Review ArticleAn introductory review on gravitational-deformation induced structures, fabrics and modeling
Introduction
Geological structures and fabrics (i.e., spatial and geometric configuration or pattern of main constituent features), originated by gravitational slope failures, have been frequently omitted/underestimated by field geologists, and the recent literature presents a variety of such examples. PS-DInSAR analyses demonstrated that Alpine valleys are significantly affected by gravitational movements (e.g., Ambrosi and Crosta, 2011). Some studies suggest that commonly more than 5% of alpine slopes are affected by deep-seated gravitational slope deformations (e.g., Agliardi et al., 2009a, Agliardi et al., 2012, Agliardi et al., 2013, Pedrazzini et al., 2011). From these observations the following questions arise: what are the similarities between tectonics and slope tectonics at a local and at regional scale? Does a real limit between processes exist? In fact, very similar patterns can be found depending on the material involved, the size and type of slope instability, and the scale of observation (Fig. 1). Numerous studies demonstrated that folds or faults in a slope can be generated by large slope deformations, which have been confused with pure tectonics. Early works by Oulianoff and Badoux (1922), Zischinsky (1969), Nemčok et al. (1972), and Záruba and Mencl (1982) have shown that deformation of rocks can result from gravitational spreading of a valley slope. In addition, deformation within soft rocks (e.g., clay, marl) shows behaviors very similar to classical tectonics (Hutchinson, 1988). Gomberg et al. (1995) noted that the landslide slip surfaces and tectonic faults are analogues. Baroň et al. (2004) mapped folds within active, deep-seated rotational landslides in the flysch belt of Outer West Carpathians, which originated due to compression along active landslide blocks. The blocks had relatively well preserved original geological structure whereas the zones of compression underwent deformation analogous to tectonic nappes. Ortner (2007) and Alsop and Marco (2012) pointed out that the orientation of the fold axis in the case of gravity driven movements, is parallel to the strike of the slope, making sometimes possible to distinguish between tectonics or slope tectonic-driven structures. Slope tectonics is the result of the interaction between rock-mass properties, type of failure mechanism and factors driving to slope destabilization. External factors influence the behavior of any slope and are often connected to the recent history of the slope. Relief controls the stress level in the slope and this is required in order to generate a potential instability, but it is clear that rock mass strength and pre-existing structures can play a major role on slope instabilities (Ambrosi and Crosta, 2006, Ambrosi and Crosta, 2011). The origin of discontinuities affecting rock masses is diverse, and can owe to stratification, foliation, faults and fractures induced by tectonic deformation, exhumation, topographic stress, loading history, valley erosion, debuttresing and consequent release of valley slopes due to glacial retreat. Nevertheless, the rock can be damaged by tectonics or gravity induced stresses, with or without the support of water pressure reducing the effective stress, or by high differential stresses. According to Selby (1982), assuming a perfectly sound granitic rock (unit weight: 25KN/m3; uniaxial compressive strength: 200 MPa) with no discontinuity, the critical height of a vertical cliff would be 9 km, a value never observed in nature. Since the early works of Heim (1932), Ampferer (1939), Stini (1941), and Terzaghi (1963), many studies have analyzed the impact of type of slope deformation on hillslope processes. However, and except by a few exceptions (Agliardi et al., 2001, Agliardi et al., 2009a, Chigira, 1992, Hutchinson, 1988, Mahr and Nemčok, 1977, Nemčok et al., 1972), only some works have addressed this issue in the broad sense. The objective of this paper is to summarize the state of the art regarding the classification of slope movements with emphasis on slope scale rock instabilities, the main gravitational deformation-induced structures and fabrics that can be found in gravitational slope failures, as well as the numerical models and their limitation to reproduce such structures.
Section snippets
Classifications
In contrast to several classifications proposed to categorize and describe slope movements (Cruden and Varnes, 1996, Heim, 1932, Hutchinson, 1988, Varnes, 1978) based on different criteria, a consistent classification of the structure and fabric, produced by those processes, is still missing. A general agreement has been found around the classification proposed by Cruden and Varnes (1996), which is based on the type of movement (i.e., falls, topples, slides, lateral spread, flow and
Gravitational-deformation induced structures
The gravitational deformation in a slope results in characteristic morphologies, structures and fabrics, which generally depend on the type of the movement, its mechanism, and also on the relative position within the landslide body. The uphill part of the movement is generally characterized by the presence of extensional features (e.g., normal faults), the lower part by compressional ones as folds and reverse faults, whereas at the sides oblique tension cracks, transcurrent shear zones and
Scale and structures
The above described structures can be found at any scale depending on the material involved, and acting loads. This has been implicitly accepted by the scientific community using analogue models for describing and justifying Riedel shears (Riedel, 1929, Skempton, 1966) both at the micro- and macro-scale. Structures in analogue models of a few tens of centimeters can mimic some of the features typical of DSGSD (Fig. 5) (Bois et al., 2012, Carrea et al., 2012). As proposed by Chigira (1992), the
Importance of the inheritance of former instabilities
The geometric configuration and the geomechanical characteristics of inherited brittle structures (e.g., persistent bedding planes, foliation, faults, or regional joint sets) are frequently identified as important predisposing factors influencing the failure mechanism of slope instabilities at all spatial scales (Agliardi et al., 2001, Ambrosi and Crosta, 2006, Bianchi Fasani et al., 2004, Bouissou et al., 2012, Nemčok et al., 1972, Saintot et al., 2011, Sartori et al., 2003, Sauchyn et al.,
Stress, strain and progressive failure
An important question in the study of rock failures concerns how failure can propagate in a slope under which controlling factors, and how it is possible to monitor or recognize its evolution. To explain failure in some soil and rock materials, the progressive failure concept has been introduced for landslides by Bjerrum (1967), and the terminology was recently extended to hard-rock instabilities (Eberhardt, 2008, Eberhardt et al., 2004). Progressive failure can occur because of stress
Modeling
Modeling of slope deformations for different geometries and materials can be achieved by analogue or numerical methods (e.g., finite differences, FDM, finite elements, FEM, or distinct elements methods, DEM; Brideau et al., 2009, Eberhardt et al., 2004, Stead et al., 2006). Nevertheless, the application of each method depends on the required model resolution needed to simulate the relevant slope, material characteristics and fabrics. Nevertheless, computer power is progressively increasing
Discussion and conclusions
It has been shown that a great analogy exists between deformation induced by regional tectonics and that due to mass movements (e.g., Fig. 1). The limit between the two classes of processes is not clearly defined especially in the case of large slope failures, and their structures can be mixed at an outcrop scale (Fig. 4, Fig. 7). Gravity represents just one of the stress components in the upper crust, but many factors can add disturbances at a local and regional scale.
The landslide community
Acknowledgments
The authors kindly acknowledge the anonymous reviewers that contributed to improve the manuscript, all the participants for stimulating discussions during the Slope Tectonics Conference held in Vienna in 2011, and all the authors of this special issue.
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