Giant Landslides Research Articles

Moraines and marls: Giant landslides of the Lago Pueyrredón valley in Patagonia, Argentina

Giant catastrophic landslides (>108 m3) dot the formerly glaciated mountain forelands of the eastern Patagonian Andes. From geomorphic mapping, sedimentological logs and radiocarbon dating, we infer the emplacement kinematics and approximate timing of giant landslides in moraines and other glacial deposits in the Lago Pueyrredón valley (LPV), Argentina. For the first time, we report in detail examples of giant low-gradient landslides with hummocky lobes derived from unconsolidated glacial deposits and weak bedrock. We find that at least 4.5 km3 of debris and weak bedrock were mobilized by slope failures in an area of ∼500 km2 since the Last Glacial Maximum (LGM; ∼25–18 ka). Nearly 90% of this landslide volume originated along the shores of, or as subaqueous failures in, a postglacial moraine-dammed meltwater lake. The larger landslides (>1 km3) detached from moraines fringing the lake, whereas other landslides displaced glacial and lake deposits either on the paleolake bed, or in a river gorge that was cut upon drainage of the glacial lake. Sequences of till, glaciofluvial and glaciolacustrine deposits overlying weak Early Miocene marlstone are mostly conducive to major landslides in the LPV. Cross-cutting relationships indicate that largest landslides in the area originated during rapid glacial lake-level drops. Distribution and internal structure of hummocks within landslide lobes suggest that these landslides were emplaced as catastrophic debris avalanches. It suggests that giant catastrophic landslides in the glacier forelands of Patagonia can result from layered weak rocks, changes in meltwater-lake levels, and possibly as a consequence of strong earthquakes linked to rapid post-glacial rebound following the retreat of the Patagonian Ice Sheet (PIS).

Landslide evolution assessment based on InSAR and real-time monitoring of a large reactivated landslide, Wenchuan, China

The upper Minjiang catchment is frequently affected by a significant number of large to giant landslides. In one such massive older landslide, approximately 2 million m3 of sediment was reactivated, threatening the safety of a pylon on the landslide and the safety of the only route that allows high-speed travel from Wenchuan to Maoxian. To trace the deformation evolution history and analyze the current deformation of the new landslide, we examined SAR data since 2007, carried out geological and UAV surveys, and designed and installed real-time monitoring equipment. This paper presents the main spatial and temporal characteristics of the landslide. We found that the landslide has experienced creep deformation in the past five years and its sensitivity to rainfall. The development process of the landslide from the formation of tensile crack at the trailing edge to the gradual pushing forward of the crack propagation was revealed. The innovation of this paper is to reveal the whole process of the spatial-temporal evolution from its beginning to large deformation of a real case of creep landslide, which can provide a very good reference for the analysis and monitoring of such creeping landslide evolution process. This representative case also shows how to combine remote sensing and in situ monitoring methods to carry out continuous monitoring and the establishment of a warning system for potential landslide sites, so as to trace the evolution history, derive activity maps and analyze development trend, which can significantly reduce landslide risk. Additional mitigation procedures can be put into place if continuous monitoring of the Minjiang River Canyon occurs, resulting in a reduction of direct losses. In particular, modeling of the slope deformation and failure, damming process, and prospective flooding can be done in advance. Therefore, it is expected that this report can strengthen the case for the deployment of systematic monitoring of potential landslide sites by integrating earth observation methods and in situ monitoring systems as an approach to reduce risk.

