Geometry and kinematics of a landslide surface in tertiary clays from 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, Spain b Investigación y Control de Calidad, S.A. Parque Tecnológico de Boecillo R-2, 47151, Boecillo, Valladolid, Spain c Departamento de Ingeniería Cartográfica y del Terreno, Facultad de Ciencias, Universidad de Salamanca, Plaza de la Merced s/n, 37008 Salamanca, Spain abstract article info Article history: Received 10 January 2008 Received in revised form 28 July 2008 Accepted 13 August 2008 Available online 2 September 2008 Keywords: Roto-translational landslide Slip surface Deformation structures Tertiary clays Duero Basin Spain The Peñalba roto-translational landslide is one of the large landslides that can be observed in the central area of the Duero Basin (Spain). Its present morphology is a semicircular-shaped depression in which more than 6 m of vertical movement have been measured. The erosion of the banks of the Duero River affecting the slope of the Peñalba Hill has incised vertical cliffs where a slip surface outcrops and can be observed. Currently, the incision level of the Duero River is located 22 m below this surface. The slip surface, on the translational zone within a large roto-translational landslide, developed within the Dueñas Facies in a single bed with 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 exceptionally well 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 similarities suggest that analogous kinematic processes would have taken place in completely different geodynamic environments. In the present work, we have adopted a structural geological–kinematic approach to explain the development 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 little attention in the scientific literature, mainly due to the lack of pre- servation frequently observed in most cases, caused by subsequent modifications of their original morphology, generally associated with erosive processes and hindering recognition in the field. In some cases fossil landslides have been recognized through location of the drainage anomalies that they caused (Mather et al., 2003), but the identification of landslide slip surfaces is very difficult. Recognition of landslide slip surfaces can be important for charac- terizing the kinematic processes that take place during landslide development because on them evidence of the origin of the instability and also its evolution may be preserved. Only when the slip surface or surfaces are preserved is it possible to describe the different structures present on or near them in order to characterize the kinematics, to study the role of the materials involved, and to understand possible reactivation mechanisms. From the point of view of kinematics, landslide slip surfaces can be considered as shear zones where movement-related structures are recorded (Wen and Aydin, 2003). From the structural geology point of view, shear zones are tabular volumes of rocks where deformation is more intense in comparison with the surrounding rocks, which should not have undergone coeval internal strain. They may be subdivided into (1) brittle shear zones, with discontinuity surfaces along which the movement takes place; (2) ductile shear zones, with ductile continuous strain along the shear zone and strain compatibility with the surrounding rocks; and (3) brittle–ductile shear zones, where ductile shear zone features appear accompanying discontinuity elements, such as faults or vein systems (Ramsay and Huber, 1987). Generally, ductile shear zones within the Earth's crust are related to high 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 the materials involved plays a crucial role in shear zone development. On shallow landslides, the main factors governing the deformation pattern developed in the sliding surfaces are not only pressure and the temperature, but also factors that control the rheology, such as the moisture content and the plasticity of the materials. For example, according to Arch et al. (1988) small differences in the moisture content strongly affect the rheology, causing deformation rates that vary by several orders of magnitude. Hence, on shallow landslides with low-plasticity dry material in the sliding zone the deformation will be brittle, whereas on high-plasticity wet material it will be ductile (Larue and Hudleston, 1987). Additionally, the failure mechanisms on shallow landslides may involve pure shear, simple shear, or a combination of both, depending Engineering Geology 104 (2009) 41–54 ⁎ Corresponding author. Tel.: +34 923 294488; fax: +34 923 29 45 02. E-mail address: myo@usal.es (M. Yenes). 0013-7952/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2008.08.008 Contents lists available at ScienceDirect Engineering Geology journal homepage: www.elsevier.com/locate/enggeo