Abstract Geomorphology, structural geology and engineering geology allow description of the main characteristics of a slope in distinct ways that can be combined to provide a complementary view of the operative slope processes. The subjects presented in this Special Publication include: slope morphology and evolution; mechanical behaviour of the material; modes of failure and collapse; influence of lithology and structural features; and the role played by controlling factors. This Slope Tectonics volume comprises a series of very different contributions that attempt to underline a multidisciplinary approach that should form the framework of slope instability studies. Slope Tectonics is adopted in this volume to mean deformation that is induced or fully controlled by the slope morphology and that generates features that can be compared to tectonic features. The stress field in a slope is the result of gravity, topography and the geological setting created by an ensemble of geodynamic processes. Active tectonics (also called neotectonics) generates a stress field that can control slope processes; a strong feedback existing between geological history, tectonics, lithology, geomorphological evolution and topography. As a consequence, a list of factors and their relative influence can be presented. Fabric induced by a local stress field within a slope: discontinuities and local faults with cataclastic bands of variable thickness; folds (Fig. 1 ), associated predominantly with brittle structures; complex failure paths (stepped or multi-surface); local failures: rock bridge failures or extensional failures (graben-like or pseudo-graben-like);

Abstract Catastrophic deep-seated landslides (DSL) are generally considered to be the result of large slope deformations also known as deep-seated gravitational slope deformation (DSGSD). This paper aims to build a synthesis of multiple studies made in the Tinée Valley (southern French Alps) to assess the geometrical, kinematical, mechanical and chronological relationships between these two gravitational processes. At the scale of the valley, data issued from geological, geomorphological and 10 Be dating indicate a clear geometrical link between DSGSD and DSL occurring at the base of the slope and suggest that gravitational slope evolution began after the glacial retreat (13 ka BP). This is supported by the example of the well-documented La Clapière slope. A continuous evolution process is characterized geometrically and temporally from geomorphic observations and analogue modelling. Coupling structural, geomorphological, physical and chronological studies allowed us to propose a four-dimensional (4D) deformation model mechanically correlated with progressive failure concept. The validity and variability of this reference site are discussed at the valley scale (taking Isola and Le Pra slope deformation as examples). It allows a rough estimation of the state of slope deformation at the valley scale to be constructed and the slope evolution with time to be considered. This 4D model could then be considered as a reference for other deep-seated gravitational slope deformations in comparable Alpine valleys.

Abstract The high density of slope failures in western Norway is due to the steep relief and to the concentration of various structures that followed protracted ductile and brittle tectonics. On the 72 investigated rock slope instabilities, 13 were developed in soft weathered mafic and phyllitic allochthons. Only the intrinsic weakness of such rocks increases the susceptibility to gravitational deformation. In contrast, the gravitational structures in the hard gneisses reactivate prominent ductile or/and brittle fabrics. At 30 rockslides along cataclinal slopes, weak mafic layers of foliation are reactivated as basal planes. Slope-parallel steep foliation forms back-cracks of unstable columns. Folds are specifically present in the Storfjord area, together with a clustering of potential slope failures. Folding increases the probability of having favourably orientated planes with respect to the gravitational forces and the slope. High water pressure is believed to seasonally build up along the shallow-dipping Caledonian detachments and may contribute to destabilization of the rock slope upwards. Regional cataclastic faults localized the gravitational structures at 45 sites. The volume of the slope instabilities tends to increase with the amount of reactivated prominent structures and the spacing of the latter controls the size of instabilities.

Abstract Structural geology has recently become a key topic in landslide research. However, the link between regional structures, their cumulative contribution to rockslide development and their significance in a spatial framework is uncertain. We examine the influence of structures on rockslide susceptibility in the Storfjorden area, a 900 km 2 fjord complex in western Norway that includes the monitored rockslide sites of Åknes and Heggursaksla. We have newly identified 52 potential rockslide sites from aerial photographs. The structural features critical for the development of large rockslides (fjord-dipping foliation, basal shear plane and breccia, extensional fracture and transfer fault) have a spatial bias in orientation. Rockslides are more likely to develop in specific fjord orientations that have favourably oriented structures. Therefore, the development of rockslides has a marked spatial distribution that we describe qualitatively with an inventory of structural features. Sites with the full plethora of features display the most movement, the largest volumes and are already under the closest scrutiny with regard to monitoring. These sites are also spatially biased, the largest clustering occurring in west Sunnylyvsfjorden. The utilization of structural criteria can show trends in spatial distribution of rockslide potential and on a regional scale can be an important tool in susceptibility analysis.

