Soft-sediment deformation (SSD) is deformation that usually occurs rapidly in unconsolidated sediment close to the surface, during or shortly after deposition, and before significant diagenesis [1]. Soft sediment deformation structures (SSDS) are features occurring in unconsolidated sediment. SSD can be induced by many natural processes, including gravity acting, overloading, unequal loading, wave-induced cyclical or impulsive stresses, shear by aqueous or other currents, storms, sudden changes in groundwater level, or earthquakes [2-9]. SSD features are known from a wide variety of depositional environments, both terrestrial: fluvial, aelolian or volcanic [9-11], or marine: shore, turbiditic, subglacial [12-14], but they are particularly well-reported from lacustrine depositional environments [15-31]. The relative abundance of seismites in lacustrine successions is explained by Sims [15] in terms of: (1) the presence of water-saturated sediments; (2) the presence of sediments with high susceptibility to liquefaction; (3) the absence of hydrodynamic and sedimentary processes able to obliterate the products of seismically-induced deformation. Liquefaction and fluidization are two common deformation mechanism for the softsediment deformation structures. Liquefaction occurs when grain weight is temporarily transferred to the pore fluid, through either the collapse of a loose grain packing or an increase in pore-fluid pressure [2,21,32]. The likely products are pervasive structures that deform existing stratification [33]. Fluidization occurs when the upwarddirected shear of fluid flowing through a porous medium counteracts the grain weight, reducing the material strength [2,9,21,24]. The process may develop new stratification [20].

SSDS such as convolute lamination, load structures, pillar structures, water-escape structures and deformed cross-bedding are common in sands contained lacustrine environments. However, very little published documentation exists for SSD generated in dammed lacustrine settings due to the relatively short life span of dammed lakes. The spatial distribution of SSD features has significant implications for the interpretation of their origins and can help in understanding the evolution of paleo-dammed lakes. On the southeastern margin of the Tibetan Plateau, the erosion rate is high and the mass movement is frequent and young strata are rarely preserved intact, thereby hindering direct studies of paleo-sesimicity by fault outcrops. Therefore, the recognition of seismically-triggered structures is an important factor in determining earthquake recurrence intervals in paleo-seismic studies [13,22,34]. In this paper we first describe the type and nature of the soft-sediment structures preserved in the Batang paleo-dammed lake sediments. Then we interpret and discuss the mechanisms of their formation. Finally, we present the implications of this evidence for regional tectonics.

The Batang-Zhongza reaches are located in the upper Jinsha River at the southeastern margin of the Tibetan Plateau (Figure 1a), and are about 90 km in length. The Jinsha River flows through the west side of Batang County from north to south, and mountains have developed along both sides of the river where the valleys are incised deeply. The average elevation of the area is greater than 2200 m; and the river is confined between steep banks (>40°), prone to rock falls, rock slides and small debris flows. Rock slides and debris flows are often seen on steep concave banks. The lithology of the exposed rocks along the valley sides are mainly Mesozoic schist, marble and limestone, granite, as well as other volcanic rocks [7].

Figure 1a: Map showing the geographic locations of the study site and regional faults.

The climate in this segment is mainly semi-arid, but changes sinificantly with height. In the lower valleys the climate is arid and hot. The average precipitation is less than 400 mm/year and the average temperature of the warmest month is 13-16°C. Due to the influence of the southwest monsoon, the rainfall is concentrated within July- September of every year. In Batang, the monthly mean temperature is 3.7°C for January and 19.5°C for July. It is due to this climate that in this area physical weathering is strong and vegetation is poorly developed.

There are two active faults systems which pass through the segment. One is the Batang fault (F1) and the other is the Jinshajiang fault zone (F2). The latter is composed of the Zengziding fault (F2-1), Zigasi- Deeqeen fault (F2-2), Benxie-Dagaiding fault (F2, F3), and Xiongsong- Suwanglongfault (F2-F4) (Figure 1a). Among these, the Xiongsong- Suwanglong fault (F2-F4) is a major one, which passes through the east side of the Wangdalong - Zhubalong area [8]. The faulted landform and chronology show that the current right-lateral strike-slip rate of the Jinsha River fault zone is 6-7 mm/a, and the vertical rate is estimated at 2-3 mm/a [35].

