Engineering Geology 104 (2009) 41–54

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Engineering Geology

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Geometry and kinematics of a landslide surface in tertiary claysfrom the Duero Basin (Spain)

M. Yenes a,⁎, S. Monterrubio a, J. Nespereira b, G. Santos c

a Departamento de Geología, Escuela Politécnica Superior de Zamora, Universidad de Salamanca, Avd. Requejo 33, 49022 Zamora, Spainb Investigación y Control de Calidad, S.A. Parque Tecnológico de Boecillo R-2, 47151, Boecillo, Valladolid, Spainc Departamento de Ingeniería Cartográfica y del Terreno, Facultad de Ciencias, Universidad de Salamanca, Plaza de la Merced s/n, 37008 Salamanca, Spain

⁎ Corresponding author. Tel.: +34 923 294488; fax: +3E-mail address: myo@usal.es (M. Yenes).

0013-7952/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.enggeo.2008.08.008

a b s t r a c t

a r t i c l e i n f o

Article history:

The Peñalba roto-translation Received 10 January 2008Received in revised form 28 July 2008Accepted 13 August 2008Available online 2 September 2008

Keywords:Roto-translational landslideSlip surfaceDeformation structuresTertiary claysDuero BasinSpain

al landslide is one of the large landslides that can be observed in the central area ofthe Duero Basin (Spain). Its presentmorphology is a semicircular-shaped depression inwhichmore than 6mofvertical movement have been measured. The erosion of the banks of the Duero River affecting the slope of thePeñalba Hill has incised vertical cliffs where a slip surface outcrops and can be observed. Currently, the incisionlevel of the Duero River is located 22 m below this surface. The slip surface, on the translational zone within alarge roto-translational landslide, developedwithin the Dueñas Facies in a single bedwith a high PI (39.0–47.8)and lower carbonate contents, would have behaved as low-shear strength surfaces (ϕ′R=18°–21.8°).At Peñalba, the slip surface and the deformation structures related to it have been exceptionallywell preserved.The structures observed are similar to those usually described in shear zones of tectonic origin: an S–C fabric,related to progressive simple shear, and C′ planes, called extensional crenulation cleavage. These similaritiessuggest that analogous kinematic processes would have taken place in completely different geodynamicenvironments. In the presentwork, wehave adopted a structural geological–kinematic approach to explain thedevelopment of the features in this roto-translational landslide.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Fossil landslides in soft continental deposits have attracted littleattention in the scientific literature, mainly due to the lack of pre-servation frequently observed in most cases, caused by subsequentmodifications of their original morphology, generally associated witherosive processes and hindering recognition in the field. In some casesfossil landslides have been recognized through location of the drainageanomalies that they caused (Mather et al., 2003), but the identificationof landslide slip surfaces is very difficult.

Recognition of landslide slip surfaces can be important for charac-terizing the kinematic processes that take place during landslidedevelopment because on them evidence of the origin of the instabilityand also its evolution may be preserved. Only when the slip surface orsurfaces are preserved is it possible to describe the different structurespresent on or near them in order to characterize the kinematics, tostudy the role of the materials involved, and to understand possiblereactivation mechanisms.

From the point of view of kinematics, landslide slip surfaces can beconsidered as shear zones where movement-related structures arerecorded (Wen and Aydin, 2003). From the structural geology point ofview, shear zones are tabular volumes of rocks where deformation is

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more intense in comparisonwith the surrounding rocks, which shouldnot have undergone coeval internal strain. They may be subdividedinto (1) brittle shear zones, with discontinuity surfaces along whichthe movement takes place; (2) ductile shear zones, with ductilecontinuous strain along the shear zone and strain compatibility withthe surrounding rocks; and (3) brittle–ductile shear zones, whereductile shear zone features appear accompanying discontinuityelements, such as faults or vein systems (Ramsay and Huber, 1987).Generally, ductile shear zones within the Earth's crust are related tohigh pressure and temperatures, whereas in upper structural levels,near the Earth's surface, rocks usually behave in a brittle manner.

Nevertheless, there are some cases in which the rheology of thematerials involved plays a crucial role in shear zone development. Onshallow landslides, the main factors governing the deformationpattern developed in the sliding surfaces are not only pressure andthe temperature, but also factors that control the rheology, such asthe moisture content and the plasticity of the materials. For example,according to Arch et al. (1988) small differences in the moisturecontent strongly affect the rheology, causing deformation rates thatvary by several orders ofmagnitude. Hence, on shallow landslideswithlow-plasticity dry material in the sliding zone the deformationwill bebrittle, whereas on high-plasticitywetmaterial it will be ductile (Larueand Hudleston, 1987).

Additionally, the failure mechanisms on shallow landslides mayinvolve pure shear, simple shear, or a combination of both, depending

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on the type of slide (rotational, translational, etc.) and on the positionof the sliding surface within it. Thus, because deformation mechan-isms vary the microstructures on the sliding surface will changeaccording to the mechanical behaviour of the materials (brittle orductile), and to the type of deformation (pure shear and/or simpleshear).

