Landslide activity in response to alpine deglaciation: the case of the Belluno Prealps (Italy)

In the Late Pleistocene, when the Piave glacier retreated from the end moraine system areas towards the Dolomitic region, several large landslides took place in the Belluno Prealps. The chronology of such landslides is mainly based on spatial relationships between mass movements and glacial or fluvial features, but poorly based on radiometric dating. The aim of this study is improving the existing data set on landslide chronology to clarify the relationship between deglaciation and landslides, that is the sensitivity of an alpine environment to climatic changes. The research is based on different types of data (geomorphological field survey, geophysical investigations, drillings, radiometric dating and pollen analysis) and focused on four large gravitational phenomena (Fadalto, Madonna del Parè, Masiere di Vedana and Marziai landslides). Different strategies were adopted in order to date mass movements, according to the environmental conditions in which they occurred. In some cases landslide debris reached a valley bottom free of ice and dammed the valley, whereas in others mass movement took place during deglaciation. Therefore investigations were addressed to define the age of landslide deposits, of lacustrine sediments upstream of accumulation zones and of glacial sediments of specific deglaciation phases. Obtaining chronological information through radiocarbon method turned out a difficult task, mainly for the scarcity of organic matter suitable for dating. Also OSL method did not give satisfactory results. Notwithstanding such dating problems, the use of different data and evidence (geomorphological, stratigraphical, geophysical, and palynological) has allowed a quite accurate definition of landslide chronology. For instance, according to pollen types and concentration it is possible to establish that the landslides occurred during the first phases of deglaciation. The main conclusions are: (a) all the examined mass movements took place between 17,000 and 15,000 years BP; (b) there is a clear relationship between landslides and the climatic changes that occurred during the last glacialinterglacial transition; (c) reaction time of slopes to glacier retreat was relatively short. Finally, we do not exclude the hypothesis that tectonics could have played a role as for slope instability during a period of glacial unloading and lithospheric rebound.