In 2018, successive large-scale landslides occurred near the village of Baige in the Jinshajiang suture zone on the Qinghai–Tibet Plateau, posing substantial threat both to the lives and property of residents and to the engineering facilities along the upper and lower reaches of the Jinsha River. In this study, field investigation data, remote sensing images, aerial photography, microanalysis of rocks, and mechanical experiments were considered to ascertain the material and geomorphic conditions of the landslides and to explore the failure mechanism and process of evolution of the landslides under the coupled effect of exogenic and endogenic factors. Located in the Jinshajiang suture zone, the Baige landslides occurred close to the western boundary fault zone. The geological structure of the slope and the formation of landslides are controlled by the specific environmental tectonic dynamics of the suture zone. Tectonic damage has dramatically influenced the strength and hydraulic characteristics of the rock mass, and the rock’s uniaxial compressive strength and resistance to water disintegration have deteriorated markedly as a result. Strong weathering of the plateau and repeated groundwater infiltration have further reduced the mechanical properties of the unique rock and soil with inferior original strength characteristics. The intensive crustal uplift and the rapid river erosion have markedly altered the free face of the damaged rock mass, thereby increasing the gravitational potential energy of the rock mass. The long-term cumulative coupled effect of tectonic movement and surface denudation represents the fundamental mechanism of the Baige landslides. The Jinshajiang suture zone continues to face the risk of further massive landslides similar to those that occurred in Baige.
Successive massive landslides occurred near the village of Baige on the right bank of the Jinsha River in Jiangda County (Tibet) in October and November 2018. Two landslides blocked the river, which formed landslide-dammed lake-outburst flood disaster chains that caused huge losses on the upper and lower reaches of the Jinsha River. The first landslide, which occurred on October 10, had a total volume of approximately 24 million m3, and it quickly blocked the Jinsha River. The barrier lake that formed subsequently flooded several roads upstream, disrupting traffic. Natural overflow of the dam, which began at 17:00 local time on October 12, did not pose great risk. On November 3, approximately 3.7 million m3 of rock mass at the back edge of the slope slipped, which scraped rock and soil from the slope surface that caused a massive landslide with a total volume of 8.7 million m3. The second landslide blocked the river again and formed a barrier dam with a height of approximately 60 m. This posed substantial threat both to the lives and property of residents along the river in Baiyu, Batang, and Lijiang and to the infrastructure of local multistage hydropower stations.
The occurrence only 23 days apart of the two landslides near Baige meant that their unique genesis, movement process, losses, and associated future risks have attracted widespread attention. Xu et al.  determined the dynamic evolution characteristics of the two landslides and the dammed lakes through on-site geological survey, interpretation of historical remote sensing images, InSAR monitoring, unmanned aerial vehicle (UAV) photography, ground deformation monitoring, and other methods. In other studies, COSI-Corr and other remote sensing analysis methods were used to obtain the slope deformation history and the time evolution characteristics of the Baige landslides [2–4]. The dynamic discrete element method , a coupled model comprising several depth-averaged equations , and depth-integrated continuum methods  were used to analyze the dynamic evolution process of the two landslides and the entire disaster chain . Zhang et al.  used erosion-based empirical equations and numerical simulation methods for timely prediction of the dam-breaching hydrograph and breach geometric parameters before the outburst of the two landslide dams, and Liu et al.  refined the outburst model on the basis of detailed field investigation following the outburst flood. In recent years, the seismic wave spectrum has been used to retrieve landslide characteristics. Historical seismic records show that many earthquakes have occurred near the landslide area. However, the most recent strong shock occurred in the nearby city of Qamdo on August 12, 2013, with a magnitude of 6.1, and had little impact on the Baige landslide . Long-term seismic data from five broadband seismic stations located at distances of 108–255 km from the site of the Baige landslides have been inverted to obtain a time series of the forces exerted on the ground by the landslides [11, 12].
Examination of the intense tectonic activity, substantial crustal uplifting, and sharp river cutting in the Baige landslide area led some studies to conclude that tectonic movement and surface erosion greatly influenced slope stability . Tian et al.  investigated the mechanisms of the Baige landslides. They found that changes in the rock mass structure and seepage conditions of the slope attributable to regional tectonic activities together with increased seepage owing to extreme precipitation triggered the landslides. Zhang et al.  indicated that the Baige landslides developed on the Jinshajiang suture zone and that active tectogenesis contributed to fractured rock mass and complex spatial distribution of lithologies that resulted in long-term creep of the slope.
