Rock Avalanches in the Tibetan Plateau of China

Abstract

China is a country with high mountainous areas widely distributed, which is characterized by high reliefs, intensive tectonic activity, fragile ecological environments, etc. Under such particular background, rock avalanches, as one of the most threatening geological disasters in the high mountainous regions, have drawn much scientific interest because of their sudden occurrence, huge volumes (>10⁶ m³), high velocities (>20 m/s), and extremely low Fahrböschung (typically 0.1 to 0.3) with destructive disasters caused. Focusing on the Tibetan Plateau of China, spatial distribution of rock avalanches in its certain area was conducted, hoping to provide insights into the controlling factors of rock avalanches’ formation. Furthermore, several typical rock avalanches were investigated in detail based on remote sensing analysis, mapping of fixed wing unmanned aerial vehicle and detailed field investigations. It is reached that toreva blocks, transverse and longitudinal ridges, ridges separated by conjugate troughs, and hummocks are common and widely distributed surficial landforms in rock avalanche deposits, which usually display in a clear sequential distribution. In the cross-section, a series of internal sedimentary structures, including jigsaw structures, inner shear zones, diapiric structures, convoluted laminations, faults, etc., were identified. Based on these detailed geological features, the possible emplacement processes and mechanisms are discussed to provide insights into the extremely high mobility of rock avalanches.

1 Introduction

In the past 20 years, the occurrence of rock avalanches has significantly increased due to the effects of climate warming, frequent seismic activities, glacier retreat, and increasing human activities in mountainous areas, which has attracted increasing scientific attention due to their ability to produce catastrophic effects (Dunning et al. 2007; Catane et al. 2008; Sun et al. 2011; Huang et al. 2012; Barth 2014; Fan et al. 2017; Strom and Abdrakhmatov 2018; Zhu et al. 2019; Friele et al. 2020). Controlled by their large volumes (>1 × 10⁶ m³), extremely high velocities (>20 m/s) and long runouts (>1 km), such geological events often affect human activities and property suddenly without warning, and can cause disasters (Legros 2002; Evans et al. 2007; Hewitt et al. 2008; Guthrie et al. 2012; Pudasaini and Miller 2013; Delaney and Evans 2014; Lucas et al. 2014; Ostermann and Sanders 2017; Zhang et al. 2020). A typical example of such disaster is the Nevados Huascaran ice-rock avalanche, which occurred in the Peruvian Andes on 31 May 1970. The detached masses fragmented greatly in propagation and transformed into a debris avalanche with a volume and average velocity of up to 10⁸ m³ and 77.8 m/s, respectively, which buried the whole Yungay city in the downstream and killed over 18,000 people (Plafker and Ericksen 1978). Another typical example is the Chamoli ice-rock avalanche occurred in 7 February 2021 in India. The area where the Chamoli ice-rock avalanche occurred is situated in the Himalayan orogeny of the Tibetan Plateau. In this case, ~23 × 10⁶ m³ of the detached ice and rock travelled downstream rapidly along the Rontigad valley and fragmented intensively. When it arrived at the Rishi Ganga valley, it formed a short-lived landslide dam 46 m thick and 800 m long. After then, the dam failed with a flood disaster chain formed, causing the damages of two hydropower stations and influencing many people, with the economic losses being over 15 billion rupees (Kropáček et al. 2021).

To illustrate the hypermobility of rock avalanches, some hypotheses have been proposed, such as air cushion (Shreve 1968), frictional-thermal related mechanisms (Habib 1975; He et al. 2015; Wang et al. 2017; Hu et al. 2019), acoustic fluidization (Melosh 1978; Johnson et al. 2016), entrainment (Hungr and Evans 2004; Iverson and Ouyang 2015; Aaron and Mcdougall 2019), dynamic fragmentation (Davies and McSaveney 2009), and self-excited vibration (Wang et al. 2015). Moreover, important research efforts focusing on field investigations (Weidinger et al. 2014; Iverson et al. 2015; Dufresne et al. 2016; Wang et al. 2018; Dufresne and Geertsema 2020; Shi et al. 2023), laboratory experiments (Iverson et al. 2004; Shea and van Wyk de Vries 2008; Manzella and Labiouse 2013; Iverson 2015; Wang et al. 2016; Pudasaini and Jaboyedoff 2020), and numerical simulations (Pudasaini and Hutter 2007; Cagnoli and Piersanti 2015; Mergili et al. 2018) have also been conducted to gain better understanding of the hypermobility of rock avalanches. However, due to the destructive capacity of rock avalanches, it is still hard to obtain effective geological evidences during the high speed movement of rock avalanches. The kinematics and dynamics of rock avalanches are still unclear and need further study.

In China, the proportion of mountainous areas in the total area of the country is up to 69% with rock avalanches commonly developed. It is reported that the number of rock avalanches occurred in 2001–2020 is up to 74 only in the western mountainous area with more than 4000 people being killed and the direct economic losses being up to tens of billions RMB. The west mountainous areas in China is one part of the Tibetan Plateau, which is the largest area with high mountainous morphologies developed with most rock avalanches occurred in this region. Therefore, considering the particularity of the geological environment of the Tibetan Plateau in China and the common distribution of rock avalanches, we focused on this area with a series of rock avalanches being studied in the past 10 years. Based on our previous works, the Himalayan Mountains of the Tibetan Plateau in China was firstly selected as a studied area with the spatial distribution of rock avalanches is presented. Then, several typical rock avalanches in the Tibetan Plateau of China were introduced in detail, hoping to provide enough field evidence that could be used in further analogue and numerical modeling of rock avalanches.

2 Geological Setting of the Studied Area

The studied area is situated in the Tibetan Plateau. For the Tibetan Plateau, it is one the youngest and highest mountainous region in the world, which is featured by a wide variety of geologies, geomorphologies, and climates. For the evolution of the Tibetan Plateau, it is formed at ~55 Ma resulted by the intensive and ongoing plate collision of India with Eurasia. Under the control of its special geological background, it is a natural laboratory for the study of major geological disasters, such as huge rockslides, rock avalanches, ice-rock avalanches, and is attracting more and more scientific attention. The data of global positioning system (GPS) indicates that convergence is still lasted in the Tibetan Plateau with numerous earthquakes being generated (Larson et al. 1999; Tapponnier et al. 2001; Zhang et al. 2004). In the past 100 years, 36 events with Ms > 7 have been reported by Advanced National Seismic System (Fig. 1).

Fig. 1

Geological map of the Tibetan Plateau with strong earthquakes being marked

As shown in Fig. 1, the Tibetan Plateau is characterized by diverse and complex patterns of active fault systems comprising predominate block bounding faults and internal faults, accommodating the continuous deformation of India-Eurasia collision (Yin and Harrison 2000; Tapponnier et al. 2001; Yin 2010). The southwestern margin of the study area is defined by the ~2400 km Himalaya frontal thrust, which lies along the south range front of the Himalayas. This fault accommodates the underthrusting of the Indian Plate beneath Tibet (Elliott et al. 2016; Whipple et al. 2016). The eastern margin of the Tibetan Plateau is marked by the 500 km, NE-SW-trending Longmen Shan thrust belt, which is dominated by thrust faults and thrust-related fold structures (Lin et al. 2014). In the north, the active left-slip Altyn Tagh fault system extending over 2000 km links the strike-slip Haiyuan fault in the east and the NW-SE-trending Pamir thrust belt in the northwest, defining the northern boundary of the Tibetan Plateau (Yin 2010). In Plateau’s interior, a series of large thrust faults and strike-slip faults trend EW to NW-SE, demarcating the boundary between large basins and ranges (e.g., Kunlun fault, Altyn Tagh fault, Haiyuan fault, Xianshuihe fault, etc.). Furthermore, a set of normal fault system are well-developed in the southern Tibet, which are attributed to the ongoing movement of roughly EW extension (Yin 2000; Blisniuk et al. 2001). These fault systems remain active and are associated with the continuous thickening and widespread viscous flow of the crust and mantle of the entire Tibetan Plateau, and time-dependent, localized shear between coherent lithospheric blocks (Yin 2010).

3 Distribution of Rock Avalanches in the Study Area

Owing to the effects of intense tectonic activities and long-term weathering, intensely fractured bedrocks are widely distributed with high topographic reliefs developed, which are thought to be particularly favorable for the generation of catastrophic bedrock landslides. Many giant rock avalanches have been reported in this area and are increasingly attracting researchers’ attention (Yuan et al. 2013; Weidinger et al. 2014; Reznichenko et al. 2017; Strom and Abdrakhmatov 2018; Wang et al. 2018, 2019a, b, 2020). Focusing on the Tibetan Plateau area in China, study on the spatial distribution of rock avalanches in the Himalayan Range is firstly conducted. Then, further studies on the characteristics of some typical rock avalanches were conducted based on our previous remote sensing and field investigations.

3.1 Spatial Distribution of Rock Avalanches in the Himalayan Range, China

The Himalayan Range is situated in the place where the Indian Plate and Eurasian Plate meet, which is characterized by high relief, intensive tectonic activity, intensive river erosion, strong glaciation, etc. (Whipple et al. 2016; Schwanghart et al. 2018; Soret et al. 2021). As the south margin of the Tibetan Plateau, the Himalayan Range is distributed in several countries with some distributed in China. Under the control of the special geological environment, rock avalanches are highly developed in this area. So, take the Himalayan Range in China as the studied area, the spatial distribution of rock avalanches is analyzed. Here, only the rock avalanches with their accumulated areas being over ≥0.2 km² are interpreted. Through remote sensing interpretation and field investigations, the total number of rock avalanches identified in the Himalayan Range of China is up to 772 with their spatial distribution being marked in Fig. 1. According to the equation proposed by Larsen et al. (2010), the volumes of these identified rock avalanches were calculated.

$$ V=\alpha \ast {A}^{\gamma } $$

(1)

Where, log α = −0.836 ± 0.015, γ = 1.332 ± 0.005, V represents the volume of rock avalanche, and A represents the accumulated area of avalanche mass. Through calculation, it is reached that the number of rock avalanches with volumes ≥1 × 10⁷ m³ is up to 485, occupying 34.5% of total number.

As shown in Fig. 1, the distribution of rock avalanches in the study area is extremely uneven in space depending on the geological environment. Figure 2 show several rock avalanches developed in the Himalayan Range. From the images of these rock avalanches, it also can be seen that different morphologies and landforms can be observed among them, which indicates that varying pregnant geological environments are also contributed to varying deposited characteristics of rock avalanches. Taking the Zhada rock avalanche (Fig. 2a) as an example, its bedrock in the source area is weakly consolidated conglomerate, fine conglomerate mixed with sandstone of Tuolin Formation in Neogene. The orientation of the stratum in this case is nearly horizontal with a series of steep fissures developed in the source area. Under the control of the steep fissure and nearly horizontal stratum, the avalanche displays a relatively lager width and shorter runout. In the deposit, the original stratigraphy is well preserved with a series of minor scarps generated. For the cases shown in Fig. 2b and c, the lithology of their source area bedrock is same. However, they present some different deposited features. To learn the possible influence of geological environments on the generation of rock avalanches in the study region, a primary study on the relationships between rock avalanches with elevation, slope angle, slope orientation, lithology, fault, river system and glaciers are analyzed here. The following results were derived primarily.

Fig. 2

Remote sensing images of several rock avalanches occurred in the Himalayan Range

• 1. From east to west, there are three densely distributed areas for rock avalanches generated in the Himalayan Range, which are the western high mountainous area belong to the Ali region (Zone I), the area within the north-south rift zone (Zone II) and the area around the Yarlung-Zangbo suture belt (Zone III). From north to south, it can be seen that the density of rock avalanches in the high Himalayan zone is relatively high.

• 2. Under the control of altitude, most of rock avalanches are distributed in the elevation of 4000–6000 m, which are usually the upstream areas of rivers or seasonal ice and snow covered areas. In the area of the eastern Himalayan tectonic syntaxis, the elevations of most rock avalanches range from 1000–3500 m, which is different from other places, indicating that other factors instead of elevation should be more contribute to the generation of rock avalanches here.

• 3. In the Himalayas, slopes with the angle ranging from 20° to 50° are most commonly distributed, which is corresponding to the distribution of deep mountainous canyons and glacial erosion valleys. In such environment, the preferred angles of slopes with rock avalanches occurred range from 20–40° mainly.

• 4. Under the influences of rainfall, sunlight and temperature, etc., the density of rock avalanches generated on the slopes facing SW and S is obviously higher than that on the N and NW facing slopes. Meanwhile, the number of rock avalanches generated on the slopes facing SW-S and N-NE is relatively higher than slopes facing other directions, which should have a certain relationship between the N-NE principal stress direction generated due to collision between the Indian and Eurasian Plates.