According to existing historical records, since 1722, seven earthquakes of M ≥ 6 have occurred in this segment and its adjacent areas [36]. Among them, the strongest earthquake, at M=7.5, occurred in 1870.

Several paleo-dammed lakes occurred as the form of single individual or a series in the Batang-Zhongza reaches of the upper Jinsha River (Figure 1b). Paleo-lake evidences consist of relict barrier bars preceded on their upstream side by lacustrine sediments (from several to tens of metres in thickness) (Figure 2a). The lake sediments occur on the upriver side of relict barrier bar deposits. The thickness of the dammed lake sediments decreases with increasing altitude. The lacustrine sediments are dominantly characterized by clear horizontal bedding, and consist mainly of fine sands, silts, and clays. The results of optical stimulated luminescence (OSL) and ¹⁴C dating data show that these dammed lakes formed during the Holocene [7,19,37] In addition, soft-sediment deformation structures are found among these paleodammed lake sediments (Figure 2b).

Figure 1b: Distribution of paleo-dammed lakes and observation points.

Figure 2: Photographs showing the paleo-dammed lake sediments in the upper Jinsha River Valley. (a) Laminated lacustrine sediments in ~30 m thick about 4.2 km downriver from Suwalong.

Figure 2b: Laminated lacustrine sediments about 3.3 km downriver from Suwalong.

The entire lacustrine succession of the upper Jinsha River valley shows many soft-sediment deformation structures at different stratigraphic levels (Table 1). In this paper, the morphologies of the soft-sediment deformation structures were studied in the field through the natural exposure profiles, or the excavation of trenches.

Table 1: Location, altitude and characteristics of soft-deformation structures in Batang area.

Based on the size and main morphological features, six different types of soft-sediment deformation structures in the dammed lake sediments have been distinguished: 1 –Folded layers, 2 – Convoluted layers, 3 – Ball-and-pillow structures, 4 –Recumbent lamination, 5 – Water escape structures, and 6 – Small-scale landslide.

Folded layers

The folded layers forming the angular uncomformity occur near the base of the clayey-silt layers of the lacustrine sediments located around the Temi Village (Figure 3a). Two normal faults occur in the deformed cross-bedding at the lower part of the S section. The deformed clayeysilt layers formed folds on both sides of the fault planes. The faulting has ruptured and slightly displaced the laminated brown clayey-silt (Figure 3b).

Figure 3: Folded layers.
(a) Photograph of laminated lacustrine sediments and two small scale normal faults.

Figure 3b: Close-up of panel.

Convoluted layers

The convoluted layer occurred in ~40 cm thick horizons at the central part of the profile (Figure 4a), and its overlying clayey silt is undeformed. The deformed beddings shaped on the interface of a fine sand layer and the underlying clayey silt layer. The deformation appears that the fine sand was curled by the circumambient clayey silt, just like the shape of a fold (Figure 4b). The convoluted layers are folded with varying amplitude, and the inclination of the axial planes of these folds varies from vertical to near horizontal.

Figure 4: Convoluted layers.
(a) Photograph showing convoluted layers, Note the folding in clayey silt dominated convoluted layers.

Figure 4b: Close-up of panel.

Ball-and-pillow structures

Ball-and-pillow structures had developed in the lacustrine sediment layers at the downstream side at distance of about 50 and 10 m from the Suwalong landslide, respectively (Figures 4a and 5). The composition of the two sedimentary layers is clayey silt layers with a thin layer of gray fine sand in the center, and the elevation of each corresponding layer of the two sediments is almost the same.

Figure 5: Photograph showing the ball-and-pillow structures.

The deformed beddings shaped distinctly on the interface of a fine sand layer and the underlying clayey silt layer at the lower part of exposed lacustrine sediments (Figure 4a). The thickness of the deformation layer is about 0.8 meter. The deformed layers are folded distinctly with varying amplitude, and the inclination of the axial planes of these folds varies from vertical to near horizontal.

