Slope tectonics: a short introduction

Abstract

Geomorphology,structuralgeologyandengineeringgeology allow description of the main character-istics of a slope in distinct ways that can be com-bined to provide a complementary view of theoperative slope processes. The subjects presentedin this Special Publication include: slope mor-phology and evolution; mechanical behaviour ofthe material; modes of failure and collapse; influ-ence of lithology and structural features; and therole played by controlling factors. This Slope Tec-tonics volume comprises a series of very differentcontributions that attempt to underline a multidisci-plinary approach thatshould form the framework ofslope instability studies.Slope Tectonics is adopted in this volume tomeandeformationthatisinducedorfullycontrolledbytheslopemorphologyandthatgeneratesfeaturesthatcanbecomparedtotectonicfeatures.Thestressfield in a slope is the result of gravity, topographyand the geological setting created by an ensembleof geodynamic processes. Active tectonics (alsocalled neotectonics) generates a stress field thatcancontrolslopeprocesses;astrongfeedbackexist-ingbetween geological history, tectonics, lithology,geomorphological evolution and topography.As a consequence, a list of factors and theirrela-tive influence can be presented.(1) Fabric induced by a local stress field within aslope:† discontinuities and local faults with cata-clastic bands of variable thickness;† folds (Fig. 1), associated predominantlywith brittle structures;† complex failure paths (stepped or multi-surface);† localfailures:rockbridgefailuresorexten-sional failures (graben-like or pseudo-graben-like);† subsidence due to weak or soluble mate-rials causing complex sliding–topplingphenomena.(2) Reactivation of pre-existing faults, disconti-nuities, joints, foliations or rock anisotropies:† surfaces characterized by residual or lowerthan peak strength;† formation of composite failure surfaces.(3) Regional tectonic movements inducing newslope morphologies:† uplift;† major fault movement;† pull-apart zones:† folding.The boundary between classical tectonics andslope deformations, especially at a large scale, hasalways been indistinct as emphasized by Antoine(1988) (Schultz-Ela 2001). In his paper Antoinediscussed mechanisms like ‘diverticulation’ def-ined by Lugeon (1943) and Badoux (1963), wherepart of Pre-Alpine nappes were reversed in geo-metry by huge landslides inverting the stratigraphy(Antoine 1988). A remarkable geometrical analogyexists between basin extensional tectonics (Wer-nicke 1981) and certain landslide spreading inclays (Voight 1973; Varnes 1978; Hutchinson1988); this is despite the fact that Wernicke’shypothesis demonstrated that Basin and Rangeregions were not produced by huge landslides, butby low-angle faulting induced by geodynamic pro-cesses. Regional extension has produced changesin topography and, as a consequence, significantgravity-induced deformations.Therefore, we suggest that the term ‘slope tec-tonics’ is justified and must be recognized as animportant component in slope deformation. Slopeinstability implies movements driven by gravitythat can produce irreversible deformations. In thepast, slopes were viewed as privileged erosionzones (De la Noe & De Margerie 1888; Strahler1950), whereas few erosion processes were attribu-ted to landslides (Young 1972). Since the work ofSimonett (1967) and Hovius et al. (1997), the link