• 5. For the control of lithology, the formations consisting of relative hard rocks interbedded with weak interlayers are the preferred strata for rock avalanches occurrence. For such formations, the appearance of the weak layers is the controlling zone for rock avalanches generation. The number of rock avalanches generated in hard rock formations is in the second place. Meanwhile, it is noticed that the slope angles are steeper in the areas composed of relative hard rocks. This indicates that such lithology usually form relatively steep slopes, which provides preferred topography condition for the generation of rock avalanches.

• 6. The distribution of rock avalanches presents a good correlation with the extensions of major faults. Under the control of the major fault systems, i.e., the main central thrust, north-south rifts, southern Tibet detachment system and the Yarlung-Zangbo suture zone, rock avalanches present a striped distribution in plan-view. The closer the distance away from fault, the more the number of rock avalanches formed.

• 7. For rock avalanches generated in the areas influenced by glaciers, the slope angle (20–50°), elevation and volume of rock avalanches are relatively higher than for those in non-glaciated areas. Meanwhile, in the glaciated areas, rock avalanches are commonly developed on the slopes facing N, W and NW. Conversely, rock avalanches generated in the non-glacial areas are mainly formed on the slopes facing S and SW. This indicates an obvious control of glacier on the generation of rock avalanches.

Through a comparison of different factors on the spatial distribution of rock avalanches in the Himalayas of China, we primary reached that the fault systems should be the most significant factor for the generation of rock avalanches. In the high Himalaya region of China, the role of glacier should be the second factor, which intensively influences the stability of the bedrocks in the source areas through the effects of unloading caused by glacier retreat and freeze-thaw cycles. For the non-glacial areas, lithology should be the second key factor, which determines the slope steepness and, thus, its stability.

3.2 Locations of the Typical Rock Avalanches

To learn the characteristics of rock avalanches developed in the Tibetan Plateau of China, several typical cases are presented for further investigation, i.e., the Luanshibao rock avalanche, Nyixoi Chongco rock avalanche, Tagarma rock avalanche and Iymek rock avalanche. As shown in Fig. 1, they are distributed in different areas of the Tibetan Plateau with the Luanshibao rock avalanche in the Sichuan West Plateau, Nyixoi Chongco rock avalanche in the South Tibet detachment system, and Tagarma and Iymek rock avalanches in the Pamir-western Himalayan syntaxis. Focusing on the deposited characteristics of these typical cases, their deposited characteristics are described in detail, hoping to provide more geological evidences for the study of rock avalanches. Figure 3 shows the remote images of the studied cases.

Fig. 3

Typical rock avalanches distributed in the Tibetan Plateau, China (a: Luanshibao rock avalanche; b: Nyixoi Chongco rock avalanche; c: Tagarma rock avalanche; d: Iymek rock avalanche)

4 Characteristics of Typical Rock Avalanche Deposits

As indicated by the deposited characteristics along the travelling paths of rock avalanches, the detached masses commonly display very significant mobility and propagate like fluid. Along their travelling paths, sequential distributions of a series of landforms and internal structures indicating fluid-like motion are commonly observed, which show typical planar zonation and vertical gradation (Dufresne et al. 2016; Strom and Abdrakhmatov 2018; Wang et al. 2018, 2019a, 2020; Mitchell et al. 2020).

According to the spatial distributions of landforms and internal structures, rock avalanche deposits can be divided into three zones in plan-view from proximal to distal areas: the source area, the transition zone and the accumulation zone (Dufresne et al. 2016; Wang et al. 2018). The source area usually has the armchair shape with a steep bedrock scarp in the upper part and a relatively gentle slope in the lower part. Sometimes, small amount of rock avalanche deposits covers the relatively gentle section. The transition zone where the sliding mass rapidly propagates is often narrow and long in plan-view. In the transition zone, large-scale toreva blocks are typically displayed, which are characterized by a relatively flat top surface and a steep scarp facing downstream. Additionally, longitudinal ridges and linear structures are common being formed by the rapid movement of rock avalanche debris. The accumulation zone is the main area where most of rock avalanche mass deposited. In plan-view, the deposits of rock avalanches often have lobate or fan-like shape. Margins of the accumulation zone are characterized by raised rims that are also called levees, and distal ridges. There are also trimlines left by moving debris along the travelling path. The frontal margins of most rock avalanches are often present in fingering deposit (digitate fronts). On the top surfaces of the deposits in the accumulation zones, clear longitudinal ridges with longitudinal groove-like troughs, transverse wave-like ridges, hummocks or conical hills, en echelon crevasse-like ridges, and X-shaped (fish scale-like) landforms with both ridges and troughs often form (Strom and Abdrakhmatov 2018; Wang et al. 2018, 2019a, 2020).

In the cross-section, the most typical deposited feature of rock avalanche deposits is inverse grading (Fig. 4). According to its vertical grain size distribution, the deposit can be divided into three facies from top to bottom: carapace facies, body facies and basal facies (Dufresne et al. 2016; Strom and Abdrakhmatov 2018; Wang et al. 2019a, 2020). The main difference among these facies is their grain size distribution. The carapace facies is defined as an assemblage of large and angular interlocking blocks concentrated near the surface of a rock avalanche deposit and is the coarsest unit in a rock avalanche deposit (Dunning 2006; Davies and McSaveney 2009; McColl and Davies 2011; Dufresne et al. 2016). Fine particles seldom occur in the carapace facies, and thus this facies is mainly clast-supported. Body facies usually forms the main part of rock avalanche deposits. This facies shows a wide range of grain sizes from fine particles of several millimeters (mm) to coarse clasts of tens of centimeters (cm). Sometimes, blocks larger than 1 m also can be seen in the body facies. The body facies shows a matrix-supported structure with coarse clasts being surrounded by large amount of fine particles. The basal facies is the bottom part of a rock avalanche deposit as shown in Fig. 4. As the direct contact zone with the substrate, the basal facies is highly fragmented and mainly composed of fine particles of mm scale or even finer particles due to intense interaction between the sliding mass and substrate.

Fig. 4

Conceptual models of the vertical structures of typical rock avalanches (Referred on Weidinger et al. (2014) and Dufresne et al. (2016))

In addition to above mentioned landforms and deposit facies, some other characteristics are also commonly observed in rock avalanche deposits, such as preserved stratigraphic sequence, jigsaw structure, multi-layered internal shear zones, etc. At the bottom of rock avalanche deposits, a series of basal deformation structures formed by intensive interaction between the sliding mass and substrate can also be observed, such as basal shear zones with folds, faults, diapiric structures, wedge intrusion, and sand dykes, etc. The variations of the grain size distribution and internal structures revealed in rock avalanche deposits from carapace facies to basal facies indicate the occurrence of different interactions among particles with varying depth. On the other hand, it also indicates the occurrence of different deformation processes and dynamic fragmentation with depth. All of these geological evidences revealed along rock avalanche paths are key insights in revealing the kinematics and dynamics of rock avalanches. In the following, detailed studies on the four cases are conducted.

4.1 Luanshibao Rock Avalanche

4.1.1 Geological Setting and General Features of the Luanshibao Rock Avalanche

The Luanshibao rock avalanche is situated in the Maoyaba basin, Litang County, Sichuan Province, China. This is regionally part of the Sichuan West Plateau. As shown in Fig. 5, the Sichuan West Plateau is on the eastern margin of the Tibetan Plateau. Due to the intensive collision between the Indian and Eurasian plates, the Sichuan West Plateau is characterized by complex geologic structures with numerous faults and folds generated. The main fault belts formed in the West Plateau include the Jinshajiang fault system (F1), Litang-Dewu fault system (F2), Xianshuihe fault system (F3), Anninghe fault system (F4), and Longmenshan thrust belt (F5) as exhibited in Fig. 5. It is reported that all of these fault belts are still active with strong earthquakes recorded in history (Tang and Han 1993; Yin and Harrison 2000). The strongest earthquake recorded in history is the Wenchuan earthquake occurred in 2008 along the Longmenshan thrust belt with abundant geological disasters (Dai et al. 2011).

Fig. 5

Simplified geological map showing major faults and earthquake activity on the Sichuan West Plateau, China

As shown in Fig. 5, the 385-km-long Litang-Dewu fault system (F2) is very active among these fault belts with 3 strong historical earthquakes (M > 7) being recorded. Xu et al. (2005) reported that F2 had an average sinistral slip rate of 4.0 ± 1.0 mm/a and a reverse slip rate being 0.1–1.8 mm/a over the past 14 ka. From west to east, F2 is further divided into four subsidiary faults that are Chalong-Maoyaba fault (F2–1), Litang-Dewu fault (F2–2), Cazhong fault (F2–3), and Mula fault (F2–4). The subsidiary fault F2–1 passes through the Maoyaba basin where the Luanshibao rock avalanche occurred as exhibited in Fig. 5. In history, two strong earthquakes with M > 7 ever occurred along this fault. The latest one is the Ms 7.3 Litang earthquake, which occurred on May 25, 1948, and caused a 70-km-long surface rupture (Tang and Han 1993). The high-level of activity along this fault system during the Quaternary contributed to the formation of the Maoyaba basin (Tang and Han 1993).

Figure 6 shows the geological setting of the Maoyaba basin, where the Luanshibao rock avalanche had occurred. For the bedrock of the studied area, there are the appearances of the Upper Triassic strata of the Qugasi (T₃q), Tumugou (T₃t), Lanashan (T₃l), and Lamaya (T₃lm) formations and intrusive rocks (γδ₅^1-d, ηγ₅^1-d, γβ₅^1-d, and δo₅^1-c) coeval with the Triassic Period (257–205 Ma). Within the basin, the Quaternary deposits, consisting of Upper Pleistocene lacustrine facies (Q_p^l) and alluvial-fluvial facies (Q_p^al - Q_p^pl) and Holocene alluvial facies (Q_h^al), accumulated (Tang and Han 1993).

Fig. 6

Geological setting of the Maoyaba basin where the Luanshibao rock avalanche had occurred

As shown in Fig. 3a, the Luanshibao rock avalanche detached from a steep slope of the Maoyaba basin northern margin. The horizontal distance and vertical drop from the head of the scarp to the toe of the avalanche deposit is 4.1 km and 821 m, respectively, yielding a fahrböschung (the ratio of the height of the highest point on the main scarp (H) to the horizontal projection of distance from this point to the tip of a rock avalanche (L) (Heim 1932)) of 0.2. This low value indicates an extremely high mobility of this rock avalanche (Cruden and Varnes 1996; Lucas et al. 2014). In plan-view, the accumulated area of the avalanche deposit is up to 4.4 km², and the estimated total volumes of the detached rock mass and accumulated debris are 39–47 × 10⁶ m³ and 64–94 × 10⁶ m³, respectively (Guo et al. 2016).

Fig. 7 shows one longitudinal profile of the avalanche with its position being marked in Fig. 4a. As shown in Fig. 7, F2–1 is located at the toe of the source area. Bounded by F2–1, the avalanche source area ranges in elevation from 4275 to 4931 m above sea level (a.s.l.) and locates on the NE hanging wall of the fault. The bedrock in this area is composed of granodiorite and biotite granite of Triassic Maohuoshan intrusion rock (γδ₅^1-d) mainly. Due to the effects of tectonic events and long-term weathering, the bedrock has been intensely fractured with three sets of preferred joints (Fig. 4a). The dip directions and dip angles of these joint sets are 0–10/40–50° (JS1), 165–175/60–70° (JS2), and 200–250/60–70° (JS3). The rest part of the avalanche is distributed in the SW footwall with the elevation ranging from 4110 to 4290 m a.s.l. For this area, the slope angle is relatively low. And the farther away from the source area, the lower the slope angle is. The substrate of this part is composed of Quaternary alluvial-fluvial facies (Q_p^al–Q_p^pl).

Fig. 7

Longitudinal profile of the Luanshibao rock avalanche

4.1.2 Sedimentary Features of the Avalanche Deposit

• (1) Plan zonation and landforms

In plan-view, the Luanshibao rock avalanche is present in a tongue shape (Fig. 8a). On the surface of the deposit, prominent well-developed morphologies, including toreva blocks, transverse and longitudinal ridges, large ridges separated by conjugate troughs, and hummocks, are spatially distributed from the proximal to the distal areas. Such regularly spatial distribution of so many landforms in one avalanche is rare seen in most reported rock avalanches. Here, the toreva blocks are associated with a massive portion composed of highly fragmented rock mass with a short runout and a steep downward slope-parallel scarp, which is located within or proximal to the source area. The observed ridges feature length-to-width ratios generally greater than 2 and heights larger than 5 m. The lineaments are characterized by length-to-width ratios greater than 2 and heights less than 5 m. Hummocks are defined as a landform with a length-to-width ratio of 1–2 (Dufresne and Davies 2009).

