Landslides, creep slow movements and rockfalls have been widely studied from modern times by using remote sensing, which goes back up to 20 years by using GPS and INSAR and almost 100 years by using aerial photographs. The use of ancient maps and even old paintings from the 19th century allow going back up to200 years. In this work, we introduce the use of lichenometry of wandering blocks to estimate the velocity of a landslide located at El Portalet (Central Pyrenees, Spain), by using these blocks (decametric size and limestone lithology) as time and spatial markers. To do this, we have used lichenometry to estimate the age of the blocks and determinate the runout distance traveled by the blocks detached from the source area, by geomorphologic markers. We have obtained the annual growth rate (GR) of the calcicolous lichen specie Aspicilia calcarea (Hoffm.) from cemeteries, historical buildings and infrastructures for the last 300 yrs. A lineal GR value of 0.45 mm/yr has been estimated. The oldest block was dated in 600 CE ± 40 yr. and the modernone in 1750 CE ±5 yr. The analysis of aerial orthophotos and field work of the study zone reveals two N- and S-oriented chutes in which wandering blocks were located: the North path (NP) and the South path (SP). Theinventory map had been carried out taking in account the wandering blocks disseminated close to 1 km in length from the source area. The lichenometry analysis yields that the block surface velocity varies from 0.3 m/yr to =0.97 m/yr for the NP and 0.2 to 0.88 m/yr for the SP. However, the behavior of both chutes is opposite. Whereas the NP accelerates during warm climate periods and decelerates in the cold ones, on the contrary, the SP velocity decelerated during Medieval Warm Period and accelerated during the Little Ice Age (LIA). Estimations of the block wandering during the Industrial Era are approximated. INSAR and GPS velocity during the last 20 yrs. shows 0.2 m/yr. Clinometers and thermometers in boreholes indicate that during the last 20 years no permafrost was detected. Instead, the rupture plane for the landslide was estimated between 13 and 14 m depth and even 25 m depth for the SP. Differential behavior of both landslides seems to be related with the presence of water, the flux-direction and the presence of natural obstacles at the foot of the landslide.

20th Congress of the International Union for Quaternary Research (INQUA). Dublin, 25th and 31st July 2019, Abstract

Instituto Geológico y Minero de España, España