Analysis of landslide stability under seismic action and subsequent rainfall: a case study on the Ganjiazhai giant landslide along the Zhaotong-Qiaojia road during the 2014 Ludian earthquake, Yunnan, China

In a strong earthquake, not only a large number of coseismic landslides are triggered, but relaxation and cracks in the rocks and soils are induced, which make these rocks and soils vulnerable to instability during subsequent rainfall; thus, strong earthquakes always have long-term effects on landslides. The geo-hazards along the Zhaotong-Qiaojia road in the 2014 Ludian earthquake (Ms 6.5) of Yunnan Province, China, were investigated, and the Ganjiazhai giant landslide was chosen as a case study. First, using the limit equilibrium analysis and Newmark method, the critical seismic intensity of the landslide before the earthquake was evaluated. Secondly, the dynamic failure process of the landslide under the measured ground motion was simulated with the discontinuous deformation analysis method. Lastly, based on stress–seepage coupling analysis and precipitation data from Ludian meteorological station, the stability of the landslide during subsequent rainfall after the earthquake was predicted. The results show that the critical seismic intensity was within degrees VIII–IX, which is consistent with the results of the earthquake damage investigation. The dynamic failure process can be divided into four stages, and four scarps formed; the potential sliding zones during the subsequent rainfall were at the first scarp and the fourth scarp, and their critical rainfall amounts were 35–40 mm and 55–60 mm, respectively. In this paper, failure process simulation and stability prediction of the landslide before, during, and after the strong earthquake are presented, which provide analysis methods for the dynamic stability of landslides in the meizoseismal area.

Postglacial Patagonian mass movement: From rotational slides and spreads to earthflows

Many of the volcanic plateau margins of the eastern, formerly glaciated, foreland of the Patagonian Andes are undermined by giant landslides (≥108 m3). One cluster of such landslides extends along the margin of the Meseta del Lago Buenos Aires (MLBA) plateau that is formed mainly by Neogene-Quaternary basalts. The dry climate is at odds with numerous >2-km long earthflows nested within older and larger compound landslides. We present a hydrological analysis, a detailed geomorphic map, interpretations of exposed landslide interiors, and radiocarbon dating of the El Mirador landslide, which is one of the largest and morphologically most representative landslide.We find that the presence of lakes on top of the plateau, causing low infiltration rates, correlates negatively with the abundance of earthflows on compound landslides along the plateau margins. Field outcrops show that the pattern of compound landslides and earthflows is likely controlled by groundwater seepage at the contact between the basalts and underlying soft Miocene molasse. Numerous peat bogs store water and sediment and are more abundant in earthflow-affected areas than in their contributing catchment areas.Radiocarbon dates indicate that these earthflows displaced metre-thick layers of peat in the late Holocene (<2.5 ka). We conclude that earthflows of the MLBA plateau might be promising proxies of past hydroclimatic conditions in the Patagonian foreland, if strong earthquakes or gradual crustal stress changes due to glacioisostatic rebound can be ruled out.

Nature and timing of a prehistoric giant landslide on the eastern margin of the Tibetan Plateau

Studying the characteristics of a prehistoric giant landslide and ascertaining its relationship with palaeo-seismic events could provide useful information on the role of regional tectonic activities in hillslope evolution. Here, a giant palaeoseismic landslide (GPSL), namely, the Luanshibao (LSB) landslide, located on the eastern margin of the Qinghai-Tibet Plateau, was investigated to ascertain its characteristics and occurrence age. The relationship between occurrence age and palaeoseismic episodes was also discussed by using 14C dating on the adjacent active faults, including the regional Yidun-Litang-Dewu (YD-LT-DW) fault zone, which crossed the LSB landslide. The dating of multiple samples acquired from palaeo-seismic trenches revealed that at least 5 palaeo-seismic episodes had occurred in the vicinity of the LSB landslide over the last 5000 years. The occurrence age of the LSB landslide (3635 ± 387 a BP) coincides with palaeo-seismic episode I, and the LSB landslide is convincingly inferred to have been triggered during episode I, which reached a seismic intensity (Chinese scale) of at least degree VII near the LSB landslide during failure. The timing of a second reactivation phase coincides with palaeo-seismic episode IV, which occurred at 2100–2200 a BP. The study could provide reference for studying the regional palaeoseismic activities and palaeo landslide evolution near the Litang area.