Fig. 8

Sketch of the Luanshibao rock avalanche (a) with statistics of huge blocks on the top surface (b)

According to the sequential distribution of these surface morphologies along the travelling path, it can be divided into three zones in plan-view that are: the source area (I), the transition zone (II), and the accumulation zone (III). In the following, detailed descriptions of the morphologies in each zone are given to explain the rock avalanche transport dynamics.

• a) Source area (I)

The source area, ranging from 4275 to 4931 m a.s.l., displays a U-shape in plan-view. This zone is characterized by a steep well-exposed scarp and a relatively gentle section with talus accumulation. The maximum longitudinal length and width of the source area are approximately 1500 m and 700 m, respectively (Fig. 3a). For the steep exposed part, its average slope is 35° with a relatively plain surface. Controlling by the preferred joint sets shown in Fig. 4a, the exposed scarp is characterized by a blocky and intensively fractured structure. For the lower relatively gentle section, its average slope angle is 20° and was partially covered by avalanche debris and post-avalanche talus.

• b) Transition zone (II)

For the transition zone, two typical landforms are well developed—toreva blocks and longitudinal ridges (Fig. 8a). As shown in Fig. 9a, two notable toreva blocks were generated next to the source area, i.e., the Level-1 (S-1) and Level-2 (S-2) toreva blocks. The S-1 toreva block ranges from 4200 to 4280 m a.s.l. and present a maximum length and width of 450 m and 150 m, respectively. In plan-view, it presents a crescent shape with its top surface inclined slightly upstream (Fig. 9b). The frontal edge of the S-1 toreva block displays a steep dipping escarpment with a maximum vertical difference of up to 65 m (Fig. 9c). In the transverse direction, the elevation of the S-1 toreva block decreases from left to right (Figs. 9a and c). Next to the S-1 toreva block is the S-2 toreva block ranging in elevation from 4175 to 4200 m a.s.l. In plan-view, the S-2 toreva block is also crescent shaped (Figs. 9a and d), with a relatively smaller size compared with the S-1 toreva block. The S-2 toreva block features a relatively gentler escarpment along its downward side (Fig. 9a). Unlike the undulating topography of the rear part of the transition zone, its frontal part is relatively flat, with several small-scale escarpments and minor lineaments sparsely distributed in the central area. The orientations of these landforms are nearly perpendicular to the main sliding direction of the rock avalanche. Among these features, the largest is the S-3 toreva block with a 10-m-high downward escarpment (Fig. 10a).

Fig. 9

Extension of the tovera blocks in the transition zone of the Luanshibao rock avalanche

Fig. 10

Extensions of S-3 toreva block (a) and both lateral ridges (b-c)

The second prominent features in zone II are two large longitudinal ridges (L-1 and L-2) in the frontal part of zone II (Fig. 8a), which are 280–340 m long and 150–180 m wide. In plan-view, they are oval in shape and oriented N10–25°E, nearly parallel to the main sliding direction of the rock avalanche (Fig. 8a). Another interesting feature is the display of two large lateral rims (LR-1 and LR-2) that stand approximately 20 m above the original topography along the left and right margins of this zone. Figures 10b and c are photographs of the accumulated features of both lateral rims captured during the field investigation. Additionally, a series of small-scale en echelon-like ridges adjacent to the right margin of zone II were also observed. The NE-SW orientations of the en echelon-like ridges are oblique to the main sliding direction of the rock avalanche, indicating the existence of lateral spreading near the right margin of the sliding mass.

• c) Accumulation zone (III)

As shown in Fig. 8a, different landforms are well developed in the accumulation zone and present sequential distribution. According to the spatial distribution of landforms in zone III, it can be further divided into three sub-zones, i.e., sub-zone III-1 with transverse and longitudinal ridges, sub-zone III-2 with large ridges separated by conjugate troughs and sub-zone III-3 with hummocks.

For sub-zone III-1, gradual changes are observed from the rear to the front. Transverse ridges are present in the rear part, and criss-crossing transverse and longitudinal ridges are present in the front part (Fig. 8a). According to the statistics, the number of prominent transverse ridges formed in the rear part is up to 18. They are mainly concentrated in the central area of sub-zone III-1 (Fig. 8a). These transverse ridges emerge 5–20 m above their surroundings. Figure 11 shows several transverse ridges photographed from different orientations. As shown in Fig. 11, the top surfaces of these transverse ridges are relatively flat with huge blocks mainly concentrated in the upstream slope.

Fig. 11

Extensions of transverse ridges in the rear part of sub-zone III-1 in the Luanshibao rock avalanche

Greatly different from the landforms present in the rear part of sub-zone III-1, a criss-crossing feature composed of transverse and longitudinal ridges is present in the front part with good continuity present (Fig. 12). Here, these criss-crossing ridges are 20–50 m wide and 30–150 m long. Differing slightly from the relief of the transverse ridges in the rear part of sub-zone III-1, these criss-crossing ridges stand 10–20 m above their surroundings. Due to the presence of these criss-crossing ridges, approximately rectangular depressions were observed in this sub-zone. Figure 12e is the photo of one depression taken during the field investigation.

Fig. 12

Extensions of criss-crossing ridges in the frontal part of sub-zone III-1 in the Luanshibao rock avalanche

To quantify the preferred direction of these ridges, statistics of their orientations were conducted with the results being graphed in Fig. 13. The orientations of the transverse ridges range from N55°W to N80°W mainly (Fig. 13a), nearly perpendicular to the sliding direction of the detached mass in this section. The longitudinal ridges are mainly parallel to the sliding direction of the detached mass, with strikes of 30–40° (Fig. 13b, including the L-1 and L-2 ridges in the transition zone).

Fig. 13

Rose diagrams of the transverse (a) and longitudinal (b) ridges in sub-zone III-1 of the Luanshibao rock avalanche with the red arrow indicating the main propagated direction

Next to sub-zone III-1, large ridges separated by conjugate troughs form sub-zone III-2. As shown in Fig. 8a, greatly different from the display of the landforms in sub-zone III-1, a series of well-developed large ridges are well developed in sub-zone III-2. Figure 14a shows an enlarged view of the extension of the conjugated ridges in this subzone. As exhibited in Fig. 14a, large ridges in this sub-zone have the parallelogram or rhomboid shape mainly. Closer to the longitudinal central line, larger the accumulated ridge is. Among these large ridges, there are deep conjugate troughs (e.g., CT-1 to CT-17 in Fig. 14a), which contributes to the formation of these parallelogram shaped ridges. Ridges in sub-zone III-2 range from 110–280 m in length and from 40–100 m in width mainly. The elevation difference between the ridges and the troughs is 15–20 m. Figure 14b-d shows three large ridges photographed during the field investigation.

Fig. 14

Extensions of parallelogram shaped ridges in sub-zone III-2 in the Luanshibao rock avalanche

Furthermore, statistics on the orientations of some of the large ridges and the deep troughs are plotted in Fig. 15. The large ridges are mainly oriented 20–40°, nearly parallel to the sliding direction of the detached mass in this section. The deep troughs have two sets of preferred orientations, i.e., 0–20° and 50–70°. With increasing distance from the longitudinal centre line of sub-zone III-2, the dimensions of the ridges and troughs decrease markedly and become difficult to recognize. Moreover, another interesting morphology in sub-zone III-2 is the presence of minor undulating topography on the top surfaces of the large ridges, as exhibited in Fig. 14b-d. This feature is similar to waves in water, with a series of alternating peaks and troughs. The orientation of the undulating topography is perpendicular to the longitudinal axes of the ridges.

Fig. 15

Rose diagrams of ridges and troughs in sub-zone III-2 of the Luanshibao rock avalanche

In the frontal part of zone III, it is an area characterized by a series of hummocks, i.e., sub-zone III-3. This sub-zone is the distal depositional area of the Luanshibao rock avalanche and is dominated by a series of small-scale, roughly circular or circular hummocks (Fig. 8a). These hummocks are 15 to 40 m in diameter and stand approximately 5–12 m above their surroundings (Fig. 16a-b). The largest hummock is the one labelled with the red dashed line in Fig. 16a. The length and width of this hummock are 45 m and 39 m, respectively, and they raise 12 m above the surrounding. In plan-view, the hummocks in sub-zone III-3 present a sparsely distributed pattern and decrease in sizes and in height-to-width ratios with increasing distance from the proximal to the distal and marginal areas. Another striking feature in sub-zone III-3 is the presence of a steep frontal edge of the deposited mass (Fig. 16c-d). It was measured that the vertical difference between the frontal edge of the deposited mass and the surface of the river is greater than 2 m. In some sections, the height of the frontal edge of the deposited mass even reaches 5.6 m.

Fig. 16

Hummocks (a-b) and frontal edge (c-d) of the Luanshibao rock avalanche

• (2) Internal deposited features

In addition to these surficial features, some typical internal deposited structures were also observed in the deposit. The first interesting feature is the layout of the carapace facies with many huge blocks (Figs. 8a and 17). In plan-view, the amount of megablocks accumulated in the right part of the rock avalanche deposit is obviously higher than that in the left part (Fig. 8a). Most of blocks are concentrated in the depressions in the right-hand parts of zone II and sub-zone III-1.

Fig. 17

Distribution of blocks in the carapace facies of the Luanshibao rock avalanche

In vertical profile, this facies is usually clast-supported with large voids. Most blocks retain the original source stratigraphy without mixing and are featured by irregular angular shapes with very sharp edges and well-developed fractures. Jigsaw structures are observed in some megablocks (Fig. 17e). On the surfaces of some megablocks, step fractures (e.g., SF in Fig. 17d and f) were common observed, indicating the occurrence of brittle fracturing during debris motion (Wang et al. 2015). The measurements of the long-axis orientations of some megablocks revealed that the orientations of most of megablocks are slightly oblique to the rock avalanche travel direction. To quantify the block size distributions on the top surface of the carapace facies, blocks with long axes of ≥2 m were sampled and counted based on our field investigations and remote sensing images. Figure 8b shows the statistical results plotted in a frequency distribution histogram. As shown in Fig. 8b, the number of blocks with long axes of ≥2 m counted in the Luanshibao rock avalanches is up to 2521. For the distribution of blocks, megablocks with long-axes of 2 to 3 m are the most common. The largest megablock is 12.5 m long and is located in zone II.

Other prominent features revealed in the Luanshibao rock avalanche deposits are the inverse grading and jigsaw structures (Wang et al. 2018; Zeng et al. 2019). During field investigation, exposures of 0.6 to 10 m in height were observed in the deposit, allowing us to examine the internal structure of its upper portion. Figure 18 shows several profiles taken during our field investigation. In the deposit, coarse-grained clast-supported breccia and blocks with a fine-grained matrix are universally distributed. The clasts are angular, with little or no rounding by abrasion. Similar to the textures of the megablocks (Fig. 17e), jigsaw structures are also observed in some excavated cross sections (Fig. 18), indicating a low disturbance in the propagation of the avalanche mass.

Fig. 18

Vertical profiles revealed in the Luanshibao rock avalanche

4.2 Nyixoi Chongco Rock Avalanche

4.2.1 Geological Setting and General Features of the Nyixoi Chongco Rock Avalanche

The Nyixoi Chongco rock avalanche is situated in the Angang graben, Nimu County, Tibetan Autonomous Region, China. The Angang graben belongs to the Yadong-Gulu rift, which is one of the North-South (N-S) trending rifts in the South Tibet detachment system (Fig. 19). The South Tibet detachment system, as one of the main structures formed by the Indian-Eurasia collision, is composed of 7 nearly parallel N-S rifts, which were initiated 8–23 Ma ago. Among them, the Yadong-Gulu rift is the youngest one in the Lhasa Terrane with its central segment being initiated 5 to 8 Ma ago. This young rift is characterized by intense tectonic activity with strong historical earthquakes (Harrison et al. 1995; Chung et al. 2005; Zhang et al. 2013). The latest strong Ms 6.3 Angang earthquake occurred in this area in 1992. Furthermore, Wu et al. (2015) conducted a series of OSL and ¹⁴C dating on samples collected from the western main boundary fault of the Angang graben and concluded that the latest prehistoric strong earthquake in this area should occur about 2.2 ka ago with its Mw being over 7.0.

Fig. 19

Regional geological setting of the Angang graben

Under the effect of historical tectonic activities, the Yadong-Gulu rift in associated with an asymmetrical normal fault system was generated with its width being 30–40 km. The Angang graben is bounded from west and east by normal faults and the western one is more developed, with a series of preserved fault triangular facets, fault scarps, and fracture zones of bedrocks, suggesting that it is an active fault with frequent, strong earthquakes. The Angang Graben is a typical half-graben situated in the central segment of the Yadong-Gulu rift, which is 3–4 km wide and 22–24 km long. It presents a generally steeper and higher relief in its western than its eastern flank, and is characterized by rugged alpine topography due to strong erosion.