Several authors have addressed structures developed in laboratorymodels simulating shear zones with different soils and rocks.They have been tackled from two different points of view: (1) usinggeotechnical techniques and parameters (e. g., Klukanova andModlitba, 1990; Dewhurst et al., 1996; Frost and Jang, 2000; Takizawaet al., 2005), (2) and using structural geology techniques (e. g.,Maltman, 1987; Arch et al., 1988; Clausen and Gabrielsen, 2002).Nevertheless, there are few references to the structures in naturallandslides (e.g., Morgenstern and Tchalenko, 1967; Larue and Hudle-ston, 1987; Fleming and Johnson, 1989; Chagira, 1992; Baum andFleming,1996; Anson and Hawkins,1999;Wen et al., 2001; Fearon andCoop, 2002; Wen and Aydin, 2003; Mather et al., 2003; Kawamuraet al., 2007), mainly due to the difficulties in locating and samplingpresent-day or fossil landslides where these structures have beenpreserved. One of the main reasons behind this phenomenon is thedilatancy behaviour commonly detected in some landslide slipsurfaces, which increases the porosity as the movement takes placeand hence increases the moisture content (e. g., Takizawa and Ogawa,1999; Anson and Hawkins, 1999; Fearon and Coop, 2002; Takizawaet al., 2005). When successive reactivations take place, the deforma-tion mechanism will change from brittle or ductile shear to plasticflows, destroying all previously generated structures (Picarelli, 1993;Wen and Aydin, 2005; Fukuoka et al., 2006).

This work focuses on study of the Peñalba roto-translationallandslide, located in the central area of theDuero Basin (Spain). Erosionof the banks of the Duero River affecting the slope of the Peñalba Hillhas incised vertical cliffs where a slip surface outcrops and can beobserved. Currently, the incision level of the Duero River is locatedapproximately 22 m below this surface (Fig. 1). This situation permitsthe observation of and hence accessibility to this landslide surface.It should be noted that on this landslide the deformation structuresare exceptionally well preserved, allowing study of the deformationmechanisms involved along the process of destabilization. Our effortshere focus on a detailed description of the geometry and kinematicsof this roto-translational landslide and a comparison of these obser-vations with those obtained from a theoretical kinematic model, de-scribed below.

2. Geological setting

Most of the area of the northern part of the Iberian Peninsulais occupied by vast tertiary basins (Fig. 1A), the Cenozoic Duero basin(97603 km2) being the largest one (Alonso-Gavilán et al., 2004). It isan endorrheic basin whose margins comprise alluvial fan depositsdeposited during the Paleogene, with lateral changes to fluvial facies,and lacustrine and palustrine facies towards the centre of the basin(Picart and Wouters, 1997). The end of the endorrheic sedimentationculminates with a carbonate formation: the Calizas del Páramo (UpperMiocene). The basin fill has a horizontal bedding and its present-daygeomorphological configuration is due to the opening of the endor-rheic basin, probably during the Upper Miocene (Santisteban et al.,1997), triggering the beginning of the draining fluvial net incision,and resulting in the generation of a carbonate plateau that occupiesthe highest areas, whereas the transition to the valleys is through theCuestas slopes.

From bottom to top the litoestratigraphic facies are (Fig. 1):Dueñas Facies (Middle-Upper Miocene). This outcrops between

700 m (the level of the Duero River) and 750–760 masl, showing avisible thickness of 50–60 m since its lowest part does not outcropin the area. Levels of clays, marly clays, marls and macrocrystaline

gypsum are the predominant lithologies. In the clays, smectiteand illite are the most abundant minerals, with also kaolinite andpalygorskite in lower proportions.

Tierra de Campos Facies (Middle Miocene). Silts, sands and clays,with some sandy and gravelly levels. Their ochre colour is easilyrecognizable between the grey, green and white colours of the otherlithologies. In the central part of the basin the facies has a thickness of30 m, decreasing towards the east down to 10 m in the Peñalba area.The clay content is very low, mainly illite and kaolinite as well as sometraces of chlorite being found. In the sandy levels quartz and feldsparare dominant. The carbonate content is below 10%, in contrast withthe 40–50% in the other lithologies.

Cuestas Facies (MiddleMiocene). This consists of marls, marly clays,gypsum, limestones and dolostones, reaching more than 80 m thick.Organic matter levels are frequent in the transition zone to the Tierrade Campos Facies.

Calizas del Páramo (Vallesiense: Upper Miocene). These materialsare composed by limestones, marly limestones and marls, 5–15 mthick. They are the carbonate levels that form the calcareous platformsof the “páramos”.

3. Strain distribution of the roto-translational landslides

Roto-translational landslides are classified as complex movements(Varnes, 1978) since they are a combination of two of the principallandslide types: at the head of the landslide, the movement isrotational (the failure surface is concave-upward), whereas at the footit is translational (the sliding surface can be considered planar orslightly undulated). These slides have been described in differentgeodynamical contexts (e. g. Fiorillo, 2003; Corominas et al., 2005; DelValle and Bonorino, 2006), and they are especially common in tertiarybasins not affected by large deformations and in which the layers aresubhorizontal, as is the case of most of tertiary basins on the IberianPeninsula (Duero, Ebro, Tajo).

In a roto-translational landslide, two different zones can be con-sidered: the depletion zone, where a decrease in the sediment load hastaken place, and the accumulation zone, where the transportedmaterials have been deposited. Regarding deformation in the landslideslip surface, there is also a zone where the failure mechanism isrotational and another one where the mechanism is translational(Fig. 2). In the zone where the failure mechanism is translational,an accumulation of sediments takes place, whereas erosion can act inthe translational and also in the rotational zone.

On the rotational zone, the slip zone of landslides can be consideredas a kind of normal fault developed under low-pressure and low-temperature conditions (Wen and Aydin, 2003), in which the type ofstructure generated depends on the material's plasticity, particle sizeand themineralogy of the clay fraction, among other factors. There arethree different types of structure found on these surfaces: (1) a clearfault surface and striation, (2) brecciated fault, and (3) a clayey breccia,such as the ones described in Shuzui (2001). Nevertheless, in mostcases the study of good-quality outcrops in this zone is always difficultbecause the non-deformed rock is only accessible at the cliff, whereasthe fault rocks have been weathered and eroded and the relicts havebeen displaced by gravity to the accumulation zone.

On the translational zone, the structures depend on two mainfactors: the spatial location within the landslide and the rheology ofthe materials.