Previous studies showed that both exogenic and endogenic forces substantially impact the formation of landslides in active tectonic zones. Surface erosion is usually the primary exogenic landslide-inducing factor, and many case studies have reported that short-term heavy rainfall infiltration [16–18] and rapid river erosion at the slope foot can facilitate slope failure . Research is ongoing regarding the influence on slope evolution of endogenic factors such as tectogenesis. For example, Brideau et al.  suggested that rock slopes are controlled mainly by discontinuous structural planes (e.g., folds, faults, and shear zones) related to regional tectonic activities. Following the study of the reactivation of the ancient Jiangdingya landslide, Guo et al.  concluded that the profoundly fractured rock mass and slope structure, formed under the influence of faulting, provided internal features conducive to landslide occurrence. Steepening of a slope through rapid crustal uplift and sharp river erosion could induce many landslides that might restore a hill to its original equilibrium state in an active tectonic area [22–24]. Carlini et al. [25, 26] found that the development and distribution of large landslides might highlight the existence of tectonic processes and could also indicate regional-scale tectonic activity.
The Baige landslides occurred in the Jinshajiang suture zone on the eastern Qinghai–Tibet Plateau. The particular geologic, geomorphic, and climatic conditions of the area resulted in the complex genesis and evolution of the landslides. Most previous related studies focused on the influence of single exogenic or endogenic factors but failed to explain clearly why successive large-scale landslides occurred without further rainfall and/or earthquake activity. Therefore, following on from previous research, this study considered detailed field investigation data, UAV photography, mechanical experiments, and microscopic analysis of rocks to analyze the geological conditions of the slope and to determine the overall characteristics of the two landslide-dammed lake-outburst flood disaster chains. Moreover, the tectonic effects of the Jinshajiang suture zone on the Baige landslides were investigated together with consideration of the slope evolution process under integration of exogenic and endogenic factors. The primary objectives of this study were to interpret why landslides occur frequently in the Jinsha River basin and to provide evidence for regional landslide risk assessment and disaster prevention.
2. Regional Setting
2.1. Geological Structure and Lithology
The site of the Baige landslides is located on the Jinshajiang suture zone, which is in the middle part of the Sanjiang tectonic belt in the east of the Qinghai–Tibet Plateau. In this complex tectonic environment, the rigid Indian plate pushed continuously in the NNE direction, causing oceanic crust to subside and the continental crust of the two plates to collide with each other on the northern side , which formed a spectacular geological tectonic landscape composed of continental crust and suture zones (Figure 1(a)). Simultaneously, the relatively plastic Qinghai–Tibet Plateau was uplifted strongly because of compression, profoundly changing the geological environment and crustal structure.
The Jinshajiang suture zone is an essential boundary between the Songpan–Ganze and North Qiangtang terranes . Since the Paleozoic, the Jinshajiang ocean has undergone a sedimentary–tectonic evolution process of expansion–subduction–closure, forming a complex tectonic belt in the suture zone comprising several faults and tectonic blocks that have undergone multiple stages of metamorphism and deformation . The strike of the suture zone is aligned almost in the N–S direction. The suture zone has a length of approximately 700 km, and its width is up to 80 km, protruding eastward in an arc. The suture zone represents a tectonic mélange, comprising rock and matrix of different ages and properties (Figures 1(b) and 1(c)). The Jinshajiang ocean basin was gradually closed because of plate collision. Thus, original oceanic crustal fragments including the ophiolite suite are retained within the suture zone. Most virgin rocks have undergone metamorphism during long-term tectonic evolution, forming gneiss, schist, phyllite, and other metamorphic rocks [30, 31]. On both sides of the tectonic zone are Mesozoic sedimentary basins where the main outcrops are terrigenous clastic rocks.
Owing to the influence of the Jinshajiang suture zone, regional faults and folds are oriented mainly in the NNW direction. In the area of the town of Boluo, typical outcropping of the western boundary fault broadly reflects the fault characteristics (Figure 1(d)), i.e., the major fault plane is curved, bands of rock debris, breccia, and powder are approximately 15–20 cm thick, and oblique striations can be seen (Figure 1(e)). The fault fracture zone, which is more than 30 m wide, incorporates several secondary fault planes aligned in the same direction as the central plane, with properties of a reverse fault. Additionally, the unconsolidated materials within the tectonic zone indicate that the fault has new activity.