As shown in Fig. 20, the Nyixoi Chongco rock avalanche is located on the west flank of the Angang graben. The bedrock in the studied area is the Paleogene monzonitic granite (E₂Kd), which is distributed in the footwall of the Yadong-Gulu rift mainly. The hanging wall is covered by Pleistocene deposit (Q_p), and Holocene alluvial (Q_h^al) and colluvial-pluvial (Q_h^col + pl) deposits mainly.

Fig. 20

Geological setting of the studied area where the Nyixoi Chongco rock avalanche is situated

In plan-view, the Nyixoi Chongco rock avalanche has an elongated tongue shape (Fig. 20). Due to topographic confinement, the movement direction of the Nyixoi Chongco rock avalanche presents an obvious turn of ~50^° toward southeast from the transition zone to the accumulation zone. The runout of the Nyixoi Chongco rock avalanche was ~4600 m with a vertical drop height of ~885 m, and thus the Farböschung (H/L) was 0.19, indicating an extremely high mobility of the avalanche mass (Cruden and Varnes 1996; Lucas et al. 2014). Through reconstruction of the topography of the source area, volume of the detached mass was assessed as ~0.28 × 10⁸ m³ with a maximum thickness of rock avalanche deposit ~50 m (Wang et al. 2020). Along the N-S extension of the rift, the Xuqu River is well-developed with some vertical profiles of the avalanche deposits being revealed, which provides opportunity for the study of its internal features.

Figure 21 is a longitudinal profile of the avalanche with its position being marked in Fig. 4b. Bounded by the normal fault, the avalanche source area ranges in elevation from 4270 to 4850 m a.s.l. and locates on the footwall of the fault, giving a height difference of 580 m. The bedrock in this area is mainly composed of biotite monzonitic granite belonging to the Konglangdong formation of the Eocene (E₂Kd). Due to the effects of tectonic events and long-term weathering, the bedrock has been intensely fractured with two sets of preferred joints generated (Fig. 4b). The dip directions and dip angles of these joint sets are 180–190°/40–50° (JS1) and 350–360°/60–70° (JS2), respectively, controlling the wedge failure of the rock mass in the source area. In the hanging wall, it is the distribution of the rest part of the avalanche with the elevation ranging from 3965 to 4270 m a.s.l. For this area, the slope angle is relatively low. And the farther away from the source area, the lower the slope angle is. The substrate of this part is composed of Quaternary colluvial-diluvial (Q₄^col- Q₄^dl) and alluvial deposits (Q₄^al).

Fig. 21

Longitudinal profile of the Nyixoi Chongco rock avalanche with different scales in vertical and horizontal axes

4.2.2 Sedimentary Features of the Avalanche Deposit

• (1) Plan zonation and landforms

As exhibited in Fig. 22, a series of landforms are sequentially distributed along the travelling path of the Nyixoi Chongco rock avalanche, including toreva blocks, longitudinal ridges, transverse ridges, en echelon ridges and hummocks. According to the spatial distribution of these landforms, the Nyixoi Chongco rock avalanche can be divided into three zones from proximal to distal part: the source area (I), the transition zone (II), and the accumulation zone (III). Field investigation reveals that scarps with heights over 15 m developed in the frontal part of the transition zone, which formed the natural boundary between the transition zone and the accumulation zone. Since the source area presents a steep topography, the transition zone and the accumulation zone present relatively gentle topography with mean slope of ~10° and < 2°, respectively. Due to post-rock avalanche erosion by the Xuqu River, the section of the avalanche deposit is revealed, which provides precious geological evidences for studies on the internal sedimentary characteristics and the rock avalanche deposit-substrate interactions.

Fig. 22

Shaded relief map of the Nyixoi Chongco rock avalanche with the main landforms marked

• a) Source area (I)

The source area of the Nyixoi Chongco rock avalanche is located on the footwall of the boundary normal fault developed in the west flank of Angang graben. Similar to the Luanshibao rock avalanche, its source area also displays a U-shape in plan-view. As exhibited in Fig. 21, this area is also characterized by a steep well-exposed scarp and a relatively gentle section with talus accumulated. Bedrock in the source area was highly fractured due to cut of those discontinuities with wedge failure developed in the source area.

• b) Transition zone (II)

The transition zone is characterized by large toreva blocks (S-1 and S-2) and large longitudinal ridges (LR) from its rear area to its frontal area as exhibited in Fig. 22. In plan-view, the S-1 toreva block is 270 m long and 100 m wide with 90 m high frontal steep escarpment (Fig. 23a). Next to S-1, it is the display of S-2 toreva block, which is 700 m in length. The height of its frontal escarpment is about 65 m as exhibited in Fig. 22b. Compared to the escarpment of S-1, the slope angle of the frontal escarpment of S-2 is relatively lower. On the top surfaces of both toreva blocks, numerous blocks are distributed (Fig. 23c), with some displaying jigsaw structures.

Fig. 23

Toreva blocks in the transition zone of the Nyixoi Chongco rock avalanche (a: S-1 toreva block; b: S-2 toreva block; c: distribution of blocks on the top surface of S-2; the red arrow indicates the movement direction of the sliding mass)

Next to both tovera blocks, it is the extensions of longitudinal ridges (LR) as shown in Fig. 22. Due to the erosion of surficial runoff, the deposited landform in the right part of zone II has been changed. So, only the landforms deposited in the middle and left part of zone II were investigated in detail during our field survey. Figure 24a shows the extension of the longitudinal ridges in zone II. It can be seen that the lengths of the longitudinal ridges all exceed 590 m, with the aspect ratio > 100. In plan-view, L-1 is in a long strip shape with its length and width being ~1000 m and ~ 120 m, respectively. L-2 presents a fan-shape from the narrow proximal to relative wide distal part with its length being ~750 m. L-3 displays an oval shape with its length and width being ~590 m and ~ 100 m, respectively. The orientation of the ridges is 100–120°, which are nearly parallel to the main moving direction of the avalanche mass. Besides these longitudinal ridges, another interesting landform was observed during our field survey, which is the distribution of numerous secondary en echelon-arranged ridges distributed on the surfaces of the longitudinal ridges (Fig. 24b-c). The secondary ridges formed an acuate angle with the main ridges.

Fig. 24

Longitudinal ridges in the transition zone of the Nyixoi Chongco rock avalanche

• c) Accumulation zone (III)

For the accumulation zone, transverse ridges, en echelon-arranged ridges, and hummocks of various sizes are sequentially developed and well-preserved. According to the spatial distribution of these landforms from the proximal to distal area of zone III, it can be further divided into three sub-zones, which are sub-zone III-1 with transverse ridges (TR) and secondary shear faulting, sub-zone III-2 with en echelon ridges (ER), and sub-zone III-3 with hummocks (HM) (Fig. 22).

The sub-zone III-1 with transverse ridges and secondary shear faulting starts from the turning part of the avalanche and comprises the main part of zone III (Fig. 22). In the rear part of this sub-zone, the transverse ridges are 30–60 m long, 15–30 m wide, and 20–30 m high. Figure 25a-b shows the extension of some transverse ridges in this sub-zone. Huge transverse ridges with length > 100 m are distributed locally. The aspect ratio of transverse ridges in this sub-zone is 2–3, with large transverse troughs developed between adjacent ridges. The ridges along with the troughs formed the ridge-trough coupled landforms (Fig. 25b). The main trend of the transverse ridges is 40–70°, which is nearly perpendicular to the movement direction of the avalanche mass. In the frontal part of sub-zone III-1, the scales of transverse ridges increase slightly with most ridges ranging from 20–70 m in length and even >100 m (Fig. 22). Meanwhile, the width of transverse ridges in this part is also larger, especially for those near the central line of the avalanche, whose width exceeds 70 m. Another interesting feature observed in the frontal part of sub-zone TR is the extensions of the secondary shear faulting on the surfaces of several large transverse ridges near the center line of the avalanche. As shown in Fig. 25c, a large transverse ridge has been cut and separated into 5 parts by several dextral secondary shear faulting. Such landform has also been reported in previous studies (Shea and van Wyk de Vries 2008; Paguican et al. 2014; Longchamp et al. 2016).

Fig. 25

Surficial topography in the accumulation zone of the Nyixoi Chongco rock avalanche (a-b: the transverse ridge-trough coupled topography present in the sub-zone TR; c: secondary shear faulting on the surfaces of transverse ridges in the sub-zone TR; d: en echelon ridges in sub-zone ER; e: hummocks in sub-zone HM)

Next to sub-zone III-1, there is a sub-zone III-2 with en echelon ridges (ER). In this sub-zone, these en echelon ridges have relatively smaller sizes in 10–60 m long, 10–20 m wide, and 20–30 m high (Fig. 22). Figure 25d shows one set of the en echelon ridges in this sub-zone with their main trends being 30–40° and 310–320°, respectively, forming an acute angle with the movement direction of the sliding mass. In this sub-zone, the scales of ridges are smaller than that of sub-zone III-1.

Similar to the Luanshibao rock avalanche, the frontal part of the Nyixoi Chongco rock avalanche is a sub-zone III-3 with multiple hummocks (HM). As shown in Fig. 22, this sub-zone is composed mainly of a series of small-scaled hummocks with their maximum diameters of 10–50 m and heights of 5–12 m, respectively. Figure 24e shows the extension of several hummocks in this sub-zone, which are present in nearly circular shapes. On the top surface of hummocks in this sub-zone, there is almost no blocks. Hummocks in sub-zone III-3 are distributed without main trend.

• (2) Internal deposited features

Apart from these landforms as described above, some typical internal deposited features were also observed. The first prominent feature is the carapace facies with numerous huge blocks (Figs. 22 and 26). As indicated by the red points (representing blocks >2 m in long axes) marked in Fig. 22, megablocks are commonly distributed along the sliding path. It is obvious that megablocks are distributed with spatial heterogeneity, as the number of megablocks on the slope of transverse ridges is much larger than on the back of the slope. Furthermore, the main orientation of the majority of megablocks (Fig. 26a-c) is nearly parallel to the main trend of the transverse ridges (i.e., perpendicular to the movement direction of the avalanche). Among these megablocks, jigsaw structures are commonly observed along the travelling path (Fig. 26d-f).

Fig. 26

Surficial features of the carapace faices of the Nyixoi Chongco rock avalanche (The movement direction of the rock avalanche is shown as the red arrow)

As described above, well-preserved outcrops were revealed due to post-avalanche river erosion, which provides invaluable geological evidences for the study of rock avalanche emplacement. Figure 27a shows that the Nyixoi Chongco rock avalanche deposit is inversely graded with the carapace facies, the body facies, and the basal facies being sequential present from the top to the bottom. The carapace facies consists of angular blocks with size mainly ranging from a dozen of centimeters to several meters. Small amount of fine particles fill the pores among blocks as matrix. Thus, the carapace facies is characterized by a clast-supported structure. The body facies contains fewer blocks yet it is composed of clasts with size ranging from several millimeters to a dozen of centimeters mainly. Clasts in the body facies are also angular and poorly rounded. In some outcrops, jigsaw structures are commonly observed in the body facies (Fig. 27b-d), indicating that the original shape of clasts was preserved after fracturing with the fragments did not separating apart or just dislocated minor. In the areas with jigsaw structures appeared, directional arrangements of fragmented clasts can be observed as exhibited in Fig. 27b-c. In one outcrop, z-shaped inner shear band was observed (Fig. 27d). The color of the inner shear band is tangerine, which is significantly different from the color of the main deposits. Grain size in this band is finer than the main deposits. Locally, even black clay scale grains are present in the inner shear band. Another interesting feature as revealed by some outcrops is the appearance of shear structures. As shown in the outcrops of Fig. 27b-d, three preferred sets of shear structures in varying scales, i.e., R shear, P shear and Y shear, can be observed, indicating the internal shearing mode in the emplacement of the avalanche mass.

