Large-scale Landsliding Research Articles

Terrace formation linked to outburst floods at the Diexi palaeo-landslide dam, upper Minjiang River, eastern Tibetan Plateau

Abstract. River terraces are frequently investigated with the aim of extracting information regarding tectonic or climate forcing on the evolution of landscapes. Terraces formed following the blockage of valleys by large-scale landsliding have received limited attention despite the high likelihood of their prevalence in landslide-dominated mountain belts. Here, we investigate the geomorphology, sedimentology, and chronology of two outstanding sets of terraces upstream of the giant river-blocking Diexi palaeo-landslide on the upper Minjiang River, eastern Tibetan Plateau. The first set occurs at Tuanjie village and has seven levels (T1–T7); the second set, at Taiping village, has three levels (T1–T3). All the terraces display a consistent sedimentary sequence comprising lacustrine muds topped by fluvial gravel sometimes capped by loess and a palaeosol. Based on field examination, lithofacies analysis, elevation data, and chronometric data (optically stimulated luminescence and radiocarbon dating), we correlate T1, T2, and T3 at Taiping with T5, T6, and T7 at Tuanjie. Our analysis suggests that two damming and three outburst events have occurred at the Diexi palaeo-landslide over the past 35 000 years. A giant landslide (> 300 m high) blocked the river before 35 ka, followed by the first outburst flood at ∼ 27 ka; the river was blocked again between 27 and 17 ka, followed by a second outburst at ∼ 17 ka. A third outburst at ∼ 12 ka was followed by gradual fluvial incision of the palaeo-dam crest to its current level. We attribute the terraces at Diexi to the recurrent blockage and outburst events, which reflect the shifting sediment transport capacity and incision at the palaeo-dam crest. Here, climatic fluctuations play a minor role in terrace formation, and tectonism plays no role at all.

A massive ancient river-damming landslide triggered by buckling failure in the upper Jinsha River, SE Tibetan Plateau

Large-scale landsliding is an extremely important geological process in shaping landscapes in the Tibetan Plateau. In this research, an ancient river-damming landslide with an estimated debris volume of 4.9 × 107 m3, located in the upper Jinsha River, SE Tibetan Plateau, was studied. The landslide once formed a dam over 60 m high and blocked the river. Lacustrine sediments, composed of silty clay with particle sizes of 0.002–0.25 mm, are intermittently distributed along both banks, extending about 6.5 km upstream. The OSL dating indicates that the lacustrine sediments have an age of 2.6 ± 0.2 ka. Detailed field investigation and theoretical analysis was performed to investigate the characteristics, potential cause and mechanism of the landslide. The results suggest that the landslide was most likely triggered by buckling of planar rock slabs under gravity. It may start as a translational sliding along the weak interlayer composed of mica schist at the upper part of the slope and then formed buckles by curving amphibolite rock beds near the slope toe. The hillslope has still been affected by gravitational deformations, with geomorphology characterized by tension cracks, buckle folds, and small landslide scars distributed on the slope surface, suggesting that the evolution of the river valley caused by buckling deformation has not achieved equilibrium.

The giant coastal landslides of Northern Chile: Tectonic and climate interactions on a classic convergent plate margin