The spatial location will determine the type of shear strain: pure,simple or a combination of both (Webb and Cooper, 1988). As shownin Fig. 2, on the translational zone there are three different types ofdeformation, which allow three subzones to be differentiated:

1. Simple shear+extensionparallel to the slip surface: the placewherethe translational fault mechanism begins. The load reduction dueto sediment removal towards the accumulation zone causes the

Fig.1. (A) Regional setting of the Peñalba landslide within the geological context of the Duero Basin. The locations of other landslides are shown by black spots. (B) Simplified geological map of the Peñalba landslide, onwhich the outcrop of theslip surface has been marked. (C) Geological cross section of the landslide (location in B) in which different overlapping instabilities are marked. 43

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Fig. 2. (A) Section for a hypothetical roto-translational landslide. In the translational zone the deformation mechanism changes along the entire surface (see text for further details).(B) Finite strain ellipsoid for simple shear (after Twiss and Mores, 1992). (C) Evolution of a subhorizontal marker (So) according to the subzone where it is located within thetranslational zone.

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simple shear mechanism to act at the same time as the horizontalextension.

2. Simple shear+translation parallel to the slip surface: in the inter-mediate part of the translational zone the sediment load over theslip surface does not change along the process and this determinesa simple shear deformation mechanism.

3. Simple shear+contraction parallel to the slip surface: this takesplace on the end of the translational zone, where the materialaccumulates. The deformation mechanism is a simple shear en-closed by a horizontal contraction. As stated by Gutiérrez-Alonsoet al. (1999) “regional extension can lead to localized contraction,and hence the development of local thrust geometries”.

Fig. 3. Scheme of elements disposition in a shear zone. (A) Riedel shear zone. (B) S–Ctype ductile shear zone. (C) Brittle–ductile shear zone.

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In all three subzones, the strain is discontinuous and a disconti-nuity surface on the bottom of the landslide accommodates theprogress of the deformation (Sanderson, 1982).

Fig. 2B depicts the different areas that are described in the finitedeformation ellipses for simple shear (Twiss andMores, 1992), includ-ing stratification (So) as a subhorizontal marker: (a) the LL sector:the finite deformation is positive and, all markers would becomeelongated; (b) the SS sector: the finite deformation is negative andthe markers in it will always be shortened; (c) the SL sector: the linesare shorter than originally, but in the final stage they have becomeelongated, and therefore boudinated folds can be recognized.

In the “simple shear+extension” subzone, So would be in theLL sector, under an extensional deformation; in the “simple shear+translation” subzone, So would meet the non-finite longitudinaldeformation lines, and therefore the bedding would not be deformed,and finally in the “simple shear+contraction” subzone So would beshortened in its final length. As a consequence, considering the de-formation on the horizontal marker at the end of the process it ispossible to establish on which sliding subzone the observations arecarried out (Fig. 2C).

Fig. 4. Idealized scheme of a shear zone from the translational section of a landslide. It is showductile fashion) and tensional cracks (T) changing their spatial position from Subzone 1 (simshear+contraction).

Regarding the rheology of the materials, in some cases the shearstrain on sliding surfaces has been described as the Riedel brittle sheartype. Nevertheless, on the translational zone the landslide movementpasses through a high-plasticity material with a highmoisture contentin most cases; under these conditions, the deformation is generallyductile or brittle–ductile, and S–C shear zones (Wen and Aydin, 2003)or foliated cataclasites (Larue and Hudleston, 1987; Shuzui, 2001) willbe created.

Regarding a Riedel shear zone (Fig. 3A), several slide surfaces maydevelop (Davis et al., 1999). One of the most important of these areR shears, synthetic to the general movement and forming a 15° angle(ϕ /2, being ϕ the angle of internal friction) with the shear zoneboundaries. R′ shears are antithetic to the general movement anddevelop at 75° (90−ϕ /2) to the boundaries. The progressive evolutionof the shear zone can generate a secondary synthetic shear system,known as P shears. Surfaces parallel to the general trace, Y shears,have also been described. Finally, tensional faults (T) can be generatedforming an angle of approximately 45° to the shear axis and parallelto the direction of the highest principal stress (σ1).

S–C type ductile shear zones (Fig. 3B) are usually developed underhigh-pressure and high-temperature conditions, creating specialrocks called S–C mylonites. These are foliated rocks with evidence ofhigh ductile deformation and contain fabric elements with asym-metric shapes (Passchier and Trouw, 1996). This kind of shear zonegenerates several foliations (mineral grain flattening or elongation)(van der Pluijm and Marshak, 2004): the S foliation, oblique to thegeneral trace of the shear, is usually the most developed one; the Cfoliation, mainly parallel to the boundaries of the shear zone, re-presents small microshears synthetic to the principal shear; finally,the C′ foliation, oblique to the boundaries and called extensionalcrenulation cleavage, reflects an extensional component in the finitestrain ellipsoid. Moreover, in a ductile shear zone it is possible to findother indicators of the shear sense, e.g.: the deviation of markers fromtheir original position towards different degrees of obliquity to theshear trace (1 in Fig. 3B), asymmetric folds, (2 in Fig. 3B), and rotatedclasts with type σ asymmetric tails (3 in Fig. 3B).

Finally, when brittle–ductile deformation (Fig. 3C) takes place theresult is the presence of rocks called foliated breccias or melange(Larue and Hudleston, 1987), or clayey breccias (Shuzui, 2001). Larueand Hudleston (1987) describe the foliated breccias as stronglyfoliated clays and embedded fragments become extended and brokeninto angular blocks which float and are rotate in the clayey matrix.The deformation in the clayey matrix displays foliations similar tothose obtained under ductile conditions (S, C and C′). Depending onthe nature and physical properties of the materials involved, theymayalso behave in amore brittle way and the C planes could be consideredas Y Riedel shears, while the C′ planes would be the R Riedel shears.