Fig. 27

Sedimentary features of the Nyixoi Chongco rock avalanche

The basal facies is the direct contact zone with the substrate. Due to the intense interaction between the sliding mass and substrate, the mean grain size in this facies is finest compared with the carapace facies and the body facies. For this facies, it consists of fine particles <2 mm with a small number of clasts mainly. Along the contact line between the basal facies and substrate, preferred alignment of clasts were observed (Fig. 28a-b and d-e), indicating the relative direction of movement of the sliding mass over the substrate. In the Nyixoi Chongco rock avalanche, the eroded depth has exceeded the thickness of the avalanche with the interaction morphology between the avalanche mass and substrate being well revealed as shown in Fig. 28. The substrate of the Nyixoi Chongco rock avalanche is a layer of alluvial deposits consisting of silty and clayey particles (finer than the rock avalanche particles) mainly, which is classified as a soft substrate (Shi et al. 2023). Under the control of such relatively soft substrate, a series of diapiric structures with varying size and morphology were generated along the interface of the basal facies and substrate (Fig. 28). For these diapiric structures, their upstream slopes and the downstream slopes are asymmetric with the downstream slopes being steeper. In some outcrops, vertical fold and recumbent fold were even generated, indicating an intensive interaction between the avalanche mass and substrate (Fig. 28g, l-m). With the generation of such structures, their original layer sequences are still well-preserved, indicating low disturbance and mixing during formation. Consisting with the preservation of the layer sequence, the boundary between the avalanche mass and substrate is sharp without obvious mixing, which also indicates a low disturbance between the basal facies and the substrate during the avalanche propagation.

Fig. 28

Diapiric structures generated along the avalanche mass and substrate of the Nyixoi Chongco rock avalanche

4.3 Tagarma Rock Avalanche

4.3.1 Geological Setting and General Features of the Tagarma Rock Avalanche

Tagarma rock avalanche is located in the Tarim Basin, south of Muztag Ata peak in the Eastern Pamir (Fig. 29). This region is in the northwestern part of the Himalayan convergence zone formed as a result of continuous collision of the Indian-Eurasian plates initiated about 55 Ma ago. For the western tectonic knot, it is one of the regions in the Tibetan Plateau experiencing the strongest tectonic stresses and exhibiting the fastest uplift and denudation rates (Robinson et al. 2004). To date, the convergence zone remains ongoing deformation with an uplift rate of 1.5 ± 0.5 mm/yr. and a shortening rate of 20–25 mm/yr. (Bufe et al. 2017). Such intensive deformation has been promoting to the striking topography, active tectonics and high seismicity in this range (Arrowsmith and Strecker 1999; Cowgill 2010; Strom 2010). Under the effect of historical intensive tectonic activities, a series of fault systems with different scales and kinematics have been generated as shown in Fig. 29. Northward, it reaches the Pamir Arc tectonic belt, and southward, extends to the Nanga Parbat-Haramush massif.

Fig. 29

Regional geological setting of the Pamir-western Himalayan syntaxis region

Among these fault systems, the Kongur Shan normal fault is the main fault system going across the studied area (Fig. 29), which is a well-known extensional system and was formed in the late Cenozoic due to the exhumation of the Muztagh Ata gneiss dome. The extension of this fault is approximately 250 km with its overall strike being NNW and the dip angle ranging from 55° to 75°. It is reported that this fault zone began to generate at 7–8 Ma and remains active as a Holocene active fault. The latest strong earthquake recorded in this region is the 1895 Tashkurgan earthquake with magnitude of Mw 7.0 (Seong et al. 2009).

Fig. 30 shows the geological setting of the Muztag Ata peak region, where the Tagarma rock avalanche occurred. The bedrock is composed of Paleoproterozoic gneiss (Pt₁B), Paleogene monzonitic granite (ηγE), Triassic monzonitic granite (ηγT), and Triassic Quartz diorite (δoT). The basin is filled by the Upper Pleistocene glacial deposits (Q_p^gl), glaciofluvial outwash deposits (Q_p^fgl), and alluvial-diluvial deposits (Q_p^pal), and Holocene alluvial facies (Q_h^al), alluvial-diluvial deposits (Q_h^pal), glacial deposits (Q_h^gl) and fluvial deposits (Q_h^fl). Due to different geological processes, the Quaternary landscapes in this region have undergone significant erosion and alteration. Divided by the Kongur Shan normal fault, bedrocks in this studied area appears in the footwall mainly with the hanging wall being covered by Quaternary deposits mainly as exhibited in Fig. 30. The footwall belongs to the Muztagh Ata gneiss dome, characterized by high relief. Gneiss underwent medium- to high-grade metamorphism during the Paleoproterozoic Era. The Tagarma rock avalanche was just one case originated from the gneiss dome. The highest peak in the area is Muztagh Ata, which stands at an elevation of 7509 m a.s.l. For the hanging wall, it is featured by gentle slopes, with the Tagarma intermontane basin formed in the downstream.

Fig. 30

Geological setting of the studied area where the Tagarma rock avalanche occurred

For the climate of this area, it falls within the domain of mid-latitude westerlies and too far to be affected by Indian summer monsoon. So, its climate is characterized by cold and aridity, with an average annual precipitation of 76 mm and temperature of 3.6 °C (Sun et al. 2006; Seong et al. 2009). The Muztagh Ata peak is covered by glacier as shown in Fig. 30.

As shown in Fig. 30, the Tagarma rock avalanche was detached from a steep southwestern slope of the Muztag Ata peak (38° 4′ 33″ N, 75° 10′ 57″ E). The source area is a high mountainous ridge with an extremely steep frontal slope on the footwall of the Kongur Shan normal fault (Fig. 31), and is composed of the Paleoproterozoic gneiss (Pt₁B). The gneiss exhibits well-developed foliations (F) and dips to the southwest at an angle of 60°. As a result of historical tectonic activities and weathering, two sets of joint structures with attitudes about 315°∠50° (JS1) and 60°∠70° (JS2) dissect the bedrock (Fig. 4c). The travel path on the hanging wall presents a relatively flat terrain in contrast to the steep source area, mainly composed of Quaternary glaciofluvial outwash deposits (Qp^gfl) (Fig. 31b). By ¹⁰Be dating of gneiss boulders sampled from the avalanche deposits, Yuan et al. (2013) reached that this event should occur approximately 6.8 ka B.P. Based on an analysis of the fault activity and climatic environment of the study region, Zhu et al. (2019) proposed that the Tagarma rock slope was stable under natural conditions through a discrete element modeling and most likely was triggered by a paleoseismic event. The mean annual rainfall of this region is only 76 mm, which is too low to trigger a landslide as large as the Tagarma rock avalanche. Glaciers have accumulated only in the high-elevation areas, and no evidence of freeze-thaw process has been found in the source area. Thus, we also agree that the Tagarma rock avalanche was a single event triggered by a strong earthquake (Wang et al. 2020).

Fig. 31

Remote image (a) and longitudinal profile (b) of the Tagarma rock avalanche

In plan-view, the Tagarma rock avalanche deposited in an elongated tongue shape (Fig. 31a). The horizontal distance (L) and vertical drop (H) from the head of the scarp to the toe of the avalanche deposit is 5430 m and 1510 m, respectively, giving rise to a fahrböschung value (H/L; Heim 1932) of 0.28. This low value indicates a hypermobility of the rock avalanche (Cruden and Varnes 1996; Lucas et al. 2014). The accumulated area of the avalanche deposit is up to 6.15 km². With the help of a fixed-wing unmanned aerial vehicle (UAV), high-resolution aerial images of the Tagarma rock avalanche were captured in July 2018, and its high-resolution DEM model was built. Based on the high-resolution DEM data, the pre-avalanche topography of the Tagarma rock avalanche was reconstructed utilizing the Topo to Raster tool in ArcMap 10.7 software (ESRI). Additionally, through a comparison of the pre- and post-avalanche topographies, it has been determined that the detached rock mass and the deposit have approximate volumes of 0.96 × 10⁸ m³ and 1.1 × 10⁸ m³, respectively. As such, the coefficient of dilatation for this landslide is calculated to be 1.15.

The debris tongue of this avalanche has a sharp and steep distal edge that exceeds 50 m in height, which is like to lava flows, surpasses the majority of rock avalanches with comparable flat travel paths (Evans et al. 2007; Dufresne et al. 2010; Basharat et al. 2012; Charrière et al. 2016; Wang et al. 2018, 2019a, b). Due to the arid climate, vegetation is rare along the avalanche path, and only two gullies resulting from upstream glacial meltwater were observed. In contrast to sediments on the left side of the source area that have been altered by gullies, landforms unaffected by such features are well-preserved on the surface of this region from proximal to distal locations. Owing to the dry climatic conditions prevalent in the research region, blocks manifest negligible levels of weathering. Furthermore, some well-preserved outcrops exposed by gully erosion were revealed, which is advantage for the study of the avalanche propagation.

4.3.2 Sedimentary Features of the Avalanche Deposit

• (1) Plan zonation and landforms

As described above, high resolution images of the rock avalanche were obtained by UAV with the resolution of the aerial images being approximately 0.1 m, which was high enough for observing the landforms of the avalanche deposit. Figure 32 is the shaded relief map of the avalanche derived from these images. As shown in Fig. 32, the spatial distributions of landforms along the travelling path can be highlighted and are easier to observe. From Fig. 32, it can be seen that an array of prominent topographic features emerge on the deposit surface, including toreva block, lateral levees, arc-shaped transverse ridges and linear longitudinal structures from proximal to distal areas. Based on the spatial distribution of these landforms, the avalanche can be divided into three zones: the source area (I), transition zone (II), and accumulation zone (III). In the following, the landforms of each zone are described in detail based on the analysis of these maps and field investigations.

Fig. 32

Shaded relief map of the Tagarma rock avalanche

• a) Source area (I)

The source area, situated at the footwall of the Kongur Shan normal fault, ranges in elevation from 4010 to 4791 m a.s.l. The fault approximately intersects with the toe of the rupture surface of the rock avalanche. In plan-view, the source area has the U-shape with its maximum length and width being 1250 m and 780 m, respectively. As shown in Fig. 33a, the source area is composed of an upper steep well-exposed scarp (I-1) and a relatively gentle section with talus accumulation (I-2). For the steep exposed part (I-1), its average slope is over 50° with the preferred joint sets being seen clearly (Fig. 33a). Cutting by the preferred joint sets, the exposed scarp is characterized by a blocky and intensively fractured structure. For the lower relatively gentle section (I-2), its average slope angle is lower than 30° and was covered by avalanche debris and post-avalanche talus.

Fig. 33

Photos showing the extension of the source area (a), toreva block (b) and right lateral ridge (c) of the Tagarma rock avalanche

• b) Transition zone (II)

As exhibited in Fig. 31a, a gully sourced from the glacier in the upstream goes across the transition zone, which destroyed the surficial features of zone II greatly. So, only the toreva block near to the source area and both lateral levees are still well-preserved. Figure 33b shows the extension of the toreva block taken during the field survey. It can be seen that the top surface of the toreva block is relatively flat with some huge blocks distributed. It is measured that the width of the toreva block is up to 572 m with the height of its frontal escarpment being up to 150 m. As exhibited in Fig. 33b, a well-preserved outcrop of the toreva block is revealed with an intact rock mass intersected by three sets of structures being observed. The orientations of joint sets in the intact rock mass are 315°/40° (JS1′), 60°/75° (JS2′) and 220°/55° (F′), respectively, which is similar to that of the bedrock in the source area. This indicates that the detached mass should be slightly fragmented and has a minimal disturbance when it reached the rear part of the transition zone.

In addition to the toreva block, the second prominent feature in the transition zone is the extension of both large lateral levees (LR-1 and LR-2). Figure 33c show the extension of the right lateral levee (LR-1). The LR-1 spans a length of roughly 1600 m and is elevated approximately 60 m above its surroundings. The length and width of LR-2 are 2000 m and 40 m, respectively. On the top surfaces of both lateral ridges, there is a widespread distribution of boulders as exhibited in Fig. 33c.

• c) Accumulation zone (III)

Compared with the transition zone, the landforms in the accumulation zone are slightly eroded and relatively well-preserved. As exhibited in Fig. 31a, the most typical surficial feature in zone III is a sequential distribution of arc-shaped transverse ridges with convex sides facing forward. Unlike linear transverse ridges observed in other rock avalanches described herein, those of the Tagarma rock avalanche exhibit curvature. This provides an ideal condition for a meticulous study of the characteristics of arc-shaped transverse ridges.

From Fig. 31a, it can be seen that megablocks within zone III exhibit alternate dense and sparse concentrations as runout increases. According to the variations in megablock concentrations and topographies in this zone, zone III can be further divided into four distinct sub-zones, namely III-1, III-2, III-3 and III-4 (Fig. 34a). In these sub-zones, the transverse ridges exhibit a stepped distribution in the longitudinal direction, featuring enormous and steep steps. Figures 34c-d are photos of the frontal escarpment of the transverse ridges with their locations being marked by yellow squares in Fig. 34a. Height of the steep step that forms the frontal edge of sub-zones III-3 and III-4 is greater than 30 m and.