Documented for the first time are an extensive suite of late Neogene giant terrestrial coastal landslides along the classic convergent margin of western South America (18° to 24° south). These are remarkable in terms of their unusual abundance and atypical setting, such failures previously being linked with oceanic volcanic edifices or over-steepened glaciated coastlines. Located within the hyper-arid Coastal Cordillera of the Atacama Desert of Northern Chile we report the presence of more than 60 individual large-scale landslides with individual volumes up to 9 km3 developed over a horizontal coastline distance of some 650 km. These landslides were emplaced as a combination of rock avalanches and multiple rotational failures. The majority terminated directly into the Pacific – likely generating significant tsunami hazard to the Chilean and south Peruvian coastline in a region which is today considered to be part of a notorious seismic gap. The proliferation and scale of these Late Neogene giant landslides in this actively uplifting, hyperarid terrain suggests they are the main geomorphic agent for relief reduction, probably triggered by megathrust earthquakes and potentially providing a unique palaeoseismic archive. The temporal and spatial distribution of these giant landslides corresponds with a period of surface steepening of the forearc wedge in the Central Andes and south to north differential uplift associated with factors such as aseismic ridge subduction. The resulting surface gradient increases, combined with the persistent climatic aridity of the region, have served to limit effective relief-reducing geomorphic processes in this oversteepened terrain to large-scale landsliding. The phenomena documented here geospatially link previously recognised large-scale slope failures from the off-shore environment and higher altitude areas of the Andean forearc, suggesting that large-scale landsliding is capable of transferring sediment on a regional scale to the off-shore Peru–Chile trench. This has implications for the friction of the subducting Nazca plate and associated seismicity and uplift.

Nature and timing of large landslides within an active orogen, eastern Pamir, China

Large-scale landsliding (involving ≫106m3 in volume) is important in landscape development in high mountains. To assess the importance of large landslides in high mountains, four large landslides (Bulunkou, Muztagh, Taheman, and Yimake) were mapped in the NE Chinese Pamir at the westernmost end of the Himalayan–Tibetan orogen and dated using 10Be terrestrial cosmogenic nuclides. The Bulunkou landslide at the southernmost end of Muji Valley is composed of ~1.7×107m3 of landslide debris and has an age of 2.0±0.1ka. The Muztagh landslide, located on the SW side of the massif Muztagh Ata, is composed of ~4.7×108m3 of debris, and has an age of 14.3±0.8ka. The Taheman landslide, located south of Muztagh Ata, is composed of ~2.6×108m3 of landslide debris and has an age of 6.8±0.2ka. The Yimake landslide, on the northern frontal range of the Pamir at the southwestern end of the Tarim basin, is composed of ~1.4×109m3 of landslide debris and has an age of 7.1±0.6ka. Two other large landslides are present in the region, the Aerpa Aigezi (on a tributary of the Gez River) and the Bile Jiyi (on the Yarkand River) landslides, and are composed of ~1.6×107m3 and ~5.2×106m3 of landslide debris, respectively. However, the Aerpa Aigezi and Bile Jiyi landslides were not studied in as much detail or dated because of their inaccessibility. Given the tectonically active nature of this region, with numerous active faults, and the morphology of the landslides, these landslides were likely triggered by earthquakes. However, other causes — including long-term increased precipitation and geologic bedrock structure — could be important contributing factors in their formation.

The late Messinian erosional surface and the subsequent reflooding in the Mediterranean: New insights from the Melilla–Nador basin (Morocco)

New field investigations in the Cape des Trois Fourches peninsula (northern Morocco) allow us to bring new insights on the late Messinian erosional surface and the overlying marine deposits. Our results show that the late Messinian erosional surface, characterized by an erosional plateau, is interrupted by an up to 70 m deep paleovalley, thought to result from a landslide. The minimum relative sea-level drop is 70 m, probably much more with regard to the indication that a large-scale landsliding has occurred. The post-erosional deposits (Cycle 3) comprise one major depositional sequence composed of two high-order sedimentary subcycles. Deposits of the first subcycle 3A filled in the erosional basin and the paleovalleys; deposits of the second subcycle 3B overflowed the paleovalleys and the erosional plateau. We found evidence of the transgressive systems tract of the deposits corresponding to the post-Messinian crisis reflooding, for which a latest Messinian onset can neither be proved nor entirely ruled out because the rare biostratigraphic data are not sufficiently conclusive. The uppermost part of the reflooding deposits (subcycle 3A) is shown to be definitely Pliocene in age based on the occurrence of Globorotalia puncticulata and the composition of the assemblage of pectinids.

