion of Science (JSPS) KAKENHI, Grant Number 17H02973 (Principal investigator: M. Chigira), the Key Research and Development Program of Sichuan Province of China (2017SZYZF0008), the R&D Program of Sichuan International Science and Technology Cooperation and Exchanges (2018HH0059) and the State Scholarship Fund, China Scholarship Council (201608050091).

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