Fig. 34

Enlarged view of the accumulation zone (a) with rose diagrams of the counted sites (b), and two enlarged views of the stepped slopes in zone III (c-d)

Corresponding to the layout of these arc-shaped transverse ridges, another notable characteristic observed in zone III is the directional distribution pattern of megablocks along the arc-shaped transverse ridges. To give a direct impression of the spatial extensions of blocks in this zone and accurately assess the distributed regularity of blocks on the top surface of each arc-shaped transverse ridge, 11 sites as highlighted by red squares in Fig. 33a were selected for statistical analysis of the preferred extensions of blocks with varying location. For each site, its size is 50 × 50 m². The rose diagrams shown in Fig. 33b depict the preferred orientations of blocks corresponding to the 11 sites. For each site, only boulders with the side length > 1 m were counted. And a total of 3676 blocks were analyzed, with roughly 300 blocks of l > 1 m in each site. As indicated in Fig. 33b, blocks in the three sites within sub-zone III-1 (i.e., No. 1′, No. 2′, and No. 3′) exhibit a predominant orientation of approximately N. Additionally, a secondary preferential orientation towards N85°W can be observed for the blocks in site No. 3′. For blocks in sites No. 4′ and No. 5′ in sub-zone III-2, their preferred orientations are 310–330° and 270–280°, respectively. For blocks in sites No. 6′ and No. 9′ in sub-zones III-3 and III-4, their preferred orientations are 300–330°. For blocks in site No. 10′, the preferred orientation is 270–280°, which is same to that of blocks in No. 5′ in sub-zone III-2. The preferred orientation of blocks in No. 11′ is 50–60°. From the extensions of blocks in each site, it can be reached that most of blocks are aligned tangentially to the trajectories of the arc-shaped transverse ridges, except sites No. 2′, No. 7′ and No. 8′.

As exhibited in Fig. 32, the third prominent feature of zone III is the extension of the linear longitudinal structures as highlighted by the red dashed lines in the shaded relief map of the Tagarma rock avalanche. In plan-view, these linear longitudinal structures display a feature of dextral shearing.

• (2) Internal deposited features

Apart from these landforms as described above, some typical internal deposited structures were also observed in the deposit. The first prominent feature is the layout of the carapace facies with many huge blocks (Figs. 35a-f). As exhibited in Figs. 35a-g, directional alignment of megablocks is evident along the entire travelling path. Clusters of superimposed megablocks exhibiting varying degrees of fragmentation are widely distributed in the carapace facies, forming what is known as an imbricate structure. And jigsaw structures were commonly observed. Based on the distribution of these megablocks, a conceptual model to illustrate how the distribution of these clusters changes with increasing runout is reached (Fig. 35g). For the avalanche mass accumulated at the rear edge of the transition zone, it displays a very low disturbance with a typical jigsaw structure can be observed in the profile of the toreva block (Fig. 35b). The orientation relationship of its internal three sets of structures essentially inherits the bedrock orientations in the source area, with only a minor degree of rotation during movement. As the runout increases, the detached rockmass undergoes further fragmentation, which is controlled by internal foliation and two sets of structures. This results in the formation of these slab-like megablocks that are several dozen meters in length. The contact surface is formed due to control exerted by internal foliation, and these megablocks are distributed in a superimposed relationship facing upstream (Fig. 35c, e-f) or downstream (Fig. 35c-d).

Fig. 35

Photos showing the extension of megablocks along the travelling path and corresponding conceptual model with the red arrow indicating the movement direction

In addition to the alignment of megablocks in the carapace facies, several typical internal sedimentary characteristics were also observed within the deposit. The first prominent feature is the inverse grading as exhibited in Fig. 36a. As shown in Fig. 36b, the carapace facies is mainly composed of angular blocks. To quantify the grain size distribution of the deposit, one sampling location of the carapace facies was selected for grain size analysis. It is reached that the content of blocks ≥10 mm in length is over 60%. Similar to the directional alignment of megablocks observed on the top surface of the accumulation zone, the orientation of megablocks can also be observed in profile, either upstream or downstream. The body facies contains nearly 80% of the deposit with its grain size ranging from mm to dozens of cm (Fig. 36c). The basal facies is primarily supported by a matrix with a relatively high content of angular and irregular fine-grained clasts (Fig. 36d). As the lowest zone of the avalanche mass, it exhibits the highest degree of fragmentation with its particle size being significantly smaller than that of the body facies and carapace facies. The content of particles <1 mm in size is over 40%, which is significantly greater than that in the body facies. Furthermore, the majority of the coarse fragments within the sediment exhibit a slab-like morphology with their upper surface oriented downstream and their inclination direction aligned with the propagation vector of the avalanche mass.

Fig. 36

Photos showing the inverse grading of the deposit and substrate layout

The substrate beneath the basal facies is composed of well-rounded pebbles and boulders, which are distinctly different from the avalanche mass (Fig. 36a). These clasts were deposited in directional arrangements either upstream or downstream (Fig. 36e-f), indicating the existence of pushing and squeezing effect from the avalanche mass. Cracks and fissures are commonly developed in the well-aligned boulders and pebbles in the substrate (Fig. 36g-h), which is consistent with the findings of Dufresne et al. (2010) and suggests strong interactions between the avalanche mass and substrate. In addition to these features, another interesting phenomenon was also observed during the field investigation. That is the extension of a nearly horizontal boundary between the basal and substrate with minimal mixing observed, which is obvious different from that observed in the Nyixoi Chongco rock avalanche.

4.4 Iymek Rock Avalanche

4.4.1 Geological Setting and General Features of the Iymek Rock Avalanche

The gigantic Iymek rock avalanche is the longest rock avalanche reported in China at present (Shi et al. 2023). It is located in the eastern foot of Pamir (Fig. 37a). As shown in Fig. 37b, an arcuate thrust belt comprising a series of actively seismogenic thrust faults dipping to south are well-developed along the margin of Pamir, i.e., the main Pamir thrust. It demarcates the boundary between the Pamir and the Tarim basin (Fu et al. 2011; Chevalier et al. 2011, 2015). This fault system, characterized by intensive tectonic activity, is the source zone of strong earthquakes with high magnitude (Arrowsmith and Strecker 1999; Zubovich et al. 2010; Fan et al. 1994; Strecker et al. 2003; Ischuk et al. 2013).

Fig. 37

Location of the Iymek rock avalanche and regional geological setting

The Main Pamir Thrust passing across the IRA deposits, is a ~ 1000 km long, seismogenic, and segmented reverse fault with low angle dipping to southwest and defines the northeast boundary of the Pamir (Figs. 37b and 38a). GPS data suggest the Main Pamir Thrust remains tectonically active and the uphill rate along it reach up to 6 mm year⁻¹ in the Alai valley. Controlled by the Main Pamir Thrust, the hanging wall is composed of bedrock mainly, including the Upper Cretaceous (K₂), Upper Carboniferous to Lower Permain (C₃-P₁), Carboniferous (C), Lower Palaeozoic (Pz₁) sedimentary and metamorphic rocks and Permian granite (γ₄³). In the footwall is the Tarim basin, representing a gentle piedmont basin filled with Upper Pleistocene to Holocene alluvial and diluvial deposits (Q_3-4^al-pl), Middle Pleistocene alluvial deposit (Q₂^al) and Pliocene to Lower Pleistocene deposit (N₂-Q₁).

Fig. 38

Geological setting of the Iymek rock avalanche (a) with a longitudinal profile of A-A′ (b)

In terms of the climatic environment, this range is located in the innermost part of the continent surrounded by high mountains, making it extreme dry. It belongs to an extremely continental climate with mean annual precipitation less than 100 mm/yr. In mountainous region immediately behind the Iymek rock avalanche, the Quaternary glaciation and deglaciation occurred repeatedly and left abundant peculiar landforms, such as regular curvilinear form, evenly truncated spurs, and wide infilled floor (Liu et al. 2014; Strom and Abdrakhmatov 2018). However, there are no signatures related to glaciation or deglaciation on both sides of the Iymek and adjoining valleys.

As shown in Fig. 38b, the Iymek peak is composed of the meta-sedimentary rocks with different degrees of metamorphism formed in the lower Palaeozoic Era, including chlorite schists, marbles and intercalated metamorphic conglomerates. The anaclinal bedrocks develop centimetric foliations (F) dipping to southwest at 15–40° and are dissected by two sets of joints with attitudes 113°∠60° (JS1) and 325°∠45° (JS2), respectively (Fig. 4d). Such nearly orthogonal discontinuities are particularly responsible for a wedge failure.

The Iymek rock avalanche detached from the steep Iymek peak most likely triggered by strong earthquake along the Main Pamir Thrust ~7.1 ka ago (Yuan et al. 2013; Shi et al. 2023). After detaching from the source area, the huge mass travelled downward rapidly along the laterally confined valley and deposited onto a gentle basin terrain as an asymmetrically elongated sheet (Fig. 39a). In plan-view, the depositional sheet is surrounded by bulging distal edges and raised margins, making the avalanche deposit clearly distinguished from adjacent alluvial deposits (Fig. 39a). From the current head to the toe, the drop height of the avalanche is 2135 m and the travelled distance is 16.3 km, which correspond to an apparent friction coefficient of 0.13. The covered area is over 48.0 km² with varying deposited thickness. It is measured that the thickest depth is over 90 m distributed in the valley section and the thinnest depth is ~10 m in the middle part of the deposit sheet. Through topography reconstruction, it is reaches that the detached volume and the depositional volume of the Iymek rock avalanche are ~0.98 km³ and 1.37 km³, respectively, yielding a dilatation coefficient of 1.39 (Shi et al. 2023).

Fig. 39

Remote sensing image showing three zones of the Iymek rock avalanche (I: the source area; II: the transition zone; III: the accumulation zone) (a) and enlarged views of the stepped slopes dividing the three zones (b-d)

Figure 38b shows one longitudinal profile of the avalanche with its position being marked in Fig. 4d. From Fig. 38b, it can be seen that the Main Pamir Thrust is located along the boundary of the transition zone and accumulation zone, which is also a terrain rapid change zone. Bounded by the Main Pamir Thrust, the avalanche source area and transition zone are mainly composed of marble, chlorite schist and conglomerate of Lower Palaeozoic. Due to the effects of tectonic events and long-term weathering, the bedrock has been intensely fractured with preferred joints developed (Fig. 4d). The accumulation zone is at the NE footwall. For this area, the slope angle is relatively low. And the farther away from the source area, the lower the slope angle is. The substrate of this part is composed of Quaternary alluvial-fluvial facies (Q_3-4^al-pl).

The arid climate allows the huge deposit sheet to be well-preserved. Subsequent river erosion yields a series of well-exposed outcrops with thickness more than 80 meters, providing a good opportunity to learn the internal characteristics of the deposit. This catastrophic emplacement leaves a splendid legacy bearing tremendous volume, well-preserved landforms, as well as spectacular exposures, which provide an excellent laboratory for studying rock avalanches.

4.4.2 Sedimentary Features of the Avalanche Deposit

• (1) Plan zonation and landforms

As exhibited in Fig. 39a, the Iymek rock avalanche presents an elongated tongue with two branches at its frontal part. On the surface of the deposit, prominent well-developed landforms, including toreva blocks, transverse and longitudinal ridges, and hummocks, are distributed from the proximal to the distal areas. Based on geomorphological and depositional characteristics, three zones can be recognised from the scarp to the distal edge (Fig. 38a): the source area (I), the transition zone (II), and the accumulation zone (III).

• a) Source area (I)

The source area of the deep-seated Iymek rock avalanche features a V-shaped scar (Fig. 39b). The source area scar is 3.0 km long, and 1.7 km wide with its elevation ranging from 2800 to 3980 m a.s.l. Through reconstructing of the pre-avalanche topography, it is derived that the original altitude of the Iymek peak was up to 4225 m a.s.l., higher than present altitude. The main scarp of the detached wedge is behind the original ridge crest, shaving the summit peak about 250 m. The southeast-facing scarp is a nearly planar surface reflecting the light of sun.

• b) Transition zone (II)

For the transition zone, the first prominent landform is the display of a large toreva block at the base of the source area. As the prominent landform near the source area, it consists of one or more faulted blocks that have slid down a little distance without disintegration and covers the toe of rupture surface. The diamond-shaped toreva block has a relatively gentle platform with a length of ~900 m in the movement direction, a maximum width of 800 m, and a vertical height of 220 m (Fig. 39b). A well-exposed outcrop reveal its internal structures efficiently, which indicates that the toreva block is composed of undisintegrated host rock mass, i.e., the grey marble overlying on the green schists bearing continuous, undistorted original foliations.

