applied sciences Article Retrospective Modeling of a Large Paleo-Landslide Related to Deglaciation in the Sierra de Urbión, Cordillera Ibérica, Spain Pablo Sanz de Ojeda 1, Eugenio Sanz Pérez 1, Rubén Galindo 1,* and Cesar Sanz Riaguas 2 1 Departamento de Ingeniería y Morfología del Terreno, Escuela Técnica Superior de Ingenieros de Caminos, Canales y Puertos, Universidad Politécnica de Madrid, C/Profesor Aranguren s/n, 28040 Madrid, Spain; [email protected] (P.S.d.O.); [email protected] (E.S.P.) 2 Desarrollos Logísticos y Fomento de Suelo S.L, (DELFOS), C/Narváez, 15, 28009 Madrid, Spain; [email protected] * Correspondence: [email protected] Abstract: Through a study of glacial geomorphology and retrospective modeling of the stability of the slopes, it has been possible to reconstruct and know the mechanism of the formation of a large landslide induced by the retreat of the glacier corresponding to the Picos de Urbión (Coordillera Ibérica, Spain) during the last glacial cycle. It is a rotational landslide of 150 Mm3 that involved a layer of lutites and clays of the Cameros Basin that outcropped on one of the slopes of the valley, and whose initial geometry was modified by the over-excavation of the glacier tongue, which reached 140 m in height. The breakage occurred when the support of the ice tongue was partially removed. The structural layout and high water table also contributed to the landslide. It is the first time that landslides associated with the deglaciations of the last glacial cycle have been retrospectively modeled, which may be of interest when applied to geomorphological sciences. Citation: Sanz de Ojeda, P.; Sanz Pérez, E.; Galindo, R.; Sanz Riaguas, Keywords: paleolandslide; deglaciation; large and rotational landslide; numerical simulation; C. Retrospective Modeling of a Large Iberian range Paleo-Landslide Related to Deglaciation in the Sierra de Urbión, Cordillera Ibérica, Spain. Appl. Sci. 2021, 11, 4277. https://doi.org/ 1. Introduction and Objectives 10.3390/app11094277 Slope movements in mountains affected by glacierism can be strongly influenced by the advance and, especially, by the retreat of glaciers. The advances of the ice through the Academic Editor: José A. Peláez valleys excavate and cause a decompression in the rock mass. However, when the glacier retreats, the slope is left without ice support, the valley is deeper, the excavated slopes Received: 15 April 2021 steeper, and decompression continues. Accepted: 5 May 2021 Steep slopes and greater than 500 m can undergo slow and continuous deformation, Published: 9 May 2021 characterized by its bulging and the appearance of lateral escarpments parallel to the level lines (“sackung”). It is a gravitational spreading or gravitational slope deformation [1–3]. Publisher’s Note: MDPI stays neutral Their formation is usually previous to the generation of large landslides [4–6]. If these with regard to jurisdictional claims in published maps and institutional affil- landslides originated natural dams and lakes upstream, the dating of these lake sediments iations. can be used to assess the risk of flooding due to the collapse of these dams [7], or for paleoclimatic investigations and geomorphological evolution studies, such as improving the information on deglaciation in mountain areas (for example, in the Pyrenees [8]). The formation of large deep landslides due to the retreat of valley glaciers after the last glacial cycle has been described in many mountainous areas, such as the Alps, Himalaya, Copyright: © 2021 by the authors. Rocky Mountains, Andes, New Zealand, etc. [9–13]. Additionally, in mountains recently Licensee MDPI, Basel, Switzerland. abandoned by glaciers, where the retreat and thinning of glaciers have been experienced This article is an open access article worldwide in recent decades as a result of global warming, it has directly affected the distributed under the terms and conditions of the Creative Commons stability of the slopes (for example, in Patagonia [14]). Attribution (CC BY) license (https:// In Spain, they are described in the Cantabrian Mountains in [15], for example or in creativecommons.org/licenses/by/ the Pyrenees [16–22]. Although in the Iberian Range the Demanda, Neila, Cebollera, and 4.0/). Moncayo mountain ranges were affected by Pleistocene glacierism, most of the time they Appl. Sci. 2021, 11, 4277. https://doi.org/10.3390/app11094277 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 4277 2 of 20 were glacier cirques with tongue of little erosive capacity and incapable of generating these phenomena. There is only one antecedent to the case presented here, where a notable example of a landslide associated with the retreat of the 5 km glacier on the north face of the Picos de Urbión is briefly described [23]. On the other hand, and as is known, the identification, recognition, inventory, and detailed study of these movements can inform us about the failure mechanisms and the causes that generated them. A good characterization of