The surface types described above do not have a fixed locationalong the translational zone related to the slip surface limit, since thestrain ellipsoid changes progressively (Fig. 2). In Fig. 4may be seen thatin Subzone 1 (“simple shear+extension”), if the material behaves in a

n as the shears (Riedel R type if the material has brittle behaviour, or C′ if it behaves in aple shear+extension) to Subzone 2 (simple shear+translation) and Subzone 3 (simple

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brittle fashion R Riedel shears appear, while if the behaviour is ductile,C′ shears are generated. In Subzone 2 (“simple shear+translation”)the situation is the same, with only a change in orientation owingto the rotation of the strain ellipse. Finally, since the strain ellipsecontinues to rotate, in Subzone 3 (“simple shear+contraction”)ductile or brittle planes would appear with an inverse movement.Muller and Martel (2000) reported the same strain ellipsoidorientations on applying the “fracture mechanics theory”, and Savageand Wasowki (2006) discussed the evolution of normal listric faultswhere flow is extensive and inverse faults where it is compressive,using the “Coulomb plastic flow model” for theoretical slides. It isimportant to note that owing to the orientation of the strain ellipsoidin Subzone 1 the tensional fractures (T) would be vertical, allowingthe water flow enter the landslide slip surface, and triggering itsreactivation; as the strain ellipse rotates, the tension fractures movetowards a more subhorizontal disposition, causing the self-arrestof the slide.

Fig. 5. Scheme of the evolution of the Peñalba slope. (A) Initial stage. (B) The river, in the fitttranslational landslide that displaces the river course. Firstly, the translational zone developskeeps fitting and a new incision on the slope causes vertical cliffs due to toppling, leavinglandslide takes place, displacing again the course of the river.

4. The Peñalba landslide

The Peñalba landslide is located in the central area of the Basin,in the Duero River Valley, in the context of arid continental climaticconditions and an annual rainfall below 500 mm. The Duero Rivercrosses area from east to west incised 160 m into the tertiary deposits(Fig. 1).

Near the study area, several recent landslides have been identified(Yenes et al., 2001). They are linked to one of the main Duero Rivertributaries, the Pisuerga River Valley (west of the area), and also tothe Duero River Valley, where large multiple landslides have beendeveloped (Peñalba, Sardón, La Curva, Los Canónigos and Valdelas-monjas; Fig. 1A), usually affecting theMiocene sediments between theupper cliff, formed by the Páramo limestone, and the bottom of thelandslides in the Dueñas Facies.

The generation of recent large landslides close to the Duero Riverhas been determinedby the interaction of several factors (Monterrubio

ing process, reaches a high-plasticity and low-shear strength layer, causing a large roto-above the clay level, and later on it keeps going above the river flood plain. (C) The riveroutcrops of the surface of the first landslide. (D) Finally, a recently a small rotational

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et al., 2001): (a) the erosion level of the river, located 160 m below thePáramo surface; (b) the strong slope created by the progressive lateralerosion of the river banks; (c) low-shear strength on the Dueñas Faciesclay levels; and (d) occasional intense precipitation events.

Sometimes, these recent landslides can be considered as “active”,whereas in others they can be considered as “dormant”, according tothe adaptation of standard landslide classification based on activityproposed by Mather et al. (2003).

The Peñalba slope (Fig. 1) is located on the convex side of ameander in the Duero River. Between the 705 masl, at which the riverflows, and the more than 850 masl at the Páramo surface, it has anaverage slope higher than 50%. On it, different landslides have becomesuperimposed over one another over time (Fig. 5). First instability ofthe slopewould take place when the fitting of the Duero river reacheda low-shear strength layer; at this moment, a large roto-translationallandslides occurs, moving forward firstly the slided mass abovethe subhorizontal clayey layer; once going over the river, it wouldmove above the flood plain clayeymaterials (Fig. 5B). Nowadays, in theDuero Basin large roto-translational landslides are being generated,following the same evolution (Yenes et al., 2008). Subsequently, thefitted of the river would keep going up to its current level. The incision

Fig. 6. (A) General view of the Peñalba slope from the south. The scarps mapped in Fig. 1 are s(C) Detail of the landslide surface (red); beneath the surface, the bedding is subhorizontalductile shear zones can be seen. (D) Lateral view of the landslide from the east. The slip surfground. Note the brittle behaviour of the layers with echelon fractures. (E) Detail of the la(For interpretation of the references to colour in this figure legend, the reader is referred to

of the river on the slope causes cliffs originated by toppling (Fig. 5C),discovering a horizontal surface, located 22 m above the river level(727 masl), bellowing to the translational zone of the first large roto-translational landslide. This situation favours unusual observation andaccessibility conditions for these types of structure. Finally, to thewestside of the studied surface, a small rotational landslide occurs, movingthe current course of the river (Fig. 5D).

The chaotic deposits located just above this surface have beeninterpreted as being related to the synsedimentary fracture movement(Picart and Wouters, 1997). The existence of synsedimentary faultsnecessarily implies that above the deformation structure new sedi-ments without deformation should appear, but this is not the case inPeñalba, where only slipped materials appears. On the other hand, theexistence of synsedimentary faults has not been found anywhere else inthe Duero Basin; the geologic context with the filling of an endorheicbasin does not favour their generation. However, once the basin wasfilled, and from the moment it became exorheic, starting the emptyingof the basin, high and potentially unstable slopes were generated,which explains the widespread distribution of such landslides.

Therefore, here they are reinterpreted as having been caused bya roto-translational landslide perhaps generated during the drainage

hown. (B) Front view of the Peñalba slip surface that emerges 22 m above the river level.and parallel, whereas the layers above are more disrupted. An oblique fold typical oface is no longer horizontal, and has gradually turned upwards to cut the surface of thendslide surface. Grooved marks indicate the direction of movement N195/horizontal.the web version of this article.)