Characteristics of features formed by gas hydrate and free gas in the continental slope and abyssal plain of the Middle Caspian Sea

Physicochemical conditions are proper to form gas hydrate and free gas in the Caspian Sea which are considered both for geohazard assessment and future possible source of energy. Gas migration, hydrate formation and decomposition cause different types of features to form in the seabed and sediments which can be affected by geological and physical conditions of sea. In this study, high-resolution marine seismoacoustic profiles were interpreted to investigate features formed by gas hydrate and free gas and assess their characteristics in the continental slope and abyssal plain of the middle Caspian Sea. A mud volcano, gas chimneys, pockmarks, gas pipes, sediment disturbances, a crater, a giant landslide and a big gentle diapir were investigated and reported in the present study. The geological and physical conditions including bathymetry, tectonic activity, sediments supply and water depth are different in the continental slope and abyssal plain, thereby the features of these two environments were compared to understand the effect of different conditions.

Current stability modelling of an incipient San Andres giant landslide on El Hierro Island, Canaries, Spain _ first attempt using limited input data.

Current stability modelling of an incipient San Andres giant landslide on El Hierro Island, Canaries, Spain _ first attempt using limited input data.

The spatial response pattern of coseismic landslides induced by the 2008 Wenchuan earthquake to the surface deformation and Coulomb stress change revealed from InSAR observations

The 2008 Mw 7.9 Wenchuan earthquake triggered plenty of coseismic giant landslides, which resulted in almost one third of total fatalities and economic losses during the event. Previous studies investigated the spatial relations between landslide distribution and topographic and seismic factors such as elevation, slope aspect, distance from rupture trace and seismic intensity. However, few studies are performed exploring the effects of coseismic surface deformation and Coulomb stress change on triggering landslides due to lack of adequate deformation observation data and stress calculation model for slope failure. In this study, we develop an envelope method to map an entire coseismic deformation field in both near- and far-field areas of seismic faults through the data fusion from InSAR and pixel offset-tracking (POT) techniques. The change in static Coulomb stress (SCS) acting on coseismic landsliding surface caused by the event is determined using the faulting model derived from the joint inversion of InSAR and GPS data, and also with the use of the elastic half-space dislocation theory and the generalized Hook’s law. The analysis suggests the spatial response pattern of seismic landslides to the coseismic ground motion and stress change, especially in the vicinity of fault rupture trace. The landslide density dramatically rises with the stress increase within the range from Yingxiu to Beichuan areas along the major surface rupture. Moving further and eastward along the fault strike, most of large landslides are triggered as the zone of positive SCS change narrows. Moreover, the high-magnitude surface displacements are possibly responsible for the giant landsliding events in the easternmost section. From the analysis of the stress transfer, the occurrence of landslides in the study area is largely controlled by the Yingxiu-Beichuan fault with overwhelming rupture length and fault slip, yet the Pengguan fault indeed shows dominance in the area between the two faults. The results show that coseismic surface deformation (derived from InSAR data in this study) and static Coulomb stress change can serve as two significant controlling factors on seismic landslide distribution and that the stress factor seems more significant in the vicinity of surface rupture.

Giant landslide triggerings and paleoprecipitations in the Central Western Andes: The aricota rockslide dam (South Peru)

The central part of the Western Andes holds an exceptional concentration of giant paleolandslides involving very large volumes of rock material (v>km3). While those gravitational slope failures are interpreted consensually as an erosional response to the geodynamic activity of the Andes (relief formation and tectonic activity), the question of their triggering mechanisms remains enigmatic. To clarify the respective roles of climatic versus seismic forcing on the Andean landslides, new temporal constraints on paleo movements are essential. Here, we focus on one of those giant slope failures, the Aricota giant rockslide that damned the Locumba valley in southern Peru. We conducted fieldwork, high-resolution DEM analysis and cosmogenic nuclide dating to decipher its development history and failure mechanisms. Our results point to the occurrence of two successive events. A giant failure mobilizing a rock volume of ca. 2km3 first produced a dam at 17.9±0.7 ka. Considering its height of ca. 600m, the Aricota rockslide dam is one of the three largest landslide dams worldwide. At 12.1±0.2 ka, a second event produced ca. 0.2 km3 of material, and the rock-avalanche debris spread out over the dam. As the chronology of those two events is pointing to the two main paleoclimatic pluvial periods in this region (Heinrich Stadial 1a and Younger Dryas), we favor the interpretation of a climatic forcing. At a regional scale, the concomitant aggradation of alluvial terraces and fan systems along the nearby valleys highlights higher regional erosion, sediments supply and mass-wasting events during those paleoprecipitation events and strengthens this conclusion.