Abstract More than 250 rock slope failures have occurred in Sogn and Fjordane County in historical times. So far, 28 sites are known where open cracks indicate that rock slope failures may occur in the future. Detailed structural and geomorphological analyses of these sites have been conducted, and form the basis for an evaluation and comparison of the unstable rock slopes. Four of these sites are described in detail herein. The main characteristics for rock slope instabilities in Sogn and Fjordane are: (1) a preferred location within relatively weak rock units, such as phyllites and weathered mafic gneisses; and (2) the development of most instabilities at convex slope breaks, which are evident as knick-points in the slope profile. Sogn and Fjordane is compared with other Norwegian regions, particularly Møre and Romsdal County, with respect to the spatial distribution of past and current rock slope instabilities. Sogn and Fjordane shows the greatest number of historical slope failures, whereas in Møre and Romsdal a larger amount of potential instabilities is observed. We propose that the larger amount of unstable rock slopes in Møre and Romsdal may be controlled by a locally high gradient of ongoing post-glacial uplift and a higher rate of neotectonic activity.

Abstract Deep-seated gravitational slope deformation (DSGSD) is a common and widespread type of large slope instability in the Alps. In the Aosta Valley region in NW Italy, DSGSDs occupy at least 13.5% of the regional territory. In this study, regional distribution analyses have been coupled with local detailed geological and geomorphological surveys of individual phenomena to detect the controlling factors, deformation processes and evolution stages of DSGSD. Data and maps from field and remote-sensing investigations have been supported by drill data and geomechanical and hydrogeochemical analyses from project studies for hydroelectric plants and tunnels. Several phenomena related to DSGSD have been studied thoroughly: gravity-induced stresses, tectonic–metamorphic setting, morphostructural relations, glacial and periglacial morphodynamics, recent tectonic evolution, hydrogeological conditions and karst phenomena have been generically indicated as controlling factors. In the studied area three of the controlling factors were crucial in differentiating the form and evolution of DSGSDs: deep dissolution, surface tectonics, and tectonostructural setting. They are presented as possible end members of a classification scheme for DSGSDs.

Abstract A giant rockslide occurred on the southern side of an Upper Tertiary shield volcano in central Nicaragua in the Holocene. The failure caused tectonic-like deformation of rock masses and changed the local stress regime. The lower, compressional part of the rockslide produced a stress field with the axis of maximum stress (σ 1 ) parallel to the displacement vector of the main body. The upper part of the rockslide was gravity-driven with σ 1 vertical and σ 3 horizontal, and oriented SE–NW. The mass tended to move SE. In the crown, the stress field had a subvertical σ 1 steeply dipping towards the west. Data at the base of the Santa Lucia Depression, where east- and west-dipping reverse and thrust faults developed, showed that the compressional stress, σ 1 , was nearly horizontal and east–west oriented, the horizontal σ 2 was north–south oriented, and the σ 3 was subvertical. These compressional conditions resulted from the collapse of the crown after the main slope failure phase. Simultaneously, along with the gravity relaxation of the main displaced mass, the slopes and mountain slopes along the main scarp depression underwent deep-seated sliding, sagging and flowing.

Abstract Åknes is an active complex large rockslide of approximately 30–40 Mm 3 located within the Proterozoic gneisses of western Norway. The observed surface displacements indicate that this rockslide is divided into several blocks moving in different directions at velocities of between 3 and 10 cm year −1 . Because of regional safety issues and economic interests this rockslide has been extensively monitored since 2004. The understanding of the deformation mechanism is crucial for the implementation of a viable monitoring system. Detailed field investigations and the analysis of a digital elevation model (DEM) indicate that the movements and the block geometry are controlled by the main schistosity (S 1 ) in gneisses, folds, joints and regional faults. Such complex slope deformations use pre-existing structures, but also result in new failure surfaces and deformation zones, like preferential rupture in fold-hinge zones. Our interpretation provides a consistent conceptual three-dimensional (3D) model for the movements measured by various methods that is crucial for numerical stability modelling. In addition, this reinterpretation of the morphology confirms that in the past several rockslides occurred from the Åknes slope. They may be related to scars propagating along the vertical foliation in folds hinges. Finally, a model of the evolution of the Åknes slope is presented.