a landslide includes an adequate investigation of the internal architecture and hydrogeology, and in the works [24–26], we can find good examples of case studies of large landslides and of the different methodolo- gies followed. When retrospective modeling is done, the initial topography before the landslide has to be reconstituted, and in this sense, the three-dimensional reconstruction of the pre- landslide topography has been carried out by several authors using different methodologies and applying different criteria [27,28]. The objectives pursued in this work are the following: (1) Characterize this great landslide from the geological and geomorphological point of view; (2) Reconstruction of the original slope and, after geomechanical characterization of the geological materials and hydrogeological conditions, analyze the failure using an appropriate numerical model in order to identify the factors and causes that have controlled the development of the landslide (back analysis); (3) Integrate the process of landslide within the context of paleo- evolution and deglaciation of the glacier of the Picos de Urbión. 2. Site Description 2.1. Study Area The Sierra de Urbión is part of the northwestern sector of the Iberian range. It is one of the highest mountain ranges in the Iberian System, in northern Spain, reaching its maximum altitude at 2228 m above sea level (Figure1). This mountain range serves as a watershed between the Duero river basin and the Ebro river basin. The north slope includes, among others, the headwaters of the Urbión river, a tributary of the Najerilla river, and which is a tributary of the Ebro, while in the southern slope has its source the river Duero. The main watershed runs from west to east and is approximately 25 km long, the highest elevation being the Picos de Urbión, although there are other holm oak peaks of 2000 m, such as Zorraquin (2105 m) or Muñalba (2073 m). To the east it continues with the Sierra de Cebollera, and to the west by the Sierra de Neila, both with peaks a little over 2000 m. The climate is Mediterranean and mountainous, with wet tendencies. Average annual rainfall is probably above 1500–1600 mm in the main watershed, falling mainly in Spring and Winter. According to Camarero and Gutiérrez [29], the minimum of precipitation is in Summer (from July and August), and the maximum from November to February. The Atlantic influence is more noticeable on the northern slope than on the southern one. In the summits a certain aridity is observed in July and August since the precipitation is less than 40 mm. The Urbión mountain range was affected by Pleistocene glacierism, which in the valley of the north face studied here left U-shaped valley up to 5 km long (Figure 2), descending the glacier to 1270 m.a.s.l. [30]. The studies on quaternary glacierism began at the beginning of the 20th century with the work of Carandell and Gómez de Llarena [31], and which continued Thornes [32], and Sanz Pérez [33]. The identification of different glacial stages was possible in the Neila and Urbión mountain range thanks to sedimentological studies in Laguna Grande and Laguna de Hornillo, respectively [34,35]. Another palynological study in the Urbión mountain range [36] covers the last 15,000 years. Appl. Sci. 2021, 11, 4277 3 of 20 Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 21 FigureFigure 1. Location 1. Location of theof the Picos Picos de de Urbi Urbiónón andand thethe Urbión Urbión river river valley. valley. Location Location of specific of specific study study area. area. 2.2. Substrate Geology 2.2. Substrate Geology The general architecture of the Urbión mountain range is resolved in a great slope The general architecture of the Urbión mountain range is resolved in a great slope whose structure is determined by the soft and generalized inclination of the stratigraphic whoseseries structure towards the is determined south, with variable by thesoft dips and between generalized 9° and 20°. inclination The front ofof thethe stratigraphicslope is ◦ ◦ seriesoriented towards to the the north, south, which with is where variable the dips oldest between layers emerge. 9 and Thus, 20 . Thegoing front up the of valley the slope is orientedof the toUrbión the north, river, whichfrom where is where the theglacier oldest ended layers up emerge. to the Picos Thus, de going Urbión up in the a valley of thenorth–south Urbión river,direction, from there where are the the following glacier ended types of up grounds to the Picosarranged de from Urbi óoldestn in ato north– southmost direction, modern [37] there (Figure are the2): (25) following Cambrian types quartzites, of grounds on which arranged (24) sandstones, from oldest con- to most modernglomerates, [37] (Figure marls and 2): (25)clays Cambrian of the Triassic quartzites, are supported on which in (24) discordance. sandstones, Above conglomerates, it ap- marlspears and a Jurassic clays of calcareous the Triassic sequence are supported consisting in of discordance.