Bol’shoi Ilek as the ilek formation stratotype of the Lower Cretaceous, and the new dinosaur and mammoth faunae site in the south-east of Western Siberia

The geological section of Bolshoi Ilek (up to 83 m high), southeast of Western Siberia, represented by Cretaceous and Quaternary systems has been reassessed. Its major part is composed with the Lower Cretaceous deposits of deltic genesis that reached 80 m thickness and characterized the Ilek Formation stratotype. During the screen washing of sand fraction the remains of fishes, lizards, turtles, crocodiloformes, dinosaurs, and mammals, typical for the Barremian - Lower Albian, have been revealed. Mesozoic strata are covered by eolian-deluvial and eluvial Quaternary deposits from 3 to 13,5 m. Three layers with mammoth fauna remains were distinguished there. The middle one is dated by 14С as ~ 22500 years ago. The observation have revealed that Bolshoi Ilek is a large landslide more than 1 square kilometre area. All of it allowed to compare it with the coeval section of Shestakovo outcrop that also exposes the part of giant landslide, caused by earthquakes more than 9 points by Richter scale, which indicates a powerful seismic event, occurred in the southeast of region at the very end of Pleistocene.

Bol’shoi Ilek as the Ilek Formation Stratotype of the Lower Cretaceous and a New Dinosaur and Mammoth Fauna Site in the Southeastern Western Siberia

This paper clarifies the structure of the Bol’shoi Ilek section (height up to 83 m), which is composed by the deposits of Cretaceous and Quaternary systems, in the southeastern part of Western Siberia. The major volume of the section comprises the Lower Cretaceous rocks of the delta complex, reaching a thickness of 80 m and characterizing the Ilek Formation stratotype. The remains of fishes, lizards, turtles, crocodiliforms, dinosaurs, and mammals typical for the Barremian–Early Albian were found for the first time when screen-washing the sand fraction. The Mesozoic rocks are superposed by Quaternary eolian–deluvial and eluvial deposits with a thickness of 3–13.5 m or more. This sequence consists of three layers with mammoth fauna remains, the middle of which is dated at 14C ~ 22 500 years ago. In addition, according to the investigation data, Bol’shoi Ilek is a huge landslide with an area of more than 1 km2. All these facts make it possible to draw an analogy with the even-aged section of the Shestakovo high bank which also partially exposes the giant landslide. Such landslides are triggered by earthquakes with a magnitude of more than nine points, which is indicative of a powerful seismic event that occurred in the southeastern part of the study region at the very end of the Pleistocene.

Emergency Disposal Solution for Control of a Giant Landslide and Dammed Lake in Yangtze River, China

Although landslide early warning and post-assessment is of great interest for mitigating hazards, emergency disposal solutions for properly handling the landslide and dammed lake within a few hours or days to mitigate flood risk are fundamentally challenging. In this study, we report a general strategy to effectively tackle the dangerous situation created by a giant dammed lake with 770 million cubic meters of water volume and formulate an emergency disposal solution for the 25 million cubic meters of debris, composed of engineering measures of floodgate excavation and non-engineering measures of reservoirs/hydropower stations operation. Such a disposal solution can not only reduce a large-scale flood (10,000-year return period, 0.01%) into a small-scale flood (10-year return period, 10%) but minimize the flood risk as well, guaranteeing no death raised by the giant landslide.