2.2. Geomorphology and Climate
The Qinghai–Tibet Plateau is the largest, highest, and youngest plateau on Earth. The geomorphic pattern of the Qinghai–Tibet Plateau has been changed dramatically by long-term orogeny and surface denudation (Figure 2(a)). Affected by continuous crustal uplift and river cutting since the Quaternary, the Sanjiang basin represents a typical alpine canyon landform (Figure 2(b)). Data on regional river erosion reveal that the rate of surface erosion in the Sanjiang area is generally high, although it has a characteristic gradient of gradual decrease eastward. In the upper reaches of the Jinsha River, the Dege–Baiyu section has the highest rate of erosion .
The typical climatic conditions of the eastern Qinghai–Tibet Plateau are mild and humid in summer and dry and cold in winter. Owing to the substantial difference in height in the Sanjiang region, temperature and precipitation both have apparent vertical variation. The mountains have low temperatures and high precipitation, whereas the valleys have high temperatures and low rainfall. With global warming, the Qinghai–Tibet Plateau has experienced notable climate change over the previous few decades and temperature, humidity, and precipitation in the Sanjiang area have all increased . Consequently, the geomorphic form and structure of the rock mass material in the area have also changed profoundly. Thus, the valley slopes have become unstable because of intense tectonic activity, surface erosion, and enhanced precipitation.
3. Field Investigation and Geological Analysis of the Landslides
3.1. Changes in Slope Topography
The characteristics of the changes in slope topography were analyzed by comparing high-resolution remote sensing images and digital elevation model data obtained before and after the occurrence of the two landslides (Figure 3). Because the gradients at the foot, main body, and rear of the slope are approximately 55°, 35°–55°, and 65°, respectively, slope topography could be characterized as the stepped ramp type before the first landslide. The relative relief between the front and rear edges of the slope was 850 m, with top and toe elevations of 3732 and 2882 m, respectively, which provided the potential energy and a free face for the occurrence of the landslides.
The elevation of the shear outlet of the first landslide was approximately 3010 m, and the elevation of the main body ranged from 3010 to 3650 m. After the first landslide, the ground surface of the back edge retreated approximately 90 m, forming a trough landform. With an overall volume of 24 million m3, the landslide accumulation formed a barrier dam that blocked the Jinsha River. The plane shape of the landslide was similar in form to an armchair, and the section shape comprised steep–gentle alternating steps. The main slide direction of the landslide was 100°, but the sliding direction turned to 75° because of the high-speed movement of the landslide controlled by the narrow terrain (Figure 4(a)).
The second landslide body was located at elevations from 3350 to 3700 m, with a shear opening elevation of 3350 m. Following the slip of the lower slope, the upper rock mass with a volume of approximately 3.7 million m3 was destroyed in the manner of a collapse. During its high-speed movement, the broken rock mass scraped the shallow rock and soil mass of the lower slope, which rapidly increased the overall volume to 8.7 million m3 and this mass was deposited across the Jinsha River, blocking the overflow channel of the existing barrier dam (Figure 4(b)).
3.2. Material Composition of the Slope
Examination of the geological map (Figure 1(b)) and on-site borehole cores [14, 35] revealed that the exposed strata within the landslide area are mainly extremely weathered gneiss and serpentinite. To obtain better understanding of the landslides, field investigations were conducted on the back edge of the landslides and on the accumulation bodies near the villages of Baige and Zexue in August 2019. First, the UAV was used to take photos of the overall morphology of the landslide area including key components of the slope body and the accumulation body. On this basis, six survey points were selected for rock mass structure measurement, strength tests, image collection, and rock and soil sampling (Figure 5).
On the main scarp of the landslide area, gray-yellow gneiss and schist are mainly exposed at B1 and B2 (Figure 5). The rocks at B1 are relatively broken with multiple sets of joints (Figure 6(a)), while the rocks are relatively hard with few joints at B2 (Figure 6(b)). The cataclastic gray-green serpentine strips outcropping at B3 (Figure 6(c)) are distributed in the middle and upper parts of the slope (Figure 6(d)), confirming that the tectonic location of the landslide area is inside the Jinshajiang suture zone.
