Tectonic thrust as conditioning element for large landslides at the front of the chain

The areas located at the transition from the geodynamic domains of the chain into the foredeep one constitute an interesting macro-region, which is characterized by the effects related to the pushing and underthrust of the chain on the foredeep deposits. The orogenesis produced a severe deformation of the internal boundary foredeep deposits as consequence of the thrusting of allochthonous nappes under these deposits (Fig. 1). In Italy, the aforementioned boundary region exists on the east side of the Apennine, where there are discontinuities caused by tectonic phenomena related to the powerful horizontal thrust of the Apennine nappe on the foredeep deposits (Patacca & Scandone, 2007). From a geological point of view, this region is characterized by Plio-Pleistocenic marine blue-grey silty-clay deposits filling the foredeep system of the Apennines outcrops, pushed and under-trusted by the action of the tectonic uplift (Patacca & Scandone, 2007) (Fig. 1). Therefore, tectonic thrusting and deformations strongly influence geomorphology of such areas and can constitute an important driving factor both for landslides involving the deformed foredeep deposits along the buried front of the chain, and also for river (Guerricchio, 1990; Galeandro at al., 2012; Guerricchio et al., 2010). For this reason, the landslides at the front of the Apennine may be related to tectonical uplift effects, which may cause progressive collapses, or they already caused huge landslides (Fig. 2). Two interesting examples of these tectonic interactions are in low Biferno valley and in Fortore valley (Fig. 2). The earlier is on the right side of lower Biferno Valley downstream Ponte Liscione Dam, where river Biferno passes from Southern Apennine to Adriatic foredeep, characterized in the order by allochthonous nappes and plio-pleistocenic deposits. The geomorphological features of the area are strongly conditioned by tectonic uplifts related to the Apennine orogenesis that generated a huge landslide with a front of about 10 km and a maximum deep of the involved material of about 200 m or more involving an area of about 40 km2 on the right side of Biferno valley, (Galeandro et al., 2012; Guerricchio et al. 2010). A fairly similar scenario was also identified in mid Fortore valley, which is located about 50 km south than Biferno. Both the scenarios share the asymmetric path of the rivers, with respect to their valleys, the presence of meanders and the particular bow-shape of their tributaries. It is noteworthy that both the mentioned scenarios are located on the buried front of the Apennine chain. It is also interesting to observe that there are several large landslides along the Adriatic coast, between Pescara and Gargano headland. The sites of the main landslides like Fossacesia, Torino di Sangro, Casalbordino and Petacciato (Guerricchio et al., 1996; Doglioni et al., 2011) are located on the buried front of Apennine (Fig. 2), whereas the buried front of the chain is particularly advanced. Therefore the stress related to the under-thrusting of advanced Apennine buried nappe can be considered one of the factors contributing to their activation.