A comprehensive global database of giant landslides on volcanic islands

The first comprehensive global database of giant landslides on volcanic islands is outlined in this report. This database comprises a total of one hundred and eighty-two entries: the Atlantic Ocean hosts seventy-five giant landslides; the Pacific Ocean hosts sixty-seven giant landslides; and the Indian Ocean hosts forty giant landslides. To determine the spatial characteristics of each giant landslide, it has been necessary to georeference published maps using ArcGIS software coupled with global DTMs. Using the georeferenced outputs, it has been possible to measure the basic morphometric characteristics of each landslide such as its length, width, perimeter, area, and fall height. Landslide volumes have been calculated with a higher degree of certainty in thirty-five cases and with a lower degree of certainty in sixty-three cases while complete outlines of the landslide area have been defined in ninety-six cases. On the basis of these data, it has been possible to interrogate relationships between potentially significant variables. The age distribution of giant landslides on volcanic islands demonstrates that more than half of the records in the database occurred during the last 0.5 Ma. This global database of giant landslides on volcanic islands is hosted on the website of the Institute of Rock Structure & Mechanics: https://www.irsm.cas.cz/ext/giantlandslides . From there, the records can be downloaded as a spreadsheet or as a kml file for interrogation in a number of geospatial software programs including ArcGIS and Google Earth. This work is part of the activities of the International Consortium on landslides, namely, its International Programme on Landslides (Project No. 212).

Giant Submarine Landslide in the South China Sea: Evidence, Causes, and Implications

Submarine landslides can be tremendous in scale. They are one of the most important processes for global sediment fluxes and tsunami generation. However, studies of prodigious submarine landslides remain insufficient. In this review paper, we compile, summarize, and reanalyze the results of previous studies. Based on this reanalysis, we discover the giant Baiyun–Liwan submarine slide in the Pearl River Mouth Basin, South China Sea. We describe three concurrent pieces of evidence from ~23 Ma to 24 Ma, the Oligocene–Miocene boundary, for this landslide: the shoreward shift of the shelf break in the Baiyun Sag, the slump deposition to the southeast, and the abrupt decrease in the accumulation rate on the lower continental slope. This landslide extends for over 250 km, and the total affected area of the slide is up to ~35,000–40,000 km2. The scale of the landslide is similar to that of the Storegga slide, which has long been considered to be the largest landslide on earth. We suggest that strike–slip movement along the Red River Fault and ridge jump of the South China Sea caused the coeval Baiyun–Liwan submarine slide. The identification of the giant landslide will promote the understanding of not only its associated geohazards but also the steep rise of the Himalayan orogeny and marine engineering. More attention needs to be paid to areas with repeated submarine landslides and offshore installations.

Role of structural inheritance in the gravitational deformation of the Monviso meta-ophiolite Complex: the Pui-Orgiera serpentinite landslide (Varaita Valley, Western Alps)

ABSTRACT Depending on its chemical and mineralogical composition, the serpentinite represents a peculiar rocky material, which plays a significant role in influencing pedogenetic weathering, vegetation ecology, geo-mechanical and tectonic processes, as well as gravitational slope failure. In serpentinite-bearing environments the response to gravitational stress is more effective than in other rock types. In the Monviso meta-ophiolite Complex (Western Alps), a close relation between landsliding and serpentinite rock occurrences is particularly evident. Our geological map (Main Map), at a scale 1:10,000, illustrates the geology and geomorphological features of the Pui-Orgiera giant (2.98 km2) complex landslide, located on the southern slope of the Monviso Massif. This map clearly documents that the characteristics and kinematics of the landslide are closely associated to the anomalous thickening and widening increase of the Baracun Shear Zone, a remnants of an intra-oceanic detachment fault which separates serpentinite and meta-intrusives from metabasalt and metasediments of the Monviso meta-ophiolite Complex.