The second typical feature of the transition zone is a well-confined valley topography in the downstream, which has a length and width of 4.0–4.8 km and 2.0 km, respectively (Fig. 40a). The open valley bottom provides the avalanche mass propagation along laterally confined valley that gradually widens downstream. Along both lateral boundaries, a pair of parallel trimlines composed of the avalanche masses adhering to the lateral bedrock ridges can be observed. The continuous trimlines extend 4 km along both valley sides and stand 100 m above the current deposit surface. Trimlines indicate the original thickness of rapidly moving debris passing here during rock avalanche motion. At end of the transition zone, complex terrain consisted of several pre-existing rocky hills as the throat of the passage lead to the moving mass split into several streams along the channels (Fig. 40b). Although most of them are covered by avalanche materials, three isolated rocky islands formed by in-situ bedrock can be identified. Figure 39c shows one profile of the in-situ island photographed during field survey. The right one forms a 55 m high steep downward escarpment comprising of metamorphic conglomerates mainly.

Fig. 40

Panorama of the transition zone comprising well-confined valley topography and trimlines (a), and its frontal complex terrain (b)

• c) Accumulation zone (III)

In the accumulation zone, the sheet is composed of two asymmetrically elongated lobes. On a low-angle (3–4°) basin surface, the left lobe extends 9 km with a width of 2.5 km and the right lobe extends 7 km with a width of 2.7 km in the 45–50° direction. Each lobe is surrounded by relatively steep lateral levees and bulging distal fronts with heights of nearly 30 m (Fig. 39d). The steep frontal rim is similar to the steep frontal rim of the Tagarma rock avalanche. Figure 41 shows an enlarged view of the accumulation zone. From Fig. 41, it can be seen that the surficial landform in zone III is a classical wave-like topography akin to other large rock avalanches, including flowbands, hummocks, as well as longitudinal and transverse ridges. According to the spatial distribution of these landforms, zone III can be further divided into six sub-zones as exhibited in Fig. 41.

Fig. 41

Enlarged view of the accumulation zone with the six sub-zones being marked and corresponding rose diagrams of ridge directions in each sub-zone. The ridges and hummocks are marked by the block short lines and the orange circles, respectively

The proximal area of zone III is characterized by a series of elongated flowbands and longitudinal ridges (the sub-zone III-P). As shown in Fig. 41, the flowbands commonly extend several hundred meters to thousand meters toward the movement direction and align with parallel longitudinal grooves. Close to the margins of both lateral sides of sub-zone III-P, a series of sub-parallel flowbands are well continuous, and can be traced up to 1.0 km with widths of 30 to 100 m. Especially, the flowbands on the right side extend more than 1.0 km and display a flexural shape to accommodate the flow-like motion of the avalanche mass (Fig. 42a-b). The right side flowbands consist of block clusters and are separated by areas lacking large blocks. The left side flowbands are relatively straight directed less than 30°, diverging from the main rock avalanche direction (45°) and deflecting to the left lateral margin, suggesting the local moving direction during rock avalanche propagation. Furthermore, some flowbands split into sub-parallel multiple bands (Fig. 41c). In the central part of sub-zone III-P, several longitudinal ridges stand tens meters above the adjacent alluvial deposit (Fig. 41d-e). These ridges stretch hundreds of meters with a preferred orientation of 40–50°, parallel to the moving direction of the avalanche mass. In terms of interior, ridges consist of intensively fragmented marble clasts, rather than of large (> 1 m) block aggregations.

Fig. 42

Morphological features in sub-zone III-P. (a) Flowbands on the right side with their corresponding satellite image (b). (c) Flowbands on the left side. (d) Longitudinal ridges with their corresponding satellite image (e)

Next to the sub-zone III-P with longitudinal landforms distributed, there are series of conjugate ridges in the left lobe, named as sub-zone III-L1. In this sub-zone, undulating topography like water waves is the most general landform (Fig. 43a-b). With the increase of runout, the scales and preferred orientations of ridges slightly change with three preferential directions. To quantify the preferred extensions of ridges in sub-zone III-L1, three areas L1–1, L1–2, and L1–3 were selected for statistical analysis with corresponding rose diagrams being graphed (Fig. 41). We found that the conjugate ridges in the III-L1–1 area strike 160–170° and 90–100°, respectively, oblique to the avalanche mass main movement direction. The angle between the both preferred sets of conjugate ridges is 60°. In addition, a set of ridges oriented at 110–130° are also well-developed in this part. In the area of III-L1–2, the X-conjugate ridges are still the prevailing ridges with their preferred extensions being of 90–100° and 180–190°, respectively. With a further increase of runout, the ridges apparently present two different prevailing orientations with one being slightly parallel to the main moving direction and another being mainly oriented at 90–130° oblique to the main moving direction.

Fig. 43

Photographs exhibiting the deposited feature of the ridges in sub-zones III-L1 (a-b) and III-R1 (c)

In the right lobe (the sub-zone III-R1), series of transverse ridges are distributed next to the sub-zone III-P. Similar to sub-zone III-L1, undulating topography like water waves is also the most general landform in sub-zone III-R1, although their shapes and scales are different. In sub-zone III-R1, it is mainly featured by a series of transverse ridges and associated depressions (Fig. 43c). The surfaces of these transverse ridges are covered by fragmented clasts and blocks. Spatially, these transverse ridges extend 50 to 370 m in length with the width and height ranging from 20 to 50 m and 2 to 10 m, respectively. The wavelengths between adjacent transverse ridges or grooves are 20 to 80 m, with the mean value of 50 m. To clarify the spatial orientation of these ridges, three areas named R1–1, R1–2, and R1–3 were selected for statistical analysis with the corresponding rose diagrams being graphed (Fig. 41). The preferred orientation of the transverse ridges in sub-zone III-R1 is 130–150°, which is perpendicular to the avalanche mass main moving direction (45°). Additionally, it is noticed that the scale and density of the transverse ridges in sub-zone III-R1 varies with runout. In the central area of sub-zone III-R1, the transverse ridges are distributed relatively uniformly and have approximately equal wavelength. As the runout increases, the transverse ridges appear to be denser with their widths being larger and the adjacent grooves being shallower, so that it is difficult to distinguish individual ridges in the distal area.

In addition to the distribution of the landforms in sub-zones III-L1 and III-R1, another prominent feature is the oblique ridges along the marginal areas in both lobes (Fig. 41), named as sub-zones III-L3 and III-R2. In both sub-zones, the oblique ridges are well extended with an en echelon alignment featured by dextral shear dislocations. These ridges can maximally stretch for about 400 m long and about 10–50 m wide. From the rose diagrams of subzones III-L3 and III-R2 (Fig. 41), it can be reached that the prevailing orientation of these oblique ridges is about East in both areas and oblique to the main moving direction (45°).

Besides, there are multiple hummocks in the medial area of the left lobe, which is named as sub-zone III-L2. In plan-view, over 1000 hummocks can be identified (Fig. 41), which is also a diagnostic morphologic feature reported at other rock avalanches. These hummocks are featured by regular or irregular circular cones and circular to elliptical shapes in plan-view. They commonly range from 10 m to 50 m in diameter and raise 2–10 m above adjacent deposit surface (Fig. 44a-b). A typical example shown Fig. 44b is a circular hummock that has a height of 4.3 m with a diameter of 14 m and consists of intensively fragmented clasts rather than large blocks. The distribution of hummocks is relatively non-uniform. Some stand alone, while others distributed closely and separated by shallow grooves, forming a large hummock or ridge.

Fig. 44

Photographs of hummocks in sub-zone III-L2 (a-b) and lateral levees in zone III (c-d)

The last significant feature in zone III is the extensions of the lateral levees in each lobe, which comprise the sheet margins in the accumulation zone (Fig. 41). A pairs of levees along the left lobe are of good continuity and well-preserved. The left levee of the left lobe extends 6.2 km with a width of 60–110 m and rises from 20–30 m above the outer side terrain. The right levee extends straightly 5.8 km with width of 80–100. Interestingly, a secondary levee with a width about 50–60 m, with the overlapping on the inside of the primary right levee is observed, forming a multiple sub-parallel levee (Fig. 44c-d). By contrast, in the right lobe, its left levee extending 3.6 km is discontinuous and broken into several segments because of subsequent erosion. The right levee segment remains only 1.5 km in the proximal area and the counterpart completely disappeared in the distal area due to intensive erosion. These levees are higher than the undulating depositional terrain surrounding them.

• (2) Internal deposited features

As shown in Fig. 45a, the Iymek rock avalanche deposit also can be generalized into three facies from the top to bottom, i.e., the carapace facies, body facies, and basal facies, forming a classical inverse grading.

Fig. 45

Three facies of the Iymek rock avalanche deposit in accumulation zone (a) with both close-up views to show the interaction features between the basal facies and underlying substrate (b-c)

• a) Carapace facies

For the carapace facies, large blocks with preserved original foliations are widely distributed on the surface (Fig. 46a-c). Directional alignment of blocks can also be observed in the Iymek rock avalanche deposit. And the foliated surfaces of most blocks are facing upstream or downstream (Fig. 46b-c), which is similar to the Tagarma rock avalanche. Through a statistical analysis on the grain size distribution of the surficial blocks, it is reached that the block size distribution of the carapace facies presents a wide span from several centimeters to the maximum up to 10 m (Fig. 46a-c). In outcrops, this facies predominantly consist of poorly sorted coarse clasts (Fig. 46d-e). Irregular angular clasts bearing very sharp edges interlock with each other, resulting in a clast-supported structure. In some outcrops, jigsaw structures with original foliations being preserved can be observed (Fig. 46e). For these jigsaw fractured blocks, they experienced disintegration, but did not separate and were commonly immersed in fragmented smaller clasts.

Fig. 46

Representative carapace facies comprising large blocks. (a) A panorama of the blocky surface of the deposit. (b-c) Surficial blocks preserved their original foliations. (d-e) Internal structures of the carapace facies

• b) Body facies

Underlying the carapace faices, it is the layout of the body facies, which constitutes the main part of rock avalanche deposits. As shown in Fig. 45a, the body facies of the Iymek rock avalanche deposit is mainly composed of clasts ranging from ultrafine powders to decametric blocks, implying an intensive but heterogeneous fragmentation. Those blocks include isolated jigsaw-fractured clasts and large isolated blocks, which still preserve their original foliations and are commonly surrounded by fine-grained matrix with a matrix-supported structure.

In the body facies, some different landslide-tectonic structures are also well developed. The first one is the preserved initial stratigraphy. As exhibited in Figs. 47a and 48a, preservation of original stratigraphic sequence composed of grey marble layer overlying green chlorite schist layer can be observed commonly throughout the Iymek rock avalanche deposit from the transition zone to accumulation zone, although the avalanche mass has experienced long runout and has been fragmented completely. Figure 47 shows an ~87 m high outcrop in the transition zone. For this outcrop, the upper part of the body facies is fragmented marble with the underlying material being fragmented chlorite schist. Both of them consist of the body facies with a clear boundary between them, indicating a low disturbance and mixing in the body facies. In the underlying chlorite schist layer, the original rock mass structure without distortion can also be observed, indicating a low disturbance of the fragmented mass. Additionally, a purple-red metamorphic conglomerates layer about 3–4 m thick sandwiched in the schist layers can be observed. In the fragmented chlorite schist, some small-scaled normal faults were generated. Similar to the outcrop shown in Figs. 47 and 48a shows another outcrop with preserved original stratigraphy in the body facies, which is at the proximal area of the accumulation zone. As the runout further increases, the underlying chlorite schist layer is invisible due to the limited erosion depth. Figure 48b-c is both enlarged views to show the contact relationships between both fragmented layers in the body facies. It can be seen that although the avalanche mass displays an increasing fragmentation with travelled distance in the accumulation zone, the contact surface between the fragmented marble layer and green schist layer is still well-defined without intensive internal mixing.

Fig. 47

Typical outcrop in the transition zone shows the preserved original bedrock stratigraphy and faults. In the schematic representations, the colour grey, green, and red represent the marble breccia, chlorite schist breccia and metamorphic conglomerate breccia, respectively

Fig. 48

Preserved original stratigraphy in the proximal area of the accumulation zone. A layer of green fragmented chlorite schist breccia was sandwiched by grey fragmented marble breccia

The second typical feature in the body facies should be the appearances of a series small-scaled normal and reverse faults as revealed by outcrops in the deposit from the transition zone to the accumulation zone. As shown in the outcrop in the transition zone (Fig. 47), several high-angle normal faults dipping to the avalanche movement direction that dislocated the metamorphic conglomerate layer are well developed. In the outcrop of the accumulation zone (Fig. 48a), several small-scaled normal faults interspersed in the body facies were also observed, which were truncated by a large low-angle thrust fault. Figure 48d-e shows enlarged views of the fault system. The combined relationship of these faults is an important geological evidence, indicating the direction of relative movement of material in the body facies. Beneath the thrust fault, a narrow wedge along the movement direction consisting of chlorite schist clasts inserted into the marble matrix and tapers off (Fig. 48e). In the distal area, numerous faults also can be identified. Compared with the transition zone, these faults have relatively smaller length but are distributed densely, and have complex interrelation. For instance, an outcrop belonging to the transverse ridge displays a set of normal and reverse faults interlace with each other and dislocate the internal layers and shear bands (Fig. 49a).

