Timing of paraglacial rock‐slope failures and denudation signatures in the Cantabrian Mountains (North Iberian Peninsula)

Abstract

Glacial erosion of hillslopes and stress changes induced by the transition from glacial to nonglacial conditions exert a strong influence on slope instability and are considered among the scope of paraglacial geomorphology. Failure mechanisms and coupling between paraglacial rock-slope failures (RSFs) and fluvial erosion are difficult to define. Here we show a preliminary spatio-temporal framework of paraglacial RSFs in a small catchment of the central Cantabrian Mountains, the San Isidro valley, with a dense concentration of RSFs. Preliminary radiocarbon dates obtained from two floodplain sequences deposited upstream from RSFs indicate that their sedimentation started as consequence of valley impoundment by RSFs after glacier retreat (after approximately 16.1 ka), consistent with the deglaciation pattern of nearby valleys. RSFs continued during the Holocene. Glacier erosion, debuttressing, and stressrelease conditions played an important role in slope destabilization as preparatory factors in all cases and probably triggered the oldest events. However, the long prefailure endurance (approximately 12 ka) between RSFs points to other factors such as rainfall and fluvial down-cutting of hillslopes as triggers for Holocene events.Postglacial fluvial incision rates of 2.2-2.5 mm a −1 were estimated along gullies carved into bedrock areas nonaffected by RSFs. These values are one order of magnitude higher than previous rates based on other geomorphological proxies (~0.2 mm a −1 ), suggesting accelerated fluvial incision following the last deglaciation. Local RSFs contributed to increase in fluvial incision rates by a factor of three. This study provides a quantitative perspective of postglacial land degradation relevant for understanding postorogenic landscape evolution.

Paraglacial processes in recently deglaciated environments

“…Rodríguez‐Rodríguez et al () examine hillslope failures and consequent valley floor changes in a glacially eroded valley in the Cantabrian Mountains, Spain. They map a very high concentration of large rock slope failures and develop a chronology for these events.…”

“…Several of the articles in this Special Issue examine different aspects of hillslope response or the effects they have on other processes and landforms in alpine locations. Rodríguez-Rodríguez et al (2018) examine hillslope failures and consequent valley floor changes in a glacially eroded valley in the Cantabrian Mountains, Spain. They map a very high concentration of large rock slope failures and develop a chronology for these events.…”

Deglaciation and postglacial evolution of the Cère Valley (Cantal, French Massif Central) based on geomorphological mapping, 36Cl surface exposure dating and glacier modelling

“…the current topography) and the pre‐landslide topography. The pre‐landslide topography was reconstructed following the methodology given in Rodríguez‐Rodríguez et al (2018).…”

Paraglacial Rock-Slope Failure Following Deglaciation in Western Norway

“…They involve substantial fragmentation of rock mass during runout and usually persist in the landscape (Hermanns and Longva 2012). Catastrophic failures during or soon after ice downwastage and retreat have been attributed to thinning and debuttressing of oversteepened glaciated rockwalls, permafrost degradation and isostatic seismic shock, while failure delayed (up to several millennia) after deglaciation (referred to as 'pre-failure endurance' by Ballantyne 2002b) may relate to long-term, progressive stress-release and strength degradation, or transient triggering mechanisms (Evans et al 1989;Sigurdsson and Williams 1991;McColl 2012;Ballantyne and Stone 2013;Steiger et al 2016;Rodríguez-Rodríguez 2018).…”