Abstract Large slope failures in fractured rocks are often controlled by the combination of pre-existing tectonic fracturing and brittle failure propagation in the intact rock mass during the pre-failure phase. This study focuses on the influence of fold-related fractures and of post-folding fractures on slope instabilities with emphasis on Turtle Mountain, located in SW Alberta (Canada). The structural features of Turtle Mountain, especially to the south of the 1903 Frank Slide, were investigated using a high-resolution digital elevation model combined with a detailed field survey. These investigations allowed the identification of six main discontinuity sets influencing the slope instability and surface morphology. According to the different deformation phases affecting the area, the potential origin of the detected fractures was assessed. Three discontinuity sets are correlated with the folding phase and the others with post-folding movements. In order to characterize the rock mass quality in the different portions of the Turtle Mountain anticline, the geological strength index (GSI) has been estimated. The GSI results show a decrease in rock mass quality approaching the fold hinge area due to higher fracture persistence and higher weathering. These observations allow us to propose a model for the potential failure mechanisms related to fold structures.

Abstract Few studies of rockslides have addressed the relationships between structures, geomorphological expression and direct evidence for movement. We employ structural geology, geomorphology and interferometric synthetic aperture radar (InSAR) to investigate the evolution of the surface features developed in response to movement of the Gamanjunni rockslide site in Troms County in northern Norway. The slide is located on a west-facing mountainside, and is bounded by two angled back scarps and a 20°–30° basal sliding plane. The volume is estimated at 24 Mm 3 and is therefore among the largest potential rockslides in Norway. InSAR provides a new method to measure the movement of potential rockslides, and thus provides a direct link between qualitative movement data and field observations. We document the relationship between variations in ground movement rates and changing back-scarp geomorphology at the Gamanjunni site as well as movement patterns within the incipient rockslide. We demonstrate that variations in InSAR documents millimetre variations in scarp displacement and that this is reflected in the evolving back-scarp geometry. We conclude that InSAR can provide important information to complement field observations. The ability of InSAR to document landslide movement patterns greatly extends our knowledge of back-scarp evolution and active landslide processes.

Abstract Evidence of deep-seated gravitational slope deformations (DSGSD) and of large prehistoric landslides is fairly widespread within the Central Apennines (Italy). These gravity-induced processes accompanied the intense Plio-Quaternary uplift phases that affected the mountain chain. In this study a multidisciplinary approach has been adopted in order to better constrain the relationship between the tectonic evolution and the gravitational morphogenesis of a typical Apennine morphostructure, such as the Caramanico Valley. For this purpose a conceptual model of the morphostructural evolution of the area has been reconstructed, on the basis of geological constraints derived by the integration of detailed geological–structural and geomorphological surveys with available literature data. Based on this evolutionary model, a multistage numerical modelling using the finite difference method code FLAC 6.0 has been performed in order to: (i) evaluate the effect of the uplift-related morphological changes of the valley–slope system; and (ii) assess the role of the horizontal/vertical stress ratio variations due to geodynamic regime shifts. The results of the numerical model show a good fit with the actual geomorphical evidence and also confirm the presence during some evolutionary stages of stress–strain conditions compatible with those necessary to produce the massive rock slope failures testified by the presence of large palaeo-landslide deposits.