On the lateral scarp of the landslide area, the on-site survey found black schistose rock, preliminarily identified as metamorphite (Figure 6(e)), the attitude of which is similar to that of the rupture surface. However, few remnants of the black schistose rock remain on the slope surface because most of it slipped. Such rocks have not been mentioned in previous studies, which suggests that the slope rock construction might not be well understood; therefore, we drilled cores at B5 for further analysis (Figure 6(f)). In the front of the landslide, the same gray-yellow gneiss was found as observed in the rear edge. The rock strata exhibit strike consistent with the direction of the main tectonic line, dip into the slope, and many unloading fissures (Figures 6(g) and 6(h)).
Fault F1 was excavated at B4 in the trailing edge because of a landslide control project. We investigated the tectonic activity characteristics of the fault and the structural features of the rock mass on both sides. The outcrop shows F1 as a reverse fault with a carbonization zone (Figure 7(a)), approximately 1–3 m wide, consisting mainly of black schist and fault gouge (Figure 7(b)).
The landslide accumulation body was found to have obvious stratification characteristics at B6 (Figure 8(a)). Gray-green serpentine (Figure 8(b)), gray-yellow gneiss (Figure 8(c)), and black flake rocks (Figure 8(d)) correspond one-to-one with the slope rocks.
Affected by factors such as tectonic action, groundwater fluctuation, and weathering, the rock mass structure is broken and its strength is low. We took samples from each survey point for subsequent petrological analysis using an optical microscope to determine the detailed structure and material composition of the landslide.
3.3. Geological Structure of the Slope
The Baige landslide slope has a complicated material composition that comprises rock formations of mainly gray-yellow schist, gneiss, black mylonite, and gray-green serpentine. Through detailed analysis of the direction of the fractures in the bedrock, 10 major structural planes of tectonic origin were identified (Figure 9), including a fault, 4 schistosities, and 5 joints (Table 1). The thrust fault F1 controls the main scarp of the landslide slope. The rear and deep parts of the slope are mainly gray-yellow cataclastic gneiss and schist, with attitude of bedding of approximately . The gray-green serpentine outcrops in the source area display rocks fragmented with multiple sets of joints. Many black mylonites probably existed in the front of the original slope; however, these rocks are now only visible in the lateral margins. The mylonite foliation S3 is inferred to be the sliding surface, providing shear conditions for the landslide. Additionally, several discontinuity sets are also developed in the slope, controlling the lateral and trailing edges of the landslide.
Owing to the short distance of movement, the accumulation body retains the original slope sequence and presents a relatively noticeable layered accumulation feature. Thus, in combination with detailed surveys of the source and accumulation areas, the material and structural characteristics of the initial slope can be restored (Figure 10).
4. Analysis of Landslide Genesis
4.1. Rocks Damaged and Metamorphosed by Plate Collision
Located in the Jinshajiang suture zone, violent tectonic movement must have greatly influenced the Baige landslides . To study the influence of tectonic activity on the slope rock mass, we sampled both the slope and the accumulation for petrological analysis (Figures 11 and 12). The sampling site numbers and rock characteristics are listed in Table 2.
The results of the microscope analysis of the rocks from different parts of the slope revealed an almost one-to-one correspondence between the accumulation rock and the slope rock, indicating that the material composition of the landslide changed slightly in the process of movement. However, the rocks of both the landslide and the accumulation have obvious features of directional arrangement, cataclasis, and mylonitization, reflecting the profound impact of plate collision on the slope rocks . In the process of violent plate collision, crustal rock mass is strongly compressed, resulting in deformation, failure, and dynamic metamorphism (Figure 13). Under the action of intense horizontal compressive stress, crustal rocks are deformed or even destroyed and structural planes develop. However, near the boundary fracture of a suture zone, the rock is subject to strong shear stress, making it prone to dynamic metamorphism and mylonite formation.