Fig. 49

Extension of faults developed in the frontal area of the accumulation zone with internal shear bands

The third prominent feature in the body facies throughout the avalanche deposit is the distribution of narrow internal shear bands composed of highly pulverized ultrafine grains. Most of shear bands have thicknesses of centimetres and are subhorizontal with little undulations. The colour of the internal shear bands is obviously different from adjacent materials, which is easy to be identified (Fig. 49). As shown in Fig. 49a, a series of internal shear bands are well generated that are displaced by a number of moderately dipping normal faults and divided into several segments. These shear bands feature undulated bands bearing thin trailing vein or present an elongated lens shape (Fig. 49b). Additionally, some shear bands display complex structures like a flame or irregularly contorted appearance, implying an intensive shearing (Fig. 49d).

The last and key deposited feature observed in the body facies should be the presence of a spectacular flame-like structure. This structureconsists of an array of undulated shear bands with upward-pointing cusps, which was exposed on a cross-section of a lateral levee (Fig. 50). As the enlarged view in Fig. 50b indicated, fragmented marble matrix is interspersed with fragmented chlorite schist material, but their boundary is still sharp without intense mixing. Some clast aggregations with lenticle shapes are immersed in the fine-grained matrix (Fig. 50d). In addition, some small shear-induced flame bands with light orange colour is immersed in the pulverized marble facies, which is only several centimetres thick and extends for about 2 m (Fig. 50e). Its appearance is intensively contorted and displays an irregularly undulated boundary, tapering off gradually towards the lateral margin. This is one of the important findings reached in this rock avalanche, which provides geological evidences to reveal the formation mechanism of lateral levees.

Fig. 50

Rolled flame structures within the lateral levee (a) with enlarged views of internal structures (b, d-e) and their schematic representations (c)

• c) Basal facies

Beneath the body facies, at rock avalanche deposits base is the basal facies, directly interacting with substrate (Fig. 51). This facies consists of the finest grains with millimetre-sized matrix compared with overlying deposit materials, implying more intensive fragmentation and pulverization due to significant shearing under overburden. From the outcrops where the substrate is exposed, it can be seen that the contact between the basal facies and substrate is considerably sharp without obvious mixing. Principal shear bands at the bottom of the avalanche deposit, characterised by well continuous, relatively planar extension and traced for several hundred meters in outcrops, can be observed, forming a distinct boundary between the deposit and substrate. Typical shear bands rooted in the basal facies (Fig. 45c) occasionally can be observed and presents a light orange colour. Additionally, structures of clastic dikes and diapiric structures due to avalanche-substrate interaction can be observed in the basal facies (Fig. 51a). An irregular wedge of diapiric intrusion squeezed into the overlying basal facies and resulted in the deformation of adjacent materials (Fig. 51a). In this outcrop, pulverised avalanche material is mainly less than 2 cm and presents a layer-like texture that curved extremely to form convoluted laminations, making the diapiric structure like an asymmetric thrusting fold. However, the axial plane is opposite to the moving direction of the avalanche mass. Different from the basal facies, the underlying substrate mainly consists of pebbles and gravels ranging from 2 to 10 cm in size, which are much coarser than the avalanche basal facies materials (Fig. 51b). These pebbles present original sedimentary features, characterized by a compact distribution with oblique long axis and a good cemented status.

Fig. 51

Diapiric structure in the basal facies (a) with enlarged view showing the contact form between the basal facies and substrate (b-c)

In the basal facies, there is another interesting feature, which provides geological evidence for the role of liquefaction in rock avalanche propagation. There are some clastic dikes generated in an outcrop near the frontal area of the accumulation zone. These spectacular clastic dikes are related to substrate liquefaction at the accumulation zone (Fig. 52). In the exposures, these clastic dikes root from the substrate (Fig. 52) and extend vertically into the overlying rock avalanche deposit with the maximum height being up to 8 m and several centimetres to 2 m in width. They commonly display elongated wedge shapes and taper off from the bottom to top. In vertical, they display continuous extension without any trace of being cut by the moving avalanche mass. In contrast, some subhorizontal shear bands within the avalanche deposit were separated and dislocated by the clastic dikes (Fig. 52d-g). These clastic dikes consist of fine grains that are mixtures of clays and sands with high circularity, and present relatively homogenous internal textures, suggesting a source of substrate. Diameters of gravels are 1–2 cm with the largest being up to 4 cm, and fine-grained sands replaced gravels with the height increasing. Furthermore, some clastic dikes exhibit well-organized internal structures indicating flow-like motion. For instance, a clastic dike that is 8 meters high and 2.5 meters wide displays a series of upward arched lineations inclined at angles of 30 to 50° relative to both clastic dike walls (Fig. 52e). Due to fine grains aligned regularly within the dikes, a similar clastic dike comprising a series of oblique and repeated laminations that are in inclined contact with the dike walls at a high angle was also observed in another outcrop (Fig. 52f). The existence of the dikes indicates that liquefaction occurred in the substrate when the avalanche mass arrived. As indicated by the relationship between the avalanche mass and dikes, it can be deduced that the avalanche mass was dislocated by the dikes with the dikes not being disturbed. This indicates that the dikes should be generated during the final accumulation stage of the avalanche mass, rather than during the propagation process.

Fig. 52

Outcrops (a, f) with enlarged views of typical clastic dikes (b-e, g)

5 Discussion

Take the area of the Himalayan range in China as an studied area, the spatial distribution of rock avalanches in the studied area is conducted with the controlling factors on the generation of rock avalanches being primarily analyzed. Furthermore, several typical rock avalanches that occurred in the Tibetan Plateau, demonstrating excellent landforms and internal deposition features, are introduced in detail, hoping to provide more geological evidences insight on rock avalanche kinematics. With these studies, it can be reached that active fault has a prominent role in the predetermining and occurrence of rock avalanches. Controlled by faults, the source areas of rock avalanches are commonly located on steep bedrock slopes with well-developed structures. The transition and accumulation zones are usually located on the rest fault wall with Quaternary deposits distributed along the travelling path. For the Quaternary deposits, they may be fine-grained fluvial materials, which can be classified as a relatively soft substrate like the Luanshibao and Nyixoi Chongco rock avalanches. Sometimes, they are composed of coarse-grained materials sourced from diluvial or fluvioglacial processes, which can be classified as a relatively hard substrate like at the Tagarma and Iymek rock avalanches. Through detailed studies on the deposited features of the four typical cases in the Tibet Plateau, similar landforms are observed in their transition zone, such as toreva blocks with steep frontal escarpments and longitudinal ridges. Different deposited features also can be observed along their accumulation zones due to variations in substrate conditions.

As argued in previous studies (Andrade and van Wyk de Vries 2010; Thompson et al. 2010; Strom 2010; Johnson et al. 2012; Paguican et al. 2014; Longchamp et al. 2016; Wang et al. 2018, 2019a, b, 2020), toreva blocks are usually products of a rapid stretching process, which commonly distributes in the rear part of the transition zone, especially at the toe of the source area. Next to the distribution of toreva blocks, it is usually the distribution of longitudinal landforms, which is attributed to an extensional stress status in propagation with a sinistral or dextral shearing occurred along the lateral margins (Valderrama et al. 2016; Wang et al. 2018). Through a comparison of the four cases, it can be reached that material conditions of the substrate have little influence on the generation of landforms in the transition zone.

For the generation of transverse ridges, it is generally the products of compression in the deceleration process of avalanche masses (Nicoletti and Parise 1996; Shea and van Wyk de Vries 2008; Dufresne and Davies 2009; Ostermann et al. 2012; Paguican et al. 2014; Yoshida 2014; Charrière et al. 2016). For the occurrence of the rapid deceleration process, it is resulted by the gradually slowing down of the slope of the travelling path. With the slowing down of the travelling path slope, the basal friction will increase, which is disadvantage for the propagation of the avalanche mass. As described above, slope angles of the accumulation zones of the Luanshibao and Nyixoi Chongco rock avalanches are both less than 2°. Yoshida (2014) argued that gentler travelling path can result in the reduction of down-sliding force and the increase of basal friction, which is disadvantageous for the emplacement of avalanche masses. In the deceleration process, different emplacement forms should occur under the influence of different substrate conditions. For the Luanshibao and Nyixoi Chongco rock avalanches with soft substrates, the increase of basal friction was relatively low and the avalanche masses still kept relatively high mobility. Lateral spreading increases obviously due to the low lateral resistance of a soft substrate. With the occurrence of lateral spreading, a series of small-scaled hummocks will be generated. And a relative thin sheet will form with its frontal leading edge usually being several meters only. Greatly different from the Luanshibao and Nyixoi Chongco rock avalanches with soft substrates, the Tagarma and Iymek rock avalanches featured by hard substrates have steep and high frontal leading edges with the thickness being up to tens of meters. Meanwhile, the degree of lateral spreading is limited and there is almost no distribution of hummocks. Through a comparison of volume, it can be seen that the volume of the Tagarma rock avalanche is similar to that of the Luanshibao and Nyixoi Chongco rock avalanches, but they have different deposited features. This indicates that volume should not the key role for the difference of the deposited features of the rock avalanches with different substrates. A comparison of the landforms and internal deposited structures of these rock avalanches indicates that the detached mass generally displays a rapid extensional movement in the transition zone. In the accumulation zone, the avalanche should display a rapid deceleration process with intensive compression in the condition of a hard substrate. When the substrate is soft, the avalanche should display a combination of longitudinal deceleration and lateral spreading in unconfined conditions.

In addition to the differential movement of rock avalanches under different substrate conditions, a series of other sheared structures can be observed in rock avalanche deposits, such as jigsaw structures, preserved original stratigraphy sequence, aligned distribution of clasts, etc. These are geological evidences of a laminar-like flow movement. Furthermore, geological evidences for the generation of lateral levees are observed in the Iymek rock avalanche, which indicates that the formation of lateral levees should be resulted from a lateral bulldozing process instead of a sorting process typical of granular flows. In the frontal area of the accumulation, the liquefaction phenomenon is also observed with the avalanche mass being dislocated, which indicated that the liquefied dikes should be generated at the final moment and have no contribution to the high mobility of the rock avalanche.

6 Conclusion

Rock avalanches, as one of the most threatening geological processes in mountainous regions, commonly cause tremendous threat to life. With this study, the sedimentary structures of four typical rock avalanches are analyzed in detail with the following conclusions being reached.

• 1. Through a comparison of different factors on the spatial distribution of rock avalanches in the Himalayas of China, we primary reached that the fault systems should be the most significant factor for their generation. For the non-glacial areas, the role of glacier should be the second factor, which intensively influences the stability of the bedrocks in the source areas. For the non-glacial areas, lithology should be the second key factor, which determines the slope steepness and, thus, its stability.

• 2. In rock avalanche deposits, some features such as jigsaw structures, preserved original stratigraphy sequence, aligned distribution of clasts are commonly developed, indicating a low disturbance of the avalanche mass in propagation. Hence, it is proposed that the propagation of rock avalanches should be a rapid motion similar to a laminar flow.

• 3. After detaching from the source area, the rock avalanche mass exhibits an extension-dominated sliding process in the transition zone. When it arrives at the accumulation zone, it fails into a compression-dominated sliding process. If the substrate is composed of soft material like water-rich fluvial deposits, a rapid radial spreading process will appear in unconfined conditions at the frontal edge due to the rapid decrease of basal friction with a relatively thin deposited sheet being formed. If the substrate is composed of hard material liking dry diluvial or fluvioglacial deposits, the moving mass will display a steep and thick leading edge due to a high increase of basal friction before motion halts.

• 4. The finding of the rolling flame structures within the raised lateral levees of the Iymek rock avalanche is the helpful geological evidence for the exploration of the formation mechanisms of lateral levee. With this evidence, it is reached that the formation of the lateral levees is associated with a lateral bulldozing movement of the avalanche mass instead of a complex mechanism related to grain self-segregation. In this process, the rear materials continuously push and plough the front decelerated materials aside, resulting in the formation of these outward extended flame-like shear bands.

• 5. The appearance of the clastic dikes in the Iymek rock avalanche suggests that the occurrence of substrate liquefaction should be caused by the high-energy shearing during emplacement. But, the localized liquefaction of the substrate is not a contributor for the high mobility of the Iymek rock avalanche. It is only a product of the strong interaction between the sliding mass and the substrate during the final accumulated stage.