Abstract Stress distribution in mountainous areas is influenced by local morphology. Valley morphology and the relationship between main and tributary valleys strongly depend on geological characteristics and evolution. They may control the evolution of slope instabilities, especially when interacting with pervasive structural features. We performed parametric three-dimensional (3D) numerical modelling of simplified slope geometries with variable slope angle (from 21° to 35°), length, combining different orientations for different slope sectors and changing attitude of pervasive planes of anisotropy (foliation, schistosity, bedding). Data used in the 3D models are the initial slope geometry, rock mass properties and internal anisotropy. We assumed Mohr–Coulomb behaviour, with the presence of ubiquitous joints and different piezometric levels. The model results show that plastic deformation initiates near the highest ridge just after deglaciation commences. A shear zone develops and propagates toward the toe of the slope, and its shape is strongly controlled by slope geometry, anisotropy and in situ stresses. The thickness of the failing mass, for model slope reliefs up to 3200 m, increases from 50 m to some hundreds of metres during glacier retreat, and it depends on geometry of slopes, anisotropy and in situ stresses. Results are compared to examples of deep-seated slope deformations from the Alps, which helps in the interpretation of such phenomena and in the understanding of their influence on valley evolution.

Abstract Deep-seated gravitational slope deformations (DSGSDs) are fairly common phenomena in the Eastern Italian Alps. The Celentino DSGSDs (Trentino–Alto Adige Region, Peio Valley) extends over an area of approximately 5 km 2 between 2400 and 1050 m a.s.l. (metres above sea level), involving metamorphic rocks. A set of parallel ridge-top trenches and antislope scarps are present in the upper part of the slope over a length of 2 km. The shear surface is inferred at 80–100 m in depth, and the involved volume is estimated to be 3.5×10 8 –4.0×10 8 m 3 . The displacement of the rock mass has diverted the Noce River for 200–250 m. Structural, geomorphological and engineering geology surveys were conducted both within and outside the DSGSDs. The evolution of the whole ridge has been modelled using the two-dimensional (2D) finite element code Phase 2 , simulating the unloading of the glacier cover and assuming a progressive damage of the rock mass. The numerical analyses demonstrate that continuum modelling can be used to examine the evolution of stresses, strains and plastic yielding, providing results consistent with field observations. Structural control due to brittle geological structures seems to be extremely important in the location, shape and extent of the DSGSDs, and post-glacial debuttressing an important predisposing factor in its development and triggering. The DSGSDs are common phenomena in areas of high relief energy, associated with orogenic history and mountain unroofing where tectonics is still active.

Abstract A cluster of exceptionally large sediment fans occurs in Val Venosta, a glacial trough in the east-central Alps, Italy. Its 59 tributary valleys generate 49 fans with volume:catchment area ratios varying across four orders of magnitude. Geomorphological and statistical analysis distinguish ‘allometric’ and ‘anomalous’ fans. Catastrophic massive slope failure origins are suggested for the anomalous cases. They comprise ‘outsize fans’ and ‘megafans’, the latter attaining 400 m cone height and 2700 m radius, and dominating the trough. Above most fans, evidence is found for source cavities of comparable volume. Reconstruction of the missing sides and heads of two tributary valleys reveals lost mountains 700 m deep. They are credible sources for the Malser Haide, a globally significant 11 km-long megafan with an estimated volume of 1650 Mm 3 , and the St Valentin outsize fans. Generally, anomalous fans occur where landslides are funnelled, comminuted and controlled through ‘debouchures’ high enough above the trough floor for conoidal deposition. Although sedimentological data are sparse, these fans may represent a new category of catastrophic slope failure outcome, mimicking conventional sediment fans of incremental origin. The Val Venosta cluster is the largest in the Alps, with concentrated glacial erosion in conducive geology among the possible factors explaining anomalous fan incidence.

Abstract Usually geomorphology, structural geology and engineering geology provide descriptions of slope instability in quite distinctive ways. This new research is based on combined approaches to providing an integrated view of the operative slope processes. ‘Slope Tectonics’ is the term adopted here to refer to those deformations that are induced or fully controlled by the slope morphology, and that generate features which can be compared to those created by tectonic activity. Such deformation can be induced by the stress field in a slope which is mainly controlled by gravity, topography and the geological setting created by the geodynamic context. The content of this book includes slope-deformation characterization using morphology and evolution, mechanical behaviour of the material, modes of failure and collapse, influence of lithology and structural features, and the role played by controlling factors. The contributions cover broad aspects of slope tectonics that attempt to underline a multidisciplinary approach, which should create a better framework for studies of slope instability.