Long-term erosion of the Nepal Himalayas by bedrock landsliding: the role of monsoons, earthquakes and giant landslides

Abstract. In active mountain belts with steep terrain, bedrock landsliding is a major erosional agent. In the Himalayas, landsliding is driven by annual hydro-meteorological forcing due to the summer monsoon and by rarer, exceptional events, such as earthquakes. Independent methods yield erosion rate estimates that appear to increase with sampling time, suggesting that rare, high-magnitude erosion events dominate the erosional budget. Nevertheless, until now, neither the contribution of monsoon and earthquakes to landslide erosion nor the proportion of erosion due to rare, giant landslides have been quantified in the Himalayas. We address these challenges by combining and analysing earthquake- and monsoon-induced landslide inventories across different timescales. With time series of 5 m satellite images over four main valleys in central Nepal, we comprehensively mapped landslides caused by the monsoon from 2010 to 2018. We found no clear correlation between monsoon properties and landsliding and a similar mean landsliding rate for all valleys, except in 2015, where the valleys affected by the earthquake featured ∼5–8 times more landsliding than the pre-earthquake mean rate. The long-term size–frequency distribution of monsoon-induced landsliding (MIL) was derived from these inventories and from an inventory of landslides larger than ∼0.1 km2 that occurred between 1972 and 2014. Using a published landslide inventory for the Gorkha 2015 earthquake, we derive the size–frequency distribution for earthquake-induced landsliding (EQIL). These two distributions are dominated by infrequent, large and giant landslides but under-predict an estimated Holocene frequency of giant landslides (&gt; 1 km3) which we derived from a literature compilation. This discrepancy can be resolved when modelling the effect of a full distribution of earthquakes of variable magnitude and when considering that a shallower earthquake may cause larger landslides. In this case, EQIL and MIL contribute about equally to a total long-term erosion of ∼2±0.75 mm yr−1 in agreement with most thermo-chronological data. Independently of the specific total and relative erosion rates, the heavy-tailed size–frequency distribution from MIL and EQIL and the very large maximal landslide size in the Himalayas indicate that mean landslide erosion rates increase with sampling time, as has been observed for independent erosion estimates. Further, we find that the sampling timescale required to adequately capture the frequency of the largest landslides, which is necessary for deriving long-term mean erosion rates, is often much longer than the averaging time of cosmogenic 10Be methods. This observation presents a strong caveat when interpreting spatial or temporal variability in erosion rates from this method. Thus, in areas where a very large, rare landslide contributes heavily to long-term erosion (as the Himalayas), we recommend 10Be sample in catchments with source areas &gt; 10 000 km2 to reduce the method mean bias to below ∼20 % of the long-term erosion.

Superheated steam, hot CO2 and dynamic recrystallization from frictional heat jointly lubricated a giant landslide: Field and experimental evidence

We used thermogravimetry to determine that the frictionally heated sliding surface reached ∼850°C at the base of the 1.16×109 m3 Daguangbao landslide, triggered during the 2008 Ms8.2 Wenchuan earthquake. Optical and electron microscope images reveal evidence of dolomite thermal decomposition, hot fluid penetrating fine cracks and dynamic recrystallization of minerals. A high-speed rotary shear apparatus simulated the rapid sliding and provided data on very low frictional resistance (friction coefficient, μ≈0.05) during shearing above 1 m/s. Electron-probe analysis of samples from the experiments showed lime and magnesia from thermal decomposition of dolomite. Some of the sheared minerals formed a dynamically recrystallized layer ∼0.1 mm thick, and its viscous resistance would have contributed to the low apparent friction during high-speed sliding. A further explanation, however, was very high pore-fluid pressure from hot supercritical CO2 (and superheated steam at a partial pressure of >10 MPa in the field situation with groundwater). These friction-reducing mechanisms are not mutually exclusive and both were present simultaneously at the landslide base.