Distribution of landslides in southwest New Zealand

This study examines the size distribution of a regional medium-scale inventory of 778 landslides in the mountainous southwest of New Zealand. The spatial density of mapped landslides per unit area can be expressed as a negative power–law function of Landslide area AL spanning three orders of magnitude (∼10−2–101 km2). Although observed in other studies on landslide inventories, this relationship is surprising, given the lack of absolute ages, and thus uncertainty about the temporal observation window encompassed by the data. Large slope failures (arbitrarily defined here as having a total affected area AL>1 km2) constitute 83% of the total affected landslide area ALT. This dominance by area affects slope morphology, where large-scale landsliding reduces slope angles below the regional modal value of hillslopes, ϕmod∼39°. More numerous smaller and shallower failures tend to be superimposed on the pre-existing relief. Empirical scaling relationships show that large landslides involve >106 m3 of material. The volumes VL of individual preserved and presumably prehistoric (i.e. pre-1840) landslide deposits equate to 100–102 years of total sediment production from shallow landsliding in the respective catchments, and up to 103 years of contemporary regional sediment yield from the mountain ranges. Their presence in an erosional landscape indicates the geomorphic importance of landslides as temporary local sediment storage.

Rock Mass Strength Assessment and Significance to Edifice Stability, Mount Rainier and Mount Hood, Cascade Range Volcanoes

—Catastrophic edifice and sector failure occur commonly on stratovolcanoes worldwide and in some cases leave telltale horseshoe-shaped calderas. Many of these failures are now recognised as having resulted from large-scale landsliding. These slides often transform into debris avalanches and lahars that can devastate populations downstream of the volcano. Research on these phenomena has been directed mainly at understanding avalanche mechanics and travel distances and related socioeconomic impacts. Few investigations have examined volcanic avalanche source characteristics. The focus of this paper is to 1) describe a methodology for obtaining rock strengths that control initial failure and 2) report results of rock mass strength testing from Mount Rainier and Mount Hood. Rock mass and shear strength for fresh and hydrothermally altered rocks were obtained by 1) utilizing rock strength and structural information obtained from field studies and 2) applying rock mechanics techniques common in mining and civil engineering to the edifice region. Rock mass and intact rock strength differences greatly in excess of one order of magnitude were obtained when comparing strength behavior of fresh and completely altered volcanic rock. The recognition and determination of marked strength differences existing on the volcano edifice and flank, when combined with detailed geologic mapping, can be used to quantify volcano stability assessment and improve hazard mitigation efforts.

LANDSLIDING IN THE LIKHU KHOLA DRAINAGE BASIN, MIDDLE HILLS OF NEPAL

A number of claims have been made for increased population pressure in the Himalaya leading to land degradation, especially landsliding. However, a lack of detailed information has meant that it has been very difficult to assess the scale of landsliding. This paper reports the results of a three-year study of landsliding in four subcatchments of the Likhu Khola drainage basin in the Middle Hills, Nepal. Over the three years (1991, 1992, and 1993), 381 slope failures were mapped with the vast majority being small, almost planar slumps on the risers of agricultural terraces. The next main category of failure was debris slides that tended to be larger. The largest failures occurred in the drier south-facing catchments. Sediment delivery ratios from the landslides in the four subcatchments varied from 38.2% to 88.4%, producing denudation rates ranging from 0.46 mm/yr to 8.1 mm/yr. The overall rate for the four subcatchments was 4.4 mm/yr. These results indicate that landsliding is a significant contributor to overall denudation but that large-scale landsliding is not a major problem in the study area. [Key words: landsliding, land use, Himalaya, Nepal.]

