Lithological and morpho-structural control on landslide distribution in the Daunia Apennines, Southern Italy.

Lithological and morpho-tectonic settings are among the most important influencing factors in the development of landslide phenomena in terms of size, spatial distribution, and pattern, especially in tectonically active sectors.

In this work, a 1,460 km² wide portion of SE Apennines, the Daunia Mountains (Apulia region), has been investigated to produce a new geomorphological (historical) landslide inventory map. This area is characterised by low gradients and clay-rich flysch formations (late Cretaceous-Miocene) that have been deformed by contractional tectonics. Daunia Apennines are notoriously prone to landslides, and this new geomorphological historical landslide inventory map reported the presence of 17,437 landslides, with an average density of about 15.6 landslides per square kilometre, excluding lowlands plain. A preliminary analysis conducted for the entire area showed the main relationships between landslides and the different tectonic units. Here, a downscaling investigation is carried out, focusing on an area of approximately 370 km², where more detailed 1:50,000 geological data are available (Carta Geologica d’Italia, CARG project).

Investigation of landslide size, type and spatial distribution within the different lithologies, shows that smaller landslides tend to develop within the siliciclastic and turbiditic sedimentary succession belonging to the San Bartolomeo Formation. On the other hand, ancient landslides with larger and heterogeneous dimensions, develop in the flysch lithologies made up of reddish thin-bedded clays and silts, interbedded with calcarenites and calcilutites layers, belonging to the Flysch Rosso and Flysch di Faeto Formations. These formations constitute most of the external ridges of the Daunia Apennine, forming the Daunia tectonic unit, which is strongly affected by the Apennine frontal thrusts system.

Similarly to what was observed in other geological settings of the Italian territory, the spatial distribution of landslides appears to be linked to the main morpho-structural lineaments of the region, and especially the spatial pattern of the largest landslides seems both passively and actively controlled by tectonic forcing, which has determined lines of weakness along the slopes. Additionally, the presence of the Apennine frontal thrust also caused topographic growth with increased local relative relief that favoured the occurrence of large landslides.

Building on such analyses, the unprecedented detail of the new geomorphological landslide inventory map, which reports a relative age estimation of landslides, will also help defining a possible landscape evolution pattern starting from evidences of the oldest slope failures that were recognized. Future work will add absolute dating constraints to such evolution pattern hypotheses which will help understand and compare past trend of landslide occurrence to the present day morpho-climatic setting.