Table 1Topographic survey

Station points Points X Y Z

P1 375156.04 4608361.15 733.44P60 375441.51 4608548.81 707.33P61 374087.93 4608027.94 719.94N19 375041.00 4608471.91 749.89N20 375055.32 4608477.00 749.62

P2 375393.99 4608562.21 734.23N35 375407.24 4608586.71 744.85N37 375234.35 4608512.46 751.15

P3 375478.05 4608612.66 744.63N29 375789.02 4608484.03 753.70N31 375556.62 4608624.43 751.78

P61 374087.93 4608027.94 719.94N47 373922.74 4608369.45 749.34N49 373755.49 4608355.76 748.53

Station points and points radiated from them. X and Y in UTM ED50 coordinates andZ in metres above sea level at Alicante, Spain.

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of the Basin. This interpretation is supported by topographical,geomorphological, structural and geotechnical evidence, addressedbelow.

The Peñalba slide (Fig. 6) shows different movement features atdifferent scales, allowing it to be stated that a mass movement hasoccurred in the zone, and also making it possible to establish itsdirection and sense. The stratification below the sliding surfaceremains subhorizontal and parallel, while above it, the layers have achaotic aspect (Fig. 6B). Nevertheless, in the zone closest to thelandslide slip plane, stratification surfaces are preserved, allowing thedefinition of an oblique fold (Fig. 6C), characteristic of the frontal zones

Fig. 7. Topographic survey. (A) Geological sketch map of the Peñalba slipped area with meacoordinates. (B) Cross section showing the lower part of Tierra de Campos Facies with the sasharp jump at slide area.

in ductile shears (Passchier and Trouw,1996). This cylindrical fold overductile shear zones has its axis parallel to the stretching lineation,indicating the direction of the movement. The sliding surface showsfriction grooves (Fig. 6E), developed as a consequence of the frictionbetween the most competent particles over the ductile clay. Onanalysing these features, a N195E/horizontal orientationwas obtainedfor the movement direction. At the eastern end of the landslide(Fig. 6D), the slip surface is no longer horizontal and gradually turnsupwards to cut the surface of the ground. This eastern end showsbrittle behaviour in the layers determined by step fractures (echelonfractures) commonly observed on the slide limbs (Sowers and Royster,1978; Fleming and Johnson, 1989; Muller and Martel, 2000). As seenin the figure, the synthetic normalmovement of these fractures agreeswith the general movement of the landslide.

4.1. Topographical measurements

As stated above, the horizontal surface at Peñalba located 22 mabove the current river level is reinterpreted here as having beencaused by a landslide probably generated during the drainage of theBasin. On this basis, together with observations of the landslide sur-face, that considered later, and the preserved geomorphologic features(semicircular depressed area, lateral terminations, etc.), a detailedtopographic survey was performed in order to gain a detailed geo-graphic frame with several control points at the bottom of the Tierrade Campos Facies, both inside and outside the landslide mass, with aview to obtaining objective data for calculating the vertical displace-ment of the landslide. The bottom of the Tierra de Campos Facieswas chosen because it has a very clear and continuous trace alongthe outcrop and is a sedimentary discontinuity limit onwhich there is

suring points and location of radiations performed. Geographic reference in UTM ED50me horizontal scale used in A. Note the gradual reduction in height to the west and the

Fig. 8. Sampled profiles: A, area around the slip surface and B, the same beds just to the east, outside the slipped area. At top, bar diagrams of carbonate contents and plasticity index for the two profiles. The samples maintain their relativeposition and the slip surface has been marked in the A profile. To the left, diagram of percentage of carbonates compared to the plasticity index of the outer profile (B). Samples were taken in an alternation of green and red clays (from samplePEX-6 until PEX-8). A decline in plasticity with increasing carbonate contents can be seen.

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Table 2Sample test results (see location in Fig. 7)

Sample Carbonates Atterberg limits Classification

LL PL PI USCS AASHTO

PEX-1 14.96 79.6 50.4 29.2 MH A-7-5PEX-2 33.43 72.7 33.4 39.3 CH A-7-5PEX-3 36.44 55.7 27.6 28.1 CH A-7-6PEX-4 13.48 57.2 35.9 21.2 MH A-7-5PEX-5 54.32 64.8 28.5 36.3 CH A-7-6PEX-6 34.1 68.6 27.5 41.1 CH A-7-6PEX-6B 60.6 54 23.4 30.6 CH A-7-6PEX-6.5 33.09 83.3 31.0 52.3 CH A-7-5PEX-K 55.9 66 25.4 40.6 CH A-7-6PEX-L 62.4 64.8 28.3 36.5 CH A-7-6PEX-7 41.96 80.3 33.1 47.2 CH A-7-5PEX-M 72.0 62.5 27.7 34.8 CH A-7-6PEX-7B 59.1 72.7 28.0 44.7 CH A-7-6PEX-8 27.67 73.9 34.1 39.8 CH A-7-5PIN-1 95.3 30.1 17.9 12.2 CL A-6PIN-2 75.6 44.5 28.9 15.6 ML A-7-6PIN-3 44.1 88.0 38.6 49.4 CH A-7-5PIN-4 55.2 33.5 20.6 12.9 CL A-6PIN-5 23.7 36.3 20.5 15.8 CL A-6PIN-6 27.2 59.5 23.1 36.4 CH A-7-6PIN-7 29.7 64.0 25.0 39.0 CH A-7-6PIN-7B 43.0 72.1 24.3 47.8 CH A-7-6PIN-8 44.1 66.0 26.4 39.6 CH A-7-5PIN-9 39.8 69.0 25.5 43.5 CH A-7-5M-1 23.2 61.0 29.0 32.0 CH A-7-5M-2 49.8 56.0 29.8 26.2 CH A-7-6M-3 42.7 44.5 24.9 19.6 CL A-7-6

Carbonates: carbonate contents inpercentages. LL: liquid limit. PL: plastic limit. PI: plasticityindex. USCS: Unified Soil Classification System. AASHTO: Soil Classification of theAmericanAssociation of State Highway and Transportation Officials.