The composition of the sedimentary layer is clayey silt layers with a thin layer of gray fine sand in the center. In the ball-and-pillow structure (Figure 5), it occurred in the centre of the clayey silt layer underneath (Figures 4b and 5). The gray fine sand appears to have fallen into the underlying clayey silt layer, and possesses a spherical or pillow-like shape with a maximum size of ~30 cm. The ball-and pillow structures show a gradual upward increase in the extent of deformation, and its overlying clayey silt is undeformed.

Recumbent lamination

A 250 cm thick lake sediment with nearly horizontal bedding exposed on the gentle slope terrain. The lake sediment consists of yellow silt sand. At the N side of the upper lake sediment layer, a ~100 cm thick debris-flow sediment layer developed, consisting of gravels, sands and clays. The lake sediment near the bottom or right side of the debris-flow body underwent distinct deformation, characterized by small-scale folds and recumbent laminae (Figures 6a and 6b).

Figure 6a: Recumbent lamination.
(a) Photograph showing the folds and broken laminae in silt sand layers.

Figure 6b: Close-up of panel.

Water-escape structures

Continous step-like lacustrine successions are well developed at the upstream side of the Xuelongnang paleolandslide dam along the Jinsha river valley. A 25 cm thick SSD layer occurred in the bottom part of exposed lacustrine sediments, which is characterized by the presence of curvilinear laminae, and mixing of clastics draped by broken silty-clay laminae (Figures 7a and 7b). The mixed clastics consisting in coarse sands and gravels were also shown as inclined or upright dykes. The silty-clay layer is underlied by a 20 cm thick brownish or yellow fine sand horizon. The material of the mixed clastics appears to originate from the underlying sand horizon.

Figure 7: Water-escape structures.
(a) Note the near-vertical conduit starts from a fine sand bed and are filled.

Figure 7b: One large-scale conduit and two small-scale conduits start from a fine sand bed and are filled by coarse particles. Note the occurrences of curvilinear laminae.

Small-scale lanslide

These affect the laminated silty-clay lake sediment layers with a very gentle dip angle (~2°C), and reach a maximum thickness of about 120 cm (Figures 8a and 8b). Slump beds are mainly represented by slide structures. In more detail, the deformation features show asymmetrical folds, slide surfaces or disorderly bedding. In certain places some gravels seems to fall randomly into the tension fissures of the deformed sediment. The base of the laminated sediment layer in this profile is undeformed.

Figure 8: Small-scale landslide.
(a) Slide structures shown by asymmetrical folds, convoluted bedding or disorderly bedding.

Figure 8b: Close-up of panel.

The presence of discrete deformation layers within the planar beds indicates episodic disturbance during the sedimentation process, which can be triggered by sediment loading, storm-currents, water waves, glaciation, rapid sedimentation or earthquakes [10,16,38-40]. Therefore, in order to identify the origin of the soft-sediment deformation structures (SSDS), some approaches such as criteria-based and trigger-based approaches are adopted to analyze SSDS [25,28,38].

Many studies have investigated the differences between seismic and non-seismic deformation structures [10,27,28,30,41,42,55]. In particular, to infer a probable seismic origin, it is necessary to exclude the influence of sedimentary processes on the unconsolidated sediments [26]. In lacustrine successions, this interpretative phase maybe easy as hydrodynamic and sedimentary processes, which are capable of inducing similar soft-sediment deformation structures, are generally absent [43].

The two small-scale normal faults accompanied by clear folding are associated with clayey-silt layers (Figure 3a). The faults are genetically related to earthquakes and are not caused by slope instability, as evidenced by the NW trending dip, which is against the slope of the exposed lake sediments.

The large-scale convoluted layers, folded in different directions, suggest that the deformation occurred at the sediment-water interface, most likely due to the shear produced by the back and forth movement of water over the water saturated laminae during an earthquake (Figure 4a). The origin of similar structures in lacustrine deposits has been attributed to an elastic-plastic response of sediment to shear-stress caused by the back and forth movement of water [15,44].

