Study of the fault propagation process in the High Agri Valley area (Southern Apennines)

The area of the High Agri Valley, located in the central part of the Southern Apennines, has been extensively studied in the past, due to the presence of important economic resources and active faults. In particular, attention was focused on the large-scale faults, affecting the allochthonous tectonic units the area, with a direction nearly parallel to the chain axis. Based on that, the previous authors identified two different fault systems located on the opposite sides of the valley. Less attention, however, has been paid to the transversely oriented faults that make up Transverse Tectonic Lines (TTL). The Agri valley is one of the NW-SE elongated basin formed during the extensional phase that, starting from lower Pliocene, affected the Southern Apennines. In the area important structures recorded the brittle and ductile deformation that involves all the tectonic units that make up the Southern Apennines thrust and fold belt. This latter results from the tectonic collision between the African and European plates in the present-day Mediterranean area. These allowed the allochthonous wedge to migrate with NE vergence on the autochthonous Apulian carbonates.

The TTL in the eastern part of the High Agri Valley appear to have similar lengths (segments rarely reach 8 km in length) and are characterized by a NE-SW orientation, nearly parallel to the main thrust vergence direction. There are few transverse fault planes directly visible in the field and most of the faults have been deduced from the displacement of stratigraphic contacts as well as from the observation of satellite images. The maximum vertical displacements in the central part of the major fault segments exceed 1500 m, thus allowing us to consider these structures of considerable importance on the scale of the Southern Apennines.

The throw profiles derive from the analysis of cut-off lines of formational tops displaced from selected faults obtained from a static 3D model. This allows us to hypothesize its growth pattern and kinematics. Most of the throw profiles of LLTs have a characteristic bell-shaped geometry with greater displacement in the central part that gradually decreases at the tips. Moreover, the observation of the hanging-wall and footwall curves of the cut-off lines of the formational tops allow to hypothesize the kinematics of the studied faults.

The analysis of LTTs and throw profiles in fault developed within highly deformed allochthonous Units can be considered as a new approach that can be proposed for further studies in fold and thrust belts. The transverse faults could be interpreted as linkage structures between segments of faults parallel to the chain axis or be confined by the latter which inhibit their lateral propagation. This could also be important in relation to the seismicity of the Southern Apennines as well as in the compartmentalization of aquifers hosting important water resources in the study area.