50 M. Yenes et al. / Engineering Geology 104 (2009) 41–54

an abrupt transition from the lacustrine sediments at the top of theDueñas Facies to the fluvio-alluvial deposits of the Tierra de CamposFacies (Picart and Wouters, 1997).

Since 2002, several topographic measurements have been takenat selected control points in order to monitor present-day movementson the Peñalba slope (Santos et al., 2005). Previously a control gridin UTM ED50 coordinates was established, constructed by geodeticmeasurements carried out with a double-frequency GPS, Leica GPSSystem 530 model with an AT 502 antenna, in relative static posi-tioning. Using the grid data, and performing radial surveys using atotal station (Leica TCR 307), significant points were located. Table 1shows the radiated points indicating the station point from whichthey were radiated, and also the resulting altitude. The final resultshave centimetre precision.

From the measurements taken at the furthest points E and W ofthe landslide area (N29, located E at an altitude of 753.70 m, and N49,to the W at an altitude of 748.53 m), it can be seen that the altitude ofthe lower surface of the Tierra de Campos Facies decreases from E toW(Fig. 7).

Closer to the slipped area, several measurements were made onthe lower surface of the Tierra de Campos Facies. The data obtainedindicate that the altitude at points N31 and N37 differs only by 63 cm,with an average altitude of 751.47 m, these points being outside thelandslide mass, one on either side, whereas at point N35 the 744.85 maltitude is inside the landslide (Fig. 7). It may therefore be concludedthat the slide caused a vertical displacement of 6.62 m at the pointmeasured.

4.2. Geotechnical properties

The slip surface studied on this paper belongs to the translationalzone of a large roto-translational landslide. On this translationalzone, the movement has been developed at the upper part of redand green marly clay, with a thickness of 5 m. In order to study thedifferent levels involved in the generation of the landslide, twodifferentprofiles were sampled to characterize the geotechnical properties ofthe soils involved (from the geotechnical point of view, the tertiarysediments involved in this paleolandslide are soils): the first one acrossthe slide surface (A in Fig. 8) and the second one in equivalentmaterialsoutside the landslide (B in Fig. 8). Sampling in the former includedsamples taken several metres below the landslide slip surface (P-1, P-2and P-3), four more samples collected several centimetres below theslip surface (PIN-7, PIN-7B, PIN-8 and PIN-9), and another six ofdifferent lithologies from several centimetres above the surface (PIN-1,PIN-2, PIN-3, PIN-4, PIN-5 and PIN-6). The same levels were analysedoutside the landslide, in an attempt to identify the lithology responsiblefor the generation of the landslide (PEX-6 to PEX-8).

The tests carried out in all the samples were (1) carbonate con-tent determination using the Calcimeter Bernard Method (UNE103200:1993: AENOR, 2002) and (2) Atterberg Limits (UNE 103103-1994, UNE 103104:1993: AENOR, 2002). Two soil classification sys-tems were used: the Unified Soil Classification System (USCS), andthe AASHTO Soil Classification System, developed by the AmericanAssociation of State Highway and Transportation Officials (ASTM D2487-83 and ASTM M 145, respectively). The results obtained areshown in Table 2. According to these, in both areas high-plasticityclays (CH group, USCS; clayey soils of A-6 and A-7 groups, accordingto the AASHTO) are the most abundant materials. Both areas aretherefore favourable to the initiation of landslides.

It is interesting to note that the carbonate content displays aninverse correlationwith the plasticity: thus, as the carbonates decreasethe plasticity index (PI) increases. It may therefore be speculatedthat the carbonates would act as a plasticity inhibitor, as stated inother studies (Hawkins andMcDonald,1992; Bouazza and Ghili,1999).In marly soils it has been reported that when the carbonate content ishigher than 25%, thewater absorption capacity of the clays is inhibited

(El Amrani Paaza et al., 2002). In this sense, in Fig. 8 samples of red andgreenmarly clay from the profile outside the landslide are representedon a PI vs carbonates plot (PEX-6 to PEX-8) and the tendency of PI toincrease as the carbonate content decreases can be seen.

In light of the above, it may be expected that the paleolandslidewould have been generated in an event involving a clay layer with alow carbonate content. Regarding the results outside the landslide,sample PEX-6.5 is the best candidate since it displays 34% of carbonates(lower than the surrounding samples) and has a PI of 52.3 (the highestvalue obtained in all the samples). Sample PEX-6.5 was collected in ared clay level located in the upper part of thick clayey strata, so it ispossible that this material could have been the factor controllingthe paleolandslide. Within the landslide, there is greater variability inthe values obtained, but it is noteworthy that the highest PI valuescorrespond to the levels around the landslide slip surface, mainly insamples located below: PIN-7: 39.0, PIN-7B: 47.8, PIN-8: 39.6 and PIN-9: 43.5. Therefore, the red clayey samples (PIN-7, PIN-7B and PIN-8)would be equivalent to sample PEX-6.5, indicating that the beds underthe paleolandslide slip surface, with a high PI and lower carbonatecontents, would have behaved as low-shear strength surfaces, abovewhich thewhole landslidemass would have moved, whereas the bedsabove would have behaved in a ductile, brittle or brittle–ductile waywhile sliding, in agreement with their plasticity properties. A moredetailed description of the behaviour in the upper beds is discussed inthe following section.