4.2. Decrease of Rock and Soil Strength
4.2.1. Uniaxial Compressive Strength Attenuation of Jointed Rocks
The structure and the condition of the joints have important influences on the mechanical properties of a rock mass, and the geological strength index (GSI) classification can be set up to address these two factors [37, 38]. However, the GSI value of rock mass in Baige is generally low (Table 3), which makes it difficult to take rock samples for mechanical testing. We considered using the Schmidt hammer test to obtain the strength of jointed rocks. A Schmidt hammer (L-type) was used to measure the uniaxial compressive strength (UCS) of the rock at B1, B2, B3, and B4 on the main scarp of the landslide. Measurements were performed 10 times at each location in accordance with the ASTM D5873 method, and the average of the rebound value was calculated [39, 40]. According to multiple empirical formulas  ; thus, the uniaxial compressive strength of the rock mass at the four locations was computed. Comparison of our calculated UCS values with those obtained by previous experiments for gneiss and serpentine [42, 43] revealed that the UCS of the rocks at B1, B2, and B3 was substantially lower than that of other similar rocks. Moreover, the original rock at B4 was transformed into fault gouge by tectonic activity and the strength of which is also lower. The strong tectonic movement and surface weathering in the region were responsible for the development of many discontinuities in the rock mass, which reduced their overall mechanical strength.
4.2.2. Change of Shear Strength of Rock and Soil
Although the strength of both gneiss and serpentine is reduced by tectonic movement and surface weathering erosion, it remains higher than that of both mylonite and fault gouge, which are formed by the strong shearing action of suture boundary faults. In the case of the Baige landslides, mylonites and fault gouges represent the weak structural belts that control slope stability. Analysis of the samples of fault gouge from the source area of the landslide revealed that the fault gouge has plasticity and low strength in its natural state, with the mechanical properties of a soil (Figure 14(a)). After drying, the strength of the fault gouge increased but the material collapsed after soaking in water for five minutes (Figure 14(b)), which proves that water has considerable influence on the mechanical strength of fault gouge. Soil direct shear tests were conducted on the fault gouge samples under different water content states to analyze the variation law of fault gouge shear strength parameters with water content. The relation curves with respect to internal friction angle, fault gouge cohesion, and water content (Figure 14(c)) indicate that fault gouge cohesion decreases rapidly with increasing water content, whereas the internal friction angle changes slightly. Overall, the strength parameters of fault gouge are sensitive to change in water content, and when the water content increases, the shear strength decreases rapidly, which is very unfavorable with regard to slope stability.
4.3. Surface Denudation Promoted Slope Failure
4.3.1. Time-Dependent Deformation and Stress Redistribution
The landforms of the Jinsha River basin on the eastern margin of the Qinghai–Tibet Plateau are mainly alpine valleys (Figure 2). Long-term tectonic uplift and rapid river erosion can easily lead to time-dependent deformation of slopes. It can be seen from satellite imagery of the Baige area that the surface rock mass of the slope underwent obvious stress relief deformation before the occurrence of the successive landslides (Figure 15(a)). River erosion has dramatically changed the free face conditions of the slope in the Baige area, causing release of lateral stress on the slope of the river valley and producing many unloading cracks in the rock mass (Figure 15(b)). Moreover, the gravitational potential energy of the rock mass has increased sharply, causing adjustment of the stress field. In the process of formation and evolution, with downcutting and lateral erosion, the valley will deepen and widen. First, the irregular unloading effect will cause redistribution of the stress in the valley slope and stress concentration near the free face. Then, with long-term external force, the shallow rock mass of the valley slope will creep, relax, and weaken. Consequently, the stress in the slope becomes further redistributed, forming a characteristic “humped” stress distribution (Figure 15(c)).
4.3.2. Rainfall Infiltration Accelerated Slope Failure
Heavy rainfall infiltration also accelerates slope failure. To analyze the triggering factors of the Baige landslides, Liu et al.  performed correlation analysis between monthly precipitation and the rate of slope deformation obtained from SAR intensity images (Figure 16). The results revealed strong correlation, although deformation acceleration was found to lag heavy rainfall slightly. Precipitation in the study area is concentrated mainly in summer (June–September). The heaviest rainfall during 2014–2018 occurred in August 2018; however, it was not until October 2018 that the rate of slope deformation increased sharply and reached its peak. In fact, infiltration of the heavy rainfall in August 2018 decreased the frictional resistance and strength of the rock and the soil mass, which eventually promoted overall failure of the slope.
