Etienne Cossart

An early Holocene age for the Vatn landslide (Skagafjörður, central northern Iceland): Insights into the role of postglacial landsliding on slope development

Recent research in northern Iceland has highlighted a significant period of rock slope instability during the early Holocene due to the combined effects of postglacial rebound, relative sea-level fall, and glacially oversteepened mountain slopes. Using the Vatn landslide (Skagafjörður, central northern Iceland) as an example, this paper focuses on this period and describes the sequence of events that led to landsliding. Geomorphic mapping, stratigraphical evidence, and both radiocarbon and tephra dating were applied. Collectively, the data acquired indicate that the landslide occurred between 11,400 and 10,790 cal. yr BP. However, while rock slope failure represents a significant disintegration of mountain slopes, this study suggests that large postglacial landslides might also play a role in arresting sediment transport from other hillslope processes rather than contributing large volumes of sediment.

Denudation rates during a postglacial sequence in Northern Iceland: example of Laxárdalur valley in the Skagafjörður area

ABSTRACT For several decades, geomorphologists have focused on the functioning of geomorphic systems after deglaciation. The relative importance of paraglacial vs. periglacial processes has been highly debated. At present, the development of dating techniques allows to contribute to this debate. We reconstruct in this paper the geomorphic evolution of Tindastóll mountain slopes in Laxárdalur valley (Skagafjörður area, central northern Iceland), where a chronological framework can be established through tephrochronology and an assemblage of dated raised beaches. Volumetric calculations of constructed and excavated landforms were created from field data and from DEM and geographical information system techniques. Collectively, our data exhibit a first stage of paraglacial landsliding during the first half of the Holocene, followed by a stage of scree and rockglacier development (during the second half of the Holocene, but before 1100u2005AD). Our estimations indicate that more than 85% of the total sediment production were due to rock slope failure, and the rate of bedrock denudation due to periglacial processes was about one half of the rate of paraglacial processes. Nevertheless, paraglacial and periglacial processes cannot be seen here as antagonistic processes: they are organized in a sequence during which periglacial processes are conditioned (enhancement of bedrock denudation rates) by fracturing and consequent mass wasting. Screes and concomitant rockglaciers were indeed preconditioned by the landslide, while areas non-affected by landslides have remained mostly intact, characterized by a very low rate of accumulation due to geomorphic (periglacial) activity.

Éditorial. Questions de géographie physique dans les montagnes tropicales : enjeux environnementaux (Topics of physical geography in tropical mountains : environmental issues)

Meme s il est difficile de definir les caracteristiques des montagnes tropicales du globe, quelques traits communs peuvent etre degages. L instabilite des versants, la sensibilite aux changements climatiques contemporains, la presence possible de fortes densites de population apparaissent comme quelques uns de ces traits, qui renforcent linteret porte par le geographe physicien sur ces montagnes.

Aléas naturels et menaces sur les axes de communication en Himalaya du Népal : la vallée de la moyenne Kali Gandaki (Natural risks and threats on circulation axes in the Nepalase Himalayas : the middle Kali Gandaki valley)

Affectes pendant la mousson par de nombreux glissements de terrain, les fonds de vallees himalayennes subissent de lourdes pertes en vies humaines et en degâts materiels. Le long de la moyenne Kali Gandaki (Nepal central), nous examinons les aleas et les risques (erosion de berges, ennoyage, ensevelissement) qui menacent la nouvelle route et les villages. La proximite de massifs englaces presente un risque supplementaire. Loin detre la cause premiere des instabilites observees, la route apparait comme un enjeu vulnerable, dont la destruction peut hypothequer le desenclavement de ce pays, lun des plus pauvres du globe, et ainsi peser sur son developpement.