The ball-and-pillow structures (Figure 5) is a type of load structure, and may form as a consequence of loading of a denser sand layer over a less dense clayey silt layer during earthquake induced liquefaction, so that fine sands would fall into the clayey silt layer in a pillow or globular shape [45]. It could not have possibly been produced by the liquefaction of collapse, as there were no large-scale colluvial deposits in its surrounding area. In addition, the upper and underneath sedimentary layers have horizontal bedding and did not generate lateral deformation or slip surface. The characteristics of softsediment deformation show a gradual upward increase in the extent of deformation, also indicating the fact that these structures are most possibly caused by seismic liquefaction [46].

The deformed lamination forms in response to shearing by debris flows. The local deformed lamination only occurred at the bottom or one side of the debris-flow sediments (Figure 6). The relationship between debris-flow facies and the occurrence of deformed lamination reveal that the trigger was flow turbulence in origin. The cohesive and silt sand sediments deform in a plastic or brittle manner [47-50].

The large-scale water-escape structures are associated with the finesand horizons (Figures 7a and 7b). Due to the high permeability, these features are produced by earthquake induced elevated fluid pressure. This pressure is released through the upward flow of fluid, liquefaction and fluidization [20,24,30,33,40] Liquefied and fluidized sediments deform as a viscous fluid. The surrounding cohesive materials behave in a plastic or brittle manner (folded or broken laminae) [28].

Small-scale landslide structures are induced by a mainly lateral driving-force system, due to the presence of a slope. In low-angle slopes deformation begins only after a drastic reduction of the shear strength of the sediments [28]. The behavior is plastic. The facies analysis performed on this succession shows that deformation occurred in a more or less flat depocentral area. We interpret the landslide structures as being seismically-induced (Figure 8a and 8b).

Finally, the presence of more than one type of SSDS, shallow lake depths and geological setting with frequent earthquakes indicates that the genesis of these features is chiefly due to paleo-sesimic events [51].

On the southeastern margin of the Tibetan Plateau, mass movement occurs frequently and the surface erosion rate is high, thus young strata are rarely preserved. Many landslide-dammed lakes are developed within deep valleys at the southeastern margin of the Tibetan Plateau. In the Batang-Zhongza reaches, based on geomorphological investigation, chronology dating and tectonic setting analysis, the landslides which had formed the landslide-dammed lakes are suggested to be due to multi-period palo-earthquakes occurring during the Holocene [8,37]. However, direct evidence for the existence of paleo-earthquakes in this area remains absent. The soft-sediment deformation structures related to earthquakes provide an opportunity to better understand the history of seismicity in the Batang area, southeastern Tibet (Table 1).

Our study reveals the widespread development of soft-sediment deformation structures in the Batang paleo-dammed lacustrine sediments. These deformation structures include small-scale normal faults, convoluted layers, ball-and-pillow structures, deformed lamination, water-escape structures, and small-scale slump.

Earthquakes commonly trigger sediment mobilization phenomena, particularly liquefaction and fluidization [36] and softsediment structures have commonly been associated with a seismic trigger. However, many other natural agents and processes can act as triggers, including waves, floods, sliding and slumping, rapid loading, bioturbation, cryoturbation, glacial drag, and groundwater movements [39,49]. In general, it is common to adopt contrasting approaches to infer a seismic or non-seismic trigger, based respectively on comparison with a set of established criteria or an assessment of all possible triggers [46].

Combining the assessment of depositional facies, potential triggers, paleoenvironmental context, and available criteria, we may conclude that the trigger agents of the deformation in the Batang area were most likely earthquakes, slides, and debris flows. The seismically-induced soft-sediment deformation structures provide new substantial evidence for the existence of active tectonics and paleo-earthquakes in this area during the Holocene [52-54].

The study is supported by the National Natural Science Foundation of China (grants 41571012, 41230743), the Open Research Fund of State Key Laboratory of Simulation and Regulation of Water Cycle in River basin (grant IWHRSKL- 201507), and the Fundamental Research Funds for the Central Universities (grant 2652015060).