5.1. Mechanism of Slope Failure Caused by Exogenic and Endogenic Effects
The unique geologic, geomorphic, and climatic environment of the Baige landslide area intensifies the exogenic and endogenic effects. In the Jinshajiang suture zone of the eastern Qinghai–Tibet Plateau (the location of the Baige landslides), the in situ stress value is relatively high and crustal uplift has been rapid because of violent plate collision. Otherwise, long-term erosion of the Jinsha River has led to the relaxation and unloading of the shallow rock mass in the valley. Affected by regional tectonics, rise and fall of groundwater, weathering, and river erosion, the slope rock mass is very broken with densely developed joints. Therefore, formation of the conditions that led to the occurrence of the Baige landslides was distinctly influenced by both intense tectonic action and surface denudation (Figure 17).
5.1.1. Tectonic Action
During collision of the tectonic plates, gneiss, mylonite, and serpentinite were formed in the deep crust where temperature and pressure were sufficiently high for the rocks to undergo plastic deformation. Moreover, plate collision also caused the regional crust to undergo strong horizontal compression and vertical uplift, which provided the elevation difference required for the formation of the landslides. Additionally, the high in situ stress of the suture zone caused fracturing of the slope rock mass, development of discontinuities, and directional arrangement of tectonic slices, which reduced the strength of the rocks and gradually formed weak structural planes that controlled the stability of the slope. The rock near the boundary fault underwent dynamic metamorphism, forming mylonite, which reduced the overall shear strength of the rock mass. Rocks of this particular type are prone to infiltration and represent weak interlayers within the slope.
5.1.2. Surface Denudation
During long-term tectonic uplifting, sharp river erosion caused the valley slope to steepen and provided the potential energy for slope instability. Meanwhile, weathering increased the discontinuities and reduced the strength of the rock mass. With continuous river erosion, lateral stress in the slope was relieved and the unloaded rock mass adjusted the local stress field, which resulted in time-dependent deformation of the slope. Furthermore, infiltration of heavy rainfall softened particular types of rock and soil (e.g., mylonite and fault gouge), which promoted slope failure.
5.2. Slope Evolution Process
Having analyzed the material composition, slope structure, and geomorphic conditions of the Baige landslide, the process of evolution of the landslide under the long-term effect of coupled tectonic activity and surface erosion can be clarified as follows (Figure 18).
Having experienced plate collision and compression, the remaining oceanic crust formed the Jinshajiang suture zone with strong rock deformation and rapid regional crustal uplift, which provided the potential energy conditions necessary for the occurrence of the landslides
Continued tectonic compression and epigenetic reforming caused deformation, failure, and dynamic metamorphism of the slope rock mass, which gradually accentuated the height difference. Meanwhile, the slope rock mass underwent gradual unloading of the free face, which led to time-dependent deformation and internal stress adjustment of the slope
Under the action of infiltration of heavy rainfall, the rock and soil of slope, especially fault gouge and mylonite, formed weak interlayers that accelerated slope deformation and gradually extended discontinuities
Finally, the discontinuities gradually developed into a sliding surface. The rock mass at the foot of the slope was cut out along a gently inclined structural plane and slipped toward the free face. The trailing edge of the rock mass of the landslide was pulled and cracked along a steeply inclined structural plane, which caused catastrophic failure. Following the slide of the lower rock mass, the upper rock mass fell and scraped the surface rock and soil in the middle and lower sections of the slope. This caused a sharp increase of the landslide volume and the massive deposit blocked the Jinsha River
5.3. Risk of Massive Landslides in the Jinshajiang Suture Zone
In the upstream region of the Jinsha River, which is where the area of the Baige landslide is located, there have been other massive rock failures such as the Suwalong landslide , Xuelongnang landslide , and Samaoding landslide . However, the denudation rates differ markedly, according to data on regional river erosion and crustal uplift [33, 48]. The erosion rate upstream of the Dege section in the upper reaches of the Jinsha River is relatively small (<0.02 mm/yr), whereas the erosion rates in the lower reaches are all >0.02 mm/yr and they are highest near Dege and Baiyu (Figure 19(a)). The rates of crustal uplift of the Jinsha River Basin are generally high and >2 mm/yr at most measuring points. Practically, concentrated rainfall and the broken rock mass within the region, coupled with sharp river erosion and the construction of hydropower stations, roads, railways, and other major projects in recent years, have led to the triggering of a large number of landslides. Analysis of the Jinsha River Basin revealed that landslides have occurred most frequently in upstream areas of the Jinsha River, especially in the Dege–Baiyu section, consistent with the variation of river erosion and crustal uplift rates (Figure 19(b)). We calculated the channel steepness index () of the upstream Jinsha River, using the Topographic Analysis Kit . Analysis of the relationship between the channel steepness index and the landslide distribution in the Baiyu section of the Jinsha River Basin suggests that the larger the value, the more frequent the occurrence of landslides (Figures 19(c) and 19(d)).