Giant landslide displacement analysis using a point cloud set conflict technique: a case in Xishancun landslide, Sichuan, China

ABSTRACTLandslides, threatening millions of human lives, are geological phenomena on earth, occurred frequently. An increasing number of techniques are being used to monitor landslide deformation. Among these, Light Detection and Ranging (LiDAR) stands out for its high efficiency and accuracy in displacement detection, particularly for giant landslides. In this work, we collected two temporal datasets of terrain laser scanning and proposed a flowchart for giant landslide displacement analysis using the point cloud set conflict（PCSC） technique. First, the terrestrial points were obtained by performing registration and off-terrain point filtering. Second, the landslide displacement field was acquired using the proposed method based on its surface roughness. The displacement results from our established methodological system are comparable with the ones of Interferometric Synthetic Aperture Radar (InSAR)-derived deformations. The differences estimated from two systems are at the centimetre level. Cross-analysis on the trigger factor with landslide occurred mechanism could be achieved based on the results as well. Therefore, this work provides a novel system to analyse the displacement of a giant landslide in the future study.

Mass movements along a rapidly uplifting river valley: an example from the upper Jinsha River, southeast margin of the Tibetan Plateau

Because of the rapid uplift of the Tibetan Plateau since the middle and late Pleistocene, the surrounding mountainous areas of the Tibetan Plateau also show a rapid uplift trend. The study area is located in the upper reaches of the Jinsha River, southeastern Tibetan Plateau. Tectonically, the study area is located in the eastern segment of the Jinshajiang suture zone, which is the weak belt of the lithosphere. Affected by the rapid uplift, steep slopes are formed by the rapid vertical down-cutting of rivers in the studied reach. Under the sustained action of the gravitational field, the deformation and failure of slopes readily occur. To better formulate development and risk mitigation plans, a mass movement inventory map at the scale of 1:50,000 was prepared for the 100 km long reach. A total of 247 mass movements of different types, sizes and relative ages were mapped via image interpretation and detailed field survey. The statistical analysis of their spatial distributions was performed using two relative concentration indexes. The correlations between the spatial distribution and six common factors were analyzed: elevation, slope angle, slope aspect, lithology, active fault, and river system. The geomorphological survey shows that the mass movements locally play an important role in the present-day landscape evolution. In addition, the geomorphic features indicate that the mapped giant landslides can be classified as very old landslides, and they very likely occurred under a different morphoclimatic environment than that of the present.

Giant landslides in the foreland of the Patagonian Ice Sheet

Quaternary glaciations have repeatedly shaped large tracts of the Andean foreland. Its spectacular large glacial lakes, staircases of moraine ridges, and extensive outwash plains have inspired generations of scientists to reconstruct the processes, magnitude, and timing of ice build-up and decay at the mountain front. Surprisingly few of these studies noticed many dozens of giant (≥108 m3) mass-wasting deposits in the foreland. We report some of the world's largest terrestrial landslides in the eastern piedmont of the Patagonian Ice Sheet (PIS) along the traces of the former Lago Buenos Aires and Lago Puyerredón glacier lobes and lakes. More than 283 large rotational slides and lateral spreads followed by debris slides, earthflows, rotational and translational rockslides, complex slides and few large rock avalanches detached some 164 ± 56 km3 of material from the slopes of volcanic mesetas, lake-bounding moraines, and river-gorge walls. Many of these landslide deposits intersect with well-dated moraine ridges or former glacial-lake shorelines, and offer opportunities for relative dating of slope failure. We estimate that >60% of the landslide volume (∼96 km3) detached after the Last Glacial Maximum (LGM). Giant slope failures cross-cutting shorelines of a large Late Glacial to Early Holocene lake (“glacial lake PIS”) likely occurred during successive lake-level drop between ∼11.5 and 8 ka, and some of them are the largest hitherto documented landslides in moraines. We conclude that 1) large portions of terminal moraines can fail catastrophically several thousand years after emplacement; 2) slopes formed by weak bedrock or unconsolidated glacial deposits bordering glacial lakes can release extremely large landslides; and 3) landslides still occur in the piedmont, particularly along postglacial gorges cut in response to falling lake levels.