Paraglacial slope failures in the Aran valley (Central Pyrenees)

Abstract

Slope failures are widespread phenomena in mid-latitude mountain environments that were glaciated during the Last Glacial Cycle. This is the case of the Aran valley, in the Upper Garonne catchment, Central Pyrenees, that included glaciers several hundred meters thick. Following postglacial warming and ice thinning, the recently deglaciated slopes were subject to intense stress readjustments - the so-called paraglacial dynamics. We have identified up to 135 major slope failures in the Aran valley, with only 10 units occurring outside the glaciated domain of the maximum ice extent of the Last Glacial. The presence of polished bedrock surfaces, till and moraine ridges next to some of these features evidence a close connection between glacial and slope processes. We have detected different types of slope failures affecting both bedrock (12 large catastrophic rock slope failures, 16 rock-slope deformation, 34 rockfalls, and 49 rockslides) and unconsolidated glacial sediments (14 slope readjustments on drift-mantled slopes). The average altitude of rock slope failures oscillates between 1551 and 1991 m, with a mean length ranging from 147 to 905 m and a width between 247 and 513 m. The affected surface is also highly variable, oscillating between 0.02 and 126.2 ha. Slope failures occur in different lithological settings, but they are most frequent in slate, lutite and limestone bedrocks. We conclude that most of the failures show a paraglacial origin, though other factors (i.e. lithology and topography) promoted slope instability.

Introduction

Landscape transition from glacial to non-glacial periods results in major changes in the geomorphological setting of mountain areas and polar regions (Serrano et al., 2018; Ruiz-Fernández et al., 2019). This shift implies a geomorphic readjustment to the new environment, which is know as the paraglacial phase (Slaymaker, 2011; Oliva et al., 2019a). The intensity of the paraglacial response decreases with time and affects deglaciated areas at timescales ranging from years to several millennia (Church and Ryder, 1972; Ballantyne, 2002). The timescale of paraglacial dynamics is debatable, though it is generally accepted that, at least, as long as glacial material is available, deglaciated areas are in the paraglacial phase (Ballantyne, 2002). Following glacial retreat, paraglacial processes are more intense on slopes from recently deglaciated cirques and valleys (Ballantyne, 2002). In these newly exposed ice-free environments, slope debuttressing triggers very active slope processes reworking both glacial sediments and affecting the stability of bedrock outcrops.

Slope response to paraglacial adjustment encompasses distinct geomorphological processes. Rock slope failures (RSF) are one of the main drivers in dismantling recently deglaciated areas and mobilizes a large amount of sediments from the high areas to the valley floors (Feuillet and Mercier, 2012; Cossart et al., 2013). This process is enhanced by the presence of permafrost increasing the activity of rock slope failures, particularly during rapid warming periods when permafrost degrades (Krautblatter et al., 2012; Etzelmüller, 2013). RSF is a general term that includes several types of displacements in bedrock, such as large catastrophic rock slope failures (large rockslides and rock avalanches), paraglacial rock-slope deformation (deep-seated gravitational slope deformations) and rapid rockfalls (Ballantyne, 2002). Rockslides can be also considered as RSF events (Jarman, 2006). Paraglacial dynamics does not only affect the bedrock, but also reworks drift-mantled slopes that can be efficiently eroded by postglacial processes (Ballantyne, 2002).

Since the Maximum Ice Extent of the Last Glacial Cycle (MIE), mid-latitude mountains such as the Pyrenees have been affected by a long-term deglaciation process and small periods of glacial readvance (Oliva et al., 2019b) that have resulted in a variety of paraglacial phenomena. This study focuses on the mapping and spatial characterization of paraglacial slope processes that reshaped the landscape of the Upper Garonne catchment since the MIE.

To date, in Iberian mountain ranges, few works have examined the spatial distribution and geographical characteristics of paraglacial adjustments following MIE glacial retreat. In the Sierra Nevada, the deglaciation of cirques promoted the development of rock glaciers during the paraglacial phase at the end of the Younger Dryas and the onset of the Holocene (Oliva et al., 2016; Palacios et al., 2016). Currently, at ca. 3000–3100 m, paraglacial dynamics is very intense in this massif, occurring in cirques that were glaciated during the Little Ice Age (LIA), with a variety of processes and landforms (rockfalls, mudflows, debris flows, development of rock glaciers and protalus lobes, etc.) subject to permafrost degradation (Gómez-Ortiz et al., 2014). In the Iberian Central Range, Last Glacial Maximum (LGM) moraines present in steep slopes are currently being eroded by water erosion, landslides and debris flows suggesting that these areas are still in the paraglacial readjustment phase (Palacios et al., 2011, 2012; Campos et al., 2018). In the Cantabrian Mountains, paraglacial dynamics following the LGM was examined through the analysis of several deposits in deglaciated slopes and glacial cirques (alluvial, drift-mantled slopes, landslides, etc.), and results revealed an intense landslide activity occurring after the glacial retreat prior to 16.1 ka (Alonso and Corte, 1992; Rodríguez-Rodríguez et al., 2018; Santos-González et al., 2018). Evidence of contemporary paraglacial dynamics has been also described in the Cantabrian Mountains, namely in the cirque bottoms that were glaciated during the LIA at elevations of 2100 m, where a wide range of processes are observed, such as pattern ground, ice mounds, debris flows, etc. (González-Trueba, 2007; Ruiz-Fernández, 2015; Serrano et al., 2018).

In the Pyrenees, glaciers during the MIE reached the piedmont in the northern valleys (Goron, 1941; Taillefer, 1953; Stange et al., 2014; Fernandes et al., 2017) but stayed within the valleys in its southern slope (Calvet, 2004; Delmas, 2015). The long-term deglaciation following the LGM was interrupted by several glacial advances (García-Ruiz et al., 2003, 2013, 2015; González-Sampériz et al., 2006; Delmas, 2015; Palacios et al., 2015b; Oliva et al., 2019b). Therefore, the spatial patterns of paraglacial adjustment are constrained by the different glacial phases. Following the deglaciation, a wide range of rock failures occurred on slopes of the Eastern Pyrenees and were categorized as catastrophic, rockslides or rock-slope deformation (Jarman et al., 2014). Glacial retreat favoured rockslope deformation during the Late Glacial and Holocene (Gutiérrez et al., 2005; McCalpin and Corominas, 2019). Catastrophic failures also occurred following the deglaciation, and some sites still record small surface displacements (Corominas et al., 2015). In addition, the distribution of some slope deposits in the Pyrenees such as debris accumulations, drift-mantled slopes and alluvial fans were also associated with paraglacial dynamics (García-Ruiz et al., 1988, 2007; Palacios et al., 2015; Fernandes et al., 2017). Rock glaciers are very abundant permafrost-related features inside the cirques in the Pyrenees, and their origin has been associated with a rapid deglaciation, having thus a paraglacial origin (Fernandes et al., 2018; Palacios et al., 2015a, 2015b; Serrano et al., 2011). Paraglacial dynamics is currently active in the Central Pyrenees approximately above 2700 m (González-García, 2014).

Taking into account the knowledge gap on the relationship of major slope processes and the paraglacial readjustment in the Central Pyrenees, this paper aims to better understand the geomorphological evidence and the patterns of paraglacial response in the Upper Garonne valley, namely the Aran valley. To do so, we target at three specific goals:

• Map the diversity of the major slope failures.

• Examine the spatial distribution of those failures and its relationship with the formerly glaciated area.

• Discuss the variety of paraglacial slope phenomena with its controlling factors of the mid-latitude mountains ranges.