It shows that tectonic uplift and river erosion are closely related to the occurrence of landslides in the upper reaches of the Jinsha River. Generally, owing to strong tectonic activity and surface erosion in the Jinshajiang suture zone, there is considerable risk of massive landslides occurring in the upper reaches of the Jinsha River. Attention should focus on the potential for massive landslides and their associated disaster chains in river sections with strong tectonic uplift and high rates of erosion.
In this study, we considered field investigation data, UAV photography, mechanical experiments, and microscopic analyses to examine the formation and evolution process of the Baige landslides. Results revealed the genetic mechanism of the large-scale Baige landslide in the absence of heavy rainfall or earthquake activity, explained why landslides occur frequently within the study area, and provided a case for further study of the geohazard characteristics of the suture zone.
The formation and evolution of the Baige landslides were controlled by the unique tectonic dynamics of the suture zone. The source area of the Baige landslides is located in a fractured rock zone of the west boundary fault of the Jinshajiang suture zone. The particular geological structure and the geomorphologic, hydrologic, and climatic conditions exacerbate the exogenic and endogenic effects of this area. Intense plate collision has caused substantial tectonic damage to the crustal rock mass, which greatly affected the strength and hydraulic properties of the slope rock mass. Additionally, the UCS and water disintegration resistance of the rock have deteriorated markedly as a result. The infiltration of heavy rainfall promoted deformation and failure of the slope by decreasing the frictional resistance and strength of the rock and soil mass.
The primary cause of the Baige landslides was the effect of coupled internal and external dynamics. On the one hand, high accumulation of tectonic stress in the suture zone has caused complex dynamic metamorphism and deformation of the rocks near the boundary fault of the Jinshajiang suture zone. The slope rock mass has become broken owing to strong tectonic effects, and the strength and hydraulic properties of the rock mass have deteriorated sharply. Thus, a weak structural zone that controls the stability of the slope gradually formed. On the other hand, long-term tectonic uplift and sharp river erosion have greatly changed the free face of the original slope. The increase of the gravitational potential energy of the rock mass has intensified the unloading deformation of the rock mass and substantially reduced the self-stability of the slope. Coupled with strong weathering of the plateau and repeated infiltration of groundwater, the mechanical properties of the special rock and soil whose original strength characteristics are inferior have been further reduced.
Disaster prevention and control in suture zones face considerable challenges. By studying the genetic mechanism and evolution process of the Baige landslides, we can assume that slope stability will be poor under similar geologic and geomorphic environmental conditions. Thus, the risk of landslide disasters in the suture zone of the eastern Qinghai–Tibet Plateau remains high. Especially in the in river sections with strong tectonic uplift and high rates of erosion, massive landslides have occurred most frequently. In this study, we established that the Jinshajiang suture zone has caused deformation, failure, and metamorphism of regional slope rocks and that local slope failure is the result of long-term coupling of tectonic activities and surface erosion. However, to fully elucidate the complexity of the coupling process, further analysis with additional data will be needed.
All the data supporting the results of this study have been presented in the paper.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
This research was supported financially by the Second Tibetan Plateau Scientific Expedition and Research Program (STEP) (Grant no. 2019QZKK0906) and the National Natural Science Foundation of China (Grant no. 41941017). Deep appreciation is offered to the Sichuan Bureau of Surveying, Mapping & Geoinformation and the Chengdu Center of the China Geological Survey for their sharing of data and technical assistance. We would like to thank prof. Yuanfu Xiao of Chengdu University of Technology and Yao Li, Hao Wang, Jiao Wang, Jian Guo, Liqin Zhou, Guotang Zhang, and Dingzhu Liu of the Institute of Mountain Hazards and Environment (CAS) for the assistance with field investigation, providing data support, and offering valuable suggestions.