Landslide-driven drainage network evolution in a pre-steady-state mountain belt: Finisterre Mountains, Papua New Guinea

The emergence of an orogen is generally characterized by an early phase of rapid surface uplift and the concomitant evolution of montane topography, leading in some cases to a steady state in which tectonic mass flux is balanced by erosion. This early phase of mountain growth is exemplified in the Finisterre Mountains of Papua New Guinea, a propagating growth fold whose catchments can be observed at a range of stages in their temporal evolution. Watersheds appear to initiate by isolated gorge incision, to expand by large-scale landsliding in a manner controlled by ground-water seepage, and to entrench by fluvial incision of landslide scars and deposits. Once a montane system of ridges and valleys is established, only rare, major landslides can modify the drainage pattern. The steady-state morphology of a mountain belt is therefore intimately related to its initial phase of growth.

Landsliding caused by Pleistocene glacial lake ponding–an example from central British Columbia

Sections cut through the Quaternary sediment fill of the Fraser River valley in central British Columbia provide evidence for large-scale landsliding during Pleistocene time. Especially notable are thick, laterally extensive diamict beds, consisting mainly of Tertiary rock debris, that occur near the base of glaciolacustrine sequences. These beds were deposited by subaqueous debris flows during one or more periods of lake ponding when advancing Pleistocene glaciers blocked the ancestral Fraser River. The association of diamict beds and glaciolacustrine sediments deposited during periods of glacier advance may indicate a genetic link between slope failure and lake filling. These observations (1) demonstrate the adverse effects of high pore pressures on the stability of slopes underlain by poorly indurated Tertiary rocks and (2) extend the known history of landslides involving these rocks back into the Pleistocene. Key words: landslides, debris flows, Pleistocene, glacial lake.

Significance of Neogene Phosphorites in Capistrano Embayment, Southern California: ABSTRACT

The post-Relizian Monterey, Capistrano, and Niguel Formations comprise about 3,280 ft (1,000 m) of phosphorite-bearing marine sediments. Primary deposition appears to have been as pelletal and nodular phosphorite forming phosphoritic shales with occasional high-grade zones ranging up to 20 to 40% P2O5 and individual beds ranging up to 7 ft (2 m) or more in thickness. Most high-grade shale beds, however, are less than 3 ft (1 m) thick and average 20 to 25% P2O5. Some of the best exposures are in Aliso and Oso Canyons within the San Juan Capistrano quadrangle. Excavations for construction have exhumed weathered phosphorites at and below the water table exposing remobilized phosphate which is readily recognizable as vivianite [Fe3(PO4)2).8H2O]. The purple (azurite colored) vivianite oxidizes on drying to a brownish color within a few weeks, thus it is rarely identified during conventional field mapping. Large-scale landsliding within the Monterey and Capistrano Formations has commonly fractured phosphorite-bearing beds and allowed mobilization and redeposition of the original phosphorite as vivianite above the basal shear plane of block-glide landslides and downstream. Water in downstream drainages has the potential for being misidentified as a pollutant from septic systems. This suggests that geochemical techniques might provide valuable methods of exploration for certain phosphate occurrences. Of several phosphorite basal conglomerates within or between the Monterey, Capistrano, and Niguel Formations, the best exposed and one of the best developed lies along the angular unconformity between the Monterey Formation and the overlying Niguel Formation on the east flank of the San Joaquin Hills, immediately north of the U.S. Geological Survey office at Laguna Niguel. This resistant phosphorite bed is composed of what appears to be nodules derived from the underlying Monterey Formation. They have been redeposited in and just below the littoral zone. Some of the phosphorite is concentrated in downslope channels within the neritic zone. Additional exploration within the Capistrano embayment would probably yield economic amounts of phosphate rock. Rapid urban expansion in this part of southern California will probably preclude further exploration and development. However, as the Neogene phosphorite-bearing formations of the Pacific slope become better known, there may be substantial incentive for exploration and development in other areas. California is one of the largest consumers of phosphate in the United States and imports almost 100% of its supply from Florida, Idaho, and other areas. Potentially, millions of dollars could be saved annually in transportation costs and a strategic commodity (petroleum) would be conserved, if local deposits could supply the California market. End_of_Article - Last_Page 1688------------