Low-shear strength clay levels within the Dueñas Facies have beenobserved previously (Monterrubio et al., 2001). These authorsobserved that most of the landslides located in the Duero Basin areof the roto-translational type on which the slip surface in the trans-lational zone develops on a low-shear strength level in the DueñasFacies, as is the case in the Peñalba landslide. This level is very difficultto sample, and it is not always possible to gather unaltered samples tocarry out the laboratory tests required to obtain strength parameters.Because of this, the samples used to characterize the mechanicalparameters of the clay level were collected in the excavations for

51M. Yenes et al. / Engineering Geology 104 (2009) 41–54

the foundation of a mini hydro-electric power station in the nearbyarea of Reinoso del Cerrato. This site, also in the Duero Basin, presents asimilar geological context to that of Peñalba, with an active landslideinvolving the same facies. Block samples from the detachment zoneof the slide were collected and trimmed at the laboratory to performdirect shear tests on the same rupture surface as the one in the land-slide. Consolidated and drained direct shear tests were done withnormal stresses of 150.7 kPa, 298 kPa and447.7 kPa,with a shear rate ofN0.0005 mm/min, suitable to ensure effective stresses to be measuredand therefore to obtain the effective strength parameters. Graphicrepresentation of the tests (Fig. 9) indicates a cohesion of 102.5 kPa anda peak angle of friction of 25°. After shearing the twofirst samples, theywere sheared manually several times to ensure reorientation of theminerals on the shear plane thus creating the residual conditions of thesoil. Each sample was sheared three or four times with increasingnormal stresses, obtaining a null residual cohesion and a residual angleof friction varying between 18° (sample with 150.7 kPa of normalstress) and 21.8° (sample with 298 kPa of normal stress).

4.3. Microstructures

Because of the special mechanical properties of the slipped sedi-ments, they have developed a plethora of microstructures that re-semble those produced in shear zones undermetamorphic conditions.This microstructural convergence indicates a kinematic link betweenthe processes involved in the development of this kind of structure,regardless of the geodynamic context, the P–T conditions, the rate ofdeformation, and the nature of the materials involved.

As previously mentioned on discussing the kinematics of roto-translational landslides, at the bottom of the landslide slip zone adiscontinuity must be created in order to accommodate the wholeof the deformation during the progress of the movement. At Peñalba,this discontinuity can be seen on a cliff (Fig. 6E), below which a high-plasticity red clay is located (samples PIN-7, 7B and 8). This levelwould have acted as a low-shear strength surface over which all thesediments became mobilised. The deformation structures preservedbelow the slip surface are scarce (only some slickensides developed indirections coherent with the general movement of the landslide).

Fig. 9. Results of consolidated an

It is just above the discontinuity surface, in the clay level, wherefriction striations developed, as seen in Fig. 10A. In the same figure,some grooves due to large, more competent particle motion (carrot-shaped marks in the terminology used in Doblas et al., 1997) can alsobe seen. These kinematic markers can be used to establish the direc-tion and sense of the movement: the striations will show the direc-tion, and the asymmetric shape of the grooves will indicate the sense(according to the position of the less incised side).

Above the striated level, there are several light-coloured clayeymarls, red clays, green clays and some levels that are dark coloured,owing to their organic material content. All of them appear sub-horizontally and parallel outside the landslide, but inside the slippedmass they are mixed, folded, in the case of the more ductile levels(clay), or fractured, in the brittle ones (marly levels). In greater detail,some deformation structures appear, indicating (a) the deformationmechanism that acted when they were generated, (b) the mechanicalbehaviour of the materials and (c) the site of observation on thetranslational zone of the landslide.

Among these structures, the existence of marly or gypsum clasts,with a more brittle behaviour than the clayey matrix where they areembedded (Fig. 10B), should be noted within the matrix. A foliationwas generated as a consequence of the landslide and it appearssurrounding the clasts in a σ-like shape, similar to the porphyroclastshapes in ductile shear zones. In classical structural studies of thesezones, these σ-like shaped objects have been used as tools to identifythe shear sense. In landslides such as the one at Peñalba they can beused to confirm that the deformation mechanism acting when theywere triggered was a simple shear with a relative displacement ofthe upper part toward the south, which is in accordance with thelandslide movement observed on the outcrops.

In strongly deformed areas, two foliations resembling the S–C typefrom ductile shear zones can be seen (Fig. 10C). The C foliation, parallelto the landslide slip surface boundaries, is conformed by smallerslipped surfaces coherent with themain one, whereas the S foliation isoblique to the principal plane of the landslide.

In the Peñalba slip surface exceptionally well preserved deformationstructures have been found, indicating an extensional component inthe finite strain ellipsoid. In Fig. 10D, planes similar to the C′ planes in

d drained direct shear tests.

52 M. Yenes et al. / Engineering Geology 104 (2009) 41–54

53M. Yenes et al. / Engineering Geology 104 (2009) 41–54

ductile shears or Rplanes in Riedel brittle shears can also be seen, aswellas the two oblique foliations, characteristic of ductile shears (C and S).

As stated above, on the more ductile levels a very penetrative andwell preserved foliation has developed. In order to carry out a detailedstudy of themineralogy and the deformation structures created duringthe movement, several thin sections in the XZ planes were studied(X, direction parallel to the longest principal axis, and Z normal to it).In polarized light microphotographs (Fig. 10E), the foliation is in-terpreted as a shape fabric with a statistically preferred orientationof all the minerals within the sediment. Also very common is theexisence of more competent clasts behaving like the porphyroclastsin a mylonitic rock, the foliation surrounding these clasts andproviding the sense of the shear. Again at microscopic scale, it can beseen that the deformation is brittle–ductile, because fracture planeswith orientations that can be associated with the C′ or R planes areobserved.

5. Discussion and conclusions

Chaotic deposits have previously been interpreted as havingbeen caused by a synsedimentary fault outcrop at the Peñalba cliffs,created by lateral incision by the River Duero. Here we consider thatthey are linked to a landslide that was triggered probably during thedenudation processes of the Duero basin. This result was obtainedon the basis of the geomorphologic features of the slope, the topo-graphic data, the geotechnical properties of the materials involvedand the deformation structures observed on the landslide slip zone atmacroscopic, mesoscopic and microscopic scales.

The location and description of fossil landslides are difficult owingto important hindrances, such as the lack of geomorphologic featuresindicating their existence (Mather et al., 2003). Nevertheless, atPeñalba the landslide slip surface and the deformation structuresrelated to it have been exceptionally well preserved.

The preserved structures of the landslide are similar to thosefrequently described in tectonic process developed on shear zones.In this sense, Larue and Hudleston (1987) described the structuresin “the Portuguese Bend landslide (California, USA)” (foliations,stretching fabrics, etc.), considering that they are undistinguishablefrom those of tectonic origin except for their metamorphic grade.Likewise, Knight (2005) described the development of small-scalethrust structures that formed in front of the landslide toe on theQuaternary glacial till coast of West Runton, north Norfolk (England),indicating that they also bore a morphological resemblance to thrustsheets found at different scales within accretion prisms (Maltman et al.,1993; Maltman 1998). Thus, the similarities found in the structures ofdifferent geodynamic environments with different pressure andtemperature conditions suggest that the kinematicmechanism involvedwould have been similar. This confirms that it is useful to applystructural geology techniques when studying processes in shallowlandslides and, conversely, that it might be useful to use geotechnicaltechniques and methodologies to study certain tectonic processes.

In the present work, we adopted a structural geological–kinematicapproach to propose a kinematic model for roto-translational land-slides according to the strain orientations on each of the areas thatcanbedistinguished on such slides. In thismodel, the translational zoneof the landslide has been divided into three subzones accordingto the finite strain ellipsoid occurring in each: Subzone 1, simple shear+extension; Subzone 2, simple shear+translation; Subzone 3, simple

Fig. 10. (A) Hand sample photograph from the basal slip surface area. Friction scratches andsurrounding a marl fragment, generating a σ-object typical of ductile shear zones. (C) Higgenerated: C planes parallel to the shear zone limits, and S planes oblique to the shear zoneplanes, C′ or R planes have been generated, indicating an extensional component of the finipatterns of all the minerals of the sediment generate the main foliation in the matrix. Thisgenerates the C′ or R extensional planes.

shear+contraction. The structures have different orientations, depend-ing on the subzone where they are located, and are conditioned by thedifferent kinematic processes involved. The observation of microstruc-tures developed around the slide surface suggests that the responseof the rocks has changed along the process of destabilization(brittle, ductile or ductile–brittle), and its disposition as referred tothe boundaries of the slide area provides information about the sub-zone where the observations were carried out.

S–C type fabrics are the most frequent deformation structures atthe Peñalba slip surface. These structures have frequently beenobtained in laboratory experiments reproducing the shear zonesgenerated when different kinds of soils are placed under stressconditions. S–C type fabric generation in rocks is due to the occurrenceof a progressive simple shear (Twiss and Mores, 1992; Passchier andTrouw, 1996), and hence it may be concluded that this has possiblybeen the main mechanism responsible for the structures currentlyseen at the Peñalba landslide.

From the structural geology point of view, C′ planes reflect anextensional component in the finite strain ellipsoid, onwhich they areknown as an extensional crenulation cleavage (Platt and Vissers,1980).The similarities between this type of tectonic foliation and the C′planes detected at the Peñalba landslide suggest that the simpleshear causing the deformation would have developed under exten-sional conditions. Accordingly, the present-day outcrop correspondsto the root zone of the landslide or to its translational zone (Subzone1: simple shear+extension). In its end zone, the shear would havebeen under compressive conditions as a consequence of the materialaccumulating at the foot of the landslide (Subzone 3: simple shear+contraction).

The model proposed may be useful to locate zones where obser-vations are to be carried out, considering the deformation structurespreserved around the slip zone. On potential or present-day land-slides, the strain on each of the slide areas will determine thekinematic process and also the probabilities of reactivation, since ifthe tension cracks are vertical (Subzone 1) they will prompt thereactivation, whereas if their position is subhorizontal (Subzone 3)they will arrest the landslide. When considering recent or potentialslides, such informationwill be a useful tool for delineating the areasfor which stabilization and corrective measures will be mostefficient.

In conclusion, the Peñalba slope is an outcrop of a landslidesubhorizontal slip surface in which the deformation structurescreated during themovement have been exceptionally well preserved.Study of the slip surface geometry and the deformation structuresdeveloped proved to be useful to settle the kinematics of this typeof roto-translational landslide. The structures found have proved tobe a useful tool for establishing the observation subzone within thetranslational zone of the landslide and for comparing them withthose predicted in a theoretical model for roto-translational landslidesdiscussed here.

Acknowledgements

Support of this work was provided by the regional governmentof “Castilla y León” through the project SA013A08. Comments andsuggestions by Dr. G. Gutiérrez-Alonso have improved the originalmanuscript. We thank Dr. G. Crosta and two anonymous reviewers forconstructive comments.

asymmetrical marks are shown (1 and 2). (B) Hand sample photograph of clayey matrixh-deformation area outcrop at slip surface in which two oblique foliations have beenlimits. (D) Hand sample from a high-deformation area in which in addition to S and Cte strain ellipsoid. (E) Crossed nicols microphotograph on an XZ plane. The orientationfoliation surrounds a more competent fragment. The progression of the deformation

54 M. Yenes et al. / Engineering